Structural and optical properties of nanocrystalline ZnO thin films derived from clear emulsion of monodispersed ZnO nan
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Structural and optical properties of nanocrystalline ZnO thin films derived from clear emulsion of monodispersed ZnO nanocrystals Masashi Inoguchi, Keigo Suzuki,a) Nobuhiko Tanaka, Keisuke Kageyama, and Hiroshi Takagi Murata Manufacturing Co., Ltd., Nagaokakyo-shi, Kyoto 617-8555, Japan (Received 13 January 2009; accepted 20 April 2009)
In this study, the dense films of well-crystallized ZnO nanocrystals were successfully prepared by direct spin-coating of the colloidal solution of ZnO nanoparticles derived from the microemulsion method. The average grain sizes in the films were reasonably controlled in the range from 6.5 to 34.3 nm by simply changing the annealing temperatures. The increase in band gap energies was found in the size region less than 13.3 nm, finally resulting in 3.47 eV for the average size of 6.5 nm. The photoluminescence spectra at room temperature showed intense ultraviolet (UV) emission with faint green luminescence. The Stokes shifts of the films were estimated to be one or two orders of magnitude smaller than those of the conventional ZnO nanocrystalline films, suggesting the well crystallization and slight amounts of lattice defects in the ZnO nanoparticles. These excellent features may be favorable to make high-performance optical application such as UV emitting devices. I. INTRODUCTION
Zinc oxide (ZnO) has attracted considerable attention because of its high potential as a semiconductor material. It has been used in various applications such as ultraviolet (UV) laser, diode, transparent conductor, gas sensor, catalyst, and phosphor. In particular, ZnO is suitable for UV emission devices because of its wide direct band gap energy (Eg) of 3.37 eV and high efficiency of excitonic emission due to the high exciton binding energy. In low-dimensional ZnO nanostructures, excellent optical properties due to the quantum size effects are expected, leading to the enhancement of the performance of UV emission devices. A large number of studies have been carried out on various types of ZnO nanostructures such as quantum dots (QDs),1–12 nanocrystalline thin films,13–22 nanowires,23,24 nanobelts,25 and nanorods.26,27 The films that consist of nanocrystals, nanorods, and nanowires are an important form of nanostructured materials in terms of the development of applications. Thus, fabrication of thin films of nanocrystalline ZnO has been intensively performed using gas-phase13,17–19,21 and liquidphase14–16,20,22 methods. The gas-phase production allows us to fabricate the films with high crystallinity. However, it is usually carried out by an environment of high temperature under a high vacuum, which requires relatively complicated equipment and high energy costs. Liquid-phase fabrication is advantageous for its convenience, but precursor solutions reported so far contain a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0277 J. Mater. Res., Vol. 24, No. 7, Jul 2009
unfavorable elements such as alkali metals. These unfavorable elements may deteriorate the
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