导线非共线的分子器件输运性质的第一性原理研究

分子器件是电子器件向小体积化发展的极限,分子器件中的电子在输运过程中体现出明显的量子效应,分子导线与分子接触的位置和导线间的角度等器件结构因素都会对分子器件的输运性质产生较大的影响.迄今为止,尚未见利用第一性原理量子输运计算方法研究导线非共线的分子器件输运性质的报道.本文以金-苯(硫醇)-金结构的分子器件为例,利用基于非平衡格林函数理论和密度泛函理论的第一性原理量子输运计算方法对其输运性质进行了系统研究,特别注重于研究随着非共线导线间导线夹角角度的变化及导线和苯(硫醇)分子接触位置的不同对器件输运性质的影响.计算表明,金导线与苯(硫醇)的接触位置及导线的夹角等器件结构细节不仅能够定量地影响金-苯(硫醇)-金分子器件的电流大小,还能够定性地改变器件的输运性质,使得部分器件结构出现负微分电阻效应.研究结果对全面理解分子器件的输运性质具有一定的指导意义.     

苯基分子器件 量子相干输运第一性原理研究

摘要：分子器件在纳米尺度下,电子的相干性将对体系的电导产生重大影响。本文基于第一性原理计算研究了苯分子连接于一维金属电极下的电荷输运性质。发现一维金电极连接下,不同的连接方式（para与meta）体系下的电导将会有显著差别,而一维铂电极连接下,体系的电导差别不大。我们通过计算电极的能带,发现金电极与铂电极在费米面处的散射态数目有差别。 当量子相干效应导致散射态局域化发生改变时,由于铂电极的通道数较多,电子依然可以通过扩展的通道输运,因此不同连接方式下的电导变化不明显。     

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.     

Investigation on the electronic transport properties of MgS nanotube based molecular devices – a first-principles study

The electronic transport properties of pure MgS nanotube based molecular devices, Mn-substituted nanotubes and Se-substituted nanotubes are investigated using density functional theory. The state of the art of this work is to study the transport properties of MgS nanotubes with substitution impurities across electrodes. The electronic transport properties are discussed in terms of device density of states and transmission spectrum of MgS nanotubes. The effects of Mn substitution and Se substitution in nanotubes are studied. The major contribution to density of states arises only from p orbitals in MgS nanotubes. The substitution effect and bias voltages also have influence in the density of states. The transmission spectrum provides information about the transmission of electrons along the nanotube. The information provided in this work gives a clear vision to fine-tune MgS nanostructures with improved transport property in nanoelectronic device fabrication.     

Device design based on the covalent homocoupling of 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.     

单根ZnO纳米线低温条件下的电子输运及光敏性质

用化学气相沉积的方法合成了ZnO纳米线，采用微栅模板法制得电极从而获得欧姆接触的单根ZnO纳米线半导体器件。通过研究60~300 K范围内的电阻变化情况，发现在整个温度区间内存在热激活和近程跳跃两种传输机制。在300，200，100 K的条件下分别测试了器件的紫外光响应和恢复情况，结果表明：低温下器件对紫外光的敏感性提高，电流的恢复时间随着温度的降低而延长。     

Ab initio study of transport properties of an all-carbon molecular switch based on C20 molecule

Choosing closed-ended armchair (5, 5) single-wall carbon nanotubes (CCNTs) as electrodes, we have investigated the electron transport properties across a carbon molecular junction consisting of a C₂₀ molecule sandwiched between two semi-infinite carbon nanotubes. It is shown that the Landauer conductance of this carbon hybrid system can be tuned within several orders of magnitude not only by varying the tube-C₂₀ distance, but more importantly by changing the orientation of the C₂₀ molecule and rotating the C₂₀ molecule or one of the tubes around the symmetry axis of the system at fixed distances. This fact could make this all-carbon molecular system a possible candidate for a nano-electronic switching device. Moreover, our study also reveals that molecular configuration selection and structural relaxation would play an important role in the design of such devices.      

Spin filtering and large magnetoresistance behaviors in carbon chain-zigzag graphene nanoribbon nanojunctions

Using the non-equilibrium Green's function method combined with the density functional theory, we investigate the electron and spin transport properties of carbon chains covalently connected with zigzag-edged graphene electrodes at finite bias with the parallel (P) and antiparallel (AP) magnetism configurations. When two zigzag-edged graphene electrodes are H2–ZGNR–H structures, spin filtering effect can be realized only with AP magnetism configuration. While one electrode is replaced with the H–ZGNR–H structure, we observe a dual spin filtering effect with above two magnetism configurations. But the spin transport properties of carbon chains can also be affected by the linking way of the carbon chain ends. Deeper analyses show that the spin-related properties are related to the electrodes, magnetism configurations, and the connection structure between electrodes and carbon chains.     

基于石墨烯电极的蒽醌分子器件开关特性

研究了基于石墨烯电极的蒽醌分子器件的开关特性.分别选取了锯齿型和扶手椅型的石墨烯纳米带作为电极,考虑蒽醌基团在氧化还原反应下的两种构型,即氢醌(HQ)分子和蒽醌(AQ)分子,构建了双电极分子结,讨论了氧化还原反应和不同的电极结构对蒽醌分子器件开关特性的影响.研究发现,无论是锯齿型石墨烯电极还是扶手椅型石墨烯电极,HQ构型的电流都明显大于AQ构型的电流,即在氧化还原反应下蒽醌分子呈现出显著的开关特性.同时,当选用锯齿型石墨烯电极时其开关比最高能达到3125,选用扶手椅型石墨烯电极时开关比最高能达到1538.此外,当HQ构型以扶手椅型石墨烯为电极时,在0.7-0.75 V之间表现出明显的负微分电阻效应.因此该系统在未来分子开关器件领域具有潜在的应用价值.     

Effect of torsion angle on electronic transport through different anchoring groups in molecular junction

By applying nonequilibrium Green's function formalism combined with first-principles density functional theory, we investigate effect of torsion angle on electronic transport properties of 4,^4′-biphenyl molecule connected with different anchoring groups (dithiocarboxylate and thiol group) to Au(111) electrodes. The influence of the HOMO-LUMO gaps and the spatial distributions of molecular orbitals on the quantum transport through the molecular device are discussed. Theoretical results show that the torsion angle plays important role in conducting behavior of molecular devices. By changing the torsion angle between two phenyl rings, namely changing the magnitude of the intermolecular coupling effect, a different transport behavior can be observed in these two systems.     

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.     

Effect of crystallographic orientations on transport properties of methylthiol-terminated permethyloligosilane molecular junction

The understanding of the influence of electrode characteristics on charge transport is essential in the field of molecular electronics. In this work, we investigate the electronic transport properties of molecular junctions comprising methylthiol-terminated permethyloligosilanes and face-centered crystal Au/Ag electrodes with crystallographic orientations of (111) and (100), based on the ab initio quantum transport simulations. The calculations reveal that the molecular junction conductance is dominated by the electronic coupling between two interfacial metal-S bonding states, which can be tuned by varying the molecular length, metal material of the electrodes, and crystallographic orientation. As the permethyloligosilane backbone elongates, although the σ conjugation increases, the decreasing of coupling induced by the increasing number of central Si atoms reduces the junction conductance. The molecular junction conductance of methylthiol-terminated permethyloligosilanes with Au electrodes is higher than that with Ag electrodes with a crystallographic orientation of (111). However, the conductance trend is reversed when the electrode crystallographic orientation varies from (111) to (100), which can be ascribed to the reversal of interfacial coupling between two metal-S interfacial states. These findings are conducive to elucidating the mechanism of molecular junctions and improving the transport properties of molecular devices by adjusting the electrode characteristics.     

First-principles study of transition metal linear monoatomic chains adsorption on boron nitride nanotube

Structural, electronic and magnetic properties of six 3d transition metals (TM=V, Cr, Mn, Fe, Co and Ni) linear monoatomic chains adsorbed on the (5,5) boron nitride nanotube (BNNT) at five different sites have been investigated by first-principle calculations. The results indicate all TM chains can be spontaneously adsorbed on the outer surface of the BNNT. The stable adsorption sites are different for different TM chains. All TM chains can be adsorbed on the N site, while the adsorption on the Z site is unstable. The dispersion character occurs in energy band curves of stable TM/BNNT systems and bring about the band gap disappearance in comparison with that of pure (5,5) BNNT. Interestingly, the TM/BNNT systems with nearly half-filled 3d metals V and Cr at H and N sites, as well as Mn at A site show a half-metal character and are usable in spintronics devices. The different electronic properties of BNNT can also be achieved through decorations of the same TM chain on different sites. The TM chain adsorbed BNNT systems exhibit high stability, promising electronic properties and high magnetic moments, which may be useful for a wide variety of next-generation nanoelectronic device components.     

电极距离对分子器件电输运特性的影响

以4,4′_二巯基联苯分子为研究对象，利用从头计算方法和弹性散射格林函数理论，研究了两个电极之间的距离对分子几何结构和电子结构以及该分子结电输运性质的影响. 计算结果表明，电极距离的不同会改变分子的几何结构和电子结构，从而影响分子体系的电输运特性. 扩展分子的平衡状态不是电子输运的最佳状态，适当调整两个电极之间距离可以改善分子的电输运特性. 关键词： 电极距离 伏 安特性 分子电子学     

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.     

Electronic transport properties of silicon chains sandwiched between graphene electrodes: first-principles study

The effects of orientation and silicon chain length on the electronic transport properties for linear silicon chains sandwiched between two graphene electrodes are investigated by using non-equilibrium Green’s functions combined with density functional theory. Our results demonstrate that the conductance of single silicon chains can hardly be affected by its orientation, as there is negligible difference between the conductance of tilted and un-tilted chains, and the conductance is impacted greatly by the length of chains, i.e. the transmission coefficient is doubled for double chains. The equilibrium conductance of single silicon chains shows even-odd oscillating behavior, and its tendency decreases with the increase of the chain length. The non-equilibrium electronic transport properties for all types of chains are also calculated, and all current–voltage curves of silicon chains show a linear character. The frontier molecular orbitals, the total and projected density of states are used to analyse the electronic transport properties for all types of chains.     

分子整流器整流特性的键桥调控效应

利用密度泛函理论和非平衡格林函数方法,研究了基于同一分子在不同空间构型下形成的三个不同分子器件的电子输运特性及整流效果. 研究表明:旋转中间苯环(键桥——π桥),能明显改变整流效果,这是因为改变了分子轨道的离域性,这一发现说明通过改变键桥的空间取向能有效地调控分子整流器的整流性质,这对于设计新型分子整流器有重要意义. 关键词： 分子整流器 键桥 密度泛函理论 非平衡格林函数     

Spin-polarized transport in all-metal mesoscopic spin-valve devices

In spintronic devices the spin of the electron as well as its charge is utilized. We have fabricated a spin-valve device consisting of two permalloy electrodes which are connected by a normal-metal strip made of aluminum. By changing the relative alignment of the magnetizations of the electrodes the resistance of the device can be controlled. The domain configurations of the electrodes are imaged by magnetic-force microscopy in external fields at room temperature. In transport measurements at temperatures between 1.8 and 150 K we identify the observed magnetoresistance as a spin-valve effect.     

High-efficiency spin filtering in salophen-based molecular junctions modulated with different transition metal atoms

Based on the density functional theory and nonequilibrium Green's function methods, we investigate the spin transport properties of the molecular junctions constructed by a homologous series of 3d transition metal(II) salophens (TM-salophens, TM = Co, Fe, Ni and Mn) sandwiched between two gold electrodes. It is found that among the four molecular junctions only Co-salophen junction can act as an efficient spin filter distinctively. The conductance through Co-salophen molecular junction is dominated by spin-down electrons. The mechanism is proposed for these phenomena.     

光致异构体开关特性的理论研究

利用弹性散射格林函数方法,对4,4-二羧基1,2-二苯乙烯分子的两种异构体与金电极构成的单分子结进行了研究. 研究表明,该类分子是通过末端羧基化学吸附于金表面的,两种分子结电导特性的差异主要是因为分子与电极的相互作用所致. 对每一种分子来说,都存在三种不同的稳定电导值,分别对应着分子末端与金表面的不同接触方式. 分子与金表面的相互作用导致分子结电子结构的变化是其电导差异的主要原因. 理论结果与实验测量结果符合得较好. 关键词： 光致异构体 伏安特性 电子输运 分子电子学