First-principle study of the electronic structure of layered Cu2Se.

Copper selenide (Cu2Se) has attracted significant attention due to the extensive applications in thermoelectric and optoelectronic devices over the last few decades. Among various phase structures of Cu2Se, layered Cu2Se exhibits unique properties, such as purely thermal phase transition, high carrier mobility, high optical absorbance and high photoconductivity. Herein, we carry out a systematic investigation for the electronic structures of layered Cu2Se with several exchange-correlation functionals at different levels through first-principle calculations. It can be found that the electronic structures of layered Cu2Se are highly sensitive to the choice of functionals, and the correction of on-site Coulomb interaction also has a noticeable influence. Comparing with the results calculated with hybrid functional and G0W0method, it is found that the electronic structures calculated with LDA +Ufunctional are relatively accurate for layered Cu2Se. In addition, the in-plane biaxial strain can lead to the transition of electronic properties from metal to semiconductor in the layered Cu2Se, attributed to the change of atomic orbital hybridization. Furthermore, we explore the spin-orbit coupling (SOC) effect of Cu2Se and find that the weak SOC effect on electronic structures mainly results from spatial inversion symmetry of Cu2Se. These findings provide valuable insights for further investigation on this compound.

Identifying and Visualizing the Edge Terminations of Single-Layer MoSe2 Island Epitaxially Grown on Au(111).

Recently, single-layer transition-metal dichalcogenides have drawn significant attention due to their remarkable physical properties in the monolayer as well as at the edges. Here, we constructed high-quality, single-layer MoSe2 islands on the Au(111) surfaces in ultrahigh vacuum by molecular beam epitaxy. All of the islands have hexagonal or triangular shapes with two kinds of well-defined edges. Scanning tunneling spectroscopy (STS) curves show notable differences in positive sample bias for the two types of edges. Density functional theory calculations for several edge configurations of MoSe2 confirm that the STS differences are attributed to the coupling between the pz orbital of Se atoms and the dxz orbital of Mo atoms, and the two types of observed edge terminations are the bare Se edge and selenium-saturated Mo edge.

Sequence of Silicon Monolayer Structures Grown on a Ru Surface: from a Herringbone Structure to Silicene.

Silicon-based two-dimensional (2D) materials are uniquely suited for integration in Si-based electronics. Silicene, an analogue of graphene, was recently fabricated on several substrates and was used to make a field-effect transistor. Here, we report that when Ru(0001) is used as a substrate, a range of distinct monolayer silicon structures forms, evolving toward silicene with increasing Si coverage. Low Si coverage produces a herringbone structure, a hitherto undiscovered 2D phase of silicon. With increasing Si coverage, herringbone elbows evolve into silicene-like honeycomb stripes under tension, resulting in a herringbone-honeycomb 2D superlattice. At even higher coverage, the honeycomb stripes widen and merge coherently to form silicene in registry with the substrate. Scanning tunneling microscopy (STM) was used to image the structures. The structural stability and electronic properties of the Si 2D structures, the interaction between the Si 2D structures and the Ru substrate, and the evolution of the distinct monolayer Si structures were elucidated by density functional theory (DFT) calculations. This work paves the way for further investigations of monolayer Si structures, the corresponding growth mechanisms, and possible functionalization by impurities.

Construction of Two-Dimensional Chiral Networks through Atomic Bromine on Surfaces.

Using atomic bromine and 2,6-diphenylanthracene (DPA), we successfully constructed and characterized the large-area 2D chiral networks on Ag(111) and Cu(111) surfaces by combining molecular beam epitaxy with scanning tunneling microscopy. The Br atoms distribute themselves periodically in the network with the maximum number of -C-H···Br hydrogen bonds. Density functional theory calculations demonstrate that the hydrogen bonds contribute to the stability of the Br-organic networks. In addition, by controlling the ratio of bromine atoms to DPA molecules, different patterns of Br-organic networks were obtained on Ag(111) surfaces. Further experiments with 2,6-di(4-cyclohexylphenyl)anthracene on Ag(111) produced analogous atomic bromine guided 2D chiral networks.

Epitaxial Growth and Air-Stability of Monolayer Antimonene on PdTe2.

Monolayer antimonene is fabricated on PdTe2 by an epitaxial method. Monolayer antimonene is theoretically predicted to have a large bandgap for nanoelectronic devices. Air-exposure experiments indicate amazing chemical stability, which is great for device fabrication. A method to fabricate high-quality monolayer antimonene with several great properties for novel electronic and optoelectronic applications is provided.

Modifying the STM tip for the ' ultimate ' imaging of the Si(111)-7×7 surface and metal-supported molecules.

We report on high-resolution STM measurements with modified probe tips. First, both the rest atoms and adatoms of a Si(111)-7×7 surface are observed simultaneously. The visibility of rest atoms is dependent upon the sample bias voltage (less than -0.7 V) and is enhanced by sharpening the tip, which is rationalized by first-principles calculations. Second, a tip with a perylene molecule adsorbed at its apex is used to discriminate the molecular states and the metal states of the underlying Ag(110) surface, which is attributable to a mismatch between the energy levels of the functionalized tip and the adsorbates on silver. Lastly, high-resolution images of iron phthalocyanine (FePc) and zinc phthalocyanine (ZnPc) molecules on Au(111) are obtained by using an O(2)-terminated tip, and the images reveal rich intramolecular features arising from molecular orbitals that are not observed when using clean metallic tips.

Anchoring of a single molecular rotor and its array on metal surfaces using molecular design and self-assembly.

Functionalizing of single molecules on surfaces has manifested great potential for bottom-up construction of complex devices on a molecular scale. We discuss the growth mechanism for the initial layers of polycyclic aromatic hydrocarbons on metal surfaces and we review our recent progress on molecular machines, and present a molecular rotor with a fixed off-center axis formed by chemical bonding. These results represent important advances in molecular-based nanotechnology.