Probing the radiative and full electromagnetic local densities of states with electron energy loss spectroscopy and cathodoluminescence spectroscopy

Abstract

Electron Energy Loss Spectroscopy (EELS) and CathodoLuminescence spectroscopy (CL) map the surface plasmon modes of metallic nanoparticles at the nanometer scale [1]. Although they yield similar results, EELS and CL differ by nature. EELS measures the energy lost by fast electrons interacting with a sample whereas CL measures the energy of the light emitted from this interaction [2]. However, the consequence of this different nature has remained unexplored because combining EELS and CL on single nanoobjects could not be achieved. We combined EELS and CL on single nanoobjects using a Scanning Transmission Electron Microscope (STEM) [3]. In a STEM, an electron beam is focused onto a nanometric probe that is scanned over the sample. At each point of the sample, the transmitted electrons are collected for EELS, while the emitted light is collected for CL; both a full EELS spectrum and a full CL spectrum are recorded. We studied small gold triangular nanoprisms as a model system (Fig. 1a). The combination of EELS and CL allows us to map the same surface plasmon modes using both techniques (Fig. 1b and c). We showed that CL only probes the dipolar mode, contrary to EELS that probes both the dipolar and quadrupolar modes. Furthermore, the dipolar mode resonates at shifted resonances in EELS and CL for prisms with edge lengths larger than 100 nm relying on carbon substrate (Fig. 2a). The magnitude of the shift relates to the mode damping. Having demonstrated that EELS and CL signals differ, we showed that the full and radiative ElectroMagnetic Local Densities Of States (EMLDOS) differ similarly (Fig. 2b) [4]. Whereas EELS measures a quantity close to the full EMLDOS projected along the electron direction [5], CL measures a quantity close to the radiative EMLDOS projected along the electron direction [4]. Contrary to EELS, CL probes only the radiative modes, which are not necessarily dipolar for object sizes beyond the quasistatic limit. The ratio of the CL resonance to the EELS resonance quantifies the radiative weight of the mode damping. If the modes are damped and induce interfering radiation, CL resonant line shapes are shifted and asymmetric compared to EELS. This work demonstrates the great interest of combining EELS and CL for plasmonics [3, 4]. The research leading to these results has received funding from the European Union Seventh Framework Programme [No. FP7/2007‐2013] under Grant Agreement No. n312483 (ESTEEM2). We gratefully aknowledge the following collaborators who significantly contributed to this work : Luiz F. Zagonel, Viktor Myroshnychenko, Benito Rodríguez‐González, Marcel Tencé, Leonardo Scarabelli, Jens Förstner, Luis M. Liz‐Marzán, F. Javier García de Abajo and Odile Stéphan.