First-principle Study Of The Effects Of Defects On The Electronic Structures And Transport Properties Of Carbon Nanotubes

Carbon nanotubes (CNTs) have become very promising materials in nanoscience.Because of the quasi-one dimensional structures and unique electric properties, CNTshave a lot of potential applications in di?erent fields, such as ?at-panel displays andnanodevices. In this dissertation, we investigate the electronic structures and transportproperties of carbon nanotubes with adsorbates and defects.Using DMol3 code based on density-functional theory, we calculated the adsorp-tion of the gas molecules CH4, CO and H2O on the tip of CNT under emission con-ditions. Since the gas molecules may be decomposed to radicals under strong electricfield, we also considered the adsorption of the CH3 and OH radicals. From the calcu-lated ionization potential and work function, we found that the H2O and the CH3 haveadvantages to field emission and the CH4, CO and OH have disadvantages. The resultsof H2O and CO are consistent with experiments, and the enhancement of current byCH4 in experiment may be due to the CH3 decomposed from the CH4.The transport properties of carbon nanotubes with a double vacancy under uniaxialstrain are studied. we first use first-principles plane-wave calculation to optimize thestructures of single-walled CNTs with a double vacancy under 0%, 3%, and 6% strains,respectively. Then, we use the single-particle Green function method to calculate theirtransport properties. It is found that di?erent strains cause di?erent local structuresnear the defect, which change the transmission functions around the Fermi energy, andthe conductance tends to be maximized under about 3% strain.Structural properties of carbon nanotubes with a double vacancy are also studiedby using a multiscale hybrid energy density method. Based on the optimized struc-ture, the single-particle Green function method is then used to investigate the transportproperty. It is found that a reconstruction of the structure occurs with an increase ofthe imposed axial force, which alters the transmission function around the Fermi en-ergy and will reduce the current. This reconstruction cannot be found by running a molecular dynamics simulation without a quantum description.We also perform simulations for O2 adsorption on a (5,5) CNT with a double va-cancy. The first-principles plane-wave calculation and single-particle Green functionmethod are used to optimize the structures and to study the transport properties, respec-tively. It is found that an O2 can be either physisorbed or chemisorbed on the defectiveCNT. The physisorption has only minor e?ects on the transport while the chemisorp-tion can improve it. For the chemisorption, the conductance of lateral and verticalconfigurations are also di?erent, which can be explained by considering the di?erentO-O distance which a?ects the energy levels of the oxygen atoms with respect to theFermi energy.