Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid-solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime-experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3 method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology.

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The ultraviolet photoemission and optical absorption spectra of pyrrole and polypyrrole are analyzed using the spectroscopically parametrized CNDO/S3 model. Calculated charge densities, bond orders, and orbital eigenvalues for pyrrole are compared with the extensive literature on this molecule. The densities of valence states are computed for pyrrole, oligomers of pyrrole, and an infinite planar polypyrrole chain. They provide a good description of ultraviolet valence electron photoemission spectra measured for gas-phase pyrrole, condensed thin films of pyrrole, and thin-film polypyrrole. The photogenerated radical cation states in polypyrrole seem to extend over four or more pyrrole units for the higher binding energy σ-electron states but are localized on fewer units for the lowest binding energy π-electron states. The relationship between the density of valence states and the energy band structure of polypyrrole is established. The energies of the first few dipole allowed optical absorption transitions are calculated for pyrrole and its oligomers via a limited configuration interaction analysis. Examination of the trends in these transition-state energies as a function of the size of the oligomer suggests that the lowest-energy optically excited states in polypyrrole are excitons which extend over at least four pyrrole units. Comparison of the model predictions for ring and linear-chain molecular conformations indicates that both are compatible with existing spectroscopic measurements.

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The ordering of Bi on a Si(111)-7&&7 surface was studied as a function of overlayer coverage and deposition conditions using low-energy electron diff'raction (LEED) and Auger electron spectroscopy.We observed a one-third-monolayer and a saturated one-monolayer phase. Both phases displayed identical J3XJ3-R30' LEED symmetries. LEED intensity data were used to differentiate between the two phases and to determine a quantitative atomic geometry via a thorough dynamical LEED analysis.The structural characterization of column-III and -V elements deposited on semiconductor substrates continues to be a major subject of study. This interest is driven in part by the desire to improve III-V epitaxy on silicon for application to the manufacture of microelectronics. ' The Si(111)surface is one of the most thoroughly studied substrates. It is known that atomic reconstruction occurs for the free surface of Si(111) and that this reconstruction can be significantly modified by the presence of adatom impurities even at trace, submonolayer concentration. The physical origin of surface reconstruction can be understood as a minimization of the surface free energy.The Si(111)-1x 1 surface with all atoms located at their ideal bulk positions is not realized since it would have too many energetically costly dangling bonds. The 2X 1 and 7X7 reconstructions are stabilized electronically by the removal of surface dangling bonds, leading to a subtle change in the surface atomic geometry. The introduction of covalently bonded adatoms introduces an additional term in the surface free energy, that passivates the surface dangling bonds. Column-III (Al, Ga, In) adatoms generally induce a J3x J3-R30' (called J3Xv 3 hereafter) reconstruction on the Si(111) surface by bonding at the T4 site directly above second-layer Si atoms. The column-V metal As, on the other hand, stabilizes a 1 x 1 surface cell by substituting for the topmost (absent) Si atom. One might expect As-like reconstructions to occur upon adsorbing other column-V elements such as Sb and Bi. Instead, both Sb and Bi adatoms induce a J3XJ3 structure; no well-ordered 1 x 1 structure has been reported. Recent x-ray-diff'raction and scanning-tunnel microscope (STM) studies show that the J3XJ3 reconstruction involves adatom trimers at each 43 x J3 site, at one monolayer (ML) coverage.This trirner model disagrees with the model of 3 -ML saturated coverage proposed earlier by Kawazu et al. However, x-ray-diffraction results are ambiguous with respect to a 180 rotation of the Si(111) surface. ' Therefore, the atomic configuration responsible for the J3XJ3 reconstruction on an atomic scale has not been determined. In this paper, we present results from low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES) studies which show that two distinct phases form the v3X&3 superlattice in the Bi/Si(111) system. The Bi coverage necessary for the completion of each phase was determined to be about -, ' and 1 ML and the substrate temperatures corresponding to each phase are -360 C an...

The intensities of 16 nonequivalent beams of normally incident positrons difFracted from the (110) surfaces of GaAs and InP are reported. The sample temperature was approximately 110 K. The intensities were measured over the energy range 30 eV~E(200 eV. The atomic geometries of GaAs(110) and InP(110) were extracted from these intensities via their comparison with the predictions of a multiplescattering model using the criterion of minimization of the x-ray R factor. The best-fit surface geometries resulting from these analyses are approximately bond-length-conserving top-layer and second-layer rotations characterized by the tilt angles (co&=28. 5'+2. 5' co2= -3.5 +3 ) for GaAs (110) and (co, =24. 5'+1.5', co2= -3. 0'+3') for InP(110). Comparable low-energy electron intensity data were obtained and analyzed for InP(110) leading to (m& = 31 +5 and co2 = -3'+3 ). Small changes 0 (Ad & 0.07 A) in the bond lengths associated with the top-layer species are characteristic of the best-fit structures, but of these, only a small contraction (hd/d~3%) of the bond between the top-layer cation and second-layer anion seems likely to lie outside the uncertainties inherent in the analysis.

PACS 62.60.+v, 63.50.+x We report measurements of the sound velocity as a function of temperature in water using the picosecond acoustic interferometry technique. We show that this method can be used to make velocity measurements on very small sample volumes.