A nanoparticle of nickel leads the creation of valuable molybdenum disulfide nanoribbons, which uniquely form in two layers, with a thin top catalyzed by the speedy nanoparticle riding at the crest of a wave-like bottom layer. Image: Ksenia Bets/Yakobson Research Group.
A nanoparticle of nickel leads the creation of valuable molybdenum disulfide nanoribbons, which uniquely form in two layers, with a thin top catalyzed by the speedy nanoparticle riding at the crest of a wave-like bottom layer. Image: Ksenia Bets/Yakobson Research Group.

It’s now possible to quickly make ultrathin nanoribbons of molybdenum disulfide (MoS2), thanks to a speedy nickel nanoparticle leading the way. Materials theorist Boris Yakobson and his team at Rice University’s George R. Brown School of Engineering collaborated with the Honda Research Institute (HRI) and others to make tightly controlled bilayer nanoribbons of MoS2, which have potential applications in quantum computing.

The researchers found that nanoparticles of nickel exposed to powders of molybdenum oxide and sodium bromide and sulfur gas in a chemical vapor deposition furnace wrangle the resulting nanoribbons into shape, constraining their width to several micrometers. At the same time, the nickel catalyzes the formation of a thinner second layer of less than 30nm, roughly equivalent to the width of the nanoparticle itself. They report their work in a paper in Science Advances.

MoS2 is a transition metal dichalcogenide, with a layer of molybdenum atoms sandwiched between layers of sulfur atoms in a crystal lattice. In bulk form, it’s valued for its mechanical strength and use as a lubricant, and in 2D form for its semiconducting properties. These make MoS2 nanoribbons a possible boon for microelectronics and catalysis, and any process that helps make them in bulk would be welcome.

“The value originates from the very narrow dimensions of the nanowires, which can serve as single-electron transistors,” said Yakobson. He described the underlying phenomenon as a Coulomb blockade, a quantization of electrical charge in which voltage rises and falls in steps rather than at a steady rate. This effect has been observed in quantum dots, in which conductance is not constant at low bias voltages.

“The characteristic current oscillations with gate voltage rise have never been observed at such a high temperature (60K or -351°F),” Yakobson said. “This suggests the further formation of quantum dots with 1nm diameter and the possibility of controlled single-photon emission, the qubits in quantum computing.”

“This is remarkable, as normally the oscillations are observed only at temperatures below 4K in 2D flakes,” added co-author and Rice graduate student Jincheng Lei.

The researchers discovered that a moist environment was necessary for atoms of sodium, molybdenum, nickel and oxygen to settle on the silicon oxide/silicon substrate, whereupon exposure to sulfur at 770°C (1418°F) prompted the formation of MoS2 bilayers. The same process without nickel yielded common MoS2 flakes.

An interesting aspect of the growth process is that the nickel, deposited as a liquified droplet, contributes to the formation of both the wide bottom ribbon and the thin top ribbon. The bottom ribbon serves as an epitaxial template as the droplet catalyzes the thin needle of MoS2 on top. The researchers found that putting a trench in the substrate could trap the nickel droplet and stop ribbon formation entirely.

Why the ribbons form at all was a matter for the Rice researchers to investigate. Through performing first-principle calculations, Yakobson and his Rice co-authors Lei and research scientist Ksenia Bets discovered that whereas noncatalytic growth is responsible for the wide bottom layer, the nickel nanoparticle is directly responsible for catalyzing the thinner ribbon on top.

“The growth in this system is quite peculiar in that it is present in two forms at the same time,” Bets explained. “In both cases, the MoS6 molecule serves as a precursor, and the extra four sulfur atoms need to be removed to form an MoS2 crystal. A very fast catalytic desulfurization on the surface of the nickel particle facilitates the first mechanism, resulting in the formation of the narrow top ribbon.

“In contrast, the second process happens directly on the edge of the bottom layer and is much slower, leading to continuous slow expansion. By depositing MoS2 on one side, the nickel particle is propelled forward, like a jet ski, and the top layer ribbon is left behind as a footprint, while the bottom layer continues to slowly expand akin to the waves behind a boat.”

The researchers also found that the vapor-liquid-solid synthesis mechanism led to a speed of growth orders of magnitude higher than the conventional growth of MoS2 flakes. As the nickel desulfurized MoS6, the energy barrier dropped significantly, leading to a bilayer ribbon that formed 23 times faster than it did in noncatalytic growth simulations. This phenomenon matched what the HRI scientists saw in their experiments.

That speed could make the ribbons more practical for incorporation into products.  “The potential applications are extremely broad,” said corresponding author Avetik Harutyunyan, senior chief scientist at HRI. “We see immediate opportunities for applications in high-speed, low-energy consumption electronics, spintronics, quantum sensing, and quantum and neuromorphic computing.”

This work aligns nicely with a recent study by Yakobson’s group, including Lei, that analyzed how MoS2 flakes flash into existence during chemical vapor deposition. Like that work, this new study could lead to techniques for making nanoribbons of other dichalcogenides.

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.