Monochromated Transmission Electron Microscope Research Articles

Direct Observation of Locally Modified Excitonic Effects within a Moiré Unit Cell in Twisted Bilayer Graphene.

Bilayer graphene, which forms moiré superlattices, possesses distinct electronic and optical properties owing to its hybridized energy band and the emergence of van Hove singularities depending on its twist angle. Extensive research has been conducted on the global characteristics of moiré superlattices induced by their long-range periodicity. However, the local properties, which differ owing to the variations in the three-dimensional atomic arrangement, within a moiré unit cell have been rarely explored. In this study, we demonstrate the highly localized excitation of carbon 1s electrons to unoccupied van Hove singularities in twisted bilayer graphene by electron energy loss spectroscopy using a monochromated transmission electron microscope. The core-level excitations associated with the van Hove singularities exhibit a systematic twist-angle dependence analogous to optical excitations. Furthermore, local variations in the core-level van Hove singularity peaks, which can originate from the core-exciton lifetimes and band modifications corresponding to the local stacking geometry within a moiré unit cell, are unambiguously corroborated.

Faceting\u2013roughening transition of a Cu grain boundary under electron-beam irradiation at 300\xa0keV

In this study, we examined the beam-irradiation effect on the structural evolution of the grain boundary (GB) in a Cu bicrystal at room temperature using a Cs-corrected, monochromated transmission electron microscope at an acceleration voltage of 300 keV. Faceting of the GB was observed at a low current density of the electron beam. With increasing current density, the GB became defaceted. The faceting–roughening transition was shown to be reversible, as the process was reversed upon decreasing the current density. The structural transition is explained by inelastic scattering effects by electron-beam irradiation.

Optimization of monochromated TEM for ultimate resolution imaging and ultrahigh resolution electron energy loss spectroscopy

The performance of a monochromated transmission electron microscope with Wien type monochromator is optimized to achieve an extremely narrow energy spread of electron beam and an ultrahigh energy resolution with spectroscopy. The energy spread in the beam is improved by almost an order of magnitude as compared to specified values. The optimization involves both the monochromator and the electron energy loss detection system. We demonstrate boosted capability of optimized systems with respect to ultra-low loss EELS and sub-angstrom resolution imaging (in a combination with spherical aberration correction).

Coupling of Surface-Plasmon-Polariton-Hybridized Cavity Modes between Submicron Slits in a Thin Gold Film

Electron energy loss spectroscopy (EELS) in a monochromated transmission electron microscope is applied to probe standing-wave-like cavity modes hybridized with surface plasmon polaritons (SPP) in rectangular submicron slits in a thin gold film. Coupling of hybridized SPP-cavity modes between two adjacent slits is studied by systematically varying the width of the metal bar d that separates the identical slits in a two-slit system. Measurements on two-slit systems with different slit lengths L and fixed width reveal energy shifts and mode splitting of the fundamental SPP cavity mode which can be generally described as a function of a dimensionless scaling parameter L/d. Numerical simulations with the Discontinuous Galerkin Time-Domain (DGTD) method confirm the experimental data and reveal insights into the underlying complex coupling mechanisms.

Probing Local Electronic Transitions in Organic Semiconductors through Energy‐Loss Spectrum Imaging in the Transmission Electron Microscope

Improving the performance of organic electronic devices depends on exploiting the complex nanostructures formed in the active layer. Current imaging methods based on transmission electron microscopy provide limited chemical sensitivity, and thus the application to materials with compositionally similar phases or complicated multicomponent systems is challenging. Here, it is demonstrated that monochromated transmission electron microscopes can generate contrast in organic thin films based on differences in the valence electronic structure at energy losses below 10 eV. In this energy range, electronic fingerprints corresponding to interband excitations in organic semiconductors can be utilized to generate significant spectral contrast between phases. Based on differences in chemical bonding of organic materials, high‐contrast images are thus obtained revealing the phase separation in polymer/fullerene mixtures. By applying principal component analysis to the spectroscopic image series, further details about phase compositions and local electronic transitions in the active layer of organic semiconductor mixtures can be explored.

EELS study of Fe- or Co-doped titania nanosheets.

Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets are Fe- and Co-doped titanium oxides, respectively, and they are synthesized by the exfoliation of lepidocrocite-type layered titanates. We have investigated these nanosheets by electron energy-loss spectroscopy (EELS) using a monochromated transmission electron microscope. The energy-loss near-edge structures (ELNESs) of Fe-L and Co-L indicate that Fe(3+) and Co(2+) ions are substituted in the octahedral sites in each nanosheet. The Ti-L edges of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets correspond to the octahedral coordination of Ti(4+) and oxygen atoms as well as an undoped titania nanosheet (Ti0.87O2). On the other hand, the electron transitions from 2p3/2 to 3d eg in Ti-L3 regions are different in each nanosheet. We have also investigated the electron-beam-induced damage of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. The results indicated that Fe(3+) ions in the Ti0.6Fe0.4O2 nanosheets were selectively reduced to Fe(2+) ions in the reduction process by electron irradiation. In contrast, the chemical shift of the Ti-L edge of the Ti0.8Co0.2O2 nanosheets indicated that Ti(4+) ions were reduced. These results suggest that the substitution of 3d metals in titania nanosheets affects their crystal and electronic structures and material properties such as their long-range atomic configuration and reduction mechanism.

Interslit Coupling via Ultrafast Dynamics across Gold-Film Hole Arrays

One-dimensional arrays of submicrometer rectangular holes in 200 nm thin gold films are investigated using electron energy loss spectroscopy combined with scanning transmission electron microscopy (STEM-EELS). Improved energy resolution, down to 0.11 eV, is accomplished in our monochromated transmission electron microscope, allowing the reliable quantification of signals at loss energies as small as 0.43 eV. The standing-wave resonances of individual holes are thus investigated at nanometer-scale spatial resolution, focusing in particular on the effect of neighboring holes. We show how the coupling between holes is facilitated by surface plasmon polaritons (SPPs) propagating on the top and bottom surfaces of the separating metal-film strips. Thus, complex spatiotemporal coupling dynamics emerges, characterized by strong interslit interactions and a phase that can be controlled by varying the width of the metal strip between adjacent holes. Applying real-space real-time numerical simulations and exploiting the short interaction time of 300 keV electrons with the thin gold film, we reveal intriguing features of these ultrafast coupling mechanisms, including unusual line-narrowing and marked SPP signal enhancements in the corresponding EEL spectra. Complementary aspects of the far-field and near-field components in the SPP eigenstates are further demonstrated. Our combined analysis effectively equips STEM-EELS with an excellent temporal resolution and further yields a consistent description of the entangled femtosecond-scale SPP dynamics.

Doping Monolayer Graphene with Single Atom Substitutions

Functionalized graphene has been extensively studied with the aim of tailoring properties for gas sensors, superconductors, supercapacitors, nanoelectronics, and spintronics. A bottleneck is the capability to control the carrier type and density by doping. We demonstrate that a two-step process is an efficient way to dope graphene: create vacancies by high-energy atom/ion bombardment and fill these vacancies with desired dopants. Different elements (Pt, Co, and In) have been successfully doped in the single-atom form. The high binding energy of the metal-vacancy complex ensures its stability and is consistent with in situ observation by an aberration-corrected and monochromated transmission electron microscope.

Direct bandgap measurements in a three-dimensionally macroporous silicon 9R polytype using monochromated transmission electron microscope

We demonstrate a three-dimensionally macroporous Si 9R polytype that exhibits a different electronic structure than bulk diamond-structured Si. Unlike the latter one which has an indirect-bandgap transition close to the zone boundary, the conduction band minimum in this material, as revealed by valence electron energy-loss spectroscopy in a monochromated transmission electron microscope, significantly shifts toward the Γ point within a range from 0.6 to 5.6 nm−1, indicating substantially less momentum transfer required to fulfill the bandgap transition.

Comparison of EFTEM and STEM EELS plasmon imaging of gold nanoparticles in a monochromated TEM

We present and compare two different imaging techniques for plasmonic excitations in metallic nanoparticles based on high energy-resolution electron energy-loss spectroscopy in a monochromated transmission electron microscope. We demonstrate that a recently developed monochromated energy-filtering (EFTEM) approach can be used in addition to the well established scanning technique to directly obtain plasmon images in the energy range below 1 eV. The EFTEM technique is described in detail, and a double experiment performed on the same, triangular gold nanoparticle compares equivalent data acquired by both techniques, respectively.

TEM and impedance spectroscopy of doped ceria electrolytes

Preliminary investigations are presented into the grain boundary structure of gadolinia doped ceria by high-resolution transmission electron microscopy (HR-TEM) imaging using a 300 kV field emission FEI TITAN 80/300 monochromated transmission electron microscope, with a spatial resolution of < 0.2 nm. Impedance spectroscopy over the temperature range 150–350 °C and frequency range 0.1 Hz–10 MHz has been used to separate the contribution to the total conductivity of the grain interior and grain boundary for ceramic samples. The samples were fabricated from powder of composition Ce 1 − x Gd x O 2 − δ , x = 0.1 (CGO10) and x = 0.2 (CGO20) which were obtained from three different commercial suppliers. These impedance measurements have been correlated with scanning electron microscopy (SEM) images of the material microstructure in order to estimate the specific grain boundary conductivity. The bricklayer model has been used to interpret the impedance data, which show wide variation in specific grain boundary conductivity between samples of the same nominal composition. No grain boundary second phases have been observed at any boundaries and the grain boundary blocking effect has been attributed to the presence of space charge layers. HR-TEM studies at these interfaces reveal defect structures, which may play a significant role in the grain boundary conductivity behaviour of these materials.

Chemical bonds in damaged and pristine low- κ materials: A comparative EELS study

Chemical bonds in low- κ dielectric materials can be studied at nanometer spatial resolution using electron energy loss spectroscopy (EELS) in a monochromated transmission electron microscope (TEM). However, it is unknown to what extent TEM sample preparation and exposure to the electron beam during the measurement alter the original chemical bond state in these materials. Therefore, we are assessing the effect of different TEM sample preparation methods and beam conditions in a comparative study on low- κ dielectric materials.

The influence of relativistic energy losses on bandgap determination using valence EELS

Since monochromated transmission electron microscopes have become available, the determination of bandgaps and optical properties using electron energy loss spectrometry (EELS) has again attracted interest. The underlying idea is very simple: below the bandgap energy no transitions can contribute to the valence EELS signal. However, the bandgap cannot be directly read out from the recorded data. Therefore the optical properties cannot be determined correctly from the low loss using the Kramers–Kronig relations. We will discuss under which conditions relativistic effects may be surpressed. It is demonstrated that scanning TEM (STEM) geometry is not applicable for most bandgap measurements.

Enhancement of resolution in core-loss and low-loss spectroscopy in a monochromated microscope

The significant enhancement of the energy resolution in the new generation of commercially available monochromated transmission electron microscopes presents new challenges in term of selecting the correct experimental conditions and understanding the various effects that can potentially influence the quality of the EELS data. In this respect we investigated the effect of point spread function of the detector and spectrum-diffraction mixing on the energy resolution and the intensity of the zero loss peak tails. Alternative approaches to improve the energy resolution by mathematical methods have been tested. By using a simple and commonly available test case (Si L 2,3 edges) we assessed the efficiency of the deconvolution algorithms to improve the resolution. The results show that the deconvolution is not always successful in improving the resolution of the core loss EELS data and the results may not always be reliable. Contrary to this, the application of the Richardson–Lucy deconvolution algorithm on some bandgap measurements data appears to be very effective. The procedure proved successful in removing the contribution of the zero-loss peak tails and allows an easier access to spectroscopic information starting at energy losses as low as of 0.5 eV with monochromated spectra and 1 eV with the non-monochromated spectra.

Accuracy of the calculated unoccupied states in GaN phases as tested by high-resolution electron energy-loss spectroscopy

The electronic structures of cubic and hexagonal phases of GaN have been investigated by high-resolution electron energy-loss spectroscopy in a monochromated transmission electron microscope. Both the $\mathrm{Ga}\text{\ensuremath{-}}{L}_{2,3}$ and N $K$-edges were measured. The data are compared to the latest versions of two different calculation schemes: band structure and multiple-scattering calculations. We have found that both methods are capable of giving results that can be compared quantitatively to the experiment. Small discrepancies with experiment could be eliminated by future developments in the implementation of these methods.

Čerenkov losses: A limit for bandgap determination and Kramers–Kronig analysis

Measuring low energy losses in semiconductors and insulators with high spatial resolution becomes attractive with the increasing availability of modern transmission electron microscopes (TEMs) equipped with monochromators, Cs correctors and energy filters. In this paper, we demonstrate that Čerenkov losses pose a limit for the interpretation of low energy loss spectra (EELS) in terms of interband transistions and bandgap determination for many materials.If the velocity of a charged particle in a medium exceeds the velocity of light, photons are emitted leading to a corresponding energy loss of a few electronvolt. Since these losses are strong for energies below the onset of interband transitions, they change the apparent loss function of semiconductors and insulators, with the risk of erroneous interpretation of spectra. We measured low energy losses of Si and GaAs with a monochromated TEM demonstrating the effect of sample thickness on Čerenkov losses. Angle resolved EELS and energy filtered diffraction patterns (taken without a monochromator) show the extremely narrow angular distribution of Čerenkov losses. The latter experiment provides a method that allows to decide whether Čerenkov radiation masks the very low loss signal in EELS.

Changes in the electronic structure of cerium due to variations in close packing

Here we use electron energy-loss spectroscopy in a monochromated transmission electron microscope with 100 meV energy resolution and 2 \AA{} spatial resolution to show that the electronic structure of face centered cubic \ensuremath{\gamma} and double hexagonal close-packed \ensuremath{\beta} cerium are considerably different, contrary to previous assumptions in literature. These results are supported by synchrotron-radiation-based x-ray absorption, multielectronic atomic spectral simulations, and local density approximation calculations, illustrating that changes in {111} stacking sequences can drive substantial electronic changes in close-packed phases of cerium that have a similar atomic volume.

Electron energy loss-near edge structure as a fingerprint for identifying chromium nitrides

Electron energy-loss near-edge structure data for the N K and the Cr L 2,3 edges of CrN and Cr 2N have been acquired in order to distinguish between these chromium nitride modifications. The N K edge spectra of these compounds have been modelled using both band structure and multiple scattering methods. We compare the results of these calculations with the experimental edges which have been recorded using a conventional transmission electron microscope (TEM) as well as a monochromated TEM (Wien filter).

EELS performance measurements on a new high energy resolution imaging filter

In this paper we characterize the spectroscopy performance of a new high resolution electron energy-loss spectrometer for transmission electron microscopy, recently installed in our lab. With the event of commercially available monochromated transmission electron microscopes, equipped with improved spectrometers, experiments became feasible that allow the study of chemical bonding and electronic structures of atoms and solids at the nanometer level with an energy resolution in the range of 0.2–0.3eV. This significant enhancement in resolution, however, can only be fully harnessed with an optimized microscope and spectrometer setup and in a suitable local environment. We attempt to quantify some of the limiting factors on our system with emphasis on spectrum mixing, spectrometer aberrations and transmissivity and we sketch proper working conditions in order to achieve the desired performance.