**2. Experimental details**

**1. Introduction**

1362 Recent Advances in Graphene Research

mains a considerable challenge.

layers may serve to functionalize graphene.

Graphene is a single layer of carbon atoms arranged in a hexagonal lattice. It is one of the few structures that are stable in two dimensions as a free-standing crystal [1, 2]. Its extraordina‐ ry properties, such as unconventional two-dimensional (2D) electron gas, high carrier mobility, half-integer quantum Hall effect at room temperature, and spin transport [1], have made graphene a very promising candidate for the design of the new generation of devices, such as ultrafast electronic circuits and photodetectors. Despite significant progress in its synthesis, the development of production methods warranting fine control over film mor‐ phology and thickness, which are both crucial to determine its electronic properties, re‐

Epitaxial graphene layers on silicon carbide (SiC) have been extensively studied due to their potential for large-scale production with a high crystalline quality [3, 4]. It is now well established that, in this system, the interaction between the graphene and its supporting substrate (SiC) can affect considerably the electronic properties of graphene. Indeed, it is known that the first carbon layer onto a SiC substrate is covalently bonded to the Si atoms of the substrate, and this "buffer layer" does not display graphitic electronic properties [5–8]. Moreover, the remaining unsaturated Si dangling bonds, at the buffer layer/SiC interface, induce a high intrinsic n-doping in graphene, which degrades the carrier mobility. These drawbacks pose a major obstacle to the integration of graphene/SiC in future electronic devices. A solution to this problem is provided by the passivation of the Si dangling bonds using dopants and/or decoupling the graphene layer from the SiC substrate. Recently, decoupling of graphene has been achieved by depositing molecules [9, 10] or atomic layers of Bi [11], Ge [12], F [13], and Li [14, 15]. Furthermore, Riedl et al. [16] have shown that hydrogen intercala‐ tion can induce such desired decoupling. More recently, it was demonstrated that the oxygen can partially decouple the buffer layer from the substrate and reduce the intrinsic electron doping [17]. Moreover, the intercalation of metal clusters or molecules between the graphene

Numerous studies have been performed to understand the intercalation of transition metals [18–20]. The study by Gierz et al. [21] claimed that the strongly interacting first carbon layer was decoupled from the SiC(0 0 0 1) substrate via gold intercalation. The shift of Dirac points due to gold intercalation was then theoretically studied by Chuang et al. [22]. A similar study performed by Premlal et al. [23], using scanning tunneling microscopy and spectroscopy (STM/ STS), concluded that the intercalated gold cluster displays a new surface reconstruction and induces a possible hole-doping effect. Nonetheless, further investigations are needed to better

clarify the effects on the electronic structure of graphene induced by intercalated Au.

In this chapter, we discuss the properties of Au-intercalated epitaxial graphene grown on 6H-SiC(0 0 0 1). Low-energy electron diffraction (LEED), low-energy electron microscopy (LEEM), transmission electron microscopy (TEM), X-ray photoemission electron microscopy (XPEEM), microspot angle-resolved photoemission spectroscopy (µ-ARPES), and micro-Raman spec‐ troscopy allowed us to study the structural and electronic properties of epitaxial graphene on SiC. In particular, we focused on the effect of Au adsorption on the local morphology, structure,

The structural and electronic properties of the graphene/SiC interface were characterized using the SPELEEM III (Elmitec GmbH) microscope operating at the Nanospectroscopy beamline of the Elettra storage ring in Trieste (Italy). This instrument combines LEEM and energy-filtered XPEEM imaging with µLEED and µARPES. The µ-spot diffraction data are typically collected from areas of diameter as small as 2 µm. Such analytical methods are well-established tools for characterizing the local morphology, thickness, corrugation, and electronic structure of single layer graphene films [24, 25].

Semi-insulating on-axis SiC(0 0 0 1) substrates were used in this study. After polishing, the samples were exposed to hydrogen etching at 1600°C in order to remove surface defects. Graphene growth was carried out by annealing the substrates at 1300–1400°C under argon and silicon fluxes. This method favors the formation of large and homogeneous domains [26, 27]. During the graphitization process, the argon partial pressure was kept below *P* = 2 × 10−5 Torr. The samples were then cooled to the room temperature and transferred ex situ to the micro‐ scopes used in these studies.

Before the measurements, the graphene samples were annealed at 600°C for 30 min in ultrahigh vacuum, in order to reduce the contamination consequent to atmosphere exposure. The Au incorporation process was carried out using a post-growth deposition method. The samples were then further annealed at 800°C for 20 min in order to favor migration of gold.

In order to vary the amount of buffer layer on the surface prepared as above, we studied two samples grown under the same conditions but with different level of graphitization. As shown in the next section, sample 1 (S1) is at a later stage of graphitization in comparison with sample 2 (S2). S1 is mainly covered by 1 ML graphene, with small areas that present the characteristic of the buffer layer or bilayer graphene. On the other side, S2 presents mainly one and two graphene layers. Both samples were fully characterized by means of LEEM, µARPES, and µLEED, before and after Au deposition, respectively. Post-growth Au deposition on previ‐ ously characterized graphene allows unraveling the effect of gold on the graphene layers. The Au 4f, Si 2p, and C 1s core-level images were recorded using two photon energies (hν = 200 and 360 eV) to tune the surface sensitivity. They were calibrated using the Au 4f7/2 component.

The TEM thin foil was prepared by focused ion beam (FIB). The surface was protected by an amorphous layer carbon deposited before the FIB process. The thin foils were prepared following the <1 1 –2 0> zone axis of the SiC substrate. We used a TEM/STEM microscope Jeol 2200FS working at 200 keV equipped with a spherical aberration (Cs) corrector on the STEM probe. The probe current was 50 pA with a probe size of 0.1 nm (FWHM). The convergence half-angle for the probe was 30 mrad, and the detection half-angles for the HAADF images were, respectively, 100 mrad (inner) and 170 mrad (outer).
