Theoretical Chemistry Study Of The Structures And Nonlinear Optical Properties Of Modified Carbon/Boron Nitride Nanotubes

Carbon nanotubes(CNTs) and boron nitride nanotubes(BNNTs) have been proposed as novel nanomaterials in a wide variety of applications because their unique physical and chemical properties were shown in many fields. With the coming of information age, design and development of nonlinear optical nano-materials is very important for optical communication, optical computer and optical process of information. Quantum chemistry is the theoretical basis for investigating chemical problems. After a long time development, quantum chemistry has already become a widely used theoretical method to understanding mechanism of functional materials. In this context, understanding the relationship between structures and properties by quantum chemical methods is very important to explore new high-performance nonlinear optical materials. It is our expectation that, new strategies could be found to adjust the nonlinear optical properties of nanotubes by theoretical investigations. Besides, on the basis of the new perspective, we can establish effective relationships between structures and properties of funcational materials, which could provide useful information for further design and preparation of nanoscale nonlinear optical materials.In the present thesis, we mainly investigate several important basic problems for understanding the structures and nonlinear optical properties of modified carbon/boron nitride nanotubes. Especially, we intensively study the covalent modification of CNTs and the non-covalent interaction between lithium atom and BNNTs. The thesis has been focus on the following five sections:1. A series of isoelectronic models were systematically investigated to explore the crucial factors for enhancing the static first hyperpolarizibility by doping the boron nitride segment into the CNT with differently connecting patterns. Density functional theory and time-dependent density functional theory studies show that the nitride-connecting pattern might increase the contribution of the boron nitride segment to the crucial transition states, which obviously increases the occupied orbital energy while the unoccupied orbital energy is slightly influenced. Correspondingly, the transition energy of nitride-connecting pattern model is smaller than that of boron-connecting pattern model. As a result, the static first hyperpolarizability of nitride-connecting pattern model is remarkably larger. The results indicate that, the N-connecting pattern of the BN segment linking to the conjugated CNT segment is a more efficient way to enhance the first hyperpolarizability of heteronanotubes.2. To further understand the modified mechanism of boron nitride segment doped CNTs, we have investigated the structure and first hyperpolarizability of a series of HA-n and isoelectronic PA-n heteronanotubes, in which HA-n is composed by a helical C-segment and BN-segment, while PA-n is composed by an arc C-segment and BN-segment. Interestingly, results show that the first hyperpolarizability of HA-n models significantly depends on the length of the tube, whereas that of PA-n shows a slightly increasing tendency. It indicates that the length of the heteronanotube is a key factor in inducing the helix topological effect. Further investigations indicate that the helix C-segment in HA-n leads to a fascinating long-range charge transfer character, which is the key factor cause the difference. It is our expectation that the new knowledge about heteronanotubes might provide more ideas for further exploring sp2-hybridized carbon segments with special topologies.3. On the basis of the above investigation, we study the lithiation effect on carbon-boron-nitride heterojunction nanotubes to further reveal the mechanism of modification by a quantum chemical investigation. Interestingly, the lithiation effect is significantly dependent on the activating segment of the heterojunction nanotubes and it is better to activating C-segment. For lithiation on the BN-segment, the nitride-connecting pattern of the boron nitride segment linking to the C-segment is a more efficient way. However, for lithiation on the C-segment, the boron-connecting pattern is also very important. Further investigations indicate that the large first hyperpolarizability originated from the charge transfer is from the C-segment to the lithium atoms in the models with lithiation on the carbon-segment.4. The non-covalent interaction between lithium atom and BNNT has been investigated by quantum chemical methods. Results show that the 2s electron of lithium atom could be effectively bound by the B atoms of BNNTs to form interesting excess electron analogues. It is worthy of note that the excess electron is mainly distributed at the B-rich edge in Li@B-BNNT. However, the distribution of excess electron in Li@N-BNNT is more diffuse and pyramidal from B-rich edge to N-rich edge. Correspondingly, the transition energy of Li@N-BNNT is obviously smaller than that of Li@B-BNNT. As results, the first hyperpolarizability of Li@N-BNNT is dramatically larger than that of Li@B-BNNT.5. On the basis of the above investigation, we study the first hyperpolarizability and stability of complexes formed by BNNT encapsulated linear excess electron compounds. Quantum chemistry investigations show the BNNT works as a protective shield to enhance the stability of the unstable linear excess electron compound. The vertical ionization potential of the excess electron compound is increased after encapsulating, which is more difficult to be oxidized. Significantly, the encapsulated complex exhibits a considerable first hyperpolarizability, which is dramatically larger than that of BNNT. Furthermore, it is easy to encapsulate from the B-rich edge rather than N-rich edge of BNNT due to its lower energy barrier. We expect that the knowledge would provide new strategy for the design and synthesis of stable high-performance nonlinear optical materials.