More optical applications for tungsten diselenide

The proof-of-concept study could lead to ultrathin, lightweight, and flexible photovoltaic cells, light emitting diodes (LEDs), and other optoelectronic devices, they claim.

Their report is one of three papers by different groups describing similar results with this material, published in the March 9 issue of Nature Nanotechnology. The MIT research was carried out by Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics, graduate students Britton Baugher and Yafang Yang, and postdoc Hugh Churchill.

Tungsten diselenide (WSe2) is part of a class of single-molecule-thick materials under investigation for possible use in new optoelectronic devices that manipulate the interactions of light and electricity. In these experiments, the MIT researchers were able to use the material to produce diodes. 

Typically, diodes are made by doping, a process of injecting other atoms into the crystal structure of a host material. By using different materials for this irreversible process, it is possible to make either of the two basic kinds of semiconducting materials, p-type or n-type.

But with the new material, either p-type or n-type functions can be obtained by bringing the thin film into very close proximity with an adjacent metal electrode, and tuning the voltage in this electrode from positive to negative. 

In their experiments, the MIT team produced a device with a sheet of WSe2 material that was electrically doped half n-type and half p-type, creating a working diode that has properties very close to the ideal, Jarillo-Herrero said in a statement.

By making diodes, it is possible to produce all three basic optoelectronic devices: photodetectors, photovoltaic cells, and LEDs; the MIT team has demonstrated all three, Jarillo-Herrero said. 

While these are proof-of-concept devices, and not designed for scaling up, the successful demonstration could point the way toward a wide range of potential uses, he said.

In principle, Jarillo-Herrero said, it should be possible to make LEDs that produce any colour — something that is difficult to do with conventional materials. And because the material is so thin, transparent, and lightweight, devices such as solar cells or displays could potentially be built into building or vehicle windows, or even incorporated into clothing, he said.

While selenium is not as abundant as silicon or other promising materials for electronics, the thinness of these sheets is a big advantage, Churchill said.  ‘It’s thousands or tens of thousands of times thinner [than conventional diode materials] so you’d use thousands of times less material [to make devices of a given size].’

In addition to the diodes the team has produced, the team has also used the same methods to make p-type and n-type transistors and other electronic components. Such transistors could have a significant advantage in speed and power consumption because they are so thin, said Jarillo-Herrero.