Anisotropy of the Raman Spectra of Nanographite Ribbons

A polarized Raman study of nanographite ribbons on a highly oriented pyrolytic graphite substrate is reported. The Raman peak of the nanographite ribbons exhibits an intensity dependence on the light polarization direction relative to the nanographite ribbon axis. This result is due to the quantum confinement of the electrons in the 1D band structure of the nanographite ribbons, combined with the anisotropy of the light absorption in 2D graphite, in agreement with theoretical predictions.

Figure 1

(a) AFM image of many ribbons parallel to each other. (b) AFM image of a nanographite ribbon near a step. The arrow indicates the position of the ribbon for reference. (c) The height profile obtained through the dotted line in (b).

Figure 2

(a) Raman spectra obtained for light incident with different polarization angles ($\theta$) with respect to the ribbon direction. The inset shows a schematic figure of the sample (horizontal gray line) showing the direction between the ribbon axis and the light polarization vector ($\overrightarrow{P}$). (b) Intensity of the $G_1$ peak versus $\theta$. The dotted line is a ${cos}^2\theta$ theoretical curve. The error bars are associated with baseline corrections. (c) Raman frequencies of the $G_2$ (triangles) and $G_1$ (squares) peaks as a function of the laser power density.

Figure 3

(a) A zigzag nanographite ribbon network with $N=8$. $\overrightarrow{L}$ and $\overrightarrow{T}$ are the longitudinal and transverse vectors that define the unit cell. (b) The reciprocal lattice of zigzag nanographite ribbon shown in (a). ${\overrightarrow{K}}_L$ and ${\overrightarrow{K}}_T$ represent reciprocal lattice vectors. (c) The $k$ dependence for the probability of absorption [$W\left(\overrightarrow{k}\right)$] near to a $K$ point, for the four different values of $\theta$. The dark region gives a large absorption coefficient. The arrows indicate the $K_T$ vectors that coincide with a maximum in the optical absorption for $\theta =0°$ and with a node in the optical absorption for $\theta =90°$ [10].