Core-shell nanoparticles are being intensively studied due to their exceptional properties like quantum dot confinement. In these nanoparticles, their optical and electronic properties can be modulated changing their dimensions [1]. In particular, materials such as CdSe/ZnS or InP/ZnS are extensively used in a variety of applications such as biochemical sensors [2], light emitting diodes [3] or photovoltaic devices [4]. In these nanoparticles, the shell has the function of avoiding the re-absorption of the light emitted by the core of the particle [5]. Recently, special attention is paid to InP/ZnS nanoparticles to replace the CdSe cores because of the harmful consequences in the environment and health due to the presence of Cd.     

In this communication, we analyse core/shell nanoparticles of different compositions by electron microscopy techniques. In particular, we have studied CdSe/ZnS and InP/ZnS nanoparticles with diameter of the core of approx. 2.7 nm and 0.6 nm thick shells. Initial analyses have been carried out by high resolution transmission electron microscopy (HRTEM). Fig 1 a) shows an HRTEM image of CdSe/ZnS nanoparticles, where one of the particles has been marked. The observed particles have been found to be very homogeneous in shape and dimensions The measured average size of the observed particles is 3 nm approx., which agrees with the designed value. However, and as it can be observed, the core and the shell cannot be distinguished with this technique. The small thickness of the shell is not expected to produce a layer with a noticeably different lattice parameter than the core. Because of this, the samples have been analysed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM), where the intensity in the images can be related to the atomic number Z of the atoms in the material. Fig. 1 b) shows an HAADF-STEM image of CdSe/ZnS nanoparticles. As it can be observed, a clear interface between the core and the shell is not noticed, but it can be seen that the edge of the particle shows smaller intensity than the core. This can be related to two reasons. On the one hand, in these particles, the Z number of the material in the shell is smaller than in the core (Z_ZnS = 46, Z_CdSe = 82). However, it should also be considered that the electron beam finds a smaller amount of material at the edge of the particle, what would cause a reduction of intensity. Because of this, the correlation of the area of reduced intensity with the ZnS shell is not straightforward. In order to investigate the possibility to distinguish the core and the shell in these nanoparticles, structures with different sizes of core and shell are being studied by HAADF-STEM. Image simulations will be carried out in order to help with the interpretation of these images, to allow the correlation of the structural characteristics of these nanoparticles with their optoelectronic properties.

Acknowledgements: This work was supported by the Spanish MINECO (projects TEC2014-53727-C2-1-R, -2-R and CONSOLIDER INGENIO 2010 CSD2009-00013) and Junta de Andalucía (PAI research group TEP-946). The research leading to these results has received co-funding from the European Union.

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