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A highly flexible and sensitive piezoresistive sensor based on MXene with greatly changed interlayer distances
QQ Academic Group: 1092348845

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[Research Background]


Similar to graphene, MXenes have excellent properties such as large specific surface area, high electrical conductivity, and good strength. Based on the above characteristics, MXenes have many important applications, such as water purification, lithium-ion batteries, and supercapacitors. However, since MXenes are prepared by selectively etching away the “A” layer elements of the MAX phase, the adjacent interlayer spacing of MXenes can be simply controlled by compression. Therefore, pressure sensors can be fabricated from MXenes materials.
At present, there are three main types of pressure sensors, piezoelectric, piezoresistive and capacitive. Among them, a piezoresistive sensor is a sensor that converts external pressure into a resistive signal. It has received extensive attention due to its low manufacturing cost, easy signal acquisition, and important applications in smart displays, skin-sensing electronic devices, and portable medical monitors.
Carbon materials and their composites are widely used in flexible and stretchable piezoresistive sensors with high sensitivity due to their macroscopic deformation properties. However, due to the shortcomings of the complex preparation process and the structure of the carbon material itself, scientists have been prompted to look for new materials or a structure with simple interlayer atomic motions to reduce costs, simplify steps, and most importantly, improve sensitivity to meet realistic requirements. Due to the wide interlayer distance of MXenes, it has high sensitivity and flexibility under external pressure, which can meet the detection requirements of human blinking, bulging cheeks and other subtle activities.

[Introduction to Achievements]

In 2017, the team of Gao Yihua, a double-appointed professor at the School of Physics of Huazhong University of Science and Technology and Wuhan National Laboratory of Optoelectronics, developed a flexible piezoelectric sensor based on Ti3C2-MXene, which has fast response, high compressibility and sensitivity, and is suitable for a A series of human responses were tested. The result was published online in Nature Communications: A highly flexible and sensitive piezoresistive sensor based on MXene with greatly changed interlayer distances

[Graphic and text guide]


Figure 1. Microstructure of the working MXenes piezoresistive sensor: (a) Under the action of external force, the distance between two adjacent MXenes interlayers in the sensor decreases. Wide spacing (Dw) between two layers is more easily compressed, while narrow spacing (Dn) has a smaller compression ratio, (b) Equivalent circuit diagram of an MXenes-based piezoresistive sensor where the total resistance decreases with decreasing distance .



Fig. 2 Typical microstructure of MXenes and in situ dynamic process under external force. (a) TEM image of the cross section of the MXenes, the inner image shows the layered structure, (b) HRTEM image of the top view of the MXenes, the inner part is the corresponding diffraction pattern, showing good hexagonal features, (c) in the original In the dynamometer, a nanoindenter supported by a tapered tip was locally applied on the FIB-treated MXenes sample. (d)-(f) Under the action of external force, the wider distance in MXenes decreases rapidly from ~12 nm, via ~3 nm to ~0 nm at 7, 9, and 10 s, respectively, (g)-(i ) The narrower distances (4 intervals) marked with red lines are 5.23 nm, 4.98 nm and 4.81 nm, respectively, (j)-(l) were analyzed using digital photomicrography software. Measured from the initial state of g, the strain values of the h and i states are 4.78 and 8.03%, respectively. a, b is 4nm, c is 200nm, d-f is 40nm, g-i is 20nm.



Figure 3. Pressure response of the MXenes sensor. (a) I-T curve at 13 kPa pressure. The current increases monotonically with increasing pressure, (b) the linear relationship of the IV curve indicates the ohmic contact between the MXenes and the interdigital electrodes, and (c) the ΔR/Roff increases sharply as a function of pressure for pressures less than 5 kPa, exceeding At 5 kPa, the growth rate slowed down. The SD of the data shown by the error bars is in the range of 1.44-8.21%, calculated by analyzing five sets of data at each pressure, (d) ΔR/Roff versus strain. GF is up to 180.1. The SD of the data represented by error bars is in the range of 0.26-1.78%, calculated by analyzing four sets of data at each strain value, (e) IT relationship at different bending angles, (f) an IT curve A partial zoom in, showing that its response time is less than 30 milliseconds. (g) When the load is added or removed, the output current and the external pressure are kept in good synchronization. (h) The resistance change rate ΔR/Roff maintains good stability.



Figure 4. MXenes sensors are used to explore various human activities. The current changes of the sensor caused by small changes such as blinking (a), cheek bulging (b) and throat swallowing (c) are precisely recorded, (d)-(f) elbow joint (d), finger (e) and Changes in sensor current caused by flexion-extension motion of the ankle (f), (g) IT curve of knee joint flexion-extension, (h) The sensor is connected in series with a Bluetooth-implanted microcircuit to convert the current signal into a mobile phone signal, (i) ) The knee motion of the Bluetooth-implanted microcircuit is very similar in amplitude and shape to that of the Agilent device, but at a faster frequency.



Figure 5 Prepared 4 x 4 μm sensor array. (a) A 4×4 MXenes sensor pixel array detects pressure distribution (5mm scale), (b) a watch is placed on the sensor array, (c) the output corresponding to each pixel is recorded and measured. The color contrast in c maps the local pressure distribution in line with the position of the watch in b.

[Summary of this article]

According to the basic characteristic that the interlayer distance changes greatly under the action of external force, a highly flexible and highly sensitive MXenes piezoresistive sensor is developed. The in situ TEM study directly clarifies the basic working mechanism of the piezoresistive sensor. The sensor has fast response speed and good stability, and has a wide range of applications in detecting tiny human activities and other weak pressures. The GF value of this paper reaches 180.1, which is significantly higher than other reports. This is the first piezoresistive sensor utilizing MXenes (Ti3C2), opening a new field for the application of other MXenes.
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