中文摘要：

采用密度泛函理论计算系统研究了单壁碳纳米管（Single-walled carbon nanotube,SWCNT）改变手性外延生长（手性指数从（n,m）变化到（n±Δ,m±Δ）,其中Δ=1和2）的热力学过程。结果表明,碳管手性变化后外延生长在热力学上都需要吸收能量,其所需吸收的能量随着管径的减小线性减小。在Δ=1的情况下,由于近扶手椅型碳管改变手性时,所引入的5～7元环对与管轴的夹角比近锯齿型碳管更大,导致5～7元环对的形成能增加,使得管径相同的近扶手椅型碳管比近锯齿型碳管在改变手性生长时需要吸收更多的能量。在Δ=2的情况下,发现只有当两个必须引入的5～7元环对相互毗邻,手性改变的外延生长所需能量最小,预测其为实验上最易于实现的碳管手性指数由（n,m）变化到（n±Δ,m±Δ）的外延生长模式。这些理论研究结果有助于深入理解SWCNTs手性变化后外延生长的热力学行为,可为基于外延生长可控制备单一手性SWCNTs提供理论依据。

英文摘要：

The energetic of the change in the chirality of single-wall carbon nanotubes（SWCNTs） during epitaxial growth from（n,m） to（n ±△,m△）（△ = 1,2） was investigated by density functional theory calculations.The calculated energies for changing the chirality of different SWCNTs show a nearly linear decrease with decreasing tube diameter.In the case of △ = 1,more energy input is needed for near armchair（nAC） SWCNTs to change their chiralities than those for near zigzag（nZZ） SWCNTs with comparable diameters,due to the larger formation energies of pentagon-heptagon defects（5,7-defects） introduced in the nAC-SWCNTs.These larger formation energies for the nAC-SWCNTs come from the larger angles between the orientation of a 5,7-defect and the tube axis than those for nZZ-SWCNTs.The topological connection oftwo adjacent 5,7-defects,which is indispensable for changing the chirality during growth in the case of △ = 2,is found to be energetically most stable.The energies needed to change chirality in the case of △ =2 are calculated to be less than twice those in the case of △ = 1 for SWCNTs with comparable diameters.These results may help us understand the change in chirality during the epitaxial growth of SWCNTs and guide the future synthesis of SWCNTs with a single-chirality.