John Weeks

Research in the Weeks Group

Research in the Weeks group has two main components. The first focuses on the static and dynamic properties of interfaces, concentrating in particular on the dynamics of steps on crystal surfaces. We study these processes theoretically using both analytical methods and computer simulations, and make contact with experiments in the Williams group (physics) and Reutt-Robey group (chemistry).

Recent collaborative work has focused on low-dimensional boundaries between phases and domains in organic thin films, which are important in charge transport and recombination. STM experiments have visualized the fluctuations of interfacial boundaries in islands of an organic thin film, acridine-9-carboxylic acid (ACA) on Ag(111). Although ACA has highly anisotropic intermolecular interactions, forming linear chains connected by strong hydrogen bonds, it forms islands that are compact in shape (see upper figure) with crystallographically distinct boundaries that have essentially identical thermodynamic and kinetic properties. The physical basis for this surprising behavior is shown to arise from significantly different substrate interactions induced by alternating orientations of successive molecules in the condensed phase. Incorporating this additional set of interactions in a lattice-gas model (lower figure) can straightforwardly reproduce the experimentally observed isotropic behavior. See C. Tao, Q. Liu, B. S. Riddick, W. G. Cullen, J. Reutt-Robey, J. D. Weeks, and E. D. Williams, "Dynamic interfaces in an organic thin film," Proc. Natl. Acad. Sci. USA 105, 16418-16425 (2008).

The second main research area focuses on the properties of nonuniform and confined fluids, particularly those with long-ranged Coulomb or dipolar interactions. We have developed a new and general approach, called Local Molecular Field (LMF) theory, that determines both the structural and thermodynamic properties of such complicated nonuniform fluids by using a simpler "mimic" system with only short ranged (essentially nearest neighbor) interactions but in an effective field that accounts for the averaged effects of the long-ranged interactions. Both qualitative reasoning and quantitative calculations are often much easier in the mimic system, and the effective field corrects major errors that can occur from simple truncations of Coulomb interactions alone.

The figure above shows the electrostatic potential felt by a test charge in water vertically confined by two model corrugated Pt(111) walls. The red curve gives the potential arising from a naïve truncation; it differs significantly from the solid black curve, given by a careful treatment of electrostatics in the full SPC/E water model. The light blue line gives results from the LMF theory. It essentially coincides with the correct results, and illustrates the general accuracy of the new LMF theory in applications to Coulomb and dipolar systems. See J. M. Rodgers and J. D. Weeks, "Interplay of local hydrogen-bonding and long-ranged dipolar forces in simulations of confined water," Proc. Natl. Acad. Sci. USA 105, 19136-19141 (2008).