Theoretical Study On The Effects Of The Field Emission From The Gas Adsorption On The Carbon Nanotubes

Carbon nanotubes (CNTs) are quasi one dimensional structure,curled from the grapheme. CNTs posse large surface areas, unique hollowstructures, excellent electrical, mechanical and thermal properties. Due totheir superior structural and performance advantages, CNTs have beenwidely used in many fields, including gas sensors, field emission, andhydrogen storage. However, properties of CNT devices are influenced bymany external factors, such as metal doping and gas adsorptions. TheCNT properties with different gas atom adsorptions on different sites andunder various coverages need to be further investigated.In this thesis, based on the first principles density functional theory,we investigated the geometrical structures, adsorption energy, workfunction and the density of states for the capped (5,5) armchair singlewall carbon nanotubes (SWCNTs). Major research results are as follows: (1) Adsorptions of single hydrogen, oxygen, and nitrogen atoms ondifferent sites of the CNT surface. The most stable sites for O, N and Hadsorptions are the second layer, the top layer and the bottom layer,respectively. The adsorption stabilities are in the order of O, N, and H.The work functions increase for oxygen atom and nitrogen atomadsorptions, but decreases for hydrogen adsorption. For oxygen andnitrogen adsorptions, the electron contributions to the valence bands aremainly from2S and2p orbits, whereas the contributions to theconduction bands are mainly from the2p orbits. The electroncontributions are all from the s orbital for hydrogen adsorption.(2) Adsorption of different atoms with various coverage for pure andNi doped SWCNTs. For both pure and Ni doped systems, H adsorptioncauses the work function drop, but O and N adsorptions result in theincreases of the work functions. Adsorption of hydrogen atoms lead to theshift of Fermi level towards the conduction bands, but towards thevalence bands with oxygen and nitrogen adsorptions.For pure SWCNTs, the electron contributions to the valence bandsare mainly from s orbits and p orbits of adsorption atoms and carbonatoms. The conduction bands are dominated by s orbital of hydrogenatom, and the p orbits of the oxygen, nitrogen atom, as well as carbonatoms.With nickel dopping, the contribution of nickel atoms are mainly from their3d orbits. The contribution of the valence bands are mainly thes orbit for hydrogen atom, the s and p orbits for the oxygen and nitrogenatoms, and3d orbit for Ni. The conduction bands are mainly dominatesby the p orbits of oxygen and nitrogen atoms.