Dissociated 1/3 〈0111〉 dislocations in Bi_2Te_3 and their relationship to seven-layer Bi_3Te_4 defects

摘要

We investigate the structure of 1/3 〈0111〉 dislocations observed in Bi_2Te_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_2Te_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_3Te_4, rather than the normal Bi_2Te_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 〈0111〉-type dislocations.