Photoluminescence spectrum using DFT for double-walled carbon nanotubes with metallic constituents

A theoretical study of the photoluminescence (PL) of double-walled carbon nanotubes (DWCNTs) using density functional theory (DFT) theory is reported in this work. The DWCNTs are of the armchair/armchair type and the structures studied have the arrangements (3,3)/(2,2), (8,8)/(4,4), (12,12)/(6,6), (16,16)/(8,8), (6,6)/(3,3), (10,10)/(5,5), (14,14)/(7,7), and (18,18)/(9,9). The PL spectra were obtained taking into account different DWCNT axial lengths ranging from 0.49 nm ≤ L ≤ 2.33 nm and their inner nanotube diameters in the range of 0.31 ≤ D_inn ≤ 1.22 nm; variations in their inter-wall separations were also considered, 0.18 ≤ D_inw ≤ 0.61 nm. Although the DWCNTs have metallic SWCNT constituents, such structures give rise to photoluminescence due mainly to both curvature effects and inter-wall interaction of the inner and outer nanotubes; these two factors modify significantly their electronic structure; besides, they also lead to these structures to exhibit the quenching effect. We realized calculations at a DFT level in which we used the generalized gradient approximation (GGA) to establish the molecular geometries and the fundamental state energies. To obtain the results of the PL spectra, the constituent SWCNTs were optimized in their ground state, with the hybrid function CAM-B3LYP, which is a mixed functional exchange and correlation, and the base set that was used is the 6-31G.