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

Yan, The effect of La doping concentration on the properties of zinc oxide ﬁlms prepared by the solegel method, J. Wang, Optical and structural properties of Sr-doped ZnO thin ﬁlms, Mate[r]

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Original Article

The effect of strontium doping on structural and morphological

properties of ZnO nanoﬁlms synthesized by ultrasonic spray pyrolysis

method

A Ouhaibia, M Ghamniaa,*, M.A Dahamnia, V Heresanub, C Fauquetb, D Tonneaub

aLaboratoire LSMC, Departement de Physique, Universite d'Oran Ahmed Ben Bella, 31100 Oran, Algeria bCentre CINaM, Campus de Luminy, Universite d'Aix-Marseille, Marseille 13009, France

a r t i c l e i n f o

Article history:

Received 20 December 2017 Received in revised form 26 January 2018 Accepted 26 January 2018 Available online 16 February 2018 Keywords:

Ultrasonic spray pyrolysis Sr-doped ZnO

Morphology study Optical properties

a b s t r a c t

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 thefirst 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 ZnOfilms 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/)

1 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 semi-conductor 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

de-vices, in solar cells, in light emitter diodes[1e5]

ZnO thinﬁlms 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),

metal-organic 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 thinﬁlms 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 thinﬁlms 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 conduc-tivity 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

Contents lists available atScienceDirect

Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

https://doi.org/10.1016/j.jsamd.2018.01.004

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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=RZn¼ 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

modi-ﬁcation 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

pyrol-ysis 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 L1 To obtain

ZnO doped with strontium (Sr), we added to the 0.3 M L1solution

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-Kasource 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

spectros-copy and photoluminescence (PL) Results and discussion

3.1 Structural and morphological characterization of ZnO and

Sr-ZnO thinﬁlms

The XRD patterns of pristine and Sr-doped ZnO are shown in

Fig 2(a) From thisﬁgure, six orientations of different intensities

can be identiﬁed: (100), (002), (101), (102), (110) and (103)

Ac-cording to JCPDS 036-1451 card, these peaks indicate the hex-agonal (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 preferen-tially 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 theﬁrst 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

to-wards high diffraction angles 2qas observed inFig 2(c) This shift

moves up till 3% of Sr-doping whereD(2q)¼ 0.12and it returns

toward low angles for and 5% XRD signal shows no additional

peak which may suggest that Sr2ỵions 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 inTable

According to the values listed inTable 1corresponding to the

(002) peak, c decreases slightly with increasing the Sr-doping con-centration 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]

D¼b0:9coslq (1)

where l is the wavelength of X-ray, b is the full-width half

maximum (FWHM) of the XRD peak, andqis 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 Sr-doping was observed to play an important role in the ZnO

struc-turedﬁlms 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 be-tween 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 mi-croscope (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

3mm 3mm 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 roughnesss(rms) or the average

Fig Simpliﬁed scheme of the ultrasonic pyrolysis technique

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roughnesssa These two roughnesses are deﬁned by the following

expressions[28]:

srmsị ẳ PN

iẳ1Zi Zavgị2 N

!1

(2)

and

sa¼ PN

iẳ1Zi Zavgị

N (3)

where N is the number of points, Ziis the number of ith point of Z and

Zavg is the average value of Z These two expressions of the

rough-nesses were treated by WsXM software and their proﬁles were

extracted from AFM images as shown inFig 3(a) and (b) Thisﬁgure

shows the examples of pristine and 5% Sr-doped ZnOﬁlms where

the roughnesssrms is determined to be and 75 nm, respectively

. . . . . . . .

Fig (a) XRD spectra of pristine and Sr-doped ZnOﬁlms (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 ZnOﬁlms

Sr-doping concentration (%) Parameter a (Å) Parameter c (Å) Grain size (Å)a Cluster size (nm)b

0 3.2506 5.2128 23.39 40

1 3.2486 5.2084 24.53 150

2 3.2466 5.2010 26.23 180

3 3.2474 5.1996 28.22 200

4 3.2516 5.2090 27.57 172

5 3.2520 5.2098 27.81 153

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The AFM characterization of the pristine ZnOﬁlm revealed ho-mogeneous and continuous surface uniformly distributed

nano-metre sized grains where the surface roughness srms is

determined to be ~4 nm The state of the surface changed under

Sr-doping effect and the ZnO ﬁlms became granular with irregular

ZnO particles and somewhat porous As shown inFig 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 concen-tration from to 3% and became large-sized grains covering partially the surface and reducing thus the roughness for 3% doping The increase and decrease of the roughness with

Sr-doping (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 Sr-doping effect In order to complete the study of the surface

morphology, the particle size of pristine and doped ZnO thinﬁlms

were evaluated by the WSxM software analysis According to the

calculated grain size and as reported inTable 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 in-creases in size of ZnO particles may be due to Sr ions that

substituted into Zn2ỵsites We recall that the Sr2ỵradius is greater

than that of Zn2ỵ, so the incorporation of Sr2ỵdistorts the lattice

and creates supplementary structure defects that are responsible for changes in the morphology and structure of ZnO surface

prob-ably 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 ZnOﬁlms

were determined from transmission measurements in the range of

300e1400 nm The transmittance spectra are shown inFig It can

Fig AFM images and roughness profile (a) Pristine ZnO films, (b) roughness profile for pure ZnO films withsrms¼ nm (c) Sr-doped ZnO at 5%, (d) roughness profile with srms¼ 75 nm

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be seen that all the ZnOﬁlms 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 agglomera-tion 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 ZnOﬁlms

using the following relation of Tauc[29]:

(ahn)2¼ A(hne Eg), (4)

whereais the absorption coefﬁcient, A is a parameter depending

on the transition probability, h Planck constant,v the frequency of

the incident photons.ais determined from the relationship:

a¼1 dln

T

(5)

T is the transmittance of theﬁlm and d is the ﬁlm thickness

In the plot of the relation (6) inFig 7, we determine the gap by

extrapolation of the linear part of the curve (ahn)2and its

inter-section with the energy axis gives the value of the bandgap

The examples are given for the undoped ZnOﬁlm and for the 5%

Sr-doped ZnO ﬁlm The determined gap values for the pristine

and Sr-doped ZnOﬁlms are listed inTable As shown inFig 8, the

band gap of ZnO is found to decrease from 3.263 to 3.264 eV for to

Fig Variations of the roughnesssrms and averagesawith Sr doping

Fig Transmittance spectra of pristine and Sr-doped ZnOﬁlms

Fig Determination of the band gap value: (a) Pristine ZnOﬁlm and (b) 5% Sr-doped ZnOﬁlm

Table

Determination of the band gap values using the relation (ahn)2¼ A (hne Eg).

Sr-doping concentration (%) Band gap value (eV)

Solid ZnO 3.370

0 3.267

1 3.263

2 3.264

3 3.285

4 3.286

5 3.285

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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 Sr2ỵin

the ZnO matrix As discussed above, the substitution of Sr2ỵ for

Zn2ỵcreates non-linear defects due to the difference in atomic size

and reduces oxygen vacancies as conﬁrmed below by the

photo-luminescence analysis

3.2.2 Photoluminescence analysis

Photoluminescence (PL) is achieved in this study to complete

the optical investigation of Sr-doped ZnOﬁlms It helps us to

un-derstand, 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 9displays 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

400 500 600 700 800 900 0 6000 12000 18000 ). u. a( yti s ne t nI Wavelength (nm) Pristine ZnO (a)

400 500 600 700 800 900 0 1000 2000 3000 ). u. a( yti s ne t nI Wavelength (nm)

1% Sr-doped ZnO

(b)

400 500 600 700 800 900 0

500 1000

1500 2% Sr-doped ZnO

). u. a( yti s ne t nI Wavelength (nm) (c)

400 500 600 700 800 900 0 600 1200 1800 ). u. a( yti s ne t nI Wavelength (nm)

3% Sr-doped ZnO

(d)

400 500 600 700 800 900

0 900 1800 2700 ). u. a( yti s ne t nI Wavelength (nm)

4% Sr-doped ZnO

(e)

400 500 600 700 800 900 0 1000 2000 3000 4000 ). u. a( yti s ne t nI Wavelength (nm)

5% Sr-doped ZnO

(f)

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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 thinﬁlms

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 ZnOﬁlms was improved for the 3% Sr-doping

4 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 thinﬁlms 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' syn-thesis The authors thank also the Algerian-French cooperation through the Tassili 14MDU915 project for the funding support References

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