Precursor-Dependent Blue-Green Photoluminescence Emission of ZnO Nanoparticles

Published: December 05, 2011

r 2011 American Chemical Society 25227 dx.doi.org/10.1021/jp208487v | J. Phys. Chem. C 2011, 115, 25227–25233

ARTICLE

pubs.acs.org/JPCC

Precursor-Dependent Blue-Green Photoluminescence Emissionof ZnO NanoparticlesErwan Rauwel,*,†,‡,§ Augustinas Galeckas,§,|| Protima Rauwel,§,|| Martin Fleissner Sunding,§,|| andHelmer Fjellv�ag†,§

†Department of Chemistry, ‡SFI-inGaP, and §SMN, University of Oslo, N-0315 Oslo, Norway

)Department of Physics, University of Oslo, N-0316 Oslo, Norway

bS Supporting Information

1. INTRODUCTION

The engineering of materials at the nanometer scale is offundamental importance as size reduction and shape modifica-tion allow tuning of their fundamental properties. Researchtargeted toward the preparation and the characterization ofnanostructured ZnO-based materials has gained particular sig-nificance over the past decade1 and has been the subject ofnumerous studies. ZnO due to its high refractive index, highthermal conductivity, and wide direct bandgap (Eg ≈ 3.37 eV)2

has a high potential in many micro-/nanoelectronic,3�5

magnetic,6 photovoltaic,7 and optical applications,8,9 where thelatter is also considered as most challenging. Even thoughelectroluminescence in ZnO was demonstrated over a decadeago,10 the integration of ZnO in light-emitting devices has beeninhibited by problems related to p-type doping of ZnO.11 Asmentioned above, the fundamental material properties can bemodified by size reduction and also depend on the particularmethod through which nanostructuring is realized.3,12�16 Forinstance, an improved crystallinity is commonly reported forZnO nanopillars as compared to the bulk material, based onconsiderably quenched defect-related emission in luminescencespectra. On the other hand, any substantial size reduction alsomeans a consequential increase of surface-to-volume ratio and asubsequent stronger surface-defect-related emission. In the caseof unsupported nanometer-size particles, the surface effects maystart to dominate the bulk properties, turning the defect-relatedluminescence into a potential light source. The native (intrinsic)defects in the crystalline structure are believed to be responsible

for the broad visible luminescence in ZnO, although certaincontroversy in particular assignments remains.17 The greenemission is commonly associated with oxygen vacancies,4,18,19

whereas the blue�violet band is linked to zinc vacancies orinterstitials.16 While the efficient red light emitters needed formany practical applications can be realized from cadmium sulfideor selenide by controlling the particle size,20�22 stable green- andblue-emitting nanocrystals are more difficult to achieve. In thisrespect, ZnO nanoparticles (NPs) are among the most promis-ing candidates because of the nontoxicity, stability in air, andtendency to aggregate due to high surface energy; more speci-fically, surface modification has proven an effective way tostabilize luminescence of NPs at room temperature.23 It was alsoshown by Meulenkamp that aging and the size of the ZnOnanoparticles prepared with zinc acetate dihydrate in an alco-holic solution are governed by the water content and thepresence of reaction products. In fact, it is demonstrated thatwater has many roles like increasing the concentration ofdissolved ZnII species and enhancing the activity of all species.23

This study has therefore reinforced our motivation to outlinethe role of water molecules in the case of the non-aqueoussol�gel route. Recently, preparation of ionic liquid crystals wasshown to be a feasible way to realize stable green and blue lightemitters.24 In the present study, we address the possibility to

Received: September 2, 2011Revised: November 4, 2011

ABSTRACT: We report on properties of ZnO nanoparticlessynthesized via non-aqueous sol�gel routes. The role of thehydrates in the zinc precursor (Zn(acac)2, xH2O) on the structureand surface termination during the synthesis is studied for the firsttime. The structural and chemical properties of the ZnO nanopar-ticles were studied by standard structural and optical characteriza-tion methods. A broad luminescence was observed from thenanoparticles stretching throughout the visible region of the spec-trum and comprising characteristic blue and green emission bandscommonly associated with intrinsic defects in ZnO. A tentativemodel is proposed to explain differences in the luminescence ofnanoparticles synthesized using different routes by taking intoaccount the role of oxygen vacancies and other native defects: mostlikely being zinc vacancies and interstitials, located near the surface of the nanoparticles.

25228 dx.doi.org/10.1021/jp208487v |J. Phys. Chem. C 2011, 115, 25227–25233

The Journal of Physical Chemistry C ARTICLE

produce ZnO nanoparticles using a non-aqueous sol�gel meth-od with an ultimate goal of integrating them in thermal imagingapplications. ZnO NPs synthesized using benzylamine and Znacetylacetonate have already been reported in the literature;25

however, nothing was mentioned on their physical properties. Percontra, in this report we present structural and optical propertiesof ZnO NPs synthesized using the benzylamine and Zn acetyla-cetonate route along with a second synthesis based on ananhydrous precursor (Zn acetate). In this way, the effects of thehydrate groups on the structural and optical properties can bedirectly examined and interrelated. First, we address the outcomeof hydrate added to the precursor on the structure and composi-tion of the NPs. Then, PL properties of the NPs are comparedwith those of bulk ZnO material and discussed considering theluminescence of different near-surface intrinsic defects createdduring the growth of the NPs with or without hydrate.

2. MATERIALS AND METHODS

Synthesis. The procedure for synthesizing ZnO NPs wascarried out in a glovebox (O2 and H2O < 1 ppm). In a typicalsynthesis, zinc acetylacetonate hydrate (1.52 mmol) (99.995%,Aldrich) or zinc acetate (2.73 mmol) (99.99%, Aldrich) wasadded to 20 mL (183 mmol) of benzyl amine (purified byredistillation (g99.5%), Aldrich). The reaction mixture wastransferred into a stainless steel autoclave and carefully sealed.

Thereafter, the autoclave was taken out of the glovebox andheated in a furnace at 200 �C for 2 days. The resulting milkysuspensions were centrifuged and the precipitates thoroughlywashed with ethanol and dichloromethane and subsequentlydried in air at 60 �C.Characterization. X-ray diffraction (XRD) patterns were

collected using a Philips X’Pert powder diffractometer equippedwith a Cu Kα1 radiation source (λ = 0.15406 nm). ScanningElectron Microscopy (SEM) images were recorded on a FEIQuanta 200FEG. Transmission electron microscopy (TEM) wascarried out on JEOL 2000FS and JEOL-2010FS TransmissionElectron Microscopes both operating at 200 kV. X-ray photo-electron spectroscopy (XPS) analysis was carried out on a KratosAnalytical Axis UltraDLD photoelectron spectrometer equippedwith a monochromated Al Kα X-ray source. The analyzersettings employed for the narrow scans allow for 0.57 eV energyresolution as determined by the full width at half-maximum of theAg 3d5/2 photoelectron peak. Low-energy electrons were usedfor charge compensation, and the energy scale was calibratedbased on the C 1s peak for adventitious carbon at 285.0 eVbinding energy (BE). Manufacturer’s sensitivity factors wereused for quantification. Thermogravimetric (TG) analyses werecarried out in flowing N2 atmosphere (15 mL/min) with aheating rate of 5 �C/min, using a Rheometric Scientific STA1500 instrument.Optical absorption properties were derived from the diffuse

reflectance measurements performed at room temperature usinga ThermoScientific EVO-600 UV�vis spectrophotometer.Photoluminescence (PL) was investigated at 8 K temperatureby employing 325 nm wavelength of a cw He�Cd laser with anoutput power of 10 mW as an excitation source. The emissionwas collected by a microscope and directed to a fiber opticspectrometer (OceanOptics USB4000, spectral resolution 2 nm).Low-temperature measurements were realized using a closed-cycle He refrigerator (Janis, Inc. CCS450).

Table 1. X-ray Diffraction Peak Values Used for ScherrerCalculation Measurements

acetylacetonate, hydrate precursor (A) acetate precursor (B)

100 002 100 002

2θ (deg) 31.77 34.40 31.93 34.58

fwhm (deg) 0.299 0.226 0.212 0.210

Ø (nm) 27.62 36.81 38.98 39.62

Figure 1. X-ray diffraction pattern of ZnO nanoparticles (a) produced using zinc acetylacetonate hydrate (type-A NPs) and (b) produced using zincacetate (type-B NPs).



3. RESULTS AND DISCUSSION

Structural and Chemical Characterizations. Zinc oxide NPsof about 30 and 40 nm diameter were synthesized using zincacetylacetonate, hydrate (hereafter referred to as type-A NPs),and zinc acetate (type-B NPs) with benzylamine, respectively.This non-aqueous sol�gel process was recently developed as apotential alternative to conventional hydrolytic routes.26 Themost important advantage of this method is the possibility tosynthesize extremely pure and highly crystalline oxide NPs at lowtemperatures and without any surfactant.25,27,28

An XRD study showed that ZnO NPs have the hexagonalwurtzite-analogous (P63mc) crystal structure (a = 3.25 Å and c =5.20 Å) with a high crystallinity.29 No signs of secondary phaseare observed using XRD or TEM, whereas TEM analysisindicates the high purity of the ZnO NPs. Figure 1 exhibits theXRD pattern for both samples A and B. The analysis of spectraillustrate that in the case of acetylacetonate precursor (type-ANPs) the NPs seem to be smaller due to broader XRD peaks. Infact, the Scherrer method30 applied to the 100 and 002 reflections(Table 1) estimated the crystallite size of 37 and 27 nm for ZnOsynthesized using acetylacetonate (type-A) and a diameter of39 nm for ZnO synthesized using acetate (type-B). The meansize of the crystallites, T, was estimated from the Scherrerequation

T ¼ 0:9λβ cos θ

where λ is the Kα radiation (λ = 0.15406 nm); θ is the Braggdiffraction angle; and β is the width of the peak at half themaximum intensity in radian. It appears that in the case of type-ANPs two values are found and correspond to two different growthrates for two different directions, implying a nonspherical parti-cle shape.The chemical composition of ZnO NPs was analyzed using

energy-dispersive spectroscopy (EDS) and demonstrates a lowcarbon content for both types of nanoparticles, estimated to beeven lower for type-A NPs, suggesting that some organic specieslinger even after rinsing the NPs.Figures 2 and 3 depict the general morphology of the ZnO

NPs revealed by using TEM and HRTEM. As previouslycalculated using the Scherrer equation, type-A ZnO NPs arecomprised of “bean-shaped” particles with an average length of30 nm and a width of 15�20 nm (Figure 2a and 2b). These

nanoparticles illustrate a good degree of monodispersion. Elec-tron diffraction (ED) performed on a selected area (Figure 2c)confirms the high crystallinity and the hexagonal P63mc struc-ture. XRD measurements, via the Scherrer equation, indicatedthat the growth direction is (002). The TEM study performed ontype-BNPs showed that the shape is mostly hexagonal with sharpedges. Figures 3a, 3b, and 3c are transmission electron micro-graphs taken at different magnifications. The aim was to firstacquire a closer look at how these nanoparticles are disposed onthe carbon grid. In the lower magnification image of Figure 3a,one observes monodispersed particles along with particles tend-ing to slightly agglomerate. The higher magnification image of acluster of particles in Figure 3b again displays agglomerated andmonodispersed particles. At an even higher magnification as inFigure 3c, the sharp facets of the particles are discernible. Theselected area diffraction pattern (Figure 3f) from one of thesezones confirms the crystallinity and the hexagonal hcp structureof the material. The high crystalline quality of these ZnOnanostructures is represented in the HRTEM image ofFigure 3e which is part of a larger nanoparticle represented inFigure 3d. The inset of Figure 3e is the power spectrumindicating that the nanoparticles are oriented along the Æ001æzone axis. Lattice parameters of a = 3.25 Å and b = 5.20 Å weremeasured for both samples. The difference in particle size wasattributed to the precursors’ ligand. In fact, longer ligands willpromote the production of smaller NPs. In the present case, theacetyl acetonate ligand has not only decreased the particlediameter but also promoted the synthesis of the elongated “bean”like shape.Thermogravimetric analysis (TGA) was performed on both

powders to characterize the thermal stability of these NPs anddetect the presence of organic contamination, if any, that couldadhere as a result of the synthesis. The typical measurementsperformed on both ZnO synthesized using zinc actetylacetonatehydrate and acetate are shown in Figures 1aS and 1bS, respec-tively (Supporting Information). The TGA measurements per-formed on type-A NPs (Figure 1aS, Supporting Information)are typical from oxide NPs synthesized using this method.31

However, only a total weight loss of about 1.80% was measuredup to 800 �C speaking for the high purity of the samples. Theweight loss that corresponds to water desorption was estimatedto be about 0.06%. In the case of type-B NPs (Figure 1bS,Supporting Information), the curve is slightly different, and asharp drop corresponding to a weight loss of about 30% is visible

Figure 2. TEMmicrographs of ZnO nanoparticles produced using zinc acetylacetonate hydrate (type-A NPs) (a) and (b) with low magnification and(c) electron diffraction pattern indicating pure hexagonal wurtzite-analogous (P63mc) crystal structure.



from 225 to 336 �C. This behavior is typical of organic solventadsorbed on the surface that desorbs abruptly at elevatedtemperatures. Such a drop has already been observed at highertemperature in the case of yttria-basedmaterials,32 but in our casethis concerns the fast desorption of remaining benzylamine sol-vent or other organic species coming from the reaction synthesis25

adsorbed on the surface of the ZnO NPs. The presence ofhydrate in the acetylacetonate precursor most probably pro-motes the desorption of these organic species onto the surface ofthe NPs, coming from the solvent during the reaction process. Inthe case of pure acetate precursor, i.e., without any hydrate in the

solution, benzylamine ligands and other organic species remainstrongly bonded to the surface on the ZnO NPs.XPS measurements were performed on both types of NPs to

compare the purity of the surface and the degree of oxidation.Figure 4 presents the XPS survey spectra, showing a higheramount of carbon in type-B NPs, thus confirming the results ofprevious EDS analysis (not shown here). Moreover, there is anotable difference in the nitrogen content between both samples:While the nitrogen 1s peak is clearly visible on the surveyspectrum of type-B NPs, it is absent in type-A NPs. This suggeststhat benzylamine molecules from the solvent and other organic

Figure 3. TEM micrographs of ZnO nanoparticles produced using zinc acetate (type-B NPs) (a) and (b) low magnification and (c) highermagnification images of ZnOnanoparticles, (d) high-resolution TEM image of a ZnOnanoparticle, (e)HRTEM image of the encircled area of (d) (insetshows power spectrum), and (f) electron diffraction pattern indicating pure hexagonal wurtzite-analogous (P63mc) crystal structure.

Figure 4. XPS survey performed on ZnO nanoparticles produced using (a) zinc acetylacetonate, hydrate (type-A NPs), and (b) zinc acetate (type-B NPs).



species remain adsorbed on the surface of the ZnO NPs in thecase of type-B NPs (without hydrate). This XPS study confirmsthe previous result obtained from TGA and reinforces the initialhypothesis about the role of hydrate during the reaction process.The hydrate in the precursor is likely to promote the desorptionof the benzylamine solvent and other organic species during thereaction. This allows obtaining very clean and pure ZnO NPswithout organic molecules on the surface. The Zn 2p3/2 photo-electron peaks can be deconvoluted with a single compo-nent for both samples, as illustrated in Figure 5a for type-ANPs. Satisfactory peak fitting is achieved. The measured Zn 2p3/2and Zn LMM peak positions differ by 0.8 eV between bothsamples, and the modified Auger parameter (AP) for Zn is,however, identical for both samples and matches reported valuesfor ZnO and, combined with the single Zn 2p3/2 photoelectronpeak, excludes the presence of Zn(OH)2 (Table 2).The differences in the peak positions can indicate that the

charge referencing based on the C 1s peak is not adapted forthese samples. Energy referencing the spectra based on theaverage Zn 2p3/2 BE for ZnO in the NIST XPS database,33

1021.8 eV BE, leads to C 1s peak positions of 285.5 eV BE for themain component in type-A NPs and 284.7 eV BE in type-B NPs.The relatively low BE in type-B NPs fits well with aromaticcarbon from the suggested presence of organic species on thesurface of the NPs in this type.34 The BE of the C 1s peak in type-A NPs might indicate partial oxidation of the organic matterpresent on the sample or that the use of the Zn 2p3/2 peak forcharge referencing does not lead to realiable results; i.e., the Zn2p3/2 BE is much lower than in type-B NPs. This could then

indicate the presence of a negative charge on Zn stemming fromoxygen vacancies. The O 1s photoelectron peaks are similar forboth samples, with a main peak at low BEs and a shoulder athigher BEs from an additional component, as shown in Figure 5bfor type-A NPs. The main peak at low BE has a typical energy foroxygen in zinc oxide (Table 2). The shoulder at a higher bindingenergy could be attributed to the presence of OH groups oradsorbed oxygen species or of oxygen atoms related to oxygenvacancies, possibly indicating different species between bothtypes. As no hydrate was used during the synthesis of type-BNPs, the presence of hydroxyl groups is not likely in the as-synthesized NPs. The presence of oxygen vacancies and oxygenspecies adsorbed on the surface certainly affects the PL responseof the material. The quantification of the elements from thedetailed scans enables the determination of the amount of Zncompared to O element (Table 3).In the case of type-A NPs, 40 at. % of Zn with 20 at. % of O

with high BE and 40 at. % of O with lower BE were measured.A lower Zn quantity was measured in the case of type-B NPs(36 at. %). This could be attributed to Zn vacancies in the samplebut can also stem from the stronger absorption of the Zn 2p3/2photoelectrons compared to the O 1s photoelectrons by theorganic matter adsorbed on the surface of the ZnO NPs.A deeper insight into the above-mentioned nonstoichiome-

tries can be attained from optical characterization of NPs in theview of the fact that most of the (intrinsic) native defects in ZnOare luminescent. Indeed, clearly contrasting optical response wasobserved from the NPs under UV excitation despite similari-ties in size, shape, and routes of synthesis. The influence of the

Figure 5. XPS spectra from ZnO nanoparticles. The lines show the deconvoluted (a) Zn 2p3/2 and (b) O 1s component for ZnO nanoparticlessynthesized using zinc acetylacetonate hydrate (type-A NPs).

Table 2. Photoelectron Peak Positions andModified Auger Parameter (AP) Values for Zn and O, with Reference Values from theNIST XPS Database33 a

Zn 2p3/2/eV BE Zn AP/eV O 1s/eV BE O AP/eV

acetylacetonate, hydrate precursor (A) 1021.3 2010.1 530.1 (531.4) 1040.2

acetate precursor (B) 1022.1 2010.1 530.9 (532.1) 1040.3

ZnO reference 1021.2 � 1022.5 2009.5 � 2011.0 529.9 � 531.2 1040.4 � 1041.3

average = 1021.8 average = 2010.1 average = 530.4 average = 1040.9

Zn(OH)2 reference 1022.7 2009.2aThe O 1s values in parentheses correspond to the high BE component of the peak.



presence of hydrate in the precursor on optical properties isdiscussed in the following section.Optical Characterizations. It is known that the higher sur-

face-to-volume ratio in NPs as compared to bulk material leads toa corresponding enhancement of surface-related emission in theoverall luminescence.35 As a first approximation, the role ofsurface effects in the present study was deduced from thecomparison of luminescence properties of NPs against those ofhigh crystallinity bulk ZnO. A summary of PL and diffuse-reflectance measurements is presented in Figure 6. One canimmediately notice that the band-edge luminescence, typicallyprevailing in the ZnO spectra, appears generally suppressed,whereas broad visible emission is considerably enhanced in thecase of nanoparticles. Among the two types of nanoparticles,optical properties of type B ZnO NPs are closer to those of bulkZnO in terms of both band edge and PL, demonstrating adistinctively direct type of optical transition in the Tauc plotwith a characteristic excitonic absorption peak and the bandgapof 3.28 eV at room temperature. In contrast, the absorption edgefor the type-A ZnONPs appears red-shifted (3.2 eV) with regardto that of ZnO, also exhibiting an absorption tail stretchedthroughout the visible range.The origin of enhanced visible emission from ZnO nanopar-

ticles is usually attributed to oxygen vacancies located predomi-nantly near the surface, the ionization state of which in turndepends on a particular charge state at the surface.36 It is knownthat ZnO exhibits a downward band bending near the surface,37

which leads to the formation of an accumulation region for

electrons (and depletion region for holes). The barrier heightand width of such a region are related to the net positive surfacecharge, which might be caused by donor-like surface states oradsorbed atoms.38 This means that any modification of thesurface charge will cause a corresponding change in the bandbending and, accordingly, can be assessed by monitoring theintensity of visible PL.The broad visible luminescence of nanoparticles is apparently

composed of several sub-bands (see Figure 6). The emissionaround 3 eV (410 nm), which dominates in the spectra of type-BNPs, is commonly associated with optical activity of zinc-relatedintrinsic defects (Zni interstitials and/or VZn vacancies).

17 Notethat quantification results from the XPS data suggest a possibleZn depletion that would indicate the presence of Zn vacanciesspecifically for the type-B NPs. Conversely, the missing blueemission in PL from type-A NPs is consistent with no indicationsof Zn deficiency close to the surface as indicated by XPS. On theother hand, green luminescence is a common feature for bothtypes of NPs and is comprised of two merged emission bandscentered at∼2.5 eV (500 nm) and∼2.2 eV (550 nm), which arebelieved to originate from double- (VO

+2) and single-charged(VO

+) oxygen vacancies, respectively.39 For type-B NPs, VO-related emission around ∼2.5 eV (500 nm) appears moreprominent presumably because of more reductive environmentduring the synthesis in the absence of hydrate. The latterassumption is based on the fact that, if present, hydrate wouldcertainly promote a better oxidation of the ZnO nanoparticlesduring the synthesis process.As already mentioned, XPS and PL measurements both point

toward some nonstoichiometry near the surface of nanoparticlesfor the synthesis route involving zinc acetate precursor. Morespecifically, there is an apparent deficit of Zn atoms on the zincsites of the ZnO lattice, and this effect is plausibly caused by thelack of hydrate, which is known to promote desorption of thebenzylamine ligands. It is therefore reasonable to associate thepromotion of zinc vacancy formation with some obstructioncaused by benzylamine on the surface during the synthesis. As afinal point, we note that besides zinc vacancies yet anotherpotential contributor, associated to zinc interstitials, to theobserved blue emission can not be ruled out, assuming that largearomatic rings adsorbed on the surface promote intersite latticeposition for Zn during the synthesis.

4. CONCLUSIONS

The high surface-to-volume ratio of the investigated ZnONPsmakes the luminescence of intrinsic defects (oxygen vacancies,zinc vacancies, and/or zinc interstitials) dominant in the overallemission spectra. The band-edge luminescence, typically prevail-ing in spectra of bulk ZnOmaterial, appears totally suppressed inthe case of nanosized particles. The employed ZnO-nanoparticlesynthesis routes using acetylacetonate hydrate and zinc acetatehave different effects on the final surface state of the NPs asevidenced from TGA measurements, XPS quantitative analyses,and PL. In fact, the role of hydrate is 2-fold: it promotes theoptimization of the oxidation during the growth process andfacilitates the desorption of the benzylamine ligands and otherorganic species during the reaction process. The residual ben-zylamine molecules on the surface promote the formation ofintrinsic defects in the form of Zn vacancies and/or interstitialZn. All measurements considered, we demonstrate the key roleof the hydrate in such reactions. Finally, without using hydrate

Table 3. Zn and O Quantification from XPS Results in Type-A and Type-B Nanoparticles

Zn at. %

O at. %

(high energy)

O at. %

(low energy)

acetylacetonate,

hydrate precursor (A)

40 20 40

acetate precursor (B) 36 22 42

Figure 6. PL spectra at 8 K from ZnO nanoparticles produced usingzinc acetylacetonate hydrate (curve #01, type-A NPs) and zinc acetate(curve #02, type-B NPs) along with the reference spectrum of bulk ZnO(gray curve). Inset shows optical band edges of the correspondingnanoparticles estimated from the diffuse-reflectance spectra (DRS)at 300 K.



during the process we were able to synthesize highly crystallineZnO NPs with a sufficient amount of luminescent centers tomake these NPs promising for device applications. Such NPs willbe integrated into devices engineered for thermal imaging andwill be the subject of future communications.

’ASSOCIATED CONTENT

bS Supporting Information. Figures 1S. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel.: (47) 228 54764. Fax: (47) 228 55565. E-mail: [email protected].

’ACKNOWLEDGMENT

Financial support from Research Council of Norway for fina-ncial support, project 176740/130, and Marie Curie (PERG05-GA-2009-249243) is acknowledged.We thankMrs. S. Aravinthanfrom inGAP, University of Oslo, for her assistance with the TGAand TDA measurements. We thank Dr. Maria Rosario fromUniversity of Aveiro, CICECO, for XRD measurements.

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