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N3.5.1

New Zinc and Cadmium Chalcogenide Structured Nanoparticles S. M. Daniels,a P. O’Briena* N. L. Picketta and J. M. Smithb a

Department of Chemistry and The Manchester Materials Science Centre, University of

Manchester, UK. E-mail: [email protected] b

School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, UK.

Abstract The growth of 2D quantum dot quantum well (QDQW) nanocrystals in which a shell of CdSe is grown onto cores of ZnS and capped with a further shell of ZnS is reported. The red shift in the interband absorption and photoluminescence spectrum of the quantum dots (QDs) indicates relocalization of carriers from confinement in the ZnS core to the CdSe shell. The change in interband absorption energy utilizing the effective mass approximation with spherical symmetry was modeled, enabling an estimate of the CdSe thicknesses grown. 1.8nm and 2.5nm ZnS cores were selected as the base on which to grow the CdSe shells. Despite the 12% lattice mismatch between ZnS and CdSe, our results indicate that we have successfully grown CdSe shells approximately three monolayers thick onto 2.5nm ZnS core. Anything beyond a single monolayer of CdSe could not be grown onto the 1.8nm core, although some success was observed by incorporating a CdS graded layer in-between the ZnS core and CdSe shell. The effect of ZnS shell thickness on photoluminescence efficiency has also been studied with optimum shell thicknesses showing quantum yields as high as 52%. Growth of these nanocrystals represents a significant step in the development of strained nanocrystalline heterostructures.

Introduction Compound semiconductor nanoparticles have been the subject of intense research over the last ten years, generated by their novel optical and electronic properties. The trend in nanomaterial synthesis, amongst others, has mainly been confined to II-VI semiconductor materials known as quantum dots where quantum-confinement effects are large and in the visible part of the spectrum. They are fundamentally important because they possess properties found in-between that of molecular and bulk states of matter. Greater understanding of the physical properties of these materials can be linked to developments in solvothermal preparative routes that allow monodisperse quantum dots to be easily prepared with high optical quality and a high degree of crystallinity.1 Zero-dimensional (0D) and one-dimensional (1D) structures have been grown and studied extensively. Colloidal 2D systems, however, are rare (thin films on the other hand have been relatively well studied). Most of the properties of 2D semiconductors have yet to be studied. The first and hence most studied system were of CdS/HgS/CdS by Eychmüller et al,2-4 grown by the substitution of Cd for Hg on the core surface to deposit one monolayer of HgS. The CdS/HgS/CdS emitted a red band-edge emission originating from the HgS layer. The poor particle quality prevented detailed study of the expected 2D semiconductor properties, such as thickness dependent absorption an