| Carbon forms a great variety of crystalline and disordered structures withdifferent properties because it is able to exist in three hybridizations, sp3, sp2, sp1.Inthis work, theoretical calculation for carbon nitride and nanodiamond are performedby atom-scale computer simulation, and the correlation between microstructure andproperties in these materials are discussed.The First part of this work is about the investigation of amorphous carbonnitride (a-CNx). a-CNx is a new type of material with excellent properties. N and Catoms have many possible bonding configurations in a-CNx and it is very necessaryto make sure of the correlation between bonding configuration and a-CNx properties.The addition of N to a-C (just about a-CNx) has three effects. First, small additions ofN can dope ta-C n type. Second, Liu and Cohen have proposed that the compoundC3N4 would show a hardness exceeding that of diamond. Considerable research hasbeen in C3N4 synthesis and it proves difficult to achieve the high N incorporation andto maintain the C bonding as sp3. The third effect of N in a-CNx is to createtopological disorder in graphitic bonding. This leads to the formation offullerene-like microstructure in a-CNx films deposited by sputtering. The films havea very high elastic recovery. In fact, there is much controversy about the microscopicmechanism in both N doping in ta-C and fullerene-like structure in a-CNx.Through first principle calculation based on density functional theory, weinvestigate N incorporation in a-C. The structure models, at densities of 2.0 and 2.9g/cm3, are generated by liquid quench method and have a full geometry optimization.It demonstrates that, in comparison with ta-C structure, N incorporation has littleeffect on the sp3 fraction and sp3 atom topology of a-CNx at density of 2.9 g/cm3 withN fraction of 3.2%, but for sp2 atoms, a clustering tendency is found. In particular, anew threefold C defect is found in a-CNx, which introduced by N auto-compensation.When N incorporation is up to 31.2%, N2 is formed in a-CNx network at density of2.0 g/cm3, but stable bonding configurations between three-fold N and C atoms are inthe network at density of 2.9 g/cm3. The later network has a fullerene-like (orsp2-rich) structure with five-fold and six-fold rings.The secod part of this work is about the first principle study of nanodiamond.Nanodiamond is the diamond particle in nanometer scale. With successful diamondsynthesis, nanodiamond has broad applications in industry. However, many issuesabout nanodiamond are still underway and they restrict the further R&D aboutnanodiamond. In 2002, Lifshitz et al. investigated the nucleation mechanism ofdiamond and Larciprete et al. found the transformation from nanodiamond to carbonnanotube. In 2003, Dahl et al. isolated large quantities of high-quality, H-terminatedC nanoparticles from petroleum and separated into monodisperse samples. Suchnanostructures, with a large variety of shapes, are likely to be used as well-definedstructural building blocks in nanoscale electromechanical systems. At the same time,Raty et al. demonstrated fullerene-like surface reconstruction in non-hydrogennanodiamond through ab initio calculations and X-ray absorption and emissionexperiments. Now, people have much interest in quantum effect of nanodiamond andits new properties, to be different from bulk diamond, have been found. This is alsothe purpose of our work.In our work about nanodiamond, we firstly investigate eight structures ofdiamondoids (a type of H-passivated nanodiamond) by performing first principlecalculation. Their diameters are from 0.5 to 1.6 nm. It demonstrates that diamondoidsare stable and very similar in structure to diamond, with all-sp3 carbon atoms.However, the C-C bond length, binding energy, surface Mulliken charge, electronaffinity, and ionization potential of diamondoid are all size dependence. The bandgaps and the highest occupied molecular orbitals (HOMO) in the electronic structureof diamondoids exhibit quantum confinement effect, but not the lowest unoccupiedmolecular orbitals (LUMO). The critical size of such quantum effect in diamondoidsis about 1.0 ~ 1.2 nm in our calculation. When the diameter of diamondoid is about1.0 ~ 1.2 nm, however, its band gap is narrower than that of bulk diamond. Weexplain this phenomenon as a cooperation of surface effect and quantum confinementeffect on the electronic structure of diamondoids.Furthermore, we perform first principle calculation about non-hydrogennanodiamond (C147). The surface carbon atoms in C147 have a fullerene-likereconstruction after geometry optimization and the atoms in the core are still indiamond structure, which is also named bucky diamond. It demonstrates that theband gap of bucky diamond is nearly zero due to the hanging bond in the core carbonatoms, where HOMO and LUMO are all localized. |
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