Properties of materials at the nanoscale (1-100 nm) can be tuned by size and shape due to quantum confinement and surface chemistry. Theoretical investigations often begin with the idealized structures, but in reality nanostructured materials are often very complicated, and the complexity may significantly modify the properties of materials studied. We are investigating (1) electronic excitations (2) interfacial charge/energy transfer (3) transport properties (4) surface oxidation, etc, in complex and realistic nanostructures/nanosystems, using a suite of modern quantum mechanical simulations tools, including the density functional theory, many-body perturbation theory using Green's functions (GW/BSE), quantum Monte Carlo, and quantum chemistry, for photovoltaic and other optoelectronic applications.
Recently, graphene has risen as a fascinating system in condensed matter physics for both fundamental science and practical applications, owing to its many unique and amazing properties originating from the massless Dirac fermions; however, the associated zero band gap has to be modified to allow a meaningful on-and-off ratio when used in field-effect transistors. Numerous attempts to open band gap have been made, among them, applying periodic potentials by surface-patterned modifications, such as graphene nanomeshes with periodic nanoscale perforation, are particularly promising; e.g., the field-effect transistors based on graphene nanomashes have been demonstrated with on-and-off ratios comparable to those of graphene nanoribbon devices. Extensive theoretical efforts have been spent to investigate such graphene structures, but the precise role of periodic perturbation on band gap opening remains unclear. We investigate the band gap opening in graphene under periodic potentials, and its optical properties and charge carrier transport and thermal properties. As well as revealing the fundamental physics involved, we computationally design a cornucopia of possibilities of structural patterning and engineering graphene for realistic electronic and optical devices.
Electron-phonon (e-ph) coupling is responsible for a wide range of phenomena, including the electric resistivity, superconductivity, the Kohn effect, the Peierls instability, etc. In semiconductors, e-ph interaction manifests itself in lattice-vibration renormalization of electronic bandstructure, phonon-assisted optical absorption and emission, energy relaxation and charge carrier transport and dynamics. All the above are crucial for realistic devices, especially at the nanoscale, where the quantum confinement effects on e-ph coupling are not clear. Although e-ph coupling and the phonon-mediated properties in bulk semiconductors have been the subjects of intense investigations, in the nanoscale regime they are not well understood theoretically or carefully measured. We explore the confined electron-phonon interaction and its effects on electronic, optical, and transport properties in semiconductor nanowires and carbon nanostructures, taking advantages of the ever-increasing computational power and the recent theoretical developments.
Although density functional theory (DFT) is most widely used in quantum mechanical computations, DFT is not without drawbacks. (1) Its accuracy depends on the exchange-correlation (XC) approximation. This can be improved by constructing better XC approximations based on results obtained from more accurate methods. (2) Its applicability on nanomaterials with more than a thousand of electrons in a unit cell. Better algorithms and simplified models are needed for large scale simulations. (3) Excitations. DFT has fundamental difficulty in treating excited properties, while more accurate methods such as GW/BSE are very expensive. We are actively pursueing a general, accurate, and efficient scheme for electronic excitation computations based on single-particle DFT and many-particle approaches, which are applicable for large and complex nanostructures.
Under extreme conditions of high pressure and temperature, materials will undergo a series of structural phase transitions, in company with dramatic changes in electronic and optical properties, most remarkably, the insulator-metal transition and the occurrance of superconductivity. The invention of Diamond Envil Cell (DEC) allows comprssing materials under extremely high pressure (exceed 300 GPa) in laboratory. This is a vastly broad research area, and our interests are in H and H-rich materials, simple metals, organic molecules and the CO2-H2O system, topological insulators, lferroeletrics and multiferroics under high pressures and temperatures. We are also interested in phase transitions at the nanoscale.
During the past two decades, revolutionary breakthroughs have occurred in the understanding of ferroelectric materials, which are widely used to make capacitors, sensors, actuators, computer memories, etc. Ferroelectrics have spontaneous electric polarization and can be switched between different polarization states by external electric field. Now first-principles approaches based on Density Functional Theory and the Berry phase theory of electric polarization allow accurate predictions of ferroelectricity and poezoelectricity. We are interested in ferroelecric materials at the nanoscale or under high pressures. Furthermore, we also study narrow-gap ferroelectric semiconductors and multiferroic materials, in which two or three of ferroelectricity, ferromagnetism, and ferroelasticity occur simultaneously. Ferroelectric materials also provide a new approach for highly-efficient photovoltaic cells without junctions.