Novel nanoscale IR-UV spectroscopies in an advanced electron microscope   

Session

PST.2 - Microscopy for the study of quantum effects and nano-optics

Authors

Mr. Luiz Tizei (1)

Affiliations

1. CNRS

Keywords

CL, EELS, phonons, strong coupling, 2D materials

Abstract text

The remarkable energy resolution and control achievable in all-optical experiments makes it a hard contender against new designs and developments in other spectroscopies, at least in the optical range. However, this high precision means that even nanometer scale changes to the object under study have a detectable spectral effect . For example, a 10 nm length change on a micrometer-long metallic rod leads to an energy shift of the order of 1 meV, roughly. Hence, the necessity of coupling high spatial resolution to high spectral resolution. Up until recently, this bridge was crossed by performing complementary electron microscopy measurements at high spatial resolution in addition to optical measurements either ex or in situ.

Yet, the use of electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) in different configurations and with increased spatial and spectral resolutions has shown that there are many benefits in using electron beams for spectroscopy, at least where nanometer scale features are relevant. More importantly, the remarkable spectral resolution in experiments with focused electron beams [1-6] achieved in the previous few years makes EELS and CL powerful companions to all-optical spectroscopies. Finally, EELS and CL in a transmission electron microscope have the added benefit of direct access to the reciprocal space. In this contribution, we will describe two groups of experiments to demonstrates our current possibilities in the reciprocal and real spaces.

First we will discuss the coupling between phonons and plasmons in nanostructures [7]. We experimentally demonstrate that the interaction between a relativistic electron and vibrational modes in nanostructures is fundamentally modified in the presence of plasmons. This could be observed due to the current developments in electron optics, making possible the generation of sub-nanometer wide sub-10 meV electron beams with 60 keV primary energy. We finely tune the energy of surface plasmons in metallic micrometer-long Ag nanowires in the vicinity of hexagonal boron nitride flakes, making it possible to monitor and disentangle both strong phonon–plasmon coupling and plasmon-driven phonon enhacement at the nanometer scale. Because of the near-field character of the electron beam–phonon interaction, optically inactive phonon modes are also observed. Besides increasing our understanding of phonon physics, our results hold great potential for investigating sensing mechanisms and chemistry in complex nanomaterials down to the molecular level.

In addition to absorption experiments, sub-nanometer electron beams allow the realization of emission experiments (CL) with nanometer-scale spatial resolution [1, 3, 8]. With this in mind, we will discuss the interest in observing the emission and absorption properties of individual nanostructures will be demonstrated. As an example, we will consider the emission (CL) and absorption (EELS) spectra of WS₂ monolayers, encapsulated by hBN.

If time allows, new electron energy gain experiments (EEGS) on plasmonic systems will be discussed.

The experiments to be presented were performed on  the ChromaTEM (TEMPOS ANR project) microscope, a modified Nion Hermes 200 with a cold sample stage. If time allows, we will present some more recent results.

References

[1] N. Yamamoto, K Araya, and F. J. G. de Abajo, Phys. Rev. B 64 (2001), p. 205419.

[2] J. Nelayah, et al, Nat. Phys. 3 (2007), p. 348.

[3] L. F. Zagonel, et al, Nano Lett. 11 (2011), p. 568.

[4] O. Krivanek, et al, Nature,  514 (2014), p. 209.

[5] L. H. G. Tizei et al, Phys. Rev. Lett. 114 (2015), p. 107601.

[6] Andrew B. Yankovich, et al, Nano Lett. 19 (2019), p. 8171.

[7] L. H. G. Tizei, et al, Nano Lett. (2020) 10.1021/acs.nanolett.9b04659.

[8] M. Kociak, L. F. Zagonel, Ultramicroscopy 176 (2017) p. 112.