Single-atom detection of light elements: Imaging or spectroscopy?

Single-atom imaging and spectroscopy at a lower accelerating voltage (~60 kV) has been largely facilitated by the development of aberration correctors for transmission electron microscopy (TEM)/ scanning TEM (STEM). Such an STEM condition will reduce beam damage and has therefore been demonstrated capable of detecting individual atoms of light elements including B, C, and N in mono-layered materials. However, other light elements such as Li, O, or F are still difficult to visualise as individual atoms by using conventional STEM/TEM imaging because their extremely weak contrast can be often smeared out by the other atoms nearby. In this paper, we demonstrate the successful detection of these 'hardly visible' atoms in the spectroscopy mode.