3D porous laser-induced graphene coated sponges for field emission devices and temperature/pressure sensors

Recent interest in flexible sensors, fueled by their affordability, wearability, lightweight design, and ease of fabrication, has driven advancements in applications and fundamental understanding. Herein, we explore the synthesis route of the three-dimensional (3D) graphene-coated sponges and investigate their mechanical and electronic transport properties. Tensile and compression tests on the graphene coated sponges demonstrate Young's modulus of around 0.075 MPa. Electrical measurements with ohmic contacts show DC conductivity as low as 0.5 S/cm. Bonding durability and wettability tests under water immersion and ultrasonic agitation confirmed the strong adhesion and enhanced hydrophobicity of the graphene coating, demonstrating its mechanical and chemical robustness. Temperature measurements reveal a non-monotonic behavior in the sponge's resistance as the temperature decreases. The resistance exhibits a pronounced peak around 250 K as the temperature drops from 295 K to 200 K, followed by a steady increase from 200 K to 77 K. Field emission measurements show a stable current and a reduction in turn-on voltage as the spacing between the anode and the emitting surface decreases, revealing a low turn-on voltage of about 13 V and a field enhancement factor of 286 at an anode-cathode distance of 300 nm. Experimental data are analyzed using the Fowler-Nordheim model, evidencing a non-monotonic dependence of the field enhancement factor on the cathode-anode separation distance in the range of 100–500 nm. The results show that such a flexible 3D graphene coated sponge can be utilized as a sensitive thermistor, a field emitter, and a pressure sensor.