Thermal gating of the single molecule conductance of alkanedithiols

Single-Molecule Conductance of Staffanes

We report the first conductance measurements of [n]staffane (bicyclopentane) oligomers in single-molecule junctions. Our studies reveal two quantum transport characteristics unique to staffanes that emerge from their strained bicyclic structure. First, though staffanes are composed of weakly conjugated C-C σ-bonds, staffanes carry a shallower conductance decay value (β=0.84±0.02 n-1) than alkane chain analogs (β=0.96±0.03 n-1) when measured with the scanning tunneling microscopy break junction (STM-BJ) technique. Staffanes are thus more conductive than other σ-bonded organic backbones reported in the literature on a per atom basis. Density functional theory (DFT) calculations suggest staffane backbones are more effective conduits for charge transport because their significant bicyclic ring strain destabilizes the HOMO-2 energy, aligning it more closely with the Fermi energy of gold electrodes as oligomer order increases. Second, the monostaffane is significantly lower conducting than expected. DFT calculations suggest that short monostaffanes sterically enforce insulating gauche interelectrode orientations over syn orientations; these steric effects are alleviated in longer staffanes. Moreover, we find that [2-5]staffane wires may accommodate axial mechanical strain by "rod-bending". These findings show for the first time how bicyclic ring strain can enhance charge transmission in saturated molecular wires. These studies showcase the STM-BJ technique as a valuable tool for uncovering the stereoelectronic proclivities of molecules at material interfaces.

Enhanced charge transport across molecule-nanoparticle-molecule sandwiches

The electrical properties of large area molecular devices consisting of gold nanoparticles (GNPs) sandwiched between a double layer of alkanedithiol linkers have been examined. These devices have been fabricated by a facile bottom-up assembly in which an alkanedithiol monolayer is first self-assembled on an underlying gold substrate followed by nanoparticle adsorption and then finally assembly of the top alkanedithiol layer. These devices are then sandwiched between the bottom gold substrates and a top eGaIn probe contact and current-voltage (I-V) curves recorded. Devices have been fabricated with 1,5-pentanedithiol, 1,6-hexanedithiol, 1,8-octanedithiol and 1,10-decanedithiol linkers. In all cases the electrical conductance of the double SAM junctions with GNPs is higher than the corresponding and much thinner single alkanedithiol SAM. Competing models for this enhanced conductance are discussed and it is suggested to have a topological origin arising from how the devices assemble or structure during the fabrication, which gives more efficient cross device electron transport pathways without the GNPs producing short circuits.

Structural Order of the Molecular Adlayer Impacts the Stability of Nanoparticle-on-Mirror Plasmonic Cavities

Immense field enhancement and nanoscale confinement of light are possible within nanoparticle-on-mirror (NPoM) plasmonic resonators, which enable novel optically activated physical and chemical phenomena and render these nanocavities greatly sensitive to minute structural changes, down to the atomic scale. Although a few of these structural parameters, primarily linked to the nanoparticle and the mirror morphology, have been identified, the impact of molecular assembly and organization of the spacer layer between them has often been left uncharacterized. Here, we experimentally investigate how the complex and reconfigurable nature of a thiol-based self-assembled monolayer (SAM) adsorbed on the mirror surface impacts the optical properties of the NPoMs. We fabricate NPoMs with distinct molecular organizations by controlling the incubation time of the mirror in the thiol solution. Afterward, we investigate the structural changes that occur under laser irradiation by tracking the bonding dipole plasmon mode, while also monitoring Stokes and anti-Stokes Raman scattering from the molecules as a probe of their integrity. First, we find an effective decrease in the SAM height as the laser power increases, compatible with an irreversible change of molecule orientation caused by heating. Second, we observe that the nanocavities prepared with a densely packed and more ordered monolayer of molecules are more prone to changes in their resonance compared to samples with sparser and more disordered SAMs. Our measurements indicate that molecular orientation and packing on the mirror surface play a key role in determining the stability of NPoM structures and hence highlight the under-recognized significance of SAM characterization in the development of NPoM-based applications.

A photo-induced spin crossover based molecular switch and spin filter operating at room temperature

Since Venkataramani et al. (Science, 2011, 331(6016), 445-448) reported reversible, room-temperature light-induced spin crossover in Ni-porphyrin functionalized with a phenylazopyridine ligand (NiTPP-PAPy), this complex has attracted the attention of many researchers due to its potential applications in molecular-based devices. In this work, we perform a detailed study, by means of DFT and WFT methodologies, focused on the deposition of NiTP-PAPy over an Au(111) surface, followed by DFT-NEGF calculations employing a gold surface and the tip of an STM as electrodes, in order to probe the deposited complex's transport properties. Our DFT calculations show that not only the metalled porphyrin is strongly adsorbed on the surface, in both the high (HS) and low spin (LS) configurations, but also, and more importantly, photoinduced switching is preserved upon adsorption, a fact that is also confirmed through WFT and TD-DFT calculations. Moreover, our DFT-NEGF calculations indicate that the current passing through the molecular junction-like systems is much higher in the HS configuration than in the LS one, along with the fact that the current calculated in the ferromagnetic junction is highly spin-polarized. These remarkable transport properties suggest that the complex could be used as a component in molecular switches based on the total current passing through the system, modulated by light irradiation, spin filters due to the spin polarization of the carriers in the HS configuration, or even in two-step rectifiers combining the two features mentioned above, all of these operating at room temperature, giving to this complex the potential to be an active element in all kinds of future spintronic devices.

Temperature-Dependent Tunneling in Furan Oligomer Single-Molecule Junctions

Two commonly observed charge transport mechanisms in single-molecule junctions are coherent tunneling and incoherent hopping. It has been generally believed that tunneling processes yield temperature-independent conductance behavior and hopping processes exhibit increasing conductance with increasing temperature. However, it has recently been proposed that tunneling can also yield temperature-dependent transport due to the thermal broadening of the Fermi energy of the contacts. In this work, we examine a series of rigid, planar furan oligomers that are free from a rotational internal degree of freedom to examine the temperature dependence of tunneling transport directly over a wide temperature range (78-300 K). Our results demonstrate conductance transition from a temperature-independent regime to a temperature-dependent regime. By examining various hopping and tunneling models and the correlation between the temperature dependence of conductance and molecular orbital energy offset from the Fermi level, we conclude thermally assisted tunneling is the dominant cause for the onset of temperature-dependent conductance in these systems.

Folding a Single-Molecule Junction

Stimuli-responsive molecular junctions, where the conductance can be altered by an external perturbation, are an important class of nanoelectronic devices. These have recently attracted interest as large effects can be introduced through exploitation of quantum phenomena. We show here that significant changes in conductance can be attained as a molecule is repeatedly compressed and relaxed, resulting in molecular folding along a flexible fragment and cycling between an anti and a syn conformation. Power spectral density analysis and DFT transport calculations show that through-space tunneling between two phenyl fragments is responsible for the conductance increase as the molecule is mechanically folded to the syn conformation. This phenomenon represents a novel class of mechanoresistive molecular devices, where the functional moiety is embedded in the conductive backbone and exploits intramolecular nonbonding interactions, in contrast to most studies where mechanoresistivity arises from changes in the molecule-electrode interface.

Thermally activated charge transport in carbon atom chains

Charge transport through single molecules is at the heart of molecular electronics for realizing the practical use of the rich quantum characteristics of electrode-molecule-electrode systems. Despite the extensive studies reported in the past, little experimental efforts have been focused on the electron transport mechanism at a temperature higher than the ambient temperature. In this work, we have reported the observation of the subtle interplay between electron tunneling and charge hopping in carbon chains connected to two Au electrodes at elevated temperatures. We measured the single-molecule conductance of Au-alkanedithiol-Au molecular junctions at various temperatures from 300 K to 420 K in vacuum. The temperature dependence of conductance suggested substantial roles of superexchange with inter-chain charge hopping under elevated temperatures for alkane chains longer than heptane. This finding provides a guide to design functional molecular junctions under practical conditions.

Solid-State Electron Transport via the Protein Azurin is Temperature-Independent Down to 4 K

Solid-state electronic transport (ETp) via the electron-transfer copper protein azurin (Az) was measured in Au/Az/Au junction configurations down to 4 K, the lowest temperature for solid-state protein-based junctions. Not only does lowering the temperature help when observing fine features of electronic transport, but it also limits possible electron transport mechanisms. Practically, wire-bonded devices-on-chip, carrying Az-based microscopic junctions, were measured in liquid He, minimizing temperature gradients across the samples. Much smaller junctions, in conducting-probe atomic force microscopy measurements, served, between room temperature and the protein's denaturation temperature (∼323 K), to check that conductance behavior is independent of device configuration or contact nature and thus is a property of the protein itself. Temperature-independent currents were observed from ∼320 to 4 K. The experimental results were fitted to a single-level Landauer model to extract effective energy barrier and electrode-molecule coupling strength values and to compare data sets. Our results strongly support that quantum tunneling, rather than hopping, dominates ETp via Az.

Charge transport in hybrid platinum/molecule/graphene single molecule junctions

The single molecule conductance of hybrid platinum/alkanedithiol/graphene junctions has been investigated with a focus on understanding the influence of employing two very different contact types. We call this an "anti-symmetric" configuration, with the two different contacts here being platinum and graphene, which respectively provide very different electronic coupling to the alkanedithiol bridge. The conductance of these junctions is experimentally investigated by using a non-contact scanning tunneling microscopy (STM) based method called the I(s) technique. These experimental determinations are supported by density functional theory (DFT) calculations. These alkanedithiol bridging molecules conduct electric current through the highest occupied molecular orbital (HOMO), and junctions formed with Pt/graphene electrode pairs are slightly more conductive than those formed with Au/graphene electrodes which we previously investigated. This is consistent with the lower work function of gold than that of platinum. The measured conductance decays exponentially with the length of the molecular bridge with a low tunneling decay constant, which has a similar value for Pt/graphene and Au/graphene electrode pairs, respectively. These new results underline the importance of the coupling asymmetry between the two electrodes, more than the type of the metal electrode itself. Importantly, the tunneling decay constant is much lower than that of alkanedithiols with the symmetrical equivalent, i.e. identical metal electrodes. We attribute this difference to the relatively weak van der Waals coupling at the graphene interface and the strong bond dipole at the Pt-S interface, resulting in a decrease in the potential barrier at the interface.

Highly Reproducible Formation of a Polymer Single-Molecule Junction for a Well-Defined Current Signal

Single-molecule devices attract much interest in the development of nanoscale electronics. Although a variety of functional single molecules for single-molecule electronics have been developed, there still remains the need to implement sophisticated functionalization toward practical applications. Given its superior functionality encountered in macroscopic materials, a polymer could be a useful building block in the single-molecule devices. Therefore, a molecular junction composed of polymer has now been created. Furthermore, an automated algorithm was developed to quantitatively analyze the tunneling current through the junction. Quantitative analysis revealed that the polymer junction exhibits a higher formation probability and longer lifetime than its monomer counterpart. These results suggest that the polymer provides a unique opportunity to design both stable and highly functional molecular devices for nanoelectronics.

The influence of nearest-neighbour interactions and assembly dynamics on the transport properties of porphyrin supramolecular assemblies on Au(111)

Here we report on the effect of local molecular organization or "tertiary structure" on the charge transport properties of thiol-tethered tetraphenylporphyrin (ZnTPPF₄-SC₅SH) nanoscale clusters of ca. 5 nm in lateral dimension embedded within a dodecanethiol (C12) monolayer on Au(111). The structure of the clusters in the mixed monolayers and their resulting transport properties were monitored by Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM) and Spectroscopy (STS). The mixed films were deposited on Au(111) for a period of one to five days, during which the lateral dimensions of the ZnTPPF₄-SC₅SH islands that were formed after one day reduced by nearly 35% on average by five days, accompanied by a noticeable depletion of the surrounding C12 monolayer. These subtle changes in mixed monolayer morphology were accompanied by drastic differences in conductance. The ZnTPPF₄-SC₅SH clusters assembled for one day exhibited highly reproducible I-V spectra with simple tunneling behavior. By three days, this evolved into bias-induced switching of conductance, with a ∼100-1000 fold increase. Furthermore, current fluctuations started to become significant, and then dominated transport across the ZnTPPF₄-SC₅SH clusters assembled over five days. Our data suggests that this evolution can be understood by slow surface diffusion, enabling the ZnTPPF₄-SC₅SH molecules to overcome initial steric hindrance in the early stages of island formation in the C12 monolayer (at day one), to reach a more energetically-favored, close-packed organization, as noted by the decrease in island size (by day three). However, when desorption of the supporting matrix of C12 became pronounced (by day five), the ZnTPPF₄-SC₅SH clusters began to lose stabilization, and stochastic switching was then observed to dominate transport in the clusters, illustrating the critical nature of the local organization on these transport properties.

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

Metal complexes are receiving increased attention as molecular wires in fundamental studies of the transport properties of metal|molecule|metal junctions. In this context we report the single-molecule conductance of a systematic series of d⁸ square-planar platinum(ii) trans-bis(alkynyl) complexes with terminal trimethylsilylethynyl (C[triple bond, length as m-dash]CSiMe₃) contacting groups, e.g. trans-Pt{C[triple bond, length as m-dash]CC₆H₄C[triple bond, length as m-dash]CSiMe₃}₂(PR₃)₂ (R = Ph or Et), using a combination of scanning tunneling microscopy (STM) experiments in solution and theoretical calculations using density functional theory and non-equilibrium Green's function formalism. The measured conductance values of the complexes (ca. 3-5 × 10^-5G₀) are commensurate with similarly structured all-organic oligo(phenylene ethynylene) and oligo(yne) compounds. Based on conductance and break-off distance data, we demonstrate that a PPh₃ supporting ligand in the platinum complexes can provide an alternative contact point for the STM tip in the molecular junctions, orthogonal to the terminal C[triple bond, length as m-dash]CSiMe₃ group. The attachment of hexyloxy side chains to the diethynylbenzene ligands, e.g. trans-Pt{C[triple bond, length as m-dash]CC₆H₂(Ohex)₂C[triple bond, length as m-dash]CSiMe₃}₂(PPh₃)₂ (Ohex = OC₆H₁₃), hinders contact of the STM tip to the PPh₃ groups and effectively insulates the molecule, allowing the conductance through the full length of the backbone to be reliably measured. The use of trialkylphosphine (PEt₃), rather than triarylphosphine (PPh₃), ancillary ligands at platinum also eliminates these orthogonal contacts. These results have significant implications for the future design of organometallic complexes for studies in molecular junctions.

Observation of Giant Conductance Fluctuations in a Protein

Proteins are insulating molecular solids, yet even those containing easily reduced or oxidized centers can have single-molecule electronic conductances that are too large to account for with conventional transport theories. Here, we report the observation of remarkably high electronic conductance states in an electrochemically-inactive protein, the ~200 kD α_Vβ₃ extracelluar domain of human integrin. Large current pulses (up to nA) were observed for long durations (many ms, corresponding to many pC of charge transfer) at large gap (>5nm) distances in an STM when the protein was bound specifically by a small peptide ligand attached to the electrodes. The effect is greatly reduced when a homologous, weakly-binding protein (α₄β₁) is used as a control. In order to overcome the limitations of the STM, the time- and voltage-dependence of the conductance were further explored using a fixed-gap (5 nm) tunneling junction device that was small enough to trap a single protein molecule at any one time. Transitions to a high conductance (~ nS) state were observed, the protein being "on" for times from ms to tenths of a second. The high-conductance states only occur above ~ 100mV applied bias, and thus are not an equilibrium property of the protein. Nanoamp two-level signals indicate the specific capture of a single molecule in an electrode gap functionalized with the ligand. This offers a new approach to label-free electronic detection of single protein molecules. Electronic structure calculations yield a distribution of energy level spacings that is consistent with a recently proposed quantum-critical state for proteins, in which small fluctuations can drive transitions between localized and band-like electronic states.

Transient electrochemistry: beyond simply temporal resolution

Transient electrochemistry is a powerful method to solve many physicochemical issues.

Single-Molecule Electronics: Chemical and Analytical Perspectives

It is now possible to measure the electrical properties of single molecules using a variety of techniques including scanning probe microcopies and mechanically controlled break junctions. Such measurements can be made across a wide range of environments including ambient conditions, organic liquids, ionic liquids, aqueous solutions, electrolytes, and ultra high vacuum. This has given new insights into charge transport across molecule electrical junctions, and these experimental methods have been complemented with increasingly sophisticated theory. This article reviews progress in single-molecule electronics from a chemical perspective and discusses topics such as the molecule-surface coupling in electrical junctions, chemical control, and supramolecular interactions in junctions and gating charge transport. The article concludes with an outlook regarding chemical analysis based on single-molecule conductance.

Electrical characterization of single molecule and Langmuir-Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

Monolayer Langmuir-Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the "STM touch-to-contact" method have been used to study the nature of metal-monolayer-metal junctions in which the pyridyl group provides the contact at both molecule-surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air-water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10(-9) mol·cm(-2). The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m(-1). The UV-vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the "STM touch-to-contact" method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I-V curves were obtained. A junction conductance of 5.17 × 10(-5) G 0 results from the analysis of the pseudolinear (ohmic) region of the I-V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I-V curve of 1 determined using the I(s) method is in good agreement with the I-V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal-molecule-metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal-monolayer-metal junctions prepared under a number of different conditions.

Binding configurations and intramolecular strain in single-molecule devices

The development of molecular-scale electronic devices has made considerable progress over the past decade, and single-molecule transistors, diodes and wires have all been demonstrated. Despite this remarkable progress, the agreement between theoretically predicted conductance values and those measured experimentally remains limited. One of the primary reasons for these discrepancies lies in the difficulty to experimentally determine the contact geometry and binding configuration of a single-molecule junction. In this Article, we apply a small-amplitude, high-frequency, sinusoidal mechanical signal to a series of single-molecule devices during junction formation and breakdown. By measuring the current response at this frequency, it is possible to determine the most probable binding and contact configurations for the molecular junction at room temperature in solution, and to obtain information about how an applied strain is distributed within the molecular junction. These results provide insight into the complex configuration of single-molecule devices, and are in excellent agreement with previous predictions from theoretical models.

Incorporating single molecules into electrical circuits. The role of the chemical anchoring group

Constructing electronic circuits containing singly wired molecules is at the frontier of electrical device miniaturisation. Understanding the behaviour of different anchoring groups is key to this goal because of their significant role in determining the properties of the junction.

Single-molecule electronics: from chemical design to functional devices

The use of single molecules in electronics represents the next limit of miniaturisation of electronic devices, which would enable to continue the trend of aggressive downscaling of silicon-based electronic devices.

Chain length and temperature dependence of alkanedithiol molecular conductance under ultra high vacuum

We report scanning tunnelling microscope (STM) measurements of the single molecule conductance of α,ω-alkanedithiols for a large range of molecular chain lengths (N = 3-10) and temperatures (180-390 K) under ultra high vacuum. Two STM-based measurement techniques were employed on molecules trapped between tip and substrate: (i) the well established current-distance or I(z) technique and (ii) a new I(V,z) technique in which the current-voltage characteristics are determined as the tip-substrate distance z is varied. Low, medium, and high conductance groups were observed for each molecular length, which were temperature independent over the range examined, consistent with off-resonance tunnelling. For N > 4 the current-voltage characteristics and conductance trend with chain length is well described using a simple rectangular tunnel barrier model with parameters in excellent agreement with previously reported values. However, both 1,3-propanedithiol (N = 3) and 1,4-butanedithiol (N = 4) show an anomalous behaviour which is qualitatively similar to, but much less pronounced than, that reported by Haiss et al. (Phys. Chem. Chem. Phys., 2009, 11, 10831) for measurements performed under air and nitrogen gas.

Tactile-feedback stabilized molecular junctions for the measurement of molecular conductance

Handling the (AFM) tip: The duration of stable molecular junctions was prolonged using a tactile feedback method in which the operator can sense the force of the AFM tip on the sample surface. The movement of the tip is adjusted accordingly, maintaining a more consistent current (i) and voltage (V), instead of having the tip move at a constant preset speed, as in the conventional setup.

Single-Molecule Conductance in a Series of Extended Viologen Molecules

Single-molecule conductance in a series of extended viologen molecules was measured at room temperature using a gold-molecule-gold scanning tunneling microscopy break junction arrangement. Conductance values for individual molecules change from 4.8 ± 1.2 nS for the shortest compound to 2.9 ± 1.0 nS for the compound with six repeating units and length of 11 nm. The latter value is almost 3 orders of magnitude higher than that reported for all-carbon-based aromatic molecular wires of comparable length. On the basis of the length of the molecules, an attenuation factor of only 0.06 ± 0.004 nm(-1) (0.006 ± 0.0004 Å(-1)) was obtained. To the best of our knowledge, this is the smallest value reported for the conductance attenuation in a series of molecular wires.

Force-conductance correlation in individual molecular junctions

Conducting atomic force microscopy is an attractive approach enabling the correlation of mechanical and electrical properties in individual molecular junctions. Here we report on measurements of gold-gold and gold-octanedithiol-gold junctions. We introduce two-dimensional histograms in the form of scatter plots to better analyze the correlation between force and conductance. In this representation, the junction-forming octanedithiol compounds lead to a very clear step in the force-conductance data, which is not observed for control monothiol compounds. The conductance found for octanedithiols is in agreement with the idea that junction conductance is dominated by a single molecule.

Molecular modeling of two-photon absorption and third-order nonlinearities of polymethine dyes for all-optical switching

Stimulated by a recent experimental report [Hales JM et al. (2010) Science 327:1485-1488], two-photon absorption and third-order optical nonlinearities of selenopyrylium- and bis(dioxaborine)-terminated polymethine dyes (called SE-7C and DOB-9C) used for all-optical switching were investigated theoretically with time-dependent DFT (TD-DFT) and response theory as well as visualized real-space analysis. The calculated results for the first hyperpolarizability and second hyperpolarizability demonstrated that the two molecules both have large third-order optical nonlinearities. Using real-space analysis, we were able to visually determine that in the one-photon absorption (OPA) process, the first singlet excited state of SE-7C and DOB-9C is an intramolecular charge transfer (ICT) excited state with strong absorption, while the second excited state of these dyes (also termed the "ICT state") shows weak absorption. However, in the two-photon absorption (TPA) process, a larger TPA absorption cross-section was predicted for the second excited state. In this paper, we describe the properties of the S2 excited state, incorporating charge transfer and the transition moment, via real-space analysis, which was very important for understanding the TPA characteristics of the S(2) state.

Single-molecule conductance determinations on HS(CH2)4O(CH2)4SH and HS(CH2)2O(CH2)2O(CH2)2SH, and comparison with alkanedithiols of the same length

The acetyl-protected, thiol-terminated ethers AcS(CH(2))(4)O(CH(2))(4)SAc and AcS(CH(2))(2)O(CH(2))(2)O(CH(2))(2)SAc have been synthesised, and a range of related scanning tunnelling microscopy (STM)-based methods have been employed to fabricate and electrically characterise gold | single molecule | gold junctions involving these molecules. The single-molecule conductance values obtained are consistently found to be substantially higher (by a factor of 2-3) than the conductances of analogous alkanedithiols of similar length (HS(CH(2))(9)SH and HS(CH(2))(8)SH, respectively). A rationalisation of these findings is suggested, namely that the lone pair electrons on the oxygen atoms are substantially closer in energy to the Fermi energy of the gold leads than are the occupied and unoccupied states of methylene chains, so that the ether oxygens behave in a manner analogous to 'wells' in a double-tunnelling-barrier system. In agreement with this suggestion, the current-voltage behaviour of the monoether can be fitted using the Simmons approach, and the barrier height is found to be significantly lower than for alkanedithiols of approximately the same length.

Long-range electron tunnelling in oligo-porphyrin molecular wires

Short chains of porphyrin molecules can mediate electron transport over distances as long as 5-10 nm with low attenuation. This means that porphyrin-based molecular wires could be useful in nanoelectronic and photovoltaic devices, but the mechanisms responsible for charge transport in single oligo-porphyrin wires have not yet been established. Here, based on electrical measurements of single-molecule junctions, we show that the conductance of the oligo-porphyrin wires has a strong dependence on temperature, and a weak dependence on the length of the wire. Although it is widely accepted that such behaviour is a signature of a thermally assisted incoherent (hopping) mechanism, density functional theory calculations and an accompanying analytical model strongly suggest that the observed temperature and length dependence is consistent with phase-coherent tunnelling through the whole molecular junction.

Detailed analysis of water structure in a solvent mediated electron tunneling mechanism

This work aims at describing the water structure characteristics that influence the electron transfer superexchange mechanism by explicitly calculating the solvent mediated conductance between the donor and acceptor in a generic pair. The method employed here is based on the non-equilibrium Green function formalism for calculating the conductance over solvent trajectories previously determined by molecular dynamics methods. A non-exponential dependence of the conductance is observed with respect to the distance between the donor and the acceptor. Local fluctuations of the solvent structure are responsible for the non-monotonic dependence, mainly due to the formation of solvent bridges that act as a molecular wire connecting the sites. This shortcutting phenomenon is observed for certain ranges of distances between the donor and acceptor in the pair. Charge on the sites strongly affects the local solvent structure and causes qualitative changes in the distance dependence of the tunneling probability.

Electrochemically controlled self-assembled monolayers characterized with molecular and sub-molecular resolution

Self-assembled organization of functional molecules on solid surfaces has developed into a powerful and sophisticated tool for surface chemistry and nanotechnology. A number of reviews on the topic have been available since the mid 1990s. This perspective article aims to focus on recent development in the investigations of electronic structures and assembling dynamics of electrochemically controlled self-assembled monolayers (SAMs) of thiol containing molecules on gold surfaces. A brief introduction is first given and particularly illustrated by a Table summarizing the molecules studied, the surface lattice structures and the experimental operating conditions. This is followed by discussion of two major high-resolution experimental methods, scanning tunnelling microscopy (STM) and single-crystal electrochemistry. In Section 3, we briefly address choice of supporting electrolytes and substrate surfaces, and their effects on the SAM structures. Section 4 constitutes the major body of the article by offering some details of recent studies for the selected cases, including in situ monitoring of assembling dynamics, molecular electronic structures, and the key external factors determining the SAM packing. In Section 5, we give examples of what can be offered by theoretical computations for the detailed understanding of the SAM electronic structures revealed by STM images. A brief summary of the current applications of SAMs in wiring metalloproteins, design and fabrication of sensors, and single-molecule electronics is described in Section 6. In the final two sections (7 and 8), we discuss the current status in understanding of electronic structures and properties of SAMs in electrochemical environments and what could be expected for future perspectives.

Electron-phonon interactions in single octanedithiol molecular junctions

We study the charge transport properties and electron-phonon interactions in single molecule junctions, each consisting of an octanedithiol molecule covalently bound to two electrodes. Conductance measurements over a wide temperature range establish tunneling as the dominant charge transport process. Inelastic electron tunneling spectroscopy performed on individual molecular junctions provides a chemical signature of the molecule and allows electron-phonon interaction induced changes in the conductance to be explored. By fitting the conductance changes in the molecular junction using a simple model for inelastic transport, it is possible to estimate the phonon damping rates in the molecule. Finally, changes in the inelastic spectra are examined in relation to conductance switching events in the junction to demonstrate how changes in the configuration of the molecule or contact geometry can affect the conductance of the molecular junction.

Charge transport through molecular switches

We review the fascinating research on charge transport through switchable molecules. In the past decade, detailed investigations have been performed on a great variety of molecular switches, including mechanically interlocked switches (rotaxanes and catenanes), redox-active molecules and photochromic switches (e.g. azobenzenes and diarylethenes). To probe these molecules, both individually and in self-assembled monolayers (SAMs), a broad set of methods have been developed. These range from low temperature scanning tunneling microscopy (STM) via two-terminal break junctions to larger scale SAM-based devices. It is generally found that the electronic coupling between molecules and electrodes has a profound influence on the properties of such molecular junctions. For example, an intrinsically switchable molecule may lose its functionality after it is contacted. Vice versa, switchable two-terminal devices may be created using passive molecules ('extrinsic switching'). Developing a detailed understanding of the relation between coupling and switchability will be of key importance for both future research and technology.

Electron transport in single molecules: from benzene to graphene

Electron movement within and between molecules--that is, electron transfer--is important in many chemical, electrochemical, and biological processes. Recent advances, particularly in scanning electrochemical microscopy (SECM), scanning-tunneling microscopy (STM), and atomic force microscopy (AFM), permit the study of electron movement within single molecules. In this Account, we describe electron transport at the single-molecule level. We begin by examining the distinction between electron transport (from semiconductor physics) and electron transfer (a more general term referring to electron movement between donor and acceptor). The relation between these phenomena allows us to apply our understanding of single-molecule electron transport between electrodes to a broad range of other electron transfer processes. Electron transport is most efficient when the electron transmission probability via a molecule reaches 100%; the corresponding conductance is then 2e(2)/h (e is the charge of the electron and h is the Planck constant). This ideal conduction has been observed in a single metal atom and a string of metal atoms connected between two electrodes. However, the conductance of a molecule connected to two electrodes is often orders of magnitude less than the ideal and strongly depends on both the intrinsic properties of the molecule and its local environment. Molecular length, means of coupling to the electrodes, the presence of conjugated double bonds, and the inclusion of possible redox centers (for example, ferrocene) within the molecular wire have a pronounced effect on the conductance. This complex behavior is responsible for diverse chemical and biological phenomena and is potentially useful for device applications. Polycyclic aromatic hydrocarbons (PAHs) afford unique insight into electron transport in single molecules. The simplest one, benzene, has a conductance much less than 2e(2)/h due to its large LUMO-HOMO gap. At the other end of the spectrum, graphene sheets and carbon nanotubes--consisting of infinite numbers of aromatic rings--have small or even zero energy gaps between the conduction and valence bands. Between these two limits are intermediate molecules with rich properties, such as perylene derivatives made of seven aromatic rings; the properties of these types of molecules have yet to be fully explored. Studying PAHs is important not only in answering fundamental questions about electron transport but also in the ongoing quest for molecular-scale electronic devices. This line of research also helps bridge the gap between electron transfer phenomena in small redox molecules and electron transport properties in nanostructures.

Stabilizing single atom contacts by molecular bridge formation

Gold-molecule-gold junctions can be formed by carefully breaking a gold wire in a solution containing dithiolated molecules. Surprisingly, there is little understanding on the mechanical details of the bridge formation process and specifically on the role that the dithiol molecules play themselves. We propose that alkanedithiol molecules have already formed bridges between the gold electrodes before the atomic gold-gold junction is broken. This leads to stabilization of the single atomic gold junction, as observed experimentally. Our data can be understood within a simple spring model.