May 3 2017
Molecular model of a WS2 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)
When a material is thinned down to a single-atom thickness, it can greatly alter that material's physical properties. For instance, graphene, the popular 2D material, has unmatched electrical conductivity and strength, unlike its bulk form, graphite.
Researchers have started examining several other 2D materials for the purposes of electronics, early cancer diagnosis, sensing, water desalination and a range of other applications. Currently, a team of Penn State researchers in the Department of Physics and the Center for Two-Dimensional and Layered Materials (2DLM) have formulated a quick, nondestructive optical technique for investigating defects in 2D materials.
In the semiconductor industry, for example, defects are important because you can control properties through defects. This is known as defect engineering. Industry knows how to control defects and which types are good for devices.
Mauricio Terrones, Professor of Physics, Penn State
To truly understand the workings of a 2D material like tungsten disulfide, which has a single atom-thick layer of tungsten sandwiched between two atomic layers of sulfur, would require a high-power electron microscope capable of seeing individual atoms and the holes, termed as 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 on the paper, which was published recently in Science Advances.
The shortcomings, according to Kabius, are an increased likelihood of damage to the delicate 2D material, the complex preparation needed for the sample, and the time involved — a whole day of instrument time to image a single sample and a week or more to deduce the results. For all those reasons, and others, researchers decide to combine TEM with another technique of looking at the sample that is faster and simpler.
The method formulated by Terrones and his team uses an optical technique, fluorescent microscopy, in which a laser of a specific wavelength is aimed on a sample. The excited electrons, pushed to a higher energy level, each discharge a photon of a longer wavelength when they drop down to a lower energy level. The longer wavelength can be measured by spectroscopy and provides information regarding the defect type and location on the sample. The team can then compare the results with visual confirmation under the TEM. Theoretical calculations also helped to confirm the optical results.
The sample should be put into a temperature-controlled specimen holder and the temperature decreased to 77 K, nearly 200 °C below zero. At this temperature, the electron-hole pairs that create the fluorescence are bound to the defect — in the case of this research a cluster of sulfur vacancies in the top layer of the sandwich — and discharge 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 research.
For the semiconductor industry, this is a quick measurement, an optical nondestructive method to evaluate defects in 2D systems. 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.
Mauricio Terrones, Professor of Physics, Penn State
With reference to this, co-author Yuanxi Wang, a postdoc in the 2DLM and a theorist, added, "Our calculations show that electrons trapped by vacancies emit light at wavelengths different than the emission from defect-free regions. Regions emitting light at these wavelengths can easily identify vacancies within samples."
Vincent Crespi, distinguished professor of physics, materials science and engineering and chemistry, Penn State, said "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."
Device applications that could be enhanced by this research include membranes with selective pore sizes for eliminating salt from water or for DNA sequencing, gas sensing when gas molecules adhere to specific vacancies and the doping of 2D materials, which is the incorporation of foreign atoms to improve properties.
Other authors on the Science Advances paper, "Optical Identification of Sulfur Vacancies: Bound Excitons at the Edges of Monolayer Tungsten Disulfide," are postdoctoral scholars Kazunori Fujisawa, Bruno Carvalho and Amber McCreary; doctoral students Simin Feng, Zhong Lin and Chanjing Zhou; and research associates Nestor Perea-Lopez and Ana Laura Elias.
This research was supported by The National Science Foundation and the U.S. Army Research Office.