Enhanced 1520 nm Photoluminescence from Er3+ Ions in Di-erbium-carbide Metallofullerenes (Er2C2)@C82 (Isomers I, II, and III) (Enhanced 1520 nm photoluminescence from Er3+ Ions in di-erbium-carbide metallofullerenes (Er2C2)@C82 (Isomers I, II, and III))