Understanding Salt-Promoted Metal Organic Chemical Vapour Deposition of Transition Metal Dichalcogenides

Scalable integrated process pathways remain a key challenge for ultrathin 2D device materials particularly for potential opto-electronic device applications. For transition metal dichalcogenides (TMDC), the use of salts to enhance chemical vapour deposition (CVD) has become widespread, yet the underpinning understanding remains limited. We employ operando scanning electron microscopy (SEM) and environmental X-ray photoelectron spectroscopy (XPS) to systematically study the role of salt-based growth promotors. We focus on WS₂ as a model system and explore surface-based mechanisms linked to sodium tungstates, NaOH and NaCl on SiO₂ and sapphire substrates in conjunction with widely used gaseous (MO)CVD precursors. We report various hot salt substrate corrosion phenomena that lead to the substrate spreading of W intermediates and significantly influence the growth mechanisms. We can directly resolve the formation and wetting behaviour of liquid droplets and show that the growth scenario particularly for mono-layers is not following a vapour-liquid-solid (VLS) model. Holistic calibration of SE contrast allows us to reveal WS₂ nucleation and in-plane growth kinetics. Our discussion brings together the wider literature including mechanisms of 1D TMDC nanotube formation.