Electrical Conduction of Superatom Thin Films Composed of Group-V-Metal-Encapsulating Silicon-Cage Nanoclusters
Takaho Yokoyama, Tatsuya Chiba, Naoyuki Hirata, Masahiro Shibuta, Atsushi Nakajima
Abstract
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 (MV = V, Nb, and Ta) encapsulating silicon (Si) cage nanoclusters (MV@Si16), 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 MV@Si16 assembled films revealed that the electrical conduction mechanism is not band transport but hopping transport with Efros–Shklovskii variable range hopping for all central MV 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 MV and is several times the radius of MV@Si16 (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.