Gengyu Cao

Buckled Germanene Formation on Pt(111)

Germanene, a 2D honeycomb lattice analogous to graphene, is fabricated on a Pt(111) surface. It exhibits a buckled configuration with a (3 × 3) superlattice coinciding with the substrates (√19 × √19) superstructure. Covalent bonds exist throughout the germanene layer. The resulting high-quality germanene enables researchers to explore the fundamentals of germanene and its potential applications.

Vector-vortex Bessel–Gauss beams and their tightly focusing properties

We demonstrate that the amplitude of vector-vortex beams has a Bessel-Gauss (BG) distribution through a rigorous vector electromagnetic analysis. We also investigate the intensity profiles in the focal plane of vector-vortex beams that are focused by a high numerical-aperture lens obeying the Helmholtz condition. Although the intensity of a vector-vortex BG beam with a topological charge n=1 is nonzero along the axis in the focal plane, the beams with n≠1 show discrete multiple spots which can be useful for optical trapping.

Molecular orientations and interfacial structure of C60 on Pt(111).

Molecular orientations and assembled structures of C(60) molecules on Pt(111) have been characterized by low-temperature scanning tunneling microscopy for coverage between 0.1 ML and 1.5 ML. At room temperature, C(60) molecules preferentially decorate the steps and nucleate into single layer islands (SLIs) with hexagonal close-packed structures upon increasing coverage. C(60) islands comprise two differently oriented C(60)∕Pt(111)-(√13 × √13) R13.9° phases, in which five types of molecular orientation of C(60) carbon cage configurations are clearly identified by the high-resolution scanning tunneling microscopy image. Further annealing treatment leads to more uniform molecular orientation without apparent aggregation of C(60) SLIs. As coverage increases above 1 ML, domains corresponding to (2√3 × 2√3) R30° superstructure appear. To explain the above transformation, an interfacial reconstruction model is proposed according to the detailed study of the molecular adsorption structures in different domains.

Coverage-Dependent Structures of Cobalt-Phthalocyanine Molecules Adsorbed on Cu(001) Surface

The morphologies, self-assembly structures, and stability of cobalt-phthalocyanines (CoPc) molecules adsorbed on Cu(001) with coverage ranging from 0.2 monolayer (ML) to 1.6 ML are investigated by ultrahigh-vacuum low-temperature scanning tunneling microscopy (UHV LT-STM) at liquid nitrogen temperature. Upon increasing the deposition of CoPc molecules various structures, such as isolated adsorption, quasi-hexagonal structure, square root(29) x square root(29) structure, are well characterized by the corresponding high-resolution STM images. The CoPc-CoPc intermolecular interaction and CoPc-substrate interfacial interaction dominate the structural evolutions. For the coverage higher than 1 ML, CoPc molecules preferentially locate on top of the molecules underneath and organize into square root(58) x square root(58) structure. As more and more CoPc molecules adsorb on the first layer, in some square root(58) x square root(58) regions molecular insertion leads to the formation of the square root(29) x square root(29) domain to effectively decrease the energy of the whole system.

First-principles study of the structural and electronic properties of MoS2–WS2 and MoS2–MoTe2 monolayer heterostructures

Using first-principles calculations, we have systematically investigated the geometric and electronic structure of MoS2–WS2 and MoS2–MoTe2 monolayer (ML) heterostructures. Analysis of the variation of the total density of states and partial density of states of the specific atoms in the interfaces demonstrates that the two heterostructures show rather different properties and different changes from the initial MoS2 ML. The MoS2–WS2 ML heterostructure is still a semiconductor with a band gap of 1.58 eV, which is smaller than that of MoS2 and WS2 MLs. However, the strong interactions between MoS2and MoTe2 at the interfacial sites induce the MoS2–MoTe2 ML heterostructure to display metallic characteristics. Our results indicate that the ML heterostructures of MoS2–WS2 and MoS2–MoTe2 are expected to be a possible way to extend the application of the transition-metal dichalcogenides. (Some figures may appear in colour only in the online journal)

Direct Evidence of Dirac Signature in Bilayer Germanene Islands on Cu(111)

Bernal-stacked bilayer germanene with a stable buckled honeycomb structure has been successfully synthesized on Cu(111). Structural and electronic characterizations as well as theoretical calculations unequivocally demonstrate for the first time the presence of a nearly linear energy dispersion in the vicinity of the Fermi energy, as expected of the Dirac signature for theoretical freestanding germanene.

Two desorption components of product CO2 in steady-state CO oxidation on Pd(110)

The velocity and angular distributions of desorbing product CO2 were studied in steady-state CO oxidation on Pd(110) by means of cross-correlation time-of-flight techniques. CO2 desorption was always collimated along the surface-normal direction. The velocity distribution curves involved two desorption components, a fast one and a slow one. The former showed translational temperatures above 2300 K and was suddenly suppressed above a critical CO pressure at which CO retarded the reaction. On the other hand, the latter showed a Maxwellian distribution at the surface temperature and was not suddenly suppressed around the critical CO pressure, decreasing slowly at higher CO pressures. Below the critical CO pressure, the fraction of this component was around 0.3, and above it, the value abruptly increased to approximately 0.9. The slow component appears to be formed on sites which are suitable for oxygen dissociation, such as structural defects.

Reaction sites working in steady-state CO oxidation on a stepped Pt(113) surface

Abstract Reaction sites working in CO oxidation at the steady state were studied over Pt(113)=(s)2(111)×(001) through analysis of the angular and velocity distributions of the desorbing product CO 2 . The site preference was found to switch from the (111) site to (001) sharply in the boundary between the active region and the inhibited zone above critical CO pressures. A large difference was found in the translational energy of CO 2 produced on each site.

Molecular orientation and lattice ordering of C60 molecules on the polar FeO/Pt(111) surface.

C(60) molecules assemble into close packing layer under the domination of the intermolecular interaction when deposited onto Pt(111)-supported FeO layer kept at 400 K. From corresponding high resolution scanning tunneling microscopy (STM) image, a kind of C(60) molecular orientational ordering stabilized by the intermolecular interaction is revealed as C(60)/FeO(111)-(√133 × √133) R17.5° structure and determined from the commensurability between the C(60) nearest-neighbor distance and the lattice of the underlying oxygen layer. Moreover, due to the inhomogeneously distributed work function of the underlying FeO layer, the C(60) molecular electronic state is periodically modulated resulting in a bright-dim STM contrast. In addition, one coincidence lattice ordering is determined as 8 × 8 superstructure with respect to the C(60) primitive cell, which overlays a 3 × 3 moiré cell of the underlying FeO layer.

Molecular orbital imaging of cobalt phthalocyanine on native oxidized copper layers using STM.

To observe molecular orbitals using scanning tunneling microscopy, well-ordered oxidized layers on Cu(001) were fabricated to screen the individual adsorbed cobalt phthalocyanine (CoPc) molecules from the electronic influence of the metal surface. Scanning tunneling microscope images of the molecule on this oxidized layer show similarities to the orbital distribution of the free molecule. The good match between the differential conductance mapping images and the calculated charge distribution at energy levels corresponding to the frontier orbitals of CoPc provides more evidence of the screening of the oxidized layer from interactions between the metal surface and supported molecules.