Xingqiang Shi

Single-Molecule Resolution of an Organometallic Intermediate in a Surface-Supported Ullmann Coupling Reaction

We have studied the organometallic intermediate of a surface-supported Ullmann coupling reaction from 4, 4″-dibromo-p-terphenyl to poly(para-phenylene) by scanning tunneling microscopy/spectroscopy and density functional theory calculations. Our study reveals at a single-molecular level that the intermediate consists of biradical terphenyl (ph)(3) units that are connected by single Cu atoms through C-Cu-C bridges. Upon further increasing the temperature, the neighboring biradical (ph)(3) units are coupled by C-C bonds forming poly(para-phenylene) oligomers while the Cu atoms are released.

Synergistic effect of 2D Ti2C and g-C3N4 for efficient photocatalytic hydrogen production

Photocatalytic water splitting is an environmentally friendly technique for hydrogen production. In this work, we report a novel photocatalyst consisting of two-dimensional (2D) titanium carbide (Ti2C) and graphitic carbon nitride (g-C3N4). We observe substantially enhanced water splitting activities due to the efficient synergistic interaction between Ti2C and g-C3N4. Optimal properties are achieved in the g-C3N4 with a loading of 0.4 wt% Ti2C with a hydrogen production rate of 47.5 μmol h−1, which is 14.4 times as much as that in the case using pure g-C3N4, and it even outperforms Pt-loaded g-C3N4. We further show that the Ti2C/g-C3N4 has high stability and good reproducibility. We expect that the Ti2C/g-C3N4 can be a photocatalyst for large scale applications because both Ti2C and g-C3N4 are low-cost, abundant, and nontoxic.

Survey of structural and electronic properties of C60 on close-packed metal surfaces

The adsorption of buckminsterfullerene (C60) on metal surfaces has been investigated extensively for its unique geometric and electronic properties. The two-dimensional systems formed on surfaces allow studying in detail the interplay between bonding and electronic structures. Recent studies reveal that C60 adsorption induces reconstruction of even the less-reactive close-packed metal surfaces. First-principles computations enable access to this important issue by providing not only detailed atomic structure but also electronic properties of the substrate–adsorbate interaction, which can be compared with various experimental techniques to determine and understand the interface structures. This review discusses in detail the ordered phases of C60 monolayers on metal surfaces and the surface reconstruction induced by C60 adsorption, with an emphasis on the different types of reconstruction resulting on close-packed metal surfaces. We show that the symmetry matching between C60 molecules and metal surfaces determines the local adsorption configurations, while the size matching between C60 molecules and the metal surface lattice determines the supercell sizes and shapes; importantly and uniquely for C60, the number of surface metal atoms within one supercell determines the different types of reconstruction that can occur. The atomic structure at the molecule–metal interface is of crucial importance for the monolayer’s electronic and transport properties: these will also be discussed for the well-defined adsorption structures, especially from the perspective of tuning the electronic structure via C60–metal interface reconstruction and via relative inter-C60 orientations.

Electron Stimulation of Internal Torsion of a Surface-Mounted Molecular Rotor

A molecular rotor which includes a central rotator group was investigated by scanning tunneling microscopy at 4.9 K as it was grafted on a Cu(111) surface via its two terminal groups. Topographs with submolecular resolution revealed several distinct molecular conformations which we attribute to different angular orientations of the rotator and which are locally stable states according to density functional theory calculations. Time-resolved tunneling current spectra showed that the rotator undergoes a torsional motion around the molecular long axis as stimulated by tunneling electrons in a one-electron process with an excitation energy threshold of 355 meV. Calculations identified an intrinsic axial vibration mode of the rotator group at 370 meV as adsorbed on the surface, which we propose to be the channel for effectively converting the tunneling electron energy into the mechanical energy of the intramolecular torsion.

Ultra-high electrocatalytic activity of VS2 nanoflowers for efficient hydrogen evolution reaction

It is a great challenge to explore cheap, abundant and eco-friendly electrocatalysts for hydrogen evolution reaction (HER). Here, we report the fabrication of VS2 nanoflowers with 1T phase by a simple hydrothermal method and their electrocatalytic performance in the HER. We find that the VS2 nanoflowers show comparable HER performance to Pt in acids, including an ultra-low onset potential of 32 mV, a Tafel slope of 34 mV dec−1 which resembles that of Pt, and a small overpotential of 58 mV (54 mV for Pt) at a current density of 10 mA cm−2. High stability and almost 100% faradaic efficiency indicate the practical application of VS2 nanoflowers in the HER. Our first-principles calculations reveal that the thermoneutral Gibbs free energy of hydrogen adsorption on both the basal and edge sites of the 1T-VS2 monolayer can be achieved under certain hydrogen coverage and the monolayer shows good conductivity, which contribute to the impressive catalytic performance of VS2 nanoflowers. We expect that the VS2 nanostructures may be applicable in electrocatalysis with high efficiency.

Conductance oscillation and quantization in monatomic Al wires

We present first-principles calculations for the transport properties of monatomic Al wires sandwiched between Al(100) electrodes. The conductance of the monatomic Al wires oscillates with the number of constituent atoms as a function of the wire length, either with a period of four atoms, for wires with the typical interatomic spacing, or with a period of six atoms, for wires with the interatomic spacing of bulk face centred cubic aluminium, indicating a dependence of the period of conductance oscillation on the interatomic spacing of the monatomic Al wires.

Strong In-Plane Anisotropies of Optical and Electrical Response in Layered Dimetal Chalcogenide

An interesting in-plane anisotropic layered dimetal chalcogenide Ta2NiS5 is introduced, and the optical and electrical properties with respect to its in-plane anisotropy are systematically studied. The Raman vibration modes have been identified by Raman spectra measurements combined with calculations of phonon-related properties. Importantly, the Ta2NiS5 flakes exhibit strong anisotropic Raman response under the angle-resolved polarized Raman spectroscopy measurements. We found that Raman intensities of the Ag mode not only depend on rotation angle but are also related to the sample thickness. In contrast, the infrared absorption with light polarized along the a axis direction is always larger than that in the c axis direction regardless of thickness under the polarization-resolved infrared spectroscopy measurements. Remarkably, the first-principles calculations combined with angle-resolved conductance measurements indicate strong anisotropic conductivity of Ta2NiS5. Our results not only prove Ta2NiS5 is a promising in-plane anisotropic 2D material but also provide an interesting platform for future functionalized electronic devices.

Switching Molecular Kondo Effect via Supramolecular Interaction

We apply supramolecular assembly to control the adsorption configuration of Co-porphyrin molecules on Au(111) and Cu(111) surfaces. By means of cryogenic scanning tunneling microscopy, we reveal that the Kondo effect associated with the Co center is absent or present in different supramolecular systems. We perform first-principles calculations to obtain spin-polarized electronic structures and compute the Kondo temperatures using the Anderson impurity model. The switching behavior is traced to varied molecular adsorption heights in different supramolecular structures. These findings unravel that a competition between intermolecular interactions and molecule-substrate interactions subtly regulates the molecular Kondo effect in supramolecular systems.

Manipulating Magnetism at Organic/Ferromagnetic Interfaces by Molecule-Induced Surface Reconstruction

Fullerenes have several advantages as potential materials for organic spintronics. Through a theoretical first-principles study, we report that fullerene C60 adsorption can induce a magnetic reconstruction in a Ni(111) surface and expose the merits of the reconstructed C60/Ni(111) spinterface for molecular spintronics applications. Surface reconstruction drastically modifies the magnetic properties at both sides of the C60/Ni interface. Three outstanding properties of the reconstructed structure are revealed, which originate from reconstruction enhanced spin-split π-d coupling between C60 and Ni(111): (1) the C60 spin polarization and conductance around the Fermi level are enhanced simultaneously, which can be important for read-head sensor miniaturization; (2) localized spin-polarized states appear in C60 with a spin-filter functionality; and (3) magnetocrystalline anisotropic energy and exchange coupling in the outermost Ni layer are reduced enormously. Surface reconstruction can be realized simply by controlling the annealing temperature in experiments.

Structural phase transition associated with van Hove singularity in 5d transition metal compound IrTe2

We have investigated the electronic states of IrTe2 by using angle-resolved photoemission spectroscopy to elucidate the origin of the structural phase transition. Both the Ir 4f and Te 4d core level spectra exhibit dramatic splitting below the phase transition temperature Ts, suggesting that there exist two inequivalent Ir and Te sites with distinctly different electronic states in the distorted phase. The band related to the saddle points at the Fermi level (EF) is strongly reconstructed, which removes the van Hove singularity from EF below Ts. The wavevector connecting the adjacent saddle points is consistent with the in-plane superstructure modulation wavevector. These results indicate that the phase transition in IrTe2 is intimately associated with the saddle points. As the van Hove singularity mainly originates from the Te px+py orbitals, the Te 5p electronic states play a dominant role in the structural phase transition.