A reversible metal-to-semiconductor transition was attained in samarium thin films covered with several thicknesses of palladium (Pd) coating during in-situ hydrogen loading by applying normal pressure of hydrogen gas at ambient temperature. The electrical change is followed by extreme variations in optical properties in the visible range, where the metallic gray color of samarium thin films switched to golden greenish tinge transparent films upon hydrogen absorption. The thickness of Pd overlayer shows a decisive role in this switching phenomena and we have studied the effect of Pd overlayer thickness using electrical, optical and scanning electron microscopy (SEM) measurements. Electrical measurements reveal that Pd overlayer on samarium films is discontinuous only for thicknesses of less than 3 nm. SEM studies indicate significant changes in surface morphology of Pd overlayer as a function of its thicknesses. The micrographs reveal a gradual variation in the average size of the spherical crystallites with increasing thickness of Pd overlayer. Cracking of Pd grains has been observed on hydrogenation if the underlying samarium film thickness is >100 nm. Results of electrical and optical changes upon hydrogenation/dehydrogenation are presented.

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