RESEARCH PAPER
Photoluminescence properties of Eu 3+-doped Cd 12x Zn x S quantum dots
Karamjit Singh ÆSunil Kumar ÆN.K.Verma ÆH.S.Bhatti
Received:7July 2008/Accepted:15January 2009/Published online:12February 2009ÓSpringer Science+Business Media B.V.2009
Abstract Eu 3?-doped Cd 1-x Zn x S (0B x B 0.5)quantum dots (QDs)have been synthesized using wet chemical precipitation method.X-ray diffraction and transmission electron microscope have been ud for the crystallographic and morphological charac-terization of synthesized nanomaterials.In order to understand the spectral characteristics of doped QDs,N 2-lar induced time resolved spectra have been recorded.Excited state lifetime values for dichro-matic emission (red and violet)attributed to 5
D 0?7F J (J =1,2)transitions of Eu 3?and host lattice transitions have been calculated from the recorded luminescence decay curves.Decay time dependence on the dopant concentration (0.01–10at.wt%of Cd 2?)has been studied in detail.
Keywords Quantum dots ÁTime resolved spectra ÁDecay time ÁSynthesis ÁSemiconductors
Introduction
In recent years,optical properties of doped micon-ductor nanocrystals have attracted great attention as electronic structure and electromagnetic fields dras-tically modified due to confinement effects.Quantum size confinement affects not only the excitonic emission in the host,but also the luminescence from the dopants.Significant attention has been paid to transition/rare earth ions doped miconductor nano-crystals to find out the potential applications in photonics and biophotonics (Bharagava et al.1994;Reisfeld et al.2000;Bol et al.2002;Erwin et al.2005).Recent studies have revealed that rare earth doped luminescent II–VI materials are promis-ing candidates for applications in optical memories and color thin film electroluminescence devices (Okamoto et al.1988;Jayaraj and Vallabhan 1991).In rare earth ions 4f electrons participate in lumines-cence.The are hardly influenced by their ligands due to the prence of 5s and 5p electrons surround-ing them.Therefore,crystal field effects obrvable in 3d transition metal ions are not feasible in the ca of rare earth ions.However,rare earth ion doped phosphors have emission in the visible range.
Higher quantum efficiency and lifetime shortening of intrinsic and extrinsic miconductor quantum
K.Singh
Department of Applied Sciences,Chitkara Institute of Engineering &Technology,Rajpura 140401,Punjab,India
S.Kumar
Department of Physics,Maharishi Markandeshwar University,Mullana 133230,Haryana,India N.K.Verma
School of Physics and Materials Science,
Thapar University,Patiala 147004,Punjab,India H.S.Bhatti (&)
Department of Physics,Punjabi University,Patiala 147002,Punjab,India e-mail:in
J Nanopart Res (2009)11:1017–1021DOI 10.1007/s11051-009-9586-1
structures due to quantum confinement effects moti-vated to synthesize and characterize rare earth doped II–VI miconductor nanocrystals.In bulk micon-ductors,due to extreme dislocation of the electron or hole,the electron-hole exchange interaction term is very small,while in molecular size nan
oparticles,due to confinement,the exchange term should be very large.Therefore,there may be a large enhancement of the oscillator strength from bulk to nanostructure materials,which enhances radiative recombination rate and caus life-time shortening.So it ems possible to design and fabricate more nsitive nsors or more efficient devices.Rare earth doped miconductor quantum dots(QDs)are good candi-dates for fast and efficient phosphors and high density optical data storage applications.But there are only few reports on rare earth doped II–VI miconductor QDs.
Doping of rare earth ions in ZnS nanoparticles has been discusd by Bhargava(1996).He pointed out that rare earth ion doped ZnS nanoparticles can be uful in producing efficient phosphor materials with a gamut of colors.ZnS host is able to produce red,green,and blue luminescence due to Tm3?, Tb2?,and Eu3?dopants.Moreover,slow trapping sites or non-radiative recombination sites can be removed in nanoparticles by appropriate surface passivation.Chen et al.(2000)also investigated Eu2?-doped ZnS nanoparticles of3nm size.Qu et al.(2002)synthesized structurally and optically stable nanoparticles of Eu3?-doped ZnS with aver-age size of about3–5nm by the chemical precipitation method adding a-methacrylic acid as the stabilizer.Papakonstantinou et al.(1998)syn-thesized Eu3?-doped ZnS nanoparticles embedded in a polymer matrix.It has been obrved that introduction of Eu3?greatly enhanced the host related photoluminescence(PL)by exciting with a 315
nm wavelength.Chowdhury and Patra(2006) studied role of Eu3?concentration and surface coating on photophysical properties of CdS:Eu3? nanocrystals.They reported that site symmetry of ions plays a very important role in the modifications of radiative and nonradiative relaxation mechanisms. Julian et al.(2006)reported one-pot sol–gel synthesis and optical characterization of Eu3?-doped CdS nanocrystals in SiO2matrices.
正方形的面积Cd1-x Zn x S as a direct band gap miconductor has attracted renewed interest for applications in solar cells,photoconductive devices,and display devices. Moreover,rare earth doped Cd1-x Zn x S QDs will be very good candidates for display and nsor applica-tions.To the best of our knowledge there is no report on the synthesis and PL studies of Eu3?-doped Cd1-x Zn x S QDs.This article reportsfirst time‘lar induced time resolved lar spectroscopy’of rare earth doped quaternary miconductor QDs.Time resolved lar spectroscopic measurements and hence,calculated decay time values are very bene-ficial to make phosphor calibration curves for future optoelectronic industrial applications.High peak power puld lar excitation is capable of exciting the short lived shallow trapping states of QDs,which ems impossible with conventional lamps. Experimental求职申请书怎么写
Eu3?-doped Cd1-x Zn x S(0B x B0.5)QDs have been synthesized using well known bottom-up syn-thesis technique wet chemical precipitation method. Analytical reagent grade chemicals:cadmium ac
etate [(CH3COO)2CdÁ2H2O],zinc acetate(C4H6O4ZnÁ2H2O),europium acetate[Eu(CH3CO2)3ÁH2O], sodium sulphide(Na2SxH2O),and polyvinyl pyrrol-idone(PVP)[(C6H9NO)n]have been ud without further purification.About0.5M solution of cad-mium acetate,zinc acetate,europium acetate,and sodium sulphide were prepared in parate beakers. Then solutions of cadmium,zinc,and europium precursors were mixed in the stoichiometric propor-tion under vigorous stirring,4mL of2%PVP aqueous solution was added to total50mL volume of reaction mixture,before drop wi addition of aqueous sodium sulphide.PVP will act as the capping agent to avoid the agglomeration of QDs.The resulting precipitates were centrifuged and dried in vacuum oven for10–12h continuously.
X-ray diffraction patterns of the synthesized samples have been recorded using a Panalytical’s X’Pert Pro Powder X-ray diffractometer with Cu K a radiation(k=1.541A˚)in the2h range20–70o. From the line broadening of the XRD diffractogram average crystallite size has been calculated using Scherrer formula(Cullity1978).Transmission elec-tron microscope(TEM)images have been recorded using JEOL JEM2000Ex.Type TEM for average particle size determination.
Time resolved luminescence spectra have been recorded using high peak power (10kW),puld N 2-lar excitation.Nanophosphors pasted on the perspex sample holder are placed at 45o to the lar beam.The phosphorescence from the phosphor at an angle of 90o to the lar beam is collected by a fast photomultiplier tube (RCA 8053PMT)through asmbly of mono-chromator (wavelength lective element)and glass slab (UV radiation filter).Decay signals were recorded in the digital storage oscilloscope coupled with PC.Excited state lifetime values have been calculated from record
ed multi-exponential decay curves.Details of the experimental t-up have been already reported by Bhatti et al.(2004,2005).
Results and discussion
Broad XRD patterns have been recorded for all the samples;one such X-ray diffractogram is shown in Fig.1.This reveals that the synthesized nanomateri-als exhibit a zinc-blende crystal structure.The three diffraction peaks correspond to (111),(220),and (311)planes of the cubic crystalline CdZnS.XRD analysis show no characteristics peaks of impurity phas.XRD peak broadening confirm the nanosize formation.Average crystallite size calculated from the recorded XRD patterns is *4nm.
Figure 2shows the transmission electron micro-graph of the Eu 3?-doped nanocrystalline Cd 0.5Zn 0.5S.Average particle size calculated from the TEM is *4nm,which is similar to the average crystallite
size calculated from XRD.So all the particles are single nanocrystals having size comparable or less than the Bohr exciton radius (Bohr exciton size varies with value of x in Cd 1-x Zn x S).
N 2-lar (k =337.1nm)excitation caus dichro-matic (violet &red)emission from Eu 3?-doped Cd 1-x Zn x S QDs.Excitation cross-ction of Eu 3?ions is negligible relative to Cd 1-x Zn x S excitation at 337.1nm.The non-radiative energy transfer process is taking place from the excited state of host to the levels of Eu 3?ions.Violet emission (k 1&430nm)corresponds to host related emission,whereas red emission (k 2&617nm)attributed to 5D 0?7F 2transition of Eu 3?ions.Orange colored very feeble emission corresponding to 5D 0?7F 1transition has also been obrved.But no ti
me resolved spectra have been recorded for orange emission due to low intensity.Chowdhury and Patra (2006)have also reported the red (614nm)and orange (590nm)emission of Eu 3?ions.Multi-exponential decay curves have been recorded for all the samples.Figure 3shows one such decay curve recorded for Eu 3?-doped Cd 1-x Zn x S QDs.Multi-exponential nat-ure of decay curves confirm the emission from multi-level trapping states.Verma et al.(2003)and Bhatti et al.(2004)have reported the detailed description about the peeling-off the multi-exponential decay curves into exponential components using peeling-off method of Bube and the calculation of excited state lifetime values.
Table 1shows decay time values for violet and red emission.Three values of decay time
corresponding
Fig.1XRD pattern of Eu 3?(10at.wt%)-doped Cd 0.5Zn 0.5S
QDs
Fig.2TEM image of Eu 3?(10at.wt%)-doped Cd 0.5Zn 0.5S QDs
to each sample for each emission wavelength have been calculated due to emission from multilevel t
rapping states.Decay time values for violet emission varies from 0.05to 2.85l s,whereas excited state lifetime values for red emission varies from 0.06to 2.72ms.Recorded results show lifetime shortening
for both the emissions with increasing concentration of Zn 2?in Cd 1-x Zn x S QDs.This is due band gap broadening with increasing concentration of zinc,which quenches the emission from deep traps,whereas the shallow trap state emission becomes dominant and on the other hand probability of energy transfer from host to Eu 3?increas with increasing concentration of zinc.Table 1shows red colored decay becomes slow with increasing con-centration of Eu 3?ions,which indicates that the Eu 3?ions at higher concentration are in lower site symmetry.
Conclusions
Eu 3?-doped Cd 1-x Zn x S QDs have been synthesized at room temperature using a simple wet chemical precipitation method.Synthesized QDs of 4nm size have zinc-blende crystal structure.Lifetimes short-ening in the dichromatic emission of Eu 3?-doped Cd 1-x Zn x S QDs have been reported with increasing concentration of Zn 2?.But the rare earth related decay time becomes slow with increa in concen-tration of Eu 3?ions due to lower site
symmetry.
Fig.3Decay curve for Eu 3?-doped Cd 1-x Zn x S QDs mea-sured at 617nm
Table 1Room temperature excited state lifetime values for Eu 3?-doped Cd 1-x Zn x S QDs Sr.No.
Sample
Decay time (l s)for 430nm emission Decay time (ms)for 617nm emission s 1
凉菜s 2s 3s 10s 20s 301.CdS:Eu 3?(0.01%)0.52 1.10 2.850.170.42 1.882.CdS:Eu 3?(0.10%)0.50 1.03 2.670.190.53 2.223.CdS:Eu 3?
口甘(1.00%)0.490.99 2.330.230.59 2.434.CdS:Eu 3?(10.00%)0.470.93 2.010.280.65 2.725.Cd 0.9Zn 0.1S:Eu 3?(0.01%)0.51 1.08 2.790.150.39 1.736.Cd 0.9Zn 0.1S:Eu 3?(0.10%)0.490.98 2.370.180.47 2.067.Cd 0.9Zn 0.1S:Eu 3?(1.00%)0.460.90 1.950.220.54 2.318.Cd 0.9Zn 0.1S:Eu
3?(10.00%)0.420.87 1.830.270.63 2.689.Cd 0.7Zn 0.3S:Eu 3?(0.01%)0.370.75 1.620.110.31 1.4910.Cd 0.7Zn 0.3S:Eu
3?
(0.10%)
0.340.69 1.390.140.36 1.6111.Cd 0.7Zn 0.3S:Eu 3?(1.00%)0.290.61 1.240.180.46 2.1012.Cd 0.7Zn 0.3S:Eu 3?(10.00%)0.240.51 1.150.210.51 2.1713.Cd 0.5Zn 0.5S:Eu 3?(0.01%)0.180.38 1.020.060.160.8314.Cd 0.5Zn 0.5S:Eu
3?
(0.10%)
0.140.300.930.090.24 1.1615.Cd 0.5Zn 0.5S:Eu 3?(1.00%)0.100.210.630.130.35 1.5416.
Cd 0.5Zn 0.5S:Eu 3?(10.00%)
0.05
0.13
0.41
0.16
0.41
1.77
Acknowledgements Authors are grateful to RSIC,Punjab University,Chandigarh and especially Mr.Jagtar Singh for crystallographic and morphological studies.
References
Bharagava RN,Gallagher D,Hong X,Nurmikko A(1994) Optical properties of mangane-doped nanocrystals of ZnS.Phys Rev Lett72:416–419.doi:10.1103/PhysRevLett.
兔宝宝提莫72.416
Bhargava RN(1996)Doped nanocrystalline materials—phys-ics and applications.J Lumin70:85–94.doi:10.1016/ 0022-2313(96)00046-4
Bhatti HS,Sharma R,Verma NK,Kumar S(2004)Lar-induced phosphorescence studies of doubly-doped CaS phosphors.Ind J Eng Mater Sci11:121–124
Bhatti HS,Sharma R,Verma NK(2005)Lar induced down-conversion parameters of singly and doubly-doped ZnS phosphors.Parmana J Phys65:541–546
Bol AA,Beek RV,Meijerink A(2002)On the incorporation of trivalent rare earth ions in II–VI miconductor nanocrys-tals.Chem Mater14:1121–1126.doi:10.1021/cm011195s Chen W,Malm JO,Zwiller V,Huang Y,Liu S,Wallenberg R, Bovin JO,Samuelson L(2000)Energy structure and fluorescence of Eu2?in ZnS:Eu2?nanoparticles.Phys Rev B61:11021–11024.doi:10.1103/PhysRevB.61.11021 Chowdhury PS,Patra A(2006)Role of dopant concentration and surface coating on photophysical properties of CdS: Eu3?nanocrystals.Phys Chem Chem Phys8:1329–1334.
doi:10.1039/b517788j Cullity BD(1978)Elements of X-ray diffraction.Addison-Wesley,Massachutts,p102
我的愿望Erwin SC,Zu L,Haftel MI,Efros AL,Kennedy TA,Norris DJ (2005)Doping miconductor nanocrystals.Nature43:91.
止血的方法
doi:10.1038/nature03832
大熊猫为什么是国宝
Jayaraj M,Vallabhan C(1991)AC thinfilm electrolumines-cent devices with rare earth doped ZnS.J Electrochem Soc138:1512–1516.doi:10.1149/1.2085817
Julian B,Planelles J,Cordoncillo E,Escribano P,Aschehoug P,Sanchez C,Viana B,Pelle F(2006)Eu3?-doped CdS nanocrystals in SiO2matrices:one-pot sol–gel synthesis and optical characterization.J Mater Chem16:4612–4618.doi:10.1039/b612519k
Okamoto K,Yoshimi T,Miura S(1988)TbOF complex cen-ters in ZnS.Thin-film electroluminescent devices.Appl Phys Lett53:678.doi:10.1063/1.99848 Papakonstantinou DD,Huang J,Lianos P(1998)Photolumi-nescence of ZnS nanoparticles doped with europium ions in a polymer matrix.J Mater Sci Lett17:1571–1573.doi:
10.1023/A:1006543205507
Qu SC,Zhou WH,Liu FQ,Chen NF,Wang ZG,Pan HY,Yu DP(2002)Photoluminescence properties of Eu3?-doped ZnS nanocrystals prepared in a water/methanol solution.
Appl Phys Lett80:3605
Reisfeld R,Gaft M,Saridarov T,Panczer G,Zelner M(2000) Nanoparticles of cadmium sulfide with eur
opium and terbium in zirconiafilms having intensified luminescence.
Mater Lett45:154–156.doi:10.1016/S0167-577X(00)000 96-3
Verma NK,Kumar S,Bhatti HS,Singh S(2003)Effect of killer impurities on lar excited doped ZnS phosphors.
J Opt32:69
