Hydration effect on the electronic transport properties of oligomeric phenylene ethynylene molecular junctions

A first-principles computational method based on the hybrid density functional theory is developed to simulate the electronic transport properties of oligomeric phenylene ethynylene molecular junctions with H 2 O molecules accumulated in the vicinity as recently reported by Na et al.[Nanotechnology 18 424001 (2007)].The numerical results show that the hydrogen bonds between the oxygen atoms of the oligomeric phenylene ethynylene molecule and H 2 O molecules result in the localisation of the molecular orbitals and lead to the lower transition peaks.The H 2 O molecular chains accumulated in the vicinity of the molecular junction can not only change the electronic structure of the molecular junctions,but also open additional electronic transport pathways.The obvious influence of H 2 O molecules on the electronic structure of the molecular junction and its electronic transport properties is thus demonstrated.     

The inelastic electron tunneling spectroscopy of edge-modified graphene nanoribbon-based molecular devices

The inelastic electron tunneling spectroscopy(IETS) of four edge-modified finite-size grapheme nanoribbon(GNR)-based molecular devices has been studied by using the density functional theory and Green's function method. The effects of atomic structures and connection types on inelastic transport properties of the junctions have been studied. The IETS is sensitive to the electrode connection types and modification types. Comparing with the pure hydrogen edge passivation systems, we conclude that the IETS for the lower energy region increases obviously when using donor–acceptor functional groups as the edge modification types of the central scattering area. When using donor–acceptor as the electrode connection groups, the intensity of IETS increases several orders of magnitude than that of the pure ones. The effects of temperature on the inelastic electron tunneling spectroscopy also have been discussed. The IETS curves show significant fine structures at lower temperatures. With the increasing of temperature, peak broadening covers many fine structures of the IETS curves.The changes of IETS in the low-frequency region are caused by the introduction of the donor–acceptor groups and the population distribution of thermal particles. The effect of Fermi distribution on the tunneling current is persistent.     

Effect of Barrier Width on Bias-Dependent Tunnelling in Ferromagnetic Junctions

We present a finite temperature theory for bias-dependent tunnelling in ferromagnetic tunnelling junctions. The effects of the barrier width d on the tunnelling magnetoresistance (TMR) and its sign change behaviour are discussed with this theory. Numerical results show that both the zero-bias TMR and the critical voltage Vc at which the TMR changes its sign decrease with the increasing barrier width for a considerably thick barrier junction. Furthermore, it is found that a minimum exists in the curve of Vc versus d if a composite junction is under oxidized.     

Dephasing of quantum tunnelling in molecular nanomagnets

Dephasing mechanism of quantum tunnelling in molecular magnets has been studied by means of the spin-coherentstate path integral in a mean field approximation. It is found that the fluctuating uncompensated transverse field from the dipolar-dipolar interaction between molecular magnets contributes a random phase to the quantum interference phase. The resulting transition rate is determined by the average tunnel splitting over the random phase. Such a dephasing process leads to the suppression of quenching due to the quantum phase interference, and to the steps due to odd resonances in hysteresis loop survived, which is in good agreement with experimental observations in molecular nanomagnets Fes and Mn12.     

Manipulation and control of a single molecular rotor on Au (111) surface

Three different methods are used to manipulate and control phthalocyanine based single molecular rotors on Au (111) surface: (1) changing the molecular structure to alter the rotation potential; (2) using the tunnelling current of the scanning tunnelling microscope (STM) to change the thermal equilibrium of the molecular rotor; (3) artificial manipulation of the molecular rotor to switch the rotation on or off by an STM tip. Furthermore, a molecular ’gear wheel’ is successfully achieved with two neighbouring molecules.     

Charge and spin-dependent thermal efficiency of polythiophene molecular junction in presence of dephasing

The charge and spin-dependent thermoelectric properties of different lengths of polythiophene in a molecular junction are investigated using the B ¨uttiker probe method within Green function formalism in linear response regime. The coupling of the molecular chain to three-dimensional ferromagnetic electrodes is described by a tight-binding model for both parallel and antiparallel spin configurations. The decrease of height of transmission probability peaks and thermoelectric coefficients are observed in the presence of the B ¨uttiker probes. The reduction is more intensive in the strong dephased chains.Results show that the spin magnetothermopower is bigger than the charge magnetothermopower due to the larger difference between the spin thermopowers with respect to the charge ones. In addition, we observed that the kind of carriers participating in the thermoelectric transport depends on the number of the thiophene rings.     

Electronic Transport in Molecular Junction Based on C20 Cages

Choosing closed-ended armchair （5, 5） single-wall carbon nanotubes （CCNTs） as electrodes, we investigate the electron transport properties across an all-carbon molecular junction consisting of C20 molecules suspended between two semi-infinite carbon nanotubes. It is shown that the conductances are quite sensitive to the number of C20 molecules between electrodes for both configuration CF1 and double-bonded models： the conductances of C20 dimers are markedly smaller than those of monomers. The physics is that incident electrons easily pass the C20 molecules and are predominantly scattered at the C20-C20 junctions. Moreover, we study the doping effect of such molecular junction by doping nitrogen atoms substitutionally. The bonding property of the molecular junction with configuration CF1 has been analysed by calculating the Mulliken atomic charges. Our results have revealed that the C atoms in N-doped junctions are more ionic than those in pure-carbon ones, leading to the fact that N-doped junctions have relatively large conductance.     

Dependence of Tunnel Resistance on the Injection Current of Magnetic Tunnel Junctions

Anomalous transport behaviour,i.e.the dependence of the tunnel resistance on the injection current,has been discovered in Ta/Co/Al2O3/FeNi tunnel junctions.The zero-field voltage-current characteristic of the magnetic tunnelling junction obeys the transport principle of the normal tunnel junction at low injection current,but it exhibits a negative resistance behaviour when the injection current is raised to the break-over current level.The physics of the restorable electric breakdown has been initially studied.     

Squeezing Effect of a Nanomechanical Resonator Coupled to a Two-Level System: an Equilibrium Approach

The squeezing effect of a nanomechanical resonator coupled to a two-level system is studied by variational calculations based on both the displaced-squeezed-state （DSS） and the displaced-oscillator-state （DOS）. The stable region of the DSS ground state at both T = 0 and T ≠ 0 and the corresponding squeezing factor are calculated. It is found that when the resonator frequency lies in （kBT,△）, where A is the tunnelling splitting of the two-level-system in the presence of dissipation, tunnelling splitting of a DSS ground state decreases with the temperature, while tunnelling splittihg of a DOS ground state increases with the temperature in low temperature region. This opposite temperature dependence can help to distinguish between the DSS and DOS ground state in the experiment.     

Coherent incoherent transition in the spin boson model with a super-ohmic bath

The temperature effect on tunnelling splitting in the spin–boson model with a super-ohmic bath is studied by the small polaron theory.The coherent–incoherent transition temperature is calculated and its dependence on dissipation strength and bare tunnelling splitting is analysed.In additional to the traditional transition point described in textbooks,a new kind of transition is found in the low dissipation region,showing different temperature dependence in the transition.The relation to the corresponding transition in the polaron–phonon system is also discussed.     

Inelastic electron tunneling spectroscopy: Intensity as a function of surface coverage

Inelastic election tunneling spectroscopy (IETS) is a sensitive technique for obtaining vibrational spectra of molecules adsorbed on an oxide surface and incorporated into a metal-oxide-metal tunnel junction. IETS energy data are used routinely. However, IETS intensities contain additional information which, for theoretical and experimental reasons, has not been used systematically. This paper examines the variation of IETS intensity with surface coverage of dopant molecules in the junction, a relationship of practical and theoretical importance. IET spectra are taken using standard experimental techniques and a liquid doping technique which allows the surface coverage to be determined independently. From an analysis of a large number of modes of benzoic acid on alumina, it is found that IETS intensity, defined in the usual way as the normalized change in conductance, $\text{Δ σ}\text{σ}$, is a nonlinear function of surface coverage. A physical model is presented which attributes this behavior to a difference in elastic tunneling conductances through empty or filled regions of the dopant layer in a junction with a fraction of a monolayer coverage. In addition, the liquid and vapor doping techniques in common use in IETS are discussed in terms of statistical mechanics and are shown to be manifestations of the same basic phenomenon.     

An IETS study of a silane coupling agent; the interaction of 3-(trimethoxysilyl)propanethiol with aluminium oxide and silver surfaces

The inelastic electron tunnelling spectrum of a silane coupling agent, 3-(trimethoxysilyl)propanethiol, is presented. Approximately monolayer quantities of this silane are present on the barrier oxide of an aluminium-aluminium oxide-metal tunnelling junction, in which the counter electrode is either lead or silver. It is deduced from the IETS spectra that the alkoxy groups of this silane condense with the surface hydroxyl groups. No interaction with the lead is observed but the silver layer clearly reacts with the thiol group of the silane molecule, since the thiol stretching vibration is absent in the spectra, and probably forms the corresponding silver thiolate. The influence of this reaction on the conformation of the bound silane is discussed.     

Simulations of inelastic electron tunneling spectroscopy of semifluorinated hexadecanethiol junctions

The inelastic electron tunneling spectroscopy (IETS) of semifluorinated hexadecanethiol junctions is theoretically studied. The numerical results show that the C-F vibration modes of semifluorinated alkanethiol series can not be detected, and the C-H stretching mode in IETS is related to the CH₂ vibration. It is demonstrated that the Raman modes are preferred over IR modes in IETS, which is in good agreement with the experimental measurements presented by Beebe et al. [Nano Lett., 2007, 7(5): 1364].      

Origin of discrepancies in inelastic electron tunneling spectra of molecular junctions

We report inelastic electron tunneling spectroscopy (IETS) of multilayer molecular junctions with and without incorporated metal nanoparticles. The incorporation of metal nanoparticles into our devices leads to enhanced IET intensity and a modified line shape for some vibrational modes. The enhancement and line-shape modification are both the result of a low lying hybrid metal nanoparticle-molecule electronic level. These observations explain the apparent discrepancy between earlier IETS measurements of alkane thiolate junctions by Kushmerick et al. [Nano Lett. 4, 639 (2004)] and Wang et al. [Nano Lett. 4, 643 (2004)].     

Inelastic electron tunneling spectroscopy of an alkanethiol self-assembled monolayer using scanning tunneling microscopy

We report inelastic electron tunneling spectroscopy (IETS) of a C8 alkanethiol self-assembled monolayer using a scanning tunneling microscope (STM). High-resolution STM IETS spectra show clear features of the C-H bending and C-C stretching modes in addition to the C-H stretching mode, which enables a precise comparison with previously reported vibrational spectroscopy, especially electron energy loss spectroscopy data. Intensity variation of vibrational peaks with tip position is discussed with the STM IETS detection mechanism.     

Tunnelling states and anharmonic resonant modes in glasses

The tunnelling states in amorphous solids are explained in terms of coupled rotations of molecular or quasimolecular groups. The elastic stresses frozen in at the glass transition lead to a broad distribution of restoring forces with a finite probability for very small and even negative values. In the latter case, the anharmonic terms give rise to an asymmetric double-minimum potential as required for the tunnelling. The model predicts strongly anharmonic resonant modes which could explain the low temperature glass anomalies above 1K. For vitreous silica, the calculated densities of tunnelling states and resonant modes agree reasonably well with measured values.     

Inelastic spectroscopy identification of STM-induced benzene dehydrogenation

Inelastic electron tunneling spectroscopy (IETS) performed with the scanning tunneling microscope (STM) has been deemed as the ultimate tool for identifying chemicals on the atomic scale. However, IETS-based chemical analysis is error-prone due to the numerous degrees of freedom of chemisorbed molecular systems. First-principles simulations of IETS are presented that, by quantitative comparison with the experimental spectra, permit one to determine the final products of an STM-induced reaction on chemisorbed benzene. Our simulations reveal that IETS possesses an enhanced sensitivity to atomic structure as compared to topographic imaging due to both its energy and space resolution.     

Silver oligomer and single fullerene electronic properties revealed by a scanning tunnelling microscope

Clusters on surfaces have been investigated with low-temperature scanning tunnelling microscopy and spectroscopy. Constant current spectra acquired on Ag oligomers and one-dimensional chains on a Ag(111) reveal a single resonance peak whose energy shifts towards the Fermi level with increasing cluster size. Next, controlled and reproducible contact between a STM tip and a C₆₀ molecule adsorbed on Cu(100) is reported. The transition from tunnelling to contact is discussed in terms of local heating of the tip-molecule junction.     

Rectification in substituted atomic wires: a theoretical insight

Recently, there have been discussions that the giant diode property found experimentally in diblock molecular junctions could be enhanced by the many-body electron correlation effect beyond the mean field theory. In addition, the effect of electron-phonon scattering on an electric current through the diode molecule, measured by inelastic tunneling spectroscopy (IETS), was found to be symmetric with respect to the voltage sign change even though the current is asymmetric. The reason for this behavior is a matter of speculation. In order to clarify whether or not this feature is limited to organic molecules in the off-resonant tunneling region, we discuss the current asymmetry effect on IETS in the resonant region. We introduced heterogeneous atoms into an atomic wire and found that IETS becomes asymmetric in this substituted atomic wire case. Our conclusion gives the other example of intrinsic differences between organic molecules and metallic wires. While the contribution of electron-phonon scattering to IETS is not affected by the current asymmetry in the former case, it is affected in the latter case. The importance of the contribution of the electron-hole excitation to phonon damping in bringing about the current asymmetry effect in IETS in the latter case is discussed.     

Ab initio analysis of electron-phonon coupling in molecular devices

We report a first principles analysis of electron-phonon coupling in molecular devices under external bias voltage and during current flow. Our theory and computational framework are based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism. Using a molecular tunnel junction of a 1,4-benzenedithiolate molecule contacted by two aluminum leads as an example, we analyze which molecular vibrational modes are most relevant to charge transport under nonequilibrium conditions. We find that the low-lying modes are most important. As a function of bias voltage, the electron-phonon coupling strength can change drastically while the vibrational spectrum changes at a few percent level.