Electron transfer in metal-molecule-metal junction composed of self-assembled monolayers of helical peptides carrying redox-active ferrocene units

Electronic properties of three kinds of helical peptides with or without redox-active ferrocene units were investigated by using scanning tunneling microscopy under ultrahigh vacuum. The currents through the helical peptides carrying ferrocene units at the molecular terminals became significantly larger than that through a reference peptide without any ferrocene units. On the other hand, ferrocene units in the middle of the peptide chain did not affect the current-voltage characteristics. These results indicate that the ferrocene units near the metal electrode should play an important role for efficient electron transfer between the metal and the peptide molecules, which process is one of the rate-determining steps for characterizing molecular conductance in metal-molecule-metal junction.     

Effects of the position of internal histidine residues on the collision-induced fragmentation of triply protonated tryptic peptides

The collision-induced dissociation spectra of a series of synthetic, tryptic peptides that differed by the position of an internal histidine residue were studied. Electrospray ionization of these peptides produced both doubly and triply protonated molecular ions. Collision-induced fragmentation of the triply protonated peptide ions had better efficiency than that of the doubly protonated ions, producing a higher abundance of product ions at lower collision energies. The product ion spectra of these triply protonated ions were dominated by a series of doubly charged y-ions and the amount of sequence information was dependent on the position of the histidine residue. In the peptides where the histidine was located towards the C-terminus of the peptide, a more extensive series of sequence specific product ions was observed. As the position of the histidine residue was moved towards the N-terminus of the peptide, systematically less sequence information was observed. The peptides were subsequently modified with diethylpyrocarbonate to manipulate the product ion spectra. Addition of the ethoxyformyl group to the N-terminus and histidine residue shifted the predominant charge state of the modified peptide to the doubly protonated form. These peptide ions fragmented efficiently, producing product ion spectra that contained more sequence information than could be obtained from the corresponding unmodified peptide.     

The Correlation of Electrochemical Measurements and Molecular Junction Conductance Simulations in β‐Strand Peptides

Understanding the electronic properties of single peptides is not only of fundamental importance, but it is also paramount to the realization of peptide‐based molecular electronic components. Electrochemical and theoretical studies are reported on two β‐strand‐based peptides, one with its backbone constrained with a triazole‐containing tether introduced by Huisgen cycloaddition (peptide 1 ) and the other a direct linear analogue (peptide 2 ). Density functional theory (DFT) and non‐equilibrium Green’s function were used to investigate conductance in molecular junctions containing peptides 3 and 4 (analogues of 1 and 2 ). Although the peptides share a common β‐strand conformation, they display vastly different electronic transport properties due to the presence (or absence) of the side‐bridge constraint and the associated effect on backbone rigidity. These studies reveal that the electron transfer rate constants of 1 and 2 , and the conductance calculated for 3 and 4 , differ by approximately one order of magnitude, thus providing two distinctly different conductance states and what is essentially a molecular switch. A definitive correlation of electrochemical measurements and molecular junction conductance simulations is demonstrated using two different charge transfer techniques. This study furthers our understanding of the electronic properties of peptides at the molecular level, which provides an opportunity to fine‐tune their molecular orbital energies through suitable structural manipulation.     

Conductance titration of single-peptide molecules

We have measured the conductance of single peptides covalently bonded to two Au electrodes via S-Au bonds by repeatedly forming a large number of molecular junctions. The conductance decreases exponentially with the peptide length, with a decay constant of beta = 0.9 +/- 0.1 A-1, suggesting that tunneling is the mechanism of electron transport in the peptides. The conductance of the peptides is sensitive to the solution pH, due to the protonation/deprotonation of the amine and carboxyl groups of the peptides, which provides titration measurements based on single-molecule conductance.     

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.     

Molecularly resolved protein electromechanical properties

Previous work has shown that protein molecules can be trapped between the conductive surfaces presented by a metal-coated AFM probe and an underlying planar substrate where their molecule-specific conductance characteristics can be assayed. Herein, we demonstrate that transport across such a derived metal-protein-electrode junction falls within three, pressure-dependent, regimes and, further, that pressure-dependent conductance can be utilized in analyzing temporal variations of protein fold. Specifically, the electronic and mechanical properties of the metalloprotein azurin have been characterized under conditions of anisotropic vertical compression through the use of a conducting atomic force microscope (CP-AFM). By utilizing the ability of azurin to chemically self-assemble on the gold surface presented either by the apex of a suitably coated AFM probe or a planar metallic surface, molecular-level transport characteristics are assayable. Under conditions of low force, typically less than 2 nN, the weak physical and electronic coupling between the protein and the conducting contacts impedes tunneling and leads to charge buildup followed by dielectric breakdown. At slightly increased force, 3-5 nN, the copper protein exhibits temporal electron occupation with observable negative differential resistance, while the redox-inactive zinc mutant does not. At imposed loads greater than 5 nN, appreciable electron tunneling can be detected even at low bias for both the redox-active and -inactive species. Dynamic current-voltage characteristics have been recorded and are well-described by a modified Simmons tunneling model. Subsequent analyses enable the electron tunneling barrier height and barrier length to be determined under conditions of quantified vertical stress. The variance observed describes, in essence, the protein's mechanical properties within the confines of the tunnel junction.     

二茂铁自组装分子结中电荷隧穿与跳跃传输的机理研究

采用硫醇自组装单层膜结合悬浮纳米线技术制备了分子结器件, 对比研究了非电活性的1-十一烷基硫醇(C11)和电活性的二茂铁己硫醇(FHT)分子结的电荷传输特性. 结合两种传输机理, 提出一种新的模型拟合了分子结的电流-电压特性, 发现了氧化还原活性中心二茂铁(Ferrocene, Fc)可以使电荷传输机理由隧穿变成隧穿与跳跃共存. 结合变温实验验证了这一机理, 并对这种混合机理出现的原因进行了分析.     

Not So Innocent After All: Interfacial Chemistry Determines Charge-Transport Efficiency in Single-Molecule Junctions

Most studies in molecular electronics focus on altering the molecular wire backbone to tune the electrical properties of the whole junction. However, it is often overlooked that the chemical structure of the groups anchoring the molecule to the metallic electrodes influences the electronic structure of the whole system and, therefore, its conductance. We synthesised electron-accepting dithienophosphole oxide derivatives and fabricated their single-molecule junctions. We found that the anchor group has a dramatic effect on charge-transport efficiency: in our case, electron-deficient 4-pyridyl contacts suppress conductance, while electron-rich 4-thioanisole termini promote efficient transport. Our calculations show that this is due to minute changes in charge distribution, probed at the electrode interface. Our findings provide a framework for efficient molecular junction design, especially valuable for compounds with strong electron withdrawing/donating backbones.     

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.     

1,8-二巯基芘分子电学特性的理论研究

Based on the first principle,electrical properties of a molecular junction consisting of pyrene-1,8-dithiol molecule and gold surface have been investigated. The cluster of three gold atoms is used to simulate the gold surface. Density functional theory is employed to obtain the electronic structures of the molecule and the extended molecule. Then the frontier orbital theory and the perturbation theory are used to determine the interaction energy between the molecule and the gold surface quantitatively. The elastic Green function method is applied to study the current-voltage properties of the molecular junction. Numerical results show that the sulfur atoms can be chemically absorbed on the gold surface and the bonding between the molecule and gold is mainly covalent-typed. The fermienergy of the extended molecular system lies between the HOMO and the LUMO and closer to the HOMO of the system. When the external applied bias is lower than 1 V,there is a current gap for the molecular junction. With the increasing of the bias,the conductance of the junction exhibits plateaus. These electrical properties are closely related with the electronic structures of the molecular junction. The extended molecular orbits have great contribution to the charge transport. Localized molecular orbits give little contribution to the current while charge transport is taken place by tunneling.     

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.     

Controlling Single Molecule Conductance by a Locally Induced Chemical Reaction on Individual Thiophene Units

Among the prerequisites for the progress of single‐molecule‐based electronic devices are a better understanding of the electronic properties at the individual molecular level and the development of methods to tune the charge transport through molecular junctions. Scanning tunneling microscopy (STM) is an ideal tool not only for the characterization, but also for the manipulation of single atoms and molecules on surfaces. The conductance through a single molecule can be measured by contacting the molecule with atomic precision and forming a molecular bridge between the metallic STM tip electrode and the metallic surface electrode. The parameters affecting the conductance are mainly related to their electronic structure and to the coupling to the metallic electrodes. Here, the experimental and theoretical analyses are focused on single tetracenothiophene molecules and demonstrate that an in situ‐induced direct desulfurization reaction of the thiophene moiety strongly improves the molecular anchoring by forming covalent bonds between molecular carbon and copper surface atoms. This bond formation leads to an increase of the conductance by about 50 % compared to the initial state.     

Currents and photocurrents in organic materials determined by the interface phenomena

The role of interface between molecular material and electrode on currents and photocurrents is considered. Mechanisms of charge carrier injection, electrode recombination and transport are discussed. Particularly thermal, excitonic, photo and tunneling injection of charge carriers, diffusion in presence of image force, interface barrier between electrode and organic materials and two organic materials, non-uniformity of electrodes and other phenomena on charge carrier injection are considered. The data presented in the review which complete theoretical considerations have been taken from previous as well as current literature. The considered phenomena are very important for the analysis of many practical problems for molecular electronic devices such as rectification of current, organic transistors, electroluminescence, photovoltaic effects and some similar problems.     

Distal charge transport in peptides

Biological systems often transport charges and reactive processes over substantial distances. Traditional models of chemical kinetics generally do not describe such extreme distal processes. In this Review, an atomistic model for a distal transport of information, which was specifically developed for peptides, is considered. Chemical reactivity is taken as the result of distal effects based on two-step bifunctional kinetics involving unique, very rapid motional properties of peptides in the subpicosecond regime. The bifunctional model suggests highly efficient transport of charge and reactivity in an isolated peptide over a substantial distance; conversely, a very low efficiency in a water environment was found. The model suggests ultrafast transport of charge and reactivity over substantial molecular distances in a peptide environment. Many such domains can be active in a protein.     

Tuning electrochemical rectification via quantum dot assemblies

A novel approach to tuning electrochemical rectification using 2D assemblies of quantum dots (QDs) is presented. Asymmetric enhancement of the oxidation and reduction currents in the presence of the Fe(CN)(6)(3-/4-) redox couple is observed upon adsorption of QDs at thiol-modified Au electrodes. The extent of the electrochemical rectification is dependent on the average QD size. A molecular blocking layer is generated by self-assembling 11-mercaptoundecanoic acid (MUA) and an ultrathin film of poly(diallyldimethylammonium chloride) (PDADMAC) on the electrode. The polycationic film allows the electrostatic adsorption of 3-mercaptopropionic acid (MPA)-stabilized CdTe QDs, generating 2D assemblies with approximately 0.4% coverage. The QD adsorption activates a fast charge transfer across the blocking layer in which the reduction process is more strongly enhanced than the oxidation reaction. The partial electrochemical rectification is rationalized in terms of the relative position of the valence (VB) and conduction band (CB) edges with respect to the redox Fermi energy (ε(redox)). Quantitative analysis of the exchange current density obtained from electrochemical impedance spectroscopy demonstrates that the enhancement of charge transport across the molecular barrier is strongly dependent on the position of the QD valence band edge relative to ε(redox). The average electron tunneling rate constant through the QD assemblies is estimated on the basis of the Gerischer model for electron transfer.     

Quantum interference enhanced thermopower in single-molecule thiophene junctions

Quantum interference(QI) effects, which offer unique opportunities to widely manipulate the charge transport properties in the molecular junctions, will have the potential for achieving high thermopower.Here we developed a scanning tunneling microscope break junction technique to investigate the thermopower through single-molecule thiophene junctions. We observed that the thermopower of 2,4-TPSAc with destructive quantum interference(DQI) was nearly twice of 2,5-TP-SAc without DQI, while the con...     

Ruthenium Tris‐bipyridine Single‐Molecule Junctions with Multiple Joint Configurations

Single‐molecule junctions are of particular interest in molecular electronics. To realize molecular electronic devices, it is crucial that functional single‐molecule junctions are connected to each other by using joint units on the atomic scale. However, good joint units have not been reported because controlling the charge transport directions through the junctions is not trivial. Here, we report a joint unit that controls and changes the charge transport directions through the junctions, by using a ruthenium–tris‐bipyridine (RuBpy) complex. The RuBpy single‐molecule junction was fabricated with scanning tunnelling microscopy‐based break junction techniques. The RuBpy single‐molecule junction showed two distinct high and low conductance states. The two states were characterized by the conductance measurement, the correlation analysis, and the comparative experiment of bipyridine (Bpy), which is the ligand unit of RuBpy. We demonstrate that the Ru complex has multiple charge transport paths, where the charge is carried vertically and horizontally through the complex depending on the path.     

Vibrational spectroscopy of individual doping centers in a monolayer organic crystal

A scanning tunneling microscope (STM) is used to study individual Ag doping centers in a monolayer of C60 molecules supported on a thin Al2O3 film grown on the NiAl(110) surface. Vibronic states of the doping centers are observed with differential conductance (dIdV) spectroscopy. The double-barrier nature of the junction results in bipolar transport: same states participate in charge transport at both bias voltage polarities. Identification of the dIdV features corresponding to bipolar conduction enables a new mode of vibrational spectroscopy with STM.     

Influence of the charge relay effect on the silanol condensation reaction as a model for silica biomineralization

The catalytic effect of various sequential peptides for silica biomineralization has been studied. In peptide sequence design, lysine (K) and histidine (H) were selected as the standard amino acids and aspartic acid (D) was selected to promote the charge relay effects, such as in the enzyme active site. Therefore, homopolypeptides (K(10) and H(10)), block polypeptides (K(5)D(5) and H(5)D(5)), and alternate polypeptides [(KD)(5) and (HD)(5)] were designed, and the dehydration reaction ability of trimethylethoxysilane was investigated as a quantitative model of silica mineralization. The catalytic activity per basic residue of alternate polypeptide was the highest because of the charge relay effects at the surface of the peptide. In silica mineralization using tetraethoxysilane, spherical silica particles were obtained, and their size is related to the catalytic activities of the peptides in the model systems. From these results, the effect of the functional group combination by the peptide sequence design enables the control of the efficiency of mineralization and preparation of specific inorganic materials.