Charge Transfer Complexation of Ta-Encapsulating Ta@Si<sub>16</sub> Superatom with C<sub>60</sub>

The tantalum-encapsulating Si<sub>16</sub> cage nanocluster superatom (Ta@Si<sub>16</sub>) has been a promising candidate for a building block of nanocluster-based functional materials. Its chemical states of Ta@Si<sub>16</sub> deposited on an electron acceptable C<sub>60</sub> fullerene film were evaluated by X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS, respectively). XPS results for Si, Ta, and C showed that Ta@Si<sub>16</sub> combines with a single C<sub>60</sub> molecule to form the superatomic charge transfer (CT) complex, (Ta@Si<sub>16</sub>)<sup>+</sup>C<sub>60</sub><sup>–</sup>. The high thermal and chemical robustness of the superatomic CT complex has been revealed by the XPS and UPS measurements conducted before and after heat treatment and oxygen exposure. Even when heated to 720 K or subjected to ambient oxygen, Ta@Si<sub>16</sub> retained its original framework, forming oxides of Ta@Si<sub>16</sub> superatom.