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A novel langasite crystal microbalance instrumentation for UV sensing application
Sensors and Actuators A 252 (2016) 16–25
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A novel langasite crystal microbalance instrumentation for UV sensing application Tridib Saha a , Ningqun Guo b , N. Ramakrishnan a,∗ a b
Electrical and Computer Systems Engineering, School of Engineering, Monash University Malaysia, 47500, Malaysia Mechanical Engineering, School of Engineering, Monash University Malaysia, 47500, Malaysia
a r t i c l e
i n f o
Article history: Received 9 June 2016 Received in revised form 28 September 2016 Accepted 17 October 2016 Available online 21 October 2016 Keywords: Langasite crystal microbalance Electrical equivalent circuit Transient response ZnO thin film UV sensing Sensor instrumentation
a b s t r a c t In this paper, we present a novel low-power UV sensor instrumentation based on ZnO thin-film and langasite crystal microbalance (LCM) composite resonator. The design of this sensor utilizes the exceptional transient response characteristics of thickness shear mode langasite crystal and UV sensitivity of annealed ZnO thin film. Our comparative transient analysis of langasite and quartz crystals shows that langasite crystal has high stability and fast relaxation times, and requires a fraction of excitation power in comparison to that of quartz. Upon investigating the equivalent circuit components, we discovered that high value of motional capacitance of LCM resulted in these contrasting transient responses in LCM and QCM. This high motional capacitance can be further attributed to the high electromechanical coupling coefficient (K2 ) of piezoelectric langasite crystal. Motivated by these observed characteristics, we proposed a novel sensor instrumentation that is simple yet effective in simultaneous measurements of the frequency, dissipation factor, and the amplitude of oscillation of a LCM. Moreover, our measurements allow for direct monitoring of any change in piezoelectricity of ZnO thin film. Our new measurement approach uses a one-shot pulse instead of conventional sinusoidal waveform to drive the crystal, which allows for simple instrumentation and requires very low input power. The transient response of the crystal shows that the crystal oscillates at its fundamental frequency and the amplitude of oscillation decays exponentially. From the recorded decay curve, the frequency of the freely oscillating crystal, and other important crystal parameters were calculated. We have utilized this mechanism to demonstrate highly sensitive ZnO thin film and LCM based UV sensor. Highest sensitivity of 35.8 ppm per W/cm2 was observed for the LCM coated with 400 nm ZnO film. Additionally, the influence of UV on the piezoelectric output of ZnO thin film was also characterized by monitoring the DC offset of the output oscillations. Our findings in this work opens scope for developing low-power acoustic sensors with significant implications towards self-powering sensors. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since Sauerbrey’s mass sensing demonstration in 1959, quartz crystal microbalance (QCM) has emerged as one of the most popular sensing techniques because of its high sensitivity, stability, portability and low cost. Also known as thickness shear mode (TSM) resonator, QCM undergoes shear displacement upon excitation and resonates at specific frequencies [1]. Maximum surface displacement occurs at each of these resonant frequencies, making the crystal sensitive to any surface perturbations. Change in the surface perturbations introduces a shift in the resonance frequency of
∗ Corresponding author. E-mail addresses: [email protected] (T. Saha), [email protected] (N. Guo), [email protected] (N. Ramakrishnan). http://dx.doi.org/10.1016/j.sna.2016.10.024 0924-4247/© 2016 Elsevier B.V. All rights reserved.
the crystal [2]. Based on this sensing principle, QCMs have been extensively used in a wide range of applications such as mass, temperature, humidity, chemical and biological sensors [3]. Although quartz work excellently in most standard applications, it is not without its limitations. High temperature operation is unsuitable for QCMs as quartz loses its piezoelectric properties above its Curie temperature of 573 ◦ C [4]. Moreover, QCM in liquid media is also limited to low viscosity solutions as the oscillations are significantly damped in high viscosity liquids [5]. Over the last decade, langasite (La3 Ga5 SiO14 ) has been in the spotlight as an alternative piezoelectric crystal to quartz [6,7]. Langasite has significantly higher electromechanical coupling coefficient, reduced phase velocities and lower acoustic losses compared to that of quartz [8]. In addition to its excellent piezoelectric and acoustic properties, langasite can also operate in high temper-
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atures of up to 900 ◦ C and does not undergo phase transformation up to its melting point of 1473 ◦ C [9]. Motivated by langasite’s high temperature stability, we recently synthesized ZnO nanostructures on LCM using a thermal evaporation technique for UV sensing application [10]. Zinc oxide is predominantly used as a sensing media because of its excellent piezoelectric, semiconducting and optical properties. With a wide band gap of 3.37 eV at room temperature and a large exciton binding energy, ZnO is well-suited for gas, pH, and bio- sensors [11–13]. Recently, there have been several reports of ZnO based acoustic wave UV sensors as well, utilizing the photoelectric properties of ZnO [14–16]. Under UV illumination, photo-induced carriers are generated that results in a shift in resonant frequency of the acoustic resonator. On the other hand, ZnO nanostructures have also been independently used as UV sensors based on photocurrentvoltage (I–V) measurements [17–19]. Very recently, Wang et al. have demonstrated the effect of UV intensity on the piezoelectric output of ZnO nanowires [20]. This relationship is particularly important to quantify the photoelectric and piezoelectric property of any material. However, to our best knowledge, there has been no reports of ZnO based acoustic wave UV sensor that allow for simultaneous measurement of frequency and piezoelectricity. Crystal microbalances are mainly operated in two different modes to monitor changes in resonant frequency [21]. In one method, the crystal is excited by a continuous sinusoidal potential and the resonance is observed under steady state conditions [22]. Oscillator circuits and impedance analysis systems employ steady-state methods to detect any changes in the fundamental frequency of the crystal. In a more recently developed method, transient decay of a freely oscillating crystal is used to determine its resonant frequency and dissipation [23]. The crystal is driven for a specific amount of time by an alternating voltage tuned to the crystal’s resonant frequency, and then the driving power is switched off. Frequency and dissipation factor of the crystal is calculated from the exponentially decaying oscillating potential across the crystal, also known as ‘ring-down’ mode oscillations. Both the steady-state and transient methods require identification of the crystal’s resonant frequency before it can be used for any measurements. However, these processes require either expensive instruments, or computationally exhausting frequency sweeps to determine the largest output signal that corresponds to the resonant frequency. Till date, quartz based crystals were only explored for the above such mentioned experiments. Being a fairly new type of TSM resonator, LCM still remains to be thoroughly investigated in terms of its mechanical and electrical properties. Accordingly, in this work we investigated transient response of LCM under single pulse excitation, and compared its responses with that of a QCM. We observed the LCM alone exhibited decaying oscillation at its fundamental frequency at the falling edge of the single pulse. Based on these characteristics, we developed a novel, low-power, single pulse driven instrumentation technique to simultaneously measure frequency, dissipation and output voltage of a LCM. To establish the significance of the technique for sensing applications, we sputtered ZnO films as sensing medium on LCM surface to detect UV light and demonstrated real-time frequency and output voltage DC offset measurement of LCM sensors. The frequency and voltage measurements allowed for real-time monitoring of the sensitivity and change in piezoelectricity of ZnO thin films. Also, Laser Doppler Vibrometry (LDV) was carried out to investigate the acoustoelectric effect on the surface displacement of ZnO coated LCM under UV illumination. Further, we delved deeper into the properties of langasite crystal to derive the impedance elements of a blank crystal as well as crystals coated with various thicknesses of ZnO thin films and related the transient response to langasite’s electromechanical coupling coefficient (K2 ). Our results show promising potential for low-power and hassle-
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Table 1 Equations to calculate equivalent circuit elements of LCM. Impedance parameters R=
1 Gmax
Series Inductance
L=
QR 2f
Series Capacitance
C=
1 2fQR
Series Resistance
.
free LCM based sensor system that can be extended to a wide range of sensing applications. 2. Experimental setup and methodology 2.1. Setup for transient response analysis Fig. 1 shows the schematic diagram of our setup. Langasite crystal microbalances (LCM) of 6 MHz ± 10 kHz fundamental frequency with gold electrodes over chrome sublayer were obtained from Fomos-Materials, Russia. LCM was mounted on a two-point contact holder and connected to the circuit. One electrode of the crystal was connected to the arbitrary waveform generator (AWG) and the other to the variable load resistor. The crystal was driven with a 0–2 V pulse with a pulse width of 5 ms. The corresponding voltage output across the load resistor was monitored using a digital oscilloscope (DSO). At the falling edge of the input pulse, a sinusoidal voltage output was observed across the resistor that decays exponentially in a ‘ring-down’ mode, similar to that of previous reports of AC driven quartz crystals. The output voltage was passed through a built-in 20 MHz low-pass filter and 16 point averaging function in the oscilloscope to improve the quality of measurements and reduce the influence of noise. The ring-down oscillations as a function of time were recorded and fitted to exponential decay function using nonlinear least-squares solver (lsqcurvefit) ‘trust-region-reflective’ algorithm in MATLAB. For each measurement, only the first 50 oscillations were considered to calculate the average frequency and the dissipation factor based on the period of oscillation and decay time constant, respectively. All measurements were carried out at room temperature of 22 ◦ C and indoor relative humidity of 55%. 2.2. Derivation of electrical equivalent circuit parameters Fundamental frequency and Q factor of LCMs were measured using a network analyser. Admittance spectra (Y) were examined to measure the series resonant frequency (fs ) and the Q factor of the crystals. Electrical equivalent circuit parameters were calculated according to the equations shown in Table 1, where Gmax is the maximum conductance, Q is the quality factor and f is the series resonant frequency of the LCM. The derived values of the impedance elements were then used to simulate the response of the LCMs under a single pulse DC input using CircuitMaker2000 software. 2.3. ZnO film preparation To serve as the UV sensing medium, zinc oxide thin film of 100 nm, 200 nm and 400 nm thicknesses were sputtered on the front side of the LCMs at 150 ◦ C. These LCMs were then rapidly annealed at 500 ◦ C for 15 mins on a hot plate to remove compressive strain in ZnO film and improve their crystallinity [24]. 2.4. Laser doppler vibrometry Polytec MSA-400 Laser Doppler Vibrometer (LDV) was used to investigate the acoustoelectric effect in a bare 6 MHz LCM and a
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Fig. 1. Schematic diagram for single pulse driven crystal oscillator circuit.
LCM coated with ZnO thin film. Continuous measurement was performed in Fast Fourier Transform (FFT) measurement mode with 12800 FFT lines and 10-point complex averaging. The generator was set to scan between 5.5 MHz and 6.5 MHz frequency range, with 10 V periodic chirp waveform. The central electrodes of the LCMs were scanned using high-density grid scan. Resonant frequency of the LCMs were determined from the frequencies at which maximum vibration displacements were observed. The LCMs were then excited using sinusoidal waveform at their respective resonant frequencies. A 365 nm 9-LED UV source was directly placed 5 cm over the LCM. A point of maximum displacement was located on the LCM surface and the point was tracked to measure the values of instantaneous surface displacement. The LCMs were then exposed to UV illumination for a specific time after which the UV source was turned off. Change in surface displacement of the uncoated and ZnO film coated LCMs were monitored in the presence and absence of UV illumination. 2.5. UV sensor measurement setup The setup described in Fig. 1 was adapted for this part of the experiment. A 365 nm 9-LED UV light source and a LCM sensor were placed 3 cm apart on an optical rail. The resonant frequency of the LCM was recorded from the oscilloscope and set as the reference value. The LCM was excited using a 2 Hz 2Vp-p pulse as per our measurement setup for real-time frequency and DC offset measurements. Subsequently, the LCM sensor was illuminated with UV light and shift in frequency and DC offset were continuously recorded until the values settled down. Next, the source was turned OFF and kept off until the shift in central frequency returned back to zero. This ON-OFF cycle was repeated a few times to establish the repeatability of the sensor. 3. Results and discussion 3.1. Transient response analysis The transient response of blank LCM and QCM in a “ring-down” decay method setup, as demonstrated by Rodahl et al. [25], are shown in Fig. 2(a) and (b), respectively. Resonance was determined by detecting the maximum voltage across the load crystal. Both quartz and langasite crystals were observed to settle down at their maximum voltages of about 0.45 Vp-p when excited at their respective resonant frequencies. As observed in Fig. 2, the graphs demonstrate a charging phenomenon as the voltages increase from
Table 2 Motional impedances of quartz and langasite crystal microbalances. Crystal
R ()
L (mH)
C (fF)
LCM QCM
16.1 31.7
3.5 62.7
201 16.1
zero to maximum. This is due to the time taken for the crystals to settle in their resonant states and produce stable oscillations. LCM was significantly faster to stabilize in comparison to QCM, taking only about 0.5 ms whereas QCM required almost 8 ms. Similarly, the time to decay for LCM oscillations in its free state was also much shorter than that of QCM. Time taken to decay to minimum voltage for LCM was about 0.2 ms compared to that of about 6 ms for QCM. Thus, LCM allows for much faster overall measurement time, and required only a fraction of driving power for the measurements in comparison to that of a QCM. The best and most convenient way to comprehend the settling and decay times of LCM and QCM is by studying the electrical equivalent circuit model of the crystals. Using a network analyzer, the admittance spectra of the crystals were examined and the impedance elements were quantified. A simple ButterworthVan Dyke (BVD) model with motional R, L, C components can be used to describe the characteristics of both quartz and langasite crystals. As the static capacitance (C0 ) only influences the electrical behavior far from resonance, only the motional components affected by the electromechanical coupling (K2 ) of the crystals have been considered. The electromechanical coupling constant (K2 ) of langasite is about 2.26 times higher than that of quartz [6]. As a result, the motional impedance elements are also significantly different for LCM, as shown in Table 2. Treating the motional component as a simple RLC circuit, the neper frequency and the damping factor were calculated, which give a better understanding of the settling and decay times of the crystals. The damping factors as ratio of resonant frequencies of unperturbed LCM and QCM were 0.38 × 10−3 and 0.05 × 10−3 , respectively. This significant difference in the damping factors (∼8 times) leads to the rapid settling and decay times of LCM. It is therefore evident from these values that the K2 value and the consequent equivalent impedance elements allow for such improved transient response of LCM. Motivated by the low-power requirement and the enhanced motional impedance elements of LCM, we devised a novel, singlepulse based frequency, dissipation and amplitude measurement technique. In place of exciting the crystal in ring-down mode setup which required AC supply of a particular frequency, we attempted
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Fig. 2. Responses of uncoated LCM and QCM resonators driven with sinusoidal voltage at corresponding resonant frequencies. The transient decay voltage is also shown when the input power is switched off (dotted line).
to excite crystal with single shot pulse. The change in output voltage was monitored across a load resistor using the setup described in Fig. 1. Fig. 3(a) shows the input voltage and the measured output of the LCM circuit. At the falling edge of the pulse input, the voltage across the load resistor is observed to be exponentially decaying sinusoid as shown in Fig. 3(b). The frequency of oscillations was measured to be 6 MHz – the fundamental frequency of a blank LCM. The output voltage was recorded and processed in MATLAB to calculate the dissipation factor, by nonlinear least-squares solver. The dissipation factor, D, was calculated to be 1.14 × 10−2 for the blank LCM. The same LCM had a dissipation factor of 1.22 × 10−4 when measured directly using a network analyzer. The discrepancy is about a factor of 100 from the ‘true’ dissipation factor, and is a direct contribution from of the 100 load resistor, RL , placed in series with the LCM. The same single pulse input was applied to a QCM using the setup described in Fig. 1. However, no oscillations were observed at the rising or falling edge of the input. To investigate the reason behind the absence of oscillations in QCM as observed for LCM, we performed simulations of their equivalent circuits using CircuitMaker2000 and studied their transient responses for single pulse excitation. As shown in Fig. 4, for a short input pulse of 2 ms of 2 Vp-p , LCM and QCM produced amplitude voltages of about 2.1 mVp-p and 210 Vp-p , respectively. The simulated output of the LCM is comparable to the experimental output (Fig. 3b), apart from the DC offset of −0.5 mV as seen for the experimental data. Thus, it is safe to assume that the simulation parameters closely emulate the experimental circuit. The simulation results further confirm that the free-oscillating voltage of the QCM is significantly lower (∼10 times) than that of the LCM. Moreover, the simulation results also show the long decay time of QCM in comparison to the LCM. Therefore, LCM is a much better-suited material for this low-power single pulse setup compared to a QCM.
3.2. UV sensing performance of proposed sensors Next, the new measurement setup was employed to investigate ZnO thin-film based LCM UV sensors. Frequency response
Table 3 Sensitivities, response and recovery times of our proposed UV sensor. Sample
Sensitivity (W/cm2 )−1
Response time (s)
Recovery time (s)
100 nm 200 nm 400 nm Our previous work [10]
18.8 ppm 26.1 ppm 35.8 ppm 0.096 ppm
26 19 24 11
89 65 49 43
characteristics of LCMs coated with 100 nm, 200 nm and 400 nm thicknesses of ZnO thin-films are shown in Fig. 5(a)–(c). Resonant frequency of each of the sensors were measured in the dark and used as reference to monitor shifts in frequency. Under 365 nm UV illumination of 0.8 mW/cm2 intensity, maximum frequency downshift of 90 kHz, 125 kHz, and 172 kHz were observed for 100 nm, 200 nm and 400 nm samples, respectively. Some residual frequency shift was observed for each of the sensors after the UV source was turned off. Sensitivity, response and recovery times were also calculated for each of the sensors to characterize their performance. Sensitivity was normalized as the ratio of maximum frequency shift over central frequency (f/f0 ) per W/cm2 of UV intensity. Response and recovery times were defined as the time taken for the resonant frequency shift to change from 10% to 90%, and from 90% to 10%, respectively. The fitted data (represented by the solid lines in Fig. 5) were used for all the calculations. Table 3 shows the calculated values for each of the parameters. Highest sensitivity of 35.8 ppm per W/cm2 was observed for the LCM coated with 400 nm ZnO film. 200 nm and 100 nm samples showed sensitivities of 26.1 and 18.8 ppm per W/cm2 , respectively. The sensitivity values obtained in the present work are several orders higher in magnitude compared to our previous work on ZnO nanowire based LCM sensor [10], where we demonstrated excellent response and recovery times as indicated in Table 3. It should be also noted that sensitivity achieved in the present work is highest among all other reports on undoped ZnO based acoustic wave UV sensors [26–28]. On the other hand, best response time of
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Fig. 3. (a) Input pulse and the corresponding response of an LCM, measured across the series load resistor. (b) Zoomed in output at the falling edge of the input pulse. The red line shows the envelope of the exponential decay of the LCM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Simulated transient response of (a) LCM and (b) QCM excited by single-pulse step.
Fig. 5. Magnitudes of downshift in resonant frequencies versus time for (a) 100 nm, (b) 200 nm and (c) 400 nm ZnO thin-film coated LCMs under one UV ON-OFF cycle.
19 s was recorded for 200 nm sample, followed by 24 s of 400 nm sample and 26 s of 100 nm sample. Recovery times were relatively longer, where LCM with 400 nm ZnO film had the fastest recovery time of 49 s, while 200 nm and 100 nm samples took 65 s and 89 s, respectively. Unlike response and recovery times, sensitivity of an acoustic wave UV sensor not only depends on the geometry and quality of the ZnO sensing layer, but also on the measurement technique. As a result, the enhanced sensitivity observed in this work can be attributed to both the sensor instrumentation as well as the annealing of the ZnO thin film. As shown in Table 3, previously reported
ZnO nanowire based LCM sensor takes almost half the time to respond to UV illumination compared to that of thin-film based sensors. The recovery times are also observed to be much slower in comparison. These differences in response and recovery times can be ascribed to the high surface-to-volume ratio in nanowires compared to that of the ZnO films used in this work [18]. The generation and recombination of photocarriers on the thin-film surface takes much longer in comparison to that of ZnO nanostructures, and consequently results in slower response and recovery times. The piezoelectric output of the ZnO coated LCM composite resonators under UV illumination can also be characterized from the
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Fig. 6. (a) Downward shift in DC offset of the few initial cycles of output oscillation (b) Shift in DC offsets in the ring-down responses of ZnO thin-film coated LCMs under varying UV intensities.
transient decay. Fig. 6a shows the transient output of a ZnO coated LCM in dark state and under UV illumination. It can be clearly seen that the few initial output oscillation cycles shift downwards when illuminated by UV. This negative DC offset of the oscillations cycles were further investigated by varying the intensity of the UV illumination. As shown in Fig. 6b, UV intensity was increased from zero to 2400 W/cm2 , and the corresponding shift in DC offset for the first oscillation cycle was measured. Sample with 100 nm ZnO thin film coated demonstrated the maximum DC shift of 3.6 mV, followed by 200 nm sample with 3.44 mV, and 400 nm with 2.6 mV. The shift in DC offset for the first few cycles of oscillations occurs because of the parallel charging effect, which is directly related to the piezoelectricity of the ZnO thin film. As the intensity of the UV illumination increases, a large amount of photogenerated electron-hole pairs are generated, creating a strong piezoelectric screening effect [29]. This screening effect results in the DC offset of the output voltage. Above a certain intensity of UV, the quantity of photogenerated electrons reaches saturation and the shift in DC offset approaches stability. As the 100 nm ZnO film has the lowest thickness, the photogenerated holes quickly migrate to the surface, leaving behind high density of carriers. Under the compressive and tensile forces from the vibration of LCM, a piezoelectric field is created in the ZnO thin film with high screening effect. This screening effect and the
consequent reduction in piezoelectricity of the ZnO films inversely affects the sensitivity of the sensors. As a result, the lowest sensitivity is observed for 100 nm sample, which also shows the highest shift in DC offset among all the samples. Negative shift in resonant frequency of the LCMs were observed under UV light due to acoustoelectric effect. When the ZnO thin film is illuminated with UV light, electron-hole pairs are photogenerated. The holes migrate to the surface and are trapped, leaving behind unpaired electrons that increase the conductivity of the ZnO layer. As a result, the shear wave velocity decreases which causes the resonant frequency to decrease as per the following equation, f0 =
v 2t
where f0 is the resonant frequency, v is the acoustic velocity and t is the thickness of the piezoelectric film. The sensitivities of the ZnO thin film coated LCM sensors can also be explained by the same equation. The thickness of the piezoelectric layer, ZnO thin film in our case, is inversely proportional to the resonant frequency. As a result, highest shift was observed for the thickest ZnO film of 400 nm. To explore the mechanism of acoustoelectric effect on the surface displacement amplitude of LCM under UV illumination, Laser
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Fig. 7. LDV snapshots of surface displacement profiles of (a) bare LCM and (b) ZnO thin-film coated LCM sensors under UV illumination. Surface displacements of ZnO thin-film coated LCM measured under one UV ON-OFF cycle is shown in (c). Tracked point, P, is denoted by the back square.
Doppler Vibrometry (LDV) study was conducted. Fig. 7(a) and (b) show LDV snapshots of the displacement amplitude measured at the surface of a bare LCM and ZnO thin-film coated LCM sensors under UV illumination, respectively. As depicted in Fig. 7(a), the vibration amplitude follows shear wave mode propagation with maximum displacement of 180 pm under the area of observation. Fig. 7(b) also shows similar pattern of wave propagation with lower displacement amplitude compared to the bare LCM. Further, to track the surface displacement of LCM for one cycle of UV ON and OFF states, an observation point, P, was fixed on the LCM electrode and surface displacement was recorded with respect to time. As shown in Fig. 7(c), surface displacement sharply drops from 84 pm to 64 pm for ZnO coated LCM sensor within 50 s of UV exposure. When the UV source was turned OFF, the surface displacement value gradually increases towards its original state. On the other hand, there was no change in surface displacement for the bare LCM under UV illumination. The observed reduction of surface displacement in ZnO thin-film coated LCM under UV illumination can be correlated to the increase in surface conductivity and decrease in wave velocity as a consequence of the acoustoelectric interference. This result is also in accordance with the shift in resonant frequency observed in Fig. 5, and further concludes that the acoustoelectric effect is the sole reason behind the observed shift. It should be noted that the recovery period is much slower than the response because of the slow adsorption of oxygen at the ZnO surface [17].
Table 4 Motional impedances of ZnO thin film and LCM composite resonators. ZnO sample
R ()
L (mH)
C (fF)
100 nm 200 nm 400 nm
10.8 37.1 39.2
2.22 1.69 1.66
316 417 423
3.3. Electrical equivalent circuit model of proposed UV sensor The equivalent circuit elements can also shed some light on the relationship between the sensitivity and thickness of the piezoelectric ZnO films. Fig. 8a shows the equivalent circuit of an unperturbed LCM, and Fig. 8b shows the LCM coated with a ZnO thin-film. The equivalent motional elements for ZnO layer such as the resistor (Rm ) and the inductor (Lm ) are placed in series with the impedance elements of LCM. On the other hand, the capacitive effect of ZnO film (Cm ) is in parallel to the capacitance of the LCM. Moreover, the parallel capacitor and resistor combination, Cp and Rp helps to model the effect of UV illumination on the piezoelectricity of the LCM sensor. The parallel capacitor includes any capacitances that arise from the electrical contacts and also from the piezoelectric screening effect. The parallel resistor is the resistance due to medium loss [27]. As shown in Table 4, as the thickness of the ZnO film increased from 100 nm to 400 nm, the total series resistance and capaci-
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avenues of low-power sensors and piezoelectric sensor characterization.
Acknowledgements This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). This work was supported by the Ministry of Higher Education, Malaysia under Grant FRGS/2/2014/SG02/MUSM/03/1. The authors also thank Monash University Malaysia 2015-CRC-ILL grant for supporting the infrastructure.
References
Fig. 8. (a) Equivalent circuit model of unperturbed LCM, and (b) LCM with ZnO thin-film under UV load.
tance values also increased from 10.8 to 39.2 and 316 fF to 423 fF. This consequent increase in R and C values is accountable for the enhanced UV sensitivity observed for composite sensors with increased film thickness. Moreover, it should also be noted that even though the mass loading increased with increasing ZnO thickness, series inductance was observed to decrease from 2.2 mH to 1.66 mH. This is in stark contrast with the literature, which states that increased mass loading leads to increased motional inductance. Therefore, it is apparent that a composite resonator comprised of a piezoelectric resonator and a piezoelectric film behaves very differently to that of those with a non-piezoelectric film. The equivalent circuit can also be used to simulate the effect of UV on the photoelectric and piezoelectric properties of ZnO film on LCM. Under UV illumination, the ZnO barrier height decreases and results in an increase in motional capacitance (Cm ). Consequently, negative shift in resonant frequency for the LCM sensor is observed. On the other hand, UV illumination also affects the parallel components of the equivalent circuit model. The piezoelectric screening effect originated by UV, causes the parallel capacitor value and subsequently the DC offset to increase as observed in Fig. 6a. Cp increases significantly with increasing UV intensity, increasing the time constant of the parallel branch. As a result, higher shift in DC offset is observed and the output oscillations require longer time to stabilize. 4. Conclusion In conclusion, we have thoroughly investigated the transient response characteristics of LCM and QCM through experiments and equivalent circuit simulations. We discovered LCM exhibits faster stability and relaxation time, and required only a fraction of input power compared to that of QCM. Based on our findings, we proposed a novel, low-power, single step driven LCM instrumentation technique to monitor frequency, dissipation and amplitude of oscillation during its transient state. We also deployed our new setup to study the sensing characteristics of ZnO thin-film based LCM sensors. Additionally, we have confirmed the relationship between UV sensitivity and acoustoelectric effect in ZnO coated LCM sensors by monitoring the surface displacements using LDV measurements. Our proposed measurement method not only allows monitoring of the resonant frequency shift of the UV sensors, but also the piezoelectric properties of the ZnO sensing layer by detecting the DC offset of the initial free oscillations. These results open up new
[1] D.S.B. Martin Jr., S.J. Ricco, A.J. Frye, G.C. Wohltjen, G.C. White, E.T. Zellers, Chapter 3—acoustic wave sensors and responses, Acoust. Wave Sens. (1997) 36–149. [2] G. Sauerbrey, Verwendung von Schwingquarzen zur Wägung dünner Schichten und zur Mikrowägung, Zeitschrift für Phys. 155 (1959) 206–222. [3] M.A.M. Kashan, V. Kalavally, H.W. Lee, N. Ramakrishnan, Resonant characteristics and sensitivity dependency on the contact surface in QCM-micropillar-based system of coupled resonator sensors, J. Phys. D: Appl. Phys. 49 (2016) 195303. [4] H. Seh, H.L. Tuller, H. Fritze, Langasite for high-temperature acoustic wave gas sensors, Sens. Actuators B Chem. 93 (2003) 169–474. [5] D. Shen, H. Zhang, Q. Kang, H. Zhang, D. Yuan, Oscillating frequency response of a langasite crystal microbalance in liquid phases, Sens. Actuators B Chem. 119 (2006) 99–104. [6] Y.-K. Yong, Y. Kim, Analysis and comparison of langasite versus quartz for a thickness shear vibratory gyroscope, 2010 IEEE International Frequency Control Symposium (IEEE) (2010) 349–354. [7] H. Cheng, L. Qin, Q.-M. Wang, High temperature langasite BAW gas sensor based on ZnO nanowire arrays, 2007 IEEE Ultrasonics Symposium Proceedings (IEEE) (2007) 1361–1364. [8] E. Chilla, C.M. Flannery, H.J. Fröhlich, U. Straube, Elastic properties of langasite-type crystals determined by bulk and surface acoustic waves, J. Appl. Phys. 90 (2001) 6084–6091. [9] H. Fritze, H.L. Tuller, Langasite for high-temperature bulk acoustic wave applications, Appl. Phys. Lett. 78 (2001) 976–977. [10] T. Saha, N.Q. Guo, N. Ramakrishnan, Zinc oxide nanostructure-based langasite crystal microbalance ultraviolet sensor, IEEE Sens. J. 16 (2016) 2964–2970. [11] P.I. Reyes, Z. Zhang, H. Chen, Z. Duan, J. Zhong, G. Saraf, Y. Lu, O. Taratula, E. Galoppini, N.N. Boustany, A ZnO nanostructure-based quartz crystal microbalance device for biochemical sensing, IEEE Sens. J. 9 (2009) 1302–1307. [12] M.R. Alenezi, S.J. Henley, N.G. Emerson, S.R.P. Silva, From 1D and 2D ZnO nanostructures to 3D hierarchical structures with enhanced gas sensing properties, Nanoscale 6 (2014) 235–247. [13] A. Kołodziejczak-Radzimska, T. Jesionowski, Zinc oxide—from synthesis to application: a review, Materials (Basel) 7 (2014) 2833–2881. [14] W.-S. Wang, T.-T. Wu, T.-H. Chou, Y.-Y. Chen, A ZnO nanorod-based SAW oscillator system for ultraviolet detection, Nanotechnology 20 (2009) 135503. [15] C. Fu, K.J. Lee, K. Lee, S.S. Yang, Low-intensity ultraviolet detection using a surface acoustic-wave sensor with a Ag-doped ZnO nanoparticle film, Smart Mater. Struct. 24 (2015) 015010. [16] X. Qiu, J. Zhu, J. Oiler, C. Yu, Z. Wang, H. Yu, Film bulk acoustic-wave resonator based ultraviolet sensor, Appl. Phys. Lett. 94 (2009) 2007–2010. [17] T. Saha, A. Achath Mohanan, V. Swamy, N. Guo, N. Ramakrishnan, An optimal thermal evaporation synthesis of c-axis oriented ZnO nanowires with excellent UV sensing and emission characteristics, Mater. Res. Bull. 77 (2016) 147–154. [18] C. Soci, A. Zhang, B. Xiang, S.A. Dayeh, D.P.R. Aplin, J. Park, X.Y. Bao, Y.H. Lo, D. Wang, ZnO nanowire UV photodetectors with high internal gain, Nano Lett. 7 (2007) 1003–1009. [19] J. Zhou, Y. Gu, Y. Hu, W. Mai, P.-H. Yeh, G. Bao, A.K. Sood, D.L. Polla, Z.L. Wang, Gigantic enhancement in response and reset time of ZnO UV nanosensor by utilizing Schottky contact and surface functionalization, Appl. Phys. Lett. 94 (2009) 191103. [20] P. Wang, Y. Fu, B. Yu, Y. Zhao, L. Xing, X. Xue, Realizing room-temperature self-powered ethanol sensing of ZnO nanowire arrays by combining their piezoelectric, photoelectric and gas sensing characteristics, J. Mater. Chem. A 3 (2015) 3529–3535. [21] K. Kanazawa, Quartz crystal microbalance as a sensor to characterize macromolecular assembly dynamics, J. Sens. 2009 (2009). [22] A.L. Kipling, M. Thompson, Network analysis method applied to liquid-phase acoustic wave sensors, Anal. Chem. 62 (1990) 1514–1519. [23] M. Rodahl, F. Höök, A. Krozer, P. Brzezinski, B. Kasemo, Quartz crystal microbalance setup for frequency and Q-factor measurements in gaseous and liquid environments, Rev. Sci. Instrum. 66 (1995) 3924–3930.
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Biographies
Tridib Saha graduated from Monash University with a Bachelor of Electrical and Computer Systems Engineering degree in 2012. In the same year, he enrolled in Monash Doctoral program and is currently working towards his PhD. His current research interests include synthesis of ZnO nanostructures on acoustic resonators and their application in sensors, optoelectronics and energy harvesting systems.
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NQ Guo joined Monash University Malaysia in November 2009 as Professor and Head of Mechanical Engineering. Prior to that, Professor Guo had been developing his academic career in Nanyang Technological University from 1992 to 2009. He obtained his B.Eng from Nanjing University of Aeronautics and Astronautics, China in 1984, and his PhD from Imperial College London, UK in 1990. Professor Guo has research interests in vibration and acoustics, stress wave propagation, application of ultrasound, piezoelectric transducer, material characterization using nondestructive testing techniques, smart materials and structures, nano-fluid etc. He has published widely with over 100 publications in journals and conferences, secured competitive research grants over $2 million and graduated more than 10 higher degree research students. Professor Guo currently serves as Head of School, School of Engineering and School of Information Technology in Monash University Malaysia. N. Ramakrishnan obtained his Bachelor’s of Engineering in Electronics and Communication from Madras University, India in 2002. Further he obtained his Master of Technology in Sensor System in 2004 from VIT University, India. He obtained PhD in Nanotechnology for his findings on coupled resonance phenomenon in surface acoustic wave devices attached with resonant structures from Indian Institute of Technology Guwahati, India in 2011. Currently he is a senior lecturer in school of Engineering, Monash University Malaysia. His research interest includes micro sensors, acoustic wavesensors, nanostructured sensing medium, UV LED lithography, and micro power energy harvesting systems.
Contents lists available at ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
A novel langasite crystal microbalance instrumentation for UV sensing application Tridib Saha a , Ningqun Guo b , N. Ramakrishnan a,∗ a b
Electrical and Computer Systems Engineering, School of Engineering, Monash University Malaysia, 47500, Malaysia Mechanical Engineering, School of Engineering, Monash University Malaysia, 47500, Malaysia
a r t i c l e
i n f o
Article history: Received 9 June 2016 Received in revised form 28 September 2016 Accepted 17 October 2016 Available online 21 October 2016 Keywords: Langasite crystal microbalance Electrical equivalent circuit Transient response ZnO thin film UV sensing Sensor instrumentation
a b s t r a c t In this paper, we present a novel low-power UV sensor instrumentation based on ZnO thin-film and langasite crystal microbalance (LCM) composite resonator. The design of this sensor utilizes the exceptional transient response characteristics of thickness shear mode langasite crystal and UV sensitivity of annealed ZnO thin film. Our comparative transient analysis of langasite and quartz crystals shows that langasite crystal has high stability and fast relaxation times, and requires a fraction of excitation power in comparison to that of quartz. Upon investigating the equivalent circuit components, we discovered that high value of motional capacitance of LCM resulted in these contrasting transient responses in LCM and QCM. This high motional capacitance can be further attributed to the high electromechanical coupling coefficient (K2 ) of piezoelectric langasite crystal. Motivated by these observed characteristics, we proposed a novel sensor instrumentation that is simple yet effective in simultaneous measurements of the frequency, dissipation factor, and the amplitude of oscillation of a LCM. Moreover, our measurements allow for direct monitoring of any change in piezoelectricity of ZnO thin film. Our new measurement approach uses a one-shot pulse instead of conventional sinusoidal waveform to drive the crystal, which allows for simple instrumentation and requires very low input power. The transient response of the crystal shows that the crystal oscillates at its fundamental frequency and the amplitude of oscillation decays exponentially. From the recorded decay curve, the frequency of the freely oscillating crystal, and other important crystal parameters were calculated. We have utilized this mechanism to demonstrate highly sensitive ZnO thin film and LCM based UV sensor. Highest sensitivity of 35.8 ppm per W/cm2 was observed for the LCM coated with 400 nm ZnO film. Additionally, the influence of UV on the piezoelectric output of ZnO thin film was also characterized by monitoring the DC offset of the output oscillations. Our findings in this work opens scope for developing low-power acoustic sensors with significant implications towards self-powering sensors. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Since Sauerbrey’s mass sensing demonstration in 1959, quartz crystal microbalance (QCM) has emerged as one of the most popular sensing techniques because of its high sensitivity, stability, portability and low cost. Also known as thickness shear mode (TSM) resonator, QCM undergoes shear displacement upon excitation and resonates at specific frequencies [1]. Maximum surface displacement occurs at each of these resonant frequencies, making the crystal sensitive to any surface perturbations. Change in the surface perturbations introduces a shift in the resonance frequency of
∗ Corresponding author. E-mail addresses: [email protected] (T. Saha), [email protected] (N. Guo), [email protected] (N. Ramakrishnan). http://dx.doi.org/10.1016/j.sna.2016.10.024 0924-4247/© 2016 Elsevier B.V. All rights reserved.
the crystal [2]. Based on this sensing principle, QCMs have been extensively used in a wide range of applications such as mass, temperature, humidity, chemical and biological sensors [3]. Although quartz work excellently in most standard applications, it is not without its limitations. High temperature operation is unsuitable for QCMs as quartz loses its piezoelectric properties above its Curie temperature of 573 ◦ C [4]. Moreover, QCM in liquid media is also limited to low viscosity solutions as the oscillations are significantly damped in high viscosity liquids [5]. Over the last decade, langasite (La3 Ga5 SiO14 ) has been in the spotlight as an alternative piezoelectric crystal to quartz [6,7]. Langasite has significantly higher electromechanical coupling coefficient, reduced phase velocities and lower acoustic losses compared to that of quartz [8]. In addition to its excellent piezoelectric and acoustic properties, langasite can also operate in high temper-
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atures of up to 900 ◦ C and does not undergo phase transformation up to its melting point of 1473 ◦ C [9]. Motivated by langasite’s high temperature stability, we recently synthesized ZnO nanostructures on LCM using a thermal evaporation technique for UV sensing application [10]. Zinc oxide is predominantly used as a sensing media because of its excellent piezoelectric, semiconducting and optical properties. With a wide band gap of 3.37 eV at room temperature and a large exciton binding energy, ZnO is well-suited for gas, pH, and bio- sensors [11–13]. Recently, there have been several reports of ZnO based acoustic wave UV sensors as well, utilizing the photoelectric properties of ZnO [14–16]. Under UV illumination, photo-induced carriers are generated that results in a shift in resonant frequency of the acoustic resonator. On the other hand, ZnO nanostructures have also been independently used as UV sensors based on photocurrentvoltage (I–V) measurements [17–19]. Very recently, Wang et al. have demonstrated the effect of UV intensity on the piezoelectric output of ZnO nanowires [20]. This relationship is particularly important to quantify the photoelectric and piezoelectric property of any material. However, to our best knowledge, there has been no reports of ZnO based acoustic wave UV sensor that allow for simultaneous measurement of frequency and piezoelectricity. Crystal microbalances are mainly operated in two different modes to monitor changes in resonant frequency [21]. In one method, the crystal is excited by a continuous sinusoidal potential and the resonance is observed under steady state conditions [22]. Oscillator circuits and impedance analysis systems employ steady-state methods to detect any changes in the fundamental frequency of the crystal. In a more recently developed method, transient decay of a freely oscillating crystal is used to determine its resonant frequency and dissipation [23]. The crystal is driven for a specific amount of time by an alternating voltage tuned to the crystal’s resonant frequency, and then the driving power is switched off. Frequency and dissipation factor of the crystal is calculated from the exponentially decaying oscillating potential across the crystal, also known as ‘ring-down’ mode oscillations. Both the steady-state and transient methods require identification of the crystal’s resonant frequency before it can be used for any measurements. However, these processes require either expensive instruments, or computationally exhausting frequency sweeps to determine the largest output signal that corresponds to the resonant frequency. Till date, quartz based crystals were only explored for the above such mentioned experiments. Being a fairly new type of TSM resonator, LCM still remains to be thoroughly investigated in terms of its mechanical and electrical properties. Accordingly, in this work we investigated transient response of LCM under single pulse excitation, and compared its responses with that of a QCM. We observed the LCM alone exhibited decaying oscillation at its fundamental frequency at the falling edge of the single pulse. Based on these characteristics, we developed a novel, low-power, single pulse driven instrumentation technique to simultaneously measure frequency, dissipation and output voltage of a LCM. To establish the significance of the technique for sensing applications, we sputtered ZnO films as sensing medium on LCM surface to detect UV light and demonstrated real-time frequency and output voltage DC offset measurement of LCM sensors. The frequency and voltage measurements allowed for real-time monitoring of the sensitivity and change in piezoelectricity of ZnO thin films. Also, Laser Doppler Vibrometry (LDV) was carried out to investigate the acoustoelectric effect on the surface displacement of ZnO coated LCM under UV illumination. Further, we delved deeper into the properties of langasite crystal to derive the impedance elements of a blank crystal as well as crystals coated with various thicknesses of ZnO thin films and related the transient response to langasite’s electromechanical coupling coefficient (K2 ). Our results show promising potential for low-power and hassle-
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Table 1 Equations to calculate equivalent circuit elements of LCM. Impedance parameters R=
1 Gmax
Series Inductance
L=
QR 2f
Series Capacitance
C=
1 2fQR
Series Resistance
.
free LCM based sensor system that can be extended to a wide range of sensing applications. 2. Experimental setup and methodology 2.1. Setup for transient response analysis Fig. 1 shows the schematic diagram of our setup. Langasite crystal microbalances (LCM) of 6 MHz ± 10 kHz fundamental frequency with gold electrodes over chrome sublayer were obtained from Fomos-Materials, Russia. LCM was mounted on a two-point contact holder and connected to the circuit. One electrode of the crystal was connected to the arbitrary waveform generator (AWG) and the other to the variable load resistor. The crystal was driven with a 0–2 V pulse with a pulse width of 5 ms. The corresponding voltage output across the load resistor was monitored using a digital oscilloscope (DSO). At the falling edge of the input pulse, a sinusoidal voltage output was observed across the resistor that decays exponentially in a ‘ring-down’ mode, similar to that of previous reports of AC driven quartz crystals. The output voltage was passed through a built-in 20 MHz low-pass filter and 16 point averaging function in the oscilloscope to improve the quality of measurements and reduce the influence of noise. The ring-down oscillations as a function of time were recorded and fitted to exponential decay function using nonlinear least-squares solver (lsqcurvefit) ‘trust-region-reflective’ algorithm in MATLAB. For each measurement, only the first 50 oscillations were considered to calculate the average frequency and the dissipation factor based on the period of oscillation and decay time constant, respectively. All measurements were carried out at room temperature of 22 ◦ C and indoor relative humidity of 55%. 2.2. Derivation of electrical equivalent circuit parameters Fundamental frequency and Q factor of LCMs were measured using a network analyser. Admittance spectra (Y) were examined to measure the series resonant frequency (fs ) and the Q factor of the crystals. Electrical equivalent circuit parameters were calculated according to the equations shown in Table 1, where Gmax is the maximum conductance, Q is the quality factor and f is the series resonant frequency of the LCM. The derived values of the impedance elements were then used to simulate the response of the LCMs under a single pulse DC input using CircuitMaker2000 software. 2.3. ZnO film preparation To serve as the UV sensing medium, zinc oxide thin film of 100 nm, 200 nm and 400 nm thicknesses were sputtered on the front side of the LCMs at 150 ◦ C. These LCMs were then rapidly annealed at 500 ◦ C for 15 mins on a hot plate to remove compressive strain in ZnO film and improve their crystallinity [24]. 2.4. Laser doppler vibrometry Polytec MSA-400 Laser Doppler Vibrometer (LDV) was used to investigate the acoustoelectric effect in a bare 6 MHz LCM and a
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Fig. 1. Schematic diagram for single pulse driven crystal oscillator circuit.
LCM coated with ZnO thin film. Continuous measurement was performed in Fast Fourier Transform (FFT) measurement mode with 12800 FFT lines and 10-point complex averaging. The generator was set to scan between 5.5 MHz and 6.5 MHz frequency range, with 10 V periodic chirp waveform. The central electrodes of the LCMs were scanned using high-density grid scan. Resonant frequency of the LCMs were determined from the frequencies at which maximum vibration displacements were observed. The LCMs were then excited using sinusoidal waveform at their respective resonant frequencies. A 365 nm 9-LED UV source was directly placed 5 cm over the LCM. A point of maximum displacement was located on the LCM surface and the point was tracked to measure the values of instantaneous surface displacement. The LCMs were then exposed to UV illumination for a specific time after which the UV source was turned off. Change in surface displacement of the uncoated and ZnO film coated LCMs were monitored in the presence and absence of UV illumination. 2.5. UV sensor measurement setup The setup described in Fig. 1 was adapted for this part of the experiment. A 365 nm 9-LED UV light source and a LCM sensor were placed 3 cm apart on an optical rail. The resonant frequency of the LCM was recorded from the oscilloscope and set as the reference value. The LCM was excited using a 2 Hz 2Vp-p pulse as per our measurement setup for real-time frequency and DC offset measurements. Subsequently, the LCM sensor was illuminated with UV light and shift in frequency and DC offset were continuously recorded until the values settled down. Next, the source was turned OFF and kept off until the shift in central frequency returned back to zero. This ON-OFF cycle was repeated a few times to establish the repeatability of the sensor. 3. Results and discussion 3.1. Transient response analysis The transient response of blank LCM and QCM in a “ring-down” decay method setup, as demonstrated by Rodahl et al. [25], are shown in Fig. 2(a) and (b), respectively. Resonance was determined by detecting the maximum voltage across the load crystal. Both quartz and langasite crystals were observed to settle down at their maximum voltages of about 0.45 Vp-p when excited at their respective resonant frequencies. As observed in Fig. 2, the graphs demonstrate a charging phenomenon as the voltages increase from
Table 2 Motional impedances of quartz and langasite crystal microbalances. Crystal
R ()
L (mH)
C (fF)
LCM QCM
16.1 31.7
3.5 62.7
201 16.1
zero to maximum. This is due to the time taken for the crystals to settle in their resonant states and produce stable oscillations. LCM was significantly faster to stabilize in comparison to QCM, taking only about 0.5 ms whereas QCM required almost 8 ms. Similarly, the time to decay for LCM oscillations in its free state was also much shorter than that of QCM. Time taken to decay to minimum voltage for LCM was about 0.2 ms compared to that of about 6 ms for QCM. Thus, LCM allows for much faster overall measurement time, and required only a fraction of driving power for the measurements in comparison to that of a QCM. The best and most convenient way to comprehend the settling and decay times of LCM and QCM is by studying the electrical equivalent circuit model of the crystals. Using a network analyzer, the admittance spectra of the crystals were examined and the impedance elements were quantified. A simple ButterworthVan Dyke (BVD) model with motional R, L, C components can be used to describe the characteristics of both quartz and langasite crystals. As the static capacitance (C0 ) only influences the electrical behavior far from resonance, only the motional components affected by the electromechanical coupling (K2 ) of the crystals have been considered. The electromechanical coupling constant (K2 ) of langasite is about 2.26 times higher than that of quartz [6]. As a result, the motional impedance elements are also significantly different for LCM, as shown in Table 2. Treating the motional component as a simple RLC circuit, the neper frequency and the damping factor were calculated, which give a better understanding of the settling and decay times of the crystals. The damping factors as ratio of resonant frequencies of unperturbed LCM and QCM were 0.38 × 10−3 and 0.05 × 10−3 , respectively. This significant difference in the damping factors (∼8 times) leads to the rapid settling and decay times of LCM. It is therefore evident from these values that the K2 value and the consequent equivalent impedance elements allow for such improved transient response of LCM. Motivated by the low-power requirement and the enhanced motional impedance elements of LCM, we devised a novel, singlepulse based frequency, dissipation and amplitude measurement technique. In place of exciting the crystal in ring-down mode setup which required AC supply of a particular frequency, we attempted
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Fig. 2. Responses of uncoated LCM and QCM resonators driven with sinusoidal voltage at corresponding resonant frequencies. The transient decay voltage is also shown when the input power is switched off (dotted line).
to excite crystal with single shot pulse. The change in output voltage was monitored across a load resistor using the setup described in Fig. 1. Fig. 3(a) shows the input voltage and the measured output of the LCM circuit. At the falling edge of the pulse input, the voltage across the load resistor is observed to be exponentially decaying sinusoid as shown in Fig. 3(b). The frequency of oscillations was measured to be 6 MHz – the fundamental frequency of a blank LCM. The output voltage was recorded and processed in MATLAB to calculate the dissipation factor, by nonlinear least-squares solver. The dissipation factor, D, was calculated to be 1.14 × 10−2 for the blank LCM. The same LCM had a dissipation factor of 1.22 × 10−4 when measured directly using a network analyzer. The discrepancy is about a factor of 100 from the ‘true’ dissipation factor, and is a direct contribution from of the 100 load resistor, RL , placed in series with the LCM. The same single pulse input was applied to a QCM using the setup described in Fig. 1. However, no oscillations were observed at the rising or falling edge of the input. To investigate the reason behind the absence of oscillations in QCM as observed for LCM, we performed simulations of their equivalent circuits using CircuitMaker2000 and studied their transient responses for single pulse excitation. As shown in Fig. 4, for a short input pulse of 2 ms of 2 Vp-p , LCM and QCM produced amplitude voltages of about 2.1 mVp-p and 210 Vp-p , respectively. The simulated output of the LCM is comparable to the experimental output (Fig. 3b), apart from the DC offset of −0.5 mV as seen for the experimental data. Thus, it is safe to assume that the simulation parameters closely emulate the experimental circuit. The simulation results further confirm that the free-oscillating voltage of the QCM is significantly lower (∼10 times) than that of the LCM. Moreover, the simulation results also show the long decay time of QCM in comparison to the LCM. Therefore, LCM is a much better-suited material for this low-power single pulse setup compared to a QCM.
3.2. UV sensing performance of proposed sensors Next, the new measurement setup was employed to investigate ZnO thin-film based LCM UV sensors. Frequency response
Table 3 Sensitivities, response and recovery times of our proposed UV sensor. Sample
Sensitivity (W/cm2 )−1
Response time (s)
Recovery time (s)
100 nm 200 nm 400 nm Our previous work [10]
18.8 ppm 26.1 ppm 35.8 ppm 0.096 ppm
26 19 24 11
89 65 49 43
characteristics of LCMs coated with 100 nm, 200 nm and 400 nm thicknesses of ZnO thin-films are shown in Fig. 5(a)–(c). Resonant frequency of each of the sensors were measured in the dark and used as reference to monitor shifts in frequency. Under 365 nm UV illumination of 0.8 mW/cm2 intensity, maximum frequency downshift of 90 kHz, 125 kHz, and 172 kHz were observed for 100 nm, 200 nm and 400 nm samples, respectively. Some residual frequency shift was observed for each of the sensors after the UV source was turned off. Sensitivity, response and recovery times were also calculated for each of the sensors to characterize their performance. Sensitivity was normalized as the ratio of maximum frequency shift over central frequency (f/f0 ) per W/cm2 of UV intensity. Response and recovery times were defined as the time taken for the resonant frequency shift to change from 10% to 90%, and from 90% to 10%, respectively. The fitted data (represented by the solid lines in Fig. 5) were used for all the calculations. Table 3 shows the calculated values for each of the parameters. Highest sensitivity of 35.8 ppm per W/cm2 was observed for the LCM coated with 400 nm ZnO film. 200 nm and 100 nm samples showed sensitivities of 26.1 and 18.8 ppm per W/cm2 , respectively. The sensitivity values obtained in the present work are several orders higher in magnitude compared to our previous work on ZnO nanowire based LCM sensor [10], where we demonstrated excellent response and recovery times as indicated in Table 3. It should be also noted that sensitivity achieved in the present work is highest among all other reports on undoped ZnO based acoustic wave UV sensors [26–28]. On the other hand, best response time of
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Fig. 3. (a) Input pulse and the corresponding response of an LCM, measured across the series load resistor. (b) Zoomed in output at the falling edge of the input pulse. The red line shows the envelope of the exponential decay of the LCM. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 4. Simulated transient response of (a) LCM and (b) QCM excited by single-pulse step.
Fig. 5. Magnitudes of downshift in resonant frequencies versus time for (a) 100 nm, (b) 200 nm and (c) 400 nm ZnO thin-film coated LCMs under one UV ON-OFF cycle.
19 s was recorded for 200 nm sample, followed by 24 s of 400 nm sample and 26 s of 100 nm sample. Recovery times were relatively longer, where LCM with 400 nm ZnO film had the fastest recovery time of 49 s, while 200 nm and 100 nm samples took 65 s and 89 s, respectively. Unlike response and recovery times, sensitivity of an acoustic wave UV sensor not only depends on the geometry and quality of the ZnO sensing layer, but also on the measurement technique. As a result, the enhanced sensitivity observed in this work can be attributed to both the sensor instrumentation as well as the annealing of the ZnO thin film. As shown in Table 3, previously reported
ZnO nanowire based LCM sensor takes almost half the time to respond to UV illumination compared to that of thin-film based sensors. The recovery times are also observed to be much slower in comparison. These differences in response and recovery times can be ascribed to the high surface-to-volume ratio in nanowires compared to that of the ZnO films used in this work [18]. The generation and recombination of photocarriers on the thin-film surface takes much longer in comparison to that of ZnO nanostructures, and consequently results in slower response and recovery times. The piezoelectric output of the ZnO coated LCM composite resonators under UV illumination can also be characterized from the
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Fig. 6. (a) Downward shift in DC offset of the few initial cycles of output oscillation (b) Shift in DC offsets in the ring-down responses of ZnO thin-film coated LCMs under varying UV intensities.
transient decay. Fig. 6a shows the transient output of a ZnO coated LCM in dark state and under UV illumination. It can be clearly seen that the few initial output oscillation cycles shift downwards when illuminated by UV. This negative DC offset of the oscillations cycles were further investigated by varying the intensity of the UV illumination. As shown in Fig. 6b, UV intensity was increased from zero to 2400 W/cm2 , and the corresponding shift in DC offset for the first oscillation cycle was measured. Sample with 100 nm ZnO thin film coated demonstrated the maximum DC shift of 3.6 mV, followed by 200 nm sample with 3.44 mV, and 400 nm with 2.6 mV. The shift in DC offset for the first few cycles of oscillations occurs because of the parallel charging effect, which is directly related to the piezoelectricity of the ZnO thin film. As the intensity of the UV illumination increases, a large amount of photogenerated electron-hole pairs are generated, creating a strong piezoelectric screening effect [29]. This screening effect results in the DC offset of the output voltage. Above a certain intensity of UV, the quantity of photogenerated electrons reaches saturation and the shift in DC offset approaches stability. As the 100 nm ZnO film has the lowest thickness, the photogenerated holes quickly migrate to the surface, leaving behind high density of carriers. Under the compressive and tensile forces from the vibration of LCM, a piezoelectric field is created in the ZnO thin film with high screening effect. This screening effect and the
consequent reduction in piezoelectricity of the ZnO films inversely affects the sensitivity of the sensors. As a result, the lowest sensitivity is observed for 100 nm sample, which also shows the highest shift in DC offset among all the samples. Negative shift in resonant frequency of the LCMs were observed under UV light due to acoustoelectric effect. When the ZnO thin film is illuminated with UV light, electron-hole pairs are photogenerated. The holes migrate to the surface and are trapped, leaving behind unpaired electrons that increase the conductivity of the ZnO layer. As a result, the shear wave velocity decreases which causes the resonant frequency to decrease as per the following equation, f0 =
v 2t
where f0 is the resonant frequency, v is the acoustic velocity and t is the thickness of the piezoelectric film. The sensitivities of the ZnO thin film coated LCM sensors can also be explained by the same equation. The thickness of the piezoelectric layer, ZnO thin film in our case, is inversely proportional to the resonant frequency. As a result, highest shift was observed for the thickest ZnO film of 400 nm. To explore the mechanism of acoustoelectric effect on the surface displacement amplitude of LCM under UV illumination, Laser
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Fig. 7. LDV snapshots of surface displacement profiles of (a) bare LCM and (b) ZnO thin-film coated LCM sensors under UV illumination. Surface displacements of ZnO thin-film coated LCM measured under one UV ON-OFF cycle is shown in (c). Tracked point, P, is denoted by the back square.
Doppler Vibrometry (LDV) study was conducted. Fig. 7(a) and (b) show LDV snapshots of the displacement amplitude measured at the surface of a bare LCM and ZnO thin-film coated LCM sensors under UV illumination, respectively. As depicted in Fig. 7(a), the vibration amplitude follows shear wave mode propagation with maximum displacement of 180 pm under the area of observation. Fig. 7(b) also shows similar pattern of wave propagation with lower displacement amplitude compared to the bare LCM. Further, to track the surface displacement of LCM for one cycle of UV ON and OFF states, an observation point, P, was fixed on the LCM electrode and surface displacement was recorded with respect to time. As shown in Fig. 7(c), surface displacement sharply drops from 84 pm to 64 pm for ZnO coated LCM sensor within 50 s of UV exposure. When the UV source was turned OFF, the surface displacement value gradually increases towards its original state. On the other hand, there was no change in surface displacement for the bare LCM under UV illumination. The observed reduction of surface displacement in ZnO thin-film coated LCM under UV illumination can be correlated to the increase in surface conductivity and decrease in wave velocity as a consequence of the acoustoelectric interference. This result is also in accordance with the shift in resonant frequency observed in Fig. 5, and further concludes that the acoustoelectric effect is the sole reason behind the observed shift. It should be noted that the recovery period is much slower than the response because of the slow adsorption of oxygen at the ZnO surface [17].
Table 4 Motional impedances of ZnO thin film and LCM composite resonators. ZnO sample
R ()
L (mH)
C (fF)
100 nm 200 nm 400 nm
10.8 37.1 39.2
2.22 1.69 1.66
316 417 423
3.3. Electrical equivalent circuit model of proposed UV sensor The equivalent circuit elements can also shed some light on the relationship between the sensitivity and thickness of the piezoelectric ZnO films. Fig. 8a shows the equivalent circuit of an unperturbed LCM, and Fig. 8b shows the LCM coated with a ZnO thin-film. The equivalent motional elements for ZnO layer such as the resistor (Rm ) and the inductor (Lm ) are placed in series with the impedance elements of LCM. On the other hand, the capacitive effect of ZnO film (Cm ) is in parallel to the capacitance of the LCM. Moreover, the parallel capacitor and resistor combination, Cp and Rp helps to model the effect of UV illumination on the piezoelectricity of the LCM sensor. The parallel capacitor includes any capacitances that arise from the electrical contacts and also from the piezoelectric screening effect. The parallel resistor is the resistance due to medium loss [27]. As shown in Table 4, as the thickness of the ZnO film increased from 100 nm to 400 nm, the total series resistance and capaci-
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avenues of low-power sensors and piezoelectric sensor characterization.
Acknowledgements This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). This work was supported by the Ministry of Higher Education, Malaysia under Grant FRGS/2/2014/SG02/MUSM/03/1. The authors also thank Monash University Malaysia 2015-CRC-ILL grant for supporting the infrastructure.
References
Fig. 8. (a) Equivalent circuit model of unperturbed LCM, and (b) LCM with ZnO thin-film under UV load.
tance values also increased from 10.8 to 39.2 and 316 fF to 423 fF. This consequent increase in R and C values is accountable for the enhanced UV sensitivity observed for composite sensors with increased film thickness. Moreover, it should also be noted that even though the mass loading increased with increasing ZnO thickness, series inductance was observed to decrease from 2.2 mH to 1.66 mH. This is in stark contrast with the literature, which states that increased mass loading leads to increased motional inductance. Therefore, it is apparent that a composite resonator comprised of a piezoelectric resonator and a piezoelectric film behaves very differently to that of those with a non-piezoelectric film. The equivalent circuit can also be used to simulate the effect of UV on the photoelectric and piezoelectric properties of ZnO film on LCM. Under UV illumination, the ZnO barrier height decreases and results in an increase in motional capacitance (Cm ). Consequently, negative shift in resonant frequency for the LCM sensor is observed. On the other hand, UV illumination also affects the parallel components of the equivalent circuit model. The piezoelectric screening effect originated by UV, causes the parallel capacitor value and subsequently the DC offset to increase as observed in Fig. 6a. Cp increases significantly with increasing UV intensity, increasing the time constant of the parallel branch. As a result, higher shift in DC offset is observed and the output oscillations require longer time to stabilize. 4. Conclusion In conclusion, we have thoroughly investigated the transient response characteristics of LCM and QCM through experiments and equivalent circuit simulations. We discovered LCM exhibits faster stability and relaxation time, and required only a fraction of input power compared to that of QCM. Based on our findings, we proposed a novel, low-power, single step driven LCM instrumentation technique to monitor frequency, dissipation and amplitude of oscillation during its transient state. We also deployed our new setup to study the sensing characteristics of ZnO thin-film based LCM sensors. Additionally, we have confirmed the relationship between UV sensitivity and acoustoelectric effect in ZnO coated LCM sensors by monitoring the surface displacements using LDV measurements. Our proposed measurement method not only allows monitoring of the resonant frequency shift of the UV sensors, but also the piezoelectric properties of the ZnO sensing layer by detecting the DC offset of the initial free oscillations. These results open up new
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Biographies
Tridib Saha graduated from Monash University with a Bachelor of Electrical and Computer Systems Engineering degree in 2012. In the same year, he enrolled in Monash Doctoral program and is currently working towards his PhD. His current research interests include synthesis of ZnO nanostructures on acoustic resonators and their application in sensors, optoelectronics and energy harvesting systems.
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NQ Guo joined Monash University Malaysia in November 2009 as Professor and Head of Mechanical Engineering. Prior to that, Professor Guo had been developing his academic career in Nanyang Technological University from 1992 to 2009. He obtained his B.Eng from Nanjing University of Aeronautics and Astronautics, China in 1984, and his PhD from Imperial College London, UK in 1990. Professor Guo has research interests in vibration and acoustics, stress wave propagation, application of ultrasound, piezoelectric transducer, material characterization using nondestructive testing techniques, smart materials and structures, nano-fluid etc. He has published widely with over 100 publications in journals and conferences, secured competitive research grants over $2 million and graduated more than 10 higher degree research students. Professor Guo currently serves as Head of School, School of Engineering and School of Information Technology in Monash University Malaysia. N. Ramakrishnan obtained his Bachelor’s of Engineering in Electronics and Communication from Madras University, India in 2002. Further he obtained his Master of Technology in Sensor System in 2004 from VIT University, India. He obtained PhD in Nanotechnology for his findings on coupled resonance phenomenon in surface acoustic wave devices attached with resonant structures from Indian Institute of Technology Guwahati, India in 2011. Currently he is a senior lecturer in school of Engineering, Monash University Malaysia. His research interest includes micro sensors, acoustic wavesensors, nanostructured sensing medium, UV LED lithography, and micro power energy harvesting systems.