Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ~ 3 G_0 to G ~ 1 G_0 (G_0 = 2e/h^2). Formation of a true one-atom point contact, with G ~ 1 G_0 and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 < E-E_F < 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear "gold molecule" between the electrodes, and a weak coupling of this "molecule" to the tips.
We investigate, by means of first-principles calculations, structural and transport properties of junctions made of symmetric dithiolated molecules placed between Au electrodes. As the electrodes are pulled apart, we find that it becomes energetically favorable that Au atoms migrate to positions between the electrode surface and thiol terminations, with junction structures alternating between symmetric and asymmetric. As a result, the calculated $\emph{IV}$ curves alternate between rectifying and non-rectifying behaviors as the electrodes are pulled apart, which is consistent with recent exper…
Utilizing Co/Al$_2$O$_3$/Co magnetic tunnel junctions (MTJs) with Co electrodes of different crystalline phases, a clear relationship between electrode structure and junction transport properties is presented. For junctions with one fcc(111) textured and one polycrystalline (poly-phase and poly-directional) Co electrode, a strong asymmetry is observed in the magnetotransport properties, while when both electrodes are polycrystalline the magnetotransport is essentially symmetric. These observations are successfully explained within a model based on ballistic tunneling between the calculated ban…
Using the embedded-atom method, the structure of small copper clusters on Au(111) electrodes has been investigated both by static and dynamic calculations. By varying the size of roughly circular clusters, the edge energy per atom is obtained; it agrees quite well with estimates based on experimental results. Small three-dimensional clusters tend to have the shape of a pyramid, whose sides are oriented in the directions of small surface energy. The presence of a cluster is found to distort the underlying lattice of adsorbed copper atoms.
Flexible solution processed memristors show different behaviour dependent on the choice of electrode material. Use of gold for both electrodes leads to switchable WORM (Write Once Read Many times) resistive devices. Use of aluminium for both electrodes increases the richness of behaviour allowing both curved and triangular memristive switching resistance memories. A comparison device with an aluminium bottom electrode and gold top electrode only exhibited significant memristive resistance switching when the aluminium electrode was the cathode, suggesting that the electrode is acting as a sourc…
For investigation of electron transport on the nanoscale, a system possessing a simple to interpret electronic structure is composed of alkane chains bridging two electrodes via end groups; to date the majority of experiments and theoretical investigations on such structures have considered thiols bonding to gold electrodes. Recently experiments show that well defined molecular conductances may be resolved if the thiol end groups are replaced by amines. In this theoretical study, we investigate the bonding of amine groups to gold clusters and calculate electron transport across the resulting t…
We address the microscopic origin of the current-induced forces by analyzing results of first principles density functional calculations of atomic gold wires connected to two gold electrodes with different electrochemical potentials. We find that current induced forces are closely related to the chemical bonding, and arise from the rearrangement of bond charge due to the current flow. We explain the current induced bond weakening/strengthening by introducing bond charges decomposed into electrode components.
The effect of anchoring group on the electrical conductance of a single molecule bridging two Au electrodes was studied using di-substituted (isocyanide (CN-), thiol (S-) or cyanide (NC-)) benzene. The conductance of a single Au/1,4-diisocyanobenzene/Au junction anchored by isocyanide via a C atom (junction with the Au-CN bond) was $3 \times 10 ^{-3} G_{0}$ ($2e^{2}/h$). The value was comparable to $4 \times 10 ^{-3} G_{0}$ of a single Au/1,4-benzenedithiol/Au junction with the Au-S bond. The Au/1,4-dicyanobenzene/Au molecular junction with the Au-NC bond did not show well-defined conductance …
Solving the atomic structure of metallic clusters is fundamental to understanding their optical, electronic, and chemical properties. We report the structure of Au$_{\text{146}}$(p-MBA)$_{\text{57}}$ at subatomic resolution (0.85 Å) using electron diffraction (MicroED) and atomic resolution by X-ray diffraction. The 146 gold atoms may be decomposed into two constituent sets consisting of 119 core and 27 peripheral atoms. The core atoms are organized in a twinned FCC structure whereas the surface gold atoms follow a C$_{2}$ rotational symmetry about an axis bisecting the twinning plane. The pro…
The process of creating an atomically defined and robust metallic tip is described and quantified using measurements of contact conductance between gold electrodes and numerical simulations. Our experiments show how the same conductance behavior can be obtained for hundreds of cycles of formation and rupture of the nanocontact by limiting the indentation depth between the two electrodes up to a conductance value of approximately $5G_{0}$ in the case of gold. This phenomenon is rationalized using molecular dynamics simulations together with density functional theory transport calculations which…
In this study, the contact resistance of various metals to chemical vapour deposited (CVD) monolayer graphene is investigated. Transfer length method (TLM) structures with varying widths and separation between contacts have been fabricated and electrically characterized in ambient air and vacuum condition. Electrical contacts are made with five metals: gold, nickel, nickel/gold, palladium and platinum/gold. The lowest value of 92 Ωμm is observed for the contact resistance between graphene and gold, extracted from back-gated devices at an applied back-gate bias of -40 V. Measurements carried ou…
A novel nanostructured electrochemical immunsensor for the determination of progesterone is reported. The approach combines the properties of gold nanoparticles with the use of a graphite-Teflon composite electrode matrix, into which gold nanoparticles are incorporated by simple physical inclusion. The antibody anti-progesterone was directly attached to the electrode surface. The immunosensor functioning is based on competitive assay between progesterone and alkaline phosphatase-labelled progesterone. Monitoring of the affinity reaction was accomplished by the electrochemical oxidation of 1-na…
We present results on electromigrated Au nanojunctions broken near the conductance quantum $77.5 μ$S. At room temperature we find that wires, initially narrowed by an actively-controlled electromigration technique down to a few conductance quanta, continue to narrow after removing the applied voltage. Separate electrodes form as mobile gold atoms continuously reconfigure the constriction. We find, from results obtained on over 300 samples, no evidence for gold cluster formation in junctions broken without an applied voltage, implying that gold clusters may be avoided by using this self-breakin…
Pairs of electrodes with nanometer separation (nano-gap) are achieved through an electromigration-induced break-junction (EIBJ) technique at room temperature. Lithographically defined gold (Au) wires are formed by e-beam evaporation over oxide coated silicon substrates silanized with (3-Mercaptopropyl)trimethoxysilane (MPTMS) and then subjected to electromigration at room temperature to create a nanometer scale gap between the two newly formed Au electrodes. The Si-O-Si covalent bond at the SiO$_2$ surface and the Au-sulphur (Au-S) bond at the top evaporated Au side, makes MPTMS as an efficien…
Finite gold nanowires containing less than 1000 atoms are studied using the molecular dynamics simulation method and embedded atom potential. Nanowires with the face-centered cubic structure and the (111) oriented cross-section are prepared at T=0 K. After annealing and quenching the structure and vibrational properties of nanowires are studied at room temperature. Several of these nanowires form multi-walled structures of lasting stability. They consist of concentrical cylindrical sheets and resemble multi-walled carbon nanotubes. Vibrations are investigated by diagonalization of the dynamica…
We report that the electrical conductance between closely-spaced gold electrodes in acid solution can be turned from off [insulating; I] to on [conducting; C] to off again by monotonically sweeping a gate voltage applied to the solution. We propose that this ICI transistor action is due to an electrochemical process dependent on nanoparticles etched from the surface of the gold electrodes. These measurements mimic closely the characteristics of nanoscale acid-gated polyaniline transistors, so that researchers should guard against misinterpreting this effect in future molecular-electronics expe…
Electrochemistry experiments have established that the capacitance of electrode-electrolyte interfaces is much larger for good metals such as gold and platinum than for carbon-based materials. Despite the development of elaborate electrode interaction potentials, to date molecular dynamics simulations were not able to capture this effect. Here we show that changing the width of the Gaussian charge distribution used to represent the atomic charges in gold is an effective way to tune its metallicity. Larger Gaussian widths lead to a capacitance of aqueous solutions (pure water and 1 molar NaCl) …
Recent DFT (density functional theory) simulations showed that metals have a hitherto overlooked symmetry termed "hidden scale invariance" [Hummel {\em et al.}, Phys. Rev. B {\bf{92}}, 174116 (2015)]. According to isomorph theory, this scaling property implies the existence of lines in the thermodynamic phase diagram, so-called isomorphs, along which structure and dynamics are invariant to a good approximation when given in properly reduced units. This means that the phase diagram becomes effectively one-dimensional with regard to several physical properties. This paper investigates consequenc…
The ab initio simulation of charged interfaces in the framework of density functional theory (DFT) is heavily employed for the study of electrochemical energy conversion processes. The capacitance is the primary descriptor for the response of the electrochemical interface. It is essentially equal to the inverse of the energy curvature as a function of electron number, and as such there appears a conflict with the fundamental principle of piecewise linearity in DFT that requires the energy curvature to be zero at fractional electron numbers, i.e. almost everywhere. To resolve this conflict, we …
Equilibrium structure, local densities of states, and electronic transport in a gold nanowire made of a four-atom chain supported by two gold electrodes, which has been imaged recently by high-resolution electron microscopy, and chemical modification of the wire via the adsorption of a methylthiol molecule, are investigated with ab-initio local density functional simulations. In the bare wire at the imaged geometry the middle two atoms dimerize, and the structure is strongly modified by the adsorption of the molecule with an accompanying increase of the ballistic conductance through the wire.
We theoretically study the electronic transport in the monolayer of dithiolated phenylene vinylene oligomeres coupled to the (111) surfaces of gold electrodes. We use non-equilibrium Green functions (NEGF) and density functional theory(DFT) implemented in the TranSIESTA package to obtain a full ab initio self-consistent description ofthe transport current through the molecular nanostructure with different electrochemical bias potentials. The calculated current-voltage characteristics (IVC) of the systems for the same contact geometry have shown a systematic decrease of the conductivity with th…
Atomic-resolution imaging of gold species at organic liquid-solid interfaces
University of Manchester（曼彻斯特大学）；National Graphene Institute, University of Manchester（曼彻斯特大学国家石墨烯研究所）；Diamond Light Source（英国钻石光源实验室）；Max Planck−Cardiff Centre on the Fundamentals of Heterogeneous Catalysis FUNCAT, Cardiff Catalysis Institute, School of Chemistry, Cardiff University（卡迪夫大学化学学院卡迪夫催化研究所马克斯·普朗克-卡迪夫异相催化基础中心）；Cardiff Catalysis Institute, School of Chemistry, Cardiff University（卡迪夫大学化学学院卡迪夫催化研究所）；School of Mathematical and Physical Sciences, University of Sheffield（谢菲尔德大学数理科学学院）；University of Oxford（牛津大学）；Henry Royce Institute for Advanced Materials, University of Manchester（曼彻斯特大学亨利·罗伊先进材料研究所）
We study the current through molecular films as a function of orientation of the molecules in the film with respect to electrodes. It may change by more than an order of magnitude, depending on the angle between the axis of the molecules and the normal to the electrode. This is a consequence of a strong directional character of $p$-orbitals that determines the conductance through the molecules. We demonstrate this result on an exactly solvable model, and present the calculations for two different experimentally accessible molecular films sandwiched between gold electrodes.
The ground state energies of Ag and Au in the face-centered cubic (FCC), body-centered cubic (BCC), simple cubic (SC) and the hypothetical diamond-like phase, and dimer were calculated as a function of bond length using density functional theory (DFT). These energies were then used to parameterize the many-body Gupta potential for Ag and Au. This parameterization over several phases of Ag and Au was performed to guarantee transferability of the potentials and to make them appropriate for studies of related nanostructures. Depending on the structure, the energetics of the surface atoms play a c…
Short-range order (SRO) in disordered alloys is typically interpreted as competition between chemical effect of negative (or positive) energy gain by mixing constituent elements and geometric effects comes from difference in effective atomic radius. Although we have a number of theoretical approaches to quantitatively estimate SRO at given temperatures, it is still unclear to systematically understand trends in SRO for binary alloys in terms of geometric character, e.g., effective atomic radius for constituents. Since chemical effect plays significant role on SRO, it has been believed that pur…
We report the effects of variation in length on the electronic structure of CdSe nanorods derived from atomic clusters and passivated by fictitious hydrogen atoms. These nanorods are augmented by attaching gold clusters at both the ends to form a nanodumbbell. The goal is to assess the changes at nanolevel after formation of contacts with gold clusters serving as electrodes and compare the results with experimental observations [PRL, 95, 056805 (2005)]. Calculations involving nanorods of length 4.6 Angstrom to 116.6 Angstrom are performed using density functional theory implemented within plan…
We have fabricated a variety of novel molecular tunnel junctions based on self-assembled-monolayers (SAM) of two-component solid-state mixtures of molecular wires (1,4 methane benzene-dithiol; Me-BDT with two thiol anchoring groups), and molecular insulator spacers (1-pentanethiol; PT with one thiol anchoring group) at different concentration ratios, r of wires/spacers, which were sandwiched between two metallic electrodes such as gold and cobalt. FTIR spectroscopy and surface titration were used, respectively to verify the formation of covalent bonds with the electrodes, and obtain the number…
Recent experimental realization [J. Am. Chem. Soc., 127 (2005) 7328] of various dithiocarbamate self assembly on gold surface opens the possibility for use of dithiocarbamate linkers to anchor molecular wires to gold electrodes. In this paper, we explore this hypothesis computationally. We computed the electron transport properties of 4,4'-bipyridine (BP), 4,4'-bipyridinium-1,1'-bis(carbodithioate) (BPBC), 4-(4'-pyridyl)-peridium-1-carbodithioate (BPC) molecule junctions based on the density functional theory and non-equilibrium Green's functions. We demonstrated that the stronger molecule-ele…
The final performance of a molecular electronic device is determined by the chemical structure of the molecular wires used in its assembly. Molecular place-exchange was used to incorporate di-thioacetate terminated molecules into ordered arrays of dodecanethiol capped gold nanoparticles. X-ray photoelectron spectroscopy confirmed successful molecular replacement. Room-temperature molecular conductance of a statistically large number of devices reveals that conductance is enhanced by up to two orders of magnitude for the di-thioacetate terminated molecules. Density functional theory transport c…
Molecular nanowires in which a single molecule bonds chemically to two metal electrodes and forms a stable electrically conducting bridge between them have been studied intensively for more than a decade. However the experimental determination of the bonding geometry between the molecule and electrodes has remained elusive. Here we demonstrate by means of ab initio calculations that inelastic tunneling spectroscopy (IETS) can determine these geometries. We identify the bonding geometries at the gold-sulfur interfaces of propanedithiolate molecules bridging gold electrodes that give rise to the…
Spurred by the increasing needs in electrochemical energy storage devices, the electrode/electrolyte interface has received a lot of interest in recent years. Molecular dynamics simulations play a proeminent role in this field since they provide a microscopic picture of the mechanisms involved. The current state-of-the-art consists in treating the electrode as a perfect conductor, precluding the possibility to analyze the effect of its metallicity on the interfacial properties. Here we show that the Thomas-Fermi model provides a very convenient framework to account for the screening of the ele…
We have investigated the contact resistance of rubrene single-crystal field-effect transistors (FETs) with Nickel electrodes by performing scaling experiments on devices with channel length ranging from 200 nm up to 300 $μ$m. We find that the contact resistance can be as low as 100 $Ω$cm with narrowly spread fluctuations. For comparison, we have also performed scaling experiments on similar Gold-contacted devices, and found that the reproducibility of FETs with Nickel electrodes is largely superior. These results indicate that Nickel is a very promising electrode material for the reproducible …
Nanomechanical shuttles transferring small groups of electrons or even individual electrons from one electrode to another offer a novel approach to the problem of controlled charge transport. Here, we report the fabrication of shuttle-junctions consisting of a 20 nm diameter gold nanoparticle embedded within the gap between two gold electrodes. The nanoparticle is attached to the electrodes through a monolayer of flexible organic molecules which play the role of springs so that when a sufficient voltage bias is applied, then nanoparticle starts to oscillate transferring electrons from one elec…
The development of portable and wearable electronics requiring only few tens of microwatts to operate fosters the search for low power energy sources. Moreover, the fast-growing demand for new sustainable energy has raised the interest in energy harvesters able to convert mechanical energy into electrical power. Triboelectric nanogenerators (TENGs) can provide such green power supply. Recent developments in TENGs technology show that they can be used in different applications ranging from wearable self-powered sensors to wind and sea wave energy harvesting. In spite of the wide number of TENGs…
We study electron transport through single-molecule junctions formed by an octanethiol molecule bonded with the thiol anchoring group to a gold electrode and the opposing methyl endgroup to a carbon tip. Using the scanning tunneling microscope based break junction technique, we measure the electrical conductance of such molecular junctions. We observe the presence of well-defined conductance plateaus during the stretching of the molecular bridge, which is the signature of the formation of a molecular junction.
We report detailed atomistic simulations combined with high-fidelity conductance calculations to probe the structural origins of conductance fluctuations in thermally evolving Au-benzene-1,4-dithiolate-Au junctions. We compare the behavior of structurally ideal junctions (electrodes with flat surfaces) to structurally realistic, experimentally representative junctions resulting from break junction simulations. The enhanced mobility of metal atoms in structurally realistic junctions results in significant changes to the magnitude and origin of the conductance fluctuations. Fluctuations are larg…
Motivated by recent experiments, we study the variability of the thermal conductance of single dithiolated alkane molecules of varying length connecting two gold electrodes. For this purpose, we examine (i) the influence of the metal-molecule contact and of the electrode orientation on the thermal conductance of straight alkane chains, (ii) the effect of molecule-internal disorder realized through torsional gauche defects and the behavior upon stretching, and (iii) the modifications resulting from temperature-dependent dynamical variations of the geometry. While we analyze the former two aspec…
We present a theoretical study of the transport characteristics of molecular junctions, where first-row diatomic molecules are attached to (001) gold and platinum electrodes. We find that the conductance of all of these junctions is of the order of the conductance quantum unit $G_0$, spelling out that they belong to the transparent regime. We further find that the transmission coefficients show wide plateaus as a function of the energy, instead of the usual sharp resonances that signal the molecular levels in the tunneling regime. We use Caroli's model to show that this is a rather generic pro…
We study the effects of gating and contact geometry on current through self-assembled monolayers of conjugated molecules strongly coupled to gold electrodes by sulfur ``anchor groups''. The current changes by more than an order of magnitude depending on the angle between the axis of the benzene-dithiolate molecules and the normal to the electrode on the less coordinated ``top site'' position. The effect of gating is also much stronger in this case compared to higher coordinated ``hollow site'' binding of the molecule on a Au(111) surface. The large hybridization of the molecular states with el…
We present a theoretical study of the conductance of atomic junctions comprising single noble gas atoms (He, Ne, Ar, Kr, and Xe) coupled to gold electrodes. The aim is to elucidate how the presence of noble gas atoms affects the electronic transport through metallic atomic-size contacts. Our analysis, based on density functional theory and including van der Waals interactions, shows that for the lightest elements (He and Ne) no significant current flows through the noble gas atoms and their effect is to reduce the conductance of the junctions by screening the interaction between the gold elect…
We present first-principles calculations for the transport properties of monoatomic Al wires sandwiched between Al(100) electrodes. The conductance of the monoatomic Al wires oscillates with the number of the constituent atoms as a function of the wire length, either with a period of four-atom for wires with the typical interatomic spacing or a period of six-atom with the interatomic spacing of the bulk fcc aluminum, indicating a dependence of the period of conductance oscillation on the interatomic distance of the monoatomic Al wires.
The kinetic energy distributions of photo-electrons emitted from gold surfaces under illumination by UV-light close to the threshold are measured and analyzed. Samples are prepared as chemically clean through Ar-Ion sputtering and then exposed to atmosphere for variable durations before Quantum Yield measurements are performed after evacuation. During measurements the bias voltage applied to the sample is varied and the resulting emission current measured. Taking the derivative of the current-voltage curve yields the energy distribution which is found to closely resemble the distribution of to…
Magnetic switching of a single molecular magnet (SMM) due to spin-polarized current flowing between ferromagnetic metallic electrodes is investigated theoretically. Magnetic moments of the electrodes are assumed to be collinear and parallel to the magnetic easy axis of the molecule. Electrons tunneling through a barrier between magnetic leads are coupled to the SMM via exchange interaction. The current flowing through the system as well as the spin relaxation times of the SMM are calculated from the Fermi golden rule. It is shown that spin of the SMM can be reversed by applying a voltage betwe…
It is often assumed that the hydrogen atoms in the thiol groups of a benzene-1,4-dithiol dissociate when Au-benzene-1,4-dithiol-Au junctions are formed. We demonstrate, by stability and transport properties calculations, that this assumption can not be made. We show that the dissociative adsorption of methanethiol and benzene-1,4-dithiol molecules on a flat Au(111) surface is energetically unfavorable and that the activation barrier for this reaction is as high as 1 eV. For the molecule in the junction, our results show, for all electrode geometries studied, that the thiol junctions are energe…
Spin crossover (SCO) molecules are promising bi-stable magnetic switches with applications in molecular spintronics. However, little is known about the switching effects of a single SCO molecule when it is confined between two metal electrodes. Here we examine the switching properties of a [Fe(III)(EtOSalPet )(NCS)] SCO molecule that is specifically tailored for surface deposition and binding to only one gold electrode in a nanogap device. Temperature dependent conductivity measurements on SCO molecule containing electromigrated gold break junctions show voltage independent telegraphic-like sw…
We have studied conductance ($\it{g}$) of butanedithiol-linked gold nanoparticle films across a percolation insulator-to-metal transition. As the transition proceeds, electrons become itinerant (i.e. Coulomb charging and kinetic effects are both significant), and films exhibit a previously unobserved zero-bias conductance peak (ZBCP). The peak is much more pronounced and easily observed using electromigration-induced break junction (BJ) contacts rather than macroscopic 4-probe electrodes. We attribute this ZBCP to quantum correlations amongst electrons, in view of other temperature- ($\it{T}$-…
