助听器排名
数学书Journal of Alloys and Compounds390(2005)
226–229任职报告
减肥早餐怎么吃
A novel red phosphor for white light emitting diodes
Yunsheng Hu a,b,Weidong Zhuang a,∗,Hongqi Ye b,Donghui Wang a,
Shusheng Zhang a,Xiaowei Huang a
a National Engineering Rearch Center for Rare Earth Materials,General Rearch Institute for Nonferrous Metals,
Grirem Advanced Materials Co.Ltd.,No.2Xinjiekouwai Street,Beijing100088,China
b College of Chemistry and Chemical Engineering,Central South University,Changsha410083,China
Received14April2004;accepted10July2004
Abstract
A novel red phosphor,CaMoO4:Eu3+,has been developed for white light emitting diodes(LEDs).The phosphor was prepared by using oxides in air and its luminescent properties have been investigated.The excitation and emission spectra indicate that this phosphor can be effectively excite
d by ultraviolet(UV)(394nm)and blue(464nm)light,and exhibits a satisfactory red performance(616nm),nicelyfitting in with the widely applied UV or blue LED chips.The novel phosphor is much more stable and has stronger emission intensity than the sulfide red phosphors ud in white LEDs.
©2004Elvier B.V.All rights rerved.
Keywords:Phosphor;Semiconductor;Luminescence
1.Introduction
Within the last veral years,much progress has been made in the art of high-brightness LEDs with various colors.Nowa-days,a white LED device has been commendably realized using YAG:Ce as a broad band yellow phosphor coated on the blue LED chip[1–3].However,there exist at least two drawbacks in this combination.Firstly,the overall efficiency decreas rapidly when lowering the correlated color temper-ature of the device.Secondly,a concern with this device is that the“White”output light has an undesirable color balance for a true color rendition,viz.,the output light is deficient in the red region of the visible light spectrum(above600nm).
So,a parate red light source may have to be ud to compensate LED for the red deficiency of the output light. On the other hand,for UV LED chip coated by red,green and blue light-emitting phosphors,a promising white light generation way,it also desiderates a kind of efficient and stable red phosphor.However,the red light emitting phosphor for white LED is commercially still limited to divalent Eu ∗Corresponding author.
E-mail address:(W.Zhuang).ion activated sulfide.Tho sulfide phosphors are chemically unstable and not desirable in efficiency.
In general,the phosphor material for white LED has to have a sufficient absorption at the emission wavelength of the blue diode,the quantum yield should be high under UV–vis excitation and the full width at half maximum of the emission band should be as small as possible in order to achieve high luminous output.Bad on the options,trivalent Europium ion(Eu3+)is the preferable choice,which is chon for the activator ion in prent work.
On the other hand,in view of the host material,for molyb-date(MoO42−),the central Mo metal ion is coordinated by four O2−ions in tetrahedral symmetry(T d).Hence,MoO42−is relatively stable and can be opted for host material.
In this paper,trivalent Europium ion(Eu3+)activated cal-cium molybdate(CaMoO4)phosphor has been developed by solid-state reaction in air,and its luminescent properties were investigated.
2.Experimental啤酒烧鱼
The phosphor CaMoO4:Eu3+was obtained by solid-state reaction in air.The starting materials MoO3(L.R.),CaO
0925-8388/$–e front matter©2004Elvier B.V.All rights rerved. doi:10.1016/j.jallcom.2004.07.063
Y.Hu et al./Journal of Alloys and Compounds 390(2005)226–229227
(A.R.)and Eu 2O 3(4N)were weighed by appropriate stoi-chiometric ratio,where mole concentration of the activator Eu ion varied from 18%to 30%.After the powders were blended and grounded thoroughly in an agate mortar,the ho-mogeneous mixture obtained was put into an alumina crucible and calcined in a muffle furnace at the temperature of 700◦C for 3h into the intentional sample.
The crystal structure of the sample was identified by X-ray diffraction (XRD),which was recorded on a MXP21V AHF-M21X X-ray diffraction running Cu K ␣radiation at 40kV and 250mA.A SPEX fluoroMAX-2fluorescence spectrom-eter was ud to detect the excitation and emission spectra of the samples.
3.Results and discussion 3.1.X-ray diffraction analysis
The XRD pattern of the Eu 3+-doped CaMoO 4sample (nominal composition Ca 0.76MoO 4:Eu 0.243+)is prented in Fig.1.Compared with the JCPDS card 29-0351,only a sin-gle pha belonged to the CaMoO 4:Eu 3+is obrved and no extraneous pha is emerged,and the doped Eu ion has little influence on the host structure.It is argued that this single pha has been developed fully through our preparation pro-cedure.
According to the JCPDS card 29-0351,CaMoO 4has a tetragonal crystal structure with space group
of I 41/a (88),and its lattice parameter is a =0.5226nm,c =1.143nm,while the corresponding lattice parameters of the sample in Fig.1are as follows:a =0.5136nm,c =1.1379nm,V =0.3093nm 3.It is concluded that the doping of Eu has lowered the lattice parameters of the phosphor,which results from that the ionic radius of Ca 2+(0.114nm)is slightly lower than that of Eu 3+(0.109nm)[4]and Eu ion is expected to occupy the Ca 2+site in this
phosphor.
Fig.1.XRD pattern of Ca 0.76MoO 4:Eu 0.24
3+
.
Fig.2.Emission spectra of Ca 0.76MoO 4:Eu 0.243+phosphor excited by 464nm and 394nm,respectively.
3.2.Excitation and emission spectra of the phosphors Figs.2and 3illustrate the emission and excitation spectra of the Ca 0.76MoO 4:Eu 0.243+phosphor,respectively.
It has reported that the unactivated CaMoO 4produces a broad green emission under UV-light excitation,and the or-ange emission at 580nm is obrved only if the excitation wavelength is longer than 320nm [5].However,as shown in Fig.2,green or orange emission is quenched and the line spectrum emission in the red light region appears by intro-ducing the activator ion Eu 3+.
The emission spectrum of CaMoO 4:Eu 3+is compod of groups of veral sharp lines,which belong to the character-istic emission of trivalent Eu ion.Each group corresponds to a transition between 5D J (J =0,1)and 7F J (J =1–4)(il-lustrated in Fig.2(a)).The weaker emission in the vicinity o
f 590nm is ascribed to the Eu 3+magnetic dipole transition 5D 0–7F 1
,which is innsitive to site symmetry.The
main Fig.3.Excitation spectrum of Ca 0.76MoO 4:Eu 0.243+phosphor (monitored in wavelength 616nm).
228Y.Hu et al./Journal of Alloys and Compounds 390(2005)226–229
emission lines around 610–620nm (612nm and 616nm)is assigned to the Eu 3+electric dipole transition of 5D 0–7F 2,induced by the lack of inversion symmetry at the Eu 3+site in CaMoO 4:Eu 3+and the break of parity lection rules,and is much stronger than that of the transition to the 7F 1state.
The emission spectrum of Ca 0.76MoO 4:Eu 0.243+phosphor excited by 394nm is also demonstrated in Fig.2(b).It is obvious that the spectrum is identical with that excited by 464nm.
The excitation spectrum of CaMoO 4:Eu 3+consists of a broad band and some sharp lines.As shown in Fig.3,the broad excitation band extending to 350nm is attributable to the host crystal.The energy absorbed by the host is trans-ferred to the Eu 3+and produces the red emission.The mech-anism of energy absorption by the host lattice is not the same in the long and short wavelength UV regions;the absorptions are related to charge-transfer process involv-ing Mo O and Ca O bands,respectively;while the nar-row peaks beyond 350nm result from the f–f transition ab-sorption of Eu 3+
ions.So
this novel phosphor can be well excited by ultraviolet (394nm)and visible light (464nm),nicely in agreement with the widely applied UV LED or blue LED chips.
It is well known that Eu 3+absorbs no or less energy for certain excitation wavelengths,and therefore the efficient Eu 3+-activated phosphors mainly depend on the absorption of host and energy transfer efficiency [6].Remarkably,MoO 42−plays an important role in this novel phosphor.It absorbs the energy then transfers to Eu 3+,which increas the excited energy of Eu 3+and enhances the emission efficiency.That is to say,the novel phosphor is a kind of efficient luminescent material.
3.3.Effect of activator Eu 3+ion concentration
In order to investigate further the significant effect of the activator ions on the luminescence properties of the novel red phosphor,the relationship between emission intensity and concentration of activator ion Eu 3+is listed in Table 1.Gen-erally,energy migration process increa the probability
Table 1艾灸关元的功效和作用
Relative intensity of some emission peaks of Ca 1−x MoO 4:Eu x 3+phosphor with different Eu 3+concentrations x
Relative intensity of some emission peaks (excited by 464nm light source)536nm
610nm 612nm 616nm 0.18 1.20.859.60.20 1.53.220.138.30.22 2.84.950.873.70.24 4.917.292.71000.26 3.43.85054.10.28 1.52.636.364.10.30
0.3
15.1
11
6.2
始字开头的成语>盈余管理
Fig.4.Emission spectra of CaMoO 4:Eu 3+and sulfide phosphors (excited by 464nm light source).
that the optical excitation is trapping at defects or impurity sites,enhancing non-radiative relaxation.T
his caus con-centration quenching,becau an increa in the activator concentration encourages such non-radiative process.On the other hand,a decrea in the activator concentration de-creas the energy stored by the ions.Conquently,there is an optimum in the activator concentration,resulting from the trade-off of the above two factors.Seeing from Table 1,the favorable mole concentration of Eu 3+in CaMoO 4:Eu 3+phosphor is 24%.
Additionally,eing from Table 1,the 536nm emission from 5D 1is quenched with the increa in the Eu 3+concen-tration due to a cross-relaxation process (5D J →7D 0)→(7F 0→7F J )[7].At the same time,when the concentration of Eu 3+is up to 0.30,the highest peak moves to 610nm,and the relative intensity decreas sharply.On the other hand,when the concentration of Eu 2O 3is beyond 0.28,the phos-phor body color is slightly yellow.
Fig.5.Emission spectra of CaMoO 4:Eu 3+and sulfide phosphors (excited by 394nm light source).
Y.Hu et al./Journal of Alloys and Compounds390(2005)226–229229
3.4.Comparison of emission characteristics of
CaMoO4:Eu and conventional sulfide phosphor
As a novel red phosphor,the prent synthesized phosphor has higher emission intensity when compared with the con-ventional sulfide ud as red phosphor in white LEDs under the same excitation intensity,as shown in Figs.4and5.The sulfide phosphor is activated by divalent Eu ion and exhibits broadband emission with the highest peak in the vicinity of 650nm.When the excitation wavelength is464nm,the for-mer is higher by15%than the latter on the relative intensity of the highest peaks;while the excitation wavelength turns to394nm,the latter’s emission intensity is only20%of the former’s.
4.Conclusions
In conclusion,CaMoO4:Eu3+has been successfully de-veloped.It can be excited efficiently by UV and visible light and emits the red light with line spectrum,which is in agree-ment with the UV and blue LED chips.Compared with the sulfides that are ud now as red phosphors in white LEDs, this novel phosphor has stronger emission intensity under the excitation of464nm blue light,and has much stronger emis-sion intensity under the excitation of394nm UV light.Most importantly,the novel phosphor is much more stable than sulfide phosphors.
Acknowledgements
This work isfinancially supported by the National Natural Science Foundation of China(under grant no.50372086)and the National Hi-Tech Rearch and Development Program of China(863program,under grant no.2001AA313140). References
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