First-Principles Study On Hydrogenated And Lithiated Two-Dimensional Germanene Functional Materials

Since graphene was successfully prepared and won the Nobel Prize in physics,two-dimensional (2D) graphene-like materials,such as silicene and germanene,have intrigued great enthusiasm.Unlike the planar 2D structure of graphene,the atoms on the two sublattices of germanene have a displacement perpendicular to the plane of the germanene surface,thus forming a low buckling honeycomb structure.Although C,Si and Ge are group-?A element in the periodic table of elements,germanene has a stronger spin-orbit coupling (SOC) effect than that in graphene and silicene,because Ge atoms have the atomic weight larger of than that of C atom and Si atom.Germanene have been reported to possess excellent physical and electronic properties,and has an broad application prospect in high-precision sensors,optoelectronic devices and other fields.In order to further uncover the electronic features of 2D germanium functional material,as well as its application in the field of nano electronics,the structure and electronic properties of 2D hydrogenated and lithiated germanene(Ge₂H₂ and Ge₂Li₂)under vertical electric field are investigated by using the first-principles calculations.The influences of electric field on stability,micro geometry,band structure and charge transfer of hydrogenated and lithiated germanene are analyzed.In addition,the adsorption properties of gas molecules(CO₂,CH₄,H₂S,H₂ and NH₃)on the surface of the 2D Ge₂Li₂ are studied.The changes of the microstructure and electronic properties of the Ge₂Li₂ 2D materials adsorbed different gas molecules are analyzed.The main contents are as follows:Using the first-principles calculations within the framework of density theory,we have studied the modulation effect of electric field on the structure and electronic properties of Ge₂H₂ and Ge₂Li₂ nanosheets with and without SOC correction calculation.Considering SOC effect,our results indicate that both hydrogenation and lithation germanene are stable under electric field.In addition,there is little variation of bond length and bulking height in Ge₂H₂ and Ge₂Li₂ nanosheets under electric field.It is also found that the the existence of SOC leads to the band gap decrease of Ge₂H₂ and Ge₂Li₂,but it does not change the dominant effect of electric field.For the two structures of Ge₂Li₂ and Ge₂H₂,the band structure changes from semiconductor to metal after the vertical electric field is applied,which is due to the electrostatic potential difference caused by Stark effect,and then resulting in the movement of the energy level from conduction band to the Fermi level.We also find that the charge distribution is mainly concentrated on the hydrogen atom under electric field,while the charge density between the two Ge atoms is weakened.In contrast,it can be validated that the transferred electrons are localized around the Ge atoms in Ge₂Li₂ under electric field.The results reveal the microstructure and electronic properties of two new functional materials,Ge₂Li₂ and Ge₂H₂,under the action of electric field,which provide a foundation for the further research on the electric field effect of the two kinds of materials and the fabrication of electronic devices based on the field effect.We have studied the adsorption of gas molecules(CO₂,CH₄,H₂ S,H₂ and NH₃)on 2D Ge₂Li₂ monolayer by using the first-principles methods.The adsorption geometries,adsorption energies,charge transfer,and band structures of above mentioned gas molecules adsorption on Ge₂Li₂ monolayer are analyzed.It is found that the adsorption of CO₂ on Ge₂Li₂ monolayer is a strong chemisorption,while other gas molecules such as CH₄,H₂ S,H₂ and NH₃ are physisorption.We have found that Li atom has obvious charge transfer to Ge atom and gas molecule.The strong covalent binding is formed between the CO₂ molecule and the nearest Ge atom in Ge₂Li₂ monolayer.This adsorption of CO₂ molecule on Ge₂Li₂ monolayer leads to a direct energy gap of 0.304?eV.Other gas molecules exhibit mainly ionic binding to the nearest Li atoms in Ge₂Li₂ monolayer,which leads to these systems present indirect energy gap.Furthermore,the work function of monolayer Ge₂Li₂ varies with the type of adsorbed gas molecules,indicating that the work function of monolayer Ge₂Li₂ can be effectively adjusted by the adsorption of gas molecules.The above studies show that the electronic properties of monolayer Ge₂Li₂ are sensitive to the adsorbed gas molecules,which means that it can be used as a potential application material in gas detection device.