Electronic Structure And Properties Of Monolayer Metal-BHT With ?-d Conjugate Bonds

Since the discovery of graphene,two-dimensional?2D?materials with atomic thickness have attracted much attention due to their unique physical and chemical properties.Recently,an exciting topic in 2D materials is the bottom-up synthesis of nanosheets based on an organic framework such as a metal organic frameworks?MOFs?using coordinate bonds and a covalent organic frameworks?COFs?using covalent bonds.Although some metal-organic coordination polymers have been discovered,there are still many materials in this family whose crystal structures are unknown and those materials functional properties are uncontrollable.In general,research on the conductive?-conjugate polymers is still in its infancy.However,conductive?-conjugate polymers are attractive and they have great application potential.They not only provide new physical properties,but also regulate these properties by combining different metals and ligands.In this thesis,the first-principles method based on density functional theory is used to calculate the more detailed properties of the experimentally prepared conductive?-conjugate polymers?M-BHT?.We have predicted the possible new M-BHT crystal structure and designed special functional M-BHT materials.First,based on the calculation of cohesive energy,phonon spectrum and quantum molecular-dynamic simulation,we have predicted a stable type of 2D metal-benzenehexathiolate?M-BHT?coordination compounds by first-principles calculation.The electronic structures of stable M-BHTs and the corresponding inorganic semiconducting materials have been compared.From the point of view of satisfying stoichiometric ratios and saturation of chemical bonds,we designed possible planar molecular structures and demonstrated that there may be two different2D M-BHTs,i.e.group II-S₆C₆ and group IV-[S₆C₆]₂.However,the cohesive energy calculation indicates that the group IV-[S₆C₆]₂ coordination compound cannot be obtained by thermodynamic equilibrium growth.In contrast,Mg₃S₆C₆ and Zn₃S₆C₆from the group II-S₆C₆ have not only thermodynamic stability,but also dynamic stability due to their phonon spectrum with no imaginary frequency.Moreover,they are still the two most stable ones when the bridge atom S of ligand BHT is replaced by the other chalcogens of O,Se and Te.Further studies indicated that Mg₃S₆C₆,and Zn₃S₆C₆ both have room temperature dynamic stability and exhibit semiconducting.The exceptional stability and relatively narrow band gap make them advantageous over their inorganic counterparts.Our findings open opportunities to search for new 2D planar conducting coordination compound for organic electronic applications.Secondly,we systematically study the electronic properties of the two-dimensional metal-organic framework semiconductors?MOFSs??M₃S₆C₆,M=Mg,Ca,Zn,Cd,Ge,and Sn?using the first principles calculations.We find that the metal d band is important in determining the hole transport properties of M ₃S₆C₆.The p-d hybridization between the metal d and S-C p bands will delocalize the wavefunction of the band edge states and reduce the effective mass.From group IIA?Mg,Ca?to IVA?Ge,Sn?to IIB?Zn,Cd?,as the p-d coupling increases,the hole effective masses dramatically decrease.Additionally,due to the fact that the conduction band minimum of group IIB?Zn,Cd?MOFSs is mainly dominated by the delocalized M s state,they also have the very small electron effective mass.Therefore,the 2D group IIB?Zn,Cd?MOFSs have excellent hole and electron effective masses,which are comparable with the conventional semiconductors and even better than the popular 2D materials WS₂and MoS₂.This result suggests that Zn₃S₆C₆ and Cd₃S₆C₆ MOFSs could be the promising 2D semiconductors for the electronic applications.Next,we have predicted that a tight kagome lattice with spin-splitting and spontaneous magnetization can be achieved in a transition metal-Mg₂-S₆C₆ alloy formed by substituting Mg with 3d transition metal atoms in nonmagnetic Mg₃-S₆C₆.It is found that most systems have ferromagnetic and semiconductor properties after replacing the metal atom,except the Ni and Cu-Mg₂-S₆C₆ systems.The exchange interactions between transition metals and organic ligands electively regulate spin polarization.The?-d coupling,one of the exchange interactions,play a crucial role in determining the spin state of the system and it will make and it will weaken the magnetism and even make it disappear.Additionally,it is also confirmed that the VMg₂-S₆C₆ can exhibit strong ferromagnetism with higher Curie temperature.Finally,we have provided a new regulation strategy which was used to study the effects of active sites and the interaction between the metal and the ligand on the catalytic activity.Through the first principle calculation,it was determined that Cu₃Se₆C₆ is a superior potential catalyst.Good electrical conductivity and the strong?-d conjugation of the metal with ligand can promote the catalytic activity,thus forming a small overpotential.In addition,the electron donating property of the Se₆C₆ligand enable the charge transfer effect,which induces the active sites of adjacent atoms in the ligand possessing more electrons,and facilitate the generation of OOH*,hence leading to catalytic activity improvement.