A collaboration between research groups at Aarhus University in Denmark and the Elettra Synchrotron in Trieste, Italy, has succeeded in synthesizing single-layer vanadium disulfide for the first time. This synthesis, which involved growing the monolayer on a gold substrate, revealed the existence of a vanadium sulfide crystal with a rectangular unit-cell structure that was different from previously observed phases in the bulk crystal. Published recently in the journal 2D Materials, these results demonstrate a way to grow two-dimensional materials with no bulk analog.

Transition metal dichalcogenides (TMDCs) are materials containing a transition metal, commonly tungsten or molybdenum, bonded to chalcogenides, such as sulfur or selenium. In recent years, researchers discovered methods for isolating atomically thin monolayers of these materials. TMDC monolayers often have additional exotic properties, such as superconductivity or a transition from a metallic to insulating state. “Single-layered transition metal dichalcogenides exhibit many complex correlated electronic states and interesting electronic or optical properties,” says Charlotte Sanders, an assistant professor at Aarhus University who led this study.

Theoretical predictions for monolayer VS₂ suggest that it could possess a metal-to-insulator transition or two-dimensional magnetic behavior. “There’s been so much attention on the TMDCs containing 4d transition metal atoms, such as molybdenum. TMDCs made with 3d transition metals, such as vanadium, have the potential to [have] much more interesting electronic correlations, because the electrons in vanadium are more localized than those in the larger transition metals,” says Chris Marianetti, an assistant professor at Columbia University in condensed-matter physics who calculates properties of materials, including VS₂, from first-principles and is not affiliated with the current work.

Single-layer vanadium disulfide, however, has been hard to isolate. Monolayers of other types of TMDCs are often exfoliated from a bulk crystal, leading to a single layer whose crystal structure matches that of the bulk crystal. Because VS₂ degrades in air, many of these exfoliation methods were unsuitable.

Sanders and her colleagues instead grew monolayers of VS₂ through a bottom-up approach by first depositing a small amount of vanadium metal on a gold substrate. The metal was then exposed to hydrogen disulfide or dimethyldisulfide to form the monolayer. The resulting monolayer exhibited a hexagonal atomic structure. After annealing at a temperature of 823 K, its structure changed to one with a rectangular unit cell with slightly less sulfur. X-ray photoelectron spectroscopy and diffraction identified that there was less sulfur present in the second phase, which could be written as V_1+xS₂. This second phase was “a completely new crystal that had never been seen or predicted before,” Sanders says. The monolayer could be reverted back into the initial phase through an intermediate phase that was sulfur-depleted but still possessed a hexagonal atomic structure.

“This work is very exciting and it could open up a new path forward to study vanadium disulfide and related compounds,” Marianetti says. The next step is to analyze the physical and electronic properties of the newly synthesized VS₂ monolayers. These properties remain an open question, because at least one of the phases observed in the experiment does not resemble those seen in bulk VS₂ crystals or those predicted by theory for two-dimensional VS₂. Sanders and her colleagues plan to study these materials with experimental methods and first-principles calculations to determine their band structure and correlated electronic behaviors.

Sanders anticipates that the bottom-up method of synthesizing two-dimensional TMDCs could offer new ways to control the structure of these materials. In exfoliation, the atomic structure of the bulk crystal plays a large role in the final lattice of the isolated monolayer. Here, substrates can stabilize crystal structures that are different from those seen in bulk crystals. Further substrates could be explored to create more interesting lattice structures. “The fact that entirely new materials can be created that do not have a three-dimensional bulk counterpart will hopefully point the way to the engineering of novel two-dimensional materials by bottom-up approaches,” Sanders says.

Read the abstract in 2D Materials (doi:10.1088/2053-1583/aad0c8).