Near-Atomistic Meso-Scale Tomographic Imaging of PbSe Quantum-Dot Super-Lattice Assemblies

Lead-chalcongenide quantum-dots have been widely studied because they show the property of multiple-exciton generation for high-energy photons. A functional device that incorporates multiple-exciton generation would have to be composed of an organized lattice of quantum-dots that are epitaxially fused at the interfaces. A perfect lattice might be expected to achieve mobility of 50 cm^2/Vs but measured is much lower. To investigate the structural origin of lower charge mobility we use high-resolution electron tomography to achieve near atomistic resolution imaging. We show atomic orientation variation for 1000s of QDs in several different ordered superlattice samples. Unlike previous studies, we study the position and alignment changes both laterally and vertically through samples with as many as 13 layers of QD particles. In addition to studying the variation in position and orientation across the sample, we also measure the thickness of epitaxial bridges between neighboring particles. Comparing the superlattice positions of the particles with the distribution of atomic lattice orientations shows that superlattice disorder is driven by the difference between the cubic structure of the atomic lattice vs the triclinic superlattice. Electron tomography proves to be a powerful tool for investigating quantum-dot superlattice samples.