Applied Materials and Surface Science Laboratory
This aspect of our research is directed towards the characterization of chemical and physical phenomena that take place on metal and semiconductor surfaces when exposed to a variety of chemicals. The deposition of organic materials for chemical functionalization is of increasing importance for the eventual commercialization of many M/NEMS products such as chemical sensors. We have studied the functionalization of Au and SiN surfaces with poly(ethyleneglycol) (PEG) self-assembled monolayers (SAMs). Such PEG SAMs have been applied to Au/SiN microcantilevers, in order to provide a coating that can adsorb humidity from the atmosphere and provide a moist environment for the sensing receptors to work.
SAMs also provide a model system for the adsorption, nucleation, and growth of organic materials on semiconductor surfaces. Using time-resolved scanning probe microscopy, our group has examined the growth dynamics of alkylsiloxane SAMS on a variety of surfaces, including Si(100) and rough surfaces of polycrystalline silicon.
Many one-dimensional bottom up nanostructures, such as carbon nanotubes and silicon nanowires are grown by the vapor-liquid-solid technique, in which a metal nanocluster is exposed to a vapor-phase precursor of the nanostructure material (such as SiCl4 for Si), which reacts on the cluster surface. The two materials alloy, and if above the eutectic temperature for the combination, the cluster melts. This liquid droplet is a preferred adsorption site and so supersaturates, forming a single-crystal nanostructure as a precipitate underneath. While, this technique has been used extensively, many fundamental features of the mechanism are still not well understood. Since gold is the most common catalyst for the growth of silicon nanowires, we have applied the techniques of surface science, such as Auger electron spectroscopy, X-ray photoelectron spectroscopy, low-energy electron diffraction, transmission electron microscopy, Raman spectroscopy, and AFM to study the interaction of gold on silicon surfaces. In this way we hope to elucidate the reason for individual nanowire orientations and thus precisely control the growth orientation of silicon nanowires. Current progress in our lab has uncovered the surface phases of gold on (111) silicon over a wide temperature range, and has shown that gold clusters undergo a “self-pinning” to silicon substrates when annealed above a certain temperature. Au surface migration on Si surface has been recently found to be a non-diffusive process, driven by the difference in surface tension of Au vs Si.