High-resolution transmission electron microscopy investigation of the nanostructure of undoped and Pt-doped nanocrystalline pulsed laser deposited SnO2 thin films

Pulsed laser deposition (PLD) was used to grow nanocrystalline SnO2 thin films onto alumina substrates. The reactive PLD process was carried out at different substrate deposition temperatures (T-d) between 20 and 600 degreesC under an oxygen background pressure of 150 mtorr. The same PLD technique was used to produce SnO2 films in situ-doped with Pt (at the level of similar to 2 at. %) through the concomitant ablation of both SnO2 target and Pt strips. Conventional and high-resolution transmission electron microscopy (HRTEM) observations have revealed that the microstructure of the PLD SnO2 films is highly sensitive to their deposition temperature. Indeed, its changes from a porous granular structure with extremely fine equiaxed grains (similar to 4 nm diameter), at T-d = 20 degreesC to a very compact and textured columnar structure characterized by SnO2 columns (similar to 25 nm diameter) composed of grains of similar to 12 nm of diameter, at T-d = 600 T. In addition, the PLD SnO2 films were found to exhibit the highest nanoporosity at T-d = 300 degreesC which also coincides with the granular-to-columnar microstructural transition. On the other hand, the microstructure of the Pt-doped SnO2 films, deposited at 300 degreesC, was found to contain a high density of defects, such as twin boundaries and edge dislocations. By combining HRTEM and EDS microanalysis, we were able to show that the Pt-dopant self-organizes into spherical nanoparticles (1 -2 nm diameter) randomly distributed at the SnO2 grain boundaries. Finally, doping the films with such platinum nanoclusters is found to affect the SnO2 nanostructure by particularly reducing the SnO2 mean grain size (from similar to 10 nm when undoped to similar to 6 nm for the doped films). (C) 2003 Elsevier Ltd. All rights reserved.