STEM Developments: Atomic-Resolution SE Imaging, Fast 4D STEM, Ultra-High Energy Resolution

A new type of secondary electron (SE) detector designed by Nion for a modern STEM combines SE detection with an atom-sized probe in the operating range 20-200 kV, clean (metal-sealed and bakable) UHV vacuum conditions, and state-of-the-art electron energy loss spectroscopy (EELS) and 4D-STEM capabilities [1,2]. Running experience reveals that common S/TEM samples (e.g. MoS₂ or metal nanoparticles on a carbon film) that give atomic-resolution annual dark field (ADF) images initially show only surface contamination in the SE signal. We use multiple methods for UHV surface cleaning, including resistive heating of the whole sample in a side entry holder (e.g. Protochips) and direct laser illumination of a small spot (20 mm x 40 mm). Direct laser illumination is particularly interesting because the laser light modulates the SE signal in some samples by changing the local charge distribution on the surface [3].<br/><br/>After cleaning, atomic-resolution SE signals are readily visible. Expanding on previous work with atomic-resolution SE imaging [4], lower operation voltage in a modern STEM avoids knock-on damage and enables atomic-resolution SE studies of 2D materials. We will show two examples: (1) combining atomic resolution ADF, SE, and 4D-STEM to study MoS₂ with intentional Vanadium dopants at 60 keV primary energy, and (2) combining atomic resolution EELS and SE analysis to study the origins of the SE signal in a model system that can tolerate very high electron doses -- monolayer graphene with Nitrogen and Boron dopants.<br/><br/>4D-STEM and especially EELS experiments on 2D materials have traditionally been complicated by slow detectors and readout noise. The latest direct detectors avoid readout noise and achieve high DQE and dynamic range with speed approaching that of traditional single-channel detectors (&gt;10,000 frames/second). Combining these detectors with powerful open-source software for smart acquisition/compression and live processing makes multi-frame (time series) 4D-STEM or EELS imaging possible, bringing significant advantages when changing the sample environment (e.g. heating, cooling, light-, or gas-injection) causes sample drift that makes longer exposures impractical.<br/><br/>Doing these experiments in an instrument capable of &lt;5 meV energy resolution [5] makes possible synergistic experiments such as mapping the absolute temperature of the sample in and around the spot illuminated by a laser using electron energy gain spectroscopy [6] and measuring the presence and local bonding configuration of hydrogen in the sample by detecting the “infrared absorption” signal of the H bonds with vibrational EELS [7]. Phonon spectroscopy of surface dopants visible in the SE signal is also very promising.<br/><br/>References<br/>[1] M.T. Hotz et al., Microsc. Microanal. 29 Suppl. 1 (2023) 2064-265<br/>[2] J. Martis, B. Plotkin-Swing et al., Proceedings 20^th IMC (Busan, 2023)<br/>[3] J. Martis, N. Dellby et al., Proceedings 20^th IMC (Busan, 2023)<br/>[4] Y. Zhu et al., Nature Materials 8 (2009) 808-812.<br/>[5] N. Dellby et al., Microsc. Microanal. 29 Suppl. 1 (2023) 626-627<br/>[6] J.C. Idrobo et al., Phys. Rev. Lett. 120 (2018) 095901<br/>[7] P. Rez et al., Nature Comm. <b>7</b> (2016) 10945