Phase plate STEM imaging using two dimensional electron detector
Phase plate STEM imaging using two dimensional electron detector
复制标题
使用二维电子探测器进行相位板 STEM 成像
DOI：
10.1017/s1431927619001156
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发表时间：
2019
影响因子：
2.8
通讯作者：
Misaki Tsubouchi and Hiroki Minoda
中科院分区：
文献类型：
--
作者：
Y. Abe;T. Umeda;M. Okamoto;R. Kosugi;S. Harada;M. Haruyama;W. Kada;O. Hanaizumi;S. Onoda;T. Ohshima;Misaki Tsubouchi and Hiroki Minoda
扫描透射电子显微镜（STEM）中的相板（PP）可以将相衬传递函数由正弦型修改为余弦型，以增强由轻元素[1]组成的生物分子的相衬。在相板STEM （P-STEM）中，我们使用了带有中心孔的碳PP （Zernike PP）。薄膜材料的平均内电位为入射电子波提供相移。我们调整碳膜的厚度，使能量为200KeV的电子波产生半π相移。由于在P-STEM中，为了获得更高的对比度图像，我们必须使用无相移的非散射电子波和有相移的散射电子波，因此应该在探测器的正上方使用一个检测孔径来选择具有适当光学条件的电子波。P-STEM条件合适的电子波落在图1中“Out of plane”表示的圆形区域。为了重建相位目标的相位分布，孔洞越小越好，因为孔洞可以增强空间频率较低的结构信息。这是传统亮场瞬变电磁法（BF）和瞬变电磁法（STEM）的弱点之一。然而，使用孔径较小的PP重建相位分布需要更小的探测器或更小的探测孔径来匹配PP的孔径，如图1所示。使用具有较小孔的PP减少了总信号，这对于可视化光束敏感样品是不希望的。最近，二维探测器被用于STEM成像来重建相位分布[3]。使用2D检测器和预标本PP的组合进行STEM成像也有报道[0]。本文将二维探测器应用于Zernike型PP。
Phase plate (PP) in Scanning Transmission Electron Microscopy (STEM) can modify phase contrast transfer function from sine type to cosine type to enhance phase contrast of biological molecules composed of light elements [1]. In phase plate STEM (P-STEM), we used a carbon PP with a center hole (Zernike PP). A mean inner potential of the film material provide a phase shift to the incident electron wave. We adjust the thickness of the carbon film to provide a half π phase shift to the electron wave with energy of 200KeV. Since we have to use the unscattered electron wave without phase shift and the scattered waves with phase shift to obtain higher contrast image in P-STEM, a detection aperture to choose electron waves with an appropriate optical condition should be used just above the detector. The electron waves with the appropriate P-STEM condition lands in the circle region indicated with “Out of plane” in Fig. 1.In order to reconstruct the phase distribution of the phase objects, smaller hole is better because structural information with lower spatial frequency can be enhanced. This is one of the weakest points in conventional bright field (BF) TEM and STEM. However, the reconstruction of phase distribution using the PP with smaller hole requires smaller detector or smaller detection aperture to match the hole size of the PP as in Fig. 1. Using the PP with smaller hole reduces the total signals and this is undesirable to visualize beam sensitive samples. Recently, a two dimensional (2D) detector has been used in STEM imaging to reconstruct phase distribution [3]. STEM imaging using combination of a 2D detector and a pre-specimen PP was also reported [4]. In the present paper we applied the 2D detector to our Zernike type PP.