Effect of Gd3+ doping on structural and optical properties of ZnO nanocrystals

Effect of Gd3þdoping on structural and optical properties of ZnO nanocrystalsDeepika Mithal*,Tapanendu KunduDepartment of Physics,Indian Institute of Technology Bombay,Powai,Mumbai400076,Indiaa r t i c l e i n f oArticle history:Received29January2017 Received in revised form23April2017Accepted26April2017 Available online27April2017Keywords:NanocrystalsSemiconductor quantum dots Optical properties PhotoluminescenceDefects a b s t r a c tTo investigate the effect of rare earth ions on the excitation energy redistribution between the UV and visible photoluminescence,the Gd3þdoped ZnO nanocrystals prepared through Sol-Gel method were studied.Different structural characterizations and spectroscopic studies were carried out to establish a correlation between the structural effect and the photoluminescence.Our study indicates that the incorporation of Gd3þin ZnO nanocrystals prepared through this synthesis takes place predominantly on the surface up to0.18mole fraction of Gd3þ.This is manifested by the observed lattice contraction due to the presence of Gd3þon the surface.This structural defect introduces various defects that modifies the photoluminescence properties.For these modifications in crystal system,the A1symmetry phonon modes of wurtzite structure are found to be gradually more Raman active than the other modes as the Gd3þconcentration is increased while the E modes are suppressed.These doping induced modification of phonon spectrum strongly influences the exciton phonon coupling and thereby the photophysical properties.Resulting crystal imperfections strongly enhance the defect band luminescence.It is shown that energy transfer from selectively excited Gd3þcenters to surface states can be effectively exploited for the sensitization of defect band luminescence in variousfields such as bio imaging,light emitting devices etc.©2017Elsevier Masson SAS.All rights reserved.1.IntroductionIn the past several years,research activities have been focused on Zinc Oxide(ZnO),a group II-VI semiconductor,for various op-toelectronic applications due to its high quantum yield both in the UV and the visible wavelength range.While the UV emission often results from the near band edge transition,the origin of the broad visible band is mainly attributed to emissions from different defect states developed in the crystal system during synthesis process.At nanoscale,space confinement modifies the band gap of the semi-conductor[1]and thus the fundamental optical transitions,ab-sorption and UV emission,can be tailored favorably for the device applications.However,this advantage is offset by the emergence of defect states depending on the synthesis process of nanocrystals.A clever control over these defect states[2,3]thus offers opportunities to develop various tunable photonic devices forfield applications[4e6].Introduction of external dopants into the ZnO nanocrystals is one such way to remold these UV and visible emissions.Several rare earth ions such as Nd3þ,Dy3þ,Tb3þ,Gd3þ,Eu3þetc.doped ZnO nanocrystals have been investigated previously in this regard [7e13].In most of these cases,the intensity of the visible emission of the doped nanocrystals has been initially increased with the dopant concentration.This enhancement of the visible emission is generally attributed to the formation of various defects such as oxygen interstitials,oxygen vacancies and e OH groups as a result of doping[10,12].For further increment of rare earth ions above a critical concentration,the intensity of visible emission has been found to quench monotonously.Not only the defect state emission of the doped ZnO nanocrytals,the enhancement of the intensity of characteristic rare earth transitions have also been observed due to the excitation energy redistribution.In Sm3þ,Eu3þand Tb3þdoped ZnOfilms,the emission from rare earth(RE)ions has been found to be less prominent under the direct excitation of these ions[14e16]. However,excitation above the band edge absorption of ZnO,sharp*Corresponding author.Department of Physics,Jamia Millia Islamia University, New Delhi110025,India.E-mail addresses:deepmithal09@,deepika@phy.iitb.ac.in(D.Mithal), tkundu@phy.iitb.ac.in(T.Kundu).Contents lists available at ScienceDirectSolid State Sciencesjournal h omepage:ww /l ocate/ss scie/10.1016/j.solidstatesciences.2017.04.0061293-2558/©2017Elsevier Masson SAS.All rights reserved.Solid State Sciences68(2017)47e54features of these rare earth ions in the visible region have been clearly observed along with the broad yellow emission of ZnO.In all these cases,the energy transfer takes place from ZnO host matrix to the rare earth ions depending on the proximity of the lowest excited state of these rare earth ions with respect to the defect states of ZnO.This energy transfer is also efficient because of the long lifetime of the visible emission[17].However,the reverse energy transfer from rare earth to ZnO host matrix has not been observed so far.From these observations,it can be inferred that rare earth ions can perturb the optical transitions of the ZnO host matrix in two ways;(i)by developing structural disorders in ZnO host matrix due to the ionic size mismatch with respect to zinc and,(ii) by invoking energy transfer processes that naturally depends upon the relative placement of the host and rare earth energy levels. These aspects play crucial role in determining the dynamics of the excitation energy distribution among the energy levels of the RE ions and host matrix.A systematic understanding of the relative contributions coming from structural disorders due to doping as well as energy transfer mechanisms is essential to tailor the optical properties of the rare earth doped ZnO nanocrystals.To decipher the role of these two aspects separately,we have investigated the Gd3þdoped ZnO nanocrystals.In the above said context,Gadolinium(Gd3þ)is interesting as its lowest excited state(6P)lies above the band edge absorption of ZnO nanocrystals reported here.Consequently,the direct interactions between the Gd3þenergy levels and the defect states are less probable.In this case,the characteristics of the visible emission would be governed by defect states arising from structural disorder only.Therefore,this is taken as a model system for understanding the correlation between the intensity of visible emission and the structural modification on incorporation of rare earth ions.Further, this system also provides possibility to excite both ZnO host nanocrystal and Gd3þdopant simultaneously to understand the energy transfer mechanism between these two systems.In view of this,a systematic structural and optical studies have been carried out on Gd3þdoped ZnO(Gd3þ:ZnO)nanocrystals over a wide dopant concentration range.The extent of change in the crystal structure on Gd3þincorporation has been investigated by different spectroscopic techniques such as UV-Vis absorption spectroscopy, X-ray diffraction(XRD),Raman spectroscopy and X-ray photo-electron spectroscopy(XPS).The photoluminescence(PL)spectra upon excitation above the band edge have been analyzed to correlate the defect mediated energy transfer mechanisms with the degree of structural disorder resulting from Gd3þdoping.The effect of Gd3þdoping on decay dynamics of the defect state emission has also been investigated using time resolved PL spectra.Photo-luminescence excitation(PLE)spectroscopy have been exploited to get an insight into the role of different energy transfer mechanisms those are operative between the nanocrystal host matrix and Gd3þdopant.2.Experimental proceduresZnO nanocrystals were synthesized by sol-gel method following the process developed by Meulenkamp et al.,[18].In this method, 1.1g of precursor zinc acetate dehydrate was refluxed at60 C temperature with50ml of ethanol.In a separateflask,0.29g of LiOH was sonicated with50ml of ethanol at room temperature for 30min.LiOH was then added drop wise to zinc acetate solution for 10min,whole mixture was left for stirring and further sonicated for one hour.For removing the reaction by-products,the sample was mixed with hexane and left for hours.This supernatant was removed and again it was dispersed in ethanol with the addition of hexane.This procedure was repeated for a number of times.The product was dried in air and collected for measurements.For the Gd3þdoping in ZnO nanocrystals,different concentrations(mol%) of Gd3þwere taken as precursors.The precursors Zinc acetate dehydrate and Gadolinium acetate hydrate were procured and used without any further purifipositions of the elements present in four prepared samples were derived from the energy dispersive X-ray spectrum.The observed ratio of Gd3þat%and Zn at%(x¼Gd/Zn)forfive prepared samples investigated here are 0.02,0.059,0.18and0.34.Crystal structure analysis of the synthesized nanocrystal was carried out by X-ray diffraction pattern from a PAN analytical X-ray powder diffractometer.To obtain the structural and the morpho-logical characteristics of these prepared samples,Transmission electron microscopic(TEM)(Model JEOL,2100F,accelerating voltage of200KV)images and the associated energy dispersive X-ray(EDX)spectra were analyzed.Absorption spectra were measured by using JASCO V570UV-Vis spectrophotometer.X-ray photoelectron spectroscopy(XPS)was carried out by Thermo VG Scientific MultiLab with the concentric hemispherical analyzer. AlK a(1486.1eV)radiation was used as an X-ray source.C1s at 284.6eV was used as the reference for the binding energy cali-bration.Micro Raman spectroscopy was performed using Horiba Jobin Yvon HR640,using excitation source(514nm)from Arþion laser.The continuous width(CW)excitation PL spectra were obtained from the drop cast samples on Si substrate by a CW He-Cd laser source(325nm).An Acton monochromator attached with a cooled CCD camera(ANDOR)system was used for the photoluminescence measurement.For the time resolved spectroscopy,a nanosecond pulsed excitation PL measurement system was developed.A second harmonic(532nm)of Q Switched LITRON Nd:YAG laser was used for pumping a tunable dye laser(Lamda Physik).The second har-monic output from the dye laser was used as an excitation source and the luminescence from the sample was collected through a double monochromator(SPEX1680)attached with a photo-multiplier tube(PMT).The signal output from the PMT was recor-ded using an oscilloscope(TDS2024)and the oscilloscope was interfaced with the computer through the labview program.This program is also developed to control the monochromator and the tunable dye laser.The time resolved photoluminescence spectra and pulsed photoluminescence excitation(PPLE)spectra were ob-tained either by varying the wavelength of the monochromator or the excitation wavelength of the dye laser,respectively.Fig.1.Absorption spectra of Gd3þ:ZnO nanocrystals with x(ratio of Gd3þat%and Zn at%)values(i)0.00,(ii)0.02,(iii)0.059,(iv)0.18and(v)0.34.The inset shows the variation of average band gap values of Gd3þ:ZnO with the corresponding x values.D.Mithal,T.Kundu/Solid State Sciences68(2017)47e54 483.Results and discussionThe UV-Vis absorption spectra of prepared undoped ZnO (x¼0.0)and doped Gd3þ:ZnO(x¼0.02to0.34)nanoparticles dispersed in ethanol solvent are shown in Fig.1.A monotonous blue shift of the absorption spectrum relative to that of undoped ZnO nanoparticles is observed with increasing concentration of Gd3þions.The average band gap values[18]obtained from these ab-sorption spectra are shown in the inset of Fig.1.It was observed that the average band gap of the undoped ZnO nanoparticles increased from3.32eV to3.49eV for x¼0.34.Mean particle size was derived from TEM images of these samples.The reduction of the mean diameter was observed with respect to the undoped ZnO nano-particles[11,12].These values for x¼0.00,0.02,0.059,0.18,0.34,are 6.7nm,6.3nm,5.9nm,5.5nm and4.4nm,respectively.Thus the observed blue shift of the absorption spectra given in Fig.1and the corresponding average band gap values can very well be attributed to this reduction of particle size due to Gd3þdoping in the ZnO nanocrystals.The XRD pattern shown in Fig.2present the effect of the incorporation of Gd3þion on the crystal structural of these ZnO nanocrystals.All the observed reflection peaks of diffraction planes for the undoped nanocrystals(indicated in Fig.2a)correspond to wurzite crystal structure of ZnO.No extra peaks related to the other crystalline phases were observed upon Gd3þincorporation.How-ever,it is observed that the intensities of the diffraction peaks gradually diminish with the increment of the Gd3þconcentration. This is accompanied by the shift and the broadening of these peaks. Fig.2b shows the extent of the shift and broadening obtained from the high resolution diffraction peak corresponding to plane(101). Both the peak shift and the full width at half maxima(FWHM)of the undoped ZnO nanocrystals increase gradually up to Gd3þcon-centration x¼0.059and remain almost constant for further increment of the Gd3þion concentration.Our observations are consistent with the previously reported Gd doped ZnO nanocrystals [11,12]and these changes are attributed to the degradation of crystallinity due to Gd3þdoping.The increased diffuseness in the electron diffraction pattern as given in supplementary information [Fig.S1]for our samples also reflects the detoriation of the crys-talinity due to the incorporation of Gd3þ.There are several mech-anisms for the observed degradation of the crystal structure.The synthesis process of these doped nanocrystals through chemical route provides several possible doping sites for the Gd3þion such as substitutional and interstitial.Gd3þion might as well be accumu-lated on the highly reactive surface of these nanocrystals.The proper doping of Gd3þion creates oxygen vacancies in the ZnO crystalline structure and it is well known that more introduction of oxygen generates stronger tensile stress in the crystal structure [19].Not only this,in case of substitutional doping,the mismatch of ionic radii between the Gd3þalso produces micro stress in the nanocrystals.Further,the accumulation of Gd3þon the surface creates the compressive hydrostatic pressure on the nanocrystals. Fig.3a shows variation of the lattice parameters a and c obtained from the XRD pattern shown in Fig.2a with respect to x.It canbeFig.2.(a)XRD Patterns for Gd3þ:ZnO nanocrystals and(b)Peak shift and broadeningin high resolution diffraction pattern corresponding to plane(101)for x values of(i)0.00,(ii)0.02,(iii)0.059,(iv)0.18and(v)0.34.Fig.3.(a)Variation in lattice parameters a and c and(b)hydrostatic pressure for Gd3þ:ZnO nanocrystals with respect to different x values.D.Mithal,T.Kundu/Solid State Sciences68(2017)47e5449seen that both a(¼3.231Å)and c(¼5.179Å)parameters decrease initially to the minimum values for x¼0.18,and then tend to in-crease upon further increment of Gd3þconcentration.A similar trend has been observed by Archana et al.[9]for Tb doped ZnO nanoparticles.The plausible explanation of the observed trend is that for low concentration of Gd3þ,the lattice contraction takes place up to a critical concentration due to the hydrostatic pressure generated by the rare earth dopants situated on the surface of the ZnO nanocrystals.Upon further increment in dopant concentration, the crystal lattice expands due to the substitution of Zn atoms in the core due to relatively large ionic radii of rare earth dopant.The hydrostatic pressure was calculated from the derived a and c pa-rameters using the relation P¼ðB0=B00Þ½ða20c0=a2c0ÞB00À1 where B0and B00are the bulk modulus and pressure derivative of bulk modulus having values183Gpa and4Gpa for ZnO,respectively.In this,a0and c0are the lattice constants for the bulk ZnO having values3.2496Åand5.2042Årespectively[10].Fig.3b shows the variation of the calculated hydrostatic pressure with respect to x. The pressure was found to vary in our case from3.02Gpa for x¼0.0e4.8Gpa for x¼0.18and decreased to4.3Gpa for x¼0.34. The observed peak shift of plane(101)shown in Fig.2b also pro-vides direct measurement of the internal compressive microstress generated due to strain and can be obtained from s st¼3*B*½ðDq101Þ*cot q101 where Dq101is the peak shift corre-sponding to the plane(101),B is the young modulus having value 143Gpa for ZnO[19].The maximum change in stress with respect to ZnO nanoparticle was found to be1.6Gpa that is in accordance with the pressure calculated using a and c parameters.Evidently,the stress originating from two distinctly different modes of Gd3þincorporation,at the surface and inside the core, develops different defects in the ZnO nanocrystals.For deeper insight into the nature of these defects,Oxygen1s XPS spectra for the ZnO and Gd3þ:ZnO nanocrystals shown in Fig.4were analyzed. The lower energy region corresponding to peak centered at 530.4eV is attributed to the O2Àions in the wurtzite structure surrounded by the Zn atoms,i.e.,oxygen atoms at the regular lattice sites[20,21].The peak at531.9eV is due to the oxygen deficient regions and these oxygen vacancies can be in the core or on the surface in the form of e OH groups.Our observations point out that due to the increment of Gd3þion,the amount of oxygen in the lattice decreases whereas number of other types of oxygen in-creases.The XPS spectra of the energy region between1180eV and 1235eV for Gd3þ:ZnO nanocrystals are given in supplementary information[Fig.S2].The observed two features at1189eV and 1220eV correspond to Gd3þ3d5/2and3d3/2states,respectively and the Zn2s state is observed at1198eV.It is to be noted that the intensity of Zn2s state is less affected than the Gd3þstates as concentration of Gd3þincreases.This implies that all Gd3þions may not be replacing the Zn atoms of the sub lattice,in accordance with our observation about contraction of lattice.Since the observed structural disorder affects the crystal sym-metry and consequently the phonon vibrations,the Raman spectra of our samples shown in Fig.5were analyzed according to the reported experimental and abinitio calculated values[22,23].The detailed analysis is given in the supplementary and the assigned peaks are marked in Fig.5.Most of the peaks in the Raman spec-trum of undoped ZnO(Fig.5i)are in agreement with the other reported values[22].However,E1modes(410cmÀ1and581cmÀ1) [23,24]are not observed in our case,instead2LO modes(1102cmÀ1, 1151cmÀ1)are found to be more Raman active.The comparison of the Raman spectra of Gd3þdoped ZnO samples brings out two major points.First,the gradual reduction in intensity of the437cmÀ1band assigned as E high2symmetry mode corresponding tooxygen sublattice is generally observed in doped ZnO nanocrystals and is attributed to the increased disorder in the crystal structure [12,25e27].However,there is a small variation of the FWHM of this band(6e8cmÀ1)observed due to doping.Second,the monotonous increment in intensity of all the A1symmetry modes with the Gd3þdoping(1102cmÀ1,1151cmÀ1,650cmÀ1-750cmÀ1)is observed in our samples.This implies that the weakening/degradation of the crystallinity results in mixing of different phonon modes and this modification increases the Raman activity of the A1modes.It is expected that this modifications profoundly influence the exciton-phonon coupling in this system and consequently all its physical properties,especially the photoemission and its dynamics(radia-tive and nonradiative processes).The PL spectra of undoped ZnO and Gd3þ:ZnO obtained under CW excitation(325nm)are presented in Fig.6.The observed PL spectra consist of one very less intense peak in UV wavelength range(350e380nm)and a broad feature in the visible wavelength range(450e675nm).The spectra presented here are normalized with respect to peak intensity of UV emission as shown in the inset of Fig.6.The observed blue shift of the peak of this band edge emission can be correlated with the observed shift in the absorp-tion spectra.As seen,the defect state emission increases up to x¼0.18(curve(iv))and decreases beyond that.Not only the change in intensity,but also the peak position of this emission shifts to-wards blue side of the spectrum till the intensity is maximum.Red shift of the peak occurs for further increment of Gd3þFig.4.Oxygen1s XPS spectra for Gd3þ:ZnO nanocrystals with x values of(i)0.00,(ii) 0.02,(iii)0.059,(iv)0.18and(v)0.34.Spectra were decomposed into two regions: Region1centered at530.4eV as and Region2centered at531.9eV.D.Mithal,T.Kundu/Solid State Sciences68(2017)47e54 50concentration.Generally,it has been shown that different parts of this broad emission contribute to the overall spectrum,originate from different defects states,mainly different kinds of oxygen va-cancies,surface defect states,Zn vacancies and interstitial both oxygen and zinc [3].To understand the contributions from these defect states in determining the overall shape of the broad emis-sion,the observed spectra 450e 675nm shown in Fig.6were deconvoluted into three basic regions.These deconvoluted spectra are shown in Fig.7.All the spectra were normalized to the peak intensity to understand the percentage contribution arising from these three regions.The deconvoluted PL spectra of ZnO nano-crystals (Fig.7(i))consists of a band centered at 565nm havingmaximum intensity corresponds to the contribution from different kinds of oxygen vacancies.The peak centered at 653nm results from the transitions from (Zn i )to (O i )[3,29].The small peak centered at 595nm is from the surface defects such as e OH groups and oxygen antisites [29].The overall small blue shift of the PL spectrum for x value of 0.02in Fig.7(ii)compared to Fig.7(i)is observed due to the blue shift of the three regions upon Gd 3þincorporation.It is observed that the contributions of both regions from oxygen vacancies and transition from Zn i to O i are almost same.A small contribution of surface defects centered at 595nm is observed and this emission increases compared to the other two regions as the Gd 3þconcentration increases.At optimum concen-tration where the PL intensity is maximum (x ¼0.18),it is seen that the contribution of surface defect dominates over the other defects.For x ¼0.34,it is observed that the contribution of region centered at 626nm originating due to the increment of transition from Zn i to O i defects relatively dominates and thus the peak of the overall spectrum gets shifted towards the red side.In case of Sn-doped ZnO nanobelts and nanorings,Ramin Youse fi[30]has shown that there is a de finite trend between the relative percentage change of green emission and the oxygen vacancies in the nonlattice observed in oxygen XPS spectrum.From detailed investigation on photocurrent in doped ZnO nanoparticles/nanocomposites,it is concluded that the enhancement of photocurrent in these doped systems is due to the observed oxygen vacancies in XPS and these compositesystemsFig.5.Raman spectra of Gd 3þ:ZnO nanocrystals with x values of (i)0.00,(ii)0.02,(iii)0.059,(iv)0.18and (v)0.34.The peak at *represents the S -M branch phonon in ZnO crystal structure (Ref.[28]).Fig.6.Photoluminescence (PL)spectra of ZnO and Gd 3þ:ZnO nanocrystals for various x values of (i)0.0(ii)0.02,(iii)0.059,(iv)0.18and (v)0.34.Spectra are normalized with respect to UV peakintensity.Fig.7.Decomposition of normalized visible spectra for x values of (i)0.0(ii)0.02,(iii)0.059,(iv)0.18and (v)0.34.The sharp peak at 650nm corresponds to the second order of excitation wavelength.D.Mithal,T.Kundu /Solid State Sciences 68(2017)47e 5451are the promising materials for photodetectors [31e 33].In light of this,we have investigated the variation of PL corresponding to normalized contribution from oxygen defects such as surface oxy-gen and antisites as shown in Fig.7(area of the peak at 595nm)with the contribution of the oxygen vacancies in XPS spectrum as shown in Fig.4(area of the peak centered at 531.9eV)and plotted in Fig.8.It is observed that there is a linear correlation between these two observed values upto the Gd concentration of 0.18.The departure from this linearity in higher concentration is obvious due to the dominant contribution in normalized PL from other than oxygen defects such as Zn i to O i as mentioned above.The overall increment of defect states emission with Gd 3þdoping can be rationalized in a way that due to the particle size reduction,the surface to volume ratio increases and this increases the defect states concentration.As Gd 3þconcentration increases,the crystallinity of the nanocrystals degrades as observed in XRD patterns and Raman spectra and this weakening in crystallinity alters the radiative transition probability.The reduction of PL in-tensity of visible emission thus results from the relative dominance of the radiative and nonradiative pathways.The above observation made for Gd 3þin ZnO is related to the structural modi fication and this insight can be extrapolated to other rare earth ions also.Thus by controlling the concentration,relative contributions of these defect states can be modi fied and hence the color of emission can be tuned.To understand the energy distribution in different time domain,the PL spectra of Gd 3þ:ZnO colloidal solutions as shown in Fig.9a were recorded immediately after excitation by the 5e 7ns 295nm laser pulse.It is seen that the UV emission is more pronounced here as compared to cw excitation (Fig.6).In all other respects,the CW and pulsed excited spectral pro files behave almost the same way.This difference in relative intensity arises due to the different population distribution in equilibrium (cw excitation)and nonequilibrium conditions (pulsed excitation).It also indicates longer relaxation time to populate defect states.Fig.9b shows the inverted output of the PMT when the UV and visible emissions were monitored upon pulsed excitation at wavelength 295nm.The decay pro files of the UV emission monitored at 370nm (Curve (ii))almost resembles the instrument response of the 5e 7ns excitation pulse (Curve (i)).Since the UV band edge emission is very fast(~picosecond)[34],this could not be resolved in our experiment.As seen in Fig.9b,decay pro file of the visible emission of Gd 3þ:ZnO consists of initially a fast component,followed by a slow and a very long decay component.It is seen that due to incorporation of Gd 3þ,the shape of the decay pro file changes.To understand the variation of these shapes,these decay pro files were deconvoluted into three exponential decay function using the instrument response function of the excitation pulse (curve (i)).The fitted values of the relative contribution and the corresponding lifetimes for these samples are given in Table 1.For undoped ZnO,the lifetime of the fast compo-nent (t 1)was calculated as 0.6ns whereas that of the slow component (t 2)was 23ns.The very long decay was having a life-time (t 3)around 0.6m s.The contribution of the veryfastFig.8.Variation of Pl corresponding to normalized contribution from oxygen defects such as surface oxygen and antisites (area of the peak at 595nm)with the contribution of the oxygen vacancies in XPS spectrum (area of the peak centered at 531.9eV).Fig.9.(a)PL spectra of Gd 3þ:ZnO upon excitation with 295nm (5e 7ns)light pulse for corresponding x values of (i)0.0(ii)0.02,(iii)0.059,(iv)0.18and (v)0.34.(b)The decay pro file upon excitation with 295nm 5e 7ns pulse for (i)Instrument response function (IRF)(ii)UV emission at 370nm ZnO.Decay pro files with x values of (i)0.0(ii)0.02,(iii)0.059,(iv)0.18and (v)0.34with monitoring wavelength at peak of visible emission.Table 1Lifetimes and their relative contributions determined from the decay pro files.x ¼(Gd 3þ/Zn)A 1t 1(ns)A2t 2(ns)A3t 3(m s)0.0(1.1%)0.60(76.3%)23.4(22.2%)~0.600.02(0.8%)0.50(80.1%)22.6(18.6%)~0.680.059(0.03%)0.55(59.4%)42.7(39.6%)~0.700.18(0.02%)0.51(44.4%)50.8(54.6%)~0.840.34(0.09%)0.57(62.1%)26.4(36.2%)~0.89D.Mithal,T.Kundu /Solid State Sciences 68(2017)47e 5452。