Chemical Functionalizations And Property Manipulation Of Two-dimensional Graphene-like Materials

In recent years,two-dimensional graphene-like materials have received increasing attention due to its application in nanodevices.Similar to bulk materials,chemical functionalizations such as doping and adsorption of extrinsic atoms,as well as defects,often exit naturally in two-dimensional materials.Intended artificial chemical functionalizations are often used to improve the physical and chemical properties of materials in order to obtain better device functionality.For example,zigzag silicene nanoribbons(ZSiNRs)have edge magnetism and can be used for silicon-based spintronic devices.Edge functionalizations can enhance or diversify their edge characterstics.Stone-Wales(SW)defect is one of the most usual and important topological defects in graphene-like materials.Although they generally degrade performance of materials,but can also be used to modulate the energy band structure of materials and improve their properties.The existence and stability of SW defects in two-dimensional materials has become a very important academic focus.In this thesis,the manipulation of spin transport via edge passivation of magnetic elements in armchair silicene nanoribbon(ASiNR)and the stability of Stone-Wales defect in two-dimensional materials with different valence electrons are studied systematically employing the first-principles simulations combining the density functional theory and the non-equilibrium Green's function method.The main aspects concerned in this thesis include:(1)Manipulation of spin transport and thermoelectric properties of edge passivated armchair silicene nanoribbon(ASiNR)by vanadium and fluorine atoms is addressed.We consider three typical device structures of ASiNR functionalized by magnetic transition metal vanadium and halogen fluorine atoms on edge.The electronic transmission spectra for the two-terminal device systems are then calculated in different configurations(ferromagnetic polarization parallel FM-P,ferromagnetic polarization antiparallel FM-AP,antiferromagnetic polarization parallel AFM-P)of electrode magnetization.The results show that these devices have spin filtering and giant magnetoresistance effects.Among them,significant spin Seebeck effects may be realized in one of the systems with antiferromagnetic configuration in the temperature range between 50 and 150 K.(2)We consider the formation path of SW defect in the plane of the two-dimensional crystal and selected the typical two-dimensional graphene-like materials containing IV,V and VI elements.The stability of SW defect and the effects of doping of these elements are discussed by calculating the kinetic energy and potential energy changes on the formation path.The results indicate that SW defect are easy to appear in group-IV materials but the structure of the SW defect in the V-group material may vary slightly while the stability decreases.It is difficult to form SW defect in group-VI crystals and doping of group-VI atoms may help eliminate SW defects in group-IV and V materials.These results give us a preliminary understanding of the effect on the stability of SW defect with different valence electronic elements,and may provide some practical information for the design and application of two-dimensional material devices in the future.