Our group studies semiconductor and metal surfaces and interfaces. We are interested in multiple facets of these systems including their electronic properties, growth morphology (sub-monolayer adsorption to thin film growth), quantum-well structures and interfacial atomic structure. Our primary techniques for investigating these systems are photoemission and X-ray diffraction (XRD).

Current Research: Electronic, Spin, and Lattice Structures and Dynamics of Nanoscale Systems

Professor Chiang's current research focuses on the physics of surfaces, interfaces, and tailored thin film structures that are promising for a wide range of scientific and technological advances in the quantum and nanoscale regimes. Measurements, modeling, and computation are performed to determine and to understand the electronic, spintronic, and atomistic behavior of selected surface-based nanoscale systems prepared by deposition, self-assembly, and artificial layering.

Electrons confined in such systems form discrete states, or quantum well states, that are sensitive to the physical dimensions, boundary conditions, and spin-orbit coupling at the interface. As a result, the electronic wave functions, total energy, charge distribution, spin texture, and density of states can exhibit substantial quantum variations as a function of system size and environment. The system's lattice responds to these changes via an electron-lattice coupling, possibly resulting in distortions and new structural phases having different symmetry types.

These effects can be pronounced at the nanoscale because of quantum coherence, interference, entanglement, and the relative ease of atomic movement at surfaces. The resulting collective behavior involving coupled electronic, spin, and lattice degrees of freedom can deviate substantially from the bulk limit, thus giving rise to ample opportunities for creating useful and emergent properties.

Professor Chiang's research is directed mainly at four areas:

  • surfaces, interfaces, and ultrathin films of nontrivial materials including topological insulators, charge density wave compounds, and other functional materials, with an emphasis upon the interplay of quantum confinement, reduced dimensions, spin texture, topological order, etc. as the film's thickness is increased from a single layer, to a double layer,...and to the thick film limit.
  • studies of dichroic and spin polarization effects associated with angle-resolved photoemission spectroscopy using linearly and circularly polarized light, which will shed light on the spin degrees of freedom that have received increasing attention because of the strong potential for spintronic applications.
  • artificially stacked materials involving different quantum phases where the interaction between topological order, superconducting pair formation, charge order, etc. in tailored structures can lead to novel behavior relevant to a fundamental understanding of complexity and emergent phenomena.
  • the physics of excitation, relaxation, and driven behavior at time scales down to the femtosecond regime, which represents an exciting frontier for condensed-matter research.

X-Ray Diffraction

The X-Ray Scattering Research Team performs Diffraction experiments on metal, insulator (C60) and semiconductor (III-V's and group IV) surfaces and interfaces. The interest in surfaces and interfaces lies not in merely determining the atomic coordinates, but in relating that structural information to the physical properties of that surface or interface. There are a number of excellent and complementary methods for obtaining this structural information from a surface; where we use the term surface to refer to a specific type of interface, specifically a vacuum-crystal interface. However, the same strengths that make these techniques well suited for surface studies impede them in interface investigations. For example, electrons used in low-energy electron diffraction (LEED) strongly interact with atoms limiting their penetration depth. This results in only a couple of techniques that can probe and determine the structure of a buried interface.

X-rays are particularly well suited for these investigations. Their characteristic wavelengths, on the order of an angstrom, make them a nearly ideal diffraction probe of atomic dimensions. In addition, their weak interaction with matter not only contributes to their unmat ched penetration power, but also allows straightforward analysis using single scattering kinematic theory. Unfortunately, this strength is also their greatest drawback for use in surface studies. However, augmentation of the low surface signal rates attributable to the X-rays' weak interaction with the "small" number of surface atoms can be achieved by using a glancing incidence geometry and/or with the higher intensity beams available at X-ray technical.

We primarily use synchrotron radiation to perform our scattering experiments. There are many advantages to using synchrotron X-rays over conventional laboratory based sources. The availability of high intensity, tunable monochromatic X-rays or broad spectrum "white" radiation permit numerous experiments once technically formidable or infeasible to become routine. These include extended X-ray absorption fine structure (EXAFS), anomalous or resonant scattering and protein crystallography. The advent of X-ray sync hrotron sources has revolutionized materials research using X-rays.

The majority of our experiments to date fall into three main categories:

  • Noble-metal semiconductor interfaces;
  • C60 encapsulated surfaces; and,
  • Transmission Diffraction and X-ray "RHEED"


Photoemission spectroscopy (PES) is a surface sensitive technique which has been applied to the study of many systems. Electrons are excited from the sample by an incoming beam of photons and subsequently analyzed by an electron energy analyzer. This information can be utilized to provide information about the chemical composition, local bonding arrangement, and electronic structure of the sample.

Photoemission spectroscopy has many inherent advantages as a surface science technique. The electrons probed typically arise from the first few layers of the sample. Electrons can be easily focused and tuned for analysis and are easily counted. The photoexcited electrons can also provide information regarding their initial state energy and momentum. Another advantage over other spectroscopic techniques using ions or atoms is that the electrons vanish after they have been detected. Finally, the impinging photon beam leaves the sample relatively unscathed, so that PES is a non-destructive technique.

The use of technical as photon sources has greatly advanced the capabilities of PES. Previously, monochromatic photons were only available from the characteristic emission lines of specific elements (e.g. Al K = 1486.6 eV or He I = 21.22 eV). Not only do technical provide access to photons of a continuously tunable energy, the intensity and energetic resolution provided by technical is unmatched. The photons available from technical arise from, as one might suspect, synchrotron radiation due to electrons held in a storage ring. A broad range of photons is produced as the electrons are bent to travel about the ring. Charged particles traveling in a nonlinear path will emit electromagnetic radiation. The various beamlines provide access to a selected energy range of these photons. They also act to focus the incoming light into a small spot onto the sample to provide maximum intensity and angular resolution.