Angle-resolved Electron Energy Loss Spectroscopy Research Articles

Examining the anisotropic behavior of the excitons in anatase TiO2 by angle-resolved electron energy-loss spectroscopy

The behavior of excitons excited in anatase crystalline materials affects the photocatalytic performance of the materials. Thus, investigating the properties of such excitons is essential for understanding the origin of their influence on photocatalytic performance. Here, angle-resolved electron energy-loss spectroscopy was applied to evaluate the size of the spatial spread of excitons in anatase TiO2. Three kinds of excitons (I–III), which were reported by polarized light absorption experiments, were experimentally identified in the electron energy-loss spectroscopy (EELS) spectra. Exciton size was evaluated from absorption intensity, which was derived by the Kramers–Kronig analysis of the EELS spectra, depending on the momentum transfer (q). The sizes of excitons I, II, and III were evaluated to be 8, 5, and 6 nm, respectively. The larger size of exciton I than that of III was with the same tendency as in the theoretical results. Exciton II, which was evaluated as a delocalized one, was evaluated to have a finite size in this experiment. The largest size of exciton I, approximately 8 nm, is the same order of the exciton diffusion lengths of the material. Therefore, exciton I should significantly influence the photocatalytic activity of anatase.

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.

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.

Layer-dependent anisotropic electronic structure of freestanding quasi-two-dimensionalMoS2

The anisotropy of the electronic transition is a well-known characteristic of low-dimensional transition-metal dichalcogenides, but their layer-thickness dependence has not been properly in- vestigated experimentally until now. Yet, it not only determines the optical properties of these low-dimensional materials, but also holds the key in revealing the underlying character of the elec- tronic states involved. Here we used both angle-resolved electron energy-loss spectroscopy and spectral analysis of angle-integrated spectra to study the evolution of the anisotropic electronic transition involving the low energy valence electrons in the freestanding MoS$_2$ layers with different thicknesses. We are able to demonstrate that the well-known direct gap at 1.8 eV is only excited by the in-plane polarized field while the out-of-plane polarized optical gap is 2.4$\pm$0.2 eV in monolayer MoS$_2$. This contrasts with the much smaller anisotropic response found for the indirect gap in the few-layer MoS$_2$ systems. In addition, we determined that the joint density of states associated with the indirect gap transition in the multilayer systems and the corresponding indirect transition in the monolayer case has a characteristic three-dimensional-like character. We attribute this to the soft-edge behavior of the confining potential and it is an important factor when considering the dynamical screening of the electric field at the relevant excitation energies. Our result provides a logical explanation, for the large sensitivity of the indirect transition to thickness variation compared with that for the direct transition, in terms of quantum confinement.

Formation of a hybrid plasmonic waveguide mode probed by dispersion measurement

Hybrid waveguides, i.e., dielectric waveguides combined with plasmonic waveguides, have great potential for concomitantly exhibiting subwavelength confinement and long range propagation, enabling a highly integrated photonic circuit. We report the characterization of hybrid waveguide modes excited in Si/SiO2/Al films, by dispersion measurement using angle-resolved electron energy-loss spectroscopy. This experiment directly verifies the formation of the hybrid waveguide mode with a strongly localized electromagnetic field in a 6-nm-thick SiO2 layer. The results clearly describe the characteristic behavior of the hybrid waveguide mode, which depends on the effective index of the constituent dielectric waveguide and the surface plasmon-polariton modes.

Direct measurement of dispersion relation for surface plasmon-polaritons on silver nanoantennas

We investigated the degree to which the dispersion relation of surface plasmon-polaritons excited on silver nanoantennas depends on length. To accomplish this, dispersion measurements for individual silver nanoantennas with lengths from 220 nm to 2.5 μm were performed using angle-resolved electron energy-loss spectroscopy (AREELS) and spatially resolved EELS (SREELS). AREELS enabled measurements of the dispersion relation extending into the high-energy region of 2.6 eV for a 2.5-μm-long silver nanoantenna, without the need to excite Fabry-Perot-type resonances. Our experiments showed that the dispersion relation of silver nanoantennas that have the same diameter is independent of their individual lengths.

Dispersion relations for coupled surface plasmon-polariton modes excited in multilayer structures

The coupled surface plasmon-polariton (SPP) modes excited in an Al/SiO2/Al multilayer structure were analyzed using angle-resolved electron energy-loss spectroscopy (AREELS) with a relativistic electron probe. The dispersion relations for the coupled SPP modes were then directly observed and compared with predicted relations obtained via calculations. Good agreement was noted between the experimental and calculated results. In the multilayer structures, the dispersion relation for the coupled SPP modes was found to be sensitive to the thickness of each film, which could be interpreted qualitatively by the electron energy-loss probability calculated for thin aluminum (Al) films and narrow Al gaps using Kröger's formula. It was demonstrated that significant differences in the excitation probability for SPPs could be observed depending on the coupling modes.

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.

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.

Doubly excited states of H2 as studied by angle-resolved electron energy loss spectroscopy in coincidence with detecting Lyman-α photons

The differential cross sections of the dissociative double excitations resulting in H(2p) formation in 80 eV electron collisions with H2 have been measured at electron scattering angles of 3, 20, 30 and 40° by means of angle-resolved electron energy loss spectroscopy in coincidence with detecting Lyman-α photons. The doubly excited states of H2 involved in this study are the optically allowed Q1 1Πu(1) and Q2 1Πu(1) states and the forbidden state at 26 eV, which has not been assigned so far and is temporarily referred to as the F state in this study. At a 3° electron scattering angle the differential cross section of the dissociative excitation to the forbidden F state is the largest among those for the three doubly excited states and decreases with increasing the electron scattering angle more rapidly than those for the allowed Q1 1Πu(1) and Q2 1Πu(1) states. The differential cross sections for the forbidden and allowed states do not differ quite so much, which may show that the forbidden F state is a dissociative state rather than an autoionizing state. The effective generalized oscillator strengths for H(2p) formation have been compared with the dipole oscillator strengths according to the limit theorem.

Experimental and theoretical study on generalized oscillator strengths of the valence-shell electronic excitations in CF4

We report an angle-resolved electron energy loss spectroscopy (EELS) study on the valence-shell electronic excitations in CF(4). Experimentally momentum-transfer-dependent generalized oscillator strengths (GOSs) or GOS profiles for low-lying electronic excitations at 12.6, 13.8, and 14.8 eV are derived from EELS spectra measured at an incident electron energy of 3 keV. We also calculate GOS profiles using theoretical wave functions at the equation-of-motion coupled cluster singles and doubles level. There are good agreements between experiment and theory except for a significant discrepancy at small momentum transfer for the 1t(l) → 3s Rydberg excitation at 12.6 eV. The experimental GOS profile for 1t(l) → 3s exhibits a shape that is typical of a dipole allowed transition, while the excitation is formally dipole forbidden. This symmetry breaking behavior is rationally accounted for by qualitatively analyzing the nature of vibronic coupling effects. For the excitation band at 13.8 eV, a shoulder and extrema are observed in the GOS profile and are then found to be mainly due to the 2(1)T(2) transition. Furthermore, the theoretical GOS profile for the 2(1)T(2) transition exhibits a remarkable oscillatory pattern; its origin is discussed by considering multicenter interference effects. For the 14.8 eV excitation band, the predominant nondipole nature of the underlying transitions are revealed and comparisons with the theoretical calculations show that major contributions to this band come from the 4t(2) → 3p excitation.

Low loss EELS and EFTEM study of Bi2Te3 based bulk and nanomaterials

ABSTRACTEnergy-filtered transmission electron microscopy (EFTEM) yields new possibilities for the investigation of Bi2Te3 based nanomaterials. Combined low-loss electron energy-loss spectroscopy (EELS) and energy-dispersive x-ray microanalysis (EDS) and energy-filtered TEM were applied on a Zeiss 912Ω TEM to investigate nanowires, thin films, and bulk materials. Multilayered Bi-Sb-Te nanowires with a diameter of 65 nm and a period of 200 nm and stoichiometric Bi2Te3 nanowires were grown by potential-pulsed electrochemical deposition. Tellurium elemental maps of the multilayered nanowires were obtained by two-window edge-jump ratio images (EJI). EDS chemical analysis showed that small Te fluctuations of 3 at.% yielded significant contrast in EJI. Energy-filtered TEM applied on nano-alloyed Bi2Te3 thin films grown by molecular beam epitaxy (MBE) revealed 10-20 nm thick Bi-rich blocking layers at grain boundaries. Plasmon spectroscopy by EELS was applied on Bi2(Te0.91Se0.09)3 bulk and yielded a plasmon energy of 16.9 eV. Finally, plasmon dispersion was measured for Bi2(Te0.91Se0.09)3 bulk by angle-resolved EELS, which yields a fingerprint of the anisotropy and the dimensionality of the electronic structure of the materials.

Differential cross sections for the dissociative single and double excitations resulting in H(2p) formation in electron–CH4 collisions at 80 eV incident electron energy

The differential cross sections for the dissociative single and double excitations resulting in H(2p) formation with the excitation energy of 19–46 eV in electron–CH4 collisions have been measured as a function of electron scattering angle in the range 4°–48° at 80 eV incident electron energy by means of angle-resolved electron energy-loss spectroscopy in coincidence with detecting Lyman-α photons. This is the first measurement of the differential cross sections for the dissociative double excitations as a function of electron scattering angle in electron–molecule collisions. Their fractions have been compared with those at the optical limit calculated from the density of the dipole oscillator strength for the emission of Lyman-α photons previously measured by our group. The dissociative double excitations in 80 eV electron collisions seem to be brought about in a very different way from those at the optical limit where they arise from the electron correlation in a methane molecule. The differential cross sections have also been discussed in terms of momentum transfer, leading to a universal curve.