“Transparent” metals: preparation and characterization of light-transmitting palladium, rhodium, and rhenium films

• Published in: Journal of electroanalytical chemistry and interfacial electrochemistry 1987-08, Vol.228 (1), p.167-178
• Main Authors: Degani, Y., Sheng, T.T., Heller, A., Aspnes, D.E., Studna, A.A., Porter, J.D.
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Metallic coatings; Metals. Metallurgy; Production techniques; Surface treatment
• ISSN: 0022-0728
• DOI: 10.1016/0022-0728(87)80105-5

We show that the principles that govern the preparation and properties of “transparent” platinum films apply also to films of palladium, rhodium and rhenium. Films of these metals, of 42–60 nm thickness, having a metal volume fraction between 0.3 and 0.5, transmit more light than equivalent, dense metal films of identical metal loadings per unit area. The films are prepared by photoelectrodeposition onto p-InP (100) photocathodes, from ∼5×10 −5 M solutions of the metal ions in 1 M HClO 4, under mass-transport limited conditions, at deposition rates of ∼ 2 nm/h. The “transparent” rhodium, rhenium and palladium films exhibit their normal catalytic behavior and have normal crystal structures. While the “transparent” rhodium and palladium films, like the bulk metals, are stable in air, the rhenium films oxidize over a period of days to form films that are either truly amorphous or consist of crystallites of less than 1 nm diameter. The spectroellipsometrically measured dielectric functions of these films in air are analyzed, in the framework of the Bruggeman effective medium approximation, to yield film thicknesses, metal volume fractions, and mean depolarization factors. The respective depolarization factors of Rh, Pd, and Re indicate dendritic, particulate, and platelet microstructures, consistent with the structures observed by transmission electron-microscopy. With the spectroellipsometrically derived, substantially divergent, depolarization factors for the different microstructures, one obtains consistent relationship among film thicknesses, metal volume fractions, and the optical properties. Absorption in tenuous films, of constant metal loading but different microstructures, tends to different finite values strongly dependent on microstructure, in the limit of infinite metal dilution.