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Research Activities Overview: Quantum size effects in metal nanosystems directly influence their performance in important applications such as electronics and catalysis. For metal systems, lithographic techniques typically do not reach nanometer length scales were quantum effects manifest. A significant challenge to novel technological developments using metal nanostructures was addressed in my laboratory: the fabrication of high-density and thermally stable nanostructure arrays having monodisperse size and shape. Noble metal/rare earth disilicide core-shell nanostructures with mean particle diameter of less than 10 nm, a narrow size distribution (<±1 nm), inter-particle spacing of ~ 10 nm, and high-density (~10^11/cm^2) have been uniquely fabricated on silicon substrates using a self-organization technique that is compatible with microelectronic manufacturing. In addition to the ability to fabricate unique bimetallic nanosystems and understand thermodynamic forces driving self-organization, the Ragan group has demonstrated the ability to measure and understand their electronic structures at nanometer resolution using scanning probe microscopy and ab-initio quantum-mechanical molecular dynamics simulations in collaboration with Ruqian Wu in the Department of Physics and Astronomy.

Results: Surface electronic structure has been identified to be an important factor in chemical activity. Our experimental data shows unique electronic surface structure on these bimetallic systems. We have measured the work function of Au on DySi2 nanowires as 3.88 eV; this is much lower than the bulk value of Au surfaces, 5.1 eV, and is attributed to electronic charge transfer between Au and nanowire surfaces. During the funding period, we have found via ab initio calculations and experiments that the work function is tunable with material and size. For example, Ag, Au Pd and Pt were predicted to have different charge transfer toward RESi2 nanowires as determined in local density of states calculations6 Charge transfer between metal atoms and nanowire surfaces leads to a surface dipole that is known to change the work function.7 Thus, the choice of noble metal can be used to tune the catalyst's work function. Our scanning Kelvin probe force microscopy (KPFM) data confirms a change in work function due to the aggregation of Pt atoms on DySi2 nanowire surfaces. We have also found that the electronic structure of the RESi2 core exhibits large changes on nanometer length scales. In situ scanning tunneling microscopy (STM)/spectroscopy (STS) yields strong evidence that the Fermi wavelength is on the order of 1 nm. For example, STS spectra of DySi2 wire having width of 3.4 nm on n-type Si(001) has nearly Ohmic (linear I-V response) behavior. In comparison, STS of wires having a width of 1 nm shows current rectification. We have also observed a lower work function of DySi2 nanowires versus larger island structures that was attributed to quantum size effects.

A transformative future goal of this research is to explore the possibility for quantifying chemical activity using basic physical parameters, such as the local work function at chemically active sites. This research is designed to significantly advance understanding of complex quantum size effects and will lay a fundamental foundation for the development of new technologies that rely on chemical activity of surfaces. For example, the Department of Energy (DOE) has identified, “…innovative synthetic techniques; novel characterization techniques; and theory, …, of catalytic pathways,” as critical research foci for nanoscale catalysts. A future direction with this described bimetallic material system is to measure charge transfer on these surfaces resulting from molecular adsorption and desorption using SPM in order to correlate material and structure with chemical activity.

Impact on Career: The ACS PRF starter grant was one of the first grants the PI, Regina Ragan, received as an Assistant Professor and has been critical in pushing her research ideas forward. The funding during the grant period provided infrastructure for advanced training of graduate students, Aniketa Shinde, and Sangyeob Lee, as well as undergraduate, Satoru Emori. Aniketa Shinde, had little experience with ab initio simulations at the beginning of the funding period and now she is highly productive. Miss Shinde recently was a finalist in the Nottingham Competition that is sponsored by the American Vacuum Society. Mr. Lee performed high-resolution scanning KPFM measurements and will be finishing his doctoral thesis at the beginning of 2009. Satoru Emori designed and built a new ultrahigh vacuum system deposition system for gas phase molecular depositions for future catalysis experimens. The research training that Mr. Emori has received led to success in obtaining competitive undergraduate fellowships and he is now attending the Massachusetts Institute of Technology as a graduate student in Materials Science.