The continuum mechanics of coherent two-phase elastic solids with mass transport

Mathematics Technical Repor

Phase Equilibrium and Nucleation in VLS-Grown Nanowires

Phase diagrams accounting for capillarity and surface stress in VLS-grown nanowires have been calculated, and linearized forms for the compositions of the solid and liquid are given. The solid−vapor interfacial energy causes a significant depression of the liquidus, and the impurity concentration in the wire decreases with decreasing wire diameter. Nucleation calculations give upper bounds on the nucleation temperature and liquid supersaturation during growth that are consistent with measurements in the Au−Ge system

Twin Plane Re-entrant Mechanism for Catalytic Nanowire Growth

A twin-plane based nanowire growth mechanism is established using Au catalyzed Ge nanowire growth as a model system. Video-rate lattice-resolved environmental transmission electron microscopy shows a convex, V-shaped liquid catalyst-nanowire growth interface for a ⟨112⟩ growth direction that is composed of two Ge {111} planes that meet at a twin boundary. Unlike bulk crystals, the nanowire geometry allows steady-state growth with a single twin boundary at the nanowire center. We suggest that the nucleation barrier at the twin-plane re-entrant groove is effectively reduced by the line energy, and hence the twin acts as a preferential nucleation site that dictates the lateral step flow cycle which constitutes nanowire growth

Evolution of NiO Island Size Distributions during the Oxidation of a Ni–5Cr Alloy: Experiment and Modeling

The classic models of metal oxidation developed by Wagner and Cabrera and Mott presuppose the existence of a planar oxide film and develop expressions for the rate at which the film thickens. Missing from those models is a description of how that initially planar film forms. Using scanning tunneling microscopy, we study the growth of NiO islands on the (100) surface of a Ni–5Cr alloy during the oxidation regime where the initial planar film is formed as oxide islands. The island height and area distributions as a function of the oxygen exposure in Langmuir (1 L = 10^–6 Torr s) are measured. Lateral island growth and thickening occur as seemingly separate processes, and after a critical thickness of ≈0.4 nm is achieved, growth is purely in the lateral direction. We develop a surface diffusion model for the evolution of the island size distribution that accounts for the lateral growth and coalescence of the NiO islands. Our results indicate that the oxygen surface diffusion screening length ξ=Dτ controls the island evolution. The screening length is found to be 0.3–0.4 nm, which suggests that the processes leading to island growth are highly localized to the island edge

In Situ Crystallization and Morphological Evolution in Multicomponent Indium Oxide Thin Films

The crystallization kinetics of Zn_0.3In_1.4Sn_0.3O₃ (ZITO-30) thin films is investigated via isothermal in situ transmission electron microscopy measurements. Extensive analysis is conducted to reveal the nucleation mechanism and growth rate at four different temperatures. The results show that the nucleation rate in this system is time-dependent and continuously decelerates following a power law decay. The crystal growth rate is constant at a given temperature, and interface-limited growth is the controlling mechanism in the kinetics of amorphous ZITO-30 crystallization. The activation energy for the overall process and interface growth are derived from the experimental data. A morphological study of the grains shows that the {100} interfaces have low mobility and are responsible for the anisotropic crystal shapes. It is found that the {111} and {100} planes of the crystal form parallel to the film–vapor interface during the nucleation process. The results demonstrate a rather complex yet tractable correlation between the experimental results and theoretical underpinning in complex multicomponent oxide thin films

In Situ Crystallization and Morphological Evolution in Multicomponent Indium Oxide Thin Films

The crystallization kinetics of Zn0.3In1.4Sn0.3O3 (ZITO-30) thin films is investigated via isothermal in situ transmission electron microscopy measurements. Extensive analysis is conducted to reveal the nucleation mechanism and growth rate at four different temperatures. The results show that the nucleation rate in this system is time-dependent and continuously decelerates following a power law decay. The crystal growth rate is constant at a given temperature, and interface-limited growth is the controlling mechanism in the kinetics of amorphous ZITO-30 crystallization. The activation energy for the overall process and interface growth are derived from the experimental data. A morphological study of the grains shows that the {100} interfaces have low mobility and are responsible for the anisotropic crystal shapes. It is found that the {111} and {100} planes of the crystal form parallel to the film–vapor interface during the nucleation process. The results demonstrate a rather complex yet tractable correlation between the experimental results and theoretical underpinning in complex multicomponent oxide thin films

Identification of an Intrinsic Source of Doping Inhomogeneity in Vapor–Liquid–Solid-Grown Nanowires

The vapor–liquid–solid (VLS) process of semiconductor nanowire growth is an attractive approach to low-dimensional materials and heterostructures because it provides a mechanism to modulate, in situ, nanowire composition and doping, but the ultimate limits on doping control are ultimately dictated by the growth process itself. Under widely used conditions for the chemical vapor deposition growth of Si and Ge nanowires from a Au catalyst droplet, we find that dopants incorporated from the liquid are not uniformly distributed. Specifically, atom probe tomographic analysis revealed up to 100-fold enhancements in dopant concentration near the VLS trijunction in both B-doped Si and P-doped Ge nanowires. We hypothesize that radial and azimuthal inhomogeneities arise from a faceted liquid–solid interface present during nanowire growth, and we present a simple model to account for the distribution. As the same segregation behavior was observed in two distinct semiconductors with different dopants, the observed inhomogeneity is likely to be present in other VLS grown nanowires

Direct Growth of Compound Semiconductor Nanowires by On-Film Formation of Nanowires: Bismuth Telluride

Bismuth telluride (Bi2Te3) nanowires are of great interest as nanoscale building blocks for high-efficiency thermoelectric devices. Their low-dimensional character leads to an enhanced figure-of-merit (ZT), an indicator of thermoelectric efficiency. Herein, we report the invention of a direct growth method termed On-Film Formation of Nanowires (OFF-ON) for making high-quality single-crystal compound semiconductor nanowires, that is, Bi2Te3, without the use of conventional templates, catalysts, or starting materials. We have used the OFF-ON technique to grow single crystal compound semiconductor Bi2Te3 nanowires from sputtered BiTe films after thermal annealing at 350 °C. The mechanism for wire growth is stress-induced mass flow along grain boundaries in the polycrystalline film. OFF-ON is a simple but powerful method for growing perfect single-crystal compound semiconductor nanowires of high aspect ratio with high crystallinity that distinguishes it from other competitive growth approaches that have been developed to date

Degeneration Behavior of Cu Nanowires under Carbon Dioxide Environment: An <i>In Situ</i>/<i>Operando</i> Study

Copper (Cu) is a catalyst broadly used in industry for hydrogenation of carbon dioxide, which has broad implications for environmental sustainability. An accurate understanding of the degeneration behavior of Cu catalysts under operando conditions is critical for uncovering the failure mechanism of catalysts and designing novel ones with optimized performance. Despite the widespread use of these materials, their failure mechanisms are not well understood because conventional characterization techniques lack the necessary time and spatial resolution to capture these complex behaviors. In order to overcome these challenges, we carried out transmission electron microscopy (TEM) with a specialized in situ gas environmental holder, which allows us to unravel the dynamic behavior of the Cu nanowires (NWs) in operando. The failure process of these nanoscale Cu catalysts under CO2 atmosphere were tracked and further rationalized based on our numerical modeling using phase-field methods

Degeneration Behavior of Cu Nanowires under Carbon Dioxide Environment: An <i>In Situ</i>/<i>Operando</i> Study

Copper (Cu) is a catalyst broadly used in industry for hydrogenation of carbon dioxide, which has broad implications for environmental sustainability. An accurate understanding of the degeneration behavior of Cu catalysts under operando conditions is critical for uncovering the failure mechanism of catalysts and designing novel ones with optimized performance. Despite the widespread use of these materials, their failure mechanisms are not well understood because conventional characterization techniques lack the necessary time and spatial resolution to capture these complex behaviors. In order to overcome these challenges, we carried out transmission electron microscopy (TEM) with a specialized in situ gas environmental holder, which allows us to unravel the dynamic behavior of the Cu nanowires (NWs) in operando. The failure process of these nanoscale Cu catalysts under CO2 atmosphere were tracked and further rationalized based on our numerical modeling using phase-field methods