Ferroelectric Gd-doped ZnO nanostructures

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Materials Chemistry and Physics 202 (2017) 56e64

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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Ferroelectric Gd-doped ZnO nanostructures: Enhanced dielectric, ferroelectric and piezoelectric properties Sahil Goel a, Nidhi Sinha b, Harsh Yadav a, Sanjay Godara a, Abhilash J. Joseph a, Binay Kumar a, * a b

Crystal Lab, Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India Department of Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Gd-doped NRs of diameter 59.85 nm were synthesised using solution method.  Crystallite size was determined using Scherrer, W-H and HRTEM methods.  Dielectric, ferroelectric and piezoelectric properties were reported first time.  High dielectric transition (Tc) was observed at 215  C.  Large d33 (45.49 pV/m) was calculated from slope of butterfly loop.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 May 2017 Received in revised form 27 July 2017 Accepted 31 August 2017

Gadolinium doped ZnO nanorods were grown by wet chemical method. Line broadening of each diffraction peak was studied to evaluate the crystallite size, lattice strain, stress and energy density. Crystallite size was estimated by Scherrer and Williamson-Hall methods. The size was found to be in good agreement with the results of transmission electron microscopy. As a result of Gadolinium doping, high remnant polarization (Pr ¼ 0.29 mC/cm2) and coercive field (Ec ¼ 16.41 kV/cm) were observed. The decrease in leakage current due to Gadolinium doping makes ZnO a remarkable candidate for ferroelectric capacitor. High ferroelectric phase transition temperature (Tc ¼ 215  C) and large piezoelectric coefficient (d33 ¼ 45.49 pm/V) were observed which makes Gadolinium doped ZnO sample useful for high temperature non-volatile ferroelectric memory applications. © 2017 Elsevier B.V. All rights reserved.

Keywords: ZnO nanoparticles Rare earth ion doping Dielectric response Piezoelectricity Ferroelectricity

1. Introduction In the microelectronics field, there is great demand of ferroelectric nanostructures with high transition temperature, high dielectric permittivity, remnant polarization, spontaneous * Corresponding author. E-mail address: [email protected] (B. Kumar). http://dx.doi.org/10.1016/j.matchemphys.2017.08.067 0254-0584/© 2017 Elsevier B.V. All rights reserved.

polarization as well as large electromechanical response. Wurtzite zinc oxide (ZnO) shows the largest electromechanical response among all the classes of tetrahedrally bonded semiconductors [1]. ZnO is relatively easy to synthesize and also structurally simple. In the last two decades, ternary II-VI (Cd-doped ZnS, Ge-doped PbTe) ferroelectric compounds have replaced perovskite compounds in memory devices [2,3]. Researchers have shown tremendous attention to ZnO nanostructures due to their application in non-

S. Goel et al. / Materials Chemistry and Physics 202 (2017) 56e64

volatile memory devices and micro electromechanical devices [4]. ZnO belongs to P63mc space group with a direct band gap (Eg) of 3.37 eV and high exciton binding energy of 60 meV that makes it an attractive material for variety of optoelectronics and electronic applications [5]. Z. Zang and X. Tang in their work have demonstrated that water-soluble ZnO nanoparticles (NPs) can be used as a labeling agent in biological fluorescent imaging [6]. Z. Zang and his co-workers in their work found that the output power of conventional LEDs increased by 58.4% when roughened with ZnO NPs [7]. The structural and dielectric properties of ZnO are greatly influenced by growth environment, purity and type of dopant. ZnO is a promising II-VI n-type semiconductor material which exhibits dielectric characteristics at low temperature and conducts at high temperature [8]. It has been reported several times that doping in ZnO improves its dielectric, ferroelectric and piezoelectric properties [9e11]. In past, several works have been reported on the enhancement of magnetic, ferroelectric properties and dielectric constant due to rare earth ion doping in BiFeO3 [12]. ZnO doped with rare earth (RE3þ) elements shows remarkable coexistence of optical, electro-chemical and semiconducting properties. The dielectric constant of Erbium (Er3þ) doped ZnO sample was found to be more than pure ZnO sample [5]. An appreciable enhancement in the dielectric constant of CoFe2O4 (CFO) ceramics was observed on doping with Gadolinium (Gd3þ) ions [13]. GaN semiconductor doped with Lanthanide impurities serve as a potential candidate for light emitting diodes (LED) [14]. However, ZnO is a better semiconductor in comparison with GaN. These reports motivated us to dope ZnO with rare earth Gd3þ ions and study the changes in dielectric, ferroelectric and piezoelectric properties in detail. There is a huge difference in the ionic radii of the rare earth dopant Gd3þ ion and the host Zn2þ ion. Hence, doping with Gd3þ ions induces large asymmetricity in electronic environment of ZnO matrix [5]. Thus dielectric, ferroelectric and piezoelectric properties of the ZnO semiconductor can be varied by incorporation of rare earth impurities. A lot of work has already been reported on effect of Gd3þ doping on photoluminescence, photocatalytic activity, optical band gap and magnetic properties of ZnO NPs [15e18]. However, as far as we know the dielectric (curie temperature determination), piezoelectric and ferroelectric properties of Gd3þ doped ZnO have not been reported yet. The present work demonstrates Gd-doped ZnO as an alternate material for large scale production of material with large dielectric constant and large piezoelectric coefficient at nanoscale. Aim of the present report is to synthesize Gadolinium doped ZnO through wet chemical solution method and to study structural, dielectric, ferroelectric, piezoelectric properties and I-V characteristics of Gd-doped ZnO in detail. The crystal structure and crystallite size were evaluated by power X-ray diffraction (XRD) and transmission electron microscopy (TEM), respectively. The crystallite size for Gd-ZnO NPs was evaluated using Scherrer, W-H and TEM methods. The present work demonstrates a strong influence of Gd doping on the dielectric response of ZnO. The room temperature ferroelectricity in Gd-ZnO was confirmed by dielectric study and polarization hysteresis curve. The electromechanical coefficient (d33) of Gd-ZnO was evaluated from the slope of butterfly (Displacement vs. voltage) loop. The results of present paper sets a benchmark for understanding the effect of rare earth ion (Gd3þ) substitution on electrical properties of ZnO. 2. Experimental details 2.1. Optimization of percentage of Gd dopant Realization of high piezoelectricity, high curie temperature and room temperature ferroelectricity in Gd doped ZnO NPs depend

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critically on the Gd doping percentage. Thus, it becomes important to pick an optimized doping percentage of Gd to study its effect on piezo/ferro/dielectric properties of ZnO NPs. The literature is abundant in reports on effect of Gd doping on photoluminescence, photocatalytic activity, optical band gap and magnetic properties of ZnO NPs. O. Opera and his coworkers have shown that 5 mol% Gd-doped ZnO exhibit largest blue green luminescence and largest photocatalytic activity against methylene blue when compared to pure ZnO and several other doping percentages of Gd3þ ions [15]. X. Ma in his work on Gd-doped ZnO has reported that 5 mol % concentration of Gd-doped ZnO nanowires exhibits the clear ferromagnetic nature and have an effective magnetic moment of 3278 mB/Gd3þ [16]. X. Ma and Z. Wang have shown that the structure of ZnO remains unchanged when the impurity density is kept low (Zn0.95Gd0.05O), whereas the structure of ZnO gets largely transformed for large doping concentration of Gd ((Zn0.90Gd0.10O) and Zn0.85Gd0.15O) [17]. Ghouri et al. have shown that the resistivity of 4% Gd-doped ZnO is higher than those of other nanocrystalline powder (3% Gd-ZnO, 2% GdZnO, 1% Gd-ZnO and 0.5% Gd-ZnO) [18]. Moreover, present report on Gd-doped ZnO is a continuation of our ongoing interest on improvement of ferro/piezo/dielectric properties of ZnO NPs by doping ZnO matrix with rare earth dopant ions (La3þ and Eu3þ) [9,11]. In two of our previous reports on rare doped ZnO, it was proved that 5 mol% lanthanum (La) and 5 mol% europium (Eu) doping in ZnO matrix increased the ferroelectric to paraelectric phase transition from 40  C for pure ZnO to 276  C and 280  C for for La-ZnO and Eu-ZnO sample, respectively [9,11]. Also, both La-ZnO and Eu-ZnO NPs were found to exhibit a giant piezoelectric coefficient (d33) equal to ~101.30 pm/V an ~43.38 pm/V, respectively. Keeping all the above mentioned results in mind, 5 mol% Gd was chosen for this study. 2.2. Gd-doped ZnO nanoparticles synthesis Gd-doped ZnO nanorods (NRs) have been synthesized using wet chemical solution route. The starting material zinc chloride and sodium hydroxide were taken in molar ratio of 1:5. Gadolinium (III) chloride hydrate was used as a source of Gd3þ dopant concentration (5 mol %). The homogenous solution of zinc chloride and gadolinium (III) chloride hydrate was prepared using distilled water as a solvent. Thereafter, the above obtained solution was mixed gently drop by drop into the sodium hydroxide solution to obtain Gd-ZnO precipitate. Finally, the white precipitate was obtained by centrifugation technique. The powder was repeatedly washed with absolute ethanol and distilled water. The powder was dried using furnace at 80  C for 5 h. Lastly, the calcinations of as prepared white precipitate was performed at 500  C for a duration of 3 h. 2.3. Characterization techniques The powder X-ray diffraction (XRD) studies on as grown pure and Gd-ZnO sample were performed at room temperature using Bruker D8 Advanced X-ray diffractometer with Cu Ka (1.5408 Å) radiation. Transmission electron microscopy (TEM) for both pure and doped ZnO was carried out using Tecnai 300 kV ultra twin (FEI) to certify the formation of nanorods (NRs). For performing TEM analysis ZnO nanopowder was dispersed in methanol and was ultrasonicated for 30 min. A drop of the above solution was poured on the carbon film placed on porous copper grid. To study electrical properties of pure and doped ZnO, the pellet form of the samples were prepared. For electrical contacts, the pellets were coated with high quality silver paste. Dielectric studies were performed for both pure and doped ZnO samples using Agilent E 4980A LCR meter in a frequency range 200 Hz to 2 MHz and

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where, bhkl is the instrument corrected full width at half maximum intensity (FWHM), q denotes the Bragg peak position, L is the crystallite size (nm), K represents the Scherrer constant (0.94), and l is equal to 1.54056 Å (wavelength of CuKa radiation). The line broadening of each diffraction peak is a combination of both sample dependent effects and instrumental broadening. The instrumental broadening was calculated by recording the diffraction pattern of a standard material (silicon). The corrected diffraction peak broadening bhkl due to sample dependent effects was determined using formula [21],

h

bhkl ¼ ðbhkl Þ2 Measured  ðbhkl Þ2 Instrumental

i1=2

(2)

Now, rewriting equation (1) as,

ln bhkl ¼ ln

Fig. 1. XRD Patterns of pure and Gd-doped ZnO NPs at room temperature.

temperature range 30  Ce240  C. The room temperature ferroelectric measurements were performed with a Precision LC ferroelectric tester (Radiant Technology; Model No. P-HVi210KSC) at a frequency of 200 Hz in Sawyer-Tower mode. A commercially available MTI-2100 FOTONIC SENSOR was used to characterize piezoelectric properties of Gd-doped ZnO NRs. IeV characteristic of pure and Gd-ZnO NPs was studied with a Keithley Source Meter (series 2400).

kl 1 þ ln cosq L

(3)

Plot was drawn with ln (1/cosq) along the x-axis and ln (bhkl) along the y-axis for the Gd-ZnO NPs (Fig. 4(a)). The slope of the fitted straight line depicted the value of crystallite size   Kl . The average crystallite size of Gd-doped ZnO NPs was L ¼ eintercept evaluated to be 42.80 nm using Scherrer method (Table 1).

3. Results and discussions 3.1. XRD and EDS analysis The room temperature XRD patterns of as prepared pure and Gd doped ZnO NPs are depicted in Fig. 1. The positions of all the observed diffraction peaks for both the NPs are in good agreement with the hexagonal wurtzite structure with P63mc space group (JCPDS card No. 36-1451). The FullProf program was used for Rietveld refinement taking starting parameters as P63mc space group and hexagonal crystal system. Good agreement between calculated and observed diffraction peaks for both pure and doped NPs was revealed by Rietveld profile (Fig. 2(a) and (b)). The refinement goodness parameter c2 was found to be 4.60 and 2.76 for pure and Gd doped ZnO, respectively. The unit cell parameters of Gd-ZnO (a ¼ 3.2524 Å, c ¼ 5.2101 Å and V ¼ 47.73 Å3) were found to be slightly greater than that of pure ZnO (a ¼ 3.2518 Å, c ¼ 5.2095 Å and V ¼ 47.70 Å3). The increase in unit cell parameter of Gd-doped ZnO suggests that Gd3þ ions (ionic radii ¼ 1.078 Å) replace the Zn2þ (ionic radii ¼ 0.74 Å) from their lattice sites [19,20]. The EDS study (Fig. 3) confirms the presence of Gd3þ dopant ions in the ZnO matrix. 3.2. Crystallite size and strain Powder XRD study can be employed to calculate the width of Bragg peak, thus determining crystallite size and lattice strain occurring due to dislocations. 3.2.1. Scherrer method The crystallite size of Gd-doped ZnO NPs was calculated by Xray peak broadening method using Debye-Scherrer formula [21]:



Kl bhkl cosq

(1)

Fig. 2. Rietveld refinement of (a) pure and (b) Gd-doped ZnO NPs at room temperature. The refinement goodness parameter c2 was computed to be 4.60 and 2.76 for pure and Gd-doped ZnO.

S. Goel et al. / Materials Chemistry and Physics 202 (2017) 56e64

bhkl ¼ 4εtanq þ

59

Kl Lcosq

(5)

By rearranging the above formula (5), we get

bhkl cosq ¼ 4εsinq þ

3.2.2. Williamson-Hall methods Both crystallite size and lattice strain (ε) contribute to line broadening (bhkl). The strain-induced Bragg peak broadening (bS) arising due to lattice distortions and imperfections is given by bS ¼ 4ε:tanq. The peak width resulting from small crystallite size Kl (bL) is evaluated by bL ¼ Lcos q (Scherrer equation). Total integral breadth at each Bragg peak position is expressed using the following Williamson-Hall (W-H) method [22]:

bhkl cosq ¼ (4)

" h2 Yhkl ¼

þ

!2 S11

h2

þ

ðhþ2kÞ 3

2

ðhþ2kÞ 3

2

þ S33

4ssinq Kl þ Yhkl L

(7)

The Young's modulus (Yhkl) for system with hexagonal crystal phase is expressed as [23]:

 2 #2 þ

al c

 4 al c

(6)

The above model (uniform deformation model; UDM) considers the isotropic nature of crystal and assumes lattice strain to be independent of crystallographic directions. The above equations are known as UDM W-H equations. Plotting the values of bhkl cosq against 4sinq, the uniform strain was estimated from the slope of the linear fit. The y-intercept of the linear fitted line depicted the value of crystallite size (L ¼ Kl/intercept). The UDM for Gd-doped ZnO NPs is shown in Fig. 4(b). In USDM (uniform stress deformation model) anisotropic nature of crystal is considered and stress (s) is related to strain (ε) by generalized Hooke's law s ¼ Yε, where Y represents the Young's modulus of elasticity. The Hooke's law is valid only for the case of small strain. Under the small strain approximation Eq. (6) can be rewritten as:

Fig. 3. EDS spectra confirming the presence of Gd in ZnO.

bhkl ¼ bS þ bL

Kl L

þ ð2S13 þ S44 Þ

h2

þ

ðhþ2kÞ 3

2

!  2 al c

Fig. 4. (a) Modified Scherrer equation plot for Gd-ZnO. The W-H analysis of Gd-ZnO nanoparticles assuming (b) UDM, (c) USDM and (d) UDEDM.

(8)

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where, S13, S11, S33 and S44 are the elastic compliances of ZnO with values 2.206  1012, 7.858  1012, 6.940  1012 and 23.57  1012 m2N1, respectively [24]. Equations (7) and (8) represent the USDM. The plot was drawn with bhkl cosq on the yaxis and 4sinq/Yhkl along the x-axis for the Gd-ZnO NPs (Fig. 4(c)). The slope of the linear fit depicted the value of uniform deformation stress and the crystallite size was estimated from the y-intercept of the linear fitted line (L ¼ Kl/intercept). The uniform deformation energy density model (UDEDM) also considers anisotropic behavior of crystal system. In UDEDM, the deformation energy density (u) was assumed to be uniform in all crystallographic directions considering deformation stress (s) as anisotropic. Using Hooke's law, the energy density (u) can be expressed in terms of lattice strain as follows: u ¼ ðε2 Yhkl Þ=2.

Equation (6) can be rewritten according to energy-strain relation as follows,



bhkl cosq ¼ 4 sin q

2u Yhkl

1=2 þ

Kl L

(9)

UDEDM plot for Gd-doped ZnO NRs was drawn with bhkl cos q along the y-axis versus 4 sin q (2/Yhkl)1/2 on the x-axis (Fig. 4(d)). The anisotropic energy density (u) and the crystallite size (L) were calculated from slope and Y-intercept of the linear fit, respectively. The stress s was calculated using s ¼ (2uYhkl)1/2. Young's modulus, Y, for Gd-doped ZnO was taken to be ~117 GPa. The value of crystallite size (L), strain (ε), stress (s) and energy density (u) calculated using different W-H methods are shown in Table 1.

Table 1 Geometrical parameters of Gd-doped ZnO nanoparticles. Scherrer method

Williamson-Hall method UDM

L (nm) 42.80

L (nm) 51.74

TEM USDM

ε  103 0.6130

L (nm) 59.00

UDEDM ε  103 0.7824

s (MPa) 91.54

L (nm) 58.01

ε  103 0.771

s (MPa) 90.17

u (KJm3) 34.745

L (nm) 59.85

Fig. 5. TEM images of (a) pure ZnO and (b) Gd-doped ZnO NRs. (c) Size histogram of pure ZnO rods showing an average diameter ~85.79 nm. (d) Size histogram of Gd-ZnO rods showing an average diameter ~59.85 nm (e) and (f) SAED ring patterns showing a ring-like pattern from ZnO diffraction planes which implies a random orientation of crystalline pure and Gd- ZnO rods.

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Fig. 6. Variation of dielectric constant ðε0 Þ vs. frequency (log f) at different temperatures for (a) pure ZnO and (b) Gd-doped ZnO nanorods. Variation of dielectric constant ðε0 Þ vs. applied temperature ( C) at various frequencies for (c) pure ZnO and (d) Gd-doped ZnO nanoparticles. The ferroelectric to paraelectric phase transition for pure ZnO is at 40  C and for Gd-ZnO is at 215  C.

3.3. TEM analysis Fig. 5(a) and (b) show the TEM images of pure and Gd-doped ZnO samples, respectively. The size distributions for pure and Gddoped samples are depicted in Fig. 5(c) and (d), respectively. TEM and size histogram analysis confirmed that both pure and Gddoped ZnO samples were found to be made of rod shaped particles. In the case of pure ZnO nanorods (NRs), the average length and diameter were estimated to be ~359.07 nm and ~85.79 nm, respectively. However, the average length and diameter of Gddoped ZnO NRs were estimated to be ~245.13 nm and ~59.85 nm, respectively. The size of the particle calculated from Scherrer method, UDM, USDM and UDEDM models is in good agreement with particle size obtained using TEM method (see Table 1). Moreover, it can be concluded that USDM and UDEDM gave a better

approximation of crystallite size when compared to UDM and Scherrer method. The selected area electron diffraction (SAED; Fig. 5(e) and (f)) patterns clearly demonstrate the well developed single crystalline wurtzite structure of both pure and Gd-ZnO particles. The miller indices of diffraction rings matched with wurtzite crystalline phase and no secondary phase was observed. Such a result was also recognized from powder XRD analysis.

3.4. Dielectric studies Fig. 6(a) and (b) depict the frequency dependence of dielectric constant ðε0 Þ for pure and Gd-doped ZnO NRs, respectively. It was observed that the value of dielectric constant ðε0 Þ decreased with increase in frequency (20 Hze2 MHz; Fig. 6(a) and (b)), at different temperatures for both the NPs. High value of dielectric constant can

Fig. 7. (a) Plot of ac conductivity vs. frequency for Gd doped ZnO. (b) Plot of ln (s) vs. 1000/T at 1 KHz for Gd-doped ZnO nanoparticles.

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be attributed to the fact that under the application of electric field the ZnO nanocrystals behave as nanodipoles and exhibit large nanoparticle density [25]. The large value of dielectric constant for Gd-doped ZnO makes it a useful candidate for charge storing applications [26]. Temperature dependence of dielectric constant ðε0 Þ for pure and Gd-ZnO NRs are shown in Fig. 6(c) and (d), respectively. The observed value of ferroelectric to paraelectric phase transition (Tc) for Gd-doped ZnO NRs (215  C) is higher than pure ZnO (40  C). With increase in frequency (200 Hze2 MHz) the dielectric peak was shifted towards higher temperature for both pure and Gd-ZnO samples (Fig. 6(c) and (d)). Increase in frequency does not allow charge carriers to align with the changing electric field which leads to decrease in polarization. Thus to restore polarization, high temperature is required. The ferroelectric phase in Gd-ZnO NRs arises due to the structural changes produced by Gd dopant in ZnO, which greatly influences its dielectric and electronic properties, leading to ferroelectricity. The phase transition in Gddoped ZnO ceramics arises due to both order-disorder characteristics of off-centered Gd ions and displacive characteristics of relative translational shifts in Zn and O sublattices [27]. The similar results have been reported in literature for (Mn, Li) co-doped ZnO nanorods [27]. The obtained value of transition temperature Tc for Gd-ZnO is higher than Tc observed for ZnO nanostructures with several other doping [9e11,28], thus highlighting its importance in the wide temperature range of ferroelectric applications. AC conductivity of Gd-ZnO was evaluated using the relation, sac ¼ 2pf tandεεo, where tand is the dielectric loss, f denotes the frequency, εo and ε are the dielectric constant of free space and sample, respectively. Fig. 7(a) displays the logarithm plot of ac conductivity variation versus frequency at various temperatures. The conductivity of the Gd-ZnO nanostructure was observed to increase with increase in temperature. The value of conductivity of the sample directly affects its dielectric, piezoelectric and ferroelectric properties. The conductivity was found to be temperature independent in the low frequency region up to 10 KHz. The temperature independent conductivity is because of the significant contribution from the dc conductivity. In higher frequency region (above 10 KHz), the contribution of ac conductivity starts to dominate. This can be justified using Jonscher power law equation, s ¼ Aus, where A is the constant, exponent 's' is the frequency dependent parameter and u is the angular frequency [29]. The Arrhenius equation was used to estimate the activation energy of the Gd-doped ZnO and is given as: sac ¼ so exp(Ea/kBT), where, kB is the Boltzmann constant and so is the pre-exponential factor. Fig. 7(b) shows the linear plot of ln sac against 1000/KT at 1 KHz. The values of activation energies in Gd-ZnO were computed to be 1.69 eV, 0.53 eV, 1.88 eV and 0.096 eV at various temperature range.

Fig. 8. (a) P-E characteristics of Gd-ZnO NRs. (b) The typical D-V butterfly loop and d33V loop for Gd-ZnO NRs.

radii difference, the off-centered positions may be occupied by Gd3þ ions after replacing the host Zn2þ ions. This results in forming permanent electric dipole moments locally thereby introducing ferroelectric behavior [9,27]. Similar results for origin of ferroelectricity due to several other doings in ZnO nanostructures have been reported earlier [9,11,27,28]. The non-zero remnant polarization in Gd-ZnO NRs makes ZnO a potential candidate for fabricating nanoscale nonvolatile ferroelectric memories devices. Fig. 8(b) presents the displacement vs. applied voltage (D-V) curve for Gd doped ZnO NRs. A typical well defined butterfly loop is obtained with a maximum displacement of ~0.073 mm at an applied voltage ~1377 V. It is well known that every point of the D-E butterfly loop provides information regarding piezoresponse of the sample under corresponding electric field. At each applied voltage, d33 was measured from the slope of corresponding butterfly loop using the law of converse piezoelectric effect: D ¼ d33V [30]. The point of intersection of butterfly loop does not exactly lie on the origin and the formula is modified accordingly to

3.5. Polarization hysteresis behavior and piezoelectric properties

D  D1 ¼ d33 ðV  V1 Þ

To further explain the results based on the dielectric phase transition studies, the polarization-electric field hysteresis of GdZnO was recorded to certify the ferroelectricity in the sample. Fig. 8(a) depicts ferroelectric polarization-applied electric field (PE) characteristics of Gd-doped ZnO NRs measured at RT (at 200 Hz). The P-E curve of Gd-ZnO shows hysteresis behavior which confirmed that ferroelectricity comes from Gd-dopants. Also from the curve one can see that the polarization does not attain saturation which suggests leaky behavior of Gd-doped ZnO sample. The values of remnant polarization (Pr) and coercive field (Ec) were found to be 0.29 mC/cm2 and 16.41 kV/cm, respectively at 1600 V. In Gd-doped ZnO NRs, the ionic radii of the host (Zn2þ) and dopant (Gd3þ) are 0.74 Å and 0.94 Å, respectively [19]. Due to this ionic

where, V1 is the applied voltage and D1 gives the displacement at intersection point [30]. The maximum of piezoelectric coefficient d33 value for Gd-doped ZnO was calculated to be 45.49 pm/V, which is very interesting as electromechanical properties of Gd-ZnO is never reported before. The d33 value for pure ZnO was reported to be 1.6 pm/V [9]. This huge enhancement in piezoelectric response (d33 value) can be attributed to the following two reasons. Firstly, the rotation of Gd-O bonds is easier than that of Zn-O bond. This is because Gd dopants exists as Gd3þ ions in Gd-ZnO [15], which have a higher positive charge than Zn2þ ions. Therefore, the noncollinear Gd-O bonds posses a stronger polarity than Zn-O and hence rotate more easily under applied electric field. Similar behavior is reported for vanadium doped ZnO nanofiber [30].

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space-charge limited conduction (SCLC) and ohmic conduction mechanism [33]. The leakage current was observed to decrease as a result of Gd-doping in ZnO nanoparticles. The dc conductivity of ZnO nanoparticles decreased from 9.4  107 to 2.6  109 mho cm1 on Gd-doping. The decrease in leakage current density can be attributed to increase in Schottky barrier height by Gd doping. 4. Conclusions In conclusion, pure and Gd-doped ZnO NRs were synthesized by wet chemical route. The XRD study confirmed the formation of Wurtzite ZnO nanostructures for both pure and doped samples. The TEM analysis showed that the pure and Gd-ZnO nanostructures were formed in nanorod shape. The diameter of uniform pure and Gd-doped ZnO NRs was found to be 85.79 nm and 59.85 nm, respectively, from TEM analysis. The average crystallite size calculated using line broadening (Scherrer and W-H methods) was in good approximation with TEM analysis. High Tc (215  C) in dielectric studies and large electromechanical response (d33 ¼ 45.49 pm/V) confirmed superior ferroelectric nature for GdZnO NRs. The leakage current was found to decrease on Gd doping. The dc conductivity decreased from 9.4  107 to 2.6  109 mho cm1 as a result of Gd-doping in ZnO nanoparticles. Our results indicate that the Gd-doping makes ZnO as a better candidate for energy harvesting devices and sensors applications. Acknowledgements We are thankful for the financial support received through the DST (EMR/2015/000385) and DRDO Project (ARMREB/MAA/2015/ 163). Sahil Goel, Abhilash J. Joseph and Harsh Yadav would like to thank CSIR, DRDO and UGC for their scholarship. Dr. Nidhi Sinha is thankful to the Principal, SGTB Khalsa College for encouragement. References

Fig. 9. (a) Leakage current density (J) variation with applied voltage (V) for pure and Gd-doped ZnO nanoparticles at room temperature. (b) Plot of Log (J) vs. Log (V) along with linear fits.

Secondly, Gd dopant in ZnO results in emergence of switchable spontaneous polarization as well as large relative permittivity which leads to large value of d33 [30,31]. The large value of d33 for Gd-ZnO opens a door for large scale production of low cost electromechanical devices replacing the existing perovskite based piezoelectric devices.

3.6. J-V characteristics In order to understand the ferroelectric behavior of the material it is necessary to study the conduction mechanism [32]. J-V characteristics of the ZnO and Gd-doped ZnO were measured by coating silver paste on ZnO pellets, which acts as a metal-semiconductormetal junction. Fig. 9(a) presents the plot of characteristics of leakage current density (J) as a function of applied electric field (E) on semi-log scale for pure and Gd-doped ZnO samples. The symmetric behavior of leakage current density about J-axis confirmed the ohmic nature of metal semiconductor junction for both the samples. Fig. 9(b) displays the plot of log J against log V. The behavior of these curves can be understood by the power law: J f Em, where m (slope of logarithmic plot) represents the nature of conduction [33]. The value of slope ‘m’ was found to be between 1 and 2 for both the samples, which can be explained on the basis of

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