Probing the Polarization of Low-Energy Excitations in 2D Materials from Atomic Crystals to Nanophotonic Arrays Using Momentum-Resolved Electron Energy Loss Spectroscopy.

Spectroscopies utilizing free electron beams as probes offer detailed information on the reciprocal-space excitations of 2D materials such as graphene and transition metal dichalcogenide monolayers. Yet, despite the attention paid to such quantum materials, less consideration has been given to the electron-beam characterization of 2D periodic nanostructures such as photonic crystals, metasurfaces, and plasmon arrays, which can exhibit the same lattice and excitation symmetries as their atomic analogues albeit at drastically different length, momentum, and energy scales. Because of their lack of covalent bonding and influence of retarded electromagnetic interactions, important physical distinctions arise that complicate interpretation of scattering signals. Here we present a fully-retarded theoretical framework for describing the inelastic scattering of wide-field electron beams from 2D materials and apply it to investigate the complementarity in sample excitation information gained in the measurement of a honeycomb plasmon array versus angle-resolved optical spectroscopy in comparison to single monolayer graphene.

Reduced-Dimensionality Al Nanocrystals: Nanowires, Nanobars, and Nanomoustaches.

Aluminum nanocrystals created by catalyst-driven colloidal synthesis support excellent plasmonic properties, due to their high level of elemental purity, monocrystallinity, and controlled size and shape. Reduction in the rate of nanocrystal growth enables the synthesis of highly anisotropic Al nanowires, nanobars, and singly twinned "nanomoustaches". Electron energy loss spectroscopy was used to study the plasmonic properties of these nanocrystals, spanning the broad energy range needed to map their plasmonic modes. The coupling between these nanocrystals and other plasmonic metal nanostructures, specifically Ag nanocubes and Au films of controlled nanoscale thickness, was investigated. Al nanocrystals show excellent long-term stability under atmospheric conditions, providing a practical alternative to coinage metal-based nanowires in assembled nanoscale devices.

Probing the Interaction between Individual Metal Nanocrystals and Two-Dimensional Metal Oxides via Electron Energy Loss Spectroscopy.

Metal nanoparticles can photosensitize two-dimensional metal oxides, facilitating their electrical connection to devices and enhancing their abilities in catalysis and sensing. In this study, we investigated how individual silver nanoparticles interact with two-dimensional tin oxide and antimony-doped indium oxide using electron energy loss spectroscopy (EELS). The measurement of the spectral line width of the longitudinal plasmon resonance of the nanoparticles in absence and presence of 2D materials allowed us to quantify the contribution of chemical interface damping to the line width. Our analysis reveals that a stronger interaction (damping) occurs with 2D antimony-doped indium oxide due to its highly homogeneous surface. The results of this study offer new insight into the interaction between metal nanoparticles and 2D materials.

Unveiling Variations in Electronic and Atomic Structures Due to Nanoscale Wurtzite and Zinc Blende Phase Separation in GaAs Nanowires.

Phase separation is an intriguing phenomenon often found in III-V nanostructures, but its effect on the atomic and electronic structures of III-V nanomaterials is still not fully understood. Here we study the variations in atomic arrangement and band structure due to the coexistence of wurtzite (WZ) and zinc blende (ZB) phases in single GaAs nanowires by using scanning transmission electron microscopy and monochromated electron energy loss spectroscopy. The WZ lattice distances are found to be larger (by ∼1%), along both the nanowire length direction and the perpendicular direction, than the ZB lattice. The band gap of the WZ phase is ∼20 meV smaller than that of the ZB phase. A shift of ∼70 meV in the conduction band edge between the two phases is also found. The direct and local measurements in single GaAs nanowires reveal important effects of phase separation on the properties of individual III-V nanostructures.

Engineering 2D Material Exciton Line Shape with Graphene/h-BN Encapsulation.

Control over the optical properties of atomically thin two-dimensional (2D) layers, including those of transition metal dichalcogenides (TMDs), is needed for future optoelectronic applications. Here, the near-field coupling between TMDs and graphene/graphite is used to engineer the exciton line shape and charge state. Fano-like asymmetric spectral features are produced in WS2, MoSe2, and WSe2 van der Waals heterostructures combined with graphene, graphite, or jointly with hexagonal boron nitride (h-BN) as supporting or encapsulating layers. Furthermore, trion emission is suppressed in h-BN encapsulated WSe2/graphene with a neutral exciton red shift (44 meV) and binding energy reduction (30 meV). The response of these systems to electron beam and light probes is well-described in terms of 2D optical conductivities of the involved materials. Beyond fundamental insights into the interaction of TMD excitons with structured environments, this study opens an unexplored avenue toward shaping the spectral profile of narrow optical modes for application in nanophotonic devices.

Subnanometer Scale Mapping of Hydrogen Doping in Vanadium Dioxide.

Hydrogen donor doping of correlated electron systems such as vanadium dioxide (VO2) profoundly modifies the ground state properties. The electrical behavior of HxVO2 is strongly dependent on the hydrogen concentration; hence, atomic scale control of the doping process is necessary. It is however a nontrivial problem to quantitatively probe the hydrogen distribution in a solid matrix. As hydrogen transfers its sole electron to the material, the ionization mechanism is suppressed. In this study, a methodology mapping the doping distribution at subnanometer length scale is demonstrated across a HxVO2 thin film focusing on the oxygen-hydrogen bonds using electron energy loss spectroscopy (EELS) coupled with first-principles EELS calculations. The hydrogen distribution was revealed to be nonuniform along the growth direction and between different VO2 grains, calling for intricate hydrogenation mechanisms. Our study points to a powerful approach to quantitatively map dopant distribution in quantum materials relevant to energy and information sciences.

Nanoscale Localized Phonons at Al2O3 Grain Boundaries.

Nanoscale defects like grain boundaries (GBs) would introduce local phonon modes and affect the bulk materials' thermal, electrical, optical, and mechanical properties. It is highly desirable to correlate the phonon modes and atomic arrangements for individual defects to precisely understand the structure-property relation. Here we investigated the localized phonon modes of Al2O3 GBs by combination of the vibrational electron energy loss spectroscopy (EELS) in scanning transmission electron microscope and density functional perturbation theory (DFPT). The differences between GB and bulk obtained from the vibrational EELS show that the GB exhibited more active vibration at the energy range of <50 meV and >80 meV, and further DFPT results proved the wide distribution of bond lengths at GB are the main factor for the emergence of local phonon modes. This research provides insights into the phonon-defect relation and would be of importance in the design and application of polycrystalline materials.

Current opinion on the prospect of mapping electronic orbitals in the transmission electron microscope: State of the art, challenges and perspectives.

The concept of electronic orbitals has enabled the understanding of a wide range of physical and chemical properties of solids through the definition of, for example, chemical bonding between atoms. In the transmission electron microscope, which is one of the most used and powerful analytical tools for high-spatial-resolution analysis of solids, the accessible quantity is the local distribution of electronic states. However, the interpretation of electronic state maps at atomic resolution in terms of electronic orbitals is far from obvious, not always possible, and often remains a major hurdle preventing a better understanding of the properties of the system of interest. In this review, the current state of the art of the experimental aspects for electronic state mapping and its interpretation as electronic orbitals is presented, considering approaches that rely on elastic and inelastic scattering, in real and reciprocal spaces. This work goes beyond resolving spectral variations between adjacent atomic columns, as it aims at providing deeper information about, for example, the spatial or momentum distributions of the states involved. The advantages and disadvantages of existing experimental approaches are discussed, while the challenges to overcome and future perspectives are explored in an effort to establish the current state of knowledge in this field. The aims of this review are also to foster the interest of the scientific community and to trigger a global effort to further enhance the current analytical capabilities of transmission electron microscopy for chemical bonding and electronic structure analysis.

Cathodoluminescence emission and electron energy loss absorption from a 2D transition metal dichalcogenide in van der Waals heterostructures.

Nanoscale variations of optical properties in transition metal dichalcogenide (TMD) monolayers can be explored with cathodoluminescence (CL) and electron energy loss spectroscopy (EELS) using electron microscopes. To increase the CL emission intensity from TMD monolayers, the MoSe2flakes are encapsulated in hexagonal boron nitride (hBN), creating van der Waals (VdW) heterostructures. Until now, the studies have been exclusively focused on scanning transmission electron microscopy (STEM-CL) or scanning electron microscopy (SEM-CL), separately. Here, we present results, using both techniques on the same sample, thereby exploring a large acceleration voltage range. We correlate the CL measurements with STEM-EELS measurements acquired with different energy dispersions, to access both the low-loss region at ultra-high spectral resolution, and the core-loss region. This provides information about the weight of the various absorption phenomena including the direct TMD absorption, the hBN interband transitions, the hBN bulk plasmon, and the core losses of the atoms present in the heterostructure. The S(T)EM-CL measurements from the TMD monolayer only show emission from the A exciton. Combining the STEM-EELS and S(T)EM-CL measurements, we can reconstruct different decay pathways leading to the A exciton CL emission. The comparison with SEM-CL shows that this is also a good technique for TMD heterostructure characterization, where the reduced demands on sample preparation are appealing. To demonstrate the capabilities of SEM-CL imaging, we also measured on a SiO2/Si substrate, quintessential in the sample preparation of two-dimensional materials, which is electron-opaque and can only be measured in SEM-CL. The CL-emitting defects of SiO2make this substrate challenging to use, but we demonstrate that this background can be suppressed by using lower electron energy.

Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates.

The recent observation of superconductivity in [Formula: see text] has raised fundamental questions about the hierarchy of the underlying electronic structure. Calculations suggest that this system falls in the Mott-Hubbard regime, rather than the charge-transfer configuration of other nickel oxides and the superconducting cuprates. Here, we use state-of-the-art, locally resolved electron energy-loss spectroscopy to directly probe the Mott-Hubbard character of [Formula: see text] Upon doping, we observe emergent hybridization reminiscent of the Zhang-Rice singlet via the oxygen-projected states, modification of the Nd 5d states, and the systematic evolution of Ni 3d hybridization and filling. These experimental data provide direct evidence for the multiband electronic structure of the superconducting infinite-layer nickelates, particularly via the effects of hole doping on not only the oxygen but also nickel and rare-earth bands.

Machine Learning Data Augmentation Strategy for Electron Energy Loss Spectroscopy: Generative Adversarial Networks.

Recent advances in machine learning (ML) have highlighted a novel challenge concerning the quality and quantity of data required to effectively train algorithms in supervised ML procedures. This article introduces a data augmentation (DA) strategy for electron energy loss spectroscopy (EELS) data, employing generative adversarial networks (GANs). We present an innovative approach, called the data augmentation generative adversarial network (DAG), which facilitates data generation from a very limited number of spectra, around 100. Throughout this study, we explore the optimal configuration for GANs to produce realistic spectra. Notably, our DAG generates realistic spectra, and the spectra produced by the generator are successfully used in real-world applications to train classifiers based on artificial neural networks (ANNs) and support vector machines (SVMs) that have been successful in classifying experimental EEL spectra.

Excitonic Effects in Energy-Loss Spectra of Freestanding Graphene.

In this work, we perform electron energy-loss spectroscopy (EELS) of freestanding graphene with high energy and momentum resolution to disentangle the quasielastic scattering from the excitation gap of Dirac electrons close to the optical limit. We show the importance of many-body effects on electronic excitations at finite transferred momentum by comparing measured EELS to ab initio calculations at increasing levels of theory. Quasi-particle corrections and excitonic effects are addressed within the GW approximation and the Bethe-Salpeter equation, respectively. Both effects are essential in the description of the EEL spectra to obtain a quantitative agreement with experiments, with the position, dispersion, and shape of both the excitation gap and the π plasmon being significantly affected by excitonic effects.

Superradiant Electron Energy Loss Spectroscopy.

We analyze the interaction between a free electron and an ensemble of identical optical emitters. The mutual coherence and correlations between the emitters can enhance the interaction with each electron and become imprinted on its energy spectrum. We present schemes by which such collective interactions can be realized. As a possible application, we investigate free-electron interactions with superradiant systems, showing how electrons can probe the ultrafast population dynamics of superradiance.

Direct Imaging of Surface Melting on a Single Sn Nanoparticle.

Despite previous studies, understanding the fundamental mechanism of melting metal nanoparticles remains one of the major scientific challenges of nanoscience. Herein, the kinetics of melting of a single Sn nanoparticle was investigated using in situ transmission electron microscopy heating techniques with a temperature step of up to 0.5 °C. We revealed the surface premelting effect and assessed the density of the surface overlayer on a tin particle of 47 nm size using a synergetic combination of high-resolution scanning transmission electron microscopy imaging and low electron energy loss spectral imaging. Few-monolayer-thick disordered phase nucleated at the surface of the Sn particle at a temperature ∼25 °C below the melting point and grew (up to a thickness of ∼4.5 nm) into the solid core with increasing temperature until the whole particle became liquid. We revealed that the disordered overlayer was not liquid but quasi-liquid with a density intermediate between that of solid and liquid Sn.

Atomic-Resolution Mapping of Localized Phonon Modes at Grain Boundaries.

Phonon scattering at grain boundaries (GBs) is significant in controlling the nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, milli-electron volt (meV) energy resolution is needed with subnanometer spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterized by the presence of 5- and 7-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides.

Imaging Photon-Induced Near-Field Distributions of a Plasmonic, Self-Assembled Vesicle by a Laser-Integrated Electron Microscope.

Plasmonic polymeric nanoassemblies offer valuable opportunities in photoconversion applications. Localized surface plasmon mechanisms behind such nanoassemblies govern their functionalities under light illumination. However, an in-depth investigation at the single nanoparticle (NP) level is still challenging, especially when the buried interface is involved, due to the availability of suitable techniques. Here, we synthesized an anisotropic heterodimer composed of a self-assembled polymer vesicle (THPG) capped with a single gold NP, enabling an 8-fold enhancement in hydrogen generation compared to the nonplasmonic THPG vesicle. We explored the anisotropic heterodimer at the single particle level by employing advanced transmission electron microscopes, including one equipped with a femtosecond pulsed laser, which allows us to visualize the polarization- and frequency-dependent distribution of the enhanced electric near fields at the vicinity of Au cap and Au-polymer interface. These elaborated fundamental findings may guide designing new hybrid nanostructures tailored for plasmon-related applications.

Direct Visualization of Subnanometer Variations in the Excitonic Spectra of 2D/3D Semiconductor/Metal Heterostructures.

The integration of metallic contacts with two-dimensional (2D) semiconductors is routinely required for the fabrication of nanoscale devices. However, nanometer-scale variations in the 2D/metal interface can drastically alter the local optoelectronic properties. Here, we map local excitonic changes of the 2D semiconductor MoS2 in contact with Au. We utilize a suspended and epitaxially grown 2D/metal platform that allows correlated electron energy-loss spectroscopy (EELS) and angle resolved photoelectron spectroscopy (nanoARPES) mapping. Spatial localization of MoS2 excitons uncovers an additional EELS peak related to the MoS2/Au interface. NanoARPES measurements indicate that Au-S hybridization decreases substantially with distance from the 2D/metal interface, suggesting that the observed EELS peak arises due to dielectric screening of the excitonic Coulomb interaction. Our results suggest that increasing the van der Waals distance could optimize excitonic spectra of mixed-dimensional 2D/3D interfaces and highlight opportunities for Coulomb engineering of exciton energies by the local dielectric environment or moiré engineering.

Hollow Ptychography: Toward Simultaneous 4D Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy.

With the recent development of high-acquisition-speed pixelated detectors, 4D scanning transmission electron microscopy (4D-STEM) is becoming routinely available in high-resolution electron microscopy. 4D-STEM acts as a "universal" method that provides local information on materials that is challenging to extract from bulk techniques. It extends conventional STEM imaging to include super-resolution techniques and to provide quantitative phase-based information, such as differential phase contrast, ptychography, or Bloch wave phase retrieval. However, an important missing factor is the chemical and bonding information provided by electron energy loss spectroscopy (EELS). 4D-STEM and EELS cannot currently be acquired simultaneously due to the overlapping geometry of the detectors. Here, the feasibility of modifying the detector geometry to overcome this challenge for bulk specimens is demonstrated, and the use of a partial or defective detector for ptycholgaphic structural imaging is explored. Results show that structural information beyond the diffraction-limit and chemical information from the material can be extracted together, resulting in simultaneous multi-modal measurements, adding the additional dimensions of spectral information to 4D datasets.

Probing the Ni(OH)2 Precursor for LiNiO2 at the Atomic Scale: Insights into the Origin of Structural Defect in a Layered Cathode Active Material.

In lithium ion batteries (LIBs), the layered cathode materials of composition LiNi1- x - y Cox Mny O2  are critical for achieving high energy densities. A high nickel content (>80%) provides an attractive balance between high energy density, long lifetime, and low cost. Consequently, Ni-rich layered oxides cathode active materials (CAMs) are in high demand, and the importance of LiNiO2 (LNO) as limiting case, is hence paramount. However, achieving perfect stoichiometry is a challenge resulting in various structural issues, which successively impact physicochemical properties and result in the capacity fade of LIBs. To better understand defect formation in LNO, the role of the Ni(OH)2  precursor morphology in the synthesis of LNO requires in-depth investigation. By employing aberration-corrected scanning transmission electron microscopy, electron energy loss spectroscopy, and precession electron diffraction, a direct observation of defects in the Ni(OH)2  precursor preparedis reported and the ex situ structural evolution from the precursor to the end product is monitored. During synthesis, the layered Ni(OH)2  structure transforms to partially lithiated (non-layered) NiO and finally to layered LNO. The results suggest that the defects observed in commercially relevant CAMs originate to a large extent from the precursors, hence care must be taken in tuning the co-precipitation parameters to synthesize defect-free Ni-rich layered oxides CAMs.

Unveiling the Coupling of Single Metallic Nanoparticles to Whispering-Gallery Microcavities.

Whispering-gallery mode resonators host multiple trapped narrow-band 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 spectroscopy with 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.