Substrate- and Time-Dependent Photoluminescence of Quantum Dots Inside the Ultrathin Polymer LbL Film Dmitry Zimnitsky, ² Chaoyang Jiang, ² Jun Xu, ‡ Zhiqun Lin, ‡ and Vladimir V. Tsukruk* ,² School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of Materials Science and Engineering, Iowa State UniVersity, Ames, Iowa 50011 ReceiVed December 20, 2006. In Final Form: January 22, 2007 The photoluminescence of CdSe/ZnS quantum dots (QDs) in different configurations at solid surfaces (glass, silicon, PDMS, and metals) is considered for three types of organization: QDs directly adsorbed on solid surfaces, separated from the solid surface by a nanoscale polymer film with different thickness, and encapsulated into a polymer film. The complete suppression of photoluminescence for QDs on conductive metal surfaces (copper, gold) indicated a strong quenching effect. The temporal variation of the photoluminescent intensity on other substrates (glass, silicon, and PDMS) can be tuned by placing the nanoscale (3-50 nm) LbL polymer film between QDs and the substrate. The photooxidation and photobleaching processes of QD nanoparticles in the vicinity of the solid surface can be tuned by proper selection of the substrate and the dielectric nanoscale polymer film placed between the substrate and QDs. Moreover, the encapsulation of QD nanoparticles into the polymer film resulted in a dramatic initial increase in the photoemission intensity due to the accelerated photooxidation process. The phenomenon of enhanced photoemission of QDs encapsulated into the ultrathin polymer film provides not only the opportunity for making flexible, ultrathin, QD-containing polymer films, transferable to any microfabricated substrate, but also improved light emitting properties. Introduction Semiconductor nanocrystals, or colloidal quantum dots (QDs), show unique size-dependent optical properties 1 and are currently of great interest for various prospective applications in opto- electronic, 2,3 photovoltaic 4 devices, optical amplifier media for telecommunication networks, 5 and for biolabeling. 6,7 The good photostability, high photoluminescence (PL) intensity, and a broad emission tunability make these QDs an excellent choice as novel chromophores. Assembling QDs at solid surfaces and interfaces is a critical stage required for their integration with solid-state devices. Moreover, processing of QDs in a combination with polymeric materials may allow the fabrication of flexible and thin luminescent materials in the form of films, fibers, and 3D items. Several fabrication techniques are widely used to make ultrathin organized nanocomposite films including spin casting, Langmuir-Blodgett (LB) deposition, and layer-by-layer (LbL) assembly. 8,9 One of the more successful approaches employed was the LbL assembly (especially spin-assisted LbL), which allows for controlled placement of various nanoparticles such as QDs between polymeric multilayers and precise adjustment of distance between the underlying surface and the array of nanoparticles confined within multilayered structures. 10-14 It is important to understand the effect of the supporting substrate in the emission properties of QD nanoparticles, because the photoemission can be quenched or enhanced by the substrate located in the close proximity to the QDs. As known, the decay of the quenching efficiency is highly sensitive to the distance, with the highest quenching occurring at 2-10 nm from the metal surfaces with quenching virtually disappearing for the distance higher than 10-20 nm. 15 According to the Fo ¨rster resonant energy transfer (FRET) theory, the energy transfer efficiency near a metal nanoparticle scales to the inverse sixth power of the distance if the nanoparticle is assumed to be a single dipole. 16,17 However, for certain combinations of metal surfaces and chromophores, a weak distance dependence (down to linear) with significant quenching extending beyond a 20 nm gap or even an increase in emission intensity can be observed. 18-21 As a step in the understanding of the QD behavior at different surfaces, Kotov and his co-workers studied the formation of QDs monolayers on silicon substrates modified with different polycations: (3-aminopropyl)-triethoxysilane (APTES), poly- ethylenimine (PEI), and poly(diallydimethylammonium) chloride (PDDA). 22 The marked difference in the structural characteristics of the QD aggregation such as an overall particle density and the variable surface distribution has been found for these surfaces. * Corresponding author. 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