单分子器件的拉伸与断裂过程第一性原理研究:末端基团效应

基于密度泛函理论,研究了含S以及含N末端基团的分子结的拉伸与断裂过程.计算结果显示,对于尖端为锥形的金电极,当末端基团为—S时,拉断分子结的作用力大小为0.,59 nN,大于H原子未解离的—SH从金电极上断裂所需的0.25 nN作用力,但明显小于—S末端从平面金电极上断裂下来的约1.5 nN的作用力.当末端基团是—NH_2或—NO_2时,分子结断裂所需拉力分别为0.45和0.33 nN.体系轨道分布表明,分子与电极通过前线占据轨道耦合后形成的扩展体系分子轨道离域性越好,拉断分子结所需的作用力越大.自然键轨道(natural bond orbital,NBO)分析显示,若分子末端与电极间未形成成键轨道,末端原子上更多的NBO净电荷可以提高分子与电极间结合的稳定性.结合我们以前的研究,可以发现,—S末端和—NH_2末端对金电极界面的微观构型具有明显的识别功能,这为精确操控并理解分子与金电极间的相互作用及界面结构提供了有用信息.     

金电极与吡啶末端连接界面结构的力学变化过程理论研究

利用基于第一性原理的绝热拉伸模拟方法计算了4,4’-二吡啶分子与不同构型金电极之间的作用过程,研究了分子在外力作用下逐渐远离金电极过程中分子与电极间界面结构特有的演化过程以及体系能量与作用力的变化特征.结果显示,分子在远离锥形电极过程中很容易出现近于垂直地连接到锥形电极第二层金原子上的特有连接构型,同时由于吡啶末端的排斥作用,电极尖端的金原子偏向一侧.分子从第二层金原子上断开并连接到尖端金原子上需要1.3—1.5 nN的拉力作用,明显大于分子从尖端电极上断开所需要的0.8—1.0 nN的作用力,从而揭示了实验中二吡啶分子结在形成过程中作用力与界面构型变化之间的对应关系.4,4’-二吡啶分子与平面金电极的作用较弱,只需要不到0.5 nN的作用力就可断开,而当分子连接到吸附在平面电极表面的孤立金原子上时,可以承受约1.7 nN的作用力.以上研究表明基于第一性原理的绝热拉伸模拟方法不仅可以揭示分子与电极之间的界面结构演化过程,而且通过作用力的计算可以很好地识别实验中分子与电极间的特有界面结构.     

单硫醇分子结的几何结构和电输运性质:压力效应与末端基团效应

利用杂化密度泛函理论,研究了以甲基、醇基、羧基为末端基团的烷烃硫醇分子与金电极形成分子结的过程,得到了分子结的几何结构与外加压力的关系. 并在此基础上,利用弹性散射格林函数方法研究了烷烃硫醇分子的电输运性质. 研究结果表明,对于C₁₁S分子来说,当两电极距离大于2.1 nm时,该分子结断裂;对于C₁₁SOH和C₁₀SCOOH来说,相应的分子结断裂的电极距离基本相同(2.15 nm). 在相同的外加压力(4.0 nN)下,C_{11关键词： 压力 末端基团 烷烃硫醇分子 电输运性质}     

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1.0V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.     

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     

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.     

硫醇自组装分子膜末端基团对其电荷输运特性的影响

在金(111)表面组装了具有不同末端基团的硫醇单层分子膜，并利用导电原子力显微镜研究了分子膜的电输运性质，发现不同末端基团的分子自组装膜的导电能力有明显差别.结合X射线光电子能谱，研究了末端基团中碳原子的结合能与相应硫醇分子电导的关系.结果表明不同末端基团分子膜导电能力的差别可归结为末端基团碳原子电子结合能的差异.结合能越高，末端基团电子的局域化程度越强，导致电子有效注入分子主链的势垒越高，从而减弱了分子膜对电子的输运能力.此外，实验还发现不同末端基团的硫醇单层分子膜有不同的表面电势，导致分子膜电流电压特性曲线的零点产生偏离. 关键词： 分子自组装膜 输运特性 末端基团 导电原子力显微镜     

金属电极对有机分子非弹性电子隧穿谱的影响

利用第一性原理计算金属电极下1,6-己二硫醇和1,4-二巯基苯分子结的非弹性电子隧穿谱,发现非弹性电子隧穿谱对金属电极的变化十分灵敏,并且非弹性电子隧穿谱的振动峰位置和强度与硫原子和金属电极表面的距离密切相关.结果表明电极材料和分子与金属成键的情况是影响分子结的非弹性电子输运的重要因素.理论分析进一步表明不同金属电极和有机分子的耦合能不同导致了谱峰强弱的调整.     

苯基分子器件量子相干输运第一性原理研究

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.     

末端基团对硫醇分子结电输运性质的影响北大核心CSCD

利用杂化密度泛函理论,研究了以甲基、醇基、羧基为末端基团的烷烃硫醇分子与金电极形成分子结的电子结构,利用弹性散射格林函数方法研究了烷烃硫醇分子的电输运性质.研究结果表明,末端基团对分子结导电能力有显著影响,其原因可归结为末端基团碳原子电子结合能的差异.结合能越高,末端基团电子的局域化程度越强,从而减弱了电子的输运能力.理论计算结果与实验结果定性定量上都比较符合.     

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.     

Signature of atomic structure in the quantum conductance of gold nanowires

We have used high resolution transmission electron microscopy to determine the structure of gold nanowires generated by mechanical stretching. Just before rupture, the contacts adopt only three possible atomic configurations, whose occurrence probabilities and quantized conductance were subsequently estimated. These predictions have shown a remarkable agreement with conductance measurements from a break junction operating in ultrahigh vacuum, corroborating the derived correlation between nanowire atomic structure and conductance behavior.     

Structural and electronic properties of gold nanowires

We have used high resolution transmission electron microscopy to determine the structure of gold nanowires generated by mechanical stretching. Just before rupture, the contacts adopt only three possible atomic configurations, whose occurrence probabilities and quantized conductance were subsequently estimated. These predictions have shown a remarkable agreement with conductance measurements from a break junction operating in ultra-high-vacuum, corroborating the derived correlation between nanowire atomic structure and conductance behavior. Received 28 November 2000     

Defect-induced conductance oscillations in short atomic chains

Electronic transport through a junction made of two gold electrodes connected with a gold chain containing a silver impurity is analyzed with a tight binding model and the density-functional theory. It is shown that the conductance depends in a simple way on the position of the impurity in the chain and the parity of the total number of atoms of the chain. For an odd chain the conductance takes on a higher value when the Ag impurity substitutes an even Au atom in the chain, and a lower one for an odd position of the Ag atom. In the case of an even chain the conductance hardly depends on the position of the Ag atom. This new kind of a defect-induced parity oscillation of the conductance is significantly more prominent than the well-known even-odd effect related to the dependence of the conductance on the parity of number of atoms in perfect chains.     

Effects of Contact Atomic Structure on Electronic Transport in Molecular Junction

Based on nonequilibrium Green＇s function and first-principles calculations, we investigate the change in molecular conductance caused by different adsorption sites with the presence of additional Au atom around the metal- molecule contact in the system that benzene sandwiched between two Au（111） leads. The motivation is the variable situations that may arise in break junction experiments. Numerical results show that the enhancement of conductance induced by the presence of additional Au is dependent on the adsorption sites of anchoring atom. When molecule is located on top site with the presence of additional Au atoms, it can increase molecular conductance remarkably and present negative differential resistance under applied bias which cannot be found in bridge and hollow sites. Furthermore, the effects of different distance between additional Au and sulfur atoms in these three adsorption sites are also discussed.     

Pulling monatomic gold wires with single molecules: an Ab initio simulation

Car-Parrinello molecular dynamics simulations demonstrate that pulling a single thiolate molecule anchored on a stepped gold surface does not preferentially break the sulfur-gold chemical bond. Instead, it is found that this process leads to the formation of a monoatomic gold nanowire, followed by breaking a gold-gold bond with a rupture force of about 1.2 nN. The simulations also indicate that previous single-molecule thiolate-gold and gold-gold rupture experiments both probe the same phenomenon, namely, the breaking of a gold-gold bond within a gold nanowire.     

High probability of single molecule junction formation with Ag electrodes

We use a scanning tunneling microscope-based break-junction technique to compare the probability of the formation of a single-molecule junction for a series of amine-terminated oligophenyl and alkane species using either Ag or Au as electrodes. For all molecules, we find that there is a significantly higher probability of junction formation when using Ag electrodes than with Au electrodes. We also find that longer molecules have a higher probability than shorter molecules to form a junction for both Ag and Au electrodes. For all molecules, the measured molecular junction length that is formed with the Ag electrodes was longer than that formed with Au electrodes. Furthermore, we can make a single atomic oxygen junction and can measure its conductance using Ag electrodes. These observations are attributed to a narrower gap of the Ag electrodes compared to that of the Au electrodes after the metal contact ruptures. Since there is a high probability of a molecular junction forming when using Ag electrodes, we can therefore perform a statistical analysis within the context of the material properties that are suitable for future molecular electronics.     

Gold–sulfur bond breaking in Zn(II) tetraphenylporphyrin molecular junctions

Quantized conductance measured from single tetraphenylporphyrin molecules with and without a central zinc(II) [Zn(II)] atom was measured using a molecular break junction (MBJ) method. From the conductance histograms we observed an additional 1.7 Å stretch for two-state conductance in a single Zn(II) tetraphenylporphyrin (ZnTPP) molecule as compared to single state conductance in a free tetraphenylporphyrin (TPP) molecule, i.e., no central Zn(II) atom. First-principles density functional calculations, using an electrode–molecule–electrode model, are completed to provide insight into the mechanisms attributed to bond stretching, and eventual bond breaking, to better understand the additional 1.7 Å of stretching observed with ZnTPP.