Defect-induced Electronic Structures Of Low-dimensional Carbon Materials

In molecular systems, if the electronic spin at the Fermi surface arenon-equilibrium, the phenomenon is called spin polarization effect, which is presentin the system with more than one unpaired electron. It is known that electron pairingcan reduce the energy of the system, but for high Z elements (such as actinides orlanthanides) or defective structures in low-dimensional carbon material, the situationmay become complicated. Delocalization effect of d or f electrons of heavy elementsmaking the contribution of electron spin parallel to system stabilization is greater thanthat of electron pairing. This process subsequently makes the system more stablewhen there are more unpaired electrons. This is an important source of exoticelectronic structures of molecular systems containing actinides and lanthanides, andalso the challenge of theoretical research. Defects or doping can introduce danglingbonds, thus result in the intrinsic spin polarization of carbon nanotubes, it is alsoconsidered to be an important mechanism for achieving one-dimensional spintronicdevices. In this thesis, we used the first-principles density functional theory to studythe property of single-walled carbon nanotubes with defects, influence of electronicextended basis functions on spin polarization of carbon adsorption defects, as well asthe interactions between single-walled carbon nanotubes with different curvature anduranium atoms.First, taking capped (5,5) carbon nanotubes as examples, we investigated effectsof four typical defects (namely, V1, V2, V3, and V4) on conformation, vibrationalmodes, and electronic structures, by using density functional theory. It is found thatthe effects on the highest peak in infrared spectra caused by different defects areconsistent, and effects on the highest peak in Raman spectra, radial mode, and breathing mode follow the same trend, namely, red shift. There are two infrared-activemodes localizing at V3and V4defects. In contrast, the influence of the defects on theelectronic structures is considerably case-dependent, with the case of V2being veryspecial, involving spin polarization and showing a localized feature. The defects leadto energy gap change up to0.1eV. By analyzing the density of states of the capped (5,5) carbon nanotube and its four defective structures, we found that at there is acharacteristic peak around-7eV which belongs to the perfect carbon nanotube.Deeper analysis of bond characteristic indicated that this peak attributes to thedelocalization contribution of p electrons of all carbon atoms. This result could befacilitate important applications for defect detections, and our conclusion is helpfulfor understanding the effects of defects on the properties of general single-walledcarbon nanotubes.Since molecular systems coming with magnetic properties are drawing attentionfrom more and more researchers, spin-polarization in molecular systems has becomean important issue, especially since the discovery of spin-polarization in carbon-basedsystems. Due to the limitations of current computational methods, related theoreticalstudies on p-electron spin-polarization of molecular systems are still debated. Withinthe spin-polarized local density approximation of density functional theory, wecompared the effects of7different basis sets (3-21G,4-31G,4-31G*,6-31G,6-31G*,6-311G,6-311G*) on the electronic states and spin orbital polarization of a (5,5)carbon nanotube containing single or two adsorbed carbon atoms as defects. What issurprising is, for the single-defect cases, using five double-splitting basis sets, thepredicted ground state results were all the same: triplet state. However, for those withtwo defects, the basis sets without spin-polarization showed the quintet was theground states, and gave the triplet state as the basis sets with spin-polarization. Thetriple-splitting basis set also reflected the same effect. Further analysis on theelectronic structures verified that this result was due to spin-polarization, whichmagnified the2s orbitals and reduced2p orbitals’ contributions from the adsorbedcarbon atoms, thereby making the energy levels between electronic states of thesystem subsequently decrease or even reverse.The adsorption between U atoms and carbon nanotubes is one of the basicproblems on the interaction of actinide elements with carbon material, which hasimportant potential applications on cleaning up nuclear waste and even the extractionof actinide materials. In this paper we clearly illustrated the interactions of inner and outer surfaces of single-walled carbon nanotube with adsorbed U atoms byfirst-principles density functional theory. The results showed that, when U atoms wereadsorbed on the carbon nanotube internal and external surfaces, both groundelectronic states were quintet but with different stable adsorption sites, namely bridgesite and hole site, respectively. The interior adsorption was more stable with theadsorption energy about1eV greater than that of the external adsorption. Theseresults can be explained by the different ground state electronic structures of these twokinds of adsorption. The electronic states of external adsorbed structure weredetermined mainly by the U atoms. On the contrary, for the electronic states of theinternal adsorbed structure, the ferromagnetic coupling between U atoms and the netspintronics on the carbon nanotube played a more important role. Compared to theexternal adsorption, there was more than1.0e charge transfer between the internalabsorbed U atom and the carbon nanotube, which facilitated more obvious donor-acceptor interactions. Further analysis on the density of states indicated that theinternal adsorption had larger bonding area between U atoms and the carbon nanotube,which also supported that the internal adsorption was more stable. This work hasprovided an important reference to the study on adsorption properties and interactionmechanism of actinide materials.We further studied the effects of curvature on the interaction between surfaces ofarmchair single-walled carbon nanotube and adsorbed U atom. The results showedthat the ground electronic states of these internal adsorption systems are all quintet,but with different stable adsorption sites. In the case of carbon nanotubes with largercurvature such as (5,5) and (6,6), the stable adsorption sites were bridge site. For thecase of smaller curvature such as (7,7) and (8,8), hole sites were the most stable. Thesmaller the curvature, the lower the interaction energy and bond order between thecarbon nanotube and U atom, indicating weaker interactions. Further analysis on theelectronic structures indicated that, as the carbon nanotube diameter increases, the roleof electrostatic and covalent interaction between the carbon nanotube and U atomdecreases, and the sources of the contribution of ground electronic states were morelocalized on the U atom. This work may provide a way to control the adsorption between carbon nanotubes and actinide atoms through adjusting the curvature of thecarbon nanotubes.