Electroluminescence and Spectral Shift of CdS Nanoparticles on Si Wafer
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Electroluminescence and Spectral Shift of CdS Nanoparticles on Si Wafer Eih-Zhe Liang1 , Ching-Fuh Lin1 , Sheng-Ming Shih, and Wei-Fang Su* Institute of Materials Science and Engineering, National Taiwan University, Taipei, Taiwan, R.O.C. 1 Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei, Taiwan, R.O.C. Abstract Preparation of CdS nanoparticles, devices fabrication, and electroluminescence properties at room temperature of CdS nanoparticles on silicon substrates are reported. A spectral shift of 86-meV of free exciton transition was observed that was due to the passivation of p-hydroxyl thiophenol molecules around nanoparticles. Controlled process conditions such as heat treatment and/or with oxygen-rich environment are experimented and found to have significant influences on emission spectra. Radiative recombination corresponding to oxygen-impurity level, 273 meV below bandgap energy, presents in samples prepared in oxygen-rich environment. In addition to such mechanism, coalescence of nanoparticles into bulk form also exists and contributes to enhanced luminescence. Introduction Low-dimensional structures including nanoparticles or quantum dots (QDs) are supposed to provide significant enhancement in the density of states, so increasing the probability of light emission. Those low-dimensional structures can be epitaxially grown on bulk materials like GaAs wafers or separately formed by chemical methods. The former way is very selective on the grown substrates. Also, QDs are usually formed with only a scarce area density. In contrast, nanoparticles formed by chemical methods have many advantages. First, they can be applied on any substrates. Second, area or volume density of the material can be very high. The process to fabricate monodispersive nanoparticles is inexpensive and facile to industrial application. Stimulated emission and optical gain had been demonstrated in CdS quantum dots by methods of optical pumping[1,2]. This encourages the employment of electrical pumping to realize efficient nanoparticle-based light emitting devices. In this work, we demonstrate the applications of CdS nanoparticles on silicon substrate. The fabrication of light emitting active layer is simply fabricated by the spin-coating technique. Electroluminescence (EL) can be easily achieved by quantum tunneling of carriers into the nanoparticles. No special care is needed, compared with thermal and chemical budgets of epitaxial technology.
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Experimental Preparation of CdS Nanoparticles Redissolvable nanoparticles powder of CdS has been synthesized by modifying Pietro’s method[3]. Cadmium acetate dihydrate (Cd(CH3COO)2 .2H2 O, 0.80 g, 3.0 mmole) was dissolved in a 20 ml mixed solvent of acetonitrile, methanol, and water with a volume ratio of 1:1:2. Another solution containing disodium sulfide nanohydrate (Na2 S.9H2 O, 0.36 g, 1.5 mmole) and p-hydroxyl thiophenol (0.56 g, 4.4 mmole) in the same solvent system was added into vigorously stirred cadmium acetate solution. The whole system was st
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