We have developed a new theoretical formalism for phonon transport in nanostructures using the nonequilibrium phonon Green's function technique and have applied it to thermal conduction in defective carbon nanotubes. The universal quantization of low-temperature thermal conductance in carbon nanotubes can be observed even in the presence of local structural defects such as vacancies and Stone-Wales defects, since the long wavelength acoustic phonons are not scattered by local defects. At room temperature, however, thermal conductance is critically affected by defect scattering since incident phonons are scattered by localized phonons around the defects. We find a remarkable change from quantum to classical features for the thermal transport through defective CNTs with increasing temperature.

The universal features of quantized thermal conductance of carbon nanotubes (CNTs) are revealed through theoretical analysis based on the Landauer theory of heat transport. The phonon-derived thermal conductance of semiconducting CNTs exhibits a universal quantization in the low temperature limit, independent of the radius or atomic geometry. The temperature dependence follows a single curve given in terms of temperature scaled by the phonon energy gap. The thermal conductance of metallic CNTs has an additional contribution from electronic states, which also exhibits quantized behavior up to room temperature.PACS numbers: 81.07. De, 44.10.+i, 63.22.+m During the last two decades, electronic transport in mesoscopic systems whose dimensions are much smaller than a mean-free-path of electrons has been extensively studied experimentally and theoretically. One of the most striking phenomena is that the electrical conductance is quantized in multiples of universal quantum, 2e2 /h, which is observed in a one-dimensional (1D) conductor formed between two reservoirs [1].In contrast, only few studies on nano-scale thermal transport phenomena, especially at low temperatures, have been performed, because the experimental observation and evaluation of thermal transport quantities under such conditions are hard to carry out. Recent advances in nanotechnology, however, have made it possible to investigate thermal transport phenomena in mesoscopic and nano-scale ballistic systems experimentally. Schwab et al. observed the universal quantum of thermal conductance, π 2 k 2 B T /3h, in nano-sized narrow wires using a sophisticated fabrication technique [2], and the value observed is consistent with the theoretical prediction proposed earlier by Rego and Kirczenow [3] and by other theoretical studies [4,5]. However, an unequivocal verification of the existence of quantized thermal conductance in other nanostructures has not been obtained thus far, in contrast to the quantization of electronic transport, which has been observed in various systems. As local heating is one of the key issues to be resolved in the development of nano-scale devices, it is highly desired to clarify the thermal properties of nanostructures.Single-wall carbon nanotubes (CNTs), which are natural quantum wires of small size and high stiffness, are considered most suitable for this purpose, having a large phonon-mean-free-path of the order of 1 µm [6,7]. Recent experiments showed that CNTs have remarkable properties of thermal transport at low temperatures, reflecting quantum effects in one dimension [7,8,9]. The low-temperature thermal conductance of CNTs is linear in the temperature T and extrapolates to zero at T = 0.This implies the existence of quantized thermal conductance in CNTs.In this Letter, we investigate the low-temperature thermal conductance in single-wall CNTs sandwiched between hot and cold heat baths, and show not only that the thermal conductance in CNTs is quantized, but also that the phonon contribution to the thermal conductance has u...

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The optical absorption of triangular nanographenes with well-defined edge structure, that is, zigzag-and armchair-type edges, is studied using the tight-binding method within the Hückel approximation. The absorption spectra of zigzag triangular nanographenes exhibit rich peak structures originating from the excitations associated with the edge states, while those of armchair ones do not. The main feature in the absorption spectra is reproduced by time-dependent density functional theory calculations within a real-time scheme. By varying the size of the nanographenes, we elucidate the role of the edge states in the optical absorption, which becomes conspicuous when the graphene size is on a nanometer scale. Our finding in this paper provides a way to capture the edge-state signature by optical experiments with visible light.

We identify peak and valley structures in the exact exchange-correlation potential of time-dependent density functional theory that are crucial for time-resolved electron scattering in a model one-dimensional system. These structures are completely missed by adiabatic approximations that, consequently, significantly underestimate the scattering probability. A recently proposed nonadiabatic approximation is shown to correctly capture the approach of the electron to the target when the initial Kohn-Sham state is chosen judiciously, and it is more accurate than standard adiabatic functionals but ultimately fails to accurately capture reflection. These results may explain the underestimation of scattering probabilities in some recent studies on molecules and surfaces.

We have performed first-principles calculations on field emission (FE) from graphitic ribbons within the time-dependent density-functional theory. An important finding is that dangling bond states localized at clean edges are major contributors to FE current. H termination makes the FE current small due to the disappearance of the dangling-bond states. FE is found not to occur from the edge state of a H-terminated zigzag ribbon even when the state is at the Fermi level. The results of the FE current from graphitic ribbons give approximately 1 microA for maximum of FE current from a circular edge of graphitic sheets with approximately 1 nm diameter of an open-ended multiwalled carbon nanotube under a high electric field of approximately 1 V/A.

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We have developed a fully self-consistent method which is suitable to examine field emission currents, on the basis of the density functional theory. In our method, the nearby counterelectrode is not necessary. By using this method, we have investigated field emission currents from a biased metallic surface represented by the jellium model. We have found that the energy barrier between the jellium and vacuum becomes lower than the Fermi energy under strong electric fields (e.g., 10 V/nm for r(s) = 4 bohr). In this situation, the slope of the Fowler-Nordheim plot becomes flatter than that under a weaker field.