Prediction And Design Of Novel Carbon Crystals By First-principles Methods

Element carbon is so active that it can form materials with itself and with almost all the elements in the periodic table showing abundant bonding configurations, such as sp, sp2 and sp3. People believe that carbon can form many allotropes beyond the traditional graphite and diamond and have discovered fullerence, nanotube and graphene. In fact, there are still many unsolved carbon allotropes reported in the literature and many potential carbon allotropes need to be explored. With the advancing of the power of computer and first-principles methods, computational alchemy is more and more popular in application for solving previously reported unknown carbons, predicting new possible carbon allotropes and designing potential multifunctional carbon materials. In this thesis, we extend and consummate the family of carbon allotropes for the purpose of providing crystal strucutures for understanding the previously reported unknown carbons and potential carbon allotropes which may be discovered in the future. We have obtained a series of innovational results:1. To understand the mechanism of the formation of superhard carbon allotropes in cold-compressing of graphite, we firstly investigated the possible up and down configurations in graphene and discovered a new set of graphane allotropes with six fundamental up and down configurations from graphene layers. Based on these fundamental configurations, we then investigated the possible interlayer hybridization configurations in graphite and proposed many potential superhard carbon allotropes for understanding the diversity of the phase transitions in the process of cold-compressing of graphite. Finally, we proposed two novel superhard carbon allotropes H-carbon and S-carbon that can be directly transformed from AB stacked graphite. Bct-BN and Z-BN were also proposed for understanding the cold-compression of h-BN.2. To explain the simple cubic carbon material discovered in 2003, we modified the crystal structure of a previously proposed cubic carbon C20-sc and obtained two new cubic carbon crystals C21-sc and C22-sc. Our first-principles calculations show that C20-sc, C21-sc and C22-sc are all dynamically and theomodynamically stable. In view of the good match in lattice constants and XRD patterns, we believe that these three cubic carbon phases co-exist in the experimental sample. Especially, C21-sc possessing both superhard and conductive properties can be considered as a muitifunctional material for applications in extreme conditions. To search for proper silicon materials for solar cell application, we applied five of our propsosed carbon allotropes to silicon and investigated their structures, stabilities, electronic and optical properties. Our results indicate that they are direct or quasi-direct semiconductors with mezzo gaps for solar light absorption and possess good dynamic and theomodynamic stabilities. Especially, M585-silicon, the only direct band gap semiconductor, with only 25 me V/atom higher than dimond silicon, is promissing to be synthesized in the future for solar cell application.3. For the purpose of searching for new carbon allotropes with special topological characteristics, we firstly proposed a nano-stitching method to modify graphene bilayers and obtained four low energy all sp2 graphene allotropes, namely “wormhole graphene” structures. Based on first-principles calculations, we found that all of these wormhole graphene allotropes are themodynamically more stable than graphdiyne and dynamically stable. Similar to carbon nanotubes and fullerences, these graphene allotropes are expected to act as two-dimensional periodic nano-capsules for encapsulating various magnetic atoms or functional clusters in design of functional carbon-based materials. Then, we turned our attentions to systematically search for potential three-dimensional all sp2 carbon allotropes and proposed a general method for such a purpose. We found about 100 three-dimensional all sp2 carbon allotropes and most of them were not proposed before. Due to the limitation of our computational resource, we took two of them(227-96 g and 24-24i) as carbon crystals and investigated their structures, stabilities and electronic properties. Our results indicate that 227-96 g and 24-24 i are dynamically stable and energetically more favorable than the experimentally synthesized graphdiyne.4. To understand the 12-fold diffraction pattern in graphite and the 5-fold twinned crystals discovered in carbon allotropes, we firstly investigated many possible stacking forms of graphite and proposed two new graphite allotropes AT-21.8 and AT-30.27 that can succesfully explain the 12-fold diffraction pattern. Furthermore, a new hexagonal graphite with-AB’C’- stacking manner was proposed, which is very stable and similar to that of the-ABC- stacked 3R-graphite. Finally, we introduced a general method to theoretically construct arbitrary n-fold twinned crystals and take FCC136 and Diamond as examples to construct 5-fold ones to understand the experimental observation of the 5-fold twinned crystals.