Thin-Film Transistor (TFT) Gas Sensor Based on Quantum Dots-Sensitized Molybdenum Disulfide
Authors: Liu Jingyao , Zhao Yunong , Hu Zhixiang , Tian Zhilai , Li Hua-Yao , Li Dehui , Liu Huan 
Published: May 2020

Introduction

NO2 primarily gets in the air from the burning of fuel. Exposure to NO2 may potentially increase susceptibility to respiratory infections, and a 5 min emergency exposure limit of 35 ppm NO2 exposure has been proposed by the American Industrial Hygiene Association [1]. There is an increasing demand to develop large-scale networking of gas sensors for achieving online NO2 monitoring with low power consumption. Semiconductor gas sensors employing high-temperature ceramics technology have attracted intense research and industry interests because of their high sensitivity and low cost. However, the high operating temperature of these sensors still sets a limit to their compatibility with CMOS technology, making it challenging to achieve gas sensor array with large-scale manufacture and application level. On the other hand, electronic gas sensors based on TFTs have shown promising results, because of the control over gate voltage can modulate the carrier concentration in the semiconductor channel by orders of magnitude, which allows the TFT gas sensors to be more sensitive and selective. 2D transition metal dichalcogenides (TMDs), typically molybdenum disulfide (MoS2) nanoflakes, are popular channel materials used for TFT-based gas sensors [2, 3], due to their unique electronic properties and large specific surface area. However, MoS2 TFT gas sensors always suffer the sluggish and weak response as well as incomplete recovery during operated at room temperature, significantly limiting their practical applications.
Herein, we proposed to construct MoS2 TFT gas sensor by sensitizing with PbS quantum dots (QDs), a highly tunable 0D nanomaterials with size-dependent bandgap and excellent solution processability [4], aiming to improve the room-temperature response and recovery kinetics. The huge amount of surface dangling bonds of QDs make them sensitive receptors for gas molecules. The hybrid PbS/MoS2 architecture benefited from excellent adsorption of the NO2 molecules to the highly active surface of PbS QDs, combined with the high mobility MoS2 nanoflakes-based TFT, thereby achieve the optimal NO2-sensing performance at room temperature.

Experimental details

Fabrication of MoS2 TFT sensors. The few-layer MoS2 nanoflakes were mechanically exfoliated from bulk crystal MoS2 onto the Si substrates covered by a 300 nm thick SiO2 layer. The substrates with interdigitated 5 nm Cr/100 nm Au as source-drain electrodes were pre-patterned by photolithography, followed by electron beam evaporation and the lift-off process. Alternatively, high quality MoS2 monolayer can be also synthesized on the above SiO2/Si substrates in a tube furnace by high purity sulfur powder and MoO3 powder, as S and Mo precursor, respectively, and then obtained MoS2 TFT with a bottom-gate bottom-contact configuration.

Sensitization of MoS2

channel with PbS QDs. Organo-hot injection method were typically employed for the synthesis of PbS QDs. Ascribe to the excellent solution processability and stable dispersibility of QDs, we use programmable electrohydrodynamic jet printing technologies as well as spin-coating methods to directly deposit QDs on the surface of MoS2 nanoflakes. Finally, we successfully fabricated the TFT sensors based on 0D PbS/2D MoS2.

Gas sensing experiments. As-fabricated PbS/MoS2 TFT sensors were measured using probe station connected with semiconductor parameter analyzer (Model B1500A, Keysight) to monitor the real-time current change, and electrical properties including current-voltage, transfer and output characteristics. All the electrical and gas sensing characterizations had been carried out in an ambient air atmosphere or inert atmosphere of N2 at room temperature (25°C). The concentration of NO2 was controlled by a mixture of NO2 (50 ppm in N2) and N2 (99.99%) flow measured by the mass flow controller.

Results and Conclusions

In conclusion, we have proposed a novel strategy to construct PbS/MoS2 TFT gas sensors for room temperature NO2 detection. Compared to the pristine MoS2 TFT sensors, the PbS/MoS2 TFT sensors exhibited higher sensitivity with fast response/recovery kinetics toward NO2. The sensing mechanism was attributed to the synergistic effect by the favorable 0D-2D interface, making combined efforts for excellent access of gas molecules to highly active PbS QDs, as well as the superb charge transport in MoS2 nanoflakes that made a great enhancement of the gas-sensing performance. The 0D-QDs with tunable bandgaps will further promote progress in the engineering of the band alignment at the 0D-2D heterojunction interface, paving a promising way to develop gas-sensing performance of 2D layered materials.

References

[1] Environmental Protection Agency (EPA), https://www.epa.gov/learn-issues/learn-about-air/ for “Air Pollution, 2013”.
[2] J. Late, Y. K. Huang, B. Liu, J. Acharya, S. N. Shirodkar, J. Luo, A. Yan, D. Charles, U. V. Waghmare, V. P. Dravid, C. N. R. Rao, Sensing Behavior of Atomically Thin-Layered MoS2 Transistors. ACS Nano 7(6), 4879-4891 (2013). doi:10.1021/nn400026u
[3] Liu, L. Chen, G. Liu, A. N. Abbas, M. Fathi, C. Zhou, High-Performance Chemical Sensing Using Schottky-Contacted Chemical Vapor Deposition Grown Monolayer MoS2 Transistors. ACS Nano 8(5), 5304−5314 (2014). doi:10.1021/nn5015215
[4] H. Liu, M. Li, O. Voznyy, L. Hu, Q. Fu, D. Zhou, Z. Xia, E. H. Sargent, J. Tang, Physically Flexible, Rapid-Response Gas Sensor Based on Colloidal Quantum Dot Solids. Adv. Mater. 26(17), 2718-2724, (2014). doi:10.1002/adma.201304366

Figure 1