Xiaoyan Li *, Georg Haberfehlner, Ulrich Hohenester, Odile Stéphan, Gerald Kothleitner, Mathieu Kociak,

• Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405 Orsay. France.
• Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria.
• Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria.
• Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405 Orsay. France.
• Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, Steyrergasse 17, 8010 Graz, Austria.
• Université Paris-Saclay, CNRS, Laboratoire de Physique des Solides, 91405 Orsay. France.

Surface phonon polaritons (SPhPs) are mixed electromagnetic and optical phonons waves that propagate at the surface of ionic materials [1]. They strongly influence the optical and thermal behavior of nanomaterials. For example, they are responsible for highly coherent emission of SiC upon heating, in stark contrast with the conventional incoherent black-body radiation [2). They also induce enhanced thermal conduction in thin membranes [3] or heat transfer between two nanosurfaces [4]. These applications rely on the nanostructuration of the electromagnetic field in the vicinity of surfaces of metamaterials or nanoparticles. Designing or even engineering the electro-magnetic local density of states (EMLDOS) for specific functionalities require therefore the unambiguous visualization of such field modulations at the nanometer scale. Recently, EELS in a scanning transmission electron microscope (STEM) made it possible to measure phonons spectra at the nanometer [5, 6], then atomic scales [7]. Nevertheless, they were restricted to 2D imaging, and not able to reveal the complete three-dimensional vectorial picture of their electromagnetic density of states. Using a highly monochromated electron beam in a scanning transmission electron microscope, we could visualize varying SPhPs signatures from nanoscale MgO cubes as a function of the beam position, energy-loss and tilt angle, see Fig 1. Following early works on plasmons [8,9], the SPhPs response was described in terms of eigenmodes and used to tomographically reconstruct the phononic surface electromagnetic fields of the object [10]. Such 3D information promises novel insights in nanoscale physical phenomena and is invaluable to the design and optimization of nanostructures for fascinating new uses. Références/References : [1] Kliewer, R. Fuchs, Theory of dynamical properties of dielectric surfaces, vol. 27 (1974). [2] J. Greffet, et al., Nature 416, 61 (2002). [3] Y. Wu, et al., Science Advances 6 (2020). [4] B. Song, et al., Nature Nanotechnology 10, 253 (2015). [5] O. L. Krivanek, et al., Nature 514, 209 (2014). [6] M. J. Lagos, et al., Nature 543, 529 (2017). [7] F. S. Hage, et al., Science 367, 1124 (2020). [8] Nicoletti et al., Nature 502, 7469 (2013). [9] Hörl et al., Nature Comm. 8, 1 (2017). [10] X. Li et al., Science 371, 6536 (2021).

Matthieu PICHER *, Yaowei Hu, Shyam Kanta Sinha, Jun Sun, Amir Khammari, Marlène Palluel, Nathalie Daro, Guillaume Chastanet, Ngoc Minh Tran, Eric Freysz, Florian Banhart,

• Institut de Physique et Chimie des Matériaux, CNRS UMR 7504, 67034 Strasbourg, France
• CNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600, Pessac, France
• Laboratoire Ondes et Matière d’aquitaine (LOMA), 33405 Talence, France

Dynamic processes at the nanoscale are not accessible by conventional TEM because they happen at much shorter timescales than the millisecond. This restriction is nowadays being overcome by developing ultrafast TEMs working with short electron pulses. In this approach, the transformation of interest is triggered by a short photonic pulse. Then, a short electron pulse probes the sample after an adjustable delay, which allows collecting pieces of information about the reaction process along its advancing at precise time steps, that is, with high temporal resolution [1,2]. Behind such a particular setup which brings into play ultrafast lasers with a transmission electron microscope stands a remarkable instrumental opportunity to study light/matter interactions within the typical high spatial resolution of electron microscopy. Here, we present two examples of investigations that have been carried out with the UTEM in Strasbourg. 1. A non-time-resolved TEM study of the laser-induced amorphization of metal nanocrystals: an infrared nanosecond laser pulse leads to fast melting of the metal nanocrystal which then dissolves carbon atoms from surrounding graphitic species, and is followed by fast cooling. This rapid quenching does not allow structural ordering and leaves a metastable amorphous metal-carbon phase. This shows how short IR pulses irradiating encapsulated metal nanocrystals can be a route towards the fabrication and stabilization of otherwise unfavorable amorphous metal or metal-carbon phases. ([3], Fig1) 2. A temporally resolved study of Spin Crossover (SCO) switching phenomena, carried out with nanosecond pulses in a stroboscopic approach. Here, individual SCO nanoparticles encapsulating gold nanorods were subjected to IR pulses, which lead to plasmonic heating of the Au rods and consequently to the thermal spin transition of the SCO associated with a significant length change. This correlated elongation was monitored with nanosecond and nanometer resolutions, and compared with time-resolved optical measurements previously acquired on a large ensemble of SCO. (Fig2) Remerciements Funding by the Agence Nationale de Recherche (ANR-11-EQPX-0041,ANR-17-CE09-0010), by the University of Strasbourg Institute of Advanced Studies (USIAS), and the METSA Institute are gratefully acknowledged. Références [1] M.Picher, K.Bücker, T.LaGrange, F.Banhart, Ultramicroscopy,188,(2018),p41 [2] SK.Sinha, A.Khammari, et al., Nature Communications,10,(2019),p3648 [3] J.Sun, SK.Sinha, et al., Carbon,161,(2020),p495-501

Understanding the molecular mechanisms underlying bubble-(bio)surfaces interactions is currently a challenge that if overcame, would allow to understand and control the various processes in which they are involved. Atomic force microscopy is a valuable tool to measure such interactions, but it is limited by the large size and instability of bubbles that can be attached on surfaces or on AFM cantilevers. To overcome these challenges, we here develop a new method to probe more accurately the interactions between bubbles and (bio)-interfaces by taking advantage of the fluidic force microscopy technology (FluidFM) that combines AFM with microfluidics. In this system, a micro-sized channel is integrated into an AFM cantilever and connected to a pressure controller system, thus creating a continuous and closed fluidic conduit that can be filled with a solution, while the tool can be immersed in a liquid environment [1]. An aperture at the end of the cantilever allows liquids to be dispensed locally. In this study, we use FluidFM in an original manner, to produce microsized bubbles of 8 µm in diameter, directly at the aperture of the microchanneled FluidFM cantilevers. For that, as shown in Figure 1 instead of liquid, the cantilever is filled with air and immersed in a liquid environment. By applying a positive pressure inside the cantilever, we succeeded in forming bubbles of controlled size directly at its aperture. Because the same pressure is maintained in the cantilever during the experiment, the dissolution of the gases from the bubble is compensated, which allows keeping the size of the bubble constant over time. After the characterization of the bubbles produced using this method, their interactions with hydrophobic surfaces were probed, showing that bubbles behave like hydrophobic surfaces. Thus they can be used to measure the hydrophobic properties of microorganisms’ surfaces, but in this case the interactions are also influenced by electrostatic forces. Finally we developed a strategy to functionalize their surface, thereby modulating their interactions with microorganisms’ surfaces. This new method provides a valuable tool to understand bubble-(bio)surfaces interactions but also to engineer them.