Setting Carriers Free: Healing Faulty Interfaces Promotes Delocalization and Transport in Nanocrystal Solids

Abstract

Superlattices of epitaxially connected nanocrystals (NCs) are model systems to study electronic and optical properties of NC arrays. Using elemental analysis and structural analysis by in situ X-ray fluorescence and grazing-incidence small-angle scattering, respectively, we show that epitaxial superlattices of PbSe NCs keep their structural integrity up to temperatures of 300 °C; an ideal starting point to assess the effect of gentle thermal annealing on the superlattice properties. We find that annealing such superlattices between 75 and 150 °C induces a marked red shift of the NC bandedge transition. In fact, the post-annealing band-edge reflects theoretical predictions on the impact of charge carrier delocalization in these epitaxial superlattices. In addition, we observe a pronounced enhancement of the charge carrier mobility and a reduction of the hopping activation energy after mild annealing. While the superstructure remains intact at these temperatures, structural defect studies through X-ray diffraction indicate that annealing markedly decreases the density of point defects and edge dislocations. This indicates that the connections between NCs in as-synthesized superlattices still form a major source of grain boundaries and defects, which prevent carrier delocalization over multiple NCs and hamper NC-to-NC transport. Overcoming the limitations imposed by interfacial defects is therefore an essential next step in the development of high-quality optoelectronic devices based on NC solids.

Structural characterization of a polycrystalline epitaxially-fused colloidal quantum dot superlattice by electron tomography

“…Higher-quality epi-SLs will in turn encourage more in-depth analysis of the tomograms, particularly with regard to disorder across length scales longer than those emphasized in this paper. 30 Finally, we note that improving the tomogram resolution by a factor of two would allow visualization of QD facets and atomic-scale defects such as edge dislocations, 58 thereby providing a comprehensive nearatomistic picture of the 3D structure of these mesoscale QD lms.…”

The Fine-Structure Constant as a Ruler for the Band-Edge Light Absorption Strength of Bulk and Quantum-Confined Semiconductors

“…To minimize the impact of dielectric screening, we analyzed the absolute absorptance of a series of QD superlattices built from PbSe or CdSe QDs. 15 − 17 Such samples combine a 0D confinement with a 2D geometry, 18 which makes that only minor corrections are needed for the screening of the electromagnetic field (see also Figure 1 ). More precisely, we calculated for the different QD samples the local field factor F , which is the ratio between the external and the local electric field using atomistic dielectric modeling (see Supporting Information S8 ) and found that | F | 2 ranges between 0.4 and 0.8.…”

Electronic properties of atomically coherent square PbSe nanocrystal superlattice resolved by Scanning Tunneling Spectroscopy

“…These height differences together with irregularities in the in-plane directions indicate the degree of structural disorder in the superlattice. We assign these imperfections to the size distribution of the building blocks and inhomogeneities in the epitaxial connections of the nanocrystals occurring in the two (001) directions of the rock-salt crystal structure [18,22,36].…”