Excitation of long-wavelength surface optical vibrational modes in films, cubes and film/cube composite system using an atom-sized electron beam

Microscopy, 2018, i3–i13 doi: 10.1093/jmicro/dfx130 Advance Access Publication Date: 23 January 2018 Article Excitation of long-wavelength surface optical vibrational modes in films, cubes and film/cube composite system using an atom-sized electron beam Maureen J. Lagos1,2,3,*, Andreas Trügler4,*, Voshadhi Amarasinghe3, Leonard C. Feldman1,2,3, Ulrich Hohenester4, and Philip E. Batson1,2,3 1 Department of Physics and Astronomy, 2Department of Materials and Science Engineering, Rutgers University, Piscataway, NJ 08854, USA, 3Institute for Advanced Materials, Devices, and Nanotechnology, Rutgers University, Piscataway, NJ 08854, USA, and 4Institute of Physics, University of Graz, Universitätsplatz 5, Graz 8010, Austria * To whom correspondence should be addressed. E-mail: (M.J.L) and andreas.truegler@ uni-graz.at (A.T) Received 18 August 2017; Editorial Decision 8 December 2017; Accepted 16 December 2017 Abstract Using spatially resolved Electron Energy-Loss Spectroscopy, we investigate the excitation of long-wavelength surface optical vibrational modes in elementary types of nanostructures: an amorphous SiO2 slab, an MgO cube, and in the composite cube/slab system. We find rich sets of optical vibrational modes strongly constrained by the nanoscale size and geometry. For slabs, we find two surface resonances resulting from the excitation of surface phonon polariton modes. For cubes, we obtain three main highly localized corner, edge, and face resonances. The response of those surface phonon resonances can be described in terms of eigenmodes of the cube and we show that the corresponding mode pattern is recovered in the spatially resolved EELS maps. For the composite cube/substrate system we find that interactions between the two basic structures are weak, producing minor spectral shifts and intensity variations (transparency behaviour), particularly for the MgO-derived modes. Key words: surface optical phonons, vibrational EELS, vibrational scattering cross-section, surface phonon polariton, infrared terahertz Introduction Finite systems sustain surface optical vibrational modes [1–3] whose properties are driven by the system shape and size. These modes play an important role in a large variety of nanoscale phenomena, such as light-phonon coupling [4], radiative heat transfer [5], heat and sound flow [6], van Der Waals friction [7], Casimir forces [8], among others. A deeper understanding of these phenomena requires fundamental experimental studies of the physical properties of surface vibrational modes in isolated nanosized objects © The Author(s) 2018. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: i3 i4 Using these remarkable capabilities, spatially resolved vibrational studies of nanostructures have began opening the door for further exploration of vibrational properties in isolated nanostructures with nanometric resolution. Electron beams can probe the spectrum of excitations of matter. Because the response of a system probed by a single keV electron is linear, this response is fully determined by the system properties in the absence of the external beam. In particular, spatially resolved electron energy loss spectroscopy (EELS) allows the mapping of inelastic scattering due to specimen excitations such as plasmons and phonons [9,20–22] in nanostructures with high spatial resolution. In these experiments, we describe the inelastic electron scattering within the local dielectric formulation. In the longwavelength limit (q → 0), keV energy electrons are deflected by small angles as a result of the small amounts of momentum transferred during the scattering. Experimentally, we can restrict scattering to long wavelengths by using a nonintersecting beam-specimen geometry or by using small apertures located near the optic axis of the microscope to restrict the angular content of the incident and scattered electrons [9,18,23–26]. Theoretically, the semi-classical dielectric theory is able to give a good description of this case, and indeed, most of the knowledge acquired about surface plasmons in nanostructures by electron scattering can be applied to interpret data of infrared phonon excitations as well. However, careful attention must be paid to ensure the validity of the theoretical methods and to avoid known situations where the local dielectric approach fails [27,28]. In this scenario, additional experimental data are required to test new theoretical models and ideas. In this paper, we present a study of the excitation of long-wavelength optical surface vibrational modes in two elementary phononic systems which exhibit surface phonon modes: amorphous silicon dioxide (a-SiO2) nanometric films and magnesium oxide (MgO) nanocubes. We explore in detail the scattering process of a fast electron with confined surface vibrational excitations in nanostructures in the dipole limit (q → 0). Surface phonon excitations in thin films have been extensively studied using many techniques, including EELS in the reflection geometry (R-EELS), and the physics of surface dipolar excitations is extensively documented [3]. Our study here focuses on the excitation of SPhP modes in the transmission geometry [29], exploring the excitation of modes which remain inactive when probing the slab in the reflection geometry. Furthermore, we performed spatially resolved vibrational studies of localized surface phonon excitations in isolated MgO nanocubes in similar manner as in [9], and in addition, we include a more detailed theoretical analysis of those localized surface excitations. We present experimental results acquired in the with high spatial resolution. In particular, experimental studies using local probes, which allow the excitation of highly localized surface and bulk vibrational modes [9] in single nanosized objects of different shapes and sizes, can provide valuable information useful for the design of more efficient infrared low-loss nanophotonic devices [10], nanophononic thermal devices [11], novel thermophotovoltaic devices [12], etc. Nanosized structures can sustain several kinds of vibrational surface modes. For instance, a surface phonon mode in a semi-infinite flat surface was predicted by Fuchs and Kliewer (FK) [13]. This mode is a longitudinal confined charge density wave oscillating along the surface with an angular frequency defined by the equation ε(ω) = −1, where ε is the bulk dielectric function within the structure. Also, for a thin slab of finite thickness, two different surface modes are generated due to the coupling between surface charges in each side of the slab surfaces through internal electric fields [14]. These self-sustained modes are the socalled surface phonon polaritons (SPhP) [2] and they will be further discussed in this work. More interestingly, the shape of the nanoscale structure plays a fundamental role in tuning the frequency of surface phonon modes, and so, (...truncated)