Nanoscale Multimodal Analysis of Organic Compounds in Beam-Sensitive Nanoparticles by Monochromated STEM-EELS

Nanomaterials are of great interest for improving existing technologies or engineering new ones. The unique properties of these emergent structures are widely studied in various fields, such as optics, electronics and medicine. As their properties depend on their sizes, local structures and chemical compositions, each nanomaterial requires deep analysis to unravel its specific behaviour. However, most of the used techniques cannot provide information on such nanosystems at the single-particle level.¹ This has led to a strong interest in electron microscopy, which can image the morphology and crystal structure of specimens with atomic resolution. Low-dose cryo-Transmission Electron Microscopy (cryo-TEM) is now commonly employed to study beam-sensitive materials such as organic and hybrid organic-inorganic materials.² In addition to imaging, the latest generation Scanning Transmission Electron Microscopes (STEM) equipped with Electron Energy Loss Spectroscopy (EELS) allow a deep characterisation of specimens down to the atomic scale.³ Thanks to the recent development of monochromated electron sources, spectral resolutions can now reach below 10 meV, allowing access from the far-IR to the X-ray range.⁴<br/>Nevertheless, the analysis of sensitive specimens remains a major challenge for spectromicroscopy. Although recent studies based on vibrational EELS have demonstrated the possibility of damage-free analysis,^5,6 most of them exhibited no or limited spatial resolution. Here, we report damage-free analysis of organic-inorganic nanoparticles at the nanoscale. It was achieved via EELS experiments using a monochromated STEM equipped with a cryo-holder and a direct electron detector (DED) in low-dose conditions (down to 10 e^-/Ų). First, the beam-induced damage was studied in the UV and soft X-ray spectral energy ranges. The DED provides simultaneous acquisition of low-loss and core-loss signals, in order to study the beam-induced chemical reactions. Secondly, the vibrational energy range was explored in the aloof and transmitted modes with a 7 meV spectral resolution. Finally, chemical maps were obtained in the three energy ranges. Here, we demonstrate that STEM-EELS provides damage-free analysis that enables efficient identification of the organic compound distributions across sensitive specimens. Our work opens new possibilities for analysing organic compounds such as polymers and biomolecules.<br/><br/>1. Chaupard, M., de Frutos, M. & Gref, R. Deciphering the Structure and Chemical Composition of Drug Nanocarriers: From Bulk Approaches to Individual Nanoparticle Characterization. <i>Particle & Particle Systems Characterization</i> <b>38</b>, 2100022 (2021).<br/>2. Danino, D. Cryo-TEM of soft molecular assemblies. <i>Current Opinion in Colloid & Interface Science</i> <b>17</b>, 316–329 (2012).<br/>3. Egerton, R. F. <i>Electron Energy-Loss Spectroscopy in the Electron Microscope</i>. (Springer Science & Business Media, 2011).<br/>4. Li, X. <i>et al.</i> Three-dimensional vectorial imaging of surface phonon polaritons. <i>Science</i> <b>371</b>, 1364–1367 (2021).<br/>5. Rez, P. <i>et al.</i> Damage-free vibrational spectroscopy of biological materials in the electron microscope. <i>Nat Commun</i> <b>7</b>, 10945 (2016).<br/>6. Collins, S. M. <i>et al.</i> Functional Group Mapping by Electron Beam Vibrational Spectroscopy from Nanoscale Volumes. <i>Nano Lett.</i> <b>20</b>, 1272–1279 (2020).