The effects of contact configurations on the rectification of dipyrimidinyl-diphenyl diblock molecular junctions

The transport properties of a conjugated dipyrimidinyl-diphenyl diblock oligomer sandwiched between two gold electrodes, as recently reported by [Diez-Perez et al. Nature Chem. 1 635 （2009）], are theoretically investigated using the fully self-consistent nonequilibrium Green＇s function method combined with density functional theory. Two kinds of symmetrical anchoring geometries are considered. Calculated current-voltage curves show that the contact structure has a strong effect on the rectification behaviour of the molecular diode. For the equilateral triangle configuration, pronounced rectification behaviour comparable to the experimental measurement is revealed, and the theoretical analysis indicates that the observed rectification characteristic results from the asymmetric shift of the perturbed molecular energy levels under bias voltage. While for the tetrahedron configuration, both rectification and negative differential conductivity behaviours are observed. The calculated results further prove the close dependence of the transporting characteristics of molecular junctions on contact configuration.

Hydration effect on the electronic transport properties of oligomeric phenylene ethynylene molecular junctions

A first-principles computational method based on the hybrid density functional theory is developed to simulate the electronic transport properties of oligomeric phenylene ethynylene molecular junctions with H2O molecules accumulated in the vicinity as recently reported by Na et al. [Nanotechnology 18 424001 （2007）]. The numerical results show that the hydrogen bonds between the oxygen atoms of the oligomeric phenylene ethynylene molecule and H2O molecules result in the localisation of the molecular orbitals and lead to the lower transition peaks. The H2O molecular chains accumulated in the vicinity of the molecular junction can not only change the electronic structure of the molecular junctions, but also open additional electronic transport pathways. The obvious influence of H2O molecules on the electronic structure of the molecular junction and its electronic transport properties is thus demonstrated.

Effect of Chirality on the Electronic Transport Properties of the Thioxanthene-Based Molecular Switch

Based on the nonequilibrium Green function method and density functional theory calculations, we theoretically investigate the effect of chirality on the electronic transport properties of thioxanthene-based molecular switch. The molecule comprises the switch which can exhibit different chiralities, that is, cis-form and trans-form by ultraviolet or visible irradiation. The results clearly reveal that the switching behaviors can be realized when the molecule converts between cis-form and trans-form. ~urthermore, the on-off ratio can be modulated by the chirality of the carbon nanotube electrodes. The maximum on-off ratio can reach 109 at 0.4 V for the armchair junction, suggesting potential applications of this type of junctions in future design of functional molecular devices.

Electronic Transport Properties of Diblock Co-Oligomer Molecule Devices Sandwiched between Nitrogen Doping Armchair Graphene Nanoribbon Electrodes

We investigate the electronic transport properties of dipyrimidinyl-diphenyl sandwiched between two armchair graphene nanoribbon electrodes using the nonequilibrium Green function formalism combined with a first-principles method based on density functional theory. Among the three models M1–M3, M1 is not doped with a heteroatom. In the left parts of M2 and M3, nitrogen atoms are doped at two edges of the nanoribbon. In the right parts, nitrogen atoms are doped at one center and at the edges of M2 and M3, respectively. Comparisons of M1, M2 and M3 show obvious rectifying characteristics, and the maximum rectification ratios are up to 42.9 in M2. The results show that the rectifying behavior is strongly dependent on the doping position of electrodes. A higher rectification ratio can be found in the dipyrimidinyl-diphenyl molecular device with asymmetric doping of left and right electrodes, which suggests that this system has a broader application in future logic and memory devices.

Effect of boron/nitrogen co-doping on transport properties of C60 molecular devices

By using nonequilibrium Green's function method and first-principles calculations, the electronic transport properties of doped C60 molecular devices were investigated. It is revealed that the C60 molecular devices show the metal behavior due to the interaction between the C60 molecule and the metal electrode. The current-voltage curve displays a linear behavior at low bias, and the currents have the relation of M1>M3>M4>M2 when the bias voltage is lower than 0.6 V. Electronic transport properties are affected greatly by the doped atoms. Negative differential resistance is found in a certain bias range for C60 and C58BN molecular devices, but cannot be observed in C59B and C59N molecular devices. These unconventional effects can be used to design novel nanoelectronic devices.

Negative differential resistance behavior in doped C_(82) molecular devices

By using the first-principle calculations and nonequilibrium Green functions method, the electronic transport properties of molecular devices constructed by C82, C80BN and C80N2 were studied. The results show that the electronic transport properties of molecular devices are affected by doped atoms. Negative differential resistance (NDR) behavior can be observed in certain bias regions for C82 and C80BN molecular devices but cannot be observed for C80N2 molecular device. A mechanism for the negative differential resistance behavior was suggested.

Electron Transport Properties of Two-Dimensional Si_1P_1 Molecular Junctions

We focus on two new 21） materials, i.e., monolayer and bilayer silicon phosphides （Sil P1）. Based on the elastic- scattering Green＇s function, the electronic-transport properties of two-dimensional monolayer and bilayer Au- Si1P1-Au molecular junctions are studied. It is found that their bandgaps are narrow （0.16eV for a monolayer molecular junction and 0.26 e V for a bilayer molecular junction）. Moreover, the calculated current-voltage char- acteristics indicate that the monolayer molecular junction provides constant output current （20 hA） over a wide voltage range, and the bilayer molecular junction provides higher current （42 hA）.

Electron Transport Properties of Two-Dimensional Monolayer Films from Au-P-Au to Au-Si-Au Molecular Junctions

We investigate the electronic-transport properties of two-dimensional monolayer films from Au-P-Au molecular junction to Au-Si-Au molecular junction using elastic scattering Green＇s function theory. In the process of replacing the P atoms with Si atoms every other line from the middle of monolayer blue phosphorus molecular structure, the substitution of Si atoms changes the properties of Au-P-Au molecular junction significantly. Interestingly, the current value has a symmetric change as a parabolic curve with the peak appearing in Au-Si_1P_1-Au molecular junction, which provides the most stable current of 15.00 nA in a wide voltage range of 0.70-2.70 V.Moreover, the current-voltage characteristics of the structures indicate that the steps tend to disappear revealing the property similar to metal when the Si atoms dominate the molecular junction.

Exploring how hydrogen at gold-sulfur interface affects spin transport in single-molecule junction

Very recently,experimental evidence showed that the hydrogen is retained in dithiol-terminated single-molecule junction under the widely adopted preparation conditions,which is in contrast to the accepted view[Nat.Chem.11351(2019)].However,the hydrogen is generally assumed to be lost in the previous physical models of single-molecule junctions.Whether the retention of the hydrogen at the gold-sulfur interface exerts a significant effect on the theoretical prediction of spin transport properties is an open question.Therefore,here in this paper we carry out a comparative study of spin transport in M-tetraphenylporphyrin-based(M=V,Cr,Mn,Fe,and Co;M-TPP)single-molecule junction through Au-SR and Au-S(H)R bondings.The results show that the hydrogen at the gold-sulfur interface may dramatically affect the spin-filtering efficiency of M-TPP-based single-molecule junction,depending on the type of transition metal ions embedded into porphyrin ring.Moreover,we find that for the Co-TPP-based molecular junction,the hydrogen at the gold-sulfur interface has no obvious effect on transmission at the Fermi level,but it has a significant effect on the spin-dependent transmission dip induced by the quantum interference on the occupied side.Thus the fate of hydrogen should be concerned in the physical model according to the actual preparation condition,which is important for our fundamental understanding of spin transport in the single-molecule junctions.Our work also provides guidance in how to experimentally identify the nature of gold-sulfur interface in the single-molecule junction with spin-polarized transport.

Device design based on the covalent homocouplingof porphine molecules

Porphine has a great potential application in molecular electronic devices.In this work,based on the density functional theory(DFT)and combining with nonequilibrium Green's function(NEGF),we study the transport properties of the molecular devices constructed by the covalent homocoupling of porphine molecules conjunction with zigzag graphene nanoribbons electrodes.We find that different couple phases bring remarkable differences in the transport properties.Different coupling phases have different application prospects.We analyze and discuss the differences in transport properties through the molecular energy spectrum,electrostatic difference potential,local density of states(LDOS),and transmission pathway.The results are of great significance for the design of porphine molecular devices in the future.

Conformational change-modulated spin transport at single-molecule level in carbon systems

Controlling the spin transport at the single-molecule level,especially without the use of ferromagnetic contacts,becomes a focus of research in spintronics.Inspired by the progress on atomic-level molecular synthesis,through firstprinciples calculations,we investigate the spin-dependent electronic transport of graphene nanoflakes with side-bonded functional groups,contacted by atomic carbon chain electrodes.It is found that,by rotating the functional group,the spin polarization of the transmission at the Fermi level could be switched between completely polarized and unpolarized states.Moreover,the transition between spin-up and spin-down polarized states can also be achieved,operating as a dual-spin filter.Further analysis shows that,it is the spin-dependent shift of density of states,caused by the rotation,that triggers the shift of transmission peaks,and then results in the variation of spin polarization.Such a feature is found to be robust to the length of the nanoflake and the electrode material,showing great application potential.Those findings may throw light on the development of spintronic devices.

并苯分子结的热电性质

通过运用非平衡格林函数方法,研究了基于并苯连接石墨烯纳米带的分子结的热电性质.主要考虑了并苯分子长度、并苯分子与石墨烯纳米带电极的接触位置对并苯分子结热电参量的影响.结果发现并苯分子结最大热电优值(ZT_(max))对应的热导中声子贡献往往占据主导地位.当并苯分子的长度增加,声子热导单调减少,最终变得与并苯分子长度几乎无关.当并苯分子与石墨烯纳米带左(右)电极的中(上)部接触时,对应的ZT_(max)是最高的,然而当并苯分子与石墨烯纳米带左(右)电极的中(中)部接触时,对应的ZT_(max)是最低的.当温度增加时,ZT_(max)有单调增加的趋势,无关于接触位置.随着并苯分子长度的增加,ZT_(max)对应化学势的位置越靠近本征费米能级.以上发现能对未来设计基于并苯分子结的热电器件提供有价值的参考.

基于不同π核扩展方式萘四甲酰基二酰亚胺有机半导体电荷传输性质的理论研究

n型半导体材料在逻辑互补电路中必不可少,但由于多数n型有机半导体材料空气稳定性差、迁移率较低,其发展相对滞后.本文基于几种典型的不同π核扩展的萘四甲酰基二酰亚胺(NDI)有机半导体材料,利用第一性原理计算分析了其电荷传输性质,分别从单分子结构特征、分子间堆积方式以及分子间相互作用等方面阐明不同核心结构对载流子传输性质的影响.结果表明,所研究分子均具有良好的空气稳定性,并表现为电子传输性.其中,长轴π扩展的分子A2具有较高的电子亲和势,其空气稳定性最好.其次,沿长轴/短轴π扩展的NDI有机半导体的重组能均降低,但二者重组能降低的起因明显不同:相比于A1分子,沿短轴π扩展的A2分子在高频区的振动受到有效的抑制,从而重组能大幅下降;而沿长轴π扩展的A3与A4分子则在高低频区的振动均受到抑制,使重组能降低.另外,不同的分子堆积对传输影响较大,发现沿长轴π扩展的A3和A4具有较小的短轴滑移,表现为二维传输材料,二维平均电子迁移率分别约为0.06与0.15cm^(2)·V^(-1)·s^(-1);沿短轴π扩展的A2分子具有较大的短轴滑移,因此表现为一维传输材料,其一维电子迁移率高达0.96cm^(2)·V^(-1)·s^(-1).

Single-chain and monolayered conjugated polymers for molecular electronics

Exploring the charge transport properties and electronic functions of molecules is of primary interest in the area of molecular electronics.Conjugated polymers（CPs） represent an attractive class of molecular candidates,benefiting from their outstanding optoelectronic properties.However,they have been less studied compared with the small-molecule family,mainly due to the difficulties in incorporating CPs into molecular junctions.In this review,we present a summary on how to fabricate CP-based singlechain and monolayered junctions,then discuss the transport behaviors of CPs in different junction architectures and finally introduce the potential applications of CPs in molecular-scale electronic devices.Although the research on CP-based molecular electronics is still at the initial stage,it is widely accepted that（1） CP chains are able to mediate long-range charge transport if their molecular electronic structures are properly designed,which makes them potential molecular wires,and（2） the intrinsic optoelectronic properties of CPs and the possibility of incorporating desirable functionalities by synthetic strategies imply the potential of employing tailor-made polymeric components as alternatives to small molecules for future molecular-scale electronics.

单分子器件电子输运性质的理论研究

对当前单分子器件的理论和实验的研究进展作了简短评述,并简要介绍电子输运理论,最后给出一个基于非平衡态格林函数电子输运理论的全自洽方法,研究单个水分子在Au(111)电极之间的输运性质的计算实例.研究结果表明由于水分子与电极之间存在较强的杂化作用,水分子的分立能级间距大,在小偏压范围内,水分子的特征已经被淹没在杂化能级之中.体系的电势变化主要发生在水分子局域区间,其电子输运行为主要是一个单通道过程.

接触构型及电极距离对4，4′二巯基二苯醚分子电输运性质的影响

利用从头计算方法和弹性散射格林函数的方法,对4,4′-二巯基二苯醚分子电输运特性的研究结果显示,分子与电极之间接触点的构型以及两电极之间的距离对4,4′-二巯基二苯醚分子的电输运性质都有很大影响,电流随电极距离的变化与耦合系数的变化存在着密切关系.分子末端硫原子处于金原子的顶位上时电流的开启电压很小,而处在金(111)面的空位上时约有1.0 V左右电流禁区.与实验结果相比,硫原子更可能处在金(111)面的空位上方.

GaN基半导体异质结构的外延生长、物性研究和器件应用

GaN基宽禁带半导体异质结构具有非常强的极化效应、高饱和电子漂移速度、高击穿场强、高于室温的居里转变温度、和较强的自旋轨道耦合效应等优越的物理性质,是发展高功率微波射频器件不可替代的材料体系,也是发展高效节能功率电子器件的主要材料体系之一,在半导体自旋电子学器件上亦有潜在的应用价值。GaN基异质结构材料、物理与器件研究已成为当前国际上半导体科学技术的前沿领域和研究热点。本文从GaN基异质结构的外延生长、物理性质及其电子器件应用三个方面对国内外该领域近年来的研究进展进行了系统的介绍和评述,并简要介绍了北京大学在该领域的研究进展。

碳纳米管的压缩屈曲机理和电子结构

用分子动力学方法模拟了碳纳米管的受压屈曲变形过程,分析了碳纳米管结构屈曲的微观机理,从能量和微观结构变化研究了碳纳米管屈曲变形的机理;用紧束缚分子动力学方法研究了碳纳米管的结构屈曲对其电子结构和输运特性的影响。在压缩过程中,随着能量的不断积累,碳纳米管局部区域原子的温度上升,结构软化,产生失稳;在压缩变形过程中,随着碳纳米管变形的发展,在结构失稳之前,原子间的相互结合作用增强,高结合能区域的电子能态分布增大;而随着失稳的发生,原子结构松弛,原子间相互结合作用减弱,导致电子能态朝低结合能区域发展。碳纳米管受压的弹性屈曲变形对其电子结构不产生较大的影响,其输运特性也不发生本质的改变。

分子构型对分子器件伏-安特性的影响

本文选取4,4′-二巯基联苯分子通过终端S原子化学吸附于两Au原子团簇形成分子结,利用从头计算方法和弹性散射格林函数理论研究了该分子两苯环之间不同位置取向对分子能级结构以及该分子结伏安特性的影响.计算结果表明苯环扭转角增加会使分子能级发生不同程度的移动,从而引起最高占据轨道(HOMO)与最低未占据轨道(LUMO)的间距增大.扭转角增大也会导致分子轨道的扩展性变差,从而使体系的导电性能降低.当扭转角为90度时,体系的导电性能最差.该工作有利于未来分子电子学器件的设计.

分子间相互作用对硫醇分子膜电输运性质的影响

利用密度泛函理论和弹性散射格林函数方法,对硫醇分子膜的电学特性进行模拟.计算结果表明,由于分子间的相互作用,导致分子膜的导电能力比单分子提高2～3个数量级.分子结的导电能力随着压力增加而增加,电极距离的变化使分子与电极以及分子间的耦合增强、分子结的链内隧穿和链间隧穿几率增大,导致电流增加.