Thinning a material down to a single-atom thickness can dramatically change that material’s physical properties. Graphene, the best known two-dimensional (2D) material, has unparalleled strength and electrical conductivity, unlike its bulk form as graphite. Studies have been conducted on hundreds of other 2D materials for the purposes of electronics, sensing, early cancer diagnosis, water desalination, and a host of other applications.

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 (inset). (Yuanxi Wang)

A fast, nondestructive optical method was developed for analyzing defects in 2D materials. In the semiconductor industry, for example, defects are important because properties can be controlled through defects, a process called defect engineering. To understand what is happening in 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, or vacancies, where the atoms are missing. The benefit of transmission electron microscopy (TEM) is that it provides a direct view of what is happening. The downsides are an increased possibility of damage to the delicate 2D material, the complex preparation required of the sample, and the time involved — usually 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 new technique uses an optical method, fluorescent microscopy, in which a laser of a specific wavelength is shone on a sample, and the excited electrons, pushed to a higher energy level, each emit a photon of a longer wavelength when the electron drops down to a lower energy level. The wavelength, or color of light, can be measured by spectroscopy, and gives information about the defect type and location on the sample. This data shows up as peaks on a graph, which is then correlated to visual confirmation under the TEM. Theoretical calculations also helped to validate the optical results. A necessary step in the process requires placing the sample in a temperature-controlled specimen holder, or stage, and lowering the temperature to 77 kelvin, almost 200 °C below 0. At this temperature, the electron-hole pairs that produce the fluorescence are bound to the defect — in the case of this work, 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 semiconductor industry, this is a quick, optical, nondestructive method to evaluate defects in 2D systems. 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.

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Tech Briefs Magazine

This article first appeared in the May, 2018 issue of Tech Briefs Magazine.

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