Materials Science and Engineering B 219 (2017) 1–9

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Materials Science and Engineering B

journal homepage: www.elsevier .com/locate /mseb

Mn-doped ZnO nanocrystals synthesized by sonochemical method:Structural, photoluminescence, and magnetic properties

http://dx.doi.org/10.1016/j.mseb.2017.02.0130921-5107/� 2017 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail address: aaelho@yahoo.com (A.A. Othman).

A.A. Othman a,⇑, M.A. Osman a, E.M.M. Ibrahim b, Manar A. Ali a, A.G. Abd-Elrahim a

aAssiut University, Faculty of Science, Department of Physics, Assiut 71516, Egyptb Sohag University, Faculty of Science, Department of Physics, Sohag 82524, Egypt

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 December 2016Received in revised form 7 February 2017Accepted 28 February 2017Available online 9 March 2017

Keywords:ZnO nanostructuresSonochemical methodOpticalPhotoluminescenceMagnetic properties

This work reports the synthesis of Mn-doped ZnO nanostructures using ice-bath assisted sonochemicaltechnique. The impact of Mn-doping on structural, morphological, optical, and magnetic properties ofZnO nanostructures is studied. The morphological study shows that the lower doped samples possessmixtures of nanosheets and nanorods while the increase in Mn content leads to improvement of an ani-sotropic growth in a preferable orientation to form well-defined edge rods at Mn content of 0.04. UV–visabsorption spectra show that the exciton peak in the UV region is blue shifted due to Mn incorporationinto the ZnO lattice. Doping ZnO with Mn ions leads to a reduction in the PL intensity due to a creation ofmore non-radiative recombination centers. The magnetic measurements show that the Mn-doped ZnOnanostructures exhibit ferromagnetic ordering at room temperature, as well as variation of the Mn con-tent can significantly affect the ferromagnetic behavior of the samples.

� 2017 Elsevier B.V. All rights reserved.

1. Introduction

Diluted magnetic semiconductors (DMSs) have attractedintense interest due to their potential applications in spintronics[1–3]. Due to the electronic spin interaction between sp-orbitalsof the host lattice and 3d-orbital of Mn-dopant, ferromagnetismcan be created [4]. ZnO has been identified as an excellent candi-date host semiconductor for supporting room-temperature ferro-magnetism when doped with a variety of 3d transition metalions, particularly Mn2+ [5,6]. In addition, ZnO semiconductor hasmany other advantages including the non-toxicity, suitability fordoping, high thermal and chemical stability, n-type semiconductorwith a wide optical band gap of 3.37 eV, and high exciton bindingenergy of 60 meV at room temperature [7]. Therefore, it has manypromising applications such as gas sensors [8], solar cells [9], lightemitting devices [10], piezoelectric devices [11], field emissiontransistors [12], Photocatalysis [13], and DNA sequence detectors[14]. Sonochemical technique is being used in preparation ofnumerous materials because of its advantages such as rapid reac-tion rate, safety, as well as uniform shape and high purity of theprepared samples. The sonochemical process causes an unusualchemical environment via the acoustic cavitation process throughthe formation, growth, and collapse of many bubbles in the

aqueous solution [15]; which leads to many unique properties inthe irradiated solution. Jung et al. [16] reported on the sonochem-ical preparation of various forms of ZnO nanostructures such ascup, rod and flower. Kandjani et al. [17] studied the effects of tem-perature and sonication power on the morphology of ZnO, andobserved that the spherical form predominated at high tempera-ture and sonication power. Bhattacharya et al. [18] preparedhexagonal ZnO nanodisks under ultrasound irradiation, theynoticed that the product was more porous when synthesized underan argon atmosphere compared to that when exposed to air. Thedoping of ZnO with the transition elements offers a feasible meansof fine tuning of the optical band gap to make use as UV detectorand light emitters [19]. Among all transition elements, dopingZnO with Mn is most encouraging because Mn at a valence stateof 2+ has the highest possible magnetic moment [20]. Recently,Mn-doped ZnO materials have attracted much interest due to thepossibility of applications to the short wave magneto-opticaldevices [21] and it was reported that, tuning the morphology ofZnO nanostructure by doping results in variations in the electricaland optical properties [22]. Furthermore, Dietl et al. [23] suggestedthat doping of ZnO by Mn element would show ferromagneticbehavior with Curie temperature above room temperature. Dekaet al. [24] demonstrated that doping ZnO lattice by Mn ions resultsin a reduction in the particle size, an increase in both of the unitcell volume and the optical band gap, as well as an exhibition offerromagnetic behavior. Li et al. [25] illustrated that Mn-doped

2 A.A. Othman et al. /Materials Science and Engineering B 219 (2017) 1–9

ZnO nanorods have crystalline wurtzite structure and preferentialgrowth direction along the c-axis, as well as exhibition of a strongnear band-edge emission and a weak deep-level emission. Moon-tragoon et al. [26] found that increasing Mn concentration leadsto a reduction in the particle size with an increase of themagnetization.

Although many research works have been carried out on Mn-doped ZnO nanostructures [27–35], a lack of using the ice-bathassisted sonochemical technique for preparing Mn-doped ZnOnanostructures has been found. Based on our previous work forCu-doped ZnO nanocrystals that showed anomalies behavior whichdepended basically on using ice-bath assisted sonochemical tech-nique [36], the present work aims to synthesize the undoped andMn-doped ZnO with various concentrations of Mn using ice-bathassisted sonochemical method at room temperature. The morphol-ogy modification induced by Mn doping that followed by specificchanges in the structure of ZnO host lattice have attracted our atten-tion to investigate the dependence of the optical, photolumines-cence, and magnetic properties of ZnO nanostructures on the Mncontent.

2. Experimental details

2.1. Materials

For preparing the samples under study, zinc acetate dihydrate[Zn(CH3COO)2�2H2O, with assay 98% purchased from Oxford labo-ratory reagent], Manganese chloride tetrahydrate [MnCl2�4H2O,with assay 97% purchased from Alpha Chemika company], andsodium hydroxide [NaOH, with assay 96% purchased from Adwic]were used as starting materials, while polyethylene glycol [PEG6000 purchased from Adwic] was used as a surfactant agent. Theabsolute ethanol was purchased from Adwic.

2.2. Samples preparation

The synthesis of Zn1�xMnxO (x = 0.00, 0.01, 0.02, 0.03, 0.04, and0.05) nanocrystals was carried out at room temperature by meansof the ice-bath assisted sonochemical method. Specifically, 2 MNaOH was dissolved in 100 ml double distilled water at room tem-perature under magnetic stirring for 5 min to form the aqueoussolution A. 0.5(1 � x) M Zn(CH3COO)2�2H2O, 0.5x M MnCl2�4H2O,and 2.5 g of PEG were dissolved in 100 ml double distilled waterunder the same conditions with magnetic stirring to form theaqueous solution B. Then, solutions A and B were mixed until thepH value reached 12.95 at 24.5 �C. After that, the final mixturesolution was irradiated with 20 kHz ultrasound with average out-put power 70W for 1 h, and the reaction temperature was stabi-lized at the room temperature using ice-bath. The obtainedprecipitate was centrifuged and washed several times with the dis-tilled water and absolute ethanol to remove the residuals, andfinally dried in air at 80 �C for 18 h. Further information was statedin our previous work [36].

2.3. Characterization

To study the crystal structure, the synthesized samples wereinvestigated by the X-ray diffraction (XRD) using PW 1700 X-raydiffractometer with Cu-Ka radiation source k = 1.5406 Å, wherethe XRD patterns were recorded over the diffraction angle (2h)range from 30� to 70� with step 0.06�. The spectra of Fourier trans-form infrared (FTIR) spectroscopy for the Zn1�xMnxO system wererecorded using NicoletTM iSTM10 FT-IR Spectrometer to identify thechemical bonding in the samples over the range of wave number4000–400 cm�1. The morphological studies of the samples wereinvestigated by a high-resolution transmission electronmicroscopy

(HRTEM) JEOL JEM 2100 microscope operating at acceleratingvoltage equals to 200 kV. UV–Vis absorption spectra were recordedat room temperature using Perkin Elmer lambda 750 spectropho-tometer in the wavelength ranges from 300 to 800 nm. The PL spec-tra of all powder samples were measured at room temperature atexcitation wavelength (kex) of 325 nm by JASCO FP-6300spectrofluorometer. The magnetic properties of all samples understudy were measured at room temperature by vibrating samplemagnetometer (VSM) using VSM-model LDJ 9600.

3. Results and discussion

3.1. Structural analysis

The XRD patterns of pure and Mn-doped ZnO nanocrystals(NCs) presented in Fig. 1(a) indicate a Wurtizite-type hexagonalstructure with space group P63mc of pure ZnO crystal with prefer-able orientation along (100), (002), (101), (102), (110), (103),(200), (112), and (201) crystalline planes that reveal good agree-ment with those of JCPDS reference cards [04-008-8198.]. In addi-tion, the high intensity of the diffraction peaks suggests the highcrystalline nature of the prepared samples. No diffraction peakscorrespond to secondary Mn-related phases or any other impuritywere detected on the XRD patterns indicating that the Mn2+ ionshave probably substituted Zn sites without changing the crystalstructure of ZnO lattice [37]. It was observed that the peak intensi-ties decreases with increasing the Mn content, as shown in Fig. 1(b), indicating that the increase in Mn content deteriorates thecrystallinity of the samples. This behavior is similar to previousreports that prepared Mn-doped ZnO by other techniques ashydrothermal and sol-gel methods [38–41]. On the other side,the increase of Mn content up to 0.02 results in: 1) a shift in thediffraction angle (2h) to higher values Fig. 1b, 2) a decrease in bothZn-O bond length (L) and unit cell volume (V), Fig. 1c, and 3) adecrease in the corresponding crystallite size as shown in Fig. 1d.This interesting feature is worth to have further discussion. Previ-ous studies illustrated that introducing divalent Mn2+ with largerionic radius (0.80 Å) than Zn2+ (0.74 Å) [42] resulted in a deteriora-tion around the dopant site which induces a compressive stressleading to an increase in the cell parameter of the host lattice.Meanwhile, the ability of Mn ions to have multiple ionizationvalence state like Mn3+ or Mn4+ with lower ionic radius of 0.58and 0.53 Å, respectively provides logical prediction of a reversecompression stress associated with the decrease of the correspond-ing cell parameter [43]. Moreover, the difference in electronegativ-ity between Zn2+ and higher ionization state of Mn+ (n = 3 or 4) alsosupports the contraction of unit cell due to the increase in theattraction to dopant sites. Similar behavior of cell parameter reduc-tion in the case Mn and Cu doping was observed and attributed tothe same mentioned reasons [44,36,45]. Further increase in Mnconcentration up to x = 0.05 caused a shift of diffraction angle tolower values due to the expansion of the lattice volume producedby the increase of distortion centers density in the lattice aroundthe Mn-dopant ions. This is in coincidenc with previous publica-tions [42,46–48]. The Zn–O bond length (L) is given by [49]:

L ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia3

2þ 1

2� z

� �2

c2

sð1Þ

where z (in the hexagonal system) is given by

z ¼ a2

3c2þ 14

ð2Þ

The values of the unit cell volume (V), and lattice parameters (aand c) of the hexagonal system was calculated using the latticeGeometry equation [50]:

Fig. 1. XRD patterns (a), the shifts of the diffraction angle of the most intensity peak (b), dependence of the unit cell volume and bond length on Mn content (c), anddependence of the crystallite size and micro-strain on Mn content (d) for the Zn1�xMnxO nanostructures.

A.A. Othman et al. /Materials Science and Engineering B 219 (2017) 1–9 3

V ¼ 0:866a2c ð3Þ

1

d2 ¼ 43

h2 þ hkþ k2

a2

!þ l2

c2ð4Þ

It was found that the values of the lattice parameters (a, c andc/a) are in a good match with those of the ICDD reference. It can beobserved from Table 1 that the determined values of (c/a) ratiosare nearly the same, indicating that the incorporation of Mn2+ ionsinto ZnO lattice doesn’t make any noticeable deformation in thelattice.

For more illustration of the effect of Mn content on the struc-ture of the ZnO crystals, the crystallite size (Dhkl), micro-strain (e)[51] and dislocation density (d) [52] of the most intense diffraction

Table 1The 2h value, crystallite size D, lattice parameters (a, c, c/a), volume V of unit cell, Zn–O benergy density per unit volume u of the Zn1�xMnxO nanopowders.

x 2h� D (nm) Lattice parameter V (Å3) L (Å) e �1

a (Å) c (Å) c/a

0 36.221 18.15 3.259 5.218 1.601 47.996 1.98299 21.20.01 36.244 16 3.2503 5.2103 1.603 47.669 1.97845 21.50.02 36.235 15.6 3.2463 5.1976 1.601 47.436 1.975254 21.90.03 36.241 18.5 3.2561 5.2221 1.603 47.948 1.982276 18.50.04 36.260 18.8 3.2634 5.2369 1.604 48.300 1.987093 18.20.05 35.156 25.2 3.2685 5.2542 1.607 48.611 1.991299 13.6

peak (101) crystalline plane of pure and Mn-doped ZnO sampleswere calculated according to the following equations: [53]

Dhkl ¼ 0:9kbhkl cos h

ð5Þ

e ¼ b cos h4

ð6Þ

d ¼ 1=D2 ð7Þwhere k is the wavelength of the incident X-ray beams(k = 1.540562 Å), bhkl is the full width at the half maximum (FWHM)corrected for the instrumental broadening of the XRD peaks, and h isthe diffraction angle.

ond length L, internal strain e and stress, dislocation density d, Young modulus Y and

0�4 d � 1015 line/m2 Y �1011 (Pascal) r � 108 (Pascal) u � 105 (J/m3)

3 3.04 3.381 7.18 7.620 3.91 3.380 7.27 7.810 4.11 3.382 7.41 8.115 2.92 3.378 6.27 5.813 2.83 3.378 6.16 5.612 1.57 3.375 4.60 3.13

4 A.A. Othman et al. /Materials Science and Engineering B 219 (2017) 1–9

Fig. 1(d) shows the influence of ZnO doping with Mn element onboth of the crystallite size and the corresponding micro-strain. It isobserved that the increase in Mn content up to 0.02 results in adecrease in the crystallite size associated with an increase in themicro-strain and the density of grain boundaries (i.e. dislocations),as shown in Table 1. This behavior can be ascribed to the inducedinhabitation of the grain growth by incorporation of Mn as animpurity, which coincides with the decrease in both the unit cellvolume and bond length (see Fig. 1(c)). Further increase of Mn con-centration leads to an increase in the crystallite size due to theexpansion of the unit cell volume caused by the mismatch betweenZn and Mn ionic radii [44]. The stress r can be determined fromHook’s law r ¼ Ye, where Y is Young’s modulus which can be cal-culated (for the hexagonal system) from the following equation[54]

Y ¼h2 þ ðhþ2kÞ2

3 þ alc

� �2� �2s11 h2 þ ðhþ2kÞ2

3

� �þ s33 al

c

� �4 þ 2s13 þ s44ð Þ h2 þ ðhþ2kÞ23

� �alc

� �2 ð8Þ

where s11, s13, s33, and s44 (the lattice compliances of ZnO) are7.858 � 10�12, �2.206 � 10�12, 6.940 � 10�12, and 23.57 � 10�12 -m2 N�1, respectively [55]. The energy density u (the parameter uis defined as the energy per unit volume of a lattice) can be calcu-lated according to Hook’s law u = (e2Yhkl)/2. The values of Y, r, u,as well as all above-mentioned parameters, of the undoped andMn-doped ZnO nanostructures, are calculated for (101) crystallineplane and listed in Table 1.

3.2. FTIR study

The purity and molecular structure of the undoped and Mn-doped ZnO samples were investigated by the FTIR spectroscopy.The FTIR spectra of Zn1�xMnxO system at room temperature areshown in Fig. 2. The characteristics peaks for all samples areassigned and tabulated in Table 2. The absorption bands between3378 and 3421 cm�1 are ascribed to O–H stretching mode [56],which indicates to an existence of water molecules on the surfaceof Zn1�xMnxO samples. The absorption peaks in the range of 1576–1636 cm�1 are attributed to Zn–O stretching mode [57]. Two peaksappear between 1384–1436 and 1030–1090 cm�1 for the undopedand Mn-doped ZnO samples. These peaks correspond to C–H bend-ing [58] and stretching mode [59], respectively and indicate to theexistence of PEG in the investigated samples. Although the FTIR is amassive analyses tool, the PEG is expected to be on the surface ofthe particles because the XRD results confirmed the pure

Fig. 2. FTIR spectra for Zn1�xMnxO nanostructures.

Wurtizite-type hexagonal structure of the prepared samples. Thepeaks in the range from 429 to 881 cm�1 (see Table 2) are attribu-ted to the metal-oxide (Zn–O or Mn–O) stretching and vibrationalmodes [18,58,60], which reveal the incorporation of Mn ions intoZnO hexagonal lattice.

3.3. Morphological study

The morphology of the undoped and Mn-doped ZnO sampleswas investigated using high-resolution transmission electronmicroscope (HRTEM). The HRTEM images of Zn1�xMnxO (x = 0,0.02, and 0.04) are shown in Fig. 3(a, b and c), respectively. Theseimages reveal that the morphology of the samples of x = 0 and0.02 Mn contents composes of mixtures of sheets and rods in nano-sized dimensions. However, the samples of higher Mn contentcomposes of pure nanorods as shown in the image of Fig. 3(c) forx = 0.04 sample as an example. To highlight the pure nanorodsmorphology, an image of x = 0.04 sample with lower magnificationhas been depicted as inset in Fig. 3c. Based on these results, it isclear that Mn addition plays a vital role in the modification ofthe morphology of ZnO NCs. ZnO nanopowders were obtainedthrough dissolution of water and starting materials by absorptionof the ultrasound energy, where the reaction inside the sonicationbath conforms to the following equations [61]

H2O ! Hþ þ OH� ð9Þ

NaOH ! Naþ þ OH� ð10Þ

ZnðCH3COOÞ2 ! Zn2þ þ 2CH3COO� ð11Þ

Zn2þ þ 4 OH� ! ZnðOH4Þ2� ð12Þ

2 ZnðOH4Þ2� ! 2 ZnOþ 4 H2Oþ O2 ð13ÞThe existence of polyethylene glycol (PEG) during the nucle-

ation process provide controlling tool on the growth of ZnO besidethe role of the ultrasound energy. The proposed mechanism forobtaining a morphology of sheets and/or rods can be summarizedby schematic diagram presented in Fig. 4. Analysis of TEM image ofthe undoped ZnO sample, Fig. 3(a), shows that the length anddiameter of the nanorods are in the range between 61–279 and21–51 nm, respectively. Incorporation of Mn2+ ions into ZnO latticeenhances an anisotropic growth in preferable orientation to formwell-defined edges rods at Mn content of 0.04, as shown in Fig. 3(b, c). it also resulted in the increase of both lengths and diametersof the nanorods to the range between 280–380 and 25–80 nm,respectively, at the Zn0.94Mn0.04O sample. This result is in a goodmatch with the above-mentioned increase of the crystallite sizeas has been discussed above in the XRD section. Note that, similarbehavior was observed by Othman et al. [36]. Analysis of theHRTEM images at higher magnification for ZnxMn1�xO (x = 0,0.02, and 0.04) are shown in Fig. 3(d-f). The data reveal that themeasured values of the interplanar d-spacing equal to 2.82 Åwhich coincides with the d-spacing of (100) crystalline plane ofthe ICDD reference of the hexagonal ZnO.

3.4. UV–vis study

Fig. 5(a) shows the UV–vis absorption spectra of the undopedand Mn-doped ZnO nanosheets and nanorods synthesized by ice-bath assisted sonochemical method. The spectra of the samplesof x = 0.00, 0.01, 0.02, 0.03, 0.04, and 0.05 show strong excitonpeaks in the UV region at 370, 368, 366, 365, 363, and 362 nm,respectively. Consequently, it can be concluded that the incorpora-tion of Mn ions into ZnO lattice leads to a blue shift of the exciton

Fig. 3. HRTEM images of undoped and Mn-doped ZnO nanocrystals. The inset ofimage (c) corresponds to the x = 0.04 sample at lower magnification.

Table 2IR peaks and their assignments for Zn1�xMnxO samples.

Assignments Wave number (cm�1)

x = 0.0 x = 0.01 x = 0.02 x = 0.03 x = 0.04 x = 0.05

O–H stretching 3393 3420 3378 3381 3421 3382Zn–O stretching 1636 1634 1635 1636 1576 1577C–H bending 1405 1384 1384 1384 1436 1432C–H stretching – – 1090 1090 – –

– – 1032 1033 – –M–O 873 881 861 862 872 880Stretching – 560 – – 545 –Mode – 429 – 429 451 –

A.A. Othman et al. /Materials Science and Engineering B 219 (2017) 1–9 5

peak. In addition, the values of the optical band gap Eoptg

� �were

determined from the first derivative of the optical absorbancespectra with respect to photon energy, Fig. 5(b); where the

Fig. 4. Schematic diagram for the preparation of pure and M

maximum in the first derivative spectra at lower energy sides rep-resent the exact value of Eopt

g [36]. The Eoptg exhibits values of 3.45,

3.47, 3.48, 3.49, 3.51, and 3.52 for x = 0.00, 0.01, 0.02, 0.03, 0.04,and 0.05 samples, respectively. Fig. 5(c) shows the Eopt

g and the cor-responding exciton energy (Eex) as functions of Mn content. Fur-thermore, the Eex values were calculated from the formulaEex = hc/kmax, where h is the Planck’s constant, c is the speed oflight, and kmax is the wavelength corresponding to the excitonpeak. Clearly, incorporation of Mn ions into ZnO lattice results inthe increase in both of the Eopt

g and Eex. This behavior can beascribed to (i) the larger band gap of MnO (4.2 eV) compared tothat of ZnO (3.34 eV) [40,41], (ii) the sp–d spin exchange interac-tion between the spin of s-orbitals of the conduction band andp-orbitals of the valence band with the spin of Mn d-orbital [62],and (iii) the Burstein-Moss (BM) effect [63]. In case of the undopedZnO nanostructures, the absorption transitions occur from the topof the valence band (VB) to the bottom of the conduction band(CB). Because of ZnO is an n-type material, the Fermi level shiftsand merges with the CB when it is heavily doped. Due to the fillingof the CB (below Fermi level) associated with the increase in carrierconcentration by doping, the absorption transitions occur from thetop of the VB to the Fermi level instead of the bottom of CB [63].Therefore, this change in the absorption transitions results in theshift of the Eopt

g to higher energies. Accordingly, BM effect leads

to the increase in the Eoptg when ZnO is doped with Mn ions. Similar

behavior of enhancement of Eoptg by Mn-doping was observed by

Hu et al. [41], and Fukumura et al. [64].

3.5. Photoluminescence (PL) study

Because the densities of the defects and oxygen vacancies affectstrongly the optical properties of the oxides nanostructures, PLanalysis is a powerful tool for estimating the different structuraldefects and optical quality of nanopowders through identificationof various trapping and recombination levels of photo-generatedcarriers. Noteworthy, the presence of the structural defects such

n-doped ZnO nanocrystals by sonochemical technique.

Fig. 5. UV–vis absorption spectra (a), the first derivative of the absorbance spectra (b), and the dependence of the optical band gap and the corresponding exciton energy onMn content (c) for Zn1�xMnxO nanostructures.

6 A.A. Othman et al. /Materials Science and Engineering B 219 (2017) 1–9

as zinc and oxygen interstitials and/or vacancies in ZnO nanosizedstructures leads to different radiative recombination transitionsbetween electrons belong to either the conduction band or trap-ping levels, and holes belong to either the valence band or trappinglevels [65,66]. Thus, the present work concerns with the investiga-tion of the influence of doping with Mn2+ ions on PL spectrum ofZnO nanopowders at excitation wavelength (kex) of 325 nm.

Based on our previous study of Cu-doped ZnO nanocrystals [36],the mechanism of radiative transition at excitation wavelength of325 nm of the undoped ZnO nanocrystals was suggested; whichthe deconvolution of the undoped ZnO emission spectrum revealbands centered at 356, 397, 430 and 515 nm. The weak UV emis-sion band at 356 nm (3.49 eV) was attributed to the band to bandradiative transition. Meanwhile, the most intense UV emissionband centered at 397 nm (3.13 eV) was ascribed to one of the fol-lowing processes: (I) exciton recombination of photo-generatedcarriers [29]; or (II) recombination of trapped electrons at shallowlevels of zinc interstitial (Zni) with photo-generated holes [67]. Inaddition, in accordance with other previously published works,the blue emission band centered at 430 nm (2.89 eV) is attributedto the recombination of trapped electrons at shallow levels of Zni

with trapped holes at zinc vacancy (VZn) [65,67]. The green emis-sion band centered at 515 nm (2.41 eV) was be ascribed to therecombination of trapped electrons at singly ionized oxygenvacancy (Vþ

O) with a photo-generated holes [68], or recombinationof trapped electrons at the deep donor level of Zni with trappedholes at singly ionized zinc vacancies ðV�

ZnÞ acceptor levels [69].The PL spectra of Mn doped ZnO samples were recorded and

depicted in Fig. 6(c). The data reveal that the increase in Mn

content causes a quenching in the PL bands intensity. This quench-ing can be attributed to the increase in non-radiative recombina-tion centers caused by Auger process [70]. According to theAuger process, the energy released by an electron recombinationis immediately absorbed by another electron where the releasedenergy is dissipated as phonons. The increase in the density ofdefects, due to the increase of the Mn content, leads to an increasein the non-radiative recombination centers which results in areduction of the emission intensity [56].

3.6. Magnetic properties

Fig. 7(a) shows the magnetic field (H) dependence of magneti-zation (M) of the Zn1�xMnxO (x = 0.01, 0.02, 0.03, 0.04, and 0.05)nanostructures. The influence of the sample holder on the mag-netic behavior was checked by investigating its magnetic proper-ties under the same conditions but no ferromagnetism wasdetected. The actual magnetization of the Zn1�xMnxO was deter-mined by subtracting the diamagnetic contribution of the sampleholder using the following equation [71]

MðHÞ ¼ MtotalðHÞ � vholderH ð14Þ

where vholder is the sample holder magnetic susceptibility [71].Consequently, the magnetism in the samples is promoted byZn1�xMnxO samples without any contribution from the sampleholder. Noteworthy, the M-H plot (not included here) of the purezinc oxide shows the typical diamagnetic behavior previously con-firmed in reference [72]. However, The M-H plots of Zn1�xMnxO

Fig. 6. Normalized PL spectra of Zn1�xMnxO nanostructures at excitation wave-length of 325 nm.

-10 -5 0 5 10

-4

-2

0

2

4

Mag

netiz

atio

n (e

mu/

g)

H (KOe)

x=0.01 x=0.02 x=0.03 x=0.04 x=0.05 x=0.1

(a)

0.01 0.02 0.03 0.04 0.053.4

3.6

3.8

4.0

4.2

4.4

Ms (

emu/

g)

Mn content

(b)

Fig. 7. The magnetization vs. the magnetic field for Zn1�xMnxO system (a), and thesaturation magnetization as a function of Mn content (b).

A.A. Othman et al. /Materials Science and Engineering B 219 (2017) 1–9 7

measured at room temperature show hysteretic behavior confirm-ing ferromagnetic ordering in the materials.

Indeed origin of room temperature ferromagnetism RTFM in theMn doped ZnO is still a puzzling issue and many explanations havebeen reported on this issue. For example, Elanchezhiyan et al. [73]reported that 5% Mn-doped ZnO thin film has the maximum ferro-magnetic moment and the ferromagnetic features decrease withthe increase of Mn content. They attributed this behavior to thedecrease in Mn-Mn distance due to the Mn ions clustering.Ramachandran et al. [74] attributed the ferromagnetic features ofthe Mn doped ZnO compounds to the role of the carrier or defectmediated mechanisms. This assumption was also supported byCong et al. who proposed that carrier induced mechanism, imply-ing the exchange interaction between local spin-polarized elec-trons (such as the electrons of Mn+2 ions) and conductiveelectrons, is the origin of the ferromagnetism in the Mn dopedZnO nanostructures [75]. Besides, Dietl et al. [76] studied the mag-netic properties of the Co-doped ZnO nanostructures. Theydescribed the origin of the RTFM in terms of a spinodal decompo-sition of an antiferromagnetic Co-rich nanocrystals in a paramag-netic matrix of ZnO: Co. They suggest that the RTFM originatesfrom uncompensated spins at the antiferromagnetic surfaces. It iswell known that, Mn oxides can be formed in four phases with var-ious magnetic features. To be specific, the antiferromagnetic Mn,MnO2 and Mn2O3 possess Neel temperatures �100, �84 and�75 K, respectively, meanwhile Mn3O4 is a ferromagnetic withCurie temperature �43 K [77]. The XRD patterns confirm that ourZn1�xMnxO nanoparticles are crystallized in typical hexagonalwurtzite structure without any Mn-related phases, which confirmthat RTFM in the samples doesn’t originate from the presence ofmanganese oxides or clusters.

The PL spectra of our samples indicates that the intensities ofthe peaks correspond to the deep level bands decrease withincreasing Mn content in the ZnO matrix, indicating a decrease inthe concentration of the Vo and Zni defects. Thus one can concludethat the RTFM in our samples doesn’t originate from the defects.On the other hand, neither antiferromagnetism nor superparamag-netism can be observed in the samples which mean that the ferro-magnetism can’t be ascribed to the spinodal decomposition[78,79]. For all these reasons, the RTFM of the samples under studymost probably arises from the substitution of Zn ions by Mn ionswhich was suggested by the aforementioned results of the XRDanalysis [72]. The scenario of the RTFM can be summarized as:the exchange interaction between the local spin polarized elec-trons (the electrons of Mn+2 ions) and the conduction electronslead to the spin polarization of conductive electrons and subse-quently, the spin-polarized conductive electrons perform anexchange interaction with local spin polarized electrons of otherMn+2, and so on. Due to this long range exchange interaction,almost all Mn+2 ions possess the same spin direction everywhereand the conduction electrons act as a medium to connect allMn+2 ions. Consequently, the material show ferromagnetic features[80–82].

The saturation magnetization Ms of the Zn1�xMnxO nanoparti-cles have been plotted in Fig. 7(b) as a function of Mn content.Ms was found to be in the range of 3.56–4.6 emu/g. These valuesare relatively higher than those reported in other literature suchas Bhowmik et al. (�0.025–0.23 emu/g) [83], Chattopadhyayet al. (�0.02–0.2 emu/g) [84] and Karmakar et al. (�0.12 emu/g)[85]. The higher values suggest the strong exchange interactionand indicate the Zn ions substitution by the Mn ions and the loweffect of Mn clusters in our samples. On the other hand, it wasfound that Ms decreases initially up to x = 0.03 and then increaseswith further increase of Mn content. According to the Ruderman-Kittel-Kasuya-Yosida RKKY theory [81,82], the decrease inmagnetization with the increase in Mn content is believed to arises

8 A.A. Othman et al. /Materials Science and Engineering B 219 (2017) 1–9

from a decrease of the Mn-Mn distance due to the clustering of theMn ions while its increase can be attributed to the increase of oxy-gen vacancies associating the Mn substitution in the lattice sites[86,87]. Therefore, it can be concluded that accurate determinationof Mn content in the Zn1-xMnxO nanoparticles is required to tunetheir ferromagnetic properties.

4. Conclusion

In summary, synthesis of Zn1�xMnxO (0 � x � 0.05) nanosheetsand nanorods by ice-bath assisted sonochemical technique hasbeen reported. The XRD results indicate that the Mn-dopingdoesn’t change the hexagonal wurtzite structure of ZnO in all sam-ples. The observed reduction in the diffraction peak intensity indi-cates that the crystal quality decreases with increasing Mn content.FTIR spectra showed that the peaks in the range smaller than881 cm�1 for the doped samples refer to the incorporation of Mnions into the ZnO hexagonal lattice. The morphological study con-firms that the increase of the Mn content leads to increase of thelengths and diameters of the nanorods. Analysis of the opticalabsorption spectra indicates that incorporation of Mn ions intoZnO lattice results in a blue shift of the exciton peak as well asan improvement of the optical band gap and the correspondingexciton peak. The photoluminescence spectra of the undoped andMn-doped ZnO nanostructures confirm that the doping with Mnions leads to the quenching of the photoluminescence intensity.Room temperature ferromagnetic ordering is obtained in the sam-ples doped with the Mn element. The ferromagnetic ordering isdue to the exchange interaction between the local spin polarizedelectrons.

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