Final Report: Nanoscale Cluster Assembly on Compliant Substrates: A New Route to Epitaxy and Nanosctructure Synthesis

Abstract

From our work at the University of Minnesota prior to 2000, we knew that buffer-layerassisted growth could be used to produce abrupt interfaces where reactions were constrained by the fact that particles derived from tens to millions of atoms were brought into contact with substrates that ranged from GaAs(110) to BiSrCaCuO superconductors [1,[10][11][12][13][14][15][16][17][18][19]. In situ scanning tunneling microscopy had demonstrated that the particles increased in size with the thickness of the buffer layer [9], and we postulated that buffer desorption somehow 'tossed the particles around' so that aggregation was possible. Through access to transmission electron microscopy in the MRL at the University of Illinois, we have been able to determine the distribution of particles delivered to amorphous carbon as a function of buffer thickness, buffer material, particle material, and warm up rate so as to reveal the physics underlying diffusion, aggregation, and coalescence. Significantly, this enhanced understanding makes it possible to design experiments that produce sizes and distributions of nanoparticles of a very wide range of materials.The first experiments were designed to determine the particle densities and shapes for comparison to Monte Carlo simulations of cluster aggregation. Graduate student Christina Haley studied Au particles on amorphous carbon that had been formed by deposition onto Xe buffers of variable thickness [2]. She compared the fractal dimension for ramified particles to simulations and determined that it was consistent with 2D diffusion-limited cluster-cluster aggregation. She also showed that both the weighted average cluster size and the cluster density had a power law dependence on the buffer thickness.When dealing with particles of nanometer dimension, it is important to realize that coalescence is driven by the very large surface energy and that particle integrity is a critical issue. We reasoned that the tendency of particles to coalesce could be altered by adsorbates that would reduce the surface energy. While the coalescence/sintering of metal particles of nanometer size had been modeled with molecular dynamic simulations and continuum models [20], clean experiments were few. Graduate student Vassil Antonov [3] investigated the coalescence of Pd nanostructures produced by BLAG under normal conditions and after coating them with an adsorbed monolayer of CO. The studies of Pd showed a behavior similar to Au, with a power law dependence of particle size on buffer thickness. He then repeated the experiments with a CO coating on the particles. He found that the CO layer was not sufficient to prevent aggregation, presumably because of irregularities in the particle shape. However, coalescence was significantly impeded, giving branched islands with thinner branches. The crossover size from compact to branched growth was reduced two-fold, indicating constraints in Pd atom diffusion related to the adsorbed CO. Another important conclusion was that solid CO performed the same function as X...