Gas-sensor property of single-molecule device:F_2 adsorbing effect

The single thiolated arylethynylene molecule with 9,10-dihydroanthracene core(denoted as TADHA) possesses pronounced negative differential conductance(NDC) behavior at lower bias regime. The adsorption effects of F_2 molecule on the current and NDC behavior of TADHA molecular junctions are studied by applying non-equilibrium Green's formalism combined with density functional theory. The numerical results show that the F_2 molecule adsorbed on the benzene ring of TADHA molecule near the electrode can dramatically suppresses the current of TADHA molecular junction. When the F_2 molecule adsorbed on the conjugated segment of 9,10-dihydroanthracene core of TADHA molecule, an obviously asymmetric effect on the current curves induces the molecular system showing apparent rectifier behavior. However, the current especially the NDC behavior have been significantly enlarged when F_2 addition reacted with triple bond of TADHA molecule.     

端基对分子器件整流性质的影响

利用基于非平衡Green函数加密度泛函理论的第一原理方法,研究了以S(Se)为端基的三并苯环分子夹在两半无限长的Au电极之间构成双探针系统的输运特性,发现体系具有较好的整流效应,最大整流系数达到6;用H取代右端同一位置的一个S（Se）,整流行为明显减弱.分析认为,这种整流是由于分子两端与电极的耦合不对称,使正负偏压下分子能级的移动和空间轨道分布不同所致.比较而言,S端基与电极的耦合导致的整流比Se强. 关键词： 电子输运 整流行为 非平衡Green函数方法     

(CH3)2和(NH2)2基团修饰的齐聚苯乙炔分子电子输运性质研究

采用密度泛函理论和非平衡格林函数相结合的方法研究了S原子作为单、双端基的(CH₃)₂-OPE (齐聚苯乙炔)和(NH₂)₂-OPE分子在金电极间的电子输运性质. 通过第一性原理优化计算获得分子部分稳定结构, 再置于Au电极之间构成两极系统, 然后再优化整个两极系统获得稳定结构. 另外, 通过非平衡格林函数方法计算了两极系统的电子输运性质. 计算结果表明, 不同的修饰基团和桥接方式可以导致两极系统的开关效应、负微分电阻行为和整流行为等不同的电子输运性质. 通过计算不同偏压下的分子体系投影轨道电子分布、透射谱、态密度, 对这些新异的电输运性质出现的机理进行了解释.     

Electrofluorescence polarity in a molecular diode

The kinetic equations describing the transmission of an electron in the molecular compound “electrode 1–molecule–electrode 2” (1M2 system) are derived using the method of a nonequilibrium density matrix. The steady-state transmission regime is considered, for which detailed analysis of the kinetics of electrofluorescence formation in systems with symmetric and asymmetric couplings between the molecule and the electrodes is carried out. It is shown that the optically active state of the molecule is formed as a result of electron hops between the molecule and each of the electrodes, as well as due to inelastic interelectrode tunneling of the electron. The electrofluorescence power for a molecular diode (asymmetric 1M2 system) depends on the polarity of the voltage bias applied to the electrodes. The polarity is explained using a model in which the optically active part of the molecule (chromophore group) is represented by the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Two mechanisms of the emergence of polarity are revealed. One mechanism is associated with nonidentical Stark shifts of the HOMO and LUMO levels relative to the Fermi levels of the electrodes. The second mechanism is associated with the fact that the rates of an electron hopping between HOMO (LUMO) and one of the electrodes are much higher than the rates of such a hopping with the other electrode. The conditions in which each mechanism can be implemented experimentally are indicated.     

电极位置和截面尺寸对分子器件输运性质的调控

研究表明分子器件的性能受器件结构搭建精度影响,分子与电极接触构型的微弱变化可能引起电输运特性较大差异.本文运用密度泛函理论和非平衡格林函数相结合的方法,研究了由金纳米线与benzene-1,4-dithiol（BDT）形成的分子结的电输运性质.通过对不同的Au-BDT接触构型输运性质的研究,发现当两电极处于对位构型时,有较好的电荷输运行为,而且比较符合制备工艺要求;当电极偏离轴线的角度不大于5°,且电极散射截面尺寸不小于4×4时,该分子结体系的电导和透射谱均比较稳定.电极截面尺寸小于4×4或者电极偏离轴线的夹角大于5°时,透射谱在费米能级附近出现不连续现象,导致体系电导降低.较小电极截面尺寸或者电极以较大角度偏离轴线将导致该分子结体系电导降低和透射谱连续性降低,主要是组成电极的金原子轨道与苯基分子轨道耦合缺失造成的.该研究为Au-BDT-Au体系设计和制备过程中电极的位置及电极截面尺寸做了科学的界定.     

Spin-dependent negative differential conductance in transport through single-molecule magnets

Transport properties are theoretically studied through an anisotropy single-molecule magnet symmetrically connected to two identical ferromagnetic leads. It is found that even though in parallel configuration of leads’ magnetizations, the total current still greatly depends on the spin polarization of leads at certain particular bias region, and thus for large polarization a prominent negative differential conductance (NDC) emerges. This originates from the joint effect of single-direction transitions and spin polarization, which removes the symmetry between spin-up and spin-down transitions. The present mechanism of NDC is remarkably different from the previously reported mechanisms. To clarify the physics of the NDC, we further monitored the shot noise spectroscopy and found that the appearance of the NDC is accompanied by the rapid decrease of Fano factor.     

Negative differential capacitance in n-GaN/p-Si heterojunctions

Negative differential capacitance (NDC) has been observed in n-GaN/p-Si heterojunctions grown by plasma assisted molecular beam epitaxy (PAMBE). The NDC is observed at low frequencies 1 and 10 kilohertz (kHz) and disappeared at a higher testing frequency of 100 kHz. The NDC is also studied with temperature and found that it has disappeared above 323 ^°C. Current-Voltage (I-V) characteristics of n-GaN /p-Si heterojunction were measured at different temperatures and are attributed to the space-charge-limited current (SCLC). A simple model involving two quantum states is proposed to explain the observed NDC behavior.     

Effective interactions in molecular dynamics simulations of lysozyme solutions

On the basis of ab-initio calculations, we predict the effect of conformation and molecule-electrode distance on transport properties of asymmetric molecular junctions for different electrode materials M (M = Au, Ag, Cu, and Pt). The asymmetry in these junctions is created by connecting one end of the biphenyl molecule to conjugated double thiol (model A) and single thiol (model B) groups, while the other end to Cu atom. A variety of phenomena viz. rectification, negative differential resistance (NDR), switching has been observed that can be controlled by tailoring the interface state properties through molecular conformation and molecule-electrode distance for various M. These properties are further analyzed by calculating transmission spectra, molecular orbitals, and orbital energy. It is found that Cu electrode shows significantly enhanced rectifying performance with change in torsion angles, as well as with increase in molecule-electrode distances than Au and Ag electrodes. Moreover, Pt electrode manifests distinctive multifunctional behavior combining switch, diode, and NDR. Thus, the Pt electrode is suggested to be a good potential candidate for a novel multifunctional electronic device. Our findings are compared with available experimental and theoretical results.     

Scanning tunneling microscopy observation of copper-phthalocyanine molecules on Si(100) and Si(111) surfaces

Molecules of Cu-phthalocyanine (CuPc) deposited on Si(100) and Si(111) surfaces have been observed by an ultra high vacuum field ion scanning tunneling microscope (FI-STM). On a Si(100) surface, STM images with four-fold symmetry are observed, which reflect the shape of the CuPc molecule. The STM pictures show that CuPc molecules are deposited with the molecular plane parallel to the substrate surface and have three kinds of adsorption configurations on the dimer-row of Si(100). The images of the CuPc are modified by the electronic state of the Si(100) surface. This behavior suggests strong interaction between the molecule and the substrate. The molecular images on the Si(111) surface have a unique bias-voltage dependence. At a sample bias of 1.6 V, the molecule looks transparent by STM, and becomes dark like a vacancy at 1.2 V. From the bias dependence, the electronic interaction between the CuPc molecule and the Si surface is discussed.     

基于石墨烯电极的Co-Salophene分子器件的自旋输运

采用基于非平衡格林函数结合第一性原理的密度泛函理论的计算方法,研究了基于锯齿型石墨纳米带电极的Co-Salophene分子器件的自旋极化输运性质.计算结果表明,当左右电极为平行自旋结构时,自旋向上的电流明显大于自旋向下的电流,自旋向下的电流在[-1V,1V]偏压下接近零,分子器件表现出优异的自旋过滤效应.与此同时,在自旋向上电流中发现负微分电阻效应.当左右电极为反平行自旋结构时,器件表现出双自旋过滤和双自旋分子整流效应.除此之外,整个分子器件还表现出较高的巨磁阻效应.通过分析器件的自旋极化透射谱、局域态密度、电极的能带结构和分子自洽投影哈密顿量,详细解释该分子器件表现出众多特性的内在机理.研究结果对设计多功能分子器件具有重要的借鉴意义.     

基于石墨烯电极的齐聚苯乙炔分子器件的整流特性

以齐聚苯乙炔分子为研究对象,采用密度泛函理论与非平衡格林函数相结合的第一性原理方法,对基于石墨烯电极的齐聚苯乙炔分子器件整流特性进行了研究,系统地分析了官能团对分子器件整流特性的影响.通过研究发现,官能团对齐聚苯乙炔分子器件整流特性影响显著,当添加失电子官能团氨基(NH_2)时出现正向整流,添加得电子官能团硝基(NO_2)时出现反向整流,当同时添加氨基和硝基官能团时,会出现正反向整流交替现象,研究结果表明通过添加不同类型的官能团能有效控制分子整流器的整流特性.     

Differential capacitance of hybrid organic/inorganic CdSe/ZnS quantum dots light-emitting device

We have studied the differential capacitance of self-assembly hybrid organic/inorganic quantum dot (QD) light-emitting devices (QD-OLEDs) at room temperature in the frequency range of 1×10³ to 1×10⁶ Hz. Self-assembly CdSe/ZnS QDs monolayer was used to fabricate trilayer hybrid QD-OLEDs on indium tin oxide coated glass substrates. We observed negative differential capacitance (NDC) in both OLED and QD-OLEDs for the first time in the intermediate frequency range of 5×10³ to 5×10⁵ Hz. The onset and frequency range of NDC are strongly depended on the applied bias voltage. This behavior is described by a mathematical model. The simulated results showed that the NDC was due to the time-dependent transient current from the QDs and organic/metal interfacial states in the device. The probable mechanisms of NDC in organic devices are discussed.     

Theoretical investigations on the orientational dependence of electron transport through porphyrin molecular wire

The effect of molecular orientation on the electron transport behavior of single porphyrin sandwiched between two gold (111) electrodes is investigated by density functional theory calculations combined with non-equilibrium Green’s function method. The results show that the porphyrin with parallel connection to gold (111) electrodes is more conductive than the porphyrin with diagonal connection to gold (111) electrodes. The mechanism of the difference of electron transport for these two molecular junctions is analyzed from the transmission spectra and the molecular projected self-consistent Hamiltonian states. It is found that the intrinsic nature of the molecule, such as the π-conjugated framework and the strength of molecule–electrode coupling, are the essential reason for generating this difference of electron transport for the two molecular systems.     

Investigation of Terminal Group Effect on Electron Transport Through Open Molecular Structures

The effect of terminal groups on the electron transport through metal-molecule-metal system has been investigated using nonequilibrium Green's function (NEGF) formalism combined with extended Huckel theory (EHT). Au-molecule-Au junctions are constructed with borazine and BCN unit structure as core molecule and sulphur (S), oxygen (O), selenium (Se) and cyano-group (CN) as terminal groups. The electron transport characteristics of the borazine and BCN molecular systems are analyzed through the transmission spectra and the current-voltage curve. The results demonstrate that the terminal groups modifying the transport behaviors of these systems in a controlled way. Our result shows that, selenium is the best linker to couple borazine to Au electrode and oxygen is the best one to couple BCN to Au electrode. Furthermore, the results of borazine systems are compared with that of BCN molecular systems and are discussed. Simulation results show that the conductance through BCN molecular systems is four times larger than the borazine molecular systems. Negative differential resistance behavior is observed with borazine-CN system and the saturation feature appears in BCN systems.     

Electrostatic spin crossover in a molecular junction of a single-molecule magnet Fe{2}

Spin crossover by means of an electric bias is investigated by spin-polarized density-functional theory calculations combined with the Keldysh nonequilibrium Green's technique in a molecular junction, where an individual single-molecule magnet Fe{2}(acpybutO)(O{2}CMe)(NCS){2} is sandwiched between two infinite Au(100) nanoelectrodes. Our study demonstrates that the spin crossover, based on the Stark effect, is achieved in this molecular junction under an electric bias but not in the isolated molecule under external electric fields. The main reason is that the polarizability of the molecular junction has an opposite sign to that of the isolated molecule, and thus from the Stark effect the condition for the spin crossover in the molecular junction is contrary to that in the isolated molecule.     

The electronic transport properties in C60 molecular devices with different contact distances

By applying non-equilibrium Green?s functions in combination with the density-functional theory, we investigate the transport behavior of molecular devices composed by metal electrode-C₆₀ molecule-metal electrode. Our results show that the electronic transport properties are affected obviously by the different contact distances between the electrodes, and the tunneling current decreases approximately exponentially at a certain bias with the increasing of contact distances. The negative differential resistance is observed and the peak-to-valley ratio can be tuned by different contact distances. The mechanisms of the contact distance effect and the negative differential resistance behavior are proposed.     

Switching behavior induced by different substituents of group in single molecular device

We investigate the electronic transport properties of photochromic azobenzene-based molecular devices with Au electrodes using non-equilibrium Green’s function and density functional theory. A reversible switching behavior between cis and trans isomerization is found in the device. In addition, the substituent of ?NH₂ on the right end hydrogen atom of azobenzene molecule reduces the switching ratio of current, consequently the disappearance of switching behavior, while the substituent of ?NO₂ improves the switching ratio of current. We discuss the different electronic transport induced by different substituents through the transmission spectra, localized density of states, molecular projected self-consistent Hamiltonian and transmission pathways. The observed polarization effect under bias is explained by the evolution of molecular projected self-consistent Hamiltonian of LUMO level. The results indicate that the electron-withdrawing group ?NO₂ substituting right terminal hydrogen of azobenzene molecule becomes a candidate for improving the performance of molecular device.     

Conduction in magnesium phthalocyanine thin films with aluminium electrodes

The various electrical properties and the nature of conduction mechanisms of magnesium phthalocyanine thin film devices with top and bottom aluminium electrodes have been investigated. The conduction mechanism was identified as injection limited essentially due to the electrode material. Even with the same electrode materials, the device showed asymmetric conduction behavior in the forward and reverse bias. In general the conduction was interpreted as a Schottky emission with barrier height Φ_s=1.07 eV for the forward bias and Φ_s=1.09 eV in the reverse bias. The effect of oxygen on the conductivity of the device has also been investigated. In the oxygen doped samples the conductivity is decreased which may be attributed to an interfacial layer between the electrode and the organic layer. Further in the oxygen doped sample while a Schottky emission is observed at lower voltages Poole-Frenkel conductivity was identified in the higher voltage region.     

Performance comparison of OSLD (Al2O3:C) and TLD (LiF:Mg,Cu,P) in accreditation proficiency testing

The U.S. Navy uses a dosimetric system, which employs the LiF:Mg,Cu,P thermoluminescence dosimeters (TLDs), developed and produced by Thermo Fisher Scientific. Every two years, the Naval Dosimetry Center (NDC) performs proficiency testing to maintain its national accreditation. Since 2007, the U.S. Navy has also tested InLight Basic - OSLN Optically Stimulated Luminescence Al₂O₃:C dosimeters (OSLD) manufactured by Landauer. In 2011 and 2013, the Naval Dosimetry Center performed proficiency testing for both systems. Here we present a comparison of the performance of TLDs (LiF:Mg,Cu,P) and OSLDs (Al₂O₃:C) in five categories of proficiency testing. The testing included irradiation with photons, neutrons, beta particles and selected mixtures of these radiations. All irradiations were performed at the Pacific Northwest National Laboratory (PNNL). The delivered doses were not reported to the NDC. The official comparison of delivered and reported doses was conducted by PNNL in terms of dose bias and its standard deviation for each category of accreditation. In total, the NDC reported to the PNNL doses for 147 dosimeters of each type (TLD and OSLD). Both NDC tested dosimetric systems have passed established limits. The comparison of OSLD and TLD system performance in each category is discussed. Advantages and disadvantages of both systems are analyzed.