Tungsten selenide (WSe2) belongs to the family of dichalcogenides – 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 or W) and X is a chalcogen (such as S, Se and Te).

When scaled down to monolayers, these materials are direct bandgap semiconductors. 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. Having a direct bandgap is also a real advantage because it is easier to make optoelectronics and photonics devices that exploit electron–hole pair excitation with direct rather than indirect gap semiconductors, which indeed silicon is. However, the problem is that as 2D dichalcogenide films become thicker, they become indirect bandgap semiconductors, and so only feebly absorb and emit light.

Now Ali Javey and colleagues have found that they can make multilayer WSe2 become direct bandgap by simply applying a tensile strain of up to 2% to it. The light emitted by the material as a consequence increases by around 35 times, so that its photoluminescence is as high as that of monolayer, unstrained WSe2.

And that is not all: in contrast to other 2D dichalcogenides such as molybdenite (MoSe2), the amount of strain needed to kick start this indirect-to-direct bandgap transition is relatively small, and easy to achieve in experiments.

The California researchers produced their multilayers by exfoliating bulk WSe2 onto SiO2/Si chips using the now-famous scotch-tape method used to produce graphene (2D carbon sheets) in the lab. They then transferred the multilayers onto a flexible substrate such as PET using the polymer PMMA as a transfer medium. They performed their experiments by flexing the PET using a two-point bending apparatus that strains the tungsten selenide.

More light emitted during a direct bandgap transition

“Applying tensile strain to WSe2 changes the distance between the atoms in this 2D material,” explains Javey. “This changes the electronic band structure of the atoms and transforms the indirect bandgap semiconductor into a direct bandgap one. Much more light is emitted during a direct bandgap transition compared with an indirect bandgap one because the former does not require a phonon (a vibration of the crystal lattice).”

The indirect-to-direct bandgap transition is possible at practical values of strain (like 2%) in WSe2 thanks to the fact that the indirect and direct bandgap values have similar values of energy at zero strain, he tells nanotechweb.org. “Similar effects may be possible in other 2D materials that have comparable direct and indirect bandgap values at zero strain.”

Strained multilayered WSe2 might be used in optoelectronics applications, such as LEDs, lasers and photodetectors, he adds. WSe2 is especially useful for light harvesting and detection, for example, where thicker films of direct bandgap material are needed.

The experiments described in this story are detailed in Nano Letters.