Lithiated Graphene Quantum Dot and its Nonlinear Optical Properties Modulated by a Single Alkali Atom: A Theoretical Perspective.

The functional modification in graphene leads to novel characteristics. We study a lithiated graphene quantum dot (LiG) and adsorption of a single alkali atom (M = Li, Na, and K) on its surface using the B3LYP-D3 method. The structures of M@LiG attain the lowest energy with M adsorbed on the terminal ring of LiG. The isomers of M@LiG are stable against dissociation into M and LiG. The frontier orbital energy gap of M@LiG is significantly reduced to 0.41-0.58 eV as compared to that of LiG (2.16 eV). There is a strong charge transfer of 0.91-0.96|e| from M to LiG in all M@LiG systems, which is slightly reduced in the lowest-energy Na@LiG structure. The CAM-B3LYP results suggest a significant increase in the dipole moment and mean polarizabilities of M@LiG due to the charge transfer and smaller energy gaps, respectively. The first static hyperpolarizability (β) value of the lowest-energy M@LiG structures becomes as large as 11.5 × 10 a.u. for M = K. Our time-dependent density functional theory (TDDFT) calculations suggest that the enormously high value of β results due to lower transition energy and higher transition dipole moment for the crucial electronic transition.