Metal-molecule Contacts Research Articles

Effects of Contact Atomic Structure on Electronic Transport in Molecular Junction

Based on nonequilibrium Green's function and first-principles calculations, we investigate the change in molecular conductance caused by different adsorption sites with the presence of additional Au atom around the metal-molecule contact in the system that benzene sandwiched between two Au(111) leads. The motivation is the variable situations that may arise in break junction experiments. Numerical results show that the enhancement of conductance induced by the presence of additional Au is dependent on the adsorption sites of anchoring atom. When molecule is located on top site with the presence of additional Au atoms, it can increase molecular conductance remarkably and present negative differential resistance under applied bias which cannot be found in bridge and hollow sites. Furthermore, the effects of different distance between additional Au and sulfur atoms in these three adsorption sites are also discussed.

Statistical Analysis of Electronic Transport Through Chemisorbed Versus Physisorbed Alkanethiol Self-Assembled Monolayers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We fabricate microscale via-hole structure molecular electronic devices using various lengths of alkanemonothiol and alkanedithiol self-assembled monolayers (SAMs) and characterize their charge transport properties. From the statistical analysis of the transport properties of these SAMs, we have found that the conductance of alkanedithiols is higher about an order of magnitude than that of alkanemonothiols, which is due to the different nature of metal–molecule contacts, i.e., physisorbed versus chemisorbed contacts. However, the transport decay coefficients that can be obtained from the length-dependent transport characterization are almost identical for the two molecular systems. </para>

Influence of metal-molecule contacts on decay coefficients and specific contact resistances in molecular junctions

The effect of metal-molecule contacts in molecular junctions is studied based on the analysis of a statistically significant number of devices and a proposed multibarrier tunneling (MBT) model, where a metal-molecule-metal junction is divided into three individual barriers: a molecular-chain body and metal-molecule contacts on either side of molecule. Using the MBT model with the statistical analysis, we could derive and distinguish decay coefficients for contact barriers $({\ensuremath{\beta}}_{1},{\ensuremath{\beta}}_{2})$, contact-dependent and contact-independent decay coefficients (${\ensuremath{\beta}}_{0}$ vs ${\ensuremath{\beta}}_{\text{body}}$), and specific contact resistances in terms of different molecular length and different natures of metal-molecule contacts.

Effect of the relative orientation of molecules on the electronic transport property of molecular devices

Based on ab initio methods and nonequilibrium Green's function theory, we have investigated the electronic transport properties of single and double 1，4-dithiolbenzene molecular devices which have different molecular relative orientations on molecule-metal contacts. Numerical results show that different molecular relative orientations have various influences on the electronic structure of molecules and current-voltage characteristics, which obviously affects the electronic transport properties of metal-molecule-metal systems. The equilibrium state of the extend molecule is not the best situation for electronic transportation. The characteristics of electronic transportation can thus be improved by adjusting the molecular relative orientation on molecule-metal contacts.

Rate-equation calculations of the current flow through two-site molecular device and DNA-based junction

Abstract Here we present the calculations of incoherent current flowing through the two-site molecular device as well as the DNA-based junction within the rate-equation approach. Selected phenomena of interest are discussed in detail. The structural asymmetry of a two-site molecule results in a rectification effect, which can be neutralized by an asymmetric voltage drop at the molecule-metal contacts due to coupling asymmetry. The results received for the poly(dG)-poly(dC) DNA molecule reveal the coupling-and temperature-independent saturation effect of the current at high voltages, establishing for short chains the inverse square distance dependence. Additionally we document the conductance peak shifting in the direction of higher voltages due to a temperature decrease.

Reversible and Controllable Switching of a Single‐Molecule Junction

This phenomen-on is caused by statisticalfluctuations in the moleculeitself or the molecule–metalcontact. The lack of controlover this type of switchingmakesitessentiallyundesira-ble for any technological ap-plication. Recent experi-ments aiming to identify thefundamental mechanisms re-sponsibleforvoltage-inducedswitching in sandwich struc-tures

Critical roles of metal-molecule contacts in electron transport through molecular-wire junctions

We study the variation of electron transmission through Au-S-benzene-S-Au junctions and related systems as a function of the structure of the Au:S contacts. For junctions with semi-infinite flat Au(111) electrodes, the highly coordinated in-hollow and bridge positions are connected with broad transmission peaks around the Fermi level, due to a broad range of transmission angles from transverse motion, resulting in high conductivity and weak dependence on geometrical variations. In contrast, for (unstable) S-adsorption on top of an Au atom, or in the hollow of a 3-Au-atom island, the transmission peaks narrow up due to suppression of large transmission angles. Such more one-dimensional situations may describe more common types of contacts and junctions, resulting in large variations in conductivity and sensitivity to bonding sites, tilting and gating. In particular, if S is adsorbed in an Au vacancy, sharp spectral features appear near the Fermi level due to essential changes of the level structure and hybridization in the contacts, admitting order-of-magnitude variations of the conductivity. Possibly such a system, can it be fabricated, will show extremely strong non-linear effects and might work as uni- or bi-directional voltage-controlled 2-terminal switches and non-linear mixing elements. Finally, density-functional-theory (DFT) based transport calculations seem relevant, being capable of describing a wide range of transmission peak structures and conductivities. Prediction and interpretation of experimental results probably require more precise modeling of realistic experimental situations.

Nanometer scale electrode separation (nanogap) using electromigration at room temperature

Pairs of electrodes with nanometer separation (nanogap) 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 MPTMS is an efficient adhesive monolayer between SiO/sub 2/ and Au. Although the Au wires are initially 2 /spl mu/m wide, gaps with length /spl sim/1 nm and width /spl sim/5 nm are observed after breaking and imaging through a field effect scanning electron microscope. This technique eliminates the presence of any residual metal interlink in the adhesion layer (chromium or titanium for Au deposition over SiO/sub 2/) after breaking the gold wire, and it is much easier to implement than the commonly used low-temperature EIBJ technique which needs to be executed at 4.2 K. Metal-molecule-metal structures with symmetrical metal-molecule contacts at both ends of the molecule are fabricated by forming a self-assembled monolayer of -dithiol molecules between the EIBJ-created Au electrodes with nanometer separation. Electrical conduction through single molecules of 1,4-Benzenedimethanethiol (XYL) is tested using the Au/XYL/Au structure with chemisorbed gold-sulfur coupling at both contacts.

Correlation between HOMO Alignment and Contact Resistance in Molecular Junctions: Aromatic Thiols versus Aromatic Isocyanides

Understanding electron transport in metal-molecule-metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal-molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (-S) or isocyanide (-CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au-CN contact is more resistive than Au-S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyanide-linked molecules. By contrast, the difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance (i.e., the beta value = 0.5 A-1).

A generalized quantum chemical approach for elastic and inelastic electron transports in molecular electronics devices

A generalized quantum chemical approach for electron transport in molecular devices is developed. It allows one to treat devices where the metal electrodes and the molecule are either chemically or physically bonded on equal footing. An extension to include the vibration motions of the molecule has also been implemented which has produced the inelastic electron-tunneling spectroscopy of molecular electronics devices with unprecedented accuracy. Important information about the structure of the molecule and of metal-molecule contacts that are not accessible in the experiment are revealed. The calculated current-voltage (I-V) characteristics of different molecular devices, including benzene-1,4-dithiolate, octanemonothiolate [H(CH2)8S], and octanedithiolate [S(CH2)8S] bonded to gold electrodes, are in very good agreement with experimental measurements.

Nanoscale patterning in application to materials and device structures

We present fabrication schemes for nanoscale molecular junctions, which allow the deposition of molecules after the fabrication steps that can uncontrollably affect the electrical properties of the molecular layers. The two techniques described here use shadow mask evaporation and nanotransfer printing. In order to make reliable contacts with the molecules (or molecular monolayers) the morphology of the contacting metals has to be optimized and controlled. We therefore characterize the surfaces of the contacting metals using scanning electron microscopy and scanning probe microscopy at various stages of the fabrication. Based on these results we developed methods to improve the morphology in order to realize more reliable metal-molecule contacts. It is shown that improvement of the surface topography of the metals indeed leads to metal-molecule-metal junctions with a very low failure rate.

First-Principles Simulations of Inelastic Electron Tunneling Spectroscopy of Molecular Electronic Devices

Inelastic electron tunneling spectroscopy (IETS) is a powerful experimental tool for studying the molecular and metal contact geometries in molecular electronic devices. A first-principles computational method based on the hybrid density functional theory is developed to simulate the IETS of realistic molecular electronic devices. The calculated spectra of a real device with an octanedithiolate embedded between two gold contacts are in excellent agreement with recent experimental results. Strong temperature dependence of the experimental IETS spectra is also reproduced. It is shown that the IETS is extremely sensitive to the intramolecular conformation and the molecule-metal contact geometry changes. With the help of theoretical calculations, it has finally become possible to fully understand and assign the complicated experimental IETS and, more importantly, provide the structural information of the molecular electronic devices.

Metal-molecule contacts

Metal-molecule contacts play a key role in determining the current-voltage ( I - V ) characteristics of a molecular junction. This article reviews specific ways that metal-molecule contacts can be controlled to impart a desired functionality, and highlights some unanswered questions.

Contact atomic structure and electron transport through molecules

Using benzene sandwiched between two Au leads as a model system, we investigate from first principles the change in molecular conductance caused by different atomic structures around the metal-molecule contact. Our motivation is the variable situations that may arise in break junction experiments; our approach is a combined density functional theory and Green function technique. We focus on effects caused by (1) the presence of an additional Au atom at the contact and (2) possible changes in the molecule-lead separation. The effects of contact atomic relaxation and two different lead orientations are fully considered. We find that the presence of an additional Au atom at each of the two contacts will increase the equilibrium conductance by up to two orders of magnitude regardless of either the lead orientation or different group-VI anchoring atoms. This is due to a resonance peak near the Fermi energy from the lowest energy unoccupied molecular orbital. In the nonequilibrium properties, the resonance peak manifests itself in a negative differential conductance. We find that the dependence of the equilibrium conductance on the molecule-lead separation can be quite subtle: either very weak or very strong depending on the separation regime.

Illustrative direct ab initio calculations of surface Raman spectra

Illustrative ab initio calculations of Raman spectra of pyridine (Pyr) and pyridine in metal-molecule complexes with silver (Ag+-Pyr, Ag5+-Pyr) have been undertaken. The Raman spectra were computed numerically in the standard Placzek approximation with the additional feature of a static external electric field, F, perturbation to the system Hamiltonian. F was allowed to change the system equilibrium geometry and symmetry as well as the polarisability, resulting in shifted vibration frequencies and changed Raman intensities. All calculations were done on a Hartree-Fock (HF) level using the 6-31+G* or the Sadlej p-VTZ basis sets for Pyr, and the Stevens Krauss (SBKJC) VDZ ECP basis set for Ag. Raman enhancement factors over 5 orders of magnitude were obtained. The enhancement was analyzed as being due to polarization and charge transfer induced by the metal-molecule contact in the absence of F and in the presence of a strong F as due to increased polarisability derivatives along the surface normal. The use of the present method to qualitatively and quantitatively predict surface Raman spectra is discussed.

Tuning current rectification across molecular junctions

We demonstrate the ability to tune the current rectification in metal–molecule–metaljunctions through control of the interaction strength of one of the two metal–moleculecontacts. Current–voltage characteristics of thiolate bound molecular wires with a nitro orpyridine termination show that the extent of current rectification in a molecular junctioncorrelates well with the extent of coupling between the chemical linker and metal electrode.

Electrical and Spectroscopic Characterization of Molecular Junctions

AbstractThe design of future molecular electronic devices requires a firm understanding of the conduction mechanisms that determine their electrical characteristics. Progress toward this goal has been hindered by complications in controlling the exact configuration and makeup of fabricated molecular junctions, thus limiting the availability of quantitative experimental data for developing cohesive theories to model and predict molecular transport. This article summarizes recent research aimed at developing well-controlled systems for comparing molecular conduction and vibrational spectra using crossed-wire and in-wire metal–molecule–metal junctions. Systematic variations in molecular structure and metal–molecule contacts show strong quantitative agreement in device properties, while spectroscopic data provide evidence that the properties are due to the molecular junction. Further investigations using these and other molecular junction test beds will provide the needed experimental data to advance fundamental understanding of molecular transport and facilitate future molecular electronics applications.

Embedding method for conductance of DNA

Using a technique based on embedding in a local-orbital formalism, the electronic structure and electron transmission properties of long biological molecules may be calculated. The electronic structure is found by adding one structural unit at a time to the molecule, and calculating an embedding potential for adding the next structural unit. At present an extended H\"uckel scheme is used to evaluate the matrix elements. The transmission is also calculated within the embedding scheme, taking the molecule-metal contacts into account. Results for the density of states and transmission are presented for several structures of DNA. The transmission is highly energy dependent, and is also greatly influenced by the orbitals to which contact is made. The implications of these calculations for conductance are discussed.

End group effect on electrical transport through individual molecules: A microscopic study

The effect on molecular transport due to chemical modification of the metal-molecule interface is investigated, using as an example the prototypical molecular device formed by attaching a p-disubstituted benzene molecule onto two gold electrodes through chemically different end groups. Using a first-principles based self-consistent matrix Green's function method, we find that depending on the end group, transport through the molecule can be mediated by either near-resonant-tunneling or off-resonant-tunneling and the conductance of the molecule varies over more than two orders of magnitude. Despite the symmetric device structure of all the molecules studied, the applied bias voltage can be dropped either equally between the two metal-molecule contacts or mostly across the source (electron-injecting) contact depending on the potential landscape across the molecular junction at equilibrium.

Electronic structure and conductance of large molecules and DNA

Using a new technique based on embedding in a local orbital formalism, the electronic structure and electron transmission properties of long biological molecules are calculated, in particular DNA. The electronic structure is found by adding one structural unit at a time to the molecule, and calculating an embedding potential for adding the next structural unit. At present, an extended Hückel scheme is used for the Hamiltonian. The transmission is also calculated within the embedding scheme, taking the molecule–metal contacts into account. The results for transmission depend greatly on the orbitals to which contact is made, and also on energy. The implications of these calculations for conductance are discussed.