Fabrication and characterization of two dimensional layered materials for printed electronic devices

Abstract

The growth of wireless communication has greatly enhanced sensor technologies, enabling their widespread integration into daily life to improve comfort, safety, and well-being. The demand for faster and smaller devices has driven the miniaturization of electronic devices. Traditional electronic devices rely on silicon and compound semiconductors, but their rigidity limits flexibility and performance. Advancements in this domain increasingly require different flexible materials with inherent mechanical strength to withstand deformation while maintaining electronic performance. Printed electronics have revolutionized prototyping by offering a cost-effective and efficient solution for device development. However, large-scale production of electronic devices such as humidity sensors is challenged by the lack of materials compatible with manufacturing processes. A recent key contribution to this field was the isolation of a single-atom-thick layer of graphite, Known as graphene. Two-dimensional (2D) layered materials like graphene, Transition Metal Dichalcogenides (TMDs), and 2D metal oxides are promising candidates for next-generation sensors due to their high surface area, flexibility, and transparency, making them ideal for future electronics. Their ultrathin layers, high crystallinity, optical transparency, and flexibility make them capable of overcoming limitations in silicon-based devices and enabling more advanced applications in flexible and printed electronics. However, to fully harness the potential of 2D materials in sensing applications, more experimental evidence is needed. This work provides an in-depth study of the design and fabrication of printed humidity sensing devices based on 2D Zinc Oxide (ZnO), Graphene Oxide (GO) and Molybdenum Disulfide (MoS2) nanosheets and the application of humidity sensors in diverse areas, such as respiratory monitoring and non-contact sensing. A low-cost flexible humidity sensor based on 2D ZnO nanosheets is developed by printing silver Interdigitated Electrodes (IDEs) on a Polyethylene Terephthalate (PET) substrate using an Epson Stylus C88+ inkjet printer. The ZnO humidity sensor achieves excellent sensing performance over a wide range of humidity levels from 11% to 97% RH, with response and recovery time of 12 seconds and 16 seconds, a sensitivity of 84.65 kΩ/%RH and hysteresis of 8.61%, highlighting its potential for sensing devices. Additionally, an ultrafast GO nanosheets-based humidity sensor was developed to detect humidity over a wide range of humidity levels from 11% to 97% RH. The sensor demonstrated a fast response time of 2 seconds and recovery time of 17 seconds, with ultra-high sensitivity (243 kΩ/%RH), low hysteresis (2.16%), excellent repeatability, long-term stability, and high flexibility (tested at bending radiuses of 4 cm, 3.5 cm, 3 cm, and 2.5 cm). The sensor exhibited excellent capabilities in monitoring human respiration, distinguishing between nose and mouth breathing, iv detecting finger movements without physical contact, and identifying basic spoken words. Furthermore, a high-performance MoS2 nanosheets-based humidity sensor is designed for respiratory monitoring and non-contact sensing. The result demonstrates the quick response and recovery times, good repeatability, reversibility, and high stability. These advancements highlight the potential of 2D materials based printed humidity sensing devices in human healthcare applications and non-contact wearable electronics.