Dissociated 1/3 0111 dislocations in [Bi.sub.2][Te.sub.3] and their relationship to seven-layer [Bi.sub.3][Te.sub.4] defects

We investigate the structure of 1/3 dislocations observed in [Bi.sub.2][Te.sub.3] nanowires. This particular type of dislocation is interesting because it has a large Burgers vector (b = 1.048 nm) with a component normal to the basal planes equal to the thickness of one full [Bi.sub.2][Te.sub.3] quintuple unit (i.e., c/3). Atomic-resolution high-angle annular dark-field scanning transmission electron microscopy observations show that the dislocations form with a complex dissociated core structure. This structure consists of two partial dislocations that separate a defected region consisting of a seven-plane-thick septuple unit, consistent with a local patch of [Bi.sub.3][Te.sub.4], rather than the normal [Bi.sub.2][Te.sub.3] quintuple layer structure. As we discuss, details of the core structure can be understood from an analysis of the crystallographic parameters of the observed partial dislocations. This analysis suggests a mechanism to accommodate the loss of tellurium through the heterogeneous nucleation and growth of seven-layer defects at 1/3 --type dislocations.