Nanoparticle thin films deposited by MAPLE for sensor applications

We report on the potentiality of the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique for the deposition of thin films of colloidal nanoparticles to be used for gas sensors based on electrical transduction mechanisms. The MAPLE technique seems very promising, since it permits a good thickness control even on rough substrates, generally used to enhance the active surface for gas adsorption. TiO2 (with a capping layer of benzyl alcohol) and SnO2 (with a capping layer of trioctylphosphine) colloidal nanoparticles were diluted in suitable solvents (0.2% concentration), frozen at liquid nitrogen temperature and ablated with a ArF (λ=193 nm) or KrF (248 nm) excimer laser. The nanoparticle thin films were deposited on silica, interdigitated alumina and <100> Si substrates and submitted to morphological (SEM-FEG), structural (XRD, FTIR), optical (UV-Vis transmission) and electrical (sensing tests) characterizations. A uniform distribution of TiO2 nanoparticles, with an average size of ~10 nm, was obtained on flat and rough substrates. The deposited TiO2 nanoparticles preserved the anatase crystalline structure, as evidenced by the XRD spectra. FTIR analysis showed that the SnO2 nanoparticles maintained the capping layer after the laser-assisted transfer process. This protective layer was removed after annealing at 400 °C. The starting nanoparticle dimensions were preserved also in this case. Electrical tests, performed on TiO2 nanoparticle films, in controlled atmosphere in presence of ethanol and acetone vapors, evidenced a high value of the sensor response even at very low concentrations (20-200 ppm in dry air). In contrast, in the case of SnO2 nanoparticle films, electrical tests to ethanol vapor presence showed poor gas sensing properties probably due to the small nanoparticle sizes and interconnections.