Size-dependent change in interband absorption and broadening of optical plasma-resonance absorption of indium particles

Abstract

Optical absorption of indium-island films, consisting of indium particles smaller than about 500 Å in diameter, has been investigated experimentally. The interband absorption, the position of which is almost constant (at about 234 nm) irrespective of particle size, was found to disappear in particle sizes below about 200 Å in diameter. This disappearance is pointed out to be similar to that in lead particles, where the disappearance is due to the effect of surface atoms. Optical plasma-resonance absorption appeared at 276–335 nm. By simulating this absorption with a Maxwell-Garnett-type effective-medium theory, we investigated the relaxation time τ and the mean free path l of conduction electrons. τ and l for particle sizes of about 120 and 160 Å in diameter are estimated to be (3.33–3.36)×10−16s and 5.79–5.85 Å, respectively, which is smaller than the bulk values (0.38×10−14s and 66.1 Å). The small τ and l are attributed to the scattering of the conduction electrons at lattice defects internal to the particles.