Photoconductivity of ZnO Nanowires Decorated with CdSe Quantum Dots

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Photoconductivity of ZnO Nanowires Decorated with CdSe Quantum Dots Dongchao Hou, Apurba Dev, Kristian Frank, Andreas Rosenauer and Tobias Voss Institute of Solid State Physics, University of Bremen, D-28359 Bremen, Germany ABSTRACT A hybrid assembly was built using ZnO nanowire (NW) arrays and colloidal CdSe quantum dots (QDs) stabilized by 3-mercaptopropionic acid (MPA). The QDs were chemically linked to the nanowires through the bonds formed between the outgoing carboxyl groups of the QD stabilizers and the zinc ions on the nanowire surface. An efficient clustering attachment of the QDs was achieved via partial removal of the stabilizers of the QDs. The photoconductivity of the NW/QD assembly was investigated by selective excitation of the CdSe QDs. Oxygen desorption from the nanowire surface enhances the photoconductivity and a model involving electron transfer between the QDs and the nanowires is proposed to explain the experimental results. INTRODUCTION Nanowire/quantum-dot (NW/QD) hybrid structures have been widely studied for potential applications in photovoltaic devices [1-4]. The combination of the tunable band gaps of the semiconductor QDs and the direct electrical pathway for charge carriers provided by the nanowires is expected to enhance the conversion efficiency. However, some practical problems, such as the low coverage of nanowires with QDs as well as the limited understanding about the charge transfer between the constituents still obstruct the improvement of the performance [1, 2, 5]. In the present work, a ZnO-NW/CdSe-QD hybrid assembly was built using ZnO nanowire arrays and MPA capped colloidal CdSe QDs. The photoconductivity of the assembly was investigated by selective excitation of the CdSe QDs. EXPERIMENTAL DETAILS The MPA capped CdSe QDs were synthesized in aqueous phase according to a method described in [6]. The reaction system was refluxed at 100 °C under nitrogen atmosphere. After synthesis, the CdSe QDs were purified by adding 2-propanol into the solution to precipitate the QDs, followed by centrifugation. The obtained QD precipitation was washed with ethanol twice and then redispersed in water at pH 9.0 adjusted by tetraethylammonium hydroxide. ZnO nanowire arrays were grown on FTO glass by a hydrothermal method. 1.3 g zinc acetate dihydrate was dissolved in 10 ml ethanol. Diethanolamine was added under stirring until the solution became transparent. The solution was then spin-coated onto the FTO glass and annealed in air at 500 °C for 1h. This coating-annealing process was repeated twice to obtain ZnO seeds on the glass sheets for the nanowire growth. 7.2 g potassium hydroxide and 4.5 g zinc nitrate hexahydrate were dissolved in 30 ml water, respectively and then mixed together. The glass sheets with ZnO seeds were put inside the mixture, and this reaction system was heated to 80 °C and kept for 3h. After synthesis, ZnO nanowire arrays were obtained on the FTO glass surface.

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To build the NW/QD assembly, first ZnO nanowires were annealed in air at 500 °C for 1 hour to remove t