A little change in temperature makes a big difference for creating the next generation of hybrid atomic-layer structures, according to scientists at Rice University, Oak Ridge National Laboratory, Vanderbilt University and Pennsylvania State University. Rice scientists led the first single-step growth of self-assembled hybrid layers made of two elements that can either be side-by-side and one-atom thick or stacked atop each other, depending on the growth temperature. The discovery could lead to what Rice materials scientist Pulickel Ajayan calls “pixel engineering”: atomically thin semiconductors with no limit to their potential for use in optoelectronic devices.
The researchers, led by Ajayan and Wu Zhou, a materials scientist at Oak Ridge, discovered the new composites when they combined the growth of two-dimensional molybdenum disulfide and tungsten disulfide through chemical vapor deposition. In this process, specific gases are heated in a furnace, where their atoms gather in an orderly fashion around a catalyst to form the crystalline material. High-temperature growth, about 850 C (1,563 F), yielded vertically stacked bilayers, with tungsten on top. Lower temperatures of about 650 C (1,202 F) caused the crystal lattices to grow side by side. The interfaces in either material are sharp and clean.
“With the advent of 2-D layered materials, people are trying to build artificial structures using graphene and now dichalcogenides as building blocks,” Ajayan says, but graphene is atomically flat and dichalcogenides are not quite flat, so there is some incompatibility when these are grown together. However, two dichalcogenides with different compositions could be compatible. “We show that depending on the conditions, we can combine two dichalcogenides to grow either in-plane hybrid or in stacks.”
The monolayer composites have small but stable band gaps, while the stacked composite layers show modified electronic properties such as enhanced photoluminescence, which will be useful for electronics that rely on optical signals. The new materials could be used for vertically stacked field-effect transistors as well as electronic devices only a few atoms thick. “The whole idea, really, is to create domains with different electronic characters within a single layer,” says Robert Vajtai of Rice University, a co-author of the study.