(Nanowerk News) Since the discovery of the remarkable properties of graphene, scientists have increasingly focused research on the many other two-dimensional materials possible, both those found in nature and concocted in the lab. However, growing high quality, crystalline 2D materials at scale has proven a significant challenge.
A pair of papers published online in two nanotechnology journals this month provide the basis for growing wafer-scale two-dimensional crystals for future electronic devices. In work led by Joan Redwing, director of the NSF-sponsored Two-Dimensional Crystal Consortium – Materials Innovation Platform, and professor of materials science and engineering and electrical engineering, Penn State, researchers developed a multistep process to make single crystal atomically-thin films of tungsten diselenide across large-area sapphire substrates.
False-colored scanning electron microscope (SEM) image of a radio-frequency field-effect transistor (RF-FET) composed of a 2-3 layer-thick epitaxially-grown tungsten diselenide (WSe2) active channel. (Image: Brian Bersch/Penn State)
“Up until now, the majority of 2D devices have been fabricated using small flakes that are exfoliated off of bulk crystals,” Redwing said. “To develop a device-ready technology, you have to be able to make devices on large-area substrates and they have to have good crystal quality.”
The process uses sapphire as the substrate because of its crystalline structure. This structure orients the film growth in a crystal pattern in a process called epitaxy. As small islands of the material form on the substrate and the substrate is heated, the islands spread out across the substrate in a uniform pattern forming a large-area film without gaps and with very few defects. The key advance was the use of gas source chemical vapor deposition to precisely control the island density and rate of spreading to achieve a single layer of the 2D material.
In a related paper, published online in the journal ACS Nano ("Realizing Large-Scale, Electronic-Grade Two-Dimensional Semiconductors"), a team led by Joshua Robinson, associate professor of materials science and engineering, Penn State, provides the foundational understanding to enable device-ready synthetic 2D semiconductors based on these epitaxial large area films in future industrial-scale electronics.
Large-scale atomically-thin 2D films by gas source chemical vapor deposition. (Image: Xiaotian Zhang/Penn State)
“The primary significance of this work is we were able to achieve an understanding of the extrinsic factors that go into having a high-quality 2D material,” Robinson said. “What we found was that even when you grow oriented crystals on a surface, there are other factor that impact the ability to get high electron mobility or fast transistors.”
In particular, they found that there is a strong interaction between the sapphire substrate and the monolayer film, with the substrate dominating the properties. To overcome these challenges, the researchers grew two or three layers, which improved the performance by factors of 20-100 times.
“This is the first real evidence of the effect of the substrate on the transport properties of 2D layers,” Robinson said.