Core—Shell Nanocrystals

Core-shell NPs exhibit unique properties with several possible applications. In the case of fluorescent saniconductor NPs, core-shell NPs help to increase the robustness and enhance the photoluminescence quantum yield as well as the probability of radioactive recombination. NPs with magnetic, plasmonic and semi-conducting properties can be used as cores or shells for manipulating the properties of these hybrid structures. Properties of either component within the hybrids (core-shell NPs) can be modulated through a conjugating component or interface. 

FIGURE 13.8 TEM images of core-sheU nanoparticles of (a) ReO -Au formed with a 5 nm ReOj particle. Inset shows ReO -Au formed over an 8 nm ReO particle, (b) ReO -TiO coreshell nanoparticle formed over a 32 nm ReOj particle with e inset showing a core-sheU nanoparticle formed over a 12 nm ReOj nanoparticle. UV-visible absorption spectra of (c) ReOj-Au core-shell nanoparticles (1 2 and 1 4). (d) ReOj-TiOj core-shell nanoparticles (1 2 and 1 4) with a 12 nm ReO particle (From Ref. 83, J. Mater. Chem., 17 (2007) 2412. 2007 Royal Society of Chemistry). 

Peng et al. [85] report the synthesis of CdSe/CdS NPs with core diameters ranging from 2.3 to 3.9nm with a shell thickness up to three monolayers. Their results indicate that in the excited state the hole is confined to the core and the electron is delocalized throughout the entire structure. ZnSe-CdS core-shell NPs are prepared via the traditional pyrolysis of organometallic precursors. The two-step synthesis involves the fabrication of 4.5-6nm ZnSe seeds followed by a subsequent deposition 

Properties of a nanocrystal can be influenced markedly by encasing it in a sheath of another material [531]. The material of the shell in such a core-shell structure can be a metal, semiconductor, or an oxide. The shell material helps to impart novel, desired properties on the nanocrystals. For example, defects prevalent in the surface states of semiconductor nanocrystals can be transferred to a buffer layer of the shell material to obtain better emission from the nanocrystals. We use the notation, core-shell to denote core-shell structures. We employ the following classification to describe the nature of the core and the shell semiconductor-semiconductor, metal-metal and metal-oxide, semiconductor-oxide and oxide-oxide. The classification is artificial in that the nanocrystals result from similar synthetic strategies. The motivation for carrying out the modification of the shell material, however, differs in each case. 

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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... 

For optical transitions localized in the core of core-shell nanocrystals, the screening factor is given by Eq. (5). If in this expression the real part of the complex dielectric function of the shell... 

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