Tunneling characteristics of Au-alkanedithiol-Au junctions formed via nanotransfer printing (nTP)

Construction of permanent metal-molecule-metal (MMM) junctions, though technically challenging, is desirable for both fundamental investigations and applications of molecule-based electronics. In this study, we employed the nanotransfer printing (nTP) technique using perfluoropolyether (PFPE) stamps to print Au thin films onto self-assembled monolayers (SAMs) of alkanedithiol formed on Au thin films. We show that the resulting MMM junctions form permanent and symmetrical tunnel junctions, without the need for an additional protection layer between the top metal electrode and the molecular layer. This type of junction makes it possible for direct investigations into the electrical properties of the molecules and the metal-molecule interfaces. Dependence of transport properties on the length of the alkane molecules and the area of the printed Au electrodes has been examined systematically. From the analysis of the current-voltage (I-V) curves using the Simmons model, the height of tunneling barrier associated with the molecule (alkane) has been determined to be 3.5 ± 0.2 eV, while the analysis yielded an upper bound of 2.4 eV for the counterpart at the interface (thiol). The former is consistent with the theoretical value of ~3.5-5.0 eV. The measured I-V curves show scaling with respect to the printed Au electrode area with lateral dimensions ranging from 80 nm to 7 μm. These results demonstrate that PFPE-assisted nTP is a promising technique for producing potentially scalable and permanent MMM junctions. They also demonstrate that MMM structures (produced by the unique PFPE-assisted nTP) constitute a reliable test bed for exploring molecule-based electronics.     

Orientation‐Controlled Single‐Molecule Junctions

The conductivity of a single aromatic ring, perpendicular to its plane, is determined using a new strategy under ambient conditions and at room temperature by a combination of molecular assembly, scanning tunneling microscopy (STM) imaging, and STM break junction (STM‐BJ) techniques. The construction of such molecular junctions exploits the formation of highly ordered structures of flat‐oriented mesitylene molecules on Au(111) to enable direct tip/π contacts, a result that is not possible by conventional methods. The measured conductance of Au/π/Au junction is about 0.1 G_o , two orders of magnitude higher than the conductance of phenyl rings connected to the electrodes by standard anchoring groups. Our experiments suggest that long‐range ordered structures, which hold the aromatic ring in place and parallel to the surface, are essential to increase probability of the formation of orientation‐controlled molecular junctions.     

Fabrication of steady junctions consisting of alpha,omega-bis(thioacetate) oligo(p-phenylene vinylene)s in nanogap electrodes

For obtaining molecular devices using metal-molecule-metal junctions, it is necessary to fabricate a steady conductive bridge-structure; that is stable chemical bonds need to be established from a single conductive molecule to two facing electrodes. In the present paper, we show that the steadiness of a conductive bridge-structure depends on the molecular structure of the bridge molecule for nanogap junctions using three types of modified oligo(phenylene vinylene)s (OPVs): alpha,omega-bis(thioacetate) oligo(phenylene vinylene) (OPV1), alpha,omega-bis(methylthioacetate) oligo(phenylene vinylene) (OPV2), and OPV2 consisting of ethoxy side chains (OPV3). We examined the change in resistance between the molecule-bridged junction and a bare junction in each of the experimental Au-OPV-Au junctions to confirm whether molecules formed steady bridges. Herein, the outcomes of whether molecules formed steady bridges were defined in terms of three types of result; successful, possible and failure. We define the ratio of the number of successful junctions to the total number of experimental junctions as successful rate. A 60% successful rate for OPV3 was higher than for the other two molecules whose successful rates were estimated to be approximately 10%. We propose that conjugated molecules consisting of methylthioacetate termini and short alkoxy side chains are well suited for fabricating a steady conductive bridge-structure between two facing electrodes.     

FTIR spectroscopy of buried interfaces in molecular junctions

We demonstrate that ATR-FTIR spectroscopy can be used to record high-quality vibrational spectra of molecules at buried interfaces in metal-molecule-silicon and metal-molecule-metal junctions. This provides quantitative information on the structure and conformation of molecules at buried interfaces, an issue of critical importance to molecular electronics. In the model systems of Au on octadecyltrichlorosilane self-assembled monolayer on Si or mecaptohexadecanoic acid multilayers on Au-covered Si, ATR-FTIR suggests that metal deposition leads to not only conformational disorder within the film but also the direct interaction of metal atoms/clusters with alkyl backbones.     

Correlation between HOMO alignment and contact resistance in molecular junctions: aromatic thiols versus aromatic isocyanides

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

The First Principle Study on the Rectification of Molecular Junctions Based on the Alkyl-chain-modified Phenyl Benzothiophene Derivative

Using density functional theory(DFT) combined with nonequilibrium Green's function investigates the electron-transport properties of several molecular junctions based on the PBTDT-CH=NH molecule, which is modified by one to four alkyl groups forming PBTDT-(CH_2)_nCH=NH. The electronic structures of the isolated molecules(thiol-ended PBTDT-(CH2)_nCH=N) have been investigated before the electron-transport calculations are performed. The asymmetric current-voltage characteristics have been obtained for the molecular junctions. Rectifying performance of Au/S-PBTDT-CH=N-S/Au molecular junction can be regulated by introducing alkyl chain. The N3 molecular junction exhibits the best rectifying effect. Its maximum rectifying ratio is 878, which is 80 times more than that of the molecular junction based on the original N molecular junction. The current-voltage(I-V) curves of all the sandwich systems in this work are illustrated by transmission spectra and molecular projection density analysis.     

Optical properties of current carrying molecular wires

We consider several fundamental optical phenomena involving single molecules in biased metal-molecule-metal junctions. The molecule is represented by its highest occupied and lowest unoccupied molecular orbitals, and the analysis involves the simultaneous consideration of three coupled fluxes: the electronic current through the molecule, energy flow between the molecule and electron-hole excitations in the leads, and the incident and/or emitted photon flux. Using a unified theoretical approach based on the nonequilibrium Green's function method we derive expressions for the absorption line shape (not an observable but a useful reference for considering yields of other optical processes) and for the current induced molecular emission in such junctions. We also consider conditions under which resonance radiation can induce electronic current in an unbiased junction. We find that current driven molecular emission and resonant light induced electronic currents in single molecule junctions can be of observable magnitude under appropriate realizable conditions. In particular, light induced current should be observed in junctions involving molecular bridges that are characterized by strong charge-transfer optical transitions. For observing current induced molecular emission we find that in addition to the familiar need to control the damping of molecular excitations into the metal substrate the phenomenon is also sensitive to the way in which the potential bias is distributed on the junction.     

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.     

The first studies of a tetrathiafulvalene-sigma-acceptor molecular rectifier

Langmuir-Blodgett monolayers of a donor-acceptor diad TTF-sigma-(trinitrofluorene) (8) with an extremely low HOMO-LUMO gap (0.3 eV) have been used to create molecular junction devices that show rectification behavior. By virtue of structural similarities and position of molecular orbitals, 8 is the closest well-studied analogue of the model Aviram-Ratner unimolecular rectifier (TTF-sigma-TCNQ). Compressing the monolayer results in aligning the molecules, and is followed by a drastic increase in the rectification ratio. The direction of rectification depends on the electrodes used and is different in n-Si/8/Ti and Au/8/C16H33S-Hg junctions. The molecular nature of such behavior was corroborated by control experiments with fatty acids and by reversing the rectification direction with changing the molecular orientation (Au/D-sigma-A versus Au/A-sigma-D).     

Effect of Asymmetric Anchoring Groups on Electronic Transport in Hybrid Metal/Molecule/Graphene Single Molecule Junctions

A combined experimental and theoretical study on molecular junctions with asymmetry in both the electrode type and in the anchoring group type is presented. A scanning tunnelling microscope is used to create the “asymmetric” Au-S-(CH₂)n-COOH-graphene molecular junctions and determine their conductance. The measurements are combined with electron transport calculations based on density functional theory (DFT) to analyze the electrical conductance and its length attenuation factor from a series of junctions of different molecular length (n). These results show an unexpected trend with a rather high conductance and a smaller attenuation factor for the Au-S-(CH₂)_n-COOH-graphene configuration compared to the equivalent junction with the “symmetrical” COOH contacting using the HOOC-(CH₂)_n-COOH series. Owing to the effect of the graphene electrode, the attenuation factor is also smaller than the one obtained for Au/Au electrodes. These results are interpreted through the relative molecule/electrode couplings and molecular level alignments as determined with DFT calculations. In an asymmetric junction, the electrical current flows through the less resistive conductance channel, similarly to what is observed in the macroscopic regime.     

Electron transport and redox reactions in carbon-based molecular electronic junctions

A unique molecular junction design is described, consisting of a molecular mono- or multilayer oriented between a conducting carbon substrate and a metallic top contact. The sp2 hybridized graphitic carbon substrate (pyrolyzed photoresist film, PPF) is flat on the scale of the molecular dimensions, and the molecular layer is bonded to the substrate via diazonium ion reduction to yield a strong, conjugated C-C bond. Molecular junctions were completed by electron-beam deposition of copper, titanium oxide, or aluminium oxide followed by a final conducting layer of gold. Vibrational spectroscopy and XPS of completed junctions showed minimal damage to the molecular layer by metal deposition, although some electron transfer to the molecular layer resulted in partial reduction in some cases. Device yield was high (>80%), and the standard deviations of junction electronic properties such as low voltage resistance were typically in the range of 10-20%. The resistance of PPF/molecule/Cu/Au junctions exhibited a strong dependence on the structure and thickness of the molecular layer, ranging from 0.13 ohms cm2 for a nitrobiphenyl monolayer, to 4.46 ohms cm2 for a biphenyl monolayer, and 160 ohms cm2 for a 4.3 nm thick nitrobiphenyl multilayer. Junctions containing titanium or aluminium oxide had dramatically lower conductance than their PPF/molecule/Cu counterparts, with aluminium oxide junctions exhibiting essentially insulating behavior. However, in situ Raman spectroscopy of PPF/nitroazobenzene/AlO(x)/Au junctions with partially transparent metal contacts revealed that redox reactions occurred under bias, with nitroazobenzene (NAB) reduction occurring when the PPF was biased negative relative to the Au. Similar redox reactions were observed in PPF/NAB/TiO(x)/Au molecular junctions, but they were accompanied by major effects on electronic behavior, such as rectification and persistent conductance switching. Such switching was evident following polarization of PPF/molecule/TiO2/Au junctions by positive or negative potential pulses, and the resulting conductance changes persisted for several minutes at room temperature. The "memory" effect implied by these observations is attributed to a combination of the molecular layer and the TiO2 properties, namely metastable "trapping" of electrons in the TiO2 when the Au is negatively biased.     

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.     

Length dependence of charge transport in nanoscopic molecular junctions incorporating a series of rigid thiol-terminated norbornylogs

Four tetrathiol-terminated norbornane homologues were synthesized and self-assembled monolayers (SAMs) of these molecules were formed on Au via adsorption from CH2Cl2. SAMs were characterized structurally via spectroscopic ellipsometry (SE), reflection-absorption infrared spectroscopy (RAIRS), Rutherford backscattering spectrometry (RBS), and X-ray photoelectron spectroscopy (XPS). Results of these analyses show that the rigid norbornylogs form monolayers that have a surface coverage slightly lower than that of alkanethiols, and that they exhibit a nonmonotonic dependence of film thickness on molecular length. Nanoscale molecular junctions incorporating these SAMs were formed and characterized electrically using conducting probe atomic force microscopy (CP-AFM). The resistances of these junctions scale exponentially with the contour length of the molecules, with beta = 0.9 A(-1), consistent with a nonresonant tunneling mechanism. Further, the resistance of norbornyl SAMs correlates well with the resistance of alkanedithiol SAMs of similar length, suggesting that the norbornyl molecules form sulfur-metal bonds on both ends of the junction.     

Highly conducting π-conjugated molecular junctions covalently bonded to gold electrodes

We measure electronic conductance through single conjugated molecules bonded to Au metal electrodes with direct Au-C covalent bonds using the scanning tunneling microscope based break-junction technique. We start with molecules terminated with trimethyltin end groups that cleave off in situ, resulting in formation of a direct covalent σ bond between the carbon backbone and the gold metal electrodes. The molecular carbon backbone used in this study consist of a conjugated π system that has one terminal methylene group on each end, which bonds to the electrodes, achieving large electronic coupling of the electrodes to the π system. The junctions formed with the prototypical example of 1,4-dimethylenebenzene show a conductance approaching one conductance quantum (G(0) = 2e(2)/h). Junctions formed with methylene-terminated oligophenyls with two to four phenyl units show a 100-fold increase in conductance compared with junctions formed with amine-linked oligophenyls. The conduction mechanism for these longer oligophenyls is tunneling, as they exhibit an exponential dependence of conductance on oligomer length. In addition, density functional theory based calculations for the Au-xylylene-Au junction show near-resonant transmission, with a crossover to tunneling for the longer oligomers.     

Study of molecular junctions with a combined surface-enhanced Raman and mechanically controllable break junction method

We have developed a combined surface-enhanced Raman spectroscopy (SERS) and break junction method to detect and characterize molecules between two microfabricated electrodes separated with a gap that can be continuously adjusted from a few angstroms to nanometers. It allows us to obtain a vibrational fingerprint of the adjustable molecular junction while performing electron transport measurements on the molecule simultaneously. This new approach will provide not only new insights into electron transport properties of molecule junctions on a chip but also the mechanism of single-molecule-SERS.     

Electron Transport through Single π‐Conjugated Molecules Bridging between Metal Electrodes

Understanding electron transport through a single molecule bridging between metal electrodes is a central issue in the field of molecular electronics. This review covers the fabrication and electron‐transport properties of single π‐conjugated molecule junctions, which include benzene, fullerene, and π‐stacked molecules. The metal/molecule interface plays a decisive role in determining the stability and conductivity of single‐molecule junctions. The effect of the metal–molecule contact on the conductance of the single π‐conjugated molecule junction is reviewed. The characterization of the single benzene molecule junction is also discussed using inelastic electron tunneling spectroscopy and shot noise. Finally, electron transport through the π‐stacked system using π‐stacked aromatic molecules enclosed within self‐assembled coordination cages is reviewed. The electron transport in the π‐stacked systems is found to be efficient at the single‐molecule level, thus providing insight into the design of conductive materials.     

Molecular tunnel junctions based on π-conjugated oligoacene thiols and dithiols between Ag, Au, and Pt contacts: effect of surface linking group and metal work function

The tunneling resistance and electronic structure of metal-molecule-metal junctions based on oligoacene (benzene, naphthalene, anthracene, and tetracene) thiol and dithiol molecules were measured and correlated using conducting probe atomic force microscopy (CP-AFM) in conjunction with ultraviolet photoelectron spectroscopy (UPS). Nanoscopic tunnel junctions (~10 nm(2)) were formed by contacting oligoacene self-assembled monolayers (SAMs) on flat Ag, Au, or Pt substrates with metalized AFM tips (Ag, Au, or Pt). The low bias (<0.2 V) junction resistance (R) increased exponentially with molecular length (s), i.e., R = R(0) exp(βs), where R(0) is the contact resistance and β is the tunneling attenuation factor. The R(0) values for oligoacene dithiols were 2 orders of magnitude less than those of oligoacene thiols. Likewise, the β value was 0.5 per ring (0.2 ?(-1)) for the dithiol series and 1.0 per ring (0.5 ?(-1)) for the monothiol series, demonstrating that β is not simply a characteristic of the molecular backbone but is strongly affected by the number of chemical (metal-S) contacts. R(0) decreased strongly as the contact work function (Φ) increased for both monothiol and dithiol junctions, whereas β was independent of Φ within error. This divergent behavior was explained in terms of the metal-S bond dipoles and the electronic structure of the junction; namely, β is independent of contact type because of weak Fermi level pinning (UPS revealed E(F) - E(HOMO) varied only weakly with Φ), but R(0) varies strongly with contact type because of the strong metal-S bond dipoles that are responsible for the Fermi level pinning. A previously published triple barrier model for molecular junctions was invoked to rationalize these results in which R(0) is determined by the contact barriers, which are proportional to the size of the interfacial bond dipoles, and β is determined by the bridge barrier, E(F) - E(HOMO). Current-voltage (I-V) characteristics obtained over a larger voltage range 0-1 V revealed a characteristic transition voltage V(trans) at which the current increased more sharply with voltage. V(trans) values were generally >0.5 V and were well correlated with the bridge barrier E(F) - E(HOMO). Overall, the combination of electronic structure determination by UPS with length- and work function-dependent transport measurements provides a remarkably comprehensive picture of tunneling transport in molecular junctions based on oligoacenes.     

Efficient conducting channels formed by the π-π stacking in single [2,2]paracyclophane molecules

The electronic transport properties of single [2,2]paracyclophane molecules directly connected to gold and platinum electrodes have been investigated both theoretically and experimentally by using first-principles quantum transport simulations and break-junction experiments. For comparison, investigations on [3,3]- and [4,4]-paracyclophanes have also been performed. Our calculations show that the strength of the π-π interaction in paracyclophanes is critically dependent on the inter-ring distance. In contrast to [4,4]paracyclophane in which the π-π interaction is very weak due to the large inter-ring distance, the π-π interaction in [2,2]- and [3,3]-paracyclophanes is rather strong and dominates the electronic transport properties. In particular, for the asymmetric Au-[2,2]paracyclophane-Au junction in which the [2,2]paracyclophane molecule is connected to each gold electrode through a Au adatom and the two Au adatoms are attached in η(1)-fashion to two carbon atoms in the benzene backbones connecting with different ethylene groups, the transmission coefficient at the Fermi level is calculated to be 1.0 × 10(-2), in excellent agreement with experiments. When the gold electrodes are replaced by platinum, the calculated transmission coefficient at the Fermi level of the symmetric Pt-[2,2]paracyclophane-Pt junction with one Pt adatom used as the linker group is increased to 0.83, demonstrating that the π-π stacking in [2,2]paracyclophane is efficient for electron transport when the molecule-electrode interfaces are electronically transparent. This is confirmed by our preliminary experimental studies on the Pt-[2,2]paracyclophane-Pt junctions, for which the low-bias junction conductance has reached 0.40 ± 0.02 G(0) (G(0) is the conductance quantum). These findings are helpful for the design of molecular electronic devices incorporating π-π stacking molecular systems.     

Effect of length and contact chemistry on the electronic structure and thermoelectric properties of molecular junctions

We present a combined experimental and computational study that probes the thermoelectric and electrical transport properties of molecular junctions. Experiments were performed on junctions created by trapping aromatic molecules between gold electrodes. The end groups (-SH, -NC) of the aromatic molecules were systematically varied to study the effect of contact coupling strength and contact chemistry. When the coupling of the molecule with one of the electrodes was reduced by switching the terminal chemistry from -SH to -H, the electrical conductance of molecular junctions decreased by an order of magnitude, whereas the thermopower varied by only a few percent. This has been predicted computationally in the past and is experimentally demonstrated for the first time. Further, our experiments and computational modeling indicate the prospect of tuning thermoelectric properties at the molecular scale. In particular, the thiol-terminated aromatic molecular junctions revealed a positive thermopower that increased linearly with length. This positive thermopower is associated with charge transport primarily through the highest occupied molecular orbital, as shown by our computational results. In contrast, a negative thermopower was observed for a corresponding molecular junction terminated by an isocyanide group due to charge transport primarily through the lowest unoccupied molecular orbital.     

Resonant photoconductance of molecular junctions formed in gold nanoparticle arrays

We investigate the photoconductance properties of oligo(phenylene vinylene) (OPV) molecules in metal-molecule-metal junctions. The molecules are electrically contacted in a two-dimensional array of gold nanoparticles. The nanoparticles in such an array are separated by only few nanometers. This allows to bridge the distance between the nanoparticles with molecules considered as molecular wires such as OPV. We report on the photoconductance of electrically contacted OPV upon resonant optical excitation of the molecules. This resonant photoconductance is sublinear in laser intensity, which suggests that trap state dynamics of the optically excited charge carriers dominate the optoelectronic response.