Edge phonon state of mono- and few-layer graphene nanoribbons observed by surface and interference co-enhanced Raman spectroscopy

Using surface and interference co-enhanced Raman scattering measurements, we detected two well-distinguished Raman bands at 1450 and $1530{\text{cm}}^{-1}$ from individual mono- and few-layer graphene nanoribbons (GNRs) prepared by chemical exfoliation and mechanical cleavage of graphite. The intensities of these two peaks strongly depend on the width and edge structure of the GNRs. Combining with first-principles calculations, the 1450 and $1530{\text{cm}}^{-1}$ Raman bands are assigned to the localized vibration of the edge atoms of zigzag and armchair GNRs terminated with H atoms, respectively. In addition, two weak peaks at $\sim 1140$ and $1210{\text{cm}}^{-1}$ are also observed, which are coupled with 1450 and $1530{\text{cm}}^{-1}$, respectively. These findings enrich the understanding on the fine structure of mono- and few-layer GNRs by Raman spectroscopy.

Figure 1

(Color online) (a) AFM image, (b) TEM image, and (c) Raman spectra of chemically derived mono-layer GNRs. The GNRs in (a) are denoted by white arrows. The inset of (a) shows the cross section taken along the black line (blue line online), indicating that the thickness and width of the measured GNR are $\sim 0.9$ and $\sim 20\text{nm}$, respectively. The inset of (c) shows the Raman spectra of three individual GNRs.

Figure 2

(Color online) (a) Optical image and (b) the corresponding AFM image of cleaved few-layer graphene and GNRs. Bottom of the left panel shows their corresponding topographic profiles. (c) $G$-band intensity normalized Raman spectra of few-layer GNRs located at A, B, and C, with the two characteristic vibration modes at 1450 and $1530{\text{cm}}^{-1}$ denoted by ${}^{\ast }$. (d) The intensity ratio of 1530 and $1450{\text{cm}}^{-1}$ to $G$ band as a function of reciprocal GNR width.

Figure 3

(Color online) The influence of the number of layers of GNRs on their Raman spectra, with the two characteristic vibration modes at 1450 and $1530{\text{cm}}^{-1}$ denoted by ${}^{\ast }$. The corresponding numbers of layers are $\sim 3$, 6, 7, and 10 from the bottom to top, based on AFM measurements.

Figure 4

(Color online) (a) AFM image of an irregular few-layer GNR, which exhibits different edge orientations at different regions. $G$-band intensity normalized Raman spectra of (b) different positions for the same GNR in (a) and of (c) different few-layer GNRs. ${}^{\ast }$ denotes the two characteristic vibration modes at 1450 and $1530{\text{cm}}^{-1}$. (d) $I_{1530}/I_{1450}$ vs $I_{\text{D}}/I_{1450}$ for the Raman spectra in (b) and (c).

Figure 5

Calculated edge phonon modes of (a) H-free 7-ZGNR, (b) H-terminated 7-ZGNR, (c) H-free 7-AGNR, and (d) H-terminated 7-AGNR. The number before the AGNR and ZGNR denotes the width of GNRs, which corresponds to the number of dimer lines across the ribbon width for AGNR and the number of zigzag chains across the ribbon width for ZGNR.

Figure 6

(Color online) (a) Raman spectra of few-layer GNRs with different intensity ratios of 1450 to $1530{\text{cm}}^{-1}$. The inset shows the enlarged spectra in the range of $1100–1300{\text{cm}}^{-1}$. (b) Raman spectra of mono-layer GNRs with different intensity ratios of 1450 to $1530{\text{cm}}^{-1}$ on Si/300 nm ${\text{SiO}}_2$ substrate. The dashed lines are used to show the positions of 1143 and $1212{\text{cm}}^{-1}$ peaks.

Figure 7

(Color online) Raman spectra of few-layer GNRs excited by laser energies of 1.58, 1.96, and 2.41 eV.