Vibrational Electron Energy Loss Spectroscopy Research Articles

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.

Linking Cause and Effect: Nanoscale Vibrational Spectroscopy of Space Weathering from Asteroid Ryugu

Airless bodies are subjected to space-weathering effects that modify the first few microns of their surface. Therefore, understanding their impact on the optical properties of asteroids is key to the interpretation of their color variability and infrared reflectance observations. The recent Hayabusa2 sample return mission to asteroid Ryugu offers the first opportunity to study these effects, in the case of the most abundant spectral type among the main-asteroid belt, C-type objects. This study employs vibrational electron energy-loss spectroscopy in the transmission electron microscope to achieve the spatial resolution required to measure the distinct mid-infrared spectral signature of Ryugu's space-weathered surface. The comparison with the spectrum of the pristine underlying matrix reveals the loss of structural -OH and C-rich components in the space-weathered layers, providing direct experimental evidence that exposure to the space environment tends to mask the optical signatures of phyllosilicates and carbonaceous matter. Our findings should contribute to rectifying potential underestimations of water and carbon content of C-type asteroids when studied through remote sensing with new-generation telescopes.

Atomic-scale origin of the low grain-boundary resistance in perovskite solid electrolyte Li0.375Sr0.4375Ta0.75Zr0.25O3

Oxide solid electrolytes (OSEs) have the potential to achieve improved safety and energy density for lithium-ion batteries, but their high grain-boundary (GB) resistance generally is a bottleneck. In the well-studied perovskite oxide solid electrolyte, Li3xLa2/3-xTiO3 (LLTO), the ionic conductivity of grain boundaries is about three orders of magnitude lower than that of the bulk. In contrast, the related Li0.375Sr0.4375Ta0.75Zr0.25O3 (LSTZ0.75) perovskite exhibits low grain boundary resistance for reasons yet unknown. Here, we use aberration-corrected scanning transmission electron microscopy and spectroscopy, along with an active learning moment tensor potential, to reveal the atomic scale structure and composition of LSTZ0.75 grain boundaries. Vibrational electron energy loss spectroscopy is applied for the first time to reveal atomically resolved vibrations at grain boundaries of LSTZ0.75 and to characterize the otherwise unmeasurable Li distribution therein. We find that Li depletion, which is a major reason for the low grain boundary ionic conductivity of LLTO, is absent for the grain boundaries of LSTZ0.75. Instead, the low grain boundary resistivity of LSTZ0.75 is attributed to the formation of a nanoscale defective cubic perovskite interfacial structure that contained abundant vacancies. Our study provides new insights into the atomic scale mechanisms of low grain boundary resistivity.

Impact of Connectivity on the Electronic Structure of N-Heterotriangulenes

N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the structural and the electronic properties of two different N-HTAs, N-HTA 550 and N-HTA 557, the latter containing an additional 7-membered ring, adsorbed on Au(111) using vibrational and electronic high-resolution electron energy loss spectroscopy in combination with state-of-the-art quantum chemical calculations. In the mono- and multilayer, both N-HTAs adopt a planar adsorption geometry with the molecular backbone oriented parallel to the gold substrate. The energies of the lowest excited electronic singlet states (S) are assigned. The optical gap (S0 → S1 transition) is found to be 3.4 eV for N-HTA 550 and 2.5 eV for N-HTA 557. Thus, the introduction of the −C═C– double bond in N-HTA 557 resulted in a pronounced decrease of the optical gap size by 0.9 eV due to the larger π-conjugated electron system compared to N-HTA 550. Structural variations or substitution patterns in N-HTAs foster the opportunity for tailoring their electronic properties.

Atomic-scale probing of heterointerface phonon bridges in nitride semiconductor

Interface phonon modes that are generated by several atomic layers at the heterointerface play a major role in the interface thermal conductance for nanoscale high-power devices such as nitride-based high-electron-mobility transistors and light-emitting diodes. Here we measure the local phonon spectra across AlN/Si and AlN/Al interfaces using atomically resolved vibrational electron energy-loss spectroscopy in a scanning transmission electron microscope. At the AlN/Si interface, we observe various interface phonon modes, of which the extended and localized modes act as bridges to connect the bulk AlN modes and bulk Si modes and are expected to boost the phonon transport, thus substantially contributing to interface thermal conductance. In comparison, no such phonon bridge is observed at the AlN/Al interface, for which partially extended modes dominate the interface thermal conductivity. This work provides valuable insights into understanding the interfacial thermal transport in nitride semiconductors and useful guidance for thermal management via interface engineering.

Simulations of spatially and angle-resolved vibrational electron energy loss spectroscopy for a system with a planar defect

Recent developments in experiments with vibrational electron energy loss spectroscopy (EELS) have revealed spectral shape variations at spatial resolutions down to sub-atomic scale. Interpretation in terms of local phonon density of states enables their qualitative understanding, yet a more detailed analysis is calling for advances in theoretical methods. In Zeiger and Rusz, Phys. Rev. Lett. 124, 025501 (2020) we have presented a frequency resolved frozen phonon multislice method for simulations of vibrational EELS. Detailed simulations for a plane wave electron beam scattering on a vibrating hexagonal boron nitride are presented in a companion manuscript (Zeiger and Rusz, arXiv:2104.03197). Here we present simulations of vibrational EELS assuming a convergent electron probe of nanometer size and atomic size on a hexagonal boron nitride structure model with a planar defect. With a nanometer beam we observe spectral shape modifications in the presence of the defect, which are correlated with local changes of the phonon density of states. With an atomic size electron beam, we observe the same, although with better contrast. In addition, we observe atomic level contrast and sub-atomic scale spectral shape modifications, which are particularly strong for small detector collection angles.

Frequency-resolved frozen phonon multislice method and its application to vibrational electron energy loss spectroscopy using parallel illumination

We explore the capabilities of the frequency-resolved frozen phonon multislice method introduced in Phys. Rev. Lett. 124, 025501 (2020) to model inelastic vibrational scattering in transmission electron microscopy. We review the method in detail and discuss advantages of using a so called hotspot thermostat instead of the $\delta$-thermostat used in our first report. We apply the method to simulate vibrational electron energy loss spectra of hexagonal boron nitride under plane wave illumination. Simulated spectroscopic information well represents the theoretical phonon bandstructure of the studied material, both in terms of energies as well as polarization vectors of individual phonon modes.

Role of Convergence and Collection Angles in the Excitation of Long- and Short-Wavelength Phonons with Vibrational Electron Energy-Loss Spectroscopy.

Current generation electron monochromators employed as attachments to scanning transmission electron microscopes (STEM) offer the ability to obtain vibrational information from materials using electron energy-loss spectroscopy (EELS). We show here that in crystals, long- and short-wavelength phonon modes can be probed simultaneously with on-axis vibrational STEM EELS. The long-wavelength phonons are probed via dipole scattering, while the short-wavelength modes are probed via impact scattering of the incident electrons. The localized character of the short-wavelength modes is demonstrated by scanning the electron beam across the edge of a hexagonal boron nitride nanoparticle. It is found that employing convergence angles that encompass multiple Brillouin zone boundaries enhances the short-wavelength phonon contribution to the vibrational energy-loss spectrum much more than that achieved by employing collection angles that encompass multiple Brillouin zone boundaries. Probing short-wavelength phonons at high spatial resolution with on-axis vibrational STEM EELS will help develop a fundamental connection between vibrational excitations and bonding arrangements at atomic-scale heterogeneities in materials.

Bond Dissociation and Reactivity of HF and H2O in a Nano Test Tube.

Molecular motion and bond dissociation are two of the most fundamental phenomena underpinning the properties of molecular materials. We entrapped HF and H2O molecules within the fullerene C60 cage, encapsulated within a single-walled carbon nanotube (X@C60)@SWNT, where X = HF or H2O. (X@C60)@SWNT represents a class of molecular nanomaterial composed of a guest within a molecular host within a nanoscale host, enabling investigations of the interactions of isolated single di- or triatomic molecules with the electron beam. The use of the electron beam simultaneously as a stimulus of chemical reactions in molecules and as a sub-angstrom resolution imaging probe allows investigations of the molecular dynamics and reactivity in real time and at the atomic scale, which are probed directly by chromatic and spherical aberration-corrected high-resolution transmission electron microscopy imaging, or indirectly by vibrational electron energy loss spectroscopy in situ during scanning transmission electron microscopy experiments. Experimental measurements indicate that the electron beam triggers homolytic dissociation of the H-F or H-O bonds, respectively, causing the expulsion of the hydrogen atoms from the fullerene cage, leaving fluorine or oxygen behind. Because of a difference in the mechanisms of penetration through the carbon lattice available for F or O atoms, atomic fluorine inside the fullerene cage appears to be more stable than the atomic oxygen under the same conditions. The use of (X@C60)@SWNT, where each molecule X is "packaged" separately from each other, in combination with the electron microscopy methods and density functional theory modeling in this work, enable bond dynamics and reactivity of individual atoms to be probed directly at the single-molecule level.

Electron- and light-induced stimulated Raman spectroscopy for nanoscale molecular mapping

We propose and theoretically analyze a new vibrational spectroscopy, termed electron- and light-induced stimulated Raman (ELISR) scattering, that combines the high spatial resolution of electron microscopy with the molecular sensitivity of surface-enhanced Raman spectroscopy. With ELISR, electron-beam excitation of plasmonic nanoparticles is utilized as a spectrally-broadband but spatially-confined Stokes beam in the presence of a diffraction-limited pump laser. To characterize this technique, we develop a numerical model and conduct full-field electromagnetic simulations to investigate two distinct nanoparticle geometries, nanorods and nanospheres, coated with a Raman-active material. Our results show the significant ($10^6$-$10^7$) stimulated Raman enhancement that is achieved with dual electron and optical excitation of these nanoparticle geometries. Importantly, the spatial resolution of this vibrational spectroscopy for electron microscopy is solely determined by the nanoparticle geometry and the plasmon mode volume. Our results highlight the promise of ELISR for simultaneous high-resolution electron microscopy with sub-diffraction-limited Raman spectroscopy, complementing advances in superresolution microscopy, correlated light and electron microscopy, and vibrational electron energy loss spectroscopy.

Accurate EELS background subtraction – an adaptable method in MATLAB

• Electron energy loss spectroscopy (EELS) requires background fitting and removal. • Scripts for background subtraction have been written in MATLAB. • The scripts can be applied to core, low, and ultra-low loss EELS. • Statistical information on the goodness-of-fit is given. • Several examples of background subtraction are presented in the main text. Electron energy-loss spectroscopy (EELS) is a technique that can give useful information on elemental composition and bonding environments. However in practice, the complexity of the background contributions, which can arise from multiple sources, can hamper the interpretation of the spectra. As a result, background removal is both an essential and difficult part of EELS analysis, especially during quantification of elemental composition. Typically, a power law is used to fit the background but this is often not suitable for many spectra such as in the low-loss region (< 50 eV) and when there are overlapping EELS edges. In this article, we present a series of scripts written in MATLAB v. R2019b that aims to provide statistical information on the model used to fit the background, allowing the user to determine the accuracy of background subtraction. The scripts were written for background subtraction of vibrational EELS in the ultralow-loss region near the zero-loss peak but can also be applied to other kinds of EEL spectra. The scripts can use a range of models for fitting, provided by the Curve Fitting Toolbox of MATLAB, and the user is able to precisely define the window for fitting as well as for edge integration. We demonstrate the advantages of using these scripts by comparing their background subtraction of example spectra to the most commonly used software, Gatan Microscopy Suite 3. The example spectra include those containing multiple scattering, multiple overlapping peaks, as well as vibrational EELS. Additionally, a comprehensive guide to using the scripts has been included in the Supplementary Information.

Functional Group Mapping by Electron Beam Vibrational Spectroscopy from Nanoscale Volumes.

Vibrational spectroscopies directly record details of bonding in materials, but spatially resolved methods have been limited to surface techniques for mapping functional groups at the nanoscale. Electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope presents a route to functional group analysis from nanoscale volumes using transmitted subnanometer electron probes. Here, we now use vibrational EELS to map distinct carboxylate and imidazolate linkers in a metal-organic framework (MOF) crystal-glass composite material. Domains <100 nm in size are observed using vibrational EELS, with recorded spatial resolution <15 nm at interfaces in the composite. This nanoscale functional group mapping is confirmed by correlated EELS at core ionization edges as well as X-ray energy dispersive spectroscopy for elemental mapping of the metal centers of the two constituent MOFs. These results present a complete nanoscale analysis of the building blocks of the MOF composite and establish spatially resolved functional group analysis using electron beam spectroscopy for crystalline and amorphous organic and metal-organic solids.

Efficient and Versatile Model for Vibrational STEM-EELS.

We introduce a novel method for the simulation of the impact scattering in vibrational scanning transmission electron microscopy electron energy loss spectroscopy simulations. The phonon-loss process is modeled by a combination of molecular dynamics and elastic multislice calculations within a modified frozen phonon approximation. The key idea is thereby to use a so-called δ thermostat in the classical molecular dynamics simulation to generate frequency dependent configurations of the vibrating specimen's atomic structure. The method includes correlated motion of atoms and provides vibrational spectrum images at a cost comparable to standard frozen phonon calculations. We demonstrate good agreement of our method with simulations and experiments for a 15nm flake of hexagonal boron nitride.

Nanoscale Probing of Adsorbates on Pt/CeO2 with Aloof-beam Vibrational Electron Energy-loss Spectroscopy

Nanoscale Probing of Adsorbates on Pt/CeO₂ with Aloof-beam Vibrational Electron Energy-loss Spectroscopy

Enhanced vibrational electron energy-loss spectroscopy of adsorbate molecules

Plasmonic excitations in metallic nanoparticles are known to depend strongly on nanoparticle size and shape. Here we explore how such excitations can be harnessed to enhance the vibrational signals from molecules adsorbed on nanoparticle surfaces, as detected using electron energy-loss spectroscopy in the scanning transmission electron microscope. We use the Born-Huang formalism in the electrostatic approximation to develop a theoretical model for electron energy-loss from an adsorbed molecule (represented by a point dipole) on the surface of a dielectric nanoparticle. We find that the adsorbate contribution to the energy-loss spectrum is approximately proportional to the square of the electric field at the adsorption site, and hence we find that the enhancement of the molecule's vibrational signal is greatest for molecules adsorbed on small, sharp nanoparticles. When the molecular frequency is near one of the nanoparticle frequencies, we generally find an asymmetric Fano-type spectrum line shape whose asymmetry is attributable to multimodal contributions. Our calculations for a molecule adsorbed on the tip of a prolate spheroidal silver nanoparticle predict that signal enhancements of several hundred times should be readily achievable, and up to several thousand times if the nanoparticle's plasmonic mode is ``tuned'' to the molecular frequency. Such enhancement effects potentially make vibrational STEM-EELS a powerful tool for the characterization of surface-functionalized nanoparticles and nanomaterials used for chemical sensing.

Spatially Resolved Vibrational Electron Energy-loss Spectroscopy Across an Abrupt SiO2/Si Interface

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Nanoscale Probing of Local Hydrogen Heterogeneity in Disordered Carbon Nitrides with Vibrational Electron Energy-Loss Spectroscopy.

In graphitic carbon nitrides, (photo)catalytic functionality is underpinned by the effect that residual hydrogen content, manifesting in amine (N-H x) defects, has on its optoelectronic properties. Therefore, a detailed understanding of the variation in the local structure of graphitic carbon nitrides is key for understanding structure-activity relationships. Here, we apply aloof-beam vibrational electron energy-loss spectroscopy in the scanning transmission electron microscope (STEM) to locally detect variations in hydrogen content in two different layered carbon nitrides with nanometer resolution. Through low dose rate TEM, we obtain atomically resolved images from crystalline and disordered carbon nitrides. By employing an aloof-beam configuration in a monochromated STEM, radiation damage can be dramatically reduced, yielding vibrational spectra from carbon nitrides to be assessed on 10's of nanometer length scales. We find that in disordered graphitic carbon nitrides the relative amine content can vary locally up to 27%. Cyano (C≡N) defects originating from uncondensed precursor are also revealed by probing small volumes, which cannot be detected by infrared absorption or Raman scattering spectroscopies. The utility of this technique is realized for heterogeneous soft materials, such as disordered graphitic carbon nitrides, in which methods to probe catalytically active sites remain elusive.

Prospects of spatial resolution in vibrational electron energy loss spectroscopy: Implications of dipolar scattering

We use scattering theory to study the spatial resolution achievable in vibrational energy loss spectroscopy using a focused beam of high-energy ($\ensuremath{\sim}100$ keV) electrons. We first outline a theory for calculating vibrational-spectroscopic images of crystalline or noncrystalline materials at nanometer spatial resolution or better (up to atomic resolution). The electron scattering and the atomic vibrations are treated quantum mechanically. Dipolar scattering from long-wavelength optical vibrations is included. We present calculated atomically resolved vibrational-spectroscopic images of a polar crystalline material (hexagonal boron nitride). For such materials, dipole scattering from long optical vibrations can give rise to a strong background in the images, which has implications for the attainable spatial resolution. We show that an annular collection geometry can significantly reduce the dipole background, thereby reducing the electron dose required to observe atomic-scale contrast.

Triisopropylsilylethynyl-Pentacene on Au(111): Adsorption Properties, Electronic Structure, and Singlet Fission Dynamics

Oligoacenes have recently received much attention because of their ability to undergo singlet fission, a process which may improve the energy conversion efficiency in organic solar cells. Here, we studied the structural and the electronic properties of triisopropylsilylethynyl-pentacene (TIPS-Pn) adsorbed on Au(111) from monolayer to multilayer coverages using multiexperimental techniques [temperature-programmed desorption, vibrational and electronic high-resolution electron energy loss spectroscopy, and energy- and time-resolved two-photon photoemission (2PPE)]. We demonstrated that well-defined TIPS-Pn molecular films can be prepared via evaporation. TIPS-Pn adopt on Au(111) a molecular adsorption geometry in which the pentacene backbone is oriented parallel to the substrate. We determined the energetic position of several unoccupied and occupied electronic states among others originating from the lowest unoccupied molecular orbital and the highest occupied molecular orbital. Femtosecond time-resolved 2...

Probing Interfacial and Surface Effects with Vibrational Electron Energy Loss Spectroscopy

Journal Article Probing Interfacial and Surface Effects with Vibrational Electron Energy Loss Spectroscopy Get access Kartik Venkatraman, Kartik Venkatraman School for the Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, USA Search for other works by this author on: Oxford Academic Google Scholar Qianlang Liu, Qianlang Liu School for the Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, USA Search for other works by this author on: Oxford Academic Google Scholar Katia March, Katia March LeRoy Eyring Center for Solid State Science, Arizona State University, Tempe, Arizona 85287-1704, USA Search for other works by this author on: Oxford Academic Google Scholar Peter Rez, Peter Rez Department of Physics, Arizona State University, Tempe, Arizona 85287-1704, USA Search for other works by this author on: Oxford Academic Google Scholar Peter Crozier Peter Crozier School for the Engineering of Matter, Transport and Energy, Arizona State University, Tempe, Arizona 85287-6106, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 1562–1563, https://doi.org/10.1017/S1431927617008479 Published: 04 August 2017