Towards Atomic-Resolution Electron Energy Loss Spectroscopy (EELS) in an Uncorrected 30kV Scanning Electron Microscope

As an era-defining technological advancement in the field of nanoscience and beyond, the effective implementation of aberration correction has allowed electron microscopy to routinely reach deep sub-angstrom-level spatial resolution. Among many impactful consequences, these developments have seen the widespread adoption of low-voltage instruments, which can maintain very high spatial resolutions thanks to their aberration correctors, even down to 20kV, especially for applications in 2-dimensional materials at the single atom level [1]. Beyond single-atom sensitivity, low-voltage operation is highly sought-after for reasons such as reduced knock-on damage to samples or increased inelastic cross-sections resulting in a high signal for spectroscopy. However, for a large number of practical materials science applications, the complexity and price of such instrumentation, especially when analytical capabilities are added, can be a drawback. In contrast, high-throughput capabilities with lower entry barriers in terms of cost and complexity, but which maintain a relatively high-resolution, can often be preferable in order to address numerous scientific questions.<br/>One possible solution is the use of (low-voltage) scanning electron microscopes (SEMs) operated in a transmission geometry – or (T)SEMs [2]. When equipped with cold field emission sources, these instruments have been shown to reach 0.2nm information transfer in bright-field STEM imaging [3], and to provide remarkable flexibility for surface and spectroscopic investigations of functional materials [4]. Here, we show how the capabilities of such a high-resolution (T)SEM can be pushed even further towards near-atomic resolution for EELS mapping. We use a Hitachi SU9000EA microscope, a low-kV (≤30kV) uncorrected (T)SEM equipped with a diffraction camera and a Hitachi electron energy-loss spectrometer developed for this instrument, which, thanks to its cold-field emitter, has a native energy resolution of ~0.3eV.<br/>In the optical configuration chosen, and at 30kV acceleration voltage, the estimated probe size was sufficient to observe 0.26nm spots in the Fourier transform of high-angle annular-dark-field STEM images of a La_1/3NbO₃ A-site deficient perovskite, a candidate high performance thermoelectric ceramic [5]. This made it possible to use EELS to map with atomic-plane resolution the location of the La cations planes in the structure, 0.8nm apart. The observed oscillations, peaks and troughs, of the integrated intensity of the La <i>M_4,5</i> edge in the linescan follow exactly those of the simultaneously acquired HAADF signal – with the darker layers corresponding to La-deficient positions, thus demonstrating plane-by-plane mapping in an SEM. The use of an edge with a high 832eV onset also highlights the applicability of EELS in this uncorrected 30kV system, even at relatively high energy losses. Other EELS applications, such as plasmonics and low primary energy core-loss (down to 3kV) will also be highlighted to further illustrate the versatility of these instruments, whose advanced capabilities as (T)SEM-EELS instruments belie their relative operational simplicity and low cost.<br/><br/>[1] U. Kaiser <i>et al.</i>, Ultramicroscopy <b>111</b> (2011), p. 1239.<br/>[2] T. Sunaoshi <i>et al.</i>, Microsc. Microanal. <b>22 </b>(S3) (2016), p. 604.<br/>[3] M. Konno <i>et al.</i>, Ultramicroscopy <b>145</b> (2014), p. 28.<br/>[4] N. Brodusch <i>et al.</i>, Ultramicroscopy <b>203</b> (2019), p. 21.<br/>[5] D. Kepaptsoglou <i>et al.</i>, Inorg. Chem. <b>57</b> (2018), p. 45.