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,
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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.