Two-electron dissociation of single molecules by atomic manipulation at room temperature

Using the tip of a scanning tunnelling microscope (STM) to mechanically manipulate individual atoms and molecules on a surface is now a well established procedure. Similarly, selective vibrational excitation of adsorbed molecules with an STM tip to induce motion or dissociation has been widely demonstrated. Such experiments are usually performed on weakly bound atoms that need to be stabilized by operating at cryogenic temperatures. Analogous experiments at room temperature are more difficult, because they require relatively strongly bound species that are not perturbed by random thermal fluctuations. But manipulation can still be achieved through electronic excitation of the atom or molecule by the electron current tunnelling between STM tip and surface at relatively high bias voltages, typically 1-5 V. Here we use this approach to selectively dissociate chlorine atoms from individual oriented chlorobenzene molecules adsorbed on a Si(111)-7 x 7 surface. We map out the final destination of the chlorine daughter atoms, finding that their radial and angular distributions depend on the tunnelling current and hence excitation rate. In our system, one tunnelling electron has nominally sufficient energy to induce dissociation, yet the process requires two electrons. We explain these observations by a two-electron mechanism that couples vibrational excitation and dissociative electron attachment steps.     

Electrically driven directional motion of a four-wheeled molecule on a metal surface

Propelling single molecules in a controlled manner along an unmodified surface remains extremely challenging because it requires molecules that can use light, chemical or electrical energy to modulate their interaction with the surface in a way that generates motion. Nature's motor proteins have mastered the art of converting conformational changes into directed motion, and have inspired the design of artificial systems such as DNA walkers and light- and redox-driven molecular motors. But although controlled movement of single molecules along a surface has been reported, the molecules in these examples act as passive elements that either diffuse along a preferential direction with equal probability for forward and backward movement or are dragged by an STM tip. Here we present a molecule with four functional units--our previously reported rotary motors--that undergo continuous and defined conformational changes upon sequential electronic and vibrational excitation. Scanning tunnelling microscopy confirms that activation of the conformational changes of the rotors through inelastic electron tunnelling propels the molecule unidirectionally across a Cu(111) surface. The system can be adapted to follow either linear or random surface trajectories or to remain stationary, by tuning the chirality of the individual motor units. Our design provides a starting point for the exploration of more sophisticated molecular mechanical systems with directionally controlled motion.     

银溶胶中水杨酸吸附行为的实验和密度泛函理论研究

从实验上获得了水杨酸(salicylic acid,SA)的常规拉曼散射(Normal Raman Scattering,NRS)光谱以及其吸附在Ag纳米颗粒上的表面增强拉曼散射(Surface-enhanced Raman Scattering,SERS)光谱.应用密度泛函理论(Density functional theory,DFT)在B3LYP/6-31+G**(C、H、O)和LANL2DZ(Ag)基组水平上对SA分子进行了结构优化,并计算了SA分子的NRS光谱以及其吸附在Ag纳米颗粒上两种不同构型体系的SERS光谱.通过理论结果与实验值对比,发现SA分子通过羧基吸附构型比羧基与羟基共同吸附构型的计算结果与实验值符合得更好.最后,利用Gauss View可视化软件对其振动模式进行了详细指认.经分析得出:在银溶胶中,SA分子是通过羧基倾斜地吸附在银纳米颗粒表面的.     

金属离子配位作用对槲皮素模板聚合物性能的影响

 采用槲皮素及槲皮素与Cu(Ⅱ)和Co(Ⅱ)的配合物为模板分子,丙烯酰胺（AM）为功能单体,在甲醇中制备了一系列印迹聚合物和非印迹聚合物,并用平衡结合实验研究了聚合物的性能,探讨了金属离子的引入对槲皮素印迹聚合物吸附性能的影响．结果表明,在印迹聚合物的聚合体系中引入金属离子Cu(Ⅱ)和Co(Ⅱ),尤其是Co(Ⅱ)后,印迹聚合物对槲皮素的吸附容量和印迹效果均有明显增加;同时如果在吸附溶液中再加入金属离子,印迹聚合物的吸附容量可以由未加金属离子时的35.99mg/g增加到69.10mg/g,印迹因子则由1.68增加到了1.94．     

Adlayer structure of TCNQ molecules on Cu(111): Anin situ STM study

In situ electrochemical scanning tunneling microscopy (STM) was employed to examine tetracyano- quinodimethane (TCNQ) layer adsorbed on Cu(111) electrode surface in 0.1 mol/L HClO₄ solution. The TCNQ molecules are found to form highly-ordered long range adlayer with a (4×4) symmetry. Each TCNQ molecule is adsorbed on Cu(111) surface in flat-lying orientation. The molecules are preferentially aligned with their long axes in the [121] direction. A structural model is proposed for the adlayer.     

Measurement of the conductance of a hydrogen molecule

Recent years have shown steady progress towards molecular electronics, in which molecules form basic components such as switches, diodes and electronic mixers. Often, a scanning tunnelling microscope is used to address an individual molecule, although this arrangement does not provide long-term stability. Therefore, metal-molecule-metal links using break-junction devices have also been explored; however, it is difficult to establish unambiguously that a single molecule forms the contact. Here we show that a single hydrogen molecule can form a stable bridge between platinum electrodes. In contrast to results for organic molecules, the bridge has a nearly perfect conductance of one quantum unit, carried by a single channel. The hydrogen bridge represents a simple test system in which to understand fundamental transport properties of single-molecule devices.     

Synergetic enhancement of photoconductivity in chlorodiane blue /oxotitanylphthalocyanine composites

The nonlinear enhancement of photosensitivity in chlorodiane blue azo (CBA)/ oxotitanylphthalocyanine composites (TiOPc) has been found. Neither evident electronic interaction nor interpenetration between chlorodiane blue and TiOPc molecules is indicated by electronic spectra and X-ray diffraction patterns. Xerographic discharge experiments suggest that the synergetic enhancement of photosensitivity results from contribution of the transition of gap states, which are induced by photoexcitation upon composite photoreceptors. Electron Spin Resonance (ESR) and X-ray Photoelectron Spectroscopy (XPS) data imply that oriented partial charge transfer between TiOPc and CBA molecules occurs when the composites are excited by incident light. This synergetic effect observed in the composite systems with weak electron donor and acceptor provides a novel strategy to design high quality photoconductive materials and devices.     

扫描隧道显微镜诱导吸附有甘氨酸的Cu(111)表面铜台阶的产生和运动

氨基酸是蛋白质的构成单元,它在金属表面的吸附为研究蛋白质同金属相互作用奠定了基础。吸附了氨基酸的金属表面会发生诸如重构改变或小面化等形貌的变化,但这些变化在吸附了氨基酸的全部金属表面都能够发生。在较大负偏压(|V_b|＞2.5V)的条件下使用扫描隧道显微镜(STM)扫描吸附有甘氨酸的Cu(111)表面,在扫描区域内原有铜台阶的形状发生了变化并产生了新的铜台阶。本文对甘氨酸在铜表面形貌变化中所起的作用进行了讨论。     

新型硅质吸附剂的合成及对黄酮的选择分离

利用分子印迹技术，以水溶性黄酮芦丁为模板剂与具有专一识别官能团的氯丙基三乙氧基硅烷反应制备出前驱物，然后将前驱物与正硅酸乙酯进行协同水解、凝胶老化和模板剂的洗脱，从而制备出对芦丁具有高效选择性的内置型硅质吸附剂．实验结果表明：该吸附剂具有较高的吸附容量，即使在有与芦丁结构相似的黄酮存在下，对芦丁依然有较高的选择性吸附能力；TEM照片表明该吸附剂具有与印迹分子相匹配的纳米孔洞，而对FTIR光谱分析表明该吸附剂在印迹识别过程有新的化学键生成．     

卤代芳香族化合物在长江底泥中吸附行为的CoMFA研究

将比较分子力场分析法 (CoMFA)移植到定量结构性质相关研究中 ,探讨了 2 8种卤代芳香族化合物在长江底泥中的吸附行为与其分子结构之间的关系 ,发现静电和空间立体效应是影响这批化合物底泥吸附的主要因素 ,疏水性质对吸附的影响不显著     

连接构型对铬卟啉分子自旋过滤效应的调控

采用第一性原理方法对不同连接构型的铬卟啉分子电子自旋输运性质进行计算分析.结果表明，对角连接构型的铬卟啉分子在0～0.2 V偏压区间范围内的自旋极化率高达95%以上，水平连接构型的导电性能比对角连接高约1个数量级.说明改变铬卟啉分子连接构型，可以改变电子自旋前线分子轨道分布和输运路径，从而实现其自旋过滤效应，对基于不同连接构型的铬卟啉分子器件的电子自旋输运性质进行有效调控.     

溶于液晶的二向色性染料有序参数的研究

研究了宾主盒的二向色性染料的有序参数(S)与其分子长度(l)、宽度等分子构型以及分子的偶极和诱导偶极的关系,发现S随l的增长而增大。当l增加到一定值时,有序参数增大缓慢,如果l超过液晶分子太大时,则出现下降趋势。而在平均分子长度为1.4 nm的E_7液晶中,染料分子长度达到2.30nm时,其有序参数将趋于饱和。染料分子的侧向取代基由于增加其分子宽度,也会使有序参数下降。宾主盒的有序参数随染料分子与液晶分子间偶极作用的增加而增大。宾主盒有序参数越大,其二色比越高,显示对比度一般也越高。     

The molecular orientation of PNBA on silver nanoparticles

The molecular orientations ofp-nitrobenzoic acid (PNBA) on the surface of silver nanoparticles and silver mirror have been investigated by using surface enhanced Raman scattering (SERS) and UV-visible absorption spectroscopy. In the mixed system of silver colloid/PNBA/ethanol, the orientation of PNBA on silver nanoparticles remarkably different from that on silver mirror was observed, and it mainly resulted from the cooperation effect of the adsorption to surface active sites on silver under the pressure of aqueous molecules. This result is of some significance for further studying surface-interfacial structures and properties of silver nanoparticles, and the technique used in this note is feasible to get the SERS spectra of some molecules insoluble in aqueous solution.     

镍基体上银和铜膜的择优取向及其价电子分析

目的研究在Ni（100）基体上沉积Ag和Cu膜的择优取向的形成机理。方法采用X-射线衍射进行分析,根据固体与分子经验电子理论（EET）的键距差（BLD）方法,计算Ni（100）晶面、Ag和Cu的33个晶面的平均电子密度及相对电子密度差。结果X-射线衍射分析结果表明,在Ni（100）基体上沉积Ag和Cu膜的主要择优取向为（111）,其次依次为（100）和（110）。计算结果表明,2个晶面的电子密度差越小,相应的择优取向越强。结论从界面处电子密度的连续原理成功地分析了在基体上沉积薄膜的择优取向的产生机理。     

Electrical generation and absorption of phonons in carbon nanotubes

The interplay between discrete vibrational and electronic degrees of freedom directly influences the chemical and physical properties of molecular systems. This coupling is typically studied through optical methods such as fluorescence, absorption and Raman spectroscopy. Molecular electronic devices provide new opportunities for exploring vibration-electronic interactions at the single molecule level. For example, electrons injected from a scanning tunnelling microscope tip into a metal can excite vibrational excitations of a molecule situated in the gap between tip and metal. Here we show how current directly injected into a freely suspended individual single-wall carbon nanotube can be used to excite, detect and control a specific vibrational mode of the molecule. Electrons tunnelling inelastically into the nanotube cause a non-equilibrium occupation of the radial breathing mode, leading to both stimulated emission and absorption of phonons by successive electron tunnelling events. We exploit this effect to measure a phonon lifetime of the order of 10 ns, corresponding to a quality factor of well over 10,000 for this nanomechanical oscillator.     

单分子二极管整流特性的第一性原理计算

在分子电子学领域中, 设计分子的结构可以实现特定的功能. 单分子二极管的整流行为是极具吸引力的器件功能之一. 研究了对称分子和非对称分子结的电子输运, 分别对应为四苯基和二嘧啶基二苯基单分子结, 二者均是共价结合到两金属电极. 与其同源对称嵌段相比, 非对称二嵌段分子表现出明显的整流行为, 且电子输运方向是从二苯基流向二嘧啶基. 利用密度泛函理论(density functional theory, DFT)和非平衡格林函数(non-equilibrium Green's function, NEGF)结合的第一性原理方法研究了单分子结的电子结构及其量子输运. 电流-电压 ($I$-$V$)曲线的非对称性可以用非对称分子二嵌段在偏压下由于电子态的局域性带来的非平衡效应进行解释. 本理论计算定性上符合其他小组的实验结果, 且尝试了不同的末端接触. 结果发现, 实验中的扫描隧道显微镜(scanning tunneling microscope, STM)针尖接触结构会一定程度地抵消非对称分子的整流效应, 而 STM 针尖接触结构的结果分析也符合之前的理论预测.     

多氯联苯辛醇—水分配系数的QSPR研究和预测

建立了多氯联苯系列分子的正辛醇—水分配系数对数-log Kow的定量结构—性质关系(QSPR)。通过它可以以分子结构参数(例如,范德华体积、偶极矩和分子椭圆度)为基础估算多氯联苯系列分子的正辛醇—水分配系数。预测了一些多氯联苯的Kow。同时研究了分子结构数据对Kow的影响,结果表明对Kow影响大的主要因素是范德华体积和分子椭圆度,而分子偶极矩的影响相对较小,说明较大的有机分子在溶解时,体积因素要比偶极距作用大。非邻位取代的氯原子在苯环中的位置对Kow影响不大。     

DDME分子在石墨(HOPG)表面吸附

用扫描隧道显微镜(STM)研究了用真空沉积法在高定向有序石墨(HOPG)上制备的1, 1dicyano-2, 2-(4-dimethylaminophenyl) ethylene(DDME)薄膜。DDME在石墨表面形成十分平整的分子柱状堆积二维有序结构。还观察到了随覆盖度降低转变成为其他二维有序结构,并对其形成机理进行了研究。     

笼型Li6Si5团簇的结构和储氢性能研究

利用密度泛函M06方法,在6-311+G(d, p)基组水平上对Si_5和Li修饰的Si_5团簇的几何结构和电子性质及储氢性能进行理论计算研究.结果表明, Si_5团簇最低能量构型为笼型结构,纯Si_5团簇不能有效吸附氢分子. Li原子的引入显著改善了Si_5团簇的储氢能力.以六个Li原子穴位修饰Si_5团簇为载体,每个Li原子周围可以有效吸附三个氢分子,其氢分子的平均吸附能为2.395 kcal/mol,储氢密度可达16.617 wt%.合适的吸附能和较高储氢密度表明Li修饰Si_5团簇有望成为理想的储氢材料.     

Atomic-scale imaging of DNA using scanning tunnelling microscopy

The scanning tunnelling microscope (STM) has been used to visualize DNA under water, under oil and in air. Images of single-stranded DNA have shown that submolecular resolution is possible. Here we describe atomic-resolution imaging of duplex DNA. Topographic STM images of uncoated duplex DNA on a graphite substrate obtained in ultra-high vacuum are presented that show double-helical structure, base pairs, and atomic-scale substructure. Experimental STM profiles show excellent correlation with atomic contours of the van der Waals surface of A-form DNA derived from X-ray crystallography. A comparison of variations in the barrier to quantum mechanical tunnelling (barrier-height) with atomic-scale topography shows correlation over the phosphate-sugar backbone but anticorrelation over the base pairs. This relationship may be due to the different chemical characteristics of parts of the molecule. Further investigation of this phenomenon should lead to a better understanding of the physics of imaging adsorbates with the STM and may prove useful in sequencing DNA. The improved resolution compared with previously published STM images of DNA may be attributable to ultra-high vacuum, high data-pixel density, slow scan rate, a fortuitously clean and sharp tip and/or a relatively dilute and extremely clean sample solution. This work demonstrates the potential of the STM for characterization of large biomolecular structures, but additional development will be required to make such high resolution imaging of DNA and other large molecules routine.