Low-Bias Conductance Mechanism of Diarylethene Isomers:a First-Principle Study

The structure-property relationship of diarylethene (DAE)-derivative molecular isomers, which involve ring-closed and ring-open forms, is investigated by employing the non-equilibrium Green's function formalism combined with density functional theory. Molecular junctions are formed by the isomers connecting to Au(111) electrodes through flanked pyridine groups. The difference in electronic structures caused by different geometry structures for the two isomers, particularly the interatomic alternative single bond and double bond of the ring-closed molecule, contributes to the vastly different low-bias conductance values. The lowest unoccupied molecular orbital (LUMO) of the isomers is the main channel for electron transport. In addition, more electrons transferred to the ring-closed molecular junction in the equilibrium condition, thereby decreasing the LUMO energy to near the Fermi energy, which may contribute to a larger conductance value at the Fermi level. Our findings are helpful for understanding the mechanism of low-bias conductance and are conducive to the design of high-performance molecular switching based on diarylethene or diarylethene-derivative molecules.     

Interpretation of stochastic events in single-molecule measurements of conductance and transition voltage spectroscopy

The first simultaneous measurements of transition voltage (V(t)) spectroscopy (TVS) and conductance (G) histograms (Guo et al., J. Am. Chem. Soc. 2011, 133, 19189) form a great case for studying stochastic effects, which are ubiquitous in molecular junctions. Here an interpretation of those data is proposed that emphasizes the different physical content of V(t) and G and reveals that fluctuations in the molecular orbital alignment have a significantly larger impact on G than initially claimed. The present study demonstrates the usefulness of corroborating statistical information on different transport properties and gives support to TVS as a valuable investigative tool.     

Dependence of single-molecule conductance on molecule junction symmetry

The symmetry of a molecule junction has been shown to play a significant role in determining the conductance of the molecule, but the details of how conductance changes with symmetry have heretofore been unknown. Herein, we investigate a naphthalenedithiol single-molecule system in which sulfur atoms from the molecule are anchored to two facing gold electrodes. In the studied system, the highest single-molecule conductance, for a molecule junction of 1,4-symmetry, is 110 times larger than the lowest single-molecule conductance, for a molecule junction of 2,7-symmetry. We demonstrate clearly that the measured dependence of molecule junction symmetry for single-molecule junctions agrees with theoretical predictions.     

Fullerene-based anchoring groups for molecular electronics

We present results on a new fullerene-based anchoring group for molecular electronics. Using lithographic mechanically controllable break junctions in vacuum we have determined the conductance and stability of single-molecule junctions of 1,4-bis(fullero[c]pyrrolidin-1-yl)benzene. The compound can be self-assembled from solution and has a low-bias conductance of 3 x 10(-4) G0. Compared to 1,4-benzenedithiol the fullerene-anchored molecule exhibits a considerably lower conductance spread. In addition, the signature of the new compound in histograms is more significant than that of 1,4-benzenediamine, probably owing to a more stable adsorption motif. Statistical analyses of the breaking of the junctions confirm the stability of the fullerene-gold bond.     

Increased Surface Density of States at the Fermi Level for Electron Transport Across Single-Molecule Junctions

Fermi's golden rule, a remarkable concept for the transition probability involving continuous states, is applicable to the interfacial electron-transporting efficiency via correlation with the surface density of states (SDOS). Yet, this concept has not been reported to tailor single-molecule junctions where gold is an overwhelmingly popular electrode material due to its superior amenability in regenerating molecular junctions. At the Fermi level, however, the SDOS of gold is small due to its fully filled d-shell. To increase the electron-transport efficiency, herein, gold electrodes are modified by a monolayer of platinum or palladium that bears partially filled d-shells and exhibits significant SDOS at the Fermi energy. An increase by 2–30 fold is found for single-molecule conductance of α,ω-hexanes bridged via common headgroups. The improved junction conductance is attributed to the electrode self-energy which involves a stronger coupling with the molecule and a larger SDOS participated by d-electrons at the electrode-molecule interfaces.     

Single molecule junctions formed via Au-thiol contact: stability and breakdown mechanism

The stability and breakdown mechanism of a single molecule covalently bound to two Au electrodes via Au-S bonds were studied at room temperature. The distance over which a molecular junction can be stretched before breakdown was measured using a scanning tunneling microscopy break junction approach as a function of stretching rate. At low stretching rates, the stretching distance is small and independent of stretching rate. Above a certain stretching rate, it increases linearly with the logarithm of stretching rate. At very high stretching rates, the stretching distance reaches another plateau and becomes insensitive to the stretching rate again. The three regimes are well described by a thermodynamic bond-breaking model. A comparative study of Au-Au atomic point contacts indicates that the breakdown of the molecular junctions takes place at Au-Au bonds near the molecule-electrode contact. By fitting the experimental data with the model, the lifetime and binding energy were extracted. Both quantities are found to have broad distributions, owing to large variations in the molecule-electrode contact geometry. Although the molecular junctions are short-lived on average, certain contact geometries are considerably more stable. Several types of stochastic fluctuations were observed in the conductance of the molecule junctions, which are attributed to the atomic level rearrangement of the contact geometry, and bond breakdown and reformation processes. The possibility of bond reformation increases the apparent lifetime of the molecular junctions.     

Contact chemistry and single-molecule conductance: a comparison of phosphines, methyl sulfides, and amines

We compare the low bias conductance of a series of alkanes terminated on their ends with dimethyl phosphines, methyl sulfides, and amines and find that junctions formed with dimethyl phosphine terminated alkanes have the highest conductance. We see unambiguous conductance signatures with these link groups, indicating that the binding is well-defined and electronically selective. This allows a detailed analysis of the single-molecule junction elongation properties which correlate well with calculations based on density functional theory.     

Hydrogen-bond-induced quantum interference in single-molecule junctions of regioisomers

Solvents can play a significant role in tuning the electrical conductance of single-molecule junctions. In this respect, protic solvents offer the potential to form hydrogen bonds with molecular backbones and induce electrostatic gating via their dipole moments. Here we demonstrate that the effect of hydrogen bond formation on conductance depends on whether transport through the junction is controlled by destructive quantum interference (DQI) or constructive quantum interference (CQI). Furthermore, we show that a protic solvent can be used to switch the conductance of single-molecule junctions between the two forms of quantum interference. To explore this possibility, two regioisomers (BIT-Zwitterion and BIT-Neutral) were synthesized and their single-molecule conductances in aprotic and protic solvents were investigated using a scanning-tunneling-microscope-based break junction technique, combined with density functional theory and quantum transport theory. We find that the protic solvent twists the geometry of BIT-Zwitterion by introducing intermolecular hydrogen bonds between the solvent and target molecule. Moreover, it increases the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule by imposing different electrostatic gating on the delocalized HOMO and localized LUMO, leading to a lower conductance compared to that in aprotic solvent. In contrast, the conductance of BIT-Neutral increases due to a transformation from DQI to CQI originating from a change from a planar to a folded conformation in the protic solvent. In addition, the stacking between the two folded moieties produces an extra through-space transport path, which further contributes to conductance. This study demonstrates that combinations of protic solvents and regioisomers present a versatile route to controlling quantum interference and therefore single-molecule conductance, by enabling control of hydrogen bond formation, electrostatic gating and through-space transport.

We demonstrate that the effect of solvent–molecule interaction through hydrogen bonding on junction conductance depends on whether transport through the junction is controlled by destructive or constructive quantum interference.     

Analytical modeling of the junction evolution in single-molecule break junctions: towards quantitative characterization of the time-dependent process

The conductance through single-molecule junctions characterized by the break junction techniques consists of the through-space tunneling and through-molecule tunneling conductance, and the existence of through-space tunneling between the electrodes makes the quantitative extraction of the intrinsic molecular signals of single-molecule junctions challenging. Here, we established an analytic model to describe the evolution of the conductance of a single molecule in break junction measurements. The experimental data for a series of oligo(aryleneethynylene) derivatives validate the proposed model, which provides a modeling insight into the conductance evolution for the opening process in a "real" break junction experiment. Further modulations revealed that the junction formation probability and rupture distance of the molecular junction, which reflect the junction stability, will significantly influence the amplitude and position of the obtained conductance peak. We further extend our model to a diffusion and a chemical reaction process, for which the simulation results show that the break junction technique offers a quantitative understanding of these time-dependent systems, suggesting the potential of break junction techniques in the quantitative characterization of physical and chemical processes at the single-molecule scale.     

Molecular conductance switch-on of single ruthenium complex molecules

Thiol-tethered Ru(II) terpyridine complexes were synthesized for a voltage-driven molecular switch and used to understand the switch-on mechanism of the molecular switches of single metal complexes in the solid-state molecular junction in a vacuum. Molecularly resolved scanning tunneling microscopy (STM) images revealed well-defined single Ru(II) complexes isolated in the highly ordered dielectric monolayer. When a negative sample-bias was applied, the threshold voltage to the high conductance state in the molecular junctions of the Ru(II) complex was consistent with the electronic energy gap between the Fermi level of the gold substrate and the lowest ligand-centered redox state of the metal complex molecule. As an active redox center leading to conductance switching in the molecule, the lowest ligand-centered redox state of Ru(II) complexes was suggested to trap an electron injected from the gold substrate. Our suggestions for a single-molecule switch-on mechanism in the solid state can provide guidance in a design that improves the charge-trapping efficiency of the ligands with different metal substrates.     

基于裂结技术的单分子尺度化学反应研究进展

分子电子学是研究单分子器件的构筑、性质以及功能调控的一门新兴学科。其中,金属/分子/金属结的构筑和表征是现阶段分子电子学的主要研究内容。裂结技术是当前分子电子学研究的主要实验方法,主要包括机械可控裂结技术和扫描隧道显微镜裂结技术。本文对裂结技术进行了介绍,并对近年来利用这些技术,在单分子尺度化学反应的检测和动力学研究,以及将这些技术与溶液环境、静电场、电化学门控等方法相结合,调控单分子器件的电输运性质等方面所取得的进展进行了概述。     

基于电化学方法研究以铜和银为电极的对苯二甲酸单分子结电导

利用基于电化学跳跃接触的扫描隧道显微镜裂结法(ECSTM-BJ), 通过现场形成金属电极, 对以Cu和Ag为电极的对苯二甲酸单分子结电导进行了测量. 研究结果表明: 利用该方法对所有数据直接线性统计即可得到很好结果; 两种电极下都存在两套高和低电导值, 其中以Cu为电极的单分子结电导高低值分别为11.5和4.0 nS, 而以Ag为电极的单分子结电导分别为10.3和3.8 nS, 高值都约为低值的3倍, 且以Cu为电极的单分子结电导要略大于以Ag为电极的电导, 可归结于电极和分子的耦合不同造成的. 与同样条件下测量得到的烷基链羧酸单分子结电导只存在一套值相比,对苯二甲酸表现出两套电导值, 反应了分子内主链对分子结电导的影响.     

金属/分子/金属结的电子输运机理与表征

金属/分子/金属结是分子电子学中的基本单元.根据电子的相位是否发生改变,分子结中的电子输运可以分为相干输运和非相干输运两类.在实验上,分子结的表征方法可以分为电学性质表征和非电学性质表征两类.本文借助能级图,首先对分子结的电子输运机理作了简明解释.在此基础上,结合文献报道和本课题组此前的工作,对分子结的一些常用电学表征方法,包括电流-电压特性曲线、电流-时间曲线、电导统计柱状图、转变电压谱、散粒噪声测试、非弹性电子隧道谱和热电效应法进行了介绍.     

Correlations between molecular structure and single-junction conductance: a case study with oligo(phenylene-ethynylene)-type wires

The charge transport characteristics of 11 tailor-made dithiol-terminated oligo(phenylene-ethynylene) (OPE)-type molecules attached to two gold electrodes were studied at a solid/liquid interface in a combined approach using an STM break junction (STM-BJ) and a mechanically controlled break junction (MCBJ) setup. We designed and characterized 11 structurally distinct dithiol-terminated OPE-type molecules with varied length and HOMO/LUMO energy. Increase of the molecular length and/or of the HOMO-LUMO gap leads to a decrease of the single-junction conductance of the linearly conjugate acenes. The experimental data and simulations suggest a nonresonant tunneling mechanism involving hole transport through the molecular HOMO, with a decay constant β = 3.4 ± 0.1 nm(-1) and a contact resistance R(c) = 40 kΩ per Au-S bond. The introduction of a cross-conjugated anthraquinone or a dihydroanthracene central unit results in lower conductance values, which are attributed to a destructive quantum interference phenomenon for the former and a broken π-conjugation for the latter. The statistical analysis of conductance-distance and current-voltage traces revealed details of evolution and breaking of molecular junctions. In particular, we explored the effect of stretching rate and junction stability. We compare our experimental results with DFT calculations using the ab initio code SMEAGOL and discuss how the structure of the molecular wires affects the conductance values.     

单分子电导测量技术及其影响因素

Molecular electronics is an important field for the application of nanotechnologies with an ultimate goal of building functional devices using single molecules or molecular arrays to realize the same functionality as macroscopic devices. To attain this goal, reliable techniques for measuring and manipulating electron transfer processes through single molecules are essential. There are various techniques and many environmental factors influencing single-molecule electronic conductance measurements. In this review, we first provide a detailed introduction and classification of the current well-accepted techniques in this field for measuring single-molecule conductance. All available techniques are summarized into two categories: the fixed junction technique and break junction technique. The break junction technique involves repeatedly forming and breaking molecular junctions by mechanically controlling a pair of electrodes moving into and out of contact in the presence of target molecules. Single-molecule conductance can be determined from the conductance plateaus that appear in typical conductance decay traces when molecules bind two electrodes during their separation process. In contrast, the fixed junction technique is to fix the distance between a pair of electrodes and measure the conductance fluctuations when a single molecule binds the two electrodes stochastically. Both techniques comprise different application methods and have been employed preferentially by different groups. Specific features of both techniques and their intrinsic advantages are compared and summarized in Section 4.     

Strong effects of molecular structure on electron transport in carbon/molecule/copper electronic junctions

Carbon/molecule/copper molecular electronic junctions were fabricated by metal deposition of copper onto films of various thicknesses of fluorene (FL), biphenyl (BP), and nitrobiphenyl (NBP) covalently bonded to flat, graphitic carbon. A "crossed-wire" junction configuration provided high device yield and good junction reproducibility. Current/voltage characteristics were investigated for 69 junctions with various molecular structures and thicknesses and at several temperatures. The current/voltage curves for all cases studied were nearly symmetric, scan rate independent, repeatable at least thousands of cycles and exhibited negligible hysteresis. Junction conductance was strongly dependent on the dihedral angle between phenyl rings and on the nature of the molecule/copper "contact". Junctions made with NBP showed a decrease in conductivity of a factor of 1300 when the molecular layer thickness increased from 1.6 to 4.5 nm. The slope of ln(i) vs layer thickness for both BP and NBP was weakly dependent on applied voltage and ranged from 0.16 to 0.24 A(-1). These attenuation factors are similar to those observed for similar molecular layers on modified electrodes used to study electrochemical kinetics. All junctions studied showed weak temperature dependence in the range of approximately 325 to 214 K, implying activation barriers in the range of 0.06 to 0.15 eV. The carbon/molecule/copper junction structure provides a robust, reproducible platform for investigations of the dependence of electron transport in molecular junctions on both molecular structure and temperature. Furthermore, the results indicate that junction conductance is a strong function of molecular structure, rather than some artifact resulting from junction fabrication.     

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.     

电化学门控调节具有平行路径的单分子电路中电子传输

电化学门控已成为一种可行且高效调节单分子电导的方法.在本研究中,我们证实了具有两个平行苯环的单分子电路中电子传输可以通过电化学门控控制.首先,我们利用STM-BJ技术以金为电极构筑了具有两条平行路径的单分子结.与单条路径的单分子结相比,两条路径的分子结由于具有增强性量子干涉效应,具有2.82倍的电导值.进一步地,我们利...     

Effect of anchoring groups on single-molecule conductance: comparative study of thiol-, amine-, and carboxylic-acid-terminated molecules

We studied the effect of anchoring groups on the conductance of single molecules using alkanes terminated with dithiol, diamine, and dicarboxylic-acid groups as a model system. We created a large number of molecular junctions mechanically and analyzed the statistical distributions of the conductance values of the molecular junctions. Multiple sets of conductance values were found in each case. The I-V characteristics, temperature independence, and exponential decay of the conductance with the molecular length all indicate tunneling as the conduction mechanism for these molecules. The prefactor of the exponential decay function, which reflects the contact resistance, is highly sensitive to the anchoring group, and the decay constant is weakly dependent on the anchoring group. These observations are attributed to different electronic couplings between the molecules and the electrodes and alignments of the molecular energy levels relative to the Fermi energy level of the electrodes introduced by different anchoring groups. For diamine and dicarboxylic-acid groups, the conductance values are sensitive to pH due to protonation and deprotonation of the anchoring groups. Further insight into the binding strengths of these anchoring groups to gold electrodes is obtained by statistically analyzing the stretching length of molecular junctions.     

基于吡嗪连接的石墨烯电极单分子场效应晶体管

In single-molecule junctions, anchoring groups that connect the central molecule to the electrodes have profound effects on the mechanical and electrical properties of devices. The mechanical strength of the anchoring groups affects the device stability, while their electronic coupling strength influences the junction conductance and the conduction polarity. To design and fabricate high-performance single-molecule devices with graphene electrodes, it is highly desirable to explore robust anchoring groups that bond the central molecule to the graphene electrodes. Condensation of ortho-phenylenediamine terminated molecules with ortho-quinone moieties at the edges of graphene generates graphene-conjugated pyrazine units that can be employed as anchoring groups for the construction of molecular junctions with graphene electrodes. In this study, we investigated the fabrication and electrical characterization of single-molecule field-effect transistors (FETs) with graphene as the electrodes, pyrazine as the anchoring groups, and a heavily doped silicon substrate as the back-gate electrode. Graphene nano-gaps were fabricated by a high-speed feedback-controlled electro-burning method, and their edges were fully oxidized; thus, there were many ortho-quinone moieties at the edges. After the deposition of phenazine molecules with ortho-phenylenediamine terminals at both ends, a large current increase was observed, indicating that molecular junctions were formed with covalent pyrazine anchoring groups. The yield of the single-molecule devices was as high as 26%, demonstrating the feasibility of pyrazine as an effective anchoring group for graphene electrodes. Our electrical measurements show that the ten fabricated devices exhibited a distinct gating effect when a back-gate voltage was applied. However, the gate dependence of the conductance varied considerably from device to device, and three types of different gate modulation behaviors, including p-type, ambipolar, and n-type conduction, were observed. Our observations can be understood using a modified single-level model that takes into account the linear dispersion of graphene near the Dirac point; the unique band structure of graphene and the coupling strength of pyrazine with the graphene electrode both crucially affect the conduction polarity of single-molecule FETs. When the coupling strength of pyrazine with the graphene electrode is weak, the highest occupied molecular orbital (HOMO) of the central molecule dominates charge transport. Depending on the gating efficiencies of the HOMO level and the graphene states, devices can exhibit p-type or ambipolar conduction. In contrast, when the coupling is strong, the redistribution of electrons around the central molecule and the graphene electrodes leads to a realignment of the molecular levels, resulting in the lowest unoccupied molecular orbital (LUMO)-dominated n-type conduction. The high yield and versatility of the pyrazine anchoring groups are beneficial for the construction of single-molecule devices with graphene electrodes.