Laser writing of metal-oxide doped graphene films for tunable sensor applications

Abstract

Flexible and wearable devices play a pivotal role in the realm of smart portable electronics due to their diverse applications in healthcare monitoring, soft robotics, human-machine interfaces, and artificial intelligence. Nonetheless, the extensive integration of intelligent wearable sensors into mass production faces challenges within a resource-limited environment, necessitating low-cost manufacturing, high reliability, stability, and multi-functionality. In this study, a cost-effective fiber laser direct writing method (fLDW) was illustrated to create highly responsive and robust flexible sensors. These sensors integrate laser-induced graphene (LiG) with mixed metal oxides on a flexible polyimide film. fLDW simplifies the synthesis of graphene, functionalization of carbon structures into graphene oxides and reduced graphene oxides, and deposition of metal-oxide nanoparticles within a single experimental laser writing setup. The preparation and surface modification of dense oxygenated graphene networks and semiconducting metal oxide nanoparticles (CuO _x , ZnO _x , FeO _x ) enables rapid fabrication of LiG/MO _x composite sensors with the ability to detect and differentiate various stimuli, including visible light, UV light, temperature, humidity, and magnetic fluxes. Further, this in situ customizability of fLDW-produced sensors allows for tunable sensitivity, response time, recovery time, and selectivity. The normalized current gain of resistive LiG/MO _x sensors can be controlled between -2.7 to 3.5, with response times ranging from 0.02 to 15 s, and recovery times from 0.04 to 6 s. Furthermore, the programmable properties showed great endurance after 200 days in air and extended bend cycles. Collectively, these LiG/MO _x sensors stand as a testament to the effectiveness of fLDW in economically mass-producing flexible and wearable electronic devices to meet the explicit demands of the Internet of Things.

Fig. 1. (a) Fabrication mechanism and schematic of LiG/MO_x sensor with fLDW. (b) XRD spectra analysis and (c) SEM images with EDS results of LiG/MO_x films.

Fig. 2. (a) Schematic of electron transport modification on LiG film. (b) Resistive metal-oxide sensing mechanism for p-type and n-type LiG. (c) XRD spectra of GO and rGO films produced through fLDW (d) Laser energy density vs. Conductivity nature of induced LiG films. (e) Laser energy density vs. change in functional groups, data educed from XPS C 1s scans. (f) Time vs. normalized current gain of P-type and N-type LiG/ZnO_x samples under 1 V bias and UV stimulus.

Fig. 3. (a) XPS C 1s scan of a high energy density 1.2 (J mm⁻³) LiG film. (b) XPS C 1s scan of low energy density 0.8 (J mm⁻³) LiG film. (c) Laser energy density of fLDW process vs. sp² fraction. (d) I–V curve of LiG films with increasing sp² C–C fraction. (e) Electron hopping mechanism on GO surface morphology.

Fig. 4. (a) Nano-particle induced crumpling of rGO mechanism in LiG/MO_x films. (b) Sensing mechanism in crumpled rGO sheets with MO nanoparticles. (c) SEM image of LiG crumpling after nanoparticle deposition, low laser energy density (left), and high laser energy density (right). Time vs. change in current output of (d) LiG/FeO_x and (e) LiG/ZnO_x with different rGO crumpling.

Fig. 5. Normalized current gain vs. metal oxide percentage extrapolated from XPS spectra of, (a) copper oxide film (b) zinc oxide and (c) iron oxide.

Fig. 6. (a) XPS Cu 2p scans of LiG/CuO_x sensor films with high CuO and Cu₂O content; (b) XPS Zn 2p scans of LiG/ZnO_x sensor films with different ZnO content; and (c) XPS Fe 2p scans of LiG/FeO_x sensor films with different FeO_x compositions.

Fig. 7. I–V curves of time vs. normalized change in current of (a) n-type LiG/CuO_x sensor (left) and p-type LiG/CuO_x (right), (b) n-type LiG/ZnO_x sensor (left) and p-type LiG/FeO_x sensor (right), and (c) n-type LiG/FeO_x sensor (left) and p-type LiG/FeO_x sensor (right).

Fig. 8. Demonstration of different sensors. (a) Schematic of LiG/MO_x sensor application on a mechanical finger. (b) Applying as electrode for LiG/ZnO_x sensors. (c) Schematic and photo of a multifunctional LiG/MO_x sensor grid.

Fig. 9. (a) LiG sensor 1 V bias without stimuli (b) temperature response of LiG sensor.

Fig. 10. UV responses of (a) p-type and (b) n-type, LiG/ZnO_x sensors.

Fig. 11. Visible light responses of (a) p-type and (b) n-type LiG/CuO_x sensors.

Fig. 12. Humidity responses of (a) p-type and (b) n-type LiG/MO_x sensors.