William G. Cullen
 

As supervisor of the Scanned Probe Microscopy (SPM) lab, I manage two state-of-the-art UHV SPM instruments: an Omicron variable-temperature scanning tunneling microscope (VTSTM) and a recently acquired JEOL 4500A AFM/STM (variable-temperature) additionally equipped with a field-emission SEM. In addition to oversight of these instruments, I am an active participant in the research effort which utilizes them. The following is a brief description of my current projects.

Thermal Evolution of Supported Crystallites (with Dougherty, Degawa, Reutt-Robey and Williams)
Pb crystallites supported on Ru(0001) have proven to be a fruitful system for studying mass transport. Previously, our group has investigated the layer-by-layer peeling mechanism by which the crystallite changes its shape in response to a sudden change in chemical potential [1]. Ongoing research on this system includes the use of time-dependent STM to extract kinetic parameters from the observed decay of mound structures and vacancy islands. In addition, we are studying a novel phenomenon in which minute quantities of surface Oxygen act to drastically reduce mass transport, freezing the crystallites in a far-from-equilibrium morphology from which they rapidly decay after interaction with the STM tip [2].

Chemical Evolution of Supported Crystallites (with Dougherty, Reutt-Robey and Williams)
Pb crystallites supported on Ru(0001) have proven to be quite resistant to reaction with adsorbed gases. However, the presence of an impurity promotes autocatalytic oxidation of the crystallite [3], a process we have observed in-situ using STM during exposure to molecular oxygen. Subsequent experiments will investigate the dynamics of oxidation and response to a stronger oxidant, NO2.

Adsorption-Induced Mass Transport (with Dougherty, Reutt-Robey, and Williams)
In contrast to the observed hindrance of surface diffusion by small amounts of Oxygen, it is anticipated that adsorbed Sulfur will act as a promoter for surface diffusion. Our experiments will seek to elucidate the mechanisms by which mass transport is affected by adsorbed S using time-dependent STM with temperature variation.

Electromigration-Induced Mass Transport (with Bondarchuk, Degawa, and Williams)
An exciting new avenue of research in surface science is that of systems driven by high current density. Experiments will investigate surface electromigration by studying step fluctuations in the presence of electrical current. A key advantage of our variable-temperature system is the ability to discriminate between real current-induced effects and thermal effects produced by Joule heating of the sample.

Structural/Electrical Fluctuations (with Dougherty and Williams)
The growth of Ag on Si(111) at low temperature presents a favorable model system to study the correlation of electrical (1/f) noise with structural fluctuations at the threshold of percolation. The unique capabilities of the JEOL system will allow in-situ deposition at low temperature, with subsequent measurement of surface morphology using SPM accompanied by electrical noise measurements.

[1] K. Thürmer, J.E. Reutt-Robey, E.D. Williams, M. Uwaha, A. Emundts, H.P. Bonzel,
Phys. Rev. Lett. 87 (2001) 186102.

[2] D.B. Dougherty, K. Thürmer, M. Degawa, W.G. Cullen, J.E. Reutt-Robey, E.D. Williams, In Preparation

[3] K. Thürmer, J.E. Reutt-Robey, E.D. Williams, Science 297 (2002) 2033.