Emission Properties of Metal Nanoparticles

Abstract

Nanoparticles emit electrons and photons when they are excited by electron injection via electric current; electromagnetic radiation via microwave fields; laser radiation in infrared, visible and synchrotron (X-ray) ranges; and electron and ion bombardment. In each case, the emission mechanism depends on characteristic length scales of the nanoparticle. If the particle size is commensurate with it or is smaller, a size dependence of emission is observed [1][2][3]. Also, it is interesting that nanoparticles may demonstrate properties absent in bulk material. Electron Emission from Metal Nanoparticles Excitation by Electron InjectionExcitation by electron injection into nanoparticles via electron tunneling (electric current flow through an ensemble of tunnel-coupled metal nanoparticles on a dielectric substrate) is accompanied by electron emission [1][2][3][4]. Electron emission is observed as soon as the conductivity deviates from an ohmic behavior. Corresponding experiments are realized "in situ", e. g., when an ensemble of gold nanoparticles is deposited onto an insulating substrate directly in the column of a transmission electron microscope [5]. The main discussion in the literature concerning the mechanism of electron emission is connected with field emission [6] and electron gas heating [7]. In the first case, it is supposed that an electric field is strongly enhanced near nanoparticles and thus sufficient for field emission because of the small radius of curvature of nanoparticles. In the second case, the electron gas heating results from an attenuation of the electron-phonon interaction in nanoparticles. If the size is much smaller than the electron mean free path, the electrons execute a periodic motion within the particle without volume scattering and are reflected specularly from the boundaries. This oscillation has the frequency o ¼ ). The larger the frequency difference for electron and phonon subsystems, the less they exchange energy with one another. Experimental studies show that the electron-phonon interaction constant observed in ten nanometer-sized gold particles is several orders of magnitude lower than in the case of bulk material [8]. The energy fed into a nanoparticle under current excitation is absorbed by its electron subsystem. If the electron-phonon interaction is sufficiently attenuated, the electron subsystem temperature increases and electrons become heated, whereas the lattice remains relatively cold. The electron emission takes place under such conditions. Electron emission provoked by electron transport has been visualized using emission electron microscopy (EEM) [9][10][11]. Figure 1a-d shows a series of EEM images of a silver nanoparticle film (caesium was used to reduce the work function). The images have been acquired at different voltages applied between the contacts U f = 0 (a), 8 (b), 10 (c), and 11 V (d). The gap with a silver nanoparticle film between the silver contacts is 5 mm. Nanoparticle emissivity considerably differs because of the spread of the particle