We investigate the interface between core and shell in zinc blende CdSebased CdSe/CdS dot-in-dot heteronanocrystals. Using X-ray diffraction and transmission electron microscopy, we show that a CdS shell grows coherently around the CdSe core. A comparison of the Raman spectrum of bare CdSe nanocrystals and CdSe/CdS heteronanocrystals indicates that the difference in lattice constant leads to compressive and tensile strain in core and shell, respectively. Concomitant continuum mechanical calculations follow this result, yet the calculated strain exceeds the experimental values. Moreover, a detailed analysis of the CdSe/CdS Raman spectra reveals the appearance of additional features upon shell growth. A comparison with pure Cd(Se,S) alloyed nanocrystals relates these features to alloy vibrations. We show that these observations point toward the presence of a mixed Cd(Se,S) layer at the CdSe/CdS interface. In this way, this work provides an experimental framework based on Raman spectroscopy to analyze in detail interfacial alloying in heteronanocrystals.
We report on the "flash" synthesis of CdSe/ CdS core−shell quantum dots (QDs). This new method, based on a seeded growth approach and using an excess of a carboxylic acid, leads to an isotropic and epitaxial growth of a CdS shell on a wurtzite CdSe core. The method is particularly fast and efficient, allowing the controllable growth of very thick CdS shells (up to 6.7 nm in the present study) in no more than 3 min, which is considerably shorter than in previously reported methods. The prepared materials present state-of-the-art properties with narrow emission and high photoluminescence quantum yields, even for thick CdS shells. Additionally, Raman analyses point to an alloyed interface between the core and the shell, which, in conjunction with the thickness of the CdS shell, results in the observed considerable reduction of the blinking rate.
We investigate the encapsulation of CdSe/CdS quantum dots (QDs) in a silica shell by in situ Raman spectroscopy and find a distinct shift of the CdS Raman signal during the first hours of the synthesis. This shift does not depend on the final silica shell thickness but on the properties of the initial core-shell QD. We find a correlation between the Raman shift rate and the speed of the silica formation and attribute this to the changing configuration of the outermost layers of the QD shell, where an interface to the newly formed silica is created. This dependence of Raman shift rate on the speed of silica formation process will give rise to many possible studies concerning the growth mechanism in the water-in-oil microemulsion, rendering in situ Raman a valuable instrument in monitoring this type of reaction.
In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright–dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.
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