Engineering Heteromaterials to Control Lithium Ion Transport Pathways.

Safe and efficient operation of lithium ion batteries requires precisely directed flow of lithium ions and electrons to control the first directional volume changes in anode and cathode materials. Understanding and controlling the lithium ion transport in battery electrodes becomes crucial to the design of high performance and durable batteries. Recent work revealed that the chemical potential barriers encountered at the surfaces of heteromaterials play an important role in directing lithium ion transport at nanoscale. Here, we utilize in situ transmission electron microscopy to demonstrate that we can switch lithiation pathways from radial to axial to grain-by-grain lithiation through the systematic creation of heteromaterial combinations in the Si-Ge nanowire system. Our systematic studies show that engineered materials at nanoscale can overcome the intrinsic orientation-dependent lithiation, and open new pathways to aid in the development of compact, safe, and efficient batteries

Lattice strain effects on the optical properties of MoS2 nanosheets.

"Strain engineering" in functional materials has been widely explored to tailor the physical properties of electronic materials and improve their electrical and/or optical properties. Here, we exploit both in plane and out of plane uniaxial tensile strains in MoS2 to modulate its band gap and engineer its optical properties. We utilize X-ray diffraction and cross-sectional transmission electron microscopy to quantify the strains in the as-synthesized MoS2 nanosheets and apply measured shifts of Raman-active modes to confirm lattice strain modification of both the out-of-plane and in-plane phonon vibrations of the MoS2 nanosheets. The induced band gap evolution due to in-plane and out-of-plane tensile stresses is validated by photoluminescence (PL) measurements, promising a potential route for unprecedented manipulation of the physical, electrical and optical properties of MoS2

GaP/GaNP Heterojunctions for Efficient Solar‐Driven Water Oxidation

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/137529/1/smll201603574_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137529/2/smll201603574.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/137529/3/smll201603574-sup-0001-S1.pd

Ultra-Sharp Nanowire Arrays Natively Permeate, Record, and Stimulate Intracellular Activity in Neuronal and Cardiac Networks

Intracellular access with high spatiotemporal resolution can enhance our understanding of how neurons or cardiomyocytes regulate and orchestrate network activity, and how this activity can be affected with pharmacology or other interventional modalities. Nanoscale devices often employ electroporation to transiently permeate the cell membrane and record intracellular potentials, which tend to decrease rapidly to extracellular potential amplitudes with time. Here, we report innovative scalable, vertical, ultra-sharp nanowire arrays that are individually addressable to enable long-term, native recordings of intracellular potentials. We report large action potential amplitudes that are indicative of intracellular access from 3D tissue-like networks of neurons and cardiomyocytes across recording days and that do not decrease to extracellular amplitudes for the duration of the recording of several minutes. Our findings are validated with cross-sectional microscopy, pharmacology, and electrical interventions. Our experiments and simulations demonstrate that individual electrical addressability of nanowires is necessary for high-fidelity intracellular electrophysiological recordings. This study advances our understanding of and control over high-quality multi-channel intracellular recordings, and paves the way toward predictive, high-throughput, and low-cost electrophysiological drug screening platforms.Comment: Main manuscript: 33 pages, 4 figures, Supporting information: 43 pages, 27 figures, Submitted to Advanced Material

Semiconductor nanowires for future electronics : growth, characterization, device fabrication, and integration

This dissertation concerns with fundamental aspects of organo-metallic vapor phase epitaxy (OMVPE) of III-V semiconductor nanowires (NWs), and their structural and electrical properties inferred from a variety of device schemes. An historical perspective on the NW growth techniques and mechanisms, and an overview of demonstrated NW devices and their performance is summarized in chapter 1. In part I of the dissertation, OMVPE synthesis of InAs NWs on SiO₂/Si and InAs (111)B surfaces is discussed and their growth mechanism is resolved. Nucleation, evolution, and the role of Au nanoparticles in the growth of InAs NWs on SiO₂/Si surfaces are presented in chapter 2. Our results indicate that In droplets can lead to InAs NW growth and that Au nanoparticles are necessary for efficient AsH₃ pyrolysis. Chapter 3 discusses the key thermodynamic and kinetic processes that contribute to the InAs NW growth on InAs (111)B surfaces. Controversy in the interpretation of III-V NW growth is overviewed. Experimental evidence on the nucleation of InAs NWs from In droplets as well as the catalytic effect of Au nanoparticles on the InAs (111)B surfaces are described. NW cessation at high growth temperatures or at increased input molar V/III ratios is explained via a switch-over from vapor-liquid-solid (VLS) NW growth to vapor-solid thin film growth, in contrast to previous interpretation of vapor-solid-solid growth of III-V NWs. The substrate-NW adatom exchange is also treated, and experimental distinction of two NW growth regimes depending on this exchange is demonstrated for the first time. Our results indicate that when growing extremely uniform InAs NWs, solid-phase diffusion of In adatoms on the NW sidewalls is the dominant material incorporation process with surface diffusion lengths of ̃ 1 [mu]m. This understanding was further utilized for the growth of axial and radial InAs- InP heterostructure NWs. Polymorphism in III-V NW crystal structure is also discussed and growth conditions that lead to its observation are summarized. In part II of the dissertation, transport coefficient extraction, field-, diameter-, and surface state-dependent transport properties, and their correlation with crystal structure in InAs NWs is presented. Chapter 4 overviews the fabrication of top-gate InAs NW field-effect transistors (NWFETs), presents a model for accurate extraction of carrier mobility and carrier concentration from NWFETs, and demonstration of high electron mobility values in InAs NWs is illustrated. Chapter 5 describes the effects of surface states on transport properties and parameter extraction from InAs NWFETs. Mobility values in excess of 10000 cm²/V·s are obtained from measurements at slow gate voltage sweep rates at which charge balance in carrier capture and emission from interface states is achieved. Chapter 6 discusses scaling effects on the NW transport properties and provides experimental evidence of ballistic electron transport over length scales of ̃ 200 nm in InAs NWs at room temperature. Diameter-dependent mobility and free carrier concentration is observed and is attributed to Fermi energy pinning in the conduction band that leads to surface electron accumulation and enhanced surface scattering. Chapter 7 discusses direct correlation of InAs NW microstructures with their transport properties. Our results show that the distinct difference observed in the subthreshold characteristics between wurtzite and zinc blende InAs NWFETs is due to the presence of spontaneous polarization charges at the WZ }0001} plane interfaces with ZB segments. Numerical simulations point out that a polarization charge density of ̃ 10¹³ cm⁻² is required to surpass surface state induced electron accumulation and result in high Ion/Ioff ratios for the WZ NWFETs. Chapter 8 presents detailed experimental studies on the gate and source-drain field-dependent transport properties in InAs NWFETs. Mobility degradation at high injection fields is observed and is attributed to enhanced phonon scattering, which was verified through electro-thermal simulations and ex-situ transmission electron microscopy (TEM) and scanning TEM compositional studies on NWs exposed to high injection fields. Chapter 9 presents a novel scheme for III-V NW integration to the standard Si mainstream utilizing ion-cut induced transferred III-V layers to SiOv(2) /Si. Vertically integrated and electrically isolated III-V NWs on Si are achieved for the first time. Key challenges related to growth and implementation of vertical devices in future technology nodes are also summarize

Physics-Based Device Models and Progress Review for Active Piezoelectric Semiconductor Devices

Piezoelectric devices transduce mechanical energy to electrical energy by elastic deformation, which distorts local dipoles in crystalline materials. Amongst electromechanical sensors, piezoelectric devices are advantageous because of their scalability, light weight, low power consumption, and readily built-in amplification and ability for multiplexing, which are essential for wearables, medical devices, and robotics. This paper reviews recent progress in active piezoelectric devices. We classify these piezoelectric devices according to the material dimensionality and present physics-based device models to describe and quantify the piezoelectric response for one-dimensional nanowires, emerging two-dimensional materials, and three-dimensional thin films. Different transduction mechanisms and state-of-the-art devices for each type of material are reviewed. Perspectives on the future applications of active piezoelectric devices are discussed

Data_Sheet_1_A universal model of electrochemical safety limits in vivo for electrophysiological stimulation.docx

Electrophysiological stimulation has been widely adopted for clinical diagnostic and therapeutic treatments for modulation of neuronal activity. Safety is a primary concern in an interventional design leveraging the effects of electrical charge injection into tissue in the proximity of target neurons. While modalities of tissue damage during stimulation have been extensively investigated for specific electrode geometries and stimulation paradigms, a comprehensive model that can predict the electrochemical safety limits in vivo doesn’t yet exist. Here we develop a model that accounts for the electrode geometry, inter-electrode separation, material, and stimulation paradigm in predicting safe current injection limits. We performed a parametric investigation of the stimulation limits in both benchtop and in vivo setups for flexible microelectrode arrays with low impedance, high geometric surface area platinum nanorods and PEDOT:PSS, and higher impedance, planar platinum contacts. We benchmark our findings against standard clinical electrocorticography and depth electrodes. Using four, three and two contact electrochemical impedance measurements and comprehensive circuit models derived from these measurements, we developed a more accurate, clinically relevant and predictive model for the electrochemical interface potential. For each electrode configuration, we experimentally determined the geometric correction factors that dictate geometry-enforced current spreading effects. We also determined the electrolysis window from cyclic-voltammetry measurements which allowed us to calculate stimulation current safety limits from voltage transient measurements. From parametric benchtop electrochemical measurements and analyses for different electrode types, we created a predictive equation for the cathodal excitation measured at the electrode interface as a function of the electrode dimensions, geometric factor, material and stimulation paradigm. We validated the accuracy of our equation in vivo and compared the experimentally determined safety limits to clinically used stimulation protocols. Our new model overcomes the design limitations of Shannon’s equation and applies to macro- and micro-electrodes at different density or separation of contacts, captures the breakdown of charge-density based approaches at long stimulation pulse widths, and invokes appropriate power exponents to current, pulse width, and material/electrode-dependent impedance.</p

Engineering Heteromaterials to Control Lithium Ion Transport Pathways.

Safe and efficient operation of lithium ion batteries requires precisely directed flow of lithium ions and electrons to control the first directional volume changes in anode and cathode materials. Understanding and controlling the lithium ion transport in battery electrodes becomes crucial to the design of high performance and durable batteries. Recent work revealed that the chemical potential barriers encountered at the surfaces of heteromaterials play an important role in directing lithium ion transport at nanoscale. Here, we utilize in situ transmission electron microscopy to demonstrate that we can switch lithiation pathways from radial to axial to grain-by-grain lithiation through the systematic creation of heteromaterial combinations in the Si-Ge nanowire system. Our systematic studies show that engineered materials at nanoscale can overcome the intrinsic orientation-dependent lithiation, and open new pathways to aid in the development of compact, safe, and efficient batteries