UV lasers convert paper into graphene circuits for optical sensors

A recent study published in npj Flexible Electronics highlights a method for creating graphene circuits on paper using ultraviolet (UV) lasers. This technique involves the direct patterning of laser-induced graphene (LIG) onto lignin-rich paper without the need for chemicals or vacuum chambers. Conducted in ambient air, this process offers a sustainable and scalable approach to electronic manufacturing, producing circuits that function as broadband optical sensors.

Traditional methods of fabricating flexible electronics often rely on plastic films or metal inks, which can present challenges such as ink bleeding and nozzle clogging. Paper, as a biodegradable alternative, typically decomposes at high temperatures, making it difficult to use with conventional lasers. However, the study demonstrates that by precisely controlling light absorption and heat dissipation, graphene can be patterned onto paper without damage.

Researchers employed an LPKF ProtoLaser U4 workstation with a pulsed, diode-pumped solid-state laser at 355 nm. The laser's parameters, including a pulse width of 20 ns and a repetition rate of 25 to 250 kHz, were optimized to match the optical properties of the paper, which absorbed 94% of the laser light. This alignment facilitated efficient energy transfer, enabling the formation of graphitic structures confirmed by Raman spectroscopy.

The study found that laser defocusing was crucial for managing energy delivery and surface heating. By offsetting the beam by approximately 6 mm, the researchers achieved a sheet resistance of about 147 Ω/sq. Optimal results were obtained with a laser power of 0.6 W, a scanning speed of 20 mm/s, and a pulse repetition rate of 200 kHz, resulting in a porous graphitic layer with a thickness of 43 µm.

The resulting circuits demonstrated mechanical stability, maintaining performance after repeated bending cycles. They also showed potential for use in flexible photodetector arrays, responding to UV, visible, and near-infrared light. Additionally, the conductive tracks functioned as actuators, capable of reaching temperatures of 122 °C and operating underwater for extended periods.

This UV laser process enables the production of paper-based electronics without wet-chemical etching, supporting sustainable manufacturing. Future research may explore integrating these circuits with other materials for applications in wearable health monitors and optical diagnostics.