Nucleation and Growth of Thin Films and Nanostructures

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This research program investigates the nucleation and growth of metal nanoparticles and ultra thin metal films on amorphous substrates. Chemical growth methods will be studied that include chemical vapor deposition and atomic layer deposition. The research will focus on measuring the concentration and elucidating the chemical nature of defect sites on amorphous substrates were nucleation is initiated or where stable metal clusters are trapped. The program will probe the chemical nature of inherent defects and defects that are purposefully generated on the substrates. Fluorescent probe molecules designed to chemically titrate different proposed defect sites will be used to measure the concentration of the defects and possibly also their spatial distribution. The way the different defects affect nucleation and metal adatom trapping will be established. The research will also explore approaches to inhibit/block the growth of established metal islands and force a higher nucleation density as a route to smooth and ultra thin continuous films. The program will involve ruthenium, cobalt and tungsten metals, and silicon dioxide, aluminum oxide, and titanium dioxide as the supports. The overall objectives are to understand and describe the surface chemistry that allows metal nucleation to occur, and to determine if it is possible to control the growth of particles to maximize the nucleation density on the oxide substrate. Bonding and reactions at the substrate surface and at the film interface will be explored. Film composition and chemical bonding will be followed using X-ray photoelectron spectroscopy and low energy ion scattering spectroscopy. A full complement of characterization facilities will be used to study the films, including fluorimetry, atomic force microscopy, X-ray scattering spectroscopy, and high resolution electron microscopy.Intellectual Merit: Metal films find applications in sensors, optics and microelectronics, and as the critical dimensions or size of the applications and systems decrease, the metal films thickness also must decrease to tens of atomic diameters at most and must have a specific microstructure. Nanoparticles are used in advanced computer memory design, catalysis and quantum computing architectures; in all cases, there is a need to maximize the particle density and uniformity. This research is expected to develop a general understanding of how metal films and metal nanoparticles nucleate on amorphous oxide surfaces and initiate island growth. Nucleation and growth concepts are common to nanoparticles and polycrystalline films. The nucleation and growth issues explored transcend the material systems selected for this project.Broader Impacts: This research is motivated by the central role ultra thin metal films and nanoparticles on amorphous substrates have in applications such as electrodes, sensors, optics, thermal barriers, catalysts and diffusion barriers. There is an extensive literature pointing to the possible role of defects in nucleation and growth, however, there are few studies on amorphous surfaces that have measured and characterized the nature of these defects. This program seeks to use chemical probes designed to titrate the different defects. The fluorescence signal intensity suggest a sensitivity to 0.001 and possibly 0.0001 monolayers with these probes. If successful, the probes could be used to explore oxides more generally. Further this program addresses and seeks to describe the interfacial and surface reactions that affect the evolution of a film as it transforms from adsorbed adatoms to nucleated islands to a coalesced, continuous film.