Quantum Confinement in Coated Semiconductor Nano-Particles
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popular in various fields. Nanoparticles is a term applied to the clusters of atoms or molecules of metals and semiconductors, ranging in size from lnm to almost several times lOnm. Dr. LE.Brus and co-workers at AT & T Bell Laboratory synthesized ZnSe, ZnS, CdSe, and PbS by the colloidal methods 1 and observed the blue shift of the bandgap in these semiconductor nanometer-size particles according to their size. The materials were investigated by a variety of optical spectroacopies; including optical absorption, Raman, photoluminescence(PL), hole-burning, etc. They also developed the molecular orbit method to deal with semiconductor nanoparticles1. Some select results also deserve mention; Drs. N. Herron and Y Wang reported the elegant use of small zeolite cavities as hosts for (CdS)4 clusters 2 , and Dr.J.H.Fendlerand colleagues discovered that CdS particles in the nanometer-size water pools of inverse micelle media were unusually stable against flocculation 3 . Now, a new challenge for scientists is the synthesis of composite nano-phase materials. Such materials have structures include a sandwich , a core-shell, a football for C60 and a nanotube of carbon 4 . In the composite nano-phase materials, Dr L. E. Brus and his colleagues synthesized ZnS/CdSe semiconductor coated nanoparticles modifying the surface by sequential addition of S2271 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
and Zn2 + in an equivalent CdSe colloidal system 5 in 1990. Dr. A. Henglein and his colleagues also synthesized composite particles ( a sandwich structure ) CdS-AgI 6 , CdS-TiO 2 6 , CdS-ZnO, CdS-
Ag2 S, Cd3 P2 -ZnO, and AgI-Ag 2 S7 by the surface modification. Dr. A. Eychmuller and his colleagues synthesized the CdS/HgS semiconductor coated nanoparticles (SCN) by the same way (ion exchange ) as ours and reported the PL peak's shift as the core-shell ratio 8 in 1993. Recently, Dr. A. Eychmuller and his colleagues have synthesized the three layered semiconductor coated nanoparticles CdS/HgS/CdS9 . In this report, we will discuss the infrared photoluminescence of the Cd-S ( VCdS ) composite vacancy in CdS nanoparticlcs and coated CdS/PbS nanoparticlcs. The infrared PL of VCdS in CdS nanoparticles appears at about 1100nm 10' 1 1. The PL of VCd.S in CdS/PbS nanoparticles shifts to longer wavelengths as the thickness of PbS shell layer increases. The shift and intensity variation of the PL peak from VCdS in CdS/PbS can be explained by a quantum-confinement model of coated nanoparticles; the cxcited cncrgy gap ( HOMO-LUMO ) is not only dcpcndent on the sizc of the particles, but also strongly dependent on the core/shell ratio of the coated nanoparticles. Furthermore the infrared PL energy gap ( LUMO- VCdS ) is also strongly dependent on the core/shell ratio. II Experiment We synthesize the CdS/PbS SCN by the ion exchange method which has been discussed in detail in our previous paper 12 , and investigated the optical properties 12 . 10cc Na 2 S (4mM) aqueous solution is injected into the mixture solution of 10cc CdC
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