Exfoliated graphene on Si/SiO 2

CLSM can simultaneously produce intensity and topography images, and has a lateral resolution that can be pushed beyond the optical diffraction limit. The depth-of-field is enhanced by digitally selecting in-focus regions from multiple images at different focal planes, enabling high resolution over larger areas. To demonstrate the potential of CLSM for the characterization of graphene, we studied exfoliated graphene transferred to Si/SiO 2 substrates. The OM images of the sample was carried out on a Nikon L200N optical microscope [see Acknowledgments] in the reflection mode using white light. Figure 1a–c are OM, CLSM intensity, and CLSM height images, respectively, of exfoliated graphene flakes on Si substrate covered by 300 nm of SiO 2 . Both the imaging techniques are performed in reflection mode and compared to the corresponding AFM image in Fig. 1d. Each graphene layer absorbs 2.3 % of the incident light37. The same region imaged with the CLSM shows a signal-to-noise ratio three times higher than OM, and provides in situ map of the height. Raman spectra were recorded at the nine different regions of the sample as indicated by red dots in Fig. 1b. For a more in-depth analysis of the layer-dependent optical properties, please see Supplementary Note 1. Analysis of points P1a and P1b matches the description of single LG, where their G-peaks (Fig. 1e) and 2D-peaks can be fitted by a single Lorentzian (FWHM of 26.8 and 28.1 cm−1, respectively) and the height ratios of G/2D peaks are ~0.7 (Fig. 1f). Ni et al.38 also reported that the G-peak height increases linearly with the number of layers up to nine layers. This is in good agreement with data shown in Fig. 1e, except for point P2b, where the unexpected behavior may be due to an overlapping of the laser spot with nearby multi-LG domains (P4b and P5). Using the layer numbers determined from the Raman analysis, we found that the CLSM relative intensity (compared to Si/SiO 2 substrate) also increases approximately linearly with the layer number (Fig. 1g).

Fig. 1 Characterization of exfoliated graphene on Si/SiO 2 by confocal laser scanning microscopy (CLSM), compared to other methods. a Optical microscopy, b CLSM intensity, and c CLSM height images. d Atomic force microscopy (AFM) images of the areas marked in (c). e G-peak intensity and f G/2D-peak intensity ratio. g CLSM relative intensity measured at the red points marked in b, as a function of the graphene layer thickness. h CLSM height measurement corrected with AFM height measurement for 1–5 graphene layers. Raman data were acquired with 514.5 nm excitation. The error bars indicate standard deviation of the measurement Full size image

Fig. 2 Optical microscopy (OM) and confocal laser scanning microscopy (CLSM) imaging of CVD graphene transferred to Si/SiO 2 substrate. a OM and b CLSM intensity images of CVD-grown graphene transferred to Si/SiO 2 substrate, showing tears and scrolls. c Raman spectra for points P1 and P2 in (b). d Raman spectrum for graphene scroll indicated by a blue circle in (b). Raman data was acquired with 514.5 nm excitation Full size image

The thickness of the first graphene layer (P1b) as measured by AFM is 1.35 ± 0.1 nm (Supplementary Table 1), with subsequent layers being measured as 0.48 nm thick (as estimated from the slope of the linear fitting of the raw AFM data in Supplementary Fig. 2). The larger thickness of the first and subsequent graphene layer(s) can be the result of contamination present between the graphene and substrate interfaces39. Although the resolution for CLSM height map is ~10 nm (as specified by the manufacturer), Fig. 1c shows that CLSM is able to distinguish the height of a single layer of graphene. The raw CLSM height values are summarized in Supplementary Table 1 (and Supplementary Note 1), where the first graphene layer is 4.72 ± 0.1 nm, with each subsequent layers being 3.68 nm thick (as estimated from the slope of the linear fitting of the raw CLSM data in Supplementary Fig. 2). Therefore, with proper correction, CLSM can be a fast and reliable method to identify the layer number of exfoliated graphene flakes (Fig. 1h). This is evident by the linearity of the CLSM and theoretical height values.

CVD graphene on Si/SiO 2

Figure 2a, b shows the OM and CLSM images, respectively, of the CVD graphene that has been transferred to Si/SiO 2 substrate. The CLSM image reveals finer structures that are not clearly visible in the OM image, such as the wrinkle at point labeled as P2 in Fig. 2b, which is about 400 nm wide. Although the Raman spectra from both points P1 and P2 display the features characteristic for single LG, there is a blue shift for the G-peak and a red shift for the 2D-peak at P2, which can be attributed to strain from the wrinkle. The Raman spectra of the scroll shows a more pronounced D-peak due to the curvature effect40. Furthermore, the graphene layers in the scroll are not tightly packed, which broadens the 2D-peak. The G/2D peak ratio is also larger due to the presence of several layers of graphene within the scroll.

EG nanoribbons on SiC

We first present the EG sample with partial coverage of graphene, as shown in Fig. 3. The sample is comprised of dense 2D nanoribbons with brighter contrast that are not fully resolved by OM (Fig. 3a), but are better resolved with CLSM (Fig. 3b). Owing to the transparent nature of the SiC substrate, OM imaging is also possible in transmission mode for this particular type of sample (Supplementary Fig. 3), where the contrast is inverted, and the darker regions are associated to graphene with each layer absorbing 2.3 % of the incident light39.

Fig. 3 Graphene nanoribbons on SiC characterized by various methods. a Optical microscopy (OM), b confocal laser scanning microscopy (CLSM) intensity, c atomic force microscopy (AFM), d Kelvin probe force microscopy (KPFM), and e scanning electron microscopy (SEM) with InLens detector, f conductive atomic force microscopy (C-AFM) images. g–h Raman maps obtained for the same area as a–f, showing the area under the g G-peak and h 2D-peak. i Representative Raman spectra for the five regions indicated in (d), where each is an average of 64 individual spectra from the mapped data. Raman data was acquired with 532 nm excitation Full size image

The darker regions in OM and CLSM intensity images are the electrically insulating interfacial layer or bare SiC, as verified with several SPM and Raman techniques (Fig. 3c–g). Imaging in the differential interference contrast (DIC) mode produces a three-dimensional visualization of the surface morphology with some sacrifice of the graphene contrast (for more details, see Supplementary Note 2 and Supplementary Fig. 3). From comparisons of CLSM, DIC, and scanning probe images such as Fig. 3c, d, we find that the graphene nanoribbons preferentially grow from the step edge toward the adjacent upper terrace. The formation of parallel nanoribbons on SiC(0001) terraces has been attributed to diffusion-limited growth25 that may accompany the decomposition of single SiC atomic layers.

The CLSM intensity image of Fig. 3b shows the remarkable level of detail produced with an acquisition time of only 10 s. Compared to the OM image in reflection mode (Fig. 3a), CLSM provides greatly enhanced lateral resolution and contrast due to the point illumination by a laser source (405 nm wavelength) and by removing the out-of-focus background light with a pinhole at the conjugate focal plane in front of the sensor (i.e., typical confocal configuration). The CLSM intensity image not only shows much higher spatial resolution and fully resolved graphene nanoribbons, but also clearly reveals thin stripes (labeled by red arrows in Fig. 3d) and patches of higher reflectivity along the step edges, indicating two-LG (2LG). Moreover, we determine the lateral resolution of the CLSM to be 150 nm by analyzing the edge spread function from EG images (Supplementary Figs. 4 and Supplementary Table 2, all of which are within Supplementary Note 3).

We obtained additional measurements with AFM, KPFM, SEM, C-AFM, and Raman microscopy, on EG nanoribbons for accurate identification of their structure. The CLSM intensity image in Fig. 3b reveals a dense row of graphene nanoribbons formed from 1LG and 2LG. The KPFM map obtained in situ with the topography (Fig. 3c) reveals substantial variations in the surface potential, with five clear distinct regions (Fig. 3d). Region 1 is assigned to the 1LG nanoribbons which vary in width from ~100 to 300 nm and lengths of up to a few micrometers. Region 2 is designated as 2LG nanoribbons, which are similar in width to 1LG, but have shorter lengths. Regions 3 and 4 are designated to IFL given that there are no topographical features between them (Fig. 3c), however, the surface potential shows significant differences in the charge between the two regions and is attributed to the close proximity of region 3 to the 1LG nanoribbons. Region 5 is designated as SiC, which is least affected by charge from nearby IFL and graphene.

When SEM imaging is carried out with an InLens detector, the parts of the sample with higher work function lead to a stronger suppression of the backscattered electrons from the surface, resulting in darker contrast in SEM image due to a lower electron intensity sensed by the detector41. The SEM is able to clearly distinguish graphene regions from IFL with high spatial resolution (Fig. 3e), but the only indication of 2LG ribbons is faint haloing of IFL regions surrounding them. Additionally, there is no differentiation between IFL and SiC42. The C-AFM map is consistent with the designation of regions 3, 4 and 5 where there was no conduction for these electrically insulating regions (Fig. 3f).

Raman spectra of the same region were also collected and fitted to create maps of the G- and 2D-peak area (Fig. 3g, h), position, width and intensity (Supplementary Note 4 and Supplementary Fig. 6). Figure 3i shows the average of 64 representative Raman spectra for each of the five regions indicated in Fig. 3d. The signature single Lorentzian 2D-peak for 1LG is observed for region 1, but broadens significantly and forms the signature shoulder for ab-stacked 2LG in region 2 (see fitting of the 2D-peak in the inset of Fig. 3i). These maps clearly show that the coverage of graphene is in excellent agreement with CLSM, AFM, KPFM, and C-AFM.

The 2D forest of graphene nanoribbons is formed in EG samples produced by face-to-graphite growth with reduced process temperatures or reduced growth times (Supplementary Fig. 7a and Supplementary Note 5). In this case, the dense 2D graphene nanoribbon forest along with its conspicuous optical contrast to the IFL patches is a characteristic of incomplete EG coverage. The graphene nanoribbons will eventually merge to form continuous graphene in a succeeding growth (shown in Supplementary Fig. 7b), and the CLSM contrast features will evolve accordingly.

Next, we investigate samples with full coverage of EG using CLSM, KPFM, C-AFM, and SEM (Fig. 4a–d, respectively). The CLSM intensity image shows that the sample is predominantly covered by 1LG (as conformed by Raman spectra; not shown) and about 10 % of the area is covered by narrow patches of bi- (2LG) and tri-LG (3LG) domains (brightest contrast). The higher intensity of the reflected light from thicker graphene layer in the CLSM image is consistent with the linear increase of reflectivity reported by Ivanov et al.42, where they measured the power of reflected laser light from the surface of EG on SiC. Comparison of Fig. 4a to the surface potential map (Fig. 4b) of the same region confirms the designation of 1LG, 2LG, and 3LG. However, the darkest lines and patches in Fig. 4a indicated by the red arrows are not as clearly apparent in the KPFM image due to the proximity effect from charging. These features are confirmed to be insulating IFL or SiC by C-AFM, where zero current is measured (Fig. 4c). Moreover, the work function of the sample can be estimated from KPFM by using Φ sample = Φ probe —eΔV CPD , where Φ probe is the work function of the probe33. Thus, brighter surface potential contrast is associated with lower work function, i.e., work function of 3LG < 2LG < 1LG.

Fig. 4 Epitaxial graphene on SiC showing interfacial layer (IFL), single layer graphene (1LG), two-layer graphene (2LG) and three-layer graphene (3LG). (a) Confocal laser scanning microscopy (CLSM), (b) Kelvin probe force microscopy (KPFM), (c) conductive atomic force microscopy (C-AFM) and (d) scanning electron microscopy (SEM) image with InLens detector Full size image

Figure 4d shows the SEM image obtained in a vacuum chamber using the InLens detector to capture backscattered electrons. The IFL/SiC regions appear brightest in the SEM image, which is consistent with the contrast of SEM image for the EG nanoribbon sample (as shown in Fig. 3e and Supplementary Fig. 4b). SEM image of IFL/SiC regions with the brightest contrast has been consistently observed in all the samples presented in this paper and in all the other samples that we have imaged, indicating that IFL/SiC has the lowest work function. 1LG appears the darkest and 2LG has contrast in between the IFL/SiC region and 1LG, indicating work function of IFL/SiC < 2LG < 1LG. Finally, during the SEM imaging, the graphene surface becomes heavily charged by the electron beam and is also exposed to hydrocarbon contamination, both causing deterioration of the image resolution. The dark spots that are tens of nanometers in size, which only appear in the SEM image that was obtained last, are likely to be the hydrocarbon contamination. In contrast to that, CLSM is noninvasive and thus does not influence the imaging quality over time.