William G. Cullen
 

As director 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 JEOL 4500A AFM/STM (variable-temperature) additionally equipped with a field-emission SEM. I have had the opportunity to apply my expertise in very productive collaborations with students and post-doctoral researchers from the University of Maryland and with external visitors. The following is a brief description of current projects.

Graphene: Structure and Electronic Properties
Since its discovery in 2004, graphene has emerged at the forefront of condensed matter research due to its unique combination of electrical, mechanical and chemical properties. As a one-atom thick material system, graphene is exquisitely sensitive to the presence of impurities adsorbed on it, and is subject to a host of extrinsic effects which govern its behavior in any particular experimental realization. Surprisingly, the apparently simple issue of graphene topography on SiO₂ remains controversial to the present day, in spite of several published studies using UHV STM. Much of the controversy derives from comparisons at inequivalent resolution (graphene may be measured with STM, whereas SiO₂ must be measured by AFM). Our recent measurements show that SiO₂ is actually slightly rougher than graphene, which leads to a natural, intuitive understanding of graphene topography on SiO₂ in terms of established physics of membrane adhesion [1].

Exfoliated graphene inherits the near-perfect atomic structure of its parent material, and is observed to be essentially free of point defects which produce intervalley scattering. We showed that ion-irradiated graphene has strong intervalley scattering with a minimum conductivity below 4e²/πh and a diverging resistivity at low temperature[2], using 4-probe transport measurements in UHV. Forthcoming experiments will probe the defect structure using scanned-probe measurements in the AFM/STM/SEM facility.

Organic electronics
Current work in organic electronics is motivated by the need to produce electronic devices that are inexpensive, flexible, and suitable for mass production in large sheet-like quantities. Our materials-based investigations of organic systems have been organized along two main thrusts: (1) STM-based characterization of molecular adlayers on single-crystal metallic substrates, which explores self-assembly of multi-component systems; and (2) AFM characterization (coupled with transport measurements) of molecular semiconductors which determines the influence of structure on electronic transport behavior.

Electromigration-Induced Mass Transport
Electromigration is the structural rearrangement of material systems driven by high electrical current density. Although studies of electromigration are generally motivated by the need to mitigate against long-term component failure in integrated circuits, our work places it in a rather unique context with respect to nanotechnology. By measuring the response of nanostructures to a driving current, we can determine the coupling of structural fluctuations to electrical properties, for example the resistance noise which is generated by structural fluctuations. Initial experiments determined the effect of electromigration bias on the fluctuation of atomic steps on a Ag(111) surface[3,4]. More recent work provides a much more dramatic observation of electromigration as manifested by the directed motion of adatom and vacancy islands on Ag(111), with unambiguous demonstration as a reversal of velocity with applied current direction[5]. A common feature of both experiments is the unexpectedly large coupling of surface structural features to electrical current, implying a substantial noise effect in nanoscale systems due to structural fluctuations.

[1] W.G. Cullen, M. Yamamoto, K. Burson, J.-H. Chen, C. Jang, L. Li, M. S. Fuhrer and E. D. Williams, Physical Review Letters 105, 215504 (2010).

[2] Jian-Hao Chen, W. G. Cullen, C. Jang, M. S. Fuhrer and E. D. Williams, Physical Review Letters 102, 236805 (2009).

[3] E. D. Williams, O. Bondarchuk, C. G. Tao, W. Yan, W. G. Cullen, P. J. Rous and T. Bole, New Journal of Physics 9, 387 (2007).

[4] O. Bondarchuk, W. G. Cullen, M. Degawa, E. D. Williams, T. Bole and P. J. Rous, Physical Review Letters 99, 206801 (2007).

[5] Chenggang Tao, W. G. Cullen and E. D. Williams, Science 328, 736 (2010).