Mosaic nanostructure of TiO2 with rutile short-range atomic order

Abstract

A nanolaminate film of six 36nm TiO2–7nm Al2O3 bilayers is sputter deposited at room temperature and examined by high resolution transmission electron microscopy (HRTEM). Neither the TiO2 nor the Al2O3 layers have long-range crystallographic order. Previous Raman spectroscopy of the nanolaminate showed that short-range atomic order in the TiO2 component is characteristic of bulk rutile. The HRTEM images of the Al2O3 layers consist entirely of random contrast speckle characteristic of a material with no atomic ordering beyond the nearest-neighbor level. However, the predominant feature in the images of the TiO2 layers is a mosaic structure, with fewer regions of random contrast speckle. The mosaic consists of four repetitive elements: (1) domains of {110} planes terminating along ⟨100⟩ directions, (2) planar faults along ⟨100⟩ directions, (3) {110} facets in steps along the [001] direction, and (4) a herringbone structure of short strands of (110) and (−110) planes on either side of a ⟨100⟩ midrib. We show how two combined growth operations can generate this nanostructure: These operations are the preferential three-dimensional growth of a rutile nucleus with a {110} habit and the formation of growth faults with 12⟨10−1⟩{011} and 12⟨10−1⟩{121} displacement vectors. The results explicitly show that TiO2 with rutile short-range atomic order self-assembles into units beyond the nearest-neighbor level. This behavior is different from oxides that are continuous random network formers, such as SiO2 and Al3O3, in which the metal-oxygen bonds are predominantly covalent.