Theoretical Research On Superhalogen Modulation And Spectral Properties For Tungsten Carbide Clusters Under Directional External Electric Field

Clusters are ideal polymers between atoms,molecules and macroscopic solid substances,and play an important role in interdisciplinary fields such as catalysis,energy and materials.In the development of cluster science,it has been found that the clusters with specific size and composition can simulate the physical and chemical properties of individual atoms in the periodic table,and such specific clusters are called“superatoms”.As an important component of superatoms,superhalogens have unique stability and higher electron affinity(EA)than halogen atoms,and have become a research hotspot in the field of chemical such as catalysts,superconductors,nonlinear optical materials and high energy density materials.Recent experimental studies have shown that some superatoms can be turned non-halogen into superhalogen by applying oriented external electric field(OEEF),and their optical and electrical properties can be greatly improved.Therefore,it is of great practical significance to design new superatoms and modulate them with excellent optical and electrical properties by external electric field.In this paper,two tungsten carbide cluster systems were designed based on density functional theory(DFT),focusing on exploring the ability of OEEF to construct superhalogens,and systematically studying the effects of OEEF on the geometric structure,superatomic properties and optical properties of tungsten carbide clusters.The main research contents of this paper are as follows:(1)The geometric structure and electronic properties of W₄C₄clusters without additional electric field were firstly studied.It was found that the optimized neutral and anionic configurations of W₄C₄clusters were quasi-cubic,and their EA value was 1.58 e V,far lower than the standard of halogen.On this basis,a directional external electric field with a maximum intensity of 0.040 Au along the+z direction is introduced.The results show that the EA value of the W₄C₄cluster in the ground state increases monotonically with the increase of OEEF without changing the structural stability and orbital symmetry.When OEEF intensity increased to 0.020 Au,EA value increased to 3.06 e V,and W₄C₄changed from non-halogen cluster to super-halogen cluster.In addition,the linear correlation between the EA value of W₄C₄cluster and the lowest unoccupied orbital(LUMO)explains that the enhancement of electron affinity energy of W₄C₄cluster is mainly due to the downward movement of LUMO energy level.Interestingly,we also found that the surface charge of W₄C₄clusters responded positively to OEEF and shifted in the specified direction.At the end of Chapter 3,we also explore the effect of OEEF on the spectral properties of W₄C₄clusters,and the results show that OEEF promotes the PES peak of W₄C₄clusters to move to a higher energy region,showing an obvious blue shift.At the same time,the absorption peak of infrared spectrum and the Raman peak of Raman spectrum shift and appear new strong peaks.These findings provide theoretical guidance for the experimental study of superhalogens.(2)Based on the above research,W₆C₆clusters were constructed by DFT method,and the effects of OEEF on the geometric structure,electronic properties as well as nonlinear optical response of W₆C₆were further explored.The constructed ground state W₆C₆is a quasi-cubic structure.Secondly,OEEF can significantly affect the electronic properties of W₆C₆clusters,especially its EA value,while maintaining the quasi-cubic structure stability.The quadratic polynomial is used to fit the OEEF strength and the EA value of W₆C₆clusters,and the fitting results can provide theoretical reference for the experiments.With the increase of OEEF,the isotropic average polarizability and the first hyperpolarizability of W₆C₆cluster increase significantly.Especially when F_+y=0.010 au,the first hyperpolarizability of W₆C₆cluster is about 6980 times of that without OEEF,indicating that OEEF can significantly enhance the nonlinear optical response of clusters.In addition,under the action of OEEF,some absorption peaks in the infrared spectrum of W₆C₆cluster showed obvious red shift behavior.The results further verify the ability of external electric fields to regulate the electronic and optical properties of superatoms,and provide guidance for the design of new nonlinear optical materials.