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Materials Letters
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Effect of Er3+ concentration on the upconversion luminescence of GdOCl:Er3+
powders with excitation of 514.5 nm
Yong Li a,⁎, Xiantao Wei b, Bihai Tong c, Yi Liu a, Qing Zhang a, Wenbin Sun a, Min Yin b
a School of Mathematics and Physics, Anhui University of Technology, Maanshan 243002, Chinab Department of Physics, University of Science and Technology of China, Hefei 230026, Chinac College of Metallurgy and Resources, Anhui University of Technology, Maanshan 243002, China
Article history:Received 27 March 2012Accepted 23 April 2012Available online 30 April 2012
Keywords:LuminescencephosphorsGdOClSolid state reactionUpconversion
GdOCl phosphors doped with different Er3+ concentration were prepared by modified solid state reaction. Thestructure and purity of the powders were analyzed by X-ray diffraction. Upconversion emissions from blue to vi-olet to ultraviolet regionswere observedwith excitation of 514.5 nm. Power-dependence analysis demonstratedthat the two-photon upconversion process populated the luminescent levels 4G11/2, 2P3/2, 2H9/2, and 4F3/2, 5/2. Theenergy distribution of upconversion spectra depends on doping concentration, and the intensity ratio betweeninvolved energy levels changed with increasing Er3+ concentration. The dependence of spectral distributionon concentration was possibly ascribed to cross relaxation process of 2P3/2+4I15/2→ 4I13/2+2H9/2. Besides thecross concentration, excited state absorption and energy transfer upconversionwere proposed to be the possiblemechanisms for upconversion.
Recently, upconversion (UC) properties of inorganic luminescentmaterials have beenwidely investigated for potential application in dif-ferent areas [1–3], especially the near infrared to visible andultraviolet UC of materials doped with rare earth ions [4]. The choiceof appropriate host lattice is one of main points to be considered. Theproperties of host material have a strong influence on the UC process[2]. Lanthanide oxychlorides (LnOCl, Ln=La, Y, Gd), an intermediatehost between oxides and chlorides, have gradually been paid muchmore attention as hosts for UC [5–7], because they combine the advan-tages of both chlorides and oxides, such as low phonon cutoff energyand high chemical stability. But aswe all know, lanthanide oxychloridesdopedwith rare earth ions have been extensively used as X-ray intensi-fying phosphors in recent years [8–10], and luminescent properties ofTb3+, Sm3+, and Eu3+ doped LaOCl phosphors were studied dueto their potential applications in field emission displays [5,11,12]. How-ever, the research on UC luminescence of LnOCl doped with rare earthions is quite rare.
Among lanthanide oxychlorides, Gadolinium oxychloride (GdOCl)has low phonon cutoff energy according to Raman spectroscopy analy-sis [7], and it is easily to be doped with upconverted rare-earth activa-tors such as Er3+ and Ho3+. Thus GdOCl host would be suitable forUC. In our previous research work [7], Er3+ doped and Yb3+–Er3+
codoped GdOCl were prepared and their infrared-to-visible UC
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properties with excitation of 980 nm were investigated. In this work,the UC properties of GdOCl powder from blue to violet to ultraviolet re-gions under excitation of 514.5 nmwere reported and physical mecha-nisms and concentration dependence of UC were discussed in detail.
2. Experimental
The Er3+-doped GdOCl powders were prepared by modified solidstate reaction. It should be noted that the modified solid state reactionis different from conventional solid state reaction and wet chemicalmethods, the advantages of the both methods are combined in the pro-cesses ofmodified solid state reaction [7]. In thefirst stage of its process,modified solid state reaction offers the molecular-level mixing, andthen the product can be obtained after heat treatment is carried out be-tween the precursors in solid state form. The preparation process is de-scribed as follow: a certain amount of Gd2O3 (purity, 99.99%) and Er2O3
(99.99%) were dissolved in dilute nitric acid under vigorous stirring.After mixing nitrate solutions with the required ratio, an excess ofNH4Cl was added to the mixed nitrate solution, and the solution wasstirred and dried in 80 °C water bath for 12 hours to get rid of H2O.The dried precursor were ground and sintered in reducing atmospheresat 1000 °C for 1 hour. The Er3+-doped GdOCl with different concentra-tions were obtained. The prepared samples appeared to be white inbody.
The phase structure was analyzed by a rotating anode X-ray diffrac-tometer (MAC Science Co. Ltd MXP18AHF) under Cu Kα radiation. Themorphology and size of sampleswere examinedwith scanning electronmicroscope (SEM). Using a 514.5 nm Ar-ion laser as an excitation
Fig. 2. SEM image of the GdOCl powders prepared at 1000 °C.
185Y. Li et al. / Materials Letters 80 (2012) 184–186
source, the UC luminescence spectra were recordedwith a LABRAM-HRlaser Raman spectrometer at room temperature.
3. Results and discussion
The structure and purity of the products are confirmed by X-ray dif-fraction (XRD) patterns. As shown in Fig. 1, the crystal structure ofsamples prepared by modified solid state reaction is consistent withJCPDS Card 85-1199, and there are no other diffraction peaks in theXRD patterns. The GdOCl has PbFCl-type structure with a space groupP4/nmm, and its structure comprises distinct covalent (GdO)nn+complexcation and Cl
–
anion layers. In this structure, the Gd3+ ion is coordinatedwith four oxygens and five chlorines forming a monocapped tetragonalantiprism as the coordination polyhedron [13]. The dopant Er3+ ionswould replace Gd3+ ions and thus have a C4ν site symmetry. The SEMimage of the GdOCl phosphor prepared at 1000 °C for 1 h is presentedin Fig. 2. It can be seen that the sample shows plate-like particles withvarious diameters from 100 to 2000 nm, and there is a small amountof agglomeration in the particles.
The GdOCl powders doped with different Er3+ concentration arepumped by the 514.5 nm Ar+ laser and the UC emission is observed,as shown in Fig. 3(a, b, c). Five emissions of Er3+ ion are observed: theluminescence in the ultraviolet region from 370 to 390 nm is assignedto 4G11/2→
4I15/2 transition and the violet emissions correspond to2P3/2→ 4I13/2 (400–405 nm) and 2H9/2→
4I15/2 (405–420 nm) transi-tions. The observed bands in the blue region can be ascribed to theemission of 4F3/2, 5/2→
4I15/2 (450–460 nm), 2P3/2→ 4I11/2 (465–480 nm). With Er3+ content increasing, the UC intensity of Er3+ emis-sion increases rapidly, and concentration quenching does not happeneven when the Er3+ content is up to 8 mol%. Actually, not only the UCintensity but also the distribution of energy in emission spectra varieswith the doping content of Er3+ ions. In order to investigate well thechanges in spectral distribution with the Er3+ concentration, the emis-sion spectra are normalized by the intensity of 2H9/2→
4I15/2 transition(408 nm) in Fig. 3(d, e, f). It is clear that different doping concentrationinfluences relative intensities of emission peaks. For example, the rela-tive intensities of 4G11/2→
4I15/2 and 2P3/2→ 4I11/2, 13/2 to 2H9/2→4I15/2
emissions increase with the Er3+ content.In order to understandwell the physicalmechanisms responsible for
the observedUC luminescence, the power dependence of the intensitiesof UC emissions is presented in Fig. 4 by a log-log plot. Generally, the UCemission intensity I is propositional to the n-th power of the laserpumping power P, which is I ∝ P n. Here, n represents the number ofpump photons absorbed per photon emitted. According to Fig. 4, thecalculated slope n values are 1.45±0.01, 1.41±0.07, and 1.66±0.02for the emissions at 387 (4G11/2→
4I15/2), 407 (2H9/2→4I15/2), and
Fig. 1. XRD patterns of GdOCl powders prepared at 1000 °C.
476 nm (2P3/2→ 4I11/2), respectively. These results indicate that a two-photon process is responsible for populating the luminescent levels4G11/2, 2H9/2, and 2P3/2.
In Fig. 5, the UC luminescence mechanisms of GdOCl:Er3+ powdersare studied. The excitation wavelength 514.5 nm is resonant withtransition of 4I15/2→ 4F7/2, 2H11/2. First, Laser brings Er3+ ions into the4F7/2 or 2H11/2 levels with assistance of phonons, which is known asground-state absorption (GSA). After the process of excited state absorp-tion (ESA) fromexcited level 2S3/2 to 4G9/2 level, the Er3+ ion relaxes non-radiatively from 4G9/2 level to the 4G11/2, 2P3/2, 2H9/2, and 4F3/2,5/2luminescent levels, and then the 4G11/2→
4I15/2, 2P3/2→ 4I11/2,13/2, 2H9/2
→ 4I15/2, and 4F3/2, 5/2→ 4I15/2 emissions occur after two photon absorp-tion. Besides ESA process, there is another process being familiar as ener-gy transfer UC (ETU) to populate 4G9/2 level. In the ETUprocesswhich canbe expressed as: 4S3/2+4S3/2→ 4I15/2+4G9/2, the Er3+ ion in 4S3/2 statenonradiatively returns to the ground state 4I15/2 and immediately trans-fers its energy to the neighboring ion in the same state and excites it tothe upper 4G9/2 state. It should be noted that ETU involves two ions andwould be dominant in samples with high Er3+ concentrations, whilethe ESA involves a single ion and thus it is the main possible UC processwhen the doping concentration is low. If only the above-mentionedprocesses exist, the spectral distribution should not depend on the dopingEr3+ content. However, the exact opposite is true. The dependence ofspectral distribution on doping concentration may be explained if oneconsiders the presence of a cross-relaxation (CR) mechanism. The CR
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 3. UC spectra and normalized UC spectra of Er3+ doped GdOCl powders with differ-ent Er3+ concentration: (a, d) 8 mol%; (b, e) 4 mol%; (c, f) 1 mol%.
Fig. 5. Energy level diagrams of Er3+ and the proposed UC mechanism.
186 Y. Li et al. / Materials Letters 80 (2012) 184–186
process can be expressed as 2P3/2+4I15/2→ 4I13/2+2H9/2. This CR processinvolves populating 2H9/2 level by bypassing the other two luminescentlevels 2P3/2 and 4G11/2, resulting in change of relative population of theinvolved energy levels. Thus the intensity ratios of 4G11/2→
4I15/2and 12P3/2→ 4I11/2, 13/2 to 2H9/2→
4I15/2 emissions decrease with increas-ing the Er3+ content. The CR process also involves the interaction be-tween Er3+ ions. With increasing the doping content, the probability ofthe CR process enhances, and its influence becomes stronger in samplesdoped with higher Er3+ concentration.
4. Conclusions
In summary, Er3+ doped GdOCl powders are prepared by modifiedsolid state reaction. UC emissions of the samples from blue to violet toultraviolet regions with 514.5 nm laser excitation are reported and dis-cussed. Power dependence analysis reveals that the blue to ultravioletemissions originate from a two-photon UC process. The Er3+ contenthas a strong impact on the energy distribution of UC spectra. The CRprocess of 2P3/2+4I15/2→ 4I13/2+2H9/2 is the possible mechanisms ofthis behavior.
Acknowledgement
This work was supported by the Provincial Natural Science Re-search Program of higher education institutions of Anhui province(KJ2012Z034), Student Research Training Program Foundation ofAnhui University of Technology, the Key Lab of Novel Thin FilmSolar Cells, CAS (KF201101), National Nature Science Foundation ofChina (11074245, 10904139, 11111120060, and 50903001), NaturalScience Foundation of Chongqing (CSTC-2010BA4009). X. T. Wei ac-knowledges financial support from the China Postdoctoral ScienceFoundation (20100480693 and 201104334).
References
[1] Liu XG,Wang F. Recent advances in the chemistry of lanthanide-doped upconversionnanocrystals. Chem Soc Rev 2009;38:976–89.
[2] Haase M, Sch fer H. Upconverting Nanoparticles. Angew Chem Int Ed 2011;50:5808–29.
[3] Chen Z, Gong W, Chen T, Li S, Wang D, Wang Q. Preparation and upconversion lu-minescence of Er3+/Yb3+ codoped Y2Ti2O7 nanocrystals. Mater Lett 2012;68:137–9.
[4] Wang F, Han Y, Lim CS, Lu Y, Wang J, Xu J, et al. Simultaneous phase and size controlof upconversion nanocrystals through lanthanide doping. Nature 2010;463:1061–5.
[5] Kort KR, Banerjee S. Shape-controlled synthesis of well-defined matlockite LnOCl(Ln: La, Ce, Gd, Dy) nanocrystals by a novel non-hydrolytic approach. Inorg Chem2011;50:5539–44.
[6] Konishi T, Shimizu M, Kameyama Y, Soga K. Fabrication of upconversion emissiveLaOCl phosphors doped with rare-earth ions for bioimaging probes. J Mater SciMater Electron 2007;18:183–6.
[7] Li Y, Wei X, Yin M. Synthesis and upconversion luminescent properties of Er3+
doped and Er3+–Yb3+ codoped GdOCl powders. J Alloys Compd 2011;509:9865–8.
[8] Rambabu U, Sainkar SR, Hussain NS, Buddhudu S. The effect of zinc ions on the fluo-rescence spectra of (La, Zn)OF:Sm3+ powder phosphors. Solid state Commun1999;110:685–90.
[9] Park JK, Lim MA, Kim CH, Park HD, Han CH, Choi SY. Luminescence properties ofLaOX (X= F, Cl, Br): Eu phosphors. J Mater Sci Lett 2003;22:477–8.
[10] Rambabu U, Annapurna K, Balaji T, Buddhudu S. Fluorescence spectra of Er3+:REOCl (RE=La, Gd, Y) powder phosphors. Mater Lett 1995;23:143–6.
[11] Li GG, Li CX, Hou ZY, Peng C, Cheng ZY, Lin J. Nanocrystalline LaOCl:Tb3+/Sm3+ aspromising phosphors for full-color field-emission displays. Opt Lett 2009;34:3833–5.
[12] Lee S-S, Park H-I, Joh C-H, Byeon S-H. Morphology-dependent photoluminescenceproperty of red-emitting LnOCl:Eu (Ln=La and Gd). J Solid State Chem 2007;180:3529–34.
[13] Lee J, Zhang Q, Saito F. Mechanochemical synthesis of LaOX (X=Cl, Br) and theirsolid state solutions. J Solid State Chem 2001;160:469–73.
