Composite Semiconductor Nanoparticles
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Composite Semiconductor Nanoparticles H.S.Zhou, I. Honma, H.Komiyama and *J.W.Haus The University of Tokyo, Department of Chemical Engineering, Tokyo, Japan; The University of Tokyo, *RCAST, Tokyo, Japan
Abstract Nanometer size semiconductor particles coated with another semiconductor can exhibit unusual and interesting phenomena associated with the redistribution of the electron and hole wavefunctions. Using the band offsets and effective-masses, the band gap as well as the wavefunctions can be altered by changing the core radius of the particles. CdS/PbS coated semiconductor nanoparticles are synthesized by ion displacing method and experimental results are discussed along with the theoretical calculation. Optical absorption as well as TEM, Electron diffraction results provides for the evidence that the particles are coated. Introduction Coated particles offer a new challenge to synthesis, but they also provide new insights in the quantum confinement of carriers. Interest in these materials is partially motivated by the development of optical properties that mimic the superior performance characteristics of atoms or molecules. Control over the coating thickness gives a further engineering degree of freedom to elucidate the underlying physics of these structures; by changing the thickness of the shell and the particle radius, the overlap of the wavefunctions and the band gap can be changed. This is distinct from the change in the wavefunctions overlap caused by the finite height of the host material; in coated particles, the outer material can be chosen to have a smaller band gap, thus favoring the shift of the wavefunction maximum from the particle center toward the outer boundary. In coated particles, we will show here for example, that the carriers can spatially separate in the materials due to the band offset and mass differences Several types of direct band gap materials will be among the carriers. selected for numerical computations and outer discussion will be based on those results, while, recently, some research works on CdS/AgI1, CdS/TiO 2 1, ZnS/CdSe 2 and ZnSe/CdSe 3 composite or coated nanoparticles have been reported; here our notation denotes the (core/shell) materials. In this paper we Mat. Res. Soc. Symp. Proc. Vol. 283. @1993 Materials Research Society
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explore the growth of one semiconductor material (PbS) on a seed of another (CdS) to form coated semiconductor nanoparticle (CSN). We will provide evidence that the CSN can be fabricated by a colloidal technique. The material is characterized by optical absorption, TEM and electron diffraction techniques and we can selectively alter the electronic properties by varying the growth technique. All the evidences indicate that our particles are coated and not, for instance, separately particles.
Theoretical calculation The theoretical analysis starts with the effective mass approximation with single bands for both the conduction and valence band carriers. The single carrier envelope wavefunctions are used in each band. We restrict our analysis
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