Nonequilibrium electronic transport of 4,4'-bipyridine molecular junction

The electronic transport properties of a 4,4'-bipyridine molecule sandwiched between two Au(111) surfaces are studied with a fully self-consistent nonequilibrium Green's-function method combined with the density-functional theory. The 4,4'-bipyridine molecule prefers to adsorb near the hollow site of the Au(111) surface and distorts slightly. The modifications on the electronic structure of the molecule due to the presence of the electrodes are described by the renormalized molecular orbitals, which correspond well to the calculated transmission peaks. The average Fermi level lies close to the lowest unoccupied renormalized molecular orbital, which determines the electronic transport property of the molecular junction under a small bias voltage. The total transmission is contributed by a single channel. The transmission peaks shift with the applied bias voltage, and this behavior depends on the spatial distribution of the renormalized molecular orbitals and the voltage drop along the molecular junction. The shape of the calculated conductance curve of the equilibrium geometric configuration reproduces the main feature of the experimental results, but the value is larger than the measured data by about 6 times. Good agreement with the experimental measurements can be obtained by elongating the molecular junction. The electronic transport behaviors depend strongly on the interface configuration.     

Dithiocarbamate anchoring in molecular wire junctions: a first principles study

Recent experimental realization [J. Am. Chem. Soc., 127 (2005) 7328] of various dithiocarbamate self-assembly on gold surface opens the possibility for use of dithiocarbamate linkers to anchor molecular wires to gold electrodes. In this paper, we explore this hypothesis computationally. We computed the electron transport properties of 4,4'-bipyridine (BP), 4,4'-bipyridinium-1,1'-bis(carbodithioate) (BPBC), 4-(4'-pyridyl)-peridium-1-carbodithioate (BPC) molecule junctions based on the density functional theory and nonequilibrium Green's functions. We demonstrated that the stronger molecule-electrode coupling associated with the conjugated dithiocarbamate linker broadens transmission resonances near the Fermi energy. The broadening effect along with the extension of the pi conjugation from the molecule to the gold electrodes lead to enhanced electrical conductance for BPBC molecule. The conductance enhancement factor is as large as 25 at applied voltage bias 1.0 V. Rectification behavior is predicted for BPC molecular wire junction, which has the asymmetric anchoring groups.     

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

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

Analysis on the contribution of molecular orbitals to the conductance of molecular electronic devices

We present a theoretical approach which allows one to extract the orbital contribution to the conductance of molecular electronic devices. This is achieved by calculating the scattering wave functions after the Hamiltonian matrix of the extended molecule is obtained from a self-consistent calculation that combines the nonequilibrium Green's function formalism with density functional theory employing a finite basis of local atomic orbitals. As an example, the contribution of molecular orbitals to the conductance of a model system consisting of a 4,4-bipyridine molecule connected to two semi-infinite gold monatomic chains is explored, illustrating the capability of our approach.     

Ab initio electron transport study of carbon and boron-nitrogen nanowires

We report first-principles calculations on the electrical transport properties of two kinds of one-dimensional nanowires: (a) a carbon nanowire (CNW) with alternating single and triple bonds and (b) a boron-nitrogen nanowire (BNNW) with equidistant bonds. We demonstrate the similarity and difference between the carbon nanowire and its boron-nitrogen analogue in the molecular orbital and transport properties, and then explore the potential innovations. The effects of molecular orbitals and nanowire-electrode coupling on the transport properties are analyzed. The cases of the nanowires sandwiched between both nanoscale and bulk electrodes are considered. It suggests that the characteristics of the transmission spectra and the current-voltage characteristics (I-V curves) are determined both by the electrodes and by the molecule as well as their coupling. In particular, the negative differential resistance (NDR) phenomenon is more apparent when the nanowires are positioned between two nanoscale electrodes. The tuning of the transport properties is also probed through the changes of nanowire-electrode separation and the inclusion of a gate voltage. These lead to dramatic variations in the equilibrium conductance, which can be understood from the shift and alignment of the molecular orbital relative to the Fermi level of the electrodes. In the analysis of the effects of nanowire-electrode separation, it shows that the equilibrium conductance has the same variation behavior as that of the projected density of states (PDOS) for CNW, while the localized molecular orbitals of BNNW result in its conductance varies differently from its PDOS. The different molecular orbital characteristics near the Fermi level of these two kinds of nanowires underlie their different transport properties.     

Transport properties of graphene nanoribbon-based molecular devices

The electronic and transport properties of an edge-modified prototype graphene nanoribbon (GNR) slice are investigated using density functional theory and Green's function theory. Two decorating functional group pairs are considered, such as hydrogen-hydrogen and NH(2)-NO(2) with NO(2) and NH(2) serving as a donor and an acceptor, respectively. The molecular junctions consist of carbon-based GNR slices sandwiched between Au electrodes. Nonlinear I-V curves and quantum conductance have been found in all the junctions. With increasing the source-drain bias, the enhancement of conductance is quantized. Several key factors determining the transport properties such as the electron transmission probabilities, the density of states, and the component of Frontier molecular orbitals have been discussed in detail. It has been shown that the transport properties are sensitive to the edge type of carbon atoms. We have also found that the accepter-donor functional pairs can cause orders of magnitude changes of the conductance in the junctions.     

分子线电子输运特性的第一性原理研究

从第一性原理出发,利用密度泛函理论研究了SH-C8H16-SH分子和金表面的相互作用,并利用分子前线轨道理论和微扰理论定量地确定了该相互作用能常数,然后,利用弹性散射格林函数方法研究了该分子与金表面形成的分子线的伏-安特性.研究结果表明,当含有硫氢官能团的有机分子化学吸附于金表面时,硫原子将与金原子形成以共价键为主的混和键,此时,扩展的分子轨道使分子线的电导呈现出欧姆特性,而对于局域的分子轨道,电子的输运只能通过隧道效应来实现.对分子线伏-安特性的计算结果显示,在零偏压附近,存在一个电流禁区,随着偏压的增加,分子线的电导呈现出平台特征.     

Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers

We report a combined experimental and computational study of several ruthenium(II) sensitizers originated from the [Ru(dcbpyH(2))(2)(NCS)(2)], N3, and [Ru(dcbpyH(2))(tdbpy)(NCS)(2)], N621, (dcbpyH(2) = 4,4'-dicarboxy-2,2'-bipyridine, tdbpy = 4,4'-tridecyl-2,2'-bipyridine) complexes. A purification procedure was developed to obtain pure N-bonded isomers of both types of sensitizers. The photovoltaic data of the purified N3 and N621 sensitizers adsorbed on TiO(2) films in their monoprotonated and diprotonated state, exhibited remarkable power conversion efficiency at 1 sun, 11.18 and 9.57%, respectively. An extensive Density Functional Theory (DFT)-Time Dependent DFT study of these sensitizers in solution was performed, investigating the effect of protonation of the terminal carboxylic groups and of the counterions on the electronic structure and optical properties of the dyes. The calculated absorption spectra are in good agreement with the experiment, thus allowing a detailed assignment of the UV-vis spectral features of the two types of dyes. The computed alignments of the molecular orbitals of the different complexes with the band edges of a model TiO(2) nanoparticle provide additional insights into the electronic factors governing the efficiency of dye-sensitized solar cell devices.     

First-principles Study of Electron Transport Through Oligoacenes

The electronic transport properties of oligoacenes sandwiched between two Au（111） surfaces with serial and parrallel configurations were investigeted by using a fully self-consistent nonequilibrium Green＇s function method combined with density functional calculations. This theoretical results show that the conductivity of oligoacenes with both sandwiched configurations at low bias voltage is mainly determined by the tail of the transmission peak from the perturbed highest occupied molecular orbital. When the molecular length increases, the zero-bias voltage conductance G（0） of oligoacenes with serial configuration neither follows Magoga＇s exponential law nor displays the even-odd oscillation effect, while the G（O） of the oligoacenes sandwiched with parallel configuration monotonically increases. The reduction of energy gaps, the alignment of the Fermi level, and the spatial distribution of the perturbed molecular orbitals are used to self-consistently explore the transport mechanism through oligoacenes.     

Tunneling currents that increase with molecular elongation

We present a model molecular system with an unintuitive transport-extension behavior in which the tunneling current increases with forced molecular elongation. The molecule consists of two complementary aromatic units (1,4-anthracenedione and 1,4-anthracenediol) hinged via two ether chains and attached to gold electrodes through thiol-terminated alkenes. The transport properties of the molecule as it is mechanically elongated in a single-molecule pulling setting are computationally investigated using a combination of equilibrium molecular dynamics simulations of the pulling with gDFTB computations of the transport properties in the Landauer limit. Contrary to the usual exponential decay of tunneling currents with increasing molecular length, the simulations indicate that upon elongation electronic transport along the molecule increases 10-fold. The structural origin of this inverted trend in the transport is elucidated via a local current analysis that reveals the dual role played by H-bonds in both stabilizing π-stacking for selected extensions and introducing additional electronic couplings between the complementary aromatic rings that also enhance tunneling currents across the molecule. The simulations illustrate an inverted electromechanical single-molecule switch that is based on a novel class of transport-extension behavior that can be achieved via mechanical manipulation and highlight the remarkable sensitivity of conductance measurements to the molecular conformation.     

Al-C60-Al分子结电子输运特性的第一性原理计算

利用基于密度泛函理论的格林函数方法, 计算了Al-C60-Al分子结的电子输运特性. 考虑了C60分子在铝电极表面的原子结构弛豫, 计算结果表明共振传导是Al-C60-Al分子结电子输运的主要特征, 在费米能级附近的电导约为1.14G0 (G0=2e2/h). 投影态密度(PDOS)分析表明, Al-C60-Al分子结的电子输运主要通过C60分子的最低空分子轨道(LUMO)和次低空分子轨道(LUMO+1)进行. 讨论了C60分子和铝电极之间距离的变化对其电子输运特性的影响.     

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.     

Dual conductance, negative differential resistance, and rectifying behavior in a molecular device modulated by side groups

We investigate the electronic transport properties for a molecular device model constructed by a phenylene ethynylene oligomer molecular with different side groups embedding in a carbon chain between two graphene electrodes. Using the first-principles method, the unusual dual conductance, negative differential resistance (NDR) behavior with large peak to valley ratio, and obvious rectifying performance are numerically observed in such proposed molecular device. The analysis of the molecular projected self-consistent Hamiltonian and the evolution of the frontier molecular orbitals (MOs) as well as transmission coefficients under various external voltage biases gives an inside view of the observed results, which suggests that the dual conductance behavior and rectifying performance are due to the asymmetry distribution of the frontier MOs as well as the corresponding coupling between the molecule and electrodes. But the NDR behavior comes from the conduction orbital being suppressed at certain bias. Interestingly, the conduction properties can be tuned by introducing side groups to the molecule and the rectification as well as the NDR behavior (peak to valley ratio) can be improved by adding different side groups in the device model.     

Single-molecule charge-transport measurements that reveal technique-dependent perturbations

We compare scanning tunneling microscopy (STM) imaging with single-molecule conductive atomic force microscopy (C-AFM) measurements by probing a series of structurally related thiol-terminated oligo(phenylenevinylene)s (OPVs) designed to have unique charge-transport signatures. When one or two methylene spacers are inserted between the thiol points of attachment and the OPV core, a systematic reduction in the imaged molecular transconductance and the current transmitted through a metal-molecule-metal junction containing the molecule is observed, indicating good agreement between STM and C-AFM measurements. However, a structure where the OPV backbone is interrupted by a [2.2]paracyclophane core has a low molecular transconductance, as determined from STM images, and a high measured single-molecule conductance. This apparent disconnect can be understood by comparing the calculated molecular orbital topology of the OPV with one thiol bound to a gold surface (the geometry in the STM experiment) with the topology of the molecule with both thiol termini bound to gold (relevant to C-AFM). In the former case, a single contact splits low-lying molecular orbitals into two discrete fragments, and in the latter case, molecular orbitals that span the entire molecule are observed. Although the difference in observed conductance between the two different measurements is resolved, the overall set of observations highlights the importance of using combined techniques to better characterize charge-transport properties relevant to molecular electronics.     

Controlling Electrical Conductance through a π‐Conjugated Cruciform Molecule by Selective Anchoring to Gold Electrodes

Tuning charge transport at the single‐molecule level plays a crucial role in the construction of molecular electronic devices. Introduced herein is a promising and operationally simple approach to tune two distinct charge‐transport pathways through a cruciform molecule. Upon in situ cleavage of triisopropylsilyl groups, complete conversion from one junction type to another is achieved with a conductance increase by more than one order of magnitude, and it is consistent with predictions from ab initio transport calculations. Although molecules are well known to conduct through different orbitals (either HOMO or LUMO), the present study represents the first experimental realization of switching between HOMO‐ and LUMO‐dominated transport within the same molecule.     

Arylthio-substituted coronenes as tailored building blocks for molecular electronics

The electron transport through molecules in molecular devices is typically influenced by the nature of the interfaces with the contacting electrodes and by the interactions between neighbouring molecules. It is a major goal of molecular electronics to adjust the electronic function of a molecular device by tailoring the intrinsic molecular properties and the interfacial and intermolecular interactions. Here, we report on the tunability of the electronic properties of coronene derivatives, namely dodecakis(arylthio)coronenes (DATCs), which are found to exhibit a three-dimensional aromatic system. Scanning tunnelling microscopy (STM), spectroscopy (STS) and simulations based on the density functional theory (DFT) are employed to characterize the structural and electronic properties of these molecules deposited on Au(111) surfaces. It is shown that modifications of the peripheral aryl-groups allow us to specifically affect the self-assembly and the charge transport characteristics of the molecules. Molecular assemblies like supramolecular wires with highly delocalized orbitals and single molecules with molecular "quantum dot" characteristics are obtained in this way.     

Sensing mechanism elucidation of a chemosensor based on a metal-organic framework selective to explosive aromatic compounds

Theoretical elucidation of the turn-off mechanism of the luminescence of a chemosensor based on a metal-organic framework (MOF) [Zn₂(OBA)₄(BYP)₂] (BYP: 4,4′-bipyridine; H₂OBA: 4,4′-oxybis[benzoic acid]), selective to nitrobenzene (NB) via quantum chemical computations, is presented. The electronic structure and optical properties of Zn-MOF were investigated through the combination of density functional theory (DFT) and time-dependent DFT methods. Our results indicate that the fluorescence emission is governed by a linker (BPY)-to-linker (OBA) charge transfer (LLCT) involving orbitals π-type. Next, the interaction with the analyte was analyzed, where very interesting results were obtained, that is, the lowest unoccupied molecular orbital is now composed of orbitals from NB, which changes the emissive state of the Zn-MOF. This suggests that the LLCT process is blocked, inducing the fluorescence quenching. Otherwise, the Morokuma-Ziegler energy decomposition and natural orbitals for chemical valence on the Zn-MOF-NB interactions were studied in detail, which illustrate the possible channels of charge transfer between Zn-MOF and NB. Finally, we believe that this proposed methodology can be applied to different chemosensor-analyte systems to evidence the molecular and electronic factors that govern the sensing mechanisms.     

二苯乙炔分子导线的电子输运性质

利用第一性原理非平衡态格林函数方法研究了不同构象下二苯乙炔分子导线的电子输运性质. 从分子轨道空间分布和透射谱等方面讨论了外加偏压下分子构象对电子传递特性的影响及内在机理. 结果表明, 随着分子扭转角的增加, 分子的LUMO-HOMO能隙增加, 透射峰显著降低; 外加偏压下, 分子的HOMO分布向低电势端移动, LUMO向高电势端移动. 电流-电压计算表明, 平面构象分子的导电性最好; 随着扭转角的增加, 分子的导电性变差; 垂直构象分子的导电性最差. 最后给出了分子导线电子传递性质与分子构象的定量关系.     

Intermolecular effect in molecular electronics

We investigate the effects of lateral interactions on the conductance of two molecules connected in parallel to semi-infinite leads. The method we use combines a Green function approach to quantum transport with density functional theory for the electronic properties. The system, modeled after a self-assembled monolayer, consists of benzylmercaptane molecules sandwiched between gold electrodes. We find that the conductance increases when intermolecular interaction comes into play. The source of this increase is the indirect interaction through the gold substrate rather than direct molecule-molecule interaction. A striking resonance is produced only 0.3 eV above the Fermi energy.