Research at DND-CAT
Surface and interface structure and composition are the primary factors controlling chemical, electronic and mechanical properties of materials in many technologically important applications. This would include catalysis, corrosion, tribology, crystal growth, surface melting, electrodeposition, surface epitaxial growth, segregation at grain boundaries, and ion-transport through biomembranes. X-rays yield atom specific information via the photoeffect, as well as structural information with subangstrom resolution via scattering. Furthermore, for investigating structures such as buried interfaces and thin films, x-rays have a distinct advantage over electron or ion beam probes, since x-rays can more readily penetrate through matter. This is due to the comparatively weak interaction that x-rays have with matter. Unfortunately, it is exactly this weak interaction that gives x-rays a disadvantage in sensitivity, which has been a major problem for determining the structure of a system as dilute as a fraction of a single atom layer. The situation has improved to a certain extent with the advent of first and second generation synchrotron x-ray radiation sources, which are several orders of magnitude more intense than conventional x-ray tube sources.
With an x-ray source as bright as the APS undulator A, which we have requested, we will be able to study a more general class of systems, including those made up of atoms with inherently weaker scattering and photoeffect cross sections, and study in better detail the effects that very dilute impurity concentrations have on the properties of surfaces, interfaces, and thin films. In addition, we can move beyond studies of static two-dimensional structures to studies of such phenomena as surface reaction kinetics with millisecond or even microsecond time resolution. With this capability, we will attain a better understanding of physical transformations and chemical reactions of evolving 2D systems in terms of their intermediate states.
An example of an area of research that would greatly benefit from the increased brightness of the APS is the area of thin film growth by molecular beam epitaxy (MBE) and by chemical vapor deposition (CVD). For device fabrication this technology can be used for growing superlattice structures consisting of thin layers of differing composition and thus differing electronic and magnetic properties. Although the importance of MBE and CVD for semiconductor and magnetic film growth has been well established, little is known on a very detailed microscopic scale about the growth process. In situ real-time x-ray diffraction, reflectivity or standing wave studies are an ideal way of studying these growth processes.
The bulk of surface science research to date has been on solid-vacuum or solid-low pressure gas interfaces. Systems involving solid-high pressure gas, solid-solid or solid-liquid interfaces, although equally important, are relatively unexplored. This is primarily because most experimental techniques for surface and interface studies involve low energy electrons or ions, which have very limited penetrating power. With APS undulator radiation, the electrodeposition process can be studied in situ. This would include studies of structure and composition of the first atom layer deposited at a potential less than that required for bulk plating, as well as the subsequent growth process. The ion distribution profile in the electrolyte solution in contact with the electrode will also be studied.
Polymer surfaces are an almost totally unexplored area, simply because of the fact that polymers are so complex. This complexity is due to the two basic facts: 1) virtually all polymeric materials are a heterogenous mix of chemical species, starting with monomer molecules and ascending in size continuously to very high values (often 106 Daltons), and 2) order that develops in polymer solids can range more or less continuously from truly liquid-like to fully three dimensional - additionally, their long chain nature causes correlations to exist in spatial orientation of successive primary bonds. In the limiting case of polymer surfaces, one expects gradients in both the relative abundance of chemical species and the degree of order. Indeed, one must expect that gradients in chemical makeup will affect what gradients develop in molecular level order. The problem is that the magnitude of such gradients is poorly known at this time.