Atomistic Simulations Of The Curved Carbon Nanotubes And Graphene Oxide

In recent two decades, carbon nanomaterials have always being the hot topics in materials, chemistry and physics. Both the fabrications of fullerene and graphene have been awarded the Nobel Prize. Meanwhile, carbon nanotubes (CNTs) have also been widely used in high-tech industries, such as automotive, aerospace, electronics, etc. Carbon nanomaterials can be in various hybridized carbon forms and show abundant physical and chemical properties and promising applications, which hugely promote the research of nanoscience and nanomaterials. In this thesis, we focused on two kinds of typical carbon nanotubes, the curved CNTs and the graphene oxide (GO), and systematically investigated their structures, thermodynamic stabilities, and mechanical and electronic properties from atomistic simulations.Introducing pentagons and heptagons into the CNTs can change the curvature and further lead to curved CNTs, such as toroidal CNTs, coiled CNTs, and zigzag-shaped CNTs. The (n, n)(n=5-7) toroidal CNT shows better thermodynamic stability than C60fullerene and its stability increases with the tubular and toroidal diameters. For the toroidal CNTs with different symmetries, the one with D6h symmetry is the most stable. On the other hand, introduction of pentagons and heptagons will make the toroidal CNTs semiconducting with a HOMO-LUMO gap ranging from0.08to0.95eV due to charge localization. In addition, it was found that B atoms are easily to be doped into heptagonal sites, while N atoms are easily to be doped into pentagonal sites. Compared with the perfect CNTs, pentagons and heptagons in the toroidal CNTs can enhance the chemical activity and are preferable for heteroatom doping. Moreover, doping of B/N is able to transfer the semiconducting toroidal CNTs into metallic.Study on the mechanical properties of the (n, n)(n=5-7) coiled CNT indicates that Young’s modulus of the coiled CNT is only a few GPa, far below that of the perfect CNT. Further study shows that the coiled CNT possesses superelasticity. Under elongation or compression, it can adjust the bond angle to keep its bond length nearly a constant, achieving an elastic tensile strain more than～55%. On the other hand, calculation of the quantum conductance demonstrates that the coiled CNTs are semiconducting with a transmission gap of0.3-0.8eV. Analysis on the electron density of states suggests that the Stone-Wales defects in metallic CNTs only lead to some defective states near the Fermi energy, but can’t open a gap. A possible reason for the transmission gap is the helical geometry of the coiled CNT, which can transfer the sp2hybridized carbons into sp3hybridization.For the (n, n) zigzag-shaped CNTs with n=5-8, the first-principles calculations suggest that the Young’s modulus is about560-650GPa and the intrinsic strength is about68～81GPa, respectively, both of which increase with the tubular diameter. Compared with the Young’s modulus (～1000GPa) and intrinsic strength (～100GPa) of the CNT, introducing pentagons and heptagons will decrease the Young’s modulus and intrinsic strength. Taking the values of graphene as the limits of infinite zigzag-shaped CNTs, we can fit simple formulae to show the relationship between the Young’s modulus/intrinsic strength and the tubular diameter. Using these formulae, Young’s modulus and intrinsic strength of the zigzag-shaped CNTs with any tubular diameters can be predicted. In addition, under tensile strain, the first fractured C-C bond is a6-7bond along the strain direction. Moreover, the tensile strain can change the localized states of the conduction and valence bands, which consequently triggers the semiconductor-metal or metal-semiconductor transition.Due to the complex stoichiometry and numerous possible arrangements of the oxygen-containing groups, so far the atomistic structure of GO is still ambiguous. Experimental characterizations demonstrate GO is amorphous, while theoretical computations suggest GO with hydroxy (-OH) and epoxy (-O-) groups aggregating and arranging in an ordered chain manner is energetically preferable. Therefore, we studied the amorphous GO models, trying to find the relationship with the ordered ones. Through comparing the amorphous and ordered models, we found that the energetically preferable amorphous GO always contains some local ordered motifs. Within these ordered motifs, epoxy and/or hydroxy groups arrange orderly and can form clusters completely consisting of epoxy or hydroxy groups, agreeing with the experimental observations. Besides, we also considered the roles of oxygen coverage and OH:O ratio on the stability, electronic structure and mechanical property of GO. With increasing the oxygen coverage and OH:O ratio, GO becomes more stable and its band gap increases accordingly. However, Young’s modulus and intrinsic strength of the GO get decreased with increasing the oxygen coverage, and becomes slightly fluctuated with increasing the OH:O ratio. On the other hand, band gap of GO becomes decreased under uniaxial tensile strain, showing significant electromechanical coupling effect. This can be explained by the weakness of C-O hybridization under uniaxial tensile strain, which can release conductive charges.