†Department of Chemistry, Govt. VYT PG Autonomous College Durg, Durg, Chhattisgarh 491001, India‡Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad,Andhra Pradesh 500007, India
*S Supporting Information
ABSTRACT: The present study focused on low temperature synthesis of Mn doped ZnO nanorod array film via chemical bathdeposition method on glass substrates. Microstructural, morphological, and optical properties of Mn doped ZnO nanorods wereinvestigated. X-ray diffraction patterns showed sharp and intense peaks, indicating the highly crystalline nature of the film. Energydispersive X-ray (EDAX) results confirmed the presence of Mn ions in ZnO nanorods. Scanning electron microscopy (SEM)pictures suggested Mn doped ZnO nanorods were well aligned and distributed throughout the surface. Vibrational analysis hasbeen carried out by Fourier transform infrared and Raman spectroscopy. Room temperature photoluminescence (PL) exhibitedthe presence of one broad defects related band in the visible region ranging 440−640 nm. Blue shifting in the absorption edgewith Mn doping was observed in absorption spectra.
■ INTRODUCTION
Synthesis and study of ZnO nanostructures are currentlyattracting researchers. A variety of ZnO nanostructures havealready being synthesized, such as, nanowires, nanobelts,nanotubes, nanoflowers, and nanorods. Among these, ZnOnanorods show unique superiority because of high surface-to-volume ratio, hence, large surface area1 and potentialapplications in solar cell2 and LEDs.3 ZnO is an n-typesemiconductor of hexagonal wurtzite structure and havingoptical transparency in the visible range.4 The visible lightemission of ZnO has great importance for white-light LEDs,5
and it can be achieved through doping of transition metal ions(TM) in ZnO lattice. Doping is the effective way to magicallymanipulate physical and optical properties of a wide band gapsemiconductor. ZnO has a relatively large band gap (3.37 eV)and high exciton binding energy (60 meV) at roomtemperature which makes it an excellent host material fordoping of TM.6 Among TM, incorporation of Mn2+ in ZnOlattice has been studied intensively. Most of the studies of Mndoped ZnO have been focused on its magnetic and spintronicproperties, due to its dilute magnetic semiconductor behavior.7
Recently, Prabhakar et al.8 studied its optical properties formultispectral photodetectors and optical switches, but photo-luminescence behavior of the Mn doped ZnO nanostructure isstill an unfold area which could limit the application of thismaterial.Several synthetic approaches have been made to synthesize
Mn doped ZnO nanorods such as plasma-enhanced chemicalvapor deposition (PECVD),9 chemical vapor deposition(CVD),10 and solution growth method.11 Chemical bathdeposition (CBD) as well as SILAR both are solution growthprocesses which have attracted special interest for researchers,due to its simplicity, inexpensive equipment, and lowdeposition temperatures, which altogether results in low cost
processes and large area of scientific and industrial applications.In addition to this, the solution growth method has anadvantage of using organic moieties as capping agents tocontrol the size of ZnO nanostructures and can avoid problemsassociated with high temperature.In spite of the fact that Mn2+ has ∼14% solubility in ZnO
lattice, incorporation of Mn in ZnO lattice via a lowtemperature method is still difficult because of higher bondenergy for Mn−O compared to Zn−O, hence more energy isrequired to replace Zn2+ by Mn2+ in ZnO lattice12 compared toa high temperature method (like chemical vapor deposition),where it is easier to diffuse the Mn2+ from vapor into the hostlattice.There are a few studies focused on solution phase synthesis
of Mn doped ZnO nanorods and nanowires. Clavel et al.13
synthesized Mn doped ZnO nanowire at high temperature upto 310 °C. Vinod et al.14 and Li et al.15 demonstratedhydrothermal and solvothermal routes to prepare Mn dopedZnO nanorods which require special experimental conditionsi.e. autoclave and maintaining the pressure during reaction. Anonaqueous sol−gel method was employed by Djerdj et al.11
which also need autoclave, temperature up to 200 °C, and longreaction time ∼3 days. Moreover these solution phase methodswere employed for the synthesis of Mn doped ZnO nanorodsor nanowires in powder form. Preparation of materials as filmshave one major advantage over powder form is that films candirectly be applicable for device fabrication. Panigrahy et al.16
synthesized Mn doped ZnO nanorods via a single pot solutiongrowth method, but the resultant nanorods was not well
Received: January 7, 2014Revised: April 4, 2014Accepted: May 13, 2014
aligned to the substrate. Well aligned and ordered nanorodsarray is usually required for solar cell application.17 Comparedto all of the above-mentioned methods, we synthesize Mndoped ZnO nanorods film at very low temperature (below 100°C), less reaction time ∼2.5 h, and no requirement of anyspecial experimental setup.Here we are reporting an effective way to incorporate Mn2+
in ZnO lattice by the modified SILAR assisted CBD method,and systematic evaluation of Mn doped ZnO nanorods weremade by structural, morphological, and photoluminescencestudies.
■ EXPERIMENTAL SECTION
Materials Used. Zinc sulfate monohydrate (ZnSO4·H2O,Merck limited, India), manganese(II) sulfate monohydrate(MnSO4·H2O, Molychem, India), triethanolamine (C6H15NO3,Finar Chemicals, India), and ammonia solution 25% (SDFCL,India) were used as precursors. All the chemicals were analyticalreagent grade and were used without further purification. Thecommercial microscopic glass slides with size 1.45 × 75 × 25mm3 were used as a substrate for the deposition of Mn dopedZnO nanorods. Before deposition, the substrates were boiled 2h in chromic acid, cleaned with single distilled water (SD),double distilled water (DD), degreased with acetone, ultra-sonically cleaned by DD, and finally dried in air.Preparation of Seed Layer. The seed layer of ZnO was
prepared by SILAR. First dipping the substrate in 8 × 10−4 Msolution of [Mn2+] for 10 s, then immediately immersing thesubstrate in an ammonium zincate solution, made up of 0.05 M[Zn2+] and NH3 for 10 s followed by dipping in hot distilled
water (90 °C) for 10 s, then ultrasonically cleaning it for 30 s,and finally drying it in air for 30 s; this process completed asingle deposition cycle. First dipping in Mn2+ solution, Mnoxide may adsorb on the surface of the substrate which acts as anucleation center for ZnO.18 We repeated this cycle up to 15times to get a thin and homogeneous film. As-prepared seedlayers were subjected to annealing at 100 °C for 1 h toeliminate impurities of zinc hydroxide.
Growth of Mn Doped ZnO Nanorods. Aqueous solutionsof 0.1 M [Zn2+], 0.01 M [Mn2+] as dopant, 0.5 mL of 16.7 Mtriethanolamine as complexing agent, and NH3 were used toprepare the film of Mn doped ZnO nanorods (see Figure 1).First 10 mL [Zn2+] solution and 0, 0.1, 0.5, 0.75, and 1 mL of[Mn2+] solution namely samples ZM0, ZM1, ZM2, ZM3, andZM4, respectively, were placed in a 50 mL beaker, and thesolution was stirred for 10 min to get a clear and homogeneoussolution. Thereafter, 0.5 mL of TEA was added withcontinuously stirring for 15 min. DD was added to make thevolume up to 40 mL and then made the solution alkali withNH3 and pH of the bath maintained ≈10. The depositiontemperature was varied from 50 to 90 °C with optimizedtemperature of 65 °C. The substrates were then placed tiltedaround 60° inside the beaker and heated up to 65 °C for 150min. The growth of nanorods takes place downward surface ofthe substrate, and this surface was chosen for characterization.After deposition, the substrates were removed, washed inrunning tap water, rinsed in DD to remove soluble impurities,and then dried in air.
Characterization Techniques. Mn doped ZnO nanorodfilms were subjected to different characterization techniques.
Figure 1. Diagramatic representation of growth of Mn doped ZnO nanorods.
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The phase purity and microstructure of films was studied byBruker D-8 Advance X-ray diffractometer with CuKα X-rayradiation (λ = 0.15496 nm). Surface morphology studies andelemental analysis of the nanorods were carried out employingHitachi S3000N SEM and LINK ISIS-300 Oxford energydispersive X-ray spectroscopy (EDAX) fitted to SEM Hitachi-S520. Optical absorption study was carried out at roomtemperature by Cary 5000 UV−vis NIR spectrophotometer.Photoluminescence (PL) measurements were performed byCary eclipse fluorescence spectrophotometer at room temper-ature. FTIR spectra were obtained using Thermo NicoletNexus 670 spectrometer with 4 cm−1 resolution. Micro-Ramanscattering measurement was carried out on Horiba Jobin-YvonLABRAM HR to investigate the effect of the Mn dopant on themicrostructures and vibration properties.Morphological Analysis. In order to investigate the effect
of Mn doping on morphology of the ZnO film, we carried outSEM analysis. The morphology of undoped and Mn dopedZnO nanorod array grown on glass substrates is shown inFigure 2a-d. Vertically well-aligned hexagonal nanorods grown
throughout the substrates can be observed clearly for undopedand Mn doped samples. The diameter of the ZnO nanorod wasestimated using ImageJ software. The average diameter forsample ZM0 was 0.26 μm, whereas for samples ZM1 and ZM2the average diameter was found to be 0.52 and 0.54 μm,respectively. A significant increment in size of nanorods uponMn doping is due to the fact that Mn2+ ionic size is larger thanthat of Zn2+. Wu et al.12 also observed such an increase indiameter of Mn doped ZnO rods; they argued that the higherbond energy of Mn−O could assist the coalescence process andlead to formation of larger-diameter nanorods. The EDAXanalysis provided precise composition of the elements presentin the material. The incorporation of Mn into ZnO nanorodswas confirmed by EDAX. The atomic percentage of Mn wasfound to be 0, 0.015, 0.22, 0.33, and 0.74% calculated forsamples ZM0, ZM1, ZM2, ZM3, and ZM4 from EDAX (SI,Figure S1). Here, it can be observed that estimated amounts of
Mn in ZnO nanorods are much lower than the amount addedof Mn source in the deposition bath; such a situation was alsonoticed by Wu et al.12
The optical images of undoped and Mn doped ZnOnanorods films are shown in Figure 3. Here one can clearlyobserve that color of the films change from white to brownishyellow, indicated Mn doping in the samples.
XRD Analysis. The X-ray diffraction patterns of undoped andMn doped ZnO nanorods have been recorded in the 2θ rangeof 20−80° (Figure 4a). All the diffraction peaks could beindexed to pure hexagonal wurtzite structure of ZnO nanorodswith the most intense (002) diffraction peak revealing thenanorods grown along the c = [0001] axis and perpendicular tothe substrate.19 No additional peaks related to Zn(OH)2, ZnO2were observed, confirming the phase purity of ZnO nanorods.There was no change found in the wurtzite structure as Mnincorporated in ZnO lattice indicated that the presence of Mndoes not alter the crystallization of ZnO. XRD data was alsoused to estimate lattice constants for pure ZnO nanorods andMn doped ZnO. The lattice parameter ‘c’ has the value 5.16974Å, calculated from the (002) diffraction plane for ZM0. Thesubstitution of Zn2+ by Mn2+ ion in ZnO lattice leads toexpansion in the unit cell, hence an increase in the latticeconstant (Figure 4b) due to the fact that Zn2+ ion in tetrahedralcoordination (radius = 0.60 Å) has smaller ionic radii ascompared to the Mn2+ ion (radius = 0.66 Å). The increase oflattice parameters is nonmonotonic i.e. sample ZM-2 showedsmaller value for ‘c’ compared to undoped ZnO nanorodswhich may be due to the phase impurity of Mn.11 Theincrement in unit cell volume with Mn doping concentrationhas been shown in Figure 4c, increasing unit cell volume(except for ZM-2) with the Mn doping percentage can becorrelated to the higher ionic radius of Mn2+.To observe the effect of the Mn2+ doping concentration on
the Zn−O bond length in the c-axis direction as well as theother three directions, the relation reported in the literature20
was employed and tabulated in Table 1. In general, anincreasing trend in bond length with respect to the Mn dopingpercentage has been observed due to incorporation of Mn inZnO lattice.We observed XRD peak shifting for Mn doped ZnO
nanorods as compared to undoped ZnO nanorods. We haveplotted a graph between XRD peak shifting of (002) planesagainst the Mn doping percentage [Figure 4d]. In general, thepeaks shifted to lower angles except sample ZM-2 which maybe due to the phase impurity of Mn. The relative intensity ofthe (002) plane is often used to estimate the degree of textureof ZnO based films. Texture is commonly defined as thedistribution of crystallographic orientation of a polycrystallinematerial. We employed a relation described in the literature21 to
Figure 2. Representation of SEM images (a) ZM0, (b) ZM1, and (c)and (d) different magnification of ZM2.
Figure 3. Optical images of different samples.
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calculate the preferred orientation of nanorods and summarizedin Table 1. The maximum degree of alignment of ZnO
nanorods is ∼86% for sample ZM1 which is well comparablewith the literature.22
Raman Analysis. Micro-Raman scattering technique is oneof the most effective methods to probe the crystalline property,disorder, and defects in the nanostructure materials which caninfluence the photoluminescence properties of Mn doped ZnOnanorods. The room temperature micro-Raman spectra of ZnOand Mn doped ZnO is shown in Figure 5 in the range of 200−800 cm−1. ZnO and Mn doped ZnO nanorods have a wurtzitestructure with space group P63mc (C
46ν) with two formula units
per primitive cell, where all the atoms occupy the C3ν sites.23
Group theory predicts that two A1, two E1, two E2, and two B1modes are present in the Raman spectra of ZnO. E2 (high)phonon mode and E2 (low) phonon modes are associated with
Figure 4. (a) XRD patterns of undoped and Mn doped ZnO nanorods films. (b) Variation of lattice constant ‘c’ with the Mn doping percentage. (c)Variation of unit cell volume with the Mn doping percentage. (d) Peak shifting (002 plane) with the Mn doping percentage.
Table 1. Calculated Zn−O Bond Lengths and RelativeIntensity Ratio for Different Samples
oxygen atoms and Zn sublattice, respectively. Gaussian multiplepeak fitting to Raman spectra gave different Raman bands asfollows: ∼224 cm−1, ∼268 cm−1, ∼324 cm−1, ∼415 cm−1, ∼437cm−1, ∼576 cm−1, ∼660 cm−1, and ∼743 cm−1. According to
the Raman selection rule, only E2 and A1 longitudinal opticalmodes (LO) are allowed vibrational modes, if incident light isexactly perpendicular to surface.24 The E2 (high) modecorresponding to a wavenumber of 437.8 cm−1 was observedin ZM0. This mode is characteristic of the wurtzite structure ofZnO (consistent with the XRD results) and can be assigned forlattice vibration of oxygen atoms. Venkatesh et al.25 suggestedthe E2 (high) mode can be used to characterize the stress inZnO lattice. For ZM1 nanorods E2 (high) modes appeared at437.1 cm−1; this red shifting of Raman modes can be ascribedto the local stress arising due to incorporation of Mn2+ ions intothe Zn2+ lattice sites. This kind of red shift in Raman spectra ofMn doped ZnO has also been observed previously.16 As weincrease in Mn doping concentration, the E2 (high) modeslightly shifted to lower energy and the peak broadenedasymmetrically (Figure 5). The reason behind this behaviorbeing similar is explained elsewhere in the literature.26 Thepeak at around 580 cm−1 in sample ZM1 arises due to theoxygen deficiency such as oxygen vacancies;27 this result is wellmatched with PL analysis. The Raman spectra also depict thepresence of Mn in ZnO nanorods. The peaks at ∼415 and∼576 cm−1 can be assigned for E1 transverse optical modes(TO) and A1 (LO) phonon modes. The origin of the A1 (LO)mode is due to the zinc interstitial defect.28 One silent mode ofZnO, B1 (low), appeared at ∼268 cm−1 due to incorporation ofdopants leading to structural disorder which can be disturbingto the translational symmetry of lattice hence activating thesilent mode.29 The second order Raman mode at ∼324 cm−1 isarising due to E2 (high)-E2 (low) multiple scatteringphenomenons. This result was also found by Gayen et al.28
and Gao et al.30 for Ni doped ZnO nanorods and ZnOnanorods, respectively. One additional mode at 224 cm−1 mightbe related to the defect induced mode.31 The peak located at660 cm−1 may be due to oxygen vacancies, zinc interstitial, and
Figure 5. Room temperature micro-Raman spectra of undoped andMn doped ZnO nanorods.
Figure 6. (a) and (b) FTIR spectra of undoped ZnO (ZM0) and Mn doped ZnO (ZM1).
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antisite oxygen defects induced Raman mode, well matchedwith that reported by Yang et al.32
In the literature, one additional Raman mode at 523−528cm−1 is usually observed for Mn doped ZnO nanopowder orthin films, but the exact region of origin of this mode is stillunclear. Rao et al.33 suggested that the peak at 525 cm−1
originated due to substitutional Mn ions or Mn-doping-induceddefects. Yadav et al.34 showed the Raman mode at 524 cm−1 inMn-doped ZnO is due to the disorder induced activated 2B1(low) silent mode of ZnO, whereas Cong et al.35 demonstratedthat the peak at 524−527 cm−1 might be due to the localvibration of the Mn ions at the Zn sites. However, acomputational study using a real-space recursion method36
stated that this vibrational mode does not originate directlyfrom the vibration of local Mn atoms but from the vibration ofZn ions in a Mn-rich local environment, where the O sitesaround Zn ions are partially occupied by Mn ions. Hu et al.37
described the peak at ∼528 cm−1 in Mn-doped ZnO is thecharacteristic Raman mode of Mn2O3 which showed thepresence of the Mn2O3 precipitate in the Mn doped ZnOsample. On the basis of their observation, we can reasonablyconclude that the absence of a peak at 523−528 cm−1 in oursample is due to the absence of the Mn2O3 precipitate in oursamples. Panigrahy et al.16 also did not observe an additionalmode at 523−524 cm−1 in Mn doped ZnO nanorods.FTIR Analysis. For the detection of various functional groups
participating in the formation of ZnO nanorods, FTIRmeasurement was carried out in the wavenumber range from400 to 4000 cm−1 using the KBr method at room temperatureshown in Figure 6a and 6b for Mn doped and undoped ZnOnanorods, respectively. A peak appeared in the lower energyregion at 419.31 cm−1 showing the Zn−O bond bendingvibration.38 The broad peak in higher energy region at 3200−3400 cm−1 is due to the stretching vibration of the O−H group,and the peak at 1631.89 cm−1 is due to O−H bendingvibration. The C−H stretching vibration band arises at around2924.87 cm−1 and depicts the presence of an alkyl group; thesepeaks revealed that the presence of an organic moiety in ZnOnanorods which may triethanol amine residues, acting ascapping agents during the growth of ZnO nanorods.UV−visible Spectra. Optical absorption spectra of
undoped and Mn doped ZnO nanorods were recorded in thewavelength range of 300 to 800 nm (Figure 7). The enlargedview of spectra is represented in the inset (wavelength range350−410 nm) for the sake of clarity of the absorption edge.The absorption edge showed a trend to shift toward the lowerwavelength with increasing doping concentration indicated theband gap enlargement of ZnO with Mn doping concentration.Such a situation was also observed by Phan et al.19 and Hao etal.39
Photoluminescence Spectra. In order to study of dopinginduced defects on the photoluminescence behavior of ZnOnanorods, emission spectra were recorded. Roughly, PL spectracan be divided into two bands namely band edge emissionwhich arises due to the recombination of free excitons (electronand hole pair) through an exciton−exciton collision process,and defects level emission in the visible region appears due tothe recombination of electron−hole related to intrinsic defects,such as oxygen vacancies (VO), zinc vacancy (VZn), interstitialZn (Zni), interstitial O (Oi), Zn antisite (ZnO), and oxygenantisites defect OZn (oxygen at zinc site).9 Photoluminescencespectra of Mn doped ZnO nanorods at room temperature havebeen recorded in the wavelength range 350−650 nm with the
340 nm excitation wavelength and are presented in Figure 8a. Itconsists of an ultraviolet (UV) emission peak, called a bandedge emission which is centered at 383.63 (3.23 eV) nm and abroad defect level (DL) band ranging from 440 to 640 nm,which can be readily resolved by Gaussian curve fitting into twobands located at 516.76 nm (2.4 eV) and 582.52 nm (2.13 eV),respectively. The visible emission mechanism in ZnO is stillunclear; it was explained that more than one source isresponsible for its origin and affected by growth andexperimental conditions.40 A representative Gauss fit deconvo-luted PL spectra of sample ZM4 is shown in Figure 8b. Visiblebands at 2.4 and 2.13 eV can be ascribed as a green band and ayellow band. The defects play an important role in the visibleemission of ZnO nanorods. The formation energy of theoxygen vacancy (Vo) is lowest among all point defect in ZnOand is the most probable candidate for the green band found inthe literature.41,42 Vanheusden et al.43 found that the greenband at 510 nm (2.43 eV) originated by recombination ofsingly charged oxygen vacancy (Vo+) and holes from thevalence band. Egelhaaf et al.44 demonstrated this green bandresult from transitions between oxygen vacancies and Znvacancies. Behara et al.45 assigned the peak at 525 nm (2.36 eV)for transition between the VoZni level and a valence band. Linet al.46 suggested a green band at 521 nm (2.38 eV) appeareddue to the electronic transition between the conduction bandand the oxygen antisite defects (Ozn) level; this green emissionband showed a close agreement with our results (2.4 eV).Another luminescence band at 2.13 eV is probably due to thecomplex defect level like VoZni rather than the single pointdefect in ZnO nanorods, which is suggested by Wei et al.47 Wealso observed that the intensity of visible emission increased forMn doped samples as compared to an undoped one, revealingthat incorporation of Mn disturbed the ZnO lattice, henceincreasing the defects concentration, which is clearly visible inthe PL spectra. A similar result was also found by Ronning etal.48 in Mn doped ZnO nanobelts. This enhancement in visibleemission has not shown a trend with the Mn dopingpercentage, indicating incorporation of Mn was not directlyrelated to any specific point defects in ZnO nanorods.
Figure 7. Absorption spectra of undoped and Mn doped ZnOnanorods. The enlarged region from 350 to 425 nm is shown in theinset.
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The intensity of a UV emission peak at ∼380 nm becamestronger with Mn doping. This can be explained by the fact thatMnO has a larger band gap compared to ZnO, hence electron/holes confine more efficiently in Mn doped ZnO nanorods, andtheir recombination gives rise to the UV emission peak.12 Theenhancement in UV emission intensity did not follow any linearrelation with the Mn doping percentage.
■ CONCLUSIONIn conclusion, we successfully synthesized high density Mndoped ZnO nanorod array film by a simple and facile lowtemperature solution growth CBD method. The synthesizedmaterials are polycrystalline in nature having wurtzite structureand preferentially oriented along the c-axis, and the maximumdegree of alignment of ZnO nanorods was found to be ∼86%for sample ZM1. SEM images confirmed the presence of ZnOnanorods. Incorporation of Mn2+ into the ZnO crystal lattice isstrongly evident from EDAX and Raman analysis. The dopingpercentage of Mn2+ was varied as 0.015, 0.22, 0.33, and 0.74%.The presence of Mn in ZnO lattice changed the bond lengthand unit cell volume which has been demonstrated by XRDanalysis. Gauss fitted PL spectra exhibited the presence of twovisible bands, one green band at 2.4 eV and another at 2.13 eV.The intensity of the visible band was found to be increased for
the Mn ion implanted ZnO nanorod which is probably due tothe fact that incorporation of Mn leads to an increase in thepoint defect concentration in ZnO nanorods.
■ ASSOCIATED CONTENT*S Supporting InformationEDAX data of Mn doped ZnO nanorods films i.e. sample ZM1,ZM2, ZM3, and ZM4. This material is available free of chargevia the Internet at http://pubs.acs.org.
■ ACKNOWLEDGMENTSA.K.S. extends his gratitude towards University GrantsCommission, India for financial assistance in the form of aproject [F.No. 40-99/2011(SR)]. The authors thank Dr. V. J.Rao for UV and PL measurements. S.P.S. thanks XII FY CSIR-INTELCOAT (CSC0114) for financial support.
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Figure 8. (a) PL spectra of undoped and Mn doped ZnO nanorods.(b) A Gauss fit deconvoluted PL spectra of sample ZM4.
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