C-3 電子能量損失能譜(EELS) - 成份映像

 C-3-5 成份映像

成份映像圖顯示某特定區域內組成元素的二維分佈情形。在TEM/STEM的分析技術中，主要獲取成份映像圖的技術有EDS成份映像圖和EELS成份映像圖二種。EDS能譜中，元素能峰和背景的強度相差甚多，亦即P/B很高，所以在EDS元素映像圖中，只要設定包含能峰的適當能窗，即使不扣除背景，就能清楚地顯示某特定元素的分布。但是在EELS能譜中，元素特性邊刃座落在一高強度的背景上，如果只設定單一能窗，則獲取的成份映像圖中可能有超過一半的訊號是背景訊號，而不是真正的元素訊號，如圖C-44的邊刃後能窗(能窗III)所蘊含的訊號。所以獲取EELS成份映像圖，除了設定包含特性邊刃的邊刃後能窗外，必須同時設定二個邊刃前的能窗，如圖C-44中的能窗I和能窗II。由二個邊刃前能窗的訊號，推算出邊刃後能窗的訊號中的背景訊號(能窗B)，扣除後，才是真正的元素訊號(能窗IV)。

Elemental maps display two dimensional distributions of elements in local interested regions. EDS mapping and EELS mapping are two main analysis techniques in TEM/STEM systems. Peaks of elements, especially major elements, in EDS spectra have high P/B ratio, elemental maps show the distributions of elements clearly once proper energy windows are set, with or without background subtraction. On the contrary, all characteristic edges fall on high background in EELS spectra, the map obtained from an energy window including part of characteristic edge will include background noise as well as true element signal, as shown in the post edge window (window III) in Figure C-44. It is necessary to set two more pre-edge windows to calculate out the background (window B) in the post edge window to obtain the true signal (window IV). 

圖C-44 EELS能譜示意圖顯示運算成份映像圖需要設定三能窗，二個邊刃前能窗: 能窗I和能窗II，和一個邊刃後能窗(能窗III)。能窗III包含背景訊號(能窗B)和真正元素訊號(能窗IV)。

圖C-45顯示一組典型TEM模式下拍攝的EELS三能窗影像和演算後的成份映像圖。圖C-45(a)為TEM明場像顯示一分析區域，此區域的組成元素包含Ti, Ni, Zr, Sb等四元素。圖C-45(b)相當於圖44中能窗IV的影像，顯示富鈦相呈一近橫躺的T字形。圖C-45(e)的邊刃後能窗影像相當於圖44中能窗III的影像，和圖C-45(c)和圖C-45(d)相比，雖然隱約顯示富鈦相的區域比周圍他相稍亮，但影像的亮度變化，會被誤認為鈦的分佈幾乎涵蓋整個區域，而且都有相當的濃度。透過二個邊刃前能窗影像算出背景影像(能窗B)，再從能窗III影像中扣除後，才能得到圖C45(b)的鈦成份映像圖。

A typical set of energy-selected images acquired in TEM mode and the final processed elemental map are shown in Figure C-45. Figure 45(a) is a TEM BF image showing an interested region being consisted of Ti, Ni, Zr, and Sb. Figure C-45(b) is the elemental map corresponding the image of energy IV in Figure 44 and indicates that the shape of the Ti-rich phase look like a lying down T. Figure C-45(e) is the Ti post-edge image corresponding to the image of window III in Figure C-44. The region of Ti-rich phase in this image is a little brighter compared with corresponding regions in Figure C-45(c) and Figure C-45(d), but is not distinguishable. A true Ti map, as shown in Figure C-45, is only obtained after background subtracted from the post-edge image.

圖C-45 EELS成分映像圖的演算。(a)明場像；(b)鈦元素成份映像圖；(c) pre-edge 1影像；(d) pre-edge 2影像；(e) post-edge影像。