Changes of coupling between the electrodes and the molecule under external bias bring negative differential resistance

We report a first-principles study of electrical transport and negative differential resistance (NDR) in a single molecular conductor consisting of a borazine ring sandwiched between two Au(100) electrodes with a finite cross section. The projected density of states (PDOS) and transmission coefficients under various external voltage biases are analyzed, and it suggests that the variation of the coupling between the molecule and the electrodes with external bias leads to NDR. Therefore, we propose that one origin of NDR in molecular devices is caused by the characteristics of both the molecule and the electrodes as well as their cooperation, not necessarily only by the inherent properties of certain species of molecules themselves. The changes of charge state of the molecule have minor effects on NDR in this device because the Mulliken population analysis shows that electron occupation variation on the molecule is very small when different external biases are applied.     

Iron-phthalocyanine molecular junction with high spin filter efficiency and negative differential resistance

We investigate the spin transport properties of iron-phthalocyanine (FePc) molecule sandwiched between two N-doped graphene nanoribbons (GNRs) based on the density functional theory and nonequilibrium Green's function methods. Our calculated results clearly reveal that the FePc molecular junction has high spin-filter efficiency as well as negative differential resistance (NDR). The zero-bias conductance through FePc molecule is dominated by the spin-down electrons, and the observed NDR originates from the bias-dependent effective coupling between the FePc molecular orbitals and the narrow density of states of electrodes. The remarkable high spin-filter efficiency and NDR are robust regardless of the edge shape and the width of GNRs, and the N-doping site in GNRs. These predictions indicate that FePc junction holds great promise in molecular electronics and spintronics applications.     

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.     

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.     

Protonation effects on electron transport through diblock molecular junctions:A theoretical study

Diblock oligomers are widely used in molecular electronics. Based on fully self-consistent nonequilib-rium Green's function method and density functional theory, we study the electron transport properties of the molecular junction with a dipyrimidinyl-diphenyl (PMPH) diblock molecule sandwiched between two gold electrodes. Effects of different kinds of molecule-electrode anchoring geometry and protona-tion of the PMPH molecule are studied. Protonation leads to both conductance and rectification en-hancements. However, the experimentally observed rectifying direction inversion is not found in our calculation. The preferential current direction is always from the pyrimidinyl to the phenyl side. Our calculations indicate that the protonation of the molecular wire is not the only reason of the rectification inversion.     

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.     

Side‐Group‐Mediated Mechanical Conductance Switching in Molecular Junctions

A key target in molecular electronics has been molecules having switchable electrical properties. Switching between two electrical states has been demonstrated using such stimuli as light, electrochemical voltage, complexation and mechanical modulation. A classic example of the latter is the switching of 4,4′‐bipyridine, leading to conductance modulation of around 1 order of magnitude. Here, we describe the use of side‐group chemistry to control the properties of a single‐molecule electromechanical switch, which can be cycled between two conductance states by repeated compression and elongation. While bulky alkyl substituents inhibit the switching behavior, π‐conjugated side‐groups reinstate it. DFT calculations show that weak interactions between aryl moieties and the metallic electrodes are responsible for the observed phenomenon. This represents a significant expansion of the single‐molecule electronics “tool‐box” for the design of junctions with electromechanical properties.     

The negative differential resistance mechanism of a molecular device based on double‐cage fluorinated fullerene C20F18(NH)2C20F18: A theoretical study

The electronic transport properties of the molecular device based on double‐cage fluorinated fullerene C₂₀F₁₈(NH)₂C₂₀F₁₈ were studied theoretically. The results show that the device exhibits two negative differential resistance (NDR) peaks in its I‐V curve. The NDR peak under low bias voltage originates from the bias‐induced alignment of the molecular orbitals, and the conduction channel being suppressed at a certain bias voltage is the main reason for the NDR peak under a relatively high bias voltage.     

Position effects of single vacancy on transport properties of single layer armchair h-BNC heterostructure

Based on certain single layer armchair h-BNC heterostructures, six molecular devices with different positions of single vacancy atoms are investigated to explain the modulating process of negative differential resistance (NDR) behaviors and rectifying performance. The results show that NDR behaviors can be observed clearly with vacancy atoms near the interface of graphene nano-ribbon and BN nano-ribbon, and rectifying performance can be enhanced obviously when there are vacancy atoms in the moiety of the BN nano-ribbon. The first-principles analysis of the microscopic nature reveals that strength of electronic transmission, evolutions of molecular orbitals and distributions of molecular states are the intrinsic responses to these transport properties.     

The First Principle Study on C-doped Armchair Boron Nitride Nanoribbon Rectifier

The electronic transport properties of armchair-edged boron nitride nanoribbons(ABNNRs) devices were investigated by the first principle calculations. The calculated results show that the ABNNR device doped with carbon atoms in one of the electrodes acts as a high performance nanoribbon rectifier. It is interesting to find that there exists a particular bias-polarity-dependent matching band between two electrodes,leading to a similar current-voltage(I-V) behavior as conventional P-N diodes. The I-V behavior presents a linear positive-bias I-V characteristic,an absolutely negligible leakage current,and a stable rectifying property under a large bias region. The results suggest that C doping might be an effective way to raise ABNNRs devices' rectifying performance.     

First-principles calculation on the conductance of a single 1,4-diisocyanatobenzene molecule with single-walled carbon nanotubes as the electrodes

The conductance of a single 1,4-diisocyanatobenzene molecule sandwiched between two single-walled carbon nanotube (SWCNT) electrodes are studied using a fully self-consistent ab initio approach which combines nonequilibrium Green's function formalism with density functional theory calculations. Several metallic zigzag and armchair SWCNTs with different diameters are used as electrodes; dangling bonds at their open ends are terminated with hydrogen atoms. Within the energy range of a few eV of the Fermi energy, all the SWCNT electrodes couple strongly only with the frontier molecular orbitals that are related to nonlocal pi bonds. Although the chirality of SWCNT electrodes has significant influences on this coupling and thus the molecular conductance, the diameter of electrodes, the distance, and the torsion angle between electrodes have only minor influences on the conductance, showing the advantage of using SWCNTs as the electrodes for molecular electronic devices.     

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.     

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.     

Frontier Orbital Control of Molecular Conductance and its Switching

For transmission of electrons through a π system, when the Landauer theory of molecular conductance is viewed from a molecular orbital (MO) perspective, there obtains a simple perturbation theoretic dependence, due to Yoshizawa and Tada, on a) the product of the orbital coefficients at the sites of electrode attachment, and b) the MO energies. The frontier orbitals consistently and simply indicate high or low transmission, even if other orbitals may contribute. This formalism, with its consequent reinforcement and/or interference of conductance, accounts for the (previously explained) difference in direct vs. cross conjugated transmission across an ethylene, as well as the comparative ON/OFF ratios in the experimentally investigated dimethyldihydropyrene and dithienylethene‐type single‐molecule switches. A strong dependence of the conductance on the site of attachment of the electrodes in a π system is an immediate extrapolation; the theory then predicts that for some specified sites the switching behavior will be inverted; i.e. the “open” molecular form of the switch will be more conductive.     

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...     

双笼氟化富勒烯分子C_(20)F_(18)(CO)_2C_(20)F_(18)电子输运性能的第一性原理研究

以双笼氟化富勒烯C_(20)F_(18)(CO)_2C_(20)F_(18)为中心分子,与Ag(100)纳米线电极连接构筑分子电子器件,通过第一性原理和非平衡格林函数相结合的方法,对器件的电子输运特性进行了研究.结果显示,在外加偏压的作用下,中心分子的前线轨道逐渐定域在分子的左侧,电子透射通道被阻塞,所对应的共振隧穿峰被压制,器件的电流-电压特性曲线在0.3~0.8V区间内表现出明显的负微分电阻(NDR)现象.     

Achieving Efficient Multichannel Conductance in Through‐Space Conjugated Single‐Molecule Parallel Circuits

Constructing single‐molecule parallel circuits with multiple conduction channels is an effective strategy to improve the conductance of a single molecular junction, but rarely reported. We present a novel through‐space conjugated single‐molecule parallel circuit (f‐4Ph‐4SMe) comprised of a pair of closely parallelly aligned p‐quaterphenyl chains tethered by a vinyl bridge and end‐capped with four SMe anchoring groups. Scanning‐tunneling‐microscopy‐based break junction (STM‐BJ) and transmission calculations demonstrate that f‐4Ph‐4SMe holds multiple conductance states owing to different contact configurations. When four SMe groups are in contact with two electrodes at the same time, the through‐bond and through‐space conduction channels work synergistically, resulting in a conductance much larger than those of analogous molecules with two SMe groups or the sum of two p‐quaterphenyl chains. The system is an ideal model for understanding electron transport through parallel π‐stacked molecular systems and may serve as a key component for integrated molecular circuits with controllable conductance.     

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.     

The First Heteropentanuclear Extended Metal‐Atom Chain: [Ni+Ru25+Ni2+Ni2+(tripyridyldiamido)4(NCS)2]

This study develops the first heteropentametal extended metal atom chain (EMAC) in which a string of nickel cores is incorporated with a diruthenium unit to tune the molecular properties. Spectroscopic, crystallographic, and magnetic characterizations show the formation of a fully delocalized Ru₂⁵⁺ unit. This [Ru₂]‐containing EMAC exhibits single‐molecule conductance four‐fold superior to that of the pentanickel complex and results in features of negative differential resistance (NDR), which are unobserved in analogues of pentanickel and pentaruthenium EMACs. A plausible mechanism for the NDR behavior is proposed for this diruthenium‐modulated EMAC.