Surface Physics
Research Group

Background

Several different scattering techniques (elastic, quasielastic, and inelastic neutron scattering, x-ray diffraction, and low-energy electron diffraction) are being employed to study the physical properties of films as thin as a single molecular layer.  Interest in these quasi two-dimensional (2D) films is based on the premise that phenomena such as crystal growth, structural phase transitions, and changes of state (e.g., melting) and molecular dynamics will be easier to understand in lower dimensional systems.  In some cases, a theory may be easier to formulate conceptually in 2D as in elucidating steps in the melting process.  In other cases, it may be easier to implement a model mathematically in quasi two-dimensional systems or to perform computer simulations of a system studied experimentally.

Over the last 40 years, we have systematically studied films of increasing complexity from monolayers of rare gas atoms and diatomic molecules to films of flexible chain molecules (alkanes) and single-supported bilayer lipid membranes.  Experimental investigation of these thin films requires some method of holding the sample.  In our experiments, the molecules are physically adsorbed on a solid surface such as the basal plane of graphite, a single-crystal metal surface, or single-crystal silicon.  The molecules interact with each other and the substrate by weak van der Waals forces, electrostatic interactions, and by hydrogen bonding rather than by chemical bonding.

We conduct our neutron scattering experiments at national facilities such as the NIST Center for Neutron Research and the Spallation Neutron Source at Oak Ridge National Laboratory as well as at the University of Missouri Research Reactor Facility.  Synchrotron x-ray sources are sufficiently intense to allow structural studies of films adsorbed on single-crystal substrates.  For this purpose, we developed a specially designed ultra-high vacuum chamber for use at the Advanced Photon Source at Argonne National Laboratory and previously at the National Synchrotron Light Source at Brookhaven National Laboratory.  Frequently, it has proved useful to complement the various scattering techniques that we have used with other surface probes such as Atomic Force Microscopy and high-resolution ellipsometry.