Mullarkey Tiarnan, Downing Clive, Jones Lewys
School of Physics, Trinity College Dublin, Dublin 2, Ireland.
Centre for Doctoral Training in the Advanced Characterisation of Materials, AMBER Centre, Dublin 2, Ireland.
Microsc Microanal. 2021 Feb;27(1):99-108. doi: 10.1017/S1431927620024721.
When characterizing beam-sensitive materials in the scanning transmission electron microscope (STEM), low-dose techniques are essential for the reliable observation of samples in their true state. A simple route to minimize both the total electron-dose and the dose-rate is to reduce the electron beam-current and/or raster the probe at higher speeds. At the limit of these settings, and with current detectors, the resulting images suffer from unacceptable artifacts, including signal-streaking, detector-afterglow, and poor signal-to-noise ratios (SNRs). In this article, we present an alternative approach to capture dark-field STEM images by pulse-counting individual electrons as they are scattered to the annular dark-field (ADF) detector. Digital images formed in this way are immune from analog artifacts of streaking or afterglow and allow clean, high-SNR images to be obtained even at low beam-currents. We present results from both a ThermoFisher FEI Titan G2 operated at 300 kV and a Nion UltraSTEM200 operated at 200 kV, and compare the images to conventional analog recordings. ADF data are compared with analog counterparts for each instrument, a digital detector-response scan is performed on the Titan, and the overall rastering efficiency is evaluated for various scanning parameters.
在扫描透射电子显微镜（STEM）中对束敏感材料进行表征时，低剂量技术对于可靠观察处于真实状态的样品至关重要。一种将总电子剂量和剂量率降至最低的简单方法是降低电子束电流和/或以更高速度对探针进行光栅扫描。在这些设置的极限情况下，以及使用当前的探测器时，所得到的图像会出现不可接受的伪像，包括信号拖尾、探测器余辉和较差的信噪比（SNR）。在本文中，我们提出了一种替代方法，即通过对散射到环形暗场（ADF）探测器的单个电子进行脉冲计数来捕获暗场STEM图像。以这种方式形成的数字图像不受拖尾或余辉等模拟伪像的影响，即使在低束电流下也能获得清晰、高信噪比的图像。我们展示了在300 kV下运行的ThermoFisher FEI Titan G2和在200 kV下运行的Nion UltraSTEM200的结果，并将这些图像与传统的模拟记录进行比较。将每种仪器的ADF数据与其模拟对应数据进行比较，在Titan上进行数字探测器响应扫描，并针对各种扫描参数评估整体光栅扫描效率。
