Electrochemical loading of hydrogen in palladium capped samarium thin film: structural, electrical, and optical properties

A 50 nm samarium film capped with a 7 nm palladium overlayer switched from a metallic to semiconducting state during ex-situ hydrogen loading via electrochemical means at room temperature. The transition is accompanied by a change in transmittance measured during hydrogen loading and the associated optical appearance. The monitoring of working electrode (WE) potential, the transmittance and chi potential difference (Δχ) has been used to identify the phases present during hydrogen loading. Deloading of hydrogen has been studied in open circuit potential condition. Glancing angle X-ray diffraction (GAXRD) studies show that the rhombohedral structure of metallic samarium film (a₀=8.989 Å) changes to hexagonal structure of the SmH_3−δ film with average lattice parameters of a=3.775 Å and c=6.743 Å. A direct optical band gap of 2.9 eV has been obtained for SmH_3−δ film and 2.0 eV for SmH_{2 ± ε} film from reflectance and transmittance data. Removal of hydrogen from SmH_3−δ leads to the formation of localized states within the band whose signature is clearly seen in transmittance and Tauc’s plot curves of SmH_{2 ± ε} film. The Hall coefficient R_H measured as a function of hydrogen concentration, changes from a metal-like value −14.23×10^-10 m³/C to −1001.1×10⁻¹⁰ m³/C for SmH_3−δ films. On unloading hydrogen, the value of R_H changes to −3.56×10⁻¹⁰ m³/C at the dihydride composition.