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Lecture

Combined TEM and XRD analysis of size, shape and microstructural defects in CdSe nanocrystals



Cadmium selenide nanocrystals (CdSe NCs) are frequently used in optoelectronic applications, as they possess unique optoelectronic properties that depend heavily on their size and morphology. Consequently, the size and shape of CdSe NCs must be controlled precisely during their synthesis in order to obtain NCs with desired physical properties. Even though the thermodynamically stable modification of CdSe possesses the wurtzitic structure (space group P6_3mc), CdSe NCs crystallize frequently in the zincblende structure (space group F-43m) or in a mixture of both structures. As a result, another critical issue of the CdSe NCs synthesis is the control of phase composition and formation of microstructural defects, as both affect the optoelectronic properties as well.


The aim of our study was to correlate the size and morphology of CdSe NCs with their phase composition and with the formation of microstructure defects and to explain the effect of these microstructure defects on the optoelectronic properties of the NCs. Transmission electron microscopy with high resolution and X-ray diffraction revealed that the CdSe NCs synthesized using hot injection at temperatures between 225°C and 250°C have a size between 3 and 10 nm and crystallize predominantly with the metastable zinc blende crystal structure. While NCs having a size smaller than 4 nm were practically defect-free, the number of planar defects increases with the NC size for larger particles. Two types of planar defects that were identified as stacking faults on the cubic lattice planes {111} were detected. Planar defects belonging to the first type are situated in the interior of the nanocrystals. When these planar defects appear randomly, then they lead to an anisotropic broadening of the X-ray diffraction lines. If the planar defects appear on every second cubic lattice plane {111}, then they represent a transition of the zinc blende structure to the thermodynamically stable wurtzitic modification of CdSe. In general, the “internal” planar defects deteriorate the photoluminescence quantum yield of the CdSe NCs. Planar defects belonging to the second type originate from the oriented attachment of CdSe NCs along the {111} crystallographic planes. These defects disturb the crystallographic coherence of attached NCs. Consequently, the clusters of NCs are not recognized as agglomerates of NCs but as separated NCs both by XRD and by photoluminescence.

Speaker:
Dipl.-Ing. Stefan Neumann
TU Bergakademie Freiberg
Additional Authors:
  • Christina Menter
    Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
  • Ahmed Mahmoud
    Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
  • Prof. Dr. Doris Segets
    University Duisburg-Essen
  • Prof. Dr. David Rafaja
    TU Bergakademie Freiberg