Cathodoluminescence of CdSe/ZnS Quantum Dot Composites
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COMPOSITES J.RODRIGUEZ-VIEJO*, J.R.HEINEs, B.O.DABBOUSI#, H.MATTOUSSIs, J.MICHELS, M.G.BAWENDI#, K.F.JENSEN*$. Massachusetts Institute of Technology, Departments of Chemistry# , Chemical Engineering* and Materials Science and Engineering$, Cambridge, Massachusetts 02139.
ABSTRACT We report cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals of 33 and 42 A diameter imbedded in a ZnS matrix. The thin film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and on the voltage and current density applied. Electron beam irradiation caused a decrease of the luminescence which may be explained by electron ionization of the quantum dots. Quenching of the cathodoluminescence at low temperatures is attributed to a more efficient trapping of the ionized electrons in deep traps inside the ZnS matrix.
INTRODUCTION Semiconductor crystallites which are small compared to the exciton Bohr radius exhibit unique optical properties due to confinement of the electronic excitations [1]. The optical properties, such as absorption and emission, can be tuned across the visible by changing the size of the nanocrystals. Highly monodisperse CdSe quantum dots organically passivated with trioctylphosphines have quantum yields between 10 and 15% [2]. Solution chemistry growth of a thin ZnS overlayer on the CdSe nanoparticles raises the quantum yield to values between 30 and 50% for particles in the size range from 20 to 60 A in diameter [3,4]. The incorporation of high luminescence quantum dots into an inorganic matrix of higher bandgap, provides a wide spectral window for exploring the quantum size effect and the ability Af electrical integration of the composites into optoelectronic devices. Danek et al. [5] have already shown the feasibility of such composites by Electrospray Organometallic Chemical Vapor Deposition (ES-OMCVD). Films of CdSe/ZnSe have been grown by this method, but their luminescence characteristics were too low to be useful in optoelectronic devices [6]. In this work, we demonstrate excellent optical properties of quantum dot composites formed by ZnS overcoated CdSe quantum dots embedded in a ZnS matrix grown by OMCVD. In particular, we show cathodoluminescence from these composites and characterize the behavior of :he CL intensity during irradiation and at low temperatures.
365 Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
EXPERIMENTAL Nearly monodisperse, highly luminescent ZnS overcoated CdSe quantum dots were prepared as described elsewhere [2]. The quantum yield of the 33 and 42 A diameter tri-n-octylphosphine oxide/tri-n-octylphosphine (TOPO/TOP) cap nanocrystals used in this work was 45%. Pyridine is a solvent compatible with the OMCVD process of ZnS. Therefore, the TOPO/TOP cap was exchanged with pyridine by floculation with hexane and redispersion in pyridine. Afterwards, the dots were mixed with acetonitrile (1:2) t
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