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

To further shrink electronic devices and lower energy consumption, the semiconductor industry is interested in using atomically thin two-dimensional (2D) materials such as graphene, a single-atom-thick layer of carbon atoms. But manufacturers need a quick and accurate method for detecting defects in these materials to determine if they’re suitable for device manufacture.

Now, a team of researchers from the US and Brazil, including from Penn State, has developed a technique to quickly and sensitively characterize defects in 2D materials. The researchers report their new technique in a paper in Nano Letters.

"People have struggled to make these 2D materials without defects," said Mauricio Terrones, professor of physics at Penn State and a co-author of the paper. "That's the ultimate goal. 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."

The solution developed by the researchers uses laser light combined with second harmonic generation (SHG), a phenomenon in which the frequency of the light shone on a material reflects at double the original frequency. To this, they added dark field imaging, a technique in which extraneous light is filtered out so that defects shine through. According to the researchers, this is the first time that dark field imaging has been used for 2D materials, and it provides three times the brightness of 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," said Leandro Mallard, a professor at the Universidade Federal de Minas Gerais in Brazil and senior author of the paper. "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."

"Crystals are made of atoms, and so the defects within crystals – where atoms are misplaced – are also of atomic size," added Vincent Crespi, distinguished professor of physics, materials science and engineering, and chemistry at Penn State and a co-author of the paper. "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."

Yuanxi Wang, assistant research professor at Penn State's Materials Research Institute, and another co-author, compared the technique to finding a particular zero on a page full of zeroes. "In the dark field, all the zeroes are made invisible so that only the defective zero stands out," he said.

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, said Terrones. Because 2D materials are flexible and can be incorporated into very small spaces, they are good candidates as multiple sensors in a smartwatch or smartphone, as well as a myriad of other places where small, flexible sensors are required.

"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," said lead author Bruno Carvalho at the Universidade Federal do Rio Grande do Norte, a former visiting scholar in Terrones' group.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.