A Confocal Raman-AFM Study of Graphene
Figure 3: Averaged Raman spectrum of the analyzed graphene flake.
Figure 2: Signal intensity along “Cross section 1” marked in Figure 1 obtained from the AFM image (top) and Raman image (bottom).
height differences are in good agreement with previously reported AFM measurements of single and bi-layers of graphene on SiO2/Si substrates [6]. A confocal Raman image was recorded
from the same sample area by acquiring a spectral array of 85 × 50 complete Raman spectra. Te averaged spectrum of all recorded spectra from the array is shown in Figure 3. At low wave numbers the characteristic Raman bands of the SiO2/Si substrate are visible. Te characteristic Raman bands for graphite/graphene are: the D-band (at about 1360 rel. 1/cm) characteristic for the breathing modes of sp2 bonded atoms in rings and the G-band (at about 1580 rel. 1/cm) due to bond stretching of sp2 bonded atoms in chains and rings [7–9]. In graphene two additional Raman bands, D′ (around 2700 rel. 1/cm) and 2D′ (around 3250 rel. 1/cm), are observed, which are both due to second-order resonant phonon Raman scattering [2]. Te image in Figure 1b shows the integrated intensity of the G-band from the 2D spectral array. It can be seen that the integrated intensity of the G-band varies proportionally with the number of graphene sheets [10, 11]. A cross section 1′ along the same sample position as in the topography image is shown in Figure 2 (bottom). Tis image already indicates that Raman microscopy, a fast, non-contact imaging method, can give insight into the composition
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of a graphene flake. A more detailed analysis of the 2D array of spectra can reveal many more details about the structure of graphene. Figure 4 shows the variation of the G-band position (top), D′-band position (middle), and D′-band width as a function of number of graphene layers along cross section 1′ together with the corresponding Raman images. Te Raman images were obtained by a Lorenz fit of the corresponding Raman band. Te position of the G-band decreases with an increasing number of graphene sheets from 1582 rel. 1/cm to 1579 rel. 1/cm due to a slightly higher frequency of the Raman active photon in graphene than in graphite [12]. Te increase of the D′-band
Figure 4: Variation of the position of the G-band (top left), D′-band (middle left), and width of D′-band (bottom left) along “Cross Section 1” marked in Figure 1 without the SiO2/Si substrate and the corresponding Raman images of the analyzed graphene flake (right column).
www.microscopy-today.com • 2011 November
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