Kinetics of Buffer-Layer-Assisted Nanostructure Growth

The bottom-up approach to nanostructure assembly has received much attention as a promising way for technology to build ever-smaller devices. In physical vapor deposition, however, thermodynamics poses stringent rules as to what material can self-assemble into three-dimensional structures on the surface of choice. On the other hand, deposition of pre-formed atomic clusters on a substrate always results in structural damage to either the substrate or the particle or both. In order to circumvent these obstacles, the technique of buffer-layer-assisted growth (BLAG) was introduced by our research group during the 1990’s. In this technique, physical vapor deposition is done on a thin film buffer of condensed rare gas solid, where three-dimensional cluster growth is possible for almost any material. Warm-up and desorption of the buffer allows the delivery of the clusters to the substrate in the ultimate of softlandings. Moreover, the clusters can diffuse and aggregate on the desorbing buffer, allowing control over particle density. In my research, I have sought to elucidate the physics behind the fast surface diffusion of these particles, consisting of as many as millions of atoms, at temperatures of only 70 – 80 K. I have extensively utilized the transmission electron microscopes available in the Center of Microanalysis of Materials at the University of Illinois at Urbana-Champaign, to get insights into the key processes involved in nano-particle diffusion. My work has brought a new understanding of how the aggregation and coalescence can be controlled and used for the synthesis of novel structures on the nanometer-scale.The effects of chemisorbed CO on the aggregation and coalescence of Pd nanometer-sized clusters during Xe buffer-layer-assisted growth were investigated, and found to impede Pd cluster-cluster, producing branched islands with thinner branches and lower profile than those of clean Pd. The extent of aggregation and the size distribution of Au nanostructures were studied as a function of the buffer composition (Xe, Kr, and Ar) and thickness, following delivery to amorphous carbon substrates. For Au clusters few-nm in size, the diffusivity varies strongly with size due to self-heating during coalescence. For large Au islands the diffusivity scales as the inverse of the contact area, in agreement with molecular dynamics simulations of fast slip-diffusion of nanocrystals on incommensurate surfaces, where motion is driven by phonons and controlled by friction between a cluster facet and the buffer surface. The exponential dependence of particle density on buffer thickness is explained by a model of competition between the rates of cluster diffusion and buffer depletion, from which the Arrhenius parameters for Ag, Au, Cu, Pd, Co, and Ni nanoparticle slip-diffusion are determined. Significantly, the Arrhenius parameters follow a Meyer-Neldel-type compensation rule which bears the signature of the phononic excitations present in solid Xe, a feature that should be a general characteristic of nanoparticle slip-diffusion. Finally, we demonstrated that BLAG can be utilized for the assembly of CdSe quantum dots and rods and their delivered in ultra-high vacuum conditions on almost any substrate, opening the door for novel IIVI quantum dot heterostructures with tunable properties.U of I Onlydissertation/thesi