Vibrational and defect states in $SnOx$ nanoparticles

We have studied $SnOx$ nanoparticles fabricated by gas-phase condensation and in-flight sintering using Raman and photoluminescence (PL) spectroscopy. We are able to identify various vibrational states of the rutile phase of the $SnOx$ crystal. By thorough analysis of the vibrational modes, we are able to determine the bond lengths of the O–O and Sn–O bonds for the substoichiometric $SnO1.5$⁠, leading, together with x-ray diffraction data, to a full characterization of the $SnO1.5$ lattice. In absorption and photoluminescence spectra, we observe a finite density of states inside the band gap due to oxygen vacancies, giving rise to a midgap luminescence peak. Our results suggest that the defect related luminescence efficiency is limited by nonradiative recombination processes and by the oxygen vacancy density. We therefore conclude that the PL intensity has a maximum around a stoichiometry of $SnO1.7$⁠.