*9.3. Surface Treatments*

In their pioneering work, van der Zande et al. [26] showed that a careful treatment of SiO2/Si substrates (2 h in H2SO4/H2O<sup>2</sup> (3:1) followed by 5 min of O<sup>2</sup> plasma) and a minimal exposure of the precursors to air during storage was sufficient to obtain large flakes up to 100 µm. From these experimental observations, it is evident that the state of the surface on which the growth is carried on is of paramount relevance for the quality of flakes. From this point of view, many approaches to optimize this surface have been studied in the past. For example, the treatment of SiO2/Si substrates with oxygen plasma resulted in layer-controlled and large-area CVD MoS<sup>2</sup> films [109].

Another interesting approach was studied recently that relied on the treatment of the SiO<sup>2</sup> substrate by a piranha etching solution prior to deposition. The authors stated that this treatment reduced the surface free energy of the substrate, making the use of promoters unnecessary [110]. An interesting method to treat surfaces to ease the transfer of flakes was proposed by Shinde et al. [111]. By etching an SiO<sup>2</sup> surface with hydrofluoric acid, the hydrophilicity is increased, allowing water to penetrate at the interface between polymercapped MoS<sup>2</sup> and the substrate, resulting in direct transfer of flakes over a large area.
