基于离子修饰簇模型的单分子结的拉曼光谱-电导关联性的理论研究

在分子电子学中,金属电极-分子-金属电极分子结的单分子电导性质强烈依赖于分子锚定基团和电极的成键性质.然而其成键性质难以在室温下获得.前期本课题组结合钓鱼模式针尖增强拉曼光谱(FM-TERS)和电导测量联用系统,在室温下同时获得单分子的拉曼光谱和电导值,进而发现了单分子电导值与拉曼强度随偏压变化同增同减的关联现象.本文尝试发展一种离子修正的簇模型并计算不同偏压下模型分子4-4′联吡啶(BPy)的分子电导和拉曼光谱,再现实验观察到的关联现象.进一步基于最高占据轨道和最低非占据轨道近似的两能级模型,从理论上推导出Au-BPy-Au分子结中单分子电导和单分子拉曼光谱的关联性函数,从而揭示了这种关联性的内在物理化学本质.     

电化学门控调节具有平行路径的单分子电路中电子传输（英文）

电化学门控已成为一种可行且高效调节单分子电导的方法。在本研究中,我们证实了具有两个平行苯环的单分子电路中电子传输可以通过电化学门控控制。首先,我们利用STM-BJ技术以金为电极构筑了具有两条平行路径的单分子结。与单条路径的单分子结相比,两条路径的分子结由于具有增强性量子干涉效应,具有2.82倍的电导值。进一步地,我们利用电化学门控对具有两个平行苯环的单分结的电导进行调控,获得了333%·V~(-1)调节比。结合DFT计算,发现在E=E_F附近的V形透射系数谱图导致了实验观测的电导门控行为。本研究揭示了具有平行路径的单分子电路的电化学门控行为,并为设计高性能分子器件的分子材料提供了新的途径。     

电化学门控调节具有平行路径的单分子电路中电子传输

电化学门控已成为一种可行且高效调节单分子电导的方法。在本研究中,我们证实了具有两个平行苯环的单分子电路中电子传输可以通过电化学门控控制。首先,我们利用STM-BJ技术以金为电极构筑了具有两条平行路径的单分子结。与单条路径的单分子结相比,两条路径的分子结由于具有增强性量子干涉效应,具有2.82倍的电导值。进一步地,我们利用电化学门控对具有两个平行苯环的单分结的电导进行调控,获得了333%·V^-1调节比。结合DFT计算,发现在E=E_F附近的V形透射系数谱图导致了实验观测的电导门控行为。本研究揭示了具有平行路径的单分子电路的电化学门控行为,并为设计高性能分子器件的分子材料提供了新的途径。     

从分子水平研究电子传递

有机分子中电子传递受到诸多因素的影响, 纳米电极、界面、环境和分子本身都是必须要系统考察的因素. 本文从理论模拟和实验研究两个方面总结了在分子尺度上电子传递研究的最新进展. 着重讨论了分子动力学方法模拟纳米电极的制备, 量子化学方法研究电场作用下的分子构象及分子电导, 另外还讨论了扫描隧道显微术和电化学方法研究单分子结的电子传递. 分子电子传递的研究不仅涉及微观的实验测量, 从宏观的实验结果通过合理的分析推导, 也可以得到微观的信息.     

低聚对亚苯基亚乙烯基分子导线电子传输机理研究

合成了12个OPV分子导线(分成乙酰巯基与氨基端基两个系列),采用导电原子力显微镜和扫描隧道显微镜-裂分结的方法对该类分子导线的电学性能进行了表征.通过分析单分子电阻与分子长度、温度以及电场的依赖关系,发现OPV分子导线的电子传输机理在临界长度为2.0 nm处发生了由隧穿传导向跳跃传导的转变.通过对比分子末端分别为巯基和氨基的单分子电导值,考察了不同的连接基团对OPV分子导线电子传输性能的影响,发现末端基团只影响分子的接触电阻,但不改变分子导线本身的电子传输机理.利用密度泛函理论和非平衡格林函数方法对OPV分子导线的电学特性进行了理论研究,结果表明分子的前线轨道能级与金电极的相对位置决定了OPV分子导线的电子传输机理.     

邻碘苯甲酸电还原脱碘的原位红外光谱研究

采用循环伏安法研究了邻碘苯甲酸在NaOH溶液中的电化学还原反应,与Pt和Ti等电极相比,Ag和Cu电极对邻碘苯甲酸具有较好的电还原活性,还原电位向正电位方向移动.通过原位红外光谱技术研究了邻碘苯甲酸在Ag和Cu电极上的电还原机理.结果表明,在电位高于-800 mV时,邻碘苯甲酸在Ag电极表面先形成吸附中间态R…I…Ag,而在Cu电极表面以负离子形式存在.随着电位的进一步负移,邻碘苯甲酸在Ag和Cu电极上均发生脱碘加氢反应,经还原得到最终产物苯甲酸.     

Al-C60-Al分子结电子输运特性的第一性原理计算

利用基于密度泛函理论的格林函数方法, 计算了Al-C60-Al分子结的电子输运特性. 考虑了C60分子在铝电极表面的原子结构弛豫, 计算结果表明共振传导是Al-C60-Al分子结电子输运的主要特征, 在费米能级附近的电导约为1.14G0 (G0=2e2/h). 投影态密度(PDOS)分析表明, Al-C60-Al分子结的电子输运主要通过C60分子的最低空分子轨道(LUMO)和次低空分子轨道(LUMO+1)进行. 讨论了C60分子和铝电极之间距离的变化对其电子输运特性的影响.     

高温水溶液pH值原位测量系统与机理

研制了以氧化钇稳定氧化锆(YSZ)陶瓷薄膜电极为pH电极、外置压力平衡式Ag/AgCl电极为参比电极的高温高压水溶液pH值原位测量系统,测量了H₃BO₃\LiOH水溶液在473.15~573.15 K范围内的pH值,并与热力学计算得到的pH值比较。 结果表明,当温度高于548.15 K时,测量系统可以实现pH值的准确测量;而低于此温度时,由于YSZ陶瓷膜内阻过大,测得的pH值与理论计算值存在偏差,且随温度的降低,测量偏差增大。 讨论了该系统的pH值测量机理。     

钛基二氧化锡电极电解过程中羟基自由基检测及电催化机理

采用浸渍-热分解方法制备了钛基二氧化钌(Ti/RuO2)和钛基二氧化锡(Ti/SnO2)两类尺寸稳定阳极电极。以扫描电子显微镜(SEM)和X射线衍射(XRD)对电极结构进行了表征。用循环伏安法(CV)比较研究了Ti/RuO2和Ti/SnO2电极直接电催化氧化苯酚性能,表明苯酚在Ti/RuO2和Ti/SnO2电极上均可发生直接电氧化反应,两种电极上氧化峰电位分别为0.96和1.43 V(vs.Ag/AgC l)。以对苯二甲酸为捕获剂,利用荧光光谱法进行了羟基自由基(.OH)检测。Ti/SnO2电极在电解过程中能够产生.OH;而Ti/RuO2电极.OH的生成极其微弱。Ti/SnO2电极电解过程中生成.OH是其具有高电催化活性的主要原因,也表明了用荧光法进行羟基自由基检测方便、灵敏,可以用于电催化过程羟基自由基的检测。     

紫罗碱衍生物分子结的电学性质理论研究

本文基于密度泛函(DFT)结合非平衡格林函数(NEGF)的方法,以具有氧化还原中心的紫罗碱衍生物(N,N′-bis(4-thioalkyl)-4,4′-bipyridinium, HS-4V4-SH)功能分子构造Au(111)/S-4V4-S/Au(111)分子结,详细分析了分子在三种价态V、V⁺和V²⁺下的电学性质与分子的几何结构和电子结构的关系。基于对三种价态透射系数分析结果表明,在零偏压下,V与V⁺的电导值比V²⁺高了两个数量级,4V4分子结的电导随两个吡啶环之间夹角的增大呈线性减小。同时,理论计算结果也表明,增加烷基链(HS-nVn-SH, n = 2 ~ 7)的数目,发现分子结电导值呈指数形式衰减,其每个亚甲基的衰减因子约为1,与烷基二硫醇分子的接近。     

Structure and Electron Transport at Metal Atomic Junctions Doped with Dichloroethylene

We have investigated the structure and electron transport at dichloroethylene-doped metal atomic junctions at low temperatures (20 K) in ultra-high vacuum, using Fe, Ni, Pd, Cu, Ag, and Au. The metal atomic junctions were fabricated using the mechanically controllable break junction technique. After introducing the dichloroethylene (DCE), the conductance behavior of Fe, Ni, and Pd junctions was considerably changed, whereas little change was observed for Cu, Ag, and Au. For the Pd and Cu junctions, a clear peak was observed in their conductance histograms, showing that the single-molecule junction was selectively formed. To investigate the structure of the metal atomic junctions further, their plateau lengths were analyzed. The length analysis revealed that the Au atomic wire was elongated, and the metal atomic wires were formed for the other transition metals: those that do not normally form metal atomic wires without DCE doping, as DCE adsorption stabilized the metal atomic states. There is a strong interaction between DCE and the metals, where DCE supports the formation of the metal atomic wire for Fe, Ni, and Pd.     

单分子电导测量技术及其影响因素

Molecular electronics is an important field for the application of nanotechnologies with an ultimate goal of building functional devices using single molecules or molecular arrays to realize the same functionality as macroscopic devices. To attain this goal, reliable techniques for measuring and manipulating electron transfer processes through single molecules are essential. There are various techniques and many environmental factors influencing single-molecule electronic conductance measurements. In this review, we first provide a detailed introduction and classification of the current well-accepted techniques in this field for measuring single-molecule conductance. All available techniques are summarized into two categories: the fixed junction technique and break junction technique. The break junction technique involves repeatedly forming and breaking molecular junctions by mechanically controlling a pair of electrodes moving into and out of contact in the presence of target molecules. Single-molecule conductance can be determined from the conductance plateaus that appear in typical conductance decay traces when molecules bind two electrodes during their separation process. In contrast, the fixed junction technique is to fix the distance between a pair of electrodes and measure the conductance fluctuations when a single molecule binds the two electrodes stochastically. Both techniques comprise different application methods and have been employed preferentially by different groups. Specific features of both techniques and their intrinsic advantages are compared and summarized in Section 4.     

Linker dependent bond rupture force measurements in single-molecule junctions

We use a modified conducting atomic force microscope to simultaneously probe the conductance of a single-molecule junction and the force required to rupture the junction formed by alkanes terminated with four different chemical link groups which vary in binding strength and mechanism to the gold electrodes. Molecular junctions with amine, methylsulfide, and diphenylphosphine terminated molecules show clear conductance signatures and rupture at a force that is significantly smaller than the measured 1.4 nN force required to rupture the single-atomic gold contact. In contrast, measurements with a thiol terminated alkane which can bind covalently to the gold electrode show conductance and force features unlike those of the other molecules studied. Specifically, the strong Au-S bond can cause structural rearrangements in the electrodes, which are accompanied by substantial conductance changes. Despite the strong Au-S bond and the evidence for disruption of the Au structure, the experiments show that on average these junctions also rupture at a smaller force than that measured for pristine single-atom gold contacts.     

基于裂结技术的单分子尺度化学反应研究进展

分子电子学是研究单分子器件的构筑、性质以及功能调控的一门新兴学科。其中,金属/分子/金属结的构筑和表征是现阶段分子电子学的主要研究内容。裂结技术是当前分子电子学研究的主要实验方法,主要包括机械可控裂结技术和扫描隧道显微镜裂结技术。本文对裂结技术进行了介绍,并对近年来利用这些技术,在单分子尺度化学反应的检测和动力学研究,以及将这些技术与溶液环境、静电场、电化学门控等方法相结合,调控单分子器件的电输运性质等方面所取得的进展进行了概述。     

The binding sites of carboxylic acid group contacting to Cu electrode

In this work, the binding sites of carboxylic acid binding to Cu electrode are explored by electrochemical jump-to-contact STM break junction. Single molecular conductance of benzene-based molecules with ending groups of carboxylic acid, carbonyl and hydroxyl are measured and compared. The conductance values of 1,4-benzenedicarboxaldehyde can be found in those of 1,4-benzenedicarboxylic acid, which shows that carboxylic acid can bind to Cu electrode through carbonyl group. Carboxylic acid can also bind to the electrode through carboxylate group, and gives out larger conductance values than those of carbonyl group. However, molecule with hydroxyl group is difficult to form single molecular junction with Cu. The current work demonstrates that the carboxylic acid can bind to the electrode through carbonyl and carboxylate groups, and a new anchoring group of carbonyl group can be used to form effective single molecular junction.     

Dependence of single-molecule conductance on molecule junction symmetry

The symmetry of a molecule junction has been shown to play a significant role in determining the conductance of the molecule, but the details of how conductance changes with symmetry have heretofore been unknown. Herein, we investigate a naphthalenedithiol single-molecule system in which sulfur atoms from the molecule are anchored to two facing gold electrodes. In the studied system, the highest single-molecule conductance, for a molecule junction of 1,4-symmetry, is 110 times larger than the lowest single-molecule conductance, for a molecule junction of 2,7-symmetry. We demonstrate clearly that the measured dependence of molecule junction symmetry for single-molecule junctions agrees with theoretical predictions.     

Analytical modeling of the junction evolution in single-molecule break junctions: towards quantitative characterization of the time-dependent process

The conductance through single-molecule junctions characterized by the break junction techniques consists of the through-space tunneling and through-molecule tunneling conductance, and the existence of through-space tunneling between the electrodes makes the quantitative extraction of the intrinsic molecular signals of single-molecule junctions challenging. Here, we established an analytic model to describe the evolution of the conductance of a single molecule in break junction measurements. The experimental data for a series of oligo(aryleneethynylene) derivatives validate the proposed model, which provides a modeling insight into the conductance evolution for the opening process in a "real" break junction experiment. Further modulations revealed that the junction formation probability and rupture distance of the molecular junction, which reflect the junction stability, will significantly influence the amplitude and position of the obtained conductance peak. We further extend our model to a diffusion and a chemical reaction process, for which the simulation results show that the break junction technique offers a quantitative understanding of these time-dependent systems, suggesting the potential of break junction techniques in the quantitative characterization of physical and chemical processes at the single-molecule scale.     

Increased Surface Density of States at the Fermi Level for Electron Transport Across Single-Molecule Junctions

Fermi's golden rule, a remarkable concept for the transition probability involving continuous states, is applicable to the interfacial electron-transporting efficiency via correlation with the surface density of states (SDOS). Yet, this concept has not been reported to tailor single-molecule junctions where gold is an overwhelmingly popular electrode material due to its superior amenability in regenerating molecular junctions. At the Fermi level, however, the SDOS of gold is small due to its fully filled d-shell. To increase the electron-transport efficiency, herein, gold electrodes are modified by a monolayer of platinum or palladium that bears partially filled d-shells and exhibits significant SDOS at the Fermi energy. An increase by 2–30 fold is found for single-molecule conductance of α,ω-hexanes bridged via common headgroups. The improved junction conductance is attributed to the electrode self-energy which involves a stronger coupling with the molecule and a larger SDOS participated by d-electrons at the electrode-molecule interfaces.     

Measurement and statistical analysis of single-molecule current-voltage characteristics, transition voltage spectroscopy, and tunneling barrier height

We report on the measurement and statistical study of thousands of current-voltage characteristics and transition voltage spectra (TVS) of single-molecule junctions with different contact geometries that are rapidly acquired using a new break junction method at room temperature. This capability allows one to obtain current-voltage, conductance voltage, and transition voltage histograms, thus adding a new dimension to the previous conductance histogram analysis at a fixed low-bias voltage for single molecules. This method confirms the low-bias conductance values of alkanedithiols and biphenyldithiol reported in literature. However, at high biases the current shows large nonlinearity and asymmetry, and TVS allows for the determination of a critically important parameter, the tunneling barrier height or energy level alignment between the molecule and the electrodes of single-molecule junctions. The energy level alignment is found to depend on the molecule and also on the contact geometry, revealing the role of contact geometry in both the contact resistance and energy level alignment of a molecular junction. Detailed statistical analysis further reveals that, despite the dependence of the energy level alignment on contact geometry, the variation in single-molecule conductance is primarily due to contact resistance rather than variations in the energy level alignment.     

基于吡嗪连接的石墨烯电极单分子场效应晶体管

In single-molecule junctions, anchoring groups that connect the central molecule to the electrodes have profound effects on the mechanical and electrical properties of devices. The mechanical strength of the anchoring groups affects the device stability, while their electronic coupling strength influences the junction conductance and the conduction polarity. To design and fabricate high-performance single-molecule devices with graphene electrodes, it is highly desirable to explore robust anchoring groups that bond the central molecule to the graphene electrodes. Condensation of ortho-phenylenediamine terminated molecules with ortho-quinone moieties at the edges of graphene generates graphene-conjugated pyrazine units that can be employed as anchoring groups for the construction of molecular junctions with graphene electrodes. In this study, we investigated the fabrication and electrical characterization of single-molecule field-effect transistors (FETs) with graphene as the electrodes, pyrazine as the anchoring groups, and a heavily doped silicon substrate as the back-gate electrode. Graphene nano-gaps were fabricated by a high-speed feedback-controlled electro-burning method, and their edges were fully oxidized; thus, there were many ortho-quinone moieties at the edges. After the deposition of phenazine molecules with ortho-phenylenediamine terminals at both ends, a large current increase was observed, indicating that molecular junctions were formed with covalent pyrazine anchoring groups. The yield of the single-molecule devices was as high as 26%, demonstrating the feasibility of pyrazine as an effective anchoring group for graphene electrodes. Our electrical measurements show that the ten fabricated devices exhibited a distinct gating effect when a back-gate voltage was applied. However, the gate dependence of the conductance varied considerably from device to device, and three types of different gate modulation behaviors, including p-type, ambipolar, and n-type conduction, were observed. Our observations can be understood using a modified single-level model that takes into account the linear dispersion of graphene near the Dirac point; the unique band structure of graphene and the coupling strength of pyrazine with the graphene electrode both crucially affect the conduction polarity of single-molecule FETs. When the coupling strength of pyrazine with the graphene electrode is weak, the highest occupied molecular orbital (HOMO) of the central molecule dominates charge transport. Depending on the gating efficiencies of the HOMO level and the graphene states, devices can exhibit p-type or ambipolar conduction. In contrast, when the coupling is strong, the redistribution of electrons around the central molecule and the graphene electrodes leads to a realignment of the molecular levels, resulting in the lowest unoccupied molecular orbital (LUMO)-dominated n-type conduction. The high yield and versatility of the pyrazine anchoring groups are beneficial for the construction of single-molecule devices with graphene electrodes.