Abstract
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.
Highlights
Single crystal surfaces14 and annealed at temperatures below the eutectic point of the Au/Ge system of ~634 K15
As the annealing temperature of the sample reaches ~634 K, part of the germanium substrate melts and forms a Au-Ge eutectic liquid, which recrystallizes during cooling of the sample to room temperature (RT)15
The structure and composition of nanostructures formed after RT deposition of 6 mono layers (ML) of Au on (2 × 1) reconstructed Ge(001) in UHV16 and subsequent post-annealing at different temperatures (below and above the eutectic melting temperature of Au/Ge (634 K)–Te) and cooling rates were investigated using a combination of electron microscopy techniques
Summary
The Au/Ge(001) samples were characterized by SEM (FEI Quanta 3D FEG) equipped with a EBSD system (EDAX TSL DigiView camera) at the Institute of Physics Jagiellonian University, Krakow, Poland, which allows structural examination from the near surface of the sample (for Au, it is of a few nanometers). High Angle Annular Dark Field (HAADF) imaging was performed in a Probe Corrected Scanning Transmission Electron Microscope (STEM) FEI Titan at the University of Antwerp, Belgium, operated at 300 kV. This imaging mode provides structural images where intensities are proportional to both the thickness and mean atomic number Z. The spectra obtained for region 1 and region 2 were power-law background subtracted in the region from 100 to 1150 eV
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