Superalkali-doped Excess Electron Donor-acceptor (ee D-A) Systems:Electronic Structures And Nonlinear Optical Properties

Nonlinear optics(NLO)developed rapidly after the invention of laser and has become an independent branch in the field of optics.Nonlinear optics is widely used in optical communication,optical computing,optical information processing,laser processing,laser medical treatment,laser printing,laser guidance and directed energy weapons,etc,at present,there are some noticeable nonlinear optical fields:such as ultrafast spectroscopy for dynamic studies,high-field laser physics and strong nonlinear optical effects,etc.The application of nonlinear optics depends on the high-performance nonlinear optical materials,in order to change the traditional inefficient"stir-fry"experimental exploration and speed up the development of new materials,material design based on first principles has been widely used in the exploration of new materials.Based on density functional theory(DFT),the electronic structure and nonlinear optical properties of the excess electron donor and acceptor(ee D-A)system consisting of superalkali M₃O(M=Li,Na and K)and the excess electron clusters Si_nAl_12-nN₁₂(n=1,2)have been studied.By replacing Al atom with Si atom in Al_nN_n system,the excess electron Si_mAl_n-mN_nstructure is more stable than that of the corresponding Al_nN_n.The compound M₃O@Si_nAl_12-nN₁₂ was designed by using the superalkali M₃O(M=Li,Na and K)and the nanocage Si_nAl_12-nN₁₂(n=1,2).Although M₃O and Si_nAl_12-nN₁₂ are both excess electron systems,they can bond stably with an interaction energy of about 5 e V,and the stability increases with the increase of the electron donating ability of the superalkali.The electronic structure shows that the superalkali M₃O transfers electrons to the Si_nAl_12-nN₁₂nanocage to form the ee D-A structure.When M₃O is bound on the top of Al atom,lone pair electrons are distributed outside Si atom;when M₃O is bound on the top of Si atom,the lone pair electrons are distributed outside Al atom.When M₃O is combined with Si atom,the structure is more stable,and the binding of Al atom to the lone pair electron is looser,which further pushes up the HOMO energy level and leads to a narrower energy gap.Weakly bound lone pair electrons and small energy gap make it possess excellent nonlinear optical properties,the first hyperpolarizability(?₀)of K₃O@Si₂Al₁₀N₁₂ reaches7.12×10⁵ au.We also calculate the frequency hyperpolarizability and Hyper-Rayleigh Scattering(HRS),and according to the two-level model and the hyperpolarizability density discuss the conditions for forming large?₀ value.We hope that our study can provide guidance for enhancing the NLO response of this unconventional ee D-A system.