Optical study of continuous thin films and island films of Ir: Interband absorption and localization of conduction electrons of Ir

Abstract

For continuous thin Ir films and Ir island films consisting of Ir particles larger than about 25 Å in diameter, optical absorption has been measured in the photon energy range of 0.5–6.5 eV. Interband absorption does not exist at about 1.1 eV. Absorption at about 3.1 eV is identified as interband absorption because its particle-size dependent change is similar to that of interband absorption of Cu particles. Interband absorption at about 4.4 eV is suggested to arise from transitions between bands 2 and 5 near L in the direction Q. Absorption due to plasma oscillations of conduction electrons is absent in the island films, showing that the conduction electrons are localized by a strong correlation interaction between the conduction electrons having strong d character. Strong d character based on large hybridization of s and d bands is responsible for the strong correlation interaction.

Optical absorption of transition-metal island films: Correlation interaction between conduction electrons of transition metals

“…[6][7][8][9] In a vacuum chamber, electron-microscopic meshes covered with a carbon film and a fused-quartz substrate (18ϫ18 ϫ0.5 mm 3 ) were placed above an evaporation source. The meshes and substrate were adjacent, and the distance from the evaporation source was the same ͑30.3 cm͒ for the meshes and substrate.…”

“…Because such absorption does not occur in metal island films, the optical study of the metal island films gives reliable data on the existence of the interband absorption in the low energy range. 9 A study 10 of optical properties of bulk Pt has reported the existence of interband absorption at about 0.7 eV. The interband absorption is not found in Figs.…”

“…In the previous study of a correlation interaction for Ir island films, 9 it was shown that d character of conduction electrons of Ir is strong because of the large s -d hybridization, and that a strong correlation interaction due to the strong d character leads to the conduction-electron localization.…”

“…This ratio is pointed out to increase with lowering n and/or with strengthening d character. For Pt, Pd, Rh, and Ir, the order of the magnitude ratio is discussed based on the correlation interaction, reflected by the broadening ͑i.e., by the half width͒ for the Pt, Pd, 6 and Rh 8 island films, and on the strong correlation interaction in Ir, 9 which results in the conductionelectron localization.…”

Metal Nanoclusters: Electronic Aspects and Physico-Chemical Characterization

“…It was suggested that since the localized valence d-electrons are much more coupled to the core levels by the Coulomb interaction than the delocalized s-, p-electrons, a decrease in the d-character of the valence band should lead to increasing core level binding energies and vice versa. Unfortunately, theoretical calculations, which could verify the size dependent change in the s-d hybridization, are still missing for nanoparticles with several hundreds or several thousands of atoms, although certain optical measurements seem to confirm the occurrence of enhanced s-d hybridization in clusters with sizes in the few nm range [93,94]. Indeed, the characteristic changes of the d-bands of transition and noble metal nanoparticles (see Section 2.1) may indicate the increase in s-d hybridization.…”