First-principles Study On Doping Modification Of Monolayer WSe₂

WSe2,as a representative member of the TMDs,the band gap is about 1.2 eV when the material is in the 2H bulk structure.When the material is stripped to monolayer,the weak van der Waals force between the layers disappears,and the original indirect band gap becomes the direct band gap to reach the maximum value of 1.8 eV,which overcomes the shortcomings of graphene zero band gap and BN wide band gap.Especially monolayer WSe2 has great potential application value in nanoelectronic devices.Chemical doping is considered to be one of the effective methods to adjust the crystal structure and atomic sorting to accurately adjust the performance of TMDs.Previous experiments and theories have proved that even if the doping concentration is very low,it will still affect the electron and magnetism of the original TMDs.However,for single atom doping,the doped atoms can easily form clusters driven by thermodynamics.In contrast,cluster codoping is more stable.We use the first-principles calculations to investigate the structural,electronic,magnetic and optical properties of Mn-X(X=O,S,Se or Te),Mn-X6(X=P,S,Cl,Br or Te),Co-X6(X=B,C,N or O)codoped monolayer WSe2.The conclusions are as follows:(1)The pure and Nn-X(X=O,S,Se or Te)codoped monolayer WSe2 systems are quantitatively examined through first-principles calculations.The lower formation energy under Se-rich condition shows that the codoped systems under Se-rich condition is more stable than W-rich condition,especially the Mn-O codoping is the most stable with the minimum formation energy-2.080 eV.Although the pure monolayer WSe2 is a nonmagnetic semiconductor,the codoped systems are all magnetic with total magnetic moments of 1 ?B due to the introduced one unpaired electron.It is worth mentioning that the Mn-Se and Mn-Te codoping make the monolayer WSe2 display magnetic half-metallic properties.Finally,the optical properties of the pure and codoped monolayer WSe2 are studied in x(parallel to the WSe2 plane)and z(perpendicular to the WSe2 plane)directions.An interesting red-shift phenomenon has been observed in the curves of imaginary part of the dielectric function and absorption spectra for the Mn-X codoping,suggesting their quite promising applications in optoelectronic devices.(2)We study the effects of Mn-X6(X=P,S,Cl,Br or Te)codoping monolayer WSe2.The smaller formation energy shows that all doping systems are more stable under W-rich condition than Se-rich condition.Mn-S6 codoped monolayer WSe2 has the smallest formation energy,which may be due to S and Se elements having similar electronegativity,while the maximum formation energy of Mn-Te6 codoping may be due to the lowest electronegativity of the Te element.Although pure monolayer WSe2 is a non-magnetic semiconductor,the Mn-P6 and Mn-Te6 codoped monolayer WSe2 become magnetic semiconductor,while the Mn-S6,Mn-Cl6 and Mn-Br6 codoped monolayer WSe2 turn into magnetic half-metal.With the decrease of spin-orbit band gap,the response of Mn-X6(X=P,S,Cl,Br or Te)codoped monolayer WSe2 to light is enhanced.In the infrared region,the spectrum of codoped WSe2 systems appears red-shift phenomenon,and the response to infrared light is significantly greater than that of pure WSe2,so they can be used for infrared light detectors.In the ultraviolet region,the codoped structures have higher absorption rate,so they can be used as absorbers for strong ultraviolet radiation.(3)We systematically investigate the Co doping and Co-X6(X=B,C,N or O)codoping monolayer WSe2.The doped structure is easier to form under Se-rich condition than W-rich condition,which can be seen from the smaller formation energy.The formation energy of Co-N6 codoping under Se-rich condition and Co-O6 codoping are negative,indicating that the doping process is exothermic and the structure after doping is stable.It is well known that the monolayer WSe2 is a non-magnetic semiconductor,while the structure after Co doping or Co-X6 codoping is magnetic.In particular,the monolayer WSe2 codoped with Co-C6,Co-N6 and Co-O6 becomes half-metal,which can be used for spintronics devices.In order to study the optical properties,we also calculated the dielectric function ?(?),refractive index n(?)and absorption coefficient ?(?)The results show that in addition to Co-O6 codoping,the absorption spectra of other doped structures appear red-shift phenomenon,and the absorption coefficient is obviously higher than that of pure monolayer WSe2,so they are potential materials for solar cells and photovoltaic applications.