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

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 (Td) between 20 and 600 °C 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 ∼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 (∼4 nm diameter), at Td=20 °C to a very compact and textured columnar structure characterized by SnO2 columns (∼25 nm diameter) composed of grains of ∼12 nm of diameter, at Td=600 °C. In addition, the PLD SnO2 films were found to exhibit the highest nanoporosity at Td=300 °C 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 °C, 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 ∼10 nm when undoped to ∼6 nm for the doped films).