Piezoelectric and opto-electrical properties of

silver-doped ZnO nanorods synthesized by low

temperature aqueous chemical method

Eiman Nour, A. Echresh, Xianjie Liu, Esteban Broitman, Magnus Willander and Omer Nour

Linköping University Post Print

N.B.: When citing this work, cite the original article.

Original Publication:

Eiman Nour, A. Echresh, Xianjie Liu, Esteban Broitman, Magnus Willander and Omer Nour,

Piezoelectric and opto-electrical properties of silver-doped ZnO nanorods synthesized by low

temperature aqueous chemical method, 2015, AIP Advances, (5), 7, 077163.

http://dx.doi.org/10.1063/1.4927510

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Postprint available at: Linköping University Electronic Press

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Piezoelectric and opto-electrical properties of silver-doped ZnO nanorodssynthesized by low temperature aqueous chemical methodE. S. Nour, A. Echresh, Xianjie Liu, E. Broitman, M. Willander, and O. Nur Citation: AIP Advances 5, 077163 (2015); doi: 10.1063/1.4927510 View online: http://dx.doi.org/10.1063/1.4927510 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Tuning the emission of ZnO nanorods based light emitting diodes using Ag doping J. Appl. Phys. 116, 193104 (2014); 10.1063/1.4902526 Synthesis of highly efficient antibacterial agent Ag doped ZnO nanorods: Structural, Raman and opticalproperties J. Appl. Phys. 115, 154308 (2014); 10.1063/1.4869736 Opto-electrical properties of Sb-doped p-type ZnO nanowires Appl. Phys. Lett. 104, 111909 (2014); 10.1063/1.4869355 Emission properties of Ag @ SiO 2 / ZnO nanorods' heterostructure AIP Conf. Proc. 1512, 732 (2013); 10.1063/1.4791246 Tunable growth of ZnO nanorods synthesized in aqueous solutions at low temperatures J. Vac. Sci. Technol. B 24, 288 (2006); 10.1116/1.2163889

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AIP ADVANCES 5, 077163 (2015)

Piezoelectric and opto-electrical properties of silver-dopedZnO nanorods synthesized by low temperature aqueouschemical method

E. S. Nour,1,a A. Echresh,1 Xianjie Liu,2 E. Broitman,2 M. Willander,1and O. Nur11Department of Science and Technology (ITN), Linköping University, Campus Norrkoping,SE-60 174 Norrkoping, Sweden2Department of Physics, Chemistry and Biology (IFM), Linköping University,SE-58183 Linköping, Sweden

(Received 8 May 2015; accepted 16 July 2015; published online 23 July 2015)

In this paper, we have synthesized Zn1−xAgxO (x = 0, 0.03, 0.06, and 0.09) nanorods(NRs) via the hydrothermal method at low temperature on silicon substrate. Thecharacterization and comparison between the different Zn1−xAgxO samples, indicatedthat an increasing Ag concentration from x = 0 to a maximum of x = 0.09; Allsamples show a preferred orientation of (002) direction with no observable changeof morphology. As the quantity of the Ag dopant was changed, the transmittances, aswell as the optical band gap were decreased. X-ray photoelectron spectroscopy dataclearly indicate the presence of Ag in ZnO crystal lattice. A nanoindentation-basedtechnique was used to measure the effective piezo-response of different concen-trations of Ag for both direct and converse effects. The value of the piezoelectriccoefficient (d33) as well as the piezo potential generated from the ZnO NRs andZn1−xAgxO NRs was found to decrease with the increase of Ag fraction. The findingin this investigation reveals that Ag doped ZnO is not suitable for piezoelectric energyharvesting devices. C 2015 Author(s). All article content, except where otherwisenoted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4927510]

I. INTRODUCTION

The semiconductor zinc oxide (ZnO) has gained a lot of interest in the research community.ZnO is a group II–VI compound semiconductor with excellent thermal and chemical stability, hasa relatively large excitonic binding energy (60 meV) and a direct wide band gap (3.37eV) at roomtemperature. In addition to its semiconducting properties, and due to the inherent crystal structure,ZnO also possesses strong piezoelectric properties.1 These characteristics make ZnO a material tobe suitable for applications in electronic, optoelectronic, electrochemical, and electromechanicaldevices such as light-emitting diodes,2 photodetectors,3 photodiodes,4 gas sensors,5 solar cells,6

piezoelectric transducers7 and so on. Modification of ZnO properties by impurity incorporation iscurrently another important issue for possible different applications. Doping in ZnO with selectiveelements offers an effective method to adjust their electrical, optical, magnetic and piezoelectricproperties, which is crucial for their practical applications.8 For ZnO, silver (Ag) is a good candidatefor adjusting its optical properties. Ag ions can act as acceptors in ZnO, existing on substitutionalZn sites or in the interstitial form.9,10 In addition, in Ag doped ZnO, the location of the acceptorlevel remains contentious.9,10

In general, the physical properties of ZnO are closely connected with the deposition method,deposition parameters, annealing treatments and doping. Due to these factors, the doping has beenwidely used to adjust the structural, electrical and optical properties of ZnO thin films.10 Ag doped

aCorresponding author E-mail: eiman.satti.osman@liu.se

2158-3226/2015/5(7)/077163/10 5, 077163-1 ©Author(s) 2015

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ZnO thin films have drawn considerable attention in many studies, e.g. as reported in Refs. 11–16.In these studies Ag doped ZnO thin films showed increase in the resistivity, in addition to enhance-ment of the UV emission, and reduction of the optical band gap.10–16 In general all these Ag dopedZnO thin film experiments were concentrated on the electrical or opto-electrical properties, but toour best knowledge, no study has been published on measuring the piezoelectric effect of Ag dopedZnO. Based on the surface roughness and the electrical resistivity, S-H Nam et al claimed thatAg doped ZnO thin films deposited by radio frequency magnetron sputtering are not appropriatefor piezoelectric devices because the films have poor crystallinity and low resistivity; interestingly,they did not measured the piezoelectrical properties of their films.17 Moreover, the published pap-ers on Ag doped ZnO nanostructures are so far very limited.18,19 Among the many possible ZnOnanostructures, well-aligned nanorods (NRs) have attracted increased interest for many applica-tions. Un-doped well aligned ZnO NRs have been grown on many substrates using both physical aswell as chemical methods.20 Recently, aqueous chemical growth (ACG) methods used to synthesizemetal oxide nanostructures have become very popular among researchers.21,22 This is due to thesimplicity of the method, the low synthesis temperature (<100◦C) which enables the use of softsubstrates, like plastic, papers etc., Although this ACG method is a low temperature approach(<100◦C) it has proven effective in controlling the morphology, structure and properties by varyingthe different growth conditions such as temperature, growth time, precursor concentration, andpreparation conditions.21–25 It will be of great interest to investigate and control the incorporation ofdifferent impurities in ZnO nanostructures using the low temperature chemical methods.

In the present work, we report on the structural, optoelectronic and piezoelectric properties ofZn1−xAgxO (x = 0, 0.03, 0.06 and 0.09) NRs synthesized using the low temperature ACG approach.The structural properties were investigated using scanning electron microscopy (SEM) and powderx-ay diffraction (XRD). The chemical state of Ag in the Zn1−xAgxO was investigated by an X-rayphotoelectron spectroscopy (XPS). The influence of the Ag doping on the optical properties of thedifferent samples was investigated using UV-VIS spectrometer. Finally, the piezoelectric propertieswere investigated using direct and inverse nano-indentation measurements.

II. EXPERIMENTAL PROCEDURE

At first, silicon substrate coated with silver was cleaned by sonication in acetone, deionizedwater, and isopropanol, respectively. Then, the substrate preparation technique developed by Greenet al 26 was used to improve the quality of the grown nanorods. For the growth of the Zn1−xAgxONRs (x = 0, 0.03, 0.06 and 0.09), an equimolar concentration (0.075 M) of hexamethylenetetramine(HMT), and a mixture of zinc nitrate hexahydrate and silver nitrate solutions were prepared andmixed together. The different Ag concentrations were obtained by mixing different volume ratios ofthe zinc nitrate hexahydrate and silver nitrate. Next, the prepared solution was poured in a beakerand the pre-treated substrates were immersed in the solution with the growth side facing downward.After that, the beaker was sealed and heated in a laboratory oven at 90o C for 6 hours. Then, thegrowth beaker was allowed to cool down to room temperature. Finally, after the growth process,the samples were rinsed with deionized water and dried with flowing nitrogen in order to removethe residual salts.

A Schottky contact on the Zn1−xAgxO NRs grown on silicon substrate (x = 0, 0.03, 0.06 and0.09), was achieved using gold (Au) contact on top of the nanorods. The crystal structure of thepure and Ag-doped ZnO NRs arrays grown hydrothermally were investigated by Powder x-raydiffraction (XRD) Philips PW 1729 diffractometer equipped with Cu-Kα radiation (λ = 1.5418 Å).The surface morphology and physical parameters were measured using Field-emission scanningelectron microscopy (FE-SEM) Gemini LEO 1550. The influence of the Ag doping on the opticalproperties of the different samples was investigated using a PerkinElmer Lambda 800/900 spec-trometer part no. BV900ND0. The direct and converse piezoelectric properties tests were performedby a nanoindentation technique using a Hysitron IT- 950- Triboindenter at room temperature, asdescribed in details in Refs. 27 and 28.

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III. RESULTS AND DISCUSSION

A. Surface morphology and structure characterization

The x-ray diffraction pattern of pure ZnO NRs and different concentration of Ag doped ZnONRs obtained at 6 hours growth durations are shown in Figure 1(a). All the x-ray diffraction spectra

FIG. 1. (a) XRD patterns all the Zn1−xAgxO NRs grown on silicon substrate (x value is as indicated). (b) The XRD patternsof the (002) diffraction peaks. (c) The FWHM and the 002 peak intensity as a function of doping concentration.

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077163-4 Nour et al. AIP Advances 5, 077163 (2015)

FIG. 2. SEM image of all the silver doped Zn1−xAgxO NRs grown on silicon substrate (x value is as indicated).

were measured using the same acquisition parameters and hence are comparable. Three diffractionpeaks are well consistent with the hexagonal phase of diffraction peaks of ZnO, and in agreementwith the JCPDS Card No. 36-1451 file. In addition to the (111) Ag diffraction peak (from thesubstrate), the x-ray diffraction spectra of the different Zn1−xAgxO NRs (x = 0, 0.03, 0.06 and 0.09),samples show diffraction peaks corresponding to the ZnO (002), (100) and (101) planes. The (002)reflection peak is intense and sharper in nature, as compared to other peaks, indicating a preferentialc-axis growth orientation all the NRs. Nevertheless its intensity is decreasing with increasing the Agconcentration. Increasing the concentration of Ag leads to decrease the crystal quality of the ZnONRs, in agreement with previous results reported by Xu et al.10 Figure 1(b) shows the XRD patternsof the (002) diffraction peaks for all samples. In this Figure there is also a clear slight shift of theangular position of the (002) peak. This shift indicates that the Ag ions replaced the Zn sites in theZnO NRs crystal matrix. Additionally and because the radius of Ag2+ ion (1.22 Å) is a greater thanthat of Zn2+ ion (0.72 Å), the increase of the number of Ag2+ ion in the Zn ions lattice sites contrac-tion the lattice parameter.13 The values of full width half maximum (FWHM) and (002) intensity forthe different Zn1−xAgxO NRs (x = 0, 0.03, 0.06 and 0.09) samples are shown in Figure 1(c). It canalso be seen that FWHM is increased by the increasing the Ag dopant concentration. The FWHMchange with the Ag incorporation demonstrates that the crystallinity of the NRs decreases withincreasing the Ag concentration, indicating that large amount of Ag atoms may inhibit the c-axispreferential growth of the ZnO NRs.15

SEM images of the different Zn1−xAgxO NRs (x = 0, 0.03, 0.06 and 0.09) are shown inFigure 2. It is clear that all the NRs grown samples have a hexagonal structure with uniform, wellaligned c-axis oriented nature. The average diameter is between 100-150 nm and the approximatelength is 1µm. As can be seen from these SEM images, the addition of the Ag does not affectthe morphology: neither the size nor the spatial distributions of the NRs have been altered by thepresence of the Ag in the ZnO matrix.

B. Electronic structure characterization

X-ray photoelectron spectroscopy was used to investigate the charge state and chemical compo-sition of Ag in the Zn1−xAgxO NRs (x = 0, 0.03, 0.06 and 0.09). Figure 3 shows the XPS spectra ofthe Ag 3d peaks (Ag3d5/2 and Ag3d3/2) for the pure ZnO (pink), Zn0.97Ag0.03O (red), Zn0.94Ag0.06O(blue) and Zn0.91Ag0.09O NRs (black). For the three doped samples, the XPS signal from Ag 3d

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FIG. 3. XPS spectrum of Ag 3d peaks for the Zn1−xAgxO NRs grown on silicon substrate (x value is as indicated). All XPSpeaks were normalized. The dashed line indicates the change of the Ag peak position with doping.

photoemission was substantially detected, for instance the peak of Ag 3d in the Zn0.91Ag0.09O NRssample appears at around 368.12 eV and 374.17 eV for Ag 3d5/2 and Ag 3d3/2, respectively. There-fore, these spectra clearly confirm the incorporation of Ag into the ZnO crystal lattice. The overallobserved XPS spectrums of Zn1−xAgxO NRs were in agreement with earlier reports.10,29–32 Besidesthe appearance of Ag in the doped film, there is a shift of Ag 3d peak upon the different dopinglevels. From Zn0.97Ag0.03O NRs to Zn0.91Ag0.09O NRs, the peak position of Ag3d5/2 downshifts tothe lower binding about 0.1 eV, as marked by the dashed line in Figure 3, where all the intensitywere normalized to show the change of peak position. Such behaviors could indicate that there ismore metallic properties of Ag in the higher doped sample than lower doped sample.

C. Performance of the device

1. Optical properties

The optical band gap of the Zn1−xAgxO NRs (x = 0, 0.03, 0.06 and 0.09) samples was deter-mined using Tauc method. From this method, the (αhν)2 plot versus hν for ZnO is as shown in

FIG. 4. Plot of (αhν)2 versus hν of the ZnO (black), Zn0.97Ag0.03O (red), Zn0.94Ag0.06O (orange) and Zn0.91Ag0.09O NRs(blue).

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FIG. 5. (a) Schematic diagram of nanoindentation instrument used for measurement of direct piezopotential. (b) Generatedpiezoelectric potential as a function of applied load.

Figure 4, and according to the equation:18,33,34

(αhν)1/r = A(hν − Eg)1/2 (1)

where α is the optical absorption coefficient of the material, hν is the photon energy, A is aconstant coefficient, Eg is the optical band gap, and the exponent r depends on the nature of thenature of the transition of the material. Band gap narrowing upon doping is a well-known generalphenomenon in semiconductors, not just in ZnO. Shallow level donor impurities create energylevels in the band gap near the conduction band edge and shallow acceptor impurities create energylevels near the valence band edge. With increase in the amount of doping, the density of states ofthese dopants increase and form a continuum of states8,9 just like in the bands, and effectively theband gap decreases. In this work, the values of optical band gap for the Zn1−xAgxO NRs (x = 0,0.03,0.06 and 0.09) samples are shown in Figure 4, calculated by the extrapolation method. Thevalues obtained were 3.30 eV (black curve), 3.26 eV (red curve), 3.22 eV (orange curve) and3.17 eV (blue curve), for the different Zn1−xAgxO NRs (x = 0, 0.03, 0.06 and 0.09) samples, respec-tively. As it can be seen, the optical band gap decreased by increasing the amount of the Ag doping.

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FIG. 6. (a) Schematic diagram of nanoindentation instrument used for measurement of converse piezoelectric under theapplied voltage. Penetration depth as a function of time (6 < t < 10) of: (b) ZnONRs (c-e) Zn1−xAgxO NRs grown on siliconsubstrate (x value is as indicated) for switch from V= 0 to V= −40V.

This might be an indication that the Ag has substituted the Zn in the lattice. This is consistent withthe observation from the x-ray results discussed above. So we can conclude that the optical bandgap of Ag doped ZnO nanostructures is strongly dependent on the lattice sites of Ag in the ZnO.Additionally, the interaction of Ag states with the ZnO host states resulted in creating energy levelsin the band gap that leads to reduce the optical band gap.

2. Piezoelectric properties

a. Direct piezoelectric effect. A Schematic diagram of nanoindentation instrument used formeasurement of converse piezoelectric under the applied voltage shown in Figure 5(a). Figure 5(b)

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shows the relation between the maximum applied force and the generated piezo potential at thepoint of maximum applied force for the four different samples. Among different reported results, A.Khan et al investigated ZnO nanowires (NWs) grown on conductive fabric. The generated outputpotential was up to 13 mV at 1000 µN applied load.35 M. Hussain et al obtained up to 3 mVat 400 µN applied load for ZnO NRs sample synthesized on FTO.36 Here from our samples, it’sobserved that when the applied load was increased up to 160 µN, the resulting piezoelectric poten-tial consistently increased for the four samples. In addition, while piezoelectric potential values ofup to 7 mV were generated for pure ZnO NRs, output values of 2, 1.9 and 1.4 mV were generatedform the Zn0.97Ag0.03O, Zn0.94Ag0.06O and Zn0.91Ag0.09O NRs, respectively. It is clear that theaddition of Ag dopant decreases substantially the generated piezoelectric voltage. Missing pointsin the Figure for samples with Zn0.94Ag0.06O and Zn0.91Ag0.09O NRs were for points that did notgenerate voltage, probably due to a short circuit.

b. Converse piezoelectric effect. The converse piezoelectric effect is measured by applying aDC voltage in the range 0 to - 40 V while there is a relatively low applied force of 15 µN applied tothe samples to allow the tip to be always in physical contact with the NRs; as shown in Figure 6(a).

In order to evaluate the performance of any piezoelectric material clamped to a substrate, themost important parameter to calculate is the effective piezoelectric coefficient deff

33. In the case ofNRs, this coefficient is directly related to the change of the longitudinal elongation ∆l when the NRsare subject to a change of the applied voltage ∆V in their c axis direction: deff

33 = ∆l/∆V.27,28,37

The converse piezoelectric effect deff33, of technological importance for the design of devices,

can be related to the “true” piezoelectric coefficient d33 of bulk material by the following relation-ship:27,28,37

d33 = deff33 (s11 + s12)/(s11 + s12 + s13) (2)

where s11, s12, and s13 are the mechanical compliances of the piezoelectric NRs.From the results are shown in Figure 6(b)-6(e) we can estimate the value of deff

33 to be∼ 130 pm/V for the pure ZnO NRs sample. A piezoelectric coefficient value of 33.2 pmV−1 fromZnO NWs grown on conductive fabric substrate was reported by A. Khan et al.35 Additionally,the value of the deff

33 for the Zn0.97Ag0.03O, Zn0.94Ag0.06O and Zn0.91Ag0.09O NRs were about8.75, 6.25 and 4 pm/V, respectively. Figure 7 illustrate the substantial decrease of the piezoelectricresponse with the addition of Ag.

Because of the non-central symmetric feature in the ZnO wurtzite structure, the cations andanions are tetrahedrally coordinated and the centers of the positive ions and negative ions overlapwith each other under strain-free conditions. When an external stress is applied, the centre of thecations and anions is displaced, and this produces a non-zero dipole moment. A constructive sum

FIG. 7. Piezoelectric coefficient as a function of doping concentration.

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of these dipole moments results in a macroscopic potential, which is the origin of piezoelectricity.38

In some materials, like AlN, the doping with one foreign element can increase the piezoelectricresponse, as demonstrated in Sc-doped AlN thin films.28 On the other hand, in ZnO thin films, thecrystalline quality decreased considerably with increasing Ag atom%.17 By introducing a very smalldoping amount, the effect will be such that; this dopant addition distorts the unit cell of the ZnOcrystal. If the doping level is further increased, there will be a high deformation of the crystallinestructure, leading to loss of the symmetry and decreasing thereafter the piezo response. Our opticaland structural results are consistent and similar to those reported in Ref. 17, nevertheless, we havenot observed any change of morphology of Ag doped ZnO nanorods.

IV. CONCLUSION

In summary, silver (Ag) doped ZnO NRs have been successfully synthesized on silicon sub-strate. Structural characterization (x-ray and SEM) indicated that the morphology of the NRs waspreserved while growing ZnO NRs samples up to 9% atom fraction of Ag doping concentration.The increase of Ag concentration result in creating an energy donor levels in the band gap whichleads to reduce the optical band gap. XPS data clearly shows the incorporation of Ag into the ZnOcrystal lattice. The direct and converse piezoelectric properties of highly c-axis oriented ZnO andAg doped ZnO NRs grown by low temperature ACG method on silicon substrate were measuredand analyzed by the nanoindentation technique. The value of the piezoelectric coefficient deff

33 wasfound to decrease from 130 pm/V to 8.75 pm/V for the pure ZnO NRs and Zn0.97Ag0.03O, respec-tively. Upon further increase of the Ag fraction the piezoelectric coefficient has slightly decreasedfurther. When the nanoindenter is used to measure the direct piezoelectric effect, the piezoelectricpotential generated values from the pure sample of ZnO NRs and for the Zn0.97Ag0.03O weredecreased from 7 mV to 2 mV, respectively. These results indicated that, even preserving thecrystallinity and electrical resistivity of the Ag doped ZnO NRs, the material is not suitable forpiezoelectric device applications.

ACKNOWLEDGMENT

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