Since 2014, I devoted an important part of my activities to the study of charge recombination processes/pathways in TiO₂, mainly via the use of variants of photoluminescence (PL) spectroscopy. This activity line was motivated by the close correlations existing between PL features and heterogeneous catalysis performances.

In fact, PL in semiconductors typically occurs in tiny (~100 nm thickness or less) sub-surface regions . Hence, its characteristics are very sensitive to surface modifications (e.g. chemical reactions, molecular adsorption). Moreover, PL intensity quantifies the recombination efficiency of photogenerated charges, which is the major limiting factor for photocatalytic efficiency.

Most of the TiO₂ applications (e.g.: environmental remediation, dye-sensitized solar cells, gas sensors, self-cleaning surfaces) rest on its photocatalytic properties and on its surface chemistry. Thus, investigating and clarifying the not completely understood basic PL mechanisms active in this material can give important information.

Modification of the surface chemistry in a semiconductor during adsorption/desorption of gas species likely results in a modulation of the PL intensity, due to the strong correlation existing between the electronic distribution at surface and in the sub-surface region and the presence of gaseous adsorbates. These effects are particularly sensitive when we deal with nanostructured semiconductors (as they have large specific surface areas) with intermediate concentration of free charges (lesser free charge implies deeper modification of the electronic arrangements caused by an external molecules, see Debye screening). PL can thus be employed as a trasduction signal for chemical sensing, in parallel to electrical conductivity. Futhermore, PL is also a useful technique for studying the semiconductor properties. Hence, implementing PL techniques in controlled environment is a good way to study the electronic properties and surface rections in nanostructured functional semiconductors.

Along the years, I had set up different photoluminescence (PL) systems: continuous wave PL (CWPL), time-resolved PL (TRPL), excitation-resolved PL (also referred to as photoluminescence excitation, or PLE), temperature-resolved PL in controlled gaseous environments. These spectroscopy tewchniques have been devoted to basic studies of photogenerated charge processes/recombination in metal oxide semiconductors (I studied in particular SnO₂, ZnO, TiO₂) and in potentialities of employing the gas-induced PL.

Apart implementing the technical stuff, i also developed the interpretations for phenomenological results. Some examples: the role oxygen vacancy states in NO₂induced quenching in tin dioxide, excitonic effects in zinc oxide.

Part of my interests are devoted to nonlinear optics and, particularly, to second harmonic generation (SHG).

In this regard, I developed an experimental setup [link] later that I used later to study the charge-transfer processes at the interface between organic semiconductors of interest in organic electronics and SiO₂ gate oxide in the framework of the projects “InterFet”, funded by Regione Campania (I was Project Leader) and of the Progetto Premiale INFN-CNR “EOS”, funded by MIUR (“fondi premiali 2012”, role of S. Lettieri: participant).

More recently, I am studying the poling of polymers induced by pyroelectric effect (see here).