Electronic Structures,transport And Optical Properties Of Several Novel Two-dimensional Materials

Two-dimensional material is a single layer of crystalline material which is made of atoms just a few nanometers thick.Electrons in these materials can move on the nanoscale in two-dimensions,but their upward movement in the third is limited by quantum mechanical effect.It is because of this quantum confinement effect that two-dimensional materials produce many novel physical properties.Since the advent of graphene in 2004,about 700 kinds of two-dimensional material through experiment and theory have been proved stable and exist.Some of these materials have been used in photovoltaic,semiconductor,electrodes and biological monitoring applications,and will play a vital role in the future of flexible electronics,high-speed optoelectronic devices,new energy materials and other fields.Based on the first-principles calculation method,several new materials such as monolayer Bi2Se3-xTex(x=0,1,2),monolayer and bilayer of ?-and tetra-AgI,and monolayer ?-BTe,have been systematically studied in this paper.We have calculated the electronic structure,transport characteristics and optical properties of these two-dimensional materials,discussed the effect of strain-engineering on their energy band structure and effective mass,and explored their internal physical mechanism.The research results of this paper mainly include as follows,(1)In this work,we systematically investigate the stability,electronic characteristics,strainengineering and carrier mobilities of monolayer Bi2Se3-xTex(x=0,1,2)by using first-principles calculations.Our calculations show that these materials have good stabilities and indirect bandgaps.The highest valence band of these materials will shift under the uniaxial strain.Their elastic modulus is smaller than other conventional materials,which indicates better flexibility.We find that the electron mobility of monolayer Bi2SeTe2 along the armchair direction is higher than that of black phosphorene,reaching 2708 cm2V-1s-1,and the electron mobility of monolayer Bi2Se3 along the zigzag direction is about 24 times larger than the hole mobility.The remarkable electron mobilities and highly anisotropic properties of these new monolayers pave the way for future applications in high-speed(opto)electronic devices.(2)Agl,the owner of many allotropes,has been systematically studied in two lattice structures,?-AgI and tetra-AgI,which belong to the hexagonal and tetragonal systems,respectively.The electronic structures,carrier mobility and optical properties of monolayer and bilayer materials are investigated by first-principles calculations.It is found that these materials are direct bandgap semiconductors,the bandgap values calculated by HSE06 hybrid functional range from 2.98 to 3.22 eV,and there is no change of semiconductor type when the biaxial strain is-6%to 6%.Bilayer?-AgI and tetra-AgI materials have higher carrier mobility than those of monolayer,and the electron mobility of monolayer and bilayer materials is much higher than the hole mobility,which is another potential candidate for high-speed electronic devices.In addition,the absorption region of monolayer ?-AgI and tetra-AgI is mainly in the ultraviolet light region,and the utilization rate of ultraviolet light is high.Therefore,it can be used in optoelectronic devices working under ultraviolet light.The light absorption region of their bilayer materials can be extended to visible light region,which may be used in semiconductor optoelectronic devices in the future.(3)The crystal structure of monolayer y-BTe has obvious anisotropy due to the existence of B-B bond in the x direction.It is verified that this material is a quasi-direct bandgap semiconductor with excellent stability and a moderate bandgap of 1.32 eV by first-principles calculations.Meanwhile,monolayer y-BTe transforms into a direct bandgap semiconductor when the biaxial tensile strain is applied between 1%and 5%,demonstrating its unique properties in strainengineering.By investigating of the optical properties,we found that it can produce a high absorption coefficient about 104 cm-1 orders of magnitude in both visible and ultraviolet light regions,and has obvious anisotropy in x and y directions.Such suitable bandgap and significant light absorption properties of monolayer y-BTe make it a potential cand idate material for solar cells,photocatalysts and other applications.