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J. Phys. Chem. C 2008, 112, 20246–20250
Sensitization and Protection of Lanthanide Ion Emission in In2O3:Eu Nanocrystal Quantum Dots Javier Vela,† Bradley S. Prall,† Pawan Rastogi,‡ Donald J. Werder,† Joanna L. Casson,† Darrick J. Williams,‡ Victor I. Klimov,† and Jennifer A. Hollingsworth*,† Chemistry DiVision and Materials Physics and Applications DiVision, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: October 27, 2008
Lanthanide ions are visible to near-infrared ultranarrowband emitters with long luminescence lifetimes and thus have potential applications in lasing, up-conversion, and bioimaging. However, lanthanide ions have specific and narrow absorption bands characterized by small cross sections, and their luminescence is vibrationally quenched in common solvents. To address these limitations, lanthanide ions were incorporated into a semiconductor nanocrystal quantum dot host matrix, where the semiconductor matrix can sensitize the lanthanide ions toward absorption and reduce solvent quenching effects. Specifically, functional, colloidal europium-doped indium(III) oxide nanocrystal quantum dots were synthesized and characterized structurally and optically. Incorporation of the dopant ions into the matrix, rather than simple adhesion to the quantum dot surfaces, was demonstrated by applying a novel chemical extraction procedure utilizing ethylenediaminetetraacetic acid followed by quantitative elemental analysis. Sensitization and protection of europium ion emission by the indium(III) oxide semiconductor host were confirmed using photoluminescence excitation and time-resolved photoluminescence spectroscopies, respectively. Introduction Lanthanide ions are stable, visible to near-infrared emitters with long luminescence lifetimes (microseconds to milliseconds).1 These characteristics make them of interest for optical amplification and lasing, where long lifetimes support the creation of the required population inversion,2 photon upconVersion, where long lifetimes support the necessary excitedstate absorption processes,2 and bioimaging, where long lifetimes permit facile discrimination between background autofluorescence (short-lived) and the lanthanide optical tag signal using simple time gating techniques.1 Unfortunately, lanthanide ions have specific, narrow absorption bands with small cross sections, and their luminescence is vibrationally quenched in protic solvents by O-H and N-H vibrations. Current research is thus focused on sensitizing lanthanide ions toward absorption and improving their emission efficiencies by isolating them from their chemical environment.1,2 Previously it has been reported that lanthanide luminescence quenching can be suppressed by doping the lanthanide ions into the crystalline lattice of rare earth nanocrystals, e.g., LaF3, NaYF4, LaPO4.2,3 However, these matrices are optically “silent” and do not sensitize lanthanide ions toward absorption. An optically active rare earth matrix, YVO4, in which the lanthanide ions can be excited by way of energy transfer from the VO43groups that are themselves optimally excited at 270 nm, has also been reported.4-6 An alternative class of matrix materials for doping lanthanide ions comprises semiconductor nanocrystal quantum dots (NQDs). Similar to YVO4, NQD matrices offer the potential to simultaneously protect and sensitize lanthanide ion emission. NQD crystal lattices afford a protective, lowenergy environment unlikely to cause vibrational quenching of
lanthanide emission. In addition, as semiconductor materials, NQDs can serve as effective “antenna materials.” They are characterized by large absorption cross sections over a wide spectral range and NQD-size tunable absorption onsets.7 This latter feature distinguishes them from the YVO4-based matrices in that the energies required to excite an NQD matrix can be tuned according to NQD composition and size over a wide spectral window. Finally, NQDs are demonstrated efficient energy transfer partners for other NQDs,8 dye molecules9 and transition metal luminescent ions.10 It has also been previously demonstrated that TiO211 and II-VI NQDs (CdSe,12 CdS13) can serve as optically active energy transfer matrices for lanthanide ion sensitization. Here, we utilize In2O3, a III2VI3 NQD, as an active matrix for protecting and sensitizing Eu3+ emission. With our system, we achieve sensitized lanthanide emission with significantly improved quantum yields compared to the previously reported NQD-based systems,11,12 and we conclusively show simultaneous protection of the lanthanide emission.12 Finally, we demonstrate a simple and effective chemical extraction procedure for providing definitive chemical evidence for lanthanide incorporation that is generalizable to other NQD/lanthanide systems or, more broadly, other NQD/dopant systems. In2O3 is a near-ideal matrix for Eu3+ ion incorporation, sensitization, and protection. Photoluminescence (PL) of our In2O3 NQDs is centered at 390 nm, nearly the position of the strongest Eu3+ absorbing transition. Further, In2O3 crystallizes in the cubic bixbyite structure, also the preferred ambient structure for Eu2O3. Lastly, In2O3 is the most ionic of the Inbased NQDs;14 therefore, replacement of In3+ in In2O3 NQDs with hard Eu3+ ions is chemically supported. Experimental Methods
* Corresponding author. E-mail:
[email protected]. † Chemistry Division. ‡ Materials Physics and Applications Division.
General Information. Indium(III) acetate (In(OAc)3, Strem, 99.99%-In), europium(III) nitrate (Eu(NO3)3•(H2O)6, Acros,
10.1021/jp8074749 CCC: $40.75 2008 American Chemical Society Published on Web 11/25/2008
Lanthanide Ion Emission in In2O3:Eu NQDs 99.5%), europium(III) acetate (Eu(OAc)3•(H2O)n, n e 4, Aldrich, 99.9%), trimethylamine-N-oxide (TMNO, Aldrich, 98%, anhydrous), oleylamine (Aldrich, tech., 70%), oleic acid (Aldrich, tech., 90%), and the disodium salt of ethylenediaminetetraacetic acid (Na2(EDTA)•(H2O)2, Fisher, ACS grade) were used as received. Anhydrous Eu(NO3)3 was obtained after quantitative thermal removal of water from Eu(NO3)•(H2O)6 at 200 °C while stirring under dynamic vacuum overnight. Octadecene (Aldrich, tech. 90%) and trioctylphosphine (TOP, Strem, g 99%) were degassed and dried over activated type 4A molecular sieves prior to use. Decanol (Acros, 99%) was degassed prior to use. Elemental analyses were obtained using inductively coupled plasma optical emission spectrometry (Galbraith Laboratories of Knoxville, TN). Synthesis of Eu-Doped In2O3 NQDs. Method I. This procedure made use of the synthesis of undoped In2O3 nanocrystals recently reported by Peng and co-workers.15 A mixture of either Eu(NO3)3 · (H2O)6 or Eu(OAc)3 · (H2O)4 (x mmol), In(OAc)3 (y mmol), (x + y ) 0.1 mmol), myristic acid (137 mg, 0.6 mmol) and octadecene (6 mL) was evacuated under dynamic vacuum for 1 h while stirring at 120 °C. The resulting colorless homogeneous mixture was then backfilled with dry argon, heated to 290 °C, and stabilized at this temperature over a 10 min period. A solution of decanol (100 µL, 0.5 mmol) in octadecene (0.5 mL) was then quickly injected, causing a sudden drop in temperature to ca. 265 °C. The mixture reached 290 °C again within 1 min after injection, and the growth reaction was continued at this temperature. Aliquots were taken at periodic intervals for analysis. Qualitatively, it was observed that longer reaction (“growth”) times (from 7 to 65 min), or postisolation annealing, led to higher Eu3+ luminescence intensities. Method II. This procedure follows from the synthesis of undoped In2O3 nanocrystals reported by Fang and co-workers.16 A mixture of Eu(NO3)3 · (H2O)6 (x mmol), In(OAc)3 (y mmol), (x + y ) 0.1 mmol), oleic acid (0.6 mL, 1.9 mmol), oleyl amine (0.5 mL, 1.5 mmol), and octadecene (6 mL) was evacuated under dynamic vacuum for 1 h while stirring at 140 °C. After backfilling with dry argon, the resulting colorless homogeneous mixture was heated to 290 °C and stabilized at this temperature over a 10 min period. A freshly made hot solution of TMNO (109 mg, 1.45 mmol) in octadecene (3 mL) was then quickly injected into the mixture, causing a sudden drop in temperature to ca. 255 °C. The reaction proceeded at 290 °C for up to 15 min. Isolation (Methods I and II). After cooling to below 60 °C, the crude mixture containing the In2O3 NQDs was opened to air and an equal volume of hexane was added. The In2O3 NQDs were precipitated by the addition of an acetone-methanol mixture (3:1 ratio by volume) and separated from the mother liquor by centrifugation and decantation. The NQDs were redissolved in hexane and centrifuged to remove a very small fraction of insoluble precipitates. EDTA Extraction Procedure for Removing Unincorporated Surface-Bound Dopants. This procedure was applied to In2O3:Eu NQDs, as well as control In2O3 NQD samples. A solution of NQDs in hexane (5 mL) was mixed with a solution of Na2(EDTA) · (H2O)2 (37 mg, 0.1 mmol) in deionized water (5 mL). The biphasic mixture was vigorously stirred at room temperature (21 °C) for 4 h, and then allowed to settle. The lighter hexane fraction was separated, filtered through a KimWipe plough in a pipet, and mixed with 5 mL of clean deioinized water. Synthesis of Eu2O3 and YVO4:Eu Nanocrystals as Eu(III) Control Samples. Eu2O3 and YVO4:Eu(5%) nanocrystals were synthesized according to recent literature procedures.5,6,17 The
J. Phys. Chem. C, Vol. 112, No. 51, 2008 20247 Eu2O3 nanocrystals were analyzed both in MeOH solution and as solid films after drop casting onto a glass slide. A saturated solution of Eu(III) nitrate in oleyl amine was prepared by gently heating and stirring oleylamine with an excess of Eu(NO3) · (H2O)6 inside a closed vessel overnight. A saturated solution of dry Eu(III) nitrate in TOP was made by gently heating and stirring dry TOP with an excess of anhydrous Eu(NO3) under a dry argon atmosphere overnight. Optical Characterizations by Steady-State and TimeResolved Spectroscopies. Steady-state absorption spectra [continuous wave (cw) excitation] were recorded on a Varian Cary5000 UV-vis-NIR spectrophotometer using 1 cm path length quartz cuvettes. cw-PL spectra were recorded on a Horiba-Jobin Yvon Nanolog fluorometer using 1 cm path length quartz cuvettes in a 90° geometry. Quantum yields were measured in hexane or toluene relative to methanolic solutions of coumarin450 (Exciton) or rhodamine590 (Exciton) by known procedures.18 Emission intensities in nonpolar solvents were compared to those obtained for nanocrystals suspended in water. Time-gated and time-resolved emission experiments were performed using the third harmonic of a 10 Hz Nd:Yag laser (Spectra Physics Quanta Ray) as the excitation source. One millimeter path length cuvettes were used. The emission was monitored at 90° and spectrally resolved using a 0.3 m spectrograph onto an intensified charge-coupled device (CCD, Roper Scientific). In the time-gated experiment, the emission spectra were collected starting 5 µs after laser excitation with a gate duration of 10 ms. The time-resolved spectra were taken with a 25 µs gate duration stepped from -0.1 to 3 ms. In2O3: Eu NQDs were studied by time-resolved PL, both as-isolated (untreated) and after EDTA treatment. To study the effect of water, both untreated and EDTA-treated samples were reanalyzed by time-resolved PL after exposure to excess water: Briefly, hexane solutions of the In2O3:Eu NQDs were stirred with an equal volume of deionized water for 12 h at room temperature. The two phases were then allowed to separate, and the water-saturated hexane phase was studied. Intrinsic (natural) Eu3+ radiative lifetimes (τR) for the 5D0 (f 7Fn) state were calculated by the method of Verhoeven,19 using the formula
1 ⁄ τR ) AMD,0 × n3 × (Itot ⁄ IMD)
(1)
where n is the refractive index of the medium, equal to 1.37 (hexane);20,21 AMD,0 is the spontaneous emission probability for the 5D0 f 7F1 transition in a vacuum, equal to 14.65 s-1; and (Itot/IMD) is the ratio of the total Eu3+ emission spectrum to the area of the 5D0 f 7F1 band.20 The corrected radiative lifetime for Eu3+ in In2O3:Eu NQDs dissolved in hexane taking into account the dielectric interface effect between In2O3 and hexane was calculated using the formula
τR-corrected ) τR[(εIn203 + 2 × εhexane) ⁄ (3 × εhexane)]2
(2)
where ε is the dielectric constant (n2). Structural Characterizations. Transmission electron microscopy (TEM) studies were done on a JEOL 2010 operating at 200 kV and a FEI Tecnai G2 30 (S) TEM operating at 300 kV. Elemental analyses were performed on the Tecnai using an EDAX EDAM III energy dispersive spectrometer (EDS) with a Si(Li) detector. FEI TIA software was used to analyze the collected EDS data. X-ray diffraction (XRD) measurements were made on a Rigaku Ultima III diffractometer with a fine line sealed Cu tube that generates KR (λ ) 1.5406 Å) X-rays. The generator is a D/MAX Ultima series with a maximum power of 3 kW. The samples were mounted on a backgroundless silicon slide. Data
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Figure 1. Representative powder XRD pattern of In2O3:Eu NQDs. Inset: The XRD peaks shift to lower 2θ values upon increasing Eu3+ content. This result is qualitatively consistent with lattice expansion expected from relative ionic radii (CN ) 6; In3+, 0.94 Å vs Eu3+, 1.09 Å). The peaks identified with an asterisk arise from Si powder that was used as an internal standard.
were collected in continuous-scan mode in parallel beam slit geometry over the 2θ range from 10-90° with a sampling width of 0.005° and a scanning speed of 0.100°/min. The divergence slit was set to 1.0 mm; the divergence height-limiting slit was set at 10 mm, and the scattering and receiving slits were set to open. Solid nanocrystal samples were mixed with a small amount of silicon powder as an internal standard. A Vegard’s law plot was prepared based on lattice parameters extracted from analysis of the (400) reflection (Figure 1, inset). Results and Discussion Demonstrating Successful Incorporation of Eu(III) Dopant into In2O3 NQDs. Given the apparent spectral, structural, and chemical compatibility between lanthanide dopant and NQD matrix in the In2O3:Eu system, we sought to prepare these Eudoped In2O3 NQDs by adapting the synthesis of undoped In2O3 NQDs (Experimental Methods).15,16 Structural characterization by XRD of thusly prepared In2O3:Eu NQDs revealed that these nanocrystals index to the In2O3 cubic pattern (Figure 1). TEM showed single-crystalline particles with some variations in size and shape depending upon Eu content. Specifically, low Eu contents (0.5-5%) resulted in irregularly faceted NQDs (11.9 ( 2.2 nm) (Figure 2a) that were smaller than undoped In2O3 NQDs (∼20 nm) obtained using similar growth conditions. It is likely that the presence of the dopant ions in the reaction mixtures slows down the rate of NQD growth, resulting in smaller particles. This growth-suppressing effect can be understood by viewing the dopant ions as impurities that can affect both nucleation and growth of the host nanocrystal. With respect to nucleation, the presence of such impurity ions at or near the surface of a small crystallite can increase the “effective specific interfacial energy,” shifting the equilibrium (solvated ions T crystalline nuclei) toward solvation.22 In addition, impurities can inhibit nanocrystal growth, as surface and near-surface impurity ions can introduce activation barriers to further propagation of the growing nanocrystal.22 At higher Eu content (15-50%), a mixture of NQD clusters and “nanoflowers” (>50 nm) was obtained (Supporting Information). It has been previously shown that In2O3 nanoflowers form by oriented attachment in a limited ligand protection regime.15 Here, high %Eu may support a dopant-induced limited ligand protection regime because Eu3+ attains higher coordination numbers than does In3+ in solution, effectively decreasing the capping ligands available for NQD stabilization. Elemental analyses were obtained for In2O3:Eu NQDs using inductively coupled plasma optical emission spectrometry
Vela et al.
Figure 2. (a) Low- (left) and high-resolution (right) TEM images of In2O3:Eu NQDs (5% Eu). (b) Absorption (Abs), PL, and PLE spectra of In2O3:Eu NQDs. (c) PL spectra of In2O3:Eu NQDs at variable excitation wavelengths from 290 nm (highest Eu3+ intensity) to 535 nm (lowest Eu3+ intensity) stepped in increments of 15 nm, with the exception of the final two energies, which are 465 and 535 nm, respectively. Inset: PL intensity as a function of Eu content (calculated % Eu from 0.5-28%). (d) PL and PLE of europium(III) nitrate/TOP Eu3+ control sample.
TABLE 1: Eu and In Content in In2O3:Eu NQDs before and after EDTA Extraction europium [Eu] in In2O3:Eu NQDs (% metals) [Eu]/[In] nitrate used ratio in in reactiona calculated experimental after EDTA extracts no. (µmol) 5b 5c 15b 30b 50b
1 2 3 4 5
5 5 15 28 50
11 5
8
44 60
36 48
0.22 0.45 1.3
All: [In + Eu] ) 0.1 mmol, 0.6 mmol myristic acid, 6 mL octadecene, 290 °C. b 500 µmol decanol, 7 min reaction. c 1.45 µmol TMNO; oleylamine and acid added. a
(Galbraith Laboratories). These results demonstrated that we were successful in incorporating Eu into In2O3 from 0.5 to 50% Eu (vs total metals). In fact, the Eu content was 6-16% higher than expected from Eu loadings used synthetically (Table 1). Closer-to-nominal Eu concentrations were obtained when NQDs were prepared using europium(III) acetate instead of europium(III) nitrate, or when oleylamine/oleic acid and TMNO were used as capping ligands and oxygen source, respectively (Table 1, no. 2), where the latter comprises a more chemically active oxygen source compared to decanol. Precursor identity has been observed previously to affect dopant incorporation rate.23 EDTA Extraction Method for Removal of Surface-Bound Dopants. We also obtained nominally truer [Eu]:[In] ratios by implementing a unique EDTA extraction procedure that we developed for the purpose of removing surface-bound Eu. Further, using this method, we were able to demonstrate that our lanthanide dopant ions were incorporated into the NQD host matrix and not primarily adsorbed to its surface. This is important because previous reports on transition metal doped II-VI NQDs have established that dopant ions tend to reside near or at the NQD surface.10,22,24,25 Such surface-associated dopant ions have traditionally been removed using a ligand exchange procedure involving pyridine, as has been reported for Mn-doped CdSe NQDs, for example.24 We initially attempted to use this approach to remove excess, unincorporated Eu3+ ions; however, we observed that Eu-doped In2O3 NQDs were insufficiently soluble in pyridine, rendering this approach
Lanthanide Ion Emission in In2O3:Eu NQDs impractical. For this reason, we subsequently developed a biphasic hexane-water EDTA surface treatment for this purpose. Critically, elemental analyses showed a 3-12% drop in Eu content in the NQDs following EDTA treatment, resulting in post-EDTA %Eu values for the Eu-doped In2O3 NQDs closer to calculated Eu content based on reaction concentrations (Table 1, nos. 1, 4-5). Further, elemental analysis of the EDTA water extracts showed the presence of both In and Eu, with the [Eu]/ [In] ratio increasing with the NQD Eu content. This result suggests partial etching of the In2O3 host matrix and is actually not surprising given that EDTA-In binding is strong, with relative thermodynamics actually favoring EDTA binding to In over Eu.26 Significantly, however, the [Eu]/[In] ratio in the extract is always substantially higher than either the calculated or the experimental [Eu]/[In] ratios for the NQDs, indicating that through this process we are indeed primarily (and successfully) removing unincorporated, surface-adsorbed “excess” Eu. Demonstrating Sensitization of Eu3+ Ion Emission by the In2O3 NQD Host. Examination of In2O3:Eu NQDs in hexane by steady-state PL spectroscopy showed a broad In2O3 NQD emission centered at 390 nm and sharp peaks typical of Eu3+derived luminescence at 588 nm (5D0f7F1), 614 nm (5D0f7F2), and 698 nm (5D0f7F4) (full-width at half-maximum ) 7-18 nm) (Figure 2b).1 An excitation wavelength (λexc) of 330 nm was used in order to avoid direct excitation of Eu3+ while permitting excitation through the In2O3 matrix. PL quantum yields ranged from 1 to 6% for In2O3 emission and from 0.1 to 0.6% for sensitized Eu3+ emission. The latter is ∼1000-fold higher than that previously reported for Tb3+ in CdSe NQDs12 and ∼10-fold higher than that previously reported for Eu3+ in TiO2 NQDs.11 The maximum PL was observed for 15-30% Eu loadings, and a clear decline was observed for higher Eu content. An even stronger dependence on reaction time was obtained: growth times of 30 min compared to 7 min provided a nearly 10-fold enhancement in Eu3+ emission. To confirm Eu3+ sensitization through the NQD host, the dependence of Eu3+ PL intensity on λexc was determined. PL spectra were recorded for variable excitation wavelengths and showed a steady increase in PL intensity for all Eu3+ peaks with increasingly shorter λexc (Figure 2c). Further, photoluminescence excitation (PLE) spectra were obtained for the In2O3:Eu NQDs at the main Eu emission wavelength (614 nm; Figure 2b). As anticipated for an energy-transfer-coupled system, the Eu3+ PLE clearly resembles the PLE spectrum of the In2O3 host, as well as its broadband absorption spectrum (Figure 2b). These observations are inconsistent with direct Eu3+ excitation that is characterized by sharp transitions at 376, 380, 395, and 464 nm (e.g., PLE for europium(III) nitrate, a simple Eu3+ salt; Figure 2d) and consistent with sensitized Eu3+ excitation. It should be noted that Eu has recently been doped into In2O3 nanocrystals by way of a different chemical synthesis approach. In contrast with our results, however, no sensitization through the In2O3 matrix was observed.27 Demonstrating Protection of Eu3+ Ion Emission by the In2O3 NQD Host. To confirm that incorporation of Eu3+ into a nanoscale In2O3 matrix effectively protects Eu3+ emission from quenching by way of interaction with O-H and N-H chemical bonds, we performed time-resolved PL spectroscopy and compared the time decay in Eu3+ emission intensity (emission wavelength, λem, ) 614 nm) for In2O3:Eu NQDs with that for different Eu3+ control samples in various solution environments (Figure 3a). In2O3:Eu NQDs displayed a biexponential timedecay with time constants of ∼0.6 ms (∼70%) and ∼0.07 ms (∼30%). Critically, with respect to our aim to protect Eu3+
J. Phys. Chem. C, Vol. 112, No. 51, 2008 20249
Figure 3. (a) PL time-decay (λem ) 614 nm, λexc ) 355 nm) for 50% (red squares) and 5% Eu (blue diamonds) In2O3:Eu NQDs, dry europium(III) nitrate/TOP (black plusses), europium(III) nitrate/oleylamine (black circles), and Eu2O3 nanocrystals/methanol (gray triangles). (b) Relative Eu3+ ion emission intensities for 5% Eu In2O3:Eu NQD solutions (7 min reaction, post-EDTA, dry or water-saturated solutions) (blue diamonds), dry europium(III) nitrate/TOP (black plusses), europium(III) nitrate/oleylamine (black circles), and Eu2O3 nanocrystals/ methanol (gray triangles) (λexc ) 350 nm, hexane solvent with the exception of Eu2O3 nanocrystals for which methanol was used). Note: Spectra were parametrized to optical densities at 350 nm, but not to relative Eu3+ concentrations. Thus, because the control samples are highly concentrated, it is expected that the relatiVe Eu3+ emission intensity for the In2O3:Eu NQDs is higher than shown here.
emission, the time decay of the In2O3:Eu NQDs was unaffected by exposure to excess water and was similar to the time decay of a solution of carefully dried europium(III) nitrate in anhydrous TOP, a “protective” ligand (Figure 3a). In contrast, solutions of europium(III) nitrate in oleylamine (N-H bearing) or in wet solvents (O-H bearing), and of trioctylphosphine oxide-capped Eu2O3 nanocrystals in methanol (O-H bearing) made by a literature procedure,17 showed much faster time decay (Figure 3a), with biexponential time-constants of ∼0.2 ms (∼30%) and ∼0.08 ms (∼70%). Faster decay constants translated into lower emission intensities for these control samples compared to the In2O3:Eu NQDs (Figure 3b). The In2O3:Eu NQD time constant was used to estimate the intrinsic quantum yield of the Eu3+ ions.19 Assuming a radiative lifetime for Eu3+ of 9.0 ms (taking into account the dielectric interface effect between In2O3 and hexane),28 we obtained moderate values of 6%. The significant difference between the intrinsic quantum yield and the sensitized quantum yield implies that In2O3 NQD emission remains favored over energy transfer to Eu3+. Nevertheless, PLE indicates that sensitization by the In2O3 host is the dominant mechanism for Eu3+ excitation. As expected, direct Eu3+ excitation is not evident in absorption spectra (clearly dominated by In2O3 absorption), nor, in general, in PLE. The PLE in Figure 2b is shown as the only observed exception (where the 395 nm Eu3+ absorption feature is just visible). Even here, the sensitization factor (i.e., the ratio of sensitized Eu3+ PLE intensity over the direct Eu3+ PLE intensity) ranges from 10-20 at λexc’s for which there is no strong Eu3+ peak (i.e., 350-330 nm, respectively). The In2O3 matrix thus provides for facile excitation at wavelengths inaccessible by direct Eu3+ excitation. Potential Utility of In2O3:Eu NQDs in Optical Applications: Fluorescent Biolabeling Example. The very low quantum yields obtained for previously reported lanthanide-doped NQD systems imply a limited utility in light-emitting applications. The new In2O3:Eu NQDs are substantially improved (10-1000-fold enhancement in quantum yields). Importantly, however, the previously reported YVO4:Eu nanocrystals are characterized by even higher quantum yields (∼15%) and have also been suggested for optical applications, such as fluorescent biolabels. However, in order to take advantage of the optically active YVO4 as a sensitizing matrix, the excitation wavelength
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Vela et al. References and Notes
Figure 4. Comparison of the excitation-energy dependence of the Eu3+ emission intensity for (a) YVO4:Eu nanocrystals and (b) In2O3:Eu NQDs. Excitation wavelengths are shown in the figure insets.
must be approximately 280 nm or “bluer.” Employing excitation wavelengths “redder” (lower energy) than this results in a quick decline in PL intensity (Figure 4a). In contrast, excitation of In2O3:Eu as far red as 400 nm provides for reasonable retention of the maximum achievable intensities afforded by this system (Figure 4b). With respect to potential application as fluorescent biolabels, the In2O3:Eu NQDs show a beneficial insensitiVity to exposure to water. Specifically, saturation of In2O3:Eu NQD hexane solutions with water did not impact PL lifetimes nor PL intensity (Figure 3a,b). In contrast, addition of water to an Eu(III) nitrate-TOP control sample led to complete suppression of Eu3+ ion emission over time. Further, the PL intensity of In2O3:Eu NQDs fully suspended in water was unchanged from that obtained for these NQDs in hexane. Conclusions We have prepared soluble In2O3:Eu NQDs, where Eu3+ luminescence is sensitized and protected by the In2O3 NQD host. The observed dopant incorporation is essentially stoichiometric. Our sensitized emission quantum yields are 10-1000 times higher than those previously reported for lanthanide-doped NQDs, while our intrinsic Eu3+ quantum yields are >5%. We have also developed an EDTA treatment that removes surfacebound Eu3+ ions and should be applicable to other doped-NQD systems. Current efforts are directed at understanding the precise sensitization mechanism, as well as any inherent barriers to higher NQD-to-lanthanide energy transfer efficiencies. Acknowledgment. Research support included the DOE Center for Integrated Nanotechnologies (J.A.H., V.I.K.), LANL Director’s Postdoctoral Fellowships (J.V., B.S.P.), and the Science Undergraduate Laboratory Internship Program (P.R.). Supporting Information Available: Additional optical and structural characterization. This material is available free of charge via the Internet at http://pubs.acs.org.
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