Bias drop and phonon emission in molecular wires

Using the recursion-transfer-matrix (RTM) method combined with the non-equilibrium Green's function (NEGF) method and density-functional theory, we perform ab initio calculations for the electron transport of molecular wires bridged between electrodes. We present an effective potential of molecular wire under a finite bias voltage and discuss the phonon emission and local heating due to inelastic electron-phonon coupling effects. We find that it is strongly dependent on contact conditions. When the contacts to electrodes are bad, excitation phonon modes at contacts become dominant for the energy dissipation.     

Electron transport phenomenon simulation through the carborane nano-molecular wire

The electron transport characteristics of a 1,10-dimethylene-1,10-dicarba-closo-decaborane (10-vertex carborane) single molecular conductor is investigated via the density functional-based non-equilibrium Green's function (DFT-NEGF) method. We consider three configurations for the molecular wire sandwiched between two Au(1 0 0) electrodes: the hollow site, top site and bridge site positions. Our results show that the energetically favorable hollow site configuration has a higher current intensity than the other configurations.The projection of the density of states (PDOS) and the transmission coefficients T(E) of the two-probe system at zero bias are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to the higher conductance for the hollow configuration.Furthermore, the transmission coefficients of the hollow system at various external voltage biases are also investigated and it shows that the broadening of the transmission coefficient spectrum with increasing of the external voltage bias indicates a strong coupling between the molecular orbitals in the carborane and the incident states from the electrodes, and thus the current increases with increases of the bias voltage.     

First-principles study of current–voltage characteristics of two-terminal carbon nanostructures

We present the first-principles investigation of the transport properties of nanotubes connected to metal electrodes under external bias potential. We have developed the technique to calculate the current–voltage (I–V) curves by using the local-density approximation in the density-functional theory. We apply this technique to Al-nanotube-Al systems with different contact geometries regarding the position, the orientation, and the distance of nanotube to the electrode. These different geometries at contact can play an important role in the transport properties. The I–V curves have the different behaviors although the nanotube is connected to the same electrode. The transmission rate from one electrode to the other electrode shows strong dependence on the contact geometry.     

Rectification effects in coherent transport through single molecules

A minimal model for coherent transport through a donor/acceptor molecular junction is presented. The two donor and acceptor sites are described by single levels energetically separated by an intramolecular tunnel barrier. In the limit of strong coupling to the electrodes a current rectification for different bias voltage polarities occurs. Contacts with recent experiments of molecular rectification are also given.     

Two-electron and four-electron periodicity in single-wall carbon nanotube quantum dots

Single quantum dots have been fabricated in single-wall carbon nanotubes and electrical transport properties have been measured at low temperature. Two- and four-electron periodicities have been clearly observed in the same sample in different gate voltage ranges. The former is an even–odd effect which originates from the spin degeneracy, while the latter is related to the additional two-fold band degeneracy. The results are discussed with the energy scales associated with the dot, and the possibility for a single spin manipulation is suggested.     

Electron transport of a quantum wire with spatially periodic spin–orbit coupling

Electron transport properties of an ideal one-dimensional (1D) quantum wire are studied including spatially periodic Rashba spin–orbit coupling (SOC) and Dresselhaus SOC. By comparing with the previous work [S.J. Gong, Z.Q. Yang, J. Phys. Condens. Matter 19 (2007) 446209], two transmission gaps appear in the transmission probability of electrons and their widths are also broadened dramatically. Moreover, it is found that their widths are sensitive not only to the strength of SOCs but also to the length ratio of SOCs segment and non-SOCs segment. In addition, a ‘circle-type’ transmission behavior has been found by tuning the strength of SOCs continuously. Our results may extend the previous work and provide an more effective method to manipulate the current in nanoelectric devices.     

Investigations of the E-O behavior in the poled polymer waveguide

The electro-optic (E-O) properties of the slab polymer waveguide induced by electric poling have been studied experimentally. The thin film waveguide was prepared by a synthesized polymer polypelargonamide and poled by applying voltage between the electrodes. I–V curves and E-O characteristics of the polymer films were in-situ measured during poling. In order to evaluate the molecular dipole orientation under the poling field, the absorption spectra of this film was observed by FTIR. The E-O performance of the polymer slab film was also estimated by the Mach–Zehnder interferometer.     

Lowering the Hartree–Fock Minimizer by Electron–Positron Pair Correlation

We prove by a simple computation that a suitable coupling to the positronic sector lowers the energy of the purely electronic minimizer of the electron–positron Hartree–Fock functional.     

Electric-field effects on photoluminescence properties in a GaAs/AlAs marginal type-I superlattice

We have investigated the electric-field effects on the cw and time-resolved photoluminescence (PL) properties in a marginal type-I GaAs/AlAs superlattice (SL) whose lowest X state (X₁) is situated in the lowest Γ(Γ₁) miniband. In the low bias voltage regime, the PL spectra reveal the transition between type-I and type-II radiative recombination processes caused by Wannier–Stark localization. In contrast, in the high bias voltage regime, the decay time of the time-resolved PL is prolonged. This is because of delayed carrier transport caused by Γ–X transfer. From these results, it was found that marginal type-I SLs present various interesting phenomena that originate from the competitive carrier transport among the Γ miniband, the localized Γ Stark–ladder states, and the X₁ state.     

Hybrid solar cells using nanorod zinc oxide electrodes and perylene monoimide–monoanhydride dyes

Hybrid solar cells have been fabricated using perylene monoimide–monoanhydride dyes with nanorod zinc oxide electrodes as electron transporting layers. We have investigated the influence of the spacer alkyl chain length of perylene monoimide–monoanhydride (PMIMA) dyes on the device performance in hybrid solar cells using nanorod zinc oxide electrodes. Nanorod zinc oxide electrodes with 50–150 nm of diameter were synthesized in the presence of PEG400 by using microwave heating method. We observed that the dyes with longer and brunched alkyl chains exhibit higher efficiencies in hybrid solar cells. We report the highest efficiency obtained with zinc oxide nanorods under standard conditions for perylene monoimide–monoanhydride derivative with PMIMA_1 that performs 400 mV open circuit voltage, 2.81 mA/cm² short-circuit current and 0.59% overall conversion efficiency.     

Analysis of the photovoltaic efficiency of a molecular solar cell based on a two-level system

A molecular solar cell is modelled as a two-level system connected to electrodes by chains of electron-transporting and hole-transporting orbitals. Light absorption and emission are simulated using the generalised Planck equation and intermolecular charge transfer using non-adiabatic Marcus theory. Quantum efficiency–voltage characteristics, open-circuit voltage and monochromatic power-conversion efficiency are calculated as a function of the following parameters: charge-separation rate, interfacial recombination rate, charge mobility, light intensity and built-in bias. We find that slow charge separation, fast recombination and low mobility all contribute to a decrease in efficiency compared to the ideal (detailed balance) limit. When charge-separation and interfacial recombination rates are related through the intermolecular coupling, maximum efficiency is achieved at some optimum, but not the maximum, charge-separation rate. Two regimes are distinguished for the open-circuit voltage: when interfacial recombination is important, V_oc varies approximately linearly with the donor–acceptor energy gap; but when recombination is insignificant, V_oc is determined by the optical gap. Including exciton binding energy in the driving force for charge separation reduces V_oc. In systems with significant recombination, V_oc first increases and then saturates with increasing light intensity. Low mobility and interfacial recombination are the main avoidable sources of loss when realistic parameters are used, but the effects of low mobility can be partly compensated by applying a built-in bias between the electrodes. PACS 72.40.+w; 73.40.Lq; 72.80.Le     

Quantum transport phenomena in disordered electron systems with spin–orbit coupling in two dimensions and below

Electron transport phenomena in disordered electron systems with spin–orbit coupling in two dimensions and below are studied numerically. The scaling hypothesis is checked by analyzing the scaling of the quasi-1D localization length. A logarithmic increase of the mean conductance is also confirmed. These support the theoretical prediction that the two-dimensional metal in systems with spin–orbit coupling has a perfect conductivity. Transport through a Sierpinski carpet is also reported.     

Energy relaxation of magnetically confined electrons in quantum cascade lasers

By measuring the light emitted from a quantum cascade laser placed in a high magnetic field, we have investigated the energy relaxation of 0D magnetically confined electrons in the active quantum wells of the structure. The experiment consists of injecting electrons by tunnelling into one upper subband level and monitoring a resonant interaction with optical phonons produced by Landau tuning of subband energy levels. For this purpose, the upper level lifetime is probed by measuring the laser intensity as a function of magnetic field, under constant current bias values. Both the laser intensity and the bias voltage oscillate periodically with the reciprocal of the field. In addition, at high magnetic fields, the current threshold goes through deep minima at antiresonance values. The lifetime is then deduced and analyzed using the strong electron–phonon coupling scheme which is typically applied to quantum dots.     

XOR gate response in a mesoscopic ring with embedded quantum dots

We address XOR gate response in a mesoscopic ring threaded by a magnetic flux . The ring, composed of identical quantum dots, is symmetrically attached to two semi-infinite one-dimensional metallic electrodes and two gate voltages, viz, V_a and V_b, are applied, respectively, in each arm of the ring which are treated as the two inputs of the XOR gate. The calculations are based on the tight-binding model and the Green’s function method, which numerically compute the conductance–energy and current–voltage characteristics as functions of the ring-electrodes coupling strengths, magnetic flux and gate voltages. Quite interestingly it is observed that, for =₀/2 (₀=ch/e, the elementary flux-quantum) a high output current (1) (in the logical sense) appears if one, and only one, of the inputs to the gate is high (1), while if both inputs are low (0) or both are high (1), a low output current (0) appears. It clearly demonstrates the XOR behavior and this aspect may be utilized in designing the electronic logic gate.     

Analytical and numerical study of coupled atomistic-continuum methods for fluids

The stability and convergence rate of coupled atomistic-continuum methods are studied analytically and numerically. These methods couple a continuum model with molecular dynamics through the exchange of boundary conditions in the continuum-particle overlapping region. Different coupling schemes, including velocity–velocity, flux–velocity, velocity–flux and flux–flux, are studied. It is found that the velocity–velocity and flux–velocity schemes are stable. The flux–flux scheme is weakly unstable. The stability of the velocity–flux scheme depends on the parameter T_c which is the length of the time interval between successive exchange of boundary conditions. It is stable when T_c is small and unstable when T_c is large. For steady-state problems, the flux–velocity scheme converges faster than the other coupling schemes.     

Spin-dependent shot noise of inelastic transport through molecular quantum dots

Here we present a theoretical analysis of the effect of inelastic electron scattering on spin-dependent transport characteristics (conductance, current–voltage dependence, magnetoresistance, shot noise spectrum, Fano factor) for magnetic nanojunction. Such device is composed of molecular quantum dot (with discrete energy levels) connected to ferromagnetic electrodes (treated within the wide-band approximation), where molecular vibrations are modeled as dispersionless phonons. Non-perturbative computational scheme, used in this work, is based on the Green's function theory within the framework of mapping technique (GFT–MT), which transforms the many-body electron–phonon interaction problem into a single-electron multi-channel scattering problem. The consequence of the localized electron–phonon coupling is polaron formation. It is shown that polaron shift and additional peaks in the transmission function completely change the shape of considered transport characteristics.     

Suppression of Josephson currents in ballistic junctions by an injection current

Control of the critical current in a superconductor/two-dimensional electron gas Josephson junction by means of an injection current is reported. The control mechanism is explained by a theoretical model, which takes ballistic transport across the junction and diffusive transport through the semiconductor wire structure into account. Measurements on a Nb-AlGaSb/InAs-Nb junction show that the strong suppression of the critical current can, in principle, be explained by the theoretical model. Deviations are due to the nonlinear current–voltage characteristics of the superconductor/two-dimensional electron gas interface and the two-dimensionality of the supercurrent transport.     

Quantum fluctuations and the Kondo effect in small quantum dots

We consider electron transport through quantum dots with large level spacing and charging energy. At low temperature and strong coupling to the leads, quantum fluctuations and the Kondo effect become important. They show up, e.g., as zero-bias anomalies in the current–voltage characteristics. We use a recently developed diagrammatic technique as well as a new real-time renormalization-group approach to describe charge and spin fluctuations. The latter gives rise to a Kondo-assisted enhancement of the current through the dot as seen in experiments.     

Temperature-dependent electrical properties of organic light-emitting diodes depending on cathodes

We have studied temperature-dependent electrical properties of organic light-emitting diodes with a variation of cathode materials; Al, LiAl, and LiF/Al. The organic light-emitting diodes emit a light by a recombination of injected charge carriers such as holes and electrons. Thus, the charge transport is affected by the injection barrier at the interface. By varying the cathode materials, the electron injection at the interface could be controlled because of the work-function change at the cathode. Temperature-dependent current–voltage luminance characteristics of the organic light-emitting diodes were measured in the temperature range from 10 to 300 K. The current-voltage characteristics were analyzed in terms of Fowler–Nordheim tunneling model, and the energy-barrier height was obtained. A measured lifetime of device with LiF/Al cathode is relatively longer than the other cathodes at room temperature: 4.5 h for Al cathode, 12.4 h for LiAl, and 29.6 h for LiF/Al. The device with LiAl and LiF/Al cathode, in the aspect of lifetime and luminous efficiency, is superior to one of other cathodes.     

Aharonov–Bohm effect in Kane-type semiconductor quantum wire

In the present study, the Aharonov–Bohm effect for bound states in Kane-type quantum wire is analysed. It is demonstrated that the wave function and the energy spectra of carriers depend on the magnetic flux. The energy and the g factor of electrons are of a periodic function of the magnetic flux. The statistical property of the orbital magnetism of electrons at low temperatures and strong magnetic fields is presented. It is shown that the magnetisation of electron oscillations depends on the magnetic flux.