Shell Nanocrystals

Coating a given ensemble of nanoparticles with another material yields core-shell nanocrystals (see above see also Refs [272,273]). In these structures, the cores may be any type of colloidal particle, such as metals, insulators, and all classes of semiconductors. Likewise, the shells may consist of type of material, including organics. At this point, attention is focused on semiconductor cores and inorganic shells, mainly of semiconductor materials. 

Early reports of ternary core-shell-shell (CSS) structures stem from Reiss et al. and Talapin etaZ. [277, 278]. In the latter case, CdSe nanocrystals stabilized with TOP and TO PO were coated with a shell of CdS or ZnSe. The second (i.e., outermost) shell was composed of ZnS in both cases. According to the authors, the main purpose of the 

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Mulvaney P, Liz-Marzan L (2003) Rational Material Design Using Au Core-Shell Nanocrystals. 226 225-246... 

Steckel JS, Zimmer JP, Coe-Sullivan S, Stott NE, Bulovic V, Bawendi MG (2004) Blue luminescence from (CdS)ZnS core-shell nanocrystals. Angew Chem Int Ed 43 2154-2158... 

A. V. and Alivisatos, A. P. (1997) Epitaxial growth of highly luminescent CdSe/CdS core/shell nanocrystals with photostability and electronic accessibility. J. Am. Chem. Soc., 119, 7019-7029. 

Banin, U., Bruchez, M., Alivisatos, A. P., Ha, T, Weiss, S. and Chemla, D. S. (1999) Evidence for a thermal contribution to emission intermittency in single CdSe/ CdS core/shell nanocrystals. J. Chem. Phys., 110, 1195-1201. 

A method in which the precursor solutions are successively injected into a cell containing the substrate and rinsed in between has been used to analyze the morphology of SILAR-grown films by atomic force microscopy (AFM).10 Recently, this approach has been applied to the growth of core/shell nanocrystals by Li et al.12... 

Li, J. J. Wang, Y. A. Guo, W. Keay, J. C. Mishima, T. D. Johnson, M. B. Peng, X. 2003. Large-scale synthesis of nearly monodisperse CdSe/CdS core/shell nanocrystals using air-stable reagents via successive ion layer adsorption and reaction. J.Am. Chem. Soc. 125 12567-12575. 

Lifshitz E et al (2006) Stable PbSe/PbS and PbSe/PbSexSl-x core-shell nanocrystal quantum dots and their applications. J Phys Chem B 110 25356-25365... 

We have shown in the past few years that due to their finite size (comparable to an average protein), CdSe-ZnS core-shell nanocrystals capped with a thin layer of dihydrolipoic acid ligands provide excellent nanoscale scaffolds ( nanoscaffolds ) for attaching several proteins on their surfaces. QD-protein conjugates were used to design multiplexed immunoassays to detect soluble toxins. 

Cao YW, Banin U (2000) Growth and properties of semiconductor core/shell nanocrystals with InAs cores. J Am Chem Soc 122 9692-9702... 

Figure 17 X-ray powder diffraction patterns for core-shell nanocrystals. In panel (A) 4.0 nm diameter CdSe QDs overcoated with (a) 0, (b) 0.65, (c) 1.3, (d) 2.6, and (e) 5.3 monolayers of ZnS shell. The thin solid lines show simulations of the data. Powder patterns for wurtzite CdSe and ZnS are included for comparison in the botton and top insets, respectively. [Adapted from (95).] (B) 3.5 nm diameter pure CdS nanocrystals (dotted), 3.9 nm diameter CdSe nanocrystals (dashed), and core-shell samples having the same 3.9 nm CdSe core and CdS shell thicknesses of (a) 0.2 nm, (b) 0.7 nm, and (c) 1.1 nm. The dashed vertical lines represent peak positions for pure CdSe the solid lines represent pure CdS. [Adapted from (100).]...

A similar result has been found using Raman spectroscopy of core-shell nanocrystals. Like XRD, Raman spectroscopy has also been widely employed to study doping of bulk semiconductors (110-112) but so far has only rarely been applied to doped semiconductor nanocrystals (70). Analogous to Vegard s law, shifts in lattice Raman vibrational energies have been found to occur with increasing dopant concentration in both the bulk and nanocrystalline materials. 

Kay, A. and M. Gratzel (2002). Dye-sensitized core-shell nanocrystals Improved efficiency of mesoporous tin oxide electrodes coated with a thin layer of an insulating oxide. Chemistry of Materials, 14(7), 2930-2935. 

From left) CdS-ZnS core-shell nanocrystals (diameter 3 nm), CdSe-ZnS of diameters 1.3,... 

Nazzal, A. Y. Wang, X. Qu, L. Yu, W. Wang, Y. Peng, X. Xiao, M., Environmental effects on photoluminescence of highly luminescent CdSe and CdSe/ZnS core/shell nanocrystals in polymer thin films, J. Phys. Chem. B 2004, 108, 5507-5515... 

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