High-Pressure Synthesis and Luminescent Properties of Cubic ZnO MgO

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High-pressure synthesis and luminescent properties of cubic ZnO/MgOnanocomposites

A. N. Baranov,1 O.O. Kurakevych,2 V. A. Tafeenko,1 P. S. Sokolov,2,3 G. N. Panin,4,5 andV. L. Solozhenko2,a1Department of Chemistry, Moscow State University, 119992 Moscow, Russia

2LPMTM-CNRS, Universit Paris Nord, 93430 Villetaneuse, France

3Department of Materials Science, Moscow State University, 119992 Moscow, Russia

4Department of Physics, Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul

100-715, Republic of Korea5Institute of Microelectronics Technology and High Purity Materials, RAS, Chernogolovka, Moscow distr.

142432 Russia

Received 20 January 2010; accepted 10 February 2010; published online 12 April 2010

The formation of the nanocrystalline rocksalt ZnO rs-ZnO has been in situ studied by x-raydiffraction with synchrotron radiation at high pressure and high temperature. A number of rs-ZnO/MgO nanocomposites with preset grain size were synthesized at 7 GPa and 800 K starting fromwurtzite ZnO nanoparticles or nanorods. The use of MgO matrix allowed us to recover metastablers-ZnO in the nanocrystalline form at ambient pressure. The cathodoluminescence measurementsdemonstrated the blue shift in the luminescence of rs-ZnO nanocrystals down to 402408 nm thatcan be attributed to the enhanced incorporation of point defects with lower activation energy. 2010 American Institute of Physics. doi:10.1063/1.3359661

I. INTRODUCTION

ZnO is a promising luminescent semiconductor with awide direct band gap 3.37 eV for wurtzite structure and ahigh exciton binding energy 60 meV,1,2 while ZnO-basednanostructured materials are attractive for the fabrication ofoptoelectronic devices operating in the blue and ultravioletspectral regions. At ambient conditions, the thermodynami-cally stable phase of ZnO has a hexagonal wurtzite structurewith fourfold tetrahedral coordination w-ZnO phase. Thehigh-pressure cubic rocksalt structure with sixfold coordina-tionrs-ZnO phaseseems promising for electronic and spin-tronics applications because of its ability to incorporatemuch higher amount of doping atoms and native point de-fects than the wurtzite ZnO.

Similarly to many other II-VI compounds with wurtzitestructure, w-ZnO undergoes phase transition under high pres-sures into the denser rs-ZnO.3 At room temperature, thephase transition of bulk w-ZnO occurs at about9GPa.4 Thetransition pressure depends on both temperature5 and crystalsize.6 Upon decompression, the reverse transition of rs-ZnOinto w-ZnO takes place at about 2 GPa at room temperature,that render impossible to recover the pure high-pressure rs-ZnO at ambient conditions. However, it has been reported

that the nanocrystalline rs-ZnO can be quenched from 15GPa and550 K down to ambient conditions in a diamondanvil cell.7 At present time it is not clear whether this stabi-lization should be attributed to the grain nanosize or to theresidual strains in the quenched sample. The existence ofrs-ZnO nanocrystals at ambient pressure has been proved bythe transmission electron microscopy TEMstudy of MgOsingle crystals coimplanted by zinc and oxygen ions.8 Theproperties of rs-ZnO are still not studied. Even rs-ZnO band

gap, the value of outmost importance for optical and elec-tronic applications, has not been experimentally determined.From theoretical calculations, the rs-ZnO indirect band gapcan vary from 2.36 to 4.51 eV.911 The only high-pressurestudy of the optical properties of rs-ZnO Ref.12indicatedthat this phase is an indirect gap semiconductor with a bandgap of 2.45 eV at 13.5 GPa.

Recently it has been shown that stable MgxZn1xO solidsolutions 0.32x0.67 with rocksalt structure can besynthesized at pressures above 7 GPa and temperaturesabove 1600 K.13

Lattice parameter of the magnesium oxide aMgO=4.2112 , space group Fm3m according to JCPDS No.450946is slightly less than that of the rocksalt zinc oxide,ars-ZnO=4.280 , space groupFm3maccording to Ref.14,so that the MgO can be used as a matrix supporting mate-rialfor metastable rocksalt ZnO nanoparticle NP.

Here we report the first successful high pressure-hightemperature synthesis of rs-ZnO nanocrystals with controlledgrain size that are stable at ambient conditions in the form ofrs-ZnO/MgO nanocomposites.

II. EXPERIMENTAL DETAILS

A. Synthesis of wurtzite ZnO/MgO nanocomposites

The precursors, i.e., w-ZnO/MgO nanocomposites, havebeen synthesized by several methods in order to preparenanocrystals of different size and/or with different form fac-tor. The size and morphology of the nanocrystals are given inthe TableI.

Sample NP10.Zinc and magnesium acetates 1:4 molarratio were dissolved in methanol and mixed with sodiumhydroxide solution in methanol. After centrifuging and wash-ing5 timesthe obtained precipitate was annealed at 670 KaElectronic mail: [email protected].

JOURNAL OF APPLIED PHYSICS 107, 073519 2010

0021-8979/2010/1077/073519/5/$30.00 2010 American Institute of Physics107, 073519-1

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for 3 h. This method allowed to produce the ZnO NP of lessthan 10 nm in size distributed in magnesium oxide matrix asshown in Fig.1a. It should be noted that MgO is consid-ered as a barrier layer preventing the agglomeration of indi-vidual ZnO particles during the thermal processing. As canbe seen in Fig.1ainset, the selected area electron diffrac-tion pattern shows reflections of both w-ZnO and MgOphases. The size of the zones of coherent scattering accord-ing to the Scherrer equation is equal to 102 nm that wellagrees with the TEM observations.

Sample NP50. The ZnO nanopowder with an averageparticle size of about 50 nm was prepared by the followingprocedure. Zinc carbonate hydroxide ZCHwas obtained byprecipitation from an aqueous solution of zinc nitrate by ex-cess of ammonium carbonate. The precipitate was centri-fuged, thoroughly washed in distilled water and freeze dried.A mixture of the ZCH with NaCl was milled for 1 h andannealed in a muffle furnace at 970 K. NaCl was washed outwith distilled water. The as-prepared ZnO nanopowder wasthen dispersed in the methanol solution of magnesium ac-etate in the ZnO/MgO molar ratio1:4 using ultrasonic bath,and mixed with the equivalent amount of sodium hydroxidesolution in methanol. After centrifuging and washing 5

timesthe residue was decomposed at 670 K for 6 h in orderto obtain 50 nm NP of ZnO in MgO matrix.Sample NP200.Monodispersed ZnO spherical particles

of 170 nm in size Fig.1cwere prepared by hydrolysis ofzinc acetate dihydrate ZnAc2 2H2O by a two-stage

process.15 0.01 mol of ZnAc22H2O was dissolved in 100 mlof diethylene glycol. The clear solution was heated up to 430K, then stirred for one hour and cooled down to room tem-perature. Resulting precipitate was washed thoroughly withdistilled water and dried. The as-prepared powder was dis-persed in the methanol solution of magnesium acetate in theZnO/MgO molar ratio1:4, and then reprecipitated by equiva-lent amount of sodium hydroxide solution in methanol. The

residue was subsequently decomposed as described above.Sample NR. w-ZnO nanorods NR each nanorod is asingle-crystal NPwere synthesized by the method describedin Ref. 16. w-ZnO NRMgO composite was prepared asdescribed above for NP200.

B. High-pressure experiments

In situ experiments up to 5 GPa have been performedusing MAX80 multianvil x-ray system at beamline F2.1,DORIS III HASYLAB-DESY. The experimental detailsand high-pressure setup have been described elsewhere.17

Energy-dispersive x-ray diffractionXRDpatterns were col-lected on a Canberra solid state Ge-detector with fixed Braggangle 2=9.261 using a white beam collimated down to100100 m2. The sample temperature was measured by aPt10%Rh-Pt thermocouple without correction for the pres-sure effect on the thermocouple emf. The samples were com-pressed to the required pressure at ambient temperature, andthen diffraction patterns were collected upon stepwise heat-ing 25-or 50-K steps. With the storage ring operating at4.44 GeV and 12030 mA, the time of data collection foreach pattern was about 3 min.

Quenching experiments at 7 GPa have been performedusing a cubic multianvil press at LPMTM-CNRS. Gold cap-sules were used to prevent contamination of the samples.

C. XRD study

Quenched samples were studied by the powder XRDusing STOE diffractometer, Cu Kradiation, PSD-detector,2130. TheFULLPROF SUITEandWINPLOTRsoftware18,19

have been used for data processing. Powder diffraction pro-files were fitted using thepseudo-Voight p-Vprofile func-tion in TCH modification.20 Each reflection profile was char-acterized by its full width at half maximum FWHMand ,a mixing parameter of Gaussian and Lorenzian componentsof the pV profile function expressed as pV=L+1-G.The HGand HLvalues are FWHM of Gaussian and Lorent-zian profiles of the Voight function, respectively. The micro-structure size and strain within FULLPROF program wastreated using instrumental function with HG_inst and HL_instparameters calculated from the diffraction pattern of LaB6standard.

D. Characterization of the recovered samples

The microstructure of the samples was characterized byTEM in the bright-field mode using JEOL JEM-2010trans-mission electron microscope operating at 200 kV. Powderedsamples were dispersed in droplets of ethanol and thenloaded on copper grids coated with a carbon film. Cathod-oluminescenceCLspectra were collected at room tempera-

TABLE I. Results of scanning electron microscopy SEMand TEM analy-sis of pristine wurtzite ZnO/MgO composites.

PrecursorAverage size of w-ZnO NP and diameter/length of NR

nm

NP10 103 TEM dataNP50 539 SEM dataNP200 1698 SEM dataNR 50/ 5001000 SEM data

FIG. 1. SEM pictures of the precursors: a w-ZnO/MgO nanocompositeNP10 sample; buniform53 nmw-ZnO NPNP50 sample; cuni-form170 nmw-ZnO particlesNP200 sample, anddw-ZnO NRNRsample. Inset: SAED pattern of NP10 sample.

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ture using XL 30S FEG high-resolution scanning electronmicroscope with a mono-CL system in the 300900 nmwavelength range.

III. RESULTS AND DISCUSSION

The ZnO-MgO system has been in situ studied at pres-sures about 5 GPa using XRD with synchrotron radiation inorder to determine the optimal temperature range for the syn-thesis of rs-ZnO/MgO nanocomposites without formation of

the corresponding solid solutions.

21

The diffraction patternsof the NP10 sample taken at 4.8 GPa in the course of tem-perature increase are shown in Fig.2.

One can see that intensities of100,002, and102lines ofw-ZnO significantly decrease with temperature increase, and200and 220lines of both rs-ZnO and MgO phases shift tohigher d-spacings. The position of the most intensive 200line versus temperature is plotted in inset. Since lattice pa-rameters of rs-ZnO and MgO phases are very close, the linesof these two phases cannot be resolved in energy-dispersiveXRD patterns taken in situ at high pressures. However, thesingular point on the dependence of200 line position versustemperature should reflect the structural/compositional alter-

ations in rs-ZnO phase upon heating, while at the same con-ditions MgO undergoes quasilinear thermal expansion only.The temperature dependence of the200line position consistsof two parts; exponential and linear ones. An exponentialincrease in the plotted values of peak position at low tem-peratures indicates the formation of the rs-ZnO phase in-creasing amount of cubic phase. The singular point separat-ing the exponential and linear parts of the curve correspondsto the complete disappearance of w-ZnO at given pressureand may be determined as the completion temperature of thew-ZnO to rs-ZnO phase transition i.e., Tc =800 K. The lin-ear part of the curve at higher temperatures reflects thermalexpansion of both MgO and rs-ZnO phases.

Figure3 shows the angle-dispersive diffraction patternsof the samples recovered from 800 K and 7 GPa. The lines of

w-ZnO phase disappeared completely and the line positionsin the diffraction patterns correspond to rs-ZnO and MgO.Some patterns exhibit additional weak lines with relative in-tensities less then 2% because of possible surface reaction ofMgO with moisture during the sample preparation. The linesof impurities have been indexed in the framework of hexago-nal unit cell with lattice parameters a=3.15 and c=4.69 using DICVOL software.22 These parameters coin-cided with those of MgOH2. No lines of other phases havebeen detected.

For the structure refinement we have used approachbased on the extraction of integrated intensities from diffrac-tion pattern after Rietveld or Le Bail fitting with subsequent

FIG. 2. Diffraction patterns of the NP10 sample taken at 4.8 GPa at differenttemperatures. Underlined indexes correspond to MgO and rs-ZnO reflec-tions, while nonunderlined indexes to w-ZnO. Asterisks denote the lines ofMgOH2 impurity. Inset: position of 200 line MgO+rs-ZnO vs

temperature.

FIG. 3. Color onlineThe observed and calculated XRD patterns of recov-ered rs-ZnO/MgO nanocomposites after the refinement procedure Rietveldmethod for rs-ZnO and MgO phases and Le Bail method for Mg OH2aNR, bNP200, cNP50, and dNP10. The set of verticalbars indicates the calculated position of Braggs peaks for ZnO, MgO andMgOH2. The discrepancies between observed and calculated profiles arevery small and all residuals indicate that the unit cell parameters are accu-rately determined Rp for the whole profile is within 3.5%4.5%.




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refinement of atomic parameters using SHELX97 software.23

Earlier this approach has been successfully used for refine-ment of atomic parameter of ZnxMg1xO solid solutions.

24

The unit cell parameters of ZnO/MgO nanocomposites, atomdisplacements Uiso as well as atom occupancy parametersand final R-factors are given in Table II. The use of theintegrated intensities for the refinement of the atom param-eters gave much better, self-consistent and reasonable resultsin comparison with Rietveld method. The values of the cellparameters well agree with those of the pure Zn and Mgoxides except NP10 sample, for which we suggest the for-

mation of cubic ZnxMg1xO solid solution with a very smallx value.According to TEM data, the recovered NP10 samples

consist of the individual ZnO and MgO particles Fig.4a;ZnO is darker than MgO. The particle size varies from 7 to13 nm for ZnO and from 20 to 40 nm for MgO, thus nochange in ZnO grain size was observed in the course ofw-ZnO to rs-ZnO phase transformation selected area elec-tron diffractionSAEDpattern in Fig.4dshows completedisappearance of w-ZnO reflections in the recoveredsample. The lattice fringes of the cores are adjusted withthose of MgO matrix reflecting the effect of rs-ZnO phasestabilization Fig. 4a. The element distribution maps re-

corded using the Scanning Transmission ElectronMicroscopy-Energy-Dispersive Spectrometry STEM-EDSmethod are shown in Figs.4band4cindicating the pres-ence of separate ZnO and MgO particles. The SAED patternof recovered NR sample Fig.4eshows the single-crystalnature of quenched rs-ZnO NR contrary to the polycrystal-line MgO matrix.

Figure5shows CL spectra of recovered rs-ZnO-basednanocomposites as well as CL spectra of the precursors. Allstarting w-ZnO/MgO precursors show blue luminescence inthe range of 410446 nm. The peaks at 446 NP10, 410

NP50, 420 NP200, and 414 nm NRcould be attributedto a donor-acceptor pair transitions.25 The pressure-inducedphase transition in w-ZnO/MgO nanocomposites leads to theremarkable changes in the luminescent properties, i.e., allrecovered rs-ZnO/MgO samples show the blue shifted lumi-nescence of lower intensity in comparison to the w-ZnO

TABLE II. Results of the Rietveld refinement of atomic parameters for the phases in the rs-ZnO/MgO composites.

a ZnO

a MgO

Uiso-metalZnO/MgO2

Uiso-oxygenZnO/MgO2 Occupancy metalin ZnO /MgO

R ZnO/MgO%

NP10 4.27562 4.23512 0.0181/0.0301 0.0143/0.0321 0.966/1.0 1.0/1.0NP50 4.282225 4.225699 0.0171/0.0201 0.0143/0.0204 1.03/1.0 1.2/1.5NP200 4.280128 4.223919 0.0182/0.0252 0.0173/0.0234 0.988/1.0 0.3/1.0NR 4.280849 4.217918 0.0192/0.0233 0.0182/0.0213 0.984/1.0 0.9/0.9

FIG. 4. TEM bright field image of the recovered NP10 sample a, elemen-tal distribution maps band cfor magnesium and zinc, respectively,andSAED patterns taken from NP10 dand NR esamples.

FIG. 5. Color onlineCL spectra of rs-ZnO/MgO composites synthesizedat 7 GPa and 800 K blue circlesin comparison with starting materials redtriangles: aNP10, bNP50, cNP200, and dNR.




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High-Pressure Synthesis and Luminescent Properties of Cubic ZnO MgO