3
Science 9 March 2001:
Vol. 291. no. 5510, pp. 1947 - 1949
DOI: 10.1126/science.1058120
Prev | Table of Contents | Next
Reports
Nanobelts of Semiconducting Oxides
Zheng Wei Pan,1 Zu Rong Dai,1 Zhong Lin Wang1,2*
Ultralong beltlike (or ribbonlike) nanostructures (so-called nanobelts) were successfully synthesized for semiconducting oxides of zinc, tin, indium, cadmium, and gallium by simply evaporating the desired commercial metal oxide powders at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have a rectanglelike cross section with typical widths of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. The beltlike morphology appears to be a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts could be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts.
1 School of Materials Science and Engineering,
2 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0245, USA.
* To whom correspondence should be addressed. E-mail: zhong.wang@mse.gatech.edu
--------------------------------------------------------------------------------
Binary semiconducting oxides, such as ZnO, SnO2, In2O3, and CdO, have distinctive properties and are now widely used as transparent conducting oxide materials (1) and gas sensors (2). For example, fluorine-doped SnO2 film is widely used in architectural glass applications because of its low emissivity for thermal infrared heat (1). SnO2 nanoparticles are regarded as one of the most important sensor materials for detecting leakage of several inflammable gases owing to their high sensitivity to low gas concentrations (2). Tin-doped indium oxide (In2O3:Sn, ITO) film is an ideal material for flat panel displays because of its high electrical conductivity and high optical transparency (1), and ZnO is regarded as an ideal alternative material for ITO because of its lower cost and easier etchability (1). The current studies of semiconducting oxides have been focused on two-dimensional films and zero-dimensional nanoparticles, which can be readily synthesized with various well-established techniques such as sputtering (for films) and sol-gel (for particles). In contrast, investigations of wirelike semiconducting oxide nanostructures are cumbersome because of the unavailability of nanowire structures.
As stimulated by the novel properties of carbon nanotubes, wirelike nanostructures have attracted extensive interest over the past decade because of their great potential for addressing some basic issues about dimensionality and space-confined transport phenomena as well as applications (3). Besides nanotubules (4, 5), many other wirelike nanomaterials, such as carbides [SiC (6-8) and TiC (6)], nitrides [GaN (9, 10) and Si3N4 (11)], compound semiconductors (12, 13), element semiconductors [Si (14-16) and Ge (14)], and oxide [Ga2O3 (17) and MgO (18)] nanowires, have been successfully fabricated. In geometrical structures, these nanostructures can be classified into two main groups: hollow nanotubes and solid nanowires, which have a common characteristic of cylindrical symmetric cross section. Here, we report another group of distinctly different semiconducting oxide nanostructures that have a rectangular cross section, in correspondence to a beltlike (or ribbonlike) morphology. The oxides with the nanobelt morphology cover cations with different valence states and materials with different crystallographic structures, and it seems to be a common structural characteristic for the family of semiconducting oxides.
Our synthesis is based on thermal evaporation of oxide powders under controlled conditions without the presence of catalyst (19). The desired oxide powders were placed at the center of an alumina tube that was inserted in a horizontal tube furnace, where the temperature, pressure, and evaporation time were controlled. In our experiments, except for the evaporation temperature, which was determined on the basis of the melting point of the oxides used, we kept the following parameters constant: evaporation time, 2 hours; chamber pressure, 300 torr; and Ar flowing rate, 50 standard cubic centimeters per minute. During evaporation, the products were deposited onto an alumina plate placed at the downstream end of the alumina tube. The as-deposited products were characterized and analyzed by x-ray diffraction (XRD) (Philips PW 1800 with Cu K radiation), scanning electron microscopy (SEM) (Hitachi S800 FEG), transmission electron microscopy (TEM) [Hitachi HF-2000 FEG at 200 kV and JEOL 4000EX high-resolution TEM (HRTEM) at 400 kV], and energy-dispersive x-ray spectroscopy (EDS).
Thermal evaporation of ZnO powders (purity: 99.99%; melting point: 1975°C) at 1400°C for 2 hours resulted in white woollike products that formed in high yield on the surface of the alumina plate. SEM observations reveal that the products consist of a large quantity of wirelike nanostructures with typical lengths in the range of several tens to several hundreds of micrometers; some of them even have lengths on the order of millimeters (Fig. 1A). EDS microanalysis and powder XRD measurement (Fig. 1B) show that the sample is wurtzite (hexagonal) structured ZnO with lattice constants of a = 3.249 Å and c = 5.206 Å, consistent with the standard values for bulk ZnO (20).