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Reliability of AlInGaP-on-Si Light-Emitting Diodes

Micro-sized light-emitting diodes (μLEDs) are emerg-ing candidates for next-generation microdisplays. To achieve high resolution, it is preferable to integrate red, green, and blue self-emissive LEDs with a Si-com-plementary metal-oxide-semiconductor (CMOS) driv-er within a single die using a monolithic CMOS-com-patible process. Therefore, fabrication of AlInGaP (for red emission) and InGaN (for blue and green emission) LEDs directly on Si substrates is of great interest. We have reported on the reliability of InGaN on Si LEDs in previous years. Similar to InGaN on Si, the mismatch in lattice constant between AlInGaP and Si is large, so it is very challenging to grow high-quality AlInGaP on Si. AlInGaP layers on germanium-on-insulator (GOI) on Si substrates with a threading dislocation density of ~1.2x10-6 cm2 have recently been made using wafer bonding and layer transfer techniques. We have conducted constant current stressing of AlInGaP-on-Si LEDs made using this process by measuring the light intensity over time. Four stages of degradation of the light emission were observed (Figure 1(a)), and the degradation was seen to be non-uniform across the devices (Figure 2). The rate and degree of degradation are seen to be strongly dependent on the stressing current. The initial increase of light emission in stage I is due to the carbonization of organic hydrocarbon residues. These carbonized residues enhance current spreading and therefore increase the light emission. The stage II and III degradation is caused by the oxidation of the top C-doped p-GaAs layer by organic residues. No structural degradation is observed in the multiple quantum well layers. Finally, as the oxidation increases the contact resistance, the applied voltage also increases to keep the stressing current constant, leading to the avalanche breakdown of the contact, which is indicated as stage IV in Figure 1.

Enhancing SiN Waveguide Optical Nonlinearity via Hybrid Gallium Sulfide Integration

Silicon nitride (SiN) has become an increasingly preva-lent material platform for integrated photonic circuits. SiN enables low-loss waveguides, and it is transparent in both visible and near-infrared (0.25 – 4.3 ). In spite of its small nonlinear index, SiN has been a par-ticularly popular platform for chip-based nonlinear photonic applications, such as supercontinuum gener-ation and frequency combs. However, if SiN’s nonlin-ear index could be increased, the intensity threshold for these useful nonlinear processes could be further reduced. The group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te) are a class of layered van der Waals materials with strong second- and third-order optical nonlinearities. Gallium sulfide (GaS) in particular has a bulk bandgap of 2.5 eV, large enough to make multi-photon absorption at telecom wavelengths negligible. In this work, we create hybrid waveguides that benefit from the low-loss processing of SiN and the large non-linear index of the group-III monochalcogenides.Mechanically exfoliated GaS crystals are transferred onto planarized SiN microring resonators. Figure 1a shows an optical microscope image of a uniform GaS flake fully covering a microring resonator and its coupling region. As the simulated optical TE mode profile in Figure 1b shows, due to the large refractive index (2.6) of GaS, the mode from the SiN core is drawn into GaS. To characterize the nonlinear properties of our hybrid waveguide structures, we measure all-optical cross-wavelength modulation in the microring resonator. We measure enhanced all-optical modulation from the GaS up to 10 MHz (limited by equipment) and measure its nonlinear index to be 10 times larger than that of SiN. This work shows the potential for future incorporation of the group-III monochalcogenides in hybrid waveguides for enhanced optical nonlinearities.

Waveguide-Integrated Mid-Infrared Photodetection Using Graphene on a Scalable Chalcogenide Glass Platform

The development of compact and fieldable mid-infra-red (IR) spectroscopy devices represents a critical chal-lenge for distributed sensing with applications from gas leak detection to environmental monitoring. Green-house gases in particular represent an opportunity for IR gas sensing technology, as many of them are rela-tively inert and cannot be detected by chemical means. Recent work has focused on mid-IR photonic integrat-ed circuit (PIC) sensing platforms and waveguide-in-tegrated mid-IR light sources and detectors based on semiconductors such as PbTe, black phosphorus, and tellurene. However, material bandgaps and reliance on SiO2 substrates limit operation to wavelengths λ < 4 μm, whereas the main absorption peaks of the most potent greenhouse gases occur at longer wavelengths. Here we overcome these challenges with a chalcogenide glass-on-CaF2 PIC architecture incorporating split-gate photothermoelectric graphene photodetectors, shown in Figure 1. Figure 2 plots the photovoltage map of our device, with a maximum responsivity of 1.5 V/W. Our design extends operation to λ = 5.2 μm with a Johnson noise-limited noise-equivalent power of 1.1 nW/Hz1/2 with no fall-off in photoresponse up to f3dB = 1 MHz and a predicted 3-dB bandwidth of f3dB > 1GHz. This mid-IR PIC platform readily extends to longer wavelengths and opens the door to applications from distributed gas sensing and portable dual comb spectroscopy to weather-resilient free space optical communications.

Imaging Transparent Objects through Dynamic Scattering Media Using Recurrent Neural Networks

Transparent objects in biological imaging and X-ray im-aging are imaged by solving inverse problems based on their diffraction intensity patterns. However, the scat-tering process induced by their complex interiors com-plicates inverse problems with a severity depending on the statistics of the refractive index gradient and contrast profiles. Recently, static neural networks were used to retrieve original information from the scatter-ing. Here, we propose a novel dynamical machine learn-ing approach to image phase objects through dynamic diffusers. The motivation of this study is to accommo-date the input with spatiotemporal dynamics, such as a temporal recording of time-varying scattering pro-files. This dynamical machine learning architecture is adopted to strengthen and exploit the correlation among adjacent scattering patterns during the train-ing and testing processes. To impart dynamics, we pro-pose a simplified dynamical model as follows. We use the on-axis rotation of a diffuser and utilize multiple speckle measurements from different angles to form a sequence of images for training. Our recurrent neu-ral network (RNN) architecture effectively discards any redundancies and enhances/filters out the static pattern, that is, the quantitative phase information of transparent objects. This method is also applicable to other imaging applications that involve any other spa-tiotemporal dynamics.

Field-Based Design of a Resonant Dielectric Antenna for Coherent Spin-Photon Interfaces

We propose a field-based design for dielectric antennas to interface diamond color centers in dielectric mem-branes with a Gaussian propagating far field. This an-tenna design enables an efficient spin-photon interface with a Purcell factor exceeding 400 and a 93% mode overlap to a 0.4 numerical aperture far-field Gaussian mode. The antenna design with the back reflector is ro-bust to fabrication imperfections, such as variations in the dimensions of the dielectric perturbations and the emitter dipole’s location. The field-based dielectric an-tenna design provides an efficient free-space interface to closely packed arrays of quantum memories for mul-tiplexed quantum repeaters, arrayed quantum sensors, and modular quantum computers.

Strategies for High-Performance Solid-State Photon Upconversion Based on Triplet Exciton Annihilation

Photon upconversion, a non-linear optical process to convert low-energy photons into higher energies, has various applications such as photovoltaics, infrared sensing, and bio-imaging. In particular, upconversion based on triplet exciton annihilation is one of the most promising approaches to achieve high efficiency at low excitation intensity for practical applications. Howev-er, the reported performance in solid-state is limited due to energy back transfer, materials aggregation, and weak optical absorption, which complicates the inte-gration with solid-state applications (Figure 1).Here, we propose strategies to improve the performance in solid-state via device structure engineering. In a green-to-blue upconverter consisting of a bilayer of an absorbing and an upconverting material, we reduce energy back transfer by inserting a blocking layer in between and mitigate aggregation by doping the absorber into a host material. The upconversion efficiency had a 7-fold enhancement, with the excitation intensity reduced by 9 times (Figure 2a). To improve optical absorption, we investigate an infrared- to-visible upconverter and integrate the upconverting layers into a Fabry-Pérot microcavity. At the resonant wavelength, infrared absorption increases 74-fold, and the threshold excitation intensity for upconversion is reduced by two orders of magnitude to a sub-solar flux (Figure 2b). Our work demonstrates the importance of device structure engineering to improve the performance of solid-state photon upconversion and offers a path towards practical applications.

Magnet Field-Switchable Laser via Optical Pumping of Rubrene

Optical imaging of magnetic fields is used in spintron-ics, magnetic resonance imaging, and radiology. Most conventional approaches to magnetic field imaging rely on expensive crystalline materials or garnets, but the cost of these materials makes them poorly suited to high-area imaging. Magnetic sensing applications may benefit from cheaper magnetically active dyes. We demonstrate that the well-studied organic mole-cule rubrene can be used to spatially resolve magnetic fields. Furthermore, we report a 460% enhancement in rubrene brightness under a 0.4-T magnetic field in a first-of-its-kind magnetic field-switchable laser. We attribute the high magnetic sensitivity of rubrene to the magnetic field dependence of singlet fission, a pro-cess whereby one spin-singlet excitation splits into two spin-triplet excitations. These results suggest that rubrene—and other organic molecules that exhibit singlet fission—are promising candidates for low-cost, high-sensitivity magnetic imaging.

Multiplexed Raman Sensors Using Swept-Source Excitation

Spontaneous Raman spectroscopy is routinely used in pharmaceutical production, chemical analysis, and the semiconductor industry for characterization of struc-tural features, strain, and doping. Standard Raman systems require dispersive spectrometers and often specialized cooled charge-coupled device (CCD) detec-tors to compensate for the low signals, making them prohibitively expensive and bulky. In this work we in-troduce and demonstrate a novel Raman system archi-tecture using a swept-source laser excitation, replac-ing the spectrometer. The laser is delivered through optical fibers to custom-made Raman probes, which are designed to be compatible with either single-mode or telecom-standard multimode optical fibers. Each probe delivers the excitation light onto a sample and collects the Raman signal, which is then detected using a narrow optical filter in front of a room-temperature high-gain Si photodiode. With a standard telecom op-tical switch, we can multiplex up to 16 channels and deploy remote probes using an optical fiber network. As an initial proof-of-concept, we present the spectra collected with our probes for both solid polystyrene and liquid urea solutions and further show that the ac-quired spectra have signal-to-noise ratios comparable to that collected with our lab-built bench top Raman system. We believe this new system, in which a single tunable laser can serve a distributed sensor network, significantly reduces the space and cost of current spectrometer-based Raman systems and promotes the use of Raman for online process control.

Hafnia-Filled Photonic Crystal Emitters for Mesoscale Thermophotovoltaic Generators

Thermophotovoltaic (TPV) systems are promising as small-scale, portable generators for power sensors, small robotic platforms, and portable computational and communication equipment. In TPV systems, an emitter at high temperature emits radiation that is then converted to electricity by a low- bandgap photo-voltaic cell. One way to increase both TPV power and efficiency is to use two-dimensional, hafnia-filled-and-capped tantalum photonic crystals (PhCs); they enable spectral tailoring of thermal radiation for a wide range of angles. However, two key features are hard to real-ize simultaneously: a uniformly filled cavity and a thin capping film. Cavity-filling leads to a capping film that is both thick and uneven, so that trying to thin the film removes hafnia from within the cavity. Here, we pres-ent a method to reduce the film roughness and better control the thickness. Improved PhCs can pave the way toward high-performance TPV micro-generators for off-the-grid applications.

High-Performance Non-Mechanically-Tunable Meta-Lens

Optical metasurfaces, i.e., ultra-thin arrays of sub-wave-length antennae, have enabled a new range of photonic devices with unprecedented functionalities in sculpt-ing wavefronts and a substantially reduced form-fac-tor. Recently special interest has been drawn to a class of so-called “active metasurfaces,” whose optical prop-erties can be modulated post-fabrication by non-me-chanical effects. A variety of tuning mechanisms have been harnessed; however, demonstrated meta-optical devices often incur narrow tuning ranges and low op-tical efficiencies. Here, we implemented an active var-ifocal meta-lens based on phase-change materials that offers 1) aberration-free performance across arbitrary optical states; 2) extremely low crosstalk of nearly 30 dB; and 3) considerably enhanced focusing efficiency exceeding 20% in both states with a clear pathway for further improvement. This advancement will further unveil a new cohort of exciting applications of active metasurfaces in imaging, sensing, display, and optical ranging.

GaN µLEDs for Microsystem Optical Communications

Electronic systems smaller than 50 µm are promising for ubiquitous sensing; however, wireless communica-tion with such systems is challenging since radio-fre-quency communication is inefficient at the micron scale. Small length scales motivate the use of optical communications for micro-devices, which must be low-power due to the size constraint on solar cell sur-face area. Here we present an analysis of blue GaN microLEDs (µLEDs) for optical communications with 50 x 50 µm2 sensor microsystems called SynCells. We analyzed µLEDs with sizes from 5 x 10 µm2 up to 150 x 150 µm2, developing a test setup that can detect an LED driven by only 1 nW (Figure 1). We found higher external quantum efficiency (EQE) for larger µLEDs; also, EQE increased with current density up to a peak value, after which we observed an efficiency droop resulting from Auger recombination. GaN µLEDs operating at maxi-mally efficient current density will be able to produce detectable optical signals at sufficiently low power for practical use in SynCells.

Large-area Optical Metasurface Fabrication using Nanostencil Lithography

Optical metasurfaces promise optical components with on-demand control of light and reduced size, weight, and power (SWaP) compared to their bulk counter-parts. However, fabrication of metasurfaces in the opti-cal spectral range often relies on electron beam lithog-raphy due to the high resolution requirements, which makes fabrication scale poorly with the device dimen-sions. Recently, deep ultraviolet (DUV) lithography has been validated as a scalable manufacturing route for optical metasurfaces. However, DUV lithographic fab-rication requires significant capital investment and is also limited to standard materials and processes avail-able in foundries.We are developing nanostencil lithography as an alternative technique for scalable, versatile, and rapid prototyping of metasurface devices. Nanostencils are nano-scale shadow masks, which allow repeated fabrication of a pattern via any anisotropic deposition process once the nanostencil is made. Previous research that used nanostencil lithography required only deposition of very thin layers (100 nm or less) through the nanostencil, while transmissive dielectric metasurfaces require significantly thicker layers, especially as the wavelength of light increases. Previous nanostencils were also limited to 1 mm by 1 mm in size. We improved previous processes for fabricating and using nanostencils, increasing the yield of nanostencil fabrication by not fully etching through the nanostencil membrane before the KOH etch and improving the consistency of metasurface fabrication by developing a resist-based spacer layer. Figure 1 outlines the resulting processes.To show the effectiveness of nanostencil lithography for large-area optical metasurface fabrication, we used 2 mm by 2 mm nanostencils to fabricate 1.5-mm-diameter PbTe metalenses on CaF2. The lenses showed diffraction-limited focusing, with a representative focal spot shown in Figure 2, and focusing efficiencies comparable to efficiencies reported in state-of-the-art large-area dielectric metalenses.

The Effect of O:N Ratio in a HfOxNy Interlayer on Triplet Energy Transfer In Singlet-Fission-Sensitized Silicon

With the climate changing, the Sun is a promising source of renewable energy. However, silicon photovol-taics, the current industry standard, are approaching their theoretical efficiency limit. A proposed method to exceed this limit is to sensitize silicon with a ma-terial that undergoes singlet exciton fission, a carrier multiplication process with the potential to reduce thermalization losses by creating two excited electrons from a single photon. Successful transfer of these two electrons to silicon could result in increased photo-current and improved device efficiency. Recently, our group demonstrated the first proof-of-concept solar cell incorporating tetracene as a singlet fission material to produce additional carriers that are transported to silicon via a thin layer of hafnium oxynitride (HfOxNy). With the aim of improving singlet-fission-sensitized silicon solar cells, our research focuses on understand-ing what properties are necessary for the transport layer between singlet fission materials and silicon and the mechanism for the energy transfer process. In this work, the defect density of a HfOxNy interlayer is varied by changing the oxygen-to-nitrogen ratio, and different interlayer thicknesses are studied to examine the role of defect states on energy transfer from tetracene to silicon. The transfer efficiency is inferred via magnetic field modulation of the silicon photoluminescence. The results form a preliminary basis for unravelling the ex-citon transfer mechanism, which will subsequently be studied using time-resolved spectroscopy. Ultimately, knowledge of interlayer material properties and in-sights into the mechanism of energy transfer to silicon may inform the design of sensitizing layers for silicon and pave the way to commercializing the use of singlet fission to boost silicon photovoltaic efficiencies.

LION: Learning to Invert 3D Objects by Neural Networks

Non-destructive three-dimensional imaging is import-ant to establish the internal structure of a 3D object non-invasively. In our work, objects of interest are in-tegrated circuits (ICs). This operation requires suffi-cient measurements for computational reconstruction, such as measurements from different angles or depths. Meanwhile, if the required acquisition time is too long, the operation may become impractical, not to mention the risk of instabilities or even damage to the samples from X-ray radiation. Reducing the number of angular views and the radiation dosage per view can be used to limit beam exposure, but low-dose data acquisition schemes yield noisy measurements that significantly reduce the quality of the reconstructed image.Our goal in this project is to reduce the acquisition time by a factor of 10 100 through augmenting image reconstruction with machine learning. In the LION approach, we embed the physics of X-ray propagation and interaction with matter into the learning process. This improves both the fidelity and the efficiency of learning. We study two types of information-starved 3D imaging: limited-angle tomography and low-dose tomography. A limited-angle tomography combined with an advanced ptychography technique, achieves high-resolution (10 nm) reconstruction with the advanced technique of recurrent neural network and generative adversarial network (see Figure 1a, b). On the other hand, low-dose tomography suffers from shot noise as the photon budget reduces (<50 photons per ray). Regardless of 20 of reduction in photon budget for 50 photons case, the physics-assisted machine learning still reconstructs ICs with high fidelity (Test PCC: 0.80) (see Figure 1c).

Light Sources and Single-Photon Detectors in Bulk CMOS

Silicon photonics realized in complementary metal-ox-ide semiconductor (CMOS) processes has transformed computing, communications, sensing, and imaging. Here, we demonstrate a chip-to-chip fiber optic link that implements both the light source and detector in bulk CMOS. A high-brightness infrared light emission in forward bias for a silicon p-n junction is implement-ed in an open-foundry CMOS process – 55 BCDLite. Emission intensity of 50 mW/cm2 light at a wavelength of 1020 nm is realized at room temperature by using a deep vertical junction with lightly doped rings.An infrared-enhanced single photon avalanche di-ode (SPAD) was designed in the same process and used to collect the 1020-nm light emission. We find that a de-tector using the p-substrate as part of the p-n junction achieves good detection (6% quantum efficiency for a 20-µm diameter device) even at a wavelength of 1020 nm. This device has a breakdown voltage of -24V and can be operated in Geiger mode to achieve photon de-tection at low light levels. The capabilities of the two devices combine to demonstrate a complete chip-to-chip optical interconnect utilizing only silicon CMOS devices.

Inference of Process Variations in Silicon Photonics from Characterization Measurements

Silicon photonics, which manipulates photons instead of electrons, shows promise for higher data rates, low-er-energy communication and information processing, biomedical sensing, and novel optically based function-ality applications such as wavefront engineering and beam steering of light. In silicon photonics, both elec-trical and optical components can be integrated on the same chip, using a shared silicon integrated circuit (IC) technology base. However, silicon photonics does not yet have mature process, device, and circuit variation models for the existing IC and photonic process steps; this lack presents a key challenge for design in this emerging industry.Our goal is to develop key elements of a robust design for manufacturability (DFM) methodology for silicon photonics. One of the key steps for the goal is to find the distribution map of process variation in the ac-tual fabrication, which is usually inferred from well-de-signed test structure measurements.In this work, we develop a Bayesian-based meth-od to infer the distribution of systematic geometric variations in silicon photonics that aims to reduce the extraction error caused by measurement noise. We ap-ply this method to characterization data from multiple silicon nitride ring resonators with different design parameters and produce the estimated spatial map of device geometric variations (e.g., waveguide width, Si3N4 on SiO2 thickness, partial etch depth), as shown in Figure 1. Our results show that this characterization scheme can serve as a good test structure for process variation inference. Since characterization measure-ments are commonly used for device optimization de-sign, our method provides an efficient alternative ap-proach to study process variation in silicon photonics without requiring separate or replicated test structure design and thus facilitates the design of high-yield sili-con photonic circuits in the future.

Integrated Photonics and Electronics for Chip-Scale Control of Trapped Ions

Trapped atomic ions are promising candidates for quan-tum information processing and quantum sensing. Current state-of-the-art trapped-ion systems require many lasers and electronics to achieve precise timing and control over quantum states. Usually, electronic signals are sent into vacuum chambers via wire feed-throughs, and laser light is focused down to a trapped ion’s location with external lenses mounted outside viewports on the chamber. These requirements lead to dense and complex setups that may be prone to drift and limit the amount of control that can be achieved.We have made recent progress toward integrating control technology into the substrate of the ion trap itself. By using a planar trap design, which is compat-ible with lithographic fabrication, we may implement other well-developed processes in order to enhance the function of the ion trap. In one experiment, we demon-strate an ion trap with integrated, complementary met-al-oxide-semiconductor-based high-voltage sources that can be used to control the motional frequency and position of a trapped ion. In another demonstration, we use photonic waveguides and diffractive grating couplers to route light around a chip and focus it onto ions trapped above the surface.Integrating controls into ion traps has the potential to increase the density of independently controllable ions on a chip in next-generation systems, but there are also many immediate practical benefits. Reducing the number of required feedthroughs allows chambers to be made more compact, which may be useful for ion-based clocks or sensors. We also show that integrated-photonic platforms help to reduce vibration-induced noise seen when using external optics, which may enable portable systems based on trapped-ion quantum information processing.

Lithiation Mechanisms of Si and Ge Thin Film Battery Electrodes

Thin film batteries (TFBs) made using complementa-ry metal-oxide-semiconductor- (CMOS) compatible materials and processes can be integrated with CMOS circuits and energy harvesting and sensing devices to produce low-cost autonomous sensors with small form factors. As part of our research on CMOS-compatible Li-ion TFBs, we are studying Si and Ge films to be used as anode electrodes. While these materials have the highest known charge capacity (8300 mA/cm3 for Si and 7300 mA/cm3 for Ge), they tend to have poor reli-ability (low cyclability) due to mechanical failures as-sociated with large volume changes. The mechanisms through which lithium is stored in these materials are also poorly understood but are known to be related to poor cyclability. We have carried out mechanistic stud-ies of reversible lithium storage in Si and Ge films us-ing both electrochemical and physical characterization techniques.Figure 1 shows current-voltage measurements made during the first three lithiation/delithiation cycles of a 315-nm-thick amorphous Si film (a cyclic voltammogram, CV). The current corresponds to Li being stored in the electrode (lithiation) or removed (delithiation). Peaks in these curves indicate accelerated charging or discharging associated with phase transitions, all of which are between amorphous phases with different stoichiometries (increasingly Li-rich for lithiation). An irreversible transition is seen in the first cycle (peak 1 in Figure 1), and two reversible transitions are seen in all three and all subsequent cycles (peaks 2-2’ and 3-3’). Through new potentiostatic and transmission electron microscopy techniques, we have established that the irreversible transition occurs through propagation of a reaction through the thickness of the film (Figure 2b) and that the reversible transitions occur through an amorphous-to-amorphous nucleation and growth process (Figure 2b), sometimes referred to as a polyamorphous phase transition. In similar experiments on Ge, we have focused on the reversible transition of a Li-rich amorphous phase to a crystalline phase, which also occurs through a nucleation and growth process. These studies have been correlated with the reliability of TFBs.

Gated Nonreciprocal Magnon Transmission from Direction-Dependent Magnetic Damping

An important application of magnetic materials in information technology is to provide nonreciprocity, which allows unidirectional signal transmission. A rep-resentative device is the two-terminal microwave isola-tor. A ferromagnet inside naturally breaks the time-re-versal symmetry and allows microwave transmission only from port 1 to port 2, while signals from port 2 to port 1 are suppressed. Despite wide applications, these conventional nonreciprocal devices suffer from their bulk volume and the difficulty of being integrated into high-density circuits. Nowadays, new mechanisms that can provide passive and directional isolation of signals are being pursued at sub-micrometer scale. Among var-ious proposals, magnons, the quanta of the collective excitation of magnetic moments, show unique poten-tial due to the tunability and the possibility for on-chip integration. So far, nonreciprocal magnon transmission has been achieved only at resonant conditions with gigahertz frequency. It is unclear if nonreciprocity can still be observed for magnons with a broad spectrum up to terahertz frequency.Here we show that using a magnetic gate, one can realize tunable nonreciprocal propagation in spin Hall effect-excited incoherent magnons, whose frequency covers the spectrum from a few gigahertz up to terahertz. We further identify the direction-dependent magnetic damping as the dominant mechanism for the nonreciprocity, which originates from the interlayer dipolar coupling and works both in the ballistic and diffusive regions of magnons. As a natural result of the chiral magnon-magnon coupling, our findings provide a general mechanism for introducing directional magnon transmission and lead to a design of passively gated magnon transistors for applications of information transmission and processing.

Gigahertz Frequency Antiferromagnetic Resonance and Strong Magnon-Magnon Coupling in Layered Crystal CrCl3

Magnon-magnon hybrid systems have recently been realized between two adjacent magnetic layers, with potential applications to hybrid quantum systems and coherent information processing. Realizing mag-non-magnon coupling within a single material requires antiferromagnetic (AFM) or ferrimagnetic materials with magnetic sublattice structures. However, con-ventional AFM resonance lies in terahertz frequencies, which require specialized techniques to probe. In this work, we realize strong magnon-magnon coupling within a single material, CrCl3. CrCl3 is a layered van der Waals AFM material, with parallel intralayer alignment and antiparallel interlayer alignment of magnetic moments (Figure 1). Because of weak anisotropy and interlayer magnetic coupling, we observe both optical and acoustic modes of AFM resonances within the range of typical microwave electronics (<20GHz), in contrast to conventional AFM resonances. By breaking rotational symmetry, we further show that strong magnon-magnon coupling with large tunable gaps can be realized between the two resonant modes (Figure 2). Our results demonstrate strong magnon-magnon coupling within a single material and establish CrCl3 as a convenient platform for studying AFM dynamics in microwave frequencies. Because CrCl3 is a van del Waals material that can be cleaved to produce air-stable monolayer thin films, these results open up the possibility to realize magnon-magnon coupling in magnetic van der Waals heterostructures by symmetry engineering.

Nanoparticle-Enhanced Microsputtered Gold Thin Films for Low-Cost, Agile Manufacturing of Interconnects

Silicon and gold are ubiquitous in the microelectronics industry—silicon as the cornerstone of semiconduc-tor devices, and gold as a material with unmatched electro-optical properties. However, gold films do not adhere well to silicon or silicon dioxide, necessitating the need for an adhesion layer made of a third material. This need increases complexity and cost. Also, rework-ing interconnects via traditional (cleanroom) technolo-gy poses challenges, e.g., thermal budget, vacuum com-patibility. In this project, we explore microplasma sputtering to implement at low-cost interconnects for agile electronics. We have shown that under proper operational conditions, a microplasma sputterer creates at room temperature and atmospheric pressure dense, highly conductive gold films with a fivefold better adhesion than the state of the art, without using an adhesion layer, annealing, or any other pre/post printing steps. If the gold film is sputtered in an atmospheric-pressure microsputterer in the presence of a fast-moving jet of air, gold nanoparticles form. The high collisionality of the atmospheric-pressure gas and high energy of the plasma facilitate nanoparticle formation, while the jet carries the nanoparticles to the substrate. The speed of the jet of air determines the size of the nanoparticles. These nanoparticles then act as an adhesion layer to allow a gold film, made of these nanoparticles and individual atoms, to adhere well to a silicon or silicon dioxide substrate. By rastering the printhead over the desired deposition area, we can interweave large nanoparticles and smaller atoms, creating a dense film (Figure 1). This process allows us to optimize adhesion, density, and conductivity simultaneously. Conductivity of the resultant films is also near-bulk (120% of bulk gold—the highest value reported for a room-temperature additive manufacturing method), allowing for their use in microelectronics.

Large-Scale 2D Perovskite/Transition Metal Dichalcogenide Heterostructure for Photodetector

Monolayer transition metal dichalcogenides (TMDCs) have been attractive nanomaterials for optoelectron-ics due to their extremely high quantum efficiency, but their atomically thin thickness prevents them from ab-sorbing sufficient light for optoelectrical applications.To improve the optoelectrical performance of TMDCs, during the past years, 2D Ruddlesden-Popper perovskites (PVSKs)/TMDC heterostructures have been demonstrated. Thanks to their high absorption coefficient, long diffusion length of charge carrier, sharp exciton emission, and high power conversion efficiency, 2D PVSKs have been used as an absorption layer for TMDCs. However, 2D PVSK/TMDC heterostructures are limited in the micrometer scale, since 2D PVSKs have been fabricated only by the tape-exfoliation method. We reported a layer resolved splitting (LRS) technique to isolate multilayer 2D materials into a monolayer in wafer scale in 2018. To improve the scalability of 2D PVSKs for large-scale application, in this work, we successfully split micrometer-thick 2D PVSKs into nanometer-thick and millimeter-width scale with the LRS technique. We also obtained 90% of photoluminescence quantum yield, which is the world’s best record to the best our knowledge. Then we plan to demonstrate large-scale 2D PVSK/TMDC heterostructures for photodetectors, which only has been previously demonstrated up to micrometer scale.

Grayscale Stencil Lithography

In this work, we demonstrate a new eBeam evaporation method with fixed single stencil shadow mask to gener-ate 2D patterns with spatial thickness variation across wafer-scale substrate. This method outperforms con-ventional approaches like the iterative photo-lithogra-phy-and-lift-off method or grayscale photolithography, due to their limitations of efficiency, material choices, and manufacturing complexity. We applied the method to create a multi-spectral reflective color filter arrays with two layers of variable thickness. It offers a broad-er design space to achieve a wide color spectrum with simple and efficient fabrication procedures. This meth-od shows potential for scaling up and high-resolution patterning, which could be widely applied in manufac-turing for optical imaging, sensing, and computing. The method takes inspiration from the “pin-hole imaging” in optics to generate the convoluted pattern of the material source and the stencil shadow mask, as shown in Figure 1. The ejection of materials in the eBeam pockets is analogous to the “light source,” which passes thorough the pin holes on the shadow masks to finally cast the “image” in terms of deposition thickness. By controlling the deposition dose (T) and the tilting angle (θ) of the substrate, we could create the “point spreading function” (PSF) of the material target passing through the shadow mask, which can create smooth deposition with less than 5-nm surface roughness. As show in Figure 2, we applied this method to create a multi-spectral color filter array and a 2D pattern of the MIT “Dome” to demonstrate the capability of this method for customizable patterning. Higher resolution deposition is also possible by combining the available micro/nano stencil lithography techniques and our grayscale stencil lithography method.

Unraveling the Correlation between Raman and Photoluminescence in Monolayer Molybdenum Disulfide through Machine Learning Models

Two-dimensional (2D) transition metal dichalcogenides (TMDs) with intense and tunable photoluminescence (PL) have opened up new opportunities for optoelec-tronic and photonic applications such as light-emitting diodes, photodetectors, and single-photon emitters. Among the standard characterization tools for 2D ma-terials, Raman spectroscopy stands out as a fast and non-destructive technique capable of probing mate-rials’ crystallinity and perturbations such as doping and strain. However, the correlation between photo-luminescence and Raman spectra in monolayer MoS2 remains elusive due to its highly nonlinear correlation. Here, we systematically explore the correlation be-tween PL signatures and Raman modes through ma-chine learning models. First, we adopt a convolution neural network, DenseNet, to predict PL by spatial Ra-man maps with relatively small pixel dimensions but deep channels. Moreover, we apply a gradient boosted trees model (XGBoost) with the Shapley additive expla-nation (SHAP) to evaluate the impact of individual Ra-man features in PL behavior, which allows us to further link the strain and doping of monolayer MoS2 with its PL behavior. Our analytical method unravels the non-linear correlations of physical or chemical properties for 2D materials and provides the knowledge for tun-ing and synthesizing 2D semiconductors for high-yield photoluminescence.

Additively Manufactured Electrospray Ion Thrusters for Cubesats

Putting satellites in orbit is very expensive: typical rocket launches cost up to hundreds of millions of US dollars, and typical per-kilogram of payload costs are up to tens of thousands of US dollars). Therefore, great interest exists to develop smaller, lighter, and cheaper space satellites with adequate performance. In partic-ular, since the 1990s, research groups across the world have been developing and launching cubesats, i.e., 1-10 Kg, a few L in volume, miniaturized, mission-focused satellites. Multi-material additive manufacturing is of great interest for fabricating cubesats, as it can mono-lithically create complex, multi-functional objects composed of freeform components made of materials matched to performance.Electrospray engines produce thrust by electrohydrodynamically ejecting charged particles from liquid propellant. Electrospray thrusters are an attractive choice for propelling cubesats because their physics favors miniaturization, e.g., their start-up voltage scales with the square root of the emitter diameter. The thrust of an electrospray emitter is very low; thus , electrospray engines have large arrays of emitters to greatly boost the thrust they can deliver.We recently demonstrated the first additively manufactured electrospray engines. Our devices are composed of large arrays of conical emitters coated by a conformal forest of zinc oxide nanowires (ZnONWs) that transport the propellant to the emitter tips (Figure 1). The ZnONWs provide a large hydraulic impedance that regulates and uniformizes the flow across the emitter array, restricting the flow rate per emitter to attain ionic emission. Our devices are also remarkable because, unlikely all the other electrospray ionic liquid engines reported in the literature, they emit only ions using the ionic liquid EMI-BF4 as propellant (Figure 2), which maximizes their specific impulse for a given bias voltage, i.e., they produce more thrust per unit of propellant flow rate. Current work focuses on optimizing device design and fabrication and on developing a multi-electrode stack to control the plume.

Low-Temperature Growth of High Quality MoS2 by Metal-Organic Chemical Vapor Deposition

The large-area and high-quality synthesis of molybde-num disulfide (MoS2) plays an important role in real-izing industrial applications of flexible, wearable, and ultimately scaled devices due to its atomically thin thickness, sizable bandgap, and dangling-bond-free interface. However, currently used synthesis of MoS2 by chemical vapor deposition (CVD) require high tem-perature and a transfer process, which limits its utili-zation in device fabrications. In this work, we achieved the direct synthesis of high-quality monolayer MoS2 by metal-organic chemical vapor deposition (MOCVD) at a low temperature of 320oC by designing the exper-imental setup for better controlling the flow rate of the organic precursors. Large single-crystal monolayer MoS2 with a domain size up to 120 μm can be obtained on SiO2/Si substrate (Figure 1). Owing to the low sub-strate temperature, the MOCVD-grown MoS2 exhibits low impurity doping and nearly unstrained properties on the growth substrate, demonstrating enhanced elec-tronic performance with high electron mobility of 68.3 cm2 V-1s-1 at room temperature. In addition, we propose a model to quantitatively analyze the shape change of the MoS2 flakes grown under different conditions, which provides an insight into the growth mechanism for optimizing growth conditions.

Self-Assembly via Defect-Mediated Metal Nanoisland Nucleation on 2D Materials

Patterning point defects, nanopores, and nanoribbons can enhance (opto-)electronic properties of two-di-mensional (2D) materials. Moreover, metal adatoms and small clusters can nucleate on point defects in 2D materials. This nucleation suggests that defect pattern-ing may be used for templated self-assembly of metal nanoislands on 2D materials, enabling applications in plasmonics and single-photon emission. Focused ion beams (FIBs) are well-suited for patterning 2D materi-als with nanometer precision and can be used for the controlled creation of point defects and sub-10-nm fea-tures. For applications that require control of the loca-tions of metal islands, the optimization of FIB irradia-tion parameters for metal nucleation is crucial. In this work, we study the structural changes that arise from FIB patterning of suspended 2D materials and the influence of patterning on metal nucleation and growth. We calibrate the irradiation parameters to achieve patterning with minimal damage to the 2D material, and the features are characterized by scanning transmission electron microscopy (STEM) (Figure 1a). Using these patterned 2D materials, we study the extent to which the defects, ion species, dose rate, and sample thickness affect the nucleation and growth of metals. Figure 1 shows representative results after the deposition of Au. At high deposition amounts, Au forms small islands around graphene nanopores, indicative of defect-mediated nucleation (Figure 1b). The templating and nucleation control presented here can be generalized to anchor other materials on 2D materials, such as Si and Ge via chemical vapor deposition or other metals via thermal and e-beam evaporation. This strategy opens routes towards the directed self-assembly of semiconducting and metallic nanoislands on 2D materials with optimized charge transfer and strong light-matter interactions.

Strain Control of Nanocatalyst Synthesis

A central theme in renewable energy technologies to-day is designing nanostructured catalysts for desired reactions. Exsolution generates stable and catalytical-ly active metal nanoparticles via phase precipitation out of a host oxide. Unlike traditional nanoparticle infiltration techniques, the nanoparticle catalysts from exsolution are anchored in the parent oxide. This strong metal-oxide interaction makes the exsolved nanoparticles more resistant against particle agglom-eration than the infiltrated ones. While exsolution is an exciting and promising pathway for generating stable oxide supported nanoparticles, rational control over the exsolved particles has yet to be achieved. In par-ticular, controlling the size and density of the exsolved nanoparticles remains a big challenge.In this work, we propose point defect formation in the oxide lattice to be the fundamental knob to control exsolution and demonstrate this approach in epitaxial La0.6Sr0.4FeO3 (LSF) thin films. By combining in-situ surface characterization and ab-initio defect modeling, we show oxygen vacancy and Schottky defects to be the primary points defects formed upon Fe0 exsolution. Lattice strain tunes the formation energy, and thus the abundance of these defects, and alters the amount and size of the resulting exsolution particles. As a result, the tensile strained LSF with a facile formation of these critical point defects results in a higher Fe0 metal concentration, a larger density of nanoparticles, and reduced particle size at its surfaces. These observations highlight the critical role of point defects in controlling the size and density of the exsolved nanoparticles on the perovskite surface. The strain-controlled synthesis of nanocatalysts can benefit a wide range of applications in clean energy conversion and fuels generation such as solid oxide cells (SOCs), chemical looping (CL), and ceramic membrane reactors (CMRs).

Controlled Cracking to Improve Mechanical Stability of RuO2 Thin-Film Li-ion Electrodes

Thin film Li-ion batteries are of interest for low-cost autonomous sensors. We have investigated high-per-formance electrode materials, such as Si and Ge for an-odes and RuO2 for cathodes, that can reversibly store high concentrations of Li. RuO2 is of particular interest as a cathode material because of its ability to revers-ibly store high concentrations of Li without requir-ing high-temperature processing, unlike conventional cathode materials. While high Li capacities are benefi-cial for high energy density, high Li concentrations lead to large volume changes, which can lead to mechanical degradation during battery cycling. In particular, re-moval of Li (delithiation) leads to tensile stresses that can cause cracking and delamination of electrodes, which can severely limit the number of times that bat-teries can be charged and discharged. Motivated by the finding that patterned small patches of Si demonstrated higher mechanical stabil-ity compared to continuous films, patterned arrays of holes with stress-raising corners were fabricated with-in sputtered RuO2 thin films (Figures 1a and b). After lithiation and delithiation, channel cracks form along the directions defined by the hole array (Figure 1c). We found that this method for controlled crack formation led to increased mechanical stability, as no delamina-tion occurred within the patterned area (Figure 2, right side), while severe delamination occurred in the unpat-terned areas (Figure 2, left side). These results may oc-cur because the formation of the controlled crack array dissipates the strain energy that would otherwise drive delamination.It was further discovered that the formation of cracks was reversible. After re-lithiation, the RuO2 patches expanded, and the channel cracks closed again (Figure 1d). The sizes of the channel cracks were con-trolled by the state of charge of the film. In addition to use for mechanical stabilization of thin film electrodes, this process has potential application for creation of channel networks with electrochemically modulated channel sizes, which might be of use in microfluidic de-vices.

Seeing Superlattices: Imaging Moiré Periods at the Nanoisland-2D Material Interface Using Scanning Transmission Electron Microscopy

Opportunities are emerging to combine van der Waals (2D) materials with (3D) metals /semiconductors to explore fundamental charge-transport phenomena at their interfaces and exploit them for devices. Recent advances in scanning transmission electron microsco-py (STEM) allow detailed analysis of atomic structure, properties, and ordering at these interfaces. We use 4D STEM and integrated differential phase contrast (iDPC) to directly image moiré periodicities arising from epitaxial growth of nanoislands on 2D materials in ultra-high vacuum. Our research explores the role of emerging microscopy techniques in unveiling the alignment and ordering of moiré superlattices and the implications of moiré periodicities for the properties of 2D/3D junctions.

Small-Molecule Assemblies Inspired by Kevlar: Aramid Amphiphile Nanoribbons

Small-molecule self-assembly offers a powerful bot-tom-up approach to producing nanostructures with high surface areas, tunable surfaces, and defined in-ternal order. Historically, the dynamic nature of these systems has limited their use to specific cases, especial-ly biomedical applications, in solvated environments. Here, we present a self-assembling small-molecule platform, the aramid amphiphile (AA), which over-comes these dynamic limitations. AAs incorporate a Kevlar-inspired domain within each molecule to pro-duce strong interactions between molecules. We ob-serve that AAs spontaneously form nanoribbons when added to water with aspect-ratios exceeding 4000:1. Ro-bust internal interactions suppress the ability of AAs to move between assemblies and result in nanoribbons with mechanical properties rivaling silk. We harness this stability to extend small-molecule assemblies to the solid-state for the first time, forming macroscopic threads that are easily handled and support 200 times their weight when dried. The AA platform offers a nov-el route to use small-molecule self-assembly to achieve aligned nanoscale materials in the solid-state

Magnetically-assisted Assembly, Alignment, and Orientation of Micro-scale Components

The use of magnetic forces to improve fluidic self-assembly of micro-components has been investigated using Maxwell 3D to model the forces between Ni thin films on semiconductor device micro-pills and Sm-Co thin films patterned on target substrates [1]. Orienting and restraining forces on pills far in excess of gravity are predicted, and it is found that the fall-off of these forces with pill-to-substrate separation can be engineered through the proper design of the Sm-Co patterns to retain only properly oriented pills[1],[2].Micro-scale hybrid assembly is a potentially important way of doing heterogeneous integration, i.e., of integrating new materials on silicon integrated circuits to obtain functionality not readily available from silicon device structures alone, and fluidic self-assembly is an attractive way to automate micro-scale assembly. A serious limitation of fluidic self-assembly, however, is the lack of a good method for holding properly assembled components in place and accurately positioned until all of the components have been assembled and permanently bonded in place. We have shown, based on our modeling, that suitably patterned magnetic films can be used to provide the forces necessary to retain, and to accurately orient and position, assembled micro-components.Our motivation for pursuing micro-scale hybrid assembly is our general interest in doing optoelectronic integration, specifically of vertical cavity surface emitting lasers (VCSELS), edge-emitting lasers (EELs), and light emitting diodes (LEDs), with state-of-the-art, commercially processed Si-CMOS integrated circuits. Our ongoing research integrating these devices on silicon described elsewhere in this report provides the context for this work and illustrates the types of applications we envision for magnetically assisted self-assembly using the results of this study.Assembly experiments to verify and demonstrate the theoretical predictions are currently in progress using two sizes of 6-µm-thick pills (50 µm by 50 µm and 50 µm by 100 µm) and a variety of magnetic thin film patterns. Recesses with different dimensions are also being studied[2].

Batch-Microfabricated RPA for Ion Energy Measurements

Plasma diagnostic tools are required in numerous fields, from experimental physics to aerospace and beyond. Various sensors exist to capture plasma saturation current and plasma potential, and will infer other properties through theory. The retarding potential analyzer (RPA) is device that will directly measure ion energy, a property of interest monitoring exterior craft conditions at hypersonic speeds. Through microfabrication, our device expands the present state-of-the-art to achieve improved mechanically enforced grid alignment, while maintaining the required micron-scale features.Advantages of enforcing alignment across successive RPA electrodes was already demonstrated in a hybrid device yielding a threefold increase in signal amplitude[1]. In utilizing microelectromechanical system (MEMS) batch-fabrication techniques, alignment precision is refined to the order of 1μm[2]. This MEMS RPA exemplifies the same modularity as the hybrid device, such that grids may be interchanged within the same housing, and otherwise incompatible fabrication techniques might be used. Figure 1 shows the schematic of a complete sensor and the corresponding six-wafer housing stack. Alignment in the assembled device is enforced by curved silicon springs in the housing (Figure 2) which allow for a slight mismatch in nominal and actual grid and housing dimensions, and can accommodate changing dimensions due to thermal expansion. Another advantage of microfabrication is the batch-processing of devices in an effort to drive down costs. MEMS RPA housings are manufactured 30 devices at a time.Measurements with our MEMS RPA have shown an additional jump in peak signal strength resulting in an order of magnitude increase over its conventional counterpart[3]. In using thicker silicon electrodes over mesh grids, the device is expected to be more robust when exposed to harsher environments. Finally, if batch-fabrication can lead to a more widely available ion energy sensor, this device could find application in monitoring micromachining processes in-situ.

Micron- and Submicron-thick Parylene Substrates for Transfer Printing and Solar Applications

Transfer printing of thin metal films enables the fabrication of both planar and suspended membrane electrodes for microelectromechanical (MEMS) sensors and actuators in an additive process. In addition, transfer-printed metal films can be used to form abrupt junctions between organic and metal layers. In contrast, conventional deposition processes such as evaporation or sputtering can cause the metal atoms to penetrate into underlying organic layers.We have developed a solvent-free transfer printing method using micron- or submicron-thick films of a robust, flexible polymer, parylene-C, as a carrier membrane. The transparent parylene films are initially deposited by chemical vapor deposition onto a rigid or semi-rigid handle substrate for ease of handling during fabrication. Metal and/or organic layers are subsequently deposited and patterned on top. The entire stack is then peeled away from the handle in a continuous sheet and transferred to the receiving substrate. After transfer, the ultrathin parylene carrier may be left in place or removed by oxygen plasma.Using this method, we have demonstrated electrostatically-actuated gold membrane-covered cavity arrays for microspeakers with larger areas (>1cm2) than previously possible using a solvent-assisted contact transfer printing method[1]. The solvent-free method has also been employed to deposit metal electrodes on top of solvent-sensitive organic layers in metal-molecule-metal structures for tunneling nanoelectromechanical switches[2].Parylene films can also serve as ultrathin, lightweight substrates for organic photovoltaics (OPVs). Their chemically inert and insoluble nature enables the use of common vapor- and solution-phase methods for thin-film deposition, including thermal evaporation and layer-by-layer spin-casting. Since the full active layer stack in an OPV is itself less than a micron thick, moving to thinner, lighter substrates could significantly reduce total weight and cost[3]. Although our current devices absorb strongly in the visible wavelength range, further materials and architecture engineering could yield a fully transparent solar cell on parylene with exceptional flexibility and versatility for optoelectronic applications.

Tunneling Nanoelectromechanical Switches Based on Organic Thin Films

With the silicon-based electronics reaching physical limits that inhibit continued improvements in device performance, much research has been directed towards nanoelectromechnical (NEM) switches as a promising alternative. NEM switches exhibit abrupt switching behavior and near-zero leakage current but suffer from large actuation voltages and failure due to stiction[1][2][3]. To overcome these challenges, this work presents a NEM switch, squitch, that operates based on electromechanical modulation of tunneling current through a nanometer-thick organic thin film (Figure 1). The switching process is initiated by the electrostatic compression of the organic film sandwiched between conductive contacts. As the organic layer is compressed, the tunneling distance is reduced, leading to an exponential increase in the tunneling current. The presence of the organic layer prevents direct contact between the electrodes. Furthermore, when the electrostatic force is removed, the deformed organic layer provides the restoring force necessary to displace the top electrode to the off-state position, helping to mitigate stiction.Theoretical analysis of two- and three-terminal devices suggests the possibility of achieving large on-off current ratio exceeding 6 orders of magnitude, sub-1-V actuation voltage, and nanosecond switching time. Various fabrication techniques have been explored to form the metal-organic-metal junction with the organic layer created mainly through use of a self-assembled monolayer of thiolated molecules. While the bottom metallic contact is commonly formed through thermal evaporation, the top electrode can be fabricated through evaporation, nanotransfer printing of a metallic thin film, or assembly of graphene sheets. An example of a metal-organic-metal junction fabricated through nanotransfer printing of a gold (Au) top electrode is shown in Figure 2. In addition to device fabrication, our current work is focused on investigating the electromechanical response of organic-based tunneling junctions through simultaneous use of electrical and optical measurements.

Hydrophobic Rare-earth Oxide Ceramics with Applications to Sustained Dropwise Condensation

Hydrophobic materials that are robust to harsh environments are needed in a broad range of applications[1][2][3]. Although durable materials such as metals and ceramics, which are generally hydrophilic, can be rendered hydrophobic by polymeric modifiers[4], these materials deteriorate in harsh environments. Here we show that a class of ceramics comprising the entire lanthanide oxide series, ranging from ceria to lutecia, is intrinsically hydrophobic (Figure 1)[5]. We attribute their hydrophobicity to their unique electronic structure, which inhibits hydrogen bonding with interfacial water molecules. We also show with surface-energy measurements that polar interactions are minimized at these surfaces and with FTIR/GATR that interfacial water molecules are oriented in the hydrophobic hydration structure. Moreover, we demonstrate that these ceramic materials promote dropwise condensation, repel impinging water droplets, and sustain hydrophobicity even after exposure to harsh environments[5]. These ceramics can also be used to fabricate superhydrophobic surfaces using various techniques. As an example, we fabricated superhydrophobic surfaces by sputtering a thin layer (~200-350 nm) of ceria onto nanograss-covered silicon microposts (Figure 2a). Water droplets display high contact angles (Figure 2b) and exhibit extreme mobility due to low contact angle hysteresis (<10°) on these surfaces. In addition, impinging water droplets completely bounce off the surface, leaving it dry (Figure 2c). Hence, we envision that this class of robust hydrophobic materials will have far-reaching technological potential in various industrial applications, where water repellency and dropwise condensation are desirable.

Electro-chemical Stimulation of Neuromuscular Systems Using Ion-selective Membranes

Spinal cord injury (SCI) leads to paralysis, decrease in quality of life, and high lifetime medical costs. Direct nerve functional electrical stimulation (FES) induces muscles to contract by electrically stimulating nerves, and it shows promise for clinical applications in restoring muscle function in SCI. FES is limited by the lack of graded response in muscle contraction and by high fatigability due to the reversal of the order in which motor units are recruited. Previous work showed that ion-selective membranes can be used to modulate Ca2+ ions in-situ, decreasing the current threshold for nerve stimulation and eliciting a more graded muscle contraction response1. This work has developed polyimide-based cuff ion-selective electrodes to enable the future application of this technique in-vivo. The developed electrodes are flexible, elastic, and conductive. In-vitro tests of the electrodes by stimulation of frog sciatic nerve reproduced the decrease in the stimulation current threshold, which had been observed in planar glass-based electrodes, in the flexible polyimide-based electrodes. Additionally, cuffing the stimulated nerves with ion-selective electrodes was more effective at decreasing current threshold than planar stimulation. This work also analyzed data on twitch width, contraction time, and relaxation time to infer effects of ion-selective electrodes on recruitment order. Stimulation with the ion-selective electrodes had higher twitch width, contraction time, and relaxation time than traditional electrical stimulation at all force levels. The difference was particularly high at low force levels, indicating an effect of calcium ion depletion on recruitment order[1].

Designing Complex Digital Systems with Scaled Nano-electro-mechanical Relays

Silicon CMOS circuits have a well-defined lower limit on their achievable energy efficiency due to sub-threshold leakage. Once this limit is reached, power constrained applications will face a cap on their maximum throughput independent of their level of parallelism. Avoiding this roadblock requires an alternative device with a steeper sub-threshold slope, i.e., lower VDD/Ion for the same Ion/Ioff. One promising class of such devices is electrostatically actuated nano-electro-mechanical (NEM) switches with nearly ideal Ion/Ioff characteristics. Although mechanical movement makes NEM switches significantly slower than CMOS, they can be useful for a wide range of VLSI applications if we reexamine traditional system- and circuit-level design techniques to take advantage of the electrical properties of the device.Basic circuit design techniques and functionality of some main building blocks of VLSI systems, such as logic, memory, and clocking structures, have been demonstrated in our previous works[1][2][3]. Recently, by employing pass-transistor logic design, we have designed and demonstrated complex relay-based arithmetic units such as multipliers (Figure 1b-c)[4]. Simulation results of an optimized 16-bit relay multiplier built in a 90-nm equivalent relay process model predict ~10x improvement in energy-efficiency over optimized CMOS designs in the 10-100 MOPS performance range. The relative performance of the multiplier enhancements are in line with what was previously predicted by a NEM relay 32-bit adder[3], suggesting that complete VLSI systems, such as a microprocessor, would expect to see similar energy/performance improvements from adopting NEM relay technology[3],[4].Since scaling is crucial for performance, energy, and total area improvement, we have developed a scaled version of the original relay, a 6-terminal relay, which is 25x smaller and offers enhanced functionality. The operation of the main building block of the NEM-relay based multiplier, the (7:3) compressor, built with these scaled devices is experimentally demonstrated. This circuit, consisting of 46 scaled relays, is the largest scaled relay circuit successfully tested to date (Figure 2).

Microfluidic Electronic Detection of Protein Biomarkers

Immunoassays use antibodies to detect protein biomarkers, with a substantial global market and significant importance for clinical practice. However, traditional immunoassays are performed in centralized laboratories using optical detection methods, which means that results take days and cannot be highly multiplexed, in turn increasing patient visits, healthcare costs, and decreasing healthcare outcomes. In our project, we are developing an all-electronic immunoassay. All-electronic immunoassays have three major benefits: 1) we can achieve high-throughput immunoassays, allowing us to detect over 100 proteins in a single sample and potentially even to measure all approved protein biomarkers; 2) we can reduce cost by taking advantage of decreasing costs of silicon electronics; and 3) patients can get results before meeting with their physicians, since impedance detection is much faster than optical readout.The basic idea of an all-electronic biosensor is that by looking into the impedance change caused by association of antigens to antibodies, we can determine if certain antigens are present in the blood. Our biosensor is illustrated in Figure 1. To facilitate the sensor design, we determined three parameters (detection sensitivity, response time and sensor dimension) to characterize the performance of the sensor. We built two models to gain insight into the relationship between the sensor design and performance. The transport model of molecular binding indicated that the response time is determined mainly by the height of the channel, and sensors with higher channels need more time to reach equilibrium but can have better signal-to-noise ratio (Figure 2a). The electric model of impedance change shows that the interfacial impedance determines the electric readout and that sensors with wide electrode can offer good signal-to-noise ratio but end up with large size (Figure 2b). Therefore, the three key performance parameters cannot be achieved simultaneously, and the sensor design needs to be optimized.Another critical issue is chemical modification of the electrode’s surface; the goal is to eventually immobilize probe proteins on the electrodes. To this end, we built alkanethiol self-assembled monolayers (SAMs)as the links between gold electrodes and probe proteins. Since creating SAMs is the first step of surface modification, it is critically important to have successful SAMs. We used different surface characterization techniques, including Fourier transform infrared spectroscopy and impedance spectroscopy. Both methods showed that the self-assembled monolayers have been successfully built (Figures 2c and 2d).

Stretchable Pressure- and Shear-sensing Skin Printed from PDMS

In the fields of robotics and prosthesis design, there is need for inexpensive, wide-area pressure- and shear-sensing arrays that can be integrated into a flexible and stretchable skin analog[1],[2]. This project seeks to meet this need by building combined pressure and shear sensors based on the well documented piezoresistive property of composites made from PDMS and carbon black (CB)[3],[4]. These sensors can be printed by extruding the uncured composite and can be molded directly into a solid PDMS membrane to form a sensing skin that is water- and heat-resistant, flexible, and stretchable. The sensor concept consists of three electrical contacts, arranged linearly, with the PDMS/CB composite deposited on top of them. When this arrangement is exposed to pressure, the resistance between the center contact and each of the edge contacts changes equally. However, when sheared, the resistance changes differently between the center contact and the two edge contacts. In this way the effects of pressure and shear can be separated; see Figure 1. In fabrication, a silver/PDMS composite is used to form the contacts, a PDMS/CB composite is used to form the sensors, and pure PDMS is used to form the bulk of the skin. Each of these materials is extruded onto a plastic membrane, which is removed after heat-curing. Due to the early point in development and fabrication, the sensors have excessive drift that makes quantitative measurements difficult. However, subtracting the resting voltages (representing the resistances of each half of the sensor) measured before shear or pressure is applied from those measured after their application and plotting these differences enables observation of the separation of pressure and shear; see Figure 2. At present this method works for a carefully selected area of the sensor and degrees of pressure and shear.

Wide-bandwidth, Low-frequency, Low-g Piezoelectric MEMS Energy Harvesters

Our group proposes the piezoelectric energy harvester based on a nonlinear resonator to address the narrow bandwidth issue of conventional energy harvesters. The wide bandwidth and significantly higher power than previously reported devices have been proved by our experiments (Fig. 1), which showed a ~20% bandwidth and a 2W/cm3 power density. Increasing number of researchers have been exploring different nonlinear resonance based designs, including beam stiffening, asymmetry structure, buckled structure, and magnetic restoring force, to name a few. The Duffing mode resonance has been observed in diverse designs, and the wide bandwidth is their shared property. However, the theoretical proof of a wide bandwidth and high-power density energy harvester is not available. By building the electromechanical model for the nonlinear system, we have found the analytical solution of the output power of a nonlinear resonance based piezoelectric energy harvester and have proved a much wider power bandwidth can be obtained by varying electrical load, and the maximum power can be achieved by matching the electrical damping to mechanical damping”. By understanding the advantages of the nonlinear design, we are making new designs to bring it closer to real applications. The two gap between the nonlinear piezoelectric energy harvesters at MEMS scale now are, the operating frequency (>1000Hz) is usually much higher than the frequencies of ambient vibrations (~100Hz), and the excitation level when testing in laboratory (4~5g) is also higher than the common vibration’s amplitude (<0.5g). By optimizing the beam composition, tuning the proof mass and redesigning the beam geometry, we anticipate that our new design will work in the frequency range between 100Hz to 200Hz at the excitation of 0.5g. The new design will be fabricated and tested soon.

Design and Fabrication of Magnetically Tunable Microstructured Surfaces

Micro- and nanostructured surfaces have broad applications ranging from liquid transport in microfluidics and cell manipulation in biological systems to light tuning in optical applications[1][2]. While significant efforts have focused on fabricating static micro/nanostructured arrays[3], uniform arrays that can be dynamically tuned have not yet been demonstrated. We present a novel fabrication process for magnetically tunable microstructured surfaces, where the tilting angle can be controlled upon application of an external magnetic field. We also demonstrate this platform for droplet and bubble manipulation in heat transfer applications.The tunable surfaces consist of ferromagnetic (Ni) pillars on a soft PDMS substrate. The pillars have a diameter of 23-35 µm, pitch of 60-70 µm, and height of 70-80 µm. We used vibrating sample magnetometry to obtain hysteresis loops of the Ni pillar arrays, which match well the properties of bulk Ni. With a field strength of 0.5±0.1 Tesla and a field angle of 60±15°, a uniform 10.5±0.5° tilting angle of the pillar arrays was observed. Meanwhile, simulations using Abaqus to determine the equilibrium positions of the pillars under different applied fields show good agreement with the experiments. We also investigated how these tunable pillars changed the contact angle of water droplets on the surface. An external magnetic field of 0.3 Tesla changed the water droplet contact angle by ~15°. Future work will focus on using these surfaces to actively transport water droplets and spread the liquid film via pillar movement. These tunable surfaces promise new fluid manipulation capability for applications in condensation, evaporation, and boiling.

Scalable 3-D Microelectrode Recording Architectures for Charac-terization of Optogenetically Modulated Neural Dynamics

Optogenetics is commonly used for precision modulation of the activity of specific neurons within neural circuits[1], but assessing the impact of optogenetic neuromodulation on the neural activity of local and global circuits remains difficult. Our collaborative team recently initiated a project (Scholvin et al., SFN 2011) to design and implement 3-D silicon-micromachined electrode arrays with customizable electrode locations, targetable to defined neural substrates distributed in a 3-D pattern throughout a neural network in the mammalian brain and compatible with simultaneous use of a variety of existing light delivery devices.We have developed a series of innovations aimed at facilitating the scalability aspect of these probes – that is, aspects of probe design that should enable them to scale to 1000 channels of neural recording or more. First, we have developed streamlined electrode fabrication methodologies that enable micromachined probes to be first fabricated using conventional silicon micromachining and then rapidly assembled into custom 3-D arrays, with semi-automated formation of the necessary electrical connections and mechanical constraints. Second, we have developed a set of surgical and insertion technologies to enable the insertion of electrode arrays with a high number of electrode shanks into the brain, while minimizing probe insertion damage. Finally, to facilitate scaling of the channel count beyond what is feasible with external amplifiers, we are exploring new approaches for integrating amplifier circuits directly on the probe arrays themselves, to remove bottlenecks associated with connecting probes to the outside world.

Aligned CNT-based Microstructures and Nano-engineered Composite Macrostructures

Carbon nanotube (CNT) composites are promising new materials for structural applications thanks to their mechanical and multifunctional properties. We have undertaken a significant experimentally based program to understand both microstructures of aligned-CNT nanocomposites and nano-engineered advanced composite macrostructures hybridized with aligned CNTs.Aligned nanocomposites are fabricated by mechanical densification and polymer wetting of aligned CNT forests[1]. Polymer wetting is driven by capillary forces that arise upon contact of the polymer with the nanostructured CNT forest[2],[3], the rate of which depends on properties of the CNT forest (e.g., volume fraction) and the polymer (viscosity, contact angle, etc.). Here the polymer is typically an unmodified aerospace-grade epoxy. CNT forests are grown to mm-heights on 1-cm2 Si substrates using a modified chemical vapor deposition process. Following growth, the forests are released from the substrate and can be handled and infiltrated. The volume fraction of the as-grown CNT forests is about 1%; however, the distance between the CNTs (and thus the volume fraction of the forest) can be varied by applying a compressive force along the two axes of the plane of the forest to give volume fractions of CNTs exceeding 20% (see Figure 1a). Variable-volume fraction-aligned CNT nanocomposites were characterized using optical, scanning electron (SEM), and transmission electron (TEM) microscopy to analyze dispersion and alignment of CNTs as well as overall morphology. Extensive mechanical property testing is underway, including 3D constitutive relations and fracture toughness.Nano-engineered composite macrostructures hybridized with aligned CNTs are prepared by placing long (>20 μm) aligned CNTs at the interface of advanced composite plies as reinforcement in the through-thickness axis of the laminate (see Figure 2). Three fabrication routes were developed: transplantation of CNT forests onto pre-impregnated plies[4] (the “nano-stitch” method), placement of detached CNT forests between two fabrics followed by subsequent infusion of matrix, and in-situ growth of aligned CNTs onto the surface of ceramic fibers followed by infusion or hand-layup[5][6][7]. Aligned CNTs are observed at the composite ply interfaces and give rise to significant improvement in interlaminar strength, toughness, and electrical properties. Extension of the CNT-based architectures to ceramic-matrix composites and towards multifunctional capabilities including structural health monitoring and deicing is underway.

Nanoporous Elements with Layer-by-layer Assembly in MEMS with a Focus on Microfluidic Bioparticle Separation

We have integrated ultra-porous (99% porous) elements (nanoporous forests of vertically aligned carbon nanotubes (VACNTs)) in MEMS, showing their use in microfluidic applications for bioparticle isolation and health diagnostics. Distinct from works where the effects of fluids on VACNT elements resulted in either structural deformation or catastrophic forest collapse[1], our approach enables creation of high aspect ratio (~1-mm) nanoporous elements and preserves their shape under flow-through conditions. Figure 1 shows a device consisting of a patterned and (wet) functionalized VACNT forest integrated into a PDMS microfluidic channel.Compared to state-of-the-art designs that exploit solid materials (e.g., silicon, PDMS) for the structural features, our nanoporous elements enable flow around and through the VACNT elements, enhancing physical interaction between the particles in the flow and the functional elements. The large surface-to-volume ratio of nanoporous materials yields a significant increase in the functional surface area (~250-500X for the layouts analyzed in our works[2] ), with permeability comparable to that of macro-scale porous materials[3], thus further promoting bioparticle capture[4]. To utilize these attributes, assembly of polymer films on individual carbon nanotubes via layer-by-layer (LbL) techniques was explored. Conformal coating surrounding the VACNTs provides the opportunity to control intra-CNT spacing as well as surface functionality.Initial work on VACNT-LbL assemblies has been performed on various geometries at the same flow conditions. FITC-PAH-SPS at appropriate pH levels was used to perform LbL. Preliminary results indicate conformal coating on both the inside and outside surfaces of the VACNT wall elements. Effects of flow conditions, other polymer systems, and surface functionalization are topics of ongoing work. Optimization of functionalizing similar polymer films on flat surfaces has been performed, with the goal of applying the same surface chemistry to CNT microfluidic elements for bioparticle capture and manipulation.

Waveguide Micro-probes for Optical Control of Excitable Cells

Optogenetics is the safe, effective delivery of light-gated membrane proteins to neurons and other excitable cells (e.g., muscle, immune cells, pancreatic cells, etc.) in an enduring fashion, thus making the cells permanently sensitive to being activated or silenced by millisecond-timescale pulses of blue and yellow light, respectively[1]. This ability to modulate neural activity with a temporal precision that approaches that of the neural code itself holds great promise for human health and for studies of brain function and interconnectivity.We have developed multiple light guide microstructures produced using standard microfabrication techniques to deliver light to activate and silence neural target regions along their length as desired[2]. Each probe is a 100- to 150-micron-wide insertable micro-structure with many miniature lightguides running in parallel and delivering light to many points along the axis of insertion (Figure 1a). Such a design maximizes the flexibility and power of optical neural control while minimizing tissue damage. We have recently created 2-D arrays of such probes (Figure 1b) so multiple colors of light can be delivered to 3-dimensional patterns in the brain, at a resolution of tens to hundreds of microns, thus furthering the causal analysis of complex neural circuits and dynamics[3].The initial light-guide structures have been fabricated from silicon oxynitride clad with silicon dioxide and tests show excellent transmission of light with no visible loss in the taper and bend regions of the patterns [2]. Significantly, the novel 90˚ bend invented to direct light laterally out the side of the narrow probe (Figure 1c) functions as designed[2]. The optical sources for initial tests with the probe are independent laser modules coupled to one end of a fiber-optic ribbon cable. The other end of the ribbon cable is butt-coupled to the inputs of the probe via a standard fiber-optic connector ferrule. This allows for increased modularity and control in initial probe testing.We are now utilizing transgenic mice, which express optogenetic activators and silencers in cortical pyramidal neurons, to demonstrate optogenetic control of neural circuits in a fashion appropriate for in vivo circuit mapping or brain machine interface prototyping. Our goal is to explore the degree to which this technology can be used to functionally map neural network connectivity over large, multi-region circuits in the brain, and ultimately to enable a new generation of neural control prosthetics.

Development of Porous Piezoresistive Materials and Its Applications for Underwater Pressure Sensors and Tactile Sensors

Microelectromechanical system (MEMS) pressure sensor arrays are gaining attention in the field of underwater navigation because they are seen as alternatives to current sonar and vision-based systems that fail to navigate unmanned undersea vehicles (UUVs) in dark, unsteady, and cluttered environments. Other advantages of MEMS pressure sensor arrays include lower power consumption and the fact that their passive nature makes them covert. This work focuses on the development of a flexible pressure sensor array for UUVs, where the sensor array is inspired by the ability of fish to form three-dimensional maps of their surroundings[1],[2]. Fish are able to decipher various pressure waves from their surroundings using the array of pressure sensors in their lateral line sensory organs that can detect minute pressure differences. Similarly, by measuring pressure variations using an engineered pressure-sensor array on the surface of an UUV, this project aims to aid UUVs in the identification and location of obstacles for navigation. The active material of the pressure sensor array is a porous polydimethylsiloxane (PDMS)-carbon black composite made out of a sugar sacrificial scaffold that shows great promise for satisfying the proposed applications. The proposed device structure is flexible, easily fabricated, cost efficient, and capable of being implemented on a large-area and curved UUV surface. Although hysteresis occurs during the electromechanical test, the piezoresistivity of this porous PDMS-carbon black composite is reversible and reproducible. Compared to its non-porous counterpart[3], this porous composite shows a six-times increase in piezoresistivity and a greatly reduced Young’s Modulus. When tested underwater, this porous composite was able to differentiate water waves that had a frequency of 1 Hz and 2 Hz, which is promising for its underwater application. This porous composite was also extended to the application of tactile sensors using a different device architecture, which showed excellent response under mechanical testing.

Continuous RBC Removal Using Spiral Channel with Trapezoidal Cross-section

Red blood cells (RBC) are the most abundant cell component in many biological fluids, including blood, bone marrow aspirate, and peritoneal aspirate. Depletion of contaminating RBCs from those samples is often an indispensable sample preparation step before the application of any clinical and diagnostic tests[1], while avoiding artificial alteration on the phenotypes of sorted cells is an important criterion for all studies. The achievement of minimal artifact is especially important in the case of removing RBCs from human blood to isolate white blood cells (WBCs), which play a key role in carrying out and mediating the immune response to various pathogens. The information extracted from the isolated WBCs would be meaningful to facilitate disease prognosis only when the key features of WBCs’ original state are not masked by the sample preparation artifacts. However, several cases have been reported that the conventional methodologies for blood cell separation on the macroscale, including differential centrifugation and selective erythrocyte lysis, could result in altered imuno-phenotype[2] or impaired viability[3] of the isolated WBCs. Meanwhile, passive continuous microfluidic separation techniques utilizing the size-dependent hydrodynamic effects[4][5][6] have been considered as an alternative approach to bypass the issues associated with macroscale blood cell separation methods.In this work, we improved the separation resolution of curvilinear microchannel while maintaining the high-throughput feature by modifying the channel cross-section to be trapezoidal rather than rectangular and demonstrated its ability for efficient RBC depletion from human blood sample with negligible effect on polymorphonuclear leukocyte (PMN) immune-phenotype as compared to selective erythrocyte lysis method[7]. To our knowledge, this is the first experimental demonstration where the asymmetry velocity field within a trapezoid spiral channel affects the inertial focusing phenomenon, indicating the feasibility of using channel cross-sectional geometry (other than width and depth) as a parameter for optimization of a curvilinear inertial microfluidic sorter.

Mechanisms for Intrinsic Stress Evolution during Deposition of Polycrystalline Films

Complex kinetic processes are involved during the growth of polycrystalline films, which is typically carried out far from equilibrium. Minor changes in processing conditions can lead to tremendous differences in surface morphology, grain structure, and residual stress in the films. This result strongly influences their performance and reliability in nano- and micro-electromechanical devices and systems (N/MEMS)[1]. Control of the residual stress is especially important in the devices based on micro-beam structures, such as electrically actuated switches and accelerometers. For example, for doubly-supported beams, an average compressive stress as small as 10 MPa can cause buckling, while a tensile stress can cause changes in the beam’s stiffness. Figure 1 shows examples of problems caused by residual stresses in released micro-structures and devices.Through in situ real-time measurements we have studied the intrinsic stress evolution of a number of materials at different homologous temperatures (deposition temperature divided by the melting temperature) for several materials (Figure 2). One general trend can be found: the stress becomes more compressive when the homologous temperature is higher. In particular, at intermediate homologous temperatures, the instantaneous stress changes from compressive to tensile during film thickening. Previous models[2][3][4] are inadequate to explain this transition behavior. By characterizing the film microstructure as a function of the film thickness, we conclude that this transition originates from the increase of grain size during film thickening, which has two consequences. First, it changes the bulk stress of the film during deposition and causes a tensile component of the instantaneous stress. Second, it changes the grain size dependence of the compressive component, the magnitude of which is controlled by the competition between adatom-2D island incorporation and adatom-GB incorporation.

MEMS Pull-in and Lift-off Simulation Using Continuation Methods

The voltages at which MEMS actuators and sensors become unstable, known as pull-in and lift-off voltages, are critical parameters for almost any design. However, current general-purpose simulators compute these critical parameters by directly sweeping the voltage, leading to either excessively large computational cost or convergence failure near the instabilities. This work applies two kinds of continuation methods to simulate the pull-in and lift-off effects.The first algorithm uses arc-length continuation algorithm to compute the multiple static solutions of a given MEMS design. Using a tangent predictor and a correction scheme, a next solution point can be calculated based on the previous solution point. This method can efficiently avoid the convergence failures of Newton iterations when a direct sweeping method is applied to solve for the static solutions[1].The second algorithm uses a different idea to generate a single-solution curve. The basic idea is that we first apply arc-length continuation until a pull-in or lift-off point is approaching. After that, a homotopy method is applied to directly solve for the next point after a sharp transition of the solution curve.Both algorithms have been implemented in a commercial MEMS+IC co-design software package, and they have been tested by various industrial MEMS design cases. Figures 1 and 2 show that the simulation results from both algorithms are the same. These results are consistent with that from a commercial simulator, CoventorWare, which utilizes detailed but time-consuming finite-element and boundary-element analysis.

Deep Trench Capacitor Drive of Unreleased Si MEMS Resonator

With frequency-quality factor products (f•Q) often exceeding 1013, MEMS resonators offer a high-Q, small footprint alternative to conventional LC tanks and off-chip crystals for clocking and wireless communication. Over the past three decades, much progress has been made in the key figures of merit of MEMS resonators including small footprint, high Q, low motional impedance, and efficient energy coupling kT2. In parallel, efforts have focused on system-level metrics including high yield, low cost, robustness, easy packaging, and integration with circuits. A key challenge in MEMS resonator design is to achieve high performance yet manufacturable devices. The unreleased deep trench (DT) resonators in this work address this challenge.Beyond the performance goals of high Q and low loss, these devices target two key features desired for monolithically integrated MEMS resonators. First, lithographic definition of resonance frequency enables a broad range of frequencies to be fabricated on a single chip. Second, unreleased bulk-acoustic resonators do not require any low-yield, complex steps to freely suspend the moving structure and are robust in harsh environments without packaging. Unreleased resonators such as the HBAR[1] have been demonstrated but have thickness-defined frequency. Lateral bulk acoustic resonators with lithographically defined frequency such as the LoBAR[2] have achieved high Q but require low d31 coupling to drive and sense resonance. Meanwhile, sidewall AlN resonators[3] excite lateral resonance with d33 coupling but still require a release step. This work provides the benefits of all of these devices with high Q, efficient dielectric transduction, lateral resonance, and no release step. The DT resonator implements deep trench capacitors as both electrostatic transducers and Acoustic Bragg Reflectors (ABRs), defined in a single mask and self-aligned (Figure 1). While ABRs provide acoustic isolation in a solid medium, the DT capacitors function as internal dielectric transducers[4], which have achieved the highest frequencies in Si to date[5]. A 3.3-GHz unreleased Si resonator is demonstrated with Q of 2057 and motional impedance RX of 1.2 kΩ (Figure 2). This realization of high-Q unreleased resonators in a bulk Si process provides a high yield, low cost, no packaging solution for on-chip clocking, wireless communication, and electromechanical signal processing.

Preventing Catastrophic Failures: Nano-engineered Multi-physics Materials for Structural Applications

Catastrophic structural failures cause many physical and personal losses, with prevention estimated at billions of dollars in savings each year. Non-destructive evaluation (NDE) techniques have been pursued and employed for damage detection of such structures to detect cracks and other damage at pre-critical levels for remediation[1],[2]. Here, a novel multi-physics approach is reported that addresses drawbacks in existing techniques by taking advantage of the effects that damage, such as a crack, has on the electric and thermal transport in a material containing a CNT network distributed in the bulk material. When a potential is applied to a nano-engineered structure (see Figure 1), electric field lines concentrate in the vicinity of cracks as electrons flow around damage, causing field concentrations and “hot spots” via Joule heating, an effect which is amplified because the heat flow is also impeded in areas of damage (e.g., across a crack face). These changes of temperature can be localized through a conventional infrared thermal camera. Low power operation (a 9V standard battery is exemplary, providing a 15C rise at 1 Watt as in Figure 2) and high spatial resolution are demonstrated that are beyond state-of-the-art levels in non-destructive evaluation. Multiple applications have been identified using this technique such as crack detection in composite components that are joined by metallic fasteners, structures having internal nonvisible damage due to impact, and in situ progressive damage monitoring during a tensile strength test. The thermal nano-engineered NDE technique demonstrated here can provide a new and effective inspection route for monitoring the next generations of safer infrastructure[3][4][5]. Further expansion on this work has yielded significant technologies in ice protection systems (IPS) for vehicle structures such as unmanned aerial systems (UAS)[6] with application to infrastructure needs such as wind turbines and bridges.

MEMS-enabled Tactile Displays for the Blind and Visually Impaired

According to the World Health Organization, more than 285 million people have visual impairments worldwide, and 39 million of those are blind. About 20% of visual impairments cannot be prevented or cured; in these cases, assistive technologies are critical to enable independent integration into professional and social settings. There is a pressing need for technologies that enable the blind and visually impaired to acquire graphical information or navigate in unstructured environments. The purpose of this project is to enable compact, rapidly refreshable tactile displays that provide information in an intuitive format as part of a broader system for situational awareness, navigation, and perception of graphical information. The overall system is a collaborative effort among MIT’s Computer Science and Artificial Intelligence and Microsystems Technology Laboratories, and researchers from Northeastern University; the tactile display is the focus of this abstract.We are developing the scientific and engineering knowledge for high-resolution displays of rapidly-updatable, vibrating tactile elements, i.e., tactels (Figure 1), using a combination of macro- and microscale batch manufacturing techniques. The proposed architecture is entirely distinct from the conventional piezoelectric bending beams of refreshable Braille readers or the Optacon[1], as well as from the actuators of electroactive polymer displays[2]. Our tactels use structures that receive in-plane displacement from piezo beams to produce amplified out-of-plane displacements that can be sensed by human hands. Current work focuses on parametric multiphysics modeling of the tactels and development of the manufacturing process and assembly approach for the MEMS displays.

Erythrocyte Deformability Correlates to Intracellular Calcium Level

Elevated intracellular calcium level ([Ca++]i) and reduced deformability in red blood cells (RBCs) are commonly associated with blood-related diseases[1][2][3] as well as in-vivo ageing[4]. The correlation between RBC deformability and [Ca++]i has been established at the bulk level, typically accompanied with changes in ATP level and RBC size[4]. It is, however, unclear whether changes in RBC deformability would correspond to [Ca++]i at the single-RBC level.In this project, we attempt to establish the connection between single RBC deformability and [Ca++]i using a microfluidic device as described[5]. Calcium ionophore A23187 was used for calcium loading, creating different levels of [Ca++]i. RBC deformability is assessed by measuring the microcirculatory velocity of RBCs in a microfluidic device with narrow gaps[5]. Simultaneous measurement of calcium intensity and transit velocity was performed while an individual RBC traversed the microchannels.

Thin Film Piezoelectric Micro-machined Ultrasonic Transducer for Medical Imaging

Ultrasound is an attractive 3D medical imaging technique because it is relatively inexpensive, portable, compact, and non-invasive. However, for 3D real time imaging to be commercially realizable, scans must be consistent and high resolution and occur at a fast acquisition rate–all factors that are inhibited by the current bulk piezoelectric transducer technology[1]. Highly manual manufacturing limits the size of current transducers to millimeter length scales and the high acoustic impedance of the bulk piezoelectric limits resolution reducing bandwidth and sensitivity[2]. At high volume, micro-fabrication is high yield and less expensive, and it would enable element miniaturization for high resolution, small form factor ultrasound probes.Our group has designed a piezoelectric micro-machined ultrasonic transducer (pMUT) that transmits acoustic signals via high frequency vibrations of a thin diaphragm. These vibrations are actuated by applying a voltage across a thin film piezoelectric lead zirconate titanate (PZT) film deposited via a sol-gel technique. For sensing, acoustic waves reflected from an imaging target strain the diaphragm generating a current signal.Device fabrication begins with growth of thermal oxide on a silicon-on-insulator wafer. The bottom electrode is then deposited via a lift-off process, and PZT is deposited and patterned with a wet etch. The lift-off process is then repeated to create the top electrode. Finally, diaphragms are released and the substrate is divided into chips (Figure 2) with a back-side deep reactive ion etch. A schematic of the fabricated device is shown in Figure 1.Through electrode size optimization, our pMUT is designed to maximize deflection, which is ideal for generating the high acoustic pressure necessary to overcome signal attenuation in deep penetration imaging[3]. In the future, we hope to incorporate the optimized pMUT transducer design in pMUT arrays with a small form factor to enable 3D real time medical imaging.

Thermal Ink Jet Printing of CNT films

Ink jet printing allows for rapid, scalable, and low-cost patterning process. It does not need a vacuum environment or toxic etching process, which facilitates the integration of ink jet printing into other micro-machining process. And ink jet printing can pattern on curved surface or 3D structure. The characteristics of this technique are advantageous in industry. Previously, we demonstrated highly repeatable and uniform PZT thin film with ink jet printing[1],[2]. With this know-how in PZT film printing, we apply carbon nanotubes to ink jet printing. Due to their mechanical, thermal conductive and electrical properties, carbon nanotubes find diverse applications. The percolation network of carbon nanotubes is a transparent and conductive material, and strong bonding among carbon nanotubes enables fabrication of a bendable, foldable, and stretchable electrode. Previous research from other groups fabricated an ink jet-printed carbon nanotubes conductor[3], but the performance was not good enough to replace the current ITO transparent electrode. In this research, we controlled the amount of carbon nanotube deposition by changing the pitch of the printed line, which determines conductivity and transparency. Electrodes of 30% transparency can be fabricated using 10-um pitch, which has 36.6 S/cm conductivity and 247 ohm/sq sheet resistance. Increasing the pitch size up to 20 um gives the electrode properties of 74% transparency, 30.8 S/cm conductivity and 1.1 Kohm/sq sheet resistance. This result shows better performance compared with previous research. Figure 1 shows the SEM image of fabricated electrodes depending on the pitch size. Printed film shows uniformly deposited carbon nanotubes percolation network within 30- um pitch size. In the future, we plan to produce devices that utilize the full capabilities of this process to achieve better performance in both conductance and transparency, which will enable applications such as touch panels, organic light-emitting diodes, and solar cells.

Switchable Piezoelectric Transduction in AlGaN/GaN MEMS Resonators

High-Q MEMS resonators, with small footprint and monolithic integration, are excellent building blocks for configurable RF systems. While these resonators provide narrow bandwidth selectivity, broad-band operation typically requires a large bank of switchable devices. This bank introduces a large capacitive load at the input due to the drive transducers. Typically, piezoelectric resonators have strong electromechanical coupling coefficients enabling low loss filters. However, they must be switched in line of the RF signal, resulting in insertion loss and reduced power handling.This work presents a new implementation of piezoelectric transduction in an AlGaN/GaN heterostructure that enables on/off switching of transduction with DC voltage applied out-of-line of the RF signal and reduces the capacitive load of the resonator by >10× when in the off state. This transducer is formed in the AlGaN, between a top Schottky electrode and a 2D electron gas (2DEG) as a second electrode[1],[2] (Figure 1). When a negative bias of -7 volts is applied to the Schottky electrode, the 2DEG is depleted. The removal of this bottom electrode suppresses electromechanical transduction and serves to reduce the drive capacitance by >10×.Mechanical resonances can be detected with a passive transducer equivalent to the drive, or with a high electron mobility transistor (HEMT) embedded in the resonator, which has been previously shown to enable sensing at higher frequencies[3],[4]. The HEMT-sensed device is illustrated in Figure 1c. The DC behavior of the embedded HEMT is shown in Figure 2a, while the measured frequency response of the resonator is illustrated in Figure 2b. Applying a negative bias to the drive transducer depletes the 2DEG and suppresses the resonance signal while reducing the drive capacitance by 13×. The resonance at 2.67 GHz has Q of 650 in air with f·Q of 1.7×1012, the highest in GaN resonators to date.

Iso-dielectric Separation of Cells and Particles

The development of new techniques to separate and characterize cells with high throughput has been essential to many of the advances in biology and biotechnology over the past few decades. We are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This method, iso-dielectric separation (IDS), uses dielectrophoresis (the force on a polarizable object[1] and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductivity (Figure 1). It is similar to iso-electric focusing, except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes[2][3][4].Sepsis is a clinical condition caused by infection and, despite state-of-the-art facilities and treatments, it has a mortality rate of ~30%. Sepsis induces inflammation and organ failure; a possible treatment would require removing inflammatory agents, such as activated neutrophils, from whole blood. We used a CLP mouse model of sepsis (Figure 2a) and PMA-activated human granulocytes (Figure 2b) to monitor electrical differences between septic blood and leukocytes. With human granulocytes we saw a shift in their average isodielectric position (IDP) at high frequencies. Based on these results we did an IDP profile of leukocytes in healthy mouse blood and established a gate for the activated leukocytes. Applying the same gate under the same conditions with blood from CLP mice (n=4), we saw an increase in the number of activated leukocytes (Figure 2c). Finally we took aliquots of the same samples from healthy and CLP mice and measured common activation biomarkers with flow cytometry. Comparing both results, we see good correlation between our estimation of activated cells and the number of activated granulocytes measured in flow cytometry.

Quantifying Particle Coatings Using High-precision Mass Measurements

Microparticles are currently used in a variety of industrial and biomedical applications and are often coated with different materials to impart functionality for applications such as drug delivery, cell extraction, and biomolecular detection. In many cases, the amount of coating affects the functionality of the particle. Label-based methods such as fluorescence are commonly used for biomolecular detection applications. However, labeling is not always practical and may not be an option in cases where a material layer is added. Although there is a wide range of label-free approaches for measuring the amount of coating on a flat surface, there are very few analogous approaches for particles.We have previously demonstrated that the suspended microchannel resonator (SMR) can weigh individual microparticles with femtogram precision. Although this level of precision is sufficient to resolve meaningful differences in coating thicknesses between populations of microparticles, such measurements have remained challenging for two reasons: (i) since the weight of the microparticle is generally many orders of magnitude larger than its coating, variation in particle mass across even the most monodisperse population can obscure the mass of the coating and (ii) sample-to-sample variations in the density of the carrier solution and density drift during the measurement of an individual sample give rise to significant differences in buoyant mass. Here we address these limitations by adjusting the density of the carrier solution to diminish the buoyant mass of the particle with respect to its coating and by monitoring solution density throughout the measurement using rapid fluid exchanges with a reference solution in an adjacent bypass[1]. This method is appropriate for polymer-based microparticles coated with materials of a different density. For a protein coating on a 3-μm polystyrene microsphere, we can resolve approximately 10% of a full layer (Figure 1).

Contact-printed MEMS Membranes

It is desirable to extend the functionality of MEMS to different form factors including large-area arrays of sensors and actuators, and to various substrate materials, by developing a means to fabricate large-area suspended thin films. Conventional photolithography-based MEMS fabrication methods limit the device array size and are incompatible with flexible polymeric substrates[1][2].A new method for additive fabrication of thin (125±15-nm-thick) gold membranes on cavity-patterned silicon dioxide substrates using contact-transfer printing is presented for MEMS applications. The deflection of these membranes, suspended over cavities in a silicon dioxide dielectric layer atop a conducting electrode, can be used to produce sounds or monitor pressure. The fabrication process employs a novel technique of dissolving an underlying organic film using acetone to transfer membranes onto the substrates. The process avoids fabrication of MEMS diaphragms via wet or deep reactive-ion etching, which in turn removes the need for etch-stops and wafer bonding. Membranes up to 0.78 mm2 in area are fabricated, and their deflection is measured using optical interferometry. The membranes have a maximum deflection of about 150 nm across 28-μm-diameter cavities, as shown in Figure 1[3]. Using the membrane deflection data, Young’s modulus of these gold films is extracted (74±17 GPa), and it is comparable to that of bulk gold. Additionally, a 15 Hz sinusoidally varying voltage of 15 V peak-to-peak amplitude is applied to the MEMS device to demonstrate that the large membrane deflection is a repeatable deflection (Figure 2).These films can be utilized in microspeakers, pressure sensors, microphones, deformable mirrors, tunable optical cavities, and large-area arrays of these devices.

Automated Parameterized Dynamical Modeling of RF MEMS Resonators

Design and optimization of novel RF micro-electro-mechanical system (MEMS) resonators such as resonant body transistors (RBT) require modeling across multiple domains including mechanical (distributed stress and elastic wave models), electrical (semiconductor devices and RF small signal models), and thermal. These domains are all cross-coupled in nonlinear ways and require lengthy finite element multi-physics analyses to solve. Due to the complexity of these structures embedded in the CMOS stack and sensed using active FETs, the finite element multi-physics simulation prevents quick, intuitive parameterization of device design. A reduced model parameterized across all three domains is therefore necessary both for rapid prototyping and for device optimization.In this work, we are currently developing an algorithm to automatically generate compact and accurate models for RF MEMS resonators from input/output measurements. In a future stage of the project, we will develop physics based models from first principles and from finite element solvers. In both cases, our compact models will be suitable for AC, DC, and RF operation of the device and allow the circuit designers to run circuit-level time-domain simulations using any commercial circuit simulator[1]. The compact models are parameterized, so that the circuit designer will be able to instantiate instantaneously models within the circuit simulator for different values of the key device parameters[2]. Key parameters included in the compact parameterized models for RF MEMS resonators are resonant frequency, quality factor, presence of spurious modes, and operating temperature. Values for the model coefficients are calibrated using measurements from MEMS resonator devices. A critically important feature of our models is to guarantee that when circuit designers change arbitrarily values for the device parameters, the compact models will always preserve the physical properties of the original device and will never cause numerical instabilities and convergence issues when connected to other blocks within the circuit simulator[1]. We can use these models for sensitivity analysis and automated design space exploration. Numerical results are presented for a hybrid MEMS-CMOS Si based resonator[3] shown in Figure 1. For these devices, we model temperature dependent transconductance[4]. Figure 2 demonstrates an excellent match between the output of our identified models and the given measured data.

A Microfluidic ‘‘Baby Machine’’ for Cell Synchronization

Common techniques used to synchronize eukaryotic cells in the cell cycle often impose metabolic stress on the cells or physically select for size rather than age. To address these deficiencies, a minimally perturbing method known as the ‘‘baby machine’’ was developed previously. In the technique, suspension cells are attached to a membrane, and as the cells divide, the newborn cells are eluted to produce a synchronous population of cells in the G1 phase of the cell cycle. However, the existing ‘‘baby machine’’ is suitable only for cells that can be chemically attached to a surface. Here, we present a microfluidic ‘‘baby machine’’ in which cells are held onto a surface by pressure differences rather than chemical attachment (Figures 1 and 2). As a result, our method can in principle be used to synchronize a variety of cell types, including cells that may have weak or unknown surface attachment chemistries. We validate our microfluidic ‘‘baby machine’’ by using it to produce a synchronous population of newborn L1210 mouse lymphocytic leukemia cells in G1 phase[1].

High-throughput Electrospinning of Nanofibers from Batch-microfabricated Arrays

Nanofibers’ unique morphological properties promise to make them a key engineering material across many disciplines. In particular, the large specific surface area of the porous webs they form make them highly desirable as multifunctional layers in protective soldier clothing; scaffolds in tissue engineering; and components in devices such as fuel cells, solar cells, and ultra-capacitors[1]. However, their integration into almost all of these technologies is unfeasible as a result of the low throughput and high cost of current production methods. The most common process for producing nanofibers involves applying strong electric fields to polar, high molecular weight polymeric liquids pumped through a syringe in what is known as electrospinning. Electrospinning is the only known technique that can generate nanofibers of arbitrary length and has tremendous versatility as it can create non-woven or aligned mats of polymer, ceramic, semiconducting, and/or metallic fibers.We implement high-throughput arrays of externally fed, batch-microfabricated electrospinning emitters that are precise, simple, and scalable. We fabricate monolithic, linear emitter arrays that consist of pointed structures etched out of silicon using DRIE and assemble these into a slotted base to form a two-dimensional array. By altering the surface chemistry and roughness of the emitters, we can modify their wetting properties to enable “hemi-wicking”[2] of fluid through the micro-texture (Figure 1). The interplay between electric, surface tension, and viscoelastic forces governs the fluid transport and fiber formation. We achieve more than 30 seconds of continuous, stable electrospinning simultaneously from 9 emitters in a two-dimensional array less than 1 cm2 using bias voltages under 15kV (Figure 2). This represents a 4-fold increase in run time compared to similar externally fed approaches[3] and a 7-fold increase in emitter density compared to state-of-the-art MEMS electrospinning sources[4]. Future work should explore denser arrays and integration of a proximal extractor electrode.

Impact of SiNx Passivation on IDS,max of AlGaN/GaN HEMTs

Dielectric films such as SiNx, SiO2, and Al2O3 play key roles in AlGaN/GaN heterostructure field-effect transistors (HFETs) both as surface passivation and as gate-insulating layers[1]. Several groups, including ours, have observed that thin SiNx (<50 nm) deposition on AlGaN/ GaN HFETs by plasma-enhanced chemical vapor deposition (PECVD) can significantly change the two-dimensional electron gas (2DEG) density (ns). The origin of this change in ns has not been carefully analyzed until now. The study of the effect of a SiNx layer on AlGaN/GaN high-electron-mobility transistors (HEMTs) has been challenging since it is difficult to decouple the effects of the SiNx-induced strain from changes in surface potential.In this work, we have investigated the impact of SiNx passivation in the transport properties of AlGaN/GaN heterostructures grown on Si(111) substrates. After the fabrication of standard AlGaN/GaN membrane HEMTs, the Si substrate underneath several devices was selectively etched away using a deep reactive ion etching system with SF6 chemistry[2]. Then, the transistors were passivated with compressive (LFSiN) and tensile (HFSiN) stress SiNx layers deposited by PECVD.A comparative study of compressive and tensile SiNx dielectrics on AlGaN/GaN grown on Si (111) shows a decrease by ~ 40 % and an increase by ~13% in current, respectively (Figure 1). At the same time, a threshold voltage (VT) shift towards 0V is observed after the LFSiN deposition, unlike with the HFSiN passivation cap layer (Figure 2). It was found that surface strain induced by the passivation layer is the main contributor to the change in ns and current density in the GaN-based devices when tensile stress SiNx is deposited, unlike the compressive stress SiNx cap layer. These results pave the way to a new degree of freedom in the design of GaN electronic devices and local strain engineering.

Cell Pairing for Studying Immunity

Cell-cell interactions are crucial for proper functioning of the immune system because direct cell-cell contacts largely govern the successful progression of adaptive immune responses. The heterogeneity inherent in these interactions plays a critical role in the functional outcome produced. Assessing the heterogeneity in the initial activation and connecting it with the endpoint function would, therefore, clarify the signaling cascades involved in the observed outcomes. Current methods to study this heterogeneity are mainly limited by the control over pairing, and thus by initiation of activation and low throughput. To remedy this situation, we developed a high-throughput microfluidic cell-pairing platform for studying the activation kinetics of immune cells. We adapted the microfluidic device from a previously developed chip for studying cell reprogramming[1] and altered device design and geometries to accommodate much smaller immune cells. The device comprises a dense array of weir-based hydrodynamic cell traps that contain a back-side single-cell trap and front-side two-cell trap (Figure 1a). Using a 3-step loading protocol (Figure 1b), we achieved 67 ± 12 % (n=18) pairing efficiencies with a range of 40-86 %, the highest ever reported for such smaller cells (Figure 1c). We used our microfluidic platform to dissect the activation kinetics of T cells from two lines of trans-nuclear mice, Trp1-hi and Trp1-lo, which recognize the identical peptide-MHC complex with markedly different affinities yet are equivalent in their ability to curtail the growth of B16 melanoma in vivo. We paired Trp1-hi/lo T cells with antigen-loaded B cells in a highly parallel and synchronous manner and measured the activation profiles through Ca2+ imaging. Our results show inherent heterogeneity within the clonal population of each cell type and indicate significant differences in the activation dynamics and activation percentages between the two lines (Figure 1d). These findings emphasize how immune cells dissociate the affinity of interaction and heterogeneity in the initial activation from a robust in vivo protective ability. Study of activation cascades in this well-defined system provides substantial insight into how variation leads to robust functional outcomes in immunity.

Electrokinetic Control of Axonal Growth

Dynamic control of axonal outgrowth holds the potential to establishing a range of in-vivo and in-vitro applications including perpiheral nerve section injury recovery, neural computers, and neural interfaces. Although many axon guidance clues like surface topography[1], biochemical[2] or external forces[3] have been investigated, those methods do not provide downscaling and dynamic control of axonal growth.We have introduced the use of AC electrokinetics to dynamically control axonal growth in cultured rat hippocampal neurons. We find that the application of modest voltages at frequencies on the order of 105 Hz can cause developing axons to be stopped adjacent to the electrodes while axons away from the electric fields exhibit uninhibited growth. By switching electrodes on or off, we can reversibly inhibit or permit axon passage across the electrodes. We make use of our dynamic control over axon elongation to create an axon-diode via an axon-lock system that consists of a pair of electrode “gates” that either permit or prevent axons from passing through as shown in Figure 1a-c. Finally, we developed a neural circuit consisting of three populations of neurons, separated by three axon-locks to demonstrate the assembly of a functional, engineered neural network as shown in Figure 1b. Action potential recordings demonstrate that the AC electrokinetic effect does not harm axons, and Ca2+ imaging demonstrated the unidirectional nature of the synaptic connections. AC electrokinetic confinement of axonal growth has potential for creating functional, configurable, and directional neural networks[4].We have extended this work to demonstrate the control of axonal growth in collagen scaffold. The scaffold is confined in a microfluidic channel of three different heights where axons can develop over electrodes. When the channel height was limited to the size range of the growth cone (~ 3 mm), axon repulsion in a 2D plane was observed. We find that developing axons in the microchannel are repelled from the electrodes and follow the field lines until lower field strength (Figure 2a). Axons that grow in the same channel but further away from the electrodes show uninhibited growth. When the channel height is in the range of the growth cone (~10 mm), axonal growth is slowed down by the AC field. Finally, when the channel height is significantly bigger than the growth cone (~ 50 mm), axons develop in 3D into the scaffold and are repelled from the electrodes in the depth of the scaffold itself, where the minimum plane height is linked to the magnitude of the electric field as presented in Figure 2a-b.This new technology provides a powerful tool to confine axonal growth and leads the way to dynamic configurable neuronal networks.

A Novel Microfluidic “Cell-based” Blood Dialysis Platform for Septic Murine Model

Sepsis is an adverse systemic inflammatory response caused by microbial infection in blood. We have reported a microfluidic approach for removal of microbes and inflammatory cellular components from whole blood, inspired by the in vivo phenomenon of leukocyte margination[1],[2]. We also developed a multiplexed blood filtration platform to demonstrate the bacteria removal capability in vivo using a septic mouse model (Figure 1). As blood flows through the margination channel, deformable red blood cells migrate to the axial center (Fahreaus effect), resulting in margination of other cell types towards the sides. Bacteria-depleted blood is collected at the center outlet and returned directly to the animal, as in a complete dialysis circuit. In vitro experiments using human blood spiked with FITC-conjugated Escherichia coli (E. coli) indicated a bacteria removal efficiency of ~70%; inflammatory cellular components (platelets and leukocytes) were also depleted by >70% (Figure 2). To mimic in vivo mouse filtration, a blood sample (~1mL, similar to mouse blood volume) spiked with fluorescent E. coli was subjected to continuous filtration in a closed loop circuit using a peristaltic pump. Experimental data obtained were in good agreement with the Monod kinetics model, achieving ~40% decrease in bacteria concentration after 30 minutes of filtration. The developed technique offers significant advantages: high throughput (~2mL/hr) and label-free separation for non-specific removal of blood-borne pathogens. The device is ideal for the mouse model: the filtration flow rate (90 mL/kg/hr) is comparable to high-volume hemofiltration (45-60 mL/kg/hr) used for humans in clinical settings. Unlike current extracorporeal blood purifications which mostly focus on cytokines removal, we hypothesize that a broad spectrum removal of bacteria and inflammatory cellular components (platelets and leukocytes) could help modulate the host inflammatory response as a blood cleansing method for sepsis treatment.

Preventing Catastrophic Failures: Nano-engineered Multi-physics Structural Damage Detection

Catastrophic structural failures cause many physical and personal losses, with prevention estimated at billions of dollars in savings each year. Non-destructive evaluation (NDE) techniques have been pursued and employed for damage detection of such structures to detect cracks and other damage at pre-critical levels for remediation [1] [2] . Here, a novel multi-physics approach is reported that addresses drawbacks in existing techniques by taking advantage of the effects that damage, such as a crack, has on the electric and thermal transport in a material containing a CNT network distributed in the bulk material. When a potential is applied to a nano-engineered structure(see Figure 1), electric field lines concentrate in the vicinity of cracks as electrons flow around damage, causing field concentrations and “hot spots” via Joule heating, an effect which is amplified because the heat flow is also impeded in areas of damage (e.g., across a crackface). These changes of temperature can be localized through a conventional infrared thermal camera. Low power operation (a 9V standard battery is exemplary, providing a 15C rise at 1 Watt as in Figure 2) and high spatial resolution are demonstrated that are beyond state-of-the-art levels in non-destructive evaluation.Multiple applications have been identified using this technique such as crack detection in composite components that are joined by metallic fasteners, structures having internal nonvisible damage due to impact, and in situprogressive damage monitoring during a tensile strength test. The thermal nano-engineered NDE technique demonstrated here can provide a new and effective inspection route for monitoring the nextgenerations of safer infrastructure [3] [4] .

Aligned CNT-based Microstructures and Nano-engineered Composite Macrostructures

Carbon nanotube (CNT) composites are promising new materials for structural applications thanks to their mechanical and multifunctional properties. We have undertaken a significant experimentally based program to understand both microstructures of aligned-CNT nanocomposites and nano-engineered advanced composite macrostructures hybridized with aligned CNTs.Aligned nanocomposites are fabricated by mechanical densification and polymer wetting of aligned CNT forests [1] . Polymer wetting is driven by capillary forces that arise upon contact of the polymer with the nanostructured CNT forest [2] [3] , the rate of which depends on properties of the CNT forest (e.g., volume fraction) and the polymer (viscosity, contact angle, etc.). Here the polymer is unmodified aerospace-grade epoxy. CNT forests are grown to mm-heights on 1-cm2 Si substrates using a modified chemical vapor deposition process. Following growth, the forests are released from the substrate and can be handled and infiltrated. The volume fraction of the as-grown CNT forests is about 1%; however, the distance between the CNTs (and thus the volume fraction of the forest) can be varied by applying a compressive force along the two axes of the plane of the forest to give volume fractions of CNTs exceeding 20% (see Figure 1a). Variable-volume fraction-aligned CNT nanocomposites were characterized using optical, scanning electron (SEM), and transmission electron (TEM) microscopy to analyze dispersion and alignment of CNTs as well as overall morphology. Extensive physical property testing is underway.Nano-engineered composite macrostructures hybridized with aligned CNTs are prepared by placing long (>20 μm) aligned CNTs at the interface of advanced composite plies as reinforcement in the through-thickness axis of the laminate (see Figure 2). Three fabrication routes were developed: transplantation of CNT forests onto pre-impregnated plies [4] (the “nano-stitch” method), placement of detached CNT forests between two fabrics followed by subsequent infusion of matrix, and in-situ growth of aligned CNTs onto the surface of ceramic fibers followed by infusion or hand-layup [5] [6] [7] . Aligned CNTs are observed at the composite ply interfaces and give rise to significant improvement in interlaminar strength, toughness, and electrical properties. Interestingly, toughness improvement has demonstrated a favorable nano-scale size effect [7] . Analysis of the multifunctional properties and nanoscale interactions between the constituents in both the nanocomposites and hybrid macrostructures is underway. A new route to fabricate these materials in a continuous way has been developed.

RF MEMS Resonators in 32-nm SOI CMOS Technology

This work presents the first hybrid RF MEMS-CMOS resonators demonstrated in silicon at the transistor level of IBM’s 32-nm SOI CMOS process, without the need for any post-processing or packaging. The unreleased, Si bulk acoustic resonators are driven capacitively and sensed using a field effect transistor (FET). MEMS-CMOS Si resonators with acoustic Bragg reflectors (ABRs) are demonstrated at 11.1 GHz with Q~18 and a footprint of 5µm × 3µm.The majority of electromechanical devices require a release step to freely suspend moving structures, which necessitate costly complex encapsulation methods and back-end-of-line (BEOL) processing of large-scale devices [1] . Development of unreleased Si-based MEMS resonators in CMOS allows integration into front-end-of-line (FEOL) processing with no post-processing or packaging. We have previously demonstrated the Resonant Body Transistor (RBT), which employs active FET sensing of acoustic vibrations [2] [3] , which amplifies the mechanical signal before parasitics. Realization of the RBT in CMOS technology leverages high fT, high-performance transistors, enabling RF-MEMS resonators at frequencies orders of magnitude higher than possible with passive devices.The hybrid MEMS-CMOS RBT presented in this work is a Si bulk-acoustic resonator with electrostatic drive formed using the gate dielectric and a body-contacted nFET sense transducer (see Figure 1). Acoustic vibrations in the unreleased resonator are confined using 7 pairs of 1D ABRs surrounding the device, which are patterned using shallow trench isolation (STI). The DC characteristics of the sense transistor are similar to standard body-contacted nFETs of the 32-nm SOI process and show no direct effect of the capacitor drive voltage on the FET behavior. The frequency response of an 11.1-GHz resonator is shown in Figure 2 for multiple bias conditions, verifying the mechanical nature of the resonance.This first demonstration of an unreleased hybrid MEMS-CMOS resonator paves the way for monolithically integrated RF MEMS frequency sources and signal processors.

Electrospray Nanoprinting on Electrospun Nanofiber Mats for Low-cost Biochemical Detection

An electrospray emitter ionizes polar liquids using high electrostatic fields. The electric field produces suction on the free surface (meniscus) of an electrically conductive liquid, and the surface tension of the liquid tends to counteract the effect of the electrostatic suction. If the electric field is larger than a certain threshold, the meniscus snaps into a conic shape called a Taylor cone [1] (see Figure 1). A Taylor cone emits charged particles from its apex due to the high electrostatic fields present there; these particles can be ions, droplets, fibers, etc., depending on the working liquid and the emitter flowrate [2] . In particular, electrospray in cone-jet mode [3] creates near-monodispersed charged droplets that can be used for many applications including mass spectrometry [4] , etching [5] , and nanosatellite propulsion [6] . In this project we are exploring electrospray in cone-jet mode as a technology to create controlled nanoimprints on electrospun nanofiber mats with liquids such as fluorescent dye and nanoparticles solutions, as an alternative technology to nano-pipetting or ink jet printing. Using a shadow mask, we have shown imprints in close agreement with the dimensions of the mask aperture (see Figure 2). The long-term goal of the project is to investigate the design space of the technology to make low-cost and low false-positive biochemical detectors by exploring the multiplexing and scaling-down limits of cone-jet mode electrospray sources using batch micro- and nanofabrication [7] .

Cell Sorting and Identification for Immunology

A major challenge in immunobiology is to better understand how the immune cells dynamically interact with each other and with their environment. Any insights in that direction can help provide a clearer picture of the immune response’s evolution and reaction to infectious diseases; to this end, the use of clinical samples is extremely valuable [1] . However, most of the current analytical techniques used to characterize cells in a sample (e.g., ELISA, flow cytometry, PCR, DNA/RNA sequencing) do not preserve the cells after the analysis, and they typically fail to give multiple parameters of interest from the same sample. In addition, those analyses return only a snapshot of the sample state at a given time, which in most cases cannot be realistically compared with the in vivo conditions. Therefore, there is a need to develop new approaches for high-throughput and multiparameter analyses on clinical samples in a time-dependent manner and with single-cell resolution, to obtain the maximum information from a clinical sample.Recently developed single-cell, multiparameter analytical platforms [2] [3] are oriented to this purpose and can be complemented with DNA/RNA analysis and sequencing to provide a complete picture of the immune cells from a sample, as illustrated in Figure 1. Nevertheless, linking the results of those different approaches requires keeping the identity of the cells all along the multiple analyses. The proposed solution consists of four steps (i) a stochastic bead-based labeling of the cells of interest in the multiparameter analytical platform, (ii) imaging of the labeled cells, (iii) an optical cell release to allow sorting of the selected cells [4] and finally, (iv) a read-out of the labels on the cells in the final platform to re-assign the cell identity.

Flexible Multi-functional Electrodes for Neural Interfacing

Interfaces with the nervous system are important for understanding basic neurobiology and for neuromedicine. We are part of a multi-university NSF Engineering Research Center (ERC) focused on sensorimotor neural engineering. One of the challenges that our team is addressing is making multi-functional interfaces with the nervous system. This work builds upon a previous collaboration developing flexible multi-site electrodes (FME) for insect flight control that directly interfaced with the animal’s central nervous system (Figure 1). The FMEs are made of two layers of polyimide with gold sandwiched in between in a split-ring geometry using standard MEMS processing (Figure 2) [1] . The FMEs have a novel split-ring design that incorporates the anatomical bi-cylinder structure of the nerve cord of the moth Manduca Sexta. Additionally, we integrated carbon nanotube (CNT)-Au nanocomposites into the FMEs to enhance the charge injection capability of the electrode.As part of the NSF ERC, we are working with collaborators to extend this work by integrating the latest knowledge on electrode design into the probes. We are also investigating addition of multi-functionality to the probes, for example integrating both sensing and actuation modalities onto the same device. This integration would allow closed-loop operation of the probes, which we believe will have applicability both to uncover the basic mechanisms behind neurological disorders as well as to serve as eventual “smart” therapeutic devices.

Cell-based Sensors for Measuring Impact of Microsystems on Cell Physiology

The use of microsystems to manipulate and study cells in microenvironments is continually increasing. However, along with such increase in usage comes a growing concern regarding the impact of these microsystems on cell physiology. In this project, we are developing a set of cell-based fluorescent sensors to measure the impact of common stresses experienced in microsystems on cell physiology. We are including stress agents commonly found in microsystems (e.g., UV exposure, heat shock, fluid flow, etc.). Each sensor is designed to respond to one particular stress agent but can also be combined for multiplexed analysis of multiple stresses at once, as might be experienced in a typical microsystem. Each sensor will use different colors to both indicate the type of sensor and the strength of the signal, to ease multiplexed analysis.We are currently developing a sensor that responds to activation of the p53 protein pathway, for generalized DNA damage analysis. Similar to the heat shock sensor we previously reported [1] [2] , which coupled fluorescent protein expression to activation of heat shock factor 1, the DNA damage sensor will couple fluorescent protein expression to activation of p53. The new sensor will have cyan (cerulean) as the constitutive color and red (RFP) as the activation color. Figure 1 shows the DNA damage sensor response after 30 min of UV exposure. One of the more relevant sources for physiological stress on cells cultured in microfluidic devices is shear stress. Construction of a shear stress sensor cell line requires an understanding and characterization of the gene expression mechanisms and mechanotransduction pathways, especially since the pathways are known to have a varying correlation towards cell types, magnitudes, and dynamics of applied stresses. Therefore, we are using a multi-flow microfluidic device (see Figure 2) that can simultaneously apply different flows to cells across a 1000× range to first understand the behavior of NIH3T3 mouse fibroblast cells under flow [3] . Specifically, these cells are seeded in 6 chambers concurrently, exposed to flow for 1-6 hours, and assayed for gene expression changes. Once they are characterized, we will construct a transfected NIH3T3 cell line with RFP expression correlating to shear stress.

Iso-dielectric Separation of Cells and Particles

The development of new techniques to separate and characterize cells with high throughput has been essential to many advances in biology and biotechnology. We are developing a novel method for the simultaneous separation and characterization of cells based upon their electrical properties. This method, iso-dielectric separation (IDS), uses dielectrophoresis (the force on a polarizable object [1] ) and a medium with spatially varying conductivity to sort electrically distinct cells while measuring their effective conductivity (Figure 1). It is similar to iso-electric focusing except that it uses DEP instead of electrophoresis to concentrate cells and particles to the region in a conductivity gradient where their polarization charge vanishes [2] [3] [4] .Sepsis is a clinical condition caused by infection; despite state-of-the-art facilities and treatments, sepsis has a mortality rate of ~30%. Sepsis induces inflammation and organ failure and a possible treatment would require removing inflammatory agents from whole blood such as activated neutrophils. Using an automated IDS system (see Figure 2a) we could see electrical differences between white and red blood cells (Figure 2b). Furthermore, we measured the electrical properties of activated vs. non-activated neutrophils (see Figure 2c). The populations show differences that indicate that the populations are amenable to efficient separation. Using the position as a classifier to determine if a neutrophil is activated or non-activated yields receiver operating characteristic (ROC) curves with high area-under-curve (AUC), which would result in good specificity (see Figure 2d).

Microfluidic Perfusion for Modulating Stem Cell Diffusible Signaling

Stem cell phenotype and function are influenced by microenvironmental cues that include cell-cell, cell-extracellular matrix (ECM), and cell-media interactions (i.e., diffusible signaling), which we can control using microscale systems. Our research focuses on cell-ECM and cell-media control of mouse embryonic stem cells (mESCs). Cells are constantly secreting and responding to soluble signals, the removal of which can be mediated by modulating flow properties at the microscale [1] . To assess the contribution of cell-secreted factors to mESC differentiation and self-renewal, we utilized a two-layer microfluidic perfusion device allowing for parallel comparison of different cell culture conditions (see Figure 1A).Our results demonstrate that mESCs do not grow in differentiation conditions with minimal autocrine signaling, even with supplementation by Fgf4, a signal that has been shown to be a crucial factor in differentiation toward a neuronal stem cell fate, while they do grow when supplemented with media saturated with soluble signals (conditioned media, CM) (see Figure 1B). Consistent with this effect, inhibiting the Fgf4 receptor does not affect growth of mESCs in differentiation conditions (as Figure 1C shows), but it does affect differentiation toward a neuronal stem cell fate (as in Figure 1D) [2] .ESCs grown under self-renewal conditions are able to proliferate without conditioned media, but they lose expression of the self-renewal marker Nanog (see Figure 2A), results that, together with signaling and downstream differentiation assays, indicate differentiation towards an epiblast-like state under conditions that had previously been shown to be sufficient for self-renewal. This differentiation can be reversed by disrupting the ECM using sodium chlorate, which affects the ability of growth factors to bind to the ECM (as Figure 2B shows). This effect is evident based on colony morphology and can be duplicated by disrupting the matrix using collagenase (see Figure 2C) [3] . Together, these results indicate the importance of diffusible cell-secreted signals for mESC growth and ECM-based signals for mESC self-renewal.

Image-based Sorting of Cells

Microfluidic approaches to cell sorting include purely dielectrophoretic (DEP) trap arrays [1] , passive hydrodynamic trap arrays with active DEP-based cell release [2] , and passive microwell arrays with optical cell release to permit sorting of non-adhered cells [3] . As in the preceding technologies, we developed an image-based single-cell sorting method that enables parallel cell sorting using a dual-photopolymerization scheme. Our approach enables simultaneously sorting multiple cells of interest following high-resolution imaging with high purity using a method that requires only common equipment at modest cost. Our overall approach was to spatially segregate cells using a microwell array, image them, and then remove desired cells from the array by encapsulating all the undesired cells in a photopolymer (see Figure 1). To demonstrate the sorting of minority populations (e.g., rare cell isolation), we mixed the GFP- and mCherry-expressing cells at a ratio of 1:100 and targeted to sort the GFP-expressing cells, while RFP-expressing cells were undesired. First, the desired GFP-expressing cells were targeted via microscopy (as Figure 2a shows). The desired GFP-expressing cell in the center was isolated from its surrounding mCherry-expressing cells by the photopolymerized PEGDA sorting well, while the undesired mCherry-expressing cells were encapsulated in the cross-linked gel (in Figure 2b, the gel is autofluorescent in the green channel). Finally, the desired GFP-expressing cells were removed by simply washing the array, leaving the undesired mCherry-expressing cell (see Figure 2c). Figure 2d shows a 1 mm × 1.4 mm region of the array after the desired cells were sorted. The two layers of microwells are evident: the trapping wells made from the photopolymerized optical adhesive and the sorting wells made from the photopolymerized PEGDA hydrogel. The overall technique requires standard equipment found in biological labs and inexpensive reagents (<$10 per experiment), encouraging widespread adoption.

Nanoporous Elements in MEMS with a Focus on Microfluidic Bioparticle Separation

We integrated ultra-porous (99% porous) elements (nanoporous forests of vertically aligned carbon nanotubes (VACNTs)) in MEMS, showing their use in microfluidic applications for bioparticle isolation and health diagnostics. Distinct from works where the effects of fluids on VACNT elements resulted in either structural deformation or catastrophic forest collapse [1] , our approach enables creation of high aspect ratio (~ 1-mm) nanoporous elements and preserves their shape under flow-through conditions. Figure 1 shows a device, consisting of a patterned and (wet) functionalized VACNT forest integrated into a PDMS microfluidic channel.Compared to state-of-the-art designs that exploit solid materials (e.g., silicon, PDMS) for the structural features, our nanoporous elements enable flow around and through the VACNT elements, enhancing physical interaction between the particles in the flow and the functional elements. Multiple device layouts demonstrated a ~7X increase in specific bioparticle capture when transitioning to VACNT porous designs [1] . The large surface-to-volume ratio of nanoporous materials yields a significant increase in the functional surface area (~250-500X for the layouts analyzed in our works [2] [3] ), with permeability comparable to that of macro-scale porous materials [4] , thus further promoting bioparticle capture.Specific isolation of bioparticles ranging over 4 orders of magnitude in size (from cells to viruses) was experimentally demonstrated (Figure 1), including the ability to perform simultaneous multiphysics, multiscale isolation on a single chip [4] . Particles smaller than the average distance between single nanotubes in the VACNT elements (~80 nm) can penetrate the elements and can be isolated using chemical affinity; simultaneously, particles larger than 80 nm cannot enter the nanoporous elements and can be isolated on the elements’ outer surfaces using both mechanical filtration and biomolecular recognition. The nanoporous elements are versatile and could provide access to underexplored sub-micron particles (e.g., proteins, exosomes).

An Analytical Approach for Characterizing the Complete Stress State in Thin-film CMOS Layered Materials and 3D MEMS Design via Postbuckling

Characterization of thin film layered materials is critical for many MEMS devices. Residual stresses from production determine both final shape and performance of microdevices and should therefore be accurately determined. Stresses are typically extracted using simple test structures (clamped beams and cantilevers, see Figures 1a-b) that allow for mean and gradient residual stress estimation [1]. However, current approaches to material characterization have two major limitations. First, their accuracy is directly proportional to their cost. This is especially true for mean compressive stress, where more accurate estimates require a larger number of different test structures. Second, they oversimplify test-structure boundary conditions by considering them to be ideal (e.g., perfectly clamped in the case of buckled beams for mean compressive stress determination [1] ). To overcome these issues, we have developed a new methodology for characterizing the complete stress state (effective mean and gradient stresses) in CMOS layered materials that also assesses non-ideality of clamped boundaries [2] [3] . The approach uses a closed-form solution of the postbuckling problem of micromachined beams including non-ideal boundaries (Figure 1). In Table 1 we show the results relative to the characterization of four different CMOS material combinations. The outcomes show mean compressive stresses ranging between -15 and -105MPa, thus demonstrating the method’s ability to characterize structures subjected to both large and small compressive stresses. This ability contrasts with traditional critical length methods that encounter difficulties in quantifying small compressive stresses due to their inability to distinguish between mean stress and gradient stress effects [2] . For the CMOS materials examined here, the accuracy was ± 2MPa for mean stresses and ±3MPa/µm for gradients. Boundary non-ideality is found to be 90% of perfectly clamped for the CMOS-released films, having such a significant effect on the extracted stresses that it must be considered. The analytical tool can also be extended to 3D MEMS design, where buckling is used to controllably place structural elements outside the wafer plane. Using this approach, we have demonstrated out-of-place architectures for applications from three-axis thermal sensing to 3D flow measurement [4] ).

Growth of Vertically Aligned Carbon Nanotubes on a Continuously Moving Substrate

Vertically-aligned carbon nanotube (CNT) arrays are grown on a moving substrate, demonstrating continuous growth of nanoscale materials with long-range order. A cold-wall chamber with an oscillating moving platform (see Figure 1) is used to locally heat a silicon growth substrate coated with a Fe/Al2O3 catalyst film for CNT growth via chemical vapor deposition. The reactant gases are introduced over the substrate through a directed nozzle to attain high-yield CNT growth [1] . Aligned multi-wall carbon nanotube (MWNT) arrays (or “forests”) with heights of ≈1 mm are achieved at substrate speeds up to 2.4 mm/s. Arrays grown on moving substrates at different velocities are studied to identify potential physical limitations of repeatable and fast growth on a continuous basis. No significant differences are noted between static and moving growth as characterized by SEM (as in Figure 2) and Raman spectroscopy, although overall growth height is marginally reduced at the highest substrate velocity. CNT arrays produced on moving substrates are also found to be comparable to those produced through well-characterized batch processes consistent with a base-growth mechanism. Growth parameters required for the moving furnace are found to differ only slightly from those used in a comparable batch process; thermal uniformity appears to be the critical parameter for achieving large-area uniform array growth.Once the parameters have been optimized, a desktop continuous will growth apparatus has been designed and implemented to grow VACNTs on silicon wafers (Figure 2), flexible sheets, and alumina fibers continuously. We have demonstrated and reported the ability to manufacture VACNT arrays in a continuous manner, significantly reducing the time spent, energy consumed, and reaction products created as compared to batch processing of these technologicallyvaluable assemblies of nanoscale materials [2] .

Flush-mounted MEMS Langmuir Probe Arrays for HF-S Band Plasma-sensing

Arrays of MEMS Langmuir probes that are flush-mountable (Figure 1) can serve as a sensorial skin on a spacecraft for fine spatial and temporal resolution of plasma phenomena. The technology can also provide diagnostics for other applications such as tokamaks and nanosatellite scientific payloads [1] . The benefits are innumerable for deeper understanding of plasma physics, which is in great need of these microprobes [2] . For instance, multiplexed microprobes that are flush-mounted on all the faces of a 3-D “tip” can allow for simultaneous capture of a detailed “whole picture” of plasma behavior in different axes at a given timescale. In addition, two or more different sensory configurations, e.g., single-, double-, triple-probe methods, etc., can be adapted into the same flat die, profiting at the same time from their individual data acquisition strengths. Protruded probes cannot offer these advantages. Another area of deployment is in the observation of electron phase-space holes, self-consistent nonlinear plasma structures that are formed from strong current- or beam-driven turbulence and found in magnetic reconnection regions, which are magnetic field topology modifiers responsible for the explosive release of magnetic energy in magnetospheric storms, solar flares, and laboratory plasmas [3] . Fast micro-Langmuir probes that work at high frequencies are indispensable for studying these plasma fluctuations. We developed a system of flush-mounted MEMS Langmuir probes and apparatus with fast timescale; i.e., shorter time compared to the timescale of reconnection events in the Versatile Toroidal Facility at MIT (Figure 2); and wide bandwidth extending across regions of magnetosphere-photosphere, i.e., considering both electron and ion plasma frequencies associated with these regions.

Silicon Field Emitter Arrays for Chip-scale Vacuum Pumping

Development of miniature vacuum pumps that can be integrated with electronic or MEMS components is necessary for producing advanced equipment such as portable analytical instruments [1] and high performance sensors [2] . The proposed approach graphically illustrated in Figure 1 is based on electron impact ionization (EEI) or field ionization (FI) of the gas molecules using nano-scale sharp silicon tips. The ionized gas molecules are then evacuated from the chamber using a strong electric field to accelerate the ions and implant them permanently into a getter medium made of Ti or Al. In the EEI mode of the operation, a positive voltage is applied between the gate and the emitter to extract electrons that are used to ionize the background gas. In the FI regime, the Si sharp tips are biased at a positive voltage with respect to the gate to extract electrons from the outer shell of the gas molecules in a quantum tunneling process. The former process occurs at electric fields in the range of 3 – 6 ×107 V/cm while the later process initiates at electric fields above 108 V/cm [3] . Despite the larger required voltage, the operation in the FI regime is mandatory since the back-streaming of the positive ions during EEI mode of operation will damage the field emitter (FE) tips at mTorr-pressure range. Although state-of-the-art field emitters have been reported [4] [5] [6] , the focus of this work is to improve the reliability of the FE or FI devices for extended operation times and large currents necessary for pumping application. Since these devices demand application of large voltages between the gate and the tip of the FE/FI, wear or breakdown of the insulating dielectric is a major issue. Finite element modeling (shown in Figure 2) has been conducted to optimize the design of the device for pumping application. A new fabrication process is also being developed for high-yield fabrication of an array with more than 300K Si FEs/FIs.

Measuring Ion Energy Distribution Using Batch-microfabricated RPAs

The need to measure particle energies arises in many applications, from calibrating electron sources for electron guns in precision microscopes to determining the efficiency of space-based ion beam thrusters. Retarding potential analyzers (RPAs) are capable of filtering particles based on their energy and have been used as early as the late 1950s and early 1960s for such purposes [1] . However, these devices maintain limited application due to stringent dimensional constraints driven by plasma Debye length. Cold dense plasmas require minute apertures and tight spacing tolerances between biasing grids that are difficult to enforce using conventional means. We suggest microelectromechanical system (MEMS) batch-fabrication techniques in order to achieve unprecedented alignment accuracy of successive electrodes while incorporating the necessary micron-scale features. Assembly to a precision of a few tens of microns has been demonstrated with a hybrid RPA (see Figure 1a) [2] . Figure 1b shows the fully MEMS-fabricated sensor inspired by in-plane assembly of high-voltage devices, which will have tolerances on the order of 1μm [3] .Augmenting the optical transparency of RPAs provides a more direct path for particles to the collector plate. Signal strength is thus improved as the effective collection area is increased. Preliminary results and comparisons between MEMS-fabricated electrodes and conventional stainless steel mesh have revealed an ameliorated signal quality. Figure 2 shows a greater than two-fold improvement in peak signal strength with the micromachined grids over the conventional RPA [2] . Currents captured by the various grids and simulations suggest the possibility of ion beam focusing and interception of ions prior to reaching the collector. Alteration of the internal dynamics of the sensor provides a cleaner signal that may lead to a better interpretation of the measurements than with models that incorporated the stochastic behavior of charged species through randomly oriented electrode apertures.

Cathode for X-ray Generation with Arrays of Individually Addressable Field Emitters Controlled by Vertical Ungated FETs

This work focuses on the design and fabrication of a cathode for a portable x-ray source. The cathode is made of an array of individually addressable electron guns, each containing double-gated field emitters. Compared to thermionic cathodes, field emission arrays operate at lower vacuum and lower temperatures, use less power and are more portable. The electron beam from each gun is extracted by a proximal gate and collimated using a distal gate before it hits an anode in a micron-sized spot that generates Bremsstrahlung x-rays. The architecture of the cathode is shown in Figure 1. Each field emitter is fabricated on top of a vertical ungated field-effect transistor (FET) [1] [2] that acts as a current source due to the velocity saturation of electrons in silicon when the voltage across the FET is above a saturation voltage. Current source-like behavior provides spatial and temporal uniformity of the output current across the emitter array; it also protects against emitter burnout and current surges. Individual addressability is achieved by fabricating the structure on SOI wafers to create electrically isolate strips of silicon. The extractor and focus gates are monolithically integrated with the cathode chip. They are patterned in strips that are orthogonal to the silicon strips, so that a single electron gun can be turned on at once. Each vertical ungated FET is a 25-μm-tall column with a 0.5-μm diameter, and emitter tip radius is in the range of 20 nm. The saturation current and saturation voltage of the silicon columns are plotted as a function of doping density in Figure 2. Wafer doping of 10-20 Ω cm provides a saturation current of 0.5 μA and an output impedance of 2×109 Ω. With 100 emitters per chip, the total output current per chip is 50 μA, corresponding to a current density of 139 μA/cm2.

Batch-Microfabricated Electrospray Arrays with Integrated Electrode Stack for Ionic Liquids

Electrospray is a process to ionize electrically conductive liquids that relies on strong electric fields; charged particles are emitted from sharp tips that serve as field enhancers to increase the electrostatic pressure on the surface of the liquid, overcome the effects of surface tension, and facilitate the localization of emission sites. Ions can be emitted from the liquid surface if the liquid is highly conductive and the emitter flowrate is low. Previous research demonstrated successful operation of massive arrays of monolithic batch-microfabricated planar electrospray arrays with an integrated extractor electrode using ionic liquids EMI-BF4and EMI-Im [1] [2] – liquids of great importance for efficient nanosatellite propulsion. The current work aims to build upon the previous electrospray array designs by increasing the density of the emitter tips, increasing the output current by custom-engineering suitable nanofluidic structures for flow control, and improving the ion optics to gain control of the plume divergence and exit velocity.The basic version of the MEMS electrospray array consists of an emitter die and an extractor die (shown in Figure 1), both made of silicon and fabricated using deep reactive ion etching. The two dies are held together using a MEMS high-voltage packaging technology based on microfabricated springs that allows precision packaging of the two components with less than 1% beam interception [3] [4] . The emitter die contains dense arrays of sharp emitter tips with as many as 1,900 emitters in 1 cm2. A voltage applied between the emitter die and the extractor electrode creates the electric field necessary to ionize the ionic liquid (see Figure 2). A nanostructured material transports the liquid from the base of the emitters to the emitter tips. The present research focuses on engineering the nanofluidic structure to attain higher emitter current while maintaining good array emission uniformity and on developing batch microfabricated advanced ion optics to control the electrospray plume.

Externally-fed, Microfabricated Electrospinning Device for Increased Throughput of Polymer Nanofibers

Electrospinning is a process in which a membrane-like web of thin fibers can be produced using high electrostatic fields and polar liquids with high viscosity. It is the only known technique that can generate continuous fibers with controlled morphology in the 10-500 nm diameter range and has tremendous versatility as it can create non-woven or well-aligned mats of polymer, ceramic, semiconductor, and/or metallic fibers using the same hardware. Electrospinning is also capable of conformally coating 3D complex shapes with ultrathin layers that have complex multi-layered structure and thickness variation across the surface. In particular, polymer electrospun fibers have been proposed to develop multi-stack functional fiber mats for protective gear, because they show high breathability, elasticity, and filtration efficiency. In addition, electrospun fibers made of the appropriate materials could also be used in flexible electronics (graphene) and in structural reinforcement against mechanical trauma. However, the production of electrospun nanofibers has very low throughput due to the small fiber diameter, which limits their applications to high-end products. In this project we are investigating the development of high-throughput electrospun nanofibers using batch-microfabricated arrays of externally fed electrospinning emitters. Externally-fed emitters are attractive, because they do not require high pressure drops as internally-fed emitters do. Also, they do not clog and can process liquids that bubble.An aspect of this project is looking into the physics of wicking to optimize the fluidic micro/nanostructures that control the emitter flow rate. For solids with intrinsic contact angles below some critical value determined by roughness geometry, it becomes energetically favorable for a droplet to completely impregnate the roughness and spread through it [1] . This process of hemi-wicking has been described in pillar arrays of varying shapes and sizes [2] [3] . For externally-fed electrospinning, we must ensure a sufficient and steady flow rate of polymer solution to avoid broken or irregular fibers. We are theoretically and experimentally investigating optimal morphologies of both the micro/nano fluid control structures and the emitter geometry to attain good array emission uniformity.

Evolution of Intrinsic Stress and Grain Structure in Polycrystalline Films for Nano/Micro-electromechanical System Applications

Controlling the intrinsic stress in polycrystalline thin films is of great importance in a wide variety of applications, especially those in which mechanical properties and reliability issues are critical, e.g., Nano-/microelectromechanical systems (N/MEMS). Using capacitance techniques, intrinsic stress can be monitored in situ and in real time during deposition processes. We do this in a UHV e-beam evaporator in which we grow FCC metal films at a range of homologous temperatures, in a range of deposition rates, and with variable vacuum conditions. These studies show an evolution to a high tensile stress during film formation (Type I behavior), or an evolution first to a tensile stress and then to a compressive stress at higher thicknesses (Type II behavior). The origin of this behavior, especially Type II behavior, is not well understood. In recent studies we have found that Pd and Ni deposited at intermediate homologous temperatures undergo a behavior intermediate to that of Type I and Type II (Figure 1), where the stress evolves from tensile to compressive and back to tensile. Transmission electron microscopy (TEM) reveals that the grain size increases during deposition at low or intermediate homologous temperatures. The grain size in Ni films deposited from 300K to 473K forms a linear relation with film thickness. Figure 2 shows representative bright field TEM images of Ni films deposited at 473K. It is known that grain growth in a constrained film leads to tensile stress. We believe that while the first tensile rise is associated with a coalescence stress, the second is associated with grain growth. Grain growth itself leads to a tensile stress and also to a lower rate at which ad-atom are trapped at boundaries to cause compressive stresses. We find that changes in the deposition conditions can modify this behavior.

Designing Complex Digital Systems with Nano-electro-mechanical Relays

Silicon CMOS circuits have a well-defined lower limit on their achievable energy efficiency due to sub-threshold leakage. Once this limit is reached, power constrained applications will face a cap on their maximum throughput independent of their level of parallelism. Avoiding this roadblock requires an alternative device with steeper sub-threshold slope – i.e., lower VDD/Ion for the same Ion/Ioff. One promising class of such devices is electro-statically actuated nano-electro-mechanical (NEM) switches with nearly ideal Ion/Ioff characteristics. Although mechanical movement makes NEM switches significantly slower than CMOS, they can be useful for a wide range of VLSI applications by reexamining traditional system- and circuit-level design techniques to take advantage of the electrical properties of the device. NEM relay circuits with pass-transistor logic design combine as many propagating electrical delays into as few mechanical delays as possible, parallelizing the tasks to do more operations in less time.Basic circuit design techniques and functionality of some main building blocks of VLSI systems, such as logic, memory, and clocking structures, have been successfully demonstrated in our previous works [1] [2] [3] .Recently, complex arithmetic units such as relay-based multipliers have been developed (Figure 1b-c) [4] . Simulation results of an optimized 16-bit relay multiplier built in a scaled relay process predicts ~10x improvement in energy-efficiency over optimized CMOS designs in the 10-100 MOPS performance range. The relative performance of the multiplier enhancements are in line with what was previously predicted by a NEM relay 32-bit adder [3] , suggesting that complete VLSI systems (e.g., a microprocessor or an ASIC) would expect to see similar energy/performance improvements from adopting NEM relay technology [3] [4] . The operation of the main building block of the MEM-relay based multiplier, the (7:3) compressor, is experimentally demonstrated. This circuit, consisting of 98 MEM-relays, is the largest MEM-relay based circuit successfully tested to date (Figure 2) [4] .

Measuring Single-cell Density

We have used a microfluidic mass sensor to measure the density of single living cells. By weighing each cell in two fluids of different densities (see Figure 1), our technique measures the single-cell mass, volume, and density of approximately 500 cells per hour with a density precision of 0.001 g mL−1. We observe that the intrinsic cell-to-cell variation in density is nearly 100-fold smaller than the mass or volume variation. As a result, we can measure changes in cell density indicative of cellular processes that would be otherwise undetectable by mass or volume measurements. Here, we demonstrate this with four examples: identifying erythrocytes infected with Plasmodium falciparum malaria in a culture, distinguishing transfused blood cells from a patient’s own blood (as in Figure 2), identifying irreversibly sickled cells in a sickle cell patient, and identifying leukemia cells in the early stages of responding to a drug treatment. These demonstrations suggest that the ability to measure single-cell density will provide valuable insights into cell state for a wide range of biological processes.

Design and Modeling of a PZT Thin-film-based Piezoelectric Micromachined Ultrasonic Transducer

Although new software techniques enable higher-resolution medical ultrasound imaging, commercial ultrasonic transducer technology has remained largely unchanged for a few decades. Current transducers are fabricated from bulk PZT using assembly steps that are labor-intensive and limit individual transducers to millimeter-sized features. With micro-fabrication technology, micro-scale transducers can be easily manufactured at very low cost, but their acoustic power and efficiency may be compromised. We revisit a piezoelectric micro-machined ultrasonic transducer (PMUT) based on a lead zirconate titanate (PZT) thin film with a view to improve acoustic performance. Our initial findings show that the inherently high piezoelectric coupling of thin-film PZT produces the deflection necessary for high acoustic pressure applications without significant power requirements or application of a DC bias voltage if the design can be optimized. With its high acoustic pressure output and small size, a PMUT could be used for deep penetration and non-invasive medical imaging, e.g., intracranial monitoring of head injuries.Our group has derived the equivalent circuit for a bimorph PMUT [1] . This configuration sandwiches a PZT between top and bottom electrodes and actuates it with an applied voltage across the electrodes. Adding a structural support layer, such as silicon, creating a multimorph device increases the model’s complexity. With separate definition of mechanical and electrical neutral axes, the equivalent circuit derivation extends to include the multimorph design [2] . With this advance, transduction behavior of the PMUT can be more accurately predicted, designs more easily optimized, and results validated with a complete model. An analytical solution for deflection based on electrode coverage has been derived and the optimum electrode coverage for maximum deflection has been determined. Based on the modeling results, fabrication of an optimized PMUT design is now underway. Our eventual goal is to incorporate PMUT elements into 1D and 2D arrays with a small form factor to enable high resolution medical imaging.

Applications of Piezoresistive Nanocomposites in Electronics

Polymer materials doped with conductive particles exhibit piezoresistive properties. These materials are fabricated such that their conductivity changes with an applied compressive force. When compressed, the formation of percolation pathways allows increased electrical conduction through tunneling between the particles. This work explores and utilizes this property of composites to fabricate various devices with the ultimate goal of developing integrated flexible systems resembling sensory skins.As a first generation of piezoresistive devices, a squeezable switch (squitch) is fabricated with a three-terminal configuration shown in Figure 1 [1] . In this study, the squitch is fabricated from a composite of polydimethylsiloxane doped with 60 wt% Ni microparticles that shows more than 5 orders of magnitude change in conductivity over a 20% strain (Figure 2). In the absence of an applied gate bias, the composite is a poor conductor. An applied gate voltage generates an electrostatic force between the source and the gate that compresses the composite, causing the squitch to conduct. To allow fabrication of reliable and reproducible devices, the composite needs to be engineered such that its mechanical properties are more stable. To achieve this goal, current research explores the effects of the type of polymer and conductive particles and the method of fabrication on the properties of the nanocomposite and performance of the squitch. The surfaces of the metal particles are chemically treated to allow better distribution in the polymer matrix while also chemically binding the particles to the polymer preventing particle migration over repeated use of the device. After the composite is optimized, future work will involve extending the squitch design to fabricate devices such as analog amplifiers, digital inverters, and various sensors and developing processes to allow large-area fabrication. The devices will then be integrated to develop artificial skins.

Nano-electromechanical System Digital Switches

Nano-electromechanical systems (NEMS) are an emerging area of research with potential applications as low-power switches for electronic circuits. The proliferation of electronics in both stationary and portable applications demands the development of more energy-efficient devices than are currently available. While solid-state silicon MOS-based transistor circuits, the dominant technology in today’s electronics, have greatly reduced their power requirements by aggressive scaling, the concurrent increase in off-state leakage current limits their energy efficiency. In contrast, microelectromechanical relays have been demonstrated with zero off-state currents and abrupt switching characteristics [1] [2] . As these and other electromechanical devices are shrunk to the nanoscale, their actuation voltages, and hence power requirements, are expected to be reduced significantly.Our group recently presented a three-terminal electromechanical switch based on a piezoresistive polymer nanocomposite as the active material [3] . The metal-polymer composite consisted of a polydimethylsiloxane polymer matrix doped with 60 wt% nickel particles. A schematic diagram of this squeezable switch, or “squitch,” is shown in Figure 1. In its initial state, the conductive metal particles are separated by the insulating polymer matrix. Thus, the active material is highly resistive, and little current flows through the device (in the “off” state). When compressed, the metal-metal distances decrease until the onset of tunneling allows current to flow from source to drain (“on” state). The first-generation squitch demonstrated transistor-like behavior with drain-source conduction modulation over 4 orders of magnitude when electromechanical force was applied. However, the large mechanical dimensions of this concept demonstration necessitated higher supply voltages than desired. Our current work focuses on incorporating the squitch concept into nanoscale devices by (a) developing improved device structures and fabrication methods and (b) exploring new materials such as ligand-coated nanoparticles and self-assembled monolayers as active materials.

MEMS Pressure-Sensor Arrays for Passive Underwater Navigation

A pressure sensor array is under development for unmanned undersea vehicles (UUV). This project is inspired by the lateral line sensory organ in fish, which enables some species to form three-dimensional maps of their surroundings [1] [2] . The canal subsystem of the organ can be described as an array of pressure-sensors [3] . The lateral line allows fish to perform a variety of actions, from tracking prey [4] to recognizing nearby objects [2] [5] . Similarly, by measuring pressure variations on the surface of an UUV, an engineered pressure-sensor array supports the identification and location of obstacles for navigation.To be compatible with the doubly-curved surface of a typical UUV hull, the pressure sensor array must be flexible. Further, it is desirable that the array be amenable to wide-area fabrication. Correspondingly, the design pursued here is fabricated primarily from a PDMS polymer, some parts of which are doped with conducting nanoparticles so as to become piezoresistive. As shown in Figure 1 below, a pressure sensor array consists of piezoresistive strain-gauges patterned onto PDMS membranes suspended over cavities formed in a PDMS substrate [6] . The resistance of each strain gauge is measured using a four-point probe array with a common current source shared by all sensors. The strain-gauge resistance can be related to the deflection of its corresponding membrane, and hence the pressure difference across the membrane. All cavities are connected together so that all pressure sensors have a common reference.During the past year, flexible pressure sensor arrays were mounted on the side of a kayak for open-water tests, as shown in Figure 2a below. The pressure measurement from one sensor is shown in Figure 2b together with measurements from nearby commercial reference sensors. The similarity of the measurements demonstrates the functionality of the PDMS pressure sensors in an uncontrolled environment.

Microfluidic Device for Characterization of Dynamic Red Blood Cell Deformability

The average diameter of human red blood cells (RBCs) is around 8µm. As RBCs circulate in the body and transport oxygen, they have to deform repeatedly in small blood capillaries. RBC deformability is therefore an important mechanical attribute for efficient oxygen delivery. Several blood related diseases such as malaria, sickle cell anemia, and sepsis are marked with significant alterations in RBC deformability [1] [2] [3] .This project studies RBC dynamic deformability using a simple, portable microfluidic device [4] . The deformability of individual RBCs can be assessed by the average velocity of RBCs passing through narrow microfluidic channels. The repeated deformations to be experienced by RBCs simulate in vivo blood capillary system. Several blood-related diseases are included in our studies.

Diffusive Transport of Acid through Mucus Hydrogels inside a Microfabricated Device

In the stomach, the biological hydrogel known as mucus protects the stomach wall from the damaging effects of strongly acidic digestive juices inside the stomach lumen. Altered mucus function is linked to gastric diseases including ulcers and cancers. The biophysical mechanisms underlying the barrier are not well understood, due partly to a lack of suitable in vitro tools.In this work, we developed an in vitro microfluidic system designed to mimic mucus secretion in the stomach (see Figure 1) [1] . In our system, mucus components are pumped continuously on-chip into an acidic flow, mimicking in vivo mucus secretion into an acidic stomach lumen. A fluorescent pH indicator added to the samples allows optical tracking of acid diffusion. Our microfluidic system is superior to in vitro macroscale techniques currently used to assay mucus function [2] . Advantages of our system include study of barrier function under secretion rather than static conditions, ability to optically measure the pH profile inside the mucus layer, and low sample volume requirement enabling experiments using difficult-to-purify mucus components.With this system, we demonstrate that continuous secretion of mucin glycoprotein, the dominant protein component of mucus, hinders the diffusion of acid (Figure 2) due to the ability of mucins to directly bind and sequester H+ (see [1] for more details). We further estimate that the barrier function resulting from direct binding of H+ to mucin constitutes a significant portion of the in vivo mucus barrier. This “mucus-secretion-on-a-chip” platform may be used to systematically study the barrier function of each mucus layer component, perform diagnostics of mucus function using small amounts of clinical sample, and test mucus-targeted drugs.

Particle Behavior inside Planar Straight and Spiral Microchannels

Although inertial force-induced lateral migration has been extensively studied for almost 50 years and has been utilized in various microchannels to perform size-based separation for cell research, the mechanism of inertial focusing is generally described as the interplay between inertial lift force and dean drag force and lacks information on particle behavior in depth direction, leaving several missing pieces from the physical understanding of inertial focusing parameter space [1] [2] [3] . Here we present an exploratory study of inertial focusing in planar straight microchannels and spiral microchannels with varying geometry to identify the regimes of particle behavior in response to flow rate and channel dimension. To gather accurate information on the depth direction of a straight channel, we fabricated a pair of straight channels with the same cross-sectional dimensions but different orientations and recorded the focusing positions of particles in the top-down images under the same conditions using these two devices, respectively. Combining the data from these two channels provides unambiguous information on the cross-sectional particle focusing positions. We also developed a polymer-casting technique to fabricate PDMS devices with smooth sidewalls through which one can observe the particle positions at the outermost loop of the planar spiral in the channel depth direction. The data gathered for the same spiral channel but from different directions allowed us to map the distribution of particles in cross-section with a simulated velocity field. With accurate information on particle positions in the cross-sections of straight and spiral channels, we would be able to relate the effect of channel dimension on the force field with the related particle- focusing behavior and identify the key parameters for the optimal design of a size-based separation device targeted at specific size range.

Removal of Pathogen and Inflammatory Components from Blood using Cell Margination

Sepsis is an adverse systemic inflammatory response caused by microbial infection in blood. In this work, we report a simple microfluidic approach for intrinsic, non-specific removal of both microbes and inflammatory cellular components (platelets and leukocytes) from whole blood, inspired by the in vivo phenomenon of leukocyte margination [1] . As blood flows through a narrow microchannel (20 × 20 µm), deformable red blood cells (RBCs) migrate axially to the channel center, resulting in margination of other cell types (bacteria, platelets and leukocytes) towards the channel sides (see Figure 1) [2] . With the use of a simple cascaded channel design, the blood samples undergo a 2-stage bacteria removal in a single pass through the device, thereby allowing higher bacterial removal efficiency. As an application for sepsis treatment, we demonstrated separation of Escherichia coli and Saccharomyces cerevisiae spiked into whole blood, achieving high removal efficiencies of ~80% and ~90%, respectively (Figure 2A). Inflammatory cellular components were also depleted by >80% in the filtered blood samples, which could help to modulate the host inflammatory response and potentially serve as a blood-cleansing method for sepsis treatment. The developed technique offers significant advantages including high throughput (~1mL/hr per channel) and label-free separation that allows non-specific removal of any blood-borne pathogens (bacteria and fungi). The continuous processing and collection mode potentially enables the return of filtered blood to the patient directly, similar to a simple and complete dialysis circuit setup. Due to design simplicity, further multiplexing is possible by increasing channel parallelization or device stacking to achieve higher throughput comparable to convectional blood dialysis systems used in clinical settings.

Waveguide Micro-probes for Optical Control of Excitable Cells

Professor Ed Boyden uses light to precisely control neural activity. His lab has invented safe, effective ways to deliver light-gated membrane proteins to neurons and other excitable cells (e.g., muscle, immune cells, pancreatic cells, etc.) in an enduring fashion, thus making the cells permanently sensitive to being activated or silenced by millisecond-timescale pulses of blue and yellow light, respectively [1] . This ability to modulate neural activity with a temporal precision that approaches that of the neural code itself holds great promise for human health, and his lab has developed animal models of epilepsy and Parkinson’s disease to explore the use of optical control to develop new therapies.We have recently developed mass-fabricatable multiple light guide microstructures produced using standard microfabrication techniques to deliver light to activate and silence neural target regions along their length as desired [2] . Each probe is a 100- to 150-micron-wide insertable micro-structure with many miniature lightguides running in parallel and delivering light to many points along the axis of insertion. Such a design maximizes the flexibility and power of optical neural control while minimizing tissue damage. We are currently developing 2-D arrays of such probes so multiple colors of light can be delivered to 3-dimensional patterns in the brain, at the resolution of tens to hundreds of microns, thus furthering the causal analysis of complex neural circuits and dynamics. Such devices will allow the substrates that causally contribute to neurological and psychiatric disorders to be systematically analyzed via causal neural control tools. Given recent efforts to test such reagents in nonhuman primates, these devices may also enable a new generation of optical neural control prosthetics, contributing directly to the alleviation of intractable brain disorders.The initial light-guide structures have been fabricated from silicon oxynitride clad with silicon dioxide, and tests show excellent transmission of light with no visible loss in the taper and bend regions of the patterns [2] . Significantly, the novel 90˚ bend invented to direct light laterally out the side of the narrow probe functions as designed [2] . The optical sources for initial tests with the probe are independent laser modules coupled to one end of a fiber-optic ribbon cable (see Figure 2). The other end of the ribbon cable is butt-coupled to the inputs of the probe via a standard fiber-optic connector ferrule. This allows for increased modularity and control in initial probe testing.We are now utilizing transgenic mice, which express optogenetic activators and silencers in cortical pyramidal neurons, to demonstrate optogenetic control of neural circuits in a fashion appropriate for in vivo circuit mapping or brain machine interface prototyping. Our goal is to explore the degree to which this technology can be used to functionally map neural network connectivity over large, multi-region circuits in the brain, and to subserve a new generation of neural control prosthetics.

Compact Parameterized Modeling of RF Nano-Electro-Mechanical (NEM) Resonators

Design and optimization of novel RF Nano-Electro-Mechanical (NEM) resonators such as Resonant Body Transistors (RBT) require modeling across multiple domains, including mechanical (distributed stress and elastic wave models), electrical (semiconductor devices and RF small signal models), and thermal. These domains are all cross-coupled in nonlinear ways and require lengthy finite element multi-physics analyses to solve. Due to the complexity of these structures embedded in the CMOS stack and sensed using active FETs, the day-long time scale of each finite element simulation prevents quick, intuitive parameterization of device design. A reduced model parameterized across all three domains is therefore necessary both for rapid prototyping and for device optimization.In this work, we are developing an algorithm to automatically generate compact models for NEM resonators. Our compact models are suitable for AC, DC and RF operation of the device and allow the circuit designers to run circuit-level time-domain simulations using any commercial circuit simulator [1] [2] . The compact models are “parameterized,” so that the circuit designer will be able to instantiate instantaneously models within the circuit simulator for different values of the key device parameters. Key resonator parameters included in the compact parameterized model are resonant frequency, quality factor, signal strength, isolation, presence of spurious modes, and operating temperature. Values for the model coefficients are calibrated using measurements from NEMS resonator devices. A critically important feature of our models is to guarantee that when circuit designers change arbitrarily values for the device parameters, the compact models will always preserve the physical properties of the original device and will never cause numerical instabilities and convergence issues when connected to other device models and circuits within the circuit simulator [3] . Figure 1 shows the layout of a Si-based NEMS-CMOS resonator. Numerical results show a great promise for our technique. We have achieved high quality fit to the measured data, as Figure 2 shows, which offered modeling challenges including the presence of noise and spurious resonant peaks.

Recombination Dynamics of Charge Carriers in Nanostructured Solar Cells

Nanostructured solar cells are attracting increasing attention as a promising photovoltaic (PV) technology [1] . Generation of free charge carriers in nanostructured PV devices occurs at the electron donor-acceptor interface, analogous to the pn-junction interface in traditional crystalline silicon solar cells. However, recombination at this interface constitutes one of the major charge carrier loss pathways. Thus characterizing and controlling recombination dynamics is critical for informing the design of novel device architectures. Recombination parameters also enable comparisons between different device architectures.In this work, we employ the transient photovoltage (TPV) technique [2] to probe recombination mechanisms under standard operating conditions in three different solar cells, as shown in Figure 1: a poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk heterojunction; a chloroaluminium phthalocyanine and fullerene (ClAlPc:C60) planar mixed heterojunction; and a lead sulfide quantum dot and zinc oxide (QD PbS:ZnO) pn-heterojunction. The normalized TPV data acquired at 0.5-sun illumination intensity are shown in Figure 2a, which compares the recombination lifetimes of charge carriers in these devices. The observed differences in carrier lifetimes may arise from variations in the respective interface morphologies: for example, the slower recombination transients observed in the ClAlPc:C60 device may be attributed to the intrinsic planarity of this particular architecture. We can also measure the charge carrier lifetime as a function of the light intensity, as shown in Figure 2b; this result confirms that recombination dynamics are faster in P3HT:PCBM and QD PbS:ZnO than in ClAlPc:C60 PV devices.