First-principles Study On The Property Modulations Of Graphynes And Molybdenum Disulfide Nanostructures

Nanomaterials have long been a hot research topic in science and technology field for their extraordinary geometric structures and physical properties. Among them, lowdimensional nanomaterials such as graphene, graphynes and molybdenum disulfide(Mo S2)have attracted tremendous attention in recent years, which are viewed as the promising candidates for application in nanoelectronic devices. In order to allow flexibility in device design and improve the device performance, a fully investigation on the physical properties and their modulations of these nanomaterials is thus highly demanded from a theoretical point of view. In this thesis, using the first-principles calculations within density-functional theory(DFT), we systematically study the mechanical, electronic,magnetic and transport properties of graphynes and Mo S2, and explore the modulation effects on their properties with different artificial approaches. The main conclusions are summarized as follow:1. Mechanical and electronic properties of graphynes(graphyne, graphdiyne, graphyne-3 and graphyne-4) under elastic strain.Due to the presence of acetylenic linkages in their architectures, graphynes is found to suffer a deterioration in materials hardness as compared with graphene. The in-plane stiffness of graphynes decreases with increasing the number of acetylenic linkages. By applying strain through various approaches, the band gaps of graphynes are significantly modified. While homogeneous tensile strain leads to an increase in the band gap, the homogeneous compressive strain as well as uniaxial tensile and compressive strains induce a reduction in it. Especially, both graphyne and graphyne-3 under different tensile strains possess direct gaps at either M or S point, whereas the band gaps of graphdiyne and graphyne-4 are always direct and located at the Γ point, irrespective of strain types.Our study proposes a new route for fabrication of novel strain-tunable nanoelectronic and optoelectronic devices.2. Electronic, magnetic and transport properties of α-graphyne nanoribbons.Both the armchair and zigzag α-graphyne nanoribbons at ground state exhibit semiconducting properties and possess the direct band gap, which decreases with increasing ribbon widths. Nonetheless, while the armchair α-graphyne nanoribbons are nonmagnetic, the zigzag ones are found to have ferromagnetic ordering at each edge and opposite spin orientation between the two edges. Under the application of transverse electric field,we also predict the existence of half-metallicity in the zigzag α-graphyne nanoribbons.Based on the two-probe systems constructed by the zigzag α-graphyne nanoribbons, we further show that the structural symmetry plays a crucial role in determining the transport behavior of the zigzag nanoribbons. Then, we successfully demonstrate the dipolar spin-filtering effect(spin-polarization nearly reaches 100%) and magnetoresistance effect(order larger than 500,000%) in this system by controlling the the direction of bias voltage and magnetization configuration of the electrodes. This study suggests new ways for designing spin-valve device and manipulating spin current, which promises application of zigzag α-graphyne nanoribbons in spintronics.3. Effects of strain and electric field on the electronic structures of monolayer Mo S2 and its armchair nanoribbons.It is found that the band gap of monolayer Mo S2 can be widely tuned by applying homogeneous tensile strain. As strain increases, the gap value undergoes a descent trend and eventually becomes zero. During this course, two important characteristic transitions can be observed, namely, direct-to-indirect transition at strain of 1% and semiconductorto-metal transition at strain of 10%. On applying the transverse and perpendicular electric fields, we also actualize the band gap modulation in the armchair Mo S2nanoribbons(AMo S2NR). It is found that the band gap of monolayer AMo S2 NR is significantly reduced and finally driven to zero under transverse field, whereas the gap modulation is absent under perpendicular field. The critical strength of transverse field for gap closure decreases as ribbon width increases. In the multilayer AMo S2 NR case, in contrast, the band gap can be effectively reduced by both transverse and perpendicular fields. Nevertheless, it seems that the two fields exhibit different modulation effects on the band gap.The critical strength of perpendicular field for gap closure decreases with increasing number of layers, while the critical strength of transverse field is almost independent of it.4. Influence of surface adsorption and substitution on the electronic and magnetic properties of monolayer Mo S2.We show that, by controlling the doping strategy as well as the concentration and species of dopant atoms, the electronic and magnetic properties of monolayer Mo S2 are remarkably engineered. Upon doping the monolayer Mo S2 can be converted from nonmagnetic semiconductor into magnetic metal or even half-metal, which extents its applications in magnetic nanodevices. Additionally, the adsorption of various gas molecules on monolayer Mo S2 is also explored. We find that all the gas molecules are weakly adsorbed on monolayer Mo S2 surface and act as charge acceptors or donors for the monolayer, resulting in charge transfer behavior between them. These findings explain the recent experiment phenomenons of Mo S2-based gas sensors and thereby provide theoretical supports for their sensing application.5. Structural and electronic properties of graphene/Mo S2 van der Waals heterostructures.Using first-principles calculations, we show that the band structures of graphene/Mo S2 van der Walls heterostructures can approximately be considered as a sum of those of each constituent layer. The π and π?subbands of graphene appear in the band gap of Mo S2, and the band structure features of graphene seems to be maintained. Nonetheless, there still exits weak electronic interaction between the graphene and Mo S2 layers, which is attributed to the interlayer charge redistribution and charge transfer between them. Because Mo S2 induces negligible modulation effect on the electronic structure of graphene, the mobility in the heterostructures would be comparable to that in the isolated graphene. The conductivity of monolayer Mo S2 thereby can be increased due to the addition of graphene. Furthermore, We demonstrate that a tunable band gap can be realized in the graphene/Mo S2 tetralayer heterostructures under perpendicular electric field, suggesting a new way for band engineering of graphene.