Researchers at the University of Washington, Massachusetts Institute of Technology (MIT), and Vienna University of Technology (TU) in Austria have all shown interesting optoelectronics results with a new two-dimensional (2-D) material called tungsten diselenide (WSe2).
The main page of the journal Nature Nanotechnology this week looks almost as though some mistake had been made with three of the top four stories highlighting “2D crystals.” But it was no mistake and foretells of a near future in which new 2-D materials are developed that will show either improved or complementary properties to that of graphene, the granddad of 2-D materials.
Tungsten diselenide belongs to a larger group of transition metal dichalcogenides that also includes molybdenum disulfide (MoS2). This family consists of materials that combine one of 15 transition metals with one of three members of the chalcogen family: sulfur, selenium, or tellurium. With only a few of these transition metals having been experimented upon, it’s likely we should see more coming down the pike.
The three research groups focused on optoelectronics applications of tungsten diselenide, but each with a slightly different emphasis.
The University of Washington scientists highlighted applications of the material for a light emitting diode (LED). The Vienna University of Technology group focused on the material’s photovoltaic applications. And, finally, the MIT group looked at all of the optoelectronic applications for the material that would result from the way it can be switched from being a p-type to a n-type semiconductor.
The MIT research, which produced diodes, bears some explaining. Diodes are typically made by doping two adjacent parts of a semiconductor, one so it has excess electrons (n-type) the other so it has excess positive charge (p-type). The doping determines which way current flows through the device. Once the semiconductor is doped, that’s it; its direction is set for life. However, what is intriguing about WSe2 is that if the researchers brought the material into close proximity with a metal electrode and tuned the voltage on that electrode from positive to negative, then the WSe2 could be switched between p-type and n-type.
When the 2-D material is made into LEDs, as the University of Washington researchers have done, it becomes the thinnest-known LED. Standard LEDs used in electronics today are at least 10 to 20 times thicker than the device developed by the Washington researchers.
The Vienna researchers showed that a WSe2 photovoltaic cell is so thin that it lets 95 percent of incident light to pass through it, but it will capture a tenth of the remaining 5 percent and convert it to electricity, resulting in a high, internal conversion efficiency. This could translate to a material that would be put in the windows of buildings, allowing light through but also capable of collecting energy.
We are likely to see more potential applications for this material outside of optoelectronics. But what may be even more intriguing is what will be the next transition metal that looks to have promising properties.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.