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A very large enhancement of Raman scattering was observed for WOx nanorods with the {001}CS structure, enabling single-molecule detection of CO molecules adsorbed on WOx nanorods through Raman scattering.

The electronic, lattice, and magnetic properties of CoSb3, Cr-doped CoSb3, and related compound thin films were calculated by using the total energy pseudopotential method. A vacuum region and slab as a thin film were periodically repeated in the supercell. We considered three slab structures. One is symmetric, another is asymmetric, and the third is unusual. They are free-standing in the supercell. The magnetic moment of doped Cr atom is large in all Cr-doped cases. Magnetic moments of several Co atoms are enhanced in the asymmetric and unusual cases. The calculation results in this study indicate that electronic states of most thin films are ferromagnetic and they are energetically more favorable than nonmagnetic cases.

We studied thin films of Fe2VAl, Fe2VAl/Si, and a related compound using the total energy pseudopotential method. The internal atoms in a supercell of the repeated slab model were fully relaxed, except for Fe2VAl/Si. The detailed electronic and magnetic properties of structurally relaxed thin films of Fe2VAl, Fe2VAl/Si, and a related compound were calculated. These films are free-standing in the supercell, except for Fe2VAl/Si, which comprises Fe2VAl and Si (substrate) layers in a supercell. A thin film comprising Fe2V, Al, and Si (substrate) layers in the supercell (denoted as Fe2V/Al/Si) was also studied. Although electronic states of bulk Fe2VAl (full-Heusler) are metallic with pseudo-gap states around the Fermi level and nonmagnetic, the calculation results for Fe2VAl, Fe2VAl/Si, and Fe2V/Al/Si thin films in this study indicate that the films have ferromagnetic properties and their ferromagnetic states are energetically more favorable than nonmagnetic states. The magnetic moments are large and enhanced in the thin films. The magnetic moments of Fe in Fe2VAl (Fe12V4Al4), Fe2VAl (Fe20V8Al8), Fe2VAl/Si, and Fe2V/Al/Si thin films are 2.07 μ B/per atom, 1.29 μ B/per atom, 1.83 μ B/per atom, and 2.22 μ B/per atom, respectively. Although thin films have net ferromagnetic properties, the magnetic moments of the up and down spin states of Fe and V are antiferromagnetically aligned.

We present theoretical calculations of the thermoelectric properties of a magnetic semi-conductor CuFeS2. The electrical contributions, namely the Seebeck coefficients, electrical conductances, and electronic thermal conductances, are investigated using first-principles calculation techniques based on density functional theory and the nonequilibrium Green's function formalism. The lattice thermal transport properties are analyzed using the nonequilbirium molecular dynamics simulation method. Using a combination of the calculated electronic contributions and the lattice thermal contribution, the thermoelectric figures of merit are calculated and analyzed in doped configurations for the theoretical prediction of the highest thermoelectic energy conversion efficiency. It is shown that dilute doping enhances the figure of merit by 2.5 times compared to 3% doping which was previously experimentally obtained to give the best thermoelectric performance among samples studied.

The thermoelectric properties of TiN/MgO surface nanostructures have been determined using first-principles calculations based on the nonequilibrium Green’s function (NEGF) method. Through structural modification of the surfaces at the atomistic level, we find that the metallic TiN thin-film layer becomes semiconducting with a small bandgap, which enhances the Seebeck coefficient, while the electrical conductivity remains high at room temperature. Hence, a much larger thermoelectric figure of merit is obtained compared with bulk. These findings indicate the possibility of designing thermoelectric devices with surface nanostructures.

Enhancing the efficiency of thermoelectric materials has been practiced extensively by either improving the power factor or reducing the lattice thermal conductivity. Magnetism, and the magnetic moment–charged carrier interactions, has been suggested to enhance the efficiency of some compounds. Nevertheless, decoupling of the magnetic and the carrier concentration–related effects has never been achieved to prove once and for all, the importance of magnetism in thermoelectricity. Herein, we report improved quality criterion of bismuth telluride upon chromium substitution. The magnetic interactions with the magnetic moment carried by Cr atoms have increased the electrons' effective mass, enhancing the thermopower. Combined with the decrease in the lattice thermal conductivity, the overall performance of these compounds has been enhanced by 25% at constant carrier concentration, an improvement seldom observed. This is a robust enhancement principle because magnetic interactions are effective at high temperatures above the transition temperature, unlike magnon drag which is dependent on ordering and typically a low temperature phenomenon. Our results indicate that taking advantage of such relatively easily implemented magnetic doping effects along with existing strategies can lead to enhanced efficiency of thermoelectric materials.

We developed and implemented a numerical code called SAKE, which stands for (simulation code for atomistic Kohn-Sham equation). We developed it for first-principle electron transport calculations based on density-functional theory and non-equilibrium Green's function formalism. First, we present the central calculation parts of the formalism of the electronic states and transport properties for open and non-equilibrium systems. We show specific computational techniques, such as the use of a complex contour integration for charge density from the density matrix, which is compared with the calculation method of summing the residues of the Fermi-Dirac distribution, as well as the efficient achievement of the self-consistent procedures. Thereafter, for applications of the present computation code, SAKE, we present first-principle calculation results of three different systems. We first analyze electronic structures of polythiophene molecular wires, compare summation techniques for the density matrix. We show thermoelectric properties of an n-type antiferromagnetic semiconductor CuFeS2 as a second application. The electrical conductance, electrical thermal conductance, and the Seebeck coefficients with carrier doping are examined, and the analytical form of the Seebeck coefficient is briefly described. For the third application, we analyze the electron transport properties of polyaniline molecular wires under structural deformations, i.e. rotations around the transport direction. The thermally averaged current-voltage characteristics are also analyzed. The results show that the current decreases as the temperature increases which are determined based on the competition between the thermal energy and the electronic energy, which increases with the rotation angle.

We present an efficient computation technique for ab-initio electron transport calculations based on density functional theory and the nonequilibrium Green’s function formalism for application to heterostructures with two-dimensional (2D) interfaces. The computational load for constructing the Green’s functions, which depends not only on the energy but also on the 2D Bloch wave vector along the interfaces and is thus catastrophically heavy, is circumvented by parallel computational techniques with the message passing interface, which divides the calculations of the Green’s functions with respect to energy and wave vectors. To demonstrate the computational efficiency of the present code, we perform ab-initio electron transport calculations of Al(100)-Si(100)-Al(100) heterostructures, one of the most typical metal-semiconductor-metal systems, and show their transmission spectra, density of states (DOSs), and dependence on the thickness of the Si layers.

Nanostructured TiN(001) dot/MgO(001)-4 × 4 and 5 × 5 superlattices based on a repeated slab model have been investigated. The electronic states of TiN(001) dot/MgO(001)-4 × 4 and 5 × 5 superlattices were obtained by using the total-energy pseudopotential method. The internal atomic coordinates in a unit cell were fully relaxed. We consider two types of TiN dot structures as rectangular and rectangular parallelepiped. They all correspond to semiconductor, although relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2 × 2 and 3 × 3 superlattices correspond to metallicity. A bandgap value of the rectangular TiN(001) dot/MgO(001) superlattice increases with increasing the size of the MgO substrate, although the difference in the band gap between 4 × 4 and 5 × 5 is small at approximately 0.01 eV. Their electronic properties depend on the shape of the TiN dot and the size of the MgO substrate. The bandgap value is corrected by using a generalized density-functional theory.

Various TiN(001)/MgO(001) superlattices have been investigated by using the total energy pseudopotential method. They are (TiN)n/(MgO)m as (TiN)1/(MgO)7, (TiN)2/(MgO)6, (TiN)3/(MgO)5, and (TiN)2/(MgO)14. “n”, “m”, and “n+m” are TiN, MgO, and TiN/MgO layer numbers of superlattices in a supercell, respectively. Their relaxed superlattice structures and electronic properties were obtained. All the calculated electronic states of TiN/MgO superlattices correspond to metallicity. Values of an electrical conductance of the TiN/MgO superlattice along the c-axis around the Fermi level are quite small although its electronic structure is metallic. A Ti (N) atom at the TiN layer in (TiN)1/(MgO)7 is replaced with Sc, V, and Nb (B, C, and O) as (ScN)1/(MgO)7, (VN)1/(MgO)7, and (NbN)1/(MgO)7 ((TiB)1/(MgO)7, (TiC)1/(MgO)7, and (TiO)1/(MgO)7). Their detailed electronic properties were investigated systematically.

Various nanostructured TiN(001)/MgO(001) superlattices based on a repeated slab model with a vacuum region have been investigated by the total energy pseudopotential method. They are rectangular and rectangular parallelepiped TiN(001) dot structures on MgO(001)-2×2 and 3×3 substrates. A rectangular TiN(001) structure on a MgO(001)-2×1 substrate has also been calculated. Their detailed electronic and internal lattice properties were investigated systematically. The internal atomic coordinates in a unit cell were fully relaxed. The rectangular TiN(001) structure on the MgO(001)-2×1 superlattice, which is not a dot owing to its periodicity, corresponds to metallicity. The electronic states of relaxed rectangular TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices are semiconducting. All relaxed rectangular parallelepiped TiN(001) dot/MgO(001)-2×2 and MgO(001)-3×3 superlattices correspond to metallicity. The electronic properties depend on the shape of the TiN dot and the size of the MgO substrate.

We analyze the Seebeck coefficients of a magnetic semiconductor CuFeS2 using first-principles calculation methods based on density functional theory. The calculated temperature dependence of the Seebeck coefficient in the antiferromagnetic phase reproduces a distinctive behavior in a bulk CuFeS2, such as a peak structure at a low temperature and weak temperature dependence around room temperature. In doped systems, almost linear temperature dependence appears. Despite not including any effect beyond the conventional spin density functional theory in our calculations, the calculated results agree qualitatively with the experimental results. These agreements indicate that the behavior of the Seebeck coefficients in CuFeS2 is mainly determined by its electronic structure.

We have studied the quantum transport properties of zigzag graphene nanoribbons under periodic or Mobius boundary conditions connected to one-dimensional leads using the nonequilibrium Green's function method with a tight-binding scheme. We reveal the existence of a current path originating from the edge states of the graphene nanoribbons. We have found that the conduction channel at the Fermi energy under the periodic boundary condition vanishes under the Mobius boundary condition, which is explained by the breaking of the AB-bipartite lattice symmetry.

The glass transition and related dynamics of two types of multilayered thin films were investigated using differential scanning calorimetry and dielectric relaxation spectroscopy, to clarify the nature of heterogeneous dynamics in thin polymer films. First, the \(\alpha \)-process of multilayered thin films of poly(2-chlorostyrene) (P2CS) and polystyrene (PS) with various geometries was investigated during annealing process. The relaxation rate of the P2CS layer increases near the upper electrode with annealing, while that near the bottom electrode remains almost constant or slightly decreases with annealing. The relaxation strength for the \(\alpha \)-process of the P2CS layer increases with annealing near the upper electrode, while it decreases near the bottom electrode. A distinct positional dependence of the \(\alpha \)-process could be observed in the multilayered films. Second, the glass transition temperature, \(T_\mathrm{g}\), and the dynamics of the \(\alpha \)- and \(\beta \)-processes for stacked thin films of poly(methyl methacrylate) (PMMA) were investigated during the annealing process. The \(T_\mathrm{g}\) and the dynamics of the \(\alpha \)-process of as-stacked PMMA thin films exhibit thin-film-like behavior. Annealing at high temperature causes the \(T_\mathrm{g}\) to increase from the reduced value, and causes the dynamics of the \(\alpha \)-process to become slower approaching those of the bulk. Contrary to the \(\alpha \)-process, the relaxation time of the \(\beta \)-process is almost equal to that of the bulk PMMA, and is unaffected by the annealing process. The fragility index increases with annealing, which suggests that the glassy state of the stacked thin films changes from strong to fragile.

We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Green's function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlattice configurations at the atomistic level.

We have investigated the electron transport properties of zigzag graphene nanoribbons (GNRs) connected to one-dimensional leads on the edge. Transport properties are calculated using the nonequilibrium Green's function method with a tight-binding scheme. We revealed that an edge current originating in the edge states exists and found that the half width of the resonant peak of the transmission spectrum decays exponentially with increasing width of the GNR.