## 6 Open Research Issues and Perspectives  
Deposition of ultra-thin and conformal insulators represents a key requirement for the fabrication of devices based on different kinds of graphene. Besides ALD, also physical deposition methods (e.g., evaporation or sputtering) have been explored to this purpose. Although the presence of active sites on graphene surface is not required for these approaches, physical deposition of insulators typically results in graphene damage and/or a reduction of the carrier mobility [56; 57]. For this reason, ALD became the preferred approach for insulators deposition on graphene, in spite of the nucleation issues. Thermal ALD of high-k dielectrics on EG presents some peculiar aspects related to the unique electrical/structural properties of the EG/SiC(0001) system, i.e., the presence of the sp\({}^{3}\) hybridized interfacial BL, responsible for a high n-type doping and compressive strain of the overlying graphene. These properties were shown to be beneficial in enhancing the Al\({}_{2}\)O\({}_{3}\) nucleation on 1 L EG, whereas a poorer nucleation was observed on 2 L or few layers EG. Hence, the availability of 100% 1L EG coverage by thermal decomposition of SiC still represents a major challenge for direct ALD on EG. CVD grown graphene on catalytic metals is another type of graphene widely used for electronics/optoelectronic applications. Typically, this material is transferred on insulating substrate and subsequently processed for devices fabrication. As the thermal ALD process commonly results in an inhomogeneous coverage, seeding layers or prefunctionalization treatments with reactive gas species are commonly employed to promote the ALD growth. However, these processes need to be carefully optimized to minimize structural damage and/or a degradation of the electrical properties (doping and mobility) of underlying graphene. Recently, [PERSON] et al. [58] demonstrated the direct thermal ALD of uniform Al\({}_{2}\)O\({}_{3}\) films on monolayer CVD graphene laying on the native metal substrates (Cu and Ni-Au). The enhanced nucleation was ascribed to the wetting transparency of monolayer graphene [59] and to the presence of peculiar polar traps at the graphene/metal interface, which promote the adsorption of water molecules (i.e., the oxygen precursor for the ALD process) on the graphene surface. Clearly, the number of graphene layers is a crucial aspect for this growth mechanism. In fact, the strength of the electrostatic interaction between the water precursor and the polar traps is weakened for multilayer graphene due to a decreased transparency, resulting in an inhomogeneous Al\({}_{2}\)O\({}_{3}\) coverage [58]. The direct growth of Al\({}_{2}\)O\({}_{3}\) or HfO\({}_{2}\) on CVD graphene residing on the native metal substrate has the advantage of providing a protective layer for graphene, avoiding the direct contact of Gr with the polymeric films typically used for the transfer process [60]. Furthermore, after transfer to an insulating substrate, the oxide layer on graphene can work a gate dielectric for graphene field effect transistors [60]. Although the ALD grown protective layer is beneficial to solve the problems related to polymeric contaminations on graphene surface, the transfer of the oxide/graphene stack from the native metal substrate to the target substrate still remains a critical step. Most studies reported so far for ALD on graphene concern the deposition of metal-oxides. However, the integration of ultra-thin films of other materials, such as nitrides (including SiN\({}_{x}\), AlN, etc. ), hexagonal boron nitride, and transition metal dichalcogenides, TMDs, (such as MoS\({}_{2}\), WS\({}_{2}\), MoSe\({}_{2}\) and WSe\({}_{2}\),) with graphene (and specifically with the EG/SiC(0001) system) is attracting an increasing interest for novel electronics/optoelectronics applications [61; 62], and ALD can play a key role in this field. To date, MOCVD and MBE are mainly employed for the integration of nitrides with graphene.