zno纳米粒子英文论文

Rapid synthesis of blue emitting ZnO nanoparticles forfluorescentapplicationsLeta T.Jule a,n,Francis B.Dejene a,Kittessa T.Roro b,Zelalem N.Urgessa c,Johannes R.Botha ca Physics Department,University of the Free State,Private Bag X13,Phuthaditjhaba9866,South Africab CSIR-Energy center,Council for Scientific and Industrial Research,PO Box395,Pretoria,South Africac Department of Physics,Nelson Mandela Metropolitan University,P.O.Box77000,Port Elizabeth6031,South Africaa r t i c l e i n f oArticle history:Received4May2016Received in revised form8June2016Accepted9June2016Available online11June2016Keywords:ZnOBlue emissionThermal decompositionZinc acetate dehydrateFluorescent applicationDonor–acceptor pair(DAP)a b s t r a c tZnO nanoparticles(NPs),with size∼16–20nm were produced using simple,cost effective and rapidsynthesis method.In this method zinc salt(typically zinc acetate dehydrate)is directly annealed in air ata temperature from°200C to°500C for2h to form ZnO(NPs).This synthesis method,only requires zincprecursor to produce NPs that can emit visible emission without external doping.X-ray diffraction(XRD)patterns confirm the prepared ZnO NPs is polycrystalline structure with wurtzite phase.The observedvariation in scanning electron microscopy(SEM)images showed spherical shape of the ZnO NPs.It wasfound that the NPs exhibited the estimated direct bandgap(E g)of3.28eV,3.29eV,3.33eV and3.39eVfor a decomposition temperature of500,400,300and°200C.Energy dispersive X-ray(EDX)analysisshowed that carbon is the only impurity at lower temperature which was most likely originated from theacetate group.The photoluminescence(PL)spectra of ZnO NPs showed the appearance of a blue emis-sion,attributed to Zn interstitials,whose intensity reduces with increase in decomposition temperatureand the underlying mechanism are discussed.For the samples prepared at°200C and300°C a tem-perature dependent PL was studied and found out that there are about three transition lines at∼3.01eV,∼3.21eV and∼3.33eV,which are ascribed to zinc vacancy(V zn),donor–acceptor pairs(DAP)and excitonsbound to structural defects respectively.It is hoped that ZnO NPs produced using this method would beideal for blue light emittingfluorescent application as it is catalyst free growth,uses simple equipment,less hazardous and easy to control particle size and morphologies by scalable temperature.&2016Elsevier B.V.All rights reserved.1.IntroductionSemiconductor nanostructures are promising candidates forfuture electronic and photonic devices.Nanostructures based onwide bandgap semiconductors such as GaN and ZnO are of parti-cular interest because of their applications in short wavelengthlight emitting devices andfield emission devices[1,2].ZnO is apotential competitor of GaN for blue and UV emission and char-acterized by direct band gap of3.37eV at room temperature withlarge exciton binding energy of60meV compared to GaN(28meV)[3–5].In addition to the UV emission ZnO is also known to emit in thevisible region[6].The photoluminescence spectrum of ZnO isnormally composed of two parts:excitonic near band edge emis-sion with energy around the band gap of ZnO and defect relateddeep level emission in the visible range.ZnO is n-type semi-conductor due to oxygen vacancies,impurities like Al,H and in-terstitial Zn ions which act as donors in ZnO lattice.These nativedefects are believed to be responsible for visible photo-luminescence.The UV emission is due to excitonic related re-combination[7,8].The exact mechanism for deep level emission isstill controversial although intrinsic defects such as oxygen va-cancies,oxygen interstitial,zinc vacancies and extrinsic impuritiesare all considered as a possible origin[9–12].However,under-standing the origin of photoluminescence in ZnO nanoparticlesand improving the emission efficiency is still a major challenge.Synthesis and characterization of zinc oxide(ZnO)nano-particles has found widespread interest during past few years dueto their unique electro optical properties,which can be employedin devices such as ultraviolet(UV)light-emitting diodes(LEDs)and blue luminescent devices[13].Searching of new methodology to synthesize ZnO NPs with auniform morphology,size and reproducibility is of greatContents lists available at ScienceDirectjournal homepage:/locate/physbPhysica B/10.1016/j.physb.2016.06.0080921-4526/&2016Elsevier B.V.All rightsreserved.n Corresponding author.E-mail address:JuleLT@ufs.ac.za(L.T.Jule).Physica B497(2016)71–77importance both for fundamental studies and practicaltion.In the past,several crystal-growth technology, explored,among which are sol-gel,spray pyrolysis,por transport,vapor-phase growth,chemical bath (CBD),direct precipitation methods and hydrothermalwhich also had the additional motivation of dopingeffort to obtain p-type material[1,14,15].However,theseinvolve a strictly controlled synthesis environment, procedures,high-temperature synthesis processes and equipment.Despite extensive research over the pastsome fundamental properties of theluminescence in the ZnO synthesized by thermalare not fully understood and rarely reported.In this paper,we therefore report on ZnO NPs thatvisible region without the requirement of additional temperature-dependent PL for samples of ZnOtemperature of°200C and°300C were discussed withderlying mechanism.Moreover,the structural and optical prop-erties of ZnO NPs synthesized via thermal decomposition method has been studied.2.Experiment2.1.Sample preparationThe zinc acetate dihydrate(Zn(CH3COOH)2Á2H2O))>99%,purity purchased from Sigma Aldrich,were used as precursor without further purification to prepare ZnO NPs.In order to determine possible decomposition temperature of zinc acetate dihydrate into ZnO NPs,samples were annealed at various temperature for2h. Four sets of5g of Zn(CH3COOH)2Á2H2O was put into sample holders,the crucible and annealed at a temperature of200,300, 400and°500C for2h in muffle furnace in air to produce the ZnO NPs.2.2.CharacterizationsThe crystal structures of the samples were determined with a Bruker AXS D8ADVANCE Discover diffractometer(XRD),with Cu Ka(1.5418)radiation.Surface morphologies and elemental com-positions were studied using a scanning electron microscope(SEM) (Shimadzu model ZU SSX-550Superscan)equipped with EDX.The optical absorption measurements were carried out in the200–600nm wavelength range using a Perkin Elmer UV/Vis Lambda20 powder Spectrophotometer.Luminescence measurements were done using a photoluminescent(PL)laser system 5.0(Photon systems,USA),which uses a He–Cd laser with an excitation wa-velength of248.6nm.3.Results and discussion3.1.Structural analysisFig.1shows the XRD pattern of ZnO NPs synthesized by ther-mal decomposition method for various annealing temperatures. XRD pattern of ZnO NPs exhibits various peaks which could be indexed according to ZnO diffraction peak(JCPDS card no.36-1451).The presence of various diffraction peaks reveals hexagonal wurtzite phase of ZnO which suggests polycrystalline nature of ZnO.The diffraction peaks at scattering angleθ()2of31.74°,34.43°, 36.25°and47.5°belongs to100,002,101and102diffraction planes from ZnO,and the other peak marked with asterisks* corresponds to zinc acetate.It can be clearly seen that by increasing the growth temperature from°200C to°500C ZnO peaks became prominent and prevailed.No other impurities are observed except for zinc acetate related peaks in the samples prepared at lower temperature,but fully decomposes for the higher temperature.The estimated lattice constants slightly in-creased as the decomposition temperature increased from°200C to°500C as summarized in Table1.The increase of the lattice parameter of ZnO NPs with increase in temperature was calculated using the equation:=+()⎛⎝⎜⎜⎞⎠⎟⎟d a c143111 101222where d is the interplanar distance,a and c are the lattice parameters.Fig.2shows the variation of FWHM as measured from dif-fraction plane101and particle size of ZnO NPs synthesized at various decomposition temperatures for annealing time of2h.The FWHM,which is an indication of the crystalline quality of the prepared ZnO NPs,decreases significantly as the decomposition temperature increases.Fig.3depicts variation of lattice parameter, a and c with increasing annealing temperature respectively con-firming the increase in grain size at higher decomposition temperature.The average crystallite size of prepared ZnO NPs can be esti-mated from the FWHM of the101diffraction peak estimated using the Debye Scherrer's equation:λβθ=() D0.9cos2 where D is the crystallite size,λis the wavelength λ(=)0.15402nm of radiation used,θis the Bragg angle andβis the full-width at half-maximum measured in degrees.The estimated average particle sizes were16.3,17.8,19.6and19.7nm2degree))ΘFig.1.XRD pattern of ZnO nanoparticles synthesized at various temperatures for 2h.Table1Measured properties of ZnO nanoparticles at various temperature.Temp.(°C)Particle diameterR070.5(nm)Lattice parameter,a(Å)Lattice parameter,c(Å)20016.3 3.245 5.29930017.8 3.247 5.30240019.6 3.249 5.30550019.7 3.250 5.307L.T.Jule et al./Physica B497(2016)71–77 72corresponding to the 200,300,400and °500C decomposition temperatures.The estimated particle sizes increases with anneal-ing temperature and it has highest value at of °500C due to nar-rowing of the diffraction peak.According to Ostwald ripening the increase in the particle size is due to the merging of the smaller particles into larger ones as suggested by Nanda et al.[16]and is a result of potential energy difference between small and large particles and can occur through solid state diffusion.The intensity ratio,()()I I 002101of the peaks presenting ()002and (101)at °500C is equalto 0.53which is higher than the corresponding standard value of 0.44of bulk hexagonal wurtzite ZnO [17]suggesting the prepared ZnO structure is preferred the (002)orientation.3.2.Morphological analysisFig.4A –C illustrates the SEM images of ZnO NPs prepared at a temperature of 300,400and °500C respectively.The SEM images clearly indicate that the surface morphology of the ZnO NPs de-pending on the synthesis temperatures.It is interesting to observe the formation of spherical ZnO NPs which is fully noted when the decomposition temperature was increased to °500C (see the inset in Fig.4C).It is observed that as decomposition temperature increases,the structural defects(dis-location)for the ZnO NPs prepared at a temperature of °200C reduces as compared to °500C causing uniform ZnO NPs which are purely spherical in shape.Fig.4D –F shows chemical stoichiometryof the ZnO nanoparticles prepared at a temperature of 300,400and °500C .The results presented shows that the prepared mate-rial contains C,O and Zn elements.The decrease in the C con-centration with increasing synthesis temperature is suggested to be as a result of ef ficient evaporation of the acetate-group.4.UV –visible spectrophotometer analysisFig.5shows room-temperature UV –visible absorption spec-trum of ZnO nanoparticles synthesized for various decomposition temperatures.The prepared ZnO NPs have band edge absorption peaks at 362.2nm,389.4nm,429.72nm and 455.32nm for sam-ples prepared at a temperature of 200,300,400and °500C re-spectively.These absorption peaks conform to the well-known intrinsic band gap absorption of ZnO.The absorption edge around 389.4nm was assigned to intrinsic band-gap absorption of ZnO due to the electron transitions from the valence band to the con-duction band →O Zn p d 23[18]and the other possible explanation are given in terms of structural defects associated with Zn inter-stitial as reported in Refs.[6,19].It is observed from the spectra that the absorbance of the samples reduces slightly with increase in decomposition temperature and the absorption edge slightly shifts to lower energy.Furthermore this red shift indicated the shrinkage effect in the band gap energy as a function of tem-perature.They obtained ZnO which exhibited a high absorption band in the UV region λ(<)380nm .Fig.6represents the relationship αν()h 2versus the photon en-ergy ν()h from which the optical band gap of the nanoparticle was determined by extrapolating the linear part of the spectrum.The characteristic's change of the band gap with increase in size of the ZnO nanostructure has been studied by the observation of the red shift in photoluminescence peak position.Thus photo-luminescence is useful for the study of quantum con finement of electrons.The direct bandgap energy of the synthesized ZnO na-noparticles was estimated using Tauc's plots relation [20].ZnO nanoparticles synthesized have estimated band gap values of 3.28eV,3.29eV,3.33eV and 3.39eV for samples prepared at a temperature of 500,400,300and °200C respectively.Thus,the estimated band gap energy of the ZnO nanoparticles was found to decrease with the increase in the decomposition temperature and similar results were obtained by Kathalingam et al.[21].It is clear that when the particle size increases,the electronic states are not discrete as a result the band gap energy reduces and the oscillator strength decreases [5,22].5.Photoluminescence analysisFig.7shows the PL spectra of ZnO NPs prepared at various annealing temperatures.In the PL spectrum,four emission bands,including band edge emission at 398.3nm (3.11eV),402.8nm (3.07eV),406.9nm (3.04eV)and 409.6nm (3.02eV)for ZnO prepared at a temperature of 500,400,300and °200C respectively were observed.The visible emission in ZnO is due to different intrinsic defects such as oxygen vacancies (V o ),zinc vacancies (V Zn ),oxygen interstitials (O i ),zinc interstitials (Zn i )and oxygen antisites (O Zn )[7,23].Band edge emission centered at around 398.3nm should be attributed to the recombination of excitons and V Zn [24,25].However the origin of violet emissions centered at 3.07eV(402.79nm),406.9nm (3.04eV)and 409.6nm (3.02eV)are ascribed to an electron transition from a shallow donor level of neutral Zn i to the top level of the valence band [26,27,9].The broad deep level emission that started from UV to the visible region 360–480nm with the maximum peak at 409.64nm for the sample prepared at °200C was observed.The decrease in PL intensity withTemperature (C )P a r t i c l e s i z e (n m )F W H M ()Fig.2.Variation of FWHM of (101)X-ray diffraction peaks and estimated particle sizes plotted against decompositiontemperature.Temperature (0C )L a t t i c e p a r a m e t e r a ,(A n g .)L a t t i c e p a r a m e t e r c ,(A )Fig.3.Variation of lattice parameter,a and c as a function of temperature.L.T.Jule et al./Physica B 497(2016)71–7773increase in decomposition temperature can be attributed to for-mation of better ZnO stoichiometry and near surface band bending caused by surface impurities [15,28,29].In samples of ZnO pre-pared at higher decomposition temperatures,the increase in grain size will reduce the relative contribution from recombination near the grains,resulting in strongly reduced violet emission.On the other hand,as the decomposition temperature increases more and more ZnO NPs with less deep defects form and as the result the transition line for the emission will decrease causing the PL in-tensity to decrease [25].The other possible explanation can be given as the existence of high dislocation density (defects)at the lower decomposition temperature.The dislocation density (δ),which represents the amount of defects in the sample is de fined as the length of dislocation lines per unit volume of the crystal andisFig.4.SEM micrograph and EDX spectrum of ZnO nanoparticles at:(A)°300C .(B)°400C .(C)°500C for 2h,and D,E and F are the corresponding EDX spectra.L.T.Jule et al./Physica B 497(2016)71–7774calculated using the equation [23]:δ=()D 132where D is the crystallite size.Calculating the dislocation density (δ)of ZnO NPs synthesized at °200C and °500C using Table 1and Eq.(3)it has value of ×−25.76104()−nm 2and ×−37.63104()−nm 2respectively,suggesting high dislocation density at °200C and the lattice imperfection decrease with increase in particle size.Moreover,defect density decreases with synthesis temperature in the present study.For samples synthesized at low temperature,a number of lattice defects which can act as radiative recombination centers are suggested.The reduction of these defect related ra-diative centers after annealing at °500C in air is likely related to formation better stoichiometric ZnO NPs.This hypothesis can be supported by the fact that surface defects strongly depend on morphology (SEM),with suppression of the emission as discussed under PL section.6.Temperature dependent PLRelative changes in state population with temperature provide evidence that PL peaks originate in the same part of the sample and that carriers are free to move between the available states.This feature can be useful because thermal quenching can hide sparse low-energy states and thermal broadening can obscure important details in the spectrum,low temperature PL experi-ments were conducted for samples prepared at 200and °300C .Fig.8and 9A depicts the temperature dependent PL spectra of synthesized ZnO NPs prepared at °200C and °300C respectively studied at a temperature of 300K,273K,173K,123K and 73K.The broadness and increase in PL emission intensity were ob-served with decreasing temperature as shown in Fig.8.It is well known that the donor –acceptor pair transition energy decreases along with the band gap energy when the temperature is in-creased causing the PL intensity to decrease.The experimental data for the temperature dependence of PL band intensity can be fitted by the following expression [14]:()=+()−βI T I Ae 14E 0a where I o is the peak intensity at temperature T ¼0K,A is assumed to be constant,E a is the activation energy of the thermal quenching process,and Kb is the Boltzmann's constant.Fig.9B is a plot of the integrated intensity of the 3.21eV transition versus reciprocal temperature for the ZnO NPs prepared at a temperature of °300C in the luminescence as one step quenching process andA b s o r b a n c e (a r b .u n i t s )Wavelength (nm)Fig.5.UV –vis absorbance spectra of ZnO nanoparticles synthesized at different annealing temperatures.(h v 101(e V /c m hv (eV))αFig.6.The optical absorption energy band gap estimated using Tauc's plot relationfor ZnO nanoparticles synthesized at different annealing temperatures.P L i n t e n s i t y (a r b .u n i t s )Wavelength (nm)Fig.7.PL emission of ZnO nanoparticles synthesized at various temperatures.P L I n t e n s i t y (a r b .u n i t s )Energy (eV)Fig.8.Temperature dependent PL emission of ZnO nanoparticles prepared at °200C .L.T.Jule et al./Physica B 497(2016)71–7775fitted by the Eq.(4).The best fit,demonstrated by the solid curve has been achieved with parameters =I 10200,=E 12.9meV a .A range of experimental values have been reported for the quench-ing of the dominant bound exciton for ZnO nanorods as 13.2meV and 13.1meV for bulk ZnO [11,25,26].Haynes'empirical rule E a ¼a þb E D can be used to calculate thedonor binding energy E D ,using the value obtained for =E 12.9meV a .Different values have been reported for the con-stants a and b in the above relation.In our opinion,the most ac-curate values have been reported by Meyer et al.[11],namely =−a 3.8meV and b ¼ing these values,E D was cal-culated to be 45.8meV.This agrees with the value reported in Ref.[25].In the prepared ZnO NPs,it seems entirely plausible that hydrogen (from precursor)is the dominant donor.Fig.8shows temperature-dependent PL for ZnO NPs prepared at a temperature of °200C ,for which five distinct deep-level emission peaks at 3.01eV,3.03eV,3.04eV,3.08eV and 3.16eV were observed which was ascribed to native defects.The tem-perature-dependent PL spectra shown in Fig.9A for sample of ZnO NPs prepared at °300C clearly depicts thermal redistribution among states compared to the ZnO sample prepared at °200C .The transition line at 3.01eV were attributed to native defect (V zn )and the near-band-edge (NBE)emission peak at 3.21eV were assigned to DAP.The peak labeled in 3.21eV gains strength with decreasing temperature,in contrast,peak labeled 3.01eV is strong at low temperature because carriers are trapped at these sites and do not have enough thermal energy to escape,but it disappears at room temperature(300K)because the states are sparse relative to the intrinsic bands.With increasing temperature,the DAP emission energy was shifted to the low energy side because carriers on DAP with small donor –acceptor distance are released into the band [14,18].The transition at 3.33eV is ascribed to excitons bound to structural defects.This transition line at 3.33eV was previously observed in various ZnO samples and tentatively ascribed to donor bound excitons (DX),acceptor bound excitons,transitions of in-trinsic point defects and excitons bound to extended structural defects.The most detail study of this transition was reported by Yalishev et al.[12]and Wagner et al.[3].In their work,the 3.33eV emission line was attributed to recombination of excitons bound to extended structural donor defect complexes which disappear at temperature of 10K.Study done by Urgessa et al.[26]on ZnO nanorods growth for temperature dependent PL also observe dominance of donor-bound exciton as possible reason for this emission.Fig.10illustrates the Commission Internationale de l'Eclairage (CIE)chromaticity diagram of ZnO NPs prepared at various growth temperatures calculated using photoluminescence data and color calculator software.The coordinates were shown deep blue region with the increase in annealing temperature indicating growth temperature play a major role in tuning the emission color of the ZnO NPs.7.ConclusionIn conclusion,the luminescent properties of ZnO NPs prepared from zinc precursor at various decomposition temperatures have been investigated.The samples of ZnO NPs synthesized at a tem-perature from 200–500°C exhibits broad visible emission.ZnO NPs with crystallite size of 16–20nm and hexagonal wurtzite structure was successfully produced.A linear increase in lattice parameters and grain size with temperature was observed.It was demonstrated that the morphology and band gap energy of ZnO NPs can be tuned with annealing temperature.The temperature dependent PL spectra of ZnO NPs prepared at a temperature of °300C shows three transition lines at 3.01eV,3.21eV and 3.33eV which are ascribed to zinc vacancy (V zn ),donor –acceptor pairs (DAP)and excitons bound to structural defects respectively.The activation and binding energy for the transition at 3.21eV were calculated to be about 12.9meV and 45.8meV respectively.De-pending on the hydrogen concentration in the precursor and these energy values,hydrogen was suggested to be the possible impurityP L I n t e n s i t y (a r b .un i t s )Energy (eV)AL o g i n t e g r a t e d i n t e n s i t y (a r b .u n i t s )100/T (K -1)Fig.9.(A).Temperature dependent PL emission of ZnO nanoparticles prepared at °300C and (B).shows thermally activated luminescence quenching of the 3.21eV emission for the ZnO NPs prepared at a temperature of °300C .Fig.10.CIE diagram for temperature dependent PL sample and ZnO prepared at various measuring temperature.L.T.Jule et al./Physica B 497(2016)71–7776acting as donor in our material.It is believed that ZnO NPs pro-duced using this method would be quick and cost effective synthesis method for blue light emittingfluorescent applications. 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