Thomas Frederiksen

Inelastic transport theory from first principles : Methodology and application to nanoscale devices

Inelastic transport theory from first principles: Methodology and application to nanoscale devices

Unified description of inelastic propensity rules for electron transport through nanoscale junctions

We present a method to analyze the results of first-principles based calculations of electronic currents including inelastic electron-phonon effects. This method allows us to determine the electronic and vibrational symmetries in play, and hence to obtain the so-called propensity rules for the studied systems. We show that only a few scattering states--namely those belonging to the most transmitting eigenchannels--need to be considered for a complete description of the electron transport. We apply the method on first-principles calculations of four different systems and obtain the propensity rules in each case.

Conductance of Alkanedithiol Single-Molecule Junctions: A Molecular Dynamics Study

We study formation and conductance of alkanedithiol junctions using density functional based molecular dynamics. The formation involves straightening of the molecule, migration of thiol end-groups, and pulling out Au atoms. Plateaus are found in the low-bias conductance traces which decrease by 1 order of magnitude when gauche defects are present. We further show that the inelastic electron tunneling spectra depend on the junction geometry. In particular, our simulations suggest ways to identify gauche defects.

Controlled Contact to a C60 Molecule

The tip of a low-temperature scanning tunneling microscope is approached towards a C60 molecule adsorbed at a pentagon-hexagon bond on Cu(100) to form a tip-molecule contact. The conductance rapidly increases to approximately 0.25 conductance quanta in the transition region from tunneling to contact. Ab-initio calculations within density functional theory and nonequilibrium Greens function techniques explain the experimental data in terms of the conductance of an essentially undeformed C60. The conductance in the transition region is affected by structural fluctuations which modulate the tip-molecule distance.

Inelastic scattering and local heating in atomic gold wires.

We present a method for including inelastic scattering in a first-principles density-functional computational scheme for molecular electronics. As an application, we study two geometries of four-atom gold wires corresponding to two different values of strain and present results for nonlinear differential conductance vs device bias. Our theory is in quantitative agreement with experimental results and explains the experimentally observed mode selectivity. We also identify the signatures of phonon heating.

Modeling inelastic phonon scattering in atomic- and molecular-wire junctions

Computationally inexpensive approximations describing electron-phonon scattering in molecular-scale conductors are derived from the nonequilibrium Greens function method. The accuracy is demonstrated with a first-principles calculation on an atomic gold wire. Quantitative agreement between the full nonequilibrium Greens function calculation and the newly derived expressions is obtained while simplifying the computational burden by several orders of magnitude. In addition, analytical models provide intuitive understanding of the conductance including nonequilibrium heating and provide a convenient way of parameterizing the physics. This is exemplified by fitting the expressions to the experimentally observed conductances through both an atomic gold wire and a hydrogen molecule.

Passing current through touching molecules.

The charge flow from a single C(60) molecule to another one has been probed. The conformation and electronic states of both molecules on the contacting electrodes have been characterized using a cryogenic scanning tunneling microscope. While the contact conductance of a single molecule between two Cu electrodes can vary up to a factor of 3 depending on electrode geometry, the conductance of the C(60)-C(60) contact is consistently lower by 2 orders of magnitude. First-principles transport calculations reproduce the experimental results, allow a determination of the actual C(60)-C(60) distances, and identify the essential role of the intermolecular link in bi- and trimolecular chains.

H-atom relay reactions in real space

Hydrogen bonds are the path through which protons and hydrogen atoms can be transferred between molecules. The relay mechanism, in which H-atom transfer occurs in a sequential fashion along hydrogen bonds, plays an essential role in many functional compounds. Here we use the scanning tunnelling microscope to construct and operate a test-bed for real-space observation of H-atom relay reactions at a single-molecule level. We demonstrate that the transfer of H-atoms along hydrogen-bonded chains assembled on a Cu(110) surface is controllable and reversible, and is triggered by excitation of molecular vibrations induced by inelastic tunnelling electrons. The experimental findings are rationalized by ab initio calculations for adsorption geometry, active vibrational modes and reaction pathway, to reach a detailed microscopic picture of the elementary processes.

Force-induced tautomerization in a single molecule

Heat transfer, electrical potential and light energy are common ways to activate chemical reactions. Applied force is another way, but dedicated studies for such a mechanical activation are limited, and this activation is poorly understood at the single-molecule level. Here, we report force-induced tautomerization in a single porphycene molecule on a Cu(110) surface at 5 K, which is studied by scanning probe microscopy and density functional theory calculations. Force spectroscopy quantifies the force needed to trigger tautomerization with submolecular spatial resolution. The calculations show how the reaction pathway and barrier of tautomerization are modified in the presence of a copper tip and reveal the atomistic origin of the process. Moreover, we demonstrate that a chemically inert tip whose apex is terminated by a xenon atom cannot induce the reaction because of a weak interaction with porphycene and a strong relaxation of xenon on the tip as contact to the molecule is formed.

Force and conductance during contact formation to a C60 molecule

Force and conductance were simultaneously measured during the formation of Cu?C60 and C60?C60 contacts using a combined cryogenic scanning tunneling and atomic force microscope. The contact geometry was controlled with submolecular resolution. The maximal attractive forces measured for the two types of junctions were found to differ significantly. We show that the previously reported values of the contact conductance correspond to the junction being under maximal tensile stress.