Theoretical Investigations on Enhancement of Nonlinear Optical Properties of Alkali and Alkaline Earth Metal Doped X12 Y12 Nanocages

Abstract

Theoretical Investigations on Enhancement of Nonlinear Optical Properties of Alkali and Alkaline Earth Metal Doped X12Y12 Nanocages Materials with nonlinear optical response find applications in laser physics, optical fibers, optical computing, optical data handling, optical wave guides and gyroscopes, optical limiters, sensors and scanners. A number of strategies have appeared in the literature to design high performance nonlinear optical materials. A few well studied strategies are bond length alternation (BLA), synthesis of octupolar molecules, molecules with diradical character electrides, and alkalides. A relatively less explored strategy is doping of organic and inorganic systems with alkali metals. Doping with alkali metal generates diffuse excess electrons which reduce the transition energies for crucial excitations. The present research aims at exploring the potential of alkali and alkaline earth metal doping on the nonlinear optical response of X12Y12 nanocages. The objective of this study is not only to find the best nano-cage for doping but also to compare alkali metal with alkaline earth metal doping (substitutional and exohedral). Prior to this study, the literature illustrated only exohedral doping of alkali metal on Al12N12 nano-cage. In this study, Al12N12, B12N12, Al12P12 and B12P12 nanocages were studied. The stabilities of exohedral and substitutional doped nanocages are evaluated through adsorption and cohesive energies, respectively. The exohedral complexes of alkali metal are quite stable as reflected from their adsorption energies; however, certain alkaline earth metal doped systems have negligible adsorption energies. Cohesive energies of substitutional doped nanocages were less than the undoped system. Regardless of the doping mode, the HOMO-LUMO gaps are reduced significantly. The H-L gaps are reduced up to 74% of the original value. The maximum change in H-L gap is observed for Ca@PtopAl12P12 where the gap is reduced to 74.4% of the pristine AlP nanocage. The decrease in H-L gap also reduced the energies for crucial excitation which leads to significantly higher hyperpolarizabilities. The hyperpolarizabilities of the doped nanocages generally range from 1.3×102 au to 7.9×105 au which are several orders of magnitude higher than the values for pristine nanocages. Two level model is also applied to rationalize the obtained hyperpolarizabilities of certain doped systems. The trends of xi hyperpolarizabilities from two level method agree with the hyperpolarizability values from DFT calculations. 2nd hyperpolarizability values, which are real measure of practical application of a system are evaluated through βvec, are also remarkably high and in the same order as first hyperpolarizabilities. The participation of diffuse excess electrons in boosting nonlinear optical response of these doped systems is confirmed through analysis of partial densities of states. The results offer new insight into structure property relationship of inorganic fullerenes in designing new high performance nonlinear optical materials.