2, Montanuniversitaet Leoben, Leoben, , Austria
Due to efficient and tunable emission properties, semiconductor core/shell nanocrystals (NCs) are of high technological importance for solid-state lighting and bio-medical applications. Optical properties of core/shell nanomaterials can be improved by engineering a graded protective shell. Relaxed lattice interfaces between core and shell materials lead to smaller density of interfacial defects and lower Auger non-radiative recombination. At the same time, the absence of an atomically defined interface makes determination of the atomic structure of graded core/shell NCs a complicated task.
In this work, we demonstrate an experimental approach to quantify nanocrystal structure, selecting graded Ag-In-Se/ZnSe core/shell nanocrystals as a proof-of-concept material. We determine the average radial distribution of elements with sub-nanometer resolution using anomalous small-angle X-ray scattering (ASAXS) analysis, supported by electron microscopy. This information is combined with retrievals of the average shape of NCs obtained from ab initio shape retrieval SAXS model and crystal structure information from wide-angle X-ray scattering (WAXS) to create atomic reconstructions of the NCs. We show that this detailed understanding of structure enables quantitative studies of solid-state diffusion in the NCs and lattice relaxation at the core/shell interface. Finally, we link these results to the luminescence efficiency of graded core/shell NCs and propose design rules for optimal shell structure and record-luminescent core/shell nanocrystals.