The Formation of Close-Packed Two-Dimensional CdSe Colloidal Films by Electric-Field-Induced Deposition
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ABSTRACT Monodispersed CdSe nanocrystals with average diameter of -5 nm were synthesized from SeSO3 2 and rich Cd 21 ionic reagents in aqueous solution. Two-dimensional (2D) close-packed colloidal CdSe films were formed on the electrode surface by electric-field induced deposition method. Transmission electron microscopy (TEM) images show that colloidal CdSe films exhibit different states, named "gas", "liquid" and "solid" states, which can be transformed by adjusting applied field (E) or deposition time (T). The experimental results demonstrate that the average nearest-neighbor distance of clusters on the electrode surface is inversely proportional to the ET product. INTRODUCTION Semiconductor nanocrystals, especially II-VI materials, such as CdSe, CdS, ZnS, etc. exhibit interesting size-dependent optical and electrical properties due to the quantum confinement effect of the carrier [1]. Over the past decade, much progress has been made in the synthesis and characterization of these semiconductor nanocrystallites. They can be fabricated by a variety of chemical methods such as colloidal solution [2], inverse micelle [3] and organometallic reagent [4]. If these nanoparticles could be assembled into ordered structures on larger length scales they may have some potential applications in electric, optical and optoelectronic devices. One convenient and economical method to assemble or deposit such materials in large scales under ambient conditions is electrophoretic deposition. Many structured colloidal nanoparticle systems formed by electrophoretic deposition have been reported, such as polystyrene bead [5] and gold colloid film [6]. However, as yet, semiconductor electrophoretic deposition systems have not been fully studied and the mechanism of two-dimensional (2D) deposition of these nanoparticles has not been clearly understood yet. In this paper we report the synthesis of CdSe nanocrystals from Cd2 + and SeS0 32 ionic reagents in aqueous solution and the formation of 2D colloid films of those CdSe nanocrystals by electrophoretic deposition. The different states of CdSe films can be transformed by adjusting the applied field (E) or deposition time (T). The relationship between average nearest-neighbor distance of clusters and the ET product is also discussed. EXPERIMENT Colloidal CdSe nanocrystals were prepared by chemical method in aqueous solution according to following reaction [7] 81 Mat. Res. Soc. Symp. Proc. Vol. 571 © 2000 Materials Research Society
Cd` + 2 OH
2 + SeS0 3
_>
CdSe + S042 + H20
(1)
First, a I L of lxi0 3M Cd 21 ion aqueous solution was made by dissolving CdCl 2 in deionized water. Under quick stirring, about 80 mL of lx10 2 M Na2SeSO 3 was then injected into the above solution. The Na 2SeSO 3 reagent was freshly prepared by dissolving Se powder in Na 2SO 3 solution under stirring and N 2 bubbling. We can control the average size of CdSe nanocrystals by changing the concentration of Cd 2' and SeS0 32- ions or the pH value of the solution. During the growth process, 0.02% gelatin was used
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