Abstract:
Atomically uniform films can be made for various overlayer-substrate combinations (such as Ag, Pb, Sb, ... on Si, Ge, Fe, ...), many of which are not even lattice matched. These films show remarkable property variations as the film thickness is built up in atomic-layer increments. The thermal stability of the film, its work function, electron-phonon coupling, superconducting transition temperature, etc. exhibit damped and modulated oscillations as the film thickness increases toward the bulk limit. The underlying physics can be understood generally in terms of the energetics of a coarsened electronic structure of thin films and more specifically in terms of a "one-dimensional shell effect" -- the quantized electronic levels in the film are progressively filled at increasing film thicknesses just like the elemental atomic shells in going through the periodic table. The phase and the amplitude of the oscillations can be tailored by surface/interface engineering that leads to changes in the surface potential and the interface Schottky barrier or band mismatch. These quantum size and confinement effects are important and observable at film thicknesses well in the realm of practical device dimensions and at room temperature, suggesting opportunities for applications. When the films are made of topologically nontrivial materials, the electron spin and its transport become relevant parameters. This talk will discuss issues related to uniform film growth, general trends in connection with reduced dimensions, surprising findings, and technology potential.
Brief Bio:
After receiving a B.S. in physics from the National Taiwan University in 1971, Professor Chiang received his Ph.D. in physics from the University of California, Berkeley in 1978. He joined the Department of Physics at the University of Illinois in 1980 after working as a postdoctoral fellow at the IBM T.J. Watson Research Center in Yorktown Heights, NY.
Professor Chiang has done seminal research on the electronic properties, lattice structure, and dynamic behavior of surfaces, interfaces, and ultrathin films. He employs molecular beam epitaxy techniques to create thin films and composite systems made of metals, semiconductors, topological insulators, superconductors, and charge-density-wave compounds, where functionality and novel properties may emerge from quantum confinement and coherent coupling among the components of the composite.
While his work focuses on basic scientific principles, many of the systems under investigation have strong potential for applications. He is credited for being the first one to create atomically uniform films of thicknesses ranging from a single layer to well over a hundred layers. Such films function as miniature electron interferometers in which electrons bounce back and forth between the two boundaries to form standing waves, also known as quantum well states. These effects allow precise measurements of the electronic wavelength and the kinetics of electron motion. Professor Chiang is an outstanding theorist who is able to develop theoretical models for his experimental results.
Early in his career, Professor Chiang did pioneering work on the application of angle-resolved and core-level photoemission to surface, thin film, and superlattice research. He was one of the first to demonstrate that atoms of single-crystal surfaces have core level binding energies different from the bulk atoms; this work led to the development of quantitative methods for surface structure analysis. He developed systematic methods for three-dimensional band structure mapping, clarified the photoemission processes in terms of bulk and surface effects, and was the first to report surface change density oscillations near defects using scanning tunneling microscopy. His research on x-ray thermal diffuse scattering for phonon mapping is now a topic in textbooks.
Prof. Chiang has conducted his research using synchrotron radiation facilities including the Synchrotron Radiation Center in Stoughton, Wisconsin, the Advanced Light Source in Berkeley, California, the Advanced Photon Source at the Argonne National Laboratory, and several international facilities. He also conducts research at the free electron laser facility LCLS in Stanford, California.