Formation and stabilization of single-crystalline metastable AuGe phases in Ge nanowires

Atomic Transport in Au-Ge Droplets: Brownian and Electromigration Dynamics

The deposition of Au on Ge(111)-sqrt[3]×sqrt[3]-Au above the eutectic temperature results in the formation of AuGe liquid droplets that reach the liquidus composition by digging a hole in the Ge substrate. The combination of low-energy electron microscopy and atomic force microscopy measurements shows that AuGe droplets randomly migrate or electromigrate under an applied electric current dragging their underneath hole. The droplet motion is due to a mass transport phenomenon based on Ge dissolution at the droplet front and Ge crystallization at its rear. At high temperature the mass transport is limited by attachment or detachment at the solid-liquid interface and the activation energy is 1.05±0.3  eV. At low temperature the effective activation energy increases as a function of the droplet radius. This behavior is attributed to the nucleation of 2D layers at the faceted liquid-solid interface.

Controlled growth of hexagonal gold nanostructures during thermally induced self-assembling on Ge(001) surface

Nano-sized gold has become an important material in various fields of science and technology, where control over the size and crystallography is desired to tailor the functionality. Gold crystallizes in the face-centered cubic (fcc) phase, and its hexagonal closed packed (hcp) structure is a very unusual and rare phase. Stable Au hcp phase has been reported to form in nanoparticles at the tips of some Ge nanowires. It has also recently been synthesized in the form of thin graphene-supported sheets which are unstable under electron beam irradiation. Here, we show that stable hcp Au 3D nanostructures with well-defined crystallographic orientation and size can be systematically created in a process of thermally induced self-assembly of thin Au layer on Ge(001) monocrystal. The Au hcp crystallite is present in each Au nanostructure and has been characterized by different electron microscopy techniques. We report that a careful heat treatment above the eutectic melting temperature and a controlled cooling is required to form the hcp phase of Au on a Ge single crystal. This new method gives scientific prospects to obtain stable Au hcp phase for future applications in a rather simple manner as well as redefine the phase diagram of Gold with Germanium.

Molten Au/Ge alloy migration in Ge nanowires

Herein, we report time-resolved in situ transmission electron microscopy observation of Au particle melting at a Ge nanowire tip, subsequent forming of Au/Ge alloy liquid, and its migrating within the Ge nanowire. The migration direction and position of the Au/Ge liquid can be controlled by the applied voltage and the migration speed shows a linear deceleration in the nanowire. In a migration model proposed, the relevant dynamic mechanisms (electromigration, thermodiffusion, and viscous force, etc.) are discussed in detail. This work associated with the liquid mass transport in the solid nanowires should provide new insights into the crystal growth, interface engineering, and fabrication of the heterogeneous nanostructure-based devices.

Formation of particles of bismuth-based binary alloys and intermetallic compounds by ultrasonic cavitation

This work reports an important understanding of the bismuth reactivity with other low m_p metals (Sn, In, Ga, and Zn).

Real-time observation of the solid-liquid-vapor dissolution of individual tin(IV) oxide nanowires

The well-known vapor-liquid-solid (VLS) mechanism results in high-purity, single-crystalline wires with few defects and controllable diameters, and is the method of choice for the growth of nanowires for a vast array of nanoelectronic devices. It is of utmost importance, therefore, to understand how such wires interact with metallic interconnects-an understanding which relies on comprehensive knowledge of the initial growth process, in which a crystalline wire is ejected from a metallic particle. Though ubiquitous, even in the case of single elemental nanowires the VLS mechanism is complicated by competing processes at multiple heterogeneous interfaces, and despite decades of study, there are still aspects of the mechanism which are not well understood. Recent breakthroughs in studying the mechanism and kinetics of VLS growth have been strongly aided by the use of in situ techniques, and would have been impossible through other means. As well as several systematic studies of nanowire growth, reports which focus on the role and the nature of the catalyst tip reveal that the stability of the droplet is a crucial factor in determining nanowire morphology and crystallinity. Additionally, a reverse of the VLS process dubbed solid-liquid-vapor (SLV) has been found to result in the formation of cavities, or "negative nanowires". Here, we present a series of heating studies conducted in situ in the transmission electron microscope (TEM), in which we observe the complete dissolution of metal oxide nanowires into the metal catalyst particles at their tips. We are able to consistently explain our observations using a solid-liquid-vapor (SLV) type mechanism in which both evaporation at the liquid-vapor interface and adhesion of the catalyst droplet to the substrate surface contribute to the overall rate.

In situ liquid cell electron microscopy of the solution growth of Au-Pd core-shell nanostructures

Using in situ liquid cell electron microscopy we investigate Pd growth in dilute aqueous Pd salt solutions containing Au nanoparticle seeds. Au-Pd core-shell nanostructures are formed via deposition of Pd(0), generated by the reduction of chloropalladate complexes by radicals, such as hydrated electrons (eaq(-)) induced by the electron beam in the solution. The size and shape of the Au seeds determine the morphology of the Pd shells, via preferential Pd incorporation in low-coordination sites and avoidance of extended facets. Analysis of the Pd incorporation on Au particles at different distances from a focused electron beam provides a quantitative picture of the growth process and shows that the growth is limited by the diffusion of eaq(-) in the solution.

Metastable crystalline AuGe catalysts formed during isothermal germanium nanowire growth

We observe the formation of metastable AuGe phases without quenching, during strictly isothermal nucleation and growth of Ge nanowires, using video-rate lattice-resolved environmental transmission electron microscopy. We explain the unexpected formation of these phases through a novel pathway involving changes in composition rather than temperature. The metastable catalyst has important implications for nanowire growth, and more broadly, the isothermal process provides both a new approach to growing and studying metastable phases, and a new perspective on their formation.