Substrate defects are key to the growth of 2D materials

The creation of two-dimensional materials large enough to be used in electronics is a challenge, despite considerable effort, but Penn State researchers have discovered a method to improve the quality of a class of two-dimensional materials, likely to generate growth at the scale of the slice. future.

The field of two-dimensional materials with unusual properties has exploded in the last 15 years since Konstantin Novoselov and Andre Geim removed a single atomic layer of carbon atoms from loose graphene with the help of a simple adhesive tape. Although much scientific work has been done on these small graphene fragments, it is difficult to grow industrial sized layers.

Among the materials being considered for next-generation electronics, a group of semiconductors called dichalcogenides of transition metals are at the forefront. TDGs have only a few atoms of thickness but are very effective at emitting light, making them candidates for optoelectronics, such as light-emitting diodes, photodetectors or emitters. single photon.

"Our ultimate goal is to manufacture monolayer films of diselenide tungsten or molybdenum disulfide, and deposit them by chemical vapor deposition so as to obtain a perfect monocrystalline layer on an entire slice," said Joan Redwing, teacher of materials. science and electronics, and director of the 2-D Crystal Consortium, a materials innovation platform of the National Science Foundation, based at Penn State.

The problem comes from the way the atoms organize when they are deposited on a standard substrate, such as sapphire. Due to the crystalline structure of TMD, they form triangles at the beginning of their propagation on the substrate. Triangles can be oriented in opposite directions, with equal probability. When they collide and merge into each other to form a continuous sheet, the limit that they form resembles a large defect that dramatically reduces the electronic and optical properties of the crystal .

"When charge carriers, such as electrons or holes, encounter this defect, called inversion domain limit, they can disperse," Redwing said. "This has been a classic problem with the growth of TMD."

In recent publications in journals ACS Nano and Physical examination B, researchers from the departments of Materials Science and Engineering, Physics, Chemistry, and Engineering and Mechanical Sciences indicate that, if the DMTs are grown on a nitride surface of hexagonal boron, at least 85% will go in the same direction. Vin Crespi, a distinguished professor of physics, materials science, engineering and chemistry, and his group conducted simulations to explain why this happened. They discovered that gaps in the hexagonal surface of boron nitride, which lacked a boron or nitrogen atom, could trap a metal atom – tungsten or molybdenum – and would serve to orient the triangles in a favored direction . The improved material exhibited increased photoluminescence emission and electron mobility an order of magnitude higher than 2D TMDs developed on sapphire.

"Our next step is to develop a process to grow hexagonal boron nitride at the scale of a wafer," said Redwing. "That's what we're working on now." It's hard to control defects and to form a single layer of crystal over a large area, and there are many groups working on it. "