Plasmonic Resonance in Metallic Nanoparticles

The optical properties of noble metal nanoparticles are dominated by the surface plasmons (SP), which are collective oscillations of the free electrons confined at the surface. Surface plasmons in noble‐metal nanoparticles have received considerable attention for the wide range of applications, ranging from surface‐enhanced Raman spectroscopy (SERS), biomolecule sensing, labeling of biomolecules, cancer therapy, plasmonic absorption enhancement in solar cells to nanophotonics. Influence of particle size on the surface plasmons is of great interest. For large particles (> 10 nm), pure classical effects related to confinement of classical electromagnetic waves at the surface of the objects are well‐known. For smaller sizes, the major effects are related to quantum physics. Optical techniques have been employed to study the optical properties of sub‐10 nm nanoparticles 1 . However in these techniques the measurements are made for a set of particles. This implies an inhomogeneous broadening of the surface plasmon resonance which largely prevents observing the influence of quantum phenomena. In order to overcome these challenges, single particles measurements have been made in the past years by electron energy loss spectroscopy (EELS) in combination with a scanning transmission electron microscope (STEM). STEM‐EELS measurements allow the study of individual particles on the atomic scale and with high spectral resolution 2 . Here we investigate the plasmonic response of individual silver and gold nanoparticles, ranging from 2 to 10 nm in diameter. We will first discuss the instrumental progresses made to improve signal to noise ratio allowing detection of surface plasmons in ultrasmall nanoparticles and data treatment to improve energy resolution. We will then illustrate how bulk and surface plasmons evolve as a function of particle size. Figure 1 shows plasmons resonances well defined for a 5 nm silver particle and weak plasmons resonances (but yet visibles after deconvolution treatment) in a 2 nm silver particle. Furthermore in this work different configurations of substrate‐matrix will be shown in order to understand their influence on the plasmonic response.