Crystallization Pathway for Metastable Hexagonal Close-Packed Gold in Germanium Nanowire Catalysts

Abstract

The recent discovery of hexagonal-close-packed (hcp) Au nanoparticles, crystallized at the tips of Ge nanowires following vapor−liquid−solid (VLS) growth, prompts interest in the mechanism of metastable phase formation in the Au catalyst. The equilibrium structure of Au is face-centered cubic, and observation of hexagonal close-packing in Au is very rare.Here we report on eutectic melting of Au nanocatalysts in the transmission electron microscope (TEM), followed by rapid quenching, and subsequent in situ annealing at intermediate temperatures. A metastable hexagonal Au−Ge alloy phase solidifies during quenching of the Au−Ge liquid in the TEM, and it is a likely precursor to formation of metastable hcp Au during nanowire post-VLS cooling. A novel aspect of the hcp Au phase formation is the rejection of quenched-in Ge from the alloy during low-temperature in situ TEM annealing, while maintaining the metastable hcp crystal structure, indicating that this is the likely pathway for hcp formation following nanowire growth. The kinetics of metastable phase formation are discussed in relation to the unique characteristics of VLS nanowire growth: Ge supersaturation during growth and undercooling of the nanoscale liquid catalyst prior to crystallization at the end of growth.

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

“…4f . Our findings are consistent with the experimentally observed germanium out-diffusion from Au-Ge metastable phase which begins at ~448K 12 and the decay takes ~5 min 15 .…”

“… 22 . Upon further cooling, the metastable alloy separates into the Au hcp phase and pure Ge due to the easily diffused Ge atoms from the β-AuGe alloy, as was seen in the case of TEM in-situ studies on nanowires 12 . The formed Au nanostructures are free of any germanium content as checked by the EELS measurements Fig.…”

“… The formation of these Au fcc crystallites leads to the germanium enrichment of the remaining liquid, so that the metastable hexagonal β-AuGe alloy is formed, similarly as directly first seen in the case of Au nanostructures supported on germanium nanorods by Marshall et al . 12 and accordingly to Okamoto et al . 15 , Sutter et al .…”

“…Recently, Marshall et al . 11 12 reported on synthesis of stable Au hcp nanoparticles on top of some germanium nanorods via quenching of Au/Ge liquid. Only ~30% of the resulted nanoparticles were made of Au hcp phase.…”

“… At the beginning of the annealing process, the deposited Au atoms aggregate into clusters due to surface diffusion of Au atoms on an Au rich surface and particle coarsening 19 . When the temperature exceeds the eutectic temperature of the system, a liquid Au-Ge droplet is formed by incorporation of Ge atoms from the substrate into the hot Au clusters, as described previously in nanowires 12 . According to the Au/Ge phase diagram, the concentration of germanium in the Au-Ge droplets is dependent on the annealing temperature 15 .…”