Rock-salt Ti1−xO → rutile TiO2−x transformation twinning via pulsed laser deposition – implications for the dense (hkl)-specific phase change and optoelectronic properties

Abstract

Defective rock-salt (R) Ti_1−xO → rutile (r) TiO_2−x transformation twinning occurred in a thin film with preferred shape and crystallographic orientation when deposited on silica glass substrates, with or without a coating of fluorine-doped tin oxide, using pulsed laser ablation of a rutile polycrystal under specific pulse energies in the range of 200 to 800 mJ per pulse in vacuum. The films were characterized by X-ray diffraction and electron microscopy and shown to contain nanocrystallites of R, r, and minor high-temperature-stabilized hexagonal Ti_1−xO all with preferred orientations adopted by close-packed planes. The R + r intimate intergrowth was found to form a butterfly twin following the optimum crystallographic orientation relationship [101]_R//[11]_r with close-packed (11)_R and (011)_r planes as the twin boundary and phase interface with a fair 2-D coincidence site lattice in accordance with the dense (111)_R-specific R → r transformation route. The R + r predominant film with aliovalent point defects and paracrystalline distribution of defect clusters showed characteristic white light absorbance, violet + green double emission, and fine photoelectron chemical properties for potential photovoltaic, photocatalytic, and optoelectronic applications, in particular for multiple temperature- and oxygen-fugacity sensors in terms of the R → r transformation.