Two-dimensional semiconducting transition metal dichalcogenides (TMDCs) such as MoS2 and WS2 are layered semiconducting films that might be used to make circuits for low-power electronics, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a wide variety of surfaces. These so-called van der Waals materials have the chemical formula MX2, where M is a transition metal (such as Mo and W) and X is a chalcogen (such as S, Se and Te). They go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers. These monolayers efficiently absorb and emit light and so could also be ideal for making a variety of optoelectronics devices such as light-emitting diodes and solar cells.

The materials are usually grown in monolayers by chemical vapour deposition and these individual layers can be stacked on top of each other to fabricate 2D heterostructures on a large scale. However, although researchers have extensively studied the electrical and optical properties of these heterostructures, they have not really looked at their elastic or mechanical characteristics until now.

Measuring elastic modulus

Elastic modulus is a basic parameter to determine the mechanical properties of materials and it will be important to measure in 2D materials when they are used in flexible and stretchable electronics and photonics applications, explains team leader Junqiao Wu of the University of California, Berkeley, and the Lawrence Berkeley National Lab. 2D materials lend themselves well to such applications thanks to their being ultrathin and very flexible.

Wu’s team measured the elastic modulus of CVD-grown monolayer MoS2 and WS2 and also looked at how the individual layers in these structures interacted with each other. The researchers did this using a method called nanoindentation that makes use of an atomic force microscope tip to push a freestanding monolayer membrane that has been clamped at its circular edge. “We can measure how the membrane deforms from how the tip moves,” says team member Kai Liu, “and also determine the force applied at the AFM tip to calculate the membrane’s elastic modulus.”

In fact, we can obtain two parameters using this method. The first is the membrane’s pretension (that is, how tightly stretched the film is) and the other is its elastic modulus, he tells

Useful information for making flexible devices

The researchers found that the MoS2 and WS2 have very similar high 2D elastic moduli of around 170 N/m – a value very close to that of “exfoliated” MoS2 obtained by the now-famous “sticky tape” method that can be used to shave off 2D monolayers from a bulk block of the parent material. Graphene – the most famous of all 2D materials – was first isolated in this way (by shaving off monolayers from bulk graphite, the material found in pencil lead).

“Our results provide very useful information for making flexible devices from these 2D structures,” says Wu. “With the nanoindentation technique we used, we could, in principle, pre-select and pre-screen 2D heterostructures that are mechanically strong.”

Spurred on by its findings, the team says that it would now like to quantify interlayer attraction and friction in various other 2D heterostructured materials, by combining first-principles theoretical modelling and more indentation experiments.

The research described in this story is published in Nano Letters.