Influence of the Atomic Structure on the Raman Spectra of Graphite Edges

A study of step edges in graphite with different atomic structures combining Raman spectroscopy and scanning probe microscopy is presented. The orientation of the carbon hexagons with respect to the edge axis, in the so-called armchair or zigzag arrangements, is distinguished spectroscopically by the intensity of a disorder-induced Raman peak. This effect is explained by applying the double resonance theory to a semi-infinite graphite crystal and by considering the one-dimensional character of the defect.

Figure 1

Raman spectra obtained in three different regions of the HOPG sample. The spectra were taken at room temperature using a JY Horiba LabRam 800 monochromator system. The laser power density on the sample was $3\times {10}^5\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ and the laser energy was 1.96 eV. The inset shows an optical image of the step and the regions where spectra $\mathbf{1}$, $\mathbf{2}$, and $\mathbf{3}$ were taken (open circles).

Figure 2

(a) AFM image of the step on the HOPG substrate where Raman spectra shown in Fig. 1 were taken. (b) AFM image of edge $\mathbf{2}$. (c) Atomic resolution STM image of the region marked by the white $\mathbf{X}$ in (b). The STM measurements were performed under ambient conditions in the constant-height mode. (d) FFT filtered image from the region marked by a white square in (c).

Figure 3

(a) Representation of a double resonance Raman process that gives rise to the $D$ band in the Stokes spectra of disordered graphite materials [3, 6]. (b) Schematic illustration of the atomic structure of the step shown in Figs. 1, 2. (c) First Brillouin zone of 2D graphite, showing the double resonant mechanism for an armchair graphite edge and the density of phonons (inset) associated with this process.

Figure 4

(a) Dependence of the $D$ band intensity on the polarization direction of incident and scattered light. The spectra were taken in region $\mathbf{1}$ (armchair edge) at room temperature using a Dilor XY triple-monochromator system. The laser power density on the sample was $3\times {10}^5\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ and the laser energy was 2.41 eV. The inset shows a schematic illustration of the optical anisotropy around the $K$ point of 2D graphite, where $\theta$ is the angle between light polarization $\overrightarrow{P}$ and the armchair edge direction.