چكيده لاتين
In this thesis, the structural, electronic and optical properties of the bulk and monolayer of XY2 (X=Cr, Mo, Zr and Y=S, Se, Te) transition metals dichalcogenides have been investigated. All the calculations in this research were done based on the density functional theory (DFT) by using the WIEN2k calculation package. In calculating the exchange-correlation energy, the generalized gradient approximation (GGA) and modified Beck-Johnson potential with generalized gradient approximation (mBJ-GGA) have been used. To investigate the structural propertice and magnetic order of bulk and monolayer XY2, total energy in terms of volume for these compounds in two structural phases of honeycomb (2H) and centered honeycomb (1T) have been calculated. Except 1T-CrTe2 in bulk and monolayer form, which is ferromagnetic in 1T and 2H structural phases, other compounds are non-magnetic. To determine the energy stability of these materials, the cohesive and formation energies were calculated; the calculated resalts show that, 2H-CrS2, 2H-CrSe2, 1T-CrTe2, 2H-MoS2, 2H-MoSe2, 2H-MoTe2, 1T-ZrS2, 1T-ZrSe2 and 1T-ZrS2 compounds are more stable. After determining the most stable structural phase of XY2 bulk and monolayer form, 2H-CrS2, 2H-CrSe2, 1T-CrTe2, 2H-MoS2, 2H-MoSe2, 2H-MoTe2, 1T-ZrS2, 1T-ZrSe2 and 1T-ZrS2 were selected as suitable candidates for further investigations. The study of electronic and optical properties of XY2 in bulk form showed that these compounds are categorized from semiconductor to metal in terms of conductivity, and optically, these compounds are good reflective and absorbers of ultraviolet waves. Since 2H-CrS2 and 2H-MoS2 in the bulk have the same crystal structur phase, in this research the structural, electronic and optical properties of CrxMo1-xS2 (x=0, 0.125, 0.25, 0.375, 0.50, 0.625, 0.75, 0.875, and 1) alloys were studied for the first time. The obtained results showed that CrxMo1-xS2 alloys with different Cr concentrations are non-magnetic and have energy stability. In addition, these alloys have an energy gap between 1.54 eV (in 2H-MoS2) and 0.68 eV (in 2H-CrS2). After investigating the physical characteristics of the bulk of these compounds and CrxMo1-xS2 alloys, we focused on the XY2 monolayers, the physical properties of these monolayers and the possibility of topological phase transition under strain and Bi and Sb impurities were investigated and the optical properties of this monolayer have been studied in the presence and absence of Bi and Sb impurities. According to the results, these monolayers (except 1T-CrTe2 which is ferromagnetic) were generally non-magnetic semiconductors, which in the presence of strain, their energy gap decreased with increasing strain. In the presence of Bi and Sb impurities, the metal monolayers remain metal, but the semiconducting monolayers transfer to metal phase due to the crossing of the maximum valence band and Fermi energy. Also, the study of band order in these monolayers showes that these compounds have normal band order in the presence of strain and impurities. So, these monolayers are normal semi-conductors or metals. Finally, the optical properties of these monolayers have been investigated in the presence and absence of Bi and Sb impurities.
