ACS PRF | ACS
All e-Annual Reports
42743-AC5
Electronic and Physical Structures of Solid and Liquid Metal Films
Thin films with thicknesses in the nanometer range can exhibit interesting and useful structures and properties markedly different from the bulk counterparts due to geometric confinement of electrons. The underlying quantum physics is a cornerstone for nanoscale science which is a broadly based interdisciplinary enterprise highly relevant to the advancement of materials, devices, and technologies including the petroleum field. While much is known about films with well ordered atomic lattice structure, very little information is available about films in the liquid state. In fact, even bulk liquid metals remain a relatively unexplored area of research, as the Bloch theorem, upon which much of the solid state physics is based, is not applicable to liquids. This project is to study the electronic and physical structures of liquid metal films, the effects of quantum confinement in such films, and the liquid-solid phase transition. Films with thicknesses ranging from a single monolayer to a thick layer (bulk limit) are examined in an effort to deduce the systematics with the film thickness and temperature as the controlling parameters. The experimental methods include angle-resolved photoemission at the Synchrotron Radiation Center and x-ray diffraction at the UNICAT beamlines, Advanced Photon Source, Argonne National Laboratory. These techniques together provide a powerful approach for the proposed research topic. Our goal is to achieve a fundamental understanding of the coupling between the electrons and the atomic structure as well as the effects of disorder, scattering, coherence, and geometrical constraint.
Efforts of this group have been focused on Pb films deposited on Si and Ge substrates. The films can be made to be fairly uniform by low temperature deposition followed by annealing. We have performed detailed angle resolved photoemission measurements to characterize the electronic structure of such films. X-ray diffraction studies have also been performed to characterize the basic atomic structure and layer relaxation. Pronounced bilayer modulations have been observed and studied in detail. The effects can be related to quantum confinement of the electrons through the Fermi surface topology.
By gradually raising the sample temperature, the Pb films undergo dramatic structural changes. Ultimately, a wetting layer, about a monolayer thick, is formed, with the rest of the Pb forming large droplets. The diffraction results are rather intriguing. It appears that for relatively thin films, the transformation temperature for major structural changes is well below the bulk transition temperature. However, whether or not the changes correspond to melting remains to be determined. The answer might very well depend on the definition of the liquid state of thin films. Because of boundary constraints, partial atomic ordering near the film-substrate interface is inevitable. The x-ray data are being analyzed in detail.
The structural information will be important for understanding the electronic structure. Specifically, a key question of interest is the scattering caused by the loss of long range order. The resulting breakdown of the Bloch theorem is of fundamental significance to the electronic structure of the liquid state. One can understand the thermal scattering in the solid state in terms of phonons. After melting, how do we understanding the scattering? The atomic motions become diffusive rather than harmonic (or anharmonic). These issues are being addressed with both experiments and modeling. We intend to submit another proposal soon to continue the work.
