First-Principles Investigations of Oxygen Vacancies on SnO2 Nanofilms

In recent years multiple critical advances in nanofabrication have allowed for the well-controlled formation of nanocrystals of the n-type semiconductor tin oxide ($SnO_2$). Because gas sensing in $SnO_2$ involves changes in surface resistivity as a function of gas concentration, the high surface-to volume ratio of $SnO_2$ nanocrystals could be leveraged to produce a gas sensor with significantly enhanced sensitivity. A key feature of the sensing mechanism is the facile formation and destruction of oxygen vacancies at (or near) the surface. In this talk I will discuss our ongoing first-principles investigations of surface oxygen vacancies in $SnO_2$ nanofilms. We have focused on vacancy formation among the so-called bridging oxygen atoms on the (110) surface of rutile $SnO_2$ as a function of vacancy concentration and film thickness and have studied the effect on local atomic and electronic structure. From a set of first-principles Density Functional Theory calculations on ordered vacancy structures, we have parametrized and tested a lattice-gas model describing vacancy-vacancy interactions. Using this model we have conducted extensive Monte Carlo simulations to investigate the oxygen vacancy phases on $SnO_2$ (110) as a function of temperature and oxygen vapor pressure.