Theoretical Study On Sensitivity Of The Doped-Carbon Nanotube

The discovery of C₆₀ molecule by Kroto et al in 1985 and subsequent synthesis of carbon nanotubes by Iijima in 1991 have opened new research opportunities in science, engineering and technology. From then on, semiconductor clusters, in particular, based on silicon, carbon and boron, have been received great attention owing to their novel qualities in electronics, optics, calorifics, even biology.Up to date, more and more semiconductor clusters have been synthesized successfully, but the information about the geometries and electronic qualities can't be fully obtained from the experiments, while theoretical methods become the most effective approach to study clusters. So in this paper, we investigate the reactions of the two-membered Si-O ring with CF_n(n=1-3) radicals by using density functional theory calculations and obtain the activation energies, reaction heats, and details of the potential energy surfaces for these reactions. On the other hand, the special qualities for larger nanocluster materials, such as carbon nanotubes (CNT) doped by Si or N atom as gas sensers in order to direct experiments, have been studied.We have obtained plentiful and substantial conclusions which are listed as follows:1. The reactions of the two-membered Si-O ring with CFn (n=1-3) radicals have been studied using density functional theory calculations at the UB3LYP/6-31G(d) level. Calculated results show that these reactions proceed via either the mechanism without C-F bond break or the mechanism with both the C-F and Si-O bonds breaks. The activation energies, reaction heats, and details of the potential energy surfaces for these reactions have been obtained. CF₂ radical was found to be the most effective etchant to Si-O bond, which is in good agreement with the corresponding experimental finding.2. The reactivities of the pristine and silicon doped (Si-doped) single-walled carbon nanotubes (CNTs) toward small gaseous molecules in the atmosphere, such as formaldehyde, carbon monoxide, and hydrogen sulfide, were studied by performing density functional theory calculations. Compared with the physisorptions on the pristine (8, 0) CNT, these small molecules present strong chemical interactions with the Si-doped (8, 0) tube. Doping intrinsic CNTs with silicon is expected to be a potential strategy for improving the property of pristine CNTs and expanding the application of CNTs in nanoscience and nanotechnology.SWCNTs as chemical sensors have attracted strong interests due to their high sensitivity and fast response time towards gas molecules. However, it is a pity that a host of toxic gaseous molecules (e.g., formaldehyde) cannot be detected using the intrinsic CNT devices, since they can not adsorb on the surface of SWCNTs. So considerable experimental and theoretical work has focused on improving the sensing performance of CNTs to various desired molecules by doping or functionalizing CNTs. To this end, we studied the reactivities of H₂CO, CO or H₂S towards both intrinsic CNTs and Si-doped CNTs using Dmol³ program. The results from the adsorption geometric structures and the binding energies as well as charge transfer indicate the intrinsic SWCNT is not sensitive to H₂CO, CO and H₂S molecules, while the Si-doped SWCNTs present strong reactivity. For Si-doped CNTs, we found that new bond are formed between the Si-CNT and H₂CO/H₂S molecule, and that the band gaps of DOSs near Fermi level disappear, which prove the conductance change of the Si-doped nanotubes after the absorption of H₂S molecules. These results are expected to provide a useful guidance for the potential applications of CNT-based materials in nanoscience and nanotechnology.3. We also studied the sensitivity of N atom-doped CNT to BH₃, and proposed that N-doped CNT can serve as the BH₃ sensor.We studied the effect of the doping of the N atom on CNT and the adsorption of BH3. The results show the doping of N atom results in the electoral structure change and the narrower band gap. Comparing with the intrinsic CNT, the electronic properties and geometric structures of the N-doped CNTs present dramatic changes after the absorption of BH3 molecule; additionally, the E_b increases about 0.1 eV. The calculated DOS and Q_T also show N-doped SWCNT changes to a conductor after the adsorption of BH₃, thus the N-doped CNTs are expected to be a potential candidate for detecting the presence of BH₃.