Microstructures at the micro- or nano-scale, introduced in pressure sensors, exhibit an inextricable link with the promotion of sensitivity and detection limit for pressure sensors due to their asymmetries and compressibility5,6,11-19. The microstructures at the micro-scale with high asymmetry can effectively improve sensitivity, including micro-pyramid arrays5,6,20-22, wrinkles23, micro-domes24,25, micro-pillar arrays11, and micro-protrusions26, as well as other assembly patterns12-15,27-29. More precisely, asymmetrical nano-structures and heterogeneities16-18,30-33 are recognized to play a key role in significantly promoting the mechano-electric transduction of various stimuli. At the smallest atom-scale, the microstructures, with atom vacancies induced by a strong electric field19, have exhibited very large piezoelectric sensitivities. Despite extensive exploration and advances, it remains difficult to achieve expected atom vacancies exhibiting large structural asymmetry while maintaining structural compressibility and stability under ambient conditions. This is the limitation and challenge of pressure sensors for achieving more excellent pressure-sensing performance.
Here, we present a piezoelectric sensor with supreme pressure-sensing performance in the sensitivity and detection limit, based on a high percentage of atom vacancies in the unique two-dimensional (2D) Na2Cl crystals within multilayered graphene oxide (GO) membrane. Different from the ordinary NaCl crystal, the 2D Na2Cl crystal has a Na:Cl atom ratio of 2:1 and contains atom vacancies where Cl atoms should be located. The atom vacancies result in a very high piezoelectric coefficient of the membrane. The periodic arrangement of these atom vacancies ensures the structural stability. Remarkably, the sensor is capable of detecting signals as low as 1 mPa (corresponding to a 25 nanonewton force), together with excellent mechanical stability. The small size, fast response, and self-powered capability of the sensor make it particularly suitable for use in biological systems and complex environments. In addition, the application scope of the sensor can be greatly extended to encompass the detection of other tiny signals including electrical, magnetic, optical, and thermal signals, by converting these signals into mechanical forces.
The membranes of piezoelectric sensors were prepared from a graphene oxide (GO) suspension and dilute NaCl solution via the spray-coating method (see Methods). Explicitly, the GO suspension (20 μL, 5 mg/mL) was sprayed on a polyethylene glycol terephthalate (PET) substrate, followed by drying at 60°C for 6 hours. The prepared GO membrane was then drop-cast with dilute NaCl solution (0.01 mol/L) and exposed to the air for 2 hours. Next, the GO suspension was sprayed on the membrane again and dried at 60°C for another 6 hours, to form multilayered GO membrane containing sodium and chloride ions, which are denoted as Na–Cl@GO membrane. Different from GO membranes directly immersed in NaCl solutions in our previous work34, this spay-coating method creates up-down asymmetry in the distribution of Na and Cl ions in the membrane. These membranes, having a thickness of ~5 μm, were removed from the substrate and cut into ~5 × 5 mm2 sections. Then, they were made into sensor devices with a platinum (Pt) film coating to serve as conductive electrode and encapsulated with insulation PI film (Fig. 1a).
The sensor exhibits supreme sensitivity for pressure sensing in the low-pressure range of 1-100 mPa. An acoustic pressure below 100 mPa was generated with the acoustic excitation of a tuning fork with a frequency of ~260 Hz. The acoustic pressure (P) of the acoustic excitation was monitored using an acoustic tester (UMM-6, Dayton Audio). The voltage response of the sensor (U) to changes in the acoustic pressure was simultaneously monitored by a digital lock-in amplifier (HF2LI, Zurich Instruments) with a sampling frequency of 10 kHz (Fig. 1c). A good correlation between the voltage output of the sensor and the acoustic pressure is shown. The sensitivity S of the piezoelectric sensor is typically defined as S = δ((U-U0)/U0)/δp35, where U0 = 0.1 mV is the initial voltage of the sensor without pressure, and U is the voltage response under the applied pressure p. The peak sensitivity reaches 3.5×106 kPa-1 (Fig. 1e), which is approximately one order of magnitude larger than the highest sensitivity of 3.8×105 kPa-1 36 obtained from the current state-of-the-art pressure sensors.
Importantly, at pressure of 1 mPa or even lower, the sensor still exhibits a clear decay in the voltage response, indicating that the sensor can effectively detect pressure variations of 1 mPa or less (Fig. 1d). This value is lower than the lowest pressure detection limit of recently reported pressure sensors, 2 mPa3. The sensor thereby demonstrates supreme pressure-sensing performance in terms of sensitivity and pressure detection limit, superior to the state-of-the-art pressure sensors reported so far (Fig. 1b). Moreover, based on the pressure detection limit of 1 mPa and the sensor size of ~5 × 5 mm2, the sensor is capable of detecting tiny force as low as 25 nanonewtons.
We attribute the observed piezoelectric behaviors to the 2D Na2Cl crystals within GO membrane in the sensor. This Na2Cl crystal was discovered in our previous work34, originating from the ion–π interactions37 between the ions and the π-conjugated system in the graphitic surface38-40. Different from the ordinary NaCl crystal, the 2D Na2Cl crystal has a Na:Cl atom ratio of 2:1 and contains atom vacancies where Cl atoms should be located (Fig. 2c); here, we refer to these anion vacancies as “ghost atoms”. These Cl atom vacancies in the crystal lead to very high asymmetry, while their periodic arrangement in the crystal ensures the structural stability.
The existence of 2D Na2Cl crystals in the sensor was confirmed by transmission electron microscopy (TEM) experiments. The high-resolution TEM image shows a square structure with a lattice spacing of 3.96 ± 0.03 Å (Fig. 2a). The enlarged image overlays well with the Na2Cl structure in our previous work 34, as shown by the inset in Fig. 2a, in which the Na and Cl atoms are shown as blue and green spheres, respectively. The dark-field TEM images demonstrate that the Na:Cl ratio is approximately 2:1 (Fig. 2b and Supplementary Fig. 4).
We then performed density functional theory (DFT) computations to illustrate the underlying physics. The structure of the 2D Na2Cl crystal based on the graphene surface (Na2Cl@graphene) is shown in Fig. 2c and 2d. Here, the graphene surface is considered as the part of GO membrane without functional groups. The electric polarization direction from Na2Cl layer to graphene layer is shown in the line profile of the interfacial differential charge densities (DCD, a measure of charge variation at the interface)) in the out-of-plane direction (Fig. 2e). By analyzing the Bader charges of the atoms, a very large spontaneous out-of-plane polarization (P0), serving as one of crucial parameters for piezoelectric materials17,41, was obtained for the Na2Cl@graphene, namely, P0 = 39.0 pC/m (see Supplementary Note 2). This value is nearly two orders of magnitude larger than the out-of-plane polarization of 0.6 pC/m for MoS2/WS218.
The induced mechanical deformation of Na2Cl@graphene under external pressure was then investigated by varying the distance between the Na atom in the upper layer of the Na2Cl crystal and the graphene surface using DFT simulations. Fig. 2f clearly shows that in the out-of-plane direction, the average displacement (h1) between the Na atom in the upper layer of the Na2Cl crystal and the atom vacancy in the bottom layer of the Na2Cl crystal is much larger than the average displacement (h2) between the Cl atom in the upper layer of the Na2Cl crystal and the Na atom in the bottom layer of the Na2Cl crystal under stress. In contrast, along the in-plane direction, the distances between adjacent atoms (dx and dy) in each layer remain approximately constant. These results indicate that the atom vacancies make the Na2Cl crystal susceptible to deformation along the direction of applied stress, while maintaining structural stability.
The very large intrinsic polarization suggests an extraordinarily large piezoelectric coefficient in the sensor42. According to the stress-output charge relation from the acoustic excitation of the tuning fork (Fig. 2g), we experimentally obtained a piezoelectric coefficient (d33) up to 6.8×105 pC/N in the range of 0.025–2 μN (corresponding to the pressure range of 1–80 mPa). This value is larger than the d33 of ~2×105 pC/N in cubic fluorite gadolinium-doped CeO2-x films obtained by rearranging oxygen vacancies under very large electric fields at millihertz frequencies19.
The famous butterfly effect, a metaphor for chaos theory that posits “Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas?”43, reflects sensitive dependence on an initial condition, with extremely small signals leading to extraordinarily large responses. To our knowledge, the airflow variation caused by a flapping butterfly has not been well measured because the variation is quite small44. Using the presented sensor, the open-circuit voltage response to the airflow variation at a 5 cm distance from a flapping butterfly was obtained, and the movement of the butterfly and the real-time motoring of voltage response were shown in the movies (Supplementary Video 1). The corresponding pressure variations above, on the side of, and below the butterfly were ~3.9 mPa, ~6.8 mPa, and ~14.7 mPa, respectively (Fig. 3b and Supplementary Fig. 12b), calculated by the conversion relation between the pressure and voltage response (Supplementary Note 9). The pressure variations of 2 mPa or even lower can be clearly distinguished by the sensor (Fig. 3b). These variations in the airflow pressures are extremely small, only approximately one millionth of the airflow pressure caused by a tornado. A wing-beat frequency of ~5 Hz was obtained (Supplementary Fig. 12c), which is consistent with the results from other studies on flapping frequency44.
The sensor also exhibits a very high sensitivity in the high-pressure range. We used a force gauge (Mark-10, FS05) to produce a series of mechanical loads in the pressure range of 0.1 to 100 kPa. The voltage response of the sensor was measured using an electrochemical workstation (CHI760E) (Supplementary Fig. 6a). The output voltage shows a cyclic and step-like response, which increases from ~0.06 V to ~0.80 V and is well correlated with the applied mechanical load (Supplementary Fig. 6a). The sensitivity of the sensor was then calculated. The sensitivity decreases from 2.0×104 kPa-1 to 1.2×102 kPa-1 when the applied pressure is varied in the range of 0.1–100 kPa (Supplementary Fig. 6b).
In addition, the sensor has excellent mechanical stability and a fast response. The peak voltage due to a mechanical pressure of 80 kPa decreases by less than 3% when the sensor is subjected to over 10000 cycles of loading and unloading (Fig. 4a). A response time (τr) of 1 μs and recovery time (τd) of 238 ms were obtained using an oscilloscope (Tektronix, MSO44 4-BW-200) (Fig. 4b). The detected response time of 1 μs is actually the limit of our measurement set-up instead of the intrinsic limit of the sensor. The excellent mechanical stability and fast response ensure its application in the area of sensitive pressure sensing.
In summary, we developed an ultrasensitive piezoelectric sensor made of multilayered Na2Cl@GO membrane prepared by spray-coating method. The sensor exhibits excellent pressure-sensing performance, including ultrahigh sensitivity and an extremely low detection pressure limit under ambient conditions. DFT computations reveal that the ultrasensitive piezoelectric response of the sensor can be attributed to the unique atom vacancies of the Cl atoms ("ghost atoms") in Na2Cl crystals. The high percentage of atom vacancies induces a very large intrinsic polarization and the spray-coating method further enhances up-down asymmetry in the distribution of Na and Cl ions in the membrane. Consequently, the sensor exhibits an extraordinarily large piezoelectric constant and sensitive response to mechanical pressure, while the periodic arrangement of the atom vacancies ensures the structural stability.
Notably, the sensor can detect nanonewton forces as low as 25 nanonewtons, based on its low pressure detection limit of ~1 mPa and device size of ~5 × 5 mm2. The extremely small airflow fluctuations at a 5 cm distance from a flapping butterfly can be clearly detected, which have long been the elusive tiny signals in the famous “butterfly effect”. Moreover, the sensitivity and the pressure detection limit can be further optimized by increasing the content of the Na2Cl crystals within GO membrane. Similarly, the size of the sensor can be even smaller while keeping the excellent pressure-sensing performance by further increasing the crystal content. Considering its excellent mechanical stability, fast response and self-powered capability, the sensor shows great potential in applications that require the high sensitivity to external mechanical stimuli and the detection of the tiny forces with only several nanonewtons, even for monitoring the very fast changed signals of systems in very small spaces such as the brain and other organs. Finally, the presented sensor can serve as a seminal sensor to detect other tiny signals, including electrical, magnetic, optical, and thermal signals, by converting these signals into mechanical forces, and thus the applications can be further greatly extended; for example, combined with a magnet, the sensor can measure the very small magnetic fields by detecting the tiny force acting on the magnet.