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ABSTRACT: This work describes jump-to-contact STM-break junction experiments leading to novel statistical distribution of last-step length associated with conductance of a single atom contact. Last-step length histograms are observed with up to five for Fe and three for Cu peaks at integral multiples close to 0.075 nm, a subatomic distance. A model is proposed in terms of gliding from a fcc hollow-site to a hcp hollow-site of adjacent atomic planes at 1/3 regular layer spacing along with tip stretching to account for the multiple subatomic step-length behavior.
Physical Chemistry Chemical Physics 04/2013; · 3.57 Impact Factor
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ABSTRACT: High quality AFM force curves are presented with detailed potential dependent layering behaviors of the ionic liquid molecules, from which charged interior and neutral exterior layers are distinguished. The electric double layer is confined within the interior layers of one to two molecular size within the potential range of up to 1 V negative of the PZC.
Chemical Communications 11/2011; 48(4):582-4. · 6.17 Impact Factor
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ABSTRACT: In this paper, we present an electrochemically driven large amplitude pH alteration method based on a serial electrolytic cell involving a hydrogen permeable bifacial working electrode such as Pd thin foil. The method allows solution pH to be changed periodically up to ±4~5 units without additional alteration of concentration and/or composition of the system. Application to the acid-base driven cyclic denaturation and renaturation of 290 bp DNA fragments is successfully demonstrated with in situ real-time UV spectroscopic characterization. Electrophoretic analysis confirms that the denaturation and renaturation processes are reversible without degradation of the DNA. The serial electrolytic cell based electrochemical pH alteration method presented in this work would promote investigations of a wide variety of potential-dependent processes and techniques.
Analytical Chemistry 06/2011; 83(12):4930-5. · 5.86 Impact Factor
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ABSTRACT: We measured single-molecule conductances for three different redox systems self-assembled onto gold by the STMBJ method and compared them with electrochemical heterogeneous rate constants determined by ultrafast voltammetry. It was observed that fast systems indeed give higher conductance. Monotonous dependency of conductance on potential reveals that large molecular fluctuations prevent the molecular redox levels to lie in between the Fermi levels of the electrodes in the nanogap configuration. Electronic coupling factors for both experimental approaches were therefore evaluated based on the superexchange mechanism theory. The results suggest that coupling is surprisingly on the same order of magnitude or even larger in conductance measurements whereas electron transfer occurs on larger distances than in transient electrochemistry.
Journal of the American Chemical Society 05/2011; 133(19):7509-16. · 9.91 Impact Factor
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Zheng Liu,
Song-Yuan Ding,
Zhao-Bin Chen,
Xiang Wang,
Jing-Hua Tian,
Jason R Anema,
Xiao-Shun Zhou,
De-Yin Wu, Bing-Wei Mao,
Xin Xu,
Bin Ren,
Zhong-Qun Tian
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ABSTRACT: The conductance of single-molecule junctions may be governed by the structure of the molecule in the gap or by the way it bonds with the leads, and the information contained in a Raman spectrum is ideal for examining both. Here we demonstrate that molecule-to-surface bonding may be characterized during electron transport by 'fishing-mode' tip-enhanced Raman spectroscopy (FM-TERS). This technique allows mutually verifiable single-molecule conductance and Raman signals with single-molecule contributions to be acquired simultaneously at room temperature. Density functional theory calculations reveal that the most significant spectral change seen for a gold-4,4'-bipyridine-gold junction results from the deformation of the pyridine ring in contact with the drain electrode at high voltage, and these calculations suggest that a stronger bonding interaction between the molecule and the drain may account for the nonlinear dependence of conductance on bias voltage. FM-TERS will lead to a better understanding of electron-transport processes in molecular junctions.
Nature Communications 01/2011; 2:305. · 7.40 Impact Factor
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ABSTRACT: The last decade has witnessed remarkable advances in interfacial electrochemistry in room-temperature ionic liquids. Although the wide electrochemical window of ionic liquids is of primary concern in this new type of solvent for electrochemistry, the unusual bulk and interfacial properties brought about by the intrinsic strong interactions in the ionic liquid system also substantially influence the structure and processes at electrode/ionic liquid interfaces. Theoretical modeling and experimental characterizations have been indispensable in reaching a microscopic understanding of electrode/ionic liquid interfaces and in elucidating the physics behind new phenomena in ionic liquids. This Minireview describes the status of some aspects of interfacial electrochemistry in ionic liquids. Emphasis is placed on high-resolution and molecular-level characterization by scanning tunneling microscopy and vibrational spectroscopies of interfacial structures, and the initial stage of metal electrodeposition with application in surface nanostructuring.
ChemPhysChem 09/2010; 11(13):2764-78. · 3.41 Impact Factor
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Jing-Hua Tian,
Yang Yang,
Xiao-Shun Zhou,
Bernd Schöllhorn,
Emmanuel Maisonhaute,
Zhao-Bin Chen,
Fang-Zu Yang,
Yong Chen,
Christian Amatore, Bing-Wei Mao,
Zhong-Qun Tian
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ABSTRACT: Atomic wires (point contacts) and molecular junctions are two fundamental units in the fields of nanoelectronics and devices. This Minireview introduces our recent approaches aiming to develop versatile methods to fabricate and characterize these unique metallic and molecular structures reliably. Electrochemical methods are coupled with mechanically controllable break junction (EC-MCBJ) or scanning tunneling microscopy (STM) break junction (EC-STMBJ) methods to fabricate metallic point contacts and metal/molecule/metal junctions. With the designed electrodeposition method, the metal of interest (e.g. Au, Cu, Fe or Pd) is deposited in a controlled way on the original electrode pair, on a chip for MCBJ or on the STM tip, to make the metallic contact. Then, various metal atomic wires and molecular junctions can be fabricated and characterized systematically. Herein, we measured the quantized conductance through the construction of histograms of these metal atomic point contacts and of single molecules including benzene-1,4-dithiol (BDT), ferrocene-bisvinylphenylmethyl dithiol (Fc-VPM), 4,4'-bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE), and 1,2-di(pyridin-4-yl)ethane (BPY-EA). Finally, we briefly discussed the future of EC-MCBJ and EC-STM for nanoelectronics and devices, for example, for the formation of heterogeneous metal-based atomic point contacts and molecular junctions.
ChemPhysChem 09/2010; 11(13):2745-55. · 3.41 Impact Factor
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ABSTRACT: The development of the instrumentation of a scanning electrochemical microscopy (SECM) is presented. The core of the SECM sensing system is constructed based on piezo-bimorph scanners, a mechanical micropositioner of multi-dimensional adjustment and ultramicroelectrodes. The control of the electrochemical cell and the SECM system is realized by a battery powered bipoteniostat and analog control circuits respectively with the control of a microcomputer work station. The demonstrations of SECM experiments are given on both a standard IDA sample and a silver electrode. Discussions on the resolution and quality of SECM image are made.
Chinese Journal of Chemistry 08/2010; 13(2):105 - 111. · 0.75 Impact Factor
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ABSTRACT: Fe electrodeposition on Au(111) and Au(100) in BMIBF(4) ionic liquid is found to form hitherto unreported shape-ordered nanoscale morphologies of pseudorods and pseudosquare rings, respectively, both composed of grains of 4-7 nm. The manner of growth of the square rings is a ring-on-ring structure with enlarging side length and slightly protruding four corners. The generality of the growth mechanism is verified by the formation of almost exactly the same shape-ordered Fe nanostructures on Pt, i.e., pseudorod structure on Pt(111) and pseudosquare rings Pt(100). These structures are explained within the framework of magnetostatic interactions of spontaneously magnetized grains under crystallographic constraint of the substrate surface, which result in an antiparallel arrangement in magnetization of the grains at pseudorods and magnetic flux closure at the pseudosquare rings. The closed magnetic flux further leads to magnetic field-enhanced growth at the four corners and the outer peripheries of the pseudosquare rings. The observed shape-ordering of the Fe thin film serves as a paradigm of magnetostatic coupling, in which the roles of ionic liquid as surfactant and magnetic media may not be underestimated. The present work adds a new dimension to electrodeposition in ionic liquid, by which new magnetic film structures may be expected.
Journal of the American Chemical Society 06/2010; 132(23):8152-7. · 9.91 Impact Factor
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ABSTRACT: Neutral inorganic molecules are generally weak in surface adsorption and intermolecular interactions. Self-assembly of such types of molecule would provide valuable information about various interactions. At electrochemical interfaces, the relative strength of these interactions may be modified through control of electrode potential and electrolyte, which may lead to the discovery of new structures and new phenomena. However, studies of this nature are as yet lacking. In this work, we consider the covalent-bound semimetal compound molecules, XCl(3) (X = Sb, Bi), as model systems of neutral inorganic molecules to investigate their self-assembly at electrochemical interfaces under a high ionic atmosphere. To fulfill such investigations, in situ STM and cyclic voltammetry are employed, and comparative experiments are performed on Au(111) in ionic liquids as well as aqueous solutions with high ionic strength. In the room temperature ionic liquid of 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF(4)), potential-dependent partial charge transfer between the Au surface and XCl(3) molecules creates a molecule-surface interaction and provides the driving force for adsorption of the molecules. Supramolecular aggregations of adsorbed XCl(3) are promoted through chlorine-based short-range intermolecular correlation under crystallographic constraint, while repulsive Coulombic interactions created between the partially charged aggregations facilitate their long-range ordering. For SbCl(3) molecules, hexagonally arranged 6- or 7-member clusters are formed at 0.08 to -0.2 V (vs Pt), which assemble into a secondary ( radical31 x radical31)R8.9 degrees structure. For BiCl(3) molecules, both the 6-membered hexagonal and 3-membered trigonal clusters are formed in the narrow potential range -0.3 to -0.35 V, and are also arranged into an ordered secondary structure. Comparative studies were performed with SbCl(3) in concentrated aqueous solutions containing 2 M HCl to simulate the strong ionic strength of the ionic liquid. Almost identical 6-/7-member clusters and long-range ( radical31 x radical31)R8.9 degrees structure are observed at -0.1 V, demonstrating the crucial role of strong ionic strength in such supramolecular aggregations. However, such supramolecular structures are modified and eventually destroyed as ionic strength is further increased by addition of NaClO(4) up to 6 M. The destructive changes of the supramolecular structures are attributed to the alteration of ion distribution in the double layer from cation-rich to anion-rich at increasing NaClO(4) concentration. This modifies and eventually breaks the balance of intermolecular and molecule-electrolyte interactions. Finally, the dynamic behavior of the SbCl(3) assembly is investigated down to molecular level. It has been demonstrated that the initial stage of assembly follows a two-dimensional nucleation and growth mechanism and has a potential-dependent rate that is closely related to the surface mobility of the SbCl(3) clusters. There is a probability that clusters can escape from an existing assembly domain or insert into a vacancy in such a domain while they can also relax with central or ring members in a dynamic fashion. These phenomena indirectly reflect the dynamic properties of cations from electrolytes at the interface. The rich information contained in the self-assembly behavior of SbCl(3) and BiCl(3) demonstrates that neutral inorganic molecules can be employed for fundamental studies of a variety of interesting issues, especially the interplay of various interfacial interactions.
Journal of the American Chemical Society 09/2009; 131(41):14728-37. · 9.91 Impact Factor
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Hai-Ming Zhang,
Zu-Kui Pei,
Zhao-Xiong Xie,
谢兆雄,
La-Sheng Long,
龙腊生, Bing-Wei Mao,
毛秉伟,
Xin Xu,
徐昕,
Lan-Sun Zheng,
郑兰荪
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ABSTRACT: A kind of robust molecular template formed between melamine (M) and cyanuric acid (CA) molecules on highly oriented pyrolytic graphite (HOPG) surfaces has been prepared by putting a droplet of hot aqueous solutions containing equal mol of M and CA molecules on preheated substrate. The CA.M SAMs exhibit features of hexagonal open networks which makes them an ideal molecular template for further processing of functional guest entities. In addition to the preparation of CA.M SAMs, SAMs of M and CA molecules alone were also studied on HOPG surfaces. Chiral structures were found in the SAMs of M molecules. National Natural Science Foundation of China [20725310, 20533030, 20525311, 20673085, 20671078]; National Basic Research Program of China [2007CB815303, 2007CB815206]; Key Scientific Project of Fujian Province of China [2005HZ01-3]; NCET from the Ministry of Education of China
07/2009;
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ABSTRACT: Ring any bells? The differential capacitance curve of Au(100) in neat [BMI]BF(4) (BMI = 1-butyl-3-methylimidazolium) ionic liquid has a bell-shaped feature (see picture). The adsorption of BMI(+) shows a disorder-order transition and depends on the structure of the surface. Ordered adsorption in a micelle-like structure stabilizes the underlying Au surface.
Angewandte Chemie International Edition 07/2009; 48(28):5148-51. · 13.45 Impact Factor
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ABSTRACT: In this paper, 2-hydroxypyridine (2-HP) and polyethyleneimine (PEI) were selected as ligand and additive, respectively, in a cyanide-free silver electroplating bath. Adsorption behaviors of 2-HP and PEI at silver electrodes were investigated by means of potential-dependent surface-enhanced Raman spectroscopy (SERS). The adsorption configuration variations of 2-HP and PEI at the silver surface with the negative shift of potential were discussed according to the SERS analysis. The possible effect of 2-HP and PEI on the silver electroplating process was also proposed. In the silver electroplating process, the SERS signal variations of 2-HP with the negative shift of potential were mainly due to the deposits’ morphology change. In the presence of PEI, fine grains and smoother surfaces are observed from the images of atomic force microscopy of the silver deposits.
05/2009;
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ABSTRACT: The STM break junction (STM-BJ) and mechanically controllable break junction (MCBJ) are the two most widely applied techniques to fabricate atomic-size nanowires for conductance measurement. However, the drawbacks of the mechanical crashing between the two electrodes of the same material in these techniques hamper its capability of application in view of the variety of metals as well as the environment to perform the measurements. In this paper, we present an electrochemical strategy for STM-BJ by establishing a chemically well-defined metallic contact through a jump-to-contact mechanism between the tip and substrate of dissimilar metals, wherein the tip is in situ and electrochemically deposited with a thin film of a foreign metal of interest. The feasibility of the approach is demonstrated by taking Cu as a model system, followed by generalizing to Pd and Fe for which the conductance has been found otherwise difficult to measure at room temperature. The preferential point-contact conductance at 1, 0.9, and 0.86 G0 was measured for Cu, Pd, and Fe, respectively. The strategy present in this work not only extends the capability of STM-BJ to create a variety of metal nanowires including magnetic nanowires for further investigations but also provides opportunities to construct metal-molecule-metal junctions with a variety of choices of metals in the junctions.
Journal of the American Chemical Society 10/2008; 130(40):13228-30. · 9.91 Impact Factor
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Small 10/2008; 4(9):1355-8. · 8.35 Impact Factor
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ABSTRACT: In this paper, a kind of robust self-assembled monolayer (SAM) directed by lateral triple hydrogen bonding interactions was prepared by a one-step reaction of cyanuric acid (CA) and melamine (M) molecules on reconstructed Au(111) surfaces in ambient conditions. The structures of the CA‚M SAMs were found to be similar to those of bulk crystals. These physisorbed SAMs, directed by triple hydrogen bonds, were found to be stable at the air/substrate interface and possessed a high thermal stability (up to 400 K). According to the high-resolution STM images, a commensurate structure with respect to the reconstructed Au(111) surfaces was established. Combined with density functional theory (DFT) calculations, the roles of the adsorbate-adsorbate and the adsorbate-substrate interactions, as well as the origin of the high stability of the CA‚M SAMs, were discussed. In addition to the preparation of the CA‚M SAMs, SAMs of melamine and cyanuric acid molecules alone were also studied on the Au(111) surfaces. Chiral structures were found in both of these SAMs. Structural models of melamine SAMs and cyanuric acid SAMs were established.
The Journal of Physical Chemistry C 02/2008; · 4.80 Impact Factor
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ABSTRACT: We report single molecule conductance measurement of 4,4‘-bipyridine (BPY), 1,2-di(pyridin-4-yl)ethene (BPY-EE), and 1,2-di(pyridin-4-yl)ethane (BPY-EA), in which the two pyridine rings sandwich the conjugated ethene group and the nonconjugated ethane group, respectively. The conductance measurements of the molecules bridged in two Au electrodes have been carried out in aqueous solutions using the scanning tunneling microscopy-based break junction (STM-BJ) technique. All three molecules show two sets of conductance values with the high conductance (HC) values about 8−10 times higher than the low conductance (LC) values. The HC values for BPY, BPY-EE, and BPY-EA are (4.7 ± 1.1) × 10-3 G0, (2.3 ± 0.9) × 10-3 G0, and (4.5 ± 2.2) × 10-4 G0, respectively, in an order of BPY > BPY-EE > BPY-EA. The decrease of conductance of BPY-EE can be attributed mainly to the length increase of the molecule, while the decrease of the conductance of BPY-EA reveals the degree of electron decoupling upon insertion of one CH2CH2 group between the two pyridyl rings in the molecule. The present work demonstrates that with pyridines as the end groups, it is possible to study the conductance of very short alkane chains.
02/2008;
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Jing-Hua Tian,
Bo Liu,
Shan Jin,
Ke Dai,
Zhao-Bin Chen,
Xiulan Li,
Huixian Ke,
Sun-Tao Wu,
Yang Yang,
Bin Ren, Bing-Wei Mao,
Nongjian Tao,
Zhong-Qun Tian
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ABSTRACT: We have developed a combined Surface-enhanced raman spectroscopy (SERS) and mechanically controllable break junction (MCBJ) method to detect and characterize molecular junctions formed by two electrochemically nanofabricated electrodes on silicon chips. The method allows us to obtain vibrational spectra of the molecular junction and perform electron transport measurement on the molecules simultaneously. The preliminary I/V characterization and SERS measurement on an asymmetric molecule, OPE-NO<sub>2</sub>, and a symmetric molecule, OPE, were conducted. This approach may provide new insights into not only electron transport in molecules, but also the enhancement mechanism in single-molecule SERS.
Nanotechnology, 2007. IEEE-NANO 2007. 7th IEEE Conference on; 09/2007
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ABSTRACT: The self-assembly of a Wilkinson type of catalyst molecule, trans-RhCl(CO)(PPh3)2, on Au(111) surfaces and its electrocatalytic properties toward the hydrogen evolution reaction (HER) are investigated by employing scanning tunneling microscopy (STM), cyclic voltammetry (CV), and X-ray photoelectron spectroscopy (XPS). The self-assembled monolayers of RhCl(CO)(PPh3)2 are prepared from either dichloromethane or aqueous solutions, but the ordered structures are observed only in atmospheric conditions after solvents evaporate. In the electrolyte solutions, disordered yet uniformly sized spherical clusters of individual molecules are observed as a result of the conformational change of the molecule by the solvation effect of water. The immobilized Rh(I) molecular clusters are electrochemically stable in a wide potential window and exhibit remarkable electrocatalytic activity toward HER in perchloric acid solutions. Several comparative experiments involving similar types of immobilized complexes containing Ru(I) and Ir(I) centers and solution species of RhCl(CO)(PPh3)2 are performed. However, none of them are found to be electroactive to HER. The Tafel slope of HER on the Rh(I) complex modified Au(111) electrode in 0.1 M HClO4 is determined to be -0.061 V, which is almost in the middle of those on bare Au(111) (-0.093 V) and Rh covered (thetaRh approximately 0.3) Au(111) (-0.034 V) electrodes. XPS measurements reveal a valence change of Rh(I) to Rh(0), and an oxidative addition and reductive elimination mechanism is suggested for the enhancement of HER.
Langmuir 07/2007; 23(12):6819-26. · 4.19 Impact Factor
