Laser-assisted spray pyrolysis process for the growth of TiO 2 and Fe 2 O 3 nanoparticle coatings
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We present a laser-assisted spray pyrolysis method to fabricate nanoparticle coatings of metal oxides. In this process, 1.5-m size droplets of a titanium- or iron-containing organometallic precursor were injected into a vacuum chamber with SF6 carrier gas. The strong absorption of a 3W CO2 laser beam focused onto the injector tip in the presence of SF6 increased the temperature of the gas and the droplets to about 300 °C. Films deposited on heated substrates with and without the CO2 laser heating were studied by atomic force microscopy. The laser heating of the droplets caused the solvent to evaporate before depositing on the substrate, leading to grain sizes that are about a factor of 3 smaller than those deposited without laser heating. By controlling the concentration of the precursor in the solvent, the average particle sizes have been tuned from 80 to 50 nm.
I. INTRODUCTION
A wide range of applications associated with thin films and coatings has led to the development of a variety of growth techniques. The characteristics of the deposited films are affected by the technique used. The parameters that gauge the quality of a film include the surface morphology, the density, and the crystallinity. Among the variety of techniques available for the growth of thin films, chemical and physical vapor deposition processes are known to produce the best quality films.1 These techniques enable precise control of the growth process, and have therefore been extensively used for the growth of films for device applications. For applications that do not require such high quality, a number of more economical techniques are available. These include electroplating,2 plasma spray,3 screen-printing,4 and spray pyrolysis.5–11 The simplicity, low capital cost of equipment, and adaptability to large-scale processing make these methods attractive for coating large surface areas. The chemical spray pyrolysis technique is particularly interesting because it can be used to form coatings of a variety of different materials. Since its initial use for the deposition of CdS films for solar cells,12 it has been successfully implemented in the fabrication of thin films of metal oxides, chalcogenides, and high temperature superconductors.13 The main component of a spray pyrolya)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0089 J. Mater. Res., Vol. 22, No. 3, Mar 2007
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sis system is an atomizer that generates microdroplets of a precursor solution dissolved in a relatively volatile solvent. The droplets, in the form of a fine spray, are carried out of a nozzle onto a heated substrate by a carrier gas that can be inert or reactive. The constituents of the droplets decompose and react on the hot substrate to form the chemical compound. The substrate temperature should be high enough to evaporate the volatile solvents. The spray nozzle is usually scanned continuously during the growth to coat a large area of the substrate. Laser pyrolysis tech
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