Structure transitions between copper-sulphate and copper-chloride UPD phases on Au(111)

Structure transitions between copper UPD adlayers on Au(111)-(1 × 1) in sulfuric acid and chloride containing electrolyte were investigated by in situ scanning tunnelling microscopy. We demonstrate that co-adsorbed sulphate ions in the (√3 × √3)R30° UPD adlayer are replaced by chloride ions and, depending on the halide coverage, a commensurate (2 × 2) or a slightly distorted (5 × 5)-like Cu-Cl UPD adlayer are formed. The stability ranges of these phases are controlled both by the electrode potential and the Cl− concentration. Phase transitions between the three UPD phases were monitored by time-resolved in situ STM. The observed structure details were attributed to mechanisms based on two-dimensional nucleation and growth processe

Charge Transport in Single Au|Alkanedithiol|Au Junctions: Coordination Geometries and Conformational Degrees of Freedom

Recent STM molecular break-junction experiments have revealed multiple series of peaks in the conductance histograms of alkanedithiols. To resolve a current controversy, we present here an in-depth study of charge transport properties of Au|alkanedithiol|Au junctions. Conductance histograms extracted from our STM measurements unambiguously confirm features showing more than one set of junction configurations. Based on quantum chemistry calculations, we propose that certain combinations of different sulfur-gold couplings and trans/gauche conformations act as the driving agents. The present study may have implications for experimental methodology: whenever conductances of different junction conformations are not statistically independent, the conductance histogram technique can exhibit a single series only, even though a much larger abundance of microscopic realizations exists.Comment: 19 pages, 9 figures, 1 table; published versio

Magic ratios for connectivity-driven electrical conductance of graphene-like molecules

Experiments using a mechanically-controlled break junction and calculations based on density functional theory demonstrate a new magic ratio rule (MRR),which captures the contribution of connectivity to the electrical conductance of graphene-like aromatic molecules. When one electrode is connected to a site i and the other is connected to a site i' of a particular molecule, we assign the molecule a magic integer Mii'. Two molecules with the same aromatic core, but different pairs of electrode connection sites (i,i' and j,j' respectively) possess different magic integers Mii' and Mjj'. Based on connectivity alone, we predict that when the coupling to electrodes is weak and the Fermi energy of the electrodes lies close to the centre of the HOMO-LUMO gap, the ratio of their conductances is equal to (Mii' /Mjj')2. The MRR is exact for a tight binding representation of a molecule and a qualitative guide for real molecules

ATR-SEIRAS study of CO adsorption and oxidation on Rh modified Au(111-25 nm) film electrodes in 0.1 M H2SO4

Rh modified Au(111-25 nm) electrodes, prepared by electron beam evaporation and galvanostatic deposition, were employed to study adsorption and electro-oxidation of CO on Rh in 0.1 M sulfuric acid solution by in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The results of ATR-SEIRAS experiments were compared with those obtained by infrared reflection absorption spectroscopy on three low-index Rh single crystal surfaces. The Rh film deposited on Au(111-25 nm) electrode consists of 3D clusters forming a highly stepped [n(111) × (111)]-like surface with narrow (111) terraces. When CO was dosed at the hydrogen adsorption potential region, CO adsorbed in both atop (COL) and bridge (COB) configurations, as well as coadsorbed water species, were detected on the Rh film electrode. A partial interconversion of spectroscopic bands due to the CO displacement from bridge to atop sites was found during the anodic potential scan, revealing that there is a potential-dependent preference of CO adsorption sites on Rh surfaces. Our data indicate that CO oxidation on Rh electrode surface in acidic media involves coadsorbed water and follows the nucleation and growth model of a Langmuir-Hinshelwood type reaction.The work was supported by the Research Center Jülich, the University of Bern, Swiss National Science Foundation (200020_144471, 200021-124643), the Spanish Ministerio de Economía y Competitividad (project CTQ2013-44083-P) and University of Alicante. QX acknowledges fellowships of the Research Center Jülich; IP acknowledges support by COST Action TD 1002; and AK acknowledges the financial support by CTI Swiss Competence Centers for Energy Research (SCCER Heat and Electricity Storage)

Dynamic breaking of a single gold bond

AbstractWhile one might assume that the force to break a chemical bond gives a measure of the bond strength, this intuition is misleading. If the force is loaded slowly, thermal fluctuations may break the bond before it is maximally stretched, and the breaking force will be less than the bond can sustain. Conversely, if the force is loaded rapidly it is more likely that the maximum breaking force is measured. Paradoxically, no clear differences in breaking force were observed in experiments on gold nanowires, despite being conducted under very different conditions. Here we explore the breaking behaviour of a single Au–Au bond and show that the breaking force is dependent on the loading rate. We probe the temperature and structural dependencies of breaking and suggest that the paradox can be explained by fast breaking of atomic wires and slow breaking of point contacts giving very similar breaking forces.</jats:p

Searching the hearts of graphene-like molecules for simplicity, sensitivity and logic

If quantum interference patterns in the hearts of polycyclic aromatic hydrocarbons (PAHs) could be isolated and manipulated, then a significant step towards realizing the potential of single-molecule electronics would be achieved. Here we demonstrate experimentally and theoretically that a simple, parameter-free, analytic theory of interference patterns evaluated at the mid-point of the HOMO-LUMO gap (referred to as M-functions) correctly predicts conductance ratios of molecules with pyrene, naphthalene, anthracene, anthanthrene or azulene hearts. M-functions provide new design strategies for identifying molecules with phase-coherent logic functions and enhancing the sensitivity of molecular-scale interferometers.Comment: in J. Am. Chem. Soc. (2015

A quantum circuit rule for interference effects in single-molecule electrical junctions

A quantum circuit rule for combining quantum interference effects in the conductive properties of oligo(phenyleneethynylene) (OPE)-type molecules possessing three aromatic rings was investigated both experimentally and theoretically. Molecules were of the type X-Y-X, where X represents pyridyl anchors with para (p), meta (m) or ortho (o) connectivities and Y represents a phenyl ring with p and m connectivities. The conductances GXmX (GXpX) of molecules of the form X-m-X (X-p-X), with meta (para) connections in the central ring, were predominantly lower (higher), irrespective of the meta, para or ortho nature of the anchor groups X, demonstrating that conductance is dominated by the nature of quantum interference in the central ring Y. The single-molecule conductances were found to satisfy the quantum circuit rule Gppp/Gpmp=Gmpm/Gmmm. This demonstrates that the contribution to the conductance from the central ring is independent of the para versus meta nature of the anchor groups

Single-molecule detection of dihydroazulene photo-thermal reaction using break junction technique

基于隧穿机制的电输运是物质世界的基本过程之一。在单分子尺度，分子结构的细微变化足以导致电学性质的显著区别，这也使通过单分子电学检测方法研究化学反应过程成为可能。在这一研究工作中，课题组将通常用于单分子电学测量的裂结技术用于单分子尺度反应动力学的表征。这一工作也为未来的合成化学和化学工程研究提供了一种新思路，即通过纳米技术构造反应微环境，可以实现化学反应速率、产物和产率的优化。 该研究工作是在洪文晶教授和丹麦哥本哈根大学Mogens B. Nielsen教授的共同指导下，通过跨学科的国际合作所完成的。其中洪文晶教授课题组负责该研究工作的实验表征和统计分析，丹麦哥本哈根大学Mogens B. Nielsen教授课题组负责分子体系的合成，Kurt V. Mikkelsen教授和Gemma C. Solomon教授课题组分别负责了该研究工作的反应动力学和电输运理论计算，这也是洪文晶教授课题组与上述研究团队的首次科研合作。我校萨本栋微纳研究院的杨扬助理教授也参与了数据分析和机理讨论的部分工作。 洪文晶教授课题组长期致力于单分子尺度下的化学反应、分子组装、分子器件电输运等方面的相关研究，开发了一系列能够在单分子尺度实现精密控制和精确测量的科学仪器。以此为基础，课题组与国内外材料化学和理论研究团队密切合作，在单分子尺度电输运的量子干涉效应、电化学调控和化学反应表征等领域进行了一系列探索。【Abstract】Charge transport by tunnelling is one of the most ubiquitous elementary processes in nature. Small structural changes in a molecular junction can lead to significant difference in the single-molecule electronic properties, offering a tremendous opportunity to examine a reaction on the single-molecule scale by monitoring the conductance changes. Here, we explore the potential of the single-molecule break junction technique in the detection of photo-thermal reaction processes of a photochromic dihydroazulene/vinylheptafulvene system. Statistical analysis of the break junction experiments provides a quantitative approach for probing the reaction kinetics and reversibility, including the occurrence of isomerization during the reaction. The product ratios observed when switching the system in the junction does not follow those observed in solution studies (both experiment and theory), suggesting that the junction environment was perturbing the process significantly. This study opens the possibility of using nano-structured environments like molecular junctions to tailor product ratios in chemical reactions.This work was generously supported by the University of Copenhagen, the Danish e-Infrastructure Cooperation, the European Union Seventh Framework Programme (FP7/2007-2013) under the ERC grant agreement no.258806, the Danish Council for Independent Research—Natural Sciences, the Carlsberg foundation, NSFC (21673195,21503179), EC FP7 ITNs ‘MOLESCO’ project numbers 606728, and the Young Thousand Talent Project of China. 研究工作得到了国家自然科学基金（21673195，21503179）、固体表面物理化学国家重点实验室、能源材料化学协同创新中心（2011-iChEM）的大力资助与支持

Synthesis and Single-Molecule Conductance Study of Redox-Active Ruthenium Complexes with Pyridyl and Dihydrobenzo[b]thiophene Anchoring Groups

The ancillary ligands 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine and 4′-(2,3-dihydrobenzo[b]thiophene)-2,2′-6′,2“-terpyridine were used to synthesize two series of mono- and dinuclear ruthenium complexes differing in their lengths and anchoring groups. The electrochemical and single-molecular conductance properties of these two series of ruthenium complexes were studied experimentally by means of cyclic voltammetry and the scanning tunneling microscopy-break junction technique (STM-BJ) and theoretically by means of density functional theory (DFT). Cyclic voltammetry data showed clear redox peaks corresponding to both the metal- and ligand-related redox reactions. Single-molecular conductance demonstrated an exponential decay of the molecular conductance with the increase in molecular length for both the series of ruthenium complexes, with decay constants of βPY=2.07±0.1 nm−1 and βBT=2.16±0.1 nm−1, respectively. The contact resistance of complexes with 2,3-dihydrobenzo[b]thiophene (BT) anchoring groups is found to be smaller than the contact resistance of ruthenium complexes with pyridine (PY) anchors. DFT calculations support the experimental results and provided additional information on the electronic structure and charge transport properties in those metal|ruthenium complex|metal junctions