Showing papers on "Conductance published in 2007"
TL;DR: It is shown explicitly that differences in local structure have a limited influence on conductance because the amine-Au bonding motif is well-defined and flexible, explaining the narrow distributions seen in the experiments.
Abstract: A combination of theory and experiment is used to quantitatively understand the conductance of single-molecule benzenediamine−gold junctions. A newly developed analysis is applied to a measured junction conductance distribution, based on 59 000 individual conductance traces, which has a clear peak at 0.0064 G0 and a width of ±47%. This analysis establishes that the distribution width originates predominantly from variations in conductance across different junctions rather than variations in conductance during junction elongation. Conductance calculations based on density functional theory (DFT) for 15 distinct junction geometries show a similar spread. We show explicitly that differences in local structure have a limited influence on conductance because the amine−Au bonding motif is well-defined and flexible, explaining the narrow distributions seen in the experiments. The minimal impact of junction structure on conductance permits an unambiguous comparison of calculated and measured conductance values an...
430 citations
TL;DR: This work studied heat transport through self-assembled monolayers of long-chain hydrocarbon molecules anchored to a gold substrate by ultrafast heating of the gold with a femtosecond laser pulse.
Abstract: At the level of individual molecules, familiar concepts of heat transport no longer apply. When large amounts of heat are transported through a molecule, a crucial process in molecular electronic devices, energy is carried by discrete molecular vibrational excitations. We studied heat transport through self-assembled monolayers of long-chain hydrocarbon molecules anchored to a gold substrate by ultrafast heating of the gold with a femtosecond laser pulse. When the heat reached the methyl groups at the chain ends, a nonlinear coherent vibrational spectroscopy technique detected the resulting thermally induced disorder. The flow of heat into the chains was limited by the interface conductance. The leading edge of the heat burst traveled ballistically along the chains at a velocity of 1 kilometer per second. The molecular conductance per chain was 50 picowatts per kelvin.
423 citations
TL;DR: In this paper, the authors measured conductance distribution for single molecule benzenediamine-gold junctions, based on 59,000 individual conductance traces recorded while breaking a gold point contact in solution, has a clear peak at 0.0064 G$_{0}$ with a width of 40%.
Abstract: The measured conductance distribution for single molecule benzenediamine-gold junctions, based on 59,000 individual conductance traces recorded while breaking a gold point contact in solution, has a clear peak at 0.0064 G$_{0}$ with a width of $\pm$ 40%. Conductance calculations based on density functional theory (DFT) for 15 distinct junction geometries show a similar spread. Differences in local structure have a limited influence on conductance because the amine-Au bonding motif is well-defined and flexible. The average calculated conductance (0.046 G$_{0}$) is seven times larger than experiment, suggesting the importance of many-electron corrections beyond DFT.
358 citations
TL;DR: It is found that junctions formed with dimethyl phosphine terminated alkanes have the highest conductance and this allows a detailed analysis of the single-molecule junction elongation properties which correlate well with calculations based on density functional theory.
Abstract: We compare the low bias conductance of a series of alkanes terminated on their ends with dimethyl phosphines, methyl sulfides, and amines and find that junctions formed with dimethyl phosphine terminated alkanes have the highest conductance. We see unambiguous conductance signatures with these link groups, indicating that the binding is well-defined and electronically selective. This allows a detailed analysis of the single-molecule junction elongation properties which correlate well with calculations based on density functional theory.
355 citations
TL;DR: The results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.
Abstract: We measure the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules. Transport through these substituted benzenes is by means of nonresonant tunneling or superexchange, with the molecular junction conductance depending on the alignment of the metal Fermi level to the closest molecular level. Electron-donating substituents, which drive the occupied molecular orbitals up, increase the junction conductance, while electron-withdrawing substituents have the opposite effect. Thus for the measured series, conductance varies inversely with the calculated ionization potential of the molecules. These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.
302 citations
TL;DR: In this paper, a nonequilibrium molecular dynamics (NEMD) approach is developed to investigate thermal transport across solid-solid interfaces and compared to the diffuse mismatch model (DMM).
289 citations
TL;DR: Internal conductance ought to be included in models of canopy photosynthesis, but before this is feasible additional data on the variation in internal conductance among and within species are urgently required.
Abstract: Internal conductance describes the movement of CO 2 from substomatal cavities to sites of carboxylation. Internal conductance has now been measured in approximately 50 species, and in all of these species it is a large limitation of photosynthesis. It accounts for somewhat less than half of the decrease in CO 2 concentrations from the atmosphere to sites of carboxylation. There have been two major findings in the past decade. First, the limitation due to internal conductance (i.e. C i -C c ) is not fixed but varies among species and functional groups. Second, internal conductance is affected by some environmental variables and can change rapidly, for example, in response to leaf temperature, drought stress or CO 2 concentration. Biochemical factors such as carbonic anhydrase or aquaporins are probably responsible for these rapid changes. The determinants of internal conductance remain elusive, but are probably a combination of leaf anatomy, morphology, and biochemical factors. In most plants, the gas phase component of internal conductance is negligible with the majority of resistance resting in the liquid phase from cell walls to sites of carboxylation. The internal conductance story is far from complete and many exciting challenges remain. Internal conductance ought to be included in models of canopy photosynthesis, but before this is feasible additional data on the variation in internal conductance among and within species are urgently required. Future research should also focus on teasing apart the different steps in the diffusion pathway (intercellular spaces, cell wall, plasmalemma, cytosol, and chloroplast envelope) since it is likely that this will provide clues as to what determines internal conductance.
261 citations
TL;DR: In this paper, the authors measured the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules, and showed that the occupied states are closest to the gold Fermi energy.
Abstract: We measure the low bias conductance of a series of substituted benzene diamine molecules while breaking a gold point contact in a solution of the molecules. Transport through these substituted benzenes is by means of nonresonant tunneling or superexchange, with the molecular junction conductance depending on the alignment of the metal Fermi level to the closest molecular level. Electron-donating substituents, which drive the occupied molecular orbitals up, increase the junction conductance, while electron-withdrawing substituents have the opposite effect. Thus for the measured series, conductance varies inversely with the calculated ionization potential of the molecules. These results reveal that the occupied states are closest to the gold Fermi energy, indicating that the tunneling transport through these molecules is analogous to hole tunneling through an insulating film.
243 citations
TL;DR: In this article, the authors derived a formula to calculate the ballistic thermal conductance of a two-dimensional system directly from the dispersion relations of phonons and electrons, and applied the method to a graphene and investigated both the temperature and the Fermi energy dependences of the ballistic conductance.
Abstract: We derive a formula to calculate the ballistic thermal conductance of a two-dimensional system directly from the dispersion relations of phonons and electrons. We apply the method to a graphene and investigate both the temperature and the Fermi energy dependences of the ballistic thermal conductance. The ballistic thermal conductance per unit length of a graphene becomes isotropic from the threefold rotational symmetry. In the intrinsic graphene where the Fermi energy crosses the Dirac point, the thermal conductance of electrons increases in proportion to ${T}^{2}$ with temperature, while the phonon conductance increases in proportion to ${T}^{1.5}$ due to the quadratic dispersion relation of the out-of-plane acoustic mode and prevails over the electron-derived conductance irrespective of temperature. As the Fermi energy is moved from the Dirac point for the gated graphenes, the thermal conductance of electrons increases monotonically and the temperature dependence changes from a ${T}^{2}$ dependence in the intrinsic graphene to a $T$-linear one at low temperatures. The electron thermal conductance of the gated graphenes dominates over the phonon contribution at low temperatures.
241 citations
TL;DR: In this paper, the mesoscopic fluctuations of the conductance of a graphene strip (width W larger than length L) in an ensemble of samples with different realizations of the random electrostatic potential landscape were studied.
Abstract: We have studied numerically the mesoscopic fluctuations of the conductance of a graphene strip (width W larger than length L), in an ensemble of samples with different realizations of the random electrostatic potential landscape. For strong disorder (potential fluctuations comparable to the hopping energy), the variance of the conductance approximates the value predicted by the Altshuler-Lee-Stone theory of universal conductance fluctuations, Var GUCF=0.12 (W/L)(2e2/h)2. For weaker disorder the variance is greatly enhanced if the potential is smooth on the scale of the atomic separation. There is no enhancement if the potential varies on the atomic scale, indicating that the absence of backscattering on the honeycomb lattice is at the origin of the anomalously large fluctuations.
120 citations
TL;DR: Ab-initio calculations within density functional theory and nonequilibrium Green's function techniques explain the experimental data in terms of the conductance of an essentially undeformed C60 molecule adsorbed at a pentagon-hexagon bond on Cu(100).
Abstract: 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 Green's 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.
TL;DR: This study reveals that the Ala-29 residue region is the primary binding site for the antiflu drug amantadine (AMT), probably because that domain is relatively spacious and hydrophobic.
TL;DR: The main findings are the main trends can be predicted quantitatively based on the scattering properties of single dopants, and in the diffusive regime the analytical predictions of the Dorokhov-Mello-Pereyra-Kumar theory are in good agreement with the ab initio calculations.
Abstract: We combine the ideas of scaling theory and universal conductance fluctuations with density-functional theory to analyze the conductance properties of doped silicon nanowires. Specifically, we study the crossover from ballistic to diffusive transport in boron or phosphorus doped Si nanowires by computing the mean free path, sample-averaged conductance $⟨G⟩$, and sample-to-sample variations $\mathrm{std}(G)$ as a function of energy, doping density, wire length, and the radial dopant profile. Our main findings are (i) the main trends can be predicted quantitatively based on the scattering properties of single dopants, (ii) the sample-to-sample fluctuations depend on energy but not on doping density, thereby displaying a degree of universality, and (iii) in the diffusive regime the analytical predictions of the Dorokhov-Mello-Pereyra-Kumar theory are in good agreement with our ab initio calculations.
TL;DR: The conductance of a single molecule transport junction is investigated in the Landauer-Imry regime of coherent tunneling transport and calculations suggest that for these dithiol molecules, transport occurs mostly through the occupied orbital manifold.
Abstract: The conductance of a single molecule transport junction is investigated in the Landauer-Imry regime of coherent tunneling transport. Utilizing aromatic systems with thiol end groups, we have calculated using density functional theory the expected conductance of junctions containing molecules with different levels of conjugation and of different lengths. The calculated variations in transport junction conductance are explained in terms of the continuity of the conjugation path between leads. Molecular conjugation describes this continuity within the molecule, and the interfacial terms (spectral densities or imaginary parts of the self-energy) describe its continuity at the molecule/metal interface. We compare the results from junction conductance calculations with isolated molecule electronic structure calculations These density functional theory calculations suggest that for these dithiol molecules, transport occurs mostly through the occupied orbital manifold. The decay of the transport with length is fo...
TL;DR: A surprisingly small energy scale is found that controls the temperature evolution of conductance and the tunneling density of states in the mixed valence regime of carbon nanotubes with doubly degenerate orbitals.
Abstract: We study the evolution of conductance regimes in carbon nanotubes with doubly degenerate orbitals (shells) by controlling the contact transparency within the same sample. For sufficiently open contacts, Kondo behavior is observed for 1, 2, and 3 electrons in the topmost shell. As the contacts are opened more, the sample enters the "mixed valence" regime, where different charge states are strongly hybridized by electron tunneling. Here, the conductance as a function of gate voltage shows pronounced modulations with a period of four electrons, and all single-electron features are washed away at low temperature. We successfully describe this behavior by a simple formula with no fitting parameters. Finally, we find a surprisingly small energy scale that controls the temperature evolution of conductance and the tunneling density of states in the mixed valence regime.
TL;DR: The conductance of polyynes is found an order of magnitude larger compared with other conjugated oligomers and with the conductance weakly dependent on the applied bias and almost independent of the length of the molecular chain, polyynes appear as nearly perfect molecular wires.
Abstract: We report a full self-consistent ab initio calculation of the current-voltage curve and the conductance of thiolate capped polyynes in contact with gold electrodes. We find the conductance of polyynes an order of magnitude larger compared with other conjugated oligomers. The reason lies in the position of the Fermi level deep in the highest occupied molecular orbital related resonance. With the conductance weakly dependent on the applied bias and almost independent of the length of the molecular chain, polyynes appear as nearly perfect molecular wires.
TL;DR: The electrical conductance of 20% Ti-doped La 0.7 Sr 0.3 Mn 0.8 Ti 0.2 O 3 (LSMO) was measured using admittance spectroscopy over a wide temperature and frequency ranges.
TL;DR: In this paper, the authors measured the thermal conductance of the phonon waveguide by the 3ω method on individual single-crystalline silicon suspended nanowires and found that the cross section of the nanowire approaches the dominant phonon wavelength in silicon which is of the order of 100nm at 1K.
Abstract: We have performed thermal conductance measurements on individual single-crystalline silicon suspended nanowires. The nanowires (130nm thick and 200nm wide) are fabricated by e-beam lithography and suspended between two separated pads on silicon on insulator substrate. We measure the thermal conductance of the phonon waveguide by the 3ω method. The cross section of the nanowire approaches the dominant phonon wavelength in silicon which is of the order of 100nm at 1K. Above 1.3K the conductance behaves as T3, but a deviation is measured at the lowest temperature which can be attributed to the reduced geometry.
TL;DR: In this paper, the authors measured low-temperature conductance in nanoscale p-channel field-effect transistors lightly doped with boron and observed a conductance modulation, which they ascribed to the trapping/detrapping of single holes by/from individual acceptors.
Abstract: The authors measured low-temperature (6–28K) conductance in nanoscale p-channel field-effect transistors lightly doped with boron. They observed a conductance modulation, which they ascribed to the trapping/detrapping of single holes by/from individual acceptors. The statistics of the appearance of the modulation in a few ten samples indicates that the number of acceptors is small, or even just one, suggesting that what the authors have observed is single-charge-transistor operation by a single-acceptor quantum dot.
TL;DR: It is shown here that the absence of a maximum in the Itunneling versus electrode potential relationship can be fitted by a "soft" gating concept and that theoretical frameworks for compact monolayers may not be directly applied under such circumstances.
Abstract: Experimental data and theoretical notions are presented for 6-[1'-(6-mercapto-hexyl)-[4,4']bipyridinium]-hexane-1-thiol iodide (6V6) "wired" between a gold electrode surface and tip in an in situ scanning tunneling microscopy configuration. The viologen group can be used to "gate" charge transport across the molecular bridge through control of the electrochemical potential and consequently the redox state of the viologen moiety. This gating is theoretically considered within the framework of superexchange and coherent two-step notions for charge transport. It is shown here that the absence of a maximum in the Itunneling versus electrode potential relationship can be fitted by a "soft" gating concept. This arises from large configurational fluctuations of the molecular bridge linked to the gold contacts by flexible chains. This view is incorporated in a formalism that is well-suited for data analysis and reproduces in all important respects the 6V6 data for physically sound values of the appropriate parameters. This study demonstrates that fluctuations of isolated configurationally "soft" molecules can dominate charge transport patterns and that theoretical frameworks for compact monolayers may not be directly applied under such circumstances.
TL;DR: In this article, the conductance of molecular junctions formed of N,N-bis(n-thioalkyl)-4,4'-bipyridinium bromides or alkanedithiols between a gold (Au) scanning tunnelling microscope tip and a Au(111)-(1? 1) electrode has been studied at electrified solid/liquid interfaces.
Abstract: The conductance of molecular junctions formed of N,N'-bis(n-thioalkyl)-4,4'-bipyridinium bromides or alkanedithiols between a gold (Au) scanning tunnelling microscope tip and a Au(111)-(1 ? 1) electrode has been studied at electrified solid/liquid interfaces. A statistical analysis based on large sets of individual current?distance traces was applied to obtain the electrical conductance of single junctions. The one-electron reduction of the viologen moiety from the dication V2+ to the radical cation state gives rise to a 50% increase of the junction conductance. Increasing the length of the alkyl spacer units leads to a tunnelling decay constant ?CH2 = 5.9?6.1?nm?1. This value is significantly lower than ?CH2 = 8.2?nm?1 estimated for molecular junctions of alkanedithiols. The difference is attributed to conformational changes within the two junctions. The contact conductance was estimated to 10??S.
TL;DR: In this paper, a Monte Carlo model is developed for the hopping conductance in arrays of quantum dots QDs, and a continuous time random walk algorithm is used to simulate all possible transitions and a nonresonant electron-hopping rate based on broadening of the energy levels through quantum fluctuations.
Abstract: A Monte Carlo model is developed for the hopping conductance in arrays of quantum dots QDs. Hopping is simulated using a continuous time random walk algorithm, incorporating all possible transitions, and using a nonresonant electron-hopping rate based on broadening of the energy levels through quantum fluctuations. Arrays of identical QDs give rise to electronic conductance that depends strongly upon level filling. In the case of low charging energy, metal insulator transitions are observed at electron occupation levels, n, that correspond to the complete filling of an S, P ,o rD shell. When the charging energy becomes comparable to the level broadening, additional minima in conductance appear at integer values of n, as a result of electron-electron repulsion. Disorder in QD diameters leads to disorder in the energy levels, resulting in washing out of the structure in the dependence of conductance on n and a net reduction in conductance. Simulation results are shown to be consistent with experimental measurements of conductance in arrays of zinc oxide and cadmium selenide QDs that have different degrees of size disorder, and the degree of size disorder is quantified. Simulations of the temperature dependence of conductance show that both Coulombic charging and size disorder can lead to activated behavior and that size disorder leads to conductance that is sublinear on an Arrhenius plot.
TL;DR: It is concluded that Mrs2p forms a high conductance Mg2+ selective channel that controls M g2+ influx into mitochondria by an intrinsic negative feedback mechanism.
TL;DR: An improved algorithm for computing approximate PageRank vectors is presented, which allows us to find a cut with conductance at most ϕ, whose small side has volume at least 2 b, in time O(2 b log2 m/ϕ 2) where m is the number of edges in the graph.
Abstract: A local graph partitioning algorithm finds a cut near a specified starting vertex, with a running time that depends largely on the size of the small side of the cut, rather than the size of the input graph. In this paper, we present a local partitioning algorithm using a variation of PageRank with a specified starting distribution. We derive a mixing result for PageRank vectors similar to that for random walks, and we show that the ordering of the vertices produced by a PageRank vector reveals a cut with small conductance. In particular, we show that for any set C with conductance Φ and volume k, a PageRank vector with a certain starting distribution can be used to produce a set with conductance . We present an improved algorithm for computing approximate PageRank vectors, which allows us to find such a set in time proportional to its size. In particular, we can find a cut with conductance at most ϕ, whose small side has volume at least 2 b , in time O(2 b log2 m/ϕ 2) where m is the number of edges in the...
TL;DR: In this paper, density functional theory based electron transport calculations are performed to explain the nature of the break-junction behavior of amine-terminated molecules, and they find that amines can be adsorbed only on the apex Au atom, while the thiolate group can be attached equally well to undercoordinated and clean Au surfaces.
Abstract: Amine-terminated molecules show well-behaved conductance in the scanning tunneling microscope break-junction experimental measurements. We performed density functional theory based electron transport calculations to explain the nature of this phenomenon. We find that amines can be adsorbed only on the apex Au atom, while the thiolate group can be attached equally well to undercoordinated and clean Au surfaces. Our calculations show that only one adsorption geometry is sterically and energetically possible for the amine-anchored junction whereas three different adsorption geometries with very distinct transport properties are almost equally probable for the thiolate-anchored junction. We calculated the conductance as a function of the junction stretching when the molecules are pulled by the scanning tunneling microscope tip from the Au electrode. Our calculations show that the stretching of the thiolate-anchored junction during its formation is accompanied by significant electrode geometry distortion. The amine-anchored junctions exhibit very different behavior—the electrode remains intact when the scanning tunneling microscope tip stretches the junction.
TL;DR: Under certain circumstances, the conductance displays an undamped, oscillatory behavior as a function of applied bias voltage, which combines unusual features of the electronic structure of graphene with the unconventional pairing symmetry found in high-Tc superconductors.
Abstract: We investigate quantum transport in a normal-superconductor graphene heterostructure, including the possibility of an anisotropic pairing potential in the superconducting region. We find that under certain circumstances, the conductance displays an undamped, oscillatory behavior as a function of applied bias voltage. Also, we investigate how the conductance spectra are affected by a d-wave pairing symmetry. These results combine unusual features of the electronic structure of graphene with the unconventional pairing symmetry found for instance in high-Tc superconductors.
TL;DR: In this article, the gate bias and temperature dependent field effect mobility and conductance of a polycrystalline pentacene thin film transistor (TFT) were analyzed to study the charge transport in the material.
Abstract: The gate bias and temperature dependent field-effect mobility and conductance of a polycrystalline pentacene thin film transistor (TFT) were analyzed to study the charge transport in the material. Since both heating and cooling can obviously change the film morphology, a relatively narrow temperature range was adopted to rule out the possible influence of structure variation on the device characterization. Both mobility and conductance values increased with the gate bias and showed a thermally activated Arrhenius-like behavior, while the threshold voltage deceased with temperature. Several models were compared, and it was found that the observations could only be well interpreted by a multiple trapping model, which suggests that the temperature and gate bias dependences should be attributed to the increased free charge carrier density. The density of trap states in the band gap was evaluated by the field-effect mobility as well as the field-effect conductance data. The results disclose a possible field-ef...
TL;DR: Several physical mechanisms which might contribute to the observed effect including thermal expansion, rectification, plasmon excitation, and photon-assisted transport are discussed, among which the two latter ones are most likely the dominating ones.
Abstract: This Letter reports on the influence of laser irradiation onto the electrical conductance of gold nanocontacts established with the mechanically controllable break-junction technique. We concentrate on the study of reversible conductance changes which can be as high as 200%. We investigate the dependence on the initial conductance of the contacts, and on the wavelength, the intensity, and the position of the laser spot with respect to the sample. Under most conditions an enhancement of the conductance is observed. Several physical mechanisms which might contribute to the observed effect including thermal expansion, rectification, plasmon excitation, and photon-assisted transport are discussed, among which the two latter ones are most likely the dominating ones.
TL;DR: In this paper, the frequency dependent of the forward and reverse bias capacitance-voltage (C-V) and conductance voltage (G/WV) measurements of Al/SiO"2/p-Si (MIS) structures are carried out in frequency range of 10kHz-10MHz.
TL;DR: In this article, the conductance of a 1,4-disubstituted isocyanide (NC) or thiol(−SH) benzene molecule anchored to two Pt electrodes was investigated.
Abstract: The authors have studied the conductance of a 1,4-disubstituted isocyanide(–NC) or thiol(–SH) benzene molecule anchored to two Pt electrodes. A single molecular junction showing a well-defined conductance value (∼3×10−2G0, G0=2e2∕h) was fabricated with the Pt electrodes. The conductance of the molecular junction was one order higher than the previously documented value using Au electrodes. These observations could be explained by differences in the local density of states of the contact metal atom at the Fermi level and the extent of the hybridization and energy difference between the molecular and metal orbitals. Further insight into the binding strengths of the metal-anchoring group bond was obtained by statistically analyzing the stretching length of the molecular junction.