A Peek at the World’s Thinnest Material

Researchers at Northwestern University have recently found an innovative method to see the world’s thinnest material, graphene. Using dye fluorescein – a known chemical, used in many fields and industries – the team introduced a new imaging technique to view this ultra-thin substance.
 Images of graphene oxide sheets deposited on a SiO2/Si substrate (Credit: Northwestern University)
Images of graphene oxide sheets
deposited on a SiO2/Si substrate
(Credit: Northwestern University)

Dye fluorescein is used annually, to dye the Chicago River green on St. Patrick’s Day; CSI departments use it on a regular basis to find latent blood stains; and now, a team of researchers from Northwestern University have shown that it could help image graphene, a one-atom thick sheet. This material is especially useful when producing low-cost carbon-based transparent and flexible electronics.

Jiaxing Huang is an assistant professor of materials science and engineering at the McCormick School of Engineering and Applied Science; he’s a member of the team, which published its research in the Journal of the American Chemical Society. “There are really no good techniques that are general enough to meet the diverse imaging needs in the research and development of this group of new materials,” he said. Since graphene and its derivatives (such as graphene oxide) are quite challenging to see, manufacturing processes tend to be expensive and time-consuming.

An example for a common technique is atomic force microscopy (AFM), which scans materials with a tiny tip. It is frequently used to obtain images of graphene materials, but it’s a slow process that can only look at small areas on smooth surfaces. Another example is scanning electron microscopy (SEM), which scans a surface with high-energy electrons; however, this method only works if the material is placed in vacuum. While there are more alternatives, none solve effectively the visibility problem. “People have proposed putting graphene materials on plastic sheets for flexible electronics, but seeing them on plastic has been very challenging,” Huang said. “If one cannot exam these materials, quality control is going to be difficult.”

One of the techniques that appealed to the team is fluorescent labeling; it has been used routinely to image biological samples, typically by using fluorescent dyes that make the objects of interest light up under a fluorescence microscope. The reason such a technique wasn’t implemented for graphene materials in the past, is a mechanism called fluorescence quenching; they can “turn off” the fluorescence of nearby dye molecules. “So we thought, how about we just put dye everywhere?” Huang reveals their innovative idea. “That way, the whole background lights up, and wherever you have graphene will be dark. It’s an inverse strategy that turns out to work beautifully.”

The results were surprisingly good. When the researchers coated a graphene sample with fluorescein and put it under a fluorescence microscope they obtained images as clear as those acquired with AFM and SEM. The major difference was that this process is much cheaper, and the instrument’s availability is much higher.

According to Huang, the new technique is named fluorescence quenching microscopy (FQM). “When (graduate student) Jaemyung first showed me the FQM images of graphene materials,” he recalls, “I was tricked by the vivid details and thought they were SEM or AFM images.” In addition, the team found that FQM can visualize graphene materials in solution. “No one has been able to demonstrate this before,” says Huang.

There are two more interesting findings; one is that the dye can also be added to photoresist materials, so that graphene sheets can be seen during photolithography, and the second is that the dye is easily washed off – without disrupting the sheets themselves. “It’s a simple and dirt-cheap method that works surprisingly well in many situations,” Huang concludes.

TFOT has also covered another research made at Northwestern University: the Graphene Paper, which is stronger than diamond, and the Innovative Technique for Controlling Graphene Nature, developed by researchers at Rensselaer Polytechnic Institute. Other related TFOT stories include an Evolutionary Material Named Graphane (with an “a”), obtained from graphene by researchers at the University of Manchester, and the Graphene Semiconductors, introduced by physicists at the University of Maryland,

For more information about the innovative method to image graphene, see Northwestern University’s website.