Hydrogen storage

 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 bottleneck, 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.






GST-based Topological insulator

 Phase-change materials like Ge-Sb-Te (GST) compounds are considered the best candidates for next-generation non-volatile memories because of their rapid and reversible cycles between the crystalline and amorphous structures. The mechanism and detailed atomic structure associated with the structural transition of GST compounds have been extensively studied, but the factors responsible for the very fast atomic rearrangement are still unknown.
 Topological insulators have an energy gap at bulk phase but contain conducting surface states that are protected from external perturbations by time-reversal symmetry. We study, through first-principles calculations, topological insulating property of the ternary chalcogen compounds commonly used in phase change memory.
 We show that the conducting properties originate from topological insulating Sb2Te3 layers in GSTs. The interface states are found to be resilient to atomic disorders but sensitive to the uniaxial strains. It is found that Ge migration, which is believed to be responsible for the amorphorization of GSTs, destroys the topological insulating order. We explore how these topological insulating properties are utilized for developing electronic devices.





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.





Optical properties of a metal alloy

 Nowadays, metal (Ti, Cr, Zr) nitrides are prefered as a coating material, because of its hardness and lustrous colour. The mechanical properties are well understood but the colours still rely on empiricism. The colour of a given material can be predicted with its dielectric function, obtained by band-structure calculation. Therefore we are trying to reproduce the colour of a metal alloy only with first-principle calculation method. And we are investigating the colour variations induced by various defects, so that one will be able to determine the ratio of impurities to produce intended colour without trial and error.