Germanium Fluoride Nanocages as Optically Transparent n-Type Materials and Their Endohedral Metallofullerene Derivatives

Carbon- and silicon-based n-type materials tend to suffer from instability of the corresponding radical anions. With DFT calculations, we explore a promising route to overcome such challenges with molecular nanocages which utilize the heavier element Ge. The addition of fluorine substituents creates large electron affinities in the range 2.5-5.5 eV and HOMO-LUMO gaps between 1.6 and 3.2 eV. The LUMOs envelop the surfaces of these structures, suggesting extensive delocalization of injected electrons, analogous to fullerene acceptors. Moreover, these Ge _n F _n inorganic cages are found to be transparent in the UV-visible region as probed with their excited states. Their capacitance, linear polarizabilities, and dielectric constants are computed and found to be on the same order of magnitude as saturated oligomers and some extended π-organics (azobenzenes). Furthermore, we explore fullerene-type endohedral isomers, i.e., cages with internal substituents or guest atoms, and find them to be more stable than the parent exohedral isomers by up to -206.45 kcal mol ^-1 . We also consider the addition of Li, He, Cs, and Bi, to probe the utility of the exo/endo cages as host-guest systems. The endohedral He/Li@F ₈ @Ge ₆₀ F ₅₂ cages are significantly more stable than their parent exohedral isomers He/Li@Ge ₆₀ F ₅₂ by -182.46 and -49.22 kcal mol ^-1 , respectively. The energy of formation of endohedral He@F ₈ @Ge ₆₀ F ₅₂ is exothermic by -10.4 kcal mol ^-1 , while Cs and Bi guests are too large to be accommodated but are stable in the exohedral parent cages. Conceivable applications of these materials include n-type semiconductors and transparent electrodes, with potential for novel energy storage modalities.