Exciton Effects in Carbon Nanotube

• Main Authors: Chun-I Chang, 張鈞奕
• Other Authors: To-Sing Li
• Format: Others
• Language: zh-TW
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/yj5246

碩士 === 崑山科技大學 === 電機工程研究所 === 93 === This thesis mainly studies the electronic properties and exciton effects of the carbon nanotube, including the effects of radius, stress, and magnetic field. In the thesis, the energy dispersion relation is calculated by the tight-binding method, and the electron effective mass is obtained. The exciton binding energy is evaluated by means of the variational method. From the simulation results, the energy gap of the nanotube is inversely proportional to the radius. When the tube radius is large, the Coulomb interaction between the electron and the hole is weak because of the large distance which causes the exciton binding energy to be weak. Consequently, the exciton binding energy decreases with increasing radius. Since stress alters the positions of the carbon atoms, it is necessary to recalculate the transfer integral, and the band structure and exciton binding energy are obtained. When stress increases, the conductivity of the nanotube is changed from semiconductor into metal. The nanotube, the conductivity is changed from semiconductor into metal when stress decreases. For nanotube, the conductivity is changed from metal into semiconductor. When energy gap is smaller than the binding energy, exciton state does not exist. Consider a uniform magnetic field parallel to the tube axis, and it changes the angular momentum from to . For metallic nanotube, the energy gap is proportional to the magnetic flux, while the semiconducting nanotube is inversely proportional to the magnetic flux. Consequently, when magnetic flux increases, the energy gap of the semiconducting nanotube is changed regularly. For semiconducting nanotube, when the magnetic flux is large, and the energy gap is smaller than the binding energy, therefore the exciton state does not exist. As to the metallic nanotube, when the magnetic flux is weak, the exciton state does not exist.