| In 2004,British physicists Geim and Novoselov successfully prepared graphene by mechanical exfoliating.This kind of two-dimensional nano material only composed of carbon atoms has been widely concerned because of its excellent properties.Subsequently,other two-dimensional materials（2D）,such as boron nitride（BN）,transition metal chalcogenides（TMDs）,two-dimensional metal oxides,phosphorene,germanium and so on,have been synthesized in succession.In addition,the electronic and optoelectronic properties of two-dimensional materials are enriched by means of heterostructure,surface modification and doping.Based on the advantages of two-dimensional materials,such as good quantum effect,mechanical strength and transparency,large specific surface area,many active sites,easy to modify and so on.It has great potential in the application of photocatalysis,supercapacitor,sensor and other fields,and it is worth our in-depth discussion.The group-Ⅲ chalcogenide monolayers（MX: M = Ga,In,X = S Se Te）is a new type of layered material with honeycomb structure,which has attractive physical and chemical properties.The band gap of these two-dimensional materials is between 2.30-3.40 e V,and the band gap begins to drop to the range of 0.97-3.05 e V with the increase of the number of layers.In addition,it also shows very high carrier mobility.For example,for bulk materials of Ga S and Ga Se,the carrier mobility is 80 cm2V-1s-1,215 cm2V-1s-1,respectively.while for single-layer In Se,it is 12700 cm2V-1s-1,which makes them have great application potential in FET.Based on the first-principles calculation,the structure and electronic properties of the group-Ⅲ chalcogenide monolayers have been systematically studied.The results show that they have either direct band gap or almost degenerate direct and indirect band gap.And their band gap can be controlled by stress or external electric field to realize the transition from semiconductor to metal.These monolayers are also predicted to be able to become ferromagnetic and semimetallic by doping.In this paper,the electronic properties,transport properties and electron energy loss spectra of the group-Ⅲ chalcogenide monolayers after gas adsorption and heterojunction construction are studied.The feasibility of the group-Ⅲ chalcogenide monolayers as gas sensor and its wide application prospect in multi-functional devices and nano photon devices.The details are as follows:Based on the density functional theory,the electronic and transport properties of Janus group-Ⅲ chalcogenide monolayers（Ga2SSe,In2SSe）were investigated for detecting CO2 and NO2 using first-principles calculations.Several parameters that affect the performance of gas sensors,such as adsorption distances,adsorption energies,charge transfers,density of states（DOS）which affect the performance of gas sensors,a detailed comparison of Janus group-Ⅲ chalcogenide monolayers has been provided with their pristine systems.It was observed that,for gas molecules with different polarities（CO2 and NO2）,Janus group-Ⅲ chalcogenide monolayers（Ga2SSe,In2SSe）have high selectivity.It was mainly ascribed to the built-in electric field caused by the out-of-plane asymmetric structure of Janus monolayer which enhances the dipole-dipole interaction between the polar gas molecule and the 2D materials.And the variation of transmission spectrums of the Janus group-Ⅲ chalcogenide monolayer before and after adsorbing molecules further proves the feasibility of this kind of material as a high-sensitivity gas sensor.The group-Ⅲ chalcogenide monolayers can serve as a suitable substrate for silicene,and the Dirac electron band properties of silicene are also fully preserved.And,the maximum open band gap can reach 179 me V at the Dirac point due to the interaction of silicene and the polar two-dimensional（2D）substrate.In addition,the electronic band structure of the heterojunction can be modulated by applying electric field and biaxial strain.As a consequence,its band gap will increase or decrease according on the direction of an external electric field.When biaxial stress is applied,its band gap will gradually change from semiconductor to metal.These results show that heterojunctions with polar 2D substrate have broad application prospects in multi-functional devices.Their electron structures can further understand by analyzing the electron energy-loss spectra（EELs）.This work can provide a good substrate for growing silicene and the modulation of electronic structure can also be applied to nanodevices and optoelectronic devices. |
