Jun 19, 2019 | Updated: 09:31 AM EDT

Substrate Defect Crucial for Growth of 2D Materials

May 09, 2019 01:12 PM EDT

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Substrate Defect Crucial for Growth of 2D Materials
(Photo : Xiaotian Zhang/Penn State)

Despite tremendous efforts, it is hugely challenging creating two-dimensional materials large enough to use in electronics. Recently, researchers at Penn State have discovered a method for improving the quality of one class of 2D materials with the potential of achieving wafer-scale growth in the future.

Since Konstantin Novoselov and Andre Geim pulled a single atom layer of carbon atoms off of bulk graphene using simple adhesive tape, the 2D material field with unusual properties has exploded for more than 15 years. Even when a significant amount of science has been conducted on these small fragments of graphene, industrial-sized layers are difficult to grow.

Among the materials envisioned for next-generation electronics, a group of semiconductors called transition metal dichalcogenides are at the forefront. TMDs are only a few atoms thick but are very efficient at emitting light which makes them candidates for optoelectronics such as light-emitting diodes, photodetectors, or single-photon emitters.

The director of Penn State's 2D Crystal Consortium, a National Science Foundation Materials Innovation Platform and professor of materials science and electronics, Joan Redwing said that their 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 they get a perfect single crystal layer over an entire wafer.

The challenge they had is the way atoms organize themselves when they are deposited on a standard substrate such as sapphire. Due to the crystal structure of TMDs, they form triangles as they begin to spread across the substrate. The triangles can be oriented in opposite directions with equal probability. When they bump and merge into one another to form a continuous sheet, the boundary they create is like a substantial defect that drastically reduces the electronic and optical properties of the crystal.

Redwing explained that when the charge carriers such as electrons or holes encounter this defect, called an inversion domain boundary; they can scatter. This situation has been a classic problem with TMD growth.

A team of researchers in Penn State's Departments of Materials Science and Engineering, Physics, Chemistry, and Engineering Science and Mechanics have shown in a recent publications in the journals ACS Nano and Physical Review B that if the TMDs are grown on a surface of hexagonal boron nitride, 85 percent or more will point in the same direction.

Distinguished professor of physics, materials science and engineering and Chemistry, Vin Crespi and his team ran simultaneously explain why this happened. In their discovery, 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 orient the triangles in a preferred direction. The improved material showed increased photoluminescence emission and an order of magnitude higher electron mobility compared to 2D TMDs grown on sapphire.

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