Electrophoretic Deposition of CdSe Nanocrystal Films on Conducting Electrodes
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Electrophoretic Deposition of CdSe Nanocrystal Films on Conducting Electrodes Mohammad A. Islama, Yuqi Xiab, Benjamin J. Kraines, and Irving P. Hermana a Department of Applied Physics and Applied Mathematics b Department of Physics Materials Research Science and Engineering Center Columbia University, New York, NY 10027, U.S.A.
ABSTRACT A dc electric field is used to attract thermally charged CdSe nanocrystals in solution to rapidly form large-area, micron-thick films of equal thickness on both electrodes. A pair of Auon-Si or conducting ITO-on-glass electrodes was submerged in the nanoparticle solution and a dc voltage was applied in a dark room. Uniform, robust, very smooth, and apparently identical films formed on both electrodes. Photoluminescence and absorption of the films showed that they are indeed made of dense arrays of individual nanocrystals. The deposition implies there are both positively and negatively thermally charged dots in solution. These high quality dense arrays of the nanoparticles could be useful in several applications.
INTRODUCTION The formation of arrays and films of nanoparticles is important for exploring properties of high densities of these dots and for applications. Semiconductor [1], metal [2] and magnetic nanocrystals [3] have been self-assembled to make quantum dot superlattices. Superlattices made from nanoparticles enable the study of the evolution of collective effects from individual dot properties. Semiconductor nanoparticles are important for their optical properties, which can be used, for example, in light emitting diodes (LEDs) [4] and in biological labeling [5]. Choosing a surface for the nanocrystal film deposition is an important consideration. For example, for electronic devices it might be necessary to deposit on unpatterned or patterned electrodes, such as Au-on-Si surfaces; whereas for LEDs it may be necessary for the surface to be conducting and transparent, such as ITO-on-glass. Currently, films of nanoparticles are formed mostly by dry casting or spin coating. These films are not very uniform. Both methods depend on the particle-particle and particle-substrate interactions as a drop of the nanoparticle solution is left to dry on a substrate. However, most suitable solvents are extremely volatile and rapid dewetting of these solvents significantly undermines the long range ordering and film quality. Consequently, most of the nanocrystal superlattices formed using these methods have domain sizes less than a few hundred nanometers [6-9]. Also, such dry cast and spun on films are unpatterned. For many applications, films of controlled thickness over controlled regions may be needed. We report here on the electrophoretic deposition of CdSe nanocrystals with diameter in the range of 2.8 nm to 4.1 nm on Au-on-Si electrodes and on ITO-on-glass electrodes to form very high quality films. This continues the work first reported by the authors in Ref. [10]. The nanocrystals were found to deposit on both the anode and the cathode, unlike in conventional electrophore
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