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.
The trapping mechanisms at the origin of the persistent photocurrent effects in GaN-based devices have been studied on
different time scales by characterizing a low barrier metal-semiconductor-metal GaN-based photodetector in the
temperature range between room temperature and 500 K. The active material of the metal-semiconductor-metal device
consists of a thin film of GaN grown by metal organic chemical vapour deposition. The Arrhenius plots obtained by the
analysis of the decay times of the photocurrent as a function of the temperature on time scales from millisecond up to
hours allowed us to calculate the activation energies of the mechanisms responsible for the persistent photocurrent. The
activation energies derived from the decay times on the time scale of hours have been attributed to gallium vacancies
(VGa), gallium antisites (GaN) and carbon impurities, whereas GaN excitonic resonances resulted to be responsible for the
persistent photocurrent on the millisecond time scale. Finally, the influence of the decay times has been correlated with
the photocurrent gain of the device, which resulted to be as high as 4.1×105 at RT and 0.85×105 at 450 K.