Surface/interface X-ray diffraction

The structural challenges presented by the two-dimensional (2D) world of surfaces and interfaces have proven formidable. In spite of the critical role that they play in such diverse sciences as catalysis, tribology, metallurgy, and electronic devices and in spite of the expected richness of two-dimensional physics of melting, magnetism, and related phase transitions, only a few surface structures are known and most of those only semiquantitatively (e.g., their symmetry). Our inability in many cases to understand atomic structure and to make the structure/properties connection in the 2D region of surfaces and interfaces has significantly inhibited progress in understanding this rich area of science. Furthermore, the synthesis of new materials has played a key role in modern industrial evolution, and this synthesis is also the foundation of contemporary materials research. Many of the new materials fabrication processes are based on crystal growth and thin film deposition techniques, such as molecular beam epitaxy (MBE), chemical vapor deposition (CVD), liquid phase epitaxy (LPE), sputtering deposition, laser ablation and electrochemical plating. To improve the development and engineering of materials based on these techniques, it is essential to have a detailed understanding of the fundamental interactions and effects pertaining to the basic surface and interface structures. This knowledge will allow scientists and engineers to predict and control material properties based on the processing parameters and resulting configurations. This knowledge will be useful also for the understanding of modern composites, ceramics, and nanophase materials in which surfaces and interfaces play a large role in determining their physical and chemical characteristics. With the advent of high-brilliance synchrotron radiation sources and improved analytical methods, surface-sensitive X-ray techniques have recently been developed to study the structure and the associated reactions/transformations at surfaces and interfaces. These synchrotron X-ray techniques have already proven to be a powerful tool to probe surfaces/interfaces beyond the capabilities of earlier experimental methods involving electrons, ions, neutrons and light. This paper will discuss the general principles and experimental method of surface/interface X-ray diffraction as well as provide several examples to demonstrate the usefulness of the techniques applied to surface/interface problems.