Mechanism of Alkali Metal Compound-Promoted Growth of Monolayer MoS₂: Eutectic Intermediates

Abstract

Monolayer MoS 2 , processing flexibility and remarkable physical properties derived from its direct bandgap feature, has been endowed to be one of the potential materials for practical applications such as integrated circuits and logic devices. Recently, a facile CVD method using alkali metal compounds as promoters attracted a lot of attention. Here, we systematically investigated the mechanism of alkali metal compoundpromoted growth of monolayer MoS 2 by CVD and proposed a eutectic intermediate model. In the presence of alkali metal compounds, large monolayer MoS 2 was obtained, regardless of the anions. However, nonalkali metal compounds did not promote the growth of monolayer MoS 2 . We proposed that the formation of eutectic intermediates, containing alkali metal molybdates and molybdenum oxides, played a crucial role in promoting the growth of monolayer MoS 2 . It is because the low melting point of eutectic intermediates could facilitate their mobility, favoring less nuclei and lateral growth. The proposal of the eutectic intermediates model could not only contribute to growing ultralarge monolayer MoS 2 and other 2D materials but also inspire new ideas about growing 2D materials based on low melting point and high mobility of eutectic intermediates.

Gas-Source CVD Growth of Atomic Layered WS2 from WF6 and H2S Precursors with High Grain Size Uniformity

“…Now we will discuss the effect of NaCl in our gas-source CVD system. Previous studies have revealed that only cations play an important role in the alkali-metal assistance method 23 , so we have only considered the effects of Na. If Cl plays an important role in the mechanism, then CVD growth of TMDs from metal chloride (e.g.…”

“…These results shed light on one mechanism by which alkali-metal assistance functions during solid-source CVD. Prominent hypotheses are that the alkali metals increase the mass-flux by forming intermediates or that they promote the lateral growth of TMD once it has nucleated 21,23,25 . We think these two effects coexist in the solid-source CVD.…”

“…Although the effect of alkali metals has not been completely clarified in terms of the chemical processes involved, several models have been proposed to explain the phenomenon. For example, the volatile metal oxychloride species and/or eutectic intermediate compounds generated during the process, such as Na 2 MoO 4 , could be affecting the chemical reaction route and enhancing the diffusion of the metal precursors 23–28 . Another hypothesis is that a catalytic effect at the edges of the forming TMDs may be promoting 2D crystal growth 22,29 .…”

Aligned monolayer MoS2 ribbons growth on sapphire substrate via NaOH-assisted chemical vapor deposition

“…A possible reason is that the introduction of NaOH enhances the lateral growth speed of MoS 2 . The eutectic intermediate containing alkali molybdate and molybdenum oxides has a low melting point, which could facilitate their mobility and lateral growth [21], weakening the epitaxial control from substrate and graphoepitaxial guide from surface steps. Thus, the triangular MoS 2 domains with large size and random arrangements were obtained when a large amount of NaOH was introduced.…”

“…The microscopic mechanism of CVD is that the precursor of gas/vapor phase nucleates and then regenerates into films on the substrate [19]. A new molten salt-assisted CVD method for growing TMDCs has recently attracted attention [20][21][22][23][24][25]. The introduction of alkali metal promoter can effectively enhance the lateral growth of the film, making it a unique method for obtaining large-size single crystals.…”