Luminescence and physical properties of copper doped CdO derived nanostructures

f b, F. Yakuphanoglu c, A. Avila-Garcı´a d, A. Tavira d, TOM key 0 M udio o, CA a r t i c l e i n f o Article history: a b s t r a c t In this paper, we studied the photoluminescence (PL), the morphological, electrical and optical with a lattice parameter of a¼4.69 A˚ was confirmed by X-ray diffraction. Copper was shown to improve morphologies like nanobelts, nanorods, nanotubes, and nano- preparation of nanostructured films used in a variety of devices Half amole of cadmiumacetate dihydrate Cd(CH3COO)2.2(H2O) supplied Contents lists available at SciVerse ScienceDirect .els Journal of Lum Journal of Luminescence 132 (2012) 2653–2658 0.3ml of theMono-Ethanolamine (C2H7NO; abridgedMEA) as stabilizer,[email protected] (M. Benhaliliba). by Himedia with purity of 99%, was dissolved in 2-Methoxyethanol (C3H8O2) at 0.5:10ml, and stirred at 60 1C for 10min. The doping precursor, copper (II) acetate anhydrous Cu(CH3COO)2, supplied by Carlo Erba reagents with purity of 98%, was added to the solution and then 0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2012.03.044 n Corresponding author. Tel.: þ213 772211491, þ213 41429212; fax: þ213 41424931. E-mail addresses: [email protected], fibers [15–18]. The sol–gel process is generally used for the The CdO films were grown by sol–gel spin coating route onto microscope glass slides (76�26) mm2 supplied by object trager Isolab. [8], chemical bath (CBD) [9], spray pyrolysis [10], sputtering [11] and thermal evaporation [12]. Many elements such as Al, Fe, F and Ga [7,13,14] can be incorporated in the CdO material as dopants. CdO nanostructures have been successfully prepared in various 2. Experimental procedure 2.1. Films growth 1. Introduction Cadmium oxide (CdO) is one of the promising II–VI compound semiconductors that have great potential for optoelectronic devices [1]. CdO belongs to a family of transparent conductive oxides (TCO) such as zinc oxide (ZnO) [2–3], indium oxide In2O3 [4] and tin oxide (SnO2) [5], materials that have prompted a substantial number of research works. CdO is n type and direct- band gap semiconductor with an energy gap of 2.3 eV [6]. It crystallizes in the cubic configuration with a lattice parameter of 4.63 A˚ [7]. CdO can be produced by many processes like sol–gel [19]. These characteristics make cadmium oxide a good candidate for solar cells, phototransistors, photodiodes, transparent electro- des and gas sensors [20,21]. This study focuses on the preparation and characterization of CdO produced by facile sol–gel spin coating and doped with copper at low proportions (2% and 3%). Copper doping effects on the structural, optical, morphological, electrical and photoluminescence properties of CdO are studied. Photoluminescence of pure and copper doped CdO at ambient is investigated. Received 22 June 2011 Received in revised form 16 January 2012 Accepted 26 March 2012 Available online 16 May 2012 Keywords: CdO thin films Spin-coating Sol–gel Crystalline structure Cu doping Photoluminescence. the optical transmittance in the short wavelength range of the visible spectrum. The optical band gap of CdO ranged between 2.49 and 2.62 eV as a result of Cu content. At room temperature, resistance fell drastically with Cu doping levels. AFM analysis of samples exhibited nano-mounts and nanowires. Finally, PL results showed a strong blue–violet emission peak at 2.80 eV. & 2012 Elsevier B.V. All rights reserved. properties of pure and copper-doped cadmium oxide. CdO films were grown by a facile sol–gel spin coating process at 1200 rpm, and doped with copper at 2 and 3%. A (1 1 1)-oriented cubic structure Luminescence and physical properties o CdO derived nanostructures M. Benhaliliba a,n, C.E. Benouis a, A. Tiburcio-Silver R.R. Trujillo e, Z. Mouffak f a Physics Department, Sciences Faculty, Oran University of Sciences and Technology US b ITT-DIEE, Apdo, Postal 20, Metepec 3, 52176, Estado de Mexico, Mexico c Firat University, Physics Department, Faculty of Sciences and Arts, 23119, Elazig, Tur d Cinvestav-IPN, Departamento de Ingenierı´a Ele´ctrica-SEES, Apdo. postal 14-740, 0700 e Centro de Investigacio´n en Dispositivos Semiconductores—BUAP 14 Sur y Av. San Cla f Department of Electrical and Computer Engineering California State University, Fresn journal homepage: www B, BP1505 Oran, Algeria e´xico, D.F., Mexico , C.U. Puebla, Pue. Me´xico , USA copper doped evier.com/locate/jlumin inescence Cu doping slightly shifts the position of diffraction intensities (2y) to higher angles. The ionic radii of Cu (II) and Cd (II) are 0.73 A˚ and 0.95 A˚, respectively, and therefore the ionic radius ratio is thus rCu(II)/rCd(II)¼0.77. Since, cooper has a smaller radius than that of cadmium, it may be easy for it to diffuse in the host lattice, while causing a slight mismatch, which induces the minor angle shift mentioned above. Fig. 2 illustrates grain size of samples grown by the spin coating route according to [1 1 1], [2 0 0] and [2 2 0] directions. X-rays peaks broadening lead to small grain size (from 8 to 1170.01) nm as listed in Table 2, this fact confirms the nanostructures characteristic of our coated films. Fig. 2B depicted the grain size according to main peaks with error bars. Grain sizes with a diameter of 23 nm and 21 nm are reported by researchers for pure coated CdO [22,24]. In addition, grain size is increased by the Cu doping level (2%). 3.2. Optical characterization Fig. 3 depicts the transmittance of CdO and CdO:Cu films within the wavelength range 200–2500 nm. The optical transmit- is shown. JCPDS card no. 05-0640 line are sketched by blue lines in the bottom. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Table 1 Peak positions, [h k l] planes, Bragg angles 2y (1), and angle shift D(2y)¼2y doped CdO–2yCdO JCPDS (1) of pure (sample A) and Cu (2% and 3%) (samples B and C) doped CdO films. Sample [h k l] Bragg angle 2y (1) Angle shift D(2y) (1) A [11 1] 32.92 – [20 0] 38.12 – [22 0] 55.06 – [31 1] 65.70 – B [11 1] 32.95 0.03 [20 0] 38.26 0.14 [22 0] 55.13 0.07 [31 1] 66.25 0.55 C [11 1] 33.08 0.16 [20 0] 38.43 0.31 [22 0] 55.30 0.24 [31 1] 66.42 0.72 M. Benhaliliba et al. / Journal of Luminescence 132 (2012) 2653–26582654 3. Results and discussion 3.1. Structural analysis Fig. 1 shows the X-rays patterns of the pure and Cu doped coated CdO films depicted against the Bragg angle that ranged from 201 to 801. A number of diffraction peaks appeared at 2y¼32.921, 38.121, 55.061, 65.701 and 69.701. These peaks are associated with [1 1 1], [2 0 0], [2 2 0], [3 1 1] and [2 2 2] planes respectively, exhibiting a pure CdO crystalline phase. The grain size G was estimated by the well-known Scherrer’s formula [3], G¼ 0:94l b cosy ð1Þ where b is the FWHM (Full-Width at Half-Maximum) of the peak, 2y is the Bragg angle and l is the X-rays wavelength. The comparison of the observed XRD patterns with the standard JCPDS data (05-0640) confirms the structure of pure CdO phase with face-centered cubic crystal structure [22]. The Bragg position for strong reflections like [1 1 1] direction was 32.921, 33.081 and 33.091 respectively for pure and doped (2% and 3%) CdO films, and then a slight angle shift, estimated at 0.03–0.161, was carefully detected as sketched in Fig. 1. Others reflection positions (2y) and their angle shifts are listed in Table 1. These results are in good agreement with those found in the literature [23,24]. It seems that the CdO films have a preferential growth along the [1 1 1] direction. Textural coefficient (TC) for four reflections, is expressed as I111/(1/4) (I111þ I200þ I220þ I311) [3] and data are tabulated according to the [1 1 1] direction. At low doping level the crystalline structure is maintained, due to slight change of TC (DTC�0.08). Similar TC was obtained by Moholkar et al. [25]. Consequently, copper doping level increases the grain was added drop by drop until the homogeneous and clear solution was obtained, then the stirring continued for 60min. The solution was then aged at ambient for one day until the gel formation. The substrates were cleaned by a soft soap solution, washed thoroughly under distilled water jets, then ultrasonically by immersion in with ethanol and finally dried with argon. The viscous solution was homogenously poured by micro- pipette on the substrate, sticking it on the stainless steel spin plate of a MTI, EQ-TC-100 Desk-top Spin Coater. The sample rotated for one minute at 1200 rpm, and then was heated under air on hot-plate at 150 1C for 10min. This process was repeated 5 times, before the film was finally annealed in air at 400 1C for 60min at the furnace. 2.2. Films characterization Bruker AXS D8 Discover diffractometer with CuKa1 radiation (l¼1.5418 A1) between 201r2yr801 was used for X-ray diffrac- tion investigations. The coated films transmittance and reflec- tance were recorded by a Shimadzu 3600 PC double beam UV– vis–NIR spectrophotometer, and the electrical resistance at ambi- ent was measured by the four point probes method. AFM analysis of the spin coated films was made by using a Quesant Model 250 system having an 80�80 mm head, operating in the wave mode in air. For the 10�10 mm square images the resolution was 300�300 pixels at a fixed scan rate of 2 Hz. All analyses were performed with the software from the WSXM system. Room temperature photoluminescence measurements was carried out in an experimental setup consisting of a 325 nm, 15 mW He–Cd laser (Kimmon type), a 0.85 m double monochromator (SPEX, model 1404), and a GaAs photon counting photomultiplier (Hamamatsu). The range explored is from 350 to 600 nm, in steps of 0.5 nm and at a speed of 0.2 s per measured point. size, while 3% copper level doping reduces it. As shown in Fig. 1, 20 30 40 50 60 70 80 0 50 100 JCPDS05-0640 In te ns ity (C PS ) CdO 2 % Cu :CdO 3 % Cu:CdO (111) (200) (220) (311) (222) 2 Degree) Fig. 1. X-rays pattern of pure and copper doped CdO grown by spin coating process at 1200 rpm, Bragg angle ranges within 20–801, indexation of main peaks tance improves with copper addition by up to 85% for CdO:3% Cu samples in the UV–vis range. Same trends are reported by Kumaravel et al. [10,26]. Subramanyam et al. have found the same trend profile of transmittance within VIS and IR spectra for the DC sputtered CdO films [27]. We remark that Cu ions improve considerably the optical transmittance mainly in visible range (see inset of Fig. 3). The average transmittance at 550 nm increases with doping level, as listed in Table 2, and confirms the previous statement. We mention that low copper level doping improves the transmittance around visible edge (750–800 nm) where transmittance of 3% Cu doped CdO approaches the pure glass transparency as can be easily seen inset of Fig. 4. The direct optical band gap (Eg) is expressed as [3], ahn¼ ðhn�EgÞ1=2 ð2Þ where Eg (eV) is the optical band gap, a (m�1) is the absorption coefficient and n (Hz) is the photon frequency. We estimate the band gap Eg from the optical transmission spectra by extrapolat- ing the linear part of the plot of (ahn)2 versus hn to a¼0 as sketched in Fig. 4A. It should be noted that Cu doping level has an influence on Eg as listed in Table 2. The values of optical band gap are assessed with an uncertainty of 0.01 eV. A direct band gap of 2.17 was reported by Ma et al. [28]. Akyuz et al.have reported a band gap found to be 2.40 eV for CdO and 2.56 eV for fluorine doped CdO [29]. Similar results were reported by Kumaravel et al. (Eg �2.53 eV) [26]. Others works exhibit an average of Eg around 2.46 eV [27], CdO doped with hydrogen revealed a gap of 2.44 eV ([30] dakheel 2012). The estimated energy gap from (ahn)2, and dT/dl (not shown here) of pure and Cu-doped CdO films are given in Table 2, from the two methods a minor discrepancy of Eg is detected. These gap values lead to a blue shift which may be explained by a Burstein–Moss effect. Our results are in well agreement with those reported in the literature [6]. Absorption profile is depicted in Fig. 4B, and the absorbance maximum is reached in the UV spectrum with the following band gaps found to be 4.356 eV, 4.264 eV and 4.265 eV respectively for pure, 2% and 3% Cu doped CdO films. Cd O2 C u C dO3 C u C dO 0 2 4 6 8 10 12 220 200 111 G ra in S iz e (n m ) CdO 2 Cu CdO 3 Cu CdO 8 9 10 11 CdO 2 Cu CdO 3 Cu CdO 7.50 8.25 9.00 9.75 CdO 2 Cu CdO 3 Cu CdO 6.00 6.75 7.50 (111) G ra in s iz e (n m ) (200) (220) Fig. 2. A. Grain size is plotted as function of the copper content according to (1 1 1), (2 0 0) and (2 2 0) planes respectively. Grain size with error bars are shown nt T ure 40 60 80 100 100 CdO 2 % Cu 3 % Cu itt an ce ( % ) M. Benhaliliba et al. / Journal of Luminescence 132 (2012) 2653–2658 2655 in Fig. 2B. Table 2 Grain size values according to [1 1 1], [2 0 0] and [2 2 0] directions, textural coefficie methods), maximum UV absorbance, and transmittance calculated at 550 nm of p Sample Grain size (nm ) TC [1 1 1] [2 0 0] [2 2 0] A 8.23 8.24 6.20 1.80 B 11.25 9.66 7.47 1.72 C 8.80 7.85 7.12 1.73 C according to [1 1 1] direction, band gap Eg (eV) (calculated from (ahn)2 and dT/dl (sample A) and Cu (2% and 3%) (samples B and C) doped CdO films. Eg (eV) Maximum absorbance (eV) T (550 nm) (%) (ahn)2 dT/dl 2.49 2.59 4.356 56 2.50 2.59 4.264 68 2.56 2.62 4.265 79 0 500 1000 1500 2000 2500 0 20 400 500 600 700 800 20 40 60 80 Glass T(λ) VIS Tr an sm Wavelength (nm) Fig. 3. Transmittance variation with photon wavelength of pure, 2% and 3% Cu doped CdO produced by spin coating at 1200 RPM, inset shows VIS transmittance profile of pure, 2% and 3% Cu doped CdO and pure glass of substrate (blue solid curve). Rapid increase in IV range (blue arrow), maximum reached in VIS range (black arrow) and highest transmittance in NIR spectrum (red arrow). (For inter- pretation of the references to color in this figure legend, the reader is referred to the web version of this article.) M. Benhaliliba et al. / Journal of Luminescence 132 (2012) 2653–26582656 3.3. Surface morphology investigation The surface morphology and the roughness of samples are sketched in Fig. 5. 2D and 3D views have dimensions of 10�10 mm. We can observe that surface of selected samples is homogenous and exhibit a little voids signed by circles in 2D image (Fig. 5 left). Moreover, a careful observation of pure CdO surface shows grains like mounts with no well defined boundaries, having a grain size around 75 nm and a height around 740 nm. The nano-mounts are agglomerated with different roughness; the bright ones (signed by arrows in Fig. 5A) attract more atoms during the growth process than the dark ones which might demonstrate voids. Similar trends were reported in literature [26,31]. While the 2% Cu doped CdO sample reveals columns like wires which are separated and grown in the same direction having an average height equals to 275 nm, and the 3% Cu doped CdO shows the same shape of wires with large nanowires density ( number of nanowires per mm2 ) and most important height (�382 nm). Here, clusters exhibit different sizes of 186 nm and 208 nm for the 2Cu% and the 3Cu% doped CdO respectively, while Deokate et al. have obtained nano-needles 0 500 1000 1500 2000 2500 0 1 2 3 4 CdO 2% Cu 3% Cu A bs or ba nc e (% ) Wavelength (nm) Fig. 4. (A) Sketch of (ahn)2 against incident photon energy (hn) of undoped CdO, 2% and 3% Cu doped CdO, (extrapolation straight lines are depicted). Inset described the error bars of calculated optical band gap Eg of pure and Cu doped CdO films. (B) The absorbance is plotted versus photon wavelength of pure and Cu (2% and 3%) doped CdO. around 2000 nm for pure and fluorine doped CdO prepared by spray pyrolysis [32]. In our case, it is apparent that copper doping level enlarges the crystallite sizes, while low copper doping level has an effect on the surface morphology of coated CdO films and tends to shape the nano-grains into nanowires. Insertion of copper ions reduces the grains as depicted in microphotographs (Fig. 5), similar trends were found by Gupta et al. [33]. Nanopar- ticles of Cu doped CdO were obtained by solid state synthesis [34]. 3.4. Resistance measurement The electrical measurements were carried a four probe mea- surement system. The electrical resistance at room temperature is sketched in Fig. 6. The electrical resistance R was calculated by using [35] R¼ k V I ð3Þ where k is a constant found to be 4.53, V is the applied voltage and I is the intensity of DC current. It is observed that copper level doping diminishes the resistance from 4 kO to 10O. This fact is due to copper, which has two level of oxidation I and II, that can substitute to cadmium sites and offers free electrons, thus improving conductivity. Grain size increases with decreasing of resistance due to low effect of boundary grains. The hall measure- ment helped evaluate parameters such as carrier concentration and mobility, resistivity and Hall coefficient of pure and Cu-doped CdO films. The sample (2% Cu doped CdO) exhibited the lower resistivity as sketched in Fig. 6. We may conclude that copper improves the electrical properties of cadmium oxide. 3.5. Photoluminescence spectra The analysis of photoluminescence (PL) spectroscopy at room temperature reveals various peaks as shown in Fig. 7. We use the photoluminescence spectroscopy to determine the band gap of semiconductors since the most common radiative transition in the semiconductor occurs between states at the bottom of the conduc- tion band and the top of the valance band [36]. The PL spectra are consisted of VIS emission peak centered around 440 nm (peak a), a distinct strong emission peaked at 580 nm (peak g) which corre- sponds to violet and orange region of electromagnetic spectrum respectively, and some weak visible emission bands in the range 475–520 nm. Moreover, the strong ones are found at 2.11 (g), 2.30 (f), 2.32(e), 2.44 (d), 2.47 (c), 2.58 (b) and 2.80 eV (a). The intense emission �442 nm (peak a) might be attributed to the combination of the electrons from the conduction band and holes from the valence band. Peaks b, c, d, e and f, which are lower than peaks a and g, are ranged in the green region from 490 nm (2.53 eV) to 570 nm (2.1 eV) which may be ascribed to defects centers. The copper doping reduces the intensity of peak emission and roughly no PL peak was shifted while some emission intensities are improved by doping like peaks b, c, d and lower peaks between peaks a and b. Consequently defects sites increase whith Cu doping which is confirmed by the PL signal. Similar ranges of PL peak positions 2.2–2.6 eV have been reported in literature [31,37,38], whereas the PL peak of pure CdO is located at 3.11 eV (�399 nm) as reported in literature [36]. The Gaussian deconvolution (GD) is expressed as I¼ I0þ(A exp (�2 ((l�lc)/w)2)/(w(p/2)1/2)), where Ic and lc are the PL intensity and the wavelength of peak center respectively, and w1 is the FWMH given by w¼w1/(ln 4)1/2 where w is the peak width. GD of the VIS spectrum exhibits the main peaks at 439.45673.345, and 592.43471.032 (nm) for pure CdO, 441.57870.027 and 587.63770.015 for 2% Cu doped CdO, 587.68570.015 and 441.58970.025 for 3% Cu doped CdO as can be seen inset of Fig. 7. Fig. 5. Atomic force microscope (AFM) topography image of coated films with Cu contents (%). AFM pictures are 10�10 mm2, grain size and voids are shown by cicles and grain size by two arrows respectively (left) of 2D view and 3D view, height is shown at left corner of 3D image, arrows show nano-mounts Fig. 5(A), and nanowires Figs. 5(B) and (C) (right). M. Benhaliliba et al. / Journal of Luminescence 132 (2012) 2653–2658 2657 0 1 2 3 2x104 4x104 6x104 8x104 105 Cu content (%) R es is tiv ity ( Ω .c m ) 0 100 200 C arrier M obility (cm ²/ V s) Fig. 6. Semilog plot of electrical resistivity (left), carrier mobility (right) versus Cu content (%). e f g 4. Conclusions CdO nanostructures have been successfully produced by a facile sol–gel route. The pure and Cu doped CdO films structural, 2D and 3D AFM views, optical, electrical and photoluminescence properties were investigated. XRD pattern reveals that coated CdO films are poly- crystalline and grow according to a preferential [1 1 1] orientation. A peak broadening reveals nanostructures formations which are con- firmed by AFMmorphology investigation. The grain size exceeds then 11 nm and textural coefficient decreases a little with copper doping level. A peak broadening reveals nanostructures formation within our coated films. High transparent coated films in VIS–IR range are obtained and low copper level doping improves the transmittance mainly in visible band edge from 56% to 79%, and exceeds the point of 90% in the IR spectrum. The band gap increases with copper level doping and as shown by a blueshift in our PL results. Low copper doping level maintains the crystalline structure, extends the grains into highly transparent nanowires and reduces the material electrical resistance. These characteristics of high VIS–IR transparency and low resistive nanowires can make our coated films, produced by a facile sol–gel route, suitable for various applications in material sciences and optoelectronics devices, such as sensors, which can be investi- gated in future work. 350 400 450 500 550 600 0 1000 2000 3000 4000 PL I nt en si ty ( a. u .) Wavelength (nm) CdO 2 % Cu:CdO 3 % Cu:CdO a b c d Fig. 7. Photoluminescence behavior against photon wavelength (range 300– 650 nm) of pure and Cu (2% and 3%) doped CdO. Inset shows the deconvolution of PL spectra (main peaks) of coated CdO films (inset1) and Cu doped films (2% and 3% Cu insets 2 and 3). 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