Session H14: Focus Session: Transport Properties of Nanostructures III: Theory and Computation I

Transport Through Single-Molecule Junctions: Interference, Thermopower, and the Role of Self-Interaction Effects

Rapid progress in experiments probing transport through single molecules has opened many interesting research directions. Our theoretical work on three such directions is highlighted here; for our calculations, we combine ab initio electronic structure with a single-particle Green function description of the electronic transport. (1) We investigate the applicability of quantum interference through single molecule rings as a control mechanism in molecular electronics. We find that the quantum interference effect is strongly dependent on the interaction between molecular pi-states and contact sigma-states. Structures made with 18-annulene could be used as quantum interference effect transistors. (2) Molecular nanojunctions may support efficient thermoelectric conversion through enhanced thermopower. We calculate the thermopower for several conjugated molecular nanojunctions connected to gold electrodes. Systematic good agreement between theory and experiment is obtained-- much better agreement than for comparable calculations of the conductance. (3) Finally, since we recognize that our treatment of transport is not fully justified, it is important to study corrections and extensions. We investigate the effect of the exchange-correlation potential in atomic chains by constructing optimized effective potentials using several functionals. Dramatic effects are caused by two factors: changes in the energy gap and the self-interaction error.   

First principles calculations of thermal phonon transport in nanostructures

As the size of the electronic devices gets smaller, the power density associated with the devices becomes a very significant aspect that requires a great attention. The devices need to be designed in such a way that the heat removal from the system is efficient, thus, it is very important to understand the heat transport at the nanoscale for better thermal management. In this talk, we will discuss recent advances in the development of efficient techniques to compute the phonon contributions to thermal conductance from first principles. We will show examples of prototypical applications, ranging from atomic chains to graphene nanoribbon junctions and constrictions This work was supported, in part, by the NERC/NIST SWAN-NRI and the DARPA/HRL CERA programs.   

Coupled Ionic and Electronic Heat Transport at the Nanoscale

In modeling thermal transport in nanoscale systems, classical molecular dynamics (MD) explicitly represents phonon modes and scattering mechanisms, but electrons and their role in energy transport are missing. Furthermore, the assumption of local equilibrium between ions and electrons often fails at the nanoscale. We have coupled MD (implemented in the LAMMPS MD package) with a partial differential equation based representation of the electrons (implemented using finite elements). The coupling between the subsystems occurs via a local version of the two-temperature model. Key parameters of the model are calculated using the Time Dependent Density Functional Theory with either explicit or implicit energy flow. We will discuss application of this work in the context of the US DOE Center for Integrated Nanotechnologies (CINT).   

Atomic-Scale Thermoelectric Refrigerator

We propose a thermoelectric cooling device based on an atomic-sized junction. Using first-principles approaches, we investigate the working conditions and the coefficient of performance (COP) of an atomic junction as an electronic refrigerator. Our research reveals that the absence of local heating and the suppression of the tunneling barrier by the bridging atoms are favorable for the operation of atomistic refrigerators. From the self-consistent DFT calculations, we show that the atomistic refrigerator may operate at temperatures below 100 K. This is a great improvement in comparison with the vacuum diode. We also investigate the impact of the phononic heat current on the capability of refrigeration in the nano-refrigerator. To minimize the adverse effects of the phononic heat current, we suggest creating a poor mechanical link between the nano-structured object and the electrodes while still allowing electrons to tunnel.   

Molecular Study of Charge Transport at the Interface between Nanostructures and Matrix in Nanocomposites

The insertion of nanostructures in a polymer matrix has opened up a myriad of possibilities for multifunctional materials. The number of opportunities is however as large as the challenges involved in the study of these materials. One of those challenges, and the focus of this work, is the nature of the interface between the matrix and the nanoinsert and how this affects important properties such electrical transport. Particularly, it has been acknowledge for CNT-polymer composites that conductivity is significant at CNT concentrations below the percolation limit owing this conductivity to tunneling transport from CNT to CNT through the polymer matrices. In this work we described a set of calculations showing that parameters that are relevant to tunneling transport can be calculated from molecular models, namely Density Functional Theory and Green's Functions. In addition, a preliminary study relevant to heat transport at the interface will be described, particularly heat generated by currents and the interface will be focused.   

Quantized Conductance and Electromigration

We present a theoretical study of the low bias electromigration wind force acting on Ag(111) nanoscale surface step edges and surface atomic wires. The electromigration wind force is determined self-consistently, within the low bias Landauer-Buttiker ballistic conduction picture. Numerical estimates of the wind force on step edges are found to agree well with recently reported thin film measurements of surface electromigration. In general, the results underscore the challenging nanoscale reliability problem posed by surface electromigration and the need for a quantum transport description of the electron wind force.   

Detection of Inelastic Electron Transport Properties in Molecular Junctions by Internal Substitutions

Doping and chemical substitutions for molecules is one of promising technique to control the $I-V$ characteristic and engineering of molecular devices. In this presentation, we propose an idea of \textit{internal} substations to detect ballistic and inelastic transport in molecular junctions. We adopt the benzene-dithiol as a template molecule and apply first principle transport calculations, which are based on nonequilibrium Green's function combined with density functional theory, to several internally substituted systems. The inelastic transport is treated within the conventional lowest order expansion (c-LOE) formalism. By comparison of substituted systems, we show systematic analyses of electron tunneling pathway for both ballistic and inelastic currents as well as electron-phonon couplings on bridge molecules. The correlation of inelastic electron tunneling spectroscopy (IETS) and Raman spectroscopy will be also discussed.   

Electronic transport properties of nano-scale Si films: an ab initio study

Using a recently developed first principles transport package, we study the electronic transport properties of Si films contacted to heavily doped n-type Si leads. The quantum transport analysis is carried out using density functional theory (DFT) combined with nonequilibrium Green's functions (NEGF). This particular combination of NEGF-DFT allows the investigation of Si films with thicknesses in the range of a few nanometers and lengths up to tens of nanometers. We calculate the conductance, the momentum resolved transmission, the potential profile and the screening length as a function of length, thickness, orientation and surface structure. Moreover, we compare the properties of Si films with and without a top surface passivation by hydrogen.   

Ion-induced quantum transport in ultrathin amorphous silicon dioxide films

Heavy-ion beams impinging on electronic devices are known to produce conducting paths in oxide thin films. Here we report the results of first-principles calculations of the effect of ion-induced atomic displacements on the current-voltage characteristics of ultrathin oxides. We use density functional theory and the recently developed ``Source and Sink'' method to calculate currents in defected amorphous silicon dioxide layers sandwiched between two Al electrodes. The resulting current-voltage characteristics show significant enhancement of the electron tunneling and are found to depend on both the spatial distribution of ion-induced defects and the distribution of the defect energy levels in the oxide band gap. The quantum transport results are used to define a percolation model using Mott defect-to-defect tunneling. The calculated currents are in agreement with experimental data.   

Calculations of Electron Transport on the Si(111) 7$\times $7 Surface

The surface conductivity of a material becomes more and more relevant as the size of electronic devices continues to shrink. This is particularly true for the 7$\times $7 reconstruction of the Si(111) surface that is believed to behave as a two-dimensional metallic conductor. When two electrodes are connected to this surface and a voltage is applied, experiments indicate that the majority of the current flows through surface states. Additionally, the potential is relatively uniform along terraces and sharp drops coincide with steps separating them, indicating that the resistance of the entire surface is dominated by contributions from such steps. The potential profile of a single step has been carefully measured but its absolute resistivity is yet to be determined due to experimental uncertainty. We have carried out first principles calculations on the conductance properties of the Si(111) 7$\times $7 surface to elucidate some of these details, which will be presented in this talk.   

Plane-wave based Electron Tunneling through Au nanojunctions

A faithful theoretical analysis of the electron tunneling across nanojunctions requires a precise description of the tunneling conductance in the vacuum region. However, most of the conductance calculations are performed using atom centered localized basis sets, which cannot adequately describe the wave function in the vacuum region and can therefore lead to erroneous results. In this work, we present tunneling conductance calculations obtained using the transport calculation method introduced in Refs.\,[1,2]. Since this method employs a plane-wave basis set, it provides accurate description for the electron wave functions in all real space. We will report results for broken Au nanojunctions with different geometries, which allows us to thoroughly investigate geometric effects on the apparent barrier height. Such quantity, which is closely related to the electron tunneling mechanism, is experimentally accessible by STM experiments or across broken nanojunctions. \\[4pt] [1] L.W. Wang, Phys. Rev. B {\bf 72}, 045417 (2005).\\[0pt] [2] A. Garcia-Lekue and L.W. Wang, Phys. Rev. B {\bf 74}, 245404 (2006).   

First-principles study of electronic transport properties of Ta$_2$O$_5$ atomic switch

The atomic switch using solid electrolyte such as Ta$_2$O$_5$ has attracted attention as a promising novel nanoscale device. In the case of the Ta$_2$O$_5$ switch, experiments shows that the precipitation of metal in the Ta$_2$O$_5$ layer plays a crucial role in forming the low resistance state of the switch. However, atomistic details of the conduction path have not been clarified yet. In this work, we have examined the electronic transport properties of the low resistance state of the Cu/Ta$_2$O$_5$/Pt atomic switch using the density functional theory (VASP code) and non-equilibrium Green's function method (ATK code). Our results show that a Cu chain bridging Cu and Pt electrodes works as a conduction path in the case of crystalline Ta$_2$O$_5$. On the other hand, preliminary results show that the conduction through similar Cu chain structures is unexpectedly low in the case of amorphous Ta$_2$O$_5$.   

Scanning Gate Microscopy of a 1D InAs/InP Nanowire Quantum Dot

One-dimensional (1D) nanowire quantum dots provide ideal systems to probe the quantum behavior of electrons. We study long, thin quantum dots (length $\sim $300nm, diameter $\sim $20nm) in an InAs/InP nanowire heterostructure. They provide an interesting system - the Coulomb blockade allows one to control the electron number and measure the energy of quantum states. The nanowire diameter is less than the Bohr radius, making nanowire dots 1D for modest electron numbers. Using a liquid-He cooled scanning gate microscope (SGM) [1], we image the nanowire's conductance as a function of tip position. The conducting SGM tip creates a movable gate to probe the system. We present conductance images of long dots, which use Coulomb blockade to probe the potential profile of the nanowire system and the effects of the metal/semiconductor contacts. \\[4pt] [1] A. Bleszynski-Jayich \textit{et. al} , Phys. Rev. B \textbf{77}, 245327 (2008).