Freestanding single-crystal complete nanorings of zinc oxide were formed via a spontaneous self-coiling process during the growth of polar nanobelts. The nanoring appeared to be initiated by circular folding of a nanobelt, caused by long-range electrostatic interaction. Coaxial and uniradial loop-by-loop winding of the nanobelt formed a complete ring. Short-range chemical bonding among the loops resulted in a single-crystal structure. The self-coiling is likely to be driven by minimizing the energy contributed by polar charges, surface area, and elastic deformation. Zinc oxide nanorings formed by self-coiling of nanobelts may be useful for investigating polar surface-induced growth processes, fundamental physics phenomena, and nanoscale devices.
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 growth kinetics, we show the success of growing nanobelt-based novel structures whose surfaces are dominated by the polarized ±(0001) facets. Owing to the positive and negative ionic charges on the zinc-and oxygen-terminated ±(0001) surfaces, respectively, a spontaneous polarization is induced across the nanobelt thickness. As a result, right-handed helical nanostructures and nanorings are formed by rolling up single-crystal nanobelts; this phenomenon is attributed to a consequence of minimizing the total energy contributed by spontaneous polarization and elasticity. The polar-surface-dominated ZnO nanobelts are likely to be an ideal system for understanding piezoelectricity and polarization-induced ferroelectricity at nanoscale; and they could have applications as onedimensional nanoscale sensors, transducers, and resonators.Zinc oxide (ZnO) is a versatile smart material that has key applications in catalysts, sensors, piezoelectric transducers, 1 transparent conductors, 2 and surface acoustic wave devices. 3The noncentral symmetry and the tetrahedral coordinated ZnO 4 unit in ZnO result in anisotropic piezoelectric properties. Structurally, the wurtzite structured ZnO crystal is described schematically as a number of alternating planes composed of four-fold coordinated O 2-and Zn 2+ ions, stacked alternatively along the c-axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (0001 h)-O polar surfaces, resulting in a normal dipole moment and spontaneous polarization as well as a divergence in surface energy. To maintain a stable structure, the polar surfaces generally have facets or exhibit massive surface reconstructions, but ZnO ( (0001) is an exception, which is atomically flat, stable, and without reconstruction. 4,5 Understanding the superior stability of the ZnO ( (0001) polar surfaces is a forefront research in today's surface physics. 6-9Nanowire-and nanotube-based materials have been demonstrated as building blocks for nanocircuits, nanosystems, 10-14 and nanooptoelectronics, 15 and they have been fabricated for a wide range of materials from metals, semiconductors, and oxides to polymers. 16 22 But these ZnO nanostructures grow along the c-axis, and the side surfaces are {011 h0} and {21 h 1 h0} due to their energies which are lower than that of (0001), resulting in vanishing dipole moment and much reduced piezoelectricity. The most desirable morphology to maximize the piezoelectric effect is to create nanostructures that preserve large area (0001) polar surfaces. 23,24 However, ZnO (0001) has a surface energy that diverges with sample size due to the surface polarization charge. Therefore, growth of (0001) surface-dominated free-standing nanostructures needs to oVercome the barrier of surface energy.In this paper, we report the free-standing ZnO nanobelts that g...
We report that the Zn-terminated ZnO (0001) polar surface is chemically active and the oxygen-terminated (000(-)1) polar surface is inert in the growth of nanocantilever arrays. Longer and wider "comblike" nanocantilever arrays are grown from the (0001)-Zn surface, which is suggested to be a self-catalyzed process due to the enrichment of Zn at the growth front. The chemically inactive (0001;)-O surface typically does not initiate any growth, but controlling experimental conditions could lead to the growth of shorter and narrower nanocantilevers from the intersections between (000(-)1)-O with +/- (01(-)10) surfaces.
Zinc oxide, an important semiconducting and piezoelectric material, has three key characteristics. First, it is a semiconductor, with a direct bandgap of 3.37 eV and a large excitation binding energy (60 meV), and exhibits near‐UV emission and transparent conductivity. Secondly, due to its non‐centrosymmetric symmetry, it is piezoelectric, which is a key phenomenon in building electro‐mechanical coupled sensors and transducers. Finally, ZnO is bio‐safe and bio‐compatible, and can be used for biomedical applications without coating. With these unique advantages, ZnO is one of the most important nanomaterials for integration with microsystems and biotechnology. Structurally, due to the three types of fastest growth directions—<0001>, <01$ \bar 1 $0>, and <2$ \bar 1 $$ \bar 1 $0>—as well as the ±(0001) polar surfaces, a diverse group of ZnO nanostructures have been grown in our laboratory. These include nanocombs, nanosaws, nanosprings, nanorings, nanobows, and nanopropellers. This article reviews our recent progress in the synthesis and characterization of polar‐surface‐induced ZnO nanostructures, their growth mechanisms, and possible applications as sensors, transducers, and resonators. It is suggested that ZnO could be the next most important nanomaterial after carbon nanotubes.
We report the controlled synthesis of free-standing ZnO nanobelts whose surfaces are dominated by the large polar surfaces. The nanobelts grow along the a axis, their large top/bottom surfaces are the Ϯ͑0001͒ polar planes, and the side surfaces are (011 0). Owing to the positive and negative ionic charges on the zinc-and oxygen-terminated Ϯ͑0001͒ surfaces, respectively, the nanobelts form multiloops of nanohelixes/nanosprings/nanospirals for the sake of reducing electrostatic energy introduced by the polar surfaces as well as balancing the difference in surface tension. The polar surface dominated ZnO nanobelts are likely to be an ideal system for understanding piezoelectricity and polarization induced phenomena at nanoscale.Considerable research effort was focused recently on quasi-one-dimensional ͑1-D͒ nanomaterials, 1 due to their potential applications as building blocks for nanocircuits, 2 nano-optoelectronics, 3 and nanosensors. 4,5 In literature, the 1-D nanomaterials are termed diversely as nanowires, nanorods, nanoribbons, nanobelts, etc. Among these terminologies, the remarkable characteristics of the nanobelts are the well-defined facets, unique growth direction, and a typical rectangular cross section. 6 Simply, nanobelt is a structurally controlled nanowire structure. It is possible that facet control on nanowire may have equivalent importance as the control over the helical angle of a single-walled carbon nanotube, which determines its semiconductor or metallic characteristic.Zinc oxide ͑ZnO͒ is a versatile smart material that has key applications in catalysts, sensors, piezoelectric transducers, 7 transparent conductor and surface acoustic wave devices. 8 ZnO has wurtzite structure which is described schematically as a number of alternating planes composed of fourfold coordinated O 2Ϫ and Zn 2ϩ ions, stacked alternatively along the c axis. ZnO has partial ionic characteristics, thus there is a net dipole moment along the c axis. For the basal planes, the ͑0001͒ plane is terminated by Zn and (0001 ) plane terminated by O, resulting in the divergence of surface energy for large polar surfaces. The other commonly observed planes of ͕011 0͖ and ͕21 1 0͖ are nonpolar planes, which have lower surface energy compared to the polar basal plane. The crystallographic anisotropy of ZnO results in anisotropic growth. Under thermodynamic equilibrium condition, the facet with higher surface energy is usually small in area, while the lower energy facets are larger. Specifically in the ZnO growth, the highest growth rate is along the c axis and the large facets are usually ͕011 0͖ and ͕21 1 0͖. 6,9,10 In this article, we show that, by controlling growth kinetics, it is possible to change the growth behavior of ZnO nanobelts.We report free-standing ZnO nanobelts whose large top/ bottom flat surfaces are the Ϯ͑0001͒ polar surfaces. As a result of surface polarization, nanohelixes are formed to reduce the electrostatic energy. This type of polar surface dominated 1-D nanostructure could have potential applications as ...
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