【作者】 彭军；

【导师】 陈克求；

【作者基本信息】 湖南大学 ， 物理学， 2017， 博士

【摘要】 随着科技的进步和社会的发展,电子器件朝着小型化和高度集成化的方向发展,不过原理性的物理限制和技术上的工艺限制,使传统的以硅、锗为基础的微电子器件遇到了前所未有的挑战。科学家们把目标瞄向体积更小的分子身上,其中有机分子具有很多良好性能而被广泛研究。本文选取分子尺度体系作为研究对象,采用了第一性原理计算方法,研究了石墨烯纳米带、硼氮纳米带、有机小分子和石墨炔纳米带的电子输运性质。具体从以下几个角度进行系统研究:中心原子替代掺杂与电极-分子的接触类型对有机小分子自旋过滤效应的影响:不同边界类型对石墨烯纳米带与硼氮纳米带构成的异质结整流行为的影响;边缘修饰对石墨炔纳米带整流及自旋过滤行为的影响;门压对基于场效应的有机单分子开关行为的调控等。主要研究内容分为以下几部分:研究了中心原子替代掺杂的salophen单分子与金电极构成的分子器件的自旋电子输运性质。在所采用的一系列作为中心替代原子的过渡族金属中,只有中心原子是Co原子时才观察到明显的自旋过滤效应,且自旋向下的电子在电导中占主导地位。进一步研究还发现,分子自身与电极的连接位置也是影响器件自旋电子输运性质的重要因素。分子与电极的连接位置发生微小改变,也会改变器件的自旋电流大小,从而实现对器件传导性的调控。研究了宽度和边界类型对石墨烯与硼氮构成的异质结(BNC)的电子输运行为的影响。计算结果表明,异质结的宽度对其整流特性几乎没有影响;而不同的边界类型对异质结整流特性影响显著。异质结中,当石墨烯中的碳原子全部与硼氮纳米条带中的氮原子相连接时,有限偏压下的最大整流比可以达到107。前线分子轨道在正偏压时空间扩展程度的强烈局域,导致正负偏压下偏压窗内输运系数的不对称分布是整流现象出现的原因。还研究了边缘完全不钝化时,边界类型对BNC异质结自旋电子输运性质的影响。观察到了完美的自旋过滤效应和磁致电阻率达到107%的磁致电阻效应。研究了对称和不对称边缘氢化的锯齿形α石墨炔纳米条带在外加磁场调控下的自旋输运性质。计算结果表明,边缘全部双氢化构型(D-D)在偏压达-到一定值之前一直表现为绝缘体特征;而纳米带的左/右两半分别单氢/双氢化构型(M-D)呈现整流特性,且与外加磁场方向无关;边缘全部单氢化构型(M-M)在外加磁场调控下,能实现从导体到半金属性的转变,进一步分析表明这取决于边缘全部单氢化的α石墨炔,其能带结构中π和π*子带是否匹配。这些结果表明,通过不同的边缘氢化及外加磁场调控,锯齿形α石墨炔纳米条带器件能够被设计成多功能的分子自旋电子器件,这对进一步提升原子级电路的集成度有非常重要的意义。研究了单个 Dibenzo[d,d’]thieno[3,2-b;4,5-b’]dithiophene 分子(DBTDT)与金电极构成的三端分子器件的电子输运行为,重点考察外加门压的影响。从计算结果来看,器件的电子输运性质受外加门压的影响很大。门压引起了最高占据态分子轨道(HOMO)向费米能级(Ef)移动,导致了 HLG的减少。然而,正、负门压对器件电子输运的作用却截然相反。施加正门压,对器件的电子输运能力起促进作用;施加负门压正好相反,会减弱器件的电子输运能力。正因如此,分子器件在高导态与低导态之间的转换,可以通过门压来调控,从而实现其分子电流的开关功能。这些计算结果将有助于理解真实情况下分子器件的电子输运过程。更多还原

【Abstract】 With technogical progress and social development,the electronic devices move towards miniaturization and high integration.Conventional microelectronic devices based on silicons and germaniums are facing the unprecedented challenge.The scientists take the goal to the molecules with smaller volume,in which organic molecules have been widely reseached owing to their good performance.By applying the first principles based on density-functional theory and the nonequilibrium Green’s functions,we systematically study the electronic structures and electron trasnsport properties of the molecular systems,inculuding graphene nanoribbons,boron nitride nanoribbons,single organic molecule and graphyne nanoribbons.We mainly discuss the effects of doping,connected sites,chemical functionalization,adsorption and gate voltage on the electron trasnsport properties of the molucualar devices.The molecular rectifying behavior and switching behaviors based on spin-filtering effect,magneto resistance effect and field effect are observed.We investigate the spin transport properties of the molecular junctions constructed by a homologous series of 3d transition metal(II)salophens 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.Further studies show that the spin electronic transport properties are also strongly dependent on the contact sites.of the molecule to the electrodes,in which the spin current will srongly response to its tiny change,so as to the controllability on the conductivity of the device.We investigate the effect of boundary types on the rectifying behaviors in heterojunction composed of zigzag graphene and hexagonal boron-nitride(BNC)hybridized nanoribbons.The results demonstrate that the rectifying behavior is strongly dependent on the boundary types,while little affected by the width of BNC hybridized nanoribbons.It is noteworthy that the maximum rectifying ratio of the system at finite bias can be high up to orders of 107 in which atoms carbon in graphene nanoribbon are totally connected with atoms nitrogen in boron-nitride nanoribbon.It is due to the asymmetry distribution of the transmission coefficient near the Ef under positive and negative bias,which finally roots in more localization of LUMOs under positive bias and the absence of PDOSs of BNNR domain at the corresponding energy region under negative bias.The effect of boundary types on the spin transport properties of bare BNC is also studied.The perfect spin filtering and magnetoresistance effect with a ratio high up to 107 is observed.We investigate the spin-dependent transport properties of zigzag—edgedα-graphyne nanoribbons(Z α GYNRs)with symmetric and asymmetric edge hydrogenations,so as their response to external magnetic fields by the non-equilibrium Green’s function method combined with the density functional theory..It is found that the symmetric model D-D shows an insulating state till the bias up to some value,while rectifying behavior can be observed in the asymmetric model M-D regardless of external magnetic fields.Moreover,the symmetric model M-M can be switched between a conducting state and a half-metallic state by tuning the magnetic fields,which arises from the matching or mismatching between the π orπ*states of the two ribbons by tuning the magnetic fields.These results indicate that these systems can be designed as multifunctional molecular d spintronic evices,which is important to further improve the level of integration of future atomic-scale circuits.We investigate the transport properties of the single molecule DBTDT sandwiched between two gold electrodes,focusing on the effects of gate voltages.It is found that the gate voltages have much influence on the device,which caused the HOMO molecular orbital moving towards the Fermi level,resulting in the reduction of the HLG.Meanwhile,the positive and negative gate voltages have the distinct effects on the transport properties,in which the former can increase the transport while the latter weaken it.Hence the device can be switched between the high and low conductive states so that it can act as a gate-controlled current switcher.All of these can be helpful to the electronic transport in real molecular devices.更多还原