Ab Initio Study Of Boron-doped Carbon Nanotubes

As a new nano material, the carbon nanotubes (CNTs) have rapidly attracted a lot of attention in materials science because of their unique physical, chemical and structure properties, as well as signification potential applications. Element doping is an effective way to improve the electronic structure and properties for CNTs. Having roughly the same atomic radius as C, B atom is a natural choice of the dopant and can be easily incorporated into the carbon network through many different techniques. In this thesis, using first principle calculations, the effects of doping B atoms on the structure and electronic properties for CNTs are explored.In the fist part, the formation energy of B-doped zigzag (n, 0) single-walled CNTs and theie stability are investigated. The calculated results show that the geometry and properties of zigzag (n, 0) CNTs only depends on the chiral index n, which is also the number of the hexagon which forms the unit of zigzag. According to their electronic structures, zigzag (n, 0) CNTs can be classified into two types, one is metals (n=3k, k is integer) and another is semiconductors (n≠3k, k is integer). It has been reported by Krasheninikov and Pan that the curves of the formation energy vs. diameter for zigzag (n, 0) CNTs are of sawtooth-like shapes due to adsorption and interstice atoms, and such periodicity is characterized by the lower formation energies of defected tubes with n being a multiple of 3 as compared to their neighboring tubes. However, the reasons why the sawtooth-like shapes appear have not been clarified in detail. In this thsis, we have investigated whether the formation energy curves of zigzag CNTs with B substitutional impurities atoms are of the same shape as those with interstice and adsorption, and try to answer above question.During the calculation, the (n, 0) tubes with n = 8-19 were chosen, and three tube units and aΓsampling point in the Brillouin zone were adopted. The configurations of tubes doped with one substitutional B atom are considered. It is found that the formation energy curve for B-doped CNTs exhibits a sawtooth-like feature of periodicity, which is the similar to results for (n, 0) tubes with adsorption and interstice. The periodicity is characterized by the lower formation energies of defected tubes with n being a multiple of 3 as compared to their neighboring tubes, and such variation of sawtooth-like feature becomes gradually weak with increasing tube diameter.In order to reveal the mechanism for this periodicity, we have calculated the average cohesive energies per atom of the perfect (n, 0) tubes. It is found that the cohesive energy curve also exhibits a periodic feature, and such periodicity is characterized by small protuberances at n - 9, 12, 15, 18 in cohesive energy curve as compared to their neighboring tubes. That is to say that the slopes at n = 9, 12, 15, 18 in cohesive energy curve of (n, 0) tube varies abruptly compared to those at other n values. For (n, 0) tube, the values of the cohesive energies can be classified into two types. One is from (n, 0) tubes with n being a multiple of 3, and another from those with the other n values. Consequently, this periodic feature results from the bonding structures of perfect (n, 0) tubes with different diameters, rather than the defects (substitutional impurity atom B) in the tubes. For (n, 0) tube, when n is a multiple of 3, p electrons are more delocalized as compared to other tubes. For (n, 0) tube, p electrons are all gradually delocalized with increasing tube diameter, and the p bonding structures gradually approach to those of graphene. For the B-doped (n, 0) tubes, the protuberances is relatively weak at n=3k (k is integer) in cohesive energy curve. Consequently, the formation energy, the difference between the cohesive energy for perfect zigzag tubes and that for defect zigzag tubes, should exhibit the same periodic features as the p bonding structures of perfect zigzag tubes. The formation energy curve of B doped zigzag tubes only enlarges this periodic feature.It is well known that there is an impurity level, acceptor level, near the top of the valence band when boron atoms are incorporated into semiconducting nanotubes. Consequently, the nanotubes are transformed from intrinsic semiconductors to P-type semiconductors. It has been reported that the formation of BC₃ nanodomains is preferred in the B-doped CNTs theoretically and experimentally when more boron atoms are incorporated into SWCNTs. Theoretically, for pure C and BC₃ compound nanotubes, the critical strain that will trigger the formation of defects will decrease as the diameter of the defect ring increases, and the barrier energy is reduced with increasing tensile strain. However, previous reports have not considered that there are two configurations in the BC₃ SWCNTs, and the unusual electronic properties of BC₃ SWCNTs have not been decribed clearly. For zigzag SWCNTs, Fuentes et al. have mentioned only one configuration which is the same as BC3₂, and Miyamoto et al. have investgate only one configuration which is the same as BC3₁. They are all not aware of the existence of two kinds of BC₃ configurations. In this thesis, (10, 0) tube is chosen as a typical one to investigation, and calculated results exhibit that both BC3₁ and BC3₂ have striking acceptor states above the top of the valence bands. The reason why the configuration BC₃ prefers to localize in the B-doped CNTs has beenexplained.In the second part in this thesis, the field emission properties of B-doped CNTs are investigated using first-principles DFT method. It can be seen from the existing field emission experimental investigations that the field emission properties of B-doped CNTs are very complicated, and up to now there exists two opposing experimental results. Some investigations demonstrate that the doping of boron atom can indeed enhance the electron field emission of CNTs, while other reports indicate that the incorporation of boron atom into the carbon network apparently increases the concentration of electron holes that become electron traps and eventually impedes the electron field emission properties. And these oppositing results exist in theoretical investigations as well. Due to the existence of the opposing results, it is therefore interesting and necessary to understand how the boron atom will affect the field emission properties of CNTs and the intrinsic field emission mechanism.We have calculated the geometrical structures and the field emission properties of B-doped capped (5, 5) CNTs, which are then compared with the field emission properties of pristine and N-doped CNTs. We directly substitute one C atom by one B atom in the hexagonal lattice of capped (5, 5) SWCNT, which can lead to five different doping positions of the doped CNTs. From the calculated results of the heat of formation we can see that the B atom is preferentially located at the CNT tip. Due to the lengthened C-B bond, the doping of boron atom will introduce an outward local structural distortion along the radial direction. Upon boron atom substitution, the work function increases dramatically, indicating that the field emission properties of B-doped CNT are impeded due to this substitution. Through analyzing the changes of the electronic structures of the CNT after substitution, we can see that both the HOMO and LUMO of B-doped CNT are lower than those of pristing CNT and the LUMO decreases more significantly. This will lead to lowering the Fermi level, and the shift of the Fermi level to the valence band, which will increase the potential barrier on the surface of the CNT tip and the work function as well. At the same time, the IPs of the B-doped CNT have also been calculated, and we can see that under the applied electric field the IP of the doped CNT is larger than that of pristine CNT. All the calculation results indicate that the field electron emission of B-doped CNTs is impeded. Moreover, the possible field emission mechanism of B-doped CNTs is discussed.