Plant growth and development are negatively affected by a wide range of external stresses, including water deficits. Especially, plants generally reduce the stomatal aperture to decrease transpiration levels upon drought stress. Advanced technologies, such as wireless communications, the Internet of things (IoT), and smart sensors have been applied to practical smart farming and indoor planting systems to monitor plants' signals effectively. In this study, we develop a flexible polyimide (PI)-based sensor for real-time monitoring of water conditions in tobacco plants. The stoma response, by which a plant adjusts to drought stress to maintain homeostasis, can be confirmed through the examination of evaporated water. Using a flexible PI-based sensor, a plant's response variation is translated into an electrical signal. The sensors are integrated with a Bluetooth (BLE) module and a processing module and show potential as smart real-time water sensors in smart farms.
MoS2 thin-film transistors (TFTs) are fabricated and
simulated to explore the NO2 gas sensing mechanism depending
on different device structures. In particular, the role of the Al2O3 passivation layer on the MoS2 channel
has been investigated. In the case of nonpassivated MoS2 TFTs, increase of off-current is observed with NO2 gas,
which has been modeled with the modulation of the effective Schottky
barrier height for holes because of the generation of in-gap states
near the valence band as NO2 gases interact with the MoS2 channel. The nonpassivated MoS2 TFTs are simulated
based on nonequilibrium Green’s function method, and the simulation
results do confirm this sensing mechanism. On the other hand, MoS2 TFTs with the Al2O3 passivation layer
have been modeled with a pseudo-double gate structure as NO2 gases on the capping layer can act like the secondary gate inducing
the positive charge state. Our quantum transport simulation shows
that the significant threshold voltage shift can be achieved with
NO2 gas, which matches the experimental observation, thereby
exhibiting a completely different sensing mechanism of the passivated
device from the nonpassivated counterpart. In addition, we also discuss
competing device parameters for the passivated MoS2 TFTs
by varying the main and the secondary gate dielectric, suggesting
co-optimization to realize high sensitivity and low power consumption
simultaneously.
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