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The stellar optoelectronic properties of metal halide perovskites provide enormous promise for next-generation optical devices with excellent conversion efficiencies and lower manufacturing costs. However, there is a long-standing ambiguity as to whether the perovskite surface/interface (e.g. structure, charge transfer or source of off-target recombination) or bulk properties are the more determining factor in device performance. We fabricated an array of CsPbI3 crystal and hybrid glass composites by sintering and globally visualised the property-performance landscape. Our findings reveal that the interface is the primary determinant of the crystal phases, optoelectronic quality, and stability of CsPbI3. In particular, the presence of a diffusion "alloying" layer is discovered to be critical for passivating surface traps, and beneficially altering the energy landscape of crystal phases. However, high-temperature sintering results in the promotion of a non-stoichiometric perovskite and excess traps at the interface, despite the short-range structure of halide is retained within the alloying layer. By shedding light on functional hetero-interfaces, our research offers the key factors for engineering high-performance perovskite devices.

Moir\'{e} twist angle underpins the interlayer interaction of excitons in twisted van der Waals hetero- and homo-structures. The influence of twist angle on the excitonic absorption of twisted bilayer tungsten diselenide (WSe$_{2}$) has been investigated using electron energy-loss spectroscopy. Atomic-resolution imaging by scanning transmission electron microscopy was used to determine key structural parameters, including the nanoscale measurement of the relative twist angle and stacking order. Detailed spectral analysis revealed a pronounced blueshift in the high-energy excitonic peak C with increasing twist angle, up to 200 meV when compared to the AA$^{\prime}$ stacking. The experimental findings have been discussed relative to first-principle calculations of the dielectric response of the AA$^{\prime}$ stacked bilayer WSe$_{2}$ as compared to monolayer WSe$_{2}$ by employing the \textit{GW} plus Bethe-Salpeter equation (BSE) approaches, resolving the origin of higher energy spectral features from ensembles of excitonic transitions, and thus any discrepancies between previous calculations. Furthermore, the electronic structure of moir\'{e} supercells spanning twist angles of $\sim$9.5-46.5$^{\circ}$ calculated by density functional theory (DFT) were unfolded, showing an uplifting of the conduction band minimum near the $Q$ point and minimal change in the upper valence band concurrently. The combined experiment/theory investigation provides valuable insight into the physical origins of high-energy absorption resonances in twisted bilayers, which enables to track the evolution of interlayer coupling from tuning of the exciton C transitions by absorption spectroscopy.

International audience; Scanning transmission electron microscopy is a crucial tool for nanoscience, achieving sub-nanometric spatial resolution in both image and spectroscopic studies. This generates large datasets that cannot be analyzed without computational assistance. The so-called machine learning procedures can exploit redundancies and find hidden correlations. Principal component analysis (PCA) is the most popular approach to denoise data by reducing data dimensionality and extracting meaningful information; however, there are many open questions on the accuracy of reconstructions. We have used experiments and simulations to analyze the effect of PCA on quantitative chemical analysis of binary alloy (AuAg) nanoparticles using energy-dispersive X-ray spectroscopy. Our results demonstrate that it is possible to obtain very good fidelity of chemical composition distribution when the signal-to-noise ratio exceeds a certain minimal level. Accurate denoising derives from a complex interplay between redundancy (data matrix size), counting noise, and noiseless data intensity variance (associated with sample chemical composition dispersion). We have suggested several quantitative bias estimators and noise evaluation procedures to help in the analysis and design of experiments. This work demonstrates the high potential of PCA denoising, but it also highlights the limitations and pitfalls that need to be avoided to minimize artifacts and perform reliable quantification.

International audience; Following the lifespan of optical excitations from their creation to decay into photons is crucial in understanding materials optical properties. Macroscopically, techniques such as the photoluminescence excitation spectroscopy provide unique information on the photophysics of materials with applications as diverse as quantum optics or photovoltaics. Materials excitation and emission pathways are affected by nanometer scale variations directly impacting devices performances. However, they cannot be directly accessed, despite techniques, such as optical spectroscopies with free electrons, having the relevant spatial, spectral or time resolution. Here, we explore optical excitation creation and decay in two representative optical devices: plasmonic nanoparticles and luminescent 2D layers. The analysis of the energy lost by an exciting electron that is coincident in time with a visible-UV photon unveils the decay pathways from excitation towards light emission. This is demonstrated for phase-locked interactions, such as in localized surface plasmons, and non-phase-locked ones, such as the light emission by individual point defects. This newly developed cathodoluminescence excitation spectroscopy images energy transfer pathways at the nanometer scale. It widens the toolset available to explore nanoscale materials.

Imaging polariton dynamics Two-dimensional (2D) materials can confine light to volumes much shorter than the wavelength, and, together, the long propagation lengths make them attractive materials for developing nanophotonic platforms. Characterizing the spatiotemporal control of 2D polariton wave packets has been hindered for the same reasons that make their potential applications exciting: They have extremely small wavelengths and are strongly confined inside the material. Kurman et al. developed a new pump-probe technique based on electron emission that provides access to the spatiotemporal dynamics of 2D polaritons. The nanometric spatial resolution and femtosecond temporal resolution will be useful for probing the excitation dynamics of these materials. Science , abg9015, this issue p. 1181

Analytical studies of nanoparticles (NPs) are frequently based on huge datasets derived from hyperspectral images acquired using scanning transmission electron microscopy. These large datasets require machine learning computational tools to reduce dimensionality and extract relevant information. Principal component analysis (PCA) is a commonly used procedure to reconstruct information and generate a denoised dataset; however, several open questions remain regarding the accuracy and precision of reconstructions. Here, we use experiments and simulations to test the effect of PCA processing on data obtained from AuAg alloy NPs a few nanometers wide with different compositions. This study aims to address the reliability of chemical quantification after PCA processing. Our results show that the PCA treatment mitigates the contribution of Poisson noise and leads to better quantification, indicating that denoised results may be reliable from the point of view of both uncertainty and accuracy for properly planned experiments. However, the initial data need to be of sufficient quality: these results can only be obtained if the signal-to-noise ratio of input data exceeds a minimal value to avoid the occurrence of random noise bias in the PCA reconstructions.

The excitonic states of transition metal dichacolgenide (TMD) monolayers are heavily influenced by their external dielectric environment based on the substrate used. In this work, various wide bandgap dielectric materials, namely hexagonal boron nitride (\textit{h}-BN) and amorphous silicon nitride (Si$_3$N$_4$), under different configurations as support or encapsulation material for WS$_2$ monolayers are investigated to disentangle the factors contributing to inhomogeneous broadening of exciton absorption lines in TMDs using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). In addition, monolayer roughness in each configuration was determined from tilt series of electron diffraction patterns by assessing the broadening of diffraction spots by comparison with simulations. From our experiments, the main factors that play a role in linewidth broadening can be classified in increasing order of importance by: monolayer roughness, surface cleanliness, and substrate-induced charge trapping. Furthermore, because high-energy electrons are used as a probe, electron beam-induced damage on bare TMD monolayer is also revealed to be responsible for irreversible linewidth increases. \textit{h}-BN not only provides clean surfaces of TMD monolayer, and minimal charge disorder, but can also protect the TMD from irradiation damage. This work provides a better understanding of the mechanisms by which \textit{h}-BN remains, to date, the most compatible material for 2D material encapsulation, facilitating the realization of intrinsic material properties to their full potential.

Récemment, les microscopies électroniques à balayage et/ou en transmission se sont imposées comme des techniques de choix pour étudier les phénomènes optiques avec des résolutions spatiales bien meilleures que la limite de diffraction optique. Les spectroscopies de perte et de gain d’énergie, ainsi que la cathodoluminescence permettent ainsi d’étudier à une échelle pertinente des excitations comme les plasmons de surface dans les nanoparticules métalliques ou les excitons dans des structures confinées quantiquement.

Structural, electronic, and chemical nanoscale modifications of transition metal dichalcogenide monolayers alter their optical properties. A key missing element for complete control is a direct spatial correlation of optical response to nanoscale modifications due to the large gap in spatial resolution between optical spectroscopy and nanometer-resolved techniques. Here, we bridge this gap by obtaining nanometer-resolved optical properties using electron spectroscopy at cryogenic temperatures, specifically electron energy loss spectroscopy for absorption and cathodoluminescence for emission, which are then directly correlated to chemical and structural information. In an h-BN/WS

Lead halide perovskite (LHP) semiconductors show exceptional optoelectronic properties. Barriers for their applications, however, lie in their polymorphism, instability to polar solvents, phase segregation, and susceptibility to the leaching of lead ions. We report a family of scalable composites fabricated through liquid-phase sintering of LHPs and metal-organic framework glasses. The glass acts as a matrix for LHPs, effectively stabilizing nonequilibrium perovskite phases through interfacial interactions. These interactions also passivate LHP surface defects and impart bright, narrow-band photoluminescence with a wide gamut for creating white light-emitting diodes (LEDs). The processable composites show high stability against immersion in water and organic solvents as well as exposure to heat, light, air, and ambient humidity. These properties, together with their lead self-sequestration capability, can enable breakthrough applications for LHPs.

Whispering-gallery mode resonators host multiple trapped narrowband circulating optical resonances that find applications in quantum electrodynamics, optomechanics, and sensing. However, the spherical symmetry and low field leakage of dielectric microspheres make it difficult to probe their high-quality optical modes using far-field radiation. Even so, local field enhancement from metallic nanoparticles (MNPs) coupled to the resonators can interface the optical far-field and the bounded cavity modes. In this work, we study the interaction between whispering-gallery modes and MNP surface plasmons with nanometric spatial resolution by using electron-beam spectroscopies in a scanning transmission electron microscope. We show that gallery modes are induced over a selective spectral range of the nanoparticle plasmons, and additionally, their polarization can be controlled by the induced dipole moment of the MNP. Our study demonstrates a viable mechanism to effectively excite high-quality-factor whispering-gallery modes and holds potential for applications in optical sensing and light manipulation., 37 pages, 13 figures

Hybrid/moiré interlayer and intralayer excitons have been realized in twisted two-dimensional transition metal chalcogenides (2D-TMD) due to variation in local moiré potential within a moiré supercell. Though moiré excitons have been detected in TMD heterostructures by macroscopic spectroscopic techniques, their spatial distribution is experimentally unknown. In the present work, using high-resolution scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS), we explore the effect of the twist angle in MoS

International audience; Rare earth (RE)-doped GaN nanowires (NWs), combining the well-defined and controllable optical emission lines of trivalent RE ions with the high crystalline quality, versatility, and small dimension of the NW host, are promising building blocks for future nanoscale devices in optoelectronics and quantum technologies. Europium doping of GaN NWs was performed by ion implantation, and structural and optical properties were assessed in comparison to thin film reference samples. Despite some surface degradation for high implantation fluences, the NW core remains of high crystalline quality with lower concentrations of extended defects than observed in ion-implanted thin films. Strain introduced by implantation defects is efficiently relaxed in NWs and the measured deformation stays much below that in thin films implanted in the same conditions. Optical activation is achieved for all samples after annealing, and while optical centers are similar in all samples, Eu3+ emission from NW samples is shown to be less affected by residual implantation damage than for the case of thin films. The incorporation of Eu in GaN NWs was further investigated by nano-cathodoluminescence and X-ray absorption spectroscopy (XAS). Maps of the Eu-emission intensity within a single NW agree well with the Eu-distribution predicted by Monte Carlo simulations, suggesting that no pronounced Eu-diffusion takes place. XAS shows that 70–80% of Eu is found in the 3+ charge state while 20–30% is 2+ attributed to residual implantation defects. A similar local environment was found for Eu in NWs and thin films: for low fluences, Eu is mainly incorporated on substitutional Ga-sites, while for high fluences XAS points at the formation of a local EuN-like next neighbor structure. The results reveal the high potential of ion implantation as a processing tool at the nanoscale.

Transmission electron microscopes (TEM) are widely used in nanotechnology research. However, it is still challenging to characterize nanoscale objects; their small size coupled with dynamical diffraction makes interpreting real- or reciprocal-space data difficult. Scanning precession electron diffraction ((S)PED) represents an invaluable contribution, reducing the dynamical contributions to the diffraction pattern at high spatial resolution. Here a detailed analysis of wurtzite InP nanowires (30–40 nm in diameter) containing a screw dislocation and an associated wire lattice torsion is presented. It has been possible to characterize the dislocation with great detail (Burgers and line vector, handedness). Through careful measurement of the strain field and comparison with dynamical electron diffraction simulations, this was found to be compatible with a Burgers vector modulus equal to one hexagonal lattice cell parameter despite the observed crystal rotation rate being larger (ca. 20%) than that predicted by classical elastic theory for the nominal wire diameter. These findings corroborate the importance of the (S)PED technique for characterizing nanoscale materials.

Silver is considered to be the king among plasmonic materials because it features low inelastic absorption in the visible and infrared (vis-IR) spectral regions compared to other metals. In contrast, copper is commonly regarded as being too lossy for plasmonic applications. Here, we experimentally demonstrate vis-IR plasmons in long copper nanowires (NWs) with quality factors that exceed a value of 60, as determined by spatially resolved, high-resolution electron energy-loss spectroscopy (EELS) measurements. We explain this counterintuitive result by the fact that plasmons in these metal wires have most of their electromagnetic energy outside the metal, and thus, they are less sensitive to inelastic losses in the material. We present an extensive set of data acquired on long silver and copper NWs of varying diameters supporting this conclusion and further allowing us to understand the relative roles played by radiative and nonradiative losses in plasmons that span a wide range of energies down to $, 13 pages, 9 figures, 56 references

Structural, electronic, and chemical nanoscale modifications of transition metal dichalcogenide monolayers alter their optical properties, including the generation of single photon emitters. A key missing element for complete control is a direct spatial correlation of optical response to nanoscale modifications, due to the large gap in spatial resolution between optical spectroscopy and nanometer resolved techniques, such as transmission electron microscopy or scanning tunneling microscopy. Here, we bridge this gap by obtaining nanometer resolved optical properties using electron spectroscopy, specifically electron energy loss spectroscopy (EELS) for absorption and cathodoluminescence (CL) for emission, which were directly correlated to chemical and structural information. In an h-BN/WS$_2$/h-BN heterostructure, we observe local modulation of the trion (X$^{-}$) emission due to tens of nanometer wide dielectric patches, while the exciton, X$_A$, does not follow the same modulation. Trion emission also increases in regions where charge accumulation occurs, close to the carbon film supporting the heterostructures. Finally, localized exciton emission (L) detection is not correlated to strain variations above 1 $\%$, suggesting point defects might be involved in their formations.

International audience; Hybrid/moiré interlayer and intralayer excitons have been realized in twisted two-dimensional transition metal chalcogenides (2D-TMD) due to variation in local moiré potential within a moiré supercell. Though moiré excitons have been detected in TMD heterostructures by macroscopic spectroscopic techniques, their spatial distribution is experimentally unknown. In the present work, using high-resolution scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS), we explore the effect of the twist angle in MoS2/WSe2 heterostructures. We observe weak interaction between the layers at higher twist angles (>5°) and stronger interaction for lower twist angles. The optical response of the heterostructure varies within the moiré supercell, with a lower energy absorption peak appearing in regions with the AA stacking.

Ultra-fast transmission electron microscopy (UTEM) combines sub-picosecond time-resolution with the versatility of TEM spectroscopies. It allows one to study the dynamics of materials properties combining complementary techniques. However, until now, time-resolved cathodoluminescence, which is expected to give access to the optical properties dynamics, was still unavailable in a UTEM. In this paper, we report time-resolved cathodoluminescence measurements in an ultrafast transmission electron microscope. We measured lifetime maps, with a 12 nm spatial resolution and sub-nanoseconds resolution, of nano-diamonds with a high density of NV center. This study paves the way to new applications of UTEM and to correlative studies of optically active nanostructures., Supplementary information at the end of the article

We use ultra-fast electron microscopy to measure the spatio-temporal dynamics of phonon-polariton wavepackets in a thin hBN sample. The electron probe enables recording the wavepacket formation, propagation, and decay, unveiling intriguing acceleration & deceleration phenomena.

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]

International audience; Andromede is a new instrument for mass spectrometry analysis of nanodomains and nano-objects present on a surface. The molecular sample information (mass and structure) is obtained from time-of-flight spectra of secondary ions emitted under the impact of nanoparticles (NPs), as A u n q + with n/q up to 200 accelerated by a 4 MV single stage electrostatic Pelletron® accelerator. Such projectiles, providing huge emission rates per impact, permit the topographic analysis of complex samples. The energy deposition of these massive gold NPs induces surprisingly large holes of about 100 nm diameter in very thin graphene foils and average sputtered volume per impact of about 106 nm3 in a thick polymer material. The use of such massive clusters associated with multianode detectors, allowing the simultaneous detection of several ions of a given mass, permits correlation studies between the emitted ions within a single impact, shedding additional light on the chemical composition and structure of the analysed sample as illustrated here for various samples from metallic surfaces to biologic molecules deposits.

Advances in the fabrication and characterization of nanowires polytypes have made crystal phase engineering a well-established tool to tailor material properties. In this review, recent progresses in the field are described, with special focus on the central role that crystal phase has in modulating the electronic and optical properties of nanowires. We start with an overview on III-V nanowires, which have been developed first and for which more experimental results already exist. Then, particular attention will be devoted to group IV polytypes which represent one of the most exciting and novel topic in the field. A survey of both theoretical and experimental efforts will be provided with the final aim of highlighting the importance of crystal phase control in materials design.

Plasmonic crystals are well known to have band structure including a band gap, enabling the control of surface plasmon propagation and confinement. The band dispersion relation of bulk crystals has been generally measured by momentum-resolved spectroscopy using far field optical techniques while the defects introduced in the crystals have separately been investigated by near field imaging techniques so far. Particularly, defect related energy levels introduced in the plasmonic band gap have not been observed experimentally. In order to investigate such a localized mode, we performed electron energy-loss spectroscopy (EELS) on a point defect introduced in a plasmonic crystal made up of flat cylinders protruding out of a metal film and arranged on a triangular lattice. The energy level of the defect mode was observed to lie within the full band-gap energy range. This was confirmed by a momentum-resolved EELS measurement of the band gap performed on the same plasmonic crystal. Furthermore, we experimentally and theoretically investigated the emergence of the defect states by starting with a corral of flat cylinders protrusions and adding sequentially additional shells of those in order to eventually form a plasmonic band-gap crystal encompassing a single point defect. It is demonstrated that a defectlike state already forms with a crystal made up of only two shells.

International audience; Electroreduction of CO2 to CO is one of the simplest ways to valorise CO2 as a source of carbon. Herein, a cheap, robust, Cu-based hybrid catalyst consisting of a polymer of Cu phthalocyanine coated on carbon nanotubes, which proved to be selective for CO production (80 % faradaic yield) at relatively low overpotentials, was developed. Polymerisation of Cu phthalocyanine was shown to have a drastic effect on the selectivity of the reaction because molecular Cu phthalocyanine was instead selective for proton reduction under the same conditions. Although the material only showed isolated Cu sites in phthalocyanine-like CuN4 coordination, in situ and operando X-ray absorption spectroscopy showed that, under operating conditions, the Cu atoms were fully converted to Cu nanoparticles, which were likely the catalytically active species. Interestingly, this restructuring of the metal sites was reversible.

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.

Polaritons are compositional light-matter quasiparticles that have enabled remarkable breakthroughs in quantum and nonlinear optics, as well as in material science. Recently, plasmon–exciton polaritons (plexcitons) have been realized in hybrid material systems composed of transition metal dichalcogenide (TMDC) materials and metal nanoparticles, expanding polaritonic concepts to room temperature and nanoscale systems that also benefit from the exotic properties of TMDC materials. Despite the enormous progress in understanding TMDC-based plexcitons using optical-based methods, experimental evidence of plexcitons formation has remained indirect and mapping their nanometer-scale characteristics has remained an open challenge. Here, we demonstrate that plexcitons generated by a hybrid system composed of an individual silver nanoparticle and a few-layer WS2 flake can be spectroscopically mapped with nanometer spatial resolution using electron energy loss spectroscopy in a scanning transmission electron microscope. Experimental anticrossing measurements using the absorption-dominated extinction signal provide the ultimate evidence for plexciton hybridization in the strong coupling regime. Spatially resolved EELS maps reveal the existence of unexpected nanoscale variations in the deep-subwavelength nature of plexcitons generated by this system. These findings pioneer new possibilities for in-depth studies of the local atomic structure dependence of polariton-related phenomena in TMDC hybrid material systems with nanometer spatial resolution.

The local density of optical states governs an emitters lifetime and quantum yield through the Purcell effect. It can be modified by a surface plasmon electromagnetic field, but such a field has a spatial extension limited to a few hundreds of nanometers, which complicates the use of optical methods to spatially probe the emitter-plasmon coupling. Here we show that a combination of electron-based imaging, spectroscopies and photon-based correlation spectroscopy enables measurement of the Purcell effect with nanometer and nanosecond spatio-temporal resolutions. Due to the large variability of radiative lifetimes of emitters embedded in nanoparticles with inhomogeneous sizes we relied on a statistical approach to unambiguously probe the coupling between nitrogen-vacancy centers (NV^0) in nanodiamonds and surface plasmons in silver nanocubes. We quantified the Purcell effect by measuring the NV^0 excited state lifetimes in a large number of either isolated nanodiamonds or nanodiamond-nanocube dimers and demonstrated a statistically significant lifetime reduction for dimers.

Here, we analyze the effect of Cr doping on WSe2 crystals. The topology and the chemistry of the doped samples have been investigated by atom-resolved scanning transmission electron microscopy combined with electron energy loss spectroscopy. Cr (measured to have formal valence 3+) occupies W sites (formal valence 4+), indicating a possible hole doping. However, single or double Se vacancies cluster near Cr atoms, leading to an effective electron doping. These defects organization can be explained by the strong binding energy of the CrW–Vse complex obtained by density functional theory calculations. In highly Cr-doped samples, a local phase transition from the 2H to the to 1T phase is observed, which has been previously reported for other electron-doped transition-metal dichalcogenides. Cr-doped crystals suffer a compressive strain, resulting in an isotropic lattice contraction and an anisotropic optical bandgap energy shift (25 meV in-plane and 80 meV out-of-plane).

With the help of electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) and boundary element method (BEM) simulations, we have shown that silver nanocross can be used to tune the plasmon modes and especially the higher order modes in a very controlled manner. Morphing a nanorod to a nanocross by growing another arm along the perpendicular bisector of a nanorod can lead to a spectacular evolution of various order modes in the EEL spectra. One can keep a mode fixed in energy while bringing the next higher order mode close to it, or even make it cross. This allows one to control the relative separation of energy of the plasmon modes. Hybridization of individual nanocross plasmon modes have also been studied by bringing two nanocrosses close together along one of their arms. An analytical approach based on first order perturbation theory and symmetry arguments based on group theory aptly fits the experimental observations.

Spatially resolved electron-energy-loss spectroscopy (EELS) is performed at diffuse interfaces between MoS2 and MoSe2 single layers. With a monochromated electron source (20 meV) we successfully probe excitons near the interface by obtaining the low loss spectra at the nanometer scale. The exciton maps clearly show variations even with a 10 nm separation between measurements; consequently, the optical band gap can be measured with nanometer-scale resolution, which is 50 times smaller than the wavelength of the emitted photons. By performing core-loss EELS at the same regions, we observe that variations in the excitonic signature follow the chemical composition. The exciton peaks are observed to be broader at interfaces and heterogeneous regions, possibly due to interface roughness and alloying effects. Moreover, we do not observe shifts of the exciton peak across the interface, possibly because the interface width is not much larger than the exciton Bohr radius.

International audience; It is generally believed that CO2 electroreduction to multi‐carbon products such as ethanol or ethylene may be catalyzed with significant yield only on metallic copper surfaces, implying large ensembles of copper atoms. Here, we report on an inexpensive Cu‐N‐C material prepared via a simple pyrolytic route that exclusively feature single copper atoms with a CuN4 coordination environment, atomically dispersed in a nitrogen‐doped conductive carbon matrix. This material achieves aqueous CO2 electroreduction to ethanol at a Faradaic yield of 55 % under optimized conditions (electrolyte: 0.1 m CsHCO3, potential: −1.2 V vs. RHE and gas‐phase recycling set up), as well as CO electroreduction to C2‐products (ethanol and ethylene) with a Faradaic yield of 80 %. During electrolysis the isolated sites transiently convert into metallic copper nanoparticles, as shown by operando XAS analysis, which are likely to be the catalytically active species. Remarkably, this process is reversible and the initial material is recovered intact after electrolysis.

The dependence of excited electron–hole state properties on the size of their host semiconducting nanostructures is the seed for a plethora of applications such as light-emitting diodes (LEDs) and photovoltaic cells. However, the inability of state-of-the art, diffraction-limited optical techniques to probe lifetime variations at the scale of individual quantum emitters precludes the full understanding of the nanostructures’ optical properties. Here, we demonstrate the measurement of the individual lifetimes of quantum emitters a few angstroms thick separated by only a few nanometers, lifting the ambiguities usually faced by diffraction-limited techniques. This relies on the ability to monitor with subnanometer precision a fast electron beam that triggers extremely localized cathodoluminescence signals further analyzed through intensity interferometry (spatially resolved time-correlated cathodoluminescence, SRTC-CL). We demonstrate SRTC-CL to be a true nanometer counterpart of time-resolved photoluminesce...

Combinations of 2D materials with different physical properties can form heterostructures with modified electrical, mechanical, magnetic, and optical properties. The direct observation of a lateral heterostructure synthesis is reported by epitaxial in‐plane graphene growth from the step‐edge of hexagonal BN (h‐BN) within a scanning transmission electron microscope chamber. Residual hydrocarbon in the chamber is the carbon source. The growth interface between h‐BN and graphene is atomically identified as largely N–C bonds. This postgrowth method can form graphene nanoribbons connecting two h‐BN domains with different twisting angles, as well as isolated carbon islands with arbitrary shapes embedded in the h‐BN layer. The electronic properties of the vertically stacked h‐BN/graphene heterostructures are investigated by electron energy‐loss spectroscopy (EELS). Low‐loss EELS analysis of the dielectric response suggests a robust coupling effect between the graphene and h‐BN layers.

International audience; We report a comprehensive study on the optical properties of cubic boron nitride (c-BN) and its optically active defects. Using electron energy-loss spectroscopy (EELS) within a monochromated scanning transmission electron microscope (STEM) on the highest-quality crystals available, we demonstrate unequivocally that the optical-gap energy of c-BN slightly exceeds 10 eV. Further theoretical analysis in the framework of the Bethe-Salpeter equation of many-body perturbation theory supports this result. The spatial localization of defect-related emissions has been investigated using nanometric resolved cathodoluminescence (nano-CL) in a STEM. By high-temperature annealing a c-BN powder, we have promoted phase transitions in nanometric domains which have been detected by the appearance of specific hexagonal-phase signatures in both EELS and CL spectra. A high number of intragap optically active centers are known in c-BN, but the literature is rather scattered and hence has been summarized here. For several emission lines we have obtained nano-CL maps which show emission spot sizes as small as few tens of nanometers. Finally, by coupling nano-CL to a Hanbury-Brown-Twiss intensity interferometer, we have addressed individual spots in order to identify the possible presence of single-photon sources. The observed CL bunching effect is compatible with a limited set of single-photon emitters and it permits obtaining emission lifetimes of the order of the nanosecond.