Thermal and Electronic Transport at the Nanoscale
CECAM-USI, Lugano, Switzerland
Recent developments in growth techniques and nanofabrication have made available nanostructures and nanostructured materials with a new range of appealing physical properties. Engineering materials to the nanoscale would open novel perspectives in renewable energy technologies (photovoltaics, thermoelectrics, fuel cells) and in electronics (semiconductor devices and organic electronics), however a complete understanding of the transport properties at the nanoscale is still lagging. In many situations modeling the performances of a material or a device requires the combined knowledge of its electronic and thermal transport characteristics. The development of accurate modeling techniques to predict thermal and electronic transport in systems with reduced dimensionality is then a high-priority need for a broad community of scientists. For example, thermoelectric materials are a typical case, where dimensionality reduction plays a crucial role in enhancing the efficiency, by tuning the interplay between thermal conductivity and electronic transport properties (electrical conductivity and Seebeck coefficient). The quest for high performance thermoelectric materials has indeed stimulated a resurgence of interest in thermal transport in bulk materials as well as in nanostructures.
The performances of nanoelectronic devices, materials for molecular electronics and molecular junctions are also extremely sensitive to temperature and to heat dissipation. A full characterization of such systems cannot be limited to the electronic transport properties only, but must include also thermal transport, and the coupling between electrons and phonons. The recent discovery of thermoelectric effects in molecular junctions is a clear example of how thermal and electronic properties cooperate in organic electronic materials. A very special case is given by carbon nanostructures, such as graphene, graphene ribbons and carbon nanotubes, which present a whole lot of peculiar transport properties both electronic and thermal. Carbon nanostructures are the real systems that best approach the mathematical ideal of one-dimensional (nanotubes) and two-dimensional (graphene) structures. In this class of systems transport properties present a wide range of variations depending on the specific structure and conditions (chirality, edges, defects, interactions with a substrate etc.): in the development of applications based on nanotubes or graphene it will be crucial to deeply understand and be able to predict electron and phonon transport. In this workshop we aim at gathering theoretical, computational, and experimental researchers from across science and engineering to discuss recent progresses in thermal and electronic transport, in systems with reduced dimensionality. This topic converges the interests of scientists and engineers that traditionally have different backgrounds and belong to communities, which share similar interests but have little communication. Electronic engineers, statistical physicists, condensed matter physicists and chemists will be involved.
The issue of thermal and electronic transport in nanostructures is crucial to several branches of science and technology that have seen significant progresses in the last decade:
In 1993 Hicks and Dresselhaus  proposed that engineering materials to the nanoscale may improve significantly their thermoelectric efficiency. This is particularly true for silicon, which in its bulk phase has a very low figure of merit. The thermoelectric efficiency rises to technologically interesting values in nanostructers, such as silicon nanowires [2,3] or in nanostructured silicon based materials, e.g. SiGe nanocomposites  and nanoporous silicon .
Carbon nanotubes, graphene and graphene ribbons display tunable electronic properties that open perspectives for an era of carbon electronics . On the other hand graphene and nanotubes are among the materials with the highest thermal conductivity [7,8], higher than that of copper. This means that carbon nanostructures may outcome traditional heat-spreading materials in electronic devices.
Progresses in synthesis and fabrication provide materials for organic electronic applications, which are now competitive with respect to traditional silicon based materials. Charge transport and recombination are the key processes that determine the efficiency of organic light emitting diodes and organic photovoltaics, and depend critically on the structure of the materials at the atomic scale and at the mesoscale .
Self assembled monolayers and single molecule junctions are special branches of organic electronics, for which interesting and somewhat surprising physical phenomena have been recently observed, e.g. thermoelectric effect . Several studies address signal transport, switching, logic, and storage at the molecular level, but the correspondence between theory and experiments is still unsatisfactory and needs improvement .
In all the abovementioned cases materials fabrication and processing, as well as accurate experimental determination of the transport properties is challenging and needs contributions from theoretical modeling in terms of understanding of basic phenomena and design of new materials. At the theoretical level phonon transport is usually studied either by lattice-dynamics based approaches, such as solving the Boltzmann transport equation  and the Green function method , or by Molecular Dynamics, in its equilibrium or non-equilibrium fashion . Adiabatic electronic transport properties are computed by solving the electronic structure of the system at quantum chemical, DFT or semi-empirical level, to compute the Landauer conductance and/or Seebeck coefficient . The calculation of ballistic conductance is usually performed through the Green Function formalism (Landauer-Buttiker) . At variance, in organic electronics systems, electronic transport is not adiabatic and charge mobility is controlled by the underlying atoms dynamics. In this case it proved successful to exploit Marcus theory of charge transfer to predict the trends of charge mobility as a function of the material morphology and the thermodynamic conditions . When traditional approaches have proved inadequate to address new problems, new methodological developments have been devised, but they often provide problem-specific solutions. The impression is that there is a need for methodological cross-validation and development of more general methods, both in the case of phonon and electron transport.
Davide Donadio (University of California Davis) - Organiser
Alan McGaughey (Carnegie Mellon University, Pittsburgh, PA) - Organiser & speaker