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A laser beam (yellow) reflects off a 2D material (orange), highlighting a grain boundary defect in the atomic lattice. Image Credit: MRI/Penn State.

By Walt Mills

Two-dimensional materials are atomically thin, the most well-known being graphene, a single-atom-thick layer of carbon atoms. The semiconductor industry is interested in using 2D materials for future electronic devices to further shrink these devices and to lower energy consumption. However, a quick and accurate method of detecting defects in 2D materials is needed in order to determine if the material is suitable for manufacturing. Now a team of researchers from Penn State, Northeastern University, Rice University and Universidade Federal de Minas Gerais in Brazil have developed a technique to quickly and sensitively characterize defects in 2D materials.

“People have struggled to make these 2D materials without defects. That’s the ultimate goal,” said Mauricio Terrones, Distinguished Professor of Physics, Chemistry and Materials Science and Engineering, Penn State. “We want to have a 2D material on a four-inch wafer with at least an acceptable number of defects, but you want to evaluate it in a quick way.”

Their solution is to use laser light combined with a phenomenon called second harmonic generation, in which the frequency of the light shone on the material reflects at double the original frequency. Add to this a technique called dark field imaging, in which extraneous light is filtered out so that defects shine through. This is the first instance in which dark field imaging has been utilized, and it provides three times the brightness as the standard bright field imaging method, making it possible to see types of defects previously invisible.

“The localization and identification of defects with the commonly used bright field second harmonic generation is limited because of interference effects between different grains of 2D materials. In this work we have shown that by the use of dark field SHG we remove the interference effects and reveal the grain boundaries and edges of semiconducting 2D materials. Such a novel technique has good spatial resolution and can image large area samples that could be used to monitor the quality of the material produced in industrial scales,” said Leandro Malard, a senior author on a recent paper in Nano Letters and a professor at the Brazilian university.

Vin Crespi, Distinguished Professor of Physics, Materials Science and Engineering, and Chemistry, Penn State, added, “Crystals are made of atoms, and so the defects within crystals – where atoms are misplaced – are also of atomic size. Usually, powerful, expensive and slow experimental probes that do microscopy using beams of electrons are needed to discern such fine details in a material. Here, we use a fast and accessible optical method that pulls out just the signal that originates from the defect itself to rapidly and reliably find out how 2D materials are stitched together out of grains oriented in different ways.”

Another coauthor compared the technique to finding a particular letter O on a page full of zeroes. “In the dark field, all the zeroes are made invisible so that only the defective O stands out,” said Yuanxi Wang, assistant research professor, Penn State.

The semiconductor industry wants to have the ability to check for defects on the production line, but 2D materials will likely be used in sensors before they are used in electronics, according to Terrones. Because 2D materials are flexible and can be incorporated in very small spaces, they are good candidates for multiple sensors in a smart watch or smart phone and the myriad of other places where small, flexible sensors are required.

According to lead author Bruno Carvalho, a visiting Ph.D. student in Terrones’ group, “The next step would be an improvement of the experimental setup to map zero dimension defects - atomic vacancies for instance - and also extend it to other 2D materials that host different electronic and structural properties."

Other co-authors on the Nano Letters paper, titled “Nonlinear Dark-Field Imaging of One-Dimensional Defects in Monolayer Dichalcogenides,” are Kuzanori Fujisawa, Tianyi Zhang, Ethan Kahn, Ismail Bilgin,  Pulickel Ajayan,  Ana de Paula, Marcos Pimenta and Swastik Kar.

Funding for this work was provided by the National Science Foundation, The Air Force Office of Scientific Research and various Brazilian funding agencies.

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