Atomic-scale laser treatment enhances laser-induced graphene for efficient electrothermal heating and water purification through improved conductivity and structure.
(Nanowerk Spotlight) Graphene’s promise as a material for electronics, energy systems, and sensors rests on its distinctive structure: a single layer of carbon atoms arranged in a hexagonal lattice. It is extremely conductive, chemically stable, and flexible. But many practical routes to making graphene struggle to preserve that ideal structure. Laser-induced graphene (LIG) emerged as a solution that offers simplicity, speed, and versatility.
By using a focused laser beam to directly convert the surface of a carbon-rich polymer, such as polyimide, into graphene-like carbon, researchers can fabricate conductive, porous materials in ambient conditions and on flexible substrates. This method eliminates the need for high-temperature furnaces or chemical processing.
However, LIG’s advantages come with trade-offs. The high energy of the laser breaks chemical bonds and drives out non-carbon elements, but the process also leaves behind a high density of defects. These include missing atoms, distorted ring structures, and disordered carbon networks.
While some of these imperfections can be useful for catalysis or sensing, they disrupt the material’s electrical continuity and limit its reliability in circuits or electrochemical devices. Methods that repair these defects—without damaging the structure or requiring harsh conditions—remain a key challenge.
Various approaches have been tested to address defects in other types of graphene, such as annealing at high temperatures or using reducing chemicals. But these techniques are often incompatible with LIG’s porous architecture or flexible substrates. A more recent attempt involved flash Joule heating in vacuum, which showed that rapid high-temperature treatment could improve LIG structure. Yet this method still required careful tuning for each sample and could not modify specific regions.
Researchers from Central South University and Ben-Gurion University now report a technique that overcomes these limitations. In a study published in Advanced Functional Materials (“Femtosecond Laser Ultrafast Atomic Scale Renovating Laser-Induced Graphene”), the team demonstrates how femtosecond laser pulses can repair atomic-scale defects in LIG with precision, efficiency, and minimal side effects. This method not only improves the material’s conductivity but also transforms its surface chemistry, enabling new applications in heating and water purification.
Schematic of femtosecond laser ultrafast atomic scale renovating LIG. a) Schematic illustration for the renovation process and conductivity changes of LIG. b) Comparison of properties of femtosecond laser renovating and flash Joule heating LIG healing technology. (Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Femtosecond lasers emit pulses lasting just a few hundred femtoseconds—one quadrillionth of a second—with very high peak energy. When applied to LIG, each pulse generates an intense but brief thermal spike, reaching temperatures as high as 2763 K in nanoseconds. This rapid heating and cooling promotes the rearrangement of carbon atoms into more stable configurations, effectively healing the disordered regions left behind by the original laser fabrication process.
The study shows that this treatment leads to substantial improvements in structural order. Raman spectroscopy data indicate that the density of lattice defects is reduced, while the proportion of electrically favorable sp²-hybridized carbon increases. X-ray photoelectron spectroscopy confirms a decrease in defect-associated sp³ bonding and a slight reduction in oxygen and nitrogen content, suggesting the removal of residual heteroatoms. High-resolution transmission electron microscopy further reveals that the treated LIG, referred to as FLR-LIG, has fewer distorted carbon rings and more regular hexagonal arrangements.
These structural changes directly translate to improved electrical performance. The resistance of untreated LIG samples was measured at 593 ohms. After femtosecond laser treatment, the resistance dropped to 118 ohms. Sheet resistance—a key parameter for thin-film materials—also fell from about 98 to 20 ohms per square. Computational simulations supported these results, showing that the treated material had a smaller bandgap and more uniform electron density distribution, both of which contribute to better conductivity.
Beyond improved electrical characteristics, the treatment also changes how the material interacts with water. Untreated LIG is highly water-repellent, with a contact angle of about 151°, making it superhydrophobic. After femtosecond laser exposure, the same material becomes superhydrophilic, with water spreading across its surface almost instantly. This shift is likely caused by a combination of structural changes and the formation of oxygen-containing functional groups during the high-temperature pulse in ambient air.
This combination of higher conductivity and water-attracting surface properties makes FLR-LIG suitable for electrothermal evaporation systems. In this application, a voltage is applied across the material to heat it through electrical resistance. The researchers tested FLR-LIG as a heating surface and showed that it could reach nearly 192°C at 4 volts—more than twice the temperature reached by untreated LIG under the same conditions. This thermal performance allowed the team to build a small-scale water evaporation platform, where heat from the LIG surface drives rapid vaporization of water.
Using this setup, the team achieved an evaporation rate of 7.91 kilograms per square meter per hour under a 4-volt input. This is among the highest rates reported for electrothermal evaporation systems operating at such low voltages. The researchers also demonstrated that the system could purify different types of water: it removed dyes like methylene blue and rhodamine B, neutralized acidic and alkaline solutions, and desalted simulated seawater. In each case, the condensed vapor was colorless, had neutral pH, and showed significantly higher electrical resistance—indicating effective removal of solutes and contaminants.
Another advantage of this method is its spatial precision. Because the femtosecond laser can be tightly focused, it allows selective renovation of specific regions within a graphene circuit. The study shows that different branches of a patterned LIG circuit can be selectively treated to modify resistance in localized areas. This enables functional tuning within a single device, which could be useful for sensors, logic components, or anti-counterfeiting applications.
The technique also proves to be efficient. While some methods require multiple laser passes or chemical steps to enhance conductivity, the femtosecond laser achieves near-optimal results with just one pass. It operates in ambient air, without the need for inert gases, high vacuum, or high-pressure chambers. The process integrates defect repair, conductivity enhancement, and surface modification in a single step.
By addressing the structural weaknesses of laser-induced graphene through an elegant physical process, this work opens the door to broader applications for LIG in energy, electronics, and environmental systems. The researchers have shown that it is possible to fine-tune both the internal atomic order and external functional properties of graphene-like materials using nothing more than light, applied with the right speed and intensity. The result is a more capable, customizable, and scalable graphene platform—ready for use in systems where performance and simplicity need to go hand in hand.
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