Epitaxial-type shell

A general requirement for the synthesis of CS NCs with satisfactory optical properties is epitaxial type shell growth. Therefore an appropriate band alignment is not the sole criterion for choice of materials but, in addition, the core and shell materials should crystallize in the same structure and exhibit a small lattice mismatch. In the opposite case, the growth of the shell results in strain and the formation of defect states at the core-shell interface or within the shell. These can act as trap states for photogenerated charge carriers and diminish the fluorescence QY.95 The structural parameters of selected semiconductor materials are summarized in Table 5.1. 

It is noteworthy that the HRTEM cannot distinguish core and shell even by combining X-ray or electron diffraction techniques for some small nanoparticles. If the shell epitaxially grows on the core in the case of two kinds of metals with same crystal type and little difference of lattice constant, the precise structure of the bimetallic nanoparticles cannot be well characterized by the present technique. Hodak et al. [153] investigated Au-core/Ag-shell or Ag-core/Au-shell bimetallic nanoparticles. They confirmed that Au shell forms on Ag core by the epitaxial growth. In the TEM observations, the core/shell structures of Ag/Au nanoparticles are not clear even in the HRTEM images in this case (Figure 7). 

In general, these methods are used for the production of nanocrystalline powders which may be further compacted via techniques such as hot-pressing [157, 158] or magnetic pulsed compaction [159, 160]. In addition, other types of nanoionic material maybe prepared, such as nanometer-thin films, using techniques including molecular beam epitaxy [161], pulsed laser deposition [162] or spin-coating methods [163]. Novel structures, such as core-shell [164—166] and multi-layered [167, 168] (so-called onion structures) materials, may also be produced in this way. 

The Mo-containing MFl-type core-shell HZSM-5-Silicalite-l s (HZ5 S1) materials with various core-shell ratios prepared by the epitaxial growth of Silicalite-1 on HZSM-5 demonstrated the high shape selectivity to aromatics and stability in methane dehydroaromatization [66]. The silicalite-1 layer covering the HZSM-5 core can eliminate the external acid sites, and thus prevent the formation of active Mo species associated with Brpnsted acid sites on the external surface of catalysts. However, the overgrowth of the Silicalite-1 shell may lead to a severe inhibition of the Mo species migration into zeolite pores and consequent anchoring on the Brpnsted acid sites of the HZSM-5 core. Therefore, the catalytic performance is dependent on the core-shell ratio. 

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