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

Atomic Scale Dynamics of Contact Formation in the Cross-Section of InGaAs Nanowire Channels

Alloyed and compound contacts between metal and semiconductor transistor channels enable self-aligned gate processes which play a significant role in transistor scaling. At nanoscale dimensions and for nanowire channels, prior experiments focused on reactions along the channel length, but the early stage of reaction in their cross sections remains unknown. Here, we report on the dynamics of the solid-state reaction between metal (Ni) and semiconductor (In_0.53Ga_0.47As), along the cross-section of nanowires that are 15 nm in width. Unlike planar structures where crystalline nickelide readily forms at conventional, low alloying temperatures, nanowires exhibit a solid-state amorphization step that can undergo a crystal regrowth step at elevated temperatures. In this study, we capture the layer-by-layer reaction mechanism and growth rate anisotropy using in situ transmission electron microscopy (TEM). Our kinetic model depicts this new, in-plane contact formation which could pave the way for engineered nanoscale transistors

Ultrashort Channel Silicon Nanowire Transistors with Nickel Silicide Source/Drain Contacts

We demonstrate the shortest transistor channel length (17 nm) fabricated on a vapor–liquid–solid (VLS) grown silicon nanowire (NW) by a controlled reaction with Ni leads on an in situ transmission electron microscope (TEM) heating stage at a moderate temperature of 400 °C. NiSi₂ is the leading phase, and the silicide–silicon interface is an atomically sharp type-A interface. At such channel lengths, high maximum on-currents of 890 (μA/μm) and a maximum transconductance of 430 (μS/μm) were obtained, which pushes forward the performance of bottom-up Si NW Schottky barrier field-effect transistors (SB-FETs). Through accurate control over the silicidation reaction, we provide a systematic study of channel length dependent carrier transport in a large number of SB-FETs with channel lengths in the range of 17 nm to 3.6 μm. Our device results corroborate with our transport simulations and reveal a characteristic type of short channel effects in SB-FETs, both in on- and off-state, which is different from that in conventional MOSFETs, and that limits transport parameter extraction from SB-FETs using conventional field-effect transconductance measurements

Gold Catalyzed Nickel Disilicide Formation: A New Solid–Liquid–Solid Phase Growth Mechanism

The vapor–liquid–solid (VLS) mechanism is the predominate growth mechanism for semiconductor nanowires (NWs). We report here a new solid–liquid–solid (SLS) growth mechanism of a silicide phase in Si NWs using in situ transmission electron microcopy (TEM). The new SLS mechanism is analogous to the VLS one in relying on a liquid-mediating growth seed, but it is fundamentally different in terms of nucleation and mass transport. In SLS growth of Ni disilicide, the Ni atoms are supplied from remote Ni particles by interstitial diffusion through a Si NW to the pre-existing Au–Si liquid alloy drop at the tip of the NW. Upon supersaturation of both Ni and Si in Au, an octahedral nucleus of Ni disilicide (NiSi₂) forms at the center of the Au liquid alloy, which thereafter sweeps through the Si NW and transforms Si into NiSi₂. The dissolution of Si by the Au alloy liquid mediating layer proceeds with contact angle oscillation at the triple point where Si, oxide of Si, and the Au alloy meet, whereas NiSi₂ is grown from the liquid mediating layer in an atomic stepwise manner. By using in situ quenching experiments, we are able to measure the solubility of Ni and Si in the Au–Ni–Si ternary alloy. The Au-catalyzed mechanism can lower the formation temperature of NiSi₂ by 100 °C compared with an all solid state reaction

Gold Catalyzed Nickel Disilicide Formation: A New Solid–Liquid–Solid Phase Growth Mechanism

The vapor–liquid–solid (VLS) mechanism is the predominate growth mechanism for semiconductor nanowires (NWs). We report here a new solid–liquid–solid (SLS) growth mechanism of a silicide phase in Si NWs using in situ transmission electron microcopy (TEM). The new SLS mechanism is analogous to the VLS one in relying on a liquid-mediating growth seed, but it is fundamentally different in terms of nucleation and mass transport. In SLS growth of Ni disilicide, the Ni atoms are supplied from remote Ni particles by interstitial diffusion through a Si NW to the pre-existing Au–Si liquid alloy drop at the tip of the NW. Upon supersaturation of both Ni and Si in Au, an octahedral nucleus of Ni disilicide (NiSi₂) forms at the center of the Au liquid alloy, which thereafter sweeps through the Si NW and transforms Si into NiSi₂. The dissolution of Si by the Au alloy liquid mediating layer proceeds with contact angle oscillation at the triple point where Si, oxide of Si, and the Au alloy meet, whereas NiSi₂ is grown from the liquid mediating layer in an atomic stepwise manner. By using in situ quenching experiments, we are able to measure the solubility of Ni and Si in the Au–Ni–Si ternary alloy. The Au-catalyzed mechanism can lower the formation temperature of NiSi₂ by 100 °C compared with an all solid state reaction

Nucleation and Atomic Layer Reaction in Nickel Silicide for Defect-Engineered Si Nanochannels

At the nanoscale, defects can significantly impact phase transformation processes and change materials properties. The material nickel silicide has been the industry standard electrical contact of silicon microelectronics for decades and is a rich platform for scientific innovation at the conjunction of materials and electronics. Its formation in nanoscale silicon devices that employ high levels of strain, intentional, and unintentional twins or grain boundaries can be dramatically different from the commonly conceived bulk processes. Here, using in situ high-resolution transmission electron microscopy (HRTEM), we capture single events during heterogeneous nucleation and atomic layer reaction of nickel silicide at various crystalline boundaries in Si nanochannels for the first time. We show through systematic experiments and analytical modeling that unlike other typical face-centered cubic materials such as copper or silicon the twin defects in NiSi₂ have high interfacial energies. We observe that these twin defects dramatically change the behavior of new phase nucleation and can have direct implications for ultrascaled devices that are prone to defects or may utilize them to improve device performance

Si Radial <i>p‑i‑n</i> Junction Photovoltaic Arrays with Built-In Light Concentrators

High-performance photovoltaic (PV) devices require strong light absorption, low reflection and efficient photogenerated carrier collection for high quantum efficiency. Previous optical studies of vertical wires arrays have revealed that extremely efficient light absorption in the visible wavelengths is achievable. Photovoltaic studies have further advanced the wire approach by employing radial <i>p-n</i> junction architectures to achieve more efficient carrier collection. While radial <i>p-n</i> junction formation and optimized light absorption have independently been considered, PV efficiencies have further opportunities for enhancement by exploiting the radial <i>p-n</i> junction fabrication procedures to form arrays that simultaneously enhance <i>both</i> light absorption and carrier collection efficiency. Here we report a concept of morphology control to improve PV performance, light absorption and quantum efficiency of silicon radial <i>p-i-n</i> junction arrays. Surface energy minimization during vapor phase epitaxy is exploited to form match-head structures at the tips of the wires. The match-head structure acts as a built-in light concentrator and enhances optical absorptance and external quantum efficiencies by 30 to 40%, and PV efficiency under AM 1.5G illumination by 20% compared to cylindrical structures without match-heads. The design rules for these improvements with match-head arrays are systematically studied. This approach of process-enhanced control of three-dimensional Si morphologies provides a fab-compatible way to enhance the PV performance of Si radial <i>p-n</i> junction wire arrays

Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale

Controlling the transport of lithium (Li) ions and their reaction with electrodes is central in the design of Li-ion batteries for achieving high capacity, high rate, and long lifetime. The flexibility in composition and structure enabled by tailoring electrodes at the nanoscale could drastically change the ionic transport and help meet new levels of Li-ion battery performance. Here, we demonstrate that radial heterostructuring can completely suppress the commonly observed surface insertion of Li ions in all reported nanoscale systems to date and to exclusively induce axial lithiation along the ⟨111⟩ direction in a layer-by-layer fashion. The new lithiation behavior is achieved through the deposition of a conformal, epitaxial, and ultrathin silicon (Si) shell on germanium (Ge) nanowires, which creates an effective chemical potential barrier for Li ion diffusion through and reaction at the nanowire surface, allowing only axial lithiation and volume expansion. These results demonstrate for the first time that interface and bandgap engineering of electrochemical reactions can be utilized to control the nanoscale ionic transport/insertion paths and thus may be a new tool to define the electrochemical reactions in Li-ion batteries

Adaptable Silicon–Carbon Nanocables Sandwiched between Reduced Graphene Oxide Sheets as Lithium Ion Battery Anodes

Silicon has been touted as one of the most promising anode materials for next generation lithium ion batteries. Yet, how to build energetic silicon-based electrode architectures by addressing the structural and interfacial stability issues facing silicon anodes still remains a big challenge. Here, we develop a novel kind of self-supporting binder-free silicon-based anodes <i>via</i> the encapsulation of silicon nanowires (SiNWs) with dual adaptable apparels (overlapped graphene (G) sheaths and reduced graphene oxide (RGO) overcoats). In the resulted architecture (namely, SiNW@G@RGO), the overlapped graphene sheets, as adaptable but sealed sheaths, prevent the direct exposure of encapsulated silicon to the electrolyte and enable the structural and interfacial stabilization of silicon nanowires. Meanwhile, the flexible and conductive RGO overcoats accommodate the volume change of embedded SiNW@G nanocables and thus maintain the structural and electrical integrity of the SiNW@G@RGO. As a result, the SiNW@G@RGO electrodes exhibit high reversible specific capacity of 1600 mAh g^–1 at 2.1 A g^–1, 80% capacity retention after 100 cycles, and superior rate capability (500 mAh g^–1 at 8.4 A g^–1) on the basis of the total electrode weight

Tailoring Lithiation Behavior by Interface and Bandgap Engineering at the Nanoscale

Controlling the transport of lithium (Li) ions and their reaction with electrodes is central in the design of Li-ion batteries for achieving high capacity, high rate, and long lifetime. The flexibility in composition and structure enabled by tailoring electrodes at the nanoscale could drastically change the ionic transport and help meet new levels of Li-ion battery performance. Here, we demonstrate that radial heterostructuring can completely suppress the commonly observed surface insertion of Li ions in all reported nanoscale systems to date and to exclusively induce axial lithiation along the ⟨111⟩ direction in a layer-by-layer fashion. The new lithiation behavior is achieved through the deposition of a conformal, epitaxial, and ultrathin silicon (Si) shell on germanium (Ge) nanowires, which creates an effective chemical potential barrier for Li ion diffusion through and reaction at the nanowire surface, allowing only axial lithiation and volume expansion. These results demonstrate for the first time that interface and bandgap engineering of electrochemical reactions can be utilized to control the nanoscale ionic transport/insertion paths and thus may be a new tool to define the electrochemical reactions in Li-ion batteries