Structural and dielectric properties of Fe doped ZnO nanoparticles

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1 Indian J Phys DOI 10.7/s ORIGINAL PAPER Structural and dielectric properties of Fe doped ZnO nanoparticles M L Dinesha, G D Prasanna, C S Naveen and H S Jayanna* Department of PG Studies and Research in Physics, Kuvempu University, Shankaraghatta , Karnataka, India Received: 17 April 12 / Accepted: 28 August 12 Abstract: Nano-sized (where x = 0.00, 0.005, 0.01, 0.02, 0.03, 0.04) powders have been prepared by solution combustion method. The powder samples have been characterized by X-ray diffraction (XRD), Fourier transform infrared spectra, scanning electron microscope (SEM) and energy dispersive analysis of X-rays (EDAX). The XRD patterns showed the formation of single phase wurtzite structure. The average particle size is found to be in the range of nm. SEM images show high porosity in the material. The dielectric studies have been performed at room temperature in the frequency range 1 khz 5 MHz and different mechanisms were used to explain the observed behavior. Keywords: PACS Nos.: Oxide materials; Solution combustion; X-ray diffraction; Dielectric studies p; f; n 1. Introduction In recent years the scientists have paid much attention to the synthesis and characterization of II VI semiconductor materials at nanometer scale, due to their great potential to test fundamental concepts of quantum mechanics [1, 2] and because of their key roles in various applications such as solid state light emitting devices (LEDs), photonics [3], nanoelectronics [4], optoelectronics and data storage. Zinc oxide (ZnO) is an n-type metal oxide semiconductor having a wide direct band gap (3.37 ev) and a large exciton binding energy of mev having properties suitable for various applications such as ultraviolet opto-electronic devices, transparent high power and high frequency electronic devices, piezo-electronic transducers and chemical gas sensors [5]. Studies of nanoscale materials are interesting because of their interesting properties and applications [6 13]. The electrical properties of ZnO are very much dependent on the composition and its structure. The ferroelectric and dielectric properties of Li doped ZnO thin films have been studied by Wang et al. [14]. Oshio et al. [15] have studied the role of Mn doping in ZnO to decide the increase/ decrease of leakage current in its film. They have also studied the dielectric properties of the ZnO:Mn film and showed the large resistivity in the bulk properties of the *Corresponding author, jayanna@gmail.com material. Han et al. [16] have studied the low frequency dielectric properties and the transverse optical phonon mode in the single crystal of ZnO. Yanhong et al. [17] have studied the surface photo-voltage of ZnO nanorods. Hsu and Huang [18] have studied the dielectric constant of ZnO doped (Zr 0.8 Sn 0.2 ) TiO 4 films, which decreases with increasing frequency. Youn et al. [19] have also supported the results of Hsu and Huang [18]. Jose and Khadar [] have studied the conductivity mechanism of ZnO Ag nanocomposites. Jung et al. [21] accounted influence of growth on the structural, optical and electrical properties of In doped ZnO nanorods. Sharma et al. [22] reported about the dielectric behavior of polyaniline-zno composite. Tripathi et al. [23] studied dielectric relaxation in thioglycerol capped ZnO nanoparticles. Recently, we reported the work on structural, electrical and magnetic properties of Fe doped and Co:Fe co-doped ZnO nanoparticles prepared by simple solution combustion method [24, 25]. In the present work, we have reported on structural, dielectric and AC conductivity measurements of (where x = 0, 0.005, 0.01, 0.02, 0.03, 0.04) nanopowders. 2. Experimental details The (where x = 0.00, 0.005, 0.01, 0.02, 0.03, 0.04) powders were prepared by solution combustion method using stoichiometric composition of metal nitrates (AR grade) as oxidizers and glycine as a fuel. The aqueous Ó 12 IACS

2 M L Dinesha et al. solution containing redox mixture was taken in a Pyrex dish and heated in a muffle furnace maintained at 0 ± 10 C. The mixture finally yielded porous and voluminous powder. The following chemical reaction takes place during the synthesis, Zn (NO 3 Þ 2 +Fe(NO 3 Þ glycine 2! Zn 1 xfe x O water þ gaseous products: The XRD patterns of powder samples were obtained by Philips X pert Pro MPD X-ray diffractometer using Cu-Ka radiation (k = Å). The average particle size was determined by using Scherrer s equation, d ¼ 0:9k=b cos h ð1þ where d is size of the particle, h is glancing angle (2h/2) and b is full width at half maxima (FWHM). FTIR spectra were recorded on a PERKIN-ELMER (model-0) spectrometer using KBr pellets. Surface morphology and compositional analysis of the samples were studied by SEM and EDAX images obtained from Hitachi model S-30N microscope. For dielectric and AC conductivity studies, the powder samples were pressed uniaxially into a pellet of thickness 1 2 mm and of diameter 10 mm by applying pressure of 1 MPa for 2 min. The pellets were sintered at 300 C for 3 h to get thermal stability. On both sides of these pellets silver paint was applied which acts as electrodes. Ohmic contacts were made using a thin copper wire and conducting silver paste. Dielectric properties as well as AC conductivity measurements of the prepared samples at room temperature were performed using PSM 1735 (NumetriQ) impedance analyzer interface (IAI) with a Kelvin fixture over the frequency range 1 khz to 5 MHz. 3. Results and discussion The XRD of powder are shown in Fig. 1. These patterns have been compared with standard JCPDS All the XRD peaks can be indexed to a wurtzite structure of ZnO. Figure 1 reveals that, there is no change in the wurtzite structure of ZnO after Fe doping. No extra peaks were found in the pattern indicating the formation of single phase. A small shift in the position of main peak (101) to the lower side of 2h value is observed for doped samples. This observation is similar to the case in transition metal (Mn, Cu, and Ni) doped ZnO studied by Ekambaram et al. [26]. This shift is mainly due to the larger radius of Fe 3? than that of Zn 2?. This indicates that the doped Fe atoms substitute Zn atoms. The particle size of the samples has been calculated using Scherrer s formula and is found to be in the range of nm. Intensity (a. u) 30 () (002) (101) 50 2 θ (degree) Since the difference in radii between Zn 2? (0. Å) and Fe 3? (0.63 Å) is considerable, significant changes in the lattice constants are expected for Fe doped samples. The calculated lattice parameters a = b and c are compared with standard values a = b = Å, and c = 5.5 Å and tabulated in Table 1. With the increase of, the lattice parameter a is found to increase, whereas c parameter is found to decrease as shown in Fig. 2. The u parameter (z-coordinate of the oxygen atoms) of wurtzite structure of increases with the increase of Fe content. Figure 3 shows the FTIR spectra of the prepared samples. The absorption band at cm -1 in FTIR spectra is associated with the characteristic vibrational mode of Zn O bonding [27]. Figure 4(a) and 4(b) show the SEM images of nanocrystalline ZnO and Fe doped ZnO, respectively. The SEM image of pure ZnO powder exhibits cluster of tiny particles having diameters in the range nm. The high porosity in Fe doped samples is attributed to the liberation of gaseous products like H 2 O vapors, CO 2 and N 2 during combustion process. The EDAX images are shown in Fig. 5 which depicts the presence of Fe and Zn atoms in the prepared samples. No traces of other elements in the spectra confirm the purity of the samples. The atomic percentages of Fe and (102) (110) (103) (0) (112) (1) Fig. 1 XRD patterns of samples for different compositions 70

3 Structural and dielectric properties of Fe Table 1 Experimental values of lattice parameters, e 0 ; e 00 ; tan d and r ac for different concentration of Fe Fe (%) a (Å) c (Å) u (Å) Atomic percentage obtained from EDAX Average particle size (nm) e 0 e 00 tan d r ac Zn Fe (1 MHz) a (Å) a (Å) c (Å) Fig. 2 Variation of lattice constants with in % Transmitance (a.u) Wavenumber (cm -1 ) Fig. 3 FTIR spectra of for different compositions Zn as obtained from EDAX are tabulated in Table 1, which are very close to the starting composition. The real (e 0 ) and imaginary (e 00 ) part of the permittivity can be calculated from the capacitance (C) and loss tangent (tand), respectively. Since the capacitor formed using Kelvin fixture Fig. 6(a) has a small capacity due to its large impedance, the equivalent circuit of the capacitor is c (Å) considered to comprise an equivalent parallel capacitance (C) and an equivalent parallel conductance (G) as shown in Fig. 6(b). The admittance (Y) of circuit and the complex admittance (Y*) of circuit in Fig. 6b can be expressed as: Y ¼ jxc ¼ jxjjc e 0 Y ¼ G þ jxc ¼ jx C G j C 0 C 0 xc 0 where C 0 is the air capacitance and x is the angular frequency. Therefore, the complex relative permittivity can be defined as e ¼ e 0 je 00 ð2þ and calculated by e 0 ¼ C ¼ tc C 0 e 0 A e 00 ¼ e 0 tand ð3þ ð4þ where C is the capacitance, t is the thickness and A is the cross-sectional area of the sample. The loss tangent or tan d is defined as the ratio of the imaginary part of the dielectric constant to the real part. tand ¼ e 00 =e 0 ð5þ The AC electrical conductivity of samples was calculated using the relation r ac ¼ 2pf e 0 e 0 tan d ð6þ where f is the frequency of the applied field. Errors of the Kelvin fixture include those due to edge capacitance on the edge electrodes (stray capacitance), residual parameters of the Kelvin fixture, such as electrical length, residual impedance, stray admittance, and air gaps caused when sandwiching the sample between the electrodes. Errors due to the residual parameters of the test fixture were effectively minimized by performing Open, Short, and known Load calibrations on the sample contact using various standard resistance test fixtures of 5, 50 X, 5, 500 kx [28]. Silver electrodes were used to minimize errors due to air gaps [29].

4 M L Dinesha et al. Fig. 4 (a) ZnO and (b) Fe doped ZnO SEM images of combustion derived nanoparticles Fig. 5 EDAX image of for different compositions (a) x = 0, (b) x = 0.005, (c) x = 0.02 and (d) x = A trace appears just left side of the large peak is due to Fe It is a well known fact that the ZnO is a polar molecule. It has permanent dipole moment and responds quickly to the applied electric AC field. Whereas, in case of Fe doped ZnO; hopping between Zn 2? and Fe 3? ions will give rise to permanent electric dipoles. The frequency dependence of e 0 at room temperature is shown in the Fig. 7. It is clear from the figure that e 0 value decreases with increase in frequency reaching a constant value at higher frequencies. The large values of dielectric constant at low frequencies in pure ZnO are due to the predominance of the species of vacancies like oxygen, grain boundary defects etc. While the decrease in dielectric constant with frequency is natural because of the fact that any species contributing to polarizability is bound to show lagging behind the applied field at higher and higher frequencies. From the observed higher value of dielectric constant of doped ZnO we can conclude that, the electron exchange between Zn 2? and Fe 3? ions is predominant at lower frequencies. At higher frequencies, the

5 Structural and dielectric properties of Fe Dielectric loss (ε'') 1 1 Fig. 6 (a) Parallel plate test fixture measurement method; and (b) equivalent circuit model of the parallel plate capacitor Dielectric constant (ε') Fig. 7 Variation of dielectric constant (e 0 ) with frequency for 0 Fig. 8 Variation of dielectric loss (e 00 ) with frequency for 1.4 tan δ dielectric constant tends to reach almost a constant value due to the fact that beyond certain frequency the electron exchange between Zn 2? and Fe 3? cannot follow the applied alternating field. The dielectric loss arises when the polarization lags behind the applied AC field and is caused by the impurities and imperfections in the crystal lattice. The frequency dependences of e 00 and tan d at room temperature are shown in the Figs. 8 and 9, respectively. It is observed that for each sample, the dielectric loss decreases substantially with increasing frequency and reaches nearly a constant value. When the frequency of the applied AC electric field is much smaller, the hopping between Zn 2? and Fe 3? predominates and the electrons follow the field. Thus large value of e 00 and hence tan d is observed at low frequency. As frequency increases hopping probability per unit time decreases continuously. This is probably due to the fact that the deep donor Fe 2? cannot follow the applied high frequency which in turn reduces hopping between Zn 2? and Fe 3?. Thus at higher frequencies of the field, the loss is minimum. Figures 10(a) (c) show variation of e 0 ; e 00 and tan d with in ZnO. It is observed that at a particular Fig. 9 Variation of dielectric loss factor (tan d) with frequency for frequency, e 0 ; e 00 and tan d values decrease with increase in. It is known that, in ZnO-based semiconductors the mechanism of electric conduction is similar to that for dielectric polarization. At lower concentration of Fe, the hopping mechanism between in Fe 3? and Zn 2? is predominant which results in higher value of e 0 and hence e 00 and tan d values. As the concentration of Fe is increased, Fe in ZnO acts as a deep donor and decreases the concentration of intrinsic donors. It impedes the conduction mechanism and does not contribute to the conduction process but provides the impedance to it and hence reduction in the dielectric constants values. The density of a material also plays an important role in the variation of dielectric constant. High porosity and low density results in low dielectric constant and dielectric losses [30]. It is also clear from the observed SEM figures that, Fe doped samples exhibit high porosity and hence e 0 ; e 00 and tan d have lower values.

6 M L Dinesha et al. (a) Dielectric constant (ε') K 10K 50K 1M 5M log σ ac (b) Dielectric loss (ε'') (c) tan δ K 10K 50K 1M 5M 1K 10K 50K 1M 5M Fig. 10 (a) Variation of dielectric constant (e 0 ) with for. (b) Variation of dielectric loss (e 00 ) with Fe concentration for. (c) Variation of dielectric loss factor (tan d) with for -6.0 The variation of AC electrical conductivity (r ac ) with frequency at room temperature is shown in Fig. 11. Itis clear that the AC electrical conductivity increases with the increase in frequency. As frequency increases, hopping between charge carriers increases which results an increase in AC conductivity value of all samples. At any frequency, conductivity is found to decrease with the increase in Fe concentration. The increase of concentration decreases the concentration of intrinsic donors. Thus hopping between Zn 2? and Fe 3? is not so ease at higher concentration of Fe. In other words Fe doping ceases hopping mechanism and hence results in a decrease in the AC electrical conductivity with increase of. 4. Conclusions Single phase Fe doped ZnO metal oxides in nanoscale were prepared by solution combustion method and structural and dielectric behavior is studied. The e 0 ; e 00 ; tan d and r ac values were found to decrease with increase of. The Fe doping depresses the concentration of intrinsic donor and impeded the conduction mechanism. Also with increase of frequency, the e 0 ; e 00 and tan d were found to decrease whereas r ac value was found to increase. This behavior was attributed to different hopping mechanisms and defects formed during synthesis. References Fig. 11 Variation of AC electrical conductivity with frequency for [1] S M Prokes and K L Wang Mater. Res. Sci. Bull (1999) [2] J Hu, T W Odom and C M Lieber Acc. Chem. Res (1999)

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