Direct role of surface oxygen vacancies in visible light emission of tin dioxide nanowires

Tin dioxide SnO2 nanowires exhibit a strong visible photoluminescence that is not observed in bulk crystalline SnO2. To explain such effect, oxygen vacancies are often invoked without clarifying if they represent the direct origin of luminescence or if their presence triggers other radiative processes. Here we report an investigation of the nature of the visible light emission in SnO2 nanowires, showing that both experimental and theoretical ab initio analyses support the first hypothesis. On the basis of photoluminescence quenching analysis and of first-principles calculations we show that surface bridging oxygen vacancies in SnO2 lead to formation of occupied and empty surface bands whose transition energies are in strong agreement with luminescence features and whose luminescence activity can be switched off by surface adsorption of oxidizing molecules. Finally, we discuss how such findings may explain the decoupling between "electrical-active" and "optical-active" states in SnO2 gas nanosensors.