| Two-dimensional(2D)materials attracted more and more attentions of scientists owing to their unique physical and chemical properties.Although graphene was widely used in the fields of electronics and new energy sources,the zero band gap seriously hinders its application in optoelectronic devices.In order to remedy this defect,many research groups began to develop and study two-dimensional semiconductor materials.Semiconductor materials with layered structure can easily produce stable two-dimensional semiconductor nanosheets,and the van der Waals interaction between layers has important influences on the properties of these nanostructures.Therefore,considerable research interests are also attracted to the 2D structures of these layered semiconductors,such as Phosphorene,hexagonal boron nitride and transition metal dichaldogenide compounds so on.Today,these 2D semiconductor materials become the research hotspot of the field of semiconductor.Bismuth tri-iodide(Bi I3)and lead iodide(Pb I2)crystals,as a member of the layered semiconductor family,are mainly used in radiation detectors.In recent years,the properties of their two-dimensional nanosheets have been widely studied by many theoretical calculations and experimental measurements.On the other hand,the bilayer nanosheets are the first system involving van der Waals interaction.Therefore,based on the density functional theory,this paper mainly investigates the optoelectronic properties of the 2D nanosheets of Bi I3 and Pb I2,and also studies the layer-and strain-dependent optoelectronic properties of Pb I2.The main conclusions drawn are as follows:1.Influences of pressure on electronic structure and optical properties of Bi I3When the hydrostatic pressure is applied to Bi I3 crystal,it leads to the decrease of the c/a ratio because the interlayer interaction in the van der Waals crystal is weaker than the ionic bonds and covalent bonds in the intralayer.The space change between atoms resulting from the change of the lattice parameters leads to the decrease of the band gap value with the increase of the pressure.The effects of pressure on the optical properties of Bi I3 show that the static dielectric constant increases with the increase of the pressure,and that the increase of the pressure makes the threshold of the imaginary part of dielectric function and the absorption spectrum red shift.In the considered pressure range,the optical properties of Bi I3 are anisotropic,and the real part of dielectric function increases with the increase of pressure when photon energy is less than 2 e V,beyond that value the change is not obvious.Moreover,our calculations also show that there is a ~105/cm absorption coefficient and a non-zero photoconductivity in the visible region.All these results indicate that Bi I3 has great potential in the application of optoelectronic devices.2.Effects of stacking patterns and interlayer coupling on the electronic structure and optical properties of bilayer Bi I3The stability of bilayer Bi I3 with different stacking patterns is studied from the view of total energy.The most stable stacking pattern is consistent with the stacking order of the bulk crystals,and the order of the balance interlayer distance of different stacking patterns using the ascending numerical sort is consistent with the order of total energy.The effects of stacking patterns on structural properties can be qualitatively explained by the Lennard-Jones potential which is followed by van der Waals interaction.Stacking patterns and interlayer coupling can effectively tune the band gap value.With the decrease of interlayer spacing,seven stacking patterns take place the energy band structural transition from indirect band gap to direct band gap,which can be attributed to the change of interaction between the bottom iodine atoms in the upper monolayer Bi I3 and the top ones in the lower monolayer Bi I3 within the bilayer Bi I3.In addition,the interlayer spacing obviously affects the optical properties of bilayer Bi I3,and the stacking pattern only has a significant effect on the absorption threshold and static dielectric constants.All these discoveries will further stir up theoretical and applied research on two-dimensional Bi I3.3.Electronic structure and optical properties of bilayer Pb I2The studies on the phonon spectra and the total energy of bilayer Pb I2 show that there are 6 stable stacking patterns for bilayer Pb I2,indicating that they can be experimentally synthesized.The most stable stacking pattern is the bulk-like order.The total energy of bilayer Pb I2 calculated by the hybrid functional is smaller than that calculated by generalized gradient approximation functional.The calculation of the electron energy band structure shows that there are two stacked patterns whose characteristics are direct band gap structure with respective band gap value of 2.59 e V and 2.60 e V.The direct band gap is beneficial to the application relative to effective light absorption.In addition,it is found that the interlayer distances can effectively tune the band gap value and the band gap structure of bilayer Pb I2,which can be attributed to the interlayer coupling enhancement resulted from the reduction of the interlayer spacing by the further detailed studies.Finally,from the research on the optical properties of the most stable stacking pattern of bilayer Pb I2,it was found that its optical property is very sensitive to the interlayer distances.With the interlayer distances being increased,the static dielectric constant decreases and the optical band gap takes place blue-shift.In addition,the calculated optical spectra including excitonic effects are dominated by the exciton states,and the calculated exciton binding energy and optical band gap value are 0.81 e V and 2.73 e V,respectively.4.Excitonic effects on layer-and strain-dependent optoelectronic properties of Pb I2The G0W0+ BSE method was used to investigate the excitonic effects on the layer-and straindependent optoelectronic properties of Pb I2 and the influence of the layer number and strain on the exciton effect is also discussed.The results show that with the increase of the number of layers,the absorbance and the absorption coefficient increase,and the calculated exciton binding energy decreases.The effect of excitonic effects on the two-dimensional Pb I2 is greater than that on Pb I2 crystal.With the increase of strains,all optical spectra take place red shift,especially in the region where the photon energy is beyond the fundamental band gap.Furthermore,the correlation between the electronic structures and optical properties is also analyzed.This work will further highlight the necessity and importance of the research on potential applications of 2D Pb I2 in optoelectronic devices. |
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