The structural and electronic properties of (ZnO) n (n = 2−18) clusters are studied using gradient-corrected density-functional theory (DFT). The starting structures are generated from empirical genetic algorithm simulations or handmade constructions with chemical intuition. The lowest-energy structures of (ZnO) n are then selected from a number of structural isomers via DFT optimization. For small clusters (n = 2−7), ring structures were found to be the most stable. Three-dimensional cage and tube structures become energetically preferable for larger clusters (n = 9−18), and the competition between cage and tube structures leads to the alternative appearance of these two types of structures as global minima. The size evolution of electronic properties for zinc oxide clusters from ring toward cage or tube is discussed.

Monolayer group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te), an emerging category of two-dimensional (2D) semiconductors, hold great promise for electronics, optoelectronics and catalysts. By first-principles calculations, we show that the phonon dispersion and Raman spectra, as well as the electronic and topological properties of monolayer MX can be tuned by oxygen functionalization. Chemisorption of oxygen atoms on one side or both sides of the MX sheet narrows or even closes the band gap, enlarges work function, and significantly reduces the carrier effective mass. More excitingly, InS, InSe, and InTe monolayers with double-side oxygen functionalization are 2D topological insulators with sizeable bulk gap up to 0.21 eV. Their lowenergy bands near the Fermi level are dominated by the p x and p y orbitals of atoms, allowing band engineering via in-plane strains. Our studies provide viable strategy for realizing quantum spin Hall effect in monolayer group-III monochalcogenides at room temperature, and utilizing these novel 2D materials for high-speed and dissipationless transport devices.

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Following our recent work which revealed that the lowest-energy structures of (ZnO)n (n=9-18) follow cage and tube structural growth patterns with stacks of small subunits of (ZnO)2 and (ZnO)3 [Wang et al., J. Phys. Chem. C 111, 4956 (2007)], we have extended the search for the most stable structures to some larger clusters, i.e., (ZnO)n (n=24, 28, 36, and 48) by using gradient-corrected density-functional theory (DFT). A number of starting configurations belonging to different structural motifs were generated from handmade constructions with chemical intuition and then optimized via DFT calculations. Within the size range studied, cage and tube structures were found to be the most preferred structural motifs for the (ZnO)n clusters.

Faced with grave climate change and enormous energy demands, effective catalysts have become more and more important due to their significant effects on reducing fossil fuels consumption. The hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) by water splitting are feasible ways to produce clean sustainable energy. Here, atomic structures and related scanning tunneling microscope images of Se defects in PtSe2 are systematically explored. The equilibrium fractions of vacancies under variable conditions are predicted in detail. In addition, it is found that the vacancies are highly kinetically stable, without recovering or aggregation. The Se vacancies in PtSe2 can dramatically enhance the HER performance, comparing to Pt(111). It is also revealed that a PtSe2 monolayer with Se vacancies is also a good OER catalyst. The excellent bipolar catalysis of Se vacancies is further confirmed by experimental measurements. A defective PtSe2 made via direct selenization of Pt foil at 773 K using a chemical vapor deposition process is produced. It is shown that the HER and OER performance of defective PtSe2 is much more efficient than Pt foils by a series of measurements. This work, with its compelling theoretical and experimental studies indicates PtSe2 with Se defects is an ideal bipolar candidate for HER and OER.

Hexagonal [0001] nonpassivated ZnO nanowires are studied with density functional calculations. The band gap and Young's modulus in nanowires which are larger than those in bulk ZnO increase along with the decrease of the radius of nanowires. We find ZnO nanowires have larger effective piezoelectric constant than bulk ZnO due to their free boundary. In addition, the effective piezo-electric constant in small ZnO nanowires doesn't depend monotonously on the radius due to two competitive effects: elongation of the nanowires and increase of the ratio of surface atoms. PACS numbers: 77.65.-j,62.25.+g,73.22.-f,61.46.-w ZnO[1] is one of the most important materials due to its three key advantages: semiconducting with a direct wide band gap of 3.37 eV and a large excitation binding energy (60 meV), piezoelec-tric due to non-central symmetry in the wurtzite structure, and biocompatible. Recently, a diversity group of ZnO nanostructures including nanowires[2], nanobelts[3], nanosprings[4], nanocombs[5], nanorings[6], nanobows[7], and nanohelices[8, 9] have been synthesized under specific growth conditions. ZnO nanos-tructures could have novel applications due to their unique physical and chemical properties arising from surface and quantum confinement. In particular, ZnO nanowires with relatively simple structures are important one-dimensional (1D) nanostructures. Experimentally , the group of Wang had synthesized well-aligned [0001] ZnO nanowires enclosed by facet {10 ¯ 10} surfaces [10, 11]. Room-temperature ultraviolet lasing[12] and piezoelectric nanogenerators based on ZnO nanowire arrays have been demonstrated[13]. Rectifying diodes of single ZnO nanobelt/nanowire-based devices [14] and a ZnO nanowire photodetector[15] were fabricated very recently. Although many studies on ZnO nanowires have been conducted, there are some important issues remained to be addressed. First, the mechanical properties, especially the Young's modulus of ZnO nanowires are on debate in the literature[16, 17, 18, 19, 20]. For instance, Chen et al. [16] showed that the Young' modulus of ZnO nanowire with diameters smaller than about 120 nm is significantly higher than that of bulk ZnO. However, the elastic modulus of vertically aligned [0001] ZnO nanowires with an average diameter of 45 nm measured by atomic force microscopy was found to be far smaller than that of bulk ZnO[17]. The second issue is about the elec-tromechanical coupling in ZnO nanowires. The effective piezoelectric coefficient of individual (0001) surface dominated ZnO nanobelts measured by piezoresponse force microscopy was found to be much larger than the value for bulk wurtzite ZnO[21]. In contrast, Fan et al. showed that the piezoelectric coefficient for ZnO nanopillar with the diameter about 300 nm is smaller than the bulk values[22]. They suggested that the reduced electrome-chanical response might be due to structural defects in the pillars[22]. Whether the electromechanical coupling is enhanced or depressed in defect-free ZnO nanowires is not clear. Thirdly, although ...

The electronic and transport properties of graphene grain boundaries (GBs) are studied using density functional theory and nonequilibrium Green's function method. Most GBs preserve the semi-metal properties of perfect graphene; however, some GBs can open a moderate band gap up to 0.5 eV, which provides a potential way for band engineering of graphene-based materials. Nonequilibrium calculations of transmission coefficients showed that the conduction channels for transport electrons at Fermi level can be totally blocked or reduced due to existence of GBs. Moreover, the detailed defect arrangements have some influence on the transport behavior of graphene GBs.

Stimulated by the experimental synthesis of BN and AlN nanotubes, there has been considerable interest in nanotubes by noncarbon elements. Our density functional calculations show that the graphitic planar structure of ZnO is relatively stable and experimentally accessible. The single-walled nanotubes of ZnO have small strain energies, less than BN tubes of the same size. Hence, under certain experimental conditions, single-walled ZnO nanotubes might be fabricated. The calculated bandgaps of single-walled ZnO nanotubes are relatively uniform (∼2 eV) and almost independent of structures, which are located within a range inaccessible to BN, AlN, or GaN nanotubes.