Quartz Surface Acoustic Wave Resonators Research Articles

Detecting low-concentration ammonia with a surface acoustic wave sensor using a silver nanoparticles–graphene–polypyrrole hybrid nanocomposite film

In this study, a surface acoustic wave (SAW) resonator coated with graphene/polypyrrole hybrid nanocomposite films decorated with silver nanoparticles (AgNPs-G/PPy) is proposed for detecting ammonia (NH3) in parts-per-billion concentrations. The AgNPs-G/PPy hybrid nanocomposite film was synthesized via in situ chemical oxidative polymerization. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the AgNPs-G/PPy hybrid nanocomposite film, confirming its successful synthesis and identifying a wrinkled multilayered structure. The AgNPs-G/PPy hybrid nanocomposite film was spin-coated onto the surface of a stress-compensated temperature-cut quartz SAW resonator having an operating frequency of 98.5 MHz to create an NH3 gas sensor. NH3 adsorption by the AgNPs-G/PPy hybrid nanocomposite film modulated the acoustic wave velocity, and the corresponding frequency shift served as a sensing signal. The synergistic interaction between the three constituent materials (AgNPs, graphene, and polypyrrole) enhanced the sensitivity, selectivity, and response speed of the sensor for NH3 detection. At room temperature, the proposed sensor exhibited a positive frequency shift of 568 Hz when exposed to 50 ppb of NH3 gas and a rapid response time of less than 60 s. In addition, the SAW sensor exhibited excellent selectivity.

A Room-Temperature Surface Acoustic Wave Ammonia Sensor Based on rGO/DPP2T-TT Composite Films.

Surface acoustic wave (SAW) sensors based on reduced graphene oxide/poly (diketopyrrolopyrrolethiophene-thieno [3,2-b]thiophene-thiophene) (rGO/DPP2T-TT) composite sensing films for the detection of ammonia were investigated at room temperature in this study. The rGO/DPP2T-TT composite films were deposited onto ST-X quartz SAW resonators by a drop-casting method. FESEM, EDS, and XRD characterizations showed that the rGO/DPP2T-TT composite film was successfully synthesized and exhibited numerous wrinkles and a rough structure, which are crucial for gas adsorption. The frequency response to 500–1400 ppb ammonia shown by the prepared SAW sensor coated with rGO/DPP2T-TT composite film increased linearly as the ammonia concentration increased. The sensor based on a rGO/DPP2T-TT composite film exhibited a positive frequency shift of 55 Hz/ppm, and its frequency response to 500 ppb ammonia was 35 Hz. The sensors thus show promising potential in detecting sub-ppm concentration levels of NH3 at room temperature, which opens up possibilities for applications in the noninvasive detection of NH3 in the breath. As a result, the rGO/DPP2T-TT composite sensor can be a good candidate for in situ medical diagnosis and indoor/outdoor environment monitoring.

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.

Cellulose nano-crystals as a sensitive and selective layer for high performance surface acoustic wave HCl gas sensors

We report that cellulose nano-crystals (CNCs) can be used as a sensitive and selective layer in surface acoustic wave (SAW) sensors for in-situ HCl gas detection. CNCs were prepared through directly hydrolysis of cotton fiber and were spin-coated on quartz SAW resonators to form the sensitive layer. The CNCs have been identified to have abundant hydroxyl groups on their surfaces, which can act as the perfect adsorption sites for H2O, which can further act as the active sites for HCl gas adsorption. The absorption of HCl on the CNCs layer, thus leads to an increase of its mass, causing negative responses of the SAW sensors. Ambient humidity and thickness of CNCs layer are found to have significant influences on the responses of the SAW sensor. With an 80-nm-thick CNCs layer, the sensor shows a response of −2 kHz to 1 ppm HCl at 25 °C and relative humidity of 50 % with an excellent selectivity and recovery characteristics.

Analysis of contributions of nonlinear material constants to temperature-induced velocity shifts of quartz surface acoustic wave resonators

In this paper, we examine the significance of the various higher-order effects regarding calculating temperature behavior from a set of material constants and their temperature coefficients. Temperature-induced velocity shifts have been calculated for quartz surface acoustic wave (SAW) resonators and the contributions of different groups of nonlinear material constants (third-order elastic constants (TOE), third-order piezoelectric constants (TOP), third-order dielectric constants (TOD) and electrostrictive constants (EL)) to the temperature-induced velocity shifts have been analyzed. The analytical methodology has been verified through the comparison of experimental and analytical results for quartz resonators. In general, the third-order elastic constants were found to contribute most significantly to the temperature-induced shifts in the SAW velocity. The contributions from the third-order dielectric constants and electrostrictive constants were found to be negligible. For some specific cases, the third-order piezoelectric constants were found to make a significant contribution to the temperature-induced shifts. The significance of each third-order elastic constant as a contributor to the temperature-velocity effect was analyzed by applying a 10% variation to each of the third-order elastic constants separately. Additionally, we have considered the issues arising from the commonly used thermoelastic expansions that provide a good but not exact description of the temperature effects on frequency in piezoelectric resonators as these commonly used expansions do not include the effects of higher-order material constants.

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.

Characterization of a Protein Using Saw Resonator

Surface Acoustic Wave (SAW) propagation on the surface of a piezoelectric crystal is used as a carrier of information. SAW Sensors are micro-electromechanical systems (MEMS) in which high frequency acoustic waves travel close to the surface of a piezoelectric substrate. Because of confined acoustic energy near the surface within the range of one acoustic wavelength, SAW devices are highly sensitive for surface perturbation such as molecular absorption or adsorption and change of viscoelastic properties. Surface acoustic wave (SAW) devices are very promising in providing a high-performance sensing platform with wireless and remote operational capabilities. Research focus is on studying methods to characterize conductance, susceptance, viscosity, and other properties of protein samples, like albumin using SAW related resonator devices quickly and inexpensively using quantities of less than one Nano-liter. Applied pressure or bending of the quartz sets up a condition where a separation of charge occurs (due to change in dipole moment density). The potential difference is proportional to the magnitude of stress. When a mass is deposited on the electrode the resultant decrease in resonant frequency can detect mass changes at nanogram level. We propose to advance the sensitivity of the characterization by use of quartz SAW resonators since they can operate at much higher frequency than bulk wave oscillators.

Nonlinear frequency shift in surface acoustic wave resonator operating as a gas sensor

The shift in the resonance frequency of a two-port quartz surface acoustic wave (SAW) resonator operating as a gas sensor without a selective layer is studied versus the power of an SAW excited in the resonator. At working frequencies of the resonator (≈389 MHz) placed in the flow of moisture-containing nitrogen gas, an anomalously large positive shift of the resonance frequency is observed as the SAW power exceeds 1 mW. This shift is one order of magnitude larger than that due to the nonlinear amplitude-frequency effect, which is known for quartz SAW resonators. Possible physical mechanisms underlying this phenomenon are analyzed. Experimental data indicate that such a shift is associated with the influence of a powerful SAW on sorption processes taking place on the active surface of the resonator rather than being a direct consequence of heating of the SAW substrate by the powerful SAW.

Analysis of Frequency Locations of the In-Harmonics in Quartz Surface Acoustic Wave Resonators Using Helmholtz Equations

Preventing spuriousness in a quartz surface acoustic wave (SAW) resonator is important for maintaining reliability in digital communication systems because it reduces phase noise quality of signals. For such a resonator, the in-harmonic modes, which are also undesirable responses in the direction of the lengthwise x-coordinate and the transversal y-coordinate, have been studied. We show that the governing equation of the resonator is given by a Helmholtz equation, and we obtain equations corresponding to the resonance frequencies. We solve them theoretically for transverse modes, and numerically using a T matrix for longitudinal modes. We obtain a frequency-mode chart of a one-port quartz SAW resonator and obtain adequate agreement of accuracy with in-harmonics that involve two-dimensional modes. Furthermore, we study mesa-structure-type SAW resonators to determine a method of controlling the frequency arrangement.

1/f frequency fluctuations in quartz acoustic resonators

The level of 1/f frequency fluctuations has been measured on numerous quartz surface acoustic wave resonators. When these results are combined with existing data for quartz bulk acoustic wave resonators it becomes clear that the 1/f process is essentially the same in both types of devices. Furthermore, the power spectral density of frequency fluctuations shows a clear power law dependence that is very close to 1/Q4u , where Qu is the unloaded Q. However, the observed dependence on loaded Q is weak.

Optimization of Quartz SAW Resonator Structure with Groove Gratings

Abstruct-A new surface acoustic wave (SAW) resonator optimum design method is proposed to reduce resonator sue. The fundamental design feature is optimization of the interdigital transducer (IDT) electrode period to reflector electrode period ratio, in order to adjust the frequency for the maximum IDT admittance to the maximum reflector reflection point. As a result of the optimized electrode period ratio, both reflector and IDT frequency responses are utilized to advantage, and good resonant Characteristics are obtained with a reduced number of reflector electrode fingers. This design method can be effectively applied for SAW resonators with groove gratings, both on reflectors and transducer, that are formed by a reactive sputter etching process. 145 MHz SAW resonators with groove gratings fabricated on a small 6 mm X 3 mm ST-cut quartz chip have provided good electrical performances, such as high Q values above 2 X lo4 and low resonance resistance in the 20-30 range.