Salt-assisted CVD

In plain words

Adding ordinary salts to the gas-phase growth of 2D films. The salt helps the metal ingredients evaporate at lower temperatures, which gives larger, more even crystals.

Going deeper

Adding an alkali halide to a chalcogenide growth turns a stubborn metal oxide into a volatile oxyhalide. Lower temperatures and larger domains follow – along with alkali and halide residue that the film keeps.

What the salt does

The difficulty in growing a metal from an oxide powder is getting the metal into the gas phase at all. Many transition metal oxides have negligible vapour pressure at the temperatures a will tolerate, so the growth is starved of metal and what does arrive nucleates unevenly.

An alkali halide changes the chemistry of the source. The halide reacts with the oxide to form metal oxyhalides – species such as molybdenum dioxydichloride – that are volatile hundreds of degrees below the oxide itself. The metal is now delivered as a gas at a controllable rate, and it arrives at the substrate where the chalcogen vapour reduces and chalcogenises it. The salt also lowers the barrier to nucleation and appears to act as a surfactant on the growing edge, both of which favour fewer, larger crystals.

Why it scaled to a library

The appeal of the method is its generality. The same idea – pair a stubborn metal source with a halide to make something volatile – works across a wide range of metals and all three common chalcogens, so a single furnace recipe, adjusted in temperature and salt, produces sulfides, selenides and tellurides, along with and stacked combinations of them. That is how salt-assisted routes were used to assemble a library of atomically thin metal chalcogenides in one body of work, including compounds nobody had previously grown as .

For the field this mattered more than any single material. It turned monolayer synthesis from a per-compound research problem into something closer to a screening exercise, which is a prerequisite for the kind of broad property surveys that theory had been predicting for years.

The residue nobody wants

The catch is that the salt does not leave cleanly. Sodium and chlorine are detectable in and on the resulting films, and alkali ions are mobile: they dope the channel, drift under a gate bias, and show up as and threshold instability in made from otherwise excellent crystals. A film with beautiful domains and a contaminated interface is not obviously better than a smaller, cleaner one.

The practical problems compound this. Salt vapour corrodes tube furnaces and cross-contaminates subsequent runs, and the amount that takes part depends on how the powder was placed, how much moisture it had absorbed and where the gas flow went – so nominally identical runs drift apart. A report is only reproducible if it states the salt, the quantity, its position relative to the source and substrate, and what the finished film contains when measured by or mass spectrometry rather than assumed.

For specialists

in which alkali halides form volatile oxyhalides with metal oxides, lowering growth temperatures and enlarging domains.

Where this comes from

A library of atomically thin metal chalcogenides Zhou et al. · Nature 556, 355 (2018) cited by 1,693

Chalcogenide, halide and pnictide

Family names for compounds, taken from the element in them that carries the negative charge. Chalcogenides contain sulfur, selenium or tellurium; halides contain fluorine, chlorine, bromine or iodine; pnictides contain phosphorus, arsenic, antimony or bismuth. Most layered crystals belong to one of these families, and the catalogue groups many of them that way.

Compounds named for their most electronegative constituent: chalcogenides of group 16 (S, Se, Te; oxides are conventionally counted apart), halides of group 17 (F, Cl, Br, I) and pnictides of group 15 (P, As, Sb, Bi; nitrides usually keep their own name). Down each group the anion grows larger and less electronegative, so bonding turns more covalent, gaps shrink and spin–orbit coupling grows. Chalcogenides give the MX₂ semiconductors and metals, halides mostly ionic, often magnetic insulators built from edge-sharing octahedra, and pnictides many metals, semimetals and the parents of the iron-based superconductors; mixed-anion compounds such as oxychalcogenides and chalcohalides combine two.

Full explanation

Substrate

The base a thin film or flake sits on – often a polished slice of silicon, glass or sapphire. For a crystal one atom thick, the substrate is not just a table: its bumps, stray charges and vibrations reach right into the sheet and change how it conducts and glows. That is why the same material can behave differently on two substrates, and why the flattest, cleanest one – boron nitride – gives the best results.

The material beneath a 2D layer. It acts on the layer through surface roughness, charged impurities, surface optical phonons, dielectric screening, strain and charge transfer, and during growth it sets orientation through epitaxy. Replacing SiO₂ with hexagonal boron nitride, which is atomically flat and nearly free of dangling bonds and charge traps, improved the mobility and charge homogeneity of graphene by close to an order of magnitude.

Full explanation

Alloy and solid solution

A crystal in which two similar kinds of atom share the same sites at random – some molybdenum sites taken by tungsten, say, or some sulfur sites by selenium – so that one crystal structure holds a mixture. By choosing the mix, its properties can be set almost anywhere between those of the two pure compounds, much as mixing two paints gives any shade between them. Chemists call it a solid solution; alloy is the everyday word.

A single-phase crystal in which two or more species share a sublattice at random, such as Mo_1−xW_xS₂ or MoS_2(1−x)Se_2x, with lattice constants close to Vegard’s law and band gaps that vary continuously with x, often with a small bowing. Complete miscibility needs similar radii, the same structure type and favourable mixing energetics; otherwise the mixture separates into phases or orders. Random disorder adds alloy scattering and inhomogeneous broadening.

Full explanation