Theoretical Studies Of The Electronic Properties Of Graphene And Doped Molybdenum Disulfide

In this thesis,we studied the electronic properties of the graphene superlattices and the doped molybdenum disulfide in detail by using the density functional theory calcualtions.The graphene superlattices can be differed by whether the Dirac points are folded to ? point or not according to the band folding analyses.In previous studies,the inversion symmetry preserved defects open bandgap in the former superlattices while they cannot change the semimetal properties in the latter ones.In this thesis,we studied the electronic properties of the latter superlattices carefully by using the density functional theory with the generalized gradient approximation,in which the defects would induce the ?-band splitting to get the ?_a1-?_a2 and ?_z1-?_z2 band sets.Detailed analyses show that these splits can be explained by the bonding symmetry breaking mechanism.In addition,the two bands group would move toward each other under uniaxial strain.The overlap between them would result in a bandgap opening in the the Brilliouin zone,turning the the semimetal into semiconductor.Furthermore the gap width could be continuously enlarged by enhancing the strain until reaching the maximum value determined by the defect density.These studies contribute to the bandstructure engineering research of the graphene-based nanomaterials,calling for further studies on both theory and experiment.For the studies of the doped molybdenum disulfides?MoS₂?,both hexagonal and rectangle unit cells were employed in our calculations.We have carefully examined the maximum doping concentration of the cobalt in MoS₂.Thanks to the presence of Co dopant,the magnetic properties could be obtained.The electronic properties of the doped MoS₂ have also been carefully investigated along with the increase of the doping concentration.The critical point for Co doping when the dispersionless band starts to appear was estimated.In the mateirals with lower doping concentration,the Co atoms should be treated as isolated doping defect.Furthermore,the adsorption of Co atoms on the side surface of MoS₂ has also been investigated.In view of the adsorption energy,the favoriable site is on the top of the Mo atoms.For the Co coveraged complex Co/MoS₂ nanostructure,the Mo S₂ sheet would be changed to the ferromagnetic half-metal,showing potential applications in the field of spintronics.In addition,we have calculated the coverage of S atoms on the Co/Mo S₂ complex sheet structure,the best adsorption site and the coverage pattern are carefully examinzed.Also,we have studied the Mn doped MoS₂,the manganese atoms can replace the Mo atoms.The calculations show that the 2H-MnS₂ structure is unstable while the 1T-MnS₂ may be realization in experiment.The latter one is a metallic magnetic material,In addition,it could be changed from metal to semiconduct or by applying strain.For the partial doping,both the hexagonal and rectangle 2H-MoS₂ doping configurations were studied.The stable sheet structures with maxiumum doping concentration according to the hexagonal and rectangle doping patterns are found to be magnetic half-metal and magnetic semiconductor material,respectively,which may find applications in the spintronic devices.In our studies,we also estimated the critical dopoing concentration at which the flat band start to appear in electronic bandstructure.