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Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method
Optical Materials 34 (2012) 1946–1953
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Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method S. Muthukumaran a,⇑, R. Gopalakrishnan b a b
PG & Research Department of Physics, H.H. The Rajah’s College, Pudukkottai 622 001, Tamilnadu, India Department of Physics, Oxford Engineering College, Tiruchirapalli 620 009, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 26 January 2012 Received in revised form 30 May 2012 Accepted 6 June 2012 Available online 29 June 2012 Keywords: Cu doped ZnO nanoparticles Co-precipitation method FTIR Photoluminescence
a b s t r a c t Cu doped ZnO (Zn1xCuxO, x = 0, 0.02, 0.04 and 0.06) nanopowders have been synthesized by co-precipitation method and annealed at 500 °C for 2 h under Ar atmosphere. The synthesized samples have been characterized by powder X-ray diffraction, energy-dispersive analysis X-ray (EDAX) spectra, UV–Visible spectrophotometer and Fourier transform infrared (FTIR) spectroscopy. The XRD measurement reveals that the prepared nanoparticles have different microstructure without changing a hexagonal wurtzite structure. The calculated average crystalline size decreases from 22.24 to 15.93 nm for x = 0 to 0.04 then reaches 26.54 nm for x = 0.06 which is confirmed by SEM micrographs. The change in lattice parameters, micro-strain, a small shift and broadening in XRD peaks and the reduction in the energy gap from 3.49 to 3.43 eV reveals the substitution of Cu2+ ions into the ZnO lattice. Hydrogenation effect improves the crystal quality and optical properties. It is proposed that Cu doping concentration limit is below 6% (0.06) molar fraction which is supported by the detailed XRD analysis and the derived structural parameters. This Cu concentration limit was proposed as below 5% by previous studies. The presence of functional groups and the chemical bonding is confirmed by FTIR spectra. PL spectra of the Zn1xCuxO system show that the shift in near band edge (NBE) UV emission from 398 to 403 nm and a shift in green band (GB) emission from 527 to 522 nm which confirms the substitution of Cu into the ZnO lattice. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a direct band gap (3.36 eV at room temperature) n-type semiconductor with large exciton binding energy of 60 meV, which have the promising applications in optoelectronics, and photonics [1–4]. It has very attracted intensive research effort due to its unique properties and versatile applications in transparent electronics, hi-tech applications, ultraviolet (UV) light emitters, piezoelectric devices, chemical sensors and spintronics [5–12]. Recently, oxide based diluted magnetic semiconductors (DMSs) and Cu doped ZnO materials have attracted intensive interest by the following two facts. First, nano zinc oxide is identified as a promising semiconducting material for exhibiting room temperature ferromagnetism (RTFM) when doped with the transition metal elements [13]. Second, the microstructure and surface geometry of the material are modified by the addition of suitable dopant. The most commonly used metallic dopants in ZnO based systems are Al, Co, Cu, Ga, Sn, etc. [14–18]. Among the different metallic doping elements Cu is important because, (i) it is a prominent luminescence activator, which can modify the luminescence of ZnO crystals by creating localized impurity levels [19], (ii) it has many ⇑ Corresponding author. Tel.: +91 04322 221558; fax: +91 04322 230490. E-mail address: [email protected] (S. Muthukumaran). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.06.004
physical and chemical properties that are similar to those of Zn, and (iii) it can change the microstructure and the optical properties of the ZnO system [16]. Different physical or chemical synthetic methods have been used to prepare the ZnO nanoparticles such as thermal decomposition, thermolysis [20], chemical vapour deposition, sol–gel [21], spray pyrolysis, precipitation [22], vapour phase oxidation [23], thermal vapour transport, condensation [24], co-precipitation [25] and hydrothermal [26]. Among the different methods, the co-precipitation is one of the most important methods to prepare the nanopowder. Co-precipitation is the name given by analytical chemists to a phenomenon whereby the fractional precipitation of a specified ion in a solution results in the precipitation not only of the target ion but also of other ions existing side by side in the solution. Cu-doped ZnO nanomaterials permit tuning of chemical and physical properties by the incorporation of the dopant in lattices of ZnO [27]. The possibility of band gap engineering is reported by Chang et al. in the doped ZnO by the barrier layers which will facilitate radiative recombination by carrier confinement [28]. The control of properties for Cu-doped ZnO and band gap engineering of nanomaterials is of utmost importance for tunable light emitting diodes (LEDs) and other optoelectronic applications [29–31].
S. Muthukumaran, R. Gopalakrishnan / Optical Materials 34 (2012) 1946–1953
Cu doped ZnO nanopowders with 40 nm size have been synthesized by solution combustion method which showed a red shift and narrowing of band gap [32]. The resistance of the material was found to decrease with doping of Cu in ZnO [33]. Even though lot of research work have been carried out on Cu doped ZnO system [16,19,32,33], most of the works are on the thin films and the comprehensive study of the structural and optical properties of Cu doped ZnO nanopowders is still scanty. Therefore, in the present investigation, Cu doped ZnO [Zn1xCuxO (x = 0, 0.02, 0.04, 0.06)] nanopowders have been synthesized by co-precipitation method and annealed at 500 °C under Ar atmosphere to improve the crystalline nature. The effect of Cu on its structural, optical and morphological properties has been studied extensively. Further, the size dependent properties of the nanoparticles are correlated with the band gap. 2. Experimental procedure 2.1. Sample preparation The high purity chemicals (>99% purity) such as Zinc (II) nitrate hexahydrate (Zn(NO3)26H2O), Copper (II) nitrate trihydrate (Cu(NO3)23H2O), Sodium carbonate (Na2CO3), and Sodium hydroxide (NaOH) were used as the precursors without further purification. For the preparation of Zn0.98Cu0.02O nanopowder, Zinc nitrate (1 M/14.427 g) and Copper nitrate (1 M/0.3624 g) were dissolved in 50 ml double distilled water and kept in magnetic stirrer for 2 h under vigorous stirring. A separate buffer solution was prepared by dissolving 4 g of sodium hydroxide and 10.6 g of sodium carbonate in 50 ml double distilled water. The pH value of the buffer solution was maintained as 4.6 for better precipitation reaction. Buffer solution was then added drop wise to the initial solution under constant stirring for 2 h at room temperature to produce a white, gelatinous precipitate. The white precipitates were filtered and washed with distilled water many times. The final precipitates were dried in oven at 80 °C for 2 h. The dried precipitates were collected and ground in an agate mortar. The same procedure was repeated for other samples preparation. Finally, the collected nanopowders were annealed at 500 °C under Ar atmosphere for 2 h followed by furnace cooling. The use of Ar atmosphere is many fold purposes such as, (i) to improve the crystallinity, i.e., order of the alignment of the atoms, (ii) to enhance the conductivity, and (iii) to reduce the secondary phase formation. The same procedure is repeated to the remaining samples synthesized with nominal compositions of Zn1xCuxO with x = 0, 0.02, 0.04, 0.06.
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4000–400 cm1 with a resolution of 1 cm1. The photoluminescence (PL) spectrums within the wavelength ranges from 350 nm to 600 nm were obtained under 325 nm line of Xe laser excitation using a fluorescence spectrophotometer (F-2500). 3. Results and discussion 3.1. XRD – structural studies The typical XRD diffraction peaks of undoped and Cu doped ZnO (Zn1xCuxO) nanopowders with x = 0, 0.02, 0.04 and 0.06 are shown in Fig. 1. The pronounced diffraction peaks in the XRD pattern clearly shows the crystalline nature with peaks corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) planes. The standard diffraction peaks shows the crystal structure of Zn1xCuxO nanopowder is hexagonal wurtzite structure (space group P63mc, 186, JCPDS data card No. 36-1451) [34] with preferred orientation along (1 0 1) plane in all the samples. This is the most stable phase of ZnO. It is evident from the XRD spectra that there are no extra peaks corresponding to Cu, oxides of Cu or Cu related secondary and impurity phases, which may be attributed to the incorporation of Cu ion into the Zn lattice site rather than interstitial ones for the lower Cu concentration (x 6 0.04). Furthermore, at higher Cu concentration (Zn0.94Cu0.06O, x = 0.06), a new phase has
2.2. Characterization techniques The crystal structure of Cu doped ZnO nanopowders were determined by powder X-ray diffraction. XRD patterns were recorded on a RigaKu C/max-2500 diffractometer using Cu Ka radiation (k = 1.54056 Å) operated at 40 kV and 100 mA in the wide angle region from 10° to 80° with a 0.1° per step (2h) at the rate of 10 s per step. The topological features and the composition of Zn, O and Cu were determined by energy dispersive X-ray spectrometer on K and L lines. The surface morphology of Cu doped ZnO nanoparticles were studied using a scanning electron microscope (SEM, JEOL JSM 6390). The UV–Visible optical absorption study of the samples was carried out with a view to explore their optical properties. The spectral absorption was determined using UV visible spectrometer (Model: lambda 35, Make: Perkin Elmer) in the wavelength ranging from 350 nm to 600 nm using cm1 quartz cuvettes at room temperature. The presence of chemical bonding in Cu doped ZnO nanoparticles (as pellets in KBr) was studied by FTIR spectrometer (Model: PerkinElmer, Make: Spectrum RX I) in the range of
Fig. 1. Powder X-ray diffraction pattern for different Cu concentrations of Zn1xCuxO nanopowders at room temperature. The inset shows the variation of FWHM and crystal size with different Cu concentrations.
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S. Muthukumaran, R. Gopalakrishnan / Optical Materials 34 (2012) 1946–1953
system which has narrow FWHM (0.1902°) than the wider FWHM (0.3050°) for x = 0.04. The observed larger particle size (26.5 nm) and the smaller FWHM value (0.1902°) for Cu = 0.06 is attributed due to the combined effect of Cu2+ incorporation at Zn2+ site along with the formation of CuO phase, i.e., the effect of Cu2+ doping is dominant and appeared only at higher doping percentages of copper. A similar Cu doping concentration limit around x = 0.05 was reported in the Cu doped ZnO system [43,44]. The micro-strain (e) can be calculated using the formula [45],
been emerged between 40° and 42° corresponding to CuO along (1 1 1) plane (matched with JCPDS data card No. 05-0661) [35], which may be due to the formation of CuO from remaining un-reacted Cu2+ ions present in the solution. These peaks are related to complex reactions involving the production of Cu ? Cu2O ? CuO during firing in air. Even though, there is no secondary phases corresponding to Cu are detected by XRD analysis for x 6 0.04, the existence of secondary phases cannot be completely excluded due to limitation of this characterization technique [36]. It means Cu may form solid solution with ZnO. The presence of similar Cu phase (segregated phase) was noticed in Zn0.95Cu0.05O nanoparticles prepared by sol–gel method in the previous literature [37]. All available reflections of the present XRD phases have been fitted with a Gaussian distribution. Single crystalline structure was noticed for x 6 0.04, whereas, at x = 0.06 in addition to Cu2+ additional CuO phase is also noticed. It is noticed from Fig. 1 that the peak intensity of (1 0 1) plane increases by the addition of Cu concentrations than the other planes and hence, the Cu2+ ion is understood to have occupied the Zn2+ without changing the crystal structure. It was evident that orientation behaviour of ZnO was strongly promoted by Cu-doping [38]. The substitution of Cu2+ interstitial would affect the concentration of the interstitial Zn, oxygen vacancies and Zn vacancies [39]. A very small shift in the peak position (D2h = 0.0799°) of the diffraction peaks to the lower angles and the broadening of diffraction peaks (Db = 0.1148°) are noticed with the increase of Cu concentrations. The noticed small change in diffraction peaks and the broadening are due to the increase of micro-strain [40]. But the line broadening may be due to the size, or micro-strain, or size and micro-strain [32]. The small shift in the peak position (D2h = 0.0799°) and the broadening of the diffraction peaks illustrates the incorporation of Cu ion into the ZnO lattice and also indicates that the crystal lattice has no obvious change by Cu doping. The similar, small magnitude peak position shift was observed by the previous studies [30,41]. The average crystal size of the samples is calculated after appropriate background correction from X-ray line broadening of the diffraction peaks of (1 0 1) plane using Debye Scherrer’s formula [25],
Average crystal size ðDÞ ¼
0:9k b cos h
Micro-strain ðeÞ ¼
b cos h 4
ð2Þ
Table 1 shows the 2h value, the full width at half maximum (FWHM, b) value, d-value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size (D) and micro-strain (e) of different Zn1xCuxO nanoparticles. At the lower Cu concentration (Cu = 0.04), the substitution of Cu2+ ions instead of Zn2+ ions at their lattice sites increases the lattice constant ‘a’ and ‘c’, and the interplanar distance ‘d’, which would leaded to the decrease of the diffraction angle compared with undoped ZnO. The similar trend of lattice constant was reported by Kulyk et al. [46] by the Cu doping which is consistent with our experimental result. With increase in Cu concentrations, the resultant compound maintains a wurtzite structure, but the d-value and the lattice parameters ‘a’ and ‘c’ are slightly increases up to x = 0.04 and then decreases for x = 0.06. The change in the lattice parameters can be ascribed by the substituted of Cu2+ ion which has a smaller ionic radius than the Zn2+ (ionic radius = 0.074 nm) sites [47] in their tetrahedral coordinates. The change in the lattice parameters could be well understood by the substitution of Cu2+ ions (ionic radius = 0.73 Å) which is larger than that of Zn2+ ions (ionic radius = 0.60 Å) [47] in their tetrahedral coordinates. The slight shift in the XRD spectrum and the change in the d-value and the lattice constants with Cu doping indicated that Cu has really doped into ZnO lattice. It is observed from Table 1 that micro-strain of the Cu doped sample is increased up to x = 0.04 (0.9448 103), then slowdown and reaches minimum (0.6121 103) for x = 0.06. At x = 0.06, the atoms trapped in the non-equilibrium position could shift to a more equilibrium position, which could release the strain and hence it is decreases. The intrinsic stress is changed monotonically with increasing the Cu concentrations due to the change in the microstructure, size and shape of the particles. Meanwhile, the crystal quality of the system with Cu = 0.06 was remarkably improved by Cu doping. It is evident that the increase of strain causes the increase of lattice constants, reduction in the particle size, and the broadening and a small shift in XRD peaks. However, the decrease of strain causes the decrease of lattice constants and increase of particle size.
ð1Þ
where k is the wave length of X-ray used (1.5405 Å), b is the angular peak width at half maximum in radian along (1 0 1) plane, and h is Bragg’s diffraction angle. The inset of Fig. 1 clearly shows the average particle size is reduced from 22.4 to 15.9 nm by the addition of Cu from 0.0 to 0.04 whereas Zn0.94Cu0.06O sample has 26.5 nm. It clearly shows the presence of nano-sized particles in the samples. The reduction in the particle size is mainly due to the distortion in the host ZnO lattice by the foreign impurity, i.e. Cu2+ which decreases the nucleation and subsequent growth rate by the addition of Cu concentrations [42]. The FWHM value shows the inverse trend as it increased from x = 0 to 0.04 (0.2229° to 0.3050°) and decreased for x = 0.06 (0.1902°). The XRD spectrum of Zn0.94Cu0.06O sample shows a Cu as secondary phase segregation, which indicates that x = 0.06 is the Cu doping concentration limit in ZnO in the present
3.2. Energy-dispersive analysis X-ray (EDAX) spectra EDAX is used to analyse the amount of Cu element in Zn1xCuxO sample. The chemical compositional analysis of the pure and Cu doped ZnO nanoparticles has been carried out using EDAX. The typical EDAX spectra of Zn1xCuxO nanoparticles with x = 0.0,
Table 1 The 2h value, FWHM value, d-value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size (D) and micro-strain (e) of different Zn1xCuxO nanoparticles. Samples
2h value (°)
FWHM (b) (°)
ZnO Zn0.99Cu0.02O Zn0.97Cu0.04O Zn0.95Cu0.06O
36.1957 36.1798 36.1375 36.2174
0.2229 0.2789 0.3050 0.1902
0
d-Value (Å A)
2.4797 2.4808 2.4880 2.4783
0
Cell parameter (Å A) a=b
c
3.2491 3.2529 3.2578 3.2519
5.2072 5.2109 5.2159 5.2121
c/a Ratio
Average crystal size (D) (nm)
Micro-strain (e) (103)
1.6027 1.6019 1.6010 1.6028
22.4 17.8 15.9 26.5
0.7107 0.8819 0.9448 0.6121
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elemental compositions of the pure and Cu doped ZnO nanoparticles are given in Table 1. It is observed from Table 2 that the average Cu/Zn weight percentage ratio is derived to be 2.04%, 3.94% and 5.82% respectively in 2%, 4% and 6% Cu doped ZnO nanoparticles. The EDAX analysis confirms the presence of Cu in the ZnO system and wt.% is very nearly equal to their nominal stoichiometry within the experimental error. Therefore, the EDAX spectra show well agreement with the experimental concentration used for Zn1xCuxO system.
Fig. 2. Energy-dispersive analysis X-ray (EDAX) pictures for different Cu concentrations of Zn1xCuxO nanopowders at room temperature.
Table 2 The quantitative analysis of the compositional elements present for the different Zn1xCuxO nanoparticles using EDAX spectra. Samples
ZnO Zn0.98Cu0.02O Zn0.96Cu0.04O Zn0.94Cu0.06O
Weight percentage of the elements (%) O
Zn
Cu
21.42 19.84 22.07 22.02
78.58 78.56 74.97 73.69
– 1.60 2.96 4.29
0.04 and 0.06 are as shown in Fig. 2. It clearly shows that the intensity of Cu increases with increasing Cu incorporation in the solution. Therefore, the addition of Cu induces a dominant effect on the optical, structural and morphological properties of ZnO. The
Fig. 3. Scanning electron microscope (SEM) pictures of ZnO nanopowders annealed at 500 °C for 2 h under Ar atmosphere. (a) Undoped ZnO, (b) Zn0.98Cu0.04O nanoparticles and (c) Zn0.94Cu0.06O nanoparticles.
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Fig. 4. UV–Visible absorption spectra of undoped and different Cu doped ZnO nanoparticles as a function of wavelength.
3.3. Scanning electron microscope (SEM) – microstructural studies SEM is one of the promising techniques for the topography study of the samples and it gives important information regarding the growth mechanism, shape and size of the particles. The surface morphology of the undoped and Cu doped ZnO nanoparticles are as shown in Fig. 3a–c. The entire SEM pictures clearly show the average size of the nanoparticles is of the order of nanometer size. The undoped ZnO nanoparticles (Fig. 3a) shows the synthesized nanoparticles are homogeneous, uniformly distributed over the surface and good connectivity between the particles containing the mixer of spheroid-like and rod-like particles in which spheroid-like particles are dominant with grain size around 10–32 nm. The grain size was decreased with the increase of Cu concentrations. Fig. 3b shows the surface morphology of the Zn0.96Cu0.04O sample which has slightly lesser grain size than the undoped ZnO. The shape of the particles is turnover from spheroid-like into the rodlike and hence rod-like particles are dominant with grain size around 10–28 nm. The further increase of Cu, x = 0.06 (Fig. 3c) shows the uneven surface morphology with large grain size around 10–45 nm. The increase of Cu doped concentrations causes the more defects and deformed lattice structures. The existence of more defects greatly de-generate the particles size and shape and hence the grain size and shape is larger for x = 0.06 than the other undoped and Cu doped samples. A good correlation is found to exist between mathematical calculations from XRD analysis and those obtained from SEM studies. 3.4. Optical study The UV–Visible optical absorption spectrum of the undoped and Cu doped ZnO have been carried out at room temperature using UV Visible spectrometer (Model: Lambda 35, Make: Perkin Elmer) from 350 to 600 nm and are shown in Fig. 4. The absorption spectra show that the absorption of the samples was decreased with increasing Cu concentrations except for x = 0.06 which has higher absorption than the other samples. The inset of Fig. 4 shows clear picture of absorption changes. The absorption edge exhibits a continuous red shift up to x 6 0.04 which may be due to the formation
of shallow levels inside the band gap by doping [32]. The typical room temperature transmittance spectra for undoped ZnO and different Cu doped ZnO are shown in Fig. 5. The transmission spectra of the Cu doped ZnO nanoparticles shows just opposite trend of the optical absorption spectra. The optical band gap was evaluated using the Tauc relation [48,49],
ahc ¼ A ðhc Eg Þn
ð3Þ
where A is a constant, hc is the incident photon energy, a is the optical absorption coefficient near the fundamental absorption edge, Eg is the energy band gap of the material and the exponent n = 1/2 for direct band gap and dipole-allowed transitions. The absorption coefficients were calculated from the optical absorption spectra. The energy band gap of the Cu doped ZnO nanoparticles can be obtained by plotting (ahc)2 versus hc and extrapolating the linear portion of the absorption edge to find the intercept with energy axis as shown in Fig. 6. The band gap is initially increases from 3.47 to 3.49 eV for lower Cu concentration (x = 0 ? 0.02) and then decreases from 3.49 to 3.43 eV as Cu content increases from 0.02 to 0.06. The noticed slight red shift (DEg = 0.06 eV) in the band gap is due to Cu doping in ZnO. The similar narrowing of band gap was observed by Diouri et al. and explained by the p–d spin-exchange interactions between the band electrons and the localized d electrons of the transition-metal ion substituting the Cu2+ ion [50]. The reduction of band gap by Cu doping is also due to the strong p–d mixing of O and Cu [51]. The red shift in the band gap is theoretically explained by Bylsma et al. using the s–d and p–d exchange interactions using the second-order perturbation theory [52]. It shows that the red shift of the band gap confirms the uniform substitution of Cu ions in the ZnO lattice. Moreover, the narrowing of band gap is due to many-body effects on the conduction and valence bands [53] which can shrink the band gap originate from electron interaction and impurity scattering. It has been attributed to the merging of an impurity band into the conduction band, thereby shrinking the band gap.
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Fig. 5. Transmittance spectra of undoped and different Cu doped ZnO nanoparticles as a function of wavelength.
Fig. 6. The (ahc)2 versus hc curves for the optical band gap determination of undoped and Cu doped ZnO nanoparticles for different Cu concentrations.
3.5. Fourier transform infrared (FTIR) studies FTIR is a technique used to obtain information about the chemical bonding in a material. It is used to identify the elemental constituents of a material. The characteristic peaks exhibited by FTIR spectra of undoped and Cu doped ZnO nanoparticles are as shown in Fig. 7. FTIR spectra for all the samples are assigned at room temperature and are listed in Table 3. The broad absorption peaks around 3455 cm1 and 1146 cm1 are attributed to normal polymeric O–H stretching vibration of H2O in Cu–Zn–O lattice [54] which may be due to moisture in the solution and the atmosphere. Another sharp peak around 1598 cm1 is attributed to H–O–H bending vibration, which is assigned to a small amount of H2O in
the ZnO nanocrystals [32]. The absorption peaks observed between 2300 and 2400 cm1 is because of the existence of CO2 molecule in air [55]. The medium to weak bands at 835 cm1 and 624 cm1 are assigned to the vibrational frequencies due to the change in the microstructural features by the addition of Cu into Zn–O lattice. The Zn–O bond is assigned to the stretching frequency at 544 cm1 for pure ZnO which is shifted to higher frequency as 547 cm1 for Cu = 0.02, 552 cm1 for Cu = 0.04 and 554 cm1 for Cu = 0.06. Copper atom is slightly lighter than Zn atom, so, according to the well established theories of vibrational modes in mixed crystals the substitution should result in an upward shift of the fundamental transverse optical phonon mode [56]. The observed upward shift in the present system is consistent with the results
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Fig. 8. Room temperature photoluminescence spectra of undoped and Cu doped ZnO nanoparticles.
Fig. 7. FTIR spectra of undoped and Cu doped ZnO nanoparticles for different Cu concentrations.
Table 3 IR peaks and their assignments for the Zn1xCuxO system. Assignments
O–H stretching Additional weak band H–O–H bending vibration Zn–O stretching
Wave number (cm1) ZnO
Zn0.98Cu0.02O
Zn0.96Cu0.04O
Zn0.94Cu0.06O
3455 [54] 2851
3454
3456
3422
2854
2845
2846
1598 [32] 544
1601
1602
1606
547
552
554
given by Singhal et al. [57]. The frequency shift towards the lower side reveals the substitution of Cu2+ ion into the Zn–O lattice. 3.6. Photoluminescence (PL) studies Generally, the densities of defects and oxygen vacancies affect significantly the optical properties of oxide nanostructures. The correlation between structure and property is investigated by PL spectra. The photoluminescence spectra of the undoped and Cu doped ZnO nanoparticles are shown in Fig. 8. It is noticed that there is no clear difference in band shift (except for x = 0.06), the only difference is the luminescence intensity. The luminescence intensity of doped ZnO is high as compared to that of undoped ZnO [35]. Pure ZnO exhibits only one peak around 398 nm
associated with near band (NB) ultra violet emission. This strong ultra violet (UV) band is corresponding to the near band-edge (NBE) emission [58,59]. For x = 0.02, the intensity of NBE band increases, and a small and broad defect related green band centred on 527 nm is formed. Zn0.96Cu0.04O sample has maximum intensity NBE band around 393 nm, and a high intensity and broad green band. The further increase of Cu (x = 0.06) shift the NBE to higher wavelength side (403 nm) and the green band is shifted to lower wavelength side (522 nm) in addition to the suppression of intensity. The intensity of UV band is more or less same but exhibits small red shift from 398 to 403 nm for x = 0.02 to 0.06. This could mainly due to the s–d and p–d exchange interactions between the band electrons and the localized ‘d’ electrons of Cu2+ ions substituting Zn2+ ions [60]. It causes a negative and a positive correction to the conduction and valence band, and hence leads to band gap narrowing [61] as noticed in Fig. 6. As regards the appearance of the dominant UV emission, it is reported that the crystal quality of the nanoparticles is improved [62], particularly hydrogenated samples have good optical properties. It is appreciable for the fabrication of nano-optoelectronic devices in the near future. The origin of green band may be ascribed to the oxygen vacancies and intrinsic defects [63,64]. The observed green emission is also due to the impurity levels correspond to the singly ionized oxygen vacancy in ZnO [65]. Vanheusden et al. described this green emission by the transition between photo-excited holes and singly ionized oxygen vacancies [66]. The increasing micro-strain as discussed in Table 1 may also induce some broadening but may not play a major role. Increase in intensity with increase of Cu concentrations is due to the effective reduction of the defects such as oxygen vacancies, and zinc vacancies [67]. Therefore, the intensity of the UV emission increases and green emission reduces with the increase Cu concentrations. The change in intensity, the shift in near
S. Muthukumaran, R. Gopalakrishnan / Optical Materials 34 (2012) 1946–1953
band edge (NBE) emission from 398 to 403 nm and a shift in green band (GB) emission from 527 to 522 nm confirm the substitution of Cu into the Zn–O lattice. 4. Conclusions
[22] [23] [24] [25] [26] [27]
Zn1xCuxO (x = 0.0, 0.02, 0.04 to 0.06) nanoparticles have been successfully synthesized by a co-precipitation method and annealed under Ar atmosphere at 500 °C. The XRD and SEM measurements revealed the hexagonal wurtzite structure without any impurities. The increase of lattice constants, micro-strain, the slight shift of XRD peaks and the reduction in the average crystalline size and the band gap indicated that Cu has really doped into the Zn–O lattice. It is proposed that Cu doping concentration limit is below 6% (0.06) molar fraction which is supported by the detailed XRD analysis and the derived structural parameters. This Cu concentration limit was proposed as below 5% in the previous studies. Hydrogenation effect improves the crystal quality and optical properties. The presence of functional groups and the chemical bonding with Cu are confirmed by FTIR spectra. PL spectra of the Zn1xCuxO system described the shift in near band edge (NBE) UV emission from 398 to 403 nm and a shift in green band (GB) emission from 527 to 522 nm which confirms the substitution of Cu into the ZnO lattice. The strong UV and weak green bands emphasized that the Cu doped ZnO has good crystal surface. Hydrogenated samples have good crystalline structure and better optical properties which are appreciable for the fabrication of nano-optoelectronic devices like tunable light emitting diode in the near future. References
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Structural, FTIR and photoluminescence studies of Cu doped ZnO nanopowders by co-precipitation method S. Muthukumaran a,⇑, R. Gopalakrishnan b a b
PG & Research Department of Physics, H.H. The Rajah’s College, Pudukkottai 622 001, Tamilnadu, India Department of Physics, Oxford Engineering College, Tiruchirapalli 620 009, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 26 January 2012 Received in revised form 30 May 2012 Accepted 6 June 2012 Available online 29 June 2012 Keywords: Cu doped ZnO nanoparticles Co-precipitation method FTIR Photoluminescence
a b s t r a c t Cu doped ZnO (Zn1xCuxO, x = 0, 0.02, 0.04 and 0.06) nanopowders have been synthesized by co-precipitation method and annealed at 500 °C for 2 h under Ar atmosphere. The synthesized samples have been characterized by powder X-ray diffraction, energy-dispersive analysis X-ray (EDAX) spectra, UV–Visible spectrophotometer and Fourier transform infrared (FTIR) spectroscopy. The XRD measurement reveals that the prepared nanoparticles have different microstructure without changing a hexagonal wurtzite structure. The calculated average crystalline size decreases from 22.24 to 15.93 nm for x = 0 to 0.04 then reaches 26.54 nm for x = 0.06 which is confirmed by SEM micrographs. The change in lattice parameters, micro-strain, a small shift and broadening in XRD peaks and the reduction in the energy gap from 3.49 to 3.43 eV reveals the substitution of Cu2+ ions into the ZnO lattice. Hydrogenation effect improves the crystal quality and optical properties. It is proposed that Cu doping concentration limit is below 6% (0.06) molar fraction which is supported by the detailed XRD analysis and the derived structural parameters. This Cu concentration limit was proposed as below 5% by previous studies. The presence of functional groups and the chemical bonding is confirmed by FTIR spectra. PL spectra of the Zn1xCuxO system show that the shift in near band edge (NBE) UV emission from 398 to 403 nm and a shift in green band (GB) emission from 527 to 522 nm which confirms the substitution of Cu into the ZnO lattice. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a direct band gap (3.36 eV at room temperature) n-type semiconductor with large exciton binding energy of 60 meV, which have the promising applications in optoelectronics, and photonics [1–4]. It has very attracted intensive research effort due to its unique properties and versatile applications in transparent electronics, hi-tech applications, ultraviolet (UV) light emitters, piezoelectric devices, chemical sensors and spintronics [5–12]. Recently, oxide based diluted magnetic semiconductors (DMSs) and Cu doped ZnO materials have attracted intensive interest by the following two facts. First, nano zinc oxide is identified as a promising semiconducting material for exhibiting room temperature ferromagnetism (RTFM) when doped with the transition metal elements [13]. Second, the microstructure and surface geometry of the material are modified by the addition of suitable dopant. The most commonly used metallic dopants in ZnO based systems are Al, Co, Cu, Ga, Sn, etc. [14–18]. Among the different metallic doping elements Cu is important because, (i) it is a prominent luminescence activator, which can modify the luminescence of ZnO crystals by creating localized impurity levels [19], (ii) it has many ⇑ Corresponding author. Tel.: +91 04322 221558; fax: +91 04322 230490. E-mail address: [email protected] (S. Muthukumaran). 0925-3467/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2012.06.004
physical and chemical properties that are similar to those of Zn, and (iii) it can change the microstructure and the optical properties of the ZnO system [16]. Different physical or chemical synthetic methods have been used to prepare the ZnO nanoparticles such as thermal decomposition, thermolysis [20], chemical vapour deposition, sol–gel [21], spray pyrolysis, precipitation [22], vapour phase oxidation [23], thermal vapour transport, condensation [24], co-precipitation [25] and hydrothermal [26]. Among the different methods, the co-precipitation is one of the most important methods to prepare the nanopowder. Co-precipitation is the name given by analytical chemists to a phenomenon whereby the fractional precipitation of a specified ion in a solution results in the precipitation not only of the target ion but also of other ions existing side by side in the solution. Cu-doped ZnO nanomaterials permit tuning of chemical and physical properties by the incorporation of the dopant in lattices of ZnO [27]. The possibility of band gap engineering is reported by Chang et al. in the doped ZnO by the barrier layers which will facilitate radiative recombination by carrier confinement [28]. The control of properties for Cu-doped ZnO and band gap engineering of nanomaterials is of utmost importance for tunable light emitting diodes (LEDs) and other optoelectronic applications [29–31].
S. Muthukumaran, R. Gopalakrishnan / Optical Materials 34 (2012) 1946–1953
Cu doped ZnO nanopowders with 40 nm size have been synthesized by solution combustion method which showed a red shift and narrowing of band gap [32]. The resistance of the material was found to decrease with doping of Cu in ZnO [33]. Even though lot of research work have been carried out on Cu doped ZnO system [16,19,32,33], most of the works are on the thin films and the comprehensive study of the structural and optical properties of Cu doped ZnO nanopowders is still scanty. Therefore, in the present investigation, Cu doped ZnO [Zn1xCuxO (x = 0, 0.02, 0.04, 0.06)] nanopowders have been synthesized by co-precipitation method and annealed at 500 °C under Ar atmosphere to improve the crystalline nature. The effect of Cu on its structural, optical and morphological properties has been studied extensively. Further, the size dependent properties of the nanoparticles are correlated with the band gap. 2. Experimental procedure 2.1. Sample preparation The high purity chemicals (>99% purity) such as Zinc (II) nitrate hexahydrate (Zn(NO3)26H2O), Copper (II) nitrate trihydrate (Cu(NO3)23H2O), Sodium carbonate (Na2CO3), and Sodium hydroxide (NaOH) were used as the precursors without further purification. For the preparation of Zn0.98Cu0.02O nanopowder, Zinc nitrate (1 M/14.427 g) and Copper nitrate (1 M/0.3624 g) were dissolved in 50 ml double distilled water and kept in magnetic stirrer for 2 h under vigorous stirring. A separate buffer solution was prepared by dissolving 4 g of sodium hydroxide and 10.6 g of sodium carbonate in 50 ml double distilled water. The pH value of the buffer solution was maintained as 4.6 for better precipitation reaction. Buffer solution was then added drop wise to the initial solution under constant stirring for 2 h at room temperature to produce a white, gelatinous precipitate. The white precipitates were filtered and washed with distilled water many times. The final precipitates were dried in oven at 80 °C for 2 h. The dried precipitates were collected and ground in an agate mortar. The same procedure was repeated for other samples preparation. Finally, the collected nanopowders were annealed at 500 °C under Ar atmosphere for 2 h followed by furnace cooling. The use of Ar atmosphere is many fold purposes such as, (i) to improve the crystallinity, i.e., order of the alignment of the atoms, (ii) to enhance the conductivity, and (iii) to reduce the secondary phase formation. The same procedure is repeated to the remaining samples synthesized with nominal compositions of Zn1xCuxO with x = 0, 0.02, 0.04, 0.06.
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4000–400 cm1 with a resolution of 1 cm1. The photoluminescence (PL) spectrums within the wavelength ranges from 350 nm to 600 nm were obtained under 325 nm line of Xe laser excitation using a fluorescence spectrophotometer (F-2500). 3. Results and discussion 3.1. XRD – structural studies The typical XRD diffraction peaks of undoped and Cu doped ZnO (Zn1xCuxO) nanopowders with x = 0, 0.02, 0.04 and 0.06 are shown in Fig. 1. The pronounced diffraction peaks in the XRD pattern clearly shows the crystalline nature with peaks corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2) and (2 0 1) planes. The standard diffraction peaks shows the crystal structure of Zn1xCuxO nanopowder is hexagonal wurtzite structure (space group P63mc, 186, JCPDS data card No. 36-1451) [34] with preferred orientation along (1 0 1) plane in all the samples. This is the most stable phase of ZnO. It is evident from the XRD spectra that there are no extra peaks corresponding to Cu, oxides of Cu or Cu related secondary and impurity phases, which may be attributed to the incorporation of Cu ion into the Zn lattice site rather than interstitial ones for the lower Cu concentration (x 6 0.04). Furthermore, at higher Cu concentration (Zn0.94Cu0.06O, x = 0.06), a new phase has
2.2. Characterization techniques The crystal structure of Cu doped ZnO nanopowders were determined by powder X-ray diffraction. XRD patterns were recorded on a RigaKu C/max-2500 diffractometer using Cu Ka radiation (k = 1.54056 Å) operated at 40 kV and 100 mA in the wide angle region from 10° to 80° with a 0.1° per step (2h) at the rate of 10 s per step. The topological features and the composition of Zn, O and Cu were determined by energy dispersive X-ray spectrometer on K and L lines. The surface morphology of Cu doped ZnO nanoparticles were studied using a scanning electron microscope (SEM, JEOL JSM 6390). The UV–Visible optical absorption study of the samples was carried out with a view to explore their optical properties. The spectral absorption was determined using UV visible spectrometer (Model: lambda 35, Make: Perkin Elmer) in the wavelength ranging from 350 nm to 600 nm using cm1 quartz cuvettes at room temperature. The presence of chemical bonding in Cu doped ZnO nanoparticles (as pellets in KBr) was studied by FTIR spectrometer (Model: PerkinElmer, Make: Spectrum RX I) in the range of
Fig. 1. Powder X-ray diffraction pattern for different Cu concentrations of Zn1xCuxO nanopowders at room temperature. The inset shows the variation of FWHM and crystal size with different Cu concentrations.
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S. Muthukumaran, R. Gopalakrishnan / Optical Materials 34 (2012) 1946–1953
system which has narrow FWHM (0.1902°) than the wider FWHM (0.3050°) for x = 0.04. The observed larger particle size (26.5 nm) and the smaller FWHM value (0.1902°) for Cu = 0.06 is attributed due to the combined effect of Cu2+ incorporation at Zn2+ site along with the formation of CuO phase, i.e., the effect of Cu2+ doping is dominant and appeared only at higher doping percentages of copper. A similar Cu doping concentration limit around x = 0.05 was reported in the Cu doped ZnO system [43,44]. The micro-strain (e) can be calculated using the formula [45],
been emerged between 40° and 42° corresponding to CuO along (1 1 1) plane (matched with JCPDS data card No. 05-0661) [35], which may be due to the formation of CuO from remaining un-reacted Cu2+ ions present in the solution. These peaks are related to complex reactions involving the production of Cu ? Cu2O ? CuO during firing in air. Even though, there is no secondary phases corresponding to Cu are detected by XRD analysis for x 6 0.04, the existence of secondary phases cannot be completely excluded due to limitation of this characterization technique [36]. It means Cu may form solid solution with ZnO. The presence of similar Cu phase (segregated phase) was noticed in Zn0.95Cu0.05O nanoparticles prepared by sol–gel method in the previous literature [37]. All available reflections of the present XRD phases have been fitted with a Gaussian distribution. Single crystalline structure was noticed for x 6 0.04, whereas, at x = 0.06 in addition to Cu2+ additional CuO phase is also noticed. It is noticed from Fig. 1 that the peak intensity of (1 0 1) plane increases by the addition of Cu concentrations than the other planes and hence, the Cu2+ ion is understood to have occupied the Zn2+ without changing the crystal structure. It was evident that orientation behaviour of ZnO was strongly promoted by Cu-doping [38]. The substitution of Cu2+ interstitial would affect the concentration of the interstitial Zn, oxygen vacancies and Zn vacancies [39]. A very small shift in the peak position (D2h = 0.0799°) of the diffraction peaks to the lower angles and the broadening of diffraction peaks (Db = 0.1148°) are noticed with the increase of Cu concentrations. The noticed small change in diffraction peaks and the broadening are due to the increase of micro-strain [40]. But the line broadening may be due to the size, or micro-strain, or size and micro-strain [32]. The small shift in the peak position (D2h = 0.0799°) and the broadening of the diffraction peaks illustrates the incorporation of Cu ion into the ZnO lattice and also indicates that the crystal lattice has no obvious change by Cu doping. The similar, small magnitude peak position shift was observed by the previous studies [30,41]. The average crystal size of the samples is calculated after appropriate background correction from X-ray line broadening of the diffraction peaks of (1 0 1) plane using Debye Scherrer’s formula [25],
Average crystal size ðDÞ ¼
0:9k b cos h
Micro-strain ðeÞ ¼
b cos h 4
ð2Þ
Table 1 shows the 2h value, the full width at half maximum (FWHM, b) value, d-value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size (D) and micro-strain (e) of different Zn1xCuxO nanoparticles. At the lower Cu concentration (Cu = 0.04), the substitution of Cu2+ ions instead of Zn2+ ions at their lattice sites increases the lattice constant ‘a’ and ‘c’, and the interplanar distance ‘d’, which would leaded to the decrease of the diffraction angle compared with undoped ZnO. The similar trend of lattice constant was reported by Kulyk et al. [46] by the Cu doping which is consistent with our experimental result. With increase in Cu concentrations, the resultant compound maintains a wurtzite structure, but the d-value and the lattice parameters ‘a’ and ‘c’ are slightly increases up to x = 0.04 and then decreases for x = 0.06. The change in the lattice parameters can be ascribed by the substituted of Cu2+ ion which has a smaller ionic radius than the Zn2+ (ionic radius = 0.074 nm) sites [47] in their tetrahedral coordinates. The change in the lattice parameters could be well understood by the substitution of Cu2+ ions (ionic radius = 0.73 Å) which is larger than that of Zn2+ ions (ionic radius = 0.60 Å) [47] in their tetrahedral coordinates. The slight shift in the XRD spectrum and the change in the d-value and the lattice constants with Cu doping indicated that Cu has really doped into ZnO lattice. It is observed from Table 1 that micro-strain of the Cu doped sample is increased up to x = 0.04 (0.9448 103), then slowdown and reaches minimum (0.6121 103) for x = 0.06. At x = 0.06, the atoms trapped in the non-equilibrium position could shift to a more equilibrium position, which could release the strain and hence it is decreases. The intrinsic stress is changed monotonically with increasing the Cu concentrations due to the change in the microstructure, size and shape of the particles. Meanwhile, the crystal quality of the system with Cu = 0.06 was remarkably improved by Cu doping. It is evident that the increase of strain causes the increase of lattice constants, reduction in the particle size, and the broadening and a small shift in XRD peaks. However, the decrease of strain causes the decrease of lattice constants and increase of particle size.
ð1Þ
where k is the wave length of X-ray used (1.5405 Å), b is the angular peak width at half maximum in radian along (1 0 1) plane, and h is Bragg’s diffraction angle. The inset of Fig. 1 clearly shows the average particle size is reduced from 22.4 to 15.9 nm by the addition of Cu from 0.0 to 0.04 whereas Zn0.94Cu0.06O sample has 26.5 nm. It clearly shows the presence of nano-sized particles in the samples. The reduction in the particle size is mainly due to the distortion in the host ZnO lattice by the foreign impurity, i.e. Cu2+ which decreases the nucleation and subsequent growth rate by the addition of Cu concentrations [42]. The FWHM value shows the inverse trend as it increased from x = 0 to 0.04 (0.2229° to 0.3050°) and decreased for x = 0.06 (0.1902°). The XRD spectrum of Zn0.94Cu0.06O sample shows a Cu as secondary phase segregation, which indicates that x = 0.06 is the Cu doping concentration limit in ZnO in the present
3.2. Energy-dispersive analysis X-ray (EDAX) spectra EDAX is used to analyse the amount of Cu element in Zn1xCuxO sample. The chemical compositional analysis of the pure and Cu doped ZnO nanoparticles has been carried out using EDAX. The typical EDAX spectra of Zn1xCuxO nanoparticles with x = 0.0,
Table 1 The 2h value, FWHM value, d-value, cell parameters ‘a’ and ‘c’, c/a ratio, average crystal size (D) and micro-strain (e) of different Zn1xCuxO nanoparticles. Samples
2h value (°)
FWHM (b) (°)
ZnO Zn0.99Cu0.02O Zn0.97Cu0.04O Zn0.95Cu0.06O
36.1957 36.1798 36.1375 36.2174
0.2229 0.2789 0.3050 0.1902
0
d-Value (Å A)
2.4797 2.4808 2.4880 2.4783
0
Cell parameter (Å A) a=b
c
3.2491 3.2529 3.2578 3.2519
5.2072 5.2109 5.2159 5.2121
c/a Ratio
Average crystal size (D) (nm)
Micro-strain (e) (103)
1.6027 1.6019 1.6010 1.6028
22.4 17.8 15.9 26.5
0.7107 0.8819 0.9448 0.6121
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elemental compositions of the pure and Cu doped ZnO nanoparticles are given in Table 1. It is observed from Table 2 that the average Cu/Zn weight percentage ratio is derived to be 2.04%, 3.94% and 5.82% respectively in 2%, 4% and 6% Cu doped ZnO nanoparticles. The EDAX analysis confirms the presence of Cu in the ZnO system and wt.% is very nearly equal to their nominal stoichiometry within the experimental error. Therefore, the EDAX spectra show well agreement with the experimental concentration used for Zn1xCuxO system.
Fig. 2. Energy-dispersive analysis X-ray (EDAX) pictures for different Cu concentrations of Zn1xCuxO nanopowders at room temperature.
Table 2 The quantitative analysis of the compositional elements present for the different Zn1xCuxO nanoparticles using EDAX spectra. Samples
ZnO Zn0.98Cu0.02O Zn0.96Cu0.04O Zn0.94Cu0.06O
Weight percentage of the elements (%) O
Zn
Cu
21.42 19.84 22.07 22.02
78.58 78.56 74.97 73.69
– 1.60 2.96 4.29
0.04 and 0.06 are as shown in Fig. 2. It clearly shows that the intensity of Cu increases with increasing Cu incorporation in the solution. Therefore, the addition of Cu induces a dominant effect on the optical, structural and morphological properties of ZnO. The
Fig. 3. Scanning electron microscope (SEM) pictures of ZnO nanopowders annealed at 500 °C for 2 h under Ar atmosphere. (a) Undoped ZnO, (b) Zn0.98Cu0.04O nanoparticles and (c) Zn0.94Cu0.06O nanoparticles.
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Fig. 4. UV–Visible absorption spectra of undoped and different Cu doped ZnO nanoparticles as a function of wavelength.
3.3. Scanning electron microscope (SEM) – microstructural studies SEM is one of the promising techniques for the topography study of the samples and it gives important information regarding the growth mechanism, shape and size of the particles. The surface morphology of the undoped and Cu doped ZnO nanoparticles are as shown in Fig. 3a–c. The entire SEM pictures clearly show the average size of the nanoparticles is of the order of nanometer size. The undoped ZnO nanoparticles (Fig. 3a) shows the synthesized nanoparticles are homogeneous, uniformly distributed over the surface and good connectivity between the particles containing the mixer of spheroid-like and rod-like particles in which spheroid-like particles are dominant with grain size around 10–32 nm. The grain size was decreased with the increase of Cu concentrations. Fig. 3b shows the surface morphology of the Zn0.96Cu0.04O sample which has slightly lesser grain size than the undoped ZnO. The shape of the particles is turnover from spheroid-like into the rodlike and hence rod-like particles are dominant with grain size around 10–28 nm. The further increase of Cu, x = 0.06 (Fig. 3c) shows the uneven surface morphology with large grain size around 10–45 nm. The increase of Cu doped concentrations causes the more defects and deformed lattice structures. The existence of more defects greatly de-generate the particles size and shape and hence the grain size and shape is larger for x = 0.06 than the other undoped and Cu doped samples. A good correlation is found to exist between mathematical calculations from XRD analysis and those obtained from SEM studies. 3.4. Optical study The UV–Visible optical absorption spectrum of the undoped and Cu doped ZnO have been carried out at room temperature using UV Visible spectrometer (Model: Lambda 35, Make: Perkin Elmer) from 350 to 600 nm and are shown in Fig. 4. The absorption spectra show that the absorption of the samples was decreased with increasing Cu concentrations except for x = 0.06 which has higher absorption than the other samples. The inset of Fig. 4 shows clear picture of absorption changes. The absorption edge exhibits a continuous red shift up to x 6 0.04 which may be due to the formation
of shallow levels inside the band gap by doping [32]. The typical room temperature transmittance spectra for undoped ZnO and different Cu doped ZnO are shown in Fig. 5. The transmission spectra of the Cu doped ZnO nanoparticles shows just opposite trend of the optical absorption spectra. The optical band gap was evaluated using the Tauc relation [48,49],
ahc ¼ A ðhc Eg Þn
ð3Þ
where A is a constant, hc is the incident photon energy, a is the optical absorption coefficient near the fundamental absorption edge, Eg is the energy band gap of the material and the exponent n = 1/2 for direct band gap and dipole-allowed transitions. The absorption coefficients were calculated from the optical absorption spectra. The energy band gap of the Cu doped ZnO nanoparticles can be obtained by plotting (ahc)2 versus hc and extrapolating the linear portion of the absorption edge to find the intercept with energy axis as shown in Fig. 6. The band gap is initially increases from 3.47 to 3.49 eV for lower Cu concentration (x = 0 ? 0.02) and then decreases from 3.49 to 3.43 eV as Cu content increases from 0.02 to 0.06. The noticed slight red shift (DEg = 0.06 eV) in the band gap is due to Cu doping in ZnO. The similar narrowing of band gap was observed by Diouri et al. and explained by the p–d spin-exchange interactions between the band electrons and the localized d electrons of the transition-metal ion substituting the Cu2+ ion [50]. The reduction of band gap by Cu doping is also due to the strong p–d mixing of O and Cu [51]. The red shift in the band gap is theoretically explained by Bylsma et al. using the s–d and p–d exchange interactions using the second-order perturbation theory [52]. It shows that the red shift of the band gap confirms the uniform substitution of Cu ions in the ZnO lattice. Moreover, the narrowing of band gap is due to many-body effects on the conduction and valence bands [53] which can shrink the band gap originate from electron interaction and impurity scattering. It has been attributed to the merging of an impurity band into the conduction band, thereby shrinking the band gap.
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Fig. 5. Transmittance spectra of undoped and different Cu doped ZnO nanoparticles as a function of wavelength.
Fig. 6. The (ahc)2 versus hc curves for the optical band gap determination of undoped and Cu doped ZnO nanoparticles for different Cu concentrations.
3.5. Fourier transform infrared (FTIR) studies FTIR is a technique used to obtain information about the chemical bonding in a material. It is used to identify the elemental constituents of a material. The characteristic peaks exhibited by FTIR spectra of undoped and Cu doped ZnO nanoparticles are as shown in Fig. 7. FTIR spectra for all the samples are assigned at room temperature and are listed in Table 3. The broad absorption peaks around 3455 cm1 and 1146 cm1 are attributed to normal polymeric O–H stretching vibration of H2O in Cu–Zn–O lattice [54] which may be due to moisture in the solution and the atmosphere. Another sharp peak around 1598 cm1 is attributed to H–O–H bending vibration, which is assigned to a small amount of H2O in
the ZnO nanocrystals [32]. The absorption peaks observed between 2300 and 2400 cm1 is because of the existence of CO2 molecule in air [55]. The medium to weak bands at 835 cm1 and 624 cm1 are assigned to the vibrational frequencies due to the change in the microstructural features by the addition of Cu into Zn–O lattice. The Zn–O bond is assigned to the stretching frequency at 544 cm1 for pure ZnO which is shifted to higher frequency as 547 cm1 for Cu = 0.02, 552 cm1 for Cu = 0.04 and 554 cm1 for Cu = 0.06. Copper atom is slightly lighter than Zn atom, so, according to the well established theories of vibrational modes in mixed crystals the substitution should result in an upward shift of the fundamental transverse optical phonon mode [56]. The observed upward shift in the present system is consistent with the results
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Fig. 8. Room temperature photoluminescence spectra of undoped and Cu doped ZnO nanoparticles.
Fig. 7. FTIR spectra of undoped and Cu doped ZnO nanoparticles for different Cu concentrations.
Table 3 IR peaks and their assignments for the Zn1xCuxO system. Assignments
O–H stretching Additional weak band H–O–H bending vibration Zn–O stretching
Wave number (cm1) ZnO
Zn0.98Cu0.02O
Zn0.96Cu0.04O
Zn0.94Cu0.06O
3455 [54] 2851
3454
3456
3422
2854
2845
2846
1598 [32] 544
1601
1602
1606
547
552
554
given by Singhal et al. [57]. The frequency shift towards the lower side reveals the substitution of Cu2+ ion into the Zn–O lattice. 3.6. Photoluminescence (PL) studies Generally, the densities of defects and oxygen vacancies affect significantly the optical properties of oxide nanostructures. The correlation between structure and property is investigated by PL spectra. The photoluminescence spectra of the undoped and Cu doped ZnO nanoparticles are shown in Fig. 8. It is noticed that there is no clear difference in band shift (except for x = 0.06), the only difference is the luminescence intensity. The luminescence intensity of doped ZnO is high as compared to that of undoped ZnO [35]. Pure ZnO exhibits only one peak around 398 nm
associated with near band (NB) ultra violet emission. This strong ultra violet (UV) band is corresponding to the near band-edge (NBE) emission [58,59]. For x = 0.02, the intensity of NBE band increases, and a small and broad defect related green band centred on 527 nm is formed. Zn0.96Cu0.04O sample has maximum intensity NBE band around 393 nm, and a high intensity and broad green band. The further increase of Cu (x = 0.06) shift the NBE to higher wavelength side (403 nm) and the green band is shifted to lower wavelength side (522 nm) in addition to the suppression of intensity. The intensity of UV band is more or less same but exhibits small red shift from 398 to 403 nm for x = 0.02 to 0.06. This could mainly due to the s–d and p–d exchange interactions between the band electrons and the localized ‘d’ electrons of Cu2+ ions substituting Zn2+ ions [60]. It causes a negative and a positive correction to the conduction and valence band, and hence leads to band gap narrowing [61] as noticed in Fig. 6. As regards the appearance of the dominant UV emission, it is reported that the crystal quality of the nanoparticles is improved [62], particularly hydrogenated samples have good optical properties. It is appreciable for the fabrication of nano-optoelectronic devices in the near future. The origin of green band may be ascribed to the oxygen vacancies and intrinsic defects [63,64]. The observed green emission is also due to the impurity levels correspond to the singly ionized oxygen vacancy in ZnO [65]. Vanheusden et al. described this green emission by the transition between photo-excited holes and singly ionized oxygen vacancies [66]. The increasing micro-strain as discussed in Table 1 may also induce some broadening but may not play a major role. Increase in intensity with increase of Cu concentrations is due to the effective reduction of the defects such as oxygen vacancies, and zinc vacancies [67]. Therefore, the intensity of the UV emission increases and green emission reduces with the increase Cu concentrations. The change in intensity, the shift in near
S. Muthukumaran, R. Gopalakrishnan / Optical Materials 34 (2012) 1946–1953
band edge (NBE) emission from 398 to 403 nm and a shift in green band (GB) emission from 527 to 522 nm confirm the substitution of Cu into the Zn–O lattice. 4. Conclusions
[22] [23] [24] [25] [26] [27]
Zn1xCuxO (x = 0.0, 0.02, 0.04 to 0.06) nanoparticles have been successfully synthesized by a co-precipitation method and annealed under Ar atmosphere at 500 °C. The XRD and SEM measurements revealed the hexagonal wurtzite structure without any impurities. The increase of lattice constants, micro-strain, the slight shift of XRD peaks and the reduction in the average crystalline size and the band gap indicated that Cu has really doped into the Zn–O lattice. It is proposed that Cu doping concentration limit is below 6% (0.06) molar fraction which is supported by the detailed XRD analysis and the derived structural parameters. This Cu concentration limit was proposed as below 5% in the previous studies. Hydrogenation effect improves the crystal quality and optical properties. The presence of functional groups and the chemical bonding with Cu are confirmed by FTIR spectra. PL spectra of the Zn1xCuxO system described the shift in near band edge (NBE) UV emission from 398 to 403 nm and a shift in green band (GB) emission from 527 to 522 nm which confirms the substitution of Cu into the ZnO lattice. The strong UV and weak green bands emphasized that the Cu doped ZnO has good crystal surface. Hydrogenated samples have good crystalline structure and better optical properties which are appreciable for the fabrication of nano-optoelectronic devices like tunable light emitting diode in the near future. References
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