C-3 電子能量損失能譜(EELS)-元素鍵結化態

 C-3-4 元素鍵結化態

原子的鍵結能和在鍵結方向的電子分布密度會因周圍原子的不同而改變。元素鍵結能的改變大概在0 ~ 7 eV的範圍內，常見的固態材料分析技術中，能解析鍵結能位移的有歐傑(AES)、電子能量損失譜(EELS)、X光吸收光譜(XAS)和X射線光電子能譜(XPS)四種。其中AES、EELS、XAS三種分析技術能同時解析電子分布密度的改變。其中，EELS和XAS的能量解析度可小於1.0 eV，而EELS的特點在於空間解析度高，可以解析小於1奈米的微區。

Both chemical bonding energy and the density of states of electrons along the bond of the atoms change with the surrounding atoms. The change in bonding energy of atoms falls in the range of 0 ~ 7 eV. There are four typical material analysis techniques for solid state materials can resolve the shift in bonding energy, they are Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), X-ray absorption spectroscopy (XAS), and X-ray photon spectroscopy (XPS). AES, EELS, and XAS also can resolve the change in the density of states of electrons. The energy resolution of both EELS and XAS is better than 1.0 eV. Besides high energy resolution, the spatial resolution of EELS can be smaller than 1.0 nm.

圖C-41顯示單晶矽、碳化矽、二氧化矽三者物質中的矽的扣除背景後的EELS特性邊刃。圖C-41(b)是圖C-41(a)的低能量區域的局部放大圖，清楚顯示三種矽L特性邊刃的起始能量的不同，元素態的矽是共價鍵，鍵結能為99 eV；碳化矽中的矽和碳接近共價鍵，矽鍵結能增強為101 eV；二氧化矽中的矽為離子鍵，其鍵結能增強為103 eV。二氧化矽中矽的近邊刃微細結構明顯和其他二者不同，顯示Si-O鍵明顯和Si-Si與Si-C鍵不同。圖C-42顯示金屬鋁、氮化鋁、三氧化二鋁，和藍寶石的鋁的扣除背景後的EELS特性邊刃，有著類似圖C-41中的變化。所以只要有足夠的資料庫，從EELS扣除背景後的元素特性邊刃，即可判斷該元素的鍵結化態(chemical bonding state)。目前最常用的EELS 特性邊刃的資料庫是Gatan 建立的EELS Atalas^[1]。

Figure C-41 shows three kinds of background subtracted Si L edges, Si of Si, Si of SiC, and Si of SiO₂. Figure C-41(b), magnification of the low energy region of Figure C-41(a), shows the different threshold energy of Si L edges, 99 eV for Si/Si, 101 eV for Si/SiC, and 103 eV for Si/SiO₂. The near edge fine structure of Si of SiO₂ is obviously different from the other two, indicating that Si-O bonds is significantly different from Si-Si and Si-C. Figure C-42 shows four kinds of background subtracted Al L edges, Al of Al, Al of Al₂O₃, Al of AlN, and Al of sapphire. All these Al L edges show similar variation with those Si L edges in Figure C-41. The chemical bonding state of any element can be identified from its background subtracted characteristic edge once data base of all elements is established. The EELS Atlas^[1] edited by Gatan is most popular at present.

圖C-41 正常化後的三種Si L特性邊刃。紅線是元素Si的Si，藍線是元素6H-SiC的Si，綠線是元素SiO₂的Si，

圖C-42 正常化後的三種Si L特性邊刃。紅線是元素Si的Si，藍線是元素6H-SiC的Si，綠線是元素SiO₂的Si，

TEM電子源能量解析度愈高，EELS的能量解析度愈高，近邊刃微細結構也會愈清楚，對應的材料電子物理特性也被解析地愈透徹。圖C-43顯示鈷用不同能量解析度的電子源解析出來的L特性邊刃微細結構。

The energy resolution of EELS increases with the energy resolution of the TEM electron beam. The near edge fine structure resolved by TEM/EELS system with better energy resolution will tell electronic properties more detail and exact. Figure C-43 shows the near edge fine structure of Co L₂₃ edge from TEM/EELS system with different energy resolution.

圖C-43 CoO內Co L₂₃的近邊刃微細結構和TEM能量分辨率的關係。(a)能量分辨率 ~ 0.8 eV；(b)能量分辨率 ~ 0.5 eV；(c)能量分辨率 ~ 0.2 eV。ref. [2] (Courtesy of FEI Dr. Bert Freitag and Dr. Peter Tiemeijer)

參考文獻

1] EELS Atlas, edited by C. C. Ahn, O. L. Krivanek. Gatan, 1983.

2] 鮑忠興和劉思謙，近代穿透式電子顯微鏡實務，第18頁，第二版，台中 (2012).