Integration of Molybdenum Disulfide with Light-Emitting Diode for Reversible NO2 Sensing
Authors: Park Jun , Choi Jungwook 
Published: July 2021

Introduction

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) semiconductors have recently been applied to gas sensing applications given their unique physical and chemical properties [1-3]. However, the reversible and reliable gas detection at room temperature is not completely successful yet due to insufficient recovery of TMDCs that adsorbed the target analytes in an air environment. In order to improve the recovery of the reacted TMDCs, application of external thermal energy and illumination of UV light during the sensing have been reported [4,5]. However, the excessive supply of thermal energy could degrade the sensing response, and the UV light source may have a harmful effect on the human body. Moreover, processes to integrate the TMDCs on desired surfaces or substrates usually require complex, time-consuming steps.

This paper reports a sensitive and reversible NO2 sensor based on 2D molybdenum disulfide (MoS2) that is integrated on a surface of light-emitting diode (LED). The MoS2 layers are locally coated on the surface of LED by simply hand-drawing from the MoS2 pellet, which can be rapidly completed. The drawn MoS2 layers serve as a NO2 sensitive material where its electrical resistance changes upon the adsorption of NO2. In parallel, visible light illumination from the LED generates excitons in the MoS2 during the sensing cycles, resulting in the increased number of electrons that can respond to the NO2 molecules, which eventually improves the sensing responses. At the same time, the increased number of holes in the MoS2 by the light illumination could combine with the electrons trapped by the NO2 molecules, thereby releasing the adsorbed NO2 and thus improving the recovery. In addition, heat dissipated by the LED, which is inevitable in solid-state lighting, provides thermal energy to promote the desorption of adsorbed NO2 molecules, further improving the recovery rate. Unlike the previous works that usually activate the sensing materials by global illumination, the MoS2-integrated LED is able to effectively improve the response and the recovery of NO2 sensing by simultaneous optical and thermal stimuli.

Results and Conclusions

The MoS2 layers are formed on the surface of LED by hand-drawing of the pellet. This process provides a simple and cost-effective way to integrate the MoS2 onto the desired surface. Raman spectrum confirms the successful coating of the MoS2 with distinguishable in-plane and out-of-plane vibration modes. It is noted that the LED can be used for its original purpose of light emission even after the MoS2 coating. The electroluminescence center of the LED is ~625 nm, which covers the B exciton transition of the MoS2 measured by UV-vis spectrophotometer. We experimentally found the optimal power to operate the LED, which leads to the largest response of the MoS2 toward the NO2. Compared to the response at dark condition (~40%) under 10 ppm NO2 exposure, the response of MoS2 reaches ~131% at 400 mW of input power of the LED. In addition, the recovery rate is much improved to ~90% under 400 mW LED illumination due to increased heat dissipation from the LED. This optimal response might originate from the balance between the exciton generation in the MoS2 and the NO2 desorption by the thermal energy. The measured sensitivity is ~10.8%/ppm at the NO2 concentration range from 1 to 10 ppm under 400 mW LED illumination, which would be enough for early detection of NO2 exposure. Thus, our approach can be applicable to the sensitive, reversible gas sensing while routine and usual lighting of the LED in daily life.

References

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