Host-Sensitized Luminescence of Nd3+ and Sm3+ Ions Incorporated

Apr 24, 2009 - A sol−gel solvothermal method was introduced to incorporate Nd3+ and Sm3+ ions into TiO2 nanoparticles. Intense and well-resolved ...
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Host-Sensitized Luminescence of Nd3+ and Sm3+ Ions Incorporated in Anatase Titania Nanocrystals Wenqin Luo, Renfu Li, and Xueyuan Chen* Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China ReceiVed: February 28, 2009; ReVised Manuscript ReceiVed: April 3, 2009

A sol-gel solvothermal method was introduced to incorporate Nd3+ and Sm3+ ions into TiO2 nanoparticles. Intense and well-resolved emission lines from Nd3+ or Sm3+ ions were realized upon excitation above the TiO2 bandgap at room temperature. The sensitized emissions of Nd3+ or Sm3+ were found to be much more efficient than a direct excitation of lanthanide ions (Ln3+). Multiple site emissions of Nd3+ and Sm3+ were detected by means of site-selective spectroscopy at 10 K. Moreover, a possible host-to-Ln3+ sensitization mechanism was proposed. 1. Introduction To achieve highly efficient multicolor luminescence of lanthanide ions (Ln3+) for practical applications such as in lightemitting diodes (LEDs), host sensitization via energy transfer (ET) from the excited host to Ln3+ is an effective way to overcome the low absorptions of parity-forbidden f-f transitions of Ln3+.1–4 While the luminescence of Ln3+ has been greatly enhanced through the nonradiative ET from the triplet state of ligands to the crystal-field (CF) state of the central ions in metal organic complexes,5–7 their poor thermal stability and weak mechanical properties have restrained them from potential applications in solid-state lasers and phosphor devices. Semiconductor nanocrystals (e.g., ZnO and TiO2) are promising candidates to be used as hosts and sensitizers for Ln3+ due to the ease to tailor their optical properties via size control.8–12 Moreover, they provide the possibility of excitation with electrical current.12 Titania is a well-known wide bandgap semiconductor and a prominent candidate as the host material of Ln3+ ions because of its good chemical, mechanical, optical, and thermal properties.13–32 Sensitized emissions of TiO2:Ln3+ have been reported in solid thin films, nanoparticles, nanosheets, nanotubes, etc.33–44 In most reported cases, however, Ln3+ was embedded in amorphous or distorted surface sites adjacent to the nanocrystalline phase. As a result, only broad emission lines of Ln3+ could be observed via host sensitization. To date, the incorporation of Ln3+ ions into the TiO2 host remains intractable because of the large mismatch of ionic radii and charge imbalance between Ln3+ and Ti4+. We anticipate that embedding various Ln3+ ions within the lattice of TiO2 nanocrystals may lead to sharp and multicolor emissions from Ln3+ to meet the increasing demands for highly sensitive biolabeling, lighting, and displays. In this work, we present a simple sol-gel solvothermal method to prepare largely monodispersed spherical TiO2 phosphors with Ln3+ (Nd3+ or Sm3+) doped into TiO2 nanolattices. These spherical particles may offer brighter luminescent performance, higher definition, and much improved screen packing.35,45 Efficient sensitized emissions of Nd3+ and Sm3+ by TiO2 host were observed both at room temperature and 10 * To whom correspondence should be addressed. Phone/fax: +86-5918764-2575. E-mail: [email protected].

K. Very sharp and intense excitation and emission lines due to the CF splitting of Nd3+ and Sm3+ in TiO2 nanocrystals are observed in the low-temperature photoluminescence (PL) experiments. Finally, a possible TiO2 host-to-Ln3+ sensitization mechanism was proposed based on the PL analysis. 2. Experimental Section 2.1. Preparation of Ln3+-Doped TiO2 Nanoparticles. Nd3+or Sm3+ -doped TiO2 nanocrystals were prepared by a sol-gel solvothermal method described previously.11 The required amount of Nd(CH3COO)3 · 6(H2O) or Sm(CH3COO)3 · 2(H2O) was dissolved in a mixture of distilled water (0.4 mL) and absolute ethanol (40 mL). During the addition of tetra(n-butyl) titanate dissolved in absolute ethanol (40 mL), the mixture was stirred vigorously with a magnetic stirrer at room temperature. After continuous stirring for 3 h, the obtained cloudy mixture was transferred to 50-mL Teflon-lined autoclaves and then subjected to solvothermal treatment for 5 h at 120 °C. Cooling to room temperature led to the white precipitates that were collected by centrifugation and washed with absolute ethanol for several times. The precipitates were dried at 60 °C for 12 h and then heat treated at 500 °C for 2 h to yield the final products. Hereafter, without special explanation, the samples TiO2:Nd3+ and TiO2:Sm3+ mentioned in the following refer to the Nd3+and Sm3+-doped TiO2 nanocrystals annealed in air at 500 °C for 2 h with the nominal dopant concentrations of 1.99 and 2.34 atom %, respectively. 2.2. Characterization. The chemical compositions of the final products were measured by induction-coupled plasma (ICP) analysis (Ultima2, Jobin Yvon). Powder X-ray diffraction (XRD) patterns were collected with a RIGAKU DMAX2500PC powder diffractometer with Cu KR1 radiation (λ ) 0.154 nm). The morphology of the samples was characterized by a JSM6700F scanning electron microscope (SEM) and a JEOL2010 transmission electron microscope (TEM) equipped with the energy dispersive X-ray spectrum (EDS). Ultraviolet-visible (UV/vis) diffuse reflectance spectra of Nd3+- and Sm3+-doped TiO2 nanocrystals were measured by a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer with BaSO4 as a reference. Emission and excitation spectra and transient decays were recorded on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with both continuous (450 W) and pulsed xenon

10.1021/jp901862k CCC: $40.75  2009 American Chemical Society Published on Web 04/24/2009

Host-Sensitized Luminescence of Nd3+ and Sm3+ Ions

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Figure 1. XRD patterns of Sm3+- or Nd3+-doped TiO2 nanocrystals.

lamps. For low-temperature measurements, samples were mounted on a closed cycle cryostat (10-350 K, DE202, Advanced Research Systems). For site-selective spectroscopy, the excitation (or emission) monochromator’s slits were set as small as possible to maximize the instrumental resolution. The best wavelength resolution is 0.05 nm. The line intensities of the measured spectra were calibrated according to the FLS920 correction curve and standard mercury lamp. 3. Results and Discussion 3.1. Morphology, Crystalline Structure, and Optical Absorption. Figure 1 shows the XRD patterns of TiO2:Nd3+ and TiO2:Sm3+ nanocrystals. The diffraction peaks of both samples can be well indexed as pure anatase phase (JCPDS No. 71-1166, space group I41/amd) and no trace of characteristic peaks was observed for other impurity phases such as rutile, brookite, samarium (neodymium) oxides, or samarium (neodymium) titanium oxides. By means of the Debye-Scherrer equation, the average sizes of TiO2:Nd3+ and TiO2:Sm3+ nanocrystals were estimated to be 13 and 11 nm, respectively. As illustrated in Figure 2a, the obtained nanocrystal ensembles have a mean size of 1 µm, which result from the aggregation of 10-20 nm anatase titania nanocrystals (Figure S1, Supporting Information). The concentrations of Nd3+ and Sm3+ introduced in TiO2 nanocrystals were determined to be 2.02 and 2.30 atom % by an ICP emission spectrometer, respectively. As schematically illustrated in Figure 2b, efficient sensitized emissions of Nd3+ and Sm3+ may be achievable upon excitation above the TiO2 bandgap. Indeed, light orange-red emission was obtained in TiO2:Sm3+ nanocrystals (Figure 2c) upon excitation above the TiO2 bandgap at 343 nm (Figure 2d). In sharp contrast, much weaker emission was observed under direct excitation to 2P5/2 of Sm3+ ions at 416 nm (Figure 2e), demonstrating that the excitation through the host-to-Sm3+ ET is much more efficient than a direct f-f excitation of Sm3+. This can also be corroborated by the experimental findings from PL and PL excitation spectra (Figure 7). The Kubelka-Munk (K-M) function F(R∞) for infinitely thick samples is usually used to convert reflectance spectra into equivalent absorption spectra,46,47 with F(R∞) ) ((1 - R∞)2)/ (2R∞) ) (K)/(S), where R∞ is the reflectance from an infinitely thick sample, K is the K-M absorption coefficient, and S is the K-M scattering coefficient. By using the reflectance of BaSO4 as a reference, R∞ can be expressed as R∞ ) (Rsample)/(RBaSO4),48 where Rsample is the reflectance of the samples obtained from UV/vis diffuse reflectance spectra. The absorption coefficient R is related to the reflectance R by the following: R ) SF(R∞)/ 2Vp, where Vp is the volume fraction of the absorbing species.49

Figure 2. (a) SEM image of TiO2:Nd3+; (b) schematic illustration of ET from the TiO2 host to Nd3+ or Sm3+ ions; (c) photograph of white TiO2:Sm3+ nanopowders; photographs of the PL from TiO2:Sm3+ (d) upon excitation above the TiO2 bandgap at 343 nm and (e) direct excitation to 2P5/2 of Sm3+ at 416 nm with Xe lamp under the same experimental settings at room temperature. To eliminate the influence of the excitation light, a 495-nm long-pass glass filter was used when taking these photos. The green background lights in panels d and e were due to fluorescence from the paper that was used to load samples upon UV light excitation.

The value of S is dependent on the particle size and the incident light wavelength. Under the condition that the particle size of a sample is not comparable to the incident light wavelength, the dependence of S on the wavelength of the incident light can be neglected, and thus F(R∞) is directly proportional to R. Following the relation for the direct bandgap semiconductor F(R∞) ∝ R ∝ (hν - Eg)1/2, where hν is the photon energy and Eg is the bandgap, the bandgap of semiconductor can be determined from the diffuse reflectance spectra. Figure 3 shows the Plots of (F(R∞))2 versus hν for direct transition of TiO2: Nd3+ (or Sm3+) nanocrystals. The bandgap energies (Eg) of the sample were determined to be 3.40 ( 0.11 and 3.34 ( 0.03 eV for Nd3+- and Sm3+-doped TiO2 nanocrystals, respectively, by the extrapolation to F(R∞)2 ) 0. The bandgap energy of our samples shifts to the blue compared to that of bulk anatase (3.2 eV)50 due possibly to the quantum confinement effect.51 Besides the bandgap absorptions, as shown in the inset of Figure 3, both samples exhibit typical f-f absorptions from the ground states of Nd3+ (4I9/2) and Sm3+ (6H5/2) to their excited multiplets of 4 F3/2 (1.41 eV), 2H9/2 + 4F5/2 (1.54 eV), 4S3/2 + 4F7/2 (1.66 eV), 2 G7/2 + 4G5/2 (2.11 eV), 4G7/2 (2.34 eV), and 2G9/2 (2.41 eV) for Nd3+ ions and 6F1/2 (0.78 eV), 6F3/2 (0.81 eV), 6H15/2 (0.84 eV), 6 F5/2 (0.90 eV), 6F7/2 (1.01 eV), 6F9/2 (1.15 eV), 6F11/2 (1.31 eV), and 4H11/2 (2.60 eV) for Sm3+ ions. 3.2. Sensitized Luminescence of TiO2:Nd3+ Nanocrystals. Figure 4 shows room temperature excitation and emission spectra of TiO2:Nd3+ nanocrystals. Strong near-infrared PL of Nd3+ was observed upon excitation above the TiO2 bandgap at 345 nm. The emission lines centered at 915, 1094, and 1384 nm correspond to the transitions from 4F3/2 to 4I9/2, 4I11/2, and 4 I13/2, respectively. This emission pattern is very different from that of Nd3+ in Nd2O3 nanocrystals52 in both the peak positions and the line shapes, and thus excludes the possibility of the

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Figure 3. Plots of F(R∞)2 versus photon energy for TiO2:Nd3+ and TiO2:Sm3+ nanoparticles. F(R∞) is the Kubelka-Munk function, with F(R∞) ) (1 - R∞)2/2R∞. The inset enlarges the regions at low energies showing the characteristic f-f absorption peaks of Nd3+ and Sm3+ ions.

Figure 4. Excitation (left) and emission (right) spectra of TiO2:Nd3+ nanocrystals at room temperature.

impurity phase of Nd2O3 in the synthesized sample. It should be noted that such efficient sensitized emissions of Nd3+ in TiO2 were not reported previously. The excitation spectrum by monitoring the 4F3/2 f 4I11/2 emission at 1094 nm exhibited an intense broad peak centered at ∼345 nm (3.59 eV) that corresponds to the bandgap absorption peak of anatase TiO2, indicating that the Nd3+ emissions can be achieved via an efficient nonradiative ET process from TiO2 to Nd3+. The excitation lines arising from f-f transitions of Nd3+ itself in the room temperature excitation spectrum are hardly observable, due to the fact that the sensitized emission is a much more efficient pathway than a direct excitation of Nd3+. In contrast, both the broadband related to the TiO2 bandgap absorption and the very sharp excitation lines from f-f transitions of Nd3+ can be observed in the 10 K excitation spectrum (Figure 5a), indicating weaker host-to-Nd3+ sensitization at 10 K. Moreover, a broadband centered at 359 nm superimposed on the shoulder of the TiO2 bandgap absorption peak (338 nm) was also observed, which could be ascribed to defects. The diminishing of the defect excitation band at room temperature may be ascribed to the much more efficient nonradiative recombination or quenching of excited defects at room temperature that restrains the ET from defects to Nd3+ ions. As shown in Figure 5b, when directly excited from the ground state (4I9/2) to the 2 G7/2 (4G5/2) states of Nd3+ at 618.4 nm at 10 K, characteristic sharp emissions from 4F3/2 to 4I9/2, 4I11/2, and 4I13/2 states were observed with the most intense emission centered at 1094.7 nm. Due to Kramers degeneracy, 5, 6, and 7 lines for the transitions from 4F3/2 to its lower 4I9/2, 4I11/2, and 4I13/2 multiplets are theoretically predicted to occur at very low temperature. As a matter of fact, 4, 5, and 2 emission lines assigned to 4I9/2,4I11/2,

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Figure 5. (a) The 10 K excitation spectra of TiO2:Nd3+ nanocrystals by monitoring the 4F3/2 f 4I11/2 emission at 1094.7 nm and (b) the 10 K emission spectra under the excitation at 338 and 618.4 nm of TiO2: Nd3+ nanocrystals, respectively. The star symbol marked in panel b denotes the newly appeared emission lines upon excitation at 338 nm. The inset shows the 10 K luminescence decay curve of Nd3+ by monitoring the4F3/2 f 4I11/2 transition at 1094.7 nm under excitation at 618.4 nm, with the experimental (dotted) and fitted (solid) results.

and 4I13/2, less than theoretical numbers, were identified under the site-selective excitation at 618.4 nm. As a result, these lines should originate from the same site of Nd3+ doped in TiO2 nanocrystals. Such fine CF splittings firmly establish that Nd3+ ions should be embedded in fairly well crystalline surroundings, namely, nanolattices. The site-selective PL decay from 4F3/2 under the excitation at 618.4 nm fits well to a single exponential function (inset of Figure 5b), verifying the homogeneous CF environment of Nd3+. The luminescence lifetime was determined to be 43 µs, a value much shorter than that of Nd3+ in YAG nanopowders (286 µs)53 and Nd2O3 nanocrystals (492 µs).52 Likewise, upon excitation above the TiO2 bandgap at 338 nm at 10 K, sharp emission lines from Nd3+ were also observed (Figure 5b). However, in this case, several new emission lines (marked with star symbols) appeared in addition to those lines observed under the direct f-f excitation, which could arise from the subset of Nd3+ ions occupying significantly different CF environments from that of the predominant site aforementioned. To investigate the influence of the dopant concentration on the PL properties of the material, the room temperature PL spectra and the 4F3/2 lifetimes (monitoring the 1094 nm emission line) of TiO2:Nd3+ nanocrystals with various Nd3+ concentrations were compared in Figure 6. All the PL spectra and the 4 F3/2 lifetime were obtained upon excitation above the TiO2 bandgap. As shown in Figure 6, with the increase of the Nd3+ concentration from 0.5 to 4 atom %, the PL intensity of the 4 F3/2 f 4I9/2 transition increased first, reaching the maximum at the concentration of 1.5 atom %, and then dropped gradually. In contrast, the luminescence lifetime of 4F3/2 decreased straightforwardly with the increasing concentration (inset of Figure 6, from 135 µs at 0.5 atom % to 60 µs at 3 atom %). Generally, the nonradiative cross-relaxation process between Ln3+ pairs is mainly responsible for the shortening of Ln3+ excited state lifetime at high concentration. The cross-relaxation rate depends critically on the distance between the Ln3+ pairs (i.e., Ln3+ concentrations). For Nd3+ ions, there exist the following cross-relaxation dipolar transitions: the energy produced by the transition of 4F3/2 f 4I15/2 of one Nd3+ ion can be resonantly transferred to another neighboring Nd3+ ion causing the 4I9/2 f 4I15/2 excitation, and then the electrons in the excited

Host-Sensitized Luminescence of Nd3+ and Sm3+ Ions

Figure 6. The PL spectra and the 4F3/2 lifetime (inset) of TiO2:Nd3+ as a function of the Nd3+ concentration under the excitation above the TiO2 bandgap at room temperature.

Figure 7. Excitation (left) and emission (right) spectra of TiO2:Sm3+ nanocrystals at room temperature.

state of 4I15/2 of Nd3+ ions are nonradiatively relaxed to the ground state of 4I9/2.54 The observed lifetime behavior shows that the cross-relaxation process occurs at all dopant concentrations studied. The predominant emission line at 1094 nm for the Nd3+ concentration of 0.5-2 atom % was quenched dramatically when the amount of Nd3+ dopant was increased up to 3 atom %, which may be attributed to the significantly enhanced cross-relaxation process at higher Nd3+ concentrations. 3.3. Sensitized Luminescence of TiO2:Sm3+ Nanocrystals. The sensitized PL was also observed in TiO2:Sm3+ nanocrystals. As shown in Figure 7, intense orange-red emission lines at 560-750 nm were detected upon excitation above the TiO2 bandgap at 343 nm at room temperature. The emission lines centered at 584.1, 612.8, 664.1, and 727.0 nm can be assigned to the de-excitation from 4G5/2 to its lower multiplets of 6H5/2, 6 H7/2, 6H9/2, and 6H11/2, respectively. The excitation spectrum by monitoring the transition of 4G5/2 f 6H7/2 at 612.8 nm exhibited a broadband centered at 343 nm (3.61 eV), which can be attributed to the bandgap absorption peak of TiO2. Similar to the case of TiO2:Nd3+, much weaker excitation lines originating from Sm3+ itself were observed due to the parity forbidden nature of f-f transitions. As shown in Figure 8a, the intensities of excitation lines from f-f transition of Sm3+ ions were much enhanced at 10 K, indicating weaker host-to-Sm3+ sensitization at 10 K. Like TiO2:Nd3+, one band centered at 360 nm superimposed on the shoulder of the TiO2 bandgap absorption peak (332 nm) was observed in the 10 K excitation spectrum, which could be ascribed to defects. As clearly shown in Figure 8b, under the direct excitation to the 2P5/2 multiplet at 416.2 nm at 10 K, at least 3, 4, 5, and 3 lines were well resolved for the transitions of 4G5/2 f 6HJ (J ) 5/2, 7/2, 9/2, 11/2) of Sm3+, respectively. These CF splittings, suggesting a noncubic

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Figure 8. (a) The 10 K excitation spectrum of TiO2:Sm3+ by monitoring the 4G5/2 f 6H7/2 emission at 613.2 nm and (b) the 10 K emission spectra upon excitation at 332 and 416.2 nm of TiO2: Sm3+ nanocrystals, respectively. The rhombus symbol marked in panel b denotes the newly appeared emission lines upon excitation at 332 nm. The inset shows the 10 K luminescence decay curve of Sm3+ by monitoring the 4G5/2 f 6H7/2 transition at 613.2 nm under the excitation at 416.2 nm, with the experimental (dotted) and fitted (solid) results.

lattice site of Sm3+, are generally consistent with theoretical values (namely, 3, 4, 5, and 6 emission lines) given by Kramers degeneracy of f5. As expected, multisite Sm3+ emissions were also observed under the excitation above the TiO2 bandgap at 332 nm. Several new emission lines (marked with rhombus symbols in Figure 8b) were found, indicative of a totally different CF environment around Sm3+ in anatase lattice. Besides, a noticeable broadband starting from 470 to 730 nm was observed under the excitation above TiO2 bandgap at 10 K, which could be associated with the defect emissions in anatase TiO2 nanocrystals.10,37 Unlike TiO2:Nd3+, the PL decay of 4G5/2 under the selective excitation at 416.2 nm deviated slightly from a single exponential function (inset of Figure 8b). Similar nonexponential decay also occurred in the sample having a low Sm3+ dopant concentration of 0.5 atom %, which thus rules out the possibility of concentration quenching effect on the decay behavior of Sm3+. Moreover, since only one site emission of Sm3+ was detected under the excitation at 416.2 nm, the possibility of a superposition of different decays from other sites can be excluded. Presumably, the nonexponential decay of Sm3+ is due to the nonradiative ET process from Sm3+ to their neighboring defects. The intrinsic luminescence lifetime of 4G5/2 was determined to be 350 µs at 10 K from a fit using the Inokuti-Hirayama model,55 which is coincidently close to that of Sm3+ doped in sol-gel TiO2 nanoparticles (346.2 µs).36 The dependence of PL intensity and the 4G5/2 lifetime (by monitoring the 612.8 nm emission line) on the Sm3+ concentration under the room temperature excitation above the TiO2 bandgap was depicted in Figure 9. With the increasing Sm3+ concentration, the PL intensity of 4G5/2 f 6H7/2 increased first, reaching the maximum at the dopant concentration of 2.3 atom %, and then decreased with further increasing of dopant concentration. In contrast, the luminescent lifetime of 4G5/2 was found to decrease with the increase of dopant concentration (from 400 µs at 0.5 atom % to 140 µs at 4 atom %) due to the enhanced cross relaxations between neighboring Sm3+ ions.56 3.4. Host-to-Ln3+ Sensitization Mechanism. Figure 10 illustrates a possible mechanism for the sensitization of TiO2 host to Ln3+. The excitation above the bandgap of titania nanoparticles leads to the transition of electrons in TiO2 nanocrystals from valence band (VB) to conduction band (CB)

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Luo et al. within the crystalline TiO2 lattice and the resulting efficient hostto-Ln3+ sensitization may find important material applications in biolabels, LEDs, displays, and optical communications. Acknowledgment. This work is supported by the Knowledge Innovation and Hundreds of Talents Program of the Chinese Academy of Sciences, the NSFC (Nos. 10774143 and 10804106), the 973 program (No. 2007CB936703), and the Science and Technology Foundation of Fujian Province (No. 2007I0024). Supporting Information Available: High-resolution TEM image and EDS pattern of TiO2:Nd3+ nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. The 4G5/2 f 6H7/2 transition intensity and the 4G5/2 lifetime of TiO2:Sm3+ as a function of the Sm3+ concentration under the excitation above the TiO2 bandgap at room temperature.

Figure 10. An energy transfer mechanism describing the sensitization of TiO2 host to Nd3+ and Sm3+ ions. The curly arrows denote multiphonon nonradiative transitions, and the straight ones denote radiative transitions.

leaving holes in VB. It is believed that the energy of a recombination of electron-hole generated in the host can transfer nonradiatively to the excited states of Ln3+ ions.57,58 In this work, an energy matching between the bandgap of TiO2 nanocrystals and excited multiplets of Nd3+ or Sm3+ allows the recombination of electron-hole pairs produced by UV light irradiation to transfer their energy to the neighboring excited states of Ln3+ (possibly the 4D1/2 state of Nd3+ and 4K15/2 of Sm3+), followed by nonradiative relaxation to 4F3/2 of Nd3+ and 4 G5/2 of Sm3+, producing near-infrared and orange-red emissions, respectively. Such host sensitization may be ascribed to a phonon-assisted ET process since relatively weak host-to-Ln3+ sensitization was observed at 10 K. Meanwhile, some portion of electrons in CB would also be captured by the defects in TiO2 nanocrystals and form the defect states. The de-excitation of electrons in defect states leads to a broadband emission at 470-730 nm (Figure 8b). Recombining the captured electrons in defect states with holes can also transit its energy to the excited multiplets of Ln3+ nearby and lead to sharp intense PL of Ln3+. A similar defect-mediated ET mechanism was proposed to interpret the PL of Ln3+ in a glasslike environment sensitized by mesostructured TiO2 thin film.33 4. Conclusions In conclusion, strong host-sensitized PL of Nd3+ and Sm3+ were realized upon excitation above the bandgap of anatase TiO2 nanocrystals at room temperature. From the viewpoint of PL spectroscopy, Ln3+ ions were effectively incorporated into TiO2 nanolattices via a chemical pathway, producing sharp and wellresolved CF transitions of Ln3+. The host sensitization may occur via phonon-assisted ET processes. The embedding of Ln3+

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