ST-cut Quartz Surface Acoustic Wave Research Articles

Surface Acoustic Wave DMMP Gas Sensor with a Porous Graphene/PVDF Molecularly Imprinted Sensing Membrane.

In this paper, surface acoustic wave (SAW) sensors containing porous graphene/PVDF (polyvinylidene fluoride) molecularly imprinted sensitive membrane for DMMP gas detection were investigated. A 433 MHz ST-cut quartz SAW resonator was used to convert gas concentration changes into frequency shifts by the sensors. The porous graphene/PVDF film was fabricated on the sensor’s surface by using the tape-casting method. DMMP molecules were adsorbed on the porous structure sensing film prepared by the 2-step method to achieve the specific recognition effect. The sensitivity of the sensor could reach −1.407 kHz·ppm−1. The response time and recovery time of the SAW sensor with porous graphene/PVDF sensing membrane were about 4.5 s and 5.8 s at the concentration of 10 ppm, respectively. The sensor has good anti-interference ability to most gases in the air.

Bacterial cellulose coated ST-cut quartz surface acoustic wave humidity sensor with high sensitivity, fast response and recovery

A Love mode surface acoustic wave (SAW) humidity sensor based on bacterial cellulose (BC) coated ST-cut quartz was developed in this study. The BC film is composed of ultrafine interwoven fibers to form a highly porous network, and its surface contains a large amount of hydroxyl groups, which significantly improve the adsorption capability of SAW sensing layer for water molecules. This results in significant mass loading effects and enhanced responsivity of the SAW sensor. The resonant frequency of the sensor changes linearly with RH at lower relative humidity (RH) values (e.g. RH < 30%), but when RH > 80%, an exponential increase in frequency shift as a function of RH is obtained due to the enhanced mass loading effect. A frequency shift of 89.8 kHz was measured using a sensor with a BC film with a thickness of 148 nm thick when the RH was increased from 30% to 93%. The frequency of the sensor can be fully shifted back to the original reading when the RH was returned back to 30%, with the response and recovery times of 12 s and 5 s, respectively. The SAW sensor also exhibits good short-term repeatability and long-term stability for humidity sensing.

Surface acoustic wave ammonia sensor based on ZnS mucosal-like nanostructures

Mucosal-like ZnS nanostructures were synthesized on surface of ST-cut quartz surface acoustic wave (SAW) device using a chemical bath deposition method for ammonia sensing applications. Results showed the SAW device with ZnS mucosal nanostructures achieved a good sensitivity, e.g., with a frequency shift of ~190 Hz upon exposure to 1 ppm ammonia gas. This is mainly attributed to the large specific surface areas and more active sites on the surfaces of the ZnS mucosal nanostructures. Selectivity of the SAW device with the ZnS mucosal nanostructure for ammonia sensing was excellent in comparisons with those for other types of gases such as hydrogen sulfide, hydrogen, nitrogen dioxide, carbon monoxide and ethanol.

Integrated sensing layer of bacterial cellulose and polyethyleneimine to achieve high sensitivity of ST-cut quartz surface acoustic wave formaldehyde gas sensor

Surface acoustic wave (SAW)-based formaldehyde gas sensor using bi-layer nanofilms of bacterial cellulose (BC) and polyethyleneimine (PEI) was developed on an ST-cut quartz substrate using sol-gel and spin coating processes. BC nanofilms significantly improve the sensitivity of PEI films to formaldehyde gas, and reduces response and recovery times. The BC films have superfine filamentary and fibrous network structures, which provide a large number of attachment sites for the PEI particles. Measurement results obtained using in situ diffuse reflectance Fourier transform infrared spectroscopy showed that the primary amino groups of PEI strongly adsorb formaldehyde molecules through nucleophilic reactions, thus resulting in a negative frequency shift of the SAW sensor due to the mass loading effect. In addition, experimental results showed that the frequency shifts of the SAW devices are determined by thickness of PEI film, concentration of formaldehyde and relative humidity. The PEI/BC sensor coated with three layers of PEI as the sensing layer showed the optimal sensing performance, which had a frequency shift of 35.6 kHz for 10 ppm formaldehyde gas, measured at room temperature and 30 % RH. The sensor also showed good selectivity and stability, with a low limit of detection down to 100 ppb.

Highly sensitive and selective Love mode surface acoustic wave ammonia sensor based on graphene oxides operated at room temperature

It is crucial to develop highly sensitive and selective sensors for ammonia, one of the most common toxic gases which have been widely used in pharmaceutical, chemical, and manufacturing industries. In this study, graphene oxide (GO) film was spin-coated onto surfaces of ST-cut quartz surface acoustic wave (SAW) devices with a resonant frequency of 200 MHz for ammonia sensing. The oxygen-containing functional groups (such as hydroxyl and epoxy ones) on the surface of GO film strongly absorb ammonia molecules and thus increase the film stiffness. This is attributed to the main ammonia sensing mechanism of the Love mode SAW devices, which show not only a positive frequency shift of 620 Hz for 500 ppb ammonia gas, but also an excellent selectivity (as compared to other gases such as H2, H2S, CO, and NO2) and a good reproducibility, operated at room temperature of 22 °C.

Highly sensitive ultraviolet sensor based on ZnO nanorod film deposited on ST-cut quartz surface acoustic wave devices

A highly sensitive surface acoustic wave (SAW) ultraviolet (UV) detector operated at room temperature was developed using a sensing layer of zinc oxide (ZnO) nanorods (NRs) grown on ST-cut quartz using a hydrothermal method. Under illumination of the UV light with a wavelength of 365 nm and an intensity of 6 μW cm−2, the resonant frequency of the SAW UV sensor based on ZnO NRs/ZnO nanofilm structure was decreased by ~200 Hz, mainly due to electroacoustic effect. The estimated enhancement in the sensitivity based on the experimental results is ~3.5 folds compared to a device with only a thin nanolayer ZnO film. Meanwhile, density of ZnO NRs over the ZnO seed layer was also investigated and an optimum design was proposed.

Colloidal quantum dot-based surface acoustic wave sensors for NO2-sensing behavior

Surface acoustic wave (SAW) sensors have great advantages in real-time and in-situ gas detection due to their wireless and passive characteristics. Using nanostructured sensing materials to enhance the SAW sensor’s responses has become a research focus in recent years. In this paper, solution-processed PbS colloidal quantum dots (CQDs) were integrated into quartz SAW devices for enhancing the performance of NO2 detection operated at room temperature. The PbS CQDs were directly spin-coated onto ST-cut quartz SAW delay lines, followed by a ligand exchange treatment using Pb(NO3)2. Upon exposure to 10 ppm of NO2 gas, the sensor coated with untreated PbS CQDs showed response and recovery times of 487 s and 302 s, and a negative frequency shift of −2.2 kHz, mainly due to the mass loading effect caused by the absorption of NO2 gas on the surface of the dense CQD film. Whereas the Pb(NO3)2-treated sensor showed fast response and recovery times of 45 s and 58 s, and a large positive frequency shift of 9.8 kHz, which might be attributed to the trapping of NO2 molecules in the porous structure and thus making the film stiffer. Moreover, the Pb(NO3)2-treated sensor showed good stability and selectivity at room temperature.

Room-Temperature Ammonia Sensor Based on ZnO Nanorods Deposited on ST-Cut Quartz Surface Acoustic Wave Devices.

Using a seed layer-free hydrothermal method, ZnO nanorods (NRs) were deposited on ST-cut quartz surface acoustic wave (SAW) devices for ammonia sensing at room temperature. For a comparison, a ZnO film layer with a thickness of 30 nm was also coated onto an ST-cut quartz SAW device using a sol-gel and spin-coating technique. The ammonia sensing results showed that the sensitivity, repeatability and stability of the ZnO NR-coated SAW device were superior to those of the ZnO film-coated SAW device due to the large surface-to-volume ratio of the ZnO NRs.

Highly sensitive room-temperature surface acoustic wave (SAW) ammonia sensors based on Co3O4/SiO2 composite films

Surface acoustic wave (SAW) sensors based on Co3O4/SiO2 composite sensing films for ammonia detection were investigated at room temperature. The Co3O4/SiO2 composite films were deposited onto ST-cut quartz SAW resonators by a sol–gel method. SEM and AFM characterizations showed that the films had porous structures. The existence of SiO2 was found to enhance the ammonia sensing property of the sensor significantly. The sensor based on a Co3O4/SiO2 composite film, with 50% Co3O4 loading, which had the highest RMS value (3.72), showed the best sensing property. It exhibited a positive frequency shift of 3500Hz to 1ppm ammonia as well as excellent selectivity, stability and reproducibility at room temperature. Moreover, a 37% decrease in the conductance of the composite film as well as a positive frequency shift of 12,500Hz were observed when the sensor was exposed to 20ppm ammonia, indicating the positive frequency shift was derived from the decrease in film conductance.

Ammonia gas sensors based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices

Surface acoustic wave (SAW) ammonia gas sensors based on ZnO/SiO2 bi-layer nanofilms on ST-cut quartz surface acoustic wave devices were fabricated and characterized. The ZnO and SiO2 layers were coated onto SAW resonators by combining a sol–gel process and a spin-coating technique. The SEM and AFM results revealed the ZnO/SiO2 bi-layer films had porous structures. The gas sensing results showed that the sensitivity of sensors was dependent on the value of sheet conductivity of the sensing films. As a result, the bi-layer nanofilms were much more sensitive than the single layer films due to their appropriate sheet conductivity, and the absolute response value was dependent on the thickness of the top ZnO layer. The sensor based on the bi-layer nanofilm with 60nm top ZnO layer showed the best gas sensing property. It exhibited a frequency shift of 2000Hz in 30ppm ammonia gas with good repeatability and stability.

Dynamic chemical vapor sensing with nanofibrous film based surface acoustic wave sensors

We report on the use of electrospun nanofibrous film for surface acoustic wave (SAW) sensors to enhance its chemical detection capability. Ultrafine (100–300 nm) polyethylene oxide (PEO) fibers with controlled thickness and porosity were electrospun-coated on the surface of a ST-cut quartz SAW sensor. The nanofibrous film provides a high surface area to volume ratio, which can not only offer more adsorption sites for vapor molecules, but also shortens the diffusion length of vapor molecules into polymer material. Compared to conventional thin film coating techniques, the nanofiber-coated SAW sensor shows a higher sensitivity and faster response (shorter adsorption/desorption times). In addition, a theoretical analysis was performed to understand SAW sensor response with nanofibrous film coating. It is concluded that the nanofiber film holds a great potential in enhancing SAW sensor performance for trace level detection of chemical analytes.

Coupling coefficient determination based on simulation and experiment for ST-Cut quartz saw delay-line response

This paper used the theoretical calculation to simulate the response of surface acoustic wave (SAW) delay-line on quartz substrate and then compared to the experimental results. The coupling coefficients affected by operation frequency as well as aperture length were built up by experimental data analysis. Based on the experimental values of these two parameters, the device-coupling coefficient was defined. This method is new and we has not found in other document. In addition, it improves the simulation results and helps the analysis process more comprehensive. This study developed ST-cut quartz SAW delay-lines with gold inter-digital transducer (IDT) operating at 39.5 and 78.9 MHz corresponding to 80 and 40 μm of the wavelength. The differences of aperture length in IDT designs were investigated to help understand the effects of this parameter on SAW sensor response. The maximum error of operating frequency is 1%; of insertion loss is 4.25 and 3.13% for bandwidth. The larger of the insertion loss error is expected owing to the result of mathematical approximation and the quality of quartz substrate. The simulation results agree with the experimental results shows that the simulation method can apply to quartz-based SAW delay-line as well as for other material based SAW delay-line applications. The results help understand more about the parameters which effect the insertion loss, operating frequency and bandwidth. It should be very useful for IDT design in specific, SAW sensor and SAW filter design in general.

Design of ST-cut quartz surface acoustic wave chemical sensors

This research investigates the design rules polymer-based ST-cut quartz surface acoustic wave (SAW) sensors used in detecting organophosphorous compounds. A complex shear modulus is used to represent express different types of polymer (glassy, glassy–rubbery, and rubbery). Different propagation directions lead to different wave propagation properties in SAW devices, such as attenuation and velocity. Hence, the choice of polymer type and wave propagation direction is a very important aspect of sensor design. Calculations indicate that the glassy–rubbery and rubbery film coated on SAW sensors very sensitively detect organophosphorous vapor. However, the glassy–rubbery film is most suitable in sensing applications, because it makes signal attenuation significantly. Although sensitivity is high when an acoustic wave propagates in the Y-direction, the X-direction is preferred at the optimum path on account of the excellent temperature stability in the X-propagation direction. Therefore, SAW vapor sensors with an appropriate chemical interface and wave propagation direction are designed to yield superior performance.