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Luminescent Properties of3C-SiC Nanocrystals’ Sol

Author: XuWei
Tutor: WuXingLong
School: Nanjing University
Course: Microelectronics and Solid State Electronics
Keywords: Silicon carbide Nanocrystals photoluminescence Hydrogen bonds
CLC: TB383
Type: Master's thesis
Year: 2012
Downloads: 40
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Abstract


Silicon carbide is useful in many fields because it has many advantages such as wide band gap, high broken electric field, fast electron drift velocity, fast thermal conductivity, good thermal stability, good mechanical strength, antioxidant, corrosion-resistant and so on. SiC nanocrystals (NCs) have many unique properties including the quantum confinement and big specific surface, which makes it different from bulk material. Surrounding environment can affect the NC properties because of the interaction between environment and NCs via chemical agents. Currently, how surrounding environment affects the NC properties has become a hot research area. For example, Fan et al. put the NCs in different solvents to observe the change of the photoluminescence (PL). Wu et al. changed the pH value of the3C-SiC suspension to observe the PL change. In this paper, we change the phase of the NC hydrosol to observe the PL change. We use chemical corrosion to prepare the SiC NCs, put the NC hydrosol in cold environment to change its phase, and finally observe the PL change during the phase transition. We also change the pH value of the NC hydrosol by HC1and NaOH to repeat the previous experiments, investigate the NC stability after the phase transition, and analyze the obtained experimental data. At last, we carry out more experiments to test and verify our inference. These results will have potential applications in quantum dot fluorescent labels, quantum dots inlaid devices and so on.1. Using the microscale SiC powder as a raw material, we etch the powder by HF and HNO3into some crossed network structures. After ultrasonic vibration and centrifugation, we obtain the3C-SiC NCs with diameters of1.5-6.5nm in water in which the NC surfaces have many-H and-OH groups. The PL measurements indicate that the PL peak is at450nm under excitation with the320nm line of a Xe lamp.2. Using the cooling holder to lower the temperature of the NC hydrosol, we observe the phase transition of the NC hydrosol. Under excitation with the350nm line of a Xe lamp, the PL spectrum of the hydrosol shows two peaks, one being at450nm as an intrinsic luminescence, the other at510nm as a surface-state luminescence. During the phase transition, the PL intensity first enhances by a factor of1.5, and then decreases by a factor of1/5. Meantime, the450nm peak has a redshift and finally stays at407nm. The modification of the NC PL mainly arises from the changes in the NC concentration and NC wrapping by ice during the phase transaction. When the hydrosol becomes dense, the NCs will reunite via the action of hydrogen bonds between them. This makes the average size of the NCs become bigger so that the PL spectrum shows redshift and becomes stronger. When the NCs are wrapped by ice, the passivation layer on them consisting of hydrogen bonds is destroyed. As a result, the PL becomes weak, especially the intrinsic luminescence.3. In order to verify our inference, we further design some experiments to rule out other factors as an origin of the PL modification. By damaging the hydrogen bonds via changing the pH value of the hydrosol, we found that the PL spectra of the acidic and alkaline hydrosols no longer change during the phase transaction. In addition, we also investigate the stability and structural change of the NCs in the water solution and during the phase transaction. We disclose that the hydrogen bonds between the NCs are not very strong. They can easily spread out when the hydrosol is dilute.

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