In the past two decades, mono-layer and poly-layer molybdenum disulfide (MoS2) has emerged as a promising material for next-generation optoelectronics due to its outstanding physical, chemical, mechanical, and electrical properties. It has high intrinsic mobility, a tunable band gap (close to that of silicon), and excellent mechanical flexibility. Traditional methods for synthesizing MoS2 films include chemical vapour deposition (CVD), physical vapour deposition, and atomic layer deposition. However, all of these suffered from limited patterning areas and possible contaminations. In addition, the patterned MoS2 has to undergo several complicated steps for processing before use. Therefore, a novel strategy for robust printing of MoS2 patterns is urgently in demand.
A team led by Dr. Xuewen Wang and Dr. Wei Huang thought they solved this problem by developing a novel inkjet printing method for making MoS2 films. The principle of this approach involves designing a versatile precursor solution that can make MoS2 in situ. To do this, the team selected ammonium thiomolybdate (NH4)2MoS4) due to its high solubility in DI water. Ethanol and propylene glycol were added to lower the surface tension and increase the viscosity of the aqueous solution. When treated with a pulsed laser, ammonium thiomolybdate was induced to MoS2. This was followed by the annealing of the film under an inert atmosphere (5:2 Argon: Hydrogen) for 30 minutes at 500 degrees Celsius. The team managed to print MoS2 patterns on polyimide (PI) substrates with an area of 150 mm × 150 mm under flat and rolling conditions.
Next, the electrical property and the stability of the synthesized film were tested. The resistance of the MoS2 film is monitored under various mechanical deformations, such as different bending radii from 4 to 54 mm and cyclic bending at a bending radius of 4 mm (corresponding to 1.25% tensile strain). It was found that the resistance of the MoS2 film decreases with a smaller bending radius because of the generated compression to the MoS2 layer. In contrast, upon the application of a maximum tensile strain of 1.25%, there is an increase in resistance. When the applied compression or tensile strain is removed, the resistance of the MoS2 film recovers to the initial values. Nevertheless, a 2% variation in resistance is observed when the MoS2 film is compressed or stretched at a minimum bending radius of 4 mm, which is better than other flexible conductors, such as metal nanowires and graphene. In addition, a similar variation was observed (2%) even after repeatedly bending the film 10000 times, indicating that the annealed MoS2 film is highly stable and durable under bending deformation.
Figure 1: The change in resistance of printed MoS2 film after 60,000 bending cycles.
This synthesized film has several potential sensing applications, including sensing body temperatures and biopotentials. To make the body temperature sensor, a silver interdigital electrode with an electrode width and spacing of 200 µm is printed on the surface of the MoS2 film by screen-printing technique. The sensor was then calibrated by recording resistance response to temperature variation from 30 to 70 °C with an interval of 0.1 °C. It can be found that the resistance of the MoS2 film linearly decreases as the temperature increases, and a linear response is obtained in the entire temperature range, which is highly desirable for high-performance temperature sensors. One of the follow-up studies that the team did was to fine-tune the detection range to 38-42 °C and then attach this sensor to the mask. Therefore, as the participant breathes, the cyclic graph indicating temperature can be obtained. From that, it is also possible to determine the breathing rate of the individual. In addition, the temperature sensor also displays outstanding sensitivity. When a small object is approaching the sensor (I.e., fingers), the resistance change is displayed as a functional over distance.
Figure 2: The application in the synthesized MoS2 films as body temperature sensor.
The biopotential sensor was constructed by fabricating a multilayer pad comprising a circular MoS2 patch, gold interdigital electrodes, and a flexible PI substrate. This can be used to record electrophysiological signals, including electrocardiography (ECG) and electromyography (EMG). The ECG signals are recorded using a commercial wireless electrical potential recorder with a sampling frequency of 512 Hz. Commonly, skin motions and body movements lead to reduced SNR due to the slippage and detachment of the electrodes from the skin. Utilizing the proposed MoS2 pads, the ECG signals are successfully recorded during exercise, suggesting a minimal influence of skin deformations on the operation of the MoS2 pads. Other applications of the biopotential measurement include measuring the rate of heartbeat and different grip strengths.
Figure 3: Various applications for the MoS2 film in sensing biopotentials.
To conclude, these authors have developed a novel method for printing flexible patterns of MoS2 patterns on a polymer substrate. This method is analogous to inkjet printing which incorporates a special precursor solution that can form MoS2 under heat treatment. Such ability has promising applications in making flexible sensors which can detect changes in body temperature and biopotential.
The finding of the work has been published on Advanced Materials: Li, W.; Xu, M.; Gao, J.; Zhang, X.; Huang, H.; Zhao, R.; Zhu, X.; Yang, Y.; Luo, L.; Chen, M.; Ji, H.; Zheng, L.; Wang, X.; Huang, W. Large-Scale Ultra-Robust MoS2 Patterns Directly Synthesized on Polymer Substrate for Flexible Sensing Electronics. Adv. Mater. 2023, 35 (8), 2207447. https://doi.org/10.1002/ADMA.202207447.
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