Researchers from Tohoku University in Japan have developed a method of cleaning graphene using femtosecond lasers, which could improve the processing of micro- and nanoscale materials.
The method builds on the team’s work from January 2023, which involved using a femtosecond laser that emitted extremely short, rapid pulses of light to micro/nanofabricate silicon nitride devices (with thicknesses ranging from 5 to 50 nanometres) without a vacuum.
Using the same approach, in Nano Letters the team describes their drilling of a multi-point hole in graphene film without damaging it. Yet, when inspecting the areas irradiated with low-energy laser pulses (which did not make holes) using an electron microscope, they found that contaminants on the graphene had also been removed. Further magnification revealed nanopores less than 10 nanometres in diameter and atomic-level defects, where several carbon atoms were missing in the crystal structures of the graphene.
(a) Schematic of the laser processing system. (b) Formation of 32 laser spots on the graphene film. (c) Image of a graphene film that has been multi-point hole-drilled. (Image: Yuuki Uesugi et al.)
Atomic defects in graphene can be both detrimental and advantageous, depending on the application. While defects sometimes downgrade certain properties, they can also introduce new functionalities or enhance specific characteristics.
Typically, nanolithography and focused ion beam methods are used to manufacture micro/nanoscale materials. However, these can require large-scale equipment, lengthy manufacturing times, and complex operations.
Laser-processed graphene film observed by scanning transmission electron microscopy. The black areas indicate through-holes. The white objects indicate surface contaminants. (Image: Yuuki Uesugi et al.)
"With proper control of the input energy and number of laser shots, we were able to execute precise machining and create holes with diameters ranging from 70 nanometres - much smaller than the laser wavelength of 520 nanometres - to over 1 millimetre," says Yuuki Uesugi, assistant professor at Tohoku University's Institute of Multidisciplinary Research for Advanced Materials.
Uesugi adds: "Observing a tendency for the density of nanopores and defects to increase proportionally with the energy and number of laser shots led us to conclude the formation of nanopores and defects could be manipulated by using a femtosecond laser irradiation. By forming nanopores and atomic-level defects in graphene, not only can electrical conductivity be controlled but also quantum-level characteristics such as spin and valley. Moreover, the contaminant removal by femtosecond laser irradiation found in this research could develop a new method for non-destructively and cleanly washing high-purity graphene."
An image obtained by high-magnification transmission electron microscopy. The red areas indicate nanopores. The blue areas indicate contaminants. Atomic defects exist in the locations indicated by the arrows. (Image: Yuuki Uesugi et al.)
Thanks to the success of the drilling experiment, the researchers now plan to establish a femtosecond laser cleaning technique and investigate atomic defect formation. They claim that further breakthroughs will have a big impact on areas such as quantum materials research and biosensor development.