Quenching Photoluminescence of Eu(III) by Cu(II) in ZnO:Eu3++Cu2+ Compounds by Solution Combustion Method

In this study Cu ²⁺+Eu ³⁺ co-doped ZnO(ZnO/Cu ²⁺+Eu ³⁺) solid solution powders were synthesized by solution combustion method using as oxidant agent zinc nitrate hexahydrate and as fuel urea; the Cu ²⁺ concentrations were 0, 1, 2, 3, 10, and 20 %Wt; the Eu ³⁺ ion concentration was fixed in 3%Wt. The samples after were annealed at 900 °C by 20 h in air. The structural results showed the largely presence of a wurtzite solid solution of Cu ²⁺+Eu ³⁺doped ZnO, at high Cu ²⁺ doping CuO and Eu ₂CuO ₄ phases are also present. Morphological properties were analyzed using scanning electron microscopy (SEM) technique. However it is important to remark that the Cu ²⁺ ions suppress the Eu ³⁺ ion photoluminescence (PL) by means of an overlap mechanism between Cu ²⁺ absorption band and Eu ³⁺emission band (e.g. ⁵D ₀→ ⁷F ₂) of the Eu ³⁺ emission spectra.

• ZnO
• Co-Doped Cu2++Eu3+
• Quenching
• Photoluminescence
• Solution
• Combustion

1. Introduction

Zinc oxide (ZnO) is actually considered to be a II-VI semiconductor material of great importance in basic science as in technologic applications due to its important properties physical and chemicals: the ZnO has a band gap energy of 3.7 eV [1] - [6] , an large exciton bonding energy of 60 meV, also has defects as O and Zn vacancies; however it is chemically and thermally stable and friendly with the environment. Due to these properties, the ZnO can be utilized in the fabrication of devices such as: electro-optical devices [7] , gas sensors [8] , catalyst [9] , piezoelectric device [10] , electro-optical [11] , photovoltaic [12] , paramagnetic [13] , etc. Several methods for the production of ZnO undoped and doped with various types of dopants (rare heart, metals, lanthanides, etc.) [14] - [17] have been described, such as, electrodeposition [18] , evaporation [19] , vapor-liquid-solid (VLS) growth [20] , metal organic catalyst assisted vapor-phase epitaxy [21] , aqueous thermal decomposition [22] , microwave activated chemical bath deposition (MW-CBD) [23] , chemical bath deposition (CBD) [24] , surfactant- assisted hydrothermal method [25] , solution combustion [26] etc. This last method is more convenient than other because it is very fast, and less expensive; it has an easier composition control, and coating can be deposited on large area etc. The introduction of Cu²⁺ ions into ZnO lattice sums or produces changes that improve the ZnO properties such as: band gap tailored, magnetic and electrical properties, passivation of defects, as p-type dopant in the original n-type ZnO semiconductor. However, actually it is well known that the co-doping with Cu²⁺ ion in Eu³⁺ doped glasses photoluminescent compounds the Cu²⁺ ion can quench the Eu³⁺ photoluminescence [27] - [29] with the increase of the Cu²⁺ ion concentration in glasses matrix; this quenching effect can be used to tuning between ultra-violet and visible emission in devise photoluminescent. In this study co-doped ZnO/Cu²⁺ + Eu³⁺solid solution powders have been synthesized by solution combustion method as a function of the Cu²⁺ ion concentration in %Wt. maintaining the Eu³⁺ ion concentration constant at a value of 3% Wt. Finally the sample was annealed at 900˚C by 20 h. Also it is demonstrated that the quenching Eu³⁺ PL by the copper ion action also occurs in ZnO matrix.

2. Experimental

The synthesis method solution combustion is simple and fast caused by the chemical reaction oxidation-reduc- tion (REDOX) highly exothermic (1200˚C) [18] between an oxidizer agent and an fuel, in this experiment Zinc nitrate hexahydrate (Zn(NO₃)₂∙6H₂) was used as oxidizer agent and urea (NH₂CONH₂) as fuel, copper chloride (CuCl₂) and Europium chloride (EuCl₃) were used as dopants agents. The next chemical reaction (REDOX) between the oxidizer agent and the fuel is an reaction highly exothermic and is the Stoichiometric reaction obtained with a equivalence ratio value of unity (i.e. f = O/F = 1) where O and F are the total oxidizing and total reduction valences of the components and the energy released by the combustion is at maximum [18] . With this reaction are obtained the follow products: ZnO, H₂O (vapor), CO₂, and N_2.

Using the atomic weight concept, the Equation (1) was translated to grams/mol and used to obtain 3 gr of ZnO powder. The Cu²⁺ ion dopant concentration values were 0, 1, 2, 3, 10, and 20 wt%. The Eu³⁺ ion dopant concentration was maintained constant at 3 Wt% with respect to 3 gr of ZnO. From the translated grams/mol Equation (1) (no shown), 8.31 gr of Zn(NO₃)₂∙6H₂O, 3.68 gr of H₂NCONH₂ plus the numerous of grams corresponding to each Cu²⁺ ion concentration plus 3 Wt% Eu³⁺ ion were mixing with 20 ml of H₂O and stirring vigorously in a flask glass and after put on a hot plate at 500˚C, after of few minutes the reactive solution boils, foams, ignites, and burns with an incandescent flame at an approximate temperature of 1200˚C [18] producing 3 gr of ZnO powders approximately. After all the samples are annealing at 900˚C by 24 h. The ZnO/Cu²⁺+Eu³⁺ samples thus obtained were structurally characterized by X-ray diffraction (XRD) technique using a Philips PW 1800 diffractometer with Cu kα radiation (1.5406 Å), the photoluminescence (PL) of the ZnO/Cu²⁺+Eu³⁺ samples was studied by means of a spectrofluorometer Fluoro Max-P that uses a xenon lampas excitation source, The wavelength excitation was of 270 nm, and finally, the morphology of the ZnO/Cu²⁺+Eu³⁺ powders was recorded using a scanning electron microscopy (SEM) JEOL JSM 840 A.

3. Results and Discussion

3.1. Structural Characterization

The Figure 1 shows the X-ray diffractograms of the ZnO/Cu²⁺+Eu³⁺ powders as a function of the Cu²⁺ ion concentration and annealing at 900˚C by 20 h. From the XRD patterns, can be observed that for the Cu²⁺ doping of 1, 2, 3, 10 and 20%Wt all the diffraction peaks can be indexed to the majority phase hexagonal wurtzite type ZnO structure for all samples (JCPDS card #89-(102)), also, for the Cu²⁺ ion concentrations of 1, 2, 3 and 10%Wt a peak at 2θ = 28.5˚ is present and is assigned to the minority Eu₂O₃ phase (JPDAS card #86-2476). The diffractograms also reveal the high crystallinity of the product for the five Cu²⁺ ion concentrations, for the higher Cu²⁺ concentrations of 10 and 20%Wt. A peak corresponding to the Eu₂CuO₄ molecules is present; effectively: in Figure 2 can be see the peak at the 2theta scale at 24˚ correspond to the Eu₂CuO₄ phase, however also in Figure 2 appear all the peaks characteristic of the zinc oxide majority phase reported in this text.

Comparatively, contrary to the observation of Iribarren et al. [17] obtained from your experiments on Cu²⁺ doped ZnO, in our diffractograms can be not observed a little shift initially toward higher angles of the principal peak (101) until 3%Wt of Cu²⁺ where an maximum is reached, after at higher Cu²⁺ concentrations the angular displacement is toward lower angles, in the first case this is due to the substitution of the Zn ion with 0.06 nm size by the smaller Cu²⁺ ion with 0.057 nm size that cause an lattice parameter shrinkage, in the second case this is attributed to an relative saturation of the Cu²⁺ ions on the samples surface, in our case the not presence of angular changes can be due to the Eu³⁺ ions present in the ZnO lattice. No diffraction peaks were detected from other impurities. Using the Scherrer formula, the average crystallite size calculated from characteristic peak (101) was 160 nm for the ZnO intrinsic and decreased to 150 nm for the ZnO/Cu²⁺+Eu³⁺ samples doped with 10 Wt% of Cu²⁺ ion concentration.

Figure 1

XRD spectra of the Cu 2+ +Eu 3+ doped ZnO as a function of the Cu 2+ ion concentration in %Wt

Figure 2

XRD diffractogram of the ZnO and Eu 2 CuO 4 compound. Cu 2+ ion concentration for this figure was of 20 wt%

3.2. Morphological Study

The Figure 3(a) shows the SEM image of the form of the Eu³⁺ doped ZnO whit out Cu²⁺ ions, it is observed that the Eu³⁺/ZnO crystallites are agglomerated forming amorphous particles of 1.5 µm of large in an approximation. The Figure 3(b) shows the SEM image of a particle of Cu²⁺+Eu³⁺ doped ZnO polycrystalline particles, it can be see that the individual crystallites are agglomerated forming amorphous particles whit 2 µm side and 3 µm of large approximated.

Figure 3

SEM images of (a) undoped ZnO and (b) Cu 2+ +Eu 3+ doped ZnO with 10%Wt Cu ions.

(b)

3.3. Photoluminescence Study

The Figure 4(a) and Figure 4(b) shows the room temperature photoluminescence spectra (PL) of the Eu³⁺ doped ZnO without the Cu²⁺ ion dopant and of the Cu²⁺+Eu³⁺ doped ZnO (ZnO:Cu²⁺+Eu³⁺) nanocrystals as a function of the Cu²⁺ ion concentration in Wt% and after both samples undoped and doped with the Cu²⁺ were annealing at 900˚C by 24 h, the samples were irradiated with an excitation wavelength of 270 nm (in the UV range). In Figure 4(a) it is observed that the PL spectra correspond to the Eu³⁺ ion emission in a ZnO matrix, in particular the most intense peak at the visible color red whit 613 nm in wavelength [27] . However it is observed from Figure 4(b) that the ZnO:Cu²⁺+Eu³⁺ samples doped with 1, 2, 3, 10, and 20 Wt% are all alike: because they does not present antypical green broad emission band from 400 nm to 600 nm centered about 516 nm [28] as is showed in Figure 4(a), this band is due to the intrinsic defects emission of ZnO host. However, in the PL spectra of the ZnO:Cu²⁺+Eu³⁺ samples two peaks at the UV region in 350 and 380 nm are present, in this the intensity of this last peak I this observer that your intensity it is decreased at an concentration of 20% Cu²⁺, this decrease is due to the capacity of Quenching of the Cu²⁺ at hide concentration the first peak is attributed to the 5d®7f europium transition, the second band is attributed to near edge band (NEB) emission [29] , other peak at the violet-blue region centered about 420 nm also is observed, this peak is due to the 5D®4F Eu³⁺ transition. From the photoluminescence spectra corresponding to the Eu³⁺ ion also appear: the peaks at 591, 613 and 630 nm are related to the direct intra-4f transitions in Eu³⁺ ions ⁵D₀®⁷F_j (j = 1, 2, 3), the most intense emission is associated to the ⁵D₀®⁷F₂ transition in the red spectral region (613 nm) and is due to a allowed electric-dipole transition with inversion antisymmetric [28] [30] , which results in a large transition probability in the crystal field [31] - [34] . The peak at 591 nm is due to the ⁵D₀®⁷F₁ transition, is an allowed magnetic-dipole transition. The peak at 579 nm is due to the forbidden ⁵D₀®⁷F₀ transition due to the same total angular momentum, indicating that some Eu³⁺ ions possibly occupy other sites as interstitial sites [35] - [38] . It is very important to note from the photoluminescence spectra that as increase the Cu²⁺ ion concentration in the ZnO:Cu²⁺+Eu³⁺ samples the intensity of the most intense transition of the Eu³⁺ ion with red wavelength of 613 nm decrease until an negligible value while the intensity of the wavelength emission at 380 nm situated in the UV region increase until reach an constant value for high Cu²⁺ ion concentrations: effectively: the Figure 5 shows the emission intensity variation of the red ⁵D₀®⁷F₂ transition of the Eu³⁺ ion and of the UV emission peak at 380 nm of the ZnO:Cu²⁺+ Eu³⁺ samples as a function of the Cu²⁺ ion concentration, the graphic shows that as the Cu²⁺ ion concentration increase the red color intensity decrease until an minimum value while the intensity of the UV emission increase until obtain an constant value. It has been reported that divalent copper ion can effectively suppress Eu³⁺ photoluminescence, in an matrix of glass this is due to the quenching effect of Cu²⁺impuritieson Eu³⁺ emission photoluminescence quenching resulting from Eu³⁺→Cu²⁺ non-radiative energy transfer in which an spectral overlap occurring between Cu²⁺ absorption and Eu³⁺ emission(e.g. ⁵D₀→⁷F₂) transition occur [28] [29] . Thus, in this paper is postulated that the quenching of the Eu³⁺ photoluminescence also can occur in ZnO matrix.

Figure 4

(a) Photoluminescence spectra of Eu 3+ doped ZnO at 3%Wt; (b) Photoluminescence spectra of Cu 2+ +Eu 3+ doped ZnO as a function of the Cu 2+ ion concentration in %Wt.

(b)Figure 5

Intensity of the emissions Wavelength 380 and 613 nm as a function of the Cu 2+ ion concentration in %Wt

4. Conclusion

In this study Cu²⁺+Eu³⁺ doped ZnO solid solution powders were synthesized by solution combustion method maintaining the Eu³⁺ ion concentration fixed at 3%Wt and after annealing at 900˚C by 24 h. From the XRD study it is confirmed that the ZnO hexagonal wurtzite structure is conserved after the introduction of the Cu²⁺ ion in the ZnO structure, and the same XRD study showed the presence of the minority phases Eu₂O₃ and Eu₂CuO₄. From photoluminescence study it is demonstrated that the Cu²⁺ ion effectively suppresses the Eu³⁺ ion photoluminescence due to the PL quenching effect of the Cu²⁺ ion on the Eu³⁺ ion.

Acknowledgements

The authors wish to thank to Adriana Tejeda (IIM) for the XRD measurements. To Omar Novelo Peralta (IIM) (by you SEM study), to M.A. Canseco Martinez and Lázaro Huerta (IIM) by your chemical analysis.

References

1. Iribarren, A., Fernandez, P. and Piqueiras, I. (2008) Catodoluminescence of ZnO:Te with ZnTe and TeO2 Doping Precursors. Superlattices and Microstructures, 43, 600-604. http://dx.doi.org/10.1016/j.spmi.2007.06.009

2. Rahmani, M.B., Keshmiri, S.H., Shafiei, M., Latham, K., Wlodarski, W., Plessis, J.D. and Kalantar-Zadeh, K. (2009) Transition from n- to p-Type of Spray Pyrolysis Deposited Cu Doped ZnO Thin Films for NO2 Sensing. Sensor Letters, 7, 1-8. http://dx.doi.org/10.1166/sl.2009.1121

3. Lee, J.-B., Lee, H.-J., Seo, S.-H. and Park, J.-S. (2001) Characterization of Undoped and Cu-Doped ZnO Films for Surface Acoustic Wave Applications. Thin Solid Films, 398-399, 641-646. http://dx.doi.org/10.1016/S0040-6090(01)01332-3

4. Gao, D.Q., Xue,, D.S., Xu, Y., Yan, Z.J. and Zhang, Z.H. (2009) Synthesis and Magnetic Properties of Cu-Doped ZnO Nanowire Arrays. Electrochimica Acta, 54, 2392-2395. http://dx.doi.org/10.1016/j.electacta.2008.10.051

5. Qi, J., Gao, D.Q., Zhang, L. and Yang, Y.H. (2010) Room-Temperature Ferromagnetism of the Amorphous Cu-Doped ZnO Thin Films. Applied Surface Science, 256, 2507-2508. http://dx.doi.org/10.1016/j.apsusc.2009.10.096

6. Lupan, O., Pauporté, T., Viana, B. and Aschehoug, P. (2011) Electrodeposition of Cu-Doped ZnO Nanowire Arrays and Heterojunction Formation with p-GaN for Color Tunable Light Emitting Diode Applications. Electrochimica Acta, 56, 10543-10549. http://dx.doi.org/10.1016/j.electacta.2011.02.004

7. Pauporté, Th. and Lincot, D. (1999) Heteroepitaxial Electrodeposition of Zinc Oxide Films on Gallium Nitride. Applied Physics Letters, 75, 3817-3891. http://dx.doi.org/10.1063/1.125466

8. Mikami, M., Eto, T., Wang, J.F., Masa, Y. and Isshiki, M. (2007) Growth of Zinc Oxide by Chemical Vapor Transport Journal of Crystal Growth, 304, 37-41.

9. Levitt, A.P. (1970) Whisquer Technology. Wiley-Interscience, New York.

10. Park, W.I., Kim, D.H., Jung, S.-W. and Yi, G.-C. (2002) Metalorganic Vapor-Phase Epitaxial Growth of Vertically Well-Aligned ZnO Nanorods. Applied Physics Letters, 80, 4232-4234. http://dx.doi.org/10.1063/1.1482800

11. Vayssieres, L., Keis, K., Lindquist, S.-E. and Hagfeldt, A. (2001) Purpose-Built Anisotropic Metal Oxide Material: 3D Highly Oriented Microrod Array of ZnO. The Journal of Physical Chemistry B, 105, 3350-3352. http://dx.doi.org/10.1021/jp010026s

12. Wang, W.-W. and Zhu, Y.-J. (2004)Shape-Controlled Synthesis of Zinc Oxide by Microwave Heating Using an Imidazolium Salt., Inorganic Chemical Communications, 7, 1003-1005. http://dx.doi.org/10.1016/j.inoche.2004.06.014

13. Zwingel, D. and Gartner, F. (1974) Paramagnetic and Optical Properties of Na-Doped ZnO Single Crystals. Solid State Communications, 14, 45-49.

14. Sridevi, D. and Rajendran, K.V. (2010) Enhanced Optical Properties La Doped ZnO Nanoparticles Department of Physics, Presidency College, Chennai-5, India. Optoelectronics and Advanced Materials—Rapid Communications, 4, 1591-1593

15. Molahasani, N., Sadjadi, M.S. and Zare, K. (2013) Correlation of Morphology and Luminescent Properties of ZnO Nano Particles to Different Surfactants via Hydrothermal Method. International Journal of Nano Dimension, 4, 161- 166.

16. Zhang, J., Liu, H., Wang, Z., Ming, N., Li, Z. and Biris, A.S. (2007) Polyvinylpyrrolidone-Directed Crystallization of ZnO with Tunable Morphology and Bandgap. Advanced Functional Materials, 17, 3897-3905. http://dx.doi.org/10.1002/adfm.200700734

17. Iribarren, A., Hernández-Rodríguez, E. and Maqueira, L. (2014) Structural, Chemical and Optical Evaluation of Cu- Doped ZnO Nanoparticles Synthesized by an Aqueous Solution Method. Materials Research Bulletin, 60, 376-381.

18. Miamani, T. and Patil, K.C. (2001) Solution Combustion Synthesis of Nanoscale Oxides and Their Composites. Materials Physics and Mechanics, 4, 134-137.

19. Chow, L., Lupan, O., Chai, G., Khallaf, H., Ono, L.K., Roldan Cuenya, B., Tiginyanu, I.M., Ursaki, V.V., Sontea, V. and Schultea, A. (2013) Synthesis and Characterization of Cu-Doped ZnO One-Dimensional Structures for Miniaturized Sensor Applications with Faster Response. Sensors and Actuators A: Physical, 189, 399-408. http://dx.doi.org/10.1016/j.sna.2012.09.006

20. Xing, G.Z., Yi, J.B., Tao, J.G., Liu, T., Wong, L.M., Zhang, Z., Li, G.P., Wang, S.J., Ding, J., Sum, T.C., Huan, C.H.A. and Wu, T. (2008) Comparative Study of Room-Temperature Ferromagnetism in Cu-Doped ZnO Nanowires Enhanced by Structural Inhomogeneity. Advanced Materials, 20, 3521-3527. http://dx.doi.org/10.1002/adma.200703149

21. Singhal, S., Kaur, J., Namgyal, T. and Sharma, R. (2012) Cu-Doped ZnO Nanoparticles: Synthesis, Structural and Electrical Properties. Physica B: Condensed Matter, 407, 1223-1226. http://dx.doi.org/10.1016/j.physb.2012.01.103

22. Oztas, M. and Bedir, M. (2008) Thickness Dependence of Structural, Electrical and Optical Properties of Sprayed ZnO:Cu Films. Thin Solid Films, 516, 1703-1709. http://dx.doi.org/10.1016/j.tsf.2007.05.018

23. Bellini, J.V., Morelli, M.R. and Kiminami, R.H.G.A. (2002) Electrical Properties of Polycrystalline ZnO:Cu Obtained from Freeze-Dried ZnO+Copper(II) Acetate Powders. Journal of Materials Science: Materials in Electronics, 13, 485- 489. http://dx.doi.org/10.1023/A:1016160204435

24. Liu, Y., Liang, H.W., Xu, L., Zhao, J.Z., Bian, J.M., Luo, Y.M., Liu, Y., Li, W.C., Wu, G.G. and Du, G.T. (2010) Cu Related Doublets Green Band Emission in ZnO:Cu Thin Films. Journal of Applied Physics, 108, 113507. http://dx.doi.org/10.1063/1.3516459

25. Tian, Y.F., Li, Y.F., He, M., Putra, I.A., Peng, H.Y., Yao, B., Cheong, S.A. and Wu, T. (2011) Bound Magnetic Polarons and p-d Exchange Interaction in Ferromagnetic Insulating Cu-Doped ZnO. Applied Physics Letters, 98, 162503. http://dx.doi.org/10.1063/1.3579544

26. Reddy, A.J., Kokila, M.K., Nagabhushana, H., Chakradhar, R.P.S., Shivakumara, C., Rao, J.L. and Nagabhushana, B.M. (2011) Structural, Optical and EPR Studies on ZnO:Cu Nanopowders Prepared via Low Temperature Solution Combustion Synthesis. Journal of Alloys and Compounds, 509, 5349-5355. http://dx.doi.org/10.1016/j.jallcom.2011.02.043

27. López-Romero, S., Quiroz-Jiménez, M.J., García, M.H. and Aguilar-Castillo, A. (2014) Bright Red Luminescence and Structural Properties of Eu3+ Ion Doped ZnO by Solution Combustion Technique. World Journal of Condensed Matter Physics, 4, 227-234.

28. Jiménez, J.A. (2015) Photoluminescence of Eu3+-Doped Glasses with Cu2+ Impurities. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 145, 482-486. http://dx.doi.org/10.1016/j.saa.2015.03.047

29. Batyaev, I.M. and Tinus, A.M. (1998) Transport of Electronic Excitation Energy in Solid-State Glassy Phosphors Activated with Europium (III) and Copper (II). A.I. Gerstsen Russian State Pedagogical University, St. Petersburg. Pis’ma Zh. Tekh. Fiz, 24, 59-61.

30. Sharma, P.K., Dutta, R.K. and Pandey, A.C. (2009) Doping Dependent Room-Temperature Ferromagnetism and Structural Properties of Dilute Magnetic Semiconductor ZnO:Cu2+ Nanorods. Journal of Magnetism and Magnetic, 321, 4001-4005. http://dx.doi.org/10.1016/j.jmmm.2009.07.066

31. Ursaki, V., Rusu, E., Zalamai, V., et al. (2004) Photoluminescence of Eu Doped ZnO Structures. Proceeding of SPIE 5822, Chisinau, 3 May 2004, 148.

32. Fujihara, S.O., Suzuqui, A. and Kimura, T. (2003) Ga-Doping Effects on Electrical and Luminescent Properties of ZnO: (La,Eu) of Red Phosphor Thin Films. Journal of Applied Physics, 94, 2411-2416. http://dx.doi.org/10.1063/1.1594817

33. Qi, J.F., Matsumoto, T., Tanaka, M. and Masumoto, Y. (2000) Europium Silicate Thin Films on Si Substrates Fabricates by a Radio Frequency Sputtering Method. Journal of Physics D: Applied Physics, 33, 2074. http://dx.doi.org/10.1088/0022-3727/33/16/321

34. Ningthoujam, R.S. and Nanoci, J. (2008) Dispersible Li+ and Eu3+ Co-Doped Nanocrystalline: ZnO Luminescence and EPR Studies. Journal of Nanoscience and Nanotechnology, 8, 3059-3062. http://dx.doi.org/10.1166/jnn.2008.152

35. Chen, P.L., Ma, X.Y. and Yang, D.R. (2007) ZnO:Eu Thin Films: Sol-Gel Derivation and Strong Photoluminescence from 5D0 → 7F0 Transition of Eu3+ Ions. Journal of Alloys and Compounds, 431, 317-320. http://dx.doi.org/10.1016/j.jallcom.2006.05.078

36. Luo, L., Gong, L., Liu, Y.F., et al. (2010) Enhanced Ultraviolet Lasing from Europium-Doped Zinc Oxide Nanocrystals. Optical Materials, 32, 1066-1070. http://dx.doi.org/10.1016/j.optmat.2010.02.032

37. Hsu, C.-H., Chen, L.-C. and Zhang, X.Y. (2014) Effect of the Cu Source on Optical Properties of CuZnO Films Deposited by Ultrasonic Spraying. Materials, 7, 1261-1270.

38. Babikier, M., Wang, D.B., Wang, J.Z., Li, Q., Sun, J.M., Yan, Y., Yu, Q.J. and Jiao, S.J. (2014) Cu-Doped ZnO Nanorod Arrays: The Effects of Copper Precursor and Concentration. Nanoscale Research Letters, 9, 199. http://dx.doi.org/10.1186/1556-276X-9-199