Electrical Conduction of Superatom Thin Films Composed of Group-V-Metal-Encapsulating Silicon-Cage Nanoclusters

Nanocluster assembled films have attracted great interest for designing nanostructured materials with unique electromagnetic properties through a bottom-up approach. Superatoms of group-5 metals (M V = V, Nb, and Ta) encapsulating silicon (Si) cage nanoclusters (M V @Si 16 ), which are synthesized by high-power impulse magnetron sputtering technique, can be efficiently generated to form assembled films. Temperature-dependent current–voltage ( I – V ) characteristics of the M V @Si 16 assembled films revealed that the electrical conduction mechanism is not band transport but hopping transport with Efros–Shklovskii variable range hopping for all central M V atoms. The results show that electrons involved in conduction are strongly correlated to localized electronic states; this correlation arises because of not only the geometrical disordering in noncrystalline assembled films but also the electronic nature of a superatomic 1H orbital with multiple nodes. The localization length depends on the specific M V and is several times the radius of M V @Si 16 (0.45 nm); it is the largest for Ta (2.2 nm) and the smallest for Nb (0.8 nm), revealing a periodicity of superatoms.