Study On The Physical Properties Of Several Advanced Two-Dimensional Materials

Two-dimensional materials are popular research topics in the field of physical,chemical and material science due to the excellent physicochemical properties.Compared with bulk materials,two-dimensional materials take on larger specific surface area,higher surface atomic utilization rate,rapid ion transmission speed,and superior flexibility and transparency,which makes it possess a broad application prospect in the fields of catalysis,super-capacitor,lithium battery,fuel cell,intelligent wearable device and flexible medical device.However,there are deficiencies in practical application for pure two-dimensional materials.Through modification such as shearing,doping,defect introduction,surface modification and other methods can induce two-dimensional materials to generate spontaneous magnetization to exhibit better performance,making it play a potential role in spintronic devices,environmental protection,aerospace and other fields.Moreover,the adsorption of transition metals on the surface of two-dimensional materials can significantly improve its hydrogen storage capacity and realize reversible absorption/release of hydrogen at room temperature,which possess significant application value in the field of new type of solid-state hydrogen storage.Therefore,it is very important to explore the magnetic mechanism and hydrogen storage mechanism of two-dimensional materials,which can provide theoretical reference for the practical application of two-dimensional materials.Based on the effective-field theory with correlations and density functional theory,the magnetic,thermodynamic and hydrogen storage properties of several advanced two-dimensional materials are studied in this thesis.The Ising model of graphdiyne-like,the decorated two-dimensional Kagome lattice and graphene is established from the microscopic quantum perspective.The factors affecting magnetization,susceptibility,internal energy,specific heat,compensation temperature,phase transition temperature and hydrogen storage performance of the system are investigated.The main contents are as follows:The magnetic and thermodynamic properties of the decorated two-dimensional Kagome lattice are studied by theoretical calculation in this thesis.Different types of magnetization curves are observed,such as P-type and Q-type curves.The P-type curve shows a depressed saturation magnetization,and the Q-type curve decreases gradually with increasing temperature.A special magnetizations curve is also observed,in which the magnetization suddenly drops to zero,corresponding to the first-order phase transition.The sublattice magnetization curve shows that the total magnetization of the system comes from the superposition of the sublattice magnetization.Multiple hysteresis loops appear on the hysteresis loops within a certain range of parameters.The hysteresis loss and coercivity of the system increase with increasing the absolute value of the ferrimagnetic exchange coupling and decrease with increasing anisotropy|D_a|（|D_b|）.Reentrant phenomenon is dug out in the phase diagram.On the dynamic magnetization curve,it is found that the magnetization changes periodically with time,the bias field provides ordered energy for the system,and the alternating field provides disordered energy for the system.The magnetic and thermodynamic properties of graphdiyne-like with mixed spins 5/2and 2 are studied in this thesis.The compensation temperature(T_comp)is observed on the magnetization curve,which increases with the increasing exchange coupling|J₂|（J₁）and decreases（increases）with increasing anisotropy D_a（D_b）.The saturation magnetization（M_s）does not change with the variation of the exchange coupling|J₂|（J₁）,but increases with increasing anisotropy D_a（D_b）.Different types of magnetization curves are also found,such as N-type,P-type and Q-type curves.A singular phenomenon is found on the susceptibility curve.The phase transition temperature（T_c）corresponding to the peak moves to high temperature with the increase of|J₂|（J₁）.Two peaks are observed on the specific heat curve.It is found in the phase diagram that the ferrimagnetic area increases with the increase of|J₂|（J₁）.When the absolute value of the anisotropy D_a are large（→-∞or+∞）,the phase transition temperature curve of the system tends to be horizontal line.These results may contribute to a better understand the magnetic mechanism of graphdiyne and improve the application value of two-dimensional carbon materials in organic magnets.The magnetic,thermodynamic and hydrogen storage properties of graphene are investigated in this thesis.It is found that defect concentration can regulated the magnetic and thermodynamic properties of graphene.The saturation magnetization of the system increases with the increase of the defect concentration.Multiple magnetization plateaus are found on the step effect curve owing to the existence of multiple spin state in the system.A sharp maximum is observed on the specific heat curve due to the thermal disturbance.Transition metal modified graphene can induce the magnetism of graphene and improve the hydrogen storage performance of graphene.The binding energy of pure graphene hydrogen storage is in the range of-0.051 eV to-0.055 eV,which belongs to physical adsorption.Fe-decorated graphene can improve the binding energy of hydrogen storage,when adsorbing 5～6 hydrogen molecules,the average binding energy is-0.572 eV/H₂ and-0.487eV/H₂ respectively,meeting the range of reversible hydrogen adsorption/release at room temperature（-0.2～-0.6 eV）.The charge density difference indicates that there is no charge transfer between carbon atoms and hydrogen molecules in pure graphene hydrogen storage,while two hydrogen molecules exist Kubas interaction with iron atoms in Fe-decorated graphene hydrogen storage,and there is also a synergy between hydrogen atoms,iron atoms and carbon atoms.The density of states shows that Fe-decorated graphene is magnetic,and the total magnetization of the system is 4.00μB,among them,the influences of the 3d orbital electrons of iron atom on the magnetic properties of the system are greatest.The obtained results can help to have a better understand the magnetic mechanism and hydrogen storage mechanism of graphene,and provide necessary theoretical support for the practical production and application of graphene.