2D materials like MoS2 are creating a flurry of interest in labs around the world because they have dramatically different electronic and mechanical properties from their 3D counterparts. This means that they could find use in a host of novel device applications, such as low-power electronic circuits, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a wide variety of surfaces.

The most well known 2D materials are graphene (which is a sheet of carbon just one atom thick) and the transition metal dichalcogenides (TMDs). These so-called van der Waals (vdW) structures have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te).

Strongly interacting sulphur-sulphur layers

Multilayered MoS2 is a semiconductor with a band gap of around 1.2 eV and is composed of stacked triatomic sheets in which each plane of transition metal molybdenum atoms is covalently bonded to, and sandwiched between, two planes of sulphur atoms. Unlike monatomic multilayered graphene, which contains sp2 hybridized carbon atoms, the sulphur-sulphur layers in multilayered MoS2 – with its d-orbital electronic states and small (6.5 A wdW interlayer gap) – strongly interact with each other. This means that the material should undergo an electronic phase transition if high pressures are applied to it.

A team led by Deji Akinwande, Jung-Fu Lin and Abhishek Singh has now confirmed that this is indeed the case and says that it can make multilayered semiconducting MoS2 metallic by applying pressures of around 19 GPa. The researchers say that the transition occurs thanks to overlapping valence and conduction bands in the material, created by sulphur-sulphur interactions as the spacing between the individual MoS2 layers reduces.

Pressurizing MoS2

Akinwande’s team obtained its results by pressurizing MoS2 in a diamond anvil cell and measuring the material’s optical, structural and electronic properties as the pressure was steadily increased up to 35 GPa. Since diamond is transparent, the researchers were able to see exactly what happened to the 2D material using techniques like optical Raman and synchrotron X-ray diffraction spectroscopy.

According to the observations, the MoS2 lattice starts to distort at applied pressures of around 10 GPa. A clear semiconducting-to–metallic transition occurs at around 19 GPa. The drastic drop in resistivity as the material undergoes this transition was confirmed by in situ temperature-dependent resistivity measurements.

Pressure could control band gap and tune optoelectronic properties

The researchers say that the findings might help to make novel MoS2 devices in which pressure can be applied to control the band gap and tune optoelectronic properties. “Applications include pressure sensors, switches and transducers for flexible or rigid electronics,” Akinwande told nanotechweb.org.

The team is now busy investigating the phenomenon in monolayer MoS2, rather than just the multilayered material, and in TMDs where sulphur-sulphur interactions are absent. “New physics, such as high-pressure superconductivity, might arise here,” said Akinwande. “What is more, hybrid graphene-TMD structures will also likely yield interesting physics under high pressure. And, who knows, since we expect that the pressure can be used to vary the amount of electricity flowing through 2D heterostructures, perhaps they could indeed reach a superconducting state under pressure?”

The current work is published in Nature Communications doi:10.1038/ncomms4731.