The effect of strontium doping on structural and morphological properties of zno nanofilms synthesized by ultrasonic spray pyrolysis method

Journal of Science: Advanced Materials and Devices (2018) 29e36 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article The effect of strontium doping on structural and morphological properties of ZnO nanofilms synthesized by ultrasonic spray pyrolysis method A Ouhaibi a, M Ghamnia a, *, M.A Dahamni a, V Heresanu b, C Fauquet b, D Tonneau b a b Laboratoire LSMC, D epartement de Physique, Universit e d'Oran Ahmed Ben Bella, 31100 Oran, Algeria Centre CINaM, Campus de Luminy, Universit e d'Aix-Marseille, Marseille 13009, France a r t i c l e i n f o a b s t r a c t Article history: Received 20 December 2017 Received in revised form 26 January 2018 Accepted 26 January 2018 Available online 16 February 2018 Pristine and strontium doped ZnO nanometric films were successfully synthesized on heated glass substrates by the ultrasonic spray pyrolysis technique The samples were characterized by means of X-ray diffraction (XRD), Atomic Force Microscope (AFM), UVevisible spectroscopy and photoluminescence (PL) X-ray diffraction patterns confirmed the hexagonal (wurtzite) structure, where the most pronounced (002) peak indicates the preferential orientation along the c-axis perpendicular to the sample surface The intensity of this peak was increased rapidly from the first doping of 1% and its position was shifted toward higher angles under Sr-doping effect For the used doping range of 1e5%, the Sr-doping at 3% attracted an especial attention At this concentration, the particular transformation in the surface morphology of doped ZnO films was observed The surface became granular and rough by expanding the crystallites' size From optical measurements, transmittance and PL spectra were found to be sensitive to Sr-doping, where two different behaviors were observed before and after 3% of Sr-doping © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Ultrasonic spray pyrolysis Sr-doped ZnO Morphology study Optical properties Introduction Pristine and doped zinc oxide (ZnO) is among the most studied materials because of its interesting characteristics such as its easy synthesis, its non toxicity, its chemical stability, its suitability for doping with different metals ZnO has several favourable properties such as good transparency, strong room temperature luminescence, high electron mobility In materials science, ZnO is a n-type semiconductor with a wide direct bandgap (3.37 eV), a large excitation binding energy (60 meV) and high transmission in the visible range For these important physical properties, ZnO is used successfully in a variety of applications such as in electronics, in optoelectronic devices, in solar cells, in light emitter diodes [1e5] ZnO thin films can be easily nanostructured and synthesized by several techniques The growth techniques must be physical as sputtering, evaporation, pulsed laser deposition, … [6e8] or chemical as solegel, chemical vapour deposition (CVD), metalorganic CVD, hydrothermal and spray pyrolysis … [9e13] Among these chemical synthesis methods, we explored in this paper, the ultrasonic spray pyrolysis technique for its low cost and especially for its simplicity to implement for fabricating oxide thin films of good qualities The crystalline quality, through the control in composition and the synthesis in large scale on substrates, is easily obtained by this technique In order to improve some physical properties of ZnO, lot of works has been carried out on the doping of ZnO thin films ZnO has often been doped with metal ions such as Mn, Al, Ni [14e16] It has been shown that the ferromagnetism, the magnetism, the performance of organic solar cells or the conductivity related to the structural, optical and electrical properties are improved after having doped ZnO It is in this context that the present paper is inscribed It is about the synthesizing and characterizing of strontium (Sr) doped ZnO A few work were carried on the strontium doped ZnO obtained by chemical synthesis or physical growth We can mention the work of K Pradeev Raj et al [17] on SreZnO obtained by the co-precipitation method, the work of Xu et al [18] who used the solegel method and the works of Raghavendra et al [19,20] who studied Sr-ZnO using the spray pyrolysis synthesize Sr is an * Corresponding author LSMC Laboratory, Oran University, 31000, Oran, Algeria E-mail address: mghamnia@yahoo.fr (M Ghamnia) Peer review under responsibility of Vietnam National University, Hanoi https://doi.org/10.1016/j.jsamd.2018.01.004 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 30 A Ouhaibi et al / Journal of Science: Advanced Materials and Devices (2018) 29e36 element which has a large cationic radius (1.18 Å) and a heavy atomic weight (87.62 g) in comparison with zinc (the ionic radius of 0.6 Å and the atomic weight of 65.4 g) Due to this size effect (the radius ratio RSr =RZn ¼ 1.96), doping with Sr is somehow difficult to obtain It induces changes in structural, morphological and optical properties of ZnO In this work, we show that the strontium doping of concentrations ranging from to 5% caused a significant modification of the surface state Experimental Pristine and Sr-doped ZnO nanofilms were synthesized using the ultrasonic spray pyrolysis technique As reported in reference [21], this technique differs slightly from the classical spray pyrolysis We dissolved zinc acetate di-hydrate (Zn (CH3OO)2, 2H2O) salt as precursor of the ZnO particles in 100 ml of methanol for obtaining a transparent solution concentred at 0.3 M LÀ1 To obtain ZnO doped with strontium (Sr), we added to the 0.3 M LÀ1 solution different amounts of strontium chloride hexahydrate (SrCl2, 6H2O) In this way, we got the following doping: 1%, 2%, 3%, 4%, and 5% The resulting aqueous solution was stirred for 24 h before spraying it onto heated glass substrates Before spraying and in order to eliminate residual contamination caused by air contact, the glass substrates were previously cleaned in diluted acetone and rinsed in deionised water for several cycles After the chemical cleaning, the samples were dried with nitrogen gas In Fig we present a simplified schematic of the ultrasonic spray pyrolysis assembled by us After having vaporized ultrasonically the solution, the vapour is sprayed and deposited on glass substrates heated at 350  C and held at a 20 cm from the spray nozzle With the deposit time, ZnO films were thus prepared The structural characteristics of the pristine and Sr-doped ZnO films were examined by x-ray diffraction using Cu-Ka source of wavelength of 1.54 Å The state of the surface morphology was characterized by AFM in a tapping mode The optical properties were studied at room temperature by using uvevisible spectroscopy and photoluminescence (PL) Results and discussion 3.1 Structural and morphological characterization of ZnO and Sr-ZnO thin films The XRD patterns of pristine and Sr-doped ZnO are shown in Fig 2(a) From this figure, six orientations of different intensities Fig Simplified scheme of the ultrasonic pyrolysis technique can be identified: (100), (002), (101), (102), (110) and (103) According to JCPDS 036-1451 card, these peaks indicate the hexagonal (wurtzite) structure of ZnO As we can observe from these spectra, the (002) and (101) planes are the most pronounced As the (002) peak is the most intense, the ZnO growth is preferentially made in this direction along the c-axis perpendicular to the sample surface But its intensity does not follow the increase in Sr-doping concentration It is observed to be increased rapidly from the first doping (1%) and then it slowly decreases (Fig 2(b)) till 3% of Sr-doping and increases again from to 5% This may be explained by the size effect of strontium (RSr/RZn ratio ¼ 1.96) which is probably at the origin of the formation of several ZnO nanocrystallite phases With Sr-doping, the (002) peak shifts towards high diffraction angles 2q as observed in Fig 2(c) This shift moves up till 3% of Sr-doping where D(2q) ¼ 0.12 and it returns toward low angles for and 5% XRD signal shows no additional peak which may suggest that Sr2ỵ ions go to the regular Zn sites in the ZnO The used concentrations of strontium from to 5% did not form a new compound and we attribute the shift of the (002) peak, the instability of its intensity and its up and down behaviour to the change of the crystallinity of Sr-ZnO The method of preparation may also contribute to this perturbation of the (002) peak but its effect is not significant in this study As reported previously in references [22e25], the difference in atomic size provokes changes in the density of defects, induces stress, lattice distortion and leads to the reduction of oxygen vacancies [25] The effect of Sr-doping is also responsible for the changes in the ZnO lattice parameters as shown in Table According to the values listed in Table corresponding to the (002) peak, c decreases slightly with increasing the Sr-doping concentration from to 3% and increases for and 5% Sr-doping This decrease/increase of the c lattice parameter is consistent with the displacement of the (002) peak and the variation of its intensity To better understand these, we determine the average grain size (D) from the XRD pattern of the (002) peak using the DebyeeScherrer's formula [26] D¼ 0:9l bcosq (1) where l is the wavelength of X-ray, b is the full-width half maximum (FWHM) of the XRD peak, and q is the diffraction angle The estimated values of ZnO particle sizes are summarized in Table It is clearly seen that the grain size of ZnO increases from to 3% of Sr-doping and decreases for 4e5% In general, the doping reduces the surface roughness and consequently the size of ZnO particles must decrease; it is not the case here and indeed, the Srdoping was observed to play an important role in the ZnO structured films The 3% Sr-doping particularly attracts our attention where we can say that the Sr-ZnO films have two behaviours delimited by the 3% doping: a behaviour for a doping situated between and 3% (where c was decreased and the grains size increased) and a second behaviour for and 5% of Sr-doping (c was increased and the grains size decreased) This is clearly observed in the AFM analysis whose results were discussed just below (Fig 4), showing that the roughness present also two behaviours delimited by the 3% Sr-doping Surface morphology was characterized with atomic force microscope (AFM, Model Dimension Edge of Bruker) operating at room temperature in a tapping mode, and the images were treated using WSxM software [27] We have used the scanning area mm  mm for the study of surface morphology AFM images were acquired with a resolution of 512  512 pixels The AFM analysis allows us to determine the surface roughness The roughness is defined either by the mean square roughness s (rms) or the average A Ouhaibi et al / Journal of Science: Advanced Materials and Devices (2018) 29e36 31 Fig (a) XRD spectra of pristine and Sr-doped ZnO films (b) Profile of the (002) peak intensity (c) Shift of the (002) peak under Sr-doping content variations Table Determination of the lattice parameters a, c and the grain size from XRD patterns for pristine and Sr-doped ZnO films Sr-doping concentration (%) Parameter a (Å) Parameter c (Å) Grain size (Å)a Cluster size (nm)b 3.2506 3.2486 3.2466 3.2474 3.2516 3.2520 5.2128 5.2084 5.2010 5.1996 5.2090 5.2098 23.39 24.53 26.23 28.22 27.57 27.81 40 150 180 200 172 153 a b From DebyeeScherrer's formula From AFM measurements roughness sa These two roughnesses are defined by the following expressions [28]: srmsị ẳ and PN iẳ1 Zi Zavgị2 N !1 (2) sa ẳ PN iẳ1 Zi À ZavgÞ N (3) where N is the number of points, Zi is the number of ith point of Z and Zavg is the average value of Z These two expressions of the roughnesses were treated by WsXM software and their profiles were extracted from AFM images as shown in Fig 3(a) and (b) This figure shows the examples of pristine and 5% Sr-doped ZnO films where the roughness srms is determined to be and 75 nm, respectively 32 A Ouhaibi et al / Journal of Science: Advanced Materials and Devices (2018) 29e36 Fig AFM images and roughness profile (a) Pristine ZnO films, (b) roughness profile for pure ZnO films with srms ¼ nm (c) Sr-doped ZnO at 5%, (d) roughness profile with srms ¼ 75 nm The AFM characterization of the pristine ZnO film revealed homogeneous and continuous surface uniformly distributed nanometre sized grains where the surface roughness srms is determined to be ~4 nm The state of the surface changed under Srdoping effect and the ZnO films became granular with irregular ZnO particles and somewhat porous As shown in Fig 4(c), the 3% Sr-concentration affected noticeably the surface morphology where ZnO nanoparticles agglomerated on the surface and formed flower-like clusters The clusters grew with increasing Sr concentration from to 3% and became large-sized grains covering partially the surface and reducing thus the roughness for 3% Srdoping The increase and decrease of the roughness with Srdoping (Fig 5) are in agreement with what we have observed on the grain size determined from the (002) XRD peak and especially on the shift of this peak at the 3% Sr-doping Overall, the change on the surface morphology of ZnO was observed as a result of the Srdoping effect In order to complete the study of the surface morphology, the particle size of pristine and doped ZnO thin films were evaluated by the WSxM software analysis According to the calculated grain size and as reported in Table 1, the cluster size is 40 nm for pristine ZnO, 150 nm for 1%, 180 nm for 2%, 200 nm for 3%, 172 nm for 4%, and 153 nm for 5% Sr-doping We also observed that the 3% Sr-doping is the limit between two different behaviours of the ZnO films In the doping range 0e3%, the cluster size increased whereas it decreased for and 5% Sr-doping The increases in size of ZnO particles may be due to Sr ions that substituted into Zn2ỵ sites We recall that the Sr2ỵ radius is greater than that of Zn2ỵ, so the incorporation of Sr2ỵ distorts the lattice and creates supplementary structure defects that are responsible for changes in the morphology and structure of ZnO surface probably composed of several phases Fig 4(b), (d) and (f) represent the variation of the z-height as a function of the Sr doping These profiles reflect well the state of the surface and its roughness 3.2 Optical properties 3.2.1 Uvevisible analysis The optical properties of the pristine and Sr-doped ZnO films were determined from transmission measurements in the range of 300e1400 nm The transmittance spectra are shown in Fig It can A Ouhaibi et al / Journal of Science: Advanced Materials and Devices (2018) 29e36 33 Fig 2D AFM images and z-height profiles for pristine and Sr-doped ZnO films (a) Pristine ZnO, (c) 3% Sr-ZnO, (e) 5% Sr-ZnO, (b), (d) and (f) represent the plots of the profile of zheight 34 A Ouhaibi et al / Journal of Science: Advanced Materials and Devices (2018) 29e36 d a ¼ ln   T (5) T is the transmittance of the film and d is the film thickness In the plot of the relation (6) in Fig 7, we determine the gap by extrapolation of the linear part of the curve (ahn)2 and its intersection with the energy axis gives the value of the bandgap The examples are given for the undoped ZnO film and for the 5% Sr-doped ZnO film The determined gap values for the pristine and Sr-doped ZnO films are listed in Table As shown in Fig 8, the band gap of ZnO is found to decrease from 3.263 to 3.264 eV for to Fig Variations of the roughness srms and average sa with Sr doping be seen that all the ZnO films show a high transmittance in the visible region The visible transmission is ranging in the interval 88e92.5% The increase of Sr-doping induces a displacement of the absorption edge towards the lower wavelengths around 385 nm in the uv region The shift in the absorption threshold may be due to the scattering of the light by the increasing of the roughness surface from to 3% Sr-doping concentration For the 4% Sr-doping, the decrease of the roughness was probably responsible for the improvement of optical transmittance and for the change of the optical gap discussed bellow A slight decrease of the transmittance yield was observed from pristine to Sr-doped ZnO This decrease can be ascribed to the effect of the incorporation of Sr-atoms which induced changes in the homogeneity of the surface morphology caused by the apparition of porous surface areas with agglomeration of some ZnO nanocrystallites as revealed from the AFM analysis The analysis of the transmission spectra allows us to access the calculation of the optical gap of pristine and Sr-doped ZnO films using the following relation of Tauc [29]: (ahn)2 ¼ A(hn e Eg), (4) where a is the absorption coefficient, A is a parameter depending on the transition probability, h Planck constant, v the frequency of the incident photons a is determined from the relationship: Fig Determination of the band gap value: (a) Pristine ZnO film and (b) 5% Sr-doped ZnO film Table Determination of the band gap values using the relation (ahn)2 ¼ A (hn e Eg) Fig Transmittance spectra of pristine and Sr-doped ZnO films Sr-doping concentration (%) Band gap value (eV) Solid ZnO 3.370 3.267 3.263 3.264 3.285 3.286 3.285 A Ouhaibi et al / Journal of Science: Advanced Materials and Devices (2018) 29e36 35 2% Sr-doping and it enhances rapidly to 3.285 eV when Sr-doping reaches 3% Eg decreases again for the 5% Sr-doping and stabilizes at 3.285 eV This behaviour at 3% Sr-doping may be attributed to the modification of structural defects caused by the presence of Sr2ỵ in the ZnO matrix As discussed above, the substitution of Sr2ỵ for Zn2ỵ creates non-linear defects due to the difference in atomic size and reduces oxygen vacancies as confirmed below by the photoluminescence analysis 3.2.2 Photoluminescence analysis Photoluminescence (PL) is achieved in this study to complete the optical investigation of Sr-doped ZnO films It helps us to understand, analyze, and refine more effectively the effect of Sr doping on the structure of these films PL measurements were recorded at room temperature in the wavelength range 200e1000 nm Fig displays PL spectra with their deconvolutions of pristine and Sr-doped ZnO films PL spectra are composed of three principal peaks for all the samples One peak appearing in the uv region is detected at 398 nm (3.11 eV) and is usually attributed to Fig Plot of Eg versus Sr-doping concentration 3000 Intensity (a.u.) Pristine ZnO 12000 6000 Intensity (a.u.) 1% Sr-doped ZnO 2000 1000 (b) (a) 400 500 600 700 Wavelength (nm) 800 900 1000 500 (c) 400 500 600 700 Wavelength (nm) 800 Wavelength (nm) 900 (d) 400 500 600 700 800 900 4000 800 900 5% Sr-doped ZnO Intensity (a.u.) Intensity (a.u.) (e) 700 800 Wavelength (nm) 900 600 700 600 900 1800 500 600 3% Sr-doped ZnO 4% Sr-doped ZnO 400 500 Wavelength (nm) 1200 2700 400 1800 2% Sr-doped ZnO 1500 0 Intensity (a.u.) Intensity (a.u.) 18000 3000 2000 1000 (f) 400 500 600 700 Wavelength (nm) Fig PL spectra: (a) pristine ZnO film, (b)e(f) Sr-doped ZnO films 800 900 36 A Ouhaibi et al / Journal of Science: Advanced Materials and Devices (2018) 29e36 the recombination of free excitons It corresponds to the near-band edge transition (NBE) of ZnO [28,29] The other two peaks appear in the visible region and are located toward 500 nm (2.48 eV) and 700 nm (1.77 eV) The blue emission (500 nm) may be due to the oxygen vacancies and results from the recombination between the electron localized at the oxygen defect and the hole in the valence band The large red emission peak detected around 700 nm is probably related to stoichiometry defect due to the technique synthesis of ZnO thin films From the PL spectra, we note that the incorporation of strontium in the host ZnO matrix reduced 25% of the PL intensity signal for all samples doped At the 3% Sr-doping, the blue emission disappeared completely and the red emission was at the lower intensity (Fig 9(d)) The intensity of PL signal enhanced again for 4% and 5% Sr doping concentrations This is in full agreement with the results observed from the XRD, AFM and uvevis measurements The Sr doping concentration at 3% is the limit where the structural and morphological changes of the system Sr-ZnO take place with respect to reducing of the density defects related to the oxygen vacancies and to the interstitial sites occupied initially by Zn2ỵ The crystalline quality of ZnO lms was improved for the 3% Sr-doping Conclusion The undoped and Sr-doped ZnO nanofilms were successfully synthesized on glass substrates via the ultrasonic spray pyrolysis technique From the X-ray diffraction analysis, the preferred (002) oriented hexagonal phase of ZnO was confirmed for all samples studied Sr-doped ZnO thin films showed an increase in intensity for this peak for Sr-doping between and 3%, whereas it decreased beyond 3% This peak was shifted toward the high diffraction angle (2q) The behaviour of Sr-doping effect before and after the 3% Sr doping were also revealed in the AFM analysis and the optical study The morphology of Sr-doped ZnO surface became rough and composed of crystallite clusters of different sizes which were enhanced in the Sr-doping range 1e3% and decreased for and 5% The transmittance signal was shifted toward low wavelengths, while the photoluminescence intensity decreased The PL peak of the blue emission near 500 nm disappeared totally at a 3% Sr-doping concentration This doping concentration is considered as a doping limit in the transformations of the Sr-doped ZnO films and on its crystalline quality improvement Acknowledgements The authors thank a lot A Ranguis from CINaM of Aix-Marseille University (France) for some experimental measurements and R Baghdad from Tiaret University (Algeria) for the samples' synthesis The authors thank also the Algerian-French cooperation through the Tassili 14MDU915 project for the 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