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International audience; Vibrational optical spectroscopies can be enhanced by surface plasmons to reach molecular-sized limits of detection and characterization. The level of enhancement strongly depends on microscopic details of the sample that are generally missed by macroscopic techniques. Here we investigate phonons in h-BN by coupling them to silver-nanowire plasmons, whose energy is tuned by modifying the nanowire length. Specifically, we use electron beam milling to accurately and iteratively change the nanowire length, followed by electron energy-loss spectroscopy to reveal the plasmon-enhanced vibrational features of h-BN. This allows us to investigate otherwise hidden bulk phonons and observe strong plasmon-phonon coupling. The new milling-and-spectroscopy technique holds great potential for resolving vibrational features in material nanostructures. Vibrational spectroscopy is key in a wide range of research areas and technological applications, from molecular fingerprinting to fundamental solid-state physics 1,2. The discovery that plasmonic structures can increase the measured vibrational signal has driven the development of ultra-sensitive analytical techniques capable of reaching single-molecule detection, such as in surface-enhanced Raman spectroscopy 3,4 (SERS) and surface-enhanced infrared absorption 5 (SEIRA). Typically, plas-monic structures are designed in advance and molecules are randomly dispersed over them, leading to strongly enhanced signals associated with those sitting on the so-called hotspots 4. Alternatively, a metallic tip can be scanned over a sample to induce SERS locally, leading to chemical mapping at sub-molecular scales, a technique known as tip-enhanced Raman scattering 6 (TERS). Vibrational electron energy-loss spectroscopy (EELS) has been performed for decades mainly as a surface technique 7 using wide beams with poor spatial resolution. The recent development of a new family of electron monochromators 8 has allowed vibration mode measurements to be performed based on EELS 9-11 down to atomic spatial resolution in bulk materials 12. Unfortunately , the signal-to-noise ratio of vibrational EELS is low for materials that are sensitive to the electron beam, thus imposing a lower bound on the volume necessary for analysis. This jeopardizes the high resolution mapping of fragile materials, such as organic molecules 13. Recently, theoretical studies 14-16 have proposed the use of infrared plasmonic fields to develop a new form of enhanced vi-brational EELS which would overcome these limitations by making the molecules interact with the beam at a distance mediated by plasmons extended in a nanoparticle. Here, we demonstrate plasmon-enhanced vibrational electron spectroscopy (PEVES) through the tailored coupling of plasmon resonances in metallic nanowires to phonon modes in h-BN thin flakes. Coupling is achieved by continuously shifting the energies of the plasmon modes of micrometer-long metallic nanowires (Fig. S1) using electron-beam controlled milling to bring them into resonance with specific h-BN vibrational modes (Fig. S2). We reveal three new effects when a plasmon-phonon resonance is encountered: 1) strong coupling between surface phonons and plasmons; 2) enhancement of the bulk vibrational EELS signal; and 3) emergence of previously geometry-forbidden dark phonon modes. EELS measures the distribution of energy losses experienced by free electrons when interacting with a target 17. The energy resolution is determined by the energy spread of the electron source, typically ∼250 meV for a cold field emitter source. This resolution can be improved down to a few meV using an electron monochromator (Fig. S3). To achieve subnanometer spatial resolution, the electron monochromator can be coupled to an electron microscope (Fig. S3). Finally, an electron spectrometer is used to acquire the spectrum of the electron beam after interaction with the sample. Here, we use such a setup implemented on a NION Hermes scanning transmission microscope (STEM), see Methods. The plasmonic silver metallic nanowires were synthesized by chemical seeded growth as detailed elsewhere 18 (Fig. 1A) and subsequently deposited, either entirely (configuration (1)) or partially (configuration (2)), on an h-BN substrate 19. In theory, extended h-BN possesses a arXiv:1905.12503v1 [cond-mat.mes-hall]

The evolution of the scanning modules for scanning transmission electron microscopes (STEM) has realized the possibility to generate arbitrary scan pathways, an approach currently explored to improve acquisition speed and to reduce electron dose effects. In this work, we present the implementation of a random scan operating mode in STEM achieved at the hardware level via a custom scan control module. A pre-defined pattern with fully shuffled raster order is used to sample the entire region of interest. Subsampled random sparse images can then be extracted at successive time frames, to which suitable image reconstruction techniques can be applied. With respect to the conventional raster scan mode, this method permits to limit dose accumulation effects, but also to decouple the spatial and temporal information in hyperspectral images. We provide some proofs of concept of the flexibility of the random scan operating mode, presenting examples of its applications in different spectro-microscopy contexts: atomically-resolved elemental maps with electron energy loss spectroscopy and nanoscale-cathodoluminescence spectrum images. By employing adapted post-processing tools, it is demonstrated that the method allows to precisely track and correct for sample instabilities and to follow spectral diffusion with a high spatial localization.

International audience; Electron microscopy has shown to be a very powerful tool to deeply analyze the chemical composition at various scales. However, many samples can not be analyzed with an acceptable signal-to-noise ratio because of the radiation damage induced by the electron beam. Particularly, electron energy loss spectroscopy (EELS) which acquires a spectrum for each spatial position requires high beam intensity. Scanning transmission electron microscopes (STEM) sequentially acquire data cubes by scanning the electron probe over the sample and record a spectrum for each spatial position. Recent works developed new acquisition procedures, which allow for partial acquisition schemes following a predetermined scan pattern. A reconstruction of the full data cube is conducted as a post-processing step. A multi-band image reconstruction procedure which exploits the spectral structure and the spatial smoothness of STEM-EELS images is explained here. The performance of the proposed scheme is illustrated thanks to experiments conducted on a realistic phantom dataset as well as real EELS spectrum-image.

International audience

International audience; Over the past two or three decades, nanoscience and nanotechnology have clearly established themselves as prominent domains in research in physics, not only because of the innovative concepts and properties that they display but also for their capacity to generate many important applications and commercial developments. As many of these new devices exhibit a range of properties (transport, optical, magnetic, catalysis) which are governed by local structural and electronic configurations, such as coordination and bonding at the atomic level, it is no surprise that new tools of investigation are constantly being developed for imaging, analyzing, understanding and controlling at the relevant scale. Among them, electron microscopy has recently demonstrated its ability to meet many of these requirements. In particular, Å-sized probes are nowadays generated by aberration correctors in a scanning transmission electron microscope (STEM) and they can investigate the electron excitation spectrum of the specimen (through electron energy-loss spectroscopy, EELS) with a typical energy resolution of 0.1–0.3 eV over a broad spectral band from the IR to the X ray domain. In the high energy range, characteristic signals due to the excitation of atomic core levels are quite useful because they identify the atoms in the analyzed volume (which can itself be as small as a single atom) and can therefore deliver atomically-resolved elemental maps. But the pixel-by-pixel recording of the fine structures beyond the characteristic threshold is much more informative and tells us how the excited atom is surrounded by its neighbors, what is its exact structural environment and its charge population. The present review focuses on this particularly exciting field, with a special interest in the types of information accessible and their signature. After summarizing the ingredients required for successful experiments (instrumental as well as theoretical), examples encountered in different situations, in particular in single layers of 2D materials and at the interfaces in oxide heterostructures, will demonstrate the present capabilities of this STEM-EELS technique.

© 2015 American Chemical Society. Plasmon modes of the exact same individual gold nanoprisms are investigated through combined nanometer-resolved electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) measurements. We show that CL only probes the radiative modes, in contrast to EELS, which additionally reveals dark modes. The combination of both techniques on the same particles thus provides complementary information and also demonstrates that although the radiative modes give rise to very similar spatial distributions when probed by EELS or CL, their resonant energies appear to be different. We trace this phenomenon back to plasmon dissipation, which affects in different ways the plasmon signatures probed by these techniques. Our experiments are in agreement with electromagnetic numerical simulations and can be further interpreted within the framework of a quasistatic analytical model. We therefore demonstrate that CL and EELS are closely related to optical scattering and extinction, respectively, with the addition of nanometer spatial resolution.

Elemental analysis of inhomogeneous materials can now routinely be performed at the nanometer level, using characteristic EELS signals. Major progress in spatial resolution and accuracy in quantification has recently been fostered by the practical implementation of the spectrum-image mode in a FEG-STEM environment. The required hardware and software have been elaborated to record, store and process these large amounts of data. In the present contribution we describe the routines which have been implemented for extracting quantitative elemental maps from spectrum-images: (i) the standard background subtraction method for which the availability of several hundreds of energy loss channels across the edge to be quantified reduces the errors and bias in background modelling and extrapolation; (ii) a non-negative multiple-least-squares routine for fitting the experimental spectrum acquired for each pixel with a linear combination of reference edges, if possible recorded during the same scan. The impact of these new tools is demonstrated in a series of situations encountered in materials science (composite materials, metallic multilayers) which all require nanometer resolution and accurate data processing of complex spectra with overlapping edges.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Electron energy-loss spectroscopy (EELS) is widely used to identify elemental compositions of materials studied by microscopy. We demonstrate that the sensitivity and spatial resolution of EELS can be extended to the single-atom limit. A chemical map for gadolinium (Gd) clearly reveals the distribution of Gd atoms inside a single chain of metallofullerene molecules (Gd@C 82 ) generated within a single-wall carbon nanotube. This characterization technique thus provides the “eyes” to see and identify individual atoms in nanostructures. It is likely to find broad application in nanoscale science and technology research.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

EELS fine structures on characteristic core edges are closely related to the bonding type and structural environment around the excited atoms. Consequently, they constitute useful hints for investigating the electronic properties at interfaces. One major difficulty then lies in the extraction of the information relevant to the involved atoms, from that due to neighbouring atoms on both sides of the selected interface seen end on by the impinging electrons. We have investigated possible ways to reach this goal, using the spectrum-line method, which consists in acquiring sequences of spectra at well defined increments (0.3 nm) while scanning a subnanometer electron probe (0.7 nm) across the interface. This approach constitutes one step forward with respect to the spatial-difference (SD) method which processes spectra acquired on two adjacent areas, one encompassing the interface, the other one on a reference area nearby. A normalized version (NSD) has been introduced to extend its field of application from grain boundaries to heterophase interfaces in order to enhance their specific ELNES contribution

The sophisticated acquisition procedures now available in time or space resolved spectroscopies, also known as spectrum-imaging modes, produce large amounts of data which require specific developments for efficient processing and information extraction. For instance, in electron energy-loss spectroscopy (EELS), a line-spectrum consists of typically one hundred spectra recorded at regular intervals when scanning the subnanometer incident electron probe across the feature of interest: interfaces, multilayers or nanostructures of various shapes and dimensions. The useful information in any of these spectra depends on many factors such as the problem under investigation, the involved energy-loss range or the signal-to-noise ratio of the different features. However it is generally contained in the spectral changes, as well in energy channel as in position along the sequence.To detect, measure and identify these variations, new methods have to be developed and the accompanying algorithms to be implemented. A first category encompasses all the routines which apply successively to all spectra in the sequence the well-known software which have been elaborated for processing individual spectra.

The shape and the arrangement of subunits in Escherichia coli F1-ATPase (ECF1) lacking the delta subunit have been explored with a high performance scanning transmission electron microscope. In tilting experiments, the ECF1 molecule appeared as a flat cylinder whose width (approx. 120 A) was about twice its height. The symmetry of front view projections of ECF1 has been investigated by computer analysis. In a population taken at random from the data bank, one third of the particles showed five-fold radial symmetry components, one third six-fold radial symmetry components and the last third no typical symmetry. The six-fold radial symmetry was consistent with a hexagonal arrangement of six large peripheric masses, which probably correspond to the three alpha and the three beta subunits of ECF1. The five-fold radial symmetry was tentatively explained by a fusion of two juxtaposed peripheric subunits. Lateral projections showed a zig-zag organization of the large masses, suggesting that the large alpha and beta subunits are located on two levels, with some degree of intercalation between the subunits of the two levels.