Material Science

Substrate absconds key to development of 2D materials

Making two-dimensional materials sufficiently vast to use in electronics is a challenge of gigantic exertion however at this point, Penn State scientists have found a strategy for improving the quality of one class of 2D materials, with potential to accomplish wafer-scale development in the future.

The field of 2D materials with surprising properties has detonated in the 15 years since Konstantin Novoselov and Andre Geim pulled a single atomic layer of carbon atoms off of bulk graphene utilizing straightforward adhesive tape. Despite the fact that a lot of science has been directed on these little fragments of graphene, industrial-sized layers are hard to develop.

Of the materials imagined for next-generation electronics, a group of semiconductors called transition metal dichalcogenides are at the forefront. TMDs are just a couple of atoms thick however are very efficient at emitting light, which makes them possibility for optoelectronics, for example, light-emitting diodes, photodetectors, or single-photon emitters.

“Our ultimate goal is to make monolayer films of tungsten diselenide or molybdenum disulfide sheets, and to deposit them using chemical vapor deposition in such a way that we get a perfect single crystal layer over an entire wafer,” said Joan Redwing, professor of materials science and electronics, and director of Penn State’s 2D Crystal Consortium, a National Science Foundation Materials Innovation Platform.

The issue originates from the manner in which atoms compose themselves when they are deposited on a standard substrate, for example, sapphire. In light of the crystal structure of TMDs, they form triangles as they spread over the substrate. The triangles can be situated in inverse ways, with equivalent likelihood. When they bump and merge into each other to form a constant sheet, the boundary they form resembles a vast deformity that radically diminishes the electronic and optical properties of the crystal.

“When the charge carriers, such as electrons or holes, encounter this defect, called an inversion domain boundary, they can scatter,” Redwing said. “This has been a classic problem with TMD growth.”

In recent publications in the journals ACS Nano and Physical Review B, scientists in Penn State’s Departments of Materials Science and Engineering, Physics, Chemistry, and Engineering Science and Mechanics demonstrate that if the TMDs are developed on a surface of hexagonal boron nitride, 85 percent or more will point a similar way. Vin Crespi, recognized professor of mphysics, materials science and engineering and Chemistry, and his group ran simulations to clarify why this occurred. They found that vacancies in the hexagonal boron nitride surface, where a boron or nitrogen atom was missing, could trap a metal atom – tungsten or molybdenum – and serve to situate the triangles a favored way. The improved material indicated expanded photoluminescence emission and an order of magnitude higher electron mobility compared to 2D TMDs grown on sapphire.

“Our next step is to develop a process to grow hexagonal boron nitride across a wafer scale,” Redwing said. “That’s what we’re working on now. It’s difficult to control defects and to grow a single crystal layer across a large surface. Many groups are working on this.”