Mapping and Directing Strain Relaxation in Connected Quantum Dot Superlattices via In Situ Heating in the STEM

The remarkable tunability of quantum dot (QD) superlattices (SLs) make them versatile candidates for advancing optoelectronic devices as well as investigating exotic electronic phenomena. Due to the complicated interactions at work during superlattice formation and oriented attachment, however, defects and disorder have so far prohibited the realization of electron delocalization and tunable miniband formation. Controlled heating offers a promising route to improve these electronic properties through defect relaxation. Here, we employ <i>in situ</i> MEMS heating in combination with atomic-resolution scanning transmission electron microscopy (STEM) and image analysis to track the evolution of epitaxial connections in square PbSe QD SLs over ~100 nm fields of view. We identify four distinct strain defect types – tensile, shear, bending and twisting – and visualize them using real space, local strain and out-of-plane orientation mapping techniques. We find that upon heating tensile and shear defects are fully relaxed, and twisting defects are reduced, but bending defects remain. We attribute the persistence of bending defects in part to constraints imposed by connections with the four neighboring QDs. To test this hypothesis, we also consider connected QD wires, a possible precursor to square SLs, wherein QDs are constrained by connections in only one dimension. These results improve our understanding of the energetics of defects present in epitaxial connections and support a route toward improved electron delocalization in connected QD SLs.