Defect Location in Laterally Aligned Tin Oxide Nanowires: The Role of Growth Direction, Interface Dimensionality, and Catalyst

Laterally aligned SnO2 nanowires, which exhibitplanargrowth along crystallographically defined directions of a sapphiresubstrate surface, are superior for bio- and gas-sensing applications.Little is known about their cross-sectional geometry and the defectdistribution within the nanowire cross section, although this substantiallydetermines the electronic properties of the nanowires. In this study,SnO2 nanowires were grown on r-plane sapphire substrates.Scanning electron microscopy (SEM) and atomic force microscopy (AFM)reveal the highly oriented growth of the SnO2 nanowirestoward the substrate edges. High-resolution transmission electronmicroscopy (HRTEM) of the NW cross section and strain mapping allowedfor analyzing the crystallographic alignment of the SnO2 NW to the sapphire substrate and the defect distribution withinthe SnO2 nanowire cross section. These techniques revealeda defect-free SnO2-Al2O3 interfaceand a high alignment of the SnO2 NW lattice toward thesapphire substrate along the NW width. The determined high defectdensity close to the nanowire surface will be discussed in comparisonto freestanding SnO2 nanowires and SnO2 thinfilms on sapphire substrates considering the differences in the growthdirection and the interface dimensionality.