Mosaic nanostructure of TiO2 with rutile short-range atomic order

A nanolaminate film of six 36 nm TiO2-7 nm 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 2 rutile nucleus with a {110} habit and the-formation of growth faults with 1/2 < 10-1 >{011} and 1/2 < 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. (c) 2006 American Vacuum Society.