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 degrees 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 SnO2 nanocrystallite formation in the amorphous film occurs at 350 degrees 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 degrees C to 10 nm at 500 degrees C (likely due to a strain-limited growth mechanism) and to grow exponentially above a second onset at 500 degrees 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 SnO2 film for gas-sensing and nanoelectronic applications.