Highly Sensitive and Flexible Capacitive Pressure Sensors Based on Vertical Graphene and Micro-Pyramidal Dielectric Layer

Abstract

Many practical applications require flexible high-sensitivity pressure sensors. However, such sensors are difficult to achieve using conventional materials. Engineering the morphology of the electrodes and the topography of the dielectrics has been demonstrated to be effective in boosting the sensing performance of capacitive pressure sensors. In this study, a flexible capacitive pressure sensor with high sensitivity was fabricated by using three-dimensional vertical graphene (VG) as the electrode and micro-pyramidal polydimethylsiloxane (PDMS) as the dielectric layer. The engineering of the VG morphology, size, and interval of the micro-pyramids in the PDMS dielectric layer significantly boosted the sensor sensitivity. As a result, the sensors demonstrated an exceptional sensitivity of up to 6.04 kPa^-1 in the pressure range of 0-1 kPa, and 0.69 kPa^-1 under 1-10 kPa. Finite element analysis revealed that the micro-pyramid structure in the dielectric layer generated a significant deformation effect under pressure, thereby ameliorating the sensing properties. Finally, the sensor was used to monitor finger joint movement, knee motion, facial expression, and pressure distribution. The results indicate that the sensor exhibits great potential in various applications, including human motion detection and human-machine interaction.

Keywords: capacitive pressure sensor; high sensitivity; micro-pyramid; vertical graphene.

Figure 1

(a) Assembly process and configuration of the flexible capacitive pressure sensor. (b) Scanning electron microscopy (SEM) image of the vertical graphene (VG) electrode. (c) SEM image of the micro-pyramidal polydimethylsiloxane (PDMS) dielectric layer. (d) Photograph of a flexible capacitive pressure sensor.

Figure 2

(a) Top-view and cross-sectional scanning electron microscopy images of the VG1, VG2, and VG3 films on quartz. (b) Raman spectra of the VG1, VG2, and VG3 films. (c) Schematic illustration of the vertical graphene (VG) based pressure sensor using a flat polydimethylsiloxane (PDMS) dielectric layer. (d) Capacitance response of the VG1-F, VG2-F, and VG3-F sensors at pressures of 0–10 kPa. (e) Capacitance response of the VG1-F sensor at pressures of 0–10 kPa for five pressure loading/release cycles.

Figure 3

(a) Schematic configuration of the micro-pyramidal capacitive pressure sensor. (b) Capacitance response of the VG1-P(20-20), VG1-F, GP-P(20-20), and GP-F sensors. (c) Real-time responses of the VG1-P(20-20) sensor to pressures of 0.2, 0.5, 1, 3, and 10 kPa. (d) Real-time response of the VG1-P(20-20) sensor to an ultralow pressure of 20 Pa. The response time upon loading and unloading is shown in the insets. (e) Capacitance response under different pressure loadings at 0–1 kPa. (f) Cyclic test of the VG1-P(20-20) sensor for 300 cycles under an applied pressure of 1 kPa; the insets show the 150th to 155th cycles.

Figure 4

(a) Finite-element simulation of the pressure distribution and deformation of the micro-pyramidal polydimethylsiloxane (PDMS) dielectric layer. (b) Finite-element analysis of the pressure distribution and deformation of the flat PDMS dielectric layer. (c) Relative capacitance variations of the VG1-P(20-20), VG1-F, GP-P(20-20), and GP-F pressure sensors under a pressure of 10 kPa. (d) Simulated electrode distance change versus pressure. (e) Simulated effective contact area change versus pressure. (f) Simulated capacitance change versus pressure.

Figure 5

(a) Sensing performance of the VG1-P(20-20) sensor for finger bending. (b) Sensing performance of the VG1-P(20-20) sensor in leg bending.

Figure 6

Application of the VG1-P(20-20) sensor to identify facial expressions: (a) amazed and calm, (b) angry, and (c) smiling.

Figure 7

(a) Schematic photo and illustration of a 3 × 3 pixel flexible capacitive pressure sensor array. (b) Photo of a pair of sunglasses on the sensor array and the corresponding signal intensity distribution. (c) Photo of a pen on the sensor array and the corresponding signal intensity distribution.