Abstract
Engineering the compositional gradient for core/shell semiconductor nanocrystals improves their optical properties. To date, however, the structure of graded core/shell nanocrystal emitters has only been qualitatively described. In this paper, we demonstrate an approach to quantify nanocrystal structure, selecting graded Ag-In-Se/ZnSe core/shell nanocrystals as a proof-of-concept material. A combination of multi-energy small-angle X-ray scattering and electron microscopy techniques enables us to establish the radial distribution of ZnSe with sub-nanometer resolution. Using ab initio shape-retrieval analysis of X-ray scattering spectra, we further determine the average shape of nanocrystals. These results allow us to generate three-dimensional, atomistic reconstructions of graded core/shell nanocrystals. We use these reconstructions to calculate solid-state Zn diffusion in the Ag-In-Se nanocrystals and the lattice mismatch between nanocrystal monolayers. Finally, we apply these findings to propose design rules for optimal shell structure and record-luminescent core/shell nanocrystals.
Highlights
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[1,2]
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 from SAXS data[15] and crystal structure information from wide-angle X-ray scattering (WAXS) to create atomic reconstructions of the NCs
In the case of graded Ag-In-Se/ZnSe core/shell NCs, Zn is chosen as the anomalous scatterer
Summary
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[1,2]. 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 from SAXS data[15] and crystal structure information from wide-angle X-ray scattering (WAXS) to create atomic reconstructions of the NCs. This information is combined with retrievals of the average shape of NCs obtained from ab initio shape retrieval from SAXS data[15] 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. The Ag-In-Se/ZnSe NCs, prepared at a higher temperature of 150 °C (Fig. 2a), exhibit a high degree of alloying due to efficient Zn diffusion into the NCs during growth This sample is fully covered with a protective ZnSe shell, showing small lattice mismatch at the core/shell interface. In accordance with this structural analysis, the luminescence efficiency of thick ZnSe shell NCs is higher than that of thin ZnSe shell NCs
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