| Summary: | 碩士 === 國立清華大學 === 材料科學工程學系 === 90 === Abstract
Si-based luminescent nanostructures have attracted much attention in recent years based on the fundamental physical interests and potential applications in optoelectronic devices. However, the luminescence mechanisms for most of these luminescent materials are not clear and definitive till now. In this experiment, isolated Si nanocrystallites (ISNs) and oxygen-containing Si nanoparticles (OCSNs) are successfully fabricated by thermal evaporation technique, and the related photoluminescence (PL) properties and mechanisms are studied and discussed.
The ISNs are prepared in pure argon and the particle size decreases with decreasing the working pressure, based on the XRD and TEM analyses. According to the FTIR and ESCA spectra, there are oxygen and hydrogen absorbed on the surface of the ISNs and the amount of absorbates increases as the particle size decreases. The ISNs exhibit a bright orange PL with a maximum position at 800 nm. The PL intensity increases with decreasing of the particle size, and no peak shift is detected. Moreover, the PL intensity increases after the specimens have been aged in air, and this orange light emission is quenched when hydrogen or air is adsorbed on the surface of the ISNs. These PL properties are interpreted in terms of the oxygen-related surface state model.
In the second part of this experiment, the OCSNs are prepared in a mixed atmosphere composed of argon and oxygen, and the particle size also decreases with decreasing the working pressure. The OCSNs with a lower oxygen content exhibit orange PL which is similar to that of the ISNs, while an intense blue-green PL is observed from those with a higher oxygen content. From the XRD, HRTEM, FTIR and ESCA analyses, the OCSNs exhibiting blue-green PL have an amorphous structure and consist mainly of Si oxide. In addition, an obvious blueshift of the absorption edge is observed in the transmission spectra when the oxygen content of the OCSNs is increased, implying that the band structure of the OCSNs is dominated by the oxygen content.
Two primary PL bands are observed in the blue-green PL spectra: the peak position of high energy band (HEB) with a stronger PL intensity is at 520 nm, while the low energy band (LEB) has a peak at 800 nm. Besides the two primary bands, there is a tiny peak at 450 nm. The intensity of PL increases with increasing the oxygen content of the OCSNs, and no peak shift is observed when the particle size is varied. Furthermore, the PL intensity is very sensitive to the ambient atmosphere. A mechanism of the oxygen-induced luminescent center is proposed to be responsible for the blue-green PL in this study, and the ambient effect could be explained by surface charge redistribution during the gas adsorption and desorption process.
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