Relative humidity sensors are widely studied under the categories of both environmental and biosensors owing to their vast reaching applications. The research on humidity sensors is mainly divided into two concentration areas including novel material development and novel device structure. Another approach focuses on the development of printed sensors with performance comparable to the sensors fabricated via conventional techniques. The major challenges in the research on relative humidity sensors include the range of detection, sensitivity (especially at lower %RH), transient response time, and dependence on temperature. Temperature dependence is one of the least studied parameters in relative humidity sensor development. In this work, relative humidity sensors were fabricated using all-printed approaches that are also compatible with mass production, resulting in low cost and easy development. Laser-induced graphene (LIG)-based printed electrodes were used as the transducers, while the 2D MoS 2 and graphene nanocomposite was used as the active layer material with the built-in property of temperature independence. The exfoliation process of 2D MoS 2 was based on wet grinding, while graphene for the active layer was obtained by scratching the graphene grown on the polyimide (PI) surface via laser ablation. The resulting sensors showed an excellent output response for a full range of 0%RH to 100%RH, having no dependence on the surrounding temperature, and excellent response and recovery times of 4 and 2 s, respectively. The developed sensors can be confidently employed for a wide range of humidity sensing applications where the temperature of the surrounding environment is not constant.

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Fabrication of conductive patterns for flexible and printed electronic devices is one of the most challenging steps in the whole process. Conductive patterns in electronic devices are used as electrodes, transducers, connecting links, and sometimes, also as the active sensing elements. Since the introduction of laser induced graphene (LIG), it has been explored to print electrodes and connecting patterns for various electronic devices and systems. This work focuses on an in-house developed laser printing system and the comparison of various electrical, chemical, and morphological properties of the resulting LIG patterns using CO2 and diode lasers. The system parameters including the laser power, relative printing speed, and the printing resolution were explored and optimized to achieve conductive patterns with varying properties suitable for different targeted applications. The fabricated patterns were characterized for their sheet resistance, surface morphology using scanning electron microscope (SEM), chemical properties using Energy Dispersive (EDS) and RAMAN spectroscopies, and physical size and resolution using optical microscopy. Continuous conductive patterns with sheet resistance in range of 11.5 Ω/□ to 43 Ω/□ were achieved using CO2 laser with a minimum achievable pattern width of ~ 180 µm while patterns with sheet resistance in range of 19 Ω/□ to 105 Ω/□ were achieved using diode laser with a minimum pattern width of ~ 190 µm. The chemical and morphological properties of CO2 laser-based patterns indicate the formation of 2D graphite sheets with high porosity and low O2 concentration while the diode laser-based patterns have a lower porosity and higher percentage of O2 indicating burning and the formation of oxides. Various applications of both types have also been discussed based on their respective properties.

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