Laser-induced graphitization (LIG) of a Mediterranean cultivation softwood: does anisotropy matter?

Laser-Induced Graphitization (LIG) is a cost-effective, scalable and versatile technique that allows the conversion of carbon-rich substrates into conductive carbonaceous layers, particularly promising in the field of printed electronics. Laser-Induced Graphitization of lignocellulosic materials (green LIG) represents an attractive choice for the development of sustainable zero-waste electronic and electrochemical devices. The focus of this study is on Cedrus sp. (cedar), a fast-growing cultivation softwood diffused in the Mediterranean area, mainly employed in arboriculture or processed in chips for biofuels. Cedar wood (dry density 0.47 g/cm3, longitudinal cut, minimum thickness 1 mm) was laser-scribed in room conditions using a conventional system. Laser power, defocusing and scribing direction have been systematically varied. Pristine and antiflame-treated wood substrates have been investigated. A comprehensive structural characterization was performed from the macro- to the nanoscale by optical microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. The thermal degradation profiles were acquired by thermogravimetry and the electrical properties determined by four-point probe multimeter. Optimized LIG parameters (laser power 8 %, writing speed 1.75 %, minimum off-set 3 mm below the focal plane) led to the conversion of cedar wood into well-organized crack-free 3D carbonaceous layers characterized by a minimum sheet resistance of about 100 Ω/sq or 40 Ω/sq by single-run lasing, respectively, perpendicularly (cellulose-rich direction) or in parallel (lignin-rich direction) to the wood grain. LIG traces derived from wood pretreated with the antiflame showed comparable electrical performance (37 Ω/sq) if lased orthogonally to the wood grain (cellulose-rich direction), the Rs value increased by one order of magnitude (329 Ω/sq) lasing in parallel to the wood grain (lignin-rich direction). Therefore, this study revealed that the control of the laser-writing direction enables the optimization of the electrical performance of green LIG products derived from wood, also significantly contributing to the mitigate the variability of the results. The electrical properties of LIG traces derived from cedar wood in room conditions pave the way to the development of differential sensors, micro-fluidic devices and antennas for near-field communication.