Understanding photodoping in metal oxide nanocrystals: a route towards the development of 0D/2D nanoscale devices for light-driven energy storage

The use of metal oxide (MO) nanocrystals (NCs) to realize the next generation electronics is motivated by their great tunability in optoelectronic properties combined with excellent stability, which make them applicable to a wide range of applications, including the possibility of energy harvesting and energy storage. This thesis explores the photodoping mechanism within MO NCs, focusing on indium tin oxide (ITO) nanocrystals in both solution and solid-state films under UV illumination. The investigation reveals that charge accumulation (photocharging) in these nanocrystals, as evidenced by localized surface plasmon resonance (LSPR) and photocurrent measurements, is attributed to charge separation induced by the presence of charged surface states. This separation results in the reduction of the depletion layer at the surface, verified both by LSPR multilayer optical modeling and numerical simulations, and by photocurrent increase in thin films. Whereas photodoping is a fast process, the verified modifications attributed to charge storage both in solution and in solid state happen on longer (minutes/hours) timescales, resulting in semi-permanent modification of LSPR and persistent photocurrent (PPC). This fact underscores a kinetic equilibrium between surface adsorbates, resulting in surface-states, and photogenerated charges. Experiments verified that oxygen presence plays a crucial role in determining the effects of photodoping, underscoring its importance in surface state modulation during photodoping, hindering photocharge accumulation. This NCs photocharging capability was exploited for realizing a heterostructure combining 2D materials (MoS2) and MO NCs to realize a more efficient charge separation thanks to materials band alignment. Large-area monolayers of MoS2 were obtained with gold-assisted mechanical exfoliation, which was improved in yield by employing substrate functionalization. Kelvin Probe Force Microscopy studies confirmed charge separation across the heterostructure, positioning the use of coupled 2D and 0D materials as a promising route for realizing light-drive supercapacitors.