Preparation of II-VI Quantum Dot Composites by Electrospray Omcvd
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M. DANEK¶1, K. F. JENSEN*, C. B. MURRAY¶, M. G. BAWENDIL Massachusetts Institute of Technology, Departments of Chemistry ¶, and Chemical Engineering t, Cambridge, Massachusetts 02139
Abstract A novel technique combining electrospray and organometallic chemical vapor deposition (OMCVD) has been developed for synthesis of VI-II quantum dot composites. CdSe nanocrystallites dispersed in a mixture of pyridine and acetonitrile were electrosprayed into the gas-phase in the growth zone of an OMCVD reactor and then incorporated into a ZnSe matrix grown by OMCVD. Hydrogen selenide and diethylzinc were used to deposit the ZnSe matrix at temperatures of 150 - 240'C. The ZnSe/CdSe quantum dot composites were characterized by visible absorption spectroscopy, room temperature photoluminescence and Auger electron spectroscopy. The optical properties of the composites are determined by the size of the original CdSe nanocrystallites.
INTRODUCTION Composites incorporating semiconductor nanocrystallites (quantum dots) in a wide-band gap semiconductor matrix are attractive materials for optoelectronic devices operating in the visible spectral region. The optical properties, such as position of the absorption edge and emission maximum, can be tuned across a wide spectral region by changing nanocrystallite size [1-3]. In addition, the presence of nanocrystallites may give rise to nonlinear optical phenomena [1-3]. The function of the semiconductor matrix is passivation of the nanocrystallite surface and Moreover, in principle the stabilization of the nanocrystallite dispersion. semiconductor matrix provides an avenue for electrical integration of the composites into optoelectronic devices. In the present work we describe new quantum dot (QD) composites consisting of CdSe nanocrystallites in a ZnSe matrix. The band gap of the CdSe nanocrystallites may be varied from -3 eV to -1.8 eV by adjusting their size from 12 A to 115 A [4]. The ZnSe matrix is ideally suited to passivate the nanocrystallite surface and provides a wide spectral window for exploitation of the expected quantum size effect, because of its large band gap (approximately 2.7 eV at room temperature) and the compatibility of the ZnSe and CdSe chemistries. We have developed a novel electrospray organometallic chemical vapor deposition (ES-OMCVD) technique to synthesize ZnSe/CdSe QD composites. This method relies on the electrohydrodynamic atomization of a nanocrystallite solution inside a OMCVD reactor. The nanocrystallites are transferred from liquid to gas-phase through the breakup of charged aerosol particles. In this respect the 275 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
performance of the electrospray technique is closely related to an electrospray ionization source used in mass spectroscopy for large and nonvolatile molecules [5]. The electrosprayed nanocrystallites are subsequently incorporated into a ZnSe matrix during OMCVD growth. EXPERIMENTAL
The schematic of the ES-OMCVD reactor used for the growth of ZnSe/CdSe QD composites is
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