Structural and magnetic properties of Co-doped ZnO thin films grown by ultrasonic spray pyrolysis method

Accepted Manuscript Structural and magnetic properties of Co-doped ZnO thin films grown by ultrasonic spray pyrolysis method K. Chebbah, R. Baghdad, N. Lemée, G. Lamura, A. Zeinert, N. Hadj-Zoubir, M. Bousmaha, M.A. Bezzerrouk, A. Belfedal, H. Bouyanfif, B. Allouche, K. Zellama PII:

S0749-6036(16)30757-1

DOI:

10.1016/j.spmi.2016.11.069

Reference:

YSPMI 4748

To appear in:

Superlattices and Microstructures

Received Date: 22 August 2016 Revised Date:

16 November 2016

Accepted Date: 17 November 2016

Please cite this article as: K. Chebbah, R. Baghdad, N. Lemée, G. Lamura, A. Zeinert, N. HadjZoubir, M. Bousmaha, M.A. Bezzerrouk, A. Belfedal, H. Bouyanfif, B. Allouche, K. Zellama, Structural and magnetic properties of Co-doped ZnO thin films grown by ultrasonic spray pyrolysis method, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2016.11.069. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Cobalt effect on microstructure and magnetic properties of ZnO thin films prepared by ultrasonic spray pyrolysis method for spintronic applications

K. Chebbah1,2, R. Baghdad1,*, N. Lemée3 , N. Hadj-Zoubir1, A. Belfedal2, M. Bousmaha1, B.

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Kharroubi1 , H. Bouyanfif3, A. Zeinert3, G. Lamura4, M. Rocca4 and K. Zellama3 1

Engineering Physics Laboratory (LGP), Material Sciences Faculty, Ibn Khaldoun University,14000, Tiaret, Algeria 2

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Physics Department, Faculty of Exact Sciences, Mustapha Stambouli University, Mascara, Algeria 3

33 rue Saint-Leu, 80039

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LPMC, UFR des Sciences, Université Jules Verne Picardie, Amiens, France

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CNR-SPIN, Corso Perrone 24, I-16152 Genova, Italy

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Physics Department, University of Genova, via Dodecaneso 33, I-16146 Genova, Italy

Abstract:

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Cobalt-doped ZnO thin films with different percentage of Co content (0, 1, 3, 5, 7, 9, 11 and 15 at%) were synthesized via a simple and versatile method i.e. ultrasonic spray pyrolysis

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under atmospheric pressure and a substrate temperature of 350°C. The structure of the asprepared samples was characterized by X-ray diffraction (XRD) and Raman spectroscopy and

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FTIR. Compared to the Raman spectra of ZnO pure films, the Co-doping effect is revealed by the presence of three additional peaks around 235, 470 and 538 cm-1. FTIR results show the decrease of the bond force constant f with increasing Co-doping. The optical properties of the samples were studied by ultra-violet visible near infrared (UV-VIS-NIR) spectroscopy. These measurements show absorption bands at approximately 570, 620 and 660 nm appearing in the spectra of the Co-doped samples in comparison with pure ZnO. These results show that cobalt ions, in the oxidation state of Co2+, replace Zn2+ ions into the ZnO lattice without changing its 1

ACCEPTED MANUSCRIPT wurtzite structure. All our samples exhibit paramagnetic behavior because of the 3d electron of Co2+. PACS: 81.15.-z; 81.20.Ka; 78.30.Fs; 61.72.-y; 75.70.-i.

Raman spectroscopy; Magnetic properties. *

Corresponding authors:

E-mail address: [email protected] Fax: 00 213 46 42 47 10

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Tel: 00 213 7 99 29 94 55

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Keywords: Cobald doped ZnO, Nanostructured thin films; Ultrasonic spray pyrolysis; FTIR;

1. Introduction

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Zinc oxide (ZnO) is a direct band gap II-VI compound semiconductor with the wurtzite type structure. Its high (3.37 eV) band gap energy and large excitonic binding energy (60 meV) at room temperature, makes it a potential candidate for (UV) optoelectronic devices such as light

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emitting diodes, laser, and photodetectors [1-5]. Recently, room temperature ferromagnetism (RTFM), transparency in visual region and piezoelectricity have generated huge interest for

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ZnO among the scientific community as a strong candidate for spin transistors and spinpolarized Light-emitting diodesapplications [6-9]. In particular, the presence of

HTFM

undiluted magnetic semiconductors (DMS) was predicted by Dietl et al. with the Zener. Among DMS, ZnO-based compounds attracted the most attention since they become FM when doped with most of the transition-metal elements [10-12]. Since magnetic Co is highly soluble in ZnO, ZnO:Co systems became soon one of the most studied DMS’s for applications requiring ferromagnetism near room temperature HTFM [13]. Some subsequent 2

ACCEPTED MANUSCRIPT theoretical works using Density functional theory (DFT), [12, 11] and experimental,[14, 15] investigations show that n-type Co-doped ZnO also possesses room temperature ferromagnetism HTFM [16] even if the published experimental results on magnetic properties

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of the ZnO up-to now remain contradictory [4]. The mechanisms responsible for ferromagnetism in such ZnO systems are yet to be clarified. To understand these mechanisms and to clear up certain structural aspects due to the introduction of the cobalt atoms in the ZnO matrix and to correlate them with their optical and

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magnetic properties, we shall owe study and understand particularly their micro-structural

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properties. Raman scattering spectroscopy is a powerful non-destructive analytic technique which can provide useful information on the structure, morphology and chemical composition of semiconductor material, as well as on the photon-electron and electron–phonon interactions occurring in these materials [17]. Several Raman studies have been performed on ZnO single crystals, as well as on nanostructured doped and undoped thin films and nanowires [18-21].

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These studies have demonstrated that non resonantly excited Raman spectra, in general, follow rather well the symmetry-dicted selection rules characteristic of wurtzite structure [18,

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19]. At the same time, they revealed a strong dependence of the spectral features upon the nature and concentration of impurities present in ZnO samples [20, 22]. Some of these

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features can be associated with local impurity vibrational modes [], while others may be due to structural changes or impurity-induced lattice defects [25], grain size or secondary phases present in doped ZnO material [26]. Besides, Fourier transform infrared spectroscopy (FTIR) which is the oldest and the most commonly used method for providing specific information on molecular structure, chemical bonding and molecular environment, and Xray diffraction are powerful tools to probe and understand the structure of our material.

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ACCEPTED MANUSCRIPT Previous studies have reported that the microstructure, optoelectronic and magnetic properties of the Co-doped ZnO (ZnO:Co) materials appear to be extremely sensitive towards the conditions of their preparation [27,28]. Cobalt-doped zinc oxide thin solid films have been obtained by chemical and physical techniques, such as ultrasonic spray [29], sol–gel [30] and

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pulsed laser deposition [31]. Among them, the ultrasonic spray pyrolysis (UPS) method has attracted our interest in the synthesis of ZnO:Co nanostructured thin films.

The ultrasonic spray pyrolysis (USP) consists in ultrasonic processor solution that

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generates fine droplets, which are directly sprayed onto the substrates. This allows a precise

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control of the deposition rates and therefore leads to the formation of uniform thin films in the nanoscale range thanks to the low flow rate capabilities and high uniformity in the droplet sizes. In addition, this method present other important advantages such as: (1) the possibility to use mixtures of solid and liquid precursors; (2) no need for specific substrates; (3) an easy

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scaling into an industrial scale process.

The aim of this work is to clarify the effect of Co doping on the microstructure, optical and magnetic properties of ZnO films grown by ultrasonic spray pyrolysis method and their

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nanostructured powders. After film depositions; ZnO:Co samples were investigated by means of SEM, X-ray diffraction (XRD), Raman scattering (RS), FTIR, ultraviolet-visible-near

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infrared (UV-VIS-NIR) spectroscopy and the magnetic measurements were performed by superconducting quantum interference device (SQUID) MPMS2 by Quantum Design.

2. Experimental details ZnO:Co films were co-deposited onto (100) oriented undoped c-Si and glass substrates for the different specific characterizations by ultrasonic spray pyrolysis technique under atmospheric 4

ACCEPTED MANUSCRIPT pressure. The starting solution was 0.3 M/L of zinc acetate (Zn(CH3COO)2.2H2O) prepared by dissolving the equivalent mass of zinc acetate in methanol. Methanol is an obvious choice because of its volatility facilitating quick transformation of the precursor mist into vapor form. A few drops of hydrochloric acid were added to stabilize the solution and prevent

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precipitation of zinc hydroxide. The acidity of the spray solution was pH = 5. The compound source of doping was cobalt chloride (CoCl2). The percentages of doping the starting solution are [Co/Zn]= 0, 1, 3, 5, 7, 9, 11 and 15 at%, and substrate temperature was fixed at 350°C.

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The deposition time was fixed for 1 hour. The obtained mixture was stirred at 60 °C for 2 hours to yield a clear and homogenous solution. The spray rate was maintained at 0.26 l/min

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and 45 KHz as ultrasonic frequency. For the synthesis of the nanostructructed powders we heat the starting solution for a first step at 80 °C which is sufficient to allow a condensation. In the second step, the obtained products were washed by distilled water for several times and in the last step we heat them in an electric oven until 350 °C with a step of 10 °C/hour.

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Lang has proposed a correlation for the size diameter prediction of droplets dp (µm), produced by ultrasonic atomization, mainly based on the frequency ?? (KHz), the surface tension σ (N.m-1) and the density (Kg/m3) ρ, given by the relation (1); [32-35]: 1/ 3

  

(1)

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 8πσ d p = 0.34 2  ρf

This correlation is only applicable when liquid phase viscosity and liquid flow rate have no effect on droplet size. However, these parameters were proven to be very important in ultrasonic atomization. In our case, the calculated value of dp is around 20 µm for the methanol + zinc acetate mixture tension of surface equal to 29.63x10-3 N.m-1, its density value is 885 kg / m3 and the ultrasonic frequency value 45 KHz. The thickness of undoped and doped ZnO films, varies between 700 and 1000 nm and was measured by a Dektak III stylus profilometer (Sloan technology). To study the structural 5

ACCEPTED MANUSCRIPT properties of our films, X-ray diffraction analyses were performed on a diffractometer (DISCOVER Bruker D4 endeavor ) in the 2θ range between 10° and 80°. The surface morphologies of the films were monitored by a field emission scanning electron microscope (FE-SEM) (Joel JSM6301F, 15 kV) and the stoichiometries of the films were determined by

collected for 50s.

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energy dispersive X-ray spectroscopy (EDX) (Oxford INCA 350 Energy) at 15 kV and The Raman spectra were recorded using a Jobin Yvon T6400 micro-

Raman spectrometer equipped with an Ar+ laser (514.5 nm) excitation and with a low power

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of about 40 mW, in order to avoid any beam-induced crystallization during measurements. Optical transmittance and reflectance spectra at room temperature of the films have been

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recorded in the wavelength range of 200-2500 nm using a double beam UV-VIS-NIR 5E Varian spectrophotometer. Magnetic measurements were performed using by superconducting

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quantum interference device (SQUID) MPMS2 by Quantum Design.

3. Results and discussion

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3.1. SEM and EDAX results

Fig. 1-a, displays the surface morphology of the ZnO:Co nanostructured thin films with Co

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doping contents of 9 at% deposited on silicon substrat. It shows the size and distribution of particles. The cross section (Fig. 1-b), shows a columnar growth and evidences a film thickness around 750 nm. One can see on the SEM micrograph (Fig.1-c), that the nanostructured powder obtained from the starting solution used for growing the same sample is composed by nanostructures looking like nanorods or full tubes, with a length of 100-300 nm. The texture and the good crystal quality of the spray deposited ZnO:Co films confirm that spray deposition allows one to grow nanostructures of high crystalline quality, compared to 6

ACCEPTED MANUSCRIPT low-temperature wet chemical methods used for the growth of ZnO nanostructures [36, 37]. In ZnO films grown on silicon substrate, the cobalt content was detected by EDAX. The EDAX analysis has shown that the amount of Co incorporation is equal to the Co concentration and also has revealed the dominance of oxygen in all the samples exhibiting

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oxygen rich stoichiometry (Fig.2-a). An example of a typical EDAX spectrum of ZnO:Co (CCo= 9 at%) with elemental compositional analysis, shown in Fig. 2-b, exhibits the presence of Co, though at equal atomic percentage as compared to the doping concentration.

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XRD results

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X-ray diffraction (XRD) patterns of undoped ZnO and Co-doped ZnO powders are shown in Fig. 3. The XRD patterns of the samples of pure and Co-doped ZnO with the different doping levels reveal the presence of a single hexagonal phase with wurtzite structure. Al-Hardan et al considers that the most energetically stable crystal plane with minimum surface free energy is

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(002) plane of wurtzite structured ZnO [38]. However, the characteristic peaks with high intensities corresponding to the planes (100), (002), (101) and lower intensities at (102), (110), (103), (200), (112) and (201) indicate that the as-deposited samples can be indexed to a

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hexagonal wurtzite structure (space group P63mc [JCPDS#79-2205]) as ZnO). We can observe that for lower Co-doping samples (CCo ≤ 5 at%), the sharp and intense peaks indicate

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that all the samples are highly crystallized, and we can see the absence of any extra reflections that ensures the phase purity of our samples. So, impurity phases such as metallic cobalt, cobalt oxides and other compounds cannot be detected. However, when Co-doped concentration exceed 5 at%, although the samples exhibit the main crystalline wurtzite structure of ZnO, some extra and very faint reflections can be noticed. We can ascribe these extra reflections to the presence of impurities like Co3o4, CoCo2O4, in our samples at the limit of detection. J. Fu et al [39], find the same extra peaks in Co-doped ZnO nanostructures 7

ACCEPTED MANUSCRIPT synthesized by the hydrothermal method. They ascribed these additional reflections to the Co(OH)2 impurity phase. The average grain size is estimated from XRD patterns, using the Scherer’s formula:

Cλ B cosθ

(1)

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D=

Where B is the full width half maximum (FWHM) (in radian), λ is the X-ray wavelength

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(CuKα = 0.154 nm), θ is the Bragg diffraction angle, and C is a correction factor taken as 0.94.

a=

λ 2 sin θ

4 2 l2  h + hk + 3 (c / a ) 2

  

and

λ 2 sin θ

4 (h 2 + hk + l 2 ) 2 3(a / c)

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c=

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The lattice parameters a and c, are also calculated using the following relation [40] (Fig. 4):

(2)

(3)

The calculated values, estimated to an accuracy of better than 0.004 Å, are summarized in

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Table I. It is also observed that the relative variations in the diffraction peaks (in the peak

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intensity, shifts, and broadening) on increasing Co-doping and could be due to micro-strain which is due to imperfections within the crystalline lattice, including vacancies, stacking faults, interstitials, etc. The slight shift of XRD peaks towards higher angle as increasing Co doping indicated that the samples are in the state of stress. The micro-strain in our samples is estimated using the relation [41]:

 c film − cbulk  cbulk

σ = −233 × 10 9 

  

Pa

(4)

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ACCEPTED MANUSCRIPT where c film is the lattice constant of the film and cbulk is the strain free constant [JCPDS#792205]. The calculated values are listed in Table I. The positive sign of strain for all samples suggests that the crystallites are in the state of compressive stress, ie compression in the unit cell. The variation of micro-strain is represented on Fig. 5. It can be found that the lattice

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parameters decrease slightly with the increasing Co content in the samples, inferring that part of Co ions (Co2+) with small ionic radii (0.72 Å) replace Zn ions (0.74 Å) in the ZnO lattice without changing the würtzite structure [42]. From the table I, we note also that the unit-cell

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volume decreases with Co doping, this variation is represented on the Fig. 6. We can attribute this variation in lattice volume to the evolution of crystal planes in different directions and

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attunes in away, suggesting a distribution of global superficial free energy which is minimal [43].

The compression of the structure of our ZnO:Co films was observed by Lim et al [44]. From tables established by Shanon et al [45], the four-fold coordinated ionic radii of Co2+ is slightly

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less than Zn2+ and result in a large solubility of Co+2 in the ZnO lattice. The crystallites size varies from 27 to 38 nm depending directly on the crystallographic axes and Co doping level and indirectly on spray rate, substrate temperature, growth atmosphere, solution concentration

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etc [46,47]. Other search groups reported similar behaviour of lattice parameters as a result of

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doping ZnO with transition metals [48-50]. Furthermore, these results suggests also, that we have not or just a little proportion of Co2+ in interstitial sites; which will induce an increase in lattice parameters, since such insertion is observed for higher coordination number for Co2+ associated to a larger ionic radius [51-53]. To calculate nearest-neighbor bond length (L) or the Zn-O bond-length, we have used the relation [54]:

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ACCEPTED MANUSCRIPT 2

1 2 1  L= a +  − u  c2 3 2 

(Å)

(5)

Where a, c are the lattice parameters and u is the positional parameter of the wurtzite structure. u is a measure of the amount by which atom is displaced with respect to the next

variation of L versus cobalt content in our samples on the Fig. 7.

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along the “c” axis. We summarized the calculate L values on the table I and we presented the

In a real ZnO crystal, the sub-lattice includes four atoms per unit cell, and every zinc atom is

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surrounded by four atoms of oxygen, or vice versa, which are coordinated at the edges of a tetrahedron and the wurtzite structure deviates from the ideal arrangement, by changing the

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c/a ratio or the u value. There have been various experimental measurements and theoretical calculations to determine the lattice parameters of

ZnO [54,55].

In these studies, the

variation in u parameter and c/a ratio ranges from 0.383 to 0.3856 Å and 1.593 to 1.6035 Å, respectively. However, the experimentally observed c/a ratios are smaller than ideal [56-58].

stability and iconicity.

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The deviation from the values expected for the ideal wurtzite crystal is probably due to lattice

The calculated bond length agrees with Zn-O bond length in the unit cell and we remark a

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slight diminution of L with increasing the rate of doping of the cobalt confirming the above results. Moreover, it is known that ionic radius of O2- is 1.21 Å, and ionic radius of the Zn2+

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is 0.74 Å. Consequently, the length of the Zn–O bond is1.95 Å. The values obtained are slightly higher and indicate the presence of the structural defects, especially oxygen vacancies.

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ACCEPTED MANUSCRIPT 3.2. Raman spectroscopy results ZnO in a wurtzite structure belongs to the space group C 64ν with two formula units in the primitive cell. The optical phonons at the Γ point in the Brillouin zone, have the following

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irreductible representation [59]: Γopt = 1A1 + 2 B1 + 1E1 + 2 E 2

(5)

where A1 and E1 modes are polar and split into transverse (TO) and longitudinal optical (LO)

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phonons, and they are all Raman and infrared active. The nonpolar E 2 modes are Raman

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active only and the B1 modes are Raman inactive. Frequencies and symmetries of Raman lines in pure bulk ZnO and their assignments are summarized in Table II [60, 61]. The Raman spectra can be used also to estimate the degree of disorder in a crystal. The structural disorder is manifested in the line shape (line width and asymmetry variation) of the

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E2 mode. The line width is sensitive to the variations in residual stress, structural damages, defects and impurities. In an ideal crystal, the spatial correlation function of the phonon is infinite and leads to q=0 momentum selection rules of Raman scattering [62]. The Raman line

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shape would then be symmetric and Lorentzian. However any kind of structural disorder in materials may destroy the symmetry and can break down the Raman selection rule leading to

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finite phonon correlation length. The finite phonon mode will lead to the broadening and asymmetry of the line shape. Fig. 8, shows the room-temperature Raman scattering spectra of the ZnO:Co (Co at 0, 1, 3, 5, 7, 9, 11, and 15 at%) film deposited onto glass substrate at 350°C. As shown, the spectra exhibit five Raman bands allowed by the symmetry selection rules characteristic of wurtzite structure crystals [63, 19].

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ACCEPTED MANUSCRIPT It was found that pure and Cobalt doped ZnO present the characteristic bulk vibration modes of zinc oxide, which are located at around 332, 410, 437, 581, and 670 cm-1, as indicated by Table II, namely, the E2 (low) and E2 (high) non-polar phonon modes, the E1 transverse optical (TO) phonons, and the A1 (LO) phonons. The E1 (LO) mode (around 584 cm-1

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according to, 587-591 cm-1 according to [19, 64]) strictly speaking, cannot be observed in a backscattering geometry as a dipole-allowed mode. Compared with the undoped ZnO film, additional vibrations modes around 235, 470 and 540 cm-1 appear that could be associated

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with Co doping.

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The broad peak at about 332 cm-1, seen in the spectrum of pure ZnO has been attributed to the second order Raman processes involving acoustic phonons. Its presence demonstrates a good quality of the samples. The characteristic 437 cm-1 peak, assigned to the non polar E2 optical phonon mode, is presented for pure and Co-doped ZnO thin films, with a similar shape. The E2 mode position values in the table III, shifts of 0.2 cm-1 towards higher frequencies with

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increasing Co doping concentration expect for the case of the sample with 15 at% cobalt content. We can attribute this shift to the preparation method.

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The presence of the intense Co-related mode at around 540 cm-1 is evidenced in all Co-doped samples. In our ultrasonic spraying grown samples, this mode appears only when Co is

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present and its intensity has been shown to increase with increasing Co concentration [65, 66]. We believe that it is an indication that Co substitutes Zn in the lattice sites and the 470 and 540 cm-1 modes are related to a local vibration involving the Co atom. In fact, the ionic radius (0.72 Å) of Co2+ is smaller than that of Zn2+ (0.74 Å) than that of O2- (1.21 Å). However, similar impurity-related Raman modes at frequencies close to 530 cm-1 have been reported for ZnO materials not containing Co [64, 25], so we cannot exclude that our 524 cm-1 mode can be related to the presence of Co in the sample only indirectly. For instance, it might be 12

ACCEPTED MANUSCRIPT associated with defects in the host lattice induced by the doping [25]. Moreover, these results are in agreement with the results reported by other groups [67, 68, 25] on Zn1−xCoxO alloys; they found an additional mode at about 530 cm-1, perhaps induced by shallow-donor defects bound on the tetrahedral Co sites.

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In order to quantify the variation of the modes A1 (LO) and 540 cm-1, we fitted them with two Gaussians (the choice of Gaussians is based on the broadening and asymmetry of the Raman line shape). Table III summarizes the detailed fitting values of the broad band (460-

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640 cm-1) of undoped and Co-doped samples. An example of such fitting is presented in Fig. 9. The anomaly enhancement of the A1 (LO) mode was interpreted as induced by oxygen

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vacancies or Zn interstitials. We can see that the full width at half maximum (FWHM) value for both A1 (LO) and 538 cm-1 peaks, increased with Co doping, suggesting damage or disorder of the crystal lattice induced by Co-doping. The variations of the FWHM of A1 (LO) mode and the FWHM of the band attributed to the signature of cobalt versus Co incorporation

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in our samples were presented on Fig. 10.

The wurtzite structure in a ideal ZnO crystal (Fig. 4), is composed of two interpenetrating hexagonal close packed (hcp) sub-lattices, each of which consists of one type of atom

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displaced with respect to each other along the threefold “c” axis, thus the irregularities such as oxygen defects along “c” axis would directly affect the displacement of ions in both A1 (LO)

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(along the “c” axis) and E1 (LO) (in a – b planes) modes which leads to changes in Raman shift [54, 69].

3.3. FTIR results The FTIR spectra of undoped and Co doped ZnO thin films recorded in the range 400-4000 cm-1 are shown in Fig. 11. The two main absorption peaks are observed between 1400 and 13

ACCEPTED MANUSCRIPT 1650 cm-1, corresponding to the symmetric and asymmetric stretching of the carboxyl group (C=O). The band in the range of 400-550 cm-1 is due to the Zn-O and (Zn, Co)-O stretching mode. There is no evidence of O-H stretching at 2550 cm-1 and O-H around 1650-1750 cm-1, suggesting that no moisture was adsorbed in the samples. The broad peak in the range 3200-

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3600 cm-1 is due to OH stretching. The deformation band of C=O at around 1000 cm-1, and absorption peaks observed ordinary between 2300 and 2400 cm-1 due to adsorbed CO2 molecules in air are absent in our FTIR spectra indicating the good quality of our films. The

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same absorption bands observed in FTIR spectra of pure and Co-doped ZnO films of the present investigation are observed in others works [70- 73].

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Fig. 12 (a) shows that the FWHM of the broad band (460-640 cm-1), which is attributed to the Zn-O and (Zn, Co)-O stretching mode, decreases with increasing cobalt content. Table IV summarizes the detailed fitting. An example of such a fitting is presented in the Fig.12 (a) inset. To correlate this decrease in the value of FWHM and the shifts in the absorption

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frequency with increasing Co doping, we suggest that the origin is the difference in the bond strength caused by the substitution of Zn2+ ions by Co2+ ions in the ZnO matrix and the creation of more oxygen defects, such as oxygen vacancies, or/and interstitial oxygen.

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However, the difference between atomic masses of Zn (65.39 amu) and Co (58.9332 amu) and the decreasing in the Zn-O bond length (L) with increasing Co-doping deduced from

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XRD measurements; so confirming this behavior. These modifications occurring in ZnO lattice weakens the strength of Zn-O bond (ie: decrease in the f value) resulting in a shift of the IR absorption band.

In order to quantify the bond force constant f, we use the relation (6) which describes the frequency of the vibrations of the diatomic molecule composed by Zn and O atoms [74]:

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ACCEPTED MANUSCRIPT ν=

1 2πc

 1 1   f  +  mZn mCo 

(cm-1)

(6)

Where f is the bond force constant and c is the light velocity.

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The calculated f values are summarized in the table IV and the Fig.12 (b). shows the decreasing of f with increasing Co-doping.

Franco et al [47] describe well this behavior. They suggest that with increasing Co-doping

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more and more Co2+ ions replace Zn2+ ions, inducing the creation of oxygen defects and thus resulting in a shift of the IR absorption band towards low wavenumbers, as observed for the

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absorption band centered at around 551 cm-1 corresponding to E1 (LO) mode (Fig.4). On the other hand the difference between molar masses of Zn and Co results in the shift of the IR absorption band towards higher wavenumbers, as observed for the absorption band centered at

3.4. Optical results

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435 cm-1 corresponding to A1 (LO) mode.

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Evidence for Co substitution in the ZnO lattice for our films can be inferred from optical absorption measurements. Fig.13 shows the optical transmission for Co-doped ZnO films. An

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undoped ZnO film is included as reference. All films have an average transmittance greater than 90% in the visible region indicating good quality of the deposited films with low scattering or absorption losses. Three absorption peaks are apparent in the doped films. These peaks are characteristic of the d–d transition levels attributed to Co+2 occupying tetrahedral lattice positions, and indicate that cobalt is substituting as Co+2 on Zn lattice sites in the films. Specifically, the peaks located at energies of 1.9 eV (656 nm), 2.0 eV (612 nm) and 2.18 eV

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ACCEPTED MANUSCRIPT (568 nm) correspond to the 4A2→2 E(G), 4A2→4T1(P) and 4A2→2A1(G), respectively [75] (Fig. 14). To examine the absorption due to the characteristic Co+2 transitions, the absorption coefficient versus cobalt concentration was plotted and the area underneath the absorption peaks between

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1.7 and 2.4 eV (the relevant range for the characteristic Co+2 transitions) was calculated. The absorption coefficient was used since it is normalized by the film thickness. The calculated integrated area as a function of cobalt is presented in Fig.15. The absorptions from the Co+2

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ions increase with the Co concentration suggesting that most of the cobalt is dissolved in the lattice.

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The optical band gap Eg can be determined from the absorption coefficient α calculated as a function of incident photon energy E (hν ) . Near the absorption edge region, the absorption spectra α (hν ) of the films have been obtained from the optical transmission and reflection measurements at 300°K. The absorption coefficient is obtained from the following equation

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[76]:

   

(6)

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4 2 2 2 1  (1 − R) + 4T R − (1 − R) α = ln d  2TR 2 

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where α is the absorption coefficient, d is the thickness of the film, R and T are the reflectance and the transmittance respectively. For a direct gap semiconductor such as ZnO the absorption coefficient has the following spectral dependence [77]:

α (hν ) = A(hν − Eg )1 / 2

(7)

where A is a constant and Eg is the band gap. From this relation the optical band gap of the films was determined by extrapolating the linear part of the spectrum (αhν ) = f (hν ) as shown in Fig 16 inset. We have summarized the calculated Eg values in table V. 16

ACCEPTED MANUSCRIPT Fig. 16 shows that the band gap energy increases (blue-shifts) with cobalt concentration and decreases from CCo = 5 at%. Some reports in the literature observe a red-shift in the band gap energy as the cobalt concentration is increased [78, 79]. The red-shift is typically attributed to the sp–d exchange between the ZnO band electrons and localized d-electrons associated with

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the doped Co+2 cations. The interaction leads to corrections in the energy bands; the conduction band is lowered and the valence band is raised causing the band gap to shrink [80]. On the other hand, other papers have reported a blue-shift in the band gap of ZnO with

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cobalt doping. Peng et al [81] reported a blue-shift in the band gap of the material and a redshift of the band tails, which is similar to our observations. Ozerov et al [82] also reported a

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blue-shift in the band gap in nanocrystalline films. Yoo et al [83] observed a blue-shift in Aland Co-co-doped ZnO films which was attributed to the Burnstein–Moss effect from an increase in the carrier concentration.

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3.5. Magnetic measurements

The magnetic behavior of cobalt doped ZnO thin films was investigated by dc-squid magnetometry: (i) isothermal magnetization and (iii) temperature dependent magnetic

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susceptibility. Isothermal magnetization at 200 K (not shown) is dominated1 by the

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diamagnetic contribution of the silicon substrate. A linear fit on the high field data was used to determine the substrate diamagnetic contribution for each sample. In figure 17 we show the molar susceptibility for all the sample under test after having subtracted the diamagnetic contribution of the Si substrate.2We remark that χn(T) has a paramagnetic-like behavior for all

1

The presence of a small ferromagnetic component already saturated at 10 KOe is present in the isothermal magnetization measurements of the all samples and of the bare substrates as well. It is likely due the presence of diluted magnetic impurities in the substrate. (௠ ି௠ )ெ(௫) 2 The molar susceptibility was extracted by the following equation߯௡ = ೘೐ೌೞ ವ಺ಲ where mmeas is the ௏(௫)ఘ

measured magnetic moment, mDIA is the Si diamagnetic contribution assumed as T-independent, M(x) is the molar mass of each sample, V(x) the volume of each film and r is the ZnO theoretical density.

17

ACCEPTED MANUSCRIPT the sample under test up to 200 K3. In this range of temperature experimental data were fitted by a Curie-Weiss model: ߯௡ =

஼ ்ିఏ

+ ߯଴

(8)

where C is the Curie constant, Θ is the Curie-Weiss temperature and χ0 is a temperature

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independent contribution that in this particular case takes into account all the residual diamagnetic/ferromagnetic contribution that have not fully removed by the background subtraction as detailed in notes 1 and 2. In the inset of figure 17 we show as example the

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Curie-Weiss fit on x=0.05 data. As in this case, for all the sample under test the agreement

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between data and fit was highly satisfactory. In figure 18 and in table VI we report the parameters resulting from the above discussed fit procedure: in the bottom panel the CurieWeiss temperature is reported. In the top panel the effective magnetic moment per Co2+ ion as extracted by the Curie constant µ EFF(x)/µ B=(C/x)1/2expressed in Bohr magnetons. From these data some comments can be pointed out: (i) we can exclude the contribution of

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cobalt oxides such as CoO, Co2O3 and Co3O4since no magnetic transitions were found up to 300 K [83]. Besides, neither metallic cobalt nor cobalt oxides phases were revealed in our samples within the detecting limits of DRX carried out on powders obtained from the starting

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solution and Raman measurements. (ii) All our samples exhibit paramagnetic behavior

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because of the 3d electron of Co2+ in agreement with several reports [84-85]. (iii) At low doping (x=0.01) the effective moment per Co2+ ion is 4.25 µ B, that is in agreement with the values obtained experimentally for the Co-“high spin configuration” that lies in the range 4.35.2 µ B. This value decreases with Co content that suggests a progressive increase of Co-Co exchange interactions. (iii) These interactions are antiferromagnetic (AF) like because of the sign of the Curie-Weiss temperature (Θ<0). The fact that this parameter increases in

3

This feature is due to the diamagnetic contribution of the substrate that slightly increases with increasing temperature as it was checked on a bare substrate.

18

ACCEPTED MANUSCRIPT magnitude with Co content suggest yet a strengthening of the above discussed AF exchange interactions. By concluding this section, we observe no traces of intrinsic ferromagnetism in any of our samples contrary to what was reported by several groups [86-90]. Very recently a muon

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spectroscopy study revealed that the room-temperature ferromagnetism of Co-ZnO seems due only to grain boundaries, and that the ferromagnetic response is highly reduced in samples with big grains (>50 nm) since the sample volume fraction occupied by grain boundaries is

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highly reduced. Since the SEM images reported in figure 1 suggest that the presence of grains sizes smaller than 50 nm is rather unlikely, this finding justifies the absence of any intrinsic

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ferromagnetic response in the magnetic properties of our samples.

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Conclusion

In conclusion, we have grown ZnO:Co nanostructured thin films on glass and silicon substrates by a simple and versatile method i.e. Ultrasonic Spray Pyrolysis , under

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atmospheric pressure, 350°C and different Co doping levels. The XRD, Raman spectroscopy and UV-Vis-NIR demonstrated that cobalt can be uniformly incorporated into the wurtzite

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structure and substitute for Zn cations sites in ZnO lattice without phase phase segregation. There is no evidence for the formation of either metallic Co or CoOx particles. Two dominant interactions exist in the case of Co2+ substituting Zn2+. One is the interactions between the d electrons of Co2+ and the s and p electrons of the host ZnO bands, which leads to the red shift of Eg with increasing Co content. The other is the interactions between the localized d electrons of Co2+, which reduce the transmittance of the doped films in the visible wavelength region. The sp-d and d-d exchanges probably induce intrinsic ferromagnetism in the Co-doped 19

ACCEPTED MANUSCRIPT ZnO films. We also suggest that the observed weak ferromagnetism by the several reports, might not originate only from the Co2+ active magnetic doping atoms, but from oxygen vacancies or could also be mediated by zinc-related defects in the ZnO lattice. All our samples exhibit paramagnetic behavior because of the 3d electron of Co2+ and we observe no

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traces of intrinsic ferromagnetism in any of our samples. Thus, a complete understanding of these systems will require further studies of the structure-magnetic property relationships.

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Acknowledgments

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The authors would like to thank Doctor B. Allouche for the assistance in SEM experiments.

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ACCEPTED MANUSCRIPT Figure captions: Figure 1: Typical SEM secondary electron images of 9 at% Co-doped ZnO and deposited at 350 °C (a);

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the cross-section (b) and nanostructured powders (c). Figure 2:

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EDAX spectra of undoped and 9 at% Co-doped ZnO deposited at 350 °C. Figure 3:

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XRD patterns obtained from ZnO thin films with different doping levels. Figure 4:

Schematic diagram of the würtzite unit cell structure of ZnO compound. L is Zn-O bond

Figure 5:

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length. E1(LO) and A1(LO) modes are along “c” axis and a-b plane respectively.

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Figure 6:

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Variation of micro-strain with different doping levels.

Unit cell volume variation versus different cobalt doping concentration. Figure 7:

The decrease of Zn-O bond length with cobalt content. Figure 8: Raman spectra of ZnO:Co films deposited onto glass substrates at different Co doping levels. 21

ACCEPTED MANUSCRIPT Figure 9: Deconvolution of cobalt contribution and A1(LO) Raman peak of the film deposited at 350°C and Co at 15 at%.

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Figure 10: Variation of FWHM of A1(LO) mode and cobalt presence versus different cobalt doping

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levels. Figure 11:

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FTIR spectra of undoped and Co-doped ZnO films deposited at 350°C. Figure 12:

Variation of the FWHM of Zn-O bond vibration (a) and bond constant force f (b); with cobalt

Figure 13:

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concentration.

Transmission spectra of undoped and Co-doped ZnO films deposited at 350°C. Inset:

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Figure 14:

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transmission and reflexion spectra of 15 at% Co-doped deposited at 350°C.

Peaks located at energies of 1.9 eV (656 nm), 2.0 eV (612 nm) and 2.18 eV (568 nm). Figure 15:

The calculated area underneath the characteristic absorption wells between 1.7 and 2.4 eV plotted as a function of cobalt concentration.

22

ACCEPTED MANUSCRIPT Figure 16: Plot of the bandgap values as a function of nominal cobalt concentration. Inset: Plots of (αhν ) 2 vs. hν for Co doped ZnO film deposited at CCo = 15 at% and TS= 350°C.

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Figure 17:

x=0.05 data set is reported as example. Figure 18:

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Temperature behavior of the magnetic susceptibility. In the inset the Curie-Weiss fit for the

Variation of the effective magnetic moment per Co2+ ion (µ eff) and the Curie-Weiss

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temperature Θ as a function of the Co content.

23

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5

7

9

11

15

47.63

47.50

47.59

27.0 37.7 28.5 27.5 30.2 25.8 24.5 27.3 24.0 27.7 30.0 25.0 29.7 30.6 24.4 33.5 38.6 29.5 31.9 34.1 26.9 27.7 33.9 26.3

2.81 2.60 2.47 2.81 2.60 2.47 2.81 2.60 2.47 2.81 2.60 2.47 2.80 2.59 2.46 2.79 2.58 2.46 2.79 2.58 2.46 2.79 2.58 2.46

Zn-O bond length L (Å)

3.249 5.207 0.0004 1.977

3.247 5.201 0.273 1.976

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0.36 0.26 0.36 0.35 0.33 0.39 0.39 0.36 0.42 0.35 0.33 0.41 0.32 0.32 0.42 0.29 0.26 0.35 0.30 0.29 0.38 0.35 0.29 0.39

Microstrain (x109 MPa)

3.249 5.205 0.094 1.977

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3

31.74 34.38 36.22 31.80 34.45 36.28 31.76 34.41 36.24 31.79 34.45 36.27 31.86 34.53 36.35 31.95 34.62 36.43 31.97 34.64 36.45 31.99 34.66 36.47

47.61

47.31

47.04

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1

(100) (002) (101) (100) (002) (101) (100) (002) (101) (100) (002) (101) (100) (002) (101) (100) (002) (101) (100) (002) (101) (100) (002) (101)

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Lattice constants (Å) a c

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Sample (hkl) 2θ peak FWHM Lattice Crystallite dhkl type position size volume CCo (°) (°) V (nm) (Å) 3 (at%) (Å )

47.00

46.95

3.250 5.204 0.140 1.977

3.243 5.192 0.675 1.9732

3.238 5.179 1.257 1.969

3.237 5.177 1.347 1.969

3.237 5.174 1.481 1.968

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Table I: Lattice parameters, volume, crystallite size calculated from the peak positions (100), (002) and (101), Micro-strain and Zn-O bond length values.

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Assignments First order Second order Frequency (cm-1)

208 334 540-670 986 1050 1084-1149

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102 379 410 439 574 591

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E2 (low) A1 (TO) E1 (TO) E2 (high) A1 (LO) E1 (LO)

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Table II: The assignments of the Raman peaks of bulk ZnO according to the study [60, 61].

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CCo ( E2(high) at%) position (cm-1)

Position (cm-1)

FWHM (cm-1)

437.8

1

438.2

3

438.2

5

438.2

7

438.2

9

438.2

11

438.2

15

436.8

23.87 ------9.581 24.83 13.66 26.42 14.74 26.64 26.20 31.01 29.50 33.40 29.73 36.00 31.64 38.67

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0

580.5 ----557.56 578.27 554.38 573.35 554.36 572.70 549.0 571.39 542.60 571.50 540.62 571.00 537.01 567.58

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Co shifted

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A1(LO)

Table III: E2 (high) position, frequencies and calculated (FWHM) values, from fitting A1

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(LO) and cobalt contribution Raman modes.

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Co (at%) 0 1 3 5 7 9 11 15 Position (cm-1) 410.7 422.8 419.7 420.4 419.0 417.0 410.8 414.6 FWHM (cm-1) 45.0 57.2 51.8 51.2 45.9 43.6 41.3 37.0 f (N/cm) 3.08 3.263 3.216 3.226 3.205 3.174 3.081 3.138

Table IV: Zn-O stretching mode position and FWHM values. The calculated bond force

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constant f (Newton/cm).

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Co (at%) 0 1 3 5 7 9 11 15 Eg (eV) 3.29 3.30 3.32 3.35 3.33 3.00 2.97 2.92

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Table V : Optical gap values.

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Table VI: The effective gyro-magnetic factor of Co

ion (µ eff) and the Curie-Weiss

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temperature Θ.

2+

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Co (at%) 0 1 3 5 7 9 11 15 µeff ( µB units) ---- 0.433 0.633 0.588 0.806 0.991 0.878 0.851 ---- -5.363 -4.889 -4.414 -4.734 -4.848 -5.292 -5.839 Θ (K)

-b-

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Figure 1

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Element Atomic % O 64,83 Zn 35,17

Figure 2

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Element Atomic % O 58,34 Zn 38,27 Co 3,39

(101)

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(102)

(103)

(110)

5

2,8x10

ZnO

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Figure 3

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Figure 4

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Figure 5

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Cobalt content (at%)

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Cobalt content (at%)

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Figure 7

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Figure 8

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Co

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Figure 9

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15 at%

Transmission (arb.units)

11 at %

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pure ZnO

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f (N/cm)

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Cobalt content (at%)

Figure 12

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Figure 13

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1308 0.95 eV

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Figure 14

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CCo= 15 at% Eg = 2.91 eV 10

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Figure 16

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103 χ n (x10 cm /mol)

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T (K)

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Figure 17

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µEFF/Co

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Figure 18

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The bond length Zn-O (L) in the unit cell diminishes with increasing the rate of doping of the cobalt.

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FTIR results show the decreasing of the bond force constant f with increasing Cobalt doping. All our samples exhibit paramagnetic behavior because of the 3d electron of Co2+.

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The observed paramagnetism might not originate only from the Co2+ active magnetic

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doping atoms, but from oxygen vacancies and/or by zinc-related defects in the ZnO

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lattice.