Efficient Energy Transfer Between CdS Quantum Dots in Layer-by-layer Self-assembled Films
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1208-O09-13
Efficient Energy Transfer Between CdS Quantum Dots in Layer-by-Layer Self-Assembled Films Kunio Shimura, DaeGwi Kim, and Masaaki Nakayama Department of Applied Physics, Osaka City University 3-3-138, Sugimoto, Osaka 558-8585, Japan ABSTRACT We have investigated efficient energy transfer (ET) between CdS quantum dots (QDs) measuring photoluminescence dynamics in layer-by-layer (LBL) self-assembled films. The assembly of negatively charged colloidal QDs and positively charged polyelectrolytes results in QD/polymer multilayers. Furthermore, to reveal how the ET rate depends on the distance between CdS QDs, we fabricated bilayer structures consisting of differently sized CdS QDs. It is experimentally verified that ET between the donor and acceptor QDs is conclusively dominated by the dipole-dipole interaction. INTRODUCTION Optical properties of semiconductor quantum dots (QDs) have been intensively investigated in the past two decades. Randomly dispersed QDs have been major target in most of the studies so far. The dynamical process of resonant energy transfer (ET) between CdSe and CdTe QDs was reported in recent years [1-4]. This opened up a new aspect in photophysics of semiconductor QDs and stimulated studies on QD-based ET processes employing QDs as energy donors in QD-bioconjugate system and QD-organic dye system as well as ET between QDs. In this work, we have investigated efficient ET between CdS QDs measuring photoluminescence dynamics in layer-by-layer (LBL) self-assembled films. EXPERIMENT The colloidal CdS QDs were prepared by injecting a H2S gas into an aqueous solution containing Cd(ClO4)2. Sodium hexametaphosphate (HMP) was used as a disperse agent. In PL spectra of CdS QDs prepared by the colloidal method, a surface-defect-related PL band has been usually observed as a main PL band. A surface modification of QDs was performed to improve their PL properties. The QD surface was modified by the addition of Cd(ClO4)2 after adjusting pH of solutions to an alkaline region: This process leads to formation of a Cd(OH)2 layer on the surface of QDs [5-6]. A photoetching treatment was performed to prepare CdS QDs with smaller size by irradiating a monochromatic light to the sample solution [7-9]. For the photoetching treatment of the QDs, a 500-W Xe lamp was used as a light source. Monochromatic light (466 nm) was obtained by using interference filters: The full-width at half of the intensity maximum of the monochromatic light was ~10 nm. After the photoetching treatment, the surface modification with a Cd(OH)2 layer was applied. The substrates of quartz used for the LBL deposition were cleaned by immersion in fresh piranha solution for 20 min. Then, the substrates were rinsed with water, and then used
immediately after cleaning. The LBL assembly was performed by sequential dipping of the quartz substrates in aqueous solutions of positively charged poly(diallydimethylammonium) chloride (PDDA, Mw=100,000-200,00) and negatively charged colloidal QDs. We performed absorption measurements at room temperat
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