Session X24: Focus Session: Quantum Transport Simulations and Computational Electronics -- Molecular Junctions

Efficient k.p method for first-principles calculation of Seebeck coefficient in quantum transport

Thermoelectric properties of molecular junctions reveal fundamental aspects of nanoscale charge transport at interfaces and are relevant to potential organic/inorganic hybrid thermoelectric materials. Quantum transport calculations typically evaluate the Seebeck coefficient S by finite differences of the transmission as a function of energy. However, in ab initio calculations this quantity is difficult to converge for realistic systems and can require very large k-grids. We derive a new analytic-derivative method to evaluate S via k.p perturbation theory, implement it in a DFT-based scattering-state transport code, and apply it to calculations of molecular junctions. This technique improves k-point convergence by avoiding critical points in the lead bandstructure and allows more efficient calculations of Seebeck coefficients.   

First-Principles Studies of Charge Dynamics in Single-Molecule Junctions at Finite Bias

Extending well-established measurements of low-bias conductance of single molecule junctions, new experiments report IV characteristics of organic molecules for biases as high as 1V [1]. Such measurements provide a unique probe of electronic properties of well-defined metal-organic nanointerfaces when driven out-of-equilibrium, and an opportunity to examine a still-missing quantitative theory of out-of-equilibrium charge dynamics at the nanoscale. Here we will present first-principles transport calculations for several amine-Au and pyridine-Au linked junctions at different levels of approximation: first mean-field, and then including electron-electron correlations at equilibrium and out-of-equilibrium. We show that incorporating electronic correlations at equilibrium already leads to a very good agreement with experiments [1], and discuss how these corrections might change out of equilibrium.\\[4pt] [1] Widawsky et al., Nanotechnology (2009).   

Understanding the Role of Direct Au-C Links to Electrodes in Single Molecule Junctions

Recent experiments have shown that use of tri-methyl tin (SnMe3) link groups results in the formation of alkane single molecule junctions with measured conductance $\sim $100 times higher than found for any other link group previously used. Further evidence points to the in-situ formation of direct Au-C bonds to the electrode. In this work we use Density-Functional Theory based calculations to study the formation and structure of junctions based on direct Au-C link bonds. Transport calculations based on Non-Equilibrium Green's Functions for benzene and alkane molecules anchored through Au-C bonds show that the alkane backbone couples more strongly to the leads, resulting in a higher transmission as compared with other link groups. In the case of benzene, however, transport is primarily through the $\sigma $ system, yielding a smaller conductance increase. Finally, we discuss corrections to the position of molecular resonances found in the DFT-based calculations and the implications for conductance.   

Phonon-assisted tunneling in two-level quantum dots or diatomic molecules

Electron-electron interactions in nanoscale systems can be significantly modified by coupling to bosonic modes (photons, phonons, and plasmons) that act as sources of dissipation and decoherence (dephasing). Photon-assisted tunneling can take place through ground and excited states of various types of quantum-dot system, while signatures of vibrational modes are seen in transport through single-molecule transistors in the Coulomb blockade and Kondo regimes. We report numerical renormalization-group results for a quantum dot or diatomic molecule that has two active levels, taking into account both intra-level and inter-level Coulomb interactions. We focus on how decoherence induced by phonon-assisted inter-level transitions affects the formation of the many-body Kondo singlet between the dot/molecule and the leads, and quantify the consequent modification of the zero-bias electrical conductance.   

Quantum current of a molecular photo-switch between two graphene sheets

Light responsive materials that reversibly change shape under alternate UV and visible irradiation have attracted much interest because they can be used as optical switches, since the isomers show different features in the dimension, HOMO-LUMO gap and transmission spectrum. In view to integrate the photo-switch in the carbon based electronics devices, the conductance of a system constituted by a photochromic molecule between two graphene electrodes is investigated. In this work the conductance of the junction formed by diarylperfluorocyclopentene between two semi-infinite graphene sheets was computed using the non-equilibrium Green's function method combined with density functional theory via the TranSiesta code. The results emphasize the role of the graphene and the molecular electronic states in the switching behaviour of this hybrid system.   

Correlation between Raman scattering and conductance in a molecular junction

Raman spectroscopy of molecular junctions is a promising diagnostic and control tool. We present a model for non-resonant Raman spectroscopy, generalizing previous considerations to strong laser pulses of arbitrary time dependence. The model paves a way to realistic simulations of Raman spectroscopy experiments in molecular conduction junctions. We demonstrate within the model that the optical properties of molecular conduction junctions are strongly correlated with the electron transport properties. Feynman diagrams responsible for such similarity are analyzed for both processes, and possible explanation for observed (anti-)correlated behavior of Stokes signal and conductance is proposed.   

Towards {\it single-atom-controlled} device

{\it Single-atom-controlled} device has been explored recently in the context of molecular junction. Here, by using a codoping model, where a cation and an anion are introduced simultaneously into the host to maintain charge neutrality, we have probed the electron transport characteristics in a strongly coupled single molecular junction. We have used 1, 12-dicarba-{\it closo}-dodecaborane inorganic molecule as a precursor and have replaced one of the vertex carbon atoms by a boron atom and simultaneously decorated it with an endohedrally doped alkali atom (Li/Na) to look into the role of dopant atoms on the conductivity. The commonly used thiolate anchoring groups are used to attach the molecule in between two gold electrodes, and a parameter free, first-principles, nonequilibrium Green's function approach is used to calculate the current-voltage characteristics. Charge transfer from the alkali atom to the host is found to have a profound effect on the electronic structure causing a dramatic change in the conductivity. Since the single alkali atom controls the behavior of electron flow in this circuit, we term this device as a {\it single-atom-controlled} device.   

Conformational and Voltage Gating in a Molecular Three Terminal Device

The effect of the conformational changes in the gate arm of a three terminal molecular device is investigated. The donor-acceptor molecular moieties connected through a ring describe the two arms, whereas a $\pi $-conjugated molecular wire is used as a gate in the proposed architecture. In the absence of the gate field, the device exhibits current switching between the non-planar and planar orientations of the $\pi $ -electron moieties with respect to each other with maximum I$_{(ON)}$/I$_{(OFF)}$ =14. When the gate field is applied, I$_{(ON)}$/I$_{(OFF)}$ ratio decreases, thus suggesting that the effects of the conformational changes in the gate arm and the applied gate field oppose each other in the architecture considered. Furthermore, the tunneling current corresponding to conformational gating in two different directions appears to exhibit oscillatory nature with a phase factor of $\pi $/2 in presence of the gate field.   

Charge transport in strongly coupled molecular junctions: `In-phase' and `out of phase' contribution to electron tunneling

We report a first principles study on the evolution of charge transport in a two-terminal molecular scale device with the increase in the length of the molecular wire build out of cubane oligomers. In particular, for wires of three different lengths, we look into the relative contribution of the `in-phase' and the `out-of-phase' components of the total electronic current under the influence of an external bias. In the low bias regime, the `out-of-phase' contribution to total current is minimal and `in-phase' or elastic tunneling of the electrons is responsible for the net electronic current. This is true irrespective of the length of the molecular spacer. In this regime, the I-V characteristics follow Ohm's law and the conductance of the wires is found to decrease exponentially with length which is in agreement with experimental results. However, after a certain `off-set' voltage, the I-V characteristics become non-linear and the `out-of-phase' tunneling starts to contribute substantially to the net current.   

Negative Differential Resistance at Low Bias: C60-Based Molecular Devices

Unlike single-C60-based devices, molecular assemblies based on two or more C60 can exhibit negative differential resistance (NDR). We evaluate electron transport properties of molecular devices built from two C60 connected by an alkane chain, using a non-equilibrium Green function technique implemented within the framework of linear-scaling DFT. We find that electronic conduction in these systems is mediated by C60's lowest unoccupied molecular orbitals (LUMOs), as in the case of a single-C60-based device. However, as the LUMOs' positions are pinned to the chemical potentials of their respective electrodes, their relative alignment shifts with applied bias and leads to an NDR at a very low bias. Furthermore, the position and magnitude of the NDR can be tuned by chemical modification of the C60s and by changing the length of the alkane linker. The flexibility and richness of C60-based molecular electronics components point to a potentially promising route for the design of molecular devices and chemical sensors.   

Charge injection and transport across metal-C60 and C60-petacene interfaces: A first-principles study

Recent experiments demonstrated that [60]fullerene (C$_{60})$ molecules adsorbed on metal surfaces provide favorable energy level alignment for both electron and hole injections in the context of light-emitting diode applications [1,2]. The efficient hole injection across C$_{60}$ layers is rather surprising, since C$_{60}$s are highly electron-accepting molecules and should behave as a hole blocking (rather than hole injection) layer. To provide a microscopic understanding of these seemingly contradictory finding, we consider Au--C$_{60}$--pentacene--C$_{60}$--Au molecular junctions using a first-principles computational approach. We find the Fermi level pinning at the Au--C$_{60}$ interfaces and the strongly configuration-dependent charge transport efficiency at the C$_{60}$--pentacene interfaces. The former finding is in agreement with a recent experimental report [2] and our earlier conclusion from the study of polymerized C$_{60}$ wires [3]. We will explain the latter observation based on the nature of charge tunneling across $\pi $--$\pi $ orbitals [1] Lee, J.Y., Appl. Phys. Lett. \textbf{88}, 073512 (2006) [2] Wang, Z.B \textit{et al.}, Appl. Phys. Lett. \textbf{95}, 043302 (2009). [3] Lee, G.I., Kang, J.K., {\&} Kim, Y.-H., J. Phys. Chem. C \textbf{112}, 7029 (2008).   

Electron correlation enhancement of the diode property of asymmetric molecule

Stimulated by the giant diode property found in tetraphenlydithiol derivative including dipyrimidinil-diphenly diblock [1], a possible mechanism of the giant diode property was investigated theoretically based on electron correlation. We found that the mean field theory (MFT) fails in describing the giant diode property, as it was confirmed by first principle calculation of ballistic electronic current through the diode molecule using GGA. Electron correlation effect on electric current taken into account within the self-consistent GW approximation using Keldysh Green's function theory was found to give the fair account of the giant diode property. We conclude that elastic electron collision beyond MFT enhances the diode property quite a lot. \\[4pt] [1] I.D\'iez-P\'erez et al, Nature Chemistry, 635 (2009).   

First Principles Study and Theoretical Analysis of a Single Molecular Diode by $p-n$ di-block molecules

The concept of a single molecular diode was first proposed by Aviram and Ratner, and there have been many studies of synthesis D-$\sigma $-A or $p-n$ di-block molecules and measurements of the current-voltage ($I-V)$ characteristics for relating molecular junctions. Recently, the $I-V $measurement in a symmetric tetraphenyl junction and non-symmetric dipyrimidinyl -diphenyl diblock junction was performed, and clear rectification was found in the latter system, which resembles the $p-n$ junction by the covalent connection between electron-deficient bypyrimidinyl and electron-rich biphenyl moieties, though an applied bias is much lower than the resonant level. In this presentation, we performed the first principles calculations of electron transport for the above tetraphenyl and dipyrimidinyl -diphenyl diblock junctions by the self-consistent nonequilibrium Green's function theory with the use of our HiRUNE program module. We carried out the systematic analysis of the rectification behavior and identified the change of electron-pathway in the bridge molecule relating to $p-n$ junction based on the first principles data. The relation between the rectifying action and molecular conformation, particularly, the torsion of diblock, will be discussed.   

Temperature influence on a molecular switching under electric field: quantum transport ab initio calculation

A molecular transistor based on torsion-angle conformation change driven by gate electric field is designed and studied using {\it ab initio} calculations. This transistor consists of a ${\rm SH-C_6H_2F(CH_3)C_6H_2(CH_3)F-SH}$ molecule sandwiched between two Au(111) electrodes, where the interaction between the molecular dipole and a gate voltage induced electric field will cause the molecule to twist along its c-axis. This twist changes the quantum conductivity of the molecule. The effect of thermal fluctuation on the molecular conformation is studied, so is the ability of the transistor to shut off its current. The advantages and challenges of using such molecular conformation change as a mechanism for transistor gating are discussed   

Vibronic- and mechanical-spin control in spin-1 molecular devices

Using numerical renormalization group calculations, we study the effect of a vibronic mode on the electronic transport through a deformable spin-1 molecular device. We analyze the experimental situation of Parks et. al. [Science 328 1370 (2010)], where it is observed that stretching the molecule introduces a static magnetic anisotropy. The device is modeled as an interacting two-level system with only one level coupled to metallic leads, in which the static anisotropy is modulated by a vibronic mode. We performed calculations of the local spectral density, which indicate that this dynamic magnetic anisotropy can counter the static effects and drive the ground state into a non Fermi-liquid phase with non-zero spectral density at the Fermi level. It also renormalizes the couplings between the molecule and the metallic leads in an anisotropic fashion, reducing the spin-1 Kondo temperature of the device.