Molecule-interface coupling effects on electronic transport in molecular wires

Molecular ensemble junctions with inter-molecular quantum interference

We report of a high yield method to form nanopore molecular ensembles junctions containing ~40,000 molecules, in which the semimetal bismuth (Bi) is a top contact. Conductance histograms of these junctions are double-peaked (bi-modal), a behavior that is typical for single molecule junctions but not expected for junctions with thousands of molecules. This unique observation is shown to result from a new form of quantum interference that is inter-molecular in nature, which occurs in these junctions since the very long coherence length of the electrons in Bi enables them to probe large ensembles of molecules while tunneling through the junctions. Under such conditions, each molecule within the ensembles becomes an interference path that modifies via its tunneling phase the electronic structure of the entire junction. This new form of quantum interference holds a great promise for robust novel conductance effects in practical molecular junctions.

Quantum interference effect in the conductance of single molecule junctions has been attracting intensive interest in recent years. Here, Li and Selzer show the presence of intermolecular quantum interference over 40,000 molecules in a molecular ensemble junction with bismuth as the top electrode.

Efficient Intermolecular Charge Transport in π-Stacked Pyridinium Dimers Using Cucurbit[8]uril Supramolecular Complexes

Intermolecular charge transport through π-conjugated molecules plays an essential role in biochemical redox processes and energy storage applications. In this work, we observe highly efficient intermolecular charge transport upon dimerization of pyridinium molecules in the cavity of a synthetic host (cucurbit[8]uril, CB[8]). Stable, homoternary complexes are formed between pyridinium molecules and CB[8] with high binding affinity, resulting in an offset stacked geometry of two pyridiniums inside the host cavity. The charge transport properties of free and dimerized pyridiniums are characterized using a scanning tunneling microscope-break junction (STM-BJ) technique. Our results show that π-stacked pyridinium dimers exhibit comparable molecular conductance to isolated, single pyridinium molecules, despite a longer transport pathway and a switch from intra- to intermolecular charge transport. Control experiments using a CB[8] homologue (cucurbit[7]uril, CB[7]) show that the synthetic host primarily serves to facilitate dimer formation and plays a minimal role on molecular conductance. Molecular modeling using density functional theory (DFT) reveals that pyridinium molecules are planarized upon dimerization inside the host cavity, which facilitates charge transport. In addition, the π-stacked pyridinium dimers possess large intermolecular LUMO-LUMO couplings, leading to enhanced intermolecular charge transport. Overall, this work demonstrates that supramolecular assembly can be used to control intermolecular charge transport in π-stacked molecules.

Comparing statistical predictions of quantum particle transit times in molecular systems to experimental measurements

The movement of quantum particles between distinct spatial regions is an essential feature of nanoscale devices. Consequently, theoretical methods for characterizing the transit time associated with this movement may aid in identifying and refining nanoscale systems with desirable transport properties. Herein, we explore the utility and range of validity of a recently reported probabilistic method for quantifying the timescale of quantum particle transit. The method is applied to intramolecular proton transfer in dicarbonyl compounds, and electron transfer in donor-bridge-acceptor molecules. Direct comparison is made between statistical predictions of proton and electron transfer times and corresponding transfer times deduced from the previously reported experimental observables. Insights provided by the method into the path of flow of probability density are discussed.

Unconventional Current Scaling and Edge Effects for Charge Transport through Molecular Clusters

Metal-molecule-metal junctions are the key components of molecular electronics circuits. Gaining a microscopic understanding of their conducting properties is central to advancing the field. In the present contribution, we highlight the fundamental differences between single-molecule and ensemble junctions focusing on the fundamentals of transport through molecular clusters. In this way, we elucidate the collective behavior of parallel molecular wires, bridging the gap between single molecule and large-area monolayer electronics, where even in the latter case transport is usually dominated by finite-size islands. On the basis of first-principles charge-transport simulations, we explain why the scaling of the conductivity of a junction has to be distinctly nonlinear in the number of molecules it contains. Moreover, transport through molecular clusters is found to be highly inhomogeneous with pronounced edge effects determined by molecules in locally different electrostatic environments. These effects are most pronounced for comparably small clusters, but electrostatic considerations show that they prevail also for more extended systems.

Quantum interference in multi-branched molecules: The exact transfer matrix solutions

We present a transfer matrix formalism for studying quantum interference in a single molecule electronic system with internal branched structures. Based on the Schrödinger equation with the Bethe ansatz and employing Kirchhoff's rule for quantum wires, we derive a general closed-form expression for the transmission and reflection amplitudes of a two-port quantum network. We show that the transport through a molecule with complex internal structures can be reduced to that of a single two-port scattering unit, which contains all the information of the original composite molecule. Our method allows for the calculation of the transmission coefficient for various types of individual molecular modules giving rise to different resonant transport behaviors such as the Breit-Wigner, Fano, and Mach-Zehnder resonances. As an illustration, we first re-derive the transmittance of the Aharonov-Bohm ring, and then we apply our formulation to N identical parity-time (PT)-symmetric potentials, connected in series as well as in parallel. It is shown that the spectral singularities and PT-symmetric transitions of single scattering cells may be observed in coupled systems. Such transitions may occur at the same or distinct values of the critical parameters, depending on the connection modes under which the scattering objects are coupled.

Shot Noise of Charge and Spin Current of a Quantum Dot Coupled to Semiconductor Electrodes

On the basis of the scattering matrix theory and nonequilibrium green function method, we have investigated the fluctuations of charge and spin current of the systems that consist of a quantum dot (QD) with a resonant level coupled to two semiconductor contacts within in alternative site (AS) and alternative bond (AB) framework, where two transverse (Bx) and longitudinal (Bz) magnetic fields are applied to the QD. It is only necessary to use the autocorrelation function to characterize the fluctuations of charge current for a twoterminal system because of the relation that is defined as Σα e Sαβ = Σβ e Sαβ = 0. Our result shows that both auto-shot noise (SLL) and cross-shot noise (SLR) are essential to characterize the fluctuations of spin current when Bx is present. Moreover, our model calculations show that the sign of the cross-shot noise of spin current is negative for all surface states of AS/QD/AS junctions, whereas it oscillates between positive and negative values for two surface states of AB/QD/AB junctions as we sweep the gate voltage.

Molecule-lead coupling at molecular junctions: relation between the real- and state-space perspectives

We present insights into the lead-molecule coupling scheme in molecular electronics junctions. Using a "site-to-state" transformation that provides direct access to the coupling matrix elements between the molecular states and the eigenstate manifold of each lead, we find coupling bands whose character depends on the geometry and dimensionality of the lead. We use a standard tight-binding model to elucidate the origin of the coupling bands and explain their nature via simple "particle-in-a-box" type considerations. We further show that these coupling bands can shed light on the charge transport behavior of the junction. The picture presented in this study is not limited to the case of molecular electronics junctions and is relevant to any scenario where a finite molecular entity is coupled to a (semi)infinite system.

Impact of Anchoring Groups on Ballistic Transport: Single Molecule vs Monolayer Junctions

Tuning the transport properties of molecular junctions by chemically modifying the molecular structure is one of the key challenges for advancing the field of molecular electronics. In the present contribution, we investigate current-voltage characteristics of differently linked metal-molecule-metal systems that comprise either a single molecule or a molecular assembly. This is achieved by employing density functional theory in conjunction with a Green's function approach. We show that the conductance of a molecular system with a specific anchoring group is fundamentally different depending on whether a single molecule or a continuous monolayer forms the junction. This is a consequence of collective electrostatic effects that arise from dipolar elements contained in the monolayer and from interfacial charge rearrangements. As a consequence of these collective effects, the "ideal" choice for an anchoring group is clearly different for monolayer and single molecule devices. A particularly striking effect is observed for pyridine-docked systems. These are subject to Fermi-level pinning at high molecular packing densities, causing an abrupt increase of the junction current already at small voltages.

Rethinking first-principles electron transport theories with projection operators: the problems caused by partitioning the basis set

We revisit the derivation of electron transport theories with a focus on the projection operators chosen to partition the system. The prevailing choice of assigning each computational basis function to a region causes two problems. First, this choice generally results in oblique projection operators, which are non-Hermitian and violate implicit assumptions in the derivation. Second, these operators are defined with the physically insignificant basis set and, as such, preclude a well-defined basis set limit. We thus advocate for the selection of physically motivated, orthogonal projection operators (which are Hermitian) and present an operator-based derivation of electron transport theories. Unlike the conventional, matrix-based approaches, this derivation requires no knowledge of the computational basis set. In this process, we also find that common transport formalisms for nonorthogonal basis sets improperly decouple the exterior regions, leading to a short circuit through the system. We finally discuss the implications of these results for first-principles calculations of electron transport.

Cooperative effects in inelastic tunneling

Several aspects of intermolecular effects in molecular conduction have been studied in recent years. These experimental and theoretical studies, made on several setups of molecular conduction junctions, have focused on the current-voltage characteristic that is usually dominated by the elastic transmission properties of such junctions. In this paper, we address cooperative intermolecular effects in the inelastic tunneling signal calculated for simple generic models of such systems. We find that peak heights in the inelastic (d(2)I/dE(2) vs E) spectrum may be affected by such cooperative effects even when direct intermolecular interactions can be disregarded. This finding suggests that comparing experimental results to calculations made on single-molecule junctions should be done with care.

Probing the conductance superposition law in single-molecule circuits with parallel paths

According to Kirchhoff's circuit laws, the net conductance of two parallel components in an electronic circuit is the sum of the individual conductances. However, when the circuit dimensions are comparable to the electronic phase coherence length, quantum interference effects play a critical role, as exemplified by the Aharonov-Bohm effect in metal rings. At the molecular scale, interference effects dramatically reduce the electron transfer rate through a meta-connected benzene ring when compared with a para-connected benzene ring. For longer conjugated and cross-conjugated molecules, destructive interference effects have been observed in the tunnelling conductance through molecular junctions. Here, we investigate the conductance superposition law for parallel components in single-molecule circuits, particularly the role of interference. We synthesize a series of molecular systems that contain either one backbone or two backbones in parallel, bonded together cofacially by a common linker on each end. Single-molecule conductance measurements and transport calculations based on density functional theory show that the conductance of a double-backbone molecular junction can be more than twice that of a single-backbone junction, providing clear evidence for constructive interference.

Inelastic tunneling spectroscopy of gold-thiol and gold-thiolate interfaces in molecular junctions: the role of hydrogen

It is widely believed that when a molecule with thiol (S-H) end groups bridges a pair of gold electrodes, the S atoms bond to the gold and the thiol H atoms detach from the molecule. However, little is known regarding the details of this process, its time scale, and whether molecules with and without thiol hydrogen atoms can coexist in molecular junctions. Here, we explore theoretically how inelastic tunneling spectroscopy (IETS) can shed light on these issues. We present calculations of the geometries, low bias conductances, and IETS of propanedithiol and propanedithiolate molecular junctions with gold electrodes. We show that IETS can distinguish between junctions with molecules having no, one, or two thiol hydrogen atoms. We find that in most cases, the single-molecule junctions in the IETS experiment of Hihath et al. [Nano Lett. 8, 1673 (2008)] had no thiol H atoms, but that a molecule with a single thiol H atom may have bridged their junction occasionally. We also consider the evolution of the IETS spectrum as a gold STM tip approaches the intact S-H group at the end of a molecule bound at its other end to a second electrode. We predict the frequency of a vibrational mode of the thiol H atom to increase by a factor ~2 as the gap between the tip and molecule narrows. Therefore, IETS should be able to track the approach of the tip towards the thiol group of the molecule and detect the detachment of the thiol H atom from the molecule when it occurs.

CHARGE TRANSPORT THROUGH THIOPHENE BITHIOL MOLECULE AS A MOLECULAR WIRE

The conductance properties of the thiophene bithiol molecular wire, a nano-wire connecting two metallic electrodes, were investigated using quantum-mechanical based methods such as Density Functional Theory, in conjunction with non-equilibrium Green's function formalism. Using the quantum mechanics methods, the Hamiltonians of the three main parts of system, i.e. the right lead, the device, the left lead and conductance properties of this molecular wire such as I-V curve, were calculated.

Molecular conduction through adlayers: cooperative effects can help or hamper electron transport

We use a one-electron, tight-binding model of a molecular adlayer sandwiched between two metal electrodes to explore how cooperative effects between molecular wires influence electron transport through the adlayer. When compared to an isolated molecular wire, an adlayer exhibits cooperative effects that generally enhance conduction away from an isolated wire's resonance and diminish conductance near such a resonance. We also find that the interwire distance (related to the adlayer density) is a key quantity. Substrate-mediated coupling induces most of the cooperative effects in dense adlayers, whereas direct, interwire coupling (if present) dominates in sparser adlayers. In this manner, cooperative effects through dense adlayers cannot be removed, suggesting an optimal adlayer density for maximizing conduction.

Molecular electronic junction transport: some pathways and some ideas

When a single molecule, or a collection of molecules, is placed between two electrodes and voltage is applied, one has a molecular transport junction. We discuss such junctions, their properties, their description, and some of their applications. The discussion is qualitative rather than quantitative, and focuses on mechanism, structure/function relations, regimes and mechanisms of transport, some molecular regularities, and some substantial challenges facing the field. Because there are many regimes and mechanisms in transport junctions, we will discuss time scales, geometries, and inelastic scattering methods for trying to determine the properties of molecules within these junctions. Finally, we discuss some device applications, some outstanding problems, and some future directions.

A core-shell strategy for constructing a single-molecule junction

Understanding the effects of intermolecular interactions on the charge-transport properties of metal/molecule/metal junctions is an important step towards using individual molecules as building blocks for electronic devices. This work reports a systematic electron-transport investigation on a series of "core-shell"-structured oligo(phenylene ethynylene) (Gn-OPE) molecular wires. By using dendrimers of different generations as insulating "shells", the intermolecular π-π interactions between the OPE "cores" can be precisely controlled in single-component monolayers. Three techniques are used to evaluate the electron-transport properties of the Au/Gn-OPE/Au molecular junctions, including crossed-wire junction, scanning tunneling spectroscopy (STS), and scanning tunneling microscope (STM) break-junction techniques. The STM break-junction measurement reveals that the electron-transport pathways are strongly affected by the size of the side groups. When the side groups are small, electron transport could occur through three pathways, including through single-molecule junctions, double-molecule junctions, and molecular bridges between adjacent molecules formed by aromatic π-π coupling. The dendrimer shells effectively prohibit the π-π coupling effect, but at the same time, very large dendrimer side groups may hinder the formation of Au-S bonds. A first-generation dendrimer acts as an optimal shell that only allows electron transport through the single-molecule junction pathway, and forbids the other undesired pathways. It is demonstrated that the dendrimer-based core-shell strategy allows the single-molecule conductance to be probed in a homogenous monolayer without the influence of intermolecular π-π interactions.

Closed-form Green functions, surface effects, and the importance of dimensionality in tight-binding metals

Closed-form expressions for all elements of a d-dimensional tight-binding metal's Green function matrix are presented and used to explore edge effects of a surface. We find that, when moving from the surface into the bulk, the number of layers passed before the surfaced substrate behaves like the bulk decreases with dimensionality. In particular, the surface of a one-dimensional substrate becomes indistinguishable from the bulk after O(10(1)-10(2)) layers, a two-dimensional substrate after O(10(1)) layers, and a three-dimensional substrate after O(10(0)) layers. Finally, the effects of substrate dimensionality on molecule-substrate interactions are discussed.

Molecular transport junctions with semiconductor electrodes: analytical forms for one-dimensional self-energies

Analytical self-energies for molecular interfaces with one-dimensional, tight-binding semiconductors are derived, along with analytical solutions to the electrode eigensystems. These models capture the fundamental differences between the transport properties of metals and semiconductors and also account for the appearance of surface states. When the models are applied to zero-temperature electrode-molecule-electrode conductance, junctions with two semiconductor electrodes exhibit a minimum bias threshold for generating current due to the absence of electrode states near the Fermi level. Molecular interactions with semiconductor electrodes additionally produce (i) non-negligible molecular-level shifting by mechanisms absent in metals and (ii) sensitivity of the transport to the semiconductor-molecule bonding configuration. Finally, the general effects of surface states on molecular transport are discussed.

Substrate-dependent properties of polydiacetylene nanowires on graphite and MoS2

Scanning tunneling microscopy (STM) has been used to study polydiacetylene (PDA) nanowires and their electronic coupling with the surface. PDA nanowires exhibit intriguing substrate-dependent electronic effects when probed at varying sample bias voltage conditions on different substrate electrode materials, in this case, highly ordered pyrolytic graphite (HOPG) and molybdenum disulfide (MoS(2)). An analysis of nanowire heights over a wide range of bias voltages shows strong polymer-substrate contact effects, the strength of which is reflected in the asymmetry of the height-voltage data on each substrate. On HOPG, PDA nanowires exhibit a decrease in height as the bias voltage magnitude is reduced, and the height is substantially greater at negative voltages than at positive voltages. On MoS(2), PDA nanowires appear with much higher contrast than on HOPG when imaged at the same negative bias conditions. At positive bias voltages on MoS(2), the nanowires are invisible in all STM images, yet the unpolymerized molecules can still be imaged. These effects are necessarily electronic in origin. Surprisingly, only the polymer nanowires exhibit any bias-dependent change; the unpolymerized molecules are imaged at all bias voltages on both substrates. Additionally, the substrate affects how the unpolymerized molecules are ordered. In some areas, the molecules are arranged such that part of the monolayer is offset from the correct threefold symmetry direction by a slight misfit angle. On HOPG, this misfit is approximately 6 degrees, while on MoS(2), it is approximately 11 degrees. Interactions with the substrate thus play a role both in electronic structure and in molecular alignment.

Ultraviolet-visible spectroelectrochemistry of chemisorbed molecular layers on optically transparent carbon electrodes

Pyrolysis of diluted commercial photoresist spun onto quartz slides yields optically transparent graphitic films. Transparent carbon electrodes approximately 6 nm thick can be reproducibly prepared, with a maximum absorbance in the ultraviolet-visible (UV-vis) range of 0.25 at 270 nm. These electrodes are sufficiently conductive for electrochemistry, enabling modification of the surface via diazonium ion reduction and spectroelectrochemistry. Good quality ultraviolet-visible absorption spectra of covalently bonded molecular layers of nitroazobenzene, nitrobiphenyl, and azobenzene, with thicknesses of 1.4-4 nm, were obtained after subtracting the spectrum of the unmodified substrate. The spectra of all three molecules immobilized on the carbon surface showed red shifts of the absorption maxima relative to a solution of free molecules, indicating substantial electronic interactions between chemisorbed molecules and the Pi system of the substrate and/or intermolecular coupling. Spectroelectrochemical measurements show that reduction of free and chemisorbed molecules produce new absorption features in the 500-800 nm range; these spectral changes are partially reversible upon repeated potential cycling. Finally, density functional calculations correlate the new bands to the formation of anion radical or "methide" species that have more extensive electron delocalization than the parent molecules. The results from this work are useful for linking structural transformations in molecular layers "buried" under conductive top contacts in a type of molecular junction to changes in the electronic properties of the junction.

Configuration-dependent interface charge transfer at a molecule-metal junction

The role of the molecule-metal interface is a key issue in molecular electronics. Interface charge transfer processes for 4-fluorobenzenethiol monolayers with different molecular orientations on Au(111) were studied by resonant photoemission spectroscopy. The electrons excited into the LUMO or LUMO+1 are strongly localized for the molecules standing up on Au(111). In contrast, an ultrafast charge transfer process was observed for the molecules lying down on Au(111). This configuration-dependent ultrafast electron transfer is dominated by an adiabatic mechanism and directly reflects the delocalization of the molecular orbitals for molecules lying down on Au(111). Theoretical calculations confirm that the molecular orbitals indeed experience a localization-delocalization transition resulting from hybridization between the molecular orbitals and metal surface. Such an orientation-dependent transition could be harnessed in molecular devices that switch via charge transfer when the molecular orientation is made to change.

Novel organic materials through control of multichromophore interactions

The function of organic semiconducting and light-harvesting materials depends on the organization of the individual molecular components. Our group has tackled the problem of through-space delocalization via the design and synthesis of bichromphoric pairs held in close proximity by the [2.2]paracyclophane core. The linear and nonlinear optical properties of these molecules provide a challenge to theory. They are also useful in delineating the problem of intermolecular contacts in molecular conductivity measurements. Another area of research described here concerns conjugated polyelectrolytes. These macromolecules combine the properties of organic semiconductors and conventional polyelectrolytes. We have used these materials in the development of optically amplified biosensors and have also incorporated them into organic optoelectronic devices. Of particular interest to us is to derive useful structure/property relationships via molecular design that address important basic scientific problems and technological challenges.

Strong Intermolecular Electronic Coupling within a Tetrathiafulvalene Island Embedded in Self-Assembled Monolayers

Electroactive tetrathiafulvalene thiol, specially designed to pursue an intermolecular electronic coupling, was embedded in an n-alkanethiol SAM matrix as islands and was studied under potential control using in situ scanning tunneling microscopy. The apparent height of the islands increased with the island size, irrespective of the oxidation state of the tetrathiafulvalene backbones. This behavior can be rationalized on the basis of the strong intermolecular electronic coupling that creates efficient intermolecular conduction paths.

Interfacial bridge-mediated electron transfer: mechanistic analysis based on electrochemical kinetics and theoretical modelling

Understanding the physical and chemical factors that control the kinetics of interfacial electron-transfer (ET) reactions is important for a large number of technological applications. The present article describes electrochemical kinetic studies of these factors, in which standard interfacial ET rate constants (k(0)(l)) have been measured for ET between substrate Au electrodes and various redox couples attached to the electrode surfaces by variable lengths (l) of oligomethylene (OM), oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) bridges, which were constituents of mixed self-assembled monolayers (SAMs). The k(0)(l) measurements employed the indirect laser-induced temperature jump (ILIT) technique, which permits the measurement of interfacial ET rates that are orders of magnitude faster than those measurable by conventional techniques using the macroelectrodes that are the most convenient substrates for the mixed SAMs. The robustness of the measured rate constants (k(0)(l)), together with the Arrhenius activation energies (E(a)(l)) and preexponential factors (A(l)), is demonstrated by their invariance with respect to several experimental system parameters (including the chemical nature and length of the diluent component of the mixed SAM). Analysis of the kinetic results demonstrates that all of the observed interfacial ET processes proceed through a common type of transition state (predominantly associated with solvent reorganization around the redox moiety) and that the actual ET step involves direct electronic tunnelling between the Au electrode and the redox moiety. However, for the full range of l investigated, a global exponential decay of A(l) is not found for any of the three types of bridges. Possible reasons for this behavior, including the role of rate determining steps associated with adiabatic mechanisms within or beyond the transition state theory framework, are discussed, and comparisons with related conductance measurements are presented.

How important is the interfacial chemical bond for electron transport through alkyl chain monolayers?

We show that the electrode/molecule chemical bond does not change the tunneling barrier for charge transport through alkyl chain monolayers sandwiched between Si and Hg electrodes. This observation can be understood if the interfacial bond mainly governs the monolayer's structure, while the electronic states due to molecule-electrode bonding do not contribute significantly to tunneling. Yet, the nature of the bond affects the Schottky barrier inside the semiconductor due to changes in the interface dipole.

Surface/interface electronic structure in C(60) anchored aminothiolate self-assembled monolayer: an approach to molecular electronics

Electronic structure in self-assembled monolayers (SAMs) of C(60) anchored 11-amino-1-undecane thiol (C(60)-11-AUT) on Au(111) was studied by means of ultraviolet photoelectron spectroscopy and hybrid density functional theory calculations. Valence band features of the molecular conformation revealed the interface electronic structure to be dominated by sigma(S-Au), localized at the thiolate anchor to Au. Formation of a localized covalent bond as a result of hybridization between N P(z) orbital of -NH(2) group of the thiolate SAM and the pi level of C(60) resulted in a symmetry change from I(h) in C(60) to C1 in C(60)-11-AUT SAM. Appearance of low, but finite amplitude surface electronic states of bonded C(60), much beyond the Fermi level, ruled out Au-C(60) end group contact. The band gap E(g) of the SAM, determined to be 2.7 eV, was drastically reduced from the insulating alkanethiol SAMs ( approximately 8.0 eV) and fell intermediate between the C(60) ground state (N electrons, 1.6 eV) and C(60) solid (N+/-1 electrons, 3.7 eV).

Simultaneous Nanoindentation and Electron Tunneling through Alkanethiol Self-Assembled Monolayers

Electrical tunnel junctions consisting of alkanethiol molecules self-assembled on Au-coated Si substrates and contacted with Au-coated atomic force microscopy tips were characterized under varying junction loads in a conducting-probe atomic force microscopy configuration. Junction load was cycled in the fashion of a standard nanoindentation experiment; however, junction conductance rather than probe depth was measured directly. The junction conductance data have been analyzed with typical contact mechanics (Derjaguin-Müller-Toporov) and tunneling equations to extract the monolayer modulus (approximately 50 GPa), the contact transmission (approximately 2 x 10(-6)), contact area, and probe depth as a function of load. The monolayers are shown to undergo significant plastic deformation under compression, yielding indentations approximately 7 Angstroms deep for maximum junction loads of approximately 50 nN. Comparison of mechanical properties for different chain lengths was also performed. The film modulus decreased with the number of carbons in the molecular chain for shorter-chain films. This trend abruptly reversed once 12 carbons were present along the backbone.

Novel molecular device based on electrostatic interactions in organic polymers

A number of researchers have reported attempts to design molecular level devices. One approach is to make use of electrostatic interactions in different parts of a polymeric molecule. This paper reports a means to achieve this by adding space charge to a molecule consisting of symmetric and asymmetric subgroups. Physically, space charge residing in a subgroup produces a dipolar charge layer thereby creating a potential trough in the polymer backbone. By lifting or lowering this potential minimum, it is possible to modify the terminal current. The effect of space charge on the potential profile in the polymer backbone was examined and the change correlated to data on carrier mobilities for OC1C10-PPV reported in the literature. Modulation of space charge in the subgroup allows the manipulation of current flow along the polymer backbone, forming the basis for the development of a molecular device. A first-order analysis suggested that such a device could have current-voltage (I-V) characteristics similar to those of a MOSFET at subthreshold, with an estimated transconductance approximately 1-2 pAV and a cutoff frequency approximately 10(15) Hz.

Probing pi-coupling in molecular junctions

Charge transport characteristics for metal-molecule-metal junctions containing two structurally related pi-conjugated systems were studied to probe pi-pi interactions in molecular junctions. The first molecule contains a typical pi-conjugated framework derived from phenylene vinylene units, whereas the second has the phenylene vinylene structure interrupted by a [2.2]paracyclophane (pCp) core. Electrochemical investigations were used to characterize the defects and packing density of self-assembled monolayers of the two molecules on gold surfaces and to enable quantitative comparison of their transport characteristics. Current-voltage measurements across molecular junctions containing the two species demonstrate that the pCp moiety yields a highly conductive break in through-bond pi-conjugation. The observed high conductivity is consistent with density functional theory calculations, which demonstrate strong through-space pi-pi coupling across the pCp moiety.

Electrical addressing of confined quantum systems for quasiclassical computation and finite state logic machines

Conduction spectroscopy measures the current I through a nanosystem as a function of the voltage V between two electrodes. The differential conductance, dI/dV, has peaks that can be assigned to resonance conditions with different electronic levels of the system. Between these increments, the current has roughly constant plateaus. We discuss how measurements of the current vs. voltage can be used to perform Boolean operations and hence construct finite state logic machines and combinational circuits. The inputs to the device are the source-drain voltage, including its sign, and a gate voltage applied in a manner analogous to optical Stark spectroscopy. As simple examples, we describe a two-state set-reset machine (a machine whose output depends on the input and also on its present state) and a full adder circuit (a circuit that requires three inputs and provides two outputs).