Weipeng Xuan

Fast Response and High Sensitivity ZnO/glass Surface Acoustic Wave Humidity Sensors Using Graphene Oxide Sensing Layer

We report ZnO/glass surface acoustic wave (SAW) humidity sensors with high sensitivity and fast response using graphene oxide sensing layer. The frequency shift of the sensors is exponentially correlated to the humidity change, induced mainly by mass loading effect rather than the complex impedance change of the sensing layer. The SAW sensors show high sensitivity at a broad humidity range from 0.5%RH to 85%RH with < 1 sec rise time. The simple design and excellent stability of our GO-based SAW humidity sensors, complemented with full humidity range measurement, highlights their potential in a wide range of applications.

High sensitivity humidity sensors using flexible surface acoustic wave devices made on nanocrystalline ZnO/polyimide substrates

Flexible ZnO/polyimide surface acoustic wave (SAW) based humidity sensors were fabricated and their performances were investigated. ZnO films deposited on polyimide substrates are (0002) oriented columnar nanocrystals with grain sizes of 50–60 nm. SAW devices with different wavelengths showed high transmission performance, and the sensors exhibited a good repeatability in response to the cyclic change in humidity from ∼5% relative humidity (RH) to ∼87% RH. The responses of the sensors to RH change increase with the increase in humidity. The sensitivity increases with the decrease in wavelength owing to the improved characteristics of the sensors with shorter wavelengths. A high sensitivity of 34.7 kHz/10% RH has been obtained from a SAW sensor with no surface treatment, demonstrating that the flexible SAW humidity sensors are very promising for applications in flexible sensors and microsystems.

Bendable transparent ZnO thin film surface acoustic wave strain sensors on ultra-thin flexible glass substrates

Flexible and transparent (FT) ZnO thin film based surface acoustic wave (SAW) devices using indium tin oxide (ITO) electrodes were fabricated on ultrathin flexible glass substrates. The influence of the annealing process and ITO thickness on the optical properties and acoustic wave power transmission properties of the devices was investigated. The performance of the devices improved significantly when the annealing temperature was raised up to 300 °C. The flexible glass based SAW devices exhibited similar power transmission performance, but have a better optical transmittance than those on rigid glass. These FT strain sensors worked well under various applied strains up to ±3000 μe with fast response time, and showed excellent linearity of resonant frequency with the change of strain with a sensitivity of ∼34 Hz μe−1. The strain sensors demonstrated excellent stability and reliability under cyclic bending. The results demonstrated great potential of applications of the FT-SAW device based strain sensors on flexible glass substrates.

Thermal annealing effect on ZnO surface acoustic wave-based ultraviolet light sensors on glass substrates

Surface acoustic wave (SAW) based ultraviolet (UV) light sensors have a high sensitivity and have been extensively studied and explored for application. However, all of them were made of piezoelectric (PE) bulk materials or PE thin films on crystalline substrates such as Si and sapphire. This paper reports the fabrication of ZnO thin film SAW UV-light sensors on glass substrates and the effect of post-deposition thermal annealing on the sensing performance. It was found that annealing at temperatures higher than 300 °C can improve the properties of ZnO films and the sensing performance of the UV-sensors remarkably. When the ZnO film annealed at 400 °C was used for sensors, the UV light induced resonant frequency shift increased more than 20 times with the response speed reduced to less than 2.4 s, much better than those made on ZnO films with lower temperature annealing.

High performance dual-wave mode flexible surface acoustic wave resonators for UV light sensing

Dual-mode flexible ZnO/polyimide surface acoustic wave (SAW)-based ultraviolet (UV) light sensors were fabricated and their performance was investigated. UV light sensing measurements showed that the responses of the dual wave modes of the sensors increase with the increase of light intensity and the frequency changes linearly with the change of light intensity. Under a 4.5 mW cm−2 UV light illumination, the resonant frequency of the Rayleigh wave decreased up to ~43 kHz, while that of the Lamb wave was approximately 76 kHz. The UV light sensitivities for the two resonant modes are 111.3 and 55.8 ppm (mW cm−2)–1, respectively. The resonant frequency, phase angle and amplitude of the two resonant modes exhibited a good repeatability in responding to cyclic change of the UV light, and an excellent stability up to a long duration of UV light exposure. The dual-mode flexible SAW resonators are simple in structure, more accurate in detection, and can be fabricated at low cost are, therefore, very promising for application in flexible sensors and electronics.

Bendable ZnO thin film surface acoustic wave devices on polyethylene terephthalate substrate

Bendable surface acoustic wave (SAW) devices were fabricated using high quality c-axis orientation ZnO films deposited on flexible polyethylene terephthalate substrates at 120 °C. Dual resonance modes, namely, the zero order pseudo asymmetric (A0) and symmetric (S0) Lamb wave modes, have been obtained from the SAW devices. The SAW devices perform well even after repeated flexion up to 2500 μe for 100 times, demonstrating its suitability for flexible electronics application. The SAW devices are also highly sensitive to compressive and tensile strains, exhibiting excellent anti-strain deterioration property, thus, they are particularly suitable for sensing large strains.

AlScN thin film based surface acoustic wave devices with enhanced microfluidic performance

This paper reports the characterization of scandium aluminum nitride (Al1−x Sc x N, x  =  27%) films and discusses surface acoustic wave (SAW) devices based on them. Both AlScN and AlN films were deposited on silicon by sputtering and possessed columnar microstructures with (0 0 0 2) crystal orientation. The AlScN/Si SAW devices showed improved electromechanical coupling coefficients (K 2, ~2%) compared with pure AlN films (<0.5%). The performance of the two types of devices was also investigated and compared, using acoustofluidics as an example. The AlScN/Si SAW devices achieved much lower threshold powers for the acoustic streaming and pumping of liquid droplets, and the acoustic streaming and pumping velocities were 2  ×  and 3  ×  those of the AlN/Si SAW devices, respectively. Mechanical characterization showed that the Youngs modulus and hardness of the AlN film decreased significantly when Sc was doped, and this was responsible for the decreased acoustic velocity and resonant frequency, and the increased temperature coefficient of frequency, of the AlScN SAW devices.

Ultrafast chemical-free cell lysis by high speed stream collision induced by surface acoustic waves

This paper reports on a surface acoustic wave (SAW) based cell lysis device on a LiNbO3 substrate by utilizing high speed collision of cells, which are accelerated by acoustic streaming. With varying working powers, cell lysis was achieved within 20 s and more than 95% lysis efficiency. The cell solution volume effect on SAW based lysis was also investigated and proved that it is not the main issue. With the CCK8 based viability test and verification of cell contents by electrophoresis, the efficient lysis results of our devices have been verified.

Biomaterial Gelatin Film Based Crossbar Structure Resistive Switching Devices

Crossbar structural resistive switching devices (memristors) are fabricated using biomaterial gelatin film as the dielectric layer. The performance of the devices and the effects of gelatin film thickness and baking temperature are investigated. Results show that the optimal gelatin film thickness for the memristors is ∼80 nm and baking temperature is ∼105 °C. The optimized memristors show a bipolar resistive switching behavior with the resistance ratio between the high-resistance state and low-resistance state over 102, the retention time over 106 s without any obvious deterioration, and excellent stability and reliability, demonstrating its good potential for applications. A conductive atomic force microscopy is used to study the conductivity of the gelatin films under various biases, and the results indicate that the conductive filaments are responsible for the resistive switching behavior of the gelatin-based memristors.

Flexible surface acoustic wave strain sensor based on single crystalline LiNbO3 thin film

A flexible surface acoustic wave (SAW) strain sensor in the frequency range of 162–325 MHz was developed based on a single crystalline LiNbO3 thin film with dual resonance modes, namely, the Rayleigh mode and the thickness shear mode (TSM). This SAW sensor could handle a wide strain range up to ±3500 μe owing to its excellent flexibility, which is nearly six times the detecting range of bulk piezoelectric substrate based SAW strain sensors. The sensor exhibited a high sensitivity of 193 Hz/ μe with a maximum hysteresis less than 1.5%. The temperature coefficients of frequency, for Rayleigh and TSM modes, were −85 and −59 ppm/ °C, respectively. No visible deterioration was observed after cyclic bending for hundreds of times, showing its desirable stability and reliability. By utilizing the dual modes, the strain sensor with a self-temperature calibrated capability can be achieved. The results demonstrate that the sensor is an excellent candidate for strain sensing.A flexible surface acoustic wave (SAW) strain sensor in the frequency range of 162–325 MHz was developed based on a single crystalline LiNbO3 thin film with dual resonance modes, namely, the Rayleigh mode and the thickness shear mode (TSM). This SAW sensor could handle a wide strain range up to ±3500 μe owing to its excellent flexibility, which is nearly six times the detecting range of bulk piezoelectric substrate based SAW strain sensors. The sensor exhibited a high sensitivity of 193 Hz/ μe with a maximum hysteresis less than 1.5%. The temperature coefficients of frequency, for Rayleigh and TSM modes, were −85 and −59 ppm/ °C, respectively. No visible deterioration was observed after cyclic bending for hundreds of times, showing its desirable stability and reliability. By utilizing the dual modes, the strain sensor with a self-temperature calibrated capability can be achieved. The results demonstrate that the sensor is an excellent candidate for strain sensing.