Subtle changes in growth temperature alter the form of a four-component alloy comprising molybdenum, tungsten, sulfur and selenium. The alloy can be tuned to alter its optical bandgap, and so could find use in solar cells and light-emitting diodes. Image: Alex Kutana/Rice University.Scientists at Rice University have discovered a two-dimensional (2D) alloy with an optical bandgap that can be tuned by the temperature used to grow the alloy.
The Rice lab of materials scientist Pulickel Ajayan grew the four-component alloy, which comprises the transition metals molybdenum and tungsten and the chalcogens sulfur and selenium, in a chemical vapor deposition furnace. They found that changes in temperature produced subtle changes in the way the atoms assembled, altering the properties that determine how the alloy absorbs and emits light.
Their experiments built upon work conducted in the lab of Rice theoretical physicist Boris Yakobson, which involved creating scores of models to predict how various combinations of the four elements should work.
This process should be of interest to engineers looking to make smaller, more-efficient devices. Because the alloy’s bandgap falls in the optical range of the electromagnetic spectrum, solar cells and light-emitting diodes (LEDs) might be the first beneficiaries. The scientists report their discovery in a paper in Advanced Materials.
The theoretical team, led by co-lead author and Rice research scientist Alex Kutana, generated 152 random models of the alloy that showed the bandgap could be tuned from 1.62 to 1.84 electron volts by varying the growth temperature from 650°C to 800°C. The experimental team, led by Sandhya Susarla, then made and tested the thermodynamically stable materials in a furnace at 50°C increments. Scientists at Oak Ridge National Laboratory led by postdoctoral researcher Jordan Hachtel produced microscope images that identified and detailed the position of each atom in the alloys.
"Labs have made 2D materials with two or three components, but we don't believe anyone has tried four," said co-author and Rice postdoctoral researcher Chandra Sekhar Tiwary. "Having four components gives us an additional degree of freedom. With fewer materials, every adjustment you make to change the bandgap turns it into a different material. That's not the case here."
"What we've made should be very useful," added Susarla, a Rice graduate student. "For applications like solar cells and LEDs, you need a material that has a broad bandgap."
Tiwary said the alloy can be tuned to cover the entire spectrum of visible light, from 400nm to 700nm wavelengths. "That's a huge range we can cover by just changing this composition," he said. "If we choose the composition correctly, we can hit the correct bandgap or correct emission point."
"These materials are arguably the most important 2D semiconductors because of their excellent optoelectronic properties and low cost," Kutana said. "Our high-throughput calculations permitted us to avoid prior assumptions about how the alloy bandgap behaved. The surprising outcome was how regular the bandgap changes were, resulting in optical properties that are both useful and predictable."
This story is adapted from material from Rice University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.