Topological insulator (TI) has a bulk-phase energy gap but contains conducting surface states that have linear energy-momentum dispersions near time-reversal invariant momenta (TRIM). These surface states are chiral and robust to external perturbations because they are protected by time-reversal symmetry. Finding new TI materials and exploring implications to device applications have been the primary focus of current research on TI. Also the change in topological insulating property and detailed emergent behaviors of the surface states when the composition and structure of TI are tailored are still yet to be investigated.
We report a discovery, through first-principles calculations, that crystalline Ge-Sb-Te (GST) phase-change materials exhibit the topological insulating property. Our calculations show that the materials become topological insulator or develop conducting surface-like interface states depending on the layer stacking sequence. It is shown that the conducting interface states originate from topological insulating Sb2Te3 layers in GSTs and can be crucial to the electronic property of the compounds. These interface states are found to be quite resilient to atomic disorders but sensitive to the uniaxial strains. We presented the mechanisms that destroy the topological insulating order in GSTs and investigated the role of Ge migration that is believed to be responsible for the amorphorization of GSTs.
Hydrogen has been considered an ideal material that can replace fossil-based fuels for its high energy-conversion efficiency and environmental-friendly nature. Among many technical and economical challenges faced by hydrogen energy, developing proper storage systems and methods has been a serious bottle neck, because hydrogen has a very low energy content by volume (about four times less than gasoline). Pressurized, liquefied and hydride forms of hydrogen have been tested for their fuel cell applications, yet none of those has met the practical criteria for the ambient operations.
Recently, metal-dispersed porous materials have been suggested as plausible candidates for hydrogen storages that possess optimal hydrogen uptake characteristics. Developing such materials must be accompanied with both physical and chemical analysis of intermolecular interactions. We currently focus on designing atomic-scale 3D hydrogen storage and other catalystic systems with investigating their atomic/electronic interaction mechanisms.
Electronic structure of metal-adsorbed or strained graphenes
Graphene is a single layer of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. And it is the basic structural element of some carbon allotropes including graphite, carbon nanotubes and fullerenes. Graphene is quite different from most conventional three-dimensional materials. Intrinsic graphene is a semi-metal or zero-gap semiconductor. And electrons and holes in graphenes near Fermi level behave like relativistic particles described by the Dirac equation for spin 1/2 particles. Experimental results from transport measurements show that graphene has a remarkable high electron mobility at room temperature, with reported values in excess of 15,000 cm2V-1s-1. In addition, the finite strip forms of graphene (graphene nanoribbons) exhibit many interesting properties. There are many methods to modify the electronic/magnetic properties of graphenes. We currently investigate about the effects on such properties by metal-adsorption or strains.