基于γ-石墨炔分子磁隧道结对称性依赖的输运性质

利用非平衡格林函数和密度泛函理论,研究不同类型γ-石墨炔分子磁隧道结(MMTJ)自旋极化输运特性的影响。磁隧道结以铁磁性的锯齿形石墨烯纳米带作电极。随着纳米带宽度变化,考虑γ-石墨炔的两种接触点,我们构造了8种有代表性的且具有不同对称性的隧道结。通过计算我们发现,对称性对磁隧道结的自旋输运起决定性作用。对于偶数碳链的锯齿形石墨烯纳米带,石墨炔的接触点位居于正中,这种结构的自旋极化输运性质远优于其它结构。比如在非常宽的偏压范围内都能达到100%的自旋极化率,且隧穿磁阻(TMR)高达3.7 × 10⁵,这表明该结构在自旋滤波器和自旋阀器件方面的应用潜力最大。与之形成对比的是,当耦合位置偏离锯齿形石墨烯纳米带的中心时,输运性质迅速变为普通电输运,相应的巨磁阻效应比最优对称结构约小4个数量级。     

Current rectification by asymmetric molecules: an ab initio study

Current rectification effect in an asymmetric molecule HCOO-C6H4-(CH2)n sandwiched between two aluminum electrodes has been studied using an ab initio nonequilibrium Green's function method. The conductance of the system decreases exponentially with the increasing number n of CH2. The phenomenon of current rectification is observed such that a very small current appears at negative bias and a sharp negative differential resistance at a critical positive bias when n>or=2. The rectification effect arises from the asymmetric structure of the molecule and the molecule-electrode couplings. A significant rectification ratio of approximately 38 can be achieved when n=5.     

Graphitic Silicon Nitride: A Metal‐Free Ferromagnet with Charge and Spin Current Rectification

As a first example, herein we show that g‐Si₄N₃ is expected to act as a metal‐free ferromagnet featuring both charge and spin current rectification simultaneously. Such rectification is crucial for envisioning devices that contain both logic and memory functionality on a single chip. The spin coherent quantum‐transport calculations on g‐Si₄N₃ reveal that the chosen system is a unique molecular spin filter, the current‐voltage characteristics of which is asymmetric in nature, which can create a perfect background for synchronous charge and spin current rectification. To shed light on this highly unusual in‐silico observation, we have meticulously inspected the bias‐dependent modulation of the spin‐polarized eigenstates. The results indicate that, whereas only the localized 2p orbitals of the outer‐ring (OR) Si atoms participate in the transmission process in the positive bias, both OR Si and N atoms contribute in the reverse bias. Furthermore, we have evaluated the spin‐polarized electron‐transfer rate in the tunneling regime, and the results demonstrate that the transfer rates are unequal in the positive and negative bias range, leading to the possible realization of a simultaneous logic–memory device.     

Light‐Induced Ion Rectification in Zigzag Nanochannels

Ion transport through nanoporous systems has attracted broad interest due to its crucial role in physiological processes in living organisms and artificial bionic devices. In this work, a nanochannel system with a zigzag inner surface was fabricated by using a two‐step anodizing technique. The rectification performance of the zigzag channels was observed by I–V measurement in KCl solution. Unlike channels with asymmetric geometry, the mechanism was analyzed based on the “point effect” of charge distribution and “shape effect” of the zigzag channel. The current rectification ratio decreases from nearly 3.0 to 1.0 when the KCl concentration increased from 0.1 mm to 100 mm . The fabrication of different nanopore systems and exploration of novel mechanisms will help to develop biomimetic membranes for practical applications.     

正三角锯齿型石墨烯的电子输运特性

利用密度泛函理论和非平衡格林函数方法, 系统研究了正三角锯齿型石墨烯的电子输运特性. 研究表明: 正三角石墨烯的电流-电压(I-V)特性及整流效应与几何尺寸、边缘吸附原子的类型密切相关, 在其边缘吸附H原子和S原子的情况下, 小的正三角石墨烯有大的电流, 但有小的整流比; 改变边缘吸附原子的类型(用O原子替换H原子), 电流增大, 但其整流效应明显变低. 分析表明, 这种整流是由于正三角石墨烯前线轨道的空间分布不对称以及在正、负偏压下分子能级的非对称移动所致. 我们的研究对于认识正三角石墨烯的基本物性(电子结构及器件应用)有重要意义.     

Odd-Even Effects on Transport Properties of Polycyclic Arene Molecular Devices with Decreasing Numbers of Benzene Rings

The electron transport properties of polycyclic aromatic hydrocarbons (PAHs) with different numbers of benzene rings tethered to narrow zigzag graphene nanoribbon (ZGNR) electrodes have been investigated. Results show that the transport properties of PAHs are dependent on whether the number of benzene rings in the width direction is odd or even. This effect is strong for narrow width PAHs, but its strength decreases as the width of the PAH is increased. PAHs with an odd number of rings exhibit poor transport properties, whereas the ones having an even number of rings show excellent transport properties coupled with a negative differential resistance (NDR) effect. Moreover, the linkage points and the structure of the molecules have a noticeable effect on the transport properties of the PAH, making the odd-even effect weaker or disappear entirely. Although the PAH with three benzene rings displays poor transport capabilities, it shows excellent rectification behavior compared to the other examined molecules. These studies present a feasible avenue for designing molecular devices with enhanced performance by the careful manipulation of the PAH molecular structure.     

Current-voltage characteristics of a homologous series of polycyclic aromatic hydrocarbons

A novel alkyl-substituted polycyclic aromatic hydrocarbon (PAH) with D(2h) symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic pi system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode-like current versus voltage (I-V) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current-voltage (I-V) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO-LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction.     

Effects of electron–electron interaction on the band structure in polydiacetylenes

The ground‐state band structure of polydiacetylenes is theoretically studied with the extensional Su–Schriffer–Heeger model supplemented by electron–electron interactions. The results show the following. First, the interval of valence bands (conduction bands) increases because of the electron–electron interactions. Second, the effect of the on‐site Coulomb energy (U) is different from that of the nearest neighbor Coulomb repulsion (V); the competition between U and V shows that U makes the bandwidth narrower and the gap broader, whereas V makes the bandwidth broader and the gap narrower. There is a critical value of U/V. Third, the whole band width (E_w) decreases when the U/V ratio is less than 1.0 and increases when the U/V ratio is greater than 1.0 at V = 2.0 eV. Thus, the ground‐state band structure is sensitive to the U/V ratio. The results also show that electron–electron interactions can play an important role in the band structure of polydiacetylenes. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1656–1661, 2000     

A constrained variational approach to the designing of low transport band gap materials: A multiobjective random mutation hill climbing method

Neutral polythiophene (PT) and polyselenophene (PSe) are semiconductors with band gaps of about 2 eV. We have proposed and implemented a constrained variational method in which total energy of neutral PT or PSe oligomers is minimized under the constraint that the band gap measured by HOMO–LUMO energy difference is also a minimum in each case. The constrained (bimodal) minimization has been carried out by an adaptive random mutation hill climbing method within the basic framework of Su‐Schrieffer‐Heeger type of model. We show that the “band‐gap constrained minimization” automatically creates electron deficient quinoid regions (QR) in the PT or PSe chains, embedded in aromatic regions (ARs), on both sides. We have investigated how the number and distribution of such QRs can reduce the band gap. Band gap constrained electronic structure calculations thus provide designing clues for low transport band gap materials based on molecular chromophores. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

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.     

Switching and rectification of a single light-sensitive diarylethene molecule sandwiched between graphene nanoribbons

The "open" and "closed" isomers of the diarylethene molecule that can be converted between each other upon photo-excitation are found to have drastically different current-voltage characteristics when sandwiched between two graphene nanoribbons (GNRs). More importantly, when one GNR is metallic and another one is semiconducting, strong rectification behavior of the "closed" diarylethene isomer with the rectification ratio >10(3) is observed. The surprisingly high rectification ratio originates from the band gap of GNR and the bias-dependent variation of the lowest unoccupied molecular orbital of the diarylethene molecule, the combination of which completely shuts off the current at positive biases. Results presented in this paper may form the basis for a new class of molecular electronic devices.     

First Principle Study on the Rectification of Molecular Junctions Based on the Thiol-ended Oligosilane

The electron transport properties of various molecular junctions based on the thiol-ended oligosilane are investigated through density functional theory combined with non-equilibrium Green's function formalism. Our calculations show that oligosilanes doped by the phenyl and-C10H6 groups demonstrate better rectifying effect and their rectification ratios are up to 15.41 and 65.13 for their molecular junctions. The current-voltage(I-V) curves of all the Au/ modified oligosilane/Au systems in this work are illustrated by frontier molecular orbitals, transmission spectra and density of states under zero bias. And their rectifying behaviors are analyzed through transmission spectra.     

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.     

Ion current rectification behavior of a nanochannel having nonuniform cross-section

Due to its versatile applications in biotechnology, ion current rectification (ICR), which arises from the asymmetric nature of the ion transport in a nanochannel, has drawn much attention, recently. Here, the ICR behavior of a pH-regulated nanochannel comprising two series connected cylindrical nanochannels of different radii is examined theoretically, focusing on the influences of the radii ratio, the length ratio, the bulk concentration, and the solution pH. The results of numerical simulation reveal that the rectification factor exhibits a local maximum with respect to both the radii ratio and the length ratio. The values of the radii ratio and the length ratio at which the local maximum in the rectification factor occur depend upon the level of the bulk salt concentration. The rectification factor also shows a local maximum as the solution pH varies. Among the factors examined, the solution pH influences the ICR behavior of the nanochannel most significantly.     

Half-metallicity in graphene nanoribbons with topological defects at edge

We report first principles studies of zigzag edged graphene nanoribbons (ZGNR) with one edge partially covered by topological defects. With increasing coverage of an edge by pentagons and heptagons, which are two of the simplest topological defects possible in a graphenic lattice, ZGNRs evolve from a magnetic semiconductor to a ferromagnetic metal. This evolution can be intermediated by a narrow bandgap half-metallic phase, upon suitable concentration and conformation of defects at the edge. Spin-frustration induced by topological defects lead to substantial lowering of magnetic ordering and localization of defect-states in the vicinity of the defects. Dispersion of bands constituted by the defect-states within the bandgap of the corresponding unmodified ZGNR, leads to availability of energy windows for spin-polarized electron transport. Driven primarily by exchange interactions, the energy window for transport of electrons near Fermi energy, is consistently wider and more prevalent for the minority spin, in the entire class of ZGNRs with discontinuous patches of topological defects at an edge. Such defects have been widely predicted and observed to be naturally present at the interfaces in polycrystalline graphene, and can even be formed through chemical and physical processes. Our approach thus may lead to a feasible strategy to manifest workable half-metallicity in ZGNRs without involving non-carbon dopants or functional groups.     

Ambipolar, high performance, acene-based organic thin film transistors

We present a high performance, ambipolar organic field-effect transistor composed of a single material. Ambipolar molecules are rare, and they can enable low-power complementary-like circuits. This low band gap, asymmetric linear acene contains electron-withdrawing fluorine atoms, which lower the molecular orbital energies, allowing the injection of electrons. While hole and electron mobilities of up to 0.071 and 0.37 cm2/V.s, respectively, are reported on devices measured in nitrogen, hole mobilities of up to 0.12 cm2/V.s were found in ambient, with electron transport quenched. These devices were fabricated on octadecyltrimethoxysilane-treated surfaces at a substrate temperature of 60 degrees C.     

Current rectification in a Langmuir-Schaefer monolayer of fullerene-bis-[4-diphenylamino-4' '-(n-ethyl-n-2' ''-ethyl)amino-1,4-diphenyl-1,3-butadiene] malonate between Au electrodes

Langmuir-Schaefer (LS) monolayer films of fullerene-bis-[4-diphenylamino-4' '-(N-ethyl-N-2' '-ethyl)amino-1,4-diphenyl-1,3-butadiene] malonate, 1, sandwiched between two Au electrodes, exhibit pronounced current asymmetries (rectification) between positive and negative bias at room temperature, with no decay of the rectification after several cycles. The device shows symmetrical through-space tunneling for a bias up to +/-3 V, and asymmetrical, unimolecular, "U" type rectifier behavior in the voltage range from +/-3.0 to +/-5.4 V, with rectification ratios up to 16.5. The rectification is ascribed to the asymmetric placement of the relevant molecular orbitals, with respect to the metallic electrodes.     

Can azulene-like molecules function as substitution-free molecular rectifiers?

The feasibility of employing azulene-like molecules as a new type of high performance substitution-free molecular rectifier has been explored using NEGF-DFT calculation. The electronic transport behaviors of metal-molecule-metal junctions consisting of various azulene-like dithiol molecules are investigated, which reveals that the azulene-like molecules exhibit high conductance and bias-dependent rectification effects. Among all the substitution-free azulene-like structures, cyclohepta[b]cyclopenta[g]naphthalene exhibits the highest rectification ratio, revealing that the all fused aromatic ring structure and an appropriate separation between the pentagon and heptagon rings are essential for achieving both high conductance and high rectification ratio. The rectification ratio can be increased by substituting the pentagon ring with electron-withdrawing group and/or the heptagon ring with electron donating groups. Further increase of the rectification ratio may also be obtained by lithium adsorption on the pentagon ring. This work reveals that azulene-like molecules may be used as a new class of highly conductive unimolecular rectifiers.     

Charge transport mediated by d-orbitals in transition metal complexes

This communication reports an asymmetric charge transport with a large rectification ratio and finely featured NDR (negative differential resistance) by d-orbitals of a neutral ruthenium(ii) complex with a C(2) axis of symmetry.     

A rectifying diode with hysteresis effect from an electroactive hybrid of carbazole-functionalized polystyrene with CdTe nanocrystals via electrostatic interaction

One of the strategies to tune current-voltage behaviors in organic diodes is to combine field-induced charge transfer processes with schottky barrier. According to this principle, a rectifying diode with hysteresis effect was fabricated utilizing a hybrid of electroactive polystyrene derivative covalently tethered with electron-donor carbazole moieties and electrostatic linked with electron-acceptor CdTe nanocrystals. Current-voltage characteristics show an electrical switching behavior with some hysteresis is only observed under a negative bias, with three orders of On/Off current ratio. The hybrid material based rectifier exhibits a rectification ratio of six and its maximum rectified output current is about 5 × 10^?5 A. The asymmetric switching is interpreted as the result of both field induced charge transfer and schottky barrier, capable of reducing the misreading of cross-bar memory. Meanwhile, chemical doping of CdTe nanocrystals instead of physical blend favor their uniform dispersion in matrix and stable operation of device.