Recent advances in 3D electron microscopy have enabled spectacular progresses in our understanding of the ultrastructure of many organisms and tissues. However visualizing the metabolism within its structural context is key to understand and identify the biological mechanisms at stake¹. In this light, correlating laterally resolved molecular information from TOF-SIMS (up to 100-400 nm lateral resolution) with structural data from electron microscopy (5 nm lateral resolution) is a promising approach as evidenced with several examples.

During a symbiotic association between two organisms, both host and symbiont metabolism undergo deep modifications². As such, the correlation of SEM and TOF-SIMS data is a useful method to understand how the two partners are remodeled structurally and metabolically. Our first results suggest that the host exerts its control through preferential allocation of some nutrients to boost symbiont productivity.

Additionally, the microalga Coelastrella sp. PCV³ has recently been shown of high interest for uranium bioremediation. Our approach reveals the how uranium is being sequestrated inside specific compartments of the cell.

[1] J. Decelle, et al. Trends Cell Biol. Vol 3 (2019), 173-188.

[2] C. Uwizeye, et al. PNAS. Vol 118 (2021), e2025252118.

[3] C. Beaulier, et al, BioRxiv (2023) 2023-06. 29.546994.

2D transition metal dichalcogenides are attracting great interest thanks to their atomic layer thickness, extraordinary (opto)electronic, chemical and thermal properties. However, it has become increasingly more difficult to characterize, chemically and electrically, this kind of materials and their interfaces. Indeed, conventional methodologies, including scanning probe microscopies, fail to capture insight in the chemical and electronic nature of the semiconductor, albeit vital to understand its impact on the semiconductor performance. Therefore, in this work, we present a unique and universal in-situ approach [1] to characterize thin WS₂ layers by combining time-of-flight secondary ion mass spectrometry (ToF-SIMS) and atomic force microscopy (AFM) in conductive mode. In this way it was possible to map chemical differences between regions of different electrical conductivity in the 2D material, thanks to the accurate chemical information achievable with ToF-SIMS combined with the atomic resolution attainable with AFM. Surprisingly, WS₂ regions of lower electrical conductivity possess a larger amount of sulfur compared to regions with higher conductivity, for which oxygen was also detected. Such difference in chemical composition likely roots from the non-homogeneously terminated sapphire starting surface, altering the WS₂ nucleation behavior and associated defect formation between neighboring sapphire terraces [2]. These resulting sapphire terrace-dependent doping effects in the WS₂ hamper its electrical conductivity. Thus, we demonstrated how accurate chemical assignment at sub-micrometer lateral resolution of atomically thin 2D semiconductors is vital to achieve a more detailed understanding on how the growth behavior affects the electrical properties.

[1] V. Spampinato et al. Anal. Chem. 92 (2020) 11413

[2] Y. Shi et al. ACS Nano 15(6) (2021) 9482

Traditionally, Focused Ion Beam (FIB) platforms are coupled with an electron beam column (FIB-SEM) to perform in-situ sample preparation (FIB milling) and electron microscopy (EM), respectively. EM images can be acquired with the FIB column (secondary electrons, SE) or with the SEM column in different modes (SE, BSE, EBSD, STEM) with the possibility to perform chemical analysis (WDS/EDX). However, the electron beam probes for EDX analyses present very large interaction volumes within the sample (several µm) and EDX does not allow the detection of light elements (H, Li, ..) and trace elements (<1% in at.).

In the recent years, the ion sources integrated in FIB platforms have considerably improved in terms of source brightness, leading to better spatial resolution and current density, and versatility of available ion species. For example, the low temperature ion source (LoTIS) working with Cs ions and reaching a brightness of 10⁷ A cm^-2 sr^-1, which is much higher than the surface ionisation Cs source (10² A cm^-2 sr^-1) available on conventional SIMS instrument [1-2], can produce nm-sized probe sizes while maintaining high ion currents. These new sources are of great interest not only for their ability to mill a variety of structures, but also to provide structural and morphologic information with (sub)-nm resolution in SE.

In this global context, we developed compact magnetic sector SIMS systems specifically designed and adapted for the ZEISS ORION NanoFab Helium Ion Microscope (HIM) [2-3], the Thermo Fisher DualBeam [4] and the ZeroK FIB:ZERO platform. These instruments are capable of producing elemental SIMS maps with sub-15 nm lateral resolution, while maintaining the performance of the FIB platform in terms of secondary electron (SE) imaging and nanomachining. The latest compact SIMS generation is equipped with a novel continuous focal plane detector (FPD). This system allows for the detection of all masses in parallel for each single pixel, resulting in acquisition times as low as 1 s to obtain a full mass spectrum or 2 min to obtain a 512 x 512 pixel SIMS image with highest signal-to-noise ratio and excellent dynamic range. [2]

This SIMS system is now operating on several multi-modal FIB platforms . Here, we will review the performance of the different FIB-SIMS instruments with a focus on new developments, showcase methodologies for high-resolution 2D and 3D chemical imaging, and give an outlook on new trends and prospects.

[1] B. Knuffman, A. V. Steele, and J. J. McClelland, J. Appl. Phys. 114, p. 044303 (2013)

[2] J. N. Audinot, P. Philipp, O. De Castro, A. Biesemeier, Q. H. Hoang, T Wirtz, Reports Prog. Phys. 84, p.105901 (2021).

[3] Dowsett and T. Wirtz, Anal. Chem. 89, 17, 8957–8965 (2017)

[4] O. De Castro, J.-N. Audinot, H. Q. Hoang, C. Coulbary, O. Bouton, R. Barrahma, A. Ost, C. Stoffels, C. Jiao, M. Dutka, M. Geryk, T, Wirtz, Analytical Chemistry, 94(30), 10754-10763 (2022).

Resonant Laser-ionization Secondary Neutral Mass Spectrometry (rL-SNMS) combines the spatial resolution of traditional static time of flight secondary ion mass spectrometry with the elemental selectivity of resonant laser ionization. A set of TiSa lasers is used to ionize only one selected element from the sputtered neutral cloud via a resonant multistep excitation scheme. For ultra-trace analysis of isotopes like ²³⁸Pu, non-resonant ionization of isobaric interferences, like ²³⁸U, are challenging to remove. Reference material is needed for an investigation of the different ionization efficiencies of relevant elements. The MetroPOEM project ^[1], aims to develop SI-traceable reference material to overcome this challenge. For rL-SNMS, such solid multi-element standards have specific requirements: homogeneity on the sub-micrometer scale with known isotopic composition. In this work we present a method for the production of homogenous U-Pu samples by fast Fe coprecipitation. The homogeneity of the samples was confirmed by ToF-SIMS and EDX. The elemental composition of the material was determined with ICP-MS measurements. The samples were used to determine the suppression ratios of non-resonant U in Pu rL-SNMS measurement. The presented method will be modified to produce homogenous sample material for other elements, and used to investigate relative ionization efficiencies for different elements.

^[1] MetroPOEM is a collaboration of 22 partners from 13 countries throughout Europe funded by EURAMET under grant number 21GRD09 https://www.npl.co.uk/euramet/metropoem