Controlled Growth of Monodisperse Nanocrystallites in Tin(IV) Oxide Nanofilms

Uniform nanocrystalline films of tin oxide have been deposited on glass and silicon substrates by a solution spin-coating method followed by annealing under flowing oxygen at different temperatures. Complete conversion of the coated tin(IV) chloride solution to tin oxide at a post-oxygen-anneal temperature greater than 120 °C is confirmed by energy-dispersive X-ray analysis and depth-profiling X-ray photoemission measurements. Transmission electron microscopy and X-ray diffraction studies further reveal that the onset of SnO 2 nanocrystallite formation in the amorphous film occurs at 350 °C. The resulting nanocrystallites that are accompanied by the observed granular structures and voids throughout the film as a result of crystallization and grain growth exhibit a narrow size distribution. The average nanocrystallite size is found to slowly increase from 7 nm at 350 °C to 10 nm at 500 °C (likely due to a strain-limited growth mechanism) and to grow exponentially above a second onset at 500 °C with a concomitant growth of the grains and a rapid increase in the roughness of the granular films. X-ray diffraction and Raman experiments further show a largely uniform depth distribution of nanocrystallites throughout the film, with a higher density near the surface. The facile physical control in the average size of the nanocrystallites by post-oxygen-anneal temperature in a desirable size regime (7−30 nm) promises a cost-effective, easily scalable fabrication method of SnO 2 film for gas-sensing and nanoelectronic applications.