The electronic configuration and structure analysis in nanomaterial with high resolution EFTEM

• Main Authors: JingYi Yan, 顏精一
• Other Authors: Ji-Jung Kai
• Format: Others
• Language: zh-TW
• Published: 2004
• Online Access: http://ndltd.ncl.edu.tw/handle/364d2p

博士 === 國立清華大學 === 工程與系統科學系 === 92 === Abstract In this work, the study is to improve the analysis of the transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) in nanomaterial system. In nanoscale, the property, structure and composition of materials will be changed due to quantum effect and surface effect. Therefore, how to understand and control these three parameters is very important to design and apply the materials in nanotechnology. The advantage of field emission TEM is owns very high spatial resolution such as 1~2Å and can do individual analysis or measurement for each nanomaterails system. Besides, combining the theoretical calculation and electron energy loss spectrum collected the energy loss electrons can reveal the material properties. Although energy filtered TEM (EFTEM) can provide two dimensional information of chemical distribution (composition), it can not give properties information. Therefore, the new electron spectroscopic imaging series technique (ESI), which named advanced ESI technique is develop to improve the energy resolution of EEL-spectra, increasing the sampling of raw data and removes the noise and article by employing three numerical methods as maximum entropy deconvolution (MEM), fast Fourier interpolation and wavelet denoising method. The EEL-spectra which extracted from the newly developed advanced ESI technique is not only provide the properties of materials but also can do the quantification analysis. The noise which caused by the environment or recording system (CCD or image plate) is one of the problems in the high resolution image especially for high angle annular dark field image (HAADF) in scanning transmission electron microscopy (STEM) and it will reduce the spatial resolution and image quality. We study the noise behavior in HAADF image and demonstrate wavelet denoising method as the best method because not only reducing the noise contribution in HAADF image but also restorimg the image quality, resolution and Z-contrast ability. In the last session of this work, combing the high resolution TEM, EELS and photoluminescence (PL) analysis, the properties of different diameter ZnO nanowires have been investigated. First, the PL-spectra of different ZnO nanowires reveal the green emission will increase while the diameter decreased and the ratio of green/UV emission is almost consisted with the surface-to-volume (S/V) ratio of nanowires. For further analysis, the bandgap measurement from surface and center region of individual nanowires was done by low loss EELS analysis technique. These results indicate that the surface effect will predominate the photoluminesence of ZnO nanowires. The oxygen vacancy is identified as green emission mechanism by comparing with Zn L2,3 core loss EEL-spectrum and theoretical calculation with different defect model by FEFF V8.2 code which based on the real space multiple scattering calculation. The quantum confinement effect which contributed in plasmon loss energy is observed while the diameter of ZnO nanowires small than 20nm and it becomes more obviously in 10nm and smaller. The relationship of the quantum confinement effect and plasmon loss energy is derived by simply quantum mechansim. The correlation of the quantum confinement effect and diameter of ZnO nanowires can be found by fitting with experimental data. The experimental results also reveal the quantum confinement effect can be observed in the weak confinement region.