Electromagnetic Properties Of Bilayer Graphene Adsorbing Gas Molecules Using External Electric Field

Graphene has excellent electrical characteristics such as high thermal conductivity,electron mobility and low electronic noise,as well as unique optical properties,high mechanical strength,and relatively stable chemical properties.In particular,the large specific surface area of graphene makes it highly sensitive to adsorption of gas molecules,so it has a wide application prospect in gas-sensitive devices.However,the physical and chemical properties of intrinsic graphene are relatively stable,and the adsorption of gas molecules is weak.Doping modification and external electric field,can significantly improve the adsorption and detection effect of graphene on gas molecules.In this study,the adsorption properties of gas molecules onto bilayer graphene(BG)have been investigated by using the first-principles calculation method based on density functional theory.The electronic structures,adsorption mechanism and gas sensitivity characteristics of gas molecules onto BG are modulated by doping transition metal(TM)atoms and electric field,then the main work and conclusions are summarized as follows:(1)For AA-stacked bilayer graphene doped Pd,Mo atoms(Pd/BG,Mo/BG),the doping of metal atom changes the electronic properties of BG,and the impurity metal atom introduce the deformation of local geometry structure.The changed outer surface of BG facilitates this region to react with approaching CO and NO molecules because of the higher chemical reactivity of doped metal atom.The most stable geometry structure of CO molecule is different from that of the NO molecule adsorption on Pd/BG.The CO molecule is approximately parallel to the graphene surface,but the NO molecules are almost perpendicular to the graphene surface.For the most stable structure,the adsorption of NO molecule is more stable than CO molecule on Pd/BG,and the adsorption energy indicate the physical adsorption property for CO but chemical adsorption property for NO.The molecules of CO and NO are approximately perpendicular to the plane of Mo/BG graphene to form the most stable configuration,both CO and NO are chemical adsorption by Mo/BG.The CO adsorbed Pd/BG system has different electronic structure from that of the NO adsorbed Pd/BG system.After the adsorbing of CO molecule,the Pd/BG system changes from semi-conductive to metallic property,and the Pd/BG-CO system is nonmagnetic,while the Pd/BG-NO system has half-metallic property with larger magnetic moment.This changing in electronic properties can be use to clarify the sensitivity of gas molecule adsorption on Pd/BG system.Our researching results can provide theoretical basis and experimental guidance for the graphene-based gas sensors or detectors.(2)The adsorption of H2S onto TM-doped(TM=V,Cr,Mn,Fe,Co,Ni)BG was significantly improved compared with adsorption onto pristine BG.An external electric field can promote the adsorption of H2S onto BG,and the stability of this adsorption increases with the intensity of the electric field.Typically,this stability is independent of the direction of the electric field(upward or downward).However,H2S is sensitive to the direction of the electric field,and a downward(negative value)electric field is more favorable for H2S adsorption onto TM-doped BG.The desorption of H2S molecules from TM-doped BG was achieved using an upward(positive value)electric field,and the maximum desorption effect occurred at an electric field intensity of approximately 0.2V/A.The adsorption of gas molecules was also observed to modify the electronic structures of the TM/BG systems.All the TM/BG-H2S systems demonstrated semiconductive characteristics with different band gaps,except for the V/BG-H2S system.The results of this study showed that TM-doped BG has potential applications to methods for the adsorption,storage,and detection of H2S gas.(3)The H2 molecules adsorption onto AA-stacked BG was investigated using external electric field.For the H2 adsorption onto the 4×4×1 intrinsic bilayer graphene,the six hydrogen molecules is the maximum number,showing physical adsorption.This result indicates that the adsorption capacity of bilayer graphene is limited for hydrogen molecules,which is not conducive to hydrogen storage.The more hydrogen molecules adsorbed,the greater the effect on the electronic structure of bilayer graphene.The external electric field can effectively regulate the average adsorption energy of BG adsorbed hydrogen molecules,making it reach the ideal average adsorption energy range(-0.2 eV?-0.7 eV),which is convenient for storage.Under the electric field,the bilayer graphene can adsorb seven hydrogen molecules,indicating that the electric field can improve the hydrogen storage capacity of graphene.The results show that the external electric field makes it possible to use graphene as a hydrogen storage material.