Wesleyan University Absorbed States Laboratory

Lab Director: Robert Rollefson

A question of fundamental concern for condensed matter physics is how the nature of condensed matter (liquids and solids) changes as one goes from three dimensional systems to two dimensional systems. While strictly two dimensional matter is impossible to achieve, if one condenses a thin film of material on a uniform solid surface, as the film is made thinner, ultimately a single atom thick, one achieves a very good approximation to strictly two dimensional matter. It is systems such as these that are the subject of our research. Specifically, we are interested in the nature of phase transitions (e.g., solid to liquid) in two dimensions, and the changes which occur as one goes from two dimensional to three dimensional matter. We are also interested in the effect of the structure of the underlying solid surface on these properties. In addition to the basic question of the nature of the phases and phase changes in two dimensions, this work has potential bearing on practical problems such as catalytic action and lubrication.

The simple determination of whether the film is in a solid or a fluid state presents a considerable experimental challenge for these very thin films. There are two probes which together have proven invaluable in this matter, neutron or x-ray diffraction and nuclear magnetic relaxation. The best indication that a system has a lattice, that is that the atoms or molecules are located in a regular array, is provided by the characteristic diffraction pattern produced. To probe the dynamics of the molecules, e.g., to determine whether they are rotating or oriented in a fixed direction, we use nuclear magnetic resonance (NMR). The relaxation of the nuclear magnetic moments after being disturbed from equilibrium is strongly influenced by the motion of the molecule in which they reside. Thus, by diffraction measurements we are able to determine if a crystal lattice exists, and by NMR relaxation we can probe the dynamics of the molecules. A third experimental probe, the adsorption isotherm, gives information about the thermodynamics of the system. In this measurement the solid substrate is held at a fixed temperature and increasing amounts of the material forming the film are added while the vapor pressure above the surface is monitored. The result is the two dimensional analog of a PV diagram for the film. By combing the results of these three different measurements it is possible to obtain a fairly detailed picture of this quasi-two-dimensional world.

Our work currently involves systems of various small molecules, for instance ethane or methane, adsorbed on uniform crystalline substrates, primarily graphite and magnesium oxide. The NMR measurements and the adsorption isotherms are performed in our lab at Wesleyan, while the diffraction work is carried out in collaboration with colleagues at Brookhaven National Laboratory, a three hour drive from Middletown.

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