Optical Properties of CdSe Nanoparticle Assemblies
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Optical Properties of CdSe Nanoparticle Assemblies F. Rafael Leon1,2, Natalia Zaitseva1, Daniele Gerion1, Thomas Huser1, Denise Krol1,2 1 Lawrence Livermore National Laboratory, 7000 East Avenue Livermore, CA 94550, U.S.A. 2 University of California at Davis 3001 EU III, One Shields Avenue Davis, CA 95616, U.S.A. ABSTRACT We report on three-dimensional fluorescence imaging of micron-size faceted crystals precipitated from solutions of CdSe nanocrystals. Such crystals have previously been suggested to be superlattices of CdSe quantum dots [1,2]. Possible applications for these materials include their use in optical and optoelectronic devices. The micron-size crystals were grown by slow evaporation from toluene solutions of CdSe nanocrystals in the range of 3-6 nm, produced by traditional wet-chemistry techniques. By using a confocal microscope with laser illumination, three-dimensional raster-scanning and synchronized hyper-spectral detection, we have generated spatial profiles of the fluorescence emission intensity and spectrum. The fluorescence data of the micro-crystals were compared with spectra of individual nanocrystals obtained from the same solution. The results do not support the assertion that these microcrystals consist of CdSe superlattices. INTRODUCTION Assemblies of CdSe nanocrystals (quantum dots) have potential applications in new optical, electronic and optoelectronic devices. The multipole interaction between neighboring dots suggests that they may prove useful in computational and data storage devices, and their tunable bandgap could allow for the construction of photodiodes and light-emitting diodes in the visible range [1]. The fluorescence emission wavelengths of the quantum dots are tunable by varying their size. Because the optical properties of the CdSe semiconductor nanoparticles strongly depend on their size, surface structure, and interactions with their environment, it is important to assess how they behave when closely-packed into micrometer-scale assemblies. It has recently been suggested [2] that microcrystals grown from solutions of CdSe nanocrystals consist of crystalline superlattices of CdSe quantum dots. For individual quantum dots to condense into an ordered lattice they must be similar enough to bond together into a regular pattern. For this reason, it was speculated that an ordered superlattice of nanocrystals would consist of similar nanocrystals and thus be more homogeneous than the precursor solution, which typically contains a distribution of nanocrystals of different sizes. In the present study we have used scanning confocal fluorescence microscopy to characterize micron-size crystals grown from solutions of CdSe nanocrystals. The fluorescence spectra of these crystals are compared to those of individual nanocrystals obtained from the same solution in order to determine whether the fluorescence of the crystals is consistent with an
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ordered superlattice of CdSe nanocrystals, and if so, how uniform the nanocrystal size distribution is inside the micro-c
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