Laser-induced monolayer graphene nanoprocess


May 26, 2023

(Nanowerk News) Graphene has revolutionized materials science since its discovery in 2004, with its high electron mobility, mechanical strength and thermal conductivity. But processing graphene at the micro/nano scale is a challenging process that often involves large-scale equipment and complex operations. Now, Tohoku University researchers have applied their simple femtosecond laser technique to layers of ultra-thin graphene atoms, enabling multi-point hole drilling without damaging the graphene film. Illustration of graphene film drilled by laser irradiation Illustration of graphene film drilled by laser irradiation. The size of the carbon atom is exaggerated and different from the actual size. (Image: Yuuki Uesugi et al.)

Discovered in 2004, graphene has revolutionized many scientific fields. It has outstanding properties such as high electron mobility, mechanical strength and thermal conductivity. Extensive time and effort has been invested in exploring its potential as a next-generation semiconductor material, leading to the development of graphene-based transistors, transparent electrodes and sensors.

But to make these devices practical applications, it is essential to have efficient processing techniques that can assemble graphene films at the micrometer and nanometer scales. Typically, micro/nanoscale materials processing and device fabrication use nanolithography and focused ion beam methods. However, this has posed a longstanding challenge for laboratory researchers because of their need for large-scale equipment, long production times and complex operations.

Back in January, Tohoku University researchers created a technique that could micro/nanofabricate thin silicon nitride devices ranging from 5 to 50 nanometers in thickness. This method uses a femtosecond laser, which emits very short and fast light waves. It turns out to be able to process thin materials quickly and easily without a vacuum environment.

By applying this method to ultra-thin graphene atomic layers, the same group has now successfully drilled multi-point holes without damaging the graphene film. Details of their breakthrough are reported in the journal Nano Letters (“Self-suspended monolayer nano graphene processing and defect formation by femtosecond laser irradiation”). graphene film is being hole-drilled by laser irradiation (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 drilled with multi-point holes. (Image: Yuuki Uesugi et al.)

“With precise control of the input energy and the number of laser shots, we can perform precise machining and create holes ranging in diameter from 70 nanometers – much smaller than the laser wavelength of 520 nanometers – to over 1 millimeter,” said Yuuki Uesugi, assistant professor at Tohoku University’s Multidisciplinary Research Institute for Advanced Materials, and co-author of this paper.

Upon closer examination of the areas irradiated with low-energy laser pulses, which do not bore holes, through high-performance electron microscopy, Uesugi and colleagues found that the contaminants on the graphene had also been removed. Further magnified observations revealed nanopores less than 10 nanometers in diameter and atomic-level defects, where some carbon atoms are missing in the graphene crystal structure. Laser-processed graphene film images were observed by scanning transmission electron microscopy Laser-processed graphene film images were observed by scanning transmission electron microscopy. The black area indicates a through hole. White matter indicates surface contaminants. (Image: Yuuki Uesugi et al.)

Atomic defects in graphene are both detrimental and beneficial, depending on the application. While defects sometimes degrade certain properties, they also introduce new functionality or enhance certain characteristics.

“Observing the trend of the density of nanopores and defects increasing in proportion to the energy and number of laser shots led us to conclude that the formation of nanopores and defects can be manipulated using femtosecond laser irradiation,” added Uesugi. “By forming atomic-level nanopores and defects in graphene, it is possible to control not only electrical conductivity but also quantum-level characteristics such as twists and valleys. In addition, the removal of contaminants by femtosecond laser irradiation discovered in this study could develop new methods for non-damaging and cleaning high-purity graphene.”

Going forward, the team aims to establish a cleaning technique using a laser and carry out detailed investigations of how to carry out atomic defect formation. Further breakthroughs will have major impacts in fields ranging from quantum materials research to biosensor development.


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