Electron and light emission from island metal films and generation of hot electrons in nanoparticles

We review experimental and theoretical works devoted to electron and photon emission from island metal films (IMFs) representing ensembles of small metal particles deposited onto a dielectric substrate and coupled via penetrable potential barriers. Electrons and photons are emitted when the films are energized by passage of current through them or by laser irradiation. In either case the primary recipient of the energy is the electron gas, which can be heated up to temperatures much higher than the particle lattice temperature. A theoretical substantiation of the model of hot electrons in nanoparticles is presented. The major physical factor that permits generation of hot electrons in IMFs is the dramatic reduction (by orders of magnitude) of the electron–lattice energy transfer in the particles whose size is smaller than the mean free path of electrons in the volume. In such particles with a ballistic motion of electrons, the energy is being lost mainly in surface scattering acts which are less effective in energy transfer than generation of volume phonons. Thus, the electron temperature can become substantially higher than the lattice temperature provided the absorbed power density is high enough and the lattice of the island is intensively cooled by the substrate. The model of hot electrons is used to interpret experimental data. Non-equilibrium electron heating in IMFs can be observed even under stationary conditions, so the island metal films basically differ in their electronic properties from continuous metal films and bulk metals where hot electrons can be obtained only for very short times (≤10−11 s). Thus, the island metal films represent an important variety of nanomaterials having rather unusual physical properties. IMFs can be utilized to fabricate cathodes having interesting application potentialities in vacuum microelectronics, information display technologies and infrared image conversion. Hot electrons generated in nanoparticles may also play a significant role in various dispersed systems exposed to energy fluxes.