Hideyuki Tsuboi

Effect of Surface Termination on Superlow Friction of Diamond Film: A Theoretical Study

We have applied molecular dynamics simulation and density functional theory calculations to analyze the effects of H and OH terminations on the frictional properties of diamond films at the atomistic and electronic levels. Molecular dynamics simulations were carried out for H-, OH-, and non-terminated diamond surfaces against an iron surface. Results of molecular dynamics simulations show that the frictional force is clearly decreased by the H or OH termination on the diamond surfaces. Moreover, results of density functional calculations show that a covalent bond is formed between Fe and C, while H- or OH-terminated diamond surfaces interact repulsively with an iron surface owing to antibonding interactions. We concluded that this interaction change between iron and diamond surfaces is the major contributing factor for achieving a low friction in H- or OH-terminated diamond.

Theoretical Study on Electronic and Electrical Properties of Nanostructural ZnO

The electronic and electrical properties of ZnO semiconductor single wall nanotube were investigated using periodic supercell approach within density functional theory combined with tight-binding quantum chemistry method. Armchair (10,10) and zigzag (10,0) nanotubes were considered. The lower strain energies required to roll up a ZnO graphitic sheet into a tube and the negative cohesive energies implied the possibility for the formation of ZnO single wall nanotubes. It was shown that the band gaps between the valence band maximum (VBM) and conduction band minimum (CBM) of nanotubes calculated by means of the two methods are similar and are larger than that of the bulk ZnO. It was found that the band gaps of ZnO nanotube are relatively insensitive to the chirality and diameter. According to the estimated electrical conductivities, the non-defect bulk and nanotube ZnO exhibited insulator properties, while they exhibited semiconductor properties when oxygen vacancies are introduced in the structures. The relative stability and band gap of fullerene-like ZnO clusters were also analyzed.

Development of Multiscale Simulator for Dye-Sensitized TiO2 Nanoporous Electrode Based on Quantum Chemical Calculation

In this study, we have developed a novel multiscale simulator for a dye-sensitized TiO2 porous electrode. In the simulator, we can estimate the properties of the dye-sensitized TiO2 porous electrode using the three-dimensional mesoscopic structure model constructed on the basis of our original porous structure simulator. The microscopic physical properties of the materials were estimated by quantum chemistry calculation using a tight-binding quantum chemical molecular dynamics program. From the calculation results, we determined the absorption coefficient and the diffusion coefficient of excited carriers used in the macroscopic simulation for photoelectrode characteristics. By using this multiscale simulator, we will be able to determine the best electrode system efficiently.

Development of A Seebeck Coefficient Prediction Simulator Using Tight-Binding Quantum Chemical Molecular Dynamics

Technologies oriented to the development of thermoelectric materials are of great interest because they can assist in directly tapping the vast reserves of currently underused thermal energy. To improve the rational design of high-performance thermoelectric materials, a new numerical procedure for prediction of Seebeck coefficient based on the electronic information from tight-binding quantum chemical molecular dynamics method, has been developed. The newly developed simulator can evaluate Seebeck coefficient theoretically. The simulator is used for the prediction of the Seebeck coefficients for Pt and Si that are the representative materials for metals and semiconductors, respectively. The results show that both coefficients are in quantitative agreement with experimental data, i.e. in the case of Pt metal the predicted value is 5.62 µV/K while the experimental is -4.45 µV/K. Similarly, in the case of Si semiconductor the predicted value is -324.48 µV/K while the experimental is 300–400 µV/K. This new developed simulator can be used to guide the rational design of high performance thermoelectric materials.

Influence of Organic Functional Groups on the Electrical Properties of Carbon Black: A Theoretical Study

In the present investigation methods based on tight-binding approximation (TBA) and density functional theory (DFT) were employed to investigate the electronic properties of carbon black. The influence of organic functional groups (OFGs) such as –OH and –CHO in the basic structural units (BSUs) of carbon black was studied. It was seen that with the substitution of OFGs in the pristine graphene layers the energy gaps decreased. The changes in energy gaps associated with the topological association of OFGs were also investigated. The change of energy gaps associated with the topological association among the OFGs and with the graphene moieties was seen to be marginal.