Resistance of atomic wires

Electroluminescence Rectification and High Harmonic Generation in Molecular Junctions

The field of molecular electronics has emerged from efforts to understand electron propagation through single molecules and to use them in electronic circuits. Serving as a testbed for advanced theoretical methods, it reveals a significant discrepancy between the operational time scales of experiments (static to GHz frequencies) and theoretical models (femtoseconds). Utilizing a recently developed time-linear nonequilibrium Green function formalism, we model molecular junctions on experimentally accessible time scales. Our study focuses on the quantum pump effect in a benzenedithiol molecule connected to two copper electrodes and coupled with cavity photons. By calculating both electric and photonic current responses to an ac bias voltage, we observe pronounced electroluminescence and high harmonic generation in this setup. The mechanism of the latter effect is more analogous to that from solids than from isolated molecules, with even harmonics being suppressed or enhanced depending on the symmetry of the driving field.

Electro-thermal transport in disordered nanostructures: a modeling perspective

Following the emergence of novel classes of atomic systems with amorphous active regions, device simulations had to rapidly evolve to devise strategies to account for the influence of disordered phases, defects, and interfaces into its core physical models. We review here how molecular dynamics and quantum transport can be combined to shed light on the performance of, for example, conductive bridging random access memories (CBRAM), a type of non-volatile memory. In particular, we show that electro-thermal effects play a critical role in such devices and therefore present a method based on density functional theory and the non-equilibrium Green's function formalism to accurately describe them. Three CBRAM configurations are investigated to illustrate the functionality of the proposed modeling approach.

Adenine based molecular junction as biosensor for detection of toxic phosgene gas

The possibility of adsorption of toxic phosgene gas (COCl₂) molecule on one of the nucleobase of DNA-adenine-has been analyzed using the first principle calculations based on density function theory. In accordance with the geometry of the nucleobase, two possible positions have been considered for effective adsorption of gas molecule. The calculations performed on adsorption energies suggest that the gas molecule is able to physisorb at both the considered positions with negligibly small values of charge transfer. The in-depth analysis of electron charge densities depicts that there is no orbital overlapping between the gas molecule and adenine. We observe a significant variation of transport properties of adenine-based molecular junction on adsorption of phosgene molecule while calculation the transport parameters at both the equilibrium as well as non-equilibrium. Also, the variation of HOMO-LUMO gap of adenine molecule on adsorption of phosgene leads to alteration of current and voltage, thus implying that adenine-based sensor can be effectively utilized to sense the presence of phosgene gas in a given environment. Small adsorption energies and recovery time suggest that the rate of desorption of phosgene is very high; thus, the proposed adenine sensor can be effectively used as a highly stable and selective reusable sensor.

SAKE: first-principles electron transport calculation code

We developed and implemented a numerical code called SAKE, which stands for (simulation code for atomistic Kohn-Sham equation). We developed it for first-principle electron transport calculations based on density-functional theory and non-equilibrium Green's function formalism. First, we present the central calculation parts of the formalism of the electronic states and transport properties for open and non-equilibrium systems. We show specific computational techniques, such as the use of a complex contour integration for charge density from the density matrix, which is compared with the calculation method of summing the residues of the Fermi-Dirac distribution, as well as the efficient achievement of the self-consistent procedures. Thereafter, for applications of the present computation code, SAKE, we present first-principle calculation results of three different systems. We first analyze electronic structures of polythiophene molecular wires, compare summation techniques for the density matrix. We show thermoelectric properties of an n-type antiferromagnetic semiconductor CuFeS2as a second application. The electrical conductance, electrical thermal conductance, and the Seebeck coefficients with carrier doping are examined, and the analytical form of the Seebeck coefficient is briefly described. For the third application, we analyze the electron transport properties of polyaniline molecular wires under structural deformations, i.e. rotations around the transport direction. The thermally averaged current-voltage characteristics are also analyzed. The results show that the current decreases as the temperature increases which are determined based on the competition between the thermal energy and the electronic energy, which increases with the rotation angle.

Transport through correlated systems with density functional theory

We present recent advances in density functional theory (DFT) for applications in the field of quantum transport, with particular emphasis on transport through strongly correlated systems. We review the foundations of the popular Landauer-Büttiker(LB)  +  DFT approach. This formalism, when using approximations to the exchange-correlation (xc) potential with steps at integer occupation, correctly captures the Kondo plateau in the zero bias conductance at zero temperature but completely fails to capture the transition to the Coulomb blockade (CB) regime as the temperature increases. To overcome the limitations of LB  +  DFT, the quantum transport problem is treated from a time-dependent (TD) perspective using TDDFT, an exact framework to deal with nonequilibrium situations. The steady-state limit of TDDFT shows that in addition to an xc potential in the junction, there also exists an xc correction to the applied bias. Open shell molecules in the CB regime provide the most striking examples of the importance of the xc bias correction. Using the Anderson model as guidance we estimate these corrections in the limit of zero bias. For the general case we put forward a steady-state DFT which is based on one-to-one correspondence between the pair of basic variables, steady density on and steady current across the junction and the pair local potential on and bias across the junction. Like TDDFT, this framework also leads to both an xc potential in the junction and an xc correction to the bias. Unlike TDDFT, these potentials are independent of history. We highlight the universal features of both xc potential and xc bias corrections for junctions in the CB regime and provide an accurate parametrization for the Anderson model at arbitrary temperatures and interaction strengths, thus providing a unified DFT description for both Kondo and CB regimes and the transition between them.

Electrical characterization of C28 fullerene junctions formed with group 1B metal electrodes

We present an atomistic theory of electronic transport through single molecular junctions based on smallest stable fullerene molecule, C₂₈. The electronic properties of single molecular junctions critically depend on the nature of electrode material. The two probe device is modeled by constraining C₂₈ between two semi-infinite metal electrodes, from group 1B of periodic table, copper, silver and gold. We have highlighted the correlated phenomena of resonant conduction and current driven dynamics in molecular junctions using extendend Huckel theory in combination with non equilibrium Green's function framework. We conclude strong dependence of conductance on transmissions, which leads to oscillating conductance spectrum. An interesting interplay between conducting channels and different degrees of spatial localization and delocalization of molecular orbitals is evinced. The physical origin of current and conductance of so-formed C₂₈ molecular junctions is discussed in detail by analysing their density of states, transmission spectra, molecular orbital analysis, rectification ratio and molecular projected self consistent Hamiltonian eigen states at different operating voltages ranging from -2V to +2V.

Self-energy matrices for electron transport calculations within the real-space finite-difference formalism

The self-energy term used in transport calculations, which describes the coupling between electrode and transition regions, is able to be evaluated only from a limited number of the propagating and evanescent waves of a bulk electrode. This obviously contributes toward the reduction of the computational expenses in transport calculations. In this paper, we present a mathematical formula for reducing the computational expenses further without using any approximation and without losing accuracy. So far, the self-energy term has been handled as a matrix with the same dimension as the Hamiltonian submatrix representing the interaction between an electrode and a transition region. In this work, through the singular-value decomposition of the submatrix, the self-energy matrix is handled as a smaller matrix, whose dimension is the rank number of the Hamiltonian submatrix. This procedure is practical in the case of using the pseudopotentials in a separable form, and the computational expenses for determining the self-energy matrix are reduced by 90% when employing a code based on the real-space finite-difference formalism and projector-augmented wave method. In addition, this technique is applicable to the transport calculations using atomic or localized basis sets. Adopting the self-energy matrices obtained from this procedure, we present the calculation of the electron transport properties of C_{20} molecular junctions. The application demonstrates that the electron transmissions are sensitive to the orientation of the molecule with respect to the electrode surface. In addition, channel decomposition of the scattering wave functions reveals that some unoccupied C_{20} molecular orbitals mainly contribute to the electron conduction through the molecular junction.

Electronic transport properties of ultra-thin Ni and Ni-C nanowires

The structures and electronic transport properties of ultra-thin Ni and Ni-C nanowires obtained from carbon nanotube (CNT) templates are theoretically investigated. C atoms tend to locate at the central positions of nanowires and are surrounded by Ni atoms. Spin polarization at the Fermi level is not responsible for the spin filtration of these nanowires. Increasing C concentration can improve the resistance of nanowires by abating the number of electronic transmission channels and the coupling of electron orbitals between Ni atoms. Moreover, with the increase of diameter, the conductance of these nanowires increases as well. This study is helpful for guiding the synthesis of nanowires with desired applications.

Conductance Quantization in Resistive Random Access Memory

The intrinsic scaling-down ability, simple metal-insulator-metal (MIM) sandwich structure, excellent performances, and complementary metal-oxide-semiconductor (CMOS) technology-compatible fabrication processes make resistive random access memory (RRAM) one of the most promising candidates for the next-generation memory. The RRAM device also exhibits rich electrical, thermal, magnetic, and optical effects, in close correlation with the abundant resistive switching (RS) materials, metal-oxide interface, and multiple RS mechanisms including the formation/rupture of nanoscale to atomic-sized conductive filament (CF) incorporated in RS layer. Conductance quantization effect has been observed in the atomic-sized CF in RRAM, which provides a good opportunity to deeply investigate the RS mechanism in mesoscopic dimension. In this review paper, the operating principles of RRAM are introduced first, followed by the summarization of the basic conductance quantization phenomenon in RRAM and the related RS mechanisms, device structures, and material system. Then, we discuss the theory and modeling of quantum transport in RRAM. Finally, we present the opportunities and challenges in quantized RRAM devices and our views on the future prospects.

First-principles calculation method for electron transport based on the grid Lippmann-Schwinger equation

We develop a first-principles electron-transport simulator based on the Lippmann-Schwinger (LS) equation within the framework of the real-space finite-difference scheme. In our fully real-space-based LS (grid LS) method, the ratio expression technique for the scattering wave functions and the Green's function elements of the reference system is employed to avoid numerical collapse. Furthermore, we present analytical expressions and/or prominent calculation procedures for the retarded Green's function, which are utilized in the grid LS approach. In order to demonstrate the performance of the grid LS method, we simulate the electron-transport properties of the semiconductor-oxide interfaces sandwiched between semi-infinite jellium electrodes. The results confirm that the leakage current through the (001)Si-SiO_{2} model becomes much larger when the dangling-bond state is induced by a defect in the oxygen layer, while that through the (001)Ge-GeO_{2} model is insensitive to the dangling bond state.

Numerical solver for first-principles transport calculation based on real-space finite-difference method

We propose an efficient procedure to obtain Green's functions by combining the shifted conjugate orthogonal conjugate gradient (shifted COCG) method with the nonequilibrium Green's function (NEGF) method based on a real-space finite-difference (RSFD) approach. The bottleneck of the computation in the NEGF scheme is matrix inversion of the Hamiltonian including the self-energy terms of electrodes to obtain the perturbed Green's function in the transition region. This procedure first computes unperturbed Green's functions and calculates perturbed Green's functions from the unperturbed ones using a mathematically strict relation. Since the matrices to be inverted to obtain the unperturbed Green's functions are sparse, complex-symmetric, and shifted for a given set of sampling energy points, we can use the shifted COCG method, in which once the Green's function for a reference energy point has been calculated the Green's functions for the other energy points can be obtained with a moderate computational cost. We calculate the transport properties of a C(60)@(10,10) carbon nanotube (CNT) peapod suspended by (10,10)CNTs as an example of a large-scale transport calculation. The proposed scheme opens the possibility of performing large-scale RSFD-NEGF transport calculations using massively parallel computers without the loss of accuracy originating from the incompleteness of the localized basis set.

Transport properties of atomic-size aluminum chains: first principles and nonequilibrium Green's function studies

First principles calculations are performed to investigate atomic structure and nonequilibrium Green's function for Al atomic scale chains transport properties.

Real-space finite-difference calculation method of generalized Bloch wave functions and complex band structures with reduced computational cost

Generalized Bloch wave functions of bulk structures, which are composed of not only propagating waves but also decaying and growing evanescent waves, are known to be essential for defining the open boundary conditions in the calculations of the electronic surface states and scattering wave functions of surface and junction structures. Electronic complex band structures being derived from the generalized Bloch wave functions are also essential for studying bound states of the surface and junction structures, which do not appear in conventional band structures. We present a novel calculation method to obtain the generalized Bloch wave functions of periodic bulk structures by solving a generalized eigenvalue problem, whose dimension is drastically reduced in comparison with the conventional generalized eigenvalue problem derived by Fujimoto and Hirose [Phys. Rev. B 67, 195315 (2003)]. The generalized eigenvalue problem derived in this work is even mathematically equivalent to the conventional one, and, thus, we reduce computational cost for solving the eigenvalue problem considerably without any approximation and losing the strictness of the formulations. To exhibit the performance of the present method, we demonstrate practical calculations of electronic complex band structures and electron transport properties of Al and Cu nanoscale systems. Moreover, employing atom-structured electrodes and jellium-approximated ones for both of the Al and Si monatomic chains, we investigate how much the electron transport properties are unphysically affected by the jellium parts.

Rethinking first-principles electron transport theories with projection operators: the problems caused by partitioning the basis set

We revisit the derivation of electron transport theories with a focus on the projection operators chosen to partition the system. The prevailing choice of assigning each computational basis function to a region causes two problems. First, this choice generally results in oblique projection operators, which are non-Hermitian and violate implicit assumptions in the derivation. Second, these operators are defined with the physically insignificant basis set and, as such, preclude a well-defined basis set limit. We thus advocate for the selection of physically motivated, orthogonal projection operators (which are Hermitian) and present an operator-based derivation of electron transport theories. Unlike the conventional, matrix-based approaches, this derivation requires no knowledge of the computational basis set. In this process, we also find that common transport formalisms for nonorthogonal basis sets improperly decouple the exterior regions, leading to a short circuit through the system. We finally discuss the implications of these results for first-principles calculations of electron transport.

Nonequilibrium thermodynamics of interacting tunneling transport: variational grand potential, density functional formulation and nature of steady-state forces

The standard formulation of tunneling transport rests on an open-boundary modeling. There, conserving approximations to nonequilibrium Green function or quantum statistical mechanics provide consistent but computational costly approaches; alternatively, the use of density-dependent ballistic-transport calculations (e.g., Lang 1995 Phys. Rev. B 52 5335), here denoted 'DBT', provides computationally efficient (approximate) atomistic characterizations of the electron behavior but has until now lacked a formal justification. This paper presents an exact, variational nonequilibrium thermodynamic theory for fully interacting tunneling and provides a rigorous foundation for frozen-nuclei DBT calculations as a lowest-order approximation to an exact nonequilibrium thermodynamic density functional evaluation. The theory starts from the complete electron nonequilibrium quantum statistical mechanics and I identify the operator for the nonequilibrium Gibbs free energy which, generally, must be treated as an implicit solution of the fully interacting many-body dynamics. I demonstrate a minimal property of a functional for the nonequilibrium thermodynamic grand potential which thus uniquely identifies the solution as the exact nonequilibrium density matrix. I also show that the uniqueness-of-density proof from a closely related Lippmann-Schwinger collision density functional theory (Hyldgaard 2008 Phys. Rev. B 78 165109) makes it possible to express the variational nonequilibrium thermodynamic description as a single-particle formulation based on universal electron-density functionals; the full nonequilibrium single-particle formulation improves the DBT method, for example, by a more refined account of Gibbs free energy effects. I illustrate a formal evaluation of the zero-temperature thermodynamic grand potential value which I find is closely related to the variation in the scattering phase shifts and hence to Friedel density oscillations. This paper also discusses the difference between the here-presented exact thermodynamic forces and the often-used electrostatic forces. Finally the paper documents an inherent adiabatic nature of the thermodynamic forces and observes that these are suited for a nonequilibrium implementation of the Born-Oppenheimer approximation.

Effect of electron-vibration interactions on the thermoelectric efficiency of molecular junctions

From first-principles approaches, we investigate the thermoelectric efficiency of a molecular junction where a benzene molecule is connected directly to the platinum electrodes. We calculate the thermoelectric figure of merit ZT in the presence of electron-vibration interactions with and without local heating under two scenarios: linear response and finite bias regimes. In the linear response regime, ZT saturates around the electrode temperature T(e) = 25 K in the elastic case, while in the inelastic case we observe a non-saturated and a much larger ZT beyond T(e) = 25 K attributed to the tail of the Fermi-Dirac distribution. In the finite bias regime, the inelastic effects reveal the signatures of the molecular vibrations in the low-temperature regime. The normal modes exhibiting structures in the inelastic profile are characterized by large components of atomic vibrations along the current density direction on top of each individual atom. In all cases, the inclusion of local heating leads to a higher wire temperature T(w) and thus magnifies further the influence of the electron-vibration interactions due to the increased number of local phonons.

QUANTUM TRANSPORT AND CURRENT-TRIGGERED DYNAMICS IN MOLECULAR TUNNEL JUNCTIONS

The modelling of nanoelectronic systems has been the topic of ever increasing activity for nearly two decades. Yet, new questions, challenges and opportunities continue to emerge. In this article we review theoretical and numerical work on two new developments in the theory of molecular-scale electronics. First we review a density functional theory analysis within the Keldysh non-equilibrium Green function formalism to predict nonlinear charge transport properties of nanoelectronic devices. Next we review a recently developed quantum mechanical formalism of current-triggered nuclear dynamics. Finally we combine these theories to describe from first principles the inelastic current and the consequent molecular dynamics in molecular heterojunctions.

Real-space calculations for electron transport properties of nanostructures

Recent developments in the fabrication and investigation of conductors of atomic dimensions have stimulated a large number of experimental and theoretical studies on these nanoscale devices. In this paper, we introduce examples presenting the efficiencies and advantages of a first-principles transport calculation scheme based on the real-space finite-difference (RSFD) formalism and the overbridging boundary-matching (OBM) method. The RSFD method does not suffer from the artificial periodicity problems that arise in methods using plane-wave basis sets or the linear dependence problems that occur in methods using atomic basis sets. Moreover, the algorithm of the RSFD method is suitable for massively parallel computers and, thus, the combination of the RSFD and OBM methods enables us to execute first-principles transport calculations using large models. To demonstrate the advantages of this method, several applications of the transport calculations in various systems ranging from jellium nanowires to the tip and surface system of scanning tunneling microscopy are presented.

Critical comparison of electrode models in density functional theory based quantum transport calculations

We study the performance of two different electrode models in quantum transport calculations based on density functional theory: parametrized Bethe lattices and quasi-one-dimensional wires or nanowires. A detailed account of implementation details in both the cases is given. From the systematic study of nanocontacts made of representative metallic elements, we can conclude that the parametrized electrode models represent an excellent compromise between computational cost and electronic structure definition as long as the aim is to compare with experiments where the precise atomic structure of the electrodes is not relevant or defined with precision. The results obtained using parametrized Bethe lattices are essentially similar to the ones obtained with quasi-one-dimensional electrodes for large enough cross-sections of these, adding a natural smearing to the transmission curves that mimics the true nature of polycrystalline electrodes. The latter are more demanding from the computational point of view, but present the advantage of expanding the range of applicability of transport calculations to situations where the electrodes have a well-defined atomic structure, as is the case for carbon nanotubes, graphene nanoribbons, or semiconducting nanowires. All the analysis is done with the help of codes developed by the authors which can be found in the quantum transport toolbox ALACANT and are publicly available.

Control of molecule-based transport for future molecular devices

In this review, possibilities to modify intentionally the electronic transport properties of metal/molecule/metal devices (MMM devices) are discussed. Here especially the influence of the metal work function, the metal-molecule interface, the molecule dipole and different tunneling mechanisms are considered. A route to evaluate the effective surface work function of metal-molecule systems is given and, based on experimental results, an exemplary estimation is performed. The electron transport across different metal-molecule interfaces is characterized by relating transmission coefficients extracted from experimentally derived molecular conductances, decay constants or tunneling barrier heights. Based on the reported results the tunneling decay constant can be assumed to be suitable to characterize intrinsic molecular electron transport properties, while the nature of the metal-molecule contacts is properly described by the transmission coefficient. A clear gradation of transmission efficiencies of metal-anchoring group combinations can be given.

Negative differential resistance in C60-based electronic devices

Unlike single-C(60)-based devices, molecular assemblies based on two or more appropriately connected C(60) molecules have the potential to exhibit negative differential resistance (NDR). In this work, we evaluate electron transport properties of molecular devices built from two C(60) molecules connected by an alkane chain, using a nonequilibrium Green function technique implemented within the framework of density functional theory. We find that electronic conduction in these systems is mediated by the lowest unoccupied molecular orbitals (LUMOs) of C(60), as in the case of a single-C(60)-based device. However, as the positions of the LUMOs are pinned to the chemical potentials of their respective electrodes, their relative alignment shifts with applied bias and leads to a NDR at a very low bias. Furthermore, the position and magnitude of the NDR can be tuned by chemical modification of the C(60) molecules. The role of the attached molecules is to shift the LUMO position and break the symmetry between the forward and reverse currents. The NDR feature can also be controlled by changing the length of the alkane linker. The flexibility and richness of C(60)-based molecular electronics components point to a potentially promising route for the design of molecular devices and chemical sensors.

Dynamical Coulomb blockade and the derivative discontinuity of time-dependent density functional theory

The role of the discontinuity of the exchange-correlation potential of density functional theory is studied in the context of electron transport and shown to be intimately related to Coulomb blockade. By following the time evolution of an interacting nanojunction attached to biased leads, we find that, instead of evolving to a steady state, the system reaches a dynamical state characterized by correlation-induced current oscillations. Our results establish a dynamical picture of Coulomb blockade manifesting itself as a periodic sequence of charging and discharging of the nanostructure.

Multiple-path Quantum Interference Effects in a Double-Aharonov-Bohm Interferometer

We investigate quantum interference effects in a double-Aharonov-Bohm (AB) interferometer consisting of five quantum dots sandwiched between two metallic electrodes in the case of symmetric dot-electrode couplings by the use of the Green's function equation of motion method. The analytical expression for the linear conductance at zero temperature is derived to interpret numerical results. A three-peak structure in the linear conductance spectrum may evolve into a double-peak structure, and two Fano dips (zero conductance points) may appear in the quantum system when the energy levels of quantum dots in arms are not aligned with one another. The AB oscillation for the magnetic flux threading the double-AB interferometer is also investigated in this paper. Our results show the period of AB oscillation can be converted from 2π to π by controlling the difference of the magnetic fluxes threading the two quantum rings.

Thermoelectric efficiency in nanojunctions: a comparison between atomic junctions and molecular junctions

Using first-principles approaches, we investigate the thermoelectric efficiency, characterized by the figure of merit ZT, in metallic atomic junctions and insulating molecular junctions. To gain insight into the properties of ZT, an analytical theory is also developed to study the dependence of ZT on lengths (l) and temperatures (T). The theory considers the combined heat current carried by electrons and phonons. We observe a characteristic temperature: T(0) = (beta/gamma(/))(1/2). When T << T(0), the electronic heat current dominates the combined heat current and ZT is proportional to T(2). When T >> T(0), the phononic heat current dominates the combined heat current and ZT tends to a saturation value. Moreover, the metallic atomic junctions and the insulating molecular junctions have opposite trend for the dependence of ZT on lengths, that is, ZT increases as the length increases for aluminum atomic junctions, while ZT decreases as the length increases for alkanethiol molecular junctions.

Electronic transport calculations for rough interfaces in Al, Cu, Ag, and Au

We present results of electronic structure and transport calculations for metallic interfaces, based on density functional theory and the non-equilibrium Green's function method. Starting from the electronic structure of smooth Al, Cu, Ag, and Au interfaces, we study the effects of different kinds of interface roughness on the transmission coefficient and the I-V characteristic. In particular, we compare prototypical interface distortions, including vacancies and metallic impurities.

Asymptotic nonequilibrium steady-state operators

We present a method for the calculation of asymptotic operators for nonequilibrium steady-state quantum systems. The asymptotic steady-state operator is obtained by averaging the corresponding operator in Heisenberg representation over infinitely long time. Several examples are considered to demonstrate the utility of our method. The results obtained within our approach are compared to those obtained within the Schwinger-Keldysh nonequilibrium Green's functions.

Electron transport in open systems from finite-size calculations: Examination of the principal layer method applied to linear gold chains

We describe the occurrence of computational artifacts when the principal layer method is used in combination with the cluster approximation for the calculation of electronic transport properties of nanostructures. For a one-dimensional gold chain, we observe an unphysical band in the band structure. The artificial band persists for large principal layers and for large buffer sizes. We demonstrate that the assumption of equality between Hamiltonian elements of neighboring layers is no longer valid and that a discontinuity is introduced in the potential at the layer transition. The effect depends on the basis set. When periodic boundary conditions are imposed and the k-space sampling is converged, the discontinuity disappears and the principal layer method can be correctly applied by using a linear combination of atomic orbitals as basis set.

The biphenyl molecule as a model transistor

We study transport and charge control in a gated 4,4'-biphenyl diradical molecular transistor using self-consistent density-functional calculations. We track both electron-like and hole-like conduction and relate it to the field dependence of current-carrying pi-derived states. Owing to the coupling between the two benzene rings, the pi-states become segregated into extended, current-carrying states and localized states. Under application of the source/drain field, along the axis of the molecule, the localized pi-states become split, while the extended states become polarized and screen the field. The localized states act like isolated islands within the molecule--while they make a substantial contribution to the density of states, they make only a small contribution to transport.

First-principles modeling of electron transport

The dimensions of electronic devices are rapidly decreasing and there is a need for a new generation of modeling tools that can accurately calculate the electrical properties of devices where atomic scale details and quantum effects are important. A promising framework for such calculations is density functional theory within the non-equilibrium Green's function formalism (NEGF-DFT). In this paper we present the basic framework and applications of the formalism. The applications include the calculation of the I-V characteristics of a single molecule connected with gold electrodes and the spin-dependent electron transport through a magneto-tunnel junction consisting of MgO layers sandwiched between Fe electrodes. For the formalism to be applied in semiconductor device modeling it needs to be able to handle many thousands of atoms. We discuss new developments and future aspects of the method important for semiconductor device modeling; in particular we show that for important classes of systems the approach scales linearly with the system size.

Simple STM tip functionalization for rapid DNA sequencing: an Ab initio Green's function study

An ab initio nonequilibrium Green's function study of the electron-transport properties of the adenine, thymine, cytosine, and guanine DNA bases located between gold electrodes has been performed. One-electron transmission spectra were calculated for both gold and sulfur-modified gold electrodes, which are model conditions of scanning tunneling microscopy (STM) experiments with the different tips. It is shown that the nature of chemical bonding between molecules and metal electrodes plays the most significant role in the overall conductance of the systems. The distance between electrodes and the size of molecules are less important, at least when both sides of the molecule form chemical contact with the electrodes. On the basis of the obtained results, a simple two-pass DNA sequencing scheme is suggested.

Nonequilibrium electronic transport of 4,4'-bipyridine molecular junction

The electronic transport properties of a 4,4'-bipyridine molecule sandwiched between two Au(111) surfaces are studied with a fully self-consistent nonequilibrium Green's-function method combined with the density-functional theory. The 4,4'-bipyridine molecule prefers to adsorb near the hollow site of the Au(111) surface and distorts slightly. The modifications on the electronic structure of the molecule due to the presence of the electrodes are described by the renormalized molecular orbitals, which correspond well to the calculated transmission peaks. The average Fermi level lies close to the lowest unoccupied renormalized molecular orbital, which determines the electronic transport property of the molecular junction under a small bias voltage. The total transmission is contributed by a single channel. The transmission peaks shift with the applied bias voltage, and this behavior depends on the spatial distribution of the renormalized molecular orbitals and the voltage drop along the molecular junction. The shape of the calculated conductance curve of the equilibrium geometric configuration reproduces the main feature of the experimental results, but the value is larger than the measured data by about 6 times. Good agreement with the experimental measurements can be obtained by elongating the molecular junction. The electronic transport behaviors depend strongly on the interface configuration.