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In this tutorial review, we present the use of electron energy loss spectroscopy (EELS) and cathodoluminescence (CL) spectroscopy for surface plasmon mapping within metallic nanoparticles. We put a special emphasis on particles that are much smaller than the wavelength of visible light. We start by introducing the concept of surface plasmons, keeping the formalism as simple as possible by focusing on the quasi-static approximation. We then make a link between optical cross-sections, EELS and CL probabilities, and the surface plasmons' physical properties. A short survey of simulation tools is given. We then present typical experimental set-ups and describe some problems frequently encountered with spectrometers. Experimental conditions for improved signal to noise ratio are discussed. Analysis techniques are discussed, especially those related to the spectral imaging mode, which is extremely useful in EELS and CL experiments. Finally, the specific range of applications of EELS and CL with respect to other nano-optic techniques is discussed, as well as the strengths and weaknesses of EELS as compared with CL. [ABSTRACT FROM AUTHOR]

Abstract: Over the past ten years, Scanning Transmission Electron Microscopes (STEM) fitted with Electron Energy Loss Spectroscopy (EELS) and/or Cathodoluminescence (CL) spectroscopy have demonstrated to be essential tools for probing the optical properties of nano-objects at sub-wavelength scales. Thanks to the possibility of measuring them at a nanometer scale in parallel to the determination of the structure and morphology of the object of interest, new challenging experimental and theoretical horizons have been unveiled. As regards optical properties of metallic nanoparticles, surface plasmons have been mapped at a scale unimaginable only a few years ago, while the relationship between the energy levels and the size of semiconducting nanostructures a few atomic layers thick could directly be measured. This paper reviews some of these highly stimulating recent developments. [Copyright &y& Elsevier]

To date, quantum sources in the ultraviolet (UV) spectral region have been obtained only in semiconductor quantum dots. Color centers in wide bandgap materials may represent a more effective alternative. However, the quest for UV quantum emitters in bulk crystals faces the difficulty of combining an efficient UV excitation/detection optical setup with the capability of addressing individual color centers in potentially highly defective materials. In this work we overcome this limit by employing an original experimental setup coupling cathodoluminescence within a scanning transmission electron microscope to a Hanbury-Brown-Twiss intensity interferometer. We identify a new extremely bright UV single photon emitter (4.1 eV) in hexagonal boron nitride. Hyperspectral cathodoluminescence maps show a high spatial localization of the emission (∼80 nm) and a typical zero-phonon line plus phonon replica spectroscopic signature, indicating a point defect origin, most likely carbon substitutional at nitrogen sites. An additional nonsingle-photon broad emission may appear in the same spectral region, which can be attributed to intrinsic defects related to electron irradiation. [ABSTRACT FROM AUTHOR]

Plasmonmodes of the exact same individual gold nanoprisms are investigatedthrough combined nanometer-resolved electron energy-loss spectroscopy(EELS) and cathodoluminescence (CL) measurements. We show that CLonly probes the radiative modes, in contrast to EELS, which additionallyreveals dark modes. The combination of both techniques on the sameparticles thus provides complementary information and also demonstratesthat although the radiative modes give rise to very similar spatialdistributions when probed by EELS or CL, their resonant energies appearto be different. We trace this phenomenon back to plasmon dissipation,which affects in different ways the plasmon signatures probed by thesetechniques. Our experiments are in agreement with electromagneticnumerical simulations and can be further interpreted within the frameworkof a quasistatic analytical model. We therefore demonstrate that CLand EELS are closely related to optical scattering and extinction,respectively, with the addition of nanometer spatial resolution. [ABSTRACT FROM AUTHOR]

• A random scan operating mode in STEM achieved at the hardware level is presented. • Random sparse hyperspectral images can be extracted in any acquisition time window. • Randomization of pixel acquisition order can reduce dose accumulation effects. • Spatial and spectral dynamics can be followed in STEM hyperspectral imaging. • Proof of concepts for EELS and nano-cathodoluminescence are provided. The evolution of the scanning modules for scanning transmission electron microscopes (STEM) allows now to generate arbitrary scan pathways, an approach currently explored to improve acquisition speed and to reduce electron dose effects. In this work, we present the implementation of a random scan operating mode in STEM achieved at the hardware level via a custom scan control module. A pre-defined pattern with fully shuffled raster order is used to sample the entire region of interest. Subsampled random sparse images can then be extracted at successive time frames, to which suitable image reconstruction techniques can be applied. With respect to the conventional raster scan mode, this method permits to limit dose accumulation effects, but also to decouple the spatial and temporal information in hyperspectral images. We provide some proofs of concept of the flexibility of the random scan operating mode, presenting examples of its applications in different spectro-microscopy contexts: atomically-resolved elemental maps with electron energy loss spectroscopy and nanoscale-cathodoluminescence spectrum images. By employing adapted post-processing tools, it is demonstrated that the method allows to precisely track and correct for sample instabilities and to follow spectral diffusion with a high spatial resolution. [ABSTRACT FROM AUTHOR]