*6.1. Molybdenum*

The literature has reported the use of two liquid precursors, sodium molybdate dihydrate (SMD—Na2MoO4·2H2O) and ammonium molybdate tetrahydrate (AMT—(NH4)6Mo7O24). It is important to remember that SMD is incompatible with alkali metals, most common metals, and oxidizing agents. These precursors are usually diluted in water (0.1–0.2 g in 10–50 mL of H2O) and mixed with a promoter such as NaOH (0.1 g in 50 mL of H2O) and a density gradient medium such as iodixanol (Opti-Prep) to facilitate the spinning and substrate adhesion process. Standard proportions for these three components are in the range 1:(1–8):0.5, and they are then spun onto the substate for 30–60 s at 2000–4000 revolutions per minute. An hydrophilic surface is required to obtain a uniform coating molybdenum precursor, so the substrate is usually treated with O<sup>2</sup> plasma and/or sodium hydroxide before the drop-casting of the solution [74].

SMD was also used in an alternative approach by embedding it into two thin pieces of glass that were fused together after heating. Thanks to the high temperature, the SMD melted and diffused through the molten glass to its surface with a dissolution–precipitation process. When the metal source reached the upper surface, it reacted with sulfur to grow MoS<sup>2</sup> on the molten glass surface. Through this original method, highly uniform and monolayer MoS<sup>2</sup> flakes on centimetre-scale glass substrates were obtained thanks to a uniform distribution of the metal precursor [75].

The liquid-phase precursor approach was used to obtain controlled doping of TMD and related heterostructures. Mixing of Mo liquid precursors along with selected dopants (Fe, Re, V) was reported to produce controllable doping in MoS<sup>2</sup> with excellent uniformity. More complex structures, such as V-doped in-plane WxMo1−xS2/MoxW1−xS2, were also obtained [76]. Liquid, metal-organic-like precursors are still not common for MoS<sup>2</sup> synthesis. Efforts are being directed towards developing suitable reagents to be used in MOCVD systems, in order to enhance process reproducibility and controllability. A

notable effort was directed towards bis(tertbutylimido)bis(dimethylamido)molybdenum (Nt Bu)2(NMe2)2Mo, which was stored in a container kept at 45 ◦C and used as molybdenum precursor with H2S the as sulfurizing agent. A rapid and scalable process using a CVD approach on 2-inch sapphire substrate was developed to synthesize, layer-by-layer, atomically thin MoS<sup>2</sup> films with good spatial homogeneity and wafer-scale uniformity [77].

#### *6.2. Sulfur*

Dimethyl disulfide (CH3SSCH3, DMDS) is an organic, flammable liquid with a garliclike odour that can be used as a S precursor. It decomposes into methanethiol (CH3SH), ethene (CH2=CH2), carbon disulfide (CS2), and H2S. Reaction by-products of this precursor pose some safety hazards (see the following section) [78].

Dodecane-1-thiol (or dodecyl mercaptan, C12H25SH) is a sulfur-containing liquid thiol that can be bubbled and delivered into the reactor by using a carrier gas in order to obtain a precise and uniform concentration of S during the process [79].
