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Front. Chem. | doi: 10.3389/fchem.2018.00672

Impact of crystal structure and particle shape on the photoluminescence intensity of CdSe/CdS core/shell nanocrystals

Lukas Ludescher1,  Dmitry Dirin2,  Maksym V. Kovalenko2,  Michael Sztucki3, Peter Boesecke3 and  Rainer T. Lechner1, 4*
  • 1University of Leoben, Austria
  • 2ETH Zürich, Switzerland
  • 3European Synchrotron Radiation Facility, France
  • 4Institute of Physics, University of Leoben, Austria

To study the influence of the chemical and crystalline composition of core/shell NCs on their photoluminescence (PL) the mean structural profile of a large ensemble of NCs has to be retrieved in atomic resolution. This can be achieved by retrieving the chemical profile of core/shell NCs using anomalous small angle x-ray scattering (ASAXS) in combination with the analysis of powder diffraction data recorded by wide angle x-ray scattering (WAXS).
In the current synchrotron based study, we investigate CdSe/CdS core/shell NCs with different core dimensions by recording simultaneously ASAXS and WAXS spectra. The CdS shells are grown epitaxial on nominal spherical CdSe cores with core diameters from around 3.5 nm to 5.5 nm. Three different CdSe shell thicknesses are realised by depositing around 4, 6 and 8 monolayers (MLs) of CdSe.
We reveal that the epitaxial core/shell structure depicts a chemical sharp interface, even after a post growth annealing step. With increasing NC diameter, however, the CdSe/CdS NCs deviate significantly from a spherical shape. Instead an elliptical particle shape with pronounced surface facets for the larger core/shell NCs is found.
In combination with the powder diffraction data we could relate this anisotropic shape to a mixture of crystal phases within the CdSe core. The smallest CdSe cores exhibit a pure hexagonal wurtzite crystal structure, whereas the larger ones also possess a cubic zincblende phase fraction. This mixed crystal phase fractions lead to a non-spherical shell growth with different thicknesses along specific crystallographic directions: The long axes are terminated by basal crystal faces parallel either to the a- or c-axis, the short axes by “tilted” pyramidal planes.
By combining these structural data with the measured PL quantum yield values, we can clearly connect the optical output of the NCs to their shape and to their shell thickness. Above 6 ML CdS shell-thickness no further increase of the PL can be observed, but for large aspect ratio values the PL is significantly decreased. The gained understanding of the internal crystal structure on CdSe/CdS NCs is general applicable for a precise tuning of the optical properties of crystalline core/shell NCs.

Keywords: nanocrystals, colloid, Core/shell, SAXS, WAXS

Received: 08 Nov 2018; Accepted: 24 Dec 2018.

Edited by:

Vladimir Lesnyak, Technische Universität Dresden, Germany

Reviewed by:

Soong Ju Oh, Korea University, Japan
Vladimir Sayevich, Los Alamos National Laboratory (DOE), United States  

Copyright: © 2018 Ludescher, Dirin, Kovalenko, Sztucki, Boesecke and Lechner. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

* Correspondence: Dr. Rainer T. Lechner, University of Leoben, Institute of Physics, Leoben, Styria, Austria, rainer.lechner@unileoben.ac.at