Physical Mechanical Properties In Graphene-like Two-dimensional Nanomaterials And Their Nanoribbons

Low-dimensional nanoscale materials have attracted a great deal of attention owing to their distinct properties and potential applications in futher nanodevices. Due to the existence of quantum effect, the low dimensional materials have distinct different physical, chemical, mechanical and stability properties compared with the original materials, which entail the materials unique functions that are not available in bulk counterparts. Although the ground properties of nanomaterials have been extensively investigated recently, the study on coupling between material properties and external physical fields is still in infancy. In this thesis, using first principles calculations based on density functional theory as well as the non-equilibrium Green’s function(NEGF) method, we systematically study the physical mechanical properties of a series of low dimensional nanomaterials, including graphene-like two-dimensional(2D) graphyne and Be O nanosheet, their one-dimensional(1D) nanoribbons and zigzag graphene nanoribbons. In particular, we get deep insight into the change in structures, electronic, magnetic and transport properties of these nanoscale materials upon the applications of external physical fields, such as electric field. Ample phenomenen from the coupling of electronics and magnetism are revealed in these materials. The findings are birefly concluded below:(1) Study of Intrinsic Electronic and Transport Properties of Graphyne Sheets and Nanoribbons. Graphynes, 2D carbon allotropes like graphene but containing doubly- and triply-bonded carbon atoms, have been proven to possess amazing electronic properties as graphene. Although the electronic, optical, and mechanical properties of graphynes and graphyne nanoribbons(NRs) have been previously studied, their electronic transport behaviors are lack of understanding. Here we report a comprehensive study of the intrinsic electronic and transport properties of four distinct polymorphs of graphyne(a, b, g, and 6,6,12-graphynes) and their nanoribbons(Gy NRs) using density functional theory coupled with non-equilibrium Green’s function(NEGF) method. Among the four graphyne sheets, the 6,6,12-graphyne displays notable directional anisotropy in the transport properties. Among the Gy NRs, those with armchair edges are nonmagnetic semiconductors whereas those with zigzag edges can be either antiferromagnetic or nonmagnetic semiconductors. Among the armchair Gy NRs, the a-Gy NRs and 6,6,12-Gy NRs exhibit distinctive behavior of negative differential resistance(NDR). On the other hand, the zigzag a-Gy NRs and zigzag 6,6,12-Gy NRs exhibit symmetry-dependent transport properties, that is, asymmetric zigzag Gy NRs behave as increasing the step-to-step distance. In particular, the energy gap decreases first toward a zero minimum and then gradually increases as the step length increases, accompanying with the rapid increase in the edge magnetization. When the step length exceeds a critical value, the ZS-GNR will be always magnetic semiconductor regardless of the step-to-step distance. We also reveal that the applied transverse electric field can enlarge the energy gap of nonmagnetic ZS-GNRs, due to the breaking of band degeneration; whereas the field-induced gap change in the magnetic ZS-GNRs is spin dependent, leading to the emergence of amazing half metallicity under certain field strengths. conductors with nearly-linear current-voltage dependence, whereas symmetric Gy NRs give very weak currents due to their band gap around the Fermi level. Such symmetry-dependent behavior stems from different coupling between π* and π subbands. Unlike a- and 6,6,12-Gy NRs, both zigzag b-Gy NRs and zigzag g-Gy NRs exhibit behavior of NDR regardless of the symmetry.(2) Study of electronic and magnetic properties of graphene-like 2D Be O sheet and its 1D nanoribbons. Since the discovery of the exotic properties of graphene, 2D layered materials such as boron nitride, metal chalcogenides, transition metal oxides, and other 2D compounds have gained renewed interest. The novel electronic and magnetic properties of Be O nanoribbons(Be O NRs) as well as their stability are investigated through extensive density functional theory calculations. Different from semiconducting graphene nanoribbons and insulating BN ribbons, all zigzag edged Be O NRs are revealed to display ferromagnetic and metallic natures independent of the ribbon width and edge passivation. The polarized electron spins in H-passivated zigzag Be O NRs are from the unpaired electrons around the weakly formed Be-H bonds, while those of bare zigzag Be O NRs are due to the 2p states of edge O atoms. In sharp contrast, all armchair Be O NRs are nonmagnetic insulators regardless of the edge passivation. In particular, all bare armchair Be O NRs have a nearly constant band gap due to a peculiar edge localization effect. Interestingly, the band gaps of all armchair Be O NRs can be markedly reduced by an applied transverse electric field and even completely closed at a critical field. The critical electric field required for gap closing decreases with increasing ribbon width, thus the results have practical importance. Further stability analysis shows that bare Be O NRs are more stable than H-passivated Be O NRs of similar ribbon widths and bare armchair Be O NRs are energetically the most favorable among all the nanoribbons.(3) Study of electronic and magnetic properties of zigzag graphene nanoribbons with periodic protruded edges. Zigzag graphene nanoribbons(ZGNR) have recently attracted much attenation owing to their outstanding electronic and magnetic properties. All these previous studies mainly focus on the ZGNRs with smooth edges. However, under typical growth or fabrication conditions, the edges of graphene ribbons are often observed to be rough and tapered by 5–10 ? steps protruded out of the edges. The electronic and magnetic properties of zigzag graphene nanoribbons with protruded steps along their edges(ZS-GNRs) are investigated by extensive density-functional theory calculations. It is shown that the electronic and magnetic properties are determined by an interesting interplay between the length of the prouded step and the distance of two adjacent steps along the ribbon edge. With a small length of the protruded eps along the edge, the system can be converted from a nonmagnetic semiconductor to metal and then to a magnetic semiconductor by...