**3. Results and discussion**

#### **3.1. Growth of graphene on SiC(0 0 0 1)**

We first address the structural properties of graphene, which was studied by low-energy electron diffraction. The typical LEED of the graphene sample can be seen in **Figure 1a**. The LEED pattern demonstrates that the graphene layer is well ordered and aligned with respect to the substrate, such that the basal plane unit vectors of graphene and SiC subtend an angle of 30°. The smallest hexagon is the result of a (6√3 × 6√3)*R* × 30° reconstruction of the interfacial layer, as are the spots lying just inside the graphene pattern.

**Figure 1.** (a) Typical LEED image of graphene on SiC(0 0 0 1) and (b) typical micro-Raman spectra of the graphene monolayer. Contributions at the G and 2D bands are observed, together with a very low signal at the defect band D.

We further characterized our graphene by Raman spectroscopy, in **Figure 1b**, we present a typical Raman spectrum, which presents the typical features of high-quality epitaxial mono‐ layer graphene. Graphene contributions were identified by three main structures: (i) the D band at 1350 cm−1, (ii) the G band (symmetric E2g phonon mode) at 1592 cm−1, and (iii) the 2D band at 2704 cm−1. For both samples, the D band, which corresponds to disorder, was weak in comparison with the G and 2D bands (double resonant electron-phonon process). The low intensity of this peak showed that there was only a small number of defect/disorder in the graphene structure. This was an indication of the high quality of the epitaxial graphene produced.

#### **3.2. Au intercalation in epitaxial graphene on SiC(0 0 0 1)**

The Au incorporation process was carried out using a post-growth deposition method. Au was deposited at a temperature of 890°C, at a rate of 10 min/layer. A total coverage of 2 ML (1 ML) was deposited on sample S1 (S2). The samples were then further annealed at 800°C for 20 min in order to favor migration of gold. We now discuss the results of LEEM, µARPES, and µLEED, characterization of both samples, before and after Au deposition.

Laterally Inhomogeneous Au Intercalation in Epitaxial Graphene on SiC(0 0 0 1): A Multimethod Electron Microscopy Study 139 5 http://dx.doi.org/10.5772/64076

**3. Results and discussion**

1384 Recent Advances in Graphene Research

produced.

**3.1. Growth of graphene on SiC(0 0 0 1)**

layer, as are the spots lying just inside the graphene pattern.

**3.2. Au intercalation in epitaxial graphene on SiC(0 0 0 1)**

characterization of both samples, before and after Au deposition.

We first address the structural properties of graphene, which was studied by low-energy electron diffraction. The typical LEED of the graphene sample can be seen in **Figure 1a**. The LEED pattern demonstrates that the graphene layer is well ordered and aligned with respect to the substrate, such that the basal plane unit vectors of graphene and SiC subtend an angle of 30°. The smallest hexagon is the result of a (6√3 × 6√3)*R* × 30° reconstruction of the interfacial

**Figure 1.** (a) Typical LEED image of graphene on SiC(0 0 0 1) and (b) typical micro-Raman spectra of the graphene monolayer. Contributions at the G and 2D bands are observed, together with a very low signal at the defect band D.

We further characterized our graphene by Raman spectroscopy, in **Figure 1b**, we present a typical Raman spectrum, which presents the typical features of high-quality epitaxial mono‐ layer graphene. Graphene contributions were identified by three main structures: (i) the D band at 1350 cm−1, (ii) the G band (symmetric E2g phonon mode) at 1592 cm−1, and (iii) the 2D band at 2704 cm−1. For both samples, the D band, which corresponds to disorder, was weak in comparison with the G and 2D bands (double resonant electron-phonon process). The low intensity of this peak showed that there was only a small number of defect/disorder in the graphene structure. This was an indication of the high quality of the epitaxial graphene

The Au incorporation process was carried out using a post-growth deposition method. Au was deposited at a temperature of 890°C, at a rate of 10 min/layer. A total coverage of 2 ML (1 ML) was deposited on sample S1 (S2). The samples were then further annealed at 800°C for 20 min in order to favor migration of gold. We now discuss the results of LEEM, µARPES, and µLEED,

**Figure 2.** LEEM image of a graphene/SiC(0 0 0 1) surface, before (left) and after (right) Au deposition, with an electron beam energy of 3.90 eV. The images for the sample 1 (2) are presented at the top (bottom) of the image.

**Figure 2a**–**d** presents LEEM images obtained on the two samples before and after Au deposi‐ tion. The image contrast arises from the thickness dependent reflectivity of the film. Recent quantum mechanical calculations of the IV reflectivity of graphene allow precise quantification of the graphene thickness on SiC, which vary considerably depending on the presence of buffer layers [28]. Measurement of the low energy electron reflectivity (not shown) of sample S1 before Au deposition (**Figure 1a**) has allowed us identifying the predominant presence of 1 monolayer (neutral gray regions) of graphene, accompanied with a small amount of the buffer layer (dark gray) and graphene bilayer (white contrast). After the deposition of Au (**Figure 2b**), the three regions exhibit the same contrast, suggesting that the overall thickness distri‐ bution of the sample was preserved by the Au deposition.

A notable difference between LEEM images acquired before and after Au deposition is the presence of small dots which are uniformly distributed over both single- and bilayers gra‐ phene. These structures have a maximum size of few tens of nanometers, with several of them close to (or below) the lateral resolution of our LEEM microscope. We interpret the dots as due to three-dimensional gold islands, as none of these features were observed on the sample before Au deposition. We had repeated the Au experiment on the second sample S2, with less Au (1 ML instead of 2 ML), on which the graphitization was incomplete. Indeed, the reflectivity curves confirm that the substrate was only partially covered by one and two monolayers of graphene. The rather large light gray contrast areas in **Figure 1c** are attributed to the buffer layer, thanks to the analysis of the reflectivity curves, extracted from these areas. The corre‐ sponding image recorded after Au deposition shows that the nanoclusters are once again observed over the entire field of view. The lower concentration can be explained by the total coverage of 1 ML of gold in comparison with 2 ML Au on S1. This set of images demonstrates that the clusters nucleate also over the buffer layer, as well as on the one and two graphene MLs.

**Figure 3.** (a) LEEM image taken with an electron energy of 3.90 eV and (b–d) its corresponding XPEEM images record‐ ed at the Au 4f, C 1s, and Si 2p core levels. The field of view is 0.80 µm wide. (b) Au 4f XPEEM image taken at a bind‐ ing energy of 84.15 eV with hν = 200 eV. The highest (lowest) contrast is presented by the yellow (red) color. (c) C 1s XPEEM image taken at a binding energy of 284.60 eV with hν = 200 eV. The highest (lowest) contrast is presented by the white (black) color. (d) Si 2p XPEEM image, taken at a binding energy of 101.75 eV with hν = 200 eV. The highest (lowest) contrast is presented by the white (black) color.

In the following, we focus on the sample S1, corresponding to **Figure 2a** and **b**. In order to investigate the chemical properties of the sample after Au deposition, we have performed XPEEM measurements [29] at the C 1s, Au 4f, and Si 2p core levels (**Figure 3b**–**d**). **Figure 3a** displays a LEEM image of the sample after gold deposition, at electron energy of 3.90 eV. This image is a zoom of **Figure 2b**, marked by a dashed yellow square. **Figure 3c** is an XPEEM image at the C 1s core level, recorded at a binding energy (BE) of 284.60 eV. The BE is chosen to correspond to the maximum of the C 1s core level emission from the graphene layers. There‐ fore, the brightest (darkest) areas correspond to the graphene (substrate) layer. Regarding the Si 2p image, for which the maximum of intensity is attributed to the SiC contribution, recorded at a BE of 101.75 eV (hν = 200 eV), one can observe that the contrast is inverted, in comparison with the C 1s XPEEM image. Indeed, a lower Si signal is due to attenuation by the graphene layers; and the corresponding zones, on the C 1s image appear bright. We verified that these two images are in agreement with the graphene thickness, evaluated in LEEM (**Figure 3a**). The shapes of the domains observed in the LEEM image can be easily recognized in the C 1s and Si 2p maps. In addition to the C and Si distributions, **Figure 2b** displays an image of the Au 4f, recorded at 84.15 eV binding energy (hν = 200 eV). The Au 4f signal seems to be more intense at the buffer layer. The shape of the previous domains is not clearly recognizable anymore, meaning that the Au can be deposited or/and intercalated independently of the surface chemistry (substrate/graphene mono- or bilayer). The LEEM/XPEEM images do not allow a clear discrimination between whether the Au nanoclusters are intercalated between adjacent graphene layers or between the graphene and the buffer layer, or if they simply overlay on the surface. The STEM data presented below will help answering this question.

layer, thanks to the analysis of the reflectivity curves, extracted from these areas. The corre‐ sponding image recorded after Au deposition shows that the nanoclusters are once again observed over the entire field of view. The lower concentration can be explained by the total coverage of 1 ML of gold in comparison with 2 ML Au on S1. This set of images demonstrates that the clusters nucleate also over the buffer layer, as well as on the one and two graphene

**Figure 3.** (a) LEEM image taken with an electron energy of 3.90 eV and (b–d) its corresponding XPEEM images record‐ ed at the Au 4f, C 1s, and Si 2p core levels. The field of view is 0.80 µm wide. (b) Au 4f XPEEM image taken at a bind‐ ing energy of 84.15 eV with hν = 200 eV. The highest (lowest) contrast is presented by the yellow (red) color. (c) C 1s XPEEM image taken at a binding energy of 284.60 eV with hν = 200 eV. The highest (lowest) contrast is presented by the white (black) color. (d) Si 2p XPEEM image, taken at a binding energy of 101.75 eV with hν = 200 eV. The highest

In the following, we focus on the sample S1, corresponding to **Figure 2a** and **b**. In order to investigate the chemical properties of the sample after Au deposition, we have performed XPEEM measurements [29] at the C 1s, Au 4f, and Si 2p core levels (**Figure 3b**–**d**). **Figure 3a** displays a LEEM image of the sample after gold deposition, at electron energy of 3.90 eV. This image is a zoom of **Figure 2b**, marked by a dashed yellow square. **Figure 3c** is an XPEEM image at the C 1s core level, recorded at a binding energy (BE) of 284.60 eV. The BE is chosen to correspond to the maximum of the C 1s core level emission from the graphene layers. There‐ fore, the brightest (darkest) areas correspond to the graphene (substrate) layer. Regarding the Si 2p image, for which the maximum of intensity is attributed to the SiC contribution, recorded at a BE of 101.75 eV (hν = 200 eV), one can observe that the contrast is inverted, in comparison

(lowest) contrast is presented by the white (black) color.

MLs.

1406 Recent Advances in Graphene Research

**Figure 4.** Top: band dispersions as a function of *k*// around the *K* point of the first Brillouin zone, obtained by µARPES at hν = 40 eV, performed before (a) and after (b) Au deposition. The Fermi level and the Dirac point are superimposed on the images. Bottom: 2D maps as a function of *kx* and *ky*, recorded for a binding energy of 0.15 eV, that is, close to the Fermi level, before (c) and after (d) gold deposition.

In order to investigate the effect of Au deposition on the electronic properties, we have performed local ARPES experiments (µARPES). **Figure 4** presents the µARPES maps, before (**Figure 4a**) and after (**Figure 4b**) Au deposition, around the K point, perpendicular to the ΓKM direction. The 2D map of the unexposed graphene surface (**Figure 4a**) presents a shift of the Dirac point of 0.4 eV below the Fermi level. This energy shift (Δ*E* = *E*D – *E*F) is nowadays well known in epitaxial graphene/SiC(0 0 0 1) [9, 17, 24] and is attributed to doping from the buffer layer [6, 7]. On the 2D µARPES map, recorded after Au deposition (**Figure 4b**), the energy difference between the Fermi level and the Dirac point is still 0.40 ± 0.02 eV. In our data, we do not observe a noticeable modulation of the Dirac point energy, contrary to what reported by Gierz et al. [21]. Nevertheless, we cannot exclude that a small shift of the Dirac point (below ~20 meV) has occurred. Moreover, Au deposition on SiC(0 0 0 1) results in p- or n-doping, depending either on the number of graphene layers, the strain at the Au/graphene or SiC/ graphene interfaces [22], or the gold coverage [21]. In our case, we average the electronic information over the region that is slightly larger than that defined by the illumination, which includes both single and bilayer graphene. The local doping effect can therefore be averaged out in our data by the presence of areas with different electronic properties where Au induces p-doping and n-doping effect, respectively. However, we reckon that these antagonistic effects have to be insignificant for both cases as the Dirac cone does not get broader or split upon Au deposition.

The 2D maps of the first Brillouin zone, recorded for a BE of 0.25 eV, are presented in **Figure 4c** and **d**, close to the Fermi level, obtained before and after Au deposition, respectively. In the constant energy plots, six weak replicas of the π and π\* states surrounding the primary states can be seen, as points around the upper spot (**Figure 4c**). Low-energy electron diffraction of graphene layers grown on the SiC substrate (not shown) displays a nearly commensurate superstructure with (6√3 × 6√3)*R* × 30° unit cell with respect to the substrate because of the difference between the graphene lattice constant of 2.46 Å and that of SiC, 3.07 Å. The replicas of the π and π\* states are brought about by scattering off this superstructure in a fashion similar to those in other incommensurate systems. The 2D image recorded after Au deposition (**Figure 4d**) shows that the superstructure around each K and K′ point persists. Even if the statistics are not as good as the one before Au adsorption, one can still clearly observe these satellites, which suggest that the Au deposition has not effectively decoupled the buffer layer from the substrate. The data in **Figure 4** demonstrate that the Au adsorption on this Gr/SiC(0 0 0 1) sample neither decouples the buffer layer from the substrate, nor alters its average doping.

In order to reach an atomic level characterization of the Au distribution we have performed sectional HR-TEM on the sample 2, after gold deposition. **Figure 5** shows TEM images of two distinct areas of the sample, recorded with a bright field (BF) and dark field mode (DF). These STEM images, allow accessing the crystallographic information. High-angle annular dark field (HAADF) contrast depends directly on the atomic number of the element: gold atoms appear very bright due to their high atomic number (so-called Z-contrast' imaging). The contrast of the graphene layer is very weak in the HAADF-STEM images due to its low atomic number (by comparison with the silicon). The graphene layer is easily visible in the bright field image (contrast of diffraction), as shown in **Figure 5b** and **e**.

Laterally Inhomogeneous Au Intercalation in Epitaxial Graphene on SiC(0 0 0 1): A Multimethod Electron Microscopy Study 143 9 http://dx.doi.org/10.5772/64076

(**Figure 4a**) and after (**Figure 4b**) Au deposition, around the K point, perpendicular to the ΓKM direction. The 2D map of the unexposed graphene surface (**Figure 4a**) presents a shift of the Dirac point of 0.4 eV below the Fermi level. This energy shift (Δ*E* = *E*D – *E*F) is nowadays well known in epitaxial graphene/SiC(0 0 0 1) [9, 17, 24] and is attributed to doping from the buffer layer [6, 7]. On the 2D µARPES map, recorded after Au deposition (**Figure 4b**), the energy difference between the Fermi level and the Dirac point is still 0.40 ± 0.02 eV. In our data, we do not observe a noticeable modulation of the Dirac point energy, contrary to what reported by Gierz et al. [21]. Nevertheless, we cannot exclude that a small shift of the Dirac point (below ~20 meV) has occurred. Moreover, Au deposition on SiC(0 0 0 1) results in p- or n-doping, depending either on the number of graphene layers, the strain at the Au/graphene or SiC/ graphene interfaces [22], or the gold coverage [21]. In our case, we average the electronic information over the region that is slightly larger than that defined by the illumination, which includes both single and bilayer graphene. The local doping effect can therefore be averaged out in our data by the presence of areas with different electronic properties where Au induces p-doping and n-doping effect, respectively. However, we reckon that these antagonistic effects have to be insignificant for both cases as the Dirac cone does not get broader or split upon Au

The 2D maps of the first Brillouin zone, recorded for a BE of 0.25 eV, are presented in **Figure 4c** and **d**, close to the Fermi level, obtained before and after Au deposition, respectively. In the constant energy plots, six weak replicas of the π and π\* states surrounding the primary states can be seen, as points around the upper spot (**Figure 4c**). Low-energy electron diffraction of graphene layers grown on the SiC substrate (not shown) displays a nearly commensurate superstructure with (6√3 × 6√3)*R* × 30° unit cell with respect to the substrate because of the difference between the graphene lattice constant of 2.46 Å and that of SiC, 3.07 Å. The replicas of the π and π\* states are brought about by scattering off this superstructure in a fashion similar to those in other incommensurate systems. The 2D image recorded after Au deposition (**Figure 4d**) shows that the superstructure around each K and K′ point persists. Even if the statistics are not as good as the one before Au adsorption, one can still clearly observe these satellites, which suggest that the Au deposition has not effectively decoupled the buffer layer from the substrate. The data in **Figure 4** demonstrate that the Au adsorption on this Gr/SiC(0 0 0 1) sample neither decouples the buffer layer from the substrate, nor alters its average doping.

In order to reach an atomic level characterization of the Au distribution we have performed sectional HR-TEM on the sample 2, after gold deposition. **Figure 5** shows TEM images of two distinct areas of the sample, recorded with a bright field (BF) and dark field mode (DF). These STEM images, allow accessing the crystallographic information. High-angle annular dark field (HAADF) contrast depends directly on the atomic number of the element: gold atoms appear very bright due to their high atomic number (so-called Z-contrast' imaging). The contrast of the graphene layer is very weak in the HAADF-STEM images due to its low atomic number (by comparison with the silicon). The graphene layer is easily visible in the bright field image

(contrast of diffraction), as shown in **Figure 5b** and **e**.

deposition.

1428 Recent Advances in Graphene Research

**Figure 5.** Cross-sectional aberration-corrected STEM images recorded in the high-angle annular dark field (HAADF) (a and d) and bright field (BF) (b and e) mode, respectively. HAADF contrast depends directly on the atomic number of the element: gold atoms appear very bright due to their high atomic number (so-called 'Z-contrast' imaging). The con‐ trast of the graphene layer is very weak in the HAADF-STEM images due to its low atomic number (in comparison with the silicon). Layer graphene is easily visible in the bright field image (contrast of diffraction). The substrate, the Au, and graphene layers can clearly be observed. A scheme of the area is also represented. (d–f) DF and BF images with the associated scenarios, for another area of the surface. The substrate is represented by a gray square-shape. The Au and carbon atoms are represented by yellow and gray spheres.

On the DF images (**Figure 5a** and **d**), the bright contrast corresponds to the Au atoms. More‐ over, the dark line above the crystalline substrate, observed on the DF image, is assigned to a graphene layer. For the first area (**Figure 5a** and **b**), combining the information extracted from the DF and BF images, respectively, we demonstrate that the Au atoms are located below the two graphene layers. These images also show that this Au insertion line stops at the step edge of the substrate, as presented in the sketch (**Figure 5c**). The BF image of the second area (**Figure 5d**) shows that two Au layers have been intercalated under a single graphene layer. Whether the graphene layer is continuous or not at the end of the Au step edge is not clear. One remaining question is how the Au penetrates into the sample. The Au atoms can penetrate the graphene layer through step edges or defects, as it has already been theoretically proposed in the case of the Si out-diffusion, on epitaxial graphene [30].

As different Au insertion mode have observed with the STEM with the HAADF imaging mode, a new set of images from another area, using once again the BF and DF modes, is presented **Figure 6**. The DF mode shows that two zones of the field of view contain Au atoms. The one on the left side shows that these atoms can pass through the graphene layer, as explained for **Figure 5**. However, the zone on the right side of the image presents a new scenario of Au adsorption. The Au atoms, as presented by the black dots of the BF, have penetrated into few layers of the substrate and a nanocluster has nucleated. The lateral size of this nanocluster can be estimated to be ~5 nm. By scanning several areas of the surface, all of these three scenarios have been observed several times, confirming that the Au deposition on the graphene layer is not a uniform process.

**Figure 6.** Cross-sectional STEM images recorded in the HAADF (top) and bright field (middle) mode, respectively. The substrate, the Au, and graphene layers can clearly be observed. A scheme of the area is also represented (bottom). The substrate is represented by a gray square shape. The Au and carbon atoms are represented by yellow and gray spheres.

The lateral distances between the gold atoms in the first layer are exactly the ones found for the silicon (or carbon) atoms of the SiC substrate underneath. Indeed, the distance measured between two adjacent Au atoms is 0.265 nm, while it is of 0.267 nm for two adjacent Si atoms in a (0 0 0 2) plane. These distances are obtained with the same value in measurements performed on numerous STEM images. The distance between two adjacent Au atoms in the second layer systematically decreases to 0.236 nm. Moreover, the distance between two adjacent gold layers is 0.255 nm, considering that the *d*0 0 0 2 inter-reticular distance measured in the SiC substrate close to the surface is 0.266 nm. This value has to be compared to the experimental value of 0.252 nm [31] for the SiC bulk. Therefore, we can conclude that the strain in the SiC substrate beneath the surface is small, even when layer or nanoclusters of gold are observed.

The results observed in TEM and LEEM shows two main scenarios of gold migration when deposited on graphene/SiC(0 0 0 1). On the one hand, 1 or 2 ML of gold can intercalate between the substrate and the graphene layers. On the other hand, some gold atoms migrate inside the substrate to form nanoclusters. In the former case, this would lead to a change in the electronic properties, while it should not in the latter case. µ-ARPES measurements (**Figure 4**) do not present any noticeable changes. Before and after gold deposition, the Dirac point is at 0.40 eV below the Fermi level, for both cases. This equivalent doping can be explained by the fact that the gold mainly clusterizes and does not intercalate homogeneously under graphene layer. Therefore, the decoupling of the buffer layer, as observed by Gierz et al. [21], is not evidenced in our case.
