Review ArticleElectron Energy Loss Spectroscopy imaging of surface plasmons at the nanometer scale
Introduction
Surface plasmons are oscillations of charges, in general electrons, at the interfaces between metallic and dielectric media, which create evanescent electromagnetic fields decaying exponentially with the distance to the boundary. They have been over the past recent years a topic of intensive research, both for fundamental challenges and for potential applications in many domains. In particular, their role for manipulating the light, guiding it along metallic stripes or condensing it into holes or gaps, at a scale significantly shorter than its wavelength, has attracted the general interest, so that a “new” field of research has emerged, known as “nanoplasmonics”, a branch of “nanophotonics”.
Among the different techniques that have been developed for mapping the distribution in energy and space of these characteristic electronic excitations and their associated fields, the use of the primary electrons in an electron microscope has demonstrated unique and specific possibilities that will be extensively discussed in the present review. Generally speaking, the high-energy electrons in the electron microscope are mostly used for imaging and analyzing with a very high spatial resolution, down to the sub-angström level, the position and nature of atoms, ions and electron clouds in thin objects, using their strong scattering probabilities. The structural information is provided by images or diffraction patterns mostly implying elastic scattering. On its side, the analytical information encompassing a very broad spectral range typically from the infrared around 1 eV up to the X-ray above 1000 eV, is generated by inelastic processes which transfer energy as well as momentum between the incident electrons and the target. Practically, spectroscopies of the transmitted electrons (EELS for Electron Energy Loss Spectroscopy) and of the emitted photons (EDX for the energy dispersive X-ray spectroscopy in the X-ray spectral domain and CL for cathodoluminescence in the near-visible domain) are involved. The impact of the EELS measurements in the low-loss domain, will be specifically addressed in the following, as they associate an unmatched spatial resolution in the nanometer range with a broad spectral range (from 0.5 eV up to 5 eV investigated with a typical 100 meV energy resolution).
In this paper, after a short look at historical landmarks that have contributed to the birth and growth of this research field, we will first summarize recent progress in instrumentation and in theory which have brought it to its present blossoming. The richness and diversity of results obtained by EELS, optionally enlightened by CL measurements, will be described with the support of practical situations, involving nanostructures of quite variable nature, shape, size and environment. They will thus emphasize our newly accumulated knowledge on the physical nature and coupling of the plasmonic modes, therefore opening new broad fields for a joint exploration of electronics and optics in nano-sized structures. Let us add that the combination of EELS and CL techniques in TEM techniques for the mapping of the optical response of nanostructures has been extensively discussed in recent reviews [1], [2], [3].
Section snippets
Historical landmarks
In the mid fifties, a burst of activity, as well theoretical as experimental, see for instance [4], [5], has clearly identified bulk plasmons, i.e. collective modes associated to longitudinal oscillations of the electron gas in solids, at the origin of the narrow and intense characteristic lines recorded at multiple values of a quantized energy Ep in an EELS spectrum of fast electrons transmitted through solid foils. Furthermore, the description of the solid in terms of the dielectric
Instrumental considerations
Measuring the energy loss suffered by the primary electrons of an electron microscope traveling through or close to the object feature under investigation provides simultaneous access to spatial and spectral information. As a matter of fact, the elementary piece (or bit) of information in the relevant 3D mixed position-energy (x,y,E) space [16] is the number of electrons having impacted over the element of area (δx,δy) at the position (x, y) on the specimen and been detected with an energy loss
Theoretical developments
A vast literature has been published to describe the nature and the physical mechanisms involved in surface plasmons and associated electric fields in general. In the present text, we will restrict ourselves to the presentation and application of theoretical tools and simulation software which have been developed to interpret the EELS results and maps of surface plasmon distribution at the nanoscale in interfaces and nanostructures.
The starting point of these modeling tools is the use of
Surface (interface) plasmons in simple geometries involving planar or spherical shapes
Surface plasmon resonances can be found at the surface of most materials, at least those exhibiting clear collective response in the bulk. Examples related to different situations involving metals, semiconductors and insulators are shown in this paragraph. However, the family of metals with a wavelength-dependent dispersion of their dielectric function prone to give resonance conditions in the IR and visible spectral domains, has been mostly investigated up-to-now, i.e. the noble metals Ag and
How the shape, size and nature of individual nanoparticles govern their surface plasmon maps
Beyond the simple geometries (plane, sphere) described there above, the newly developed techniques of fabrication, either bottom-up or top-down, have generated the creation of metallic nanostructures exhibiting many different types of shapes. Because of their ability for locally exalting EM fields in particular, the collective excitation modes in these nano-objects have been extensively investigated experimentally as well as theoretically over the past few years. In this effort, electron beam
Resonant surface plasmons on nanoparticles in interaction
In real systems designed for applications in photonics, it is important to build and couple many (or at least two) individual nano-objects, thus opening the way to manipulate light at the nanometer scale with metal nanostructures as nano-optical components.
Summary and perspectives
In this review we have demonstrated how much the techniques derived from EELS spectroscopy in an electron microscope (both STEM-EELS and EFTEM) nowadays constitute a most efficient tool to investigate the characteristic electron excitations and accompanying electromagnetic fields at surfaces and interfaces. This has been supported with examples involving quite diversified situations in 1, 2 and 3 dimensions, probing interactions at the ultimate sub-nm scale and covering a spectral domain
Acknowledgments
This review basically relies on studies realized in the STEM group at the Laboratoire de Physique des Solides in Orsay, over typically three decades. Consequently many students, post-docs and collaborators have been involved and have realized successive developments which have all contributed to this new plasmonic “euphoria” (as it has been designed by Archie Howie in one recent publication): let us first mention, thank and congratulate the involved Ph.D. students starting from 1985 Mustapha
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