Synthesis of gold-cadmium selenide co-colloids
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Synthesis of gold-cadmium selenide co-colloids Rekha Nayak Department of Materials, University of Oxford, Oxford, United Kingdom
Jane Galsworthy Department of Inorganic Chemistry, University of Oxford, Oxford, United Kingdom
Peter Dobson Department of Engineering Science, University of Oxford, Oxford, United Kingdom
John Hutchison Department of Materials, University of Oxford, Oxford, United Kingdom (Received 17 September 1997; accepted 6 January 1998)
Semiconductor-metal co-colloids of CdSeyAu have been prepared by various synthetic pathways. Their microstructure, including that of Au–CdSe(TOPO) co-colloid in a core-shell structure, has been examined by high resolution transmission electron microscopy (HRTEM) and found to be well defined within the 10 nm size range. The optical absorption spectra of the colloids and of various synthesis stages have been obtained.
I. INTRODUCTION
Nanoparticulate semiconductors or “quantum dots” have interesting optical properties including size tunable band gaps and enhanced nonlinear optical properties that can potentially be harnessed for applications such as flat panel displays, optical switches, and lasers.1,2 The composite structure of the semiconductor quantum dots with either a “coating” or “core” of certain metals confers novel optical properties due to the local field effect of the metal and can further enhance the nonlinear optical properties of the semiconductor nanoparticle giving, for example, intrinsic optical bistability for such core-shell structures.3,4 To date there have been few reports of core-shell metal-semiconductor composite colloids. Honma et al. claimed to have synthesized silver colloidal particles surrounded by a shell of CdS,5 via an aqueous route, but evidence for this was not convincing. On repeating their work, we have reproduced their optical absorption spectra, but high resolution transmission electron microscopy (HRTEM) studies (Fig. 1) of the colloid has revealed evidence only for discrete Ag and Ag2 S particles and little evidence for CdS. This result highlights the problems of generating such particles in that the desired “end result” is often forbidden on chemical thermodynamic grounds. In this particular case it is not surprising that under these conditions Ag2 S forms with the dissolution of CdS because the solubility product of the former is an order of magnitude smaller. Later work by the same group, Zhou et al. has shown that heteroparticles containing a core of Au2 S surrounded by a shell of Au could be synthesized.6 The preparative route involves the synthesis of Au2 S and their subsequent J. Mater. Res., Vol. 13, No. 4, Apr 1998
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reduction over tens of minutes to generate a skin of Au around the Au2 S core. However, this route is difficult to control, since it is hard to avoid complete reduction of the original sulphide colloid to Au unless large (. 40 nm) Au2 S particles are used. This work represents an important landmark in the
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