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Sensors and Actuators B 208 (2015) 1–6

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical

jo ur nal home page: www.elsev ier .com/ locate /snb

aser-grown ZnO nanowires for room-temperature SAW-sensorpplications

urel Marcu, Cristian Viespe ∗

ational Institute for Laser, Plasma and Radiation Physics, Laser Department, Atomistilor 409, Magurele, Bucharest, Romania

r t i c l e i n f o

rticle history:eceived 9 July 2014eceived in revised form 31 October 2014ccepted 31 October 2014vailable online 8 November 2014

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 avapour–liquid–solid (VLS) technique using pulsed laser deposition (PLD) as the particle source. The fab-ricated sensors were “delay-line” type (quartz substrate; ∼69.4-MHz central frequency). The nanowirelength and diameter were controlled by the growth time and temperature, respectively. The sensorresponse at room temperature to various hydrogen (H2) concentrations was recorded for different ZnO

eywords:ydrogennO nanowireLD-VLSAW sensors

morphologies and nanowire thicknesses and compared with the performance of a thin-film sensor witha comparable amount of ZnO material. The sensor response depended on the ZnO volume and the mor-phology of the active surface. An increase in the ZnO volume enhanced the frequency shift for the sameH2 concentration, while the larger surface area of the longer nanowires enhanced the sensor response tolow H2 concentrations, enabling detection of concentrations as low as 0.01%.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Hydrogen has many applications in a variety of industries,ncluding metallurgy, chemicals, pharmaceuticals, petroleum, food,nd electronics [1–3]. The current research trend in hydrogen sen-or technology has focused on the development of sensors withow limits of detection at room temperature. The high surface-rea-to-volume ratio of nanomaterials increases sensor sensitivityeyond the limitations of conventional planar-film devices, and haseen demonstrated for various gas-detection morphologies (e.g. asanofibres [4,5], nanoparticles [6–9], nanotubes [6,10], nanoporouslms [11], nanowires [12–14], and nanorods [15]) deposited on theensor active area.

The SAW sensor has numerous advantages as low cost, high sen-itivity, compact size and fast response. SAW sensor has provenood performances in hydrogen detection, using sensitive films ofifferent material as ZnO [15,16,7], WO3 [17], Pd [11], polyaniline4].

ZnO, in particular, is sensitive to many gases, including H215], CO [16], NH3 [18], and NO [18], and offers great potential

or low-concentration detection. In this paper, we propose a ZnOanostructure-based surface acoustic wave (SAW) sensor (Fig. 1), inhich 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).

ttp://dx.doi.org/10.1016/j.snb.2014.10.141925-4005/© 2014 Elsevier B.V. All rights reserved.

a vapour–liquid–solid (VLS) technique with a pulsed laser deposi-tion (PLD) particle source. In contrast to previous studies that useda different growth technique [13], the direct growth of the ZnOnanowires resulted in a (quasi) perpendicular orientation of thenanowires on the substrate surface and better coupling betweenthe structures and the quartz substrate.

The VLS technique, particularly the laser-assisted one, is capableof producing nanowires with good crystallinity and structure prop-erties [19]. ZnO nanowires in particular can be produced almostwithout any structural defects [20].

2. Materials and methods

ZnO single-crystal [0 0 0 1] nanowires were grown on the activesensor surface of a SAW sensor via VLS-PLD. The fabricated sen-sors were “delay-line” type (quartz substrate; ∼69.4-MHz centralfrequency). A brief description of the technique is presented inFig. 2a.

A ZnO sintered powder target was ablated with Nd:YAG irradia-tion at a 500-kHz repetition rate in the presence of oxygen (O2)(ambient pressure: 1 Pa) to form a plasma. Au was used as theliquid catalyst. Incoming particles from the pulsed laser ablationplume (I) and surface diffused particles (D) were collected in the

catalyst droplets. After reaching a critical concentration in the liq-uid (by overcoming the desorption rate (E) [21]), these particleswere deposited at the bottom of the catalyst drops to form theZnO nanowires. The growth temperature of the ZnO nanowires

2 A. Marcu, C. Viespe / Sensors and Actuators B 208 (2015) 1–6

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Fig. 1. Surface acoustic wave (SAW) sensor with ZnO nanowires grown u

as 800 ◦C. A plume reflection technique was used to filter thelume, due to the presence of large clusters and droplets in the

aser plume known to inhibit VLS growth [22]. Fig. 2b shows achematic diagram of the experimental setup. Among the variousltering techniques, plume reflection was preferred because it has

higher deposition rate than the classical “eclipse” or helical mask23–25] techniques.

More details regarding the ZnO nanowire growth techniquean be found in the literature (e.g. high repetition rates foreposition [26], catalyst limitations [27], and plasma-reflectionlume-filtering techniques [28,29]).

In the present experiment, the length and diameter of the ZnOanowires could be varied as needed (Fig. 3). Using the VLS growthechnique, the nanowire diameter was determined by the cata-yst droplet size. The growth time and the number of laser pulsesetermined the nanowire length for a fixed diameter.

For tuning the nanowire diameters, the VLS growth wasuppressed by changing the substrate temperature to room tem-erature (RT). At RT, the catalyst droplet became solid and thedatom surface diffusion (D) process (Fig. 2a) drastically decreased,ransforming the growth into “classic” thin-film deposition in aLD system. Fig. 4 shows a comparison between the as-grown ZnOanowire morphology having a length of ∼888 nm (used for the2 SAW sensor described later in the text), before and after auxil-ary deposition of additional ZnO layers; more detailed informationegarding the properties of the structures can be found elsewhere

30].

The elementary sensors, based on quartz substrate, created “delay-line” type SAW device, with a centre frequency of9.4 MHz. To reduce the effect of spurious SAW reflections from

Fig. 2. (a) Experimental setup and description of the VLS growth mechanism. (b) S

vapour–liquid–solid (VLS) technique and pulsed laser deposition (PLD).

the edges of the piezoelectric substrate we have chosen to cutthe quart substrate in parallelogram geometry [31]. The SAW sen-sor consisted of two-port resonators with 50 electrodes pairs, andhad an acoustic aperture and periodicity of 2500 and 45.2 �m,respectively. Interdigital transducers (IDTs) were produced usingradio-frequency (RF) magnetron sputtering and standard pho-tolithographic techniques. The thickness of the IDT deposited onthe ST-X quartz substrate consisted of 150-nm-thick gold and 10-nm-thick chromium coatings [6,8–10]. For IDTs fabrication appliedin SAW sensors, gold and aluminium are the most commonly usedmetal. The main disadvantage of Al is that it is easily corrode. Thus,we have chosen in our experiments Au for IDTs fabrication due toits inertness and resistance to corrosion. To assure the adhesion ofgold on quartz substrate, a chromium underlayer was used [32].The ZnO nanostructures were grown directly on the sensor activesurface area (size: 8 cm2).

The gas detection system consisted of a mass flow controller,which mixed the gases from two cylinders: one cylinder contained2% H2 and 98% synthetic air, and the second cylinder contained100% synthetic air (Fig. 5). To minimise flow-induced frequencydeviation, the gas flow remained constant at 0.5 L min−1, regardlessof the H2 concentration.

The oscillating circuit consisted of SAW quartz devices, aphase shifter (PSV-70-360-S), bandpass filter (Anatech Elec-tronics B9336), and an amplifier (DHPV-100 FEMTO). Thefrequency deviation was detected with a Pendulum CNT-91

Timer/Counter/Analyser and TimeView 2.1 software. All the elec-tronic components have an impedance of 50 �, and also theimpedance of quartz devices was matched to the external circuit(50 �), by adding necessary inductances. To measure the signal

chematic diagram of the experimental setup for plasma reflection and PLD.

A. Marcu, C. Viespe / Sensors and Actuators B 208 (2015) 1–6 3

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ig. 3. Scanning electron microscopy (SEM) images of the ZnO nanowires grown on tS2), (c) ∼1217 nm (S3) and (d) 1635 nm (S4). (e) ZnO thin film (∼170 nm).

ttenuation and phase, and also to determine the optimal value ofhe inductors for quartz devices, a network/spectrum/impedancenalyzer (Agilent 4396B) with a transmission/reflection kit (Agilent7512 A/B) was used.

. Results and discussion

Using the above mentioned techniques, four SAW sensors of var-

ous lengths were fabricated (Fig. 3); the nanowire lengths were514, 888, 1217, and 1635 nm, referred to as Sensors S1–S4, respec-

ively. All four nanowires had the same diameter of ∼30 nm. Whileeasuring their response to various H2 concentrations, we noticed

Fig. 4. SEM images of the S2 ZnO nanowire SAW-sensor active surface. (a

W sensors as deposited, having a nanowire length of: (a) ∼514 nm (S1), (b) ∼888 nm

that only two sensors, with nanowire lengths of 514 (S1) and888 (S2) nm, were actually oscillating, while sensors with longernanowire lengths (1217 (S3) and 1635 (S4) nm) did not. To con-firm that this effect was strictly due to the nanowires grown on theactive surface area, the ZnO nanowires from one non-oscillatingsensor sensitive area were removed, resulting in oscillation onceagain 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) wasdeposited using the same setup and deposition conditions as the1217-nm ZnO nanowire sensor (i.e. the S3 sensor). The resultingZnO thin-film sensor was used to measure the frequency shift for

) Before and b) after auxiliary deposition of additional ZnO layers.

4 A. Marcu, C. Viespe / Sensors and Actuators B 208 (2015) 1–6

Fig. 5. Experimental setup for SAW-sensor frequency shift measurements for hydrogen (H2) detection.

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with that of the film sensor having a similar volume of ZnO materialon the active surface, the observed frequency shift for the nanowiresensor was four times larger. Further increase in the nanowire

Fig. 6. Frequency shift de

arious H2 concentrations, along with two nanowire-based sensors.he dependence of the frequency shift on the H2 concentration ishown in Fig. 6.

The sensors response (frequency shift) is given by change of theass loading of the film with high surface area and even of the

hange of conductivity [15]. Since the sensor response is almostinear with the concentration in the film (see Fig. 6), we considerhe mass loading is the dominant sensing mechanism [32], as inur experiment was obtained. The sensitivity and limit of detectionthree times the noise level divided by the sensitivity) are listed inable 1 for the SAW sensors.

Our results indicated that the thin-film sensor could be usedffectively for concentrations approaching 0.2%, and that theanowire sensors were capable of measuring concentrationspproximately one order of magnitude lower. Also, despite theependence of the sensor sensitivity on the volume of the materialf the active surface, the frequency shift increased with volume forhe same surface morphologies (e.g. S1 and S2). Comparing differ-nt surface morphologies, the sensor behaviour depended on the

ctive surface layer configuration. The nanowire form of the S2 sen-or (∼888-nm length) provided a 20–30% greater shift in frequency,ith about 33% less material than the thin-film sensor. To examine

able 1ensitivity and limit of detection of the SAW sensors.

SAW – Sensor Sensitivity (�f/c) (Hz/ppm) Limit of detection (ppm)

S1 0.032 3942S2 0.062 2253Film 0.010 4241

nce on H2 concentration.

the dependence of the frequency shift on the amount of materialfor a specific nanowire length, the nanowire sensor response wasmeasured as the nanowire diameter increased by successive depo-sition of additional ZnO layers. The performance of the nanowiresensor for a given diameter was compared with the response fromthe 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

Fig. 7. Frequency shift dependence on the (estimated) ZnO film thickness for 2% H2

concentration detection.

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hickness decreased the relative shift difference two-fold, indicat-ng a tendency towards saturation. This saturation tendency wasisible for both investigated nanowire lengths with an increase inhe nanowire diameter. Thus, our results suggest that there is anptimal length: diameter ratio for comparable amounts of material,ith a better response observed for shorter, thicker wires around

he 180-nm thin-film thickness estimated equivalent value.In terms of sensor sensitivity, the S2 sensor exhibited the

est performance among those tested (Table 1). The sensitivityepended on both the ZnO material volume and morphology. Inur study, S2 had twice as much material as S1, and the sensitivityas twice as large; this result suggests a proportional relationship

etween the sensitivity and the volume of ZnO material for theame morphology. However, the film sensor had approximatelywice as much material as S1, but the sensitivity was three timesess. The key factor in this case seems to be the surface specific area.

The observed quasi-linear increase in the frequency shift forhe thin film, with an increase in volume due to additional layers,urther supports the proportionality between sensitivity and vol-me for the thin-film morphology (Fig. 7). Also, the frequency shiftesponse to the nanowire morphology tended to reach saturation.or the thin-film sensor, although the volume increased, the sur-ace remained approximately the same. For the nanowire sensor,he wires were not perfectly aligned (bending and touching some-imes, with variable distances in between); thus, covering the wiresith additional ZnO layers increased the likelihood that the wires

ouched, thus decreasing their specific area. This decrease in thepecific area of the nanowire sensor with increasing ZnO coveragevolume) may saturate the frequency shift over time.

The S2 sensor exhibited the best performance among thoseested, with detection capabilities on the order of 2250 ppm.eyond these numbers, our experimental results showed thathe nanowire sensors could be used effectively to measure con-entrations on the order of ∼0.01%, close to the performanceemonstrated by spherical sensors requiring sophisticated fabri-ation techniques [33]. In the same experiments, the thin filmould actually be used for only about one order higher H2 con-entration. Our previous investigations on much thicker ZnO filmsndicated fairly large frequency shifts [25]; however, the measur-ble concentration was roughly in the same sensitivity range of.1%. We can understand this through the much larger surface spe-ific area and better response of the nanowires for amortisationf the SAWs. Thus, the nanowire sensor configuration was moreffective and demonstrated better performance for measuring lowoncentrations of H2, compared with the thin-film sensor. How-ver, for higher concentrations of H2, the nanowire sensors tend toecame saturated; thus, the thin-film sensors are recommended inhis case.

The reproducibility of the sensor based on ZnO nanowires wasested by exposing the devices to 1% H2 gas for 10 cycles. Theecorded fluctuations of the frequency shift were less than 10%.

. Conclusion

In conclusion, we fabricated and tested for the first time SAWensors for RT H2 detection with ZnO nanowires grown directly onhe sensor active surface using the VLS-PLD method. This techniquensured better coupling between the structures and the substrateurface. Also, a quasi-parallel alignment of the sensitive struc-ures maximised the sensor’s active area. The ZnO-nanowire sensoresponse was much better than that for a thin film for comparable

mounts of material and allowed for the detection of H2 concen-rations down to 0.01%. The sensor’s sensitivity depended linearlyn the ZnO deposited volume on the active surface area, as well asn the growth morphology. For the nanowire-based SAW sensors,

[

ctuators B 208 (2015) 1–6 5

the frequency shift became saturated more quickly than that forthe thin-film sensors and even stopped oscillating with excessiveamounts of deposited material. This behaviour of the nanowire-based SAW sensors makes them more appropriate for lower H2concentration measurements.

Acknowledgement

This work was supported by the program Partnerships in prior-ity area - PN II, under support from Romanian National Authority forScientific, CNDI–UEFISCDI, project number PN-II-PT-PCCA-2011-3.2-0762 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 fromPolitehnica University of Bucharest, Romania and Ph.D. degree in Electrical Engi-neering from the same university in 2001. He is senior researcher in LaserDepartment at the National Institute for Laser Plasma and Radiation Physics. Hisresearch interests are in laser-matter interactions, special pulsed laser depositiontechniques and nanostructures fabrication for applications in sensors and biology.

Cristian Viespe received his B.S. and M.S. in electrical engineering and physics fromPolitehnica University of Bucharest, Romania, in 2005 and 2007 respectively. In

2010 he received his Ph.D. in physics at the same university with work “SurfaceAcoustic Wave Sensors”. He is currently a senior researcher at National Institute ofLaser, Plasma and Radiation Physics at Laser Department. His main research interestsinclude sensors/biosensors devices, and develop of nanomaterials with applicationsin sensors/biosensors.