Detection of invisible phonon modes in individual defect-free carbon nanotubes by gradient-field Raman scattering

Yang Feng^{1, 3, 4}, Ji Yinglu^{2, ‡}, Zhang Xiao^{1, 4}, Fan Qingxia^{1, 4}, Zhang Nan^{1, 4}, Gu Xiaogang^{1, 3, 4}, Xiao Zhuojian^{1, 3, 4}, Zhang Qiang^{1, 4}, Wang Yanchun^{1, 3}, Wu Xiaochun^{2, 4}, Li Junjie¹, Zhou Weiya^{1, 3, 4, †}

(color online) (a) SEM images of as-grown SWNTs, the right enlarged image shows the SWNT, in which the coordinate illustrates the x and y direction defined in this paper. The scale bars are and , respectively. (b) Raman spectra of the SWNT with the laser along the y-axis polarization. (c)–(f) G-band maps and D-band maps of the SWNT excited by 514 nm laser and 785 nm laser, where the double dashed lines denote the position of the SWNT. The scale bars in these maps are 500 nm.