Theoretical Study For Amorphous Carbon, Amorphous Carbon Nitride And Nanodiamond

Carbon forms a great variety of crystalline and disordered structures with different properties because it is able to exist in three hybridizations, sp~2, sp~2, sp~1 In this work, theoretical calculation for amorphous carbon, amorphous carbon nitride, and nanodiamond are performed by atom-scale computer simulation, and the correlation between microstructure and properties in these materials are discussed.First part in this work is about the simulation of amorphous carbon (a-C). Sometimes, experimental characterization techniques can not give exact information about a-C due to its disordered nature. So, molecular dynamics modeling has recently become a powerful tool to give a good complement in investigation of a-C films. Furthermore, the excellent mechanical, optical and electronic properties of a-C films strongly depend on deposition conditions, and molecular dynamics simulation of a-C film growth is very helpful to provide a unique insight into the deposition process. Since the large scale in a-C deposition modeling, both computing precision and efficiency must be considered in the simulation. Up to now, a-C deposition simulation in the report are mainly performed by empirical potential and two-center tight-binding model, say nothing of first principle. In this work, we investigate the microstructure of periodic a-C structure and a-C films by environment-dependent tight-binding (EDTB) molecular dynamics. It is the first time that EDTB is applied into the simulation of a-C film growth and it gives a better description of a-C films than previous methods.The periodic structural models of a-C at densities of 2.0, 2.3, 2.6, 2.9, and 3.2 g/cm3 are investigated using EDTB molecular dynamics. The networks are generated by liquid quench method. It demonstrates that the low-density a-C has a significant sp2 fraction and a little sp1 fraction of carbon atoms. Furthermore, it shows as graphite-like layered structure with a narrow band gap. The high-density a-C has a significant sp3 fraction and no sp1 hybridized carbon atoms. It has a diamond-like structure with a broad band gap. Radial distribution function of these five models gives a further proof of such density-dependent structure. In particular, the fraction of sp2 (or sp3) atoms is found to have a linear dependence on the density of the network.a-C films deposited on diamond are also simulated by EDTB molecular dynamics. The substrate temperature is 300 K and the energies of impinging carbon atoms are 10 eV and 40 eV. It demonstrats that the deposited carbon films have obvious disordered structures. a-C film deposited at 10 eV has a loose network with a large number of sp2 carbon atoms. In comparison, a-C film deposited at 40 eV has a compact network. In the bulk region of this film, the sp3 fraction is up to 80% and forms into a diamond-like structure. The bulk properties of these computed a-C films, both at 10 eV and 40 eV, as well as their sp2-rich surface layers agree qualitatively with experiment. The reason of structural difference in a-C films at 10 eV and 40 eV is concluded that impinging atoms with different energy have different deposited processes during the film growth.The second part of this work is about the investigation of amorphous carbon nitride (a-CNx). a-CNx is a new type of material with excellent properties. N and C atoms have many possible bonding configurations in a-CNx and it is very necessary to 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 of N can dope ta-C n type. Second, Liu and Cohen have proposed that the compound C3N4 would show a hardness exceeding that of diamond. Considerable research has been in C3N4 synthesis and it proves difficult to achieve the high N incorporation and to maintain the C bonding as sp3. The third effect of Nin a-CNx is to create topological disorder in graphitic bonding. This leads to the formation of fullerene-like microstructure in a-CNx films deposited by sputtering. The films have a very high elastic recovery. In fact, there is much controversy about the microscopic mechanism in both N doping in ta-C and fullerene-like structure in a-CNx.Through first principle calculation based on density functional theory, we investigate N incorporation in a-C. The structure models, at densities of 2.0 and 2.9 g/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 little effect on the sp3 fraction and sp3 atom topology of a-CNx at density of 2.9 g/cm3 with N fraction of 3.2%, but for sp2 atoms, a clustering tendency is found. In particular, a new 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 of 2.0 g/cm3, but stable bonding configurations between three-fold N and C atoms are in the network at density of 2.9 g/cm3. The later network has a fullerene-like (or sp2-rich) structure with five-fold and six-fold rings.The third part of this work is about the first principle study of nanodiamond. Nanodiamond is the diamond particle in nanometer scale. With successful diamond synthesis, nanodiamond has broad applications in industry. However, many issues about nanodiamond are still underway and they restrict the further R&D about nanodiamond. In 2002, Lifshitz et al investigated the nucleation mechanism of diamond and Larciprete et al. found the transformation from nanodiamond to carbon nanotube. In 2003, Dahl et al. isolated large quantities of high-quality, H-terminated C nanoparticles from petroleum and separated into monodisperse samples. Such nanostructures, with a large variety of shapes, are likely to be used as well-defined structural building blocks in nanoscale electromechanical systems. At the same time, Raty et al. demonstrated fullerene-like surface reconstruction in non-hydrogen nanodiamond through ab initio calculations and X-ray absorption and emission experiments. Now, people have much interest in quantum effect ofnanodiamond and its new properties, to be different from bulk diamond, have been found continuously. This is also the purpose of our work.In our work about nanodiamond, we firstly investigate eight structures of diamondoids (a type of H-passivated nanodiamond) by performing first principle calculation. Their diameters are from 0.5 to 1.6 nm. It demonstrates that diamondoids are stable and very similar in structure to diamond, with all-sp carbon atoms. However, the C-C bond length, binding energy, surface Mulliken charge, electron affinity, and ionization potential of diamondoid are all size dependence. The band gaps and the highest occupied molecular orbitals (HOMO) in the electronic structure of diamondoids exhibit quantum confinement effect, but not the lowest unoccupied molecular orbitals (LUMO). The critical size of such quantum effect in diamondoids is about 1.0 ~ 1.2 nm in our calculation. When diameter of diamondoid is about 1.0 ~ 1.2 nm, however, its band gap is narrower than that of bulk diamond. We explain this phenomenon as a cooperation of surface effect and quantum confinement effect on the electronic structure of diamondoids.Furthermore, we perform first principle calculation about non-hydrogen nanodiamond (C147). The surface carbon atoms in C147 have a fiillerene-like reconstruction after geometry optimization and the atoms in the core are still in diamond structure, which is also named bucky diamond. It demonstrates that the band gap of bucky diamond is nearly zero due to the hanging bond in the core carbon atoms, where HOMO and LUMO are all localized.