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Wang H.,Peking University | Jiang Z.,Peking University | Wang Y.,Peking University | Wang Y.,Beida Information Research BIR | And 3 more authors.
ChemPhysChem | Year: 2016

The atomic structure and electronic transport properties of Au–mesitylene–Au molecular junctions formed from a mesitylene monolayer without any anchoring groups are investigated by employing the non-equilibrium Green′s function formalism combined with density functional theory. The intermolecular and adsorbate–substrate interactions are described by the non-local optB88 van der Waals functional. Two types of Au–mesitylene–Au molecular junctions are constructed, in which either an isolated mesitylene molecule or a mesitylene molecule embedded into a monolayer lying flat on one electrode surface is in contact with an atomic protrusion of the other electrode surface. The calculated low-bias conductance values of these two junctions are both in quantitative agreement with the reported experimental values [S. Afsari, Z. Li, and E. Borguet, Angew. Chem. Int. Ed. 2014, 53, 9771; Angew. Chem. 2014, 126, 9929]. This indicates that the measured conductance is intrinsic at the single-molecule Au–mesitylene–Au junction and that the intermolecular interactions in the mesitylene monolayer have little effect. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Bai M.L.,Peking University | Wang M.L.,Peking University | Hou S.M.,Peking University | Hou S.M.,Beida Information Research BIR
Wuli Huaxue Xuebao/ Acta Physico - Chimica Sinica | Year: 2015

The transition voltage of copper-vacuum-copper tunneling junctions with atomic protrusions on the electrode surface was investigated using the non-equilibrium Green’s function formalism combined with density functional theory. Our calculations show that the transition voltages of Cu-vacuum-Cu junctions with atomically sharp electrodes are mainly determined by the local density of state (LDOS) of the 4p atomic orbitals of the protrusion, and are thus sensitive to the electrode orientation and the variation of the atomic configurations of surface protrusions. For Cu-vacuum-Cu junctions with (111)-oriented electrodes, the transition voltages were calculated to be about 1.40 and 2.40 V when the atomic protrusions were chosen to be one Cu adatom or a copper cluster with four atoms arranged in a pyramid configuration, respectively. The transition voltages of Cu-vacuum-Cu junctions with (100)-oriented electrodes were more different. When the atomic protrusion on the Cu(100) surface was a copper cluster with five atoms arranged in a pyramid configuration, the transition voltage was 1.70 V. In contrast, no transition voltage was observed for Cuvacuum- Cu junctions with one Cu adatom attached to the Cu(100) electrode surface even when the bias exceeded 1.80 V, which is caused by the LDOS of the 4p atomic orbitals of the Cu adatom on the Cu(100) surface being too extended. These results demonstrate the advantages of transition voltage spectroscopy as a tool for analyzing the electronic transport properties of metal-vacuum-metal tunneling junctions. © 2015, Editorial office of Acta Physico-Chimica Sinica.

Bai M.,Peking University | Bai M.,Trinity College Dublin | Cucinotta C.S.,Trinity College Dublin | Jiang Z.,Peking University | And 8 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

We explain how the electrical current flow in a molecular junction can modify the vibrational spectrum of the molecule by renormalizing its normal modes of oscillations. This is demonstrated with first-principles self-consistent transport theory, where the current-induced forces are evaluated from the expectation value of the ionic momentum operator. We explore here the case of H2 sandwiched between two Au electrodes and show that the current produces stiffening of the transverse translational and rotational modes and softening of the stretching modes along the current direction. Such behavior is understood in terms of charge redistribution, potential drop, and elasticity changes as a function of the current. © 2016 American Physical Society.

Xiang A.,Peking University | Wang M.,Peking University | Wang H.,Peking University | Sun H.,Peking University | And 3 more authors.
Chemical Physics | Year: 2016

We studied the origin of the transition voltage of octanedithiol (C8DT) molecules sandwiched between two gold electrodes. Au-C8DT-Au molecular junctions were fabricated using the feedback controlled electromigration technique. The conductance measurement and the inelastic electron tunneling spectrum validate that the observed current-voltage characteristics indeed originate from the C8DT molecule in the junction. The transition voltage of Au-C8DT-Au junctions is determined to be about 0.95 V, which cannot be interpreted using the energy difference between the highest occupied molecular orbital or lowest unoccupied molecular orbital of the alkyl chain and the Fermi level of electrodes. Instead, we relate the measured transition voltage to the Au-S bonds, which are about 1.4 eV below the gold Fermi level. Our interpretation is supported by the ultraviolet photoelectron spectroscopy measurements and first-principles quantum transport simulations. These findings provide convincing evidence that the Au-S occupied states play a significant role in the charge transport of Au-alkanedithiol-Au molecular devices. © 2015 Elsevier B.V. All rights reserved.

Zhang X.,Peking University | Li N.,Peking University | Liu L.,Peking University | Gu G.,Peking University | And 7 more authors.
Chemical Communications | Year: 2016

Sierpiński triangle fractals were constructed on both Ag(111) and symmetry-mismatched fourfold Ag(100) surfaces through chemical reaction between H3PH molecules and Fe atoms under vacuum. Density functional theory calculations revealed that the fractals were stabilized by the strong coordination interaction between Fe and O atoms. In comparison, pure H3PH molecules formed fractals via moderately strong hydrogen bonds only on Ag(111), not on Ag(100). © 2016 The Royal Society of Chemistry.

Karan S.,University of Kiel | Li N.,Peking University | Zhang Y.,Peking University | He Y.,Peking University | And 14 more authors.
Physical Review Letters | Year: 2016

All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising only C, H, and O atoms, is investigated on a Au(111) substrate using scanning tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are switched to a number of states, three of which carry a localized spin as evidenced by conductance spectroscopy in high magnetic fields. The spin of a single molecule may be reversibly switched on and off without affecting its neighbors. We suggest that ReA on Au is readily converted to a radical by the abstraction of an electron. © 2016 American Physical Society.

Gu G.-C.,Peking University | Li N.,Peking University | Zhang X.,Peking University | Hou S.-M.,Peking University | And 3 more authors.
Wuli Huaxue Xuebao/ Acta Physico - Chimica Sinica | Year: 2016

Self-similar fractals have been extensively investigated because of their importance in mathematics and aesthetics. Chemists have attempted to synthesize various molecular fractal structures through sophisticated design. But because of poor solubility, synthesis of defect-free fractals with large sizes in solution usually proves difficult. Recently, we reported the formation of extended and defect-free Sierpiński triangle fractals by halogen or coordination bonds on surfaces under ultrahigh vacuum conditions. Their growth mechanism has been systematically studied by scanning tunneling microscopy. Using 4,4’”-dibromo-1,1’:3,,1”:4”,1’”-quaterphenyl molecules, a series of Sierpiński triangles were successfully prepared on Ag(111) through self-assembly. A slow cooling rate is crucial for growing fractals of higher order. These fractals are only observed below liquid-nitrogen temperature because of the weak interactions in halogen bonds. More stable metal-organic Sierpiński triangles were fabricated by depositing 4,4”-dicyano-1,1’:3’,1”-terphenyl molecules and Fe atoms on Au (111) and annealing at around 100 °C for 10 min. The fractals are stabilized through coordination interaction between Fe atoms and N atoms in molecules. Density functional theory calculations revealed their imaging mechanism. Monte Carlo simulations displayed the formation process of surface-supported fractal structures. Three-fold nodes are believed to dominate the structure formation of Sierpiński triangles. ©Editorial office of Acta Physico-Chimica Sinica.

Jiang Z.,University of Science and Technology Beijing | Wang H.,University of Science and Technology Beijing | Sanvito S.,Trinity College Dublin | Hou S.,University of Science and Technology Beijing | Hou S.,Beida Information Research BIR
Physical Review B - Condensed Matter and Materials Physics | Year: 2016

The evolution of the atomic structure and the vibrational and electronic transport properties of gold atomic junctions incorporating molecular and atomic hydrogen upon elongation have been investigated with the nonequilibrium Green's function formalism combined with density functional theory. Our calculations show that for the case of gold junctions doped with a single H2 molecule the low-bias conductance drops rapidly with the electrodes' separation, while it remains almost constant if a single H atom replaces the molecule. In contrast, when one considers two H atoms adsorbed on a gold monatomic chain forming an Au-H-Au-H-Au double-bridge structure, the low-bias conductance increases first and then shows a plateau upon stretching the junction, in perfect agreement with experiments on gold nanocontacts in hydrogen environment. Furthermore, also the distribution of the calculated vibrational energies of the two H atoms is consistent with the experimental result in the low-conductance region, demonstrating clear evidence that hydrogen molecules can dissociate on stretched gold monatomic chains. These findings are helpful to improve our understanding of the structure-property relation of gold nanocontacts and also provide a new prospect for gold nanowires being used as chemical sensors and catalysts. © 2016 American Physical Society.

Jiang Z.,University of Science and Technology Beijing | Wang H.,University of Science and Technology Beijing | Sanvito S.,Trinity College Dublin | Hou S.,University of Science and Technology Beijing | Hou S.,Beida Information Research BIR
Journal of Chemical Physics | Year: 2015

Inelastic electron tunneling spectroscopy (IETS) of a single hydrogen atom on the Cu(100) surface in a scanning tunneling microscopy (STM) configuration has been investigated by employing the non-equilibrium Green's function formalism combined with density functional theory. The electron-vibration interaction is treated at the level of lowest order expansion. Our calculations show that the single peak observed in the previous STM-IETS experiments is dominated by the perpendicular mode of the adsorbed H atom, while the parallel one only makes a negligible contribution even when the STM tip is laterally displaced from the top position of the H atom. This propensity of the IETS is deeply rooted in the symmetry of the vibrational modes and the characteristics of the conduction channel of the Cu-H-Cu tunneling junction, which is mainly composed of the 4s and 4pz atomic orbitals of the Cu apex atom and the 1s orbital of the adsorbed H atom. These findings are helpful for deepening our understanding of the propensity rules for IETS and promoting IETS as a more popular spectroscopic tool for molecular devices. © 2015 AIP Publishing LLC.

PubMed | Beida Information Research BIR, Maria Curie Sklodowska University, CNRS Toulouse Center for Materials Elaboration and Structural Studies, SPURc and Huazhong University of Science and Technology
Type: Journal Article | Journal: ACS nano | Year: 2015

Recent studies demonstrate that simple functional molecules, which usually form two-dimensional (2D) crystal structures when adsorbed on solid substrates, are also able to self-assemble into ordered openwork fractal aggregates. To direct and control the growth of such fractal supramolecules, it is necessary to explore the conditions under which both fractal and crystalline patterns develop and coexist. In this contribution, we study the coexistence of Sierpiski triangle (ST) fractals and 2D molecular crystals that were formed by 4,4-dihydroxy-1,1:3,1-terphenyl molecules on Au(111) in ultrahigh vacuum. Growth competition between the STs and 2D crystals was realized by tuning substrate and molecular surface coverage and changing the functional groups of the molecular building block. Density functional theory calculations and Monte Carlo simulations are used to characterize the process. Both experimental and theoretical results demonstrate the possibility of steering the surface self-assembly to generate fractal and nonfractal structures made up of the same molecular building block.

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