Optical Properties and Local Structure of Cu(II) Dopant in ZnSe Nanoparticles
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Optical Properties and Local Structure of Cu(II) Dopant in ZnSe Nanoparticles Thaddeus J. Norman Jr,1 Donny Magana,1 Frank Bridges2 and Jin Z. Zhang1 Department of Chemistry, University of California Santa Cruz, Santa Cruz, CA 95064, U.S.A. 2 Department of Physics, University of California Santa Cruz, Santa Cruz, CA 95064, U.S.A. 1
ABSTRACT Cu(II) doped ZnSe nanoparticles were synthesized using molecular cluster precursors. The Cu(II) dopant had the effect of quenching the ZnSe band edge emission, yet only weak emission from Cu(II) centers was observed. An X-ray Absorption Fine Structure (XAFS) experiment was performed on the Cu(II) doped ZnSe nanoparticles. It was determined that the Cu(II) occupies a tetrahedral site in the lattice in an environment similar to that of Cu in bulk Cu2Se, and is likely to have substituted for Zn(II) in the lattice. INTRODUCTION Semiconductor nanomaterials have attracted much attention recently because of their many novel and improved properties compared to corresponding bulk materials. The properties are derived from the spatial confinement of electrons and huge surface-tovolume ratio1-5. These new properties may be exploited in emerging technologies including advanced sensors, large area displays, solar-cells, detectors, lasers, and other opto-electronic devices6,7. For example, the strong and size-dependent optical emission of many semiconductor nanoparticles makes them promising candidates for use as fluorescent tags in the study of biological systems. The optical properties are strongly influenced by the surface and thus chemical modification of the surface has been used to control and improve the optical properties. An alternative and powerful technique for controlling and modifying optical properties is the doping of semiconductor nanoparticles with metal, and sometimes non-metal, ions. This often leads to the creation of new emission properties due to the dopant and alteration of the optical properties of the host semiconductor. Doped semiconductors with dimension on the micron or large scale constitute a large portion of commercial phosphor materials. Doped semiconductors on the nanometer scales present an attractive system for studying effects of doping in spatially confined materials. They are also useful for understanding the effects of trap states since the dopant can be considered as an impurity introduced intentionally to create electronic states within the bandgap, termed trap states. Since most dopant ions have well-defined, characteristic emissions, the emission spectrum can be used conveniently as a signature to study the influence of trap states on the materials properties. One of the most studied doped semiconductor nanoparticle systems is Mn(II) doped ZnS8-12. Upon excitation of the ZnS nanoparticle energy is transferred to the Mn(II) dopant, promoting the ion to an excited state. Relaxation of the Mn(II) ion results in the production of yellow emission centered at ~585 nm. The electronic relaxation processes or emission lifetimes do not appear to be sub
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