Process for Producing Layered 2-D Materials Determines Their Electronic Properties

By Dexter Johnson

http://spectrum.ieee.org/nanoclast/semiconductors/materials/process-for-producing-layered-2d-materials-determines-their-electronic-properties

This year, we’ve seen the emergence of different types of transistors being produced entirely from layered two-dimensional (2-D) materials featuring the dichalcogenides, tungsten diselenide (WSe2) and molybdenum disulfide(MoS2). Researchers at Argonne National Laboratory in Illinois produced a transparent thin-film transistor (TFT) with WSe2 as the semiconducting layer, graphene for the electrodes, and hexagonal boron nitride as the insulator. At about the same time, researchers at Lawrence Berkeley National Laboratory in California built an all 2-D transistor that took the shape of a field emission transistor (FET), with MoS2 as the semiconducting layer.

Now, in a collaborative effort, researchers at Rice University, Oak Ridge National Laboratory, Vanderbilt University, and Pennsylvania State University have developed a novel method for producing these hybrid layered 2-D structures. Their technique, they report, provides a high degree of control on how the resulting devices perform.

In research published in the journal Nature Materials, the researchers demonstrated how, by altering the temperature the materials are exposed to during the chemical vapor deposition (CVD) process used to produce these 2-D layered devices, they could yield either an in-plane monolayer composite, which has a small but stable band gap, or a stacked layered hybrid, which exhibits enhanced photoluminescence. At high temperatures, the researchers got vertically stacked bilayers of MoS2 and WSe2, with the tungsten on top. At lower temperatures, the two 2-D materials grew side by side.

“With the advent of 2-D layered materials, people are trying to build artificial structures using graphene and now dichalcogenides as building blocks,” said Pulickel Ajayan of Rice University in a press release. “We show that depending on the conditions, we can combine two dichalcogenides to grow either in-plane hybrid or in stacks.”

“What’s even more interesting is that the layered structure has a particular lock-in stacking order,” said Wu Zhou of Oak Ridge National Laboratory in the news release. “When you stack 2-D materials by transferring layers, there’s no way to control their orientation to one another. That impacts their electronic properties. In this paper, we demonstrate that in a certain window, we can get a particular stacking order during growth, with a particular orientation.”

Ajayan has characterized the development as “pixel engineering” because atomically thin semiconductors could be manipulated in production so that their potential uses in optoelectronics are almost limitless.

“We should be able to tweak certain regions to control certain functions, like light or terahertz emission,” said Rice's Robert Vajtai, another of the study's coauthors, in the release. “The whole idea, really, is to create domains with different electronic characters within a single layer.”