Molecular Wire Junctions Research Articles

Nanoparticle Linker‐Controlled Molecular Wire Devices Based on Double Molecular Monolayers

Highly conductive molecular wires are an important component for realizing molecular electronic devices and have to be explored in terms of interactions between molecules and electrodes in their molecular junctions. Here, new molecular wire junctions are reported to enhance charge transport through gold nanoparticle (AuNP)-linked double self-assembled monolayers (SAMs) of cobalt (II) bis-terpyridine molecules (e.g., Co(II)(tpyphS)2 ). Electrical characteristics of the double-SAM devices are explored in terms of the existence of AuNP. The AuNP linker in the Co(II)(tpyphS)2 -AuNP-Co(II)(tpyphS)2 junction acts as an electronic contact that is transparent to electrons. The weak temperature dependency of the AuNP-linked molecular junctions strongly indicates sequential tunneling conduction through the highest occupied molecular orbitals (HOMOs) of Co(II)(tpyphS)2 molecules. The electrochemical characteristics of the AuNP-Co(II)(tpyphS)2 SAMs reveal fast electron transfer through molecules linked by AuNP. Density functional theory calculations reveal that the molecular HOMO levels are dominantly affected by the formation of junctions. The intermolecular charge transport, controlled by the AuNP linker, can provide a rational design for molecular connection that achieves a reliable electrical connectivity of molecular electronic components for construction of molecular electronic circuits.

Direct mapping of electrical noise sources in molecular wire-based devices

We report a noise mapping strategy for the reliable identification and analysis of noise sources in molecular wire junctions. Here, different molecular wires were patterned on a gold substrate, and the current-noise map on the pattern was measured and analyzed, enabling the quantitative study of noise sources in the patterned molecular wires. The frequency spectra of the noise from the molecular wire junctions exhibited characteristic 1/f2 behavior, which was used to identify the electrical signals from molecular wires. This method was applied to analyze the molecular junctions comprising various thiol molecules on a gold substrate, revealing that the noise in the junctions mainly came from the fluctuation of the thiol bonds. Furthermore, we quantitatively compared the frequencies of such bond fluctuations in different molecular wire junctions and identified molecular wires with lower electrical noise, which can provide critical information for designing low-noise molecular electronic devices. Our method provides valuable insights regarding noise phenomena in molecular wires and can be a powerful tool for the development of molecular electronic devices.

Theoretical Study of Quantum Conductance of Conjugated and Nonconjugated Molecular Wire Junctions

Electron transport through molecular junctions has been widely investigated experimentally and theoretically. Unfortunately, there exists discrepancy on the single molecular conductance between theoretical calculations and experimental measurements. In this paper, first-principle density functional theory combined with nonequilibrium Green’s function approach is employed; we studied electronic structures, molecular lengths, and interfacial interactions of three kinds of molecular junctions, alkanedithiols, oligo(1,4-phenylene-ethynylene)s, and 1,4-benzene-di(n-alkylthiol) (BDnT), embedding in nanogaps of gold electrodes. First, our approach can accurately describe the binding interaction between the thiol group and gold electrode so that the conductance of alkanedithiol in a gold junction can be well predicted. We found that a previous underestimation of HOMO–LUMO gaps in the junction system leads to the overestimated conductance for conjugate molecules with sulfur atoms binding to gold electrodes. In the...

Anomalous length-independent frontier resonant transmission peaks in armchair graphene nanoribbon molecular wires

Molecular wires constitute the building blocks for nanoscale interconnects. However, the exponential decrease in conductance with wire length severely limits their applications. We predict, using first principles calculations, that armchair graphene nanoribbon (AGNR) wires, connected by transverse zigzag edges to wider AGNR electrodes, can exhibit anomalous resonant transmission peaks that are nearly independent of the wire length. We propose a new model to explain the unusual length independence of peak energies from the locally repeating property of the wavefunction in the middle-AGNR. We further uncover that this locally repeating pattern originates from states of a perfect AGNR with infinite length. The pattern can be well preserved when the AGNR is connected to wider AGNR leads because of the zigzag edges serving as electron sources and drains. The length independence of peak widths results from the zigzag edges absorbing most of the wavefunction renormalization as the length increases, so that the coupling strength to the electrodes does not change significantly. These anomalous properties arising from intrinsic wavefunction properties of the AGNRs are in sharp contrast to typical transmission properties of traditional molecular wire junctions, which suggests promising potential application as “molecular wire” interconnects in nano-electronics.

Single Molecular Resistive Switch Obtained via Sliding Multiple Anchoring Points and Varying Effective Wire Length

A single molecular resistive (conductance) switch via control of anchoring positions was examined by using a molecule consisting of more than two same anchors. For this purpose, we adopted the covered quaterthiophene (QT)-based molecular wire junction. The QT-based wire consisted of two thiophene ring anchors on each side; thus, shift of anchors was potentially possible without a change in the binding modes and distortion of the intramolecular structure. We observed three distinct conductance states by using scanning tunneling microscope-based break junction technique. A detailed analysis of the experimental data and first-principles calculations revealed that the mechanism of the resistive switch could be explained by standard length dependence (exponential decay) of conductance. Here, the length is the distance between the anchoring points, i.e., length of the bridged π-conjugated backbone. Most importantly, this effective tunneling length was variable via only controlling the anchoring positions in the same molecule. Furthermore, we experimentally showed the possibility of a dynamic switch of anchoring positions by mechanical control. The results suggested a distinct strategy to design functional devices via contact engineering.

Interfacial Design of Anisotropic Conductive Adhesive Based Interconnects Using Molecular Wires and Understanding of Their Electrical Conduction

Anisotropic conductive adhesives (ACAs) have been considered a promising interconnect material for next generation high performance devices. However, high joint resistance and low current carrying capability of ACA interconnects have been the limitations to utilizing ACAs in high power devices. In this study, we have introduced conjugated dithiols into ACA formulations to create molecular wire junctions between conductive fillers and metal pads as a means to facilitate the electron transport through the ACA joints. With the introduction of molecular wires, there is evidence of measured improvements in both the electrical conductivity and current carrying capability. The factors leading to these improvements in electrical properties are also discussed.

Cumulene molecular wire conductance from first principles

We present first principles calculations of current-voltage characteristics (IVC) and conductance of Au(111):S2-cumulene-S2:Au(111) molecular wire junctions with realistic contacts. The transport properties are calculated using full self-consistent ab initio NEGF-DFT methods under external bias. The conductance of the cumulene wires shows oscillatory behavior depending on the number of carbon atoms (double bonds). Among all conjugated oligomers, we find that cumulene wires with odd number of carbon atoms yield the highest conductance with metallic-like ballistic transport behavior. The reason is the high density of states in broad LUMO levels spanning the Fermi level of the electrodes. The transmission spectrum and the conductance depend only weakly on applied bias, and the IVC is nearly linear over a bias region from +1 to -1 V. Cumulene wires are therefore potential candidates for metallic connections in nanoelectronic applications.

Enhanced Electrical Properties of Anisotropic Conductive Adhesive With $\pi$-Conjugated Self-Assembled Molecular Wire Junctions

We have investigated the electrical properties of anisotropic conductive adhesive (ACA) joint using submicrometer-sized (~500 nm in diameter) silver (Ag) particle as conductive filler with the effect of π-conjugated self-assembled molecular wires. The ACAs with submicrometer-sized Ag particles have higher current carrying capability (~3400 mA) than those with micro-sized Au-coated polymer particles (~2000 mA) and Ag nanoparticles (~2500 mA). More importantly, by construction of π-conjugated self-assembled molecular wire junctions between conductive particles and integrated circuit (IC)/substrate, the electrical conductivity has increased by one order of magnitude and the current carrying capability of ACAs has improved by 600 mA. The crucial factors that govern the improved electrical properties are discussed based on the study of alignments and thermal stability of molecules on the submicrometer-sized Ag particle surface with surface-enhanced Raman spectroscopy (SERS), providing a fundamental understanding of conduction mechanism in ACA joints and guidelines for the formulation of high-performance ACAs in electronic packaging industry.

Laser-induced currents along molecular wire junctions

The treatment of the previous paper is extended to molecular wires. Specifically, the effect of electron-vibrational interactions on the electronic transport induced by femtosecond omega+2omega laser fields along unbiased molecular nanojunctions is investigated. For this, the photoinduced vibronic dynamics of trans-polyacetylene oligomers coupled to macroscopic metallic leads is followed in a mean-field mixed quantum-classical approximation. A reduced description of the dynamics is obtained by introducing projective lead-molecule couplings and deriving an effective Schrodinger equation satisfied by the orbitals in the molecular region. Two possible rectification mechanisms are identified and investigated. The first one relies on near-resonance photon-absorption and is shown to be fragile to the ultrafast electronic decoherence processes introduced by the wire's vibrations. The second one employs the dynamic Stark effect and is demonstrated to be highly efficient and robust to electron-vibrational interactions.

Systematic Study on Quantum Confinement and Waveguide Effects for Elastic and Inelastic Currents in Atomic Gold Wire: Importance of the Phase Factor for Modeling Electrodes

Quantum confinement of the electrodes is an important issue for electron transport through molecular or atomic wire junctions. To assess the importance of waveguide effects by quantum confinement of the electrodes, we have calculated elastic and inelastic conductance and inelastic electron tunneling spectra of atomic gold wire with gold electrodes for several models. The results show the quite important role of the phase factors between the modeled electrodes and the contact region.

Erratum: Modeling inelastic phonon scattering in atomic- and molecular-wire junctions [Phys. Rev. B72, 201101(R) (2005)

Erratum: Modeling inelastic phonon scattering in atomic- and molecular-wire junctions [Phys. Rev. B<b>72</b>, 201101(R) (2005)

Propensity rules for inelastic electron tunneling spectroscopy of single-molecule transport junctions

Using a perturbative approach to simple model systems, we derive useful propensity rules for inelastic electron tunneling spectroscopy (IETS) of molecular wire junctions. We examine the circumstances under which this spectroscopy (that has no rigorous selection rules) obeys well defined propensity rules based on the molecular symmetry and on the topology of the molecule in the junction. Focusing on conjugated molecules of C(2h) symmetry, semiquantitative arguments suggest that the IETS is dominated by a(g) vibrations in the high energy region and by out of plane modes (a(u) and b(g)) in the low energy region. Realistic computations verify that the proposed propensity rules are strictly obeyed by medium to large-sized conjugated molecules but are subject to some exceptions when small molecules are considered. The propensity rules facilitate the use of IETS to help characterize the molecular geometry within the junction.

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.

Dithiocarbamate Anchoring in Molecular Wire Junctions: A First Principles Study

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 nonequilibrium Green's functions. We demonstrated that the stronger molecule-electrode coupling associated with the conjugated dithiocarbamate linker broadens transmission resonances near the Fermi energy. The broadening effect along with the extension of the pi conjugation from the molecule to the gold electrodes lead to enhanced electrical conductance for BPBC molecule. The conductance enhancement factor is as large as 25 at applied voltage bias 1.0 V. Rectification behavior is predicted for BPC molecular wire junction, which has the asymmetric anchoring groups.

Molecular Signatures in the Transport Properties of Molecular Wire Junctions: What Makes a Junction “Molecular”?

The simplest component of molecular electronics consists of a single-molecule transport junction: a molecule sandwiched between source and drain electrodes, with or without a third gate electrode. In this Concept article, we focus on how molecules control transport in metal-electrode molecular junctions, and where the molecular signatures are to be found. In the situation where the molecule is relatively short and the gap between injection energy and molecular eigenstates is large, transport occurs largely by elastic tunneling, stochastic switching is common, and the vibronic signature can be found using inelastic electron tunneling spectroscopy (IETS). As the energy gaps for injection become smaller, one begins to see stronger molecular signatures - these include Franck-Condon-like structures in the current/voltage characteristic and strong vibronic interactions, which can lead to hopping behavior at the polaron limit. Conformational changes induced by the strong electric field lead to another strong manifestation of the molecular nature of the junction. We overview some of this mechanistic landscape, focusing on significant effects of switching (both stochastic and controlled by the electric field) and of molecular vibronic coupling.

Modeling inelastic phonon scattering in atomic- and molecular-wire junctions

Computationally inexpensive approximations describing electron-phonon scattering in molecular-scale conductors are derived from the non-equilibrium Green's function method. The accuracy is demonstrated with a first principles calculation on an atomic gold wire. Quantitative agreement between the full non-equilibrium Green's function calculation and the newly derived expressions is obtained while simplifying the computational burden by several orders of magnitude. In addition, analytical models provide intuitive understanding of the conductance including non-equilibrium heating and provide a convenient way of parameterizing the physics. This is exemplified by fitting the expressions to the experimentally observed conductances through both an atomic gold wire and a hydrogen molecule.

Dephasing effects in molecular junction conduction: An analytical treatment

A Keldysh nonequilibrium Green's function approach is used in an analytic treatment of transport in oligomeric molecular wire junctions in the presence of dephasing effects. Both the dephasing and molecule-electrode interactions are treated using the self-energy formalism, and limiting analytic forms are obtained for the current as a function of dephasing strength, length, injection energy, molecular hopping integral, and temperature. Dephasing due to the interaction between electrons and thermal phonons is investigated in detail, and we relate the phenomenological dephasing parameter to the inner-sphere reorganization energy. In the single-site limit, we observe transitions in the spectral function from Lorentzian to semicircular as the dephasing strength increases, while for the extended chain we observe both tunneling and hopping behavior. In the limit of strong dephasing, the overall resistance can be expressed as the sum of the contact resistance and a chain-length-dependent hopping term.

Effect of alkali-metal adsorption on the conductance of a molecular device

We show by first-principles density functional calculations that the adsorption of potassium on the platinum electrodes of a benzene dithiolate molecular wire junction will modify the energy-level alignments of the device. But near the Fermi energy, the energy levels, which correspond to sulfur orbitals, are quite inert to the work function change of the electrode due to a strong terminal atom/metal bonding. Our results demonstrate unambiguously that while the coadsorption of potassium on the electrodes may influence the conductance of a molecular device, it cannot change the conductance in a drastic way as the transmission peak near the Fermi level shifts only slightly with the introduction of potassium atoms.

Theoretical investigation on electron transport through an organic molecule: effect of the contact structure.

Knowing how the contact geometry influences the conductance of a molecular wire junction requires both a precise determination of the molecule/metallic-electrode interface structure and an evaluation of the conductance for different contact geometries with a fair accuracy. With a greatly improved method to solve the Lippmann-Schwinger equation, we are able to include at least one atomic layer of each electrode into the extended molecule. The artificial effect of the jellium model used for the electrodes is therefore significantly reduced. Our first-principles calculations on the transport properties of a single benzene dithiolate molecule sandwiched between Au(111) surfaces show that the transmission of the bridge site contact, which is the most stable adsorption configuration in equilibrium, displays different features from those of other configurations, and that the inclusion of the surface layers of Au electrodes into the extended molecule shifts and broadens the transmission peaks due to a stronger and more realistic S-Au bonding. We discuss the geometry dependence of the transport properties by analyzing the density of states of the molecular orbitals.

Adsorption of benzene thiolate on the (111) surface of M ( M=Pt, Ag, Cu) and the conductance of M/benzene dithiolate/ M molecular junctions: a first-principles study

To seek more conductive and robust molecular wire junction than that based on Au electrodes, we studied the adsorption of a benzene thiolate (–S–C 6H 5) on the (111) surface of Pt, Ag, and Cu, and the zero-bias conductance of a benzene dithiolate (–S–C 6H 4–S–) between Pt, Ag, or Cu electrodes by density functional theory. For Pt, the fcc–hollow site is found to be the most stable adsorption site, different from noble metal cases where the fcc–bridge site has the largest adsorption energy. The binding energy of a benzene thiolate on Pt(111) is larger than on noble metal surfaces. We evaluated the transmission coefficients by solving the Lippmann–Schwinger equation and determined the conductance of the molecular wire junction with the Landauer formula. At zero bias, the conductance for a molecular wire supported on Pt contact is higher than those supported on noble metals.