Copper-doped Al₁₂N₁₂ nano-cages: potential candidates for nonlinear optical materials

DFT calculations have been performed to study geometric, electronic and NLO properties of copper-doped Al₁₂N₁₂ nano-cages. Doping of copper significantly reduces HOMO–LUMO gap of the nano-cages. The most prominent change in E_g is observed for Cu@R₆ (copper at the center of the six-membered ring), where E_g is reduced by 52% of the original value. Total and partial densities of states have been plotted for all the structures revealing that a new HOMO has appeared between the original frontier molecular orbitals of Al₁₂N₁₂. Polarizabilities and hyperpolarizabilities show manifold increase (α = 418 au and β₀ = 1.8 × 10⁴ au for Cu@R₆) than pure Al₁₂N₁₂. TD-DFT calculations have been performed to obtain crucial excited states to account for the high hyperpolarizability values. The hyperpolarizability trend estimated from the two-level method and DFT calculations correlates nicely. The hyperpolarizability trend is justified nicely from the decreased E_g. These findings designate such doped nano-cages as excellent candidates for their potential applications in electronic devices.