Laser scribing of graphene oxide using Bessel beam for humidity sensing

Humidity sensors serve crucial functions in environmental monitoring, holding significant importance in enhancing both the quality of living environments and ensuring the integrity of production environments. Due to extensive surface area of graphene and its derivatives, the electron transport in graphene is particularly sensitive to the presence of adsorbed molecules, making its properties highly responsive to environmental factors and can be used as the core humidity sensitive material for humidity sensors. Graphene oxide, reduced graphene oxide and graphene derivatives have better hydrophilicity due to the presence of oxygen-containing functional groups on its surface, which leads to more easy to adsorb water molecules; As rGO exhibits p-type semiconducting behavior, and absorbed water molecules on the surface can act as electron donors, greatly tuning the electrical conductivity of the above material, which enables it to detect and quantify the relative humidity level of the outside world, which has attracted wide attention. Due to GO has a low electrical conductivity, electrodes with higher conductivity are often introduced as the conductive structure of the sensor. Laser scribing can realize the patterning of GO in one step, and prepare a reduced graphite oxide electrode with decent electrical conductivity in the fabrication process of GO-based humidity sensors, which is expected to realize the efficient, low-cost and flexible fabricating of graphene humidity sensor. In the fabrication process of large scale and large area, the defocusing phenomenon of focusing laser can affect the performance of laser scribing electrode.

The research group proposed the one-step fabrication of a humidity sensor based on graphene oxide (GO) using Bessel beam laser scribing. The diffraction-free Bessel beam was employed to precisely laser scribe the graphene oxide, and it was demonstrated that with optimized laser power, the line width, resistance, and sheet resistance of the resulting reduced graphene oxide (rGO) electrode remained stable within a defocusing distance of ±1.00 mm, thereby enhancing fabrication stability under defocusing conditions. Moreover, defocusing was found to effectively reduce the ablation region during fabrication and improve the uniformity of the electrode film within this defocusing range. The study also investigated the temperature and humidity responses of the electrodes, particularly those fabricated with typical defocusing settings, and discussed the related mechanisms. It was observed that adsorbed water molecules served as electron donors, reducing the density of holes and consequently increasing electrode resistance at low humidity levels. At high humidity levels, water molecules underwent ionization, where the proton conduction mechanism predominated, leading to a significant decrease in electrode resistance. Building upon these findings, GO humidity sensors were successfully fabricated in a one-step process, exhibiting a high linear humidity response with sensitivity reaching up to 0.0140/% RH and maintaining 0.0093/% RH at a 1mm defocusing condition. These results underscore the feasibility of this technology for fabricating humidity sensors.dergo ionization, proton conduction mechanism dominates, and electrode resistance decreases significantly at high humidity. On this basis, the GO humidity sensors were one-step fabricated, showing a high linear humidity response, sensitivity up to 0.0140/%RH, and maintaining 0.0093/%RH at 1mm defocusing condition, demonstrating the feasibility of this technology for humidity sensing fabricating.

In summary, this paper explores the application of Bessel beam laser scribing of graphene oxide for humidity sensing. The study investigates the impact of defocusing parameters on the electrode characteristics, providing optimal process parameters for Bessel laser scribing of GO. It is demonstrated that maintaining a defocusing distance within ±1.00 mm stabilizes the line width, resistance, and sheet resistance of the reduced graphene oxide electrode, thereby enhancing process stability in large-scale fabrication. Additionally, appropriate defocusing is found to effectively reduce the ablation range of graphene oxide thin films. Furthermore, the study examines the temperature and humidity responses of the electrodes, particularly those fabricated with typical defocusing settings, and discusses the relevant mechanisms, showcasing the feasibility of these electrodes for humidity sensing applications. The findings presented in this paper offer valuable insights for the rapid, flexible, and cost-effective fabrication of graphene-based humidity sensors.