Laser-grown ZnO nanowires for room-temperature SAW-sensor applications
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Sensors and Actuators B 208 (2015) 1–6
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Laser-grown ZnO nanowires for room-temperature SAW-sensor applications Aurel Marcu, Cristian Viespe ∗ National Institute for Laser, Plasma and Radiation Physics, Laser Department, Atomistilor 409, Magurele, Bucharest, Romania
a r t i c l e
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Article history: Received 9 July 2014 Received in revised form 31 October 2014 Accepted 31 October 2014 Available online 8 November 2014 Keywords: Hydrogen ZnO nanowire PLD-VLS SAW sensors
a b s t r a c t ZnO nanowires were grown on the active sensor surface of a surface acoustic wave (SAW) sensor via a vapour–liquid–solid (VLS) technique using pulsed laser deposition (PLD) as the particle source. The fabricated sensors were “delay-line” type (quartz substrate; ∼69.4-MHz central frequency). The nanowire length and diameter were controlled by the growth time and temperature, respectively. The sensor response at room temperature to various hydrogen (H2 ) concentrations was recorded for different ZnO morphologies and nanowire thicknesses and compared with the performance of a thin-film sensor with a comparable amount of ZnO material. The sensor response depended on the ZnO volume and the morphology of the active surface. An increase in the ZnO volume enhanced the frequency shift for the same H2 concentration, while the larger surface area of the longer nanowires enhanced the sensor response to low H2 concentrations, enabling detection of concentrations as low as 0.01%. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen has many applications in a variety of industries, including metallurgy, chemicals, pharmaceuticals, petroleum, food, and electronics [1–3]. The current research trend in hydrogen sensor technology has focused on the development of sensors with low limits of detection at room temperature. The high surfacearea-to-volume ratio of nanomaterials increases sensor sensitivity beyond the limitations of conventional planar-film devices, and has been demonstrated for various gas-detection morphologies (e.g. as nanofibres [4,5], nanoparticles [6–9], nanotubes [6,10], nanoporous films [11], nanowires [12–14], and nanorods [15]) deposited on the sensor active area. The SAW sensor has numerous advantages as low cost, high sensitivity, compact size and fast response. SAW sensor has proven good performances in hydrogen detection, using sensitive films of different material as ZnO [15,16,7], WO3 [17], Pd [11], polyaniline [4]. ZnO, in particular, is sensitive to many gases, including H2 [15], CO [16], NH3 [18], and NO [18], and offers great potential for low-concentration detection. In this paper, we propose a ZnO nanostructure-based surface acoustic wave (SAW) sensor (Fig. 1), in which the nanowires were grown directly on a quartz substrate via
∗ Corresponding author. Tel.: +40 21 4574027; fax: +40 21 4574027. E-mail address: viespe@ifin.nipne.ro (C. Viespe). http://dx.doi.org/10.1016/j.snb.2014.10.141 0925-4005/© 2014 Elsevier B.V. All rights reserved.
a vapour–liquid–solid (VLS) technique with a pulsed laser deposition (PLD) particle source. In contrast to previous studies that used a different growth technique [13], the direct growth of the ZnO nanowires resulted in a (quasi) perpendicular orientation of the nanowires on the substrate surface and better coupling between the structures and the quartz substrate. The VLS technique, particularly the laser-assisted one, is capable of producing nanowires with good crystallinity and structure properties [19]. ZnO nanowires in particular can be produced almost without any structural defects [20].
2. Materials and methods ZnO single-crystal [0 0 0 1] nanowires were grown on the active sensor surface of a SAW sensor via VLS-PLD. The fabricated sensors were “delay-line” type (quartz substrate; ∼69.4-MHz central frequency). A brief description of the technique is presented in Fig. 2a. A ZnO sintered powder target was ablated with Nd:YAG irradiation at a 500-kHz repetition rate in the presence of oxygen (O2 ) (ambient pressure: 1 Pa) to form a plasma. Au was used as the liquid catalyst. Incoming particles from the pulsed laser ablation plume (I) and surface diffused particles (D) were collected in the catalyst droplets. After reaching a critical concentration in the liquid (by overcoming the desorption rate (E) [21]), these particles were deposited at the bottom of the catalyst drops to form the ZnO nanowires. The growth temperature of the ZnO nanowires
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Fig. 1. Surface acoustic wave (SAW) sensor with ZnO nanowires grown using a vapour–liquid–solid (VLS) technique and pulsed laser deposition (PLD).
was 800 ◦ C. A plume reflection technique was used to filter the plume, due to the presence of large clusters and droplets in the laser plume known to inhibit VLS growth [22]. Fig. 2b shows a schematic diagram of the experimental setup. Among the various filtering techniques, plume reflection was preferred because it has a higher deposition rate than the classical “eclipse” or helical mask [23–25] techniques. More details regarding the ZnO nanowire growth technique can be found in the literature (e.g. high repetition rates for deposition [26], catalyst limitations [27], and plasma-reflection plume-filtering techniques [28,29]). In the present experiment, the length and diameter of the ZnO nanowires could be varied as needed (Fig. 3). Using the VLS growth technique, the nanowire diameter was determined by the catalyst droplet size. The growth time and the number of laser pulses determined the nanowire length for a fixed diameter. For tuning the nanowire diameters, the VLS growth was suppressed by changing the substrate temperature to room temperature (RT). At RT, the catalyst droplet became solid and the adatom surface diffusion (D) process (Fig. 2a) drastically decreased, transforming the growth into “classic” thin-film deposition in a PLD system. Fig. 4 shows a comparison between the as-grown ZnO nanowire morphology having a length of ∼888 nm (used for the S2 SAW sensor described later in the text), before and after auxiliary deposition of additional ZnO layers; more detailed information regarding the properties of the structures can be found elsewhere [30]. The elementary sensors, based on quartz substrate, created a “delay-line” type SAW device, with a centre frequency of 69.4 MHz. To reduce the effect of spurious SAW reflections from
the edges of the piezoelectric substrate we have chosen to cut the quart substrate in parallelogram geometry [31]. The SAW sensor consisted of two-port resonators with 50 electrodes pairs, and had an acoustic aperture and periodicity of 2500 and 45.2 m, respectively. Interdigital transducers (IDTs) were produced using radio-frequency (RF) magnetron sputtering and standard photolithographic techniques. The thickness of the IDT deposited on the ST-X quartz substrate consisted of 150-nm-thick gold and 10nm-thick chromium coatings [6,8–10]. For IDTs fabrication applied in SAW sensors, gold and aluminium are the most commonly used metal. The main disadvantage of Al is that it is easily corrode. Thus, we have chosen in our experiments Au for IDTs fabrication due to its inertness and resistance to corrosion. To assure the adhesion of gold on quartz substrate, a chromium underlayer was used [32]. The ZnO nanostructures were grown directly on the sensor active surface area (size: 8 cm2 ). The gas detection system consisted of a mass flow controller, which mixed the gases from two cylinders: one cylinder contained 2% H2 and 98% synthetic air, and the second cylinder contained 100% synthetic air (Fig. 5). To minimise flow-induced frequency deviation, the gas flow remained constant at 0.5 L min−1 , regardless of the H2 concentration. The oscillating circuit consisted of SAW quartz devices, a phase shifter (PSV-70-360-S), bandpass filter (Anatech Electronics B9336), and an amplifier (DHPV-100 FEMTO). The frequency deviation was detected with a Pendulum CNT-91 Timer/Counter/Analyser and TimeView 2.1 software. All the electronic components have an impedance of 50 , and also the impedance of quartz devices was matched to the external circuit (50 ), by adding necessary inductances. To measure the signal
Fig. 2. (a) Experimental setup and description of the VLS growth mechanism. (b) Schematic diagram of the experimental setup for plasma reflection and PLD.
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Fig. 3. Scanning electron microscopy (SEM) images of the ZnO nanowires grown on the SAW sensors as deposited, having a nanowire length of: (a) ∼514 nm (S1), (b) ∼888 nm (S2), (c) ∼1217 nm (S3) and (d) 1635 nm (S4). (e) ZnO thin film (∼170 nm).
attenuation and phase, and also to determine the optimal value of the inductors for quartz devices, a network/spectrum/impedance analyzer (Agilent 4396B) with a transmission/reflection kit (Agilent 87512 A/B) was used. 3. Results and discussion Using the above mentioned techniques, four SAW sensors of various lengths were fabricated (Fig. 3); the nanowire lengths were ∼514, 888, 1217, and 1635 nm, referred to as Sensors S1–S4, respectively. All four nanowires had the same diameter of ∼30 nm. While measuring their response to various H2 concentrations, we noticed
that only two sensors, with nanowire lengths of 514 (S1) and 888 (S2) nm, were actually oscillating, while sensors with longer nanowire lengths (1217 (S3) and 1635 (S4) nm) did not. To confirm that this effect was strictly due to the nanowires grown on the active surface area, the ZnO nanowires from one non-oscillating sensor sensitive area were removed, resulting in oscillation once again of the SAW sensor. To determine the influence of morphology, a SAW sensor with a thin-film layer (hereafter, referred to as the thin-film sensor) was deposited using the same setup and deposition conditions as the 1217-nm ZnO nanowire sensor (i.e. the S3 sensor). The resulting ZnO thin-film sensor was used to measure the frequency shift for
Fig. 4. SEM images of the S2 ZnO nanowire SAW-sensor active surface. (a) Before and b) after auxiliary deposition of additional ZnO layers.
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Fig. 5. Experimental setup for SAW-sensor frequency shift measurements for hydrogen (H2 ) detection.
Fig. 6. Frequency shift dependence on H2 concentration.
various H2 concentrations, along with two nanowire-based sensors. The dependence of the frequency shift on the H2 concentration is shown in Fig. 6. The sensors response (frequency shift) is given by change of the mass loading of the film with high surface area and even of the change of conductivity [15]. Since the sensor response is almost linear with the concentration in the film (see Fig. 6), we consider the mass loading is the dominant sensing mechanism [32], as in our experiment was obtained. The sensitivity and limit of detection (three times the noise level divided by the sensitivity) are listed in Table 1 for the SAW sensors. Our results indicated that the thin-film sensor could be used effectively for concentrations approaching 0.2%, and that the nanowire sensors were capable of measuring concentrations approximately one order of magnitude lower. Also, despite the dependence of the sensor sensitivity on the volume of the material of the active surface, the frequency shift increased with volume for the same surface morphologies (e.g. S1 and S2). Comparing different surface morphologies, the sensor behaviour depended on the active surface layer configuration. The nanowire form of the S2 sensor (∼888-nm length) provided a 20–30% greater shift in frequency, with about 33% less material than the thin-film sensor. To examine
the dependence of the frequency shift on the amount of material for a specific nanowire length, the nanowire sensor response was measured as the nanowire diameter increased by successive deposition of additional ZnO layers. The performance of the nanowire sensor for a given diameter was compared with the response from the thin-film sensor, for an ambient H2 concentration of 2% (Fig. 7). The frequency shift corresponding to a thin-film thickness of ∼180 nm increased six-fold, depending on the grown morphology. Comparing the 880-nm-long nanowire sensor (S2) performance with that of the film sensor having a similar volume of ZnO material on the active surface, the observed frequency shift for the nanowire sensor was four times larger. Further increase in the nanowire
Table 1 Sensitivity and limit of detection of the SAW sensors. SAW – Sensor
Sensitivity (f/c) (Hz/ppm)
Limit of detection (ppm)
S1 S2 Film
0.032 0.062 0.010
3942 2253 4241
Fig. 7. Frequency shift dependence on the (estimated) ZnO film thickness for 2% H2 concentration detection.
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thickness decreased the relative shift difference two-fold, indicating a tendency towards saturation. This saturation tendency was visible for both investigated nanowire lengths with an increase in the nanowire diameter. Thus, our results suggest that there is an optimal length: diameter ratio for comparable amounts of material, with a better response observed for shorter, thicker wires around the 180-nm thin-film thickness estimated equivalent value. In terms of sensor sensitivity, the S2 sensor exhibited the best performance among those tested (Table 1). The sensitivity depended on both the ZnO material volume and morphology. In our study, S2 had twice as much material as S1, and the sensitivity was twice as large; this result suggests a proportional relationship between the sensitivity and the volume of ZnO material for the same morphology. However, the film sensor had approximately twice as much material as S1, but the sensitivity was three times less. The key factor in this case seems to be the surface specific area. The observed quasi-linear increase in the frequency shift for the thin film, with an increase in volume due to additional layers, further supports the proportionality between sensitivity and volume for the thin-film morphology (Fig. 7). Also, the frequency shift response to the nanowire morphology tended to reach saturation. For the thin-film sensor, although the volume increased, the surface remained approximately the same. For the nanowire sensor, the wires were not perfectly aligned (bending and touching sometimes, with variable distances in between); thus, covering the wires with additional ZnO layers increased the likelihood that the wires touched, thus decreasing their specific area. This decrease in the specific area of the nanowire sensor with increasing ZnO coverage (volume) may saturate the frequency shift over time. The S2 sensor exhibited the best performance among those tested, with detection capabilities on the order of 2250 ppm. Beyond these numbers, our experimental results showed that the nanowire sensors could be used effectively to measure concentrations on the order of ∼0.01%, close to the performance demonstrated by spherical sensors requiring sophisticated fabrication techniques [33]. In the same experiments, the thin film could actually be used for only about one order higher H2 concentration. Our previous investigations on much thicker ZnO films indicated fairly large frequency shifts [25]; however, the measurable concentration was roughly in the same sensitivity range of 0.1%. We can understand this through the much larger surface specific area and better response of the nanowires for amortisation of the SAWs. Thus, the nanowire sensor configuration was more effective and demonstrated better performance for measuring low concentrations of H2 , compared with the thin-film sensor. However, for higher concentrations of H2 , the nanowire sensors tend to became saturated; thus, the thin-film sensors are recommended in this case. The reproducibility of the sensor based on ZnO nanowires was tested by exposing the devices to 1% H2 gas for 10 cycles. The recorded fluctuations of the frequency shift were less than 10%.
4. Conclusion In conclusion, we fabricated and tested for the first time SAW sensors for RT H2 detection with ZnO nanowires grown directly on the sensor active surface using the VLS-PLD method. This technique ensured better coupling between the structures and the substrate surface. Also, a quasi-parallel alignment of the sensitive structures maximised the sensor’s active area. The ZnO-nanowire sensor response was much better than that for a thin film for comparable amounts of material and allowed for the detection of H2 concentrations down to 0.01%. The sensor’s sensitivity depended linearly on the ZnO deposited volume on the active surface area, as well as on the growth morphology. For the nanowire-based SAW sensors,
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the frequency shift became saturated more quickly than that for the thin-film sensors and even stopped oscillating with excessive amounts of deposited material. This behaviour of the nanowirebased SAW sensors makes them more appropriate for lower H2 concentration measurements.
Acknowledgement This work was supported by the program Partnerships in priority area - PN II, under support from Romanian National Authority for Scientific, CNDI–UEFISCDI, project number PN-II-PT-PCCA-20113.2-0762 and project LAPLAS 3 PN 09 39 01 03.
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Biographies Aurelian Marcu received his B.S. and M.S. degrees in electrical engineering from Politehnica University of Bucharest, Romania and Ph.D. degree in Electrical Engineering from the same university in 2001. He is senior researcher in Laser Department at the National Institute for Laser Plasma and Radiation Physics. His research interests are in laser-matter interactions, special pulsed laser deposition techniques and nanostructures fabrication for applications in sensors and biology. Cristian Viespe received his B.S. and M.S. in electrical engineering and physics from Politehnica University of Bucharest, Romania, in 2005 and 2007 respectively. In 2010 he received his Ph.D. in physics at the same university with work “Surface Acoustic Wave Sensors”. He is currently a senior researcher at National Institute of Laser, Plasma and Radiation Physics at Laser Department. His main research interests include sensors/biosensors devices, and develop of nanomaterials with applications in sensors/biosensors.
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