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

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.