B/N掺杂类直三角石墨烯纳米带器件引起的整流效应

基于密度泛函理论结合非平衡格林函数的方法, 研究了硼(氮)非对称掺杂类直三角石墨烯纳米带器件的电子输运性能. 计算结果表明: 单个硼或氮原子取代类直三角石墨烯纳米带顶点的碳原子后, 增强了体系的电导能力, 并且出现了新颖的整流效应. 分析表明: 这是由于硼氮掺杂类直三角石墨烯纳米带器件在正负偏压下分子能级的移动方向和前线分子轨道空间分布的不对称而产生的. 最重要的是, 当左右类直三角石墨烯纳米带的顶端原子同时被硼和氮掺杂后, 体系的整流效应显著增强, 而且出现负微分电阻效应.     

The spin-dependent electronic transport properties of M(dcdmp)2 (M = Cu,Au, Co,Ni) molecular devices based on zigzag graphene nanoribbon electrodes

The spin-dependent electronic transport properties of M(dcdmp)₂ (M = Cu, Au, Co, Ni; dcdmp = 2,3-dicyano-5,6-dimercaptopyrazyne) molecular devices based on zigzag graphene nanoribbon (ZGNR) electrodes were investigated by density functional theory combined nonequilibrium Green's function method (DFT-NEGF). Our results show that the spin-dependent transport properties of the M(dcdmp)₂ molecular devices can be controlled by the spin configurations of the ZGNR electrodes, and the central 3d-transition metal atom can introduce a larger magnetism than that of the nonferrous metal one. Moreover, the perfect spin filtering effect, negative differential resistance, rectifying effect and magnetic resistance phenomena can be observed in our proposed M(dcdmp)₂ molecular devices.     

B/N掺杂对于石墨烯纳米片电子输运的影响

选用锯齿(zigzag)型石墨烯纳米片为研究对象, Au作为电极, 分子平面与Au的(111)面垂直, 并通过末端S原子化学吸附于金属表面, 构成两种分子器件: 一种是在纳米片的边缘掺杂N(B)原子, 发现电流-电压具有非线性行为, 但是整流系数较小, 特别是掺杂较多时, 整流具有不稳定性; 另一种是用烷链把两个石墨烯片连接, 在烷链附近和石墨烯片的边缘进行N(B)掺杂, 发现在烷链附近掺杂具有较大的整流, 但是掺杂的原子个数和位置会影响整流性能. 研究表明: 整流主要为正负电压下分子能级的移动方向和空间轨道分布不同导致. 部分体系中的负微分电阻现象主要由于偏压导致能级移动和透射峰形态的改变, 并且在某些偏压下主要透射通道被抑制而引起. 关键词： 石墨烯纳米片 电子输运 整流行为 非平衡格林函数方法     

Spin-dependent transport properties of debrominated tetrabromopolyaromatic with Cu or Co doping embedded between zigzag graphene nanoribbon electrodes

Using density functional theory combined with non-equilibrium Green's function method, we investigate the spin-dependent transport properties of debrominated tetrabromopolyaromatic (D-TBPA) molecules embedded between zigzag graphene nanoribbon electrodes, and the effects of copper and cobalt side doping have also been considered. Our results show that the copper doping can insert new energy levels around the Fermi Level and keep spin degeneration of band structure, the cobalt doping can also induce spin splitting. The results on spin transport properties of D-TBPAs embedded into zigzag graphene nanoribbon electrodes show that these systems exist spin filtering and negative differential resistance behaviors. Corresponding physical mechanism on the spin-dependent transport property has been revealed according to the frontier molecular orbital characteristics.     

Electronic transport properties of zigzag carbon- and boron-nitride-nanotube heterostructures

Using first-principles density functional theory and non-equilibrium Green's function formalism for quantum transport calculation, we have investigated the electronic transport properties of heteronanotubes by joining a zigzag (6,0) carbon nanotube and a zigzag (6,0) boron nitride nanotube with different atomic compositions and joint configurations. Our results show that the atomic composition and joint configuration affect strongly the electronic transport properties. Obvious negative differential resistance behavior and large rectifying behavior are obtained in the heterostructure with certain composition and joint configuration. Moreover, tube length and tube radius can affect strongly the observed NDR and rectifying behaviors. The observed negative differential resistance and rectifying behaviors are explained in terms of the evolution of the transmission spectrum with applied bias combined with molecular projected self-consistent Hamiltonian states analysis.     

Spin currents and magnetoresistance of graphene-based magnetic junctions

Using the tight-binding approximation and the nonequilibrium Green’s function approach, we investigate the coherent spin-dependent transport in planar magnetic junctions consisting of two ferromagnetic (FM) electrodes separated by a graphene flake (GF) with zigzag or armchair interfaces. It is found that the electron conduction strongly depends on the geometry of contact between the GF and the FM electrodes. In the case of zigzag interfaces, the junction demonstrates a spin-valve effect with high magnetoresistance (MR) ratios and shows negative differential resistance features for a single spin channel at positive gate voltage. In the case of armchair interfaces, the current-voltage characteristics behave linearly at low bias voltages and hence, both spin channels are in on state with low MR ratios.     

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.     

以石墨烯为电极的有机噻吩分子整流器的设计及电输运特性研究

传统硅基半导体器件受到了量子尺寸效应的限制,发展分子电子学器件有可能解决这一难题.本文提出了由石墨烯电极和有机噻吩分子相结合构造分子器件的思想,建构了"石墨烯-噻吩分子-石墨烯"结构的分子器件,并运用非平衡态格林函数结合密度泛函理论的方法研究了其电输运特性.系统地分析了电子给体"氨基"和电子受体"硝基"两种取代基的位置对有机噻吩分子电输运的影响.计算表明,有机噻吩二聚物被"氨基"和"硝基"取代后会产生明显的负微分电阻效应和整流效应.进一步对产生这些效应的物理机制进行分析,发现氨基的位置可以调整负微分电阻的强弱,硝基的位置可以改变整流的方向.     

Contact position and width effect of graphene electrode on the electronic transport properties of dehydrobenzoannulenne molecule under bias

By applying nonequilibrium Green?s function formalism in combination with density functional theory, we have investigated the electronic transport properties of dehydrobenzoannulenne molecule attached to different positions of the zigzag graphene nanoribbons (ZGNRs) electrode. The different contact positions are found to drastically turn the transport properties of these systems. The negative differential resistance (NDR) effect can be found when the ZGNRs electrodes are mirror symmetry under the xz midplane, and the mechanism of NDR has been explained. Moreover, parity limitation tunneling effect can be found in a certain symmetry two-probe system and it can completely destroy electron tunneling process. The present findings might be useful for the application of ZGNRs-based molecular devices.     

非对称结构对黑磷烯器件电子输运的影响

利用不同宽度的锯齿型黑磷烯纳米带构建了非对称结构的纳米器件.第一性原理计算结果表明,非对称结构器件I-V曲线展现表现出整流效应,最大整流比达到1.01×10~6.器件的不同非对称结构导致了不同整流行为.我们通过传输谱和透射本征态讨论得到,器件中心区域和电极的耦合变化导致了本征态的变化,从而产生不同的电子输运性质.结果为基于黑磷烯二维材料异质结整流器件的设计提供了一种可行性方案.     

Tuning spin-filtering,rectifying, and negative differential resistance by hydrogenation on topological edge defects of zigzag silicene nanoribbons

By first-principles calculations, we propose three heterojunction nanodevices based on zigzag silicene nanoribbons with different edge-hydrogenated topological line defects. The devices all present excellent spin-filtering properties with 100% spin polarization as well as remarkable rectifying effect (with rectification ratio around 10²) and negative differential resistance behaviors. Our findings shed new light on the design of silicon-based nanodevices with intriguing spintronic applications.     

Engineering of carbon-based superlight spin filter with negative differential resistance

Based on density functional theory and non-equilibrium Green's function, we investigate the edge hydrogenation and oxidation effects on the spin transport of devices consisting of a zigzag C₂N nanoribbon (ZC₂NNR) embedded in zigzag graphene nanoribbons in parallel (P) and antiparallel (AP) spin configurations. The results show that device with edge hydrogenation exhibits dual spin filtering effect in AP spin configuration and obvious negative differential resistance in both P and AP spin configuration. By substituting oxygen for hydrogen as passivation atoms of ZC₂NNR, the spin filtering efficiency is as high as 100% in the P spin configuration, and the negative differential resistance is largely enhanced with a peak to valley ratio in excess of 4×10³. Our theoretical studies suggest that zigzag C₂N nanoribbon modulated by edge substitution has great potential in the design of future multifunctional spin devices.     

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.     

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

We investigate the eleetronic transport properties of dipyrimidinyl-diphenyl sandwiched between two armchair graphene nanoribbon electrodes using the nonequilibrium Green function formalism combined with a firstprinciples method based on density functional theory. Among the three models M1-M3, Ml 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 rectifhcation 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.     

(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.     

Theoretical investigation on current–voltage characteristics in all-carbon molecular device with different contact geometries

Applying nonequilibrium Green's functions in combination with the first-principles density-functional theory, we investigate electronic transport properties of an all-carbon molecular device consisting of one phenalenyl molecule and two zigzag graphene nanoribbons. The results show that the electronic transport properties are strongly dependent on the contact geometry and device's currents can drop obviously when the connect sites change from second-nearest sites from the central atom of the molecule (S site) to third-nearest sites from the central atom of the molecule (T site). More importantly, the negative differential resistance behavior is only observed on the negative bias region when the molecule connects the graphene nanoribbons through two T sites.     

Rippling and the external electric field on conductance in corrugated graphene nanoribbons molecular device

Molecular devices constructed using corrugated graphene nanoribbons (GNRs) are proposed in the paper. Recursive Green's function calculations show that the intrinsic ripples in graphene and the external electric field energy play important roles on the electron transport properties. Negative differential resistance is observed in zigzag corrugated GNRs. With the wavelength of the ripples decreasing, both the zigzag and armchair corrugated GNRs exhibit ON/OFF characteristics. On applying external electric field, current decreases dramatically in zigzag corrugated GNRs. These findings show that corrugated GNRs can be used to design functional nanoscale devices.     

Electronic properties and conductance suppression of defected and doped zigzag graphene nanoribbons

By using the first-principles calculation based on density functional theory, we investigate the electronic structures and transport properties of the defected and doped zigzag graphene nanoribbons (ZGNRs). The effects of multivacancies defects and impurities have been considered. The results show that band structures of ZGNRs can be tuned strongly and currents drop drastically due to the defect and impurities. Moreover, the notable suppression of conductance can be found near the Fermi level, leading to the negative differential resistance (NDR) behavior under low bias. This effect presents a possibility in novel nanoelectronics devices application.     

The effects of spin-filter and negative differential resistance on Fe-substituted zigzag graphene nanoribbons

Using the first-principle calculations, we investigate the spin-dependent transport properties of Fe-substituted zigzag graphene nanoribbons (ZGNRs). The substituted ZGNRs with single or double Fe atoms, distributing symmetrically or asymmetrically on both edges, are considered. Our results show Fe-substitution can significantly change electronic transport of ZGNRs, and the spin-filter effect and negative differential resistance (NDR) can be observed. We propose that the distribution of the electronic spin-states of ZGNRs can be modulated by the substituted Fe and results in the spin-polarization, and meanwhile the change of the delocalization of the frontier molecular orbitals at different bias may be responsible for the NDR behavior.     

Graphene-like Be_3X_2(X=C,Si, Ge,Sn):A new family of two-dimensional topological insulators

Using first-principle calculations, we predict a new family of stable two-dimensional(2 D) topological insulators(TI),monolayer Be_3 X_2(X = C,Si, Ge, Sn) with honeycomb Kagome lattice. Based on the configuration of Be_3 C_2, which has been reported to be a 2 D Dirac material, we construct the other three 2 D materials and confirm their stability according to their chemical bonding properties and phonon-dispersion relationships. Because of their tiny spin-orbit coupling(SOC)gaps, Be_3 C_2 and Be_3 Si_2 are 2 D Dirac materials with high Fermi velocity at the same order of magnitude as that of graphene.For Be3 Ge2 and Be_3 Sn_2,the SOC gaps are 1.5 meV and 11.7 meV, and their topological nontrivial properties are also confirmed by their semi-infinite Dirac edge states. Our findings not only extend the family of 2 D Dirac materials, but also open an avenue to track new 2 DTI.