Johnston-Peck Aaron C, DuChene Joseph S, Roberts Alan D, Wei Wei David, Herzing Andrew A
Materials Measurement Lab, National Institute of Standards Technology, Gaithersburg, MD 20899, USA.
Department of Chemistry and Center for Nanostructured Electronic Materials, University of Florida, Gainesville, FL 32611, USA.
Ultramicroscopy. 2016 Nov;170:1-9. doi: 10.1016/j.ultramic.2016.07.002. Epub 2016 Jul 6.
Beam damage caused by energetic electrons in the transmission electron microscope is a fundamental constraint limiting the collection of artifact-free information. Through understanding the influence of the electron beam, experimental routines may be adjusted to improve the data collection process. Investigations of CeO indicate that there is not a critical dose required for the accumulation of electron beam damage. Instead, measurements using annular dark field scanning transmission electron microscopy and electron energy loss spectroscopy demonstrate that the onset of measurable damage occurs when a critical dose rate is exceeded. The mechanism behind this phenomenon is that oxygen vacancies created by exposure to a 300keV electron beam are actively annihilated as the sample re-oxidizes in the microscope environment. As a result, only when the rate of vacancy creation exceeds the recovery rate will beam damage begin to accumulate. This observation suggests that dose-intensive experiments can be accomplished without disrupting the native structure of the sample when executed using dose rates below the appropriate threshold. Furthermore, the presence of an encapsulating carbonaceous layer inhibits processes that cause beam damage, markedly increasing the dose rate threshold for the accumulation of damage.
透射电子显微镜中高能电子引起的束流损伤是限制无伪像信息收集的一个基本限制因素。通过了解电子束的影响，可以调整实验程序以改进数据收集过程。对CeO的研究表明，电子束损伤的积累不存在临界剂量。相反，使用环形暗场扫描透射电子显微镜和电子能量损失谱的测量表明，当超过临界剂量率时，就会出现可测量的损伤。这种现象背后的机制是，当样品在显微镜环境中重新氧化时，由300keV电子束照射产生的氧空位会被积极地消除。因此，只有当空位产生速率超过恢复速率时，束流损伤才会开始积累。这一观察结果表明，当使用低于适当阈值的剂量率执行时，剂量密集型实验可以在不破坏样品天然结构的情况下完成。此外，封装碳质层的存在抑制了导致束流损伤的过程，显著提高了损伤积累的剂量率阈值。
