Angle-resolved Electron Energy Loss Research Articles

Plasma-relevant fast electron impact study of difluoromethane

Difluoromethane () is a commonly used etching gas in manufacturing very-large-scale integrated circuits as the fluorine source. Electron impact excitation cross sections for are important input data for modeling the reactive etching plasmas. In this work, we present the experimental generalized oscillator strengths for the , , and transitions on the absolute scale. The measurement was realized by employing the crossed-beam based relative flow technique at an angle-resolved electron energy loss spectrometer, operating at an incident energy of 1500 eV and a resolution of 80 meV. The dipole-forbidden transition at around 9.7 eV is identified for the first time. The measured data are fitted with the well-known Lassettre formula to yield analytical expressions and optical oscillator strengths at for dipole-allowed transitions. The analytical expressions enable integrations over momentum transfer to obtain the Born integral cross sections for electron impact excitations from the threshold to 5000 eV. The integral cross sections for dipole-allowed transitions are scaled to a more reliable level by using the BE-scaling method. The experimental data can benchmark theoretical calculations and supplement the molecular database for plasma modelers.

Single scan STEM-EMCD in 3-beam orientation using a quadruple aperture

The need to acquire multiple angle-resolved electron energy loss spectra (EELS) is one of the several critical challenges associated with electron magnetic circular dichroism (EMCD) experiments. If the experiments are performed by scanning a nanometer to atomic-sized electron probe on a specific region of a sample, the precision of the local magnetic information extracted from such data highly depends on the accuracy of the spatial registration between multiple scans. For an EMCD experiment in a 3-beam orientation, this means that the same specimen area must be scanned four times while keeping all the experimental conditions same. This is a non-trivial task as there is a high chance of morphological and chemical modification as well as non-systematic local orientation variations of the crystal between the different scans due to beam damage, contamination and spatial drift. In this work, we employ a custom-made quadruple aperture to acquire the four EELS spectra needed for the EMCD analysis in a single electron beam scan, thus removing the above-mentioned complexities. We demonstrate a quantitative EMCD result for a beam convergence angle corresponding to sub-nm probe size and compare the EMCD results for different detector geometries.

Oscillator strengths and cross sections of the valence-shell excitations of CH[formula omitted]I studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CH3I are important for studying molecular photodissociation and monitoring the destruction of ozone in the Earth’s atmosphere. Based on the relative flow technique, absolute generalized oscillator strengths of the valence-shell excitations in CH3I have been determined by using an angle-resolved electron energy loss spectrometer. The measurement was carried out at an incident electron energy of 1.5 keV and an energy resolution of 80 meV. Optical oscillator strengths for the dipole-allowed transitions have been obtained at the zero limit of the squared momentum transfer, providing an independent cross check with the previous experimental results. The integral cross sections from the excitation threshold to 5000 eV have also been obtained by means of the BE-scaling method. The present experimental oscillator strengths and cross sections can supplement the fundamental spectroscopic data of CH3I and will play an important role in the studies of molecular photodissociation dynamics.

Oscillator strengths and cross sections of the valence shell excitations in nitrous oxide studied by high-energy electron scattering

Generalized oscillator strengths (GOSs) of the valence shell excitations to the A1Σ−+B1Δ, C1Π, D1Σ+ and 21Π states in nitrous oxide have been determined by employing an angle-resolved electron energy loss spectrometer operated at an incident energy of 1500 eV and an energy resolution of about 80 meV. Detailed comparisons with the previous data indicate that the higher-order Born terms play a significant role in the GOSs of C1Π and D1Σ+ states at either low impact energies or large momentum transfers, and the GOSs of A1Σ−+B1Δ and C1Π states measured with the gas cell may suffer from the pressure effect. The measured GOS data are fitted with the Lassettre formula to determine the corresponding optical oscillator strengths (OOSs) and calculate the integral cross sections (ICSs) based on the BE-scaling method. The oscillator strengths and cross sections in this work provide a cross check on the previous data and can serve as a benchmark for testing the developed theoretical models and computational codes.

Oscillator strengths and cross sections of high Rydberg states in ammonia studied by fast electron scattering

We report the experimental generalized oscillator strengths (GOSs) for transitions from the ground state to the Rydberg states , , , and an ‘undefined’ state in ammonia. The experiment was performed using an angle-resolved electron energy loss spectrometer operated at an incident energy of 1500 eV and an energy resolution of about 80 meV. The experimental observation confirms the recent theoretically predicted appreciable vibrational effects on the GOS for the transition (2021 J. Phys. B: At. Mol. Opt. Phys. 54 135202). The measured GOS data are fitted with the Lassettre formula to determine the corresponding optical oscillator strengths for dipole-allowed transitions. Furthermore, the fitted curves enable the integrations of GOSs over the momentum transfer squares to obtain the Born excitation cross sections. The integral cross sections are scaled to an accurate scale by using the BE-scaling method. The data in this work can supplement the molecular database and deepen our understanding of the scattering mechanism.

Valence-shell electronic excitations of CF4 studied by fast electron scattering

Oscillator strengths and cross sections of the valence-shell excitations in CF4 are of crucial importance in modeling the etching plasma and monitoring the greenhouse effect. In this work, generalized oscillator strengths of the valence-shell excitations in CF4 were measured by using an angle-resolved electron energy loss spectrometer, operating at an incident electron energy of 1.5 keV and a high energy resolution of 80 meV. The experimental GOSs are analytically fitted with the well-known Lassettre formula, and, thus, the optical oscillator strengths are determined at the limit of squared momentum transfer K2=0. The analytical GOS also enables an integration over K2, determining the Born cross sections with respect to the incident electron energy from the excitation threshold to 5000 eV. The BE-scaled integral cross sections for dipole-allowed transitions are obtained by adopting the BE-scaling method. Oscillator strengths and cross sections of the valence-shell excitations in CF4 provide an independent cross-check to the previous experimental and theoretical works. Moreover, the cross-checked data supplement and benchmark the molecular database for use in plasma models.

Plasma-relevant fast electron impact study of trifluoromethane

Abstract Trifluoromethane is an important etching gas in semiconductor industries as the fluorine source in reactive etching plasma. Electron impact excitations into the dissociation states are essential for plasma modeling. By employing an angle-resolved electron energy loss spectrometer, the generalized oscillator strengths (GOSs) for the valence-shell excited states have been determined at an absolute scale utilizing the crossed-beam based relative flow technique. The measurement was performed with an incident electron energy of 1500 eV at an energy resolution of about 70 meV. The corresponding optical oscillator strengths are also obtained by extrapolating the measured GOSs to the limit of zero momentum transfer. The extrapolated data and fitted linewidths are used to reproduce the photoabsorption spectra and compare with the available experimental and theoretical results. In addition, the integral cross sections from the excitation threshold to 5000 eV of the associated excited states have been derived by using the BE-scaling method. The resulted data can not only be used in plasma models but also serve as a benchmark for further theoretical calculations.

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.

Angle-Resolved Electron Energy Loss Spectroscopy

Angle-Resolved Electron Energy Loss Spectroscopy

Adsorption of Azobenzene on Hexagonal Boron Nitride Nanomesh Supported by Rh(111).

Adsorption properties of azobenzene, the prototypical molecular switch, were investigated on a hexagonal boron nitride (h-BN) monolayer (“nanomesh”) prepared on Rh(111). The h-BN layer was produced by decomposing borazine (B3N3H6) at 1000–1050 K. Temperature-programmed desorption (TPD) studies revealed that azobenzene molecules adsorbed on the “wire” and “pore” regions desorb at slightly different temperatures. Angle-resolved high-resolution electron energy loss spectroscopy (HREELS) measurements demonstrated that the first molecular layer is characterized predominantly by an adsorption geometry with the molecular plane parallel to the surface. Scanning tunneling microscopy (STM) indicated a clear preference for adsorption in the pores, manifesting a templating effect, but in some cases one-dimensional molecular stripes also form, implying attractive molecule–molecule interaction. Density functional theory (DFT) calculations provided further details regarding the adsorption energetics and bonding and confirmed the experimental findings that the molecules adsorb with the phenyl rings parallel to the surface, preferentially in the pores, and indicated also the presence of an attractive molecule–molecule interaction.

Energy losses and transition radiation in graphene traversed by a fast charged particle under oblique incidence

We perform fully relativistic calculations of the energy loss channels for a charged particle traversing a single layer of graphene under oblique incidence in a setting pertinent to a scanning transmission electron microscope (STEM), where we distinguish between the energy deposited in graphene in the form of electronic excitations (Ohmic loss) and the energy emitted in the far field in the form of transition radiation (TR). Our formulation of the problem uses a definition of two in-plane, dielectric functions of graphene, which describe the longitudinal and transverse excitation processes that contribute separately to those two energy loss channels. Using several models for the electric conductivity of graphene as the input in those dielectric functions enables us to discuss the effects of oblique incidence on several processes in a broad range of frequencies, from the terahertz (THz) to the ultraviolet (UV). In particular, at the THz frequencies, we demonstrate that the nonlocal effect in the graphene's conductivity is not important in the retarded regime, and we show that the longitudinal and transverse contributions to the emitted TR spectra exhibit strongly anisotropic angular patterns that are readily distinguishable in a cathodoluminescence measurement in a STEM. Moreover, we explore the possibility of exciting the so-called transverse mode in the optical response of graphene at the mid-infrared (MIR) range of frequencies by means of a fast charged particle under oblique incidence. Finally, we demonstrate that, aside from the usual high-energy peaks in the longitudinal contribution to the Ohmic energy loss in the MIR to the UV frequency range, there may arise strongly directional features in the in-plane distribution of the transverse contribution to the Ohmic energy loss for an oblique trajectory, which could be possibly observed via momentum- and angle-resolved electron energy loss spectroscopy of graphene in STEM.

Electron-impact electronic-state excitation of para-benzoquinone.

Angle resolved electron energy loss spectra (EELS) for para-benzoquinone (C6H4O2) have been recorded for incident electron energies of 20, 30, and 40 eV. Measured differential cross sections (DCSs) for electronic band features, composed of a combination of energetically unresolved electronic states, are subsequently derived from those EELS. Where possible, the obtained DCSs are compared with those calculated using the Schwinger multichannel method with pseudopotentials. These calculations were performed using a minimum orbital basis single configuration interaction framework at the static exchange plus polarisation level. Here, quite reasonable agreement between the experimental cross sections and the theoretical cross sections for the summation of unresolved states was observed.

Angle-resolved electron energy loss spectroscopy in hexagonal boron nitride

Electron energy loss spectra have been measured on hexagonal boron nitride single crystals employing a novel electron energy loss spectroscopic set-up composed by an electron microscope equipped with a monochromator and an in-column filter. This set-up provides high-quality energy-loss spectra and allows also for the imaging of energy-filtered diffraction patterns. These two acquisition modes provide complementary pieces of information, offering a global view of excitations in reciprocal space. As an example of the capabilities of the method we show how easily the core loss spectra at the $K$ edges of boron and nitrogen can be measured and imaged. Low losses associated to interband and/or plasmon excitations are also measured. This energy range allows us to illustrate that our method provides results of quality comparable to those obtained from non resonant X-ray inelastic scattering, but with advantageous specificities such as an enhanced sensitivity at low q and a much higher simplicity and versatility that makes it well adapted to the study of two-dimensional materials and related heterostructures. Finally, by comparing theoretical calculations against our measures, we are able to relate the range of applicability of ab initio calculations to the anisotropy of the sample and assess the level of approximation required for a proper simulation of our acquisition method.

Dispersion and damping of the interband π plasmon in graphene grown on Cu(111) foils

By means of angle-resolved electron energy loss spectroscopy, we have measured the interband π plasmon in high-quality graphene grown on peeled-off epitaxial Cu(111) foils. Experimental loss spectra have been reproduced by means of a hydrodynamic model. The dispersion relation of the plasmon frequency shows a nearly-flat dispersion up to a critical wave-vector of 0.3 Å−1. We propose that the observed behavior could be originated by confinement effects in ripples in the strained graphene sheet. Strain also limits the dispersion at higher momenta, as a consequence of the increased effective mass of charge carriers. The analysis of momentum dependence of the line-width and of the inverse quality factor indicates that damping processes are dominated by decay in electron-hole pairs via indirect interband transitions.

J_{eff} Description of the Honeycomb Mott Insulator α-RuCl_{3}.

Novel ground states might be realized in honeycomb lattices with strong spin-orbit coupling. Here we study the electronic structure of α-RuCl_{3}, in which the Ru ions are in a d^{5} configuration and form a honeycomb lattice, by angle-resolved photoemission, x-ray photoemission, and electron energy loss spectroscopy supported by density functional theory and multiplet calculations. We find that α-RuCl_{3} is a Mott insulator with significant spin-orbit coupling, whose low energy electronic structure is naturally mapped onto J_{eff} states. This makes α-RuCl_{3} a promising candidate for the realization of Kitaev physics. Relevant electronic parameters such as the Hubbard energy U, the crystal field splitting 10 Dq, and the charge transfer energy Δ are evaluated.

Vibronic effects on the low-lying electronic excitations in CO2 induced by electron impact

We report an angle-resolved electron energy loss spectroscopy (EELS) study on the valence-shell electronic excitations of CO2. Experimentally, momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles for low-lying electronic excitations have been derived from EELS spectra measured at incident electron energy of 3 keV. Theoretically, we have calculated GOS profiles using wave functions at the equation-of-motion coupled-cluster singles and doubles level. In the calculation, vibronic effects are taken into account by evaluating the electronic transition amplitudes along the individual normal coordinates. The calculation satisfactorily reproduces the experiment for the (1)Πg and (1)Δu transitions and reveals prominent roles of strong coupling between the associated mixed-Rydberg-valence and valence excited states through the bending vibration of CO2. Vibronic effects on GOS profiles have also been examined for the (1)Σu (+), (1)Πu, and 2(1)Πg transitions.

Interaction of surface and interface plasmons in extremely thin Al films on Si(111)

The collective electronic excitations in Al thin films with thickness down to mono-atomic layer were studied by scanning tunneling microscopy and angle-resolved high resolution electron energy loss spectroscopy. Clear evidences for a coupling of the Al surface plasmon and Al/Si interface plasmon were observed for the film thickness below 3 ML, which induces a splitting of the normal Al surface plasmon mode. The experimental results can be well explained by a classical model for surface plasmon excitations.

Manipulation of plasmon electron–hole coupling in quasi-free-standing epitaxial graphene layers

We have investigated the plasmon dispersion in quasi-free-standing monolayer graphene (QFMLG) and epitaxial monolayer graphene (MLG) layers by means of angle resolved electron energy loss spectroscopy. We have shown that various intrinsic p- and n-doping levels in QFMLG and MLG, respectively, do not lead to different overall slopes of the sheet plasmon dispersion, contrary to theoretical predictions. Only the coupling of the plasmon to single particle interband transitions becomes obvious in the plasmon dispersion by characteristic points of inflections, which coincide with the location of the Fermi level above or below the Dirac point. Further evidence is given by thermal treatment of the QFML graphene layer with gradual desorption of intercalated hydrogen, which shifts the chemical potential toward the Dirac point. From a detailed analysis of the plasmon dispersion, we deduce that the interaction strength between the plasmon and the electron–hole pair excitation is increased by about 30% in QFMLG compared to MLG, which is attributed to a modified dielectric environment of the graphene film.

Acoustic Surface Plasmon Dispersion on Nanostructured Cu(111)

In the present paper, the acoustic surface plasmon (ASP) dispersion is investigated on nanostructured Cu(111) by angle-resolved high-resolution electron energy loss spectroscopy (HREELS). The surface is prepared by ion bombardment at different crystal temperatures and ion fluences, following the recipes given by J. Naumann et al. (Surf Sci 388:212–219, 1997). We demonstrate that, at room temperature, the ASP survives to the disordering introduced by sputtering. The slope of the dispersion curve is thereby slightly reduced with respect to the one measured on pristine Cu(111), in accord with the disorder induced up-shift of the Shockley surface state. When ion bombardment is performed at 393 K and the surface restructures at the nanoscale forming hexagonal holes, an additional sharp loss is observed at 0.87 eV for wave vectors around 0.15 A−1, while the ASP loss gradually faints off with sputtering dose. Eventually, HREELS shows evidence only for the interband transition from the bottom of the Shockley surface state to the Fermi level.

Evidence for acoustic-like plasmons on epitaxial graphene on Pt(111)

The dispersion and the damping of the sheet plasmon in a graphene monolayer grown on Pt(111) have been studied by using angle-resolved electron energy loss spectroscopy. We found that the dispersion relation of the plasmon mode confined in the graphene sheet is linear, as a consequence of the screening by the metal substrate. Present results demonstrate that the presence of an underlying metal substrate could have striking consequences on the plasmon propagation even in the case of a system which exhibits a weak graphene-substrate interaction. Moreover, we found that Landau damping essentially occurs via interband excitations starting above the Fermi wave vector. On the contrary, intraband transitions do not have a significant influence on the collective mode.