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.

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Zinc oxide nanostructures: growth, properties and applications

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

TiO2 nanotubes: synthesis and applications.

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.

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General Route to Vertical ZnO Nanowire Arrays Using Textured ZnO Seeds

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.

ZnO nanowire and nanobelt platform for nanotechnology

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...

http://www.nanoscience.gatech.edu/zlwang//paper/2003/03_NL_1.pdf

Spontaneous Polarization-Induced Nanohelixes, Nanosprings, and Nanorings of Piezoelectric Nanobelts

Understanding wet surfaces and water/solid interfaces is of crucial importance in such diverse fields as corrosion, catalysis, mineralogy, and geology. For H2O, the delicate interplay between chemical bonding, van der Waals forces, and hydrogen bonding gives rise to complex phenomena such as complete dissociation, partial dissociation at defects, multilayer formation, and wetting. [1] Recently, an intriguing intermediate scenario was advanced, in which the interaction between the water molecules results in a partial dissociation of water on perfect surfaces, leading to superlattices with long-range order. [2–5] In view of possible failures of common theoretical approaches and severe experimental difficulties [5] this proposal has become the subject of a highly controversial debate. [5–9] For a given surface a consistent picture can only be obtained by using a broad array of experimental and computational tools in concert. Applying such a strategy, we unambiguously demonstrate that water forms a simple, unusually stable two-dimensional superstructure on defectfree zinc oxide surfaces. Diffraction (He-atom scattering (HAS), low-energy electron diffraction (LEED)), real-space (scanning tunneling microscopy (STM)), and thermodynamic (He thermal desorption spectroscopy (He-TDS)) data show that water forms a (2 4 1) superstructure with long-range order that exists up to temperatures close to the boiling point of water. Car–Parrinello ab initio simulations [10] reveal a superlattice in which every second water molecule is dissociated. The unusual thermal stability of this superstructure is a result of a favorable “key–lock” type structural arrangement in conjunction with hydrogen bonding. The computed struc

Partial dissociation of water leads to stable superstructures on the surface of zinc oxide.

Exposure of the mixed-terminated $\mathrm{ZnO}(10\overline{1}0)$ surface to atomic hydrogen at room temperature is found to lead to drastic changes of the electrical properties. The insulator surface is found to become metallic. By employing several experimental techniques (electron energy loss spectroscopy, He-atom scattering, and scanning tunneling microscopy) together with ab initio electronic structure calculations we demonstrate that a low-temperature ($1\ifmmode\times\else\texttimes\fi{}1$) phase with two H atoms in the unit cell transforms upon heating to another ($1\ifmmode\times\else\texttimes\fi{}1$) phase with only one H atom per unit cell. The odd number of electrons added to the surface per unit cell gives rise to partially filled surface states and thus a metallization of the surface.

Hydrogen induced metallicity on the ZnO(1010) surface.

A reinvestigation of the O-terminated, polar surface of zinc oxide, O-ZnO(0001\ifmmode\bar\else\textasciimacron\fi{}), using scattering of thermal energy He atoms reveals the presence of a (1\ifmmode\times\else\texttimes\fi{}3) reconstruction for the clean surface. This finding is at variance with the results of previous works, where the same surface was found to be unreconstructed. Results for the hydrogen saturated H(1\ifmmode\times\else\texttimes\fi{}1) O-ZnO(0001\ifmmode\bar\else\textasciimacron\fi{}) surface, which has also been investigated, suggest that the presence of H atoms has a strong impact on the structure of the O-terminated ZnO surfaces.

Stability of the polar surfaces of ZnO: A reinvestigation using He-atom scattering

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.

Stabilization of polar ZnO surfaces: validating microscopic models by using CO as a probe molecule.

The adsorption of H2O on the oxygen-terminated polar surface of ZnO, ZnO(000-1), has been studied by He atom scattering (HAS), low-energy electron diffraction (LEED), adsorption probability measurements, He atom reflectivity measurements as a function of exposure and surface temperature (He atom thermal desorption measurements, “He-TDS”), and X-ray photoelectron spectroscopy (XPS). The clean O−ZnO(000-1) surface is characterized by an ordered (1 × 3) oxygen vacancy structure which converts to a (1 × 1) hydrogen (OH)-terminated structure upon dissociative H2O adsorption, even at adsorption temperatures as low as TS = 200 K. The formation of the OH-species is accompanied by the formation of a shoulder in the XPS O 1s line. A detailed investigation of the coverage dependence of the H2O adsorption probability indicates the presence of a distinct precursor state. The initial trapping probability is S0 = 0.8 ± 0.1. The most probable microscopic adsorption mechanism which is consistent with the obtained data is ...

M. Kunat

Papers

Partial dissociation of water leads to stable superstructures on the surface of zinc oxide.

Hydrogen induced metallicity on the ZnO(1010) surface.

Stability of the polar surfaces of ZnO: A reinvestigation using He-atom scattering

Stabilization of polar ZnO surfaces: validating microscopic models by using CO as a probe molecule.

The Interaction of Water with the Oxygen-Terminated, Polar Surface of ZnO