Molecular model of a tungsten disulfide triangular monolayer targeted with a green laser (hv'). Red light (hv) is emitted from the edges, where defects consisting of sulfur vacancies are located. Electron-hole pairs are bound at the vacancy site (see inset). Image: Yuanxi Wang, Penn State.A team of researchers in the Department of Physics and the Center for Two-Dimensional and Layered Materials (2DLM) at Penn State has developed a fast, non-destructive optical method for analyzing defects in 2D materials. They report this novel method in a paper in Science Advances.
"In the semiconductor industry, for example, defects are important because you can control properties through defects," said Mauricio Terrones, professor of physics, materials science and engineering and chemistry. "This is known as defect engineering. Industry knows how to control defects and which types are good for devices."
To really understand what is going on in a 2D material like tungsten disulfide, which comprises a single atom-thick layer of tungsten sandwiched between two atomic layers of sulfur, requires a high-power electron microscope capable of seeing individual atoms and the holes, called vacancies, where the atoms are missing.
"The benefit of transmission electron microscopy (TEM) is that you get an image and you can see directly what is going on – you get direct evidence," said Bernd Kabius, staff scientist at Penn State's Materials Research Institute, an expert in TEM and a co-author of the paper.
The downsides to TEM, according to Kabius, are an increased possibility of damaging the delicate 2D material, complex sample preparation processes, and the time involved – an entire day of instrument time to image a single sample and a week or more to interpret the results. For those reasons, and others, researchers would like to combine TEM with another method of looking at the sample that is simpler and faster.
The technique developed by Terrones and his team employs fluorescent microscopy, which involves shining laser light at a specific wavelength on a sample. In this novel technique, electrons excited by the laser light are pushed to a higher energy level, and then each emit a photon of a longer wavelength when they subsequently drop back down to a lower energy level. The longer wavelength can be measured by spectroscopy to provide information on the type and location of defects in the sample. The team can then correlate the results with visual images produced by the TEM; theoretical calculations can also help to validate the optical results.
The sample must be placed in a temperature-controlled specimen holder and the temperature lowered to 77K, almost 200°C below zero. At this temperature, the electron-hole pairs that produce the fluorescence are bound to the defect – in this case, a group of sulfur vacancies in the top layer of the sandwich – and emit a signal stronger than the pristine areas of the material.
"For the first time, we have established a direct relationship between the optical response and the amount of atomic defects in two-dimensional materials," said Victor Carozo, former postdoctoral scholar in Terrones' lab and first author of the work.
"For the semiconductor industry, this is a quick measurement, an optical non-destructive method to evaluate defects in 2D systems," added Terrones. "The important thing is that we were able to correlate our optical method with TEM and also with atomistic simulations. I think this method can be very helpful in establishing a protocol for characterization of 2D crystalline materials."
"Our calculations show that electrons trapped by vacancies emit light at wavelengths different than the emission from defect-free regions," said Yuanxi Wang, a postdoc in the 2DLM and a theorist. "Regions emitting light at these wavelengths can easily identify vacancies within samples."
"We can establish not just an empirical correlation between the presence of certain defects and modified light emission, but also identify the reason for that correlation through first-principles calculations," said Vincent Crespi, professor of physics, materials science and engineering and chemistry.
This novel analytical technique could lead to advances in various technologies. These include membranes with selective pore sizes for removing salt from water or for DNA sequencing, gas sensing when gas molecules bind to specific vacancies and the doping of 2D materials.
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.