Nanoscale momentum-resolved vibrational spectroscopy

Hage, Fredrik S.; Nicholls, Rebecca J.; Yates, Jonathan R.; McCulloch, D. G.; Lovejoy, Tracy C.; Dellby, Niklas; Krivanek, Ondrej L.; Refson, Keith; Ramasse, Quentin M.

doi:10.1126/sciadv.aar7495

Cited by 137 publications

(122 citation statements)

References 47 publications

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“…the beam interacts via the electromagnetic (EM) field with sample areas that are far away from the actual beam position. 13,14 Unless direct impact scattering (when the beam can interact with both optical and acoustic lattice vibrations 7,15,16 ) takes place, most of the EEL vibrational signal arises purely from the excitation of optical phonons. The energy-filtered EEL signal corresponding to optical phonon losses enables us to achieve (sub)nanometric spatial resolution only if electrons scattered through significant angles are collected.…”

Section: Introductionmentioning

confidence: 99%

Vibrational electron energy loss spectroscopy in truncated dielectric slabs

et al. 2018

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Specially designed instrumentation for electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) makes it possible to probe very low-loss excitations in matter with a focused electron beam. Here we study the nanoscale interaction of fast electrons with optical phonon modes in silica. In particular we analyze the spatial dependence of EEL spectra in two geometrical arrangements: a free-standing truncated slab of silica and a slab with a junction between silica and silicon. In both cases, we identify different loss channels, involving polaritonic and non-polaritonic contributions to the total electron energy loss, and obtain the corresponding energyfiltered maps. Furthermore, we present a comparison of the theoretical simulations for a silica-silicon junction with experimental results, and discuss the spatial resolution attainable from the energyfiltered map considering optical phonon excitations in a conventional experimental arrangement.

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Section: Introductionmentioning

confidence: 99%

Vibrational electron energy loss spectroscopy in truncated dielectric slabs

et al. 2018

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“…Recent instrumental developments have improved the energy resolution of electron energy loss spectroscopy down to 4.2 meV [7,8]. This unique combination of high spatial and energy resolution opened doors to unprecedented experiments such as temperature measurement at the nanoscale [9, 10], identification and mapping of isotopically labeled molecules [11], position-and momentum-resolved mapping of phonon modes [12,13], mapping of bulk and surface modes of nanocubes [14], or investigations of the nature of polariton modes in van der Waals crystals [15].Inelastic electron scattering on atomic vibrations in a polar material consists of two major contributions, namely, impact scattering and dipolar scattering [16,17]. The latter stems from a long ranged interaction between the beam electron and an oscillating dipole moment generated by the atomic vibrations.…”

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confidence: 99%

Efficient and Versatile Model for Vibrational STEM-EELS

Zeiger

Rusz

2020

Phys. Rev. Lett.

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We introduce a novel method for the simulation of the impact scattering in vibrational scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) 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 the cost comparable to standard frozen phonon calculations. We demonstrate good agreement of our method with simulations and experiments for a 15 nm flake of hexagonal boron-nitride (hBN).Excellent spatial resolution is the highlight of the transmission electron microscope [1-6], allowing atomically resolved elemental mapping and chemistry. Recent instrumental developments have improved the energy resolution of electron energy loss spectroscopy down to 4.2 meV [7,8]. This unique combination of high spatial and energy resolution opened doors to unprecedented experiments such as temperature measurement at the nanoscale [9, 10], identification and mapping of isotopically labeled molecules [11], position-and momentum-resolved mapping of phonon modes [12,13], mapping of bulk and surface modes of nanocubes [14], or investigations of the nature of polariton modes in van der Waals crystals [15].Inelastic electron scattering on atomic vibrations in a polar material consists of two major contributions, namely, impact scattering and dipolar scattering [16,17]. The latter stems from a long ranged interaction between the beam electron and an oscillating dipole moment generated by the atomic vibrations. It can be used to perform damage-free vibrational spectroscopy in an aloof beam geometry [18,19]. High-angle impact scattering on the other hand is quite localized scattering on the atomic potential and allows for high resolution imaging and spectroscopy [17,[20][21][22].From a theoretical point of view, there is a limited number of approaches one can follow to simulate vibrational spectroscopy and imaging. Dwyer presents a treatment of both single impact and single dipolar scattering including a detailed phonon dispersion from a density functional theory calculation in a multislice scheme [23]. Forbes et al. introduced a quantum excitation of phonons model, which allows for the inclusion of thermal diffuse scattering into multislice image simulations [24]. The thermal vibrations are modeled within the Einstein model for quick image calculations, neglecting correlations in atomic movements and dispersion effects. Such an approach is not able to simulate vibrational spectra but in principle one could use different models of atomic vibrations to generate the necessary configurations. In a more recent work, Forbes et al. described a method to model the vibrational sector of the electron energy loss spectrum [25...

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“…Unfortunately, the energy resolution of most EF-TEM apparatuses (> 200 meV [36]) is largely inadequate to detect gapless excitations. Nevertheless, recent technological advancements have been decisive to improve the energy resolution up to 18-50 meV [37,38] with the next target to reach 5 meV [39]. Consequently, it is expected that in a few years the measurement of the dispersion relation of plasmonic modes along the tilt axis (Γ − A) will be experimentally feasible.…”

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Novel Undamped Gapless Plasmon Mode in a Tilted Type-II Dirac Semimetal

2020

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We predict the existence of a novel long-lived gapless plasmon mode in a type-II Dirac semimetal (DSM). This gapless mode arises from the out-of-phase oscillations of the density fluctuations in the electron and the hole pockets of a type-II DSM. It originates beyond a critical wave-vector along the direction of the tilt axis, owing to the momentum separation of the electron and hole pockets. A similar out-of-phase plasmon mode arises in other multi-component charged fluids as well, but generally it is Landau damped and lies within the particle-hole continuum. In the case of a type-II DSM, the open Fermi surface prohibits low-energy finite momentum single-particle excitations, creating a 'gap' in the particle-hole continuum. The gapless plasmon mode lies within this particle-hole continuum gap and, thus, it is protected from Landau damping.

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Nanoscale momentum-resolved vibrational spectroscopy

Abstract: A widely applicable method for accessing phonon dispersions of materials at high spatial resolution is demonstrated.

Cited by 137 publications

References 47 publications

Vibrational electron energy loss spectroscopy in truncated dielectric slabs

Vibrational electron energy loss spectroscopy in truncated dielectric slabs

Efficient and Versatile Model for Vibrational STEM-EELS

Novel Undamped Gapless Plasmon Mode in a Tilted Type-II Dirac Semimetal

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