A Comparison of the Photophysical Properties of Thiolate-Capped CdS Quantum Dots with Thiolate-Capped CdS Molecular Clus
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and accepting groups have on the properties of CdS nanoparticles, CdS-R (where R= the para-substituent on the thiolate). The results are compared to those obtained for crystallographically characterized anionic molecular clusters [4] [CdloS4 (SPh) 1 6]4 - (A) and [CdloS4(I) 4(SC6 H4 Me)12] 4 - (B). We reasoned that the spatially separated electron-hole pair would be sensitive to the electronic nature of the capping ligand of the nanoparticles and would also similarly effect the smaller Cdlo clusters. This supposition was not unwarranted, as the electronic nature of surface adsorbates has been shown to tremendously influence the optical properties of CdSe single crystals [5]. EXPERIMENTAL SECTION Reanents: Cd(N0 3)2 .4(H 2 0) (J. T. Baker Chemical Co.), anhydrous Na 2 S (Alfa) and sublimated elemental sulfur (Mallinckrodt, U.S.P.) were used as received. Thiophenol, 4-nitrothiophenol, 4-aminothiophenol, 4-methoxythiophenol and 4-chlorothiophenol were obtained from Aldrich. Reagent grade MeOH, HPLC grade MeCN and DMSO were purchased from Fisher
247 Mat. Res. Soc. Symp. Proc. Vol. 571 * 2000 Materials Research Society
and used without further purification. Water for all experiments was deionized and purified
prior to use (Continental Water Systems). Synthesis: A modified procedure [6] was used to prepare thiophenol-passivated CdS nanoparticles. 1.54 g of Cd(N0 3)2 .4(H 2 0) were dissolved in MeOH/MeCN (4:1) to obtain a 0.1 M stock solution (A). A 0.05 M stock solution (B) was prepared by dissolving 0.0975 g of Na2 S in MeOH/ H 20 (1:1). Stock solutions (C) were made by dissolving each of the thiophenols (256.0 1tL C6 HASH, 0.3130 g 4-NH 2-C6 H4 SH, 307.5 ýtL 4-MeO-C 6 H4 SH, 0.3616 g 4-CI-C 6 H4 SH or 0.3878 g 4-NO 2-C6 H4 SH) in 25 mL MeCN to give final thiol concentrations of 0.1 M. 25 mL of (B) and 25 mL of (C) were then mixed together at 25 'C as N2 was bubbled through the system, followed by 50 mL of (A) with stirring. A pale yellow
precipitate formed immediately in all cases, the solid was collected by filtration, washed with MeOH, and dried under dynamic vacuum. The synthesis of the Cdlo clusters has been reported previously [4]. Briefly, a methanolic solution of Cd(N0 3 )2 .4(H 2 0) was added to a stirred solution of thiolate and Et 3N in MeOH at room temperature followed by Et 4NX (X = Cl, Br, or I) in the same solvent. After mixing, the solvent was removed en vacuo. The residue was dissolved in MeCN and powdered sulfur was added. The product crystallizes out of solution upon slow addition of acetone. Instrumentation: Absorption spectra were acquired on a Perkin-Elmer Lambda 14 ultraviolet-visible spectrophotometer. Steady-state luminescence spectra of the nanoparticles were obtained at 4 nm resolution with a SLM-Aminco 8100 spectrofluorometer, with excitation at 350 nm (390 nm excitation for the Cdj 0 cluster, B). Emission lifetimes were obtained by a previously reported experimental procedure [7]. The shortest lifetime reliably deconvoluted from our system is -2 ns. Transmission electron microscopy was
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