| SWNTs have outstanding mechanical, electrical and opto- electronic properties. Since SWNTs already outperform classical materials such as organic polymers and semiconductors, they have attracted increasing attention from people. SWNTs research today has sprung to an astonishing scale and is truly multi- and inter- discinplinary: engineers are developing next-generation composites, elec-tronic devices, and adsorbents based on nanotubes; chemists are exploring nanotubes as containers for molecules and ions and as nanoscale reactors; biologists see nanotubes as potential shuttles for organ-selective drug delivery and other therapeutic and diagnostic purposes. No matter we focus on the fundamental or the applied research of SWNTs,we have to face the interactions of SWNTs with their environment. They can be classified into two categories: covalent and noncovalent interaction. In contrast to covalent interaction, the noncovalent interaction can functionalize the SWNTs without destroying their geometric and electronic structures, so many recent works have focus on the noncovalent functionalization of SWNTs. The first-principle study of noncovalent functionalization of SWNTs is important and time consuming, because the system usually contains over a hundred atoms. For a deeper insight into the noncovalent interactions of SWNTs with molecules, a series of complexes between aromatic organic molecule and SWNT has been extensively investigated with a LDA type of DFT. Moreover, the effects of external electric field on the SWNT complexes were studied. At last, we proposed a method to construct a SWNT-based molecular rectifier.The results were as follows:Firstly, we showed that intermolecular charge transfer occurs between DDQ/DDQp/BN/PBN and SWNTs. The amount of transferred charge is closely related to the electron affinities and ionization potentials of the aromatic molecules, it also affects the binding energies between absorbed molecules and the SWNTs. Although DDQ and DDQp have very different molecular dipoles, this kind of difference does not influence the CT phenomenon and the system binding energy significantly. DDQ doped SWNT(10,0) exhibits the electrical properties of conductors, whereas DDQp doped SWNT(10,0) belongs to p-type semi-conductor. The binding energy of BN and PBN absorbed on SWNTs differs greatly. We also found BN/PBN doped SWNT(10,0) shows two very different band structures and DOS patterns: the BN-SWNT(10,0) shows the characteristics of n-type semi-conductors whereas the PBN-SWNT(10,0) has the characteristics of p-type semi-conductors. Secondly, we discovered that the binding energy of the DDQ/DDQp-SWNT complexes varies under different external electric field. The absorbed molecules will take a preferred orientation on the surface of SWNTs when the complexes were exposed to an external electric field. The CT process between absorbed molecules and SWNTs is very sensitive to the existence of electric field. When DDQ-SWNT(10,0) was exposed to different external electric field, it always behave like conductors. Whereas the DDQp-SWNT(10,0) will behave like conductors or p-type semi-conductors responding to different external electric fields.At last, we investigated a series of DDQp-doped SWNT(10,0) with different doping concentration. Along with a higher DDQp doping concentration, the hybrid phenomenon of DDQp's HOMO orbitals and SWNT's valent bands gets more obvious, and the DDQp-SWNT(10,0) still behave as a p-type semi-conductor with a tiny energy gap(less than 0.2eV). Interestingly, when the concentration of DDQp is low, the lowest LUMO band of DDQp almost lies on the Fermi-level and becomes very flat, whereas the valent band of SWNT(10,0) was below the Fermi-level 0.2eV. Since the electric conductance is proportional to the densities of state, such a DOS pattern of DDQp-SWNT(10,0) implies that the complex concerned above have a non-symmestry I-V curve. In anthor word, the DDQp-SWNT(10,0) may be used as a molecular rectifier. |
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