Shadi A. Dayeh

Anisotropic Swelling and Fracture of Silicon Nanowires during Lithiation

We report direct observation of an unexpected anisotropic swelling of Si nanowires during lithiation against either a solid electrolyte with a lithium counter-electrode or a liquid electrolyte with a LiCoO(2) counter-electrode. Such anisotropic expansion is attributed to the interfacial processes of accommodating large volumetric strains at the lithiation reaction front that depend sensitively on the crystallographic orientation. This anisotropic swelling results in lithiated Si nanowires with a remarkable dumbbell-shaped cross section, which develops due to plastic flow and an ensuing necking instability that is induced by the tensile hoop stress buildup in the lithiated shell. The plasticity-driven morphological instabilities often lead to fracture in lithiated nanowires, now captured in video. These results provide important insight into the battery degradation mechanisms.

In situ atomic-scale imaging of electrochemical lithiation in silicon

In lithium-ion batteries, the electrochemical reaction between the electrodes and lithium is a critical process that controls the capacity, cyclability and reliability of the battery. Despite intensive study, the atomistic mechanism of the electrochemical reactions occurring in these solid-state electrodes remains unclear. Here, we show that in situ transmission electron microscopy can be used to study the dynamic lithiation process of single-crystal silicon with atomic resolution. We observe a sharp interface (~1 nm thick) between the crystalline silicon and an amorphous Li(x)Si alloy. The lithiation kinetics are controlled by the migration of the interface, which occurs through a ledge mechanism involving the lateral movement of ledges on the close-packed {111} atomic planes. Such ledge flow processes produce the amorphous Li(x)Si alloy through layer-by-layer peeling of the {111} atomic facets, resulting in the orientation-dependent mobility of the interfaces.

Ultrafast electrochemical lithiation of individual Si nanowire anodes.

Using advanced in situ transmission electron microscopy, we show that the addition of a carbon coating combined with heavy doping leads to record-high charging rates in silicon nanowires. The carbon coating and phosphorus doping each resulted in a 2 to 3 orders of magnitude increase in electrical conductivity of the nanowires that, in turn, resulted in a 1 order of magnitude increase in charging rate. In addition, electrochemical solid-state amorphization (ESA) and inverse ESA were directly observed and characterized during a two-step phase transformation process during lithiation: crystalline silicon (Si) transforming to amorphous lithium-silicon (Li(x)Si) which transforms to crystalline Li(15)Si(4) (capacity 3579 mAh·g(-1)). The ultrafast charging rate is attributed to the nanoscale diffusion length and the improved electron and ion transport. These results provide important insight in how to use Si as a high energy density and high power density anode in lithium ion batteries for electrical vehicle and other electronic power source applications.

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 via 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.

Precise Semiconductor Nanowire Placement Through Dielectrophoresis

We demonstrate the ability to precisely control the alignment and placement of large numbers of InAs nanowires from solution onto very narrow, prepatterned electrodes using dielectrophoresis. An understanding of dielectrophoretic behavior associated with such electrode geometries is essential to development of approaches for assembly of intricate nanowire systems. The influence of signal frequency and electrode design on nanowire manipulation and placement is examined. Signal frequencies in the range of 10 MHz are found to yield high percentages of aligned nanowires on electrodes with dimensions similar to that of the nanowire. Strategies for further improvement of nanowire alignment are suggested and analyzed.

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.

Direct observation of nanoscale size effects in Ge semiconductor nanowire growth.

Progress in the synthesis of semiconductor nanowires (NWs) has prompted intensive inquiry into understanding the science of their growth mechanisms and ultimately the technological applications they promise. We present new results for the size-dependent growth kinetics of Ge NWs and correlate the results with a direct experimental measurement of the Gibbs-Thomson effect, a measured increase in the Ge solute concentration in liquid Au-Ge droplets with decreasing diameter. This nanoscale-dependent effect emerges in vapor-liquid-solid Ge NW growth and leads to a decrease in the NW growth rate for smaller diameter NWs under a wide range of growth conditions with a cutoff in growth at sufficiently small sizes. These effects are described quantitatively by an analytical model based on the Gibbs-Thomson effect. A comprehensive treatment is provided and shown to be consistent with experiment for the effect of NW growth time, temperature, pressure, and doping on the supersaturation of Ge in Au, which determines the growth rate and critical cutoff diameter for NW growth. These results support the universal applicability of the Gibbs-Thomson effect to sub-100 nm diameter semiconductor NW growth.

Influence of surface states on the extraction of transport parameters from InAs nanowire field effect transistors

The capacitive effects of interface trap states in top-gated InAs nanowire field effect transistors and their influence on the experimental extraction of transport parameters are discussed. Time resolved transfer characteristics exhibit transient behavior indicating surface state trapping and detrapping with long characteristic time constants of 45s. Varying gate voltage sweep rate results in a time-dependent extrinsic transconductance; a reduced gate voltage sweep rate leads to a charge neutral interface, reduced interface state capacitance, higher measured transconductance, and minimal hysteresis. These results demonstrate that measurements with a charge neutralized or passivated surface are key to extract intrinsic nanowire transport parameters.

Growth, defect formation, and morphology control of germanium-silicon semiconductor nanowire heterostructures.

By the virtue of the nature of the vapor-liquid-solid (VLS) growth process in semiconductor nanowires (NWs) and their small size, the nucleation, propagation, and termination of stacking defects in NWs are dramatically different from that in thin films. We demonstrate germanium-silicon axial NW heterostructure growth by the VLS method with 100% composition modulation and use these structures as a platform to understand how defects in stacking sequence force the ledge nucleation site to be moved along or pinned at a single point on the triple-phase circumference, which in turn determines the NW morphology. Combining structural analysis and atomistic simulation of the nucleation and propagation of stacking defects, we explain these observations based on preferred nucleation sites during NW growth. The stacking defects are found to provide a fingerprint of the layer-by-layer growth process and reveal how the 19.5° kinking in semiconductor NWs observed at high Si growth rates results from a stacking-induced twin boundary formation at the NW edge. This study provides basic foundations for an atomic level understanding of crystalline and defective ledge nucleation and propagation during [111] oriented NW growth and improves understanding for control of fault nucleation and kinking in NWs.

Direct observation of ballistic and drift carrier transport regimes in InAs nanowires

Conductive atomic force microscopy has been used to characterize distance-dependent electron transport behavior in InAs nanowires grown by metal-organic chemical vapor deposition. Using a conducting diamond-coated tip as a local electrical probe in an atomic force microscope, the resistance of the InAs nanowire has been measured as a function of electron transport distance within the nanowire. Two regimes of transport behavior are observed: for distances of ∼200nm or less, resistance independent of electron transport distance, indicative of ballistic electron transport, is observed; for greater distances, the resistance is observed to increase linearly with distance, as expected for conventional drift transport. These observations are in very good qualitative accord with the Landauer formalism for mesoscopic carrier transport, and the resistance values derived from these measurements are in good quantitative agreement with carrier concentrations and mobilities determined in separate experiments. These res...