Picher Matthieu, Bücker Kerstin, LaGrange Thomas, Banhart Florian
Université de Strasbourg, CNRS, Institut de Physique et Chimie des Matériaux, UMR 7504, Strasbourg 67034, France.
Interdisciplinary Centre for Electron Microscopy (CIME), École Polytechnique Fédérale de Lausanne (EPFL), Lausanne 1015, Switzerland.
Ultramicroscopy. 2018 May;188:41-47. doi: 10.1016/j.ultramic.2018.03.006. Epub 2018 Mar 13.
We implement a parametric study with single electron pulses having a 7 ns duration to find the optimal conditions for imaging, diffraction, and electron energy-loss spectroscopy (EELS) in the single-shot approach. Photoelectron pulses are generated by illuminating a flat tantalum cathode with 213 nm nanosecond laser pulses in a 200 kV transmission electron microscope (TEM) with thermionic gun and Wehnelt electrode. For the first time, an EEL spectrometer is used to measure the energy distribution of single nanosecond electron pulses which is crucial for understanding the ideal imaging conditions of the single-shot approach. By varying the laser power, the Wehnelt bias, and the condenser lens settings, the optimum TEM operation conditions for the single-shot approach are revealed. Due to space charge and the Boersch effect, the energy width of the pulses under maximized emission conditions is far too high for imaging or spectroscopy. However, by using the Wehnelt electrode as an energy filter, the energy width of the pulses can be reduced to 2 eV, though at the expense of intensity. The first EEL spectra taken with nanosecond electron pulses are shown in this study. With 7 ns pulses, an image resolution of 25 nm is attained. It is shown how the spherical and chromatic aberrations of the objective lens as well as shot noise limit the resolution. We summarize by giving perspectives for improving the single-shot time-resolved approach by using aberration correction.
我们使用持续时间为7纳秒的单电子脉冲进行参数研究，以找到单次成像、衍射和电子能量损失谱（EELS）的最佳条件。在一台配备热离子枪和韦内尔特电极的200 kV透射电子显微镜（TEM）中，用213纳米纳秒激光脉冲照射扁平钽阴极来产生光电子脉冲。首次使用EEL光谱仪测量单纳秒电子脉冲的能量分布，这对于理解单次方法的理想成像条件至关重要。通过改变激光功率、韦内尔特偏压和聚光镜设置，揭示了单次方法的最佳TEM操作条件。由于空间电荷和博尔施效应，在最大发射条件下脉冲的能量宽度对于成像或光谱分析来说过高。然而，通过将韦内尔特电极用作能量过滤器，脉冲的能量宽度可以降低到2 eV，不过会牺牲强度。本研究展示了用纳秒电子脉冲获取的首批EEL光谱。使用7纳秒脉冲时，可实现25纳米的图像分辨率。展示了物镜的球差和色差以及散粒噪声如何限制分辨率。我们通过给出使用像差校正改进单次时间分辨方法的前景来进行总结。
