Metastability of Au−Ge Liquid Nanocatalysts: Ge Vapor–Liquid–Solid Nanowire Growth Far below the Bulk Eutectic Temperature

Abstract

The vapor–liquid–solid mechanism of nanowire (NW) growth requires the presence of a liquid at one end of the wire; however, Au-catalyzed Ge nanowire growth by chemical vapor deposition can occur at ∼100 °C below the bulk Au−Ge eutectic. In this paper, we investigate deep sub-eutectic stability of liquid Au−Ge catalysts on Ge NWs quantitatively, both theoretically and experimentally. We construct a binary Au−Ge phase diagram that is valid at the nanoscale and show that equilibrium arguments, based on capillarity, are inconsistent with stabilization of Au−Ge liquid at deep sub-eutectic temperatures, similar to those used in Ge NW growth. Hot-stage electron microscopy and X-ray diffraction are used to test the predictions of nanoscale phase equilibria. In addition to Ge supersaturation of the Au−Ge liquid droplet, which has recently been invoked as an explanation for deep sub-eutectic Ge NW growth, we find evidence of a substantial kinetic barrier to Au solidification during cooling below the nanoscale Au−Ge eutectic temperature.