Research focus: 2-D material promises new generation of optoelectronics devices

Technology News | March 11, 2014

By eeNews Europe

Optoelectronics

The material in question is called tungsten diselenide (WSe₂), which is part of a class of single-molecule-thick materials under investigation by a number of research teams worldwide to see whether the material can be beneficial in creating a series of new optoelectronic devices – ones that can manipulate the interactions of light and electricity.

In the March 9 issue of Nature Nanotechnology presented papers by three different research groups which described their results using WSe₂.

MIT focus on WSe₂
The MIT researchers were able to use the material to produce diodes, the basic building block of modern electronics.

A MIT research team, which comprised Pablo Jarillo-Herrero, the Mitsui Career Development Associate Professor of Physics, graduate students Britton Baugher and Yafang Yang, and postdoc Hugh Churchill, used WSe₂ which was a few atoms thick to create devices that can harness or emit light.

Diodes are typically made by ‘doping’, which is 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.

The MIT team’s experimental setup: Electricity was supplied to a tiny piece of tungsten selenide (small rectangle at center) through two gold wires (from top left and right), causing it to emit light (bright area at center), demonstrating its potential as an LED material.
(Image courtesy of Britt Baugher and Hugh Churchill)

But with WSe₂, either p-type or n-type functions can be obtained just by bringing the ultrathin film into very close proximity with an adjacent metal electrode, and tuning the voltage in this electrode from positive to negative. As a result the material can easily and instantly be switched from one type to the other, which is rarely the case with conventional semiconductors.

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 according to Jarillo-Herrero that are: “very close to the ideal” .

By making diodes, it is possible to produce all three basic optoelectronic devices – LEDs, photodetectors and photovoltaic cells. The MIT team has demonstrated all three in the form of proof-of-concept devices.

“It’s known how to make very large-area materials” of this type, explained Churchill. “There’s no reason you wouldn’t be able to do it on an industrial scale.”

In principle, the MIT researchers believe WSe₂ can be engineered to produce different bandgap values which should make it possible to develop LEDs that produce any color – which is often difficult to achieve with conventional materials. Owing to the material being so thin, transparent, and lightweight, devices such as solar cells or displays will be able to be built into buildings, vehicle windows, or even clothing.

The new material is thousands or tens of thousands of times thinner than conventional diode materials which makes the material a viable commercial alternative to silicon.

Although the field of 2-D materials is still in its infancy the efficiency performance of the WSE₂devices paves the way for new applications which require small optoelectronic elements.

Microscope image shows the teams experimental setup.
(Image courtesy of Hugh Churchill and Felice Frankel)

University of Washington focus on WSe₂

Researchers from the University of Washington (UW) are also focusing on WSe₂ and their project has enabled them to claim to have built the thinnest-known LED that can be used as a source of light energy in electronics.

“We are able to make the thinnest-possible LEDs, only three atoms thick yet mechanically strong. Such thin and foldable LEDs are critical for future portable and integrated electronic devices,” explained Xiaodong Xu, a UW assistant professor in materials science and engineering and in physics.

Xu co-authored a paper about the technology along with Jason Ross, a UW materials science and engineering graduate student.

The layers of the 2-D LED and how it emits light
(Source: University of Washington)

Three-dimensional LEDs are typically 10 to 20 times thicker than the LEDs being developed by the University of Washington team.

The research team is working on more efficient ways to create the thin LEDs and looking at what happens when two-dimensional materials are stacked in different ways. Additionally, these materials have been shown to react with polarized light in new ways that no other materials can, and researchers also will continue to pursue those applications.

Vienna University of Technology focus on WSe₂

A team of researchers at the Vienna University of Technology has also been experimenting with tungsten diselenide and their work is focused at the development of ultrathin flexible solar cells.

The Austrian research team has been studying graphene-like materials that can arranged in ultrathin layers in a way that could build solar cells.

The Vienna University of Technology chose to use tungsten diselenide because it comprises a single layer of tungsten atoms that are connected by selenium atoms above and below the tungsten plane. Similar to graphene, WSe₂ absorbs light but the latter material can also be used to create electrical power.

As much as 95% of light passes through the WSe₂ layer but a tenth of the remaining five percent, which is absorbed by the material, can be converted into electrical power which means that WSe₂-based solar cell layers could in future be integrated into glass facades that could allow part of the light into the building while creating electricity at the same time.