Simultaneous acquisition of real and reciprocal space information in transmission SEM

Abstract text (incl. figure legends and references)

In the recent years, a variety of transmission diffraction setups entered the field of scanning electron microscopy (SEM) to prove the worthwhile implementation of diffraction analysis in combination with lower energetic electrons and the analytical capabilities of a SEM [1-3]. The broad accessibility and simpler setup facilitate such developments compared to the more complex transmission electron microscopy (TEM). Moreover, the large microscope chamber offers a straightforward implementation and enough space for unique in situ systems [4-5]. Even state-of-the-art direct electron detection cameras combined with the popular 4D-STEM technique are available for SEM´s with the benefits of reduced knock-on damage and increased contrast [2]. However, in relation to in situ experiments, where a continuous observation of the occurring phenomena is indispensable, these cameras are not suitable for such an experiment, even with the fastest acquisition rate.

Based on the limitations regarding parallel acquisition of real and reciprocal space information in situ, we introduce a unique setup in SEM, which solves the current restrictions. Figure 1a shows schematically the concept of our developed system, which is similar to our existing diffraction setup (LEND) [3]. The basis is a fluorescent screen positioned below the sample and an in-chamber mounted CMOS [WJ1] camera responsible for recording of the generated diffraction patterns on the screen. To enable the additional acquisition of the real space signal, a flexible STEM detector is installed below the fluorescent screen, which contains a concentric hole allowing the electrons to pass through and contribute to image formation. Figure 1b depicts the setup installed in our SEM. The combination of a simple integrating detector (STEM) with the parallel acquisition of the averaged diffraction patterns offers high frame rates linked to the corresponding reciprocal space information. In addition, all occurring signals in SEM can be collected.

Here, we investigate a thin MoS2 flake covered with gold nanoparticles as model system for acquiring the four simultaneously available signals in one run (Figure 2). The sample topography is depicted by secondary electrons (SE), where all small particles and surface contamination are visible. Back-scattered electrons (BSE) offer elemental contrast, which facilitates to distinguish between the gold particles and MoS2. The inner sample structure can be investigated by bright-field (BF) STEM and the acquired diffraction pattern reveal the present crystal structure of MoS2. Besides BF STEM imaging, the fluorescent screen, together with the STEM detector, can be moved to switch to dark-field (DF) imaging by selecting a suitable diffraction spot.

In the future, we are planning to perform dedicated heating experiments while acquiring real and reciprocal space information in situ. An upgrade of the current setup to an in situ chip-based heating stage will expand strongly the analytical potential of this concept. The real time monitoring of an ongoing process by STEM and diffraction signal will help to reveal appearing phenomena and mechanism in several material aspects [6].

[1] 10.1016/j.ultramic.2018.09.006

[2] 10.1016/j.ultramic.2020.113137

[3] 10.1016/j.ultramic.2020.112956

[4] 10.1017/S143192762100951X

[5] 10.1017/S1431927621003974

[6] The authors acknowledge funding by the German Research Foundation (DFG) via the Research Training Group GRK 1896