Solution Precipitation of CdSe Quantum Dots
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SOLUTION PRECIPITATION OF CdSe QUANTUM DOTS CHERIE R. KAGAN AND MICHAEL J. CIMA Massachusetts Institute of Technology, Ceramics Processing Research Laboratory, 77 Massachusetts Avenue, Cambridge, MA 02139 ABSTRACT Synthesis of CdSe quantum dots with a high degree of monodispersity is achieved by nucleation from a supersaturated solution followed by growth to the desired particle size. The effects of temperature on the kinetic mechanisms of nucleation and growth were observed. A reaction vessel equipped with a low thermal mass internal heating element enabled controlled ramping of the solution temperature during the reaction. Nanocrystallite diameter is determined by the reaction time and the solution temperature during particle growth. A method was developed to fabricate -l gm thick glass films containing -3 vol% CdSe quantum dots. A sol was prepared by mixing a silica organosol with a nanocrystallite dispersion of CdSe and was applied to amorphous quartz substrates by spin-coating. The sols were dried at elevated temperatures in a nitrogen atmosphere. Optical absorbance and fluorescence measurements of the glass film were used to characterize the optical properties of the embedded nanocrystallites. Comparison of the excitonic absorbance of the quantum dot dispersion and the doped glass film shows that particle monodispersity is maintained upon incorporation into the dielectric matrix. Stokes shifts in the band-to-band fluorescence relative to the film absorbance were measured. Shifts in the wavelength of the excitonic absorbance and fluorescence were observed upon incorporation of the quantum dots into the glass film and upon heat treating the glass film to elevated temperatures. INTRODUCTION Quantum size effects in semiconductor quantum dots alter the bulk electronic structure and optical properties of these materials. Effects of quantum confinement are observed as blue shifts in the absorbance and fluorescence spectra, associated with an increase in the effective energy gap, as the particle radius is decreased to sizes comparable to the free carrier and excitonic bohr radii 1 . The size dependent optical properties of semiconductor nanocrystallites enable the fundamental study of matter as it evolves from a molecular solid to a bulk material. Quantum size effects also enhance the nonlinear optical properties of the clusters. Incorporation of semiconductor quantum dots in a low dielectric glass matrix demonstrate large third order resonant and nonresonant nonlinearities and fast response times 2 ,3. Quantum confined nanocrystallites embedded in thin film glass matrices are promising composites for ultrafast photonic switching applications. Particle monodispersity is characterized by the size distribution, stoichiometry, structural quality, surface coordination, and shape of the crystallites. Effects of polydispersity wash out the discrete structure observed in optical measurements, masking the effects of quantum confinement on allowed energy transitions. Preparation of glasses containing particles with a high deg
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