Transition metal doping: a new and effective approach for remarkably high nonlinear optical response in aluminum nitride nanocages

Abstract

The exohedral doping of a first row transition metal atom (M) on Al₁₂N₁₂ nanocages was performed for assessing the geometric, thermodynamic, electronic and nonlinear optical properties of the doped materials. The transition metals were doped at different positions on the Al₁₂N₁₂ nanocages to yield M@x-Al₁₂N₁₂ (where M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and x = b₆₄, b₆₆, r₆, and r₄). The doped nanocages carried transition metal atoms located over the Al–N bond (b₆₄/b₆₆ sites) or above the six (r₆) and four membered ring (r₄). A spin-polarized DFT study revealed that the most stable spin was monotonically increased to a sextet for Mn@x-Al₁₂N₁₂, and then decreased gradually to a singlet for Zn@x-Al₁₂N₁₂. The transition metals atoms were chemisorbed on the Al₁₂N₁₂ nanocages, as revealed from the very high binding enthalpies (−16 to −64 kcal mol⁻¹). The NBO charges and bond orders were analyzed to rationalize the strength and nature of the bonding between the transition metal and nanocage. It was found that, regardless of the doping position and atomic number, the transition metal atoms could significantly lower the HOMO–LUMO gap (E_H–L), by up to 40% of that of pure Al₁₂N₁₂. By applying the long-range separated method, the first hyperpolarizability (β₀) values were calculated. The NLO response of the transition-metal-doped nanocages was comparable to those of their alkali-metal-doped analogs, which is quite remarkable because alkali metal atoms are one of the best inducers of the NLO response. The best NLO response (β₀ = 1.85 × 10⁴ a.u.) was calculated for Cu@r₆-Al₁₂N₁₂. These remarkable outcomes promote transition-metal-atom-doped Al₁₂N₁₂ nanocages as potential applicants for the design of high-performance NLO materials.