The Electronic Structures And Optical Properties Of Graphene-like Two-dimensional Materials

Since the discovery of two-dimensional material graphene,it has been a hot research topic in physics,chemistry,materials science and other fields,due to its extraordinary characteristics.Because silicene and graphene have many similar properties,such as they are both semi-metals with zero band gap,silicene has also received many attentions.However,the zero band gaps of graphene and silicene restrict their applications in some important fields of optoelectronics.Hence,graphene-like two-dimensional materials with semiconducting properties are very important.Experimental and theoretical studies demonstrated that fully hydrogenated graphene and fully hydrogenated silicane can open their band gaps.The monolayer MoS2,on the other hand,has also drawn great attentions due to its two-dimensional nature and a direct band gap of 1.90 eV.In this dissertation,by employing the first-principles method,the electronic and optical properties of the three graphene-like two-dimensional materials,i.e.,the graphane,silicane,MoS2 monolayer and multilayers as well as their hetero-bilayers are systematically investigated.In the part I of this dissertation,the electronic and optical properties of monolayer graphane and silicane are studied by both the HSE06 and the PBE functionals.It has been demonstrated that graphane opens a direct band gap of 3.43 eV,while the band gap of silicane is an indirect one of 2.19 eV,as calculated by using the PBE functional.Our HSE06 results show that graphane has a direct band gap of 4.49 eV,while silicane has an indirect band gap of 2.94 eV.Once the electronic structures of a material are obtained,the optical properties can then be studied.In graphane,HSE06 results show that the imaginary part of the dielectric function ?2(?)has three peaks at 11.56 eV,13.25 eV and 14.33 eV for E ?X,while there are only two peaks for E?Z,lying at 7.70 eV and 15.03 eV.In silicane,HSE06 results show that ?2(?)has two peaks at 4.56 eV and 8.55 eV for E?X,while there is only one main peak at 8.68 eV for E?Z.Generally speaking,the outlines of ?2(?)of the graphane(or silicane)calculated by the PBE and HSE06 methods are quite similar.However,the ?2(?)curves by the HSE06 method have shifted to the higher energy end,as compared to the results of PBE method.This is due to the fact that the HSE06 method gives a larger band gap for graphane(or silicane).Moreover,the band structures have also been employed to analyze the interband transitions,which explains the peaks in the imaginary part of the dielectric functionIn the part II,we have systematically investigated the electronic and optical properties of bilayer graphane,bilayer silicane and bilayer MoS2,as well as the graphane/MoS2 and silicane/MoS2 hetero-bilayers.The calculations indicate that the interlayer interactions of all the bilayer systems are mainly of the characteristics of van der Waals forces,due to the fully hydrogenation in the graphane and silicane Generally speaking,the differences of the electronic and optical properties between the monolayer and homo-bilayer systems are not significant,owing to the weak interactions between the two homo-layers.Both the graphane/MoS2 and silicane/MoS2 hetero-bilayers belong to the type-II heterostructure.In this case,the spatial separation of the electron and hole pairs can reduce the recombination probability of electron-hole pairs and therefore enhance the photocatalytic efficiency.In graphane/MoS2 hetero-bilayer,the imaginary part of the dielectric function ?2(?)has two main peaks which are mainly associated with the MoS2 layer.In the ?2(?)of silicane/MoS2 hetero-bilayer,four prominent peaks are observed where two peaks are mainly contributed from the MoS2 layer and the other two mainly from the silicane layer.Due to the broader light absorption ranges and the separation of the electron-hole pairs,both the graphane/MoS2 and silicane/MoS2 hetero-bilayers are expected to be good candidates for solar energy conversion.In the part ?.the electronic and optical properties of monolayer,multilayer and bulk MoS2 have been systematically studied.Compared to the band structures of monolayer MoS2,multilayers and bulk MoS2 show splitting of bands mainly around the ?-point.On the other hand,splitting of bands in the vicinity of K-point are tiny,owing to the weak interlayer coupling around point K.With the increase of the layer number N.the interlayer coupling increases,meanwhile,the intra-layer quantum confinement decreases which result in the decrease of band gaps of the systems.Our results demonstrate that the differences between the electronic and optical properties for monolayer and multilayers MoS2 are significant However,as the layer number is increased to two,the electronic and optical properties of MoS2 multilayers start to exhibit those of bulk.When the number of MoS2 layers increased to 4,it is found that(?)of multilayer MoS2 systems were almost indistinguishable from bulk.It should be noted that the Po-type Van Hove singularities in these systems lead to the occurrence of long platforms in both the joint density of states(JDOS)and the(?)of the monolayer,multilayer and bulk MoS2.At the beginnings of these long platforms,one,two and three small steps appear in the monolayer,bilayer and trilayer MoS2,respectively.With the layer number N further increases,the number of small steps increases correspondingly and the width of the small steps then decreases.Thus,the small steps are then not obvious near the threshold energy since the width of the small steps is too narrow.The starting energy of the long platform of multilayer and bulk MoS2 depends on the minimum direct energy gap near point K,while the ending energy of the long platform depends on the minimum direct energy gap at point ?.In conclusion,the present results can help understand the electronic and optical properties of graphene-like two-dimensional materials,which should be significant for developing the optoelectronic devices.