Multi-band photoluminescence in TiO2 nanoparticles-assembled films produced by femtosecond pulsed laser deposition

We present an analysis of the photoluminescence (PL) properties of nanostructured titania (TiO2) thin films produced by femtosecond pulsed laser deposition. Up to four PL bands are evidenced and analyzed, corresponding to radiative transitions ranging from the blue/violet to the near-infrared. The PL analysis was carried out on as-grown and post-growth annealed samples in both above-bandgap and below-bandgap excitation conditions, and allowed evidencing definite correlations between surface area, crystal phase and PL efficiency. An interpretation of the various PL components is proposed in terms of various electron and/or hole states, according to the different structural characteristics and crystal phase of the nanoparticles-assembled films. In particular, similarities between the PL activity of as-grown amorphous nanoparticles and crystalline anatase nanoparticles highlight the role of specific surface states, offering interesting insights into the possibility of exploiting amorphous TiO2 nanoparticles for PL-based applications in which surface states play an active role.