Polar-surface dominated ZnO nanobelts and the electrostatic energy induced nanohelixes, nanosprings, and nanospirals

Abstract

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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