Accessing extinction and emission properties of gold nanoprisms at the nanometer scale A Losquin 1 , LF Zagonel 1,2 , B Rodríguez-González 3 , LM Liz-Marzan 3 , O Stéphan 1 and M Kociak 1 1. Laboratoire de Physique des Solides, Université Paris Sud, CNRS, Orsay, France 2. Brazilian Nanotechnology National Laboratory, CNPEM, 13083-970, Campinas, Brazil 3. Departamento de Quimica Fisica, Universidade de Vigo and Unidad Asociada, Vigo, Spain arthur.losquin@u-psud.fr Keywords: Surface Plasmons, Electron Energy Loss Spectroscopy, Cathodoluminescence The optical properties of metal nanoparticles are governed by their surface plasmon (SP) modes, which are resonant electromagnetic fields associated to the collective oscillations of their conduction electrons along the boundaries. Both the resonant energies and the electric field patterns associated to these SP modes strongly depend on numerous parameters, such as the material itself, its dielectric environment, and the size and shape of the nano-object. They therefore require spectroscopic techniques with sub-wavelength spatial resolution in order to be completely understood. Over the last few years, the highly focused electron probe of a Scanning Transmission Electron Microscope (STEM), acting as a virtual white light photon source, has successfully demonstrated its potential ability to map SP modes in nanoparticles with nanometer spatial resolution. Indeed, the nanometric analogue to extinction spectroscopy, the Electron Energy Loss Spectroscopy (EELS), has been of great interest for studying SP resonances in highly symmetric nano-objects [1], thanks to a relatively high signal to noise ratio. In parallel, cathodoluminescence (CL) has proved to be very powerful when applied in analyzing the light emission induced from SP in nanoparticles with a unique spectral resolution [2]. Moreover, both extinction and emission mappings may provide complementary as well as precious information for the physics of SP when compared using these two techniques on a single nano-object. Figure 1 shows a STEM High Angle Angular Dark Field (HAADF) image of a gold (bright) nanoprism relying on a (dark) carbon grid, together with EELS and CL spectra for a given probe position at the top tip of the nanoprism. In EELS mode, the fast electron beam is spectrally analyzed through transmission of the sample, giving access to Electron Energy Loss (EEL) probability spectra (in blue). As the incident electron beam acts as a white light source, all types of excitations are probed, including “bright” as well as “dark” SP modes. In CL mode, light coming from deexcitation of the locally excited radiative SP modes is dispersed to obtain Electron Induced Radiation Emission (EIRE) probability spectra (in red). As can be seen, CL displays clear redshift as compared to EELS, mostly due to retardation effects [3]. Hyperspectral imaging data is obtained by raster scanning the incident electron probe over the sample to investigate the related SP modes. Figure 2 shows spatially resolved EEL and EIRE maps, filtered at the relevant peak wavelengths. This evidences the dipolar character of the mode, similar to dipolar SP modes previously observed by EELS for silver nanoprisms [1]. Furthermore, it is now well established that these maps display the spatial variations of a quantity which is related to the total [4] (for EELS) and radiative [5] (for EIRE) Electromagnetic Local Density of States (EMLDOS). As the contribution of several SP modes may blur the EMLDOS patterns, we fit on each pixel a Gaussian function around the peak positions to obtain the amplitude weight of the dipolar mode. Results are given on figure 3, demonstrating the coherent character of the mode over the three tips, despite a maximum in photon emission at the upper tip. By comparing both spatially resolved EELS and CL on a single nano-object, we therefore gain insight on its complete optical properties at the nanoscale. This includes discerning its “dark” and “bright” modes, and comparing emission and absorption properties. [6]