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Graphene is an atom-thick form of carbon with such useful properties, it is currently being touted as a kind of ‘wonder material’.
One big issue with graphene is the chemical vapor deposition process typically used to create it is somewhat complicated and costly. Researchers at Rice University recently addressed this issue of process by creating graphene through the simple use of a laser.
Rice University scientists first made laser-induced graphene in 2014 when a standard laser was used to burn the exterior of a polyimide plastic sheet. The straightforward process produced much more graphene, and at a lower cost, than chemical vapor deposition.
Laser-induced graphene produced by this method was not the standard two-dimensional slice of graphene, but a porous froth of connected flakes around 20 microns thick. The laser did not cut right through, so the foam stayed connected to its plastic base.
Upon inspection with a microscope, laser-induced graphene did not appear like normal graphene, an atom-scale grid of polygons. Instead, it was a pile of interlocked flakes with five-, six- and seven-member rings of atoms. Generally, five- and seven-atom rings are thought to be defects. However, the rings ended up being features in laser-induced graphene, not defects.
A standard sheet of graphene is filled with six-member rings. Every now and then, a winding band of 5-and 7-member rings may show up, but laser-induced graphene is packed with 5- and 7-member rings. It’s an odd structure, and yet these structures are effective at trapping electrons.
As a kind of bonus, the laser process allowed for the fabrication of detailed designs. As a demonstration, the Rice team created both millimeter-sized owls, which is the school’s mascot, and microscale supercapacitors with their laser. Using polyimide sheets, the process was done completely under normal room conditions with a rapid writing sequence. The team noted that this process is well-suited for scalable, industrial use.
The laser-induced graphene made by the Rice team wasn’t as conductive as copper. However, it can readily be used as a supercapacitor, a device that blends the fast-charging, storage of a capacitor with the greater energy storage of a battery.
The Rice researchers found that the five- and seven-member formations made their graphene 'more metallic', strengthening its capacity to store electricity. Using theoretical techniques and computations, the team found the minimal density of available states in laser-induced graphene is vital for the layer capacitance and rises dramatically as a result of topological defects.
At first, the team's method only worked with polyimide. The Rice scientists said they tried 15 different polymers and discovered only two might be useful. Of those two, polyimide was found to be the best.
Incidentally, the Rice lab said they weren't even trying to make graphene at first. The fact that they stumbled upon a form of "defective" graphene that is so useful is a “gift from nature.”
Enhancing Laser-induced Graphene
In February 2019, Rice researchers announced that they had significantly expanded the potential applications of laser-induced graphene by incorporating it as part of a composite material.
By combining laser-induced graphene with cement, plastic, rubber, wax or other materials, the lab was able to create composite materials with a broad range of potential applications. The team said their novel materials might be used in various electronics, medical care, de-icing applications, and antimicrobial resistance.
To create the composite materials, the scientists poured or hot-pressed a second thin layer of material over laser-induced graphene on a polyimide sheet. When the additive layer cooled and hardened, the researchers peeled the polyimide away, leaving behind embedded graphene flakes.
These materials preserved the connectivity of laser-induced graphene flakes. In the lab, the composites heated rapidly and consistently when a voltage was applied. This result indicated the composite materials has massive potential in de-icing or heating applications.
In addition to this development, Rice researchers also discovered that they weren’t limited to using just polyimide as a source material. In 2017, they announced that they were able to make laser-induced graphene from wood, and in 2018, the team announced they had accomplished the same feat using toast.
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