*5.1. Molybdenum*

The most commonly used powders for molybdenum are MoO3, available in high purity (99.98%). The vapour pressure of MoO<sup>3</sup> was reported in [65]. A quantity of 10–200 mg of MoO<sup>3</sup> powders is placed in a quartz crucible near the substrate in the high temperature zone of the reactor. There are several approaches in positioning the substrate; some groups place the substrate facing down directly above the powders, while others place it facing up immediately after the crucible. It should be noted that some impurities might be present in these solid precursors. Robertson et al. [66] showed that Cr and V atoms were found in CVD grown MoS2, most probably from elements present in solid precursors, causing some charge trapping in the grown 2D material. The issue of unintentional doping is very relevant in 2D materials; in [67], the effect of unintentional carbon doping during CVD was discovered and thoroughly discussed.

Precursors such as molybdenum hexacarbonyl (Mo(CO)6) and molybdenum (V) chloride (MoCl5) are mostly used for ALD and MOVPE. Mo(CO)<sup>6</sup> is used with H2O or ozone to deposit thin MoO<sup>3</sup> films through ALD, to be sulfurized in a subsequent step. Mo(CO)<sup>6</sup> has a vapour pressure of about 0.10–0.15 mbar at room temperature and is very volatile (melting point of 150 ◦C, boiling point of 156 ◦C) [68].

Molybdenum (V) chloride (MoCl5) powders were also used to grow MoS<sup>2</sup> with S powders [69,70]. However, MoCl<sup>5</sup> is air sensitive and toxic, so its use poses more hazards than that of MoO<sup>3</sup> powders.

Deposition of a thin Mo layer prior to growth could also be included in the solid precursor category. Electron-beam evaporation or sputtering are used to obtain a very thin and controlled Mo layer on the substrate, which is converted to MoS<sup>2</sup> because of the exposure of the surface to S vapours. The final MoS<sup>2</sup> layers are dependent on the thickness of the initial Mo layer, so it is important to have very precise control over the metal thickness and good homogeneity. MoO<sup>3</sup> deposited by evaporation is an alternative approach to metallic Mo [71], since the oxide has a lower evaporation temperature and is easier to sulfurize.

The use of a Mo metal foil was suggested to obtain large flakes on 6-inch substrates [48]. The foil was placed 10 mm above a soda–lime glass substrate and using a mixture of Ar and O<sup>2</sup> as carrier, flakes with later size higher than 400 µmm were obtained. Reactive MoO<sup>2</sup> was produced thanks to the oxidation of Mo foil by the O<sup>2</sup> carrier, and S powders were used as an S precursor. The dimensions of triangular MoS<sup>2</sup> flakes were adjusted by controlling the distance between the Mo foil precursor and the substrate.
