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Investigation on structural, optical and dielectric properties of Co doped ZnO nanoparticles synthesized by gel-combustion route
Materials Science and Engineering B 177 (2012) 428–435
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Investigation on structural, optical and dielectric properties of Co doped ZnO nanoparticles synthesized by gel-combustion route Sajid Ali Ansari, Ambreen Nisar, Bushara Fatma, Wasi Khan ∗ , A.H. Naqvi Centre of Excellence in Materials Science (Nanomaterials), Department of Applied Physics, Z.H. College of Engg. & Tech., Aligarh Muslim University, Aligarh 202 002, India
a r t i c l e
i n f o
Article history: Received 18 October 2011 Received in revised form 22 January 2012 Accepted 24 January 2012 Available online 5 February 2012 Keywords: ZnO nanoparticles Gel-combustion XRD Dielectric constants Impedance spectroscopy TEM
a b s t r a c t We report the synthesis of Co doped ZnO nanoparticles by combustion method using citric acid as a fuel for 0%, 1%, 3%, 5% and 10% of Co doping. The structural, optical and dielectric properties of the samples were studied. Crystallite sizes were obtained from the X-ray diffraction (XRD) patterns whose values are decreasing with increase in Co content up to 5%. The XRD analysis also ensures that ZnO has a hexagonal (wurtzite) crystal structure and Co2+ ions were successfully incorporated into the lattice positions of Zn2+ ions. The TEM image shows the average particle size in the range of 10–20 nm for 3% Co doped ZnO nanoparticles. The energy band gap as obtained from the UV–visible spectrophotometer was found gradually increasing up to 5% of Co doping. The dielectric constants (ε , ε ), dielectric loss (tan ı) and ac conductivity ( ac ) were studied as the function of frequency and composition, which have been explained by ‘Maxwell Wagner Model’. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor is a very important topic and several researchers have got interest in the doped and co-doped semiconductor nanocrystals, semiconductor–dielectric nanocomposites, semiconductor–polymer nanocomposites, two-component nanocomposites, etc. ZnO was selected as a model system and is versatile n-type direct wide band II–VI semiconductor having band gap of ∼3.37 eV, has attracted enormous attention because of potential applications in electro-optic, acousto-optic, ultraviolet (UV) light emitters, chemical sensors, piezoelectric materials and high power optoelectronic devices [1–4]. With proper doping such as aluminum and gallium, ZnO can be made electrically conductive and transparent in the visible spectral region such that it can be used as the transparent conductive electrodes in solar cells and flat panel displays [5–7]. It is well known from present semiconductor technologies that the incorporation of impurities or defects into semiconductor lattices is the primary means of controlling electrical conductivity, and may also have an immense effect on the optical, luminescent, magnetic, or other physical properties of the semiconductor. For example, cadmium (Cd) doping can decrease the band gap to as low as ∼3.0 eV, whereas magnesium (Mg) doping can increase the band gap to as high as ∼4.0 eV and pure stoichiometric ZnO is an
∗ Corresponding author. Tel.: +91 571 2700042; fax: +91 571 2700042. E-mail address: [email protected] (W. Khan). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.01.022
insulator, the conductivity of ZnO can be turned over 10 orders of magnitude with only relatively small changes in the concentrations of native or non-native defects such as interstitial zinc or aluminum. A dielectric material has an arrangement of electric charge carriers that can be displaced by an electric field. The charges become polarized to compensate for the electric field such that the positive and negative charges move in opposite directions. The materials of high dielectric constant find numerous applications in microelectronics. Dielectric behavior is one of the most important properties of material, which markedly depends on the preparation condition. Many researchers have investigated the dielectric properties of ZnO system [8,9]. AC measurements are important means for studying the dynamic properties, like conductance, capacitance, dielectric constant and dielectric loss tangent of the semiconducting and dielectric materials. They provide information about the interior of the material in the region of relatively low conductivity. Moreover, ZnO is one of the potential candidate in the field of optoelectronics due to its high excitonic binding energy (∼60 meV), which depends on the dielectric constant of the materials. A reduction in dielectric constant, which is observed here, increases coulomb interaction energy between electron and holes and it may cause an enhancement in excitonic binding energy. A very elegant experimental method to study transport in grain interior (bulk) and at interfaces is impedance spectroscopy, which has emerged over the past several years as a powerful technique for the electrical characterization. In the present paper we report the synthesis of Zn1−x Cox O nanoparticles by a low-temperature gel-combustion method and
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characterized by using XRD, FETEM, EDS, UV–visible and LCR measurements. Present method of sample preparation ensures high chemical homogeneity due to the aqueous solution mixture of initial reagents and, as a result, favors the production of desired phases. Among the soft-chemistry techniques, gel-combustion method requires relative shorter reaction times and lower temperatures. At present, this method has been used in organic and inorganic synthesis [10]. Due to the large number of publications, it is not possible to provide an exhaustive overview; instead we have chosen just a representative number of instructive literature examples to elucidate some of the major aspects of Co doped ZnO. In addition, we will present theoretical physical models, which describe and explain conditions in obtaining dielectric properties. Our main interest in Co doped ZnO is to understand the effect of grain size on its electrical properties when the grains are small enough to be depleted by traps on the grain surfaces and at the grain boundaries. 2. Experimental details The Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) nanoparticles were synthesized by a gel-combustion route. All chemicals used in this work were analytical grade reagents without any further purification. In typical synthesis process, the required molar ratios of Zn(NO3 )2 ·6H2 O, Co(NO3 )2 ·6H2 O, and citric acid (C6 H8 O7 ) were completely dissolved in a 100 ml beaker to obtain a 50 ml aqueous solution. The aqueous solution was then stirred for about 2 h at 120 ◦ C in order to mix the solution uniformly and evaporates under constant stirring. When the water was completely evaporated, the solution then converts into gel form. The gel was subsequently swelling into foam like and undergoes a strong self-propagating combustion reaction to give a fine powder. This fine powder was grinded for 30 min and annealed at 600 ◦ C for 3 h to improve the ordering.
Fig. 1. XRD patterns of pure and Co doped ZnO nanoparticles. The inset displays the shifting and broadening of (0 0 2) and (1 0 1) peaks for increasing Co content.
3. Characterization techniques
˚ ˇ is the full where is the X-ray wavelength (Cu K␣ = 1.5418 A), width at half maximum of the most intense peak and is the peak position [13]. Using above equation we evaluated crystallite size for different samples and listed in Table 1. It is evident that the FWHM gradually increases with the increase in doping content up to 5% while it lowers for the 10% Co concentration, which intern decreases the crystallite size and unit cell volume up to the same content of Co. It depends on the reason that the growth of ZnO grains is determined by the movement and the diffusion of Zn2+ or Znx i . However, in the Co doped ZnO, Co may exist as the grain boundary which can enhance the energy barrier of the movement and the diffusion of Zn2+ or Znx i and increases the electrical resistivity, therefore restrain the growth of ZnO grains [14,15]. Doped Co is acting as an electrical dopant at initial doping concentration but as an impurity at higher doping concentration. Existence of impurities increases the electrical conductivity because it can diffuse [16]. Thus, favors the phenomenon of grain growth in case of 10% Co doped ZnO.
The calcined nanopowders were characterized for crystal phase identification by X-ray diffraction (XRD) in the 2 range of 20–80◦ ˚ operated (Rigaku Miniflex II) with Cu K␣ radiations ( = 1.5418 A) at voltage of 30 kV and current of 15 mA. Microstructural analysis of 3% Co doped sample was done using a field-emission electron microscope (FETEM-JEM 2100F). UV–visible spectra of Co doped ZnO nanopowders were performed in the range 250–800 nm using Perkin Elmer Spectrophotometer. Dielectric and impedance spectroscopy measurements were carried out in frequency range of 75 kHz to 7 MHz using LCR meter (Model: Agilent-4285A). The pellets as obtained were coated on the adjacent faces with the silver paste, thereby forming geometry of parallel plate capacitor. 4. Results and discussion 4.1. Structural and morphological analysis Fig. 1 shows the XRD patterns of undoped and Co doped ZnO nanocrystalline powders for different Co concentrations, sintered in air at 600 ◦ C for 3 h. The patterns were indexed using PowderX software and all peaks were well matched with hexagonal structure of ZnO using the standard data (JCPDS-36-1451) [11], without any other impure phases regardless of dopant concentrations. Thus, the wurtzite structure not modified by the addition of Co ion into the ZnO matrix indicates that the Co dopant ought to be incorporated into the lattice as substitutional ion [12]. In order to study the effect of Co doping, a careful analysis of the XRD peaks indicate that there is a significant shifting and broadening in (0 0 2) and (1 0 1) peaks
position toward higher 2 value with increasing of Co content as shown in the inset of Fig. 1. No considerable changes in the lattice parameters are found for different Co doping concentrations but to be reported it is decreasing up to 5% of Co concentration and further increases for 10%. Since the ionic radius of Co2+ is close to that of Zn2+ , the change in full width at half maxima (FWHM) is due to particle size variation. Changes in the FWHM are in accordance with the particle size which was calculated from the Debye–Scherrer’s formula. D=
0.9 ˇ cos
Table 1 Variation of crystallite size, lattice parameters, unit cell volume and optical band gap with doping concentration. Co conc. (%)
0 1 3 5 10
FWHM
0.5201 0.5326 0.5431 0.5536 0.4312
D (nm)
16.0 15.7 15.4 15.1 19.3
Eg (eV)
3.50 3.55 3.56 3.57 3.46
Lattice constant (Å) a=b
c
3.300 3.299 3.298 3.295 3.301
5.203 5.202 5.202 5.201 5.204
Unit cell volume (Å3 )
49.069 49.030 49.000 48.902 49.108
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Fig. 2. EDS spectra of 3% Co doped ZnO nanoparticles. Insets show the FETEM image and histogram of particle size distribution for the same.
The morphological and structural studies were investigated using FETEM and displayed in the inset of Fig. 2 for 3% Co doped ZnO. This image clearly exhibits a hexagonal shape of the particles with highly monodispersity. The average particle size of these nanoparticles were estimated by considering the minimum and maximum diameter of large number of particles and the particle size are found to be in the range of 10–20 nm as shown in the inset of Fig. 2 by particle distribution in histogram. To check the chemical composition of the material, an energy dispersive X-ray (EDX) spectroscopy analysis was performed. Fig. 2 shows the EDS spectra of 3% Co doped ZnO sample, which confirms the presence of Zn and Co ions in the matrix. These results are consistent with the XRD data. 4.2. Optical properties UV–visible absorption spectroscopy is a powerful technique to explore the optical properties of semiconducting nanoparticles. The absorbance is expected to depend on several factors such as band gap, oxygen deficiency, surface roughness and impurity centers [17]. The UV–visible spectra have been shown in Fig. 3, which attributes that strong UV absorption is characteristic of all measured samples, which attains a plateau above 380 nm. The optical band gap of the nanopowders was determined by applying the Tauc relationship [18] as given below: ˛h = B(h − Eg )
up to 5% Co doping, whereas it decreases for 10% Co content sample as shown in Table 1. This is in good agreement with the quantum confinement effects of nanoparticles [19]. The optical absorption spectra and the calculated band gap as they are relatively higher as compared to their bulk counterpart exhibits a blue shift in the absorption band edge which could be attributed to well known quantum size effect of semiconductor indicating that the band gap increases as the particle size approaches to nano regime. Therefore the increase in band gap in the present study can also be explained on the basis of decrease in the lattice parameters for the Co content up to 5% which is due to ˚ as compared to Zn2+ (ionic smaller size of Co2+ (ionic radii ∼72 A) ˚ but as the doping of Co is made 10%, the band gap radii ∼74 A) decreases as the lattice parameter increases. The increase in the band gap can also be studied on the basis of Moss–Burstein effect [20]. When Fermi level shifts close to the conduction band due
n
where ˛ is the absorption coefficient (˛ = 2.303A/t, here A is the absorbance and t is the thickness of the cuvett), B is a constant, h is Planck’s constant, is the photon frequency, and Eg is the optical band gap. The value of n = 1/2, 3/2, 2 or 3 depending on the nature of the electronic transition responsible for absorption and n = 1/2 for direct band gap semiconductor. An extrapolation of the linear region of a plot of (˛ h)2 on the Y-axis versus photon energy (h) on the X-axis gives the value of the optical band gap (Eg ) as shown in Fig. 4 and tabulated in Table 1 for all samples. The calculated band gap of nanopowder of ZnO was found to be 3.50 eV while it is 3.37 eV in case of the bulk ZnO which shows blue shift. It is evident from the graphs that the direct band gap for all samples increasing gradually
Fig. 3. Absorption spectra of pure and Co doped ZnO.
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Fig. 4. Plots of (˛ hv)2 versus photon energy (hv) for all samples.
to increase in carrier concentration, the lower energy transitions are blocked and the value of the band gap increases for doping up to 5%. Further the decrease in band gap when concentration level increases to 10% may be due to formation of new orbitals of Co mixed and to the transitions between partially forbidden valence band to conduction band.
It is clear that all composition exhibits dielectric dispersion as ε and ε values of the samples show a decrease with increase in frequency. The value of ε decreases faster than ε and becomes closer in the high frequency range. The decrease in dielectric constant is
4.3. Dielectric properties 4.3.1. Dielectric constant The dielectric constant is represented by: ε = ε − jε The first term is the real part of dielectric constant and describes the stored energy while the second term is the imaginary part of dielectric constant, which describes the dissipated energy. The effects of frequency on dielectric constants (real & imaginary) for all samples are shown in Figs. 5 and 6. The dielectric constants ε and ε of the materials have been calculated by the relation: Cp t Aεo
(1)
ε = ε × loss
(2)
ε =
Fig. 5. Variation of real part of dielectric constant with frequency and composition.
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Fig. 6. Variation of imaginary part of dielectric constant with frequency and composition.
rapid at low frequency and becomes slow at higher frequencies, approaching to frequency independent behavior. This result holds good enough as we increase the doping of Co from 0% to 10% [21]. The dielectric dispersion curve can be explained on the basis of Koop’s theory [22], which is based on the Maxwell–Weigner model for the homogeneous double structure [23,24]. According to this model, a dielectric medium is assumed to be made of well conducting grains which are separated by poorly conducting (or resistive) grain boundaries. Under the application of external electric field, the charge carriers can easily migrate the grains but are accumulated at the grain boundaries. This process can produce large polarization and high dielectric constant. The small conductivity of grain boundary contributes to the high value of dielectric constant at low frequency. The higher value of dielectric constant can also be explained on the basis of interfacial/space charge polarization due to inhomogeneous dielectric structure. The inhomogeneities present in the system may be porosity and grain structure. The polarization decreases with the increase in frequency and then reaches a constant value due to the fact that beyond a certain frequency of external field the hopping between different metal ions (Zn2+ , Co2+ ) cannot follow the alternating field. The large value of dielectric constant at lower frequency is due to the predominance of the effect like grain boundary defects, presence of oxygen vacancies, etc. [25], while the decrease in dielectric constant with frequency is natural because of the fact that any species contributing to polarizability is found to show lagging behind the applied field at higher and higher frequencies [26].
4.3.2. Dielectric losses Loss tangent or loss factor (tan ı) represents the energy dissipation in the dielectric system. It is considered to be caused by domain wall resonance. At higher frequencies the losses are found to be low since domain wall motion is inhibited and magnetization is forced to change rotation. Fig. 7 shows the variation in dielectric loss factor with frequency at room temperature. It is observed that tan ı decreases with the increase in frequency for all the compositions which may be due to the space charge polarization. According, to relation (2) tan ı (loss) is proportional to the imaginary part of dielectric constant, so exhibits similar dispersion behavior. It is also noticeable that loss is maximum when there is no doping
Fig. 7. Variation of dielectric loss with frequency and composition.
but it decreases when Co is incorporated into ZnO and gradually decreases in the higher frequency regime. Hence, we can conclude that these Co doped samples show the capability to be used in high frequency device applications. None of the plot display small peaks in the higher frequency region which represents the relaxation processes or loss peaks. The peaking behavior occurs when the migrating (hopping) frequency of the localized electric charge carrier approximately equals to that of frequency of the applied ac field. 4.3.3. AC conductivity Fig. 8 shows the variation in ac conductivity with frequency for different compositions at room temperature. The ac conductivity gradually increases for all compositions with the increase in frequency. Generally, the total conductivity [27] is the summation of the band and the hopping parts: tot. = 0 (T ) + (ω, T ) Here the first term in the R.H.S. is dc conductivity due to the band conduction which is frequency independent. The second term is
Fig. 8. Variation of ac conductivity with frequency for all compositions.
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Fig. 9. Logarithmic variance of ac conductivity with angular frequency for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
the pure ac conductivity due to migration of electric charge carriers between the metal ions. It has been reported that the ac conductivity gradually increases with the increase in frequency of applied ac field because the increase in frequency enhances the migration of electron. It is also clear from Fig. 8 that conductivity is maximum for the case of pure ZnO and found to be less with the doping of Co ions into ZnO matrix. This may be attributed to the fact that the dopant of Co introduces the defect ions (such as zinc interstitials and oxygen vacancies) in the ZnO system. These defects tend to segregate at the grain boundaries due to diffusion process resulting from sintering and cooling processes. Thus, doping increases the defect ions which facilitates the formation of grain boundary defect barrier leading to blockage to the flow of charge carriers. This in turn decreases the conductivity of the system on doping. Fig. 9 shows the variation of ln ac versus ln ω from where the slopes can also be easily calculated. 4.3.4. Impedance analysis There are two methods that can be used to measure polarization: ac method and a dc method [28]. The polarization resistance determined from ac measurements could be different from dc measurements. An impedance spectroscopy method is widely used to characterize the electrical properties of materials and their interfaces with electronically conducting electrodes. This complex impedance spectroscopic technique enables us to separate the resistive (real) and reactive (imaginary) components of the electrical parameters and hence provides a clear picture of the material properties. The complex formalism of the impedance is given by the relation: Z ∗ = Z − iZ = Rs −
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Fig. 10. Variation of resistive part of impedance with frequency for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
Fig. 10 shows the variation of the resistive part of impedance (Z ) with frequency. It has been clearly viewed from the pattern itself that Z decreases with the increase in frequency for all the compositions. The decrement in the real part of impedance (Z ) with the rise in frequency may be due to the increase in ac conductivity with rise in frequency. It is observed that Z has higher values at lower frequencies and decreases monotonically with the rise in frequency and attains a constant value at higher frequency part. Since Z has strong frequency dependence in the lower regime then this could be attributed to the fact that low frequency region corresponds to high resistivity due to effectiveness of the resistive grain boundaries in this region and shows independent behavior in the higher frequency region. Z increases with doping due to increase in barrier height explained later. Fig. 11 shows the variation of reactive part of impedance (Z ) with frequency, which indicates the same behavior as that of Z . The Z increases with the increase in dopant concentration due to capacitance of grain boundary (Cgb ) as
1 jωCs
The electrical phenomenon due to bulk material, grain boundary and interfacial phenomenon appears in the form of arc of a semicircle, when components of impedance are plotted in a complex argand planes (Nyquist plots). This plot can also be displayed in terms of any of the four possible complex formalisms, the permittivity (ε*), the admittance (Y*), the electric modulus (M*), the impedance (Z*) and dielectric loss (tan ı) or dissipation factor. They are related to one another [29] as follows: tan ı =
ε M Z Y = = = ε M Z Y
Fig. 11. Variation of reactive part of impedance with frequency for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
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Table 2 Variation of different impedance parameters as a function of doping concentration. Co conc. (%)
Rgb ()
Cgb (nF)
gb (×10−6 s)
0 1 3 5 10
156.73 135.81 278.24 368.10 515.38
11.77 13.81 7.30 5.52 3.99
1.844 1.876 2.031 2.032 2.056
tabulated in Table 2. This can be attributed to the fact that Z is inversely proportional to capacitance by the relation: 1 jωC
Z =
Moreover, Z and Z are given by the relations [30]. Z =
Rg 1 + (ωg2 Cg2 Rg2 )
Z =
Rg2 ωg Cg 1 + (ωg2 Cg2 Rg2 )
+
+
Rgb 2 C 2 R2 ) 1 + (ωgb gb gb 2 ω C Rgb gb gb 2 C 2 R2 ) 1 + (ωgb gb gb
where Rg , Rgb , Cg , Cgb are the resistance and capacitance of grain and grain boundary respectively, while ωg and ωgb are the frequencies at the peaks of the circular arc for the grain and grain boundary respectively. The capacitance and the relaxation times ( g , gb ) can be calculated for the grain and grain boundary by the relations: Cg =
Cgb =
1 Rg ωg 1 Rgb ωgb
g = Rg Cg gb = Rgb Cgb These parameters are obtained by analyzing the impedance data on nonlinear least square (NLLS) fit method and listed in Table 2, which exhibits that grain boundary resistance Rgb increases while the capacitance Cgb decreases with increase in dopant content (except for 0% Co doping). This is due to introduction of Co ion in ZnO matrix, which increases the defect ions concentration tending to segregation at grain boundaries forming grain boundary defect barriers [31]. As dopant increases barrier height increase and Cgb is inversely proportional to barrier height given in relation below, [32] so it decreases with increase in Co content. Cgb =
Fig. 12. Nyquist plots for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
to the flow of electrons. Smaller grains also imply a smaller graingrain surface contact area and therefore, a reduced electron flow. In pure ZnO, the grain size is maximum and decreases with increasing Co content, resulting in larger number of grain boundaries in all samples. Therefore the grain boundary contribution becomes dominant and grain contribution is not resolved. Due to this fact, the single circular arc is being observed in Cole–Cole plots. Moreover, it can be seen from Fig. 12 that total impedance increases with doping content which is in well agreement with conductivity analysis as the conductivity decreases with increase in Co content and is maximum for pure ZnO.
5. Conclusion
n1/2 1/2
˚B
Here Cgb is the grain boundary capacitance, n is the concentration of charge carriers and ˚B is the barrier height. Fig. 12 shows the complex impedance plots of pure and Co doped ZnO. Generally, the grains are effective in high frequency region while the grain boundaries are effective in low frequency region. Thus the circular arc appearing in the high frequency region corresponds to grain contribution while low frequency region corresponds to grain boundary contribution [33]. It is evident from the plot itself that all the samples show single circular arc behavior, which suggests the predominance of grain boundary resistance over the grain resistances in these samples. It has been effectively discussed in the literature that the resistivity of a polycrystalline material in general, increases with decreasing grain size [34,35]. Accordingly, smaller grains imply a larger number of insulating grain boundaries which act as a barrier
We reported structural, optical, dielectric and impedance properties of pure and Co2+ modified ZnO compounds, prepared by gel-combustion method using citric acid as the fuel. Structural studies suggest that the crystal system of parent compound remains same (hexagonal crystal system) even up to 10% Co incorporation. The crystallite size decreases with the increase in Co concentration up to 5% Co doping. The TEM image shows the average particle size in the range of 10–20 nm for 3% Co doped ZnO nanoparticles. Energy band gap varies from 3.46 eV to 3.57 eV with Co doping and all values are higher as compared to bulk ZnO (3.37 eV). The dielectrics studies revealed that the dielectric constants and dielectric loss exhibits the normal dielectric behavior, i.e. decreases with increase in frequency and concentration which has been explained in the light of Maxwell–Wagner model. AC conductivity increases with increase in frequency. Complex impedance analysis shows single circular arc corresponding to pure and Co doped ZnO, suggesting the dominance of grain boundary resistance in all the samples.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Investigation on structural, optical and dielectric properties of Co doped ZnO nanoparticles synthesized by gel-combustion route Sajid Ali Ansari, Ambreen Nisar, Bushara Fatma, Wasi Khan ∗ , A.H. Naqvi Centre of Excellence in Materials Science (Nanomaterials), Department of Applied Physics, Z.H. College of Engg. & Tech., Aligarh Muslim University, Aligarh 202 002, India
a r t i c l e
i n f o
Article history: Received 18 October 2011 Received in revised form 22 January 2012 Accepted 24 January 2012 Available online 5 February 2012 Keywords: ZnO nanoparticles Gel-combustion XRD Dielectric constants Impedance spectroscopy TEM
a b s t r a c t We report the synthesis of Co doped ZnO nanoparticles by combustion method using citric acid as a fuel for 0%, 1%, 3%, 5% and 10% of Co doping. The structural, optical and dielectric properties of the samples were studied. Crystallite sizes were obtained from the X-ray diffraction (XRD) patterns whose values are decreasing with increase in Co content up to 5%. The XRD analysis also ensures that ZnO has a hexagonal (wurtzite) crystal structure and Co2+ ions were successfully incorporated into the lattice positions of Zn2+ ions. The TEM image shows the average particle size in the range of 10–20 nm for 3% Co doped ZnO nanoparticles. The energy band gap as obtained from the UV–visible spectrophotometer was found gradually increasing up to 5% of Co doping. The dielectric constants (ε , ε ), dielectric loss (tan ı) and ac conductivity ( ac ) were studied as the function of frequency and composition, which have been explained by ‘Maxwell Wagner Model’. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor is a very important topic and several researchers have got interest in the doped and co-doped semiconductor nanocrystals, semiconductor–dielectric nanocomposites, semiconductor–polymer nanocomposites, two-component nanocomposites, etc. ZnO was selected as a model system and is versatile n-type direct wide band II–VI semiconductor having band gap of ∼3.37 eV, has attracted enormous attention because of potential applications in electro-optic, acousto-optic, ultraviolet (UV) light emitters, chemical sensors, piezoelectric materials and high power optoelectronic devices [1–4]. With proper doping such as aluminum and gallium, ZnO can be made electrically conductive and transparent in the visible spectral region such that it can be used as the transparent conductive electrodes in solar cells and flat panel displays [5–7]. It is well known from present semiconductor technologies that the incorporation of impurities or defects into semiconductor lattices is the primary means of controlling electrical conductivity, and may also have an immense effect on the optical, luminescent, magnetic, or other physical properties of the semiconductor. For example, cadmium (Cd) doping can decrease the band gap to as low as ∼3.0 eV, whereas magnesium (Mg) doping can increase the band gap to as high as ∼4.0 eV and pure stoichiometric ZnO is an
∗ Corresponding author. Tel.: +91 571 2700042; fax: +91 571 2700042. E-mail address: [email protected] (W. Khan). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.01.022
insulator, the conductivity of ZnO can be turned over 10 orders of magnitude with only relatively small changes in the concentrations of native or non-native defects such as interstitial zinc or aluminum. A dielectric material has an arrangement of electric charge carriers that can be displaced by an electric field. The charges become polarized to compensate for the electric field such that the positive and negative charges move in opposite directions. The materials of high dielectric constant find numerous applications in microelectronics. Dielectric behavior is one of the most important properties of material, which markedly depends on the preparation condition. Many researchers have investigated the dielectric properties of ZnO system [8,9]. AC measurements are important means for studying the dynamic properties, like conductance, capacitance, dielectric constant and dielectric loss tangent of the semiconducting and dielectric materials. They provide information about the interior of the material in the region of relatively low conductivity. Moreover, ZnO is one of the potential candidate in the field of optoelectronics due to its high excitonic binding energy (∼60 meV), which depends on the dielectric constant of the materials. A reduction in dielectric constant, which is observed here, increases coulomb interaction energy between electron and holes and it may cause an enhancement in excitonic binding energy. A very elegant experimental method to study transport in grain interior (bulk) and at interfaces is impedance spectroscopy, which has emerged over the past several years as a powerful technique for the electrical characterization. In the present paper we report the synthesis of Zn1−x Cox O nanoparticles by a low-temperature gel-combustion method and
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characterized by using XRD, FETEM, EDS, UV–visible and LCR measurements. Present method of sample preparation ensures high chemical homogeneity due to the aqueous solution mixture of initial reagents and, as a result, favors the production of desired phases. Among the soft-chemistry techniques, gel-combustion method requires relative shorter reaction times and lower temperatures. At present, this method has been used in organic and inorganic synthesis [10]. Due to the large number of publications, it is not possible to provide an exhaustive overview; instead we have chosen just a representative number of instructive literature examples to elucidate some of the major aspects of Co doped ZnO. In addition, we will present theoretical physical models, which describe and explain conditions in obtaining dielectric properties. Our main interest in Co doped ZnO is to understand the effect of grain size on its electrical properties when the grains are small enough to be depleted by traps on the grain surfaces and at the grain boundaries. 2. Experimental details The Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) nanoparticles were synthesized by a gel-combustion route. All chemicals used in this work were analytical grade reagents without any further purification. In typical synthesis process, the required molar ratios of Zn(NO3 )2 ·6H2 O, Co(NO3 )2 ·6H2 O, and citric acid (C6 H8 O7 ) were completely dissolved in a 100 ml beaker to obtain a 50 ml aqueous solution. The aqueous solution was then stirred for about 2 h at 120 ◦ C in order to mix the solution uniformly and evaporates under constant stirring. When the water was completely evaporated, the solution then converts into gel form. The gel was subsequently swelling into foam like and undergoes a strong self-propagating combustion reaction to give a fine powder. This fine powder was grinded for 30 min and annealed at 600 ◦ C for 3 h to improve the ordering.
Fig. 1. XRD patterns of pure and Co doped ZnO nanoparticles. The inset displays the shifting and broadening of (0 0 2) and (1 0 1) peaks for increasing Co content.
3. Characterization techniques
˚ ˇ is the full where is the X-ray wavelength (Cu K␣ = 1.5418 A), width at half maximum of the most intense peak and is the peak position [13]. Using above equation we evaluated crystallite size for different samples and listed in Table 1. It is evident that the FWHM gradually increases with the increase in doping content up to 5% while it lowers for the 10% Co concentration, which intern decreases the crystallite size and unit cell volume up to the same content of Co. It depends on the reason that the growth of ZnO grains is determined by the movement and the diffusion of Zn2+ or Znx i . However, in the Co doped ZnO, Co may exist as the grain boundary which can enhance the energy barrier of the movement and the diffusion of Zn2+ or Znx i and increases the electrical resistivity, therefore restrain the growth of ZnO grains [14,15]. Doped Co is acting as an electrical dopant at initial doping concentration but as an impurity at higher doping concentration. Existence of impurities increases the electrical conductivity because it can diffuse [16]. Thus, favors the phenomenon of grain growth in case of 10% Co doped ZnO.
The calcined nanopowders were characterized for crystal phase identification by X-ray diffraction (XRD) in the 2 range of 20–80◦ ˚ operated (Rigaku Miniflex II) with Cu K␣ radiations ( = 1.5418 A) at voltage of 30 kV and current of 15 mA. Microstructural analysis of 3% Co doped sample was done using a field-emission electron microscope (FETEM-JEM 2100F). UV–visible spectra of Co doped ZnO nanopowders were performed in the range 250–800 nm using Perkin Elmer Spectrophotometer. Dielectric and impedance spectroscopy measurements were carried out in frequency range of 75 kHz to 7 MHz using LCR meter (Model: Agilent-4285A). The pellets as obtained were coated on the adjacent faces with the silver paste, thereby forming geometry of parallel plate capacitor. 4. Results and discussion 4.1. Structural and morphological analysis Fig. 1 shows the XRD patterns of undoped and Co doped ZnO nanocrystalline powders for different Co concentrations, sintered in air at 600 ◦ C for 3 h. The patterns were indexed using PowderX software and all peaks were well matched with hexagonal structure of ZnO using the standard data (JCPDS-36-1451) [11], without any other impure phases regardless of dopant concentrations. Thus, the wurtzite structure not modified by the addition of Co ion into the ZnO matrix indicates that the Co dopant ought to be incorporated into the lattice as substitutional ion [12]. In order to study the effect of Co doping, a careful analysis of the XRD peaks indicate that there is a significant shifting and broadening in (0 0 2) and (1 0 1) peaks
position toward higher 2 value with increasing of Co content as shown in the inset of Fig. 1. No considerable changes in the lattice parameters are found for different Co doping concentrations but to be reported it is decreasing up to 5% of Co concentration and further increases for 10%. Since the ionic radius of Co2+ is close to that of Zn2+ , the change in full width at half maxima (FWHM) is due to particle size variation. Changes in the FWHM are in accordance with the particle size which was calculated from the Debye–Scherrer’s formula. D=
0.9 ˇ cos
Table 1 Variation of crystallite size, lattice parameters, unit cell volume and optical band gap with doping concentration. Co conc. (%)
0 1 3 5 10
FWHM
0.5201 0.5326 0.5431 0.5536 0.4312
D (nm)
16.0 15.7 15.4 15.1 19.3
Eg (eV)
3.50 3.55 3.56 3.57 3.46
Lattice constant (Å) a=b
c
3.300 3.299 3.298 3.295 3.301
5.203 5.202 5.202 5.201 5.204
Unit cell volume (Å3 )
49.069 49.030 49.000 48.902 49.108
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Fig. 2. EDS spectra of 3% Co doped ZnO nanoparticles. Insets show the FETEM image and histogram of particle size distribution for the same.
The morphological and structural studies were investigated using FETEM and displayed in the inset of Fig. 2 for 3% Co doped ZnO. This image clearly exhibits a hexagonal shape of the particles with highly monodispersity. The average particle size of these nanoparticles were estimated by considering the minimum and maximum diameter of large number of particles and the particle size are found to be in the range of 10–20 nm as shown in the inset of Fig. 2 by particle distribution in histogram. To check the chemical composition of the material, an energy dispersive X-ray (EDX) spectroscopy analysis was performed. Fig. 2 shows the EDS spectra of 3% Co doped ZnO sample, which confirms the presence of Zn and Co ions in the matrix. These results are consistent with the XRD data. 4.2. Optical properties UV–visible absorption spectroscopy is a powerful technique to explore the optical properties of semiconducting nanoparticles. The absorbance is expected to depend on several factors such as band gap, oxygen deficiency, surface roughness and impurity centers [17]. The UV–visible spectra have been shown in Fig. 3, which attributes that strong UV absorption is characteristic of all measured samples, which attains a plateau above 380 nm. The optical band gap of the nanopowders was determined by applying the Tauc relationship [18] as given below: ˛h = B(h − Eg )
up to 5% Co doping, whereas it decreases for 10% Co content sample as shown in Table 1. This is in good agreement with the quantum confinement effects of nanoparticles [19]. The optical absorption spectra and the calculated band gap as they are relatively higher as compared to their bulk counterpart exhibits a blue shift in the absorption band edge which could be attributed to well known quantum size effect of semiconductor indicating that the band gap increases as the particle size approaches to nano regime. Therefore the increase in band gap in the present study can also be explained on the basis of decrease in the lattice parameters for the Co content up to 5% which is due to ˚ as compared to Zn2+ (ionic smaller size of Co2+ (ionic radii ∼72 A) ˚ but as the doping of Co is made 10%, the band gap radii ∼74 A) decreases as the lattice parameter increases. The increase in the band gap can also be studied on the basis of Moss–Burstein effect [20]. When Fermi level shifts close to the conduction band due
n
where ˛ is the absorption coefficient (˛ = 2.303A/t, here A is the absorbance and t is the thickness of the cuvett), B is a constant, h is Planck’s constant, is the photon frequency, and Eg is the optical band gap. The value of n = 1/2, 3/2, 2 or 3 depending on the nature of the electronic transition responsible for absorption and n = 1/2 for direct band gap semiconductor. An extrapolation of the linear region of a plot of (˛ h)2 on the Y-axis versus photon energy (h) on the X-axis gives the value of the optical band gap (Eg ) as shown in Fig. 4 and tabulated in Table 1 for all samples. The calculated band gap of nanopowder of ZnO was found to be 3.50 eV while it is 3.37 eV in case of the bulk ZnO which shows blue shift. It is evident from the graphs that the direct band gap for all samples increasing gradually
Fig. 3. Absorption spectra of pure and Co doped ZnO.
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Fig. 4. Plots of (˛ hv)2 versus photon energy (hv) for all samples.
to increase in carrier concentration, the lower energy transitions are blocked and the value of the band gap increases for doping up to 5%. Further the decrease in band gap when concentration level increases to 10% may be due to formation of new orbitals of Co mixed and to the transitions between partially forbidden valence band to conduction band.
It is clear that all composition exhibits dielectric dispersion as ε and ε values of the samples show a decrease with increase in frequency. The value of ε decreases faster than ε and becomes closer in the high frequency range. The decrease in dielectric constant is
4.3. Dielectric properties 4.3.1. Dielectric constant The dielectric constant is represented by: ε = ε − jε The first term is the real part of dielectric constant and describes the stored energy while the second term is the imaginary part of dielectric constant, which describes the dissipated energy. The effects of frequency on dielectric constants (real & imaginary) for all samples are shown in Figs. 5 and 6. The dielectric constants ε and ε of the materials have been calculated by the relation: Cp t Aεo
(1)
ε = ε × loss
(2)
ε =
Fig. 5. Variation of real part of dielectric constant with frequency and composition.
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Fig. 6. Variation of imaginary part of dielectric constant with frequency and composition.
rapid at low frequency and becomes slow at higher frequencies, approaching to frequency independent behavior. This result holds good enough as we increase the doping of Co from 0% to 10% [21]. The dielectric dispersion curve can be explained on the basis of Koop’s theory [22], which is based on the Maxwell–Weigner model for the homogeneous double structure [23,24]. According to this model, a dielectric medium is assumed to be made of well conducting grains which are separated by poorly conducting (or resistive) grain boundaries. Under the application of external electric field, the charge carriers can easily migrate the grains but are accumulated at the grain boundaries. This process can produce large polarization and high dielectric constant. The small conductivity of grain boundary contributes to the high value of dielectric constant at low frequency. The higher value of dielectric constant can also be explained on the basis of interfacial/space charge polarization due to inhomogeneous dielectric structure. The inhomogeneities present in the system may be porosity and grain structure. The polarization decreases with the increase in frequency and then reaches a constant value due to the fact that beyond a certain frequency of external field the hopping between different metal ions (Zn2+ , Co2+ ) cannot follow the alternating field. The large value of dielectric constant at lower frequency is due to the predominance of the effect like grain boundary defects, presence of oxygen vacancies, etc. [25], while the decrease in dielectric constant with frequency is natural because of the fact that any species contributing to polarizability is found to show lagging behind the applied field at higher and higher frequencies [26].
4.3.2. Dielectric losses Loss tangent or loss factor (tan ı) represents the energy dissipation in the dielectric system. It is considered to be caused by domain wall resonance. At higher frequencies the losses are found to be low since domain wall motion is inhibited and magnetization is forced to change rotation. Fig. 7 shows the variation in dielectric loss factor with frequency at room temperature. It is observed that tan ı decreases with the increase in frequency for all the compositions which may be due to the space charge polarization. According, to relation (2) tan ı (loss) is proportional to the imaginary part of dielectric constant, so exhibits similar dispersion behavior. It is also noticeable that loss is maximum when there is no doping
Fig. 7. Variation of dielectric loss with frequency and composition.
but it decreases when Co is incorporated into ZnO and gradually decreases in the higher frequency regime. Hence, we can conclude that these Co doped samples show the capability to be used in high frequency device applications. None of the plot display small peaks in the higher frequency region which represents the relaxation processes or loss peaks. The peaking behavior occurs when the migrating (hopping) frequency of the localized electric charge carrier approximately equals to that of frequency of the applied ac field. 4.3.3. AC conductivity Fig. 8 shows the variation in ac conductivity with frequency for different compositions at room temperature. The ac conductivity gradually increases for all compositions with the increase in frequency. Generally, the total conductivity [27] is the summation of the band and the hopping parts: tot. = 0 (T ) + (ω, T ) Here the first term in the R.H.S. is dc conductivity due to the band conduction which is frequency independent. The second term is
Fig. 8. Variation of ac conductivity with frequency for all compositions.
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Fig. 9. Logarithmic variance of ac conductivity with angular frequency for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
the pure ac conductivity due to migration of electric charge carriers between the metal ions. It has been reported that the ac conductivity gradually increases with the increase in frequency of applied ac field because the increase in frequency enhances the migration of electron. It is also clear from Fig. 8 that conductivity is maximum for the case of pure ZnO and found to be less with the doping of Co ions into ZnO matrix. This may be attributed to the fact that the dopant of Co introduces the defect ions (such as zinc interstitials and oxygen vacancies) in the ZnO system. These defects tend to segregate at the grain boundaries due to diffusion process resulting from sintering and cooling processes. Thus, doping increases the defect ions which facilitates the formation of grain boundary defect barrier leading to blockage to the flow of charge carriers. This in turn decreases the conductivity of the system on doping. Fig. 9 shows the variation of ln ac versus ln ω from where the slopes can also be easily calculated. 4.3.4. Impedance analysis There are two methods that can be used to measure polarization: ac method and a dc method [28]. The polarization resistance determined from ac measurements could be different from dc measurements. An impedance spectroscopy method is widely used to characterize the electrical properties of materials and their interfaces with electronically conducting electrodes. This complex impedance spectroscopic technique enables us to separate the resistive (real) and reactive (imaginary) components of the electrical parameters and hence provides a clear picture of the material properties. The complex formalism of the impedance is given by the relation: Z ∗ = Z − iZ = Rs −
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Fig. 10. Variation of resistive part of impedance with frequency for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
Fig. 10 shows the variation of the resistive part of impedance (Z ) with frequency. It has been clearly viewed from the pattern itself that Z decreases with the increase in frequency for all the compositions. The decrement in the real part of impedance (Z ) with the rise in frequency may be due to the increase in ac conductivity with rise in frequency. It is observed that Z has higher values at lower frequencies and decreases monotonically with the rise in frequency and attains a constant value at higher frequency part. Since Z has strong frequency dependence in the lower regime then this could be attributed to the fact that low frequency region corresponds to high resistivity due to effectiveness of the resistive grain boundaries in this region and shows independent behavior in the higher frequency region. Z increases with doping due to increase in barrier height explained later. Fig. 11 shows the variation of reactive part of impedance (Z ) with frequency, which indicates the same behavior as that of Z . The Z increases with the increase in dopant concentration due to capacitance of grain boundary (Cgb ) as
1 jωCs
The electrical phenomenon due to bulk material, grain boundary and interfacial phenomenon appears in the form of arc of a semicircle, when components of impedance are plotted in a complex argand planes (Nyquist plots). This plot can also be displayed in terms of any of the four possible complex formalisms, the permittivity (ε*), the admittance (Y*), the electric modulus (M*), the impedance (Z*) and dielectric loss (tan ı) or dissipation factor. They are related to one another [29] as follows: tan ı =
ε M Z Y = = = ε M Z Y
Fig. 11. Variation of reactive part of impedance with frequency for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
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Table 2 Variation of different impedance parameters as a function of doping concentration. Co conc. (%)
Rgb ()
Cgb (nF)
gb (×10−6 s)
0 1 3 5 10
156.73 135.81 278.24 368.10 515.38
11.77 13.81 7.30 5.52 3.99
1.844 1.876 2.031 2.032 2.056
tabulated in Table 2. This can be attributed to the fact that Z is inversely proportional to capacitance by the relation: 1 jωC
Z =
Moreover, Z and Z are given by the relations [30]. Z =
Rg 1 + (ωg2 Cg2 Rg2 )
Z =
Rg2 ωg Cg 1 + (ωg2 Cg2 Rg2 )
+
+
Rgb 2 C 2 R2 ) 1 + (ωgb gb gb 2 ω C Rgb gb gb 2 C 2 R2 ) 1 + (ωgb gb gb
where Rg , Rgb , Cg , Cgb are the resistance and capacitance of grain and grain boundary respectively, while ωg and ωgb are the frequencies at the peaks of the circular arc for the grain and grain boundary respectively. The capacitance and the relaxation times ( g , gb ) can be calculated for the grain and grain boundary by the relations: Cg =
Cgb =
1 Rg ωg 1 Rgb ωgb
g = Rg Cg gb = Rgb Cgb These parameters are obtained by analyzing the impedance data on nonlinear least square (NLLS) fit method and listed in Table 2, which exhibits that grain boundary resistance Rgb increases while the capacitance Cgb decreases with increase in dopant content (except for 0% Co doping). This is due to introduction of Co ion in ZnO matrix, which increases the defect ions concentration tending to segregation at grain boundaries forming grain boundary defect barriers [31]. As dopant increases barrier height increase and Cgb is inversely proportional to barrier height given in relation below, [32] so it decreases with increase in Co content. Cgb =
Fig. 12. Nyquist plots for Zn1−x Cox O (x = 0.0, 0.01, 0.03, 0.05 and 0.1) samples.
to the flow of electrons. Smaller grains also imply a smaller graingrain surface contact area and therefore, a reduced electron flow. In pure ZnO, the grain size is maximum and decreases with increasing Co content, resulting in larger number of grain boundaries in all samples. Therefore the grain boundary contribution becomes dominant and grain contribution is not resolved. Due to this fact, the single circular arc is being observed in Cole–Cole plots. Moreover, it can be seen from Fig. 12 that total impedance increases with doping content which is in well agreement with conductivity analysis as the conductivity decreases with increase in Co content and is maximum for pure ZnO.
5. Conclusion
n1/2 1/2
˚B
Here Cgb is the grain boundary capacitance, n is the concentration of charge carriers and ˚B is the barrier height. Fig. 12 shows the complex impedance plots of pure and Co doped ZnO. Generally, the grains are effective in high frequency region while the grain boundaries are effective in low frequency region. Thus the circular arc appearing in the high frequency region corresponds to grain contribution while low frequency region corresponds to grain boundary contribution [33]. It is evident from the plot itself that all the samples show single circular arc behavior, which suggests the predominance of grain boundary resistance over the grain resistances in these samples. It has been effectively discussed in the literature that the resistivity of a polycrystalline material in general, increases with decreasing grain size [34,35]. Accordingly, smaller grains imply a larger number of insulating grain boundaries which act as a barrier
We reported structural, optical, dielectric and impedance properties of pure and Co2+ modified ZnO compounds, prepared by gel-combustion method using citric acid as the fuel. Structural studies suggest that the crystal system of parent compound remains same (hexagonal crystal system) even up to 10% Co incorporation. The crystallite size decreases with the increase in Co concentration up to 5% Co doping. The TEM image shows the average particle size in the range of 10–20 nm for 3% Co doped ZnO nanoparticles. Energy band gap varies from 3.46 eV to 3.57 eV with Co doping and all values are higher as compared to bulk ZnO (3.37 eV). The dielectrics studies revealed that the dielectric constants and dielectric loss exhibits the normal dielectric behavior, i.e. decreases with increase in frequency and concentration which has been explained in the light of Maxwell–Wagner model. AC conductivity increases with increase in frequency. Complex impedance analysis shows single circular arc corresponding to pure and Co doped ZnO, suggesting the dominance of grain boundary resistance in all the samples.
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