DFT studies of single and multiple alkali metals doped C₂₄ fullerene for electronics and nonlinear optical applications

The geometric, electronic and nonlinear properties of exohedral and endohedral single and multiple alkali metal (Li, Na and K) atom doped C₂₄ fullerene are studied. First, the most stable orientations at the most stable spin state are evaluated. Complexes with odd metal atoms are stable at doublet spin state and complexes with even number of metal atoms are stable at singlet spin state. Thermodynamic analysis shows that Li₄C₂₄ among all complexes with highest thermodynamic stability has interaction energy of −190.78 kcal mol⁻¹. The energy gaps (G_H-L) are fairly reduced in single and multi-doped cages, and the lowest energy gap is observed for K₄C₂₄ complex. NBO analysis is performed to validate the charge transfer from alkali metal toward C₂₄. The largest amount of charge (0.95 |e|) transfer is monitored in exohedral K₂C₂₄ complex where the highest charge transfer is for potassium (K) metal. Total density of states (TDOS) spectra of doped complexes justify the involvement of alkali metals and nanocage in new HOMO formation for the excess electrons. First hyperpolarizability is descriptor of NLO properties of single and multi-doped complexes are calculated. It is observed that doping of alkali metal atoms (Li, Na and K) greatly enhances the first hyperpolarizability. Among all the complexes of C₂₄, Na₃C₂₄ shows the highest hyperpolarizability value of 2.74 × 10⁵ au. The results of this study are a guideline for the computational designing of highly efficient and thermodynamically stable complexes for the optical and optoelectronic technologies.