Three gas-phase molecules react at high temperatures during chemical vapor deposition to form molybdenum disulfide. In this illustration, molybdenum atoms are purple, oxygen atoms are red and sulfur atoms are yellow. Image: Jincheng Lei/Rice University.
Three gas-phase molecules react at high temperatures during chemical vapor deposition to form molybdenum disulfide. In this illustration, molybdenum atoms are purple, oxygen atoms are red and sulfur atoms are yellow. Image: Jincheng Lei/Rice University.

Materials theorist Boris Yakobson at Rice University, together with graduate student Jincheng Lei and alumnus Yu Xie, has unveiled how a popular 2D material, molybdenum disulfide (MoS2), flashes into existence during chemical vapor deposition (CVD).

Knowing how the CVD process works will give scientists and engineers a way to optimize the bulk manufacture of MoS2 and other valuable 2D materials classed as transition metal dichalcogenides (TMDs), semiconducting crystals that could find use in next-generation electronics.

Their study, reported in a paper in ACS Nano, focuses on MoS2's 'pre-history', meaning what happens in a CVD furnace once all the solid ingredients are in place. CVD, often associated with graphene and carbon nanotubes, has been exploited to make a variety of 2D materials, and involves getting solid precursors to sublimate into gases that react together in the presence of catalysts. The chemistry dictates which molecules fall out of the gas and settle on a substrate made of copper or silicone to assemble into a 2D crystal.

The problem has been that once the furnace cranks up, it's impossible to see or measure the complicated chain of reactions in the chemical stew in real time.

"Hundreds of labs are cooking these TMDs, quite oblivious to the intricate transformations occurring in the dark oven," said Yakobson, a professor of materials science and nanoengineering and a professor of chemistry. "Here, we're using quantum-chemical simulations and analysis to reveal what's there, in the dark, that leads to synthesis."

Yakobson's theories often lead experimentalists to make his predictions come true (for example, boron buckyballs). This time, his lab determined the path molybdenum oxide (MoO3) and sulfur powder take to deposit an atomically thin lattice of MoS2 onto a surface.

The short answer is that it takes three steps. First, the solids are sublimated through heating to change them from solid to gas, which includes producing what Yakobson called a 'beautiful' ring-molecule, trimolybdenum nonaoxide (Mo3O9). Second, the molybdenum-containing gases react with sulfur atoms under high heat, up to 4040°F. Third, molybdenum and sulfur molecules fall to the surface, where they crystallize into the jacks-like lattice that is characteristic of TMDs.

What happens in the middle step was of most interest to the researchers. The lab's simulations showed that a trio of main gas-phase reactants are the prime suspects in making MoS2 – sulfur, the ring-like Mo3O9 molecules that form in sulfur's presence and the subsequent hybrid of MoS6 that forms the crystal, releasing excess sulfur atoms in the process.

Lei said their molecular dynamics simulations showed the activation barriers that must be overcome to move the process along, usually in picoseconds.

"In our molecular dynamics simulation, we find that this ring is opened by its interaction with sulfur, which attacks oxygen connected to the molybdenum atoms. The ring becomes a chain, and further interactions with the sulfur molecules separate this chain into molybdenum sulfide monomers. The most important part is the chain breaking, which overcomes the highest energy barrier."

That realization could help labs streamline the process. "If we can find precursor molecules with only one molybdenum atom, we would not need to overcome the high barrier of breaking the chain," Lei said.

According to Yakobson, the findings from this study could apply to other TMDs. "The findings raise oftentimes empirical nanoengineering to become a basic science-guided endeavor, where processes can be predicted and optimized," he said, noting that while the chemistry has been generally known since the discovery of TMD fullerenes in the early 90s, understanding the specifics will further the development of 2D synthesis.

"Only now can we 'sequence' the step-by-step chemistry involved," Yakobson said. "That will allow us to improve the quality of 2D material, and also see which gas side-products might be useful and captured on the way, opening opportunities for chemical engineering."

This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.