Surface properties of SnO₂ nanolayers prepared by spin-coating and thermal oxidation

In this work, comparative studies of the surface morphology and surface chemistry of SnO₂ nanolayers prepared by spin coating with subsequent thermal oxidation (SCTO) in the temperature range of 400-700 °C using scanning electron microscopy (SEM), atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS) methods, are presented. The SEM images show that SCTO SnO₂ nanolayers contain partly connected irregular structures strongly dependent on the final oxidation temperature, with interconnected single grains of longitudinal shape and size, resulting in a flatter surface morphology with respect to the commonly used three-dimensional (3D) SnO₂ thin films. In turn, AFM studies additionally confirm that SCTO SnO₂ nanolayers after post-oxidation annealing at higher temperatures contain isolated grains of average lateral dimensions in the range of 20-50 nm having a rather flat surface morphology of average surface roughness defined by the root mean square factor at the level of ∼2 nm. From the XPS experimental research it can be concluded that, for our SCTO SnO₂ samples, a slight surface nonstoichiometry defined by the relative [O]/[Sn] concentration at the level of 1.8-1.9 is observed, also depending on the final post-oxidation temperature, being an evident contradiction to recently published literature using x-ray diffraction data. Moreover, XPS experiments show that there is also a permanent small amount of carbon contamination present at the surface of internal grains of our SCTO SnO₂ nanolayers, creating an undesired potential barrier for interactions with gaseous species when they are used as the active materials for gas sensing devices.