Nucleation and growth of metals on carbon surfaces
This thesis work presents an investigation of the basic interaction between metals and the carbon surfaces HOPG and amorphous carbon. This work was motivated by the discovery of a family of metal nanowires which grow as single crystals protruding substantially perpendicular to a substrate, where the substrate is held at elevated temperature (800-1100 K). The most prolific growth is seen for Cu on amorphous carbon substrates. The fabrication and properties of these wires have been pioneered by our collaborator, Dr. Gunther Richter, at the Max Planck Institute for Intelligent Systems in Stuttgart, Germany. They have potential uses in nanoscale mechanical/electrical devices, as chemical/optical sensors and, in the case of magnetic wires, non-rare-earth permanent magnets and high density magnetic storage media. We aim to develop an understanding of the nucleation and growth of these structures, with the ultimate goal of being able to fine-tune their growth with respect to aspect ratio, density, and orientation.
HOPG provides a good starting point for our investigation of NW growth because it is a flat, homogeneous surface with a simple atomic arrangement that can be easily analyzed with STM. Determining the basic energetic parameters for the Cu/HOPG system could ultimately prove useful for modeling nanowire growth. Diffusion barrier (Ed) and critical nucleus size (i) can be extracted from systems exhibiting homogeneous nucleation based on the dependence of island density on temperature and flux, respectively. We present experiments which determine the extent to which homogeneous nucleation occurs in this system. In fact, we find that Cu island nucleation, under the conditions of our experiments, is mediated by defects that are created during the Cu deposition process itself.
Since nanowire growth occurs at elevated temperature, we also explore the Cu/HOPG system at elevated temperatures (300-1300 K) and address the issues of coarsening, desorption, and possible intercalation in this system. We find that coarsening begins at temperatures of 600 K - 700 K, and desorption at 800 K - 900 K.
To determine the differences or similarities between the model carbon substrate (HOPG), and the actual form used in nanowire growth (amorphous carbon), we investigate the interaction between Cu, Ag, and amorphous carbon. We explore changes in the Cu/amorphous carbon surface as a function of coverage and temperature. Ag nanowire samples are annealed to remove the Ag, and then scanned to determine the affect of nanowire growth on the underlying substrate morphology. We identify holes in the amorphous carbon which have the same number density as the metal nanostructures, and were probably caused by growth of the nanostructures.
This work concludes with the growth of metal nanowires on various substrates by MBE and magnetron sputtering, including nanowires of magnetic materials Fe and Ni. Nanowire growth was done both at MPI Stuttgart and at the Ames Laboratory. Long nanowires are grouped as bundles on the surface. We interpret this to mean that growth occurs--at least in part--by incorporation of metal atoms at the base of the nanowire.