Nav: Home

Substrate defects key to growth of 2D materials

May 09, 2019

Creating two-dimentional materials large enough to use in electronics is a challenge despite huge effort but now, Penn State researchers have discovered a method for improving the quality of one class of 2D materials, with potential to achieve wafer-scale growth in the future.

The field of 2D materials with unusual properties has exploded in the 15 years since Konstantin Novoselov and Andre Geim pulled a single atomic layer of carbon atoms off of bulk graphene using simple adhesive tape. Although a great amount of science has been conducted on these small fragments of graphene, industrial-sized layers are difficult to grow.

Of 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.

"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 problem comes from the way atoms organize themselves when they are deposited on a standard substrate, such as sapphire. Because of 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 form is like a large defect that drastically reduces 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, researchers in Penn State's Departments of Materials Science and Engineering, Physics, Chemistry, and Engineering Science and Mechanics show that if the TMDs are grown on a surface of hexagonal boron nitride, 85 percent or more will point in the same direction. Vin Crespi, distinguished professor of physics, materials science and engineering and Chemistry, and his group ran simulations to explain why this happened. 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 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.

"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."
-end-
This work occurred in the facilities of the 2D Crystal Consortium Materials Innovation Platform, a National Science Foundation-supported national-user facility focused on advancing the synthesis of 2D materials. The user facility is open to all U.S. universities, government laboratories and industries.

Contributing authors on the ACS Nano paper, titled "Defect-controlled Nucleation and Orientation of WSe2 on hBN: A Route to Single-crystal Epitaxial Monolayers," include Xiaotian Zhang and Fu Zhang, graduate students at the time the research was conducted; and professors Nasim Alem, Mauricio Terrones and Saptarshi Das, as well as Crespi and Redwing.

Contributing authors on the Physical Review B paper, titled "Full Orientation of Epitaxial MoS2 on hBN Assisted by Substrate Defects," include lead authors Fu Zhang, research assistant professor, and recent doctoral graduate Yuanxi Wang.

Penn State

Related Engineering Articles:

Engineering the meniscus
Damage to the meniscus is common, but there remains an unmet need for improved restorative therapies that can overcome poor healing in the avascular regions.
Artificially engineering the intestine
Short bowel syndrome is a debilitating condition with few treatment options, and these treatments have limited efficacy.
Reverse engineering the fireworks of life
An interdisciplinary team of Princeton researchers has successfully reverse engineered the components and sequence of events that lead to microtubule branching.
New method for engineering metabolic pathways
Two approaches provide a faster way to create enzymes and analyze their reactions, leading to the design of more complex molecules.
Engineering for high-speed devices
A research team from the University of Delaware has developed cutting-edge technology for photonics devices that could enable faster communications between phones and computers.
Breakthrough in blood vessel engineering
Growing functional blood vessel networks is no easy task. Previously, other groups have made networks that span millimeters in size.
Next-gen batteries possible with new engineering approach
Dramatically longer-lasting, faster-charging and safer lithium metal batteries may be possible, according to Penn State research, recently published in Nature Energy.
What can snakes teach us about engineering friction?
If you want to know how to make a sneaker with better traction, just ask a snake.
Engineering a plastic-eating enzyme
Scientists have engineered an enzyme which can digest some of our most commonly polluting plastics, providing a potential solution to one of the world's biggest environmental problems.
A new way to do metabolic engineering
University of Illinois researchers have created a novel metabolic engineering method that combines transcriptional activation, transcriptional interference, and gene deletion, and executes them simultaneously, making the process faster and easier.
More Engineering News and Engineering Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Climate Mindset
In the past few months, human beings have come together to fight a global threat. This hour, TED speakers explore how our response can be the catalyst to fight another global crisis: climate change. Guests include political strategist Tom Rivett-Carnac, diplomat Christiana Figueres, climate justice activist Xiye Bastida, and writer, illustrator, and artist Oliver Jeffers.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Speedy Beet
There are few musical moments more well-worn than the first four notes of Beethoven's Fifth Symphony. But in this short, we find out that Beethoven might have made a last-ditch effort to keep his music from ever feeling familiar, to keep pushing his listeners to a kind of psychological limit. Big thanks to our Brooklyn Philharmonic musicians: Deborah Buck and Suzy Perelman on violin, Arash Amini on cello, and Ah Ling Neu on viola. And check out The First Four Notes, Matthew Guerrieri's book on Beethoven's Fifth. Support Radiolab today at Radiolab.org/donate.