First-Principles Study On The Optoelectronic Properties Of Two-Dimensional Carbon Materials And Their Application In Gas Separation

Two-dimensional carbon materials have attracted great interest from both the academic and industrial communities due to their unique electronic and optical properties.The focus of material research has been on how to control and optimize the properties of two-dimensional carbon materials by altering their structure.Previous studies have shown that atomic doping and defects in two-dimensional carbon materials can significantly change their electronic and optical properties.Graphane,the fully hydrogenated form of graphene,has been widely studied in recent years due to its ease of synthesis and reversible,adjustable crystal structure.The optical properties of graphane have also been a focus of attention.In this paper,first-principles calculations were used to study the effects of defects and doping on the optoelectronic properties of graphane by constructing vacancies of different sizes and simultaneously doping with different elements such as oxygen and sulfur.Furthermore,helium（He）has a wide range of applications in many scientific and industrial fields.The shortage of He resources and the increasing demand for He make He separation of utmost importance.Graphane’s simple synthesis and stable crystal structure make it a potential ideal two-dimensional material for constructing gas separation membranes.This thesis will study the regulation of electronic and optical properties of graphane by vacancy defects and element doping（oxygen and sulfur atoms）,and apply porous graphane to effectively screen He gas.The specific work is as follows:（1）By constructing defects of different sizes and introducing atom doping（oxygen and sulfur atoms）in the graphene lattice,the electronic and optical properties of graphene can be tuned.The computational results of this study show that defects of different sizes and doped atoms can be stably embedded in graphene.By controlling the size of the defects,the electronic properties can be tuned within a certain range,transforming it from an insulator to a direct or indirect bandgap semiconductor.In addition,in terms of optical property regulation,these two factors also have a significant impact on graphene’s light absorption ability,leading to a decrease in the absorption coefficient and a blue shift in the absorption peak.Therefore,it can be seen that defects and element doping have a significant influence on the geometric structure,electronic performance,and optical properties of graphene.（2）Using first-principles calculations,the application of four different sized crown ether graphane-n（CG-n,n=3,4,5,6）membranes in helium（He）separation was investigated.The calculated results show that among 11 gases（He,Ne,Ar,H₂,CO,NO,NO₂,N₂,CO₂,SO₂ and CH₄）,He has the lowest energy barrier when passing through the CG-n membrane,with values of 4.55 eV,1.05 eV,0.53 eV,and0.01 eV,respectively.The lower energy barriers of the latter two can achieve He sieving,especially CG-6,which has the lowest reported energy barrier so far.The He selectivity of CG-n membranes at different temperatures was calculated,and CG-5 had a very high He selectivity（relative to the other 10 gas molecules）within the temperature range of 0-600 K.Additionally,the study found that the pore size of the membrane,the molecular diameter of the gas,and the type of gas molecule have a synergistic effect on the He separation performance of crown ether graphane-n membranes.Therefore,crown ether graphane-n membranes with suitable pore sizes（CG-5 and CG-6）are a class of potential high-selectivity and high-performance He separation membranes.（3）Using first-principles calculations,the application of sulfur ether graphane-n（SG-6）in He separation was investigated.The calculated results show that among five gases（He,Ne,Ar,N₂,and CH₄）,He has the lowest energy barrier when passing through the SG-6 membrane,with values of 0.20 eV,0.50eV,1.52 eV,1.97 eV,and 3.12 eV,respectively.He can pass through the membrane with a lower energy barrier than that of CG-5,which is also a recently reported low value.The electronic distribution on the sulfur atoms at the hole edge of sulfur-doped graphane with defects is more localized than that on the oxygen atoms at the hole edge of crown ether graphane,effectively reducing the energy barrier for gas molecules passing through the SG-6 membrane.The He selectivity of SG-6 membrane at different temperatures was calculated,and SG-6 had a very high He selectivity（relative to the other four gas molecules）within the temperature range of 0-600 K.Additionally,the study found that the pore size of the membrane,the molecular diameter of the gas,and the type of gas molecule have a synergistic effect on the He separation performance of sulfur-doped graphane-n membranes.Therefore,sulfur-doped porous graphane membranes（SG-6）are a class of potential high-selectivity and high-performance He separation membranes.This work is expected to inspire experimental researchers to develop other graphene-based two-dimensional separation membranes for better He separation.