Structural changes in quantum dots core-shell CdSe/ZnS by thermal treatment

(1) Instituto de Física, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, Brazil
(2) Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, Brazil
(3) Institut National des Sciences Appliquées de Lyon, 20, avenue Albert Einstein, F-69621 , Lyon, France
(4) Karlsruhe Institute of Technology, Engesserstr. 15, 76131, Karlsruhe, Baden-Wuerttemberg, Germany

Core/shell semiconductor nanocrystals have been used for some time by the technological industry in a wide range of applications, such as LEDs and biosensors. However, the structural characterization of such materials, due to their small dimensions, represents a serious challenge, specially when they are modified via thermal annealing or ion irradiation. In the present work, we used the medium energy ion scattering (MEIS) analysis technique, combined with auxiliary techniques: transmission electronic microscopy (TEM), Rutherford backscattering spectrometry (RBS) and photoluminescence (PL), in order to characterize CdSe/ZnS commercially-available core-shell nanocrystals.   Through the use of the auxiliary techniques, we were able to check the best set of annealing parameters to study the stability of the QDs and the mobility of the relevant atomic species: annealing at 400 celsius for 5 minutes. A much longer and/or warmer annealing tend to lead to an almost completely deterioration of the QD; on the other hand a much shorter and/or colder annealing tend to result in negligible changes in the nanocrystal structure.  MEIS is an ion-beam technique, capable to obtain depth-profiling with sub-nanometric resolution, under the right conditions. This technique has been steadily optimized to the analysis of nanostructured materials, so it is an important tool in probing the structure of quantum-dots, among other materials $^{[1]}$. We used the MEIS simulation PowerMeis software $^{[2]}$ to create hypothetical energy-loss distributions for a wide range of tentative QDs structures; these simulated distributions were then compared to the experimental results in order to identify the most-probable structure of all. This result was then used as feedback and the procedure was repeated until a satisfying agreement was found between simulation and experiment.  We have found the core of the quantum-dots remains a stoichiometric CdSe crystal, as in the non-thermally treated samples; however the core diameter shrinks from ~5.2 nm down to approximately 4.2 nm, in agreement to TEM and PL results. We have found an excess of cadmium, located in the shell region of the QD, resulting in a CdSe/CdZnS structure. The shell symmetry seems to depart from a spherical shape, in agreement to what one would expect as the result of a possible “melting” behaviour.  

[1] M. A. Sortica, P. L. Grande, C. Radtke, L. Almeida, R. Debastiani, J. F. Dias, A. Hentz, Applied Physics Letters, 101 (2012) 023110.
[2] M. A. Sortica, P. L. Grande, G. Machado and L. Miotti, Journal of Applied Physics 106, 1 (2009).