STRUCTURE AND FREE CARRIER PARAMETERS IN ITO NANOCRYSTALS PROBED BY SOLID STATE NMR AND OPTICAL SPECTROSCOPY

In the last decades, plasmonic nanocrystals (NCs) have been subject of intense research due to their strong optical response. Indeed, the electric field of light induces a coherent oscillation of conduction electrons, known as "Local Surface Plasmon Resonance" (LSPR), which leads to a strong absorption peak typically in the UV-Vis - near IR (NIR) regions [1,2]. Among plasmonic NCs, degenerately doped semiconductors are particularly interesting for infrared plasmonics. In these NCs conduction electrons are generated by introducing aliovalent dopants. Among this class of materials, Tin-doped Indium Oxide (ITO) is an n-type semiconductor in which the aliovalent doping consists in the partial substitution of In3+ cations in the bixbyite In2O3 crystal structure with Sn4+. ITO NCs show a resonant peak in the NIR region, tunable by varying the dopant content (Sn%) [3]. Thanks to their infrared plasmonic properties, ITO NCs have been proposed in many different fields such as magnetoplasmonics and smart materials activated by NIR light [4,5]. The correlation between free electron parameters and the presence of dopant-related structural defects is an important task in the rationalization of the optical properties of these materials, and it is still far from being completely understood. Solid State NMR (SSNMR) appears particularly attractive for this scope [6-8], as the presence of free electrons affects several NMR properties of the nuclei. In this work, we present a SSNMR investigation on ITO NCs stabilized with oleic acid, containing an increasing amount of Sn. 119Sn SSNMR spectra and spin-counting experiments allowed us to identify different Sn species, correlated with different electronic properties. Further information was obtained by measuring 119Sn spin-lattice relaxation times (T1) at different temperatures. Optical and magneto-optical spectroscopies were also employed to extract free electrons par