Molecular conductance: chemical trends of anchoring groups

Combining density functional theory calculations for molecular electronic structure with a Green function method for electron transport, we calculate from first principles the molecular conductance of benzene connected to two Au leads through different anchoring atoms-S, Se, and Te. The relaxed atomic structure of the contact, different lead orientations, and different adsorption sites are fully considered. We find that the molecule-lead coupling, electron transfer, and conductance all depend strongly on the adsorption site, lead orientation, and local contact atomic configuration. For flat contacts the conductance decreases as the atomic number of the anchoring atom increases, regardless of the adsorption site, lead orientation, or bias. For small bias this chemical trend is, however, dependent on the contact atomic configuration: an additional Au atom at the contact with the (111) lead changes the best anchoring atom from S to Se, although for large bias the original chemical trend is recovered.     

Models of electrodes and contacts in molecular electronics

Bridging the difference in atomic structure between experiments and theoretical calculations and exploring quantum confinement effects in thin electrodes (leads) are both important issues in molecular electronics. To address these issues, we report here, by using Au-benzenedithiol-Au as a model system, systematic investigations of different models for the leads and the lead-molecule contacts: leads with different cross sections, leads consisting of infinite surfaces, and surface leads with a local nanowire or atomic chain of different lengths. The method adopted is a nonequilibrium Green's-function approach combined with density-functional theory calculations for the electronic structure and transport, in which the leads and molecule are treated on the same footing. It is shown that leads with a small cross section will lead to large oscillations in the transmission function T(E), which depend significantly on the lead structure (orientation) because of quantum waveguide effects. This oscillation slowly decays as the lead width increases, with the average approaching the limit given by infinite surface leads. Local nanowire structures around the contacts induce moderate fluctuations in T(E), while a Au atomic chain (including a single Au apex atom) at each contact leads to a significant conductance resonance.     

In silico engineering of tailored ink-binding ability at molecular printboards.

Molecular recognition between guest ink molecules and beta-cyclodextrin (beta-CD) cavities at self-assembled monolayers provides a molecular printboard for nanopatterning applications. We recently used molecular dynamics (MD) simulations to describe the specificity of ink-printboard binding and here extend the simulations to include charged cyclodextrin hosts, necessary to broaden the chemistry of molecular printboards and bind charged inks such as the ferrocenium cation. Shifting to high pH, or alternatively grafting a charged sidearm onto beta-CD, created three distinct types of anionic beta-CD cavity and we used electronic structure calculations and MD simulations to measure host-guest charge transfer and binding strengths. We find that steric recognition of uncharged organic molecules is retained at the charged printboards, and that improved guest-host electrostatic contacts can strengthen binding of larger inks while penalising small inks, enhancing the level of discrimination. A prudent choice of complementary host-guest shape and charge states thus provides a means of tuning both ink binding strength and specificity at molecular printboards.     

Controlling the conductance of atomically thin metal wires with electrochemical potential

We report on the study of quantum transport in atomically thin Au wires suspended between two Au electrodes by modulating the electrochemical potential of the wires in various electrolytes. The potential modulation induces a conductance modulation with a phase shift that is always approximately 180 degrees, meaning that an increase in the potential always causes a decrease in the conductance. The amplitude of the induced conductance modulation, however, depends on several parameters. First, it depends on the atomic configurations of the individual wires. Second, the relative amplitude, defined as the ratio of the conductance modulation amplitude to the conductance, decreases as the diameter of the wire increases. Third, it depends on whether anion adsorption is present. In the absence of anion adsorption, it is approximately 0.55G(0) (G(0) = 2e(2)/h) per V of potential modulation, for a wire with conductance quantized near 1G(0). This double layer charging-induced conductance modulation can be attributed to a change in the effective diameter of the wire. In the presence of anion adsorption, the amplitude is much larger (e.g., approximately 1.6G(0)/V when I(-) adsorption takes place) and correlates well with the strength of the adsorption, which is due to the scattering of conduction electrons by the adsorbed anions.     

Conductance of single alkanedithiols: conduction mechanism and effect of molecule-electrode contacts

The conductance of single alkanedithiols covalently bound to gold electrodes has been studied by statistical analysis of repeatedly created molecular junctions. For each molecule, the conductance histogram reveals two sets of well-defined peaks, corresponding to two different conductance values. We have found that (1) both conductance values decrease exponentially with the molecular length with an identical decay constant, beta approximately equal to 0.84 A(-1), but with a factor of 5 difference in the prefactor of the exponential function. (2) The current-voltage curves of the two sets can be fit with the Simmons tunneling model. (3) Both conductance values are independent of temperature (between -5 and 60 degrees C) and the solvent. (4) Despite the difference in the conductance, the forces required to break the molecular junctions are the same, 1.5 nN. These observations lead us to believe that the conduction mechanism in alkanedithiols is due to electron tunneling or superexchange via the bonds along the molecules, and the two sets of conductance peaks are due to two different microscopic configurations of the molecule-electrode contacts.     

Molecular adsorption onto metallic quantum wires.

We have studied the adsorption of mercaptopropionic acid, 2,2'-bipyridine, and dopamine onto electrochemically fabricated Cu nanowires. The nanowires are atomically thin with conductance quantized near integer multiples of 2e(2)/h. Upon molecular adsorption, the quantized conductance decreases to a fractional value, due to the scattering of the conduction electrons by the adsorbates. The decrease is as high as 50% for the thinnest nanowires whose conductance is at the lowest quantum step, and smaller for thicker nanowires with conductance at higher quantum steps. The adsorbate-induced conductance changes depend on the binding strengths of the molecules to the nanowires, which are in the order of mercaptopropionic acid, 2,2'-bipyridine, and dopamine, from strongest to weakest. The sensitive dependence of the quantized conductance on molecular adsorption may be used for molecular detection.     

Spin-related electronic pathway through single molecule on Au(111)

Spin properties of organic molecules have attracted great interest for their potential applications in spintronic devices and quantum computing. Fe-tetraphenyl porphyrin (FeTPP) is of particular interest for its robust magnetic properties on metallic substrates. FeTPP is prepared in vacuum via on-surface synthesis. Molecular structure and spin-related transport properties are characterized by low-temperature scanning tunneling microscope and spectroscopy at 0.5 K. Density functional theory calculations are performed to understand molecular adsorption and spin distribution on Au(111). The molecular structure of FeTPP is distorted upon adsorption on the substrate. Spin excitations of FeTPP are observed on the Fe atom and high pyrrole groups in differential conductance spectra. The calculated spin density distribution indicates that the electron spin of FeTPP is mainly distributed on the Fe atom. The atomic transmission calculation indicates that electrons transport to substrate is mediated through Fe atom, when the tip is above the high pyrrole group.     

Carbon nanotube, graphene, nanowire, and molecule-based electron and spin transport phenomena using the nonequilibrium Green's function method at the level of first principles theory

Based on density functional theory, we have developed a program code to investigate the electron transport characteristics for a variety of nanometer scaled devices in the presence of an external bias voltage. We employed basis sets comprised of linear combinations of numerical type atomic orbitals, particularly focusing on k-point sampling for the realistic modeling of the bulk electrode. The scheme coupled with the matrix version of the nonequilibrium Green's function method enables calculation of the transmission coefficients at a given energy and voltage in a self-consistent manner as well as the corresponding current-voltage (I-V) characteristics. This scheme has advantages because it is applicable to large systems, easily transportable to different types of quantum chemistry packages, and extendable to time-dependent phenomena or inelastic scatterings. It has been applied to diverse types of practical electronic devices such as carbon nanotubes, graphene nanoribbons, metallic nanowires, and molecular electronic devices. The quantum conductance phenomena for systems involving quantum point contacts and I-V curves for a single molecule in contact with metal electrodes using the k-point sampling method are described.     

Tungsten devices in analytical atomic spectrometry

Tungsten devices have been employed in analytical atomic spectrometry for approximately 30 years. Most of these atomizers can be electrically heated up to 3000 °C at very high heating rates, with a simple power supply. Usually, a tungsten device is employed in one of two modes: as an electrothermal atomizer with which the sample vapor is probed directly, or as an electrothermal vaporizer, which produces a sample aerosol that is then carried to a separate atomizer for analysis. Tungsten devices may take various physical shapes: tubes, cups, boats, ribbons, wires, filaments, coils and loops. Most of these orientations have been applied to many analytical techniques, such as atomic absorption spectrometry, atomic emission spectrometry, atomic fluorescence spectrometry, laser excited atomic fluorescence spectrometry, metastable transfer emission spectroscopy, inductively coupled plasma optical emission spectrometry, inductively coupled plasma mass spectrometry and microwave plasma atomic spectrometry. The analytical figures of merit and the practical applications reported for these techniques are reviewed. Atomization mechanisms reported for tungsten atomizers are also briefly summarized. In addition, less common applications of tungsten devices are discussed, including analyte preconcentration by adsorption or electrodeposition and electrothermal separation of analytes prior to analysis. Tungsten atomization devices continue to provide simple, versatile alternatives for analytical atomic spectrometry.     

Nonlinear enhancement of oxygen evolution in thylakoid membranes: modeling the effect of light intensity and beta-cyclodextrin concentration

Electron transport through photosystem II, measured as oxygen evolution (OE), was investigated in isolated thylakoid membranes treated with beta-cyclodextrin (beta-CD, a cyclic oligosaccharide constituted of seven alpha-d-glucose residues linked by alpha-1,4 glycosidic bonds) and irradiated with white light of variable intensity. First, we found that the light-response curves of oxygen evolution are well fitted with a hyperbolic function, the shape of which is not affected by the beta-CD concentration. Second, we showed that under conditions of irradiation with white light of saturating intensity ( approximately 5000 mumol of photons/m(2).s) beta-CD enhances the oxygen evolution in the thylakoid membranes according to a sigmoid function displaying a sharp inflection point, or transition. Unexpectedely, this beta-CD effect is not observed at irradiances of less than approximately 300 mumol of photons/m(2).s. We attempted a theoretical analysis of the combined effect of irradiance and beta-CD concentration on oxygen evolution (OE(th)). For this purpose, the effect of irradiance (I) was modeled with a hyperbola (i) and the beta-CD concentration (C) contribution with a Hill equation, that is, a sigmoid function (ii). The mathematical simulations generated the following general expressions: (i) OE(th) = [OE(max)(0) G(1)(C)]I/[L(1/2)(0) G(2)(C) + I] and (ii) G(i)()(C) = 1 + p[C(n)()/(K(1/2)(n)() + C(n)())], where OE(max)(0) is the OE maximum (OE(max)) in the absence of beta-CD, L(1/2)(0) is the photon flux density giving OE(max)/2 in the absence of beta-CD, G(1)(C) or G(2)(C) is obtained from G(i)()(C) where i is 1 or 2, n is the Hill coefficient, p is a parameter to account for the beta-CD-mediated maximum OE increase, and K(1/2) is the beta-CD concentration giving half-maximal OE activity. The results of the calculations yielded the expression (iii) OE(th) = 151[1 + 3.3C(4.8)/(13.1(4.8) + C(4.8))]I/{97.5[1 + 5.2C(7.8)/(14.8(7.8) + C(7.8))] + I} which agrees well with the experimental data for a broad range of I and C. Note that, for C = 0, eq iii reverts to the light-response curve of oxygen evolution in the absence of beta-CD. We conclude that eq iii is a good approximation of the combined effect of irradiance and beta-CD concentration, meaning that the model has a significant value for predicting the outcome of associated photochemical and biochemical reactions.     

Control of the water permeability of polyelectrolyte multilayers by deposition of charged paraffin particles

We present a new way to protect polyelectrolyte multilayers from water, consisting in the adsorption and subsequent fusing of charged wax particles atop a multilayer. The formation of the wax layer is demonstrated by different techniques such as ellipsometry, contact angle measurements, and atomic force microscopy. The diffusion of water in protected and unprotected multilayers is studied by in situ neutron reflectometry. Whereas a top layer of wax crystals already allows substantial reduction of the diffusion, the fusion of this top layer leads to the dominating exclusion of water from the multilayers when dipped in water. This method opens up new interesting avenues for polyelectrolyte multilayers in practical applications where permeability of water, ions, or hydrophilic drugs is an issue.     

参杂缺陷石墨烯的高分子复合材料导热特性分子动力学模拟

Polymers are widely used advanced materials composed of macromolecular chains, which can be found in materials used in our daily life. Polymer materials have been employed in many energy and electronic applications such as energy harvesting devices, energy storage devices, light emitting and sensing devices, and flexible energy and electronic devices. The microscopic morphologies and electrical properties of the polymer materials can be tuned by molecular engineering, which could improve the device performances in terms of both the energy conversion efficiency and stability. Traditional polymers are usually considered to be thermal insulators owing to their amorphous molecular chains. Graphene-based polymeric materials have garnered significant attention due to the excellent thermal conductivity of graphene. Advanced polymeric composites with high thermal conductivity exhibit great potential in many applications. Therefore, research on the thermal transport behaviors in graphene-based nanocomposites becomes critical. Vacancy defects in graphene are commonly observed during its fabrication. In this work, the effects of vacancy defects in graphene on thermal transport properties of the graphene-polyethylene nanocomposite are comprehensively investigated using molecular dynamics (MD) simulation. Based on the non-equilibrium molecular dynamics (NEMD) method, the interfacial thermal conductance and the overall thermal conductance of the nanocomposite are taken into consideration simultaneously. It is found that vacancy defects in graphene facilitate the interfacial thermal conductance between graphene and polyethylene. By removing various proportions of carbon atoms in pristine graphene, the density of vacancy defects varies from 0% to 20% and the interfacial thermal conductance increases from 75.6 MW·m⁻²·K⁻¹ to 85.9 MW·m⁻²·K⁻¹. The distinct enhancement in the interfacial thermal transport is attributed to the enhanced thermal coupling between graphene and polyethylene. A higher number of broken sp² bonds in the defective graphene lead to a decrease in the structure rigidity with more low-frequency (< 15 THz) phonons. The improved overlap of vibrational density states between graphene and polyethylene at a low frequency results in better interfacial thermal conductance. Moreover, the increase in the interfacial thermal conductance induced by vacancy defects have a significant effect on the overall thermal conductance (from 40.8 MW·m⁻²·K⁻¹ to 45.6 MW·m⁻²·K⁻¹). In addition, when filled with the graphene layer, the local density of polyethylene increases on both sides of the graphene. The concentrated layers provide more aligned molecular arrangement, which result in better thermal conductance in polyethylene. Further, the higher local density of the polymer near the interface provides more atoms for interaction with the graphene, which leads to stronger effective interactions. The relative concentration is insensitive to the density of vacancy defects. The reported results on the thermal transport behavior of graphene-polyethylene composites provide reasonable guidance for using graphene as fillers to tune the thermal conduction of polymeric composites.     

Current density analysis of electron transport through molecular wires in open quantum systems

The current density in molecular wires connected to contacts is investigated within the nonequilibrium Green's function formalism combined with the Landauer approach. Energy-dependent and total current density through a series of molecular junctions are calculated in real space representation. A rich variety of current patterns including pronounced ring currents (“vortices”) are found even in the defect-free minimal building blocks of molecular devices. The influences of contact positions, functional groups as well as atomic defects on the transport properties are examined systematically for prototypical ortho-, meta-, and para-substituted benzenes as well as heteroaromatic systems. It is found that substitutional functional groups mainly shift the molecular levels and retain characteristic transport channels, while a significant change of electronic pathways and conductance is induced by hetero-aromaticity. The current distribution is used to calculate the static magnetic field distribution in the carbon-based conductors. © 2017 Wiley Periodicals, Inc.     

Nanowires: size evolution,reversibility, and one-atom contacts

Size-evolutionary patterns of mechanical and electronic transport properties in junctions are discussed, showing formation of nanowires upon narrowing, regardless of the initial dimensions of the junctions. Mechanical and conductance reversibility of such nanowires in elongation and compression cycles is studied. Ab-initio localspin- density functional based molecular dynamics simulations show formation of a stable “one-atom” contact in a sodium wire at 189 K, with the atomic structure near the apex of the nanowire correlated to that of two pentagonal pyramidal Na₇ clusters joined by the apex atom. The average electronic conductance through the one-atom contact, at 189 K, is estimated to be ～4.5 (2e ²/h).     

An atomistic model and key parameters for devising single molecular nanowire sensors

Based on one impurity model Hamiltonian describing a nanowire upon adsorption of a molecule, we obtain an analytical formula of the conductance which is governed clearly by modulating key parameters. The formula shows that the conductance change in nanowire upon adsorption of a molecule is mainly controlled by three factors, electron hopping between adsorbed molecule and nanowire, chemical potential, and the change of atomic configurations of the nanowires near the adsorption site. Conductance is very sensitive to the choice of these key parameters; therefore, a proper nanowire system that renders matched chemical potential as well as hopping strength between the nanowire and the adsorbed molecule should be devised for the sensor applications. Our model calculations give similar conductance features to the conductance obtained by the first principle calculations for a singe-molecule-adsorbed molecular wire. It is worthy of note that the system can be in antiresonance, which is characterized by a quick drop in conductance when a molecule is adsorbed on the nanowires.     

Microcantilever sensing and actuation with end-grafted stimulus-responsive elastin-like polypeptides

Stimulus-responsive elastin-like polypeptides (ELPs) grafted onto surfaces are of significant technical interest because they can be exploited for force generation, in sensing applications, or as molecular switches with tunable properties. Changes in the conformational state of grafted ELPs, induced by a phase transition or changes in osmotic pressure, lead to significant changes in the surface stress in the ELP graft layer and translate into detectable changes in microcantilever deflection. In this study, we investigate the conformational mechanics of ELPs in response to changes in solution pH and ionic strength using atomic force microscopy (AFM) microcantilever deflection and quartz crystal microbalance (QCM) measurements. We show that the use of genetically encoded, surface-grafted ELPs is exciting for cantilever actuation and sensing because commonly available microfabricated cantilever springs offer a simple and nonintrusive way to detect changes in solvent type, temperature, and pH, promising great potential for sensing applications in microfluidic devices.     

石墨烯在自供能传感系统中的应用

随着现代社会智能化的加速发展,传感系统中传感器的数量、密度和分布范围不断增加,传统的供能方式难以满足如此复杂多变的传感器供能需求,从周围环境中收集能量并转化为电能的自供能传感器件是解决这一难题的有效途径。石墨烯不仅具有优异的传感性能,而且在各种能源器件中有广泛的应用,这为基于石墨烯的自供能传感器件设计提供了便利。近年来,人们已经研究和发展了多种多样的石墨烯自供能传感器件。本文基于自供能器件的基本能量供给原理,包括电化学供能、光伏供能、摩擦电供能、水伏供能以及热电、压电、热释电等其它供能,分别介绍了石墨烯在自供能传感器件中的应用,并展望了基于石墨烯的自供能传感器件的未来发展、挑战和前景。     

Conjugated thiol linker for enhanced electrical conduction of gold-molecule contacts

Single-molecule electrical conduction studies are used to evaluate how the molecular linking unit influences the tunneling efficiency in metal-molecule-metal (m-M-m) junctions. This work uses conducting-probe atomic force microscopy (CP-AFM) to compare the molecular conduction of two pi-bonded molecules: one with a single thiol linker, and another with a conjugated double thiol linker at both ends of the molecules. The results demonstrate that the molecule with conjugated double thiol linkers displays higher conduction in gold-molecule-gold junctions than nonconjugated single thiol-gold contacts.     

Spectroelectrochemical Behavior of Polycrystalline Gold Electrode Modified by Reverse Micelles

The increasing demand for raising the reliability of electronic contacts has led to the development of methods that protect metal surfaces against atmospheric corrosion agents. This severe problem implies an important economic cost annually but small amounts of corrosion inhibitors can control, decrease or avoid reactions between a metal and its environment. In this regard, surfactant inhibitors have displayed many advantages such as low price, easy fabrication, low toxicity and high inhibition efficiency. For this reason, in this article, the spectroelectrochemical behavior of polycrystalline gold electrode modified by reverse micelles (water/polyethyleneglycol-dodecylether (BRIJ 30)/n-heptane) is investigated by atomic force microscopy (AFM), potentiodynamic methods and electrochemical impedance spectroscopy (EIS). Main results indicate a strong adsorption of a monolayer of micelles on the gold substrate in which electron tunneling conduction is still possible. Therefore, this method of increasing the corrosion resistance of gold contacts is usable only in conditions of long-term storage but not in the operation of devices with such contacts. In this regard, the micelle coating must be removed from the surface of the gold contacts before use. Finally, the aim of the present work is to understand the reactions occurring at the surfactant/metal interface, which may help to improve the fabrication of novel electrodes.     

Dipole preserving and polarization consistent charges

A method for estimating dipole preserving and polarization consistent (DPPC) charges is described, which reproduces exactly the molecular dipole moment as well as the local, atomic hybridization dipoles determined from the corresponding wave function and can yield accurate molecular polarization. The method is based on a model described by Thole and van Duijnen and a new feature is introduced to treat molecular polarization. Thus, the DPPC method offers a convenient procedure to describe molecular polarization in applications using semiempirical models and ab initio molecular orbital theory with relatively small basis functions such as 6‐31+G(d,p) or without inclusion of electron correlation; these methods tend to underestimate molecular polarizability. The trends of the DPPC partial atomic charges are found to be in good accord with those of the CM2 model, a class IV charge analysis method that has been used in a variety of applications. The DPPC method is illustrated to mimic the correct molecular polarizability in a water dimer test case and in water‐halide ion complexes using the explicit polarization (X‐Pol) potential with the Austin model 1 Hamiltonian. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011