Xueyuan Chen-Optical Spectroscopy of Rare Earth Ion-Doped-JNN2010

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Copyright © 2010 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol. 10, 1482–1494, 2010

Optical Spectroscopy of Rare Earth Ion-DopedTiO2 Nanophosphors

Xueyuan Chen∗ and Wenqin LuoKey Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the

Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China;State Key Laboratory of Structural Chemistry, Fuzhou, Fujian 350002, China

Trivalent rare-earth (RE3+� ion-doped TiO2 nanophosphors belong to one kind of novel optical mate-rials and have attracted increasing attention. The luminescence properties of different RE3+ ions invarious TiO2 nanomaterials have been reviewed. Much attention is paid to our recent progresseson the luminescence properties of RE3+ (RE= Eu, Er, Sm, Nd) ions in anatase TiO2 nanoparticlesprepared by a sol–gel-solvothermal method. Using Eu3+ as a sensitive optical probe, three signif-icantly different luminescence centers of Eu3+ in TiO2 nanoparticles were detected by means ofsite-selective spectroscopy at 10 K. Based on the crystal-field (CF) splitting of Eu3+ at each site,C2v and D2 symmetries were proposed for Eu3+ incorporated at two lattice sites. A structural modelfor the formation of multiple sites was proposed based on the optical behaviors of Eu3+ at differentsites. Similar multi-site luminescence was observed in Sm3+- or Nd3+-doped TiO2 nanoparticles.In Eu3+-doped TiO2 nanoparticles, only weak energy transfer from the TiO2 host to the Eu3+ ionswas observed at 10 K due to the mismatch of energy between the TiO2 band-gap and the Eu3+

excited states. On the contrary, efficient host-sensitized luminescences were realized in Sm3+- orNd3+-doped anatase TiO2 nanoparticles due to the match of energy between TiO2 band-gap andthe Sm3+ and Nd3+ excited states. The excitation spectra of both Sm3+- and Nd3+-doped samplesexhibit a dominant broad peak centered at ∼340 nm, which is associated with the band-gap ofTiO2, indicating that sensitized emission is much more efficient than direct excitation of the Sm3+

and Nd3+ ions. Single lattice site emission of Er3+ in TiO2 nanocrystals can be achieved by modi-fying the experimental conditions. Upon excitation by a Ti: sapphire laser at 978 nm, intense greenupconverted luminescence was observed. The characteristic emission of Er3+ ions was obtainedboth in the ultraviolet-visible (UV-vis) and near-infrared regions through the high-resolution exper-iments at 10 K. The CF experienced by Er3+ in TiO2 nanocrystal was systematically studied bymeans of the energy level fitting.

Keywords: Rare-Earths, Anatase TiO2, Luminescence, Nanocrystals.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14822. Photoluminescence Properties of RE3+:TiO2 Nanocrystals . . . . 1483

2.1. Eu3+-Doped TiO2 Nanocrystals . . . . . . . . . . . . . . . . . . . . . 14832.2. Er3+-Doped TiO2 Nanocrystals . . . . . . . . . . . . . . . . . . . . . 14892.3. Sm3+-Doped TiO2 Nanocrystals . . . . . . . . . . . . . . . . . . . . . 14922.4. Other RE3+ Ion-Doped TiO2 Nanocrystals . . . . . . . . . . . . . 1492

3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1493Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1493References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1493

1. INTRODUCTION

Trivalent rare-earth (RE3+� ion-doped semiconductornanophosphors belongs to one kind of novel optical

∗Author to whom correspondence should be addressed.

materials that has potential applications in the fields ofoptical communications, optoelectronic devices, flat paneldisplays, and biosensors.1–7 Titania is a well-known wideband-gap semiconductor (band-gap of 3.2 eV for anatase)and a good candidate to be used as the host material ofRE ions because of its good mechanical, optical, and ther-mal properties. One of the advantages offered by thesematerials is the ability to tailor their optical properties viasize control and to achieve highly efficient luminescencethrough the sensitization by the host. Moreover, they pro-vide the possibility of excitation with electrical current.Due to a large mismatch in ionic radius between RE3+

and Ti4+ and the charge imbalance, it is believed that theincorporation of RE3+ ions into the TiO2 lattice is verydifficult via a direct chemical way. As a consequence, inmost cases, only broadened luminescence lines of RE3+

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Chen and Luo Optical Spectroscopy of Rare Earth Ion-Doped TiO2 Nanophosphors

ions were observed due to poor incorporation of RE3+ ionsinto the titania lattice. How to effectively accommodateRE3+ ions in TiO2 systems is a precondition to achievinghigh luminescence performance of this kind of materials.In the past few years, many efforts have been made tosynthesize RE3+ ion-doped TiO2 nanomaterials in order torealize intense host-sensitized RE3+ emissions.8–19 Theseresults indicated that RE3+ ions could be introduced intothe TiO2 nanocrystals and energy transfer (ET) from thehost to RE3+ ions might be achievable if the synthesismethod is well designed.In this paper, photoluminescence (PL) properties of

RE3+ ions in TiO2 nanocrystals have been reviewed. Muchattention is focused on a new approach recently devel-oped by us to incorporate RE3+ ions in TiO2 nanolat-tices. Recent progress on the optical spectroscopy of RE3+

ions (RE = Eu, Sm, Nd and Er) in anatase TiO2 nano-crystals prepared by the sol–gel-solvothermal method hasbeen addressed in detail.

2. PHOTOLUMINESCENCE PROPERTIES OFRE3+:TiO2 NANOCRYSTALS

2.1. Eu3+-Doped TiO2 Nanocrystals

Eu3+ is a well known excellent red luminescence centerand an ideal optical probe for fundamental understanding

Xueyuan Chen received his B.Sc. degree in Material Chemistry from University of Scienceand Technology of China, Hefei, China, in 1993 and his Ph.D. in Physical Chemistry fromFujian Institute of Research on the Structure of Matter (FJIRSM), Chinese Academy ofSciences, Fuzhou, China, in 1998. His Ph.D. work focused on the optical spectroscopy andcrystal-field analysis of rare earth doped laser crystals. He received an outstanding Presidentaward from the Chinese Academy of Sciences in 1997. After receiving his doctorate hejoined FJIRSM as a research scientist and conducted research in the fields of solid-state laserand luminescent materials from 1998 to 2001. From 2001 to 2005, he was a postdoctoralresearch associate at the Chemistry Division of Argonne National Laboratory, U.S. Depart-ment of Energy, where he studied the photophysics and photochemistry of heavy elementswith an emphasis on the laser spectroscopy of lanthanide-doped nanomaterials. In 2005, he

joined the faculty at FJIRSM, where he is currently research professor of Optoelectronic Materials Chemistry and Physics.Chen’s recent research interest is focused on the synthesis, characterization, and optical spectroscopy of rare earth ionsembedded in the low-dimensional nanomaterials including insulator, semiconductor and core-shell nanocrystals. Dr. Chenhas authored more than 60 journal publications, six book chapters, co-author of a book on the spectroscopy of laser andluminescent materials, and seven Chinese patents related to solid-state lasers and nanotechnology.

Wenqin Luo received his B.Sc. degree in Chemistry from Xiamen University, Xiamen,China, in 2002 and his M.S. degree in Inorganic Chemistry from Fujian Institute of Researchon the Structure of Matter (FJIRSM), Chinese Academy of Sciences, China, in 2005. He iscurrently pursuing his Ph.D. in Materials Chemistry and Physics at FJIRSM. His researchcenters on synthesis, characterization, and optical spectroscopy of rare earth ions dopedsemiconductor nanocrystals. He has authored or co-authored more than 10 SCI journalpapers on rare earth spectroscopy.

of nano-phenomena. Thus, among various RE3+ ions,Eu3+-doped TiO2 nanomaterials have gained the mostextensive attention.8–12�20–28 Eu3+-doped TiO2 thin filmshave been prepared by the sol–gel method on silicon andcorning glass substrates.8�20 The substrates of the thin filmswere found to significantly affect the crystallization andPL performance of the samples. The films grown on glasswere amorphous, and those grown on Si were polycrys-talline. Upon the excitation by a 325 nm laser beam, Eu3+

ions embedded in both samples exhibited broad emissionlines from the 5D0 →7FJ (J = 0�1�2�3�4) (Fig. 1). More-over, for the same nominal concentration of Eu3+ and sim-ilar film thickness, a higher PL intensity was observedfor the film deposited on Si than that on glass substrate.A possible explanation of this effect was ascribed to thenon-negligible contribution from the reflected light of thePL signal at the TiO2/Si interface, which was not presentin the transparent TiO2/glass interface.

8 However, the con-tribution of better crystallinity of the sample deposited onSi to the improvement of PL intensity should also be takeninto account.A cubic mesostructured matrix of titania with a three-

dimensional array of embedded anatase nanocrystals wasused to accommodate RE3+ ions, and the sensitized RE3+

emissions by the TiO2 host were obtained.9�29 The exci-tation and emission spectra of Eu3+ (8 mol%) ion-dopedmesoporous titania films are shown in Figures 2(a and b).

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Optical Spectroscopy of Rare Earth Ion-Doped TiO2 Nanophosphors Chen and Luo

Fig. 1. PL spectra of the TiO2:Eu film deposited on a silicon substrateobtained under the excitation at 325 nm both at RT and 12 K. Similarresults were obtained for films deposited on glass. Reprinted with per-mission from [8], A. Conde-Gallardo et al., Appl. Phys. Lett. 78, 3436(2001). © 2001, American Institute of Physics.

Upon excitation above the titania band-gap, intense redemissions due to the intra-4f transitions of Eu3+ wereobserved. The obtained emission lines were unresolved asa result of the location of Eu3+ ions in amorphous titaniaregions. The excitation spectra obtained by monitoring the5D0 →7F2 transition of Eu3+ at 614 nm exhibited a broadband at 330 nm, which was associated with the band-gapof titania films, whereas excitation lines from the intra-4ftransitions of Eu3+ could not be detected, indicating thatthe main contribution to the excitation was from the band-gap of titania. Figure 3 shows a photograph of a silica film(left) and a titania film (right) both doped with 8 mol%Eu3+ and excited with an ultraviolet (UV) lamp at 300 nm.Strong red emission from the europium ion was observedfrom the titania film. In contrast, the silica film, which is

Fig. 2. PL excitation (a) and PL (b) spectra of a cubic mesoporous TiO2

thin film doped with 8 mol% Eu3+. Reprinted with permission from [9],K. L. Frindell et al., Angew. Chem. Int. Ed. 41, 959 (2002). © 2002,Wiley-VCH Verlag GmbH & Co. KGaA.

Fig. 3. Photograph of mesoporous silica (left) and titania (right) films,both doped with 8 mol% Eu3+. The films were excited with a UV lamp at300 nm. Reprinted with permission from [9], K. L. Frindell et al., Angew.Chem. Int. Ed. 41, 959 (2002). © 2002, Wiley-VCH Verlag GmbH &Co. KGaA.

unable to absorb at this wavelength, showed no red Eu3+

emission.A similar amorphous-crystalline two-phase structure

was also used to accommodate Eu3+ ions in monodispersespherical mesoporous TiO2 particles prepared by a sim-ple nonionic surfactant-assisted soft-chemistry method.15

The room temperature (RT) PL spectra of the Eu3+-dopednanoparticles with different heat treatments are comparedin Figure 4. Under UV excitation at 360 nm, broad emis-sion lines from the excited state of 5D0 to the 7F1,

7F2, and7F3 states can be observed. As shown in Figure 4, the sam-ple calcined at 400 �C shows more efficient luminescencethan the as-made sample without further calcination. Animportant factor for the increase of the sample’s lumines-cence intensity may be the formation of titania nanocrys-tallites in the pore walls under calcination, which play the

Fig. 4. RT PL spectra of the mesoporous Eu-doped TiO2 phosphor par-ticles (a) before calcination, (b) after 400 �C calcination, and (c) after500 �C calcination. (The inset is the emission photograph of the cor-responding samples.) Reprinted with permission from [15], J. B. Yinet al., Appl. Phys. Lett. 90, 113112 (2007). © 2007, American Instituteof Physics.

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role of sensitizer to transfer the absorbed excitation energyto Eu3+ ions. When the sample was further annealed at500 �C, the PL intensity was found to decrease. This maybe due to the fact that the amorphous titania was com-pletely transformed into anatase, and thus, the previouslywell dispersed Eu3+ ions tended to aggregate, and the PLof Eu3+ was quenched.Layered titanate oxides intercalated with hydrated Eu3+

ions have been synthesized by electrostatic self-assemblydeposition (ESD) and the layer-by-layer assembly (LBL)method.21�23 ET from TiO layer to intercalated Eu3+ ionshas been achieved in this material. The interlayer waterthat contributed to ET plays a key role in the sensi-tized emission of Eu3+ ions. As shown in Figure 5, theexcitation spectra obtained by monitoring the 5D0 →7F2

transition at 614 nm exhibited a broad band at 250–350 nm, which was associated with the band-gap of thetitanate nanosheet. In addition, the peak at 395 nm dueto the 7F0 →5L6 intra-4f transition of Eu3+ ions wasalso observed. Upon excitation above the TiO band-gap at300 nm, emission lines at 570, 593, and 614 nm assignedto the transitions from 5D0 to 7F0,

7F1, and7F2, respec-

tively, were detected. The intensities of the excitationand emission peaks of the as-deposited Eu/TiO film werestronger than those of Eu/TiO film treated at 100 �C for1 h due to the elimination of some of the interlayer waterby heat treatment. Emission intensity and layer distancewere also found to decrease with the decrease in humidityfor the same reason, as illustrated in Figure 5.Xin et al. reported an exfoliation-restacking route to

accommodate RE3+ ions in the lamellar aggregates oftitania nanosheets.30 The PL spectra of ex-Ti0�91O2/H,ex-Ti0�91O2/Tb and ex-Ti0�91O2/Eu (“ex- ” means “exfoli-ated”) samples are shown in Figure 6. Under the excitationat 250 nm, the ex-Ti0�91O2/Eu sample emitted light both

Fig. 5. RT excitation (�em = 614 nm) and emission (�ex = 300 nm)spectra of Eu/TiO films of as-deposited, treated at 100 �C for 1 h andunder 5% humidity for 2 days, respectively. Reprinted with permissionfrom [23], S. Ida et al., J. Phys. Chem. B 110, 23881 (2006). © 2006,American Chemical Society.

Fig. 6. PL and PL excitation spectra of (a) ex-Ti0�91O2/H, (b) ex-Ti0�91O2/Tb, and (c) ex-Ti0�91O2/Eu. The excitation spectra were measuredby monitoring the emissions at 395, 544, and 612 nm for ex-Ti0�91O2/H,ex-Ti0�91O2/Tb, and ex-Ti0�91O2/Eu, respectively. The excitation wave-lengths are also shown. Reprinted with permission from [30], H. Xinet al., Appl. Phys. Lett. 85, 4187 (2004). © 2004, American Institute ofPhysics.

from the Ti0�91O2 host and Eu3+ ions, whereas only Ti0�91O2

host emission was detected for the ex-Ti0�91O2/Tb sam-ple. Xin et al. ascribed the broad band peaking at 250 nm(Fig. 6(c)) to the Ti0�91O2 host absorption. However, theband-gap of Ti0�91O2 host was determined to be 380 nm byultraviolet-visible (UV-vis) absorption spectra, which devi-ates significantly from 250 nm. Instead, the charge trans-fer band between Eu3+ and O2− ions (namely, an electrontransferring from the O2− (2P6� orbital to the empty orbitalof Eu3+ (4f 6�) usually locates at 200–290 nm.27 Thereforeit is possible that the observed excitation band between220 and 380 nm resulted from the Eu3+−O2− charge trans-fer in ex-Ti0�91O2/Eu. However, their conclusion on the ETfrom the Ti0�91O2 nanosheet to Eu3+ ions on the basis ofthe observation of Eu3+ emissions under the excitation at250 nm requires more solid evidence.Recently, Wang et al. synthesized Eu3+-doped nanocrys-

talline titania microspheres by ultrasonic spray pyrolysis



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Fig. 7. (a) SEM image. (b) TEM image. (c) SAED pattern recordedon the sphere shown in (b) of Eu3+ (8 mol%) doped nanocrystallineTiO2 microspheres. (d) PL spectra of the spheres doped with varyingamounts of Eu3+ under the excitation at 330 nm. (e) Corresponding PLexcitation spectra by monitoring the 614 nm emission. The samples wereheat treated at 400 �C. Reprinted with permission from [18], L. Li et al.,Adv. Mater. 20, 903 (2008). © 2008, Wiley-VCH Verlag GmbH & Co.KGaA.

(USP) and the solvent evaporation-induced self-assemblymethod.18 As shown in Figure 7, the TiO2:Eu

3+ (8 mol%)sample heated at 400 �C shows spherical morphology withdiameters ranging from 0.1 to 1.4 �m. The spherical parti-cles exhibit porous structures (Fig. 7(b)) and anatase poly-crystalline nature (Fig. 7(c)). As shown in Figure 7(d),emission bands centered at 590, 614, 651, and 698 nm cor-responding to the characteristic 5D0 →7FJ (J = 1�2�3�4)transitions of Eu3+ were observed under the excitationat 330 nm. These lines are inhomogeneously broadened,suggesting that Eu3+ ions were located in much distortedcrystal field (CF) environments. Due to the porous struc-ture of the sample, high doping concentration of Eu3+

up to 16 mol% can be achieved. The excitation spectraobtained by monitoring the 5D0 →7F2 transition at 614 nm(Fig. 7(e)) exhibited a broad band at around 330 nm, whichwas associated with the band-gap absorption of TiO2 host,verifying an effective ET from the TiO2 host to the Eu3+

ions.Li and coworkers have reported the luminescence

properties of Eu3+-doped TiO2 nanocrystals synthesizedby Ar/O2 radio frequency thermal plasma oxidation ofliquid precursor mists.11 The obtained products werethe mixtures of the anatase (30–36 nm) and rutile(64–83 nm) polycrystallinity. The PL and PL excitation(PLE) spectra of 0.5 mol% Eu3+-doped and pure TiO2

nanocrystals are shown in Figure 8. The excitation spec-trum of TiO2:Eu 0.5 mol% by monitoring the 5D0 →7F2

transition at 617 nm (Fig. 8(a)) exhibited a broad bandcentered at 360 nm in addition to the characteristicf –f absorptions of Eu3+ at 416, 467, and 538 nm. Thebroad peak at 360 nm could be ascribed to TiO2 band-gapabsorption, suggesting that Eu3+ ions can be effectively

Fig. 8. Excitation and emission spectra of the nanopowders: (a) excita-tion spectrum obtained by monitoring the 617 nm emission of 0.5 mol%Eu3+-doped TiO2; (b–d) emission spectra with sample composition andexcitation wavelength indicated. Direct comparison of the emission inten-sities can be made among parts (b–d). Reprinted with permission from[11], J.-G. Li et al., J. Phys. Chem. B 110, 1121 (2006). © 2006,American Chemical Society.

excited through the TiO2 host lattice. Upon excitationabove the TiO2 band-gap at 360 nm, the Eu3+-dopednanopowder exhibits characteristic emissions in the rangeof 550–750 nm with the dominant emission at 617 nm.Besides, broad emission lines at around 400 nm arisingfrom TiO2 host (Fig. 8(b)) were also detected. Subse-quently, the defect-mediated PL dynamics of Eu3+-dopedTiO2 nanocrystals was revealed at the single-particle orsingle-aggregate.26 As shown in Figure 9(A), a numberof spots with various intensities were observed. Interest-ingly, a dramatic increase in PL intensity was observedon changing the atmosphere from air to Ar during laserirradiation of the samples. Figure 9(B) shows the typicalPL spectra of an individual luminescent spot below thediffraction limit of about 150 nm for TiO2:Eu nanopar-ticles in ambient conditions. Characteristic Eu3+ emis-sions from 5D0 to 7F1 (590 nm) and 7F2 (615 nm)were identified. As shown in Figure 9(C), a wide R dis-tribution was obtained, where R is the relative inten-sity (area) ratio of 5D0 →7F2 to 5D0 →7F1, suggestingthat the local environment around the doped Eu3+ ionsin TiO2 is quite different between individual nanoparti-cles. As shown in Figure 9(D), growth of a broad PLband in the visible region (500–750 nm) originating fromthe oxygen-vacancy-related defects was observed during



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(A)

(B)

(D) (E)

(C)

Fig. 9. (A) Typical PL image observed during 405 nm laser excitation of TiO2:Eu3+ nanoparticles (or aggregates) under air (left) and Ar (right)

atmospheres. (B) Typical PL spectra of individual TiO2:Eu3+ nanoparticles (or aggregates) in ambient air. (C) Histograms of the R values obtained

for single TiO2:Eu3+ nanoparticles (or aggregates) spin-coated on the cover glass (dry sample; RH ≈ 40%) (a), embedded in a PVA film (b), and

surface-modified nanoparticles embedded in a PS film (c). Average R values were determined from the Gaussian fits. (D) Time evolutions of thePL spectra obtained during 405 nm laser excitation for a single undoped TiO2 nanoparticle (or aggregate) under an Ar atmosphere. The dotted linesindicate the Gaussian distributions fitted to the spectra (see text for details). (E) Trajectories of PL intensities under Ar (a) and air (b) atmospheres (bintime is 33 ms). Solid lines indicate the kinetic traces calculated by Eq. (1). Reprinted with permission from [26], T. Tachikawa et al., Angew. Chem.Int. Ed. 47, 5348 (2008). © 2008, Wiley-VCH Verlag GmbH & Co. KGaA.

405-nm laser excitation for a single undoped TiO2

nanoparticle (or aggregate) under an Ar atmosphere. Thekinetics of color centers under visible-light irradiation wasstudied. As shown in Figure 9(E), the observed trajectoriesof the PL intensity were fitted well by Eq. (1):

N�t�= 1k+/k−− �k+/k−−1/N0� exp�−k+t�

(1)

where N is the number of color centers, k+ is the rate offormation of color centers, k− is the rate of deactivation ofcolor centers, and N0 is the number of color centers thatexist prior to irradiation. The k+ value was determined tobe 0�05± 0�01 s−1 for the photoactivation and deactiva-tion processes, while the k− values were 0�5±0�2 for the

photoactivation process and 15±5 s−1 for the deactivationprocess. The remarkable difference in k− was ascribed tothe different oxygen concentration in the gas phase.Recently, we have provided spectroscopic evidence of

the multiple site structure of Eu3+ ions incorporated inthe 8–12 nm TiO2 nano-aggregates.25 By means of site-selective spectroscopy at 10 K, as shown in Figure 10,three kinds of luminescence sites of Eu3+ were identi-fied. Site I exhibits broadened fluorescence lines with mostintense emission at 613.3 nm similar to that of Eu3+ ionin amorphous phase, which is ascribed to the distorted lat-tice sites near the surface. Sites II and III exhibit sharpemission and excitation lines with the most intense emis-sion lines at 616.7 for site II and 618.1 nm for site III,



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600 650 700

In

tens

ity (

arb.

uni

ts)

Wavelength (nm)

10K Emission

(X8)

(d)

(c)

(b)

(a)

7F07F1

7F27F3

7F4

Fig. 10. The 10 K emission spectra of Eu3+:TiO2 (2 mol%) annealedat 400 �C, with (a) �exc = 343�0 nm, corresponding to the band-gap exci-tation; (b) �exc = 464�6 nm for site I; (c) �exc = 470�7 nm for site II;and (d) �exc = 472�1 nm for site III. Color photographs of Eu3+:TiO2

and Eu3+:Y2O3 (2 mol%) are compared in (e) and (f), respectively. Bothnanophosphors were excited with a xenon lamp at 465–472 nm underthe same experimental condition. To eliminate the influence of excitationlight, a 495-nm long-pass glass filter was used when taking these photos.Reprinted with permission from [25], W. Q. Luo et al., J. Phys. Chem.C 112, 10370 (2008). © 2008, American Chemical Society.

which are ascribed to the lattice site with ordered crys-talline environment. The value of full width at half maxi-mum (FWHM) height of sites II and III is much smallerthan that of site I, decreasing from ∼9.0 nm (the 613-nmpeak) to 0.58 nm (the 617-nm peak). As shown inFigure 10(a), Eu3+ luminescence from sites II and III plusother minor sites emission can be observed when excitedabove TiO2 band-gap at 343 nm, indicating a weak ETfrom TiO2 host to Eu3+ ions. The weak ET can be under-stood because of the energy mismatch between TiO2 band-gap and Eu3+ ion excited states. Since no energy level ofEu3+ is located in an energy range approximately from28,500 to 31,000 cm−1, this non-resonant host-to-Eu3+ ETcan only be accomplished with the assistance of latticephonons.31 By codoping Sm3+ ions in TiO2:Eu

3+ nano-crystal, where Sm3+ acts as an energy bridge between TiO2

and Eu3+ ions, the ET from TiO2 to Eu3+ can be greatlyenhanced.32 The PL intensity of Eu3+:TiO2 is compara-ble to that of Eu3+:Y2O3 (2 mol%) nanophosphors (pre-pared by the sol–gel combustion method) under the xenonlight excitation as shown in the inset of Figure 10. Thedecay curves of 5D0 for Eu3+ at sites I, II, and III areplotted in Figure 11. The three curves can be well fittedwith a single exponential, and the 5D0 lifetimes of 0.37,0.33, and 0.39 ms for sites I, II, and III were determined,respectively.Eu3+ ion is a sensitive optical probe to detect local

symmetry around it. By carefully analyzing the high res-olution emission and excitation spectra of Eu3+ ions atthree sites, the local symmetries of three sites can be

0.0 0.5 1.0 1.5 2.0 2.5 3.0

10

100

1000

τSiteIII = (0.39 ms)

τSite II = (0.33 ms)

Inte

nsity

(ar

b. u

nits

)

Time (ms)

Exp.

Fit

τSite I = (0.37 ms) T = 10 K

Fig. 11. The 10 K luminescence decays of 5D0 for Eu3+ at sites I, II,and III respectively. Reprinted with permission from [25], W. Q. Luoet al., J. Phys. Chem. C 112, 10370 (2008). © 2008, American ChemicalSociety.

determined. The intensity ratio of 5D0 →7F2 lines of Eu3+

with electric-dipole (ED) nature to that of 5D0 →7F1 lineswith magnetic-dipole (MD) nature can provide some struc-tural information such as distortion of ligand environmentand site symmetry of local environment around Eu3+ ions.As shown in Figure 10, the ED transitions of Eu3+ ionsat the three sites are all much stronger than the MDones, suggesting that Eu3+ ions occupy low-symmetry siteswithout an inversion center. Ti4+ ions sit at a D2d site inthe anatase lattice. The substitution of Ti4+ with the largerEu3+ leads to a descent of the intrinsic D2d to a lowersite symmetry (S4, C2v, or D2�, according to the branchingrules of the 32 point groups.33 Theoretically, if Eu3+ ionssituate at the D2d or S4 lattice sites, only two lines for theJ = 0 to J = 1 transition and three lines (S4� or two lines(D2d� for the J = 0 to J = 2 CF transition are allowed orobservable.34 However, three lines for the 5D0 →7F1 tran-sition and four lines for the 5D0 →7F2 transition of Eu3+

ions at Site II can be clearly identified. Moreover, accord-ing to the ED selection rule, the 5D0 →7F0 (0-0) transitionis only allowed in the following 10 site symmetries, Cs ,C1, Cn, and Cnv (n= 2�3�4�6).35�36 The appearance of the0-0 line suggests that Eu3+ ions at site II may occupy ahighest symmetry of C2v. In contrast, for Eu3+ at site III,the absence of 0-0 emission and three resolved lines fromthe emission of 5D0 →7F1 plus three lines from the emis-sion of 5D0 →7F2 indicate a possible D2 site symmetryfor site III. Furthermore, according to the high resolutionspectra, Eu3+ ions at site II (or III) are very likely locatedat a distorted C2v (or D2� site, since one of the 5D0 →7F2

lines was observed to be split into two neighboring peaksfor site II (or III). As for Eu3+ at site I, it resides in a disor-dered environment, and therefore, should have the lowestsite symmetry C1. Consistently, we observed all three linesfor 5D0 →7F1 of Eu3+ at site I.



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A possible model describing the lattice distortion fromD2d site symmetry to C2v or D2 is depicted in Figure 12.Recently, a charge transfer vibronic exciton (CTVE) modelwas proposed to interpret the multi-site formation inEu3+:BaFCl crystals.36 Similarly, an oxygen vacancy orself-trapping CTVE may be formed to compensate forthe charge imbalance when a trivalent Eu substitutes forthe tetravalent Ti in the anatase, accompanied by a latticerelaxation. As shown in Figure 12(b), the lattice expan-sion occurs evenly at various directions, and an oxygenvacancy is created around the Eu3+ ion at site II, thus a C2v

site symmetry is formed. For site III, however, an oxygenvacancy is not physically formed; instead, the charge com-pensation may be collectively accomplished by CTVE. Asshown in Figure 12(c), two vertical symmetric planes �v

at the original Ti4+ location were broken due to the unevenlattice expansion along specific directions when Ti4+ wasreplaced by Eu3+, thus a D2 site symmetry was formed.The thermal stability of Eu3+ at sites I, II, and III was

investigated by site-selective spectroscopy experiments atRT. Figure 13 shows the influence of annealing tempera-tures on the emission intensities of the 5D0 →7F2 transi-tion, which is highly sensitive to the local environment ofEu3+ at three different sites. Multiple sites can be observedfor the sample annealed at a temperature up to 700 �C.The emission intensity of Eu3+ at sites I and III increasedgradually with the annealing temperature increased up to600 �C at the expense of the decrease of emission inten-sity of Eu3+ at site II, illustrating a site transformation ofsite II to sites I and III. When the annealing temperature

Fig. 12. Illustration of the site symmetry of (a) pure anatase nano-crystal, where Ti4+ occupies a D2d symmetry; (b) Eu3+-doped anatasenanocrystal, where Eu3+ occupies a C2v symmetry (Site II); (c) Eu3+-doped anatase nanocrystal, where Eu3+ occupies a D2 symmetry (Site III).The six nearest neighboring oxygens are labeled O1 to O6. The top-viewprojection plane of these nearest neighboring atoms and the positions ofsymmetry operators (C2 and �v� are schematically plotted. Lattice expan-sion is vividly represented by slightly moving outward O1, O2, O3, O4,O5, and O6 in (b) and (c).

400 500 600 700

Site I

Site II

Site III

Annealing temperature (ºC)

Inte

nsity

(a.

u.)

Fig. 13. Influence of the annealing temperature on the emission inten-sities of 5D0 →7F2 transitions of Eu3+ at different sites in Eu3+:TiO2

(2 mol%) nanocrystals.

increased above 700 �C, only emission lines from site Icould be observed. The above facts indicate that Eu3+ ionsat sites II and III in the samples annealed at high temper-ature are unstable, and may be expelled from the lattice tothe grain or surface sites.

2.2. Er3+-Doped TiO2 Nanocrystals

Er-doped semiconductors are one of the promising mate-rials for optical communication systems because the f –ftransition of Er3+ ions is primarily featured by the near-infrared (NIR) photoemission at 1.5 �m,16 which lies inthe minimum loss region of silica-based optical fibers. Theluminescences of Er3+ ions in TiO2 have been studied bymany research groups.13�16�17�37 Komuro et al.17 observedthe resolved visible and NIR emission lines of Er3+ ionsin TiO2 thin films prepared by the KrF excimer laser abla-tion technique rather than wet processing. As shown inFigure 14, both the PL excitation (PLE) spectra obtainedby monitoring the 4I15/2 →4S3/2 transition at 564 nm andthe 4I15/2 →4F9/2 transition at 665 nm exhibited the max-imum absorption peaks at ∼400 nm, resulting from theband-tail absorption of the anatase TiO2 host, indicativeof an energy transfer process from the TiO2 host to Er3+

ions. The PL spectrum under over-band-gap excitation atRT is shown in Figure 14. The weak and broad PL band atabout 400 nm can be ascribed to the band-tail PL from theTiO2 host. Besides the broad band, several intense sharpemission lines were also observed at 525, 564, and 665 nmcorresponding to the transitions from the excited states of2H11/2,

4S3/2, and4F9/2 to the ground state of 4I15/2, respec-

tively. The NIR emission spectra of Er3+ both at RT and20 K under excitation using He–Cd laser are shown inFigure 15. Er-related emission at 1.534 �m originatingfrom 4I13/2 →4I15/2 transition can be observed even at RT.With the decreasing experimental temperature at 20 K, the



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Fig. 14. Visible PL and PLE spectra of the TiO2:Er thin films at RT.The PLE (564) and PLE (665) spectra are monitored at 564 and 665 nm,respectively. Reprinted with permission from [17], S. Komuro et al., Appl.Phys. Lett. 81, 4733 (2002). © 2002, American Institute of Physics.

main peak position of 1.534 �m did not change but theFWHM decreased from 4.6 to 2.2 meV. The NIR emissionat 1.54 �m was also observed in Er-doped TiO2 nanopar-ticle and thin film prepared by hydrothermal method.16

The IR-to-visible upconversion of Er3+ in TiO2 nanopar-ticles is of particular interest due to their potential appli-cations in biolabels, lasers, and displays.13�38�39 Patra et al.reported the fluorescence upconversion properties of Er3+-doped TiO2 nanocrystals prepared by a sol-emulsion-gelmethod13 Figure 16 shows the upconverted fluorescencespectra of Er3+-doped TiO2 nanocrystals annealed at dif-ferent temperatures irradiated with a diode laser at 975 nm.The peak located at near 480 nm was due to an artifact,

Fig. 15. Er-related 1.54 �m emission spectra from the TiO2:Er thinfilms observed at (a) RT and (b) 20 K. Reprinted with permission from[17], S. Komuro et al., Appl. Phys. Lett. 81, 4733 (2002). © 2002,American Institute of Physics.

Fig. 16. Upconverted fluorescence emission spectra of the 0.25 mol%Er3+-doped TiO2 nanocrystals sintered at 500, 800, and 1,000 �C, respec-tively. Reprinted with permission from [13], A. Patra et al., Chem. Mater.15, 3650 (2003). © 2003, American Chemical Society.

not generated in the sample. Both green (∼550 nm) andred (∼660 nm) upconverted emissions were observed in allthree samples. The upconversion luminance values were97.35, 207.5, and 120.5 Cd/m2 for the 500, 800, and1,000 �C sintered samples, respectively. It is clear fromthe absolute luminance values and the spectra shown inFigure 16 that the optimum upconversion was attained forthe samples sintered at 800 �C, in which both rutile andanatase titania phases were present. The green and redupconverted emissions in the range of 520–570 nm (2H11/2,4S3/2 →4I15/2� and 640–690 nm (4F9/2 →4I15/2� were alsoobserved for the Er3+–Yb3+ co-doped TiO2 nanocrystals.

40

Recently, we reported the detailed study of PL andPL dynamics of anatase Er3+:TiO2 nanocrystals in theNIR region around 1.5 �m.41 By modifying the afore-mentioned synthesis condition for preparing TiO2:Eunanocrystals,25 single site emissions of Er3+ in TiO2

with a nominal dopant concentration of 0.75 mol% wereachieved.41 Figure 17 shows the 10 K excitation spectrumof Er3+-doped TiO2 nanocrystals by monitoring the NIR4I13/2 →4I15/2 emission at 1532.6 nm. Abundant sharp exci-tation lines centered at 380.6, 407.6, 489.4, 523.4, 550.5,and 654.0 nm can be assigned to the direct excitation fromground state of 4I15/2 to the upper excited states of 4G11/2,2H9/2,

4F7/2,2H11/2,

4S3/2, and4F9/2, respectively. Fine CF

splitting of the excited states of Er3+ can be easily iden-tified, indicating that Er3+ ions were incorporated into aregular TiO2 nanocrystal lattice. It should be noted that abroad excitation band centered at 358 nm was observed,which was associated with the band-gap of anatase TiO2

nanocrystals, indicating the ET from TiO2 host to Er3+

ions. According to the Kramers degeneracy for the f 11

configuration, two excitation lines from the lowest CFlevel of 4I15/2 to 4S3/2 are theoretically expected for Er3+



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300 400 500 600 7000

200

400

600

544 548 552 556 560

4G11/2 ←

2H9/2 ←

4F7/2 ←

2H11/2 ←

4I15/2

4F9/2 ←

4I15/2

4S3/2 ←

4I15/2

4I15/2

4I15/2

4I15/24I15/2

4F3/2,5/2 ←

Inte

nsity

(a.

u.)

Wavelength (nm)

T = 10 K

4S3/2←4I15/2

Fig. 17. Excitation spectrum of Er3+:TiO2 nanocrystals at 10 K, andthe inset enlarges the excitation lines for the transition of 4I15/2 →4S3/2�

Reprinted with permission from [41], C. Y. Fu et al., Opt. Lett. 33, 953(2008). © 2008, Optical Society of America.

ions sit at a lattice site at low temperature. As clearlyseen in the inset of Figure 17, there are only two linesassigned to the excited state of 4S3/2 (with an energy gapof 18 cm−1� and no trace of CF splitting due to anothersite can be observed. The CF splitting of other excitedstates in Figure 17 was also in good agreement with the-oretical analysis, indicating that the doped Er3+ ions werevery likely located at the same site in TiO2 nanocrystals.As shown in Figures 18(a and b), similar NIR emis-

sion lines were obtained upon excitation either to the2H11/2 state of Er3+ or above the TiO2 band-gap, sug-gesting a homogeneous CF environment for the dopedEr3+ ions. When excited above the band-gap energy, asshown in Figure 18(c), a broad band (peaking at ∼550 nm)attributed to defects was observed. These defects may berelated to oxygen vacancies originating from the substi-tuting Er3+ for Ti4+�11 In addition, sharp emission linesfrom 4S3/2 →4I15/2 were observed being superimposed onthe broad band. The eight CF levels of 4I15/2 were exper-imentally determined to be located at 0, 15, 95, 166, 210,378, 454, and 504 cm−1 according to the 4S3/2 →4I15/2emission at 10 K. Interestingly, self absorption lines corre-sponding to the hypersensitive transition of 4I15/2 →2H11/2

were observed due to the large rank-2 reduced matrix ele-ments (RMEs) of the unit tensor of the transition, whichcould result in strong absorptions. Similar self absorptionlines were reported in Er3+-doped Gd2O3 nanocrystals.42

Figure 18(d) shows the upconversion luminescence of Er3+

under a 976 nm laser excitation at RT. An intense greenemission due to the transition from 2H11/2 and its thermallycoupled 4S3/2 states to 4I15/2 were observed. As illustratedin the inset of Figure 18, the fluorescence decay of 4I13/2was slightly deviated from a single exponential under theexcitation of 523.4 nm, which may be caused by a non-radiative ET process from Er3+ to the neighboring defectsthat have close energy levels to the 4I13/2 state. Assuming

1500 1520 1540 1560 1580 16000

100

200

300

0 1 2 3 4 5 6

10

100

1000

400 450 500 550 600 650 7000

100

200

300

400 450 500 550 600 650 700

0

40

80

120

t (ms)

Wavelength (nm)

Inte

nsity

(a.

u.)

(b)(a)

T=10 K

4I13/2 → 4I15/2

t = 1.56 ms

2H11/2 ← 4I15/2

4S3/2 → 4I15/2T = 10 K

(c)

RT

(d)4F9/2 → 4I15/2

2H11/2, 4S3/2 → 4I15/2

Fig. 18. NIR (a) (b), visible (c), and upconverted (d) luminescencespectra of Er3+:TiO2 nanocrystals, where (a–c) were measured under the523.4, 358, and 358 nm excitation at 10 K, respectively and (d) wasmeasured under the 976 nm laser excitation at RT. The inset shows theluminescence decay curve of 4I13/2 state of Er3+, with the experimental(dotted) and fitted (solid) results. Reprinted with permission from [41],C. Y. Fu et al., Opt. Lett. 33, 953 (2008). © 2008, Optical Society ofAmerica.

the electric dipole–dipole interaction between donor andacceptor, the decay curve can be well fitted by the Inokuti-Hirayama model:43

I �t�= I0 exp�−t/0−C �t/0�1/2 (2)

where I is the time dependent PL intensity, I0 is the ini-tial intensity, C is a freely varied parameter, t is the time,and 0 is the intrinsic luminescence lifetime. The intrin-sic fluorescence lifetime of 4I13/2 was fitted to be 1.56 msat 10 K. In our latest work, systematic CF analysis ofEr3+ in TiO2 nanocrystals was carried out. CF levels below27000 cm−1 of Er3+ at the lattice site of TiO2 nanocrystalswere experimentally determined. These levels were ana-lyzed in terms of 14 free-ion CF parameters at the C2v site,and energy-level fitting yielded a final standard deviationof 25.1 cm−1, showing good agreement between the exper-imental and calculated values. A set value of CF parame-ters was obtained with B20 = 45, B22 = 152, B40 = 2483,B42 =−1050, B44 =−481, B60 =−143, B62 = 248, B64 =395, and B66 = 124 (in units of cm−1�, which was neverreported for Er3+ ions in TiO2 nanocrystals. Using theparameters obtained during the energy-level fitting, the CFstrength of Er3+ in TiO2 was determined to be 549 cm−1,which was close to that in Y2O3 (S = 569 cm−1� with C2

symmetry.44 but doubling that in YVO4 (S = 279 cm−1�



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with D2d symmetry,45 indicative of a very strong CF inter-action experienced by Er3+ ions. This set of CF parametersmay provide a basis for other RE ions doped in similarsurroundings to predict the CF splittings and to probe theiroptical behaviors, which is currently not understood.

2.3. Sm3+-Doped TiO2 Nanocrystals

The sensitized emissions of Sm3+ by the TiO2 hosthave been achieved in thin films46�47 and nanocrystallinepowders.19�48 Lange et al. prepared TiO2 thin films byusing the atomic layer deposition technique and implantedthem with Sm3+ ions. A band-gap excitation of the TiO2

(anatase) films annealed at 600 �C led to intense resolvedSm3+ emission lines, indicating that Sm3+ ions were likelyincorporated into regular positions in a TiO2 lattice.46

Strong orange red emission has been obtained in Sm3+-doped nanocrystalline TiO2 powders fabricated by the sol–gel method due to the effective ET from the TiO2 host toSm3+ ions.19 The band-gap of TiO2 can be tailored by co-doping Bi3+ and Zr4+ ions. Upon excitation above the TiO2

band-gap at 355 nm, slightly resolved emission lines fromthe 4G5/2 state to 6H5/2� 7/2� 9/2 multiplets were detected,indicating that Sm3+ ions were probably located in a quiteregular CF environment.Recently, efficient sensitization emissions of Sm3+ ions

in TiO2 host were realized in Sm3+-doped TiO2 nano-crystals prepared by the sol–gel-solvothermal method.49

Upon the excitation above the TiO2 band-gap at around340 nm at RT, as shown in Figure 19(a), intense sen-sitized emissions from Sm3+ ions were observed. Uponexcitation above the TiO2 band-gap at 10 K (Fig. 19(b)),sharp emission lines related to CF transitions from 4G5/2

to 6H5/2,6H7/2,

6H9/2, and4H11/2 multiples of Sm3+ cen-

tered at 584.1, 612.8, 664.1, and 727.0 nm were identi-fied, indicating that Sm3+ ions were incorporated into a

550 600 650 7000

20

40

60

80

100

120

Inte

msi

ty (

a.u.

)

Wavelength (nm)

RT

10 K

6H9/2

6H11/2

6H7/2

4G5/2 → 6H5/2

(a)

(b)

Fig. 19. Emission spectra of Sm3+ ion-doped TiO2 nanoparticlesannealed at 500 �C for 2 h upon excitation above TiO2 band-gap at (a)RT and (b) 10 K.

regular environment at the TiO2 nanocrystal matrix. Dueto Kramers degeneracy for the f 5 configuration, 3, 4, 5,and 6 emission lines from the lowest CF level of 4G5/2

to 6H5/2,6H7/2,

6H9/2, and6H11/2, respectively, are theo-

retically expected for Sm3+ ions located at a lattice site.As clearly identified in Figure 19(b), more emission lines(namely, at least 5, 6, 7, and 4 lines for the transitions from4G5/2 to 6H5/2,

6H7/2,6H9/2, and

6H11/2, respectively) thanexpected were seen, indicating that at least two differentCF environments around Sm3+ ions exist in the samples.

2.4. Other RE3+ Ion-Doped TiO2 Nanocrystals

The luminescence of several RE3+ (Sm3+, Eu3+, Yb3+,Nd3+, and Er3+� ions in mesoporous titania thin films withnanocrystalline walls have been studied by Stucky andcoworkers.29 Upon excitation above the TiO2 band-gap,the sensitized emissions from the RE3+ ions were demon-strated, indicative of an ET process from the TiO2 host tothe RE3+ ions. However, due to the fact that RE3+ ionswere embedded in an amorphous environment, only rela-tively broad emission lines of RE3+ ions were observed.Domaradzkli and coworkers reported the optical emissionsfrom Eu, Tb, and Nd luminescence centers in TiO2 pre-pared by the magnetron sputtering method.50 Upon exci-tation above the TiO2 band-gap, an efficient ET to theCF states of Eu3+, Tb3+, and Nd3+ ions could occur, andgreen, red, and NIR luminescence were observed, respec-tively. Jia et al. reported the observation of Tb3+ emis-sions in Tb3+-doped titania films fabricated by a sol–gelmethod.51 In that work, ET from the TiO2 host to the Tb3+

ions was also achieved. Moreover, the PL intensity of Tb3+

was found to be improved significantly after co-dopingwith Ce3+ ions.

900 1000 1100 1200 1300 14000

20

40

60

80

100

120

140

(b)

RT

10 K

Inte

nsity

(a.

u.)

Wavelength (nm)

4I11/24F3/2 → 4I9/2

4I13/2

(a)

Fig. 20. Emission spectra of Nd3+ ion-doped TiO2 nanoparticlesannealed at 500 �C for 2 h upon excitation above the TiO2 band-gap at(a) RT and (b) 10 K.



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In our recent work, strong and sharp PL of Nd3+

ions in anatase TiO2 nanocrystal prepared by the sol–gel-solvothermal method was achieved via an ET from theTiO2 host to the Nd3+ions.49 Upon the excitation abovethe TiO2 band-gap at around 339 nm at RT, as shownin Figure 20(a), intense and sharp sensitized NIR emis-sions from the transitions of 4F3/2 →4I9/2,

4I11/2 and 4I13/2of Nd3+ ions were observed at 914, 1094, and 1385nm, respectively. Similar to the Eu3+, Sm3+-doped TiO2

nanoparticles we synthesized, multiple site emissions werealso detected in Nd3+-doped TiO2 nanoparticles. As shownin Figure 20(b), upon excitation above the TiO2 band-gapat 339 nm at 10 K, at least 9, 7, and 4 emission lines for thetransitions from the lowest CF level of 4F3/2 to

4I9/2,4I11/2,

and 4I13/2, respectively, can be clearly identified, more thanthat theoretically expected in view of Kramers degeneracyfor f 3 configuration (namely, 5, 6, and 7 emissions for4F3/2 (1) to

4I9/2,4I11/2, and

4I13/2, respectively) . Undoubt-edly, this indicates more than one luminescence center ofNd3+ in TiO2 nanocrystals.

3. CONCLUSIONS

RE3+ ion-doped anatase TiO2 nanoparticles are promisingmaterials for phosphor devices. The luminescence proper-ties of different RE3+ ion-doped TiO2 nanoparticles werereviewed. ET from the TiO2 host to the RE3+ ions wasobserved in many TiO2 nanomaterials, indicating that TiO2

is a good host as well as sensitizer for RE3+ ions. However,in most cases, only unresolved PL lines were observeddue to the poor incorporation of RE3+ ions into the TiO2

lattice. A facile sol–gel solvothermal approach was intro-duced to incorporate RE3+ (RE = Eu, Er, Sm, and Nd)ions into TiO2 nanocrystals. Intense and resolved emissionlines of RE3+ ions were thus achieved. It is evidenced thatRE3+ ions were embedded in ordered nanocrystal latticeenvironments. Multiple site emissions of Eu3+, Sm3+, andNd3+ in TiO2 nanoparticles were detected by means of siteselective spectroscopy. Using Eu3+ as an optical probe,the relationship between the luminescence properties andlocal structure of Eu3+ ions were discussed. Single latticesite emission of Er3+ in TiO2 nanocrystals was achievedby modifying the experimental conditions. Upon excitationby a Ti: sapphire laser at 978 nm, intense green upcon-verted luminescence was also observed. The characteristicemission of Er3+ ions was achieved both in the UV-visand infrared regions through the high-resolution opticalspectroscopy at 10 K. For the nanophosphors we synthe-sized, efficient ETs from the TiO2 host to Er3+, Sm3+, andNd3+ ions were realized at RT. The lattice incorporationof RE3+ ions in TiO2nanocrystals and efficient host-to-REET obtained in these systems is of great significance forfurther material applications in the fields of optics andelectronics.Although some advances have been made in RE3+-

doped TiO2 nanocrystals, to gain deep insight into the

understanding of chemistry and physics of RE3+ ions inTiO2 nanocrystals, there is still much work to be done inthe future, such as the engineering of TiO2 band-gap viasize control to achieve more efficient host-to-RE3+ PL, andsystematic CF analysis of local environment and distortionaround RE ions.

Acknowledgments: This work is supported by theKnowledge Innovation and Hundreds of Talents Pro-grams of the Chinese Academy of Sciences (CAS),Instrument Developing Project of CAS (No. YZ200712),the NSFC (Nos. 10504032 and 10774143), the NationalHigh-Tech R&D Program of China (863 Program) (No.2009AA03Z430), the 973 program (No. 2007CB936703),Fujian Provincial Science Fund for Distinguished YoungScholars (No. 2009J06030), and the Key Project of Sci-ence and Technology of Fujian Province (No. 2007I0024).

References and Notes

1. A. A. Bol, R. van Beek, and A. Meijerink, Chem. Mater. 14, 1121(2002).

2. W. Chen, J. Z. Zhang, and A. G. Joly, J. Nanosci. Nanotechnol.4, 919 (2004).

3. X. Y. Chen, W. Q. Luo, Y. S. Liu, and G. K. Liu, J. Rare Earths25, 515 (2007).

4. S. C. Erwin, L. J. Zu, M. I. Haftel, A. L. Efros, T. A. Kennedy, andD. J. Norris, Nature 436, 91 (2005).

5. P. A. Tanner and L. X. Yu, J. Nanosci. Nanotechnol. 8, 1307 (2008).6. S. Geburt, D. Stichtenoth, S. Muller, W. Dewald, C. Ronning,

J. Wang, Y. Jiao, Y. Y. Rao, S. K. Hark, and Q. Li, J. Nanosci.Nanotechnol. 8, 244 (2008).

7. P. Salas, N. Nava, C. Angeles-Chavez, E. De la Rosa, and L. A.Diaz-Torres, J. Nanosci. Nanotechnol. 8, 6431 (2008).

8. A. Conde-Gallardo, M. Garcia-Rocha, I. Hernandez-Calderon, andR. Palomino-Merino, Appl. Phys. Lett. 78, 3436 (2001).

9. K. L. Frindell, M. H. Bartl, A. Popitsch, and G. D. Stucky, Angew.Chem. Int. Ed. 41, 959 (2002).

10. C. W. Jia, E. Q. Xie, J. G. Zhao, Z. W. Sun, and A. H. Peng, J. Appl.Phys. 100, 023529 (2006).

11. J. G. Li, X. H. Wang, K. Watanabe, and T. Ishigaki, J. Phys. Chem.B 110, 1121 (2006).

12. B. K. Moon, I.-M. Kwon, H. K. Yang, H. J. Seo, J. H. Jeong, S. S.Yi, and J. H. Kim, Colloid Surf. A-Physicochem. Eng. Asp. 313–314,82 (2008).

13. A. Patra, C. S. Friend, R. Kapoor, and P. N. Prasad, Chem. Mater.15, 3650 (2003).

14. J. W. Stouwdam and F. C. J. M. van Veggel, Chemphyschem 5, 743(2004).

15. J. B. Yin, L. Q. Xiang, and X. P. Zhao, Appl. Phys. Lett. 90, 113112(2007).

16. S. Jeon and P. V. Braun, Chem. Mater. 15, 1256 (2003).17. S. Komuro, T. Katsumata, H. Kokai, T. Morikawa, and X. Zhao,

Appl. Phys. Lett. 81, 4733 (2002).18. L. Li, C. K. Tsung, Z. Yang, G. D. Stucky, L. D. Sun, J. F. Wang,

and C. H. Yan, Adv. Mater. 20, 903 (2008).19. L. Y. Hu, H. W. Song, G. H. Pan, B. Yan, R. F. Qin, Q. L. Dai, L. B.

Fan, S. W. Li, and X. Bai, J. Lumin. 127, 371 (2007).20. R. Palomino-Merino, A. Conde-Gallardo, M. Garcia-Rocha,

I. Hernandez-Calderon, V. Castano, and R. Rodriguez, Thin SolidFilms 401, 118 (2001).

21. Y. Matsumoto, U. Unal, Y. Kimura, S. Ohashi, and K. Izawa, J. Phys.Chem. B 109, 12748 (2005).



IP : 158.132.160.61Sun, 13 Jun 2010 23:13:07R

EVIEW


22. M. Bettinelli, A. Speghini, D. Falcomer, M. Daldosso, V. Dallacasa,and L. Romano, J. Phys-Condens. Matter. 18, S2149 (2006).

23. S. Ida, U. Unal, K. Izawa, O. Altuntasoglu, C. Ogata, T. Inoue,K. Shimogawa, and Y. Matsumoto, J. Phys. Chem. B 110, 23881(2006).

24. Q. G. Zeng, Z. J. Ding, and Z. M. Zhang, J. Lumin. 118, 301(2006).

25. W. Q. Luo, R. F. Li, G. K. Liu, M. R. Antonio, and X. Y. Chen,J. Phys. Chem. C 112, 10370 (2008).

26. T. Tachikawa, T. Ishigaki, J. G. Li, M. Fujitsuka, and T. Majima,Angew. Chem. Int. Ed. 47, 5348 (2008).

27. Q. G. Zeng, Z. M. Zhang, Z. J. Ding, Y. Wang, and Y. Q. Sheng,Scripta Mater. 57, 897 (2007).

28. B. Chi, E. S. Victorio, and T. Jin, Nanotechnology 17, 2234 (2006).29. K. L. Frindell, M. H. Bartl, M. R. Robinson, G. C. Bazan,

A. Popitsch, and G. D. Stucky, J. Solid State Chem. 172, 81 (2003).30. H. Xin, R. Z. Ma, L. Z. Wang, Y. S. Ebina, K. Takada, and T. Sasaki,

Appl. Phys. Lett. 85, 4187 (2004).31. W. T. Carnall, G. L. Goodman, K. Rajnak, and R. S. Rana, J. Chem.

Phys. 90, 3443 (1989).32. W. Q. Luo, R. F. Li, Y. S. Liu, and X. Y. Chen, J. Nanosci. Nano-

technol. 10, 1693 (2010).33. P. H. Butler, Point Group Symmetry Application: Method and

Tables, Plenum, New York (1981).34. Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen,

J. Phys. Chem. C 113, 2309 (2009).35. X. Y. Chen and G. K. Liu, J. Solid State Chem. 178, 419 (2005).36. X. Y. Chen, W. Zhao, R. E. Cook, and G. K. Liu, Phys. Rev. B

70, 205122 (2004).

37. L. Y. Bai, X. Y. Su, and M. K. Lei, J. Inorg. Mater. 21, 1085 (2006).38. A. Bahtat, M. Bouazaoui, M. Bahtat, and J. Mugnier, Opt. Commun.

111, 55 (1994).39. A. Bahtat, M. Bouazaoui, M. Bahtat, C. Garapon, B. Jacquier, and

J. Mugnier, J. Non-Cryst. Solids 202, 16 (1996).40. Q. K. Shang, H. Yu, X. G. Kong, H. D. Wang, X. Wang, Y. J. Sun,

Y. L. Zhang, and Q. H. Zeng, J. Lumin. 128, 1211 (2008).41. C. Y. Fu, J. S. Liao, W. Q. Luo, R. F. Li, and X. Y. Chen, Opt. Lett.

33, 953 (2008).42. X. Y. Chen, E. Ma, and G. K. Liu, J. Phys. Chem. C 111, 10404

(2007).43. M. Inokuti and F. Hirayama, J. Chem. Phys. 43, 1978 (1965).44. N. C. Chang, J. B. Gruber, R. P. Leavitt, and C. A. Morrison,

J. Chem. Phys. 76, 3877 (1982).45. J. A. Capobianco, P. Kabro, F. S. Ermeneux, R. Moncorge,

M. Bettinelli, and E. Cavalli, Chem. Phys. 214, 329 (1997).46. S. Lange, I. Sildos, V. Kiisk, and J. Aarik, Mater. Sci. Eng. B-Solid

State Mater. Adv. Technol. 112, 87 (2004).47. V. Kiisk, I. Sildos, S. Lange, V. Reedo, T. Tatte, M. Kirm, and

J. Aarik, Appl. Surf. Sci. 247, 412 (2005).48. C. M. Gao, H. W. Song, L. Y. Hu, G. H. Pan, R. F. Qin, F. Wang,

Q. L. Dai, L. B. Fan, L. N. Liu, and H. H. Liu, J. Lumin. 128, 559(2008).

49. W. Q. Luo, R. F. Li, and X. Y. Chen, J. Phys. Chem. C (2009),in press.

50. D. Kaczmarek, J. Domaradzki, A. Borkowska, A. Podhorodecki,J. Misiewicz, and K. Sieradzka, Opt. Appl. 37, 433 (2007).

51. C. W. Jia, E. Q. Xie, A. H. Peng, R. Jiang, F. Ye, H. F. Lin, andT. Xu, Thin Solid Films 496, 555 (2006).

Received: 30 November 2008. Accepted: 27 March 2009.


Xueyuan Chen-Optical Spectroscopy of Rare Earth Ion-Doped-JNN2010

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