Stabilization of polar ZnO surfaces: validating microscopic models by using CO as a probe molecule.
TL;DR: Combining several types of first-principles calculations with advanced molecular beam methods, this study is able to provide conclusive evidence that the polar O-terminated surface of ZnO is either reconstructed or hydrogen covered.
Abstract: The determination of the structure of inhomogeneous metal-oxide surfaces represents a formidable task. With the present study, we demonstrate that using the binding energy of a probe molecule, CO, is a reliable tool to validate structural models for such complex surfaces. Combining several types of first-principles calculations with advanced molecular beam methods, we are able to provide conclusive evidence that the polar O-terminated surface of ZnO is either reconstructed or hydrogen covered. This finding has important consequences for the ongoing discussion regarding the stabilization mechanism of the electrostatically unstable ("Tasker type 3") polar ZnO surfaces.
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TL;DR: In this paper, a review of various nanostructures of ZnO grown by the solid-vapour phase technique and their corresponding growth mechanisms is presented. And the application of nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.
Abstract: Zinc oxide is a unique material that exhibits semiconducting and piezoelectric dual properties. Using a solid–vapour phase thermal sublimation technique, nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages of ZnO have been synthesized under specific growth conditions. These unique nanostructures unambiguously demonstrate that ZnO probably has the richest family of nanostructures among all materials, both in structures and in properties. The nanostructures could have novel applications in optoelectronics, sensors, transducers and biomedical sciences. This article reviews the various nanostructures of ZnO grown by the solid–vapour phase technique and their corresponding growth mechanisms. The application of ZnO nanobelts as nanosensors, nanocantilevers, field effect transistors and nanoresonators is demonstrated.
3,361 citations
TL;DR: A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented and the nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered Nanorod-polymer solar cells.
Abstract: A method for growing vertical ZnO nanowire arrays on arbitrary substrates using either gas-phase or solution-phase approaches is presented. A ∼10 nm-thick layer of textured ZnO nanocrystals with their c axes normal to the substrate is formed by the decomposition of zinc acetate at 200−350 °C to provide nucleation sites for vertical nanowire growth. The nanorod arrays made in solution have a rod diameter, length, density, and orientation desirable for use in ordered nanorod−polymer solar cells.
1,437 citations
TL;DR: In this paper, a review mainly focuses on the rational synthesis, structure analysis, novel properties and unique applications of zinc oxide nanowires and nanobelts in nanotechnology.
Abstract: Semiconducting zinc oxide nanowires (NWs) and nanobelts (NBs) are a unique group of quasi-one-dimensional nanomaterial. This review mainly focuses on the rational synthesis, structure analysis, novel properties and unique applications of zinc oxide NWs and NBs in nanotechnology. First, we will discuss rational design of synthetic strategies and the synthesis of NWs via vapor phase and chemical growth approaches. Secondly, the vapor–solid process for synthesis of oxide based nanostructures will be described in details. We will illustrate the polar surface dominated growth phenomena, such as the formation of nanosprings, nanorings and nanohelices of single-crystal zinc oxide. Third, we will describe the unique and novel electrical, optoelectronic, field emission, and mechanical properties of individual NWs and NBs. Finally, we will illustrate some novel devices and applications made using NWs as ultra-sensitive chemical and biological nanosensors, solar cell, light emitting diodes, nanogenerators, and nano-piezotronic devices. ZnO is ideal for nanogenerators for converting nano-scale mechanical energy into electricity owing to its coupled piezoelectric and semiconductive properties. The devices designed based on this coupled characteristic are the family of piezotronics, which is a new and unique group of electronic components that are controlled by external forces/pressure.
1,082 citations
TL;DR: In this article, the divergence of the surface energy due to intrinsic polarization is controlled by controlling the grasps of the ZnO nanostructures, and the divergence can be further reduced.
Abstract: Growth of (0001) facet-dominated, free-standing, piezoelectric zinc oxide (ZnO) nanostructures is challenged by the divergence of the surface energy due to intrinsic polarization. By controlling gr...
1,059 citations
TL;DR: In this article, a review of the available experimental and theoretical information for zinc oxide surfaces is presented, with a focus on the interaction of hydrogen on the surface of oxide surfaces. But the authors do not discuss the physical properties of these surfaces.
747 citations
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TL;DR: A simple derivation of a simple GGA is presented, in which all parameters (other than those in LSD) are fundamental constants, and only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked.
Abstract: Generalized gradient approximations (GGA’s) for the exchange-correlation energy improve upon the local spin density (LSD) description of atoms, molecules, and solids. We present a simple derivation of a simple GGA, in which all parameters (other than those in LSD) are fundamental constants. Only general features of the detailed construction underlying the Perdew-Wang 1991 (PW91) GGA are invoked. Improvements over PW91 include an accurate description of the linear response of the uniform electron gas, correct behavior under uniform scaling, and a smoother potential. [S0031-9007(96)01479-2] PACS numbers: 71.15.Mb, 71.45.Gm Kohn-Sham density functional theory [1,2] is widely used for self-consistent-field electronic structure calculations of the ground-state properties of atoms, molecules, and solids. In this theory, only the exchange-correlation energy EXC › EX 1 EC as a functional of the electron spin densities n"srd and n#srd must be approximated. The most popular functionals have a form appropriate for slowly varying densities: the local spin density (LSD) approximation Z d 3 rn e unif
146,533 citations
TL;DR: In this paper, a direct difference method for the computation of molecular interactions has been based on a bivariational transcorrelated treatment, together with special methods for the balancing of other errors.
Abstract: A new direct difference method for the computation of molecular interactions has been based on a bivariational transcorrelated treatment, together with special methods for the balancing of other errors. It appears that these new features can give a strong reduction in the error of the interaction energy, and they seem to be particularly suitable for computations in the important region near the minimum energy. It has been generally accepted that this problem is dominated by unresolved difficulties and the relation of the new methods to these apparent difficulties is analysed here.
19,483 citations
TL;DR: For the molecules Be2, F2, and P2 of Table I, the unrestricted Hartree-Fock solution breaks the singlet spin symmetry, even though the density functional solutions do not.
Abstract: For the molecules Be2, F2, and P2 of Table I, the unrestricted Hartree-Fock solution breaks the singlet spin symmetry, even though the density-functional solutions do not. For these broken-symmetry solutions, the UHF atomization energies become 17, 220, and 141 kcalymol, respectively, and the mean absolute error of all the UHF atomization energies becomes 69.8 kcalymol. The PBE correlation energy of the two-electron ions of nuclear charge Z ! ` should be corrected to 20.0479 hartree, consistent with the PBE value v 0.046644 stated in the Letter. The quoted value 20.0482 hartree was obtained from the more refined v 0.046920 of G. G. Hoffman, Phys. Rev. B 45, 8730 (1992). Reference [6] should have been “A. C. Scheiner, J. Baker, and J. W. Andzelm, J. Comput. Chem. (to be published)”.
11,883 citations
TL;DR: In this paper, a dipole moment in the repeat unit perpendicular to the surface in an ionic crystal is demonstrated and the surfaces of any ionic or partly ionic material are classified into three types.
Abstract: When there is a dipole moment in the repeat unit perpendicular to the surface in an ionic crystal, lattice sums in the electrostatic energy diverge and the calculated surface energy is infinite. The cause of this divergence is demonstrated and the surfaces of any ionic or partly ionic material are classified into three types. Type 1 is neutral with equal numbers of anions and cations on each plane and type 2 is charged but there is no dipole moment perpendicular to the surface because of the symmetrical stacking sequence. Both these surfaces should have modest surface energies and may be stable with only limited relaxations of the ions in the surface region. The type 3 surface is charged and has a dipole moment in the repeat unit perpendicular to the surface. This surface can only be stabilised by substantial reconstruction. These conclusions are important for the analysis of the surface structure of ionic crystals.
1,563 citations
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TL;DR: In this article, the authors summarize the present understanding of polar oxide surfaces and examine fundamental issues regarding their stability, such as relaxation effects, change of covalency in the surface layers, partial filling of surface states and stoichiometry variations.
Abstract: In the light of recent experimental as well as theoretical studies, we summarize our present understanding of polar oxide surfaces and examine fundamental issues regarding their stability. The focus is on the surface atomic configurations (relaxations, reconstructions, non-stoichiometry, etc) obtained under specific preparation conditions and their associated electronic structure. We discuss several mechanisms at work on polar surfaces, such as relaxation effects, change of covalency in the surface layers, partial filling of surface states, and stoichiometry variations, and try to assess their actual efficiency for cancelling the polarity.
679 citations