Latychevskaia Tatiana
Paul Scherrer Institute, Forschungsstrasse 111, 5232 Villigen, Switzerland; Department of Physics, University of Zurich, Winterthurerstrasse 190, 8057 Zurich, Switzerland.
Ultramicroscopy. 2023 Nov;253:113807. doi: 10.1016/j.ultramic.2023.113807. Epub 2023 Jul 5.
Low-energy electrons (20-300eV) hold the promise for low-dose, non-destructive, high-resolution imaging, but at the price of challenging data analysis. This study provides theoretical considerations and models for the quantitative analysis of experimental data observed in low-energy electron transmission microscopy and in-line holography. The scattering of low-energy electrons and the imaging parameters, such as the inelastic mean free path, point spread function, depth of focus, and resolution, are quantitatively described. It is shown that unlike high-energy electrons (20-300 keV), low-energy electrons (20-300eV) introduce a large phase shift into the probing electron waves. Using the projected potentials formalism, the maximal phase shift acquired by a 120eV electron wave scattered by a carbon atom is theoretically estimated to be 5.03 radian and experimentally measured to be between 3 and 7.5 radian. The point spread function evaluated for low-energy electrons shows that they diffract much stronger than high-energy electrons, and that only very thin objects of up to 3Å in thickness can be imaged in focus. Thus, when imaging an object of finite thickness, such as a macromolecule, the obtained image will always be blurred due to the out-of-focus signal. This can provide an explanation for a long-standing problem of limited resolution in low-energy electron holography of macromolecules. As for imaging of a macromolecule's structure, it is shown that the amplitude of the wavefront reconstructed from the sample's hologram provides the best match to the projected potential distribution of the macromolecule. To evaluate the absorption properties, the inelastic mean free path (IMFP) is considered. The IMFP values calculated from theoretical models agree with the measured values. The IMFP of about 5Å was measured by transmission through graphene of 50-200eV electrons. This result implies that the internal structure of only very thin samples can be imaged in transmission mode. A simple method to quantitatively evaluate the absorption of a specimen from its in-line hologram without the need to reconstruct the hologram is presented.
低能电子（20 - 300电子伏特）有望实现低剂量、无损、高分辨率成像，但代价是数据分析具有挑战性。本研究为低能电子透射显微镜和同轴全息术中观察到的实验数据的定量分析提供了理论考量和模型。定量描述了低能电子的散射以及成像参数，如非弹性平均自由程、点扩散函数、焦深和分辨率。结果表明，与高能电子（20 - 300千电子伏特）不同，低能电子（20 - 300电子伏特）会给探测电子波引入较大的相移。使用投影势形式理论，理论估计由碳原子散射的120电子伏特电子波获得的最大相移为5.03弧度，实验测量值在3至7.5弧度之间。针对低能电子评估的点扩散函数表明，它们的衍射比高能电子强得多，并且只有厚度达3埃的非常薄的物体才能清晰成像。因此，当对有限厚度的物体（如大分子）成像时，由于离焦信号，所获得的图像总会模糊。这可以解释大分子低能电子全息术中分辨率受限这一长期存在的问题。至于大分子结构的成像，结果表明从样品全息图重建的波前振幅与大分子的投影势分布最匹配。为评估吸收特性，考虑了非弹性平均自由程（IMFP）。从理论模型计算出的IMFP值与测量值相符。通过50 - 200电子伏特电子透过石墨烯测得的IMFP约为5埃。该结果意味着只有非常薄的样品的内部结构才能在透射模式下成像。提出了一种无需重建全息图即可从同轴全息图定量评估样品吸收的简单方法。
