Assembly of CdSe/CdS Quantum Dots on Au Surfaces for Photoreception
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Assembly of CdSe/CdS Quantum Dots on Au Surfaces for Photoreception Jing Tang1, Henrik Birkedal2, Eric W. McFarland3 and Galen D. Stucky1, 2 1
Materials Department, 2Department of Chemistry and Biochemistry, 3Department of Chemical Engineering, University of California, Santa Barbara, CA, 93106, USA. ABSTRACT CdSe/CdS core/shell quantum dots have been synthesized and assembled onto prefunctionalized gold surfaces by either hydrogen bonding or covalent bonds through different functional groups. Control of the conditions during the deposition process allows producing a high coverage of quantum dots via molecular linkages. The quantum-dot surface is highly photoactive and is used in a surface sensitized Schottky barrier photovoltaic structure as the photoreception component. Atomic force microscopy (AFM) and X-ray photoelectron Spectroscopy (XPS) are used to characterize and confirm the morphology and linkage of the assemblies on Au surfaces. The electron transfer from the quantum-dot layer to the Schottky barrier device is examined by measuring the current-voltage (IV) curve of such photovoltaic devices under simulated sun light. INTRODUCTION The self-assembly of semiconductor quantum dots (QDs) onto conducting substrates has been studied intensively due to the potential role for such assemblies in the fabrication of optoelectronic devices and chemical sensors. [1-4] QDs may be used as an alternative to the organic dyes used as photoreceptors in surface sensitized Schottky barrier photovoltaic structures. [2-3] The uniquely tunable optical properties of QDs and their high resistance to photo-bleaching will offer a QD-based device advantages over the corresponding dye-system. In order to achieve efficient light absorption and charge transfer, high coverage and strong binding of the QDs to the substrates are crucial factors and will affect the final device performance. The QD device structures are typically fabricated utilizing a bifunctional self-assembled monolayer (SAM) on the conducting surface as a molecular linker. By selecting appropriate linker functional groups, semiconductor QDs can be deposited onto the substrate either covalently or by non-covalent interactions such as hydrogen bonding or electrostatic attraction. [5] Studies on the electron transfer over such molecular linker have been reported for various systems, including different molecular types, structures and lengths. [1] Here, we present two readily controlled methods to assemble water-soluble citrate-capped CdSe/CdS core/shell QDs onto Au conducting substrates. In the first method, Au surfaces were pre-functionalized by a thiolcarboxylic acid SAM. The subsequent assembly of QDs was afforded by hydrogen bonding formed between the carboxylic capping agents on the QD surface and the SAM layer. In the second method, the Au surfaces were instead pre-functionalized by a benzenedithiol SAM, leading to a covalent linkage in the ensuing assembly process. These procedures are found to be applicable for large-area device fabrication with a high and h
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