The Theoretical Study Of Several Carbon Cluster Solids

Since fullerenes were successfully achieved in experiments, extensive research attentions have been paid to many kinds of exohedral fullerenes, heterofullerenes, endohedral fullerenes and the fullerene solids, as well as their novel properties. In this dissertation, carbon cluster solids are constructed based on several fullerenes or carbon clusters. Their electronic structures, mechanical, vibrational and thermodynamic properties are systematically investigated, and some interesting results are obtained. The dissertation consists of five chapters.In chapter1, we first introduce the study of fullerenes and their derivatives, and then review the researches on the fullerene solids.In the second chapter, we introduce density functional theory which is widely used in theoretical calculations, and then describe the method of constructing carbon cluser solids basing on carbon clusters.In the third chapter, using carbon tetrahedron and single carbon atom, we construct four kinds of C4solids. Basing on the first principles calculations, we find their cohesive energies, mass densities, bulk moduli, mechanical and electronic properties are essentially related with the components of the C4units in these solids. With the increase of the component of the C4units in these solids, the cohesive energy increases, and the mass density, the bulk modulus and the band gap decrease. This provides a new way for regulating the mechanical and electronic properties of these systems by changing the component of the C4units. In the obtained Raman spectra for these allotropes, some typical Raman active modes displaying evident vibrational features of C4units are revealed. This may provide a valuable reference for experiments to detect these new solids. By doping B, N or Si into the most stable solid, we find that the B-doped system can either transforms into conductor or keep the feature of the semiconductor, depending on the component of B. In the B-doped semi-conductive system, some impurity levels appear in the band gap. For N-doped systems, we find not only some impurity levels appear in the band gap, but also the N-doped systems exhibit magnetic properties. Compared to that of the perfect system, the band gaps of the Si-doped systems increase a little.In the fourth chapter, we propose four kinds of C58solids with C58(C3v) isomer. The bulk moduli of these solids are smaller than that of diamond, but their specific heat at constant volume are higher than that of diamond within the temperature range from0to1000K. In addition, the former three solids display magnetic features and the local magnetic moment mainly distributes the first-neighbor of the atoms locating on the edges of adjacent pentagons. The calculated formation energies and electronic structures of these solids reveal that the most stable solid (T-I) is a semiconductor with a direct band gap of0.12eV, and the other solids display the conductive feature. The Raman spectra analysis indicates that the typical Raman active modes of the isolated C58still remain in the Raman spectra of T-I, but more Raman active modes are found. Applying strain on T-I, we find that T-I can transform from semiconductor into metal. In the case of tensile strain, the semiconductor-metal transformation occurs, accompanying with structural transformation. When the tensile strain reaches a certain value, the structure of T-I converts into grapheme-like structure consisting of C58cages. Moreover, we find the band gap of T-I can be regulated by doping H, F or Cl in T-I. The Li-doped T-I can convert from the semiconductor to metal. For the grapheme-like structure consisting of C58, the common defects, vacancies energetically prefer to locate at the sites with the largest pyramidalization angle.In chapter5, we construct three kinds of C50solids with C50(D5h). The calculations show that these solids are softer than the diamond. Two of them are semiconductors with indirect band gaps of0.055and0.338eV and the other one is metal. The formation processes of all these solids are endothermic. The phonon spectra calculations confirm that these solids are virtually stable, indicating that they are new metastable configurations of carbon. Within the temperature range from0to1000K, their specific heat at constant volume differs little, being similar to that of T-I (C58solid) and higher than that of diamond. In order to provide some evidence for the experiment, we also simulate the X-ray diffraction spectra. Doping B and N in the most stable solid respectively, we find that the B-doped system transforms into metal and the N-doped system still remains semiconductor, the band gap of which reaches0.469eV. The formation energies of the B-doped and N-doped systems are-1.152and-1.090eV/cage respectively. Moreover, the lattice vectors of the B-doped and N-doped systems differ little from those of the perfect system. This makes it possible that if the doped system and the perfect system connect along some crystal orientations, the interface can form metal-semiconductor heteroj unction or semiconductor heterojunction. When the cages contain Mg or Ca atoms, the doped systems are both metal, and their formation energies are3.236and0.943eV/cage respectively. Similar to those of the B-doped and N-doped systems, the lattice vectors of the Mg-doped and Ca-doped systems differ little from those of the perfect system, which also makes metal-semiconductor heterojunction possible.