Reproducible measurement of single-molecule conductivity

The Current Status of Nanoelectronic Devices

Over the last thirty years, many new physical phenomena have been discovered and suggested for use in data processing devices. Many, but not all, of these devices are described as "nanoelectronic", because one or more of their characteristic dimensions lie in the size range ≈1–100 nm. However, any new idea has a long way to go before it can achieve any practical value. It is necessary to develop and test a series of increasingly complicated structures. Starting from (1) the basic physical effect and its theoretical description, one must design and fabricate prototypes of (2) a single device, then (3) a simple circuit, then (4) functional units such as assemblies of logic gates or blocks of memory cells, and eventually (5) electronic chips. This article provides an overview of the development status of some of these device technologies.

THEORETICAL STUDY OF THE ELECTRON TRANSPORT THROUGH THE CYSTEINE AMINO ACID NANOMOLECULAR WIRE

In this paper, we study the electrical transport and Negative Differential Resistance (NDR) in a single molecular conductor consisting of a cysteine sandwiched between two Au (111) electrodes via the Density Functional Theory-based Nonequilibrium Green's Function (DFT-NEGF) method. We show that (surprisingly, despite their apparent simplicity, these Au /cysteine/ Au nanowires are shown to be a convenient NDR device) the smallest two-terminal molecular wire can exhibit NDR behavior to date. Experiments with a conventional or novel self-assembled monolayer (SAM) are proposed to test these predictions.

The projected density of states (PDOSs) and transmission coefficients T(E) under various external voltage biases are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to NDR.

QUANTUM TRANSPORT AND CURRENT-TRIGGERED DYNAMICS IN MOLECULAR TUNNEL JUNCTIONS

The modelling of nanoelectronic systems has been the topic of ever increasing activity for nearly two decades. Yet, new questions, challenges and opportunities continue to emerge. In this article we review theoretical and numerical work on two new developments in the theory of molecular-scale electronics. First we review a density functional theory analysis within the Keldysh non-equilibrium Green function formalism to predict nonlinear charge transport properties of nanoelectronic devices. Next we review a recently developed quantum mechanical formalism of current-triggered nuclear dynamics. Finally we combine these theories to describe from first principles the inelastic current and the consequent molecular dynamics in molecular heterojunctions.

Revealing the molecular structure of single-molecule junctions in different conductance states by fishing-mode tip-enhanced Raman spectroscopy

The conductance of single-molecule junctions may be governed by the structure of the molecule in the gap or by the way it bonds with the leads, and the information contained in a Raman spectrum is ideal for examining both. Here we demonstrate that molecule-to-surface bonding may be characterized during electron transport by 'fishing-mode' tip-enhanced Raman spectroscopy (FM-TERS). This technique allows mutually verifiable single-molecule conductance and Raman signals with single-molecule contributions to be acquired simultaneously at room temperature. Density functional theory calculations reveal that the most significant spectral change seen for a gold-4,4′-bipyridine-gold junction results from the deformation of the pyridine ring in contact with the drain electrode at high voltage, and these calculations suggest that a stronger bonding interaction between the molecule and the drain may account for the nonlinear dependence of conductance on bias voltage. FM-TERS will lead to a better understanding of electron-transport processes in molecular junctions.

The conductance of single-molecule junctions is affected by the structure of the molecule and how it is bound to the electrodes, which may be examined using Raman spectroscopy. Liu et al. have developed 'fishing-mode' tip-enhanced Raman spectroscopy, which allows the simultaneous determination of conductance and Raman spectra.

Controllable formation and TEM spatial visualization of cross-linked gold nanoparticle spherical aggregates

Gold nanoparticles (NPs) were assembled and cross-linked into spherical aggregates by colloidal emulsion evaporation and ligand exchanging. The optical absorption, conventional transmission electron microscopy (TEM) and bright field (BF) TEM tomography confirm that the cross-linking of the pre-condensed aggregates generates high stability and compactness.

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.

Hybrid organic/inorganic molecular heterojunctions based on strained nanomembranes

In this work, we combine self-assembly and top-down methods to create hybrid junctions consisting of single organic molecular monolayers sandwiched between metal and/or single-crystalline semiconductor nanomembrane based electrodes. The fabrication process is fully integrative and produces a yield loss of less than 5% on-chip. The nanomembrane-based electrodes guarantee a soft yet robust contact to the molecules where the presence of pinholes and other defects becomes almost irrelevant. We also pioneer the fabrication and characterization of semiconductor/molecule/semiconductor tunneling heterojunctions which exhibit a double transition from direct tunneling to field emission and back to direct tunneling, a phenomenon which has not been reported previously.

Conductance histogram evolution of an EC-MCBJ fabricated Au atomic point contact

This work presents a study of Au conductance quantization based on a combined electrochemical deposition and mechanically controllable break junction (MCBJ) method. We describe the microfabrication process and discuss improved features of our microchip structure compared to the previous one. The improved structure prolongs the available life of the microchip and also increases the success rate of the MCBJ experiment. Stepwise changes in the current were observed at the last stage of atomic point contact breakdown and conductance histograms were constructed. The evolution of 1G0 peak height in conductance histograms was used to investigate the probability of formation of an atomic point contact. It has been shown that the success rate in forming an atomic point contact can be improved by decreasing the stretching speed and the degree that the two electrodes are brought into contact. The repeated breakdown and formation over thousands of cycles led to a distinctive increase of 1G0 peak height in the conductance histograms, and this increased probability of forming a single atomic point contact is discussed.

Unambiguous one-molecule conductance measurements under ambient conditions

One of the challenging goals of molecular electronics is to wire exactly one molecule between two electrodes. This is generally nontrivial under ambient conditions. We describe a new and straightforward protocol for unambiguously isolating a single organic molecule on a metal surface and wiring it inside a nanojunction under ambient conditions. Our strategy employs C(60) terminal groups which act as molecular beacons allowing molecules to be visualized and individually targeted on a gold surface using an scanning tunneling microscope. After isolating one molecule, we then use the C(60) groups as alligator clips to wire it between the tip and surface. Once wired, we can monitor how the conductance of a purely one molecule junction evolves with time, stretch the molecule in the junction, observing characteristic current plateaus upon elongation, and also perform direct I-V spectroscopy. By characterizing and controlling the junction, we can draw stronger conclusions about the observed variation in molecular conductance than was previously possible.

Electrochemical fabrication of metal/organic/metal junctions for molecular electronics and sensing applications

A simple electrochemical approach was used for fabricating electrode/metal nanowire/(molecule or polymer)/electrode junctions for sensing or molecular electronics applications. The procedure for fabricating these molecule-based devices involves electropolymerization of phenol or chemisorption of alkanethiols on one set of electrodes (E1) and electrodeposition of Ag metal nano/microwires on a second electrode (E2) which is ∼5 μm away from E1. Under appropriate deposition conditions, Ag nanowires grow from E2 and cross over to E1, forming a E1/(molecule or polymer)/Ag nanowire (NW)/E2 junction. The junction resistance was controlled by (1) electrodepositing polyphenol of varied densities on E1 and (2) assembling alkanethiols of different chain lengths on E1. Ag NWs at high resistance E1/polyphenol/Ag NW/E2 junctions functionalized with Pd monolayer protected clusters (MPCs) responded fast and reversibly to H(2) concentrations as low as 0.11% in a nitrogen carrier gas by a resistance decrease, likely due to volume expansion of the Pd nanoparticles, demonstrating the use of these electrochemically fabricated junctions for gas sensing applications.

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.

Universal scaling of the charge transport in large-area molecular junctions

Charge transport through alkanes and para-phenylene oligomers is investigated in large-area molecular junctions. The molecules are self-assembled in a monolayer and contacted with a top electrode consisting of poly(3,4-ethylenedioxythiophene)-poly(4-styrenesulfonic acid) (PEDOT:PSS). The complete set of J(V,T) characteristics of both saturated and π-conjugated molecules can be described quantitatively by a single equation with only two fit parameters. The derived parameters, in combination with a variation of the bulk conductivity of PEDOT:PSS, demonstrate that the absolute junction resistance is factorized with that of PEDOT:PSS.

Directionally oriented LB films of an OPE derivative: assembly, characterization, and electrical properties

Langmuir films have been fabricated from 4-[4'-(4''-thioacetyl-phenyleneethynylene)-phenyleneethynylene]-aniline (NOPES) after cleavage of the thioacetyl protecting group. Characterization by surface pressure vs area per molecule isotherms and Brewster angle microscopy reveal the formation of a high quality monolayer at the air-water interface. One layer Langmuir-Blodgett (LB) films were readily fabricated by the transfer of the NOPES Langmuir film onto solid substrates. X-ray photoelectron spectroscopy (XPS), surface polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), and quartz crystal microbalance (QCM) experiments conclusively demonstrate the formation of one layer LB films in which the functional group associated with binding to the substrate can be tailored by the film transfer conditions. Using LB methods this molecule could be transferred to gold samples with either the amine or thiol group attached to the gold surface. The amine group is directly attached to the gold substrate (Au-NH(2)-OPE-SH) when the substrate is initially immersed in the subphase and withdrawn during the transfer process; in contrast, monomolecular films in which the thiolate group is attached to the gold substrate (Au-S-OPE-NH(2)) are obtained when the substrate is initially out of the subphase and immersed during the transfer process. The morphology of these films was analyzed by atomic force microscopy (AFM), showing the formation of homogeneous layers. Film homogeneity was confirmed by cyclic voltammetry, which revealed a large passivation of gold electrodes covered by NOPES monolayers. Electrical properties for both polar orientated junctions have been investigated by scanning tunnelling microscopy (STM), with both orientations featuring a nonrectifying behavior.

The shear flow processing of controlled DNA tethering and stretching for organic molecular electronics

DNA has been recently explored as a powerful tool for developing molecular scaffolds for making reproducible and reliable metal contacts to single organic semiconducting molecules. A critical step in the process of exploiting DNA-organic molecule-DNA (DOD) array structures is the controlled tethering and stretching of DNA molecules. Here we report the development of reproducible surface chemistry for tethering DNA molecules at tunable density and demonstrate shear flow processing as a rationally controlled approach for stretching/aligning DNA molecules of various lengths. Through enzymatic cleavage of λ-phage DNA to yield a series of DNA chains of various lengths from 17.3 μm down to 4.2 μm, we have investigated the flow/extension behavior of these tethered DNA molecules under different flow strengths in the flow-gradient plane. We compared Brownian dynamic simulations for the flow dynamics of tethered λ-DNA in shear, and found our flow-gradient plane experimental results matched well with our bead-spring simulations. The shear flow processing demonstrated in our studies represents a controllable approach for tethering and stretching DNA molecules of various lengths. Together with further metallization of DNA chains within DOD structures, this bottom-up approach can potentially enable efficient and reliable fabrication of large-scale nanoelectronic devices based on single organic molecules, therefore opening opportunities in both fundamental understanding of charge transport at the single molecular level and many exciting applications for ever-shrinking molecular circuits.

An MCBJ case study: The influence of π-conjugation on the single-molecule conductance at a solid/liquid interface

π-Conjugation plays an important role in charge transport through single molecular junctions. We describe in this paper the construction of a mechanically controlled break-junction setup (MCBJ) equipped with a highly sensitive log I-V converter in order to measure ultralow conductances of molecular rods trapped between two gold leads. The current resolution of the setup reaches down to 10 fA. We report single-molecule conductance measurements of an anthracene-based linearly conjugated molecule (AC), of an anthraquinone-based cross-conjugated molecule (AQ), and of a dihydroanthracene-based molecule (AH) with a broken conjugation. The quantitative analysis of complementary current-distance and current-voltage measurements revealed details of the influence of π-conjugation on the single-molecule conductance.

Evidence for a near-resonant charge transfer mechanism for double-stranded peptide nucleic acid

We present evidence for a near-resonant mechanism of charge transfer in short peptide nucleic acid (PNA) duplexes obtained through electrochemical, STM break junction (STM-BJ), and computational studies. A seven base pair (7-bp) PNA duplex with the sequence (TA)(3)-(XY)-(TA)(3) was studied, in which XY is a complementary nucleobase pair. The experiments showed that the heterogeneous charge transfer rate constant (k(0)) and the single-molecule conductance (σ) correlate with the oxidation potential of the purine base in the XY base pair. The electrochemical measurements showed that the enhancement of k(0) is independent, within experimental error, of which of the two PNA strands contains the purine base of the XY base pair. 7-bp PNA duplexes with one or two GC base pairs had similar measured k(0) and conductance values. While a simple superexchange model, previously used to rationalize charge transfer in single stranded PNA (Paul et al. J. Am. Chem. Soc. 2009, 131, 6498-6507), describes some of the experimental observations, the model does not explain the absence of an enhancement in the experimental k(0) and σ upon increasing the G content in the duplexes from one to two. Moreover, the superexchange model is not consistent with other studies (Paul et al. J. Phys. Chem. B 2010, 114, 14140), that showed a hopping charge transport mechanism is likely important for PNA duplexes longer than seven base pairs. A quantitative computational analysis shows that a near-resonant charge transfer regime, wherein a mix of superexchange and hopping mechanisms are expected to coexist, can rationalize all of the experimental results.

Recognition tunneling measurement of the conductance of DNA bases embedded in self-assembled monolayers

The DNA bases interact strongly with gold electrodes, complicating efforts to measure the tunneling conductance through hydrogen-bonded Watson Crick base pairs. When bases are embedded in a self-assembled alkane-thiol monolayer to minimize these interactions, new features appear in the tunneling data. These new features track the predictions of density-functional calculations quite well, suggesting that they reflect tunnel conductance through hydrogen-bonded base pairs.

Charge transfer enhancement in the SERS of a single molecule

We measured the surface-enhanced Raman scattering (SERS) of individual gold nanoparticle-4-aminobenzenethiol (ABT)-gold film junctions to investigate the charge-transfer (CT) enhancement of the SERS signals. Despite the mild electromagnetic field enhancement (∼10(5)) and high surface density of the ABT-molecules (∼240 molecules/hotspot) at the junctions, we observed the clear spectral and temporal signatures of CT-enhanced single-molecule SERS (SM-SERS). The result reveals that only a small fraction of the molecules at the junction has a significant CT-enhancement of 10(1)∼10(3), whereas the rest of the molecules are nearly CT-inactive. Furthermore, the result also proves that overall (charge-transfer and electromagnetic) enhancement of 10(6)∼10(8) is sufficient to observe the SM-SERS of an electronically off-resonant molecule, which disproves the widespread belief that a minimum enhancement of ∼10(14) is required for SM-SERS.

Identifying diversity in nanoscale electrical break junctions

The realization of molecular-scale electronic devices will require the development of novel strategies for controlling electrical properties of metal/molecule/metal junctions, down to the single molecule level. Here, we show that it is possible to exert chemical control over the formation of metal/molecule...molecule/metal junctions in which the molecules interact by pi-stacking. The tip of an STM is used to form one contact, and the substrate the other; the molecules are conjugated oligophenyleneethynylenes (OPEs). Supramolecular pi-pi interactions allow current to flow through the junction, but not if bulky tert-butyl substituents on the phenyl rings prevent such interactions. For the first time, we find evidence that pi-stacked junctions can form even for OPEs with two thiol contacts. Furthermore, we find evidence for metal|molecule|metal junctions involving oligophenyleneethynylene monothiols, in which the second contact must be formed by the interaction of the pi-electrons of the terminal phenyl ring with the metal surface.

Manipulating localized molecular orbitals by single-atom contacts

We have fabricated atom-molecule contacts by attachment of single Cu atoms to terpyridine side groups of bis-terpyridine tetra-phenyl ethylene molecules on a Cu(111) surface. By means of scanning tunneling microscopy, spectroscopy, and density functional calculations, we have found that, due to the localization characteristics of molecular orbitals, the Cu-atom contact modifies the state localized at the terpyridine side group which is in contact with the Cu atom but does not affect the states localized at other parts of the molecule. These results illustrate the contact effects at individual orbitals and offer possibilities to manipulate orbital alignments within molecules.

The fabrication and characterization of adjustable nanogaps between gold electrodes on chip for electrical measurement of single molecules

This work reports on a new method to fabricate mechanically controllable break junctions (MCBJ) with finely adjustable nanogaps between two gold electrodes on solid state chips for characterizing electron transport properties of single molecules. The simple, low cost, robust and reproducible fabrication method combines conventional photolithography, chemical etching and electrodeposition to produce suspended electrodes separated with nanogaps. The MCBJ devices fabricated by the method can undergo many cycles in which the nanogap width can be precisely and repeatedly varied from zero to several nanometers. The method improves the success rate of the MCBJ experiments. Using these devices the electron transport properties of a typical molecular system, commercially available benzene-1,4-dithiol (BDT), have been studied. The I-V and G-V characteristic curves of BDT and the conductance value for a single BDT molecule established the excellent device suitability for molecular electronics research.

Recognition tunneling

Single molecules in a tunnel junction can now be interrogated reliably using chemically functionalized electrodes. Monitoring stochastic bonding fluctuations between a ligand bound to one electrode and its target bound to a second electrode ('tethered molecule-pair' configuration) gives insight into the nature of the intermolecular bonding at a single molecule-pair level, and defines the requirements for reproducible tunneling data. Simulations show that there is an instability in the tunnel gap at large currents, and this results in a multiplicity of contacts with a corresponding spread in the measured currents. At small currents (i.e. large gaps) the gap is stable, and functionalizing a pair of electrodes with recognition reagents (the 'free-analyte' configuration) can generate a distinct tunneling signal when an analyte molecule is trapped in the gap. This opens up a new interface between chemistry and electronics with immediate implications for rapid sequencing of single DNA molecules.

On the thermodynamic stability of alpha,omega-alkanedithiols self-assembled monolayers on unreconstructed and reconstructed Au(111)

A comparative study on the thermodynamic stability of the lying down (LD) and standing up (SU) phases of alpha,omega-butanedithiol (BDT) on unreconstructed (U) and on reconstructed (R) Au(111) surfaces is presented. The R surface is made of dithiol-Au adatom units. Density functional calculations (DFT) allow the estimation of the adsorption energy of the LD and SU BDT phases on both substrates. Surface free energies based on the DFT calculations show the coverage of the clean Au(111) surface by the LD phase, and the LD to SU phase transition as the chemical potential of the BDT molecule is increased. The LD and SU phases are more stable on R than on U substrates, suggesting that the Au(111) surface should reconstruct upon BDT adsorption. The stability analysis is extended to longer alpha,omega-dithiols. Results reveal that the LD to SU phase transition is favored as the hydrocarbon chain length of the dithiol molecule is increased. Changes in the hydrogen pressure affect the formation of the LD phase, while they have only minor effects on the LD to SU phase transitions. Our calculations explain the influence of the number of carbon atoms in the hydrocarbon chains, hydrogen pressure and dithiol pressure (or concentration) on dithiol adsorption, and phase transitions. This information is relevant to control the coverage, reactivity, and surface chemistry of the alpha,omega-dithiol self-assembled monolayers on Au surfaces.

The impact of E-Z photo-isomerization on single molecular conductance

The single molecule conductance of the E and Z isomers of 4,4'-(ethene-1,2-diyl)dibenzoic acid has been determined using two scanning tunneling microscopy (STM) methods for forming molecular break junctions [the I(s) (I = current and s is distance) method and the in situ break junction technique]. Isomerization leads to significant changes in the electrical conductance of these molecules, with the Z isomer exhibiting a higher conductance than the E isomer. Isomerization is achieved directly on the gold surface through photoirradiation, and the STM is used to determine conductance before and after irradiation; reversible switching between the two isomers could be achieved through irradiation of the surface bound species at different wavelengths. In addition, three groups of molecular conductance values [A ("low"), B ("medium"), and C ("high")] have been measured for these carboxylate-terminated molecules. The origin of these conductance groups as well as the increase of the conductance for the Z isomer have been analyzed by comparing the length of the molecules extended in the gap, derived from molecular modeling, with the experimentally observed break-off distance for both isomers.