Near-Infrared-to-Near-Infrared Downshifting and Near-Infrared-to

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Near-Infrared-to-Near-Infrared Downshifting and Near-Infrared-toVisible Upconverting Luminescence of Er3+-Doped In2O3 Nanocrystals Qingbo Xiao, Haomiao Zhu, Datao Tu, En Ma, 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 S Supporting Information *

ABSTRACT: Luminescent lanthanide-doped nanoparticles (NPs) excitable in the near-infrared (NIR) spectral region are highly desired as new optical bioprobes in the fields of biological assays and medical imaging. Er3+-doped In2O3 NPs, synthesized via a facile sol−gel solvothermal method, exhibit intense and wellresolved NIR-to-NIR downshifting (DS) and NIR-to-visible upconversion (UC) dual-mode luminescence upon NIR excitation at 808 and 980 nm, respectively. Forty-one crystalfield levels below 23 000 cm−1 were identified for Er3+ at a single lattice site of C2 symmetry in In2O3 NPs by means of highresolution site-selective NIR spectroscopy at 10 K and room temperature. Furthermore, the luminescence dynamics for the UC emissions were systematically investigated, and various UC processes were clearly distinguished based on excited state dynamics and rate equation analysis. Mediated by the long-lived intermediate states of 4I11/2 and 4I13/2, the decay times of 4F9/2 (or 4S3/2) due to energy transfer upconversion (ETU) were found to be approximately 1 order of magnitude larger than that of DS luminescence, namely, increasing from ∼20 μs to ∼0.51 ms (or ∼0.24 ms). It was revealed that the contribution from the ETU process to the UC luminescence of 4S3/2 and 4F9/2 increased significantly along with the increase of the ETU probability of Er3+ at high doping concentration. NIR-to-NIR DS luminescence of Ln3+-doped NPs can be excited by a regular light source (like xenon) with a relatively low power density, which may provide less possible damage in bioapplications. To date, most of the previous studies on NIR-to-NIR DS luminescence were restricted to NIR emissions of Nd3+ at ∼1060 nm,19−21 and NIR-to-NIR DS luminescence at 750−1100 nm for other Ln3+ in NPs such as Er3+ has been rarely reported. It is anticipated that efficient NIR luminescence of the 4I13/2 → 4I15/2 transition at ∼980 nm can be achieved in Er3+-doped NPs upon direct excitation of 4I11/2 at 808 nm. In view of their respective merits from NIR-to-visible UC and NIR-to-NIR DS luminescence, the achievement of dual-modal luminescence of Er3+ in a single kind of NP may promote their new applications as multifunctional nanoprobes such as deep tissue bioimaging with NIR-to-NIR DS luminescence as well as photodynamic therapy with NIR-to-visible UCL. Cubic sesquioxide In2O3, a well-known wide bandgap semiconductor, is recognized as one of the promising host candidates for both UC and DS luminescence through Er3+ doping. NIR luminescence of Er3+ in In2O3 nano- and microstructures has been occasionally reported before.22−25

1. INTRODUCTION Recently the research of trivalent erbium (Er3+)-doped inorganic nanoparticles (NPs) has become prominent in the fields of biological sensing and medical imaging owing to their unique photoluminescence (PL) properties upon near-infrared (NIR) excitation.1−5 Compared to conventional ultraviolet (UV) excitation, NIR excitation within the spectral range of 750− 1100 nm is highly desirable in view of its intrinsic advantages in biological applications such as deep light penetration, low photodamage, and low autofluorescence to biological specimens. Upon 980 nm excitation, efficient NIR-to-visible upconversion (UC) of Er3+ in NPs also allows high spatial resolution for in vivo observation due to the large apparent anti-Stokes shift.6−13 Moreover, the UC luminescence (UCL) lifetime for the excited states of Er3+ may be prolonged to the millisecond range mediated by long-lived intermediate levels of 4I11/2 and 4I13/2, which thus enables the UCL to be further distinguished from the short-lived background luminescence by utilizing the timeresolved PL (TRPL) detection. Different from the NIR-to-visible UCL, the NIR-to-NIR UC3,14−18 and downshifting (DS) luminescence of lanthanide (Ln3+)-doped NPs with both excitation and emission in the spectral region of 750−1100 nm can provide a high signal-to-noise ratio within the optical transparency window of biological tissues. Compared to the UC excitation that involves laser-induced multiphoton processes, © XXXX American Chemical Society

Received: March 28, 2013 Revised: April 29, 2013

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dx.doi.org/10.1021/jp4030552 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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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 NIR-to-NIR DS and NIR-to-visible UCL spectra were measured upon laser excitation at 808 and 980 nm at RT, respectively, with a pump power of ∼320 mW (power density ∼6 W/cm2) provided by a mode-locked picosecond Ti:sapphire laser (700−1000 nm, 80 MHz, pulse width ≤1.5 ps, Tsunami, Spectra-Physics). TRPL spectra, NIR DS, and visible UCL lifetimes at room temperature (RT) were measured with a customized UV to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable midband OPO pulse laser as the excitation source (410−2400 nm, 10 Hz, pulse width ≤5 ns, Vibrant 355II, OPOTEK).

However, a vast of majority of previous efforts were devoted to the 4I13/2 → 4I15/2 transition of Er3+ at around 1.54 μm for potential application in optical communications. To date, no effort toward the NIR-to-NIR DS and NIR-to-visible UC luminescence of Er3+ in In2O3 NPs has been attempted. Moreover, although the UC phenomenon of Er3+ in inorganic materials such as fluorides and oxides has been extensively investigated for the past decades, the well-established UCL mechanism, particularly to distinguish various UC processes for Er3+-doped NPs based on UCL dynamics and rate equation analysis, is still lacking. With the tremendous advances in the controlled synthesis, surface modification, and assembly of inorganic NPs such as In2O3, it is urgent to gain more insights into the UCL mechanism that remains nearly untouched in these nanosystems to optimize their optical performance for versatile bioapplications. For example, Vetrone et al. reported enhanced UC red emission relative to the green emission for Er3+-doped Y2O3 NPs with increasing concentration of Er3+, which resulted from the interionic cross-relaxation in populating the 4F9/2 state that bypassed the green-emitting states.26 By contrast, such strong cross-relaxation was not observed in In2O3:Er3+ NPs, and the enhanced UC red emission was ascribed to efficient energy transfer upconversion (ETU) of Er3+ at high concentration based on the steady-state and transient UC and DS spectroscopic evidence. Therefore, a comprehensive survey of dual-mode luminescent properties of Er3+ in In2O3 NPs under NIR excitation is of vital importance for their further applications. Herein, we report for the first time both the NIR-to-NIR DS and NIR-to-visible UC emissions for Er3+-doped In2O3 NPs (∼20 nm) upon excitation at 808 and 980 nm, respectively. More than 40 crystal-field (CF) levels of Er3+ ions, which occupy a single lattice site with site symmetry of C2, are identified by means of high-resolution site-selective spectroscopy at 10 and 300 K. Furthermore, the UCL mechanism of Er3+ in In2O3 NPs is revealed based on the UC/DS spectra and PL dynamics. With the aid of theoretical modeling on the observed luminescence dynamics, the contributions from different UC processes that are dependent on the dopant concentration are distinguished for the green and red emitting states of Er3+ in In2O3 NPs.

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure Characterization. Er3+ ions doped with In2O3 NPs were synthesized via a facile solvothermal method. The TEM image shows that In2O3:Er3+ NPs are irregularly spherical with the diameters ranging from 16 to 22 nm (Figure 1a). The XRD patterns of In2O3:Er3+ samples

2. EXPERIMENTAL SECTION Er3+ ions doped with In2O3 NPs, with nominal concentrations of 0.5, 2.0, and 4.0 atom %, respectively, were prepared via a simple solvothermal method similar to the report elsewhere.27 For better crystallinity and enhanced luminescence of In2O3:Er3+ NPs, the as-grown samples (dried at 60 °C for 12 h) were further annealed at 600 °C for 2 h to yield the final products. The precise concentrations of Er3+ were determined to be 0.6, 2.5, and 4.4 atom % by the Ultima2 ICP optical emission spectrometer. For comparison, the submicrometer counterparts of In2O3:Er3+ were obtained by annealing the as-grown samples at 900 °C for 2 h. Powder X-ray diffraction (XRD) patterns were collected on a PANalytical X’Pert PRO powder diffractometer with Cu Kα1 radiation (λ = 0.154 nm). The morphology of the samples was characterized by a JEOL-2010 transmission electron microscope (TEM). The ultraviolet−visible (UV/vis) diffuse reflectance spectrum of In2O3:Er3+ NPs was measured by a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer with BaSO4 as a reference. PL spectra and lifetimes were measured on an Edinburgh Instruments FLS920 spectrofluorimeter equipped with both continuous (450 W) and pulsed xenon lamps. For lowtemperature measurements, samples were mounted on a closed

Figure 1. (a) TEM image of In2O3:Er3+ NPs, (b) XRD pattern, and (c) plot of F(R)2 versus photon energy of In2O3:Er3+ samples. The inset of (c) enlarges the regions at low energies showing the characteristic intra4f absorption peaks of Er3+ ions in In2O3:Er3+ NPs .

can be well indexed as cubic bixbyite In2O3 (JCPDS No. 71-2194, space group Ia3), and no trace of other characteristic peaks was observed for impurity phases such as Er2O3 (Figure 1b). By means of the Debye−Scherrer equation, the average sizes of In2O3:Er3+ NPs and the submicrometer counterparts were estimated to be ∼22 and 95 nm, respectively. The Kubelka−Munk function F(R) is usually used to determine the bandgap of the semiconductor according to the diffuse reflectance spectra,28,29 with F(R) = (1 − R)2/2R, and R is the reflectance from an infinitely thick sample. Using the reflectance of BaSO4 as a reference, R can be expressed as R = Rsample/RBaSO4, where Rsample is the reflectance of the samples obtained from Ultraviolet−visible (UV/vis) diffuse reflectance spectra. For the direct bandgap semiconductor, F(R) ∝ (hv − Eg)1/2, where hv is the photon energy and Eg is the bandgap energy. Figure 1c shows the plot of F(R)2 versus hv for direct transition of In2O3:Er3+ samples. The bandgap energies of B



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In2O3:Er3+ NPs and the submicrometer counterparts were determined to be 3.57 and 3.27 eV, by the extrapolation to F(R)2 = 0, which are in good agreement with those observed in In2O3:Eu3+.27 Besides the strong bandgap absorption of In2O3 NPs, two typical f−f absorption peaks of Er3+ were also observed (inset of Figure 1c), which can be assigned to the transitions from the ground state 4I15/2 to 4F9/2 (1.86 eV) and 2H11/2 (2.37 eV), respectively. 3.2. Near-Infrared-to-Near-Infrared Downshifting Luminescence. As shown in Figure 2a and 2b, upon direct

theoretically predicted for 4I15/2 of Er3+ at the C2 site due to Kramers degeneracy (J + 1/2). To further probe the fine CF splittings experienced by Er3+ in In2O3 NPs, high-resolution excitation spectra in UV to NIR regions were measured at 10 K.

Figure 2. (a,b) NIR DS emission spectra of In2O3:Er3+ NPs at RT and 10 K. I, II, and III represent the hot-band transitions originating from the upper sublevels of 4I13/2 (or 4I11/2) to 4I15/2, respectively. (c,d) DS luminescence decays of 4I11/2 and 4I13/2 at 10 K and RT.

Figure 3. (a) 10 K excitation spectrum of In2O3:Er3+ NPs by monitoring the 4I13/2 → 4I15/2 transition at 1551.4 nm of Er3+, (b−g) enlargement of the high-resolution excitation peaks in different spectral regions, and peaks marked by circles are hot bands originating from the second lowest level of the ground state; (h) schematic energy diagrams of Er3+ ions and the bandgap of In2O3 samples.

excitation from ground state 4I15/2 to 4I9/2 at 808 nm, wellresolved emission peaks centered at 981.1 and 1551.4 nm of Er3+ were observed in the NIR region at RT and 10 K, which are attributed to the radiative transitions from the excited states of 4 I11/2 and 4I13/2 to 4I15/2, respectively. The full width at halfmaximum (fwhm) of the most intense peak centered at 1551.4 nm for the 4I13/2 → 4I15/2 transition decreases from 4.2 nm at RT to 1.4 nm at 10 K, which is about 7 times narrower than that of the 1.54 μm peak of Er3+ in In2O3 films.25 These emission peaks are completely different from that of cubic Er2O3 in terms of line positions and shapes,30 suggesting the incorporation of Er3+ ions in the lattice site of In2O3 NPs instead of the formation of Er2O3 clusters. It should be noted that the sharp and intense NIR emissions of 4I11/2 → 4I15/2 around 980 nm had not been observed in In2O3:Er3+ NPs before. Eight CF levels of 4I15/2 were determined to be 0, 33, 74, 92, 136, 192, 285, and 572 cm−1 according to the transitions from the lowest sublevels of 4I11/2 (10201 cm−1) and 4I13/2 (6520 cm−1) to sublevels of 4I15/2 at 10 K. Owing to the population distribution of Er3+ among the sublevels of 4I11/2 and 4I13/2 that obeys the Boltzmann distribution law, transitions from upper sublevels of 4I11/2 (10396 cm−1) and 4I13/2 (6587, 6699, and 6910 cm−1) to 4I15/2 can also be observed at RT (or 10 K) as assigned in Figure 2a and 2b. From the viewpoint of crystallography, there are two distinct sites available for Er3+ in cubic In2O3, a low symmetry site of C2 and a centrosymmetric site of S6. For Er3+ at the S6 site, only the magnetic-dipole (MD) transition is allowed with the condition of J = 0, ±1 (0 → 0 forbidden). The appearance of the 4I11/2 → 4I15/2 emission indicates that the NIR emissions are originated from Er3+ ions at the C2 site of In2O3 NPs. The eight emission lines assigned to the 4I11/2 → 4I15/2 transition of Er3+ in In2O3 NPs at 10 K agree well with the number of CF level splittings

As shown in Figure 3a, by monitoring the transition of 4I11/2 → 4 I15/2 at 981.1 nm, sharp peaks centered at 453.6, 489.2, 519.2, 566.7, 664.5, and 812.7 nm were observed in the excitation spectrum at 10 K, which can be assigned to the characteristic excitation of Er3+ from 4I15/2 to the excited states of 4F3/2, 4F5/2, 4 F7/2, 2H11/2, 4S3/2, 4F9/2, and 4I9/2, respectively. The enlarged high-resolution excitation peaks in different spectral regions are shown in Figure 3b−g. The excitation lines of different multiplets agree well with the expected degeneracy by careful inspection, thus verifying the single lattice site of C2 for Er3+ ions in In2O3 NPs. In addition to the excitation lines from the lowest CF level of 4I15/2, a number of hot bands with an energy gap of 33 cm−1 appear in the lower energy side. These hot bands originate from the transitions from the second lowest CF level of 4I15/2, which is very close in energy to the lowest CF level and thus can be partially populated at 10 K according to the Boltzmann distribution law. Through the high-resolution excitation and emission spectra at 10 K and RT, a total of 41 energy levels of Er3+ at the C2 site were identified and summarized in Table 1. For the In2O3:Er3+ NPs, no broad UV band corresponding to the bandgap excitation of In2O3 NPs was observed in the excitation spectrum, indicating the absence of energy transfer from the In2O3 host to Er3+ in In2O3:Er3+ NPs. In sharp contrast, an intense broad UV band centered at ∼379 nm was observed in the excitation spectrum of the submicrometer counterparts (Figure S1, Supporting Information). Upon bandgap excitation of In2O3 at 379 nm, intense NIR emissions were also observed in the emission spectrum, indicating an efficient ET from the In2O3 host to Er3+ ions for the submicrometer counterparts. As illustrated in Figure 3h, the conduction band energy of the submicrometer counterparts (26 396 cm−1) is close to the energy C



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Table 1. Energy Levels of Er3+ at the C2 Site of In2O3 Nanocrystals multiplet 4

I15/2

4

I13/2

4

I11/2

4

I9/2

4

F9/2

energy [cm−1] 0 33 74 92 136 192 285 572 6520 6587 6699 6910 10201 10396 12074 12146 12307 12524 12650 15124 15202

multiplet

4

S3/2

2

H11/2

4

F7/2

4

F5/2

4

F3/2

−

0.44 ms can be derived from the PL decays of 4I11/2 and 4I13/2 at 10 K, respectively, by fitting the initial stage in decay curves. The appearance of the rise time implies relatively slow nonradiative relaxation from the upper levels to 4I11/2 (or 4I13/2) at 10 K. The rise time of 4I13/2 is close to the PL lifetime of 4I11/2 (0.65 ms), indicating that the DS emission from 4I13/2 is predominantly populated from 4I11/2 at 10 K. The PL decays from 4I11/2 and 4 I13/2 of Er3+ at higher dopant concentration (4.4 atom %) were also measured at RT (not shown), and the corresponding PL lifetimes were fitted to be 0.52 and 3.82 ms, respectively. It can be seen that the effect of concentration quenching is insignificant for 4 I11/2 or 4I13/2 with the Er3+ concentration varying from 0.6 to 4.4 atom %, which is similar to that reported previously.36 3.3. Upconversion Luminescence Spectra. The NIR-tovisible UCL spectra of In2O3 NPs doped with various concentrations of Er3+ were measured upon excitation from 4 I15/2 to 4I11/2 of Er3+ at RT. As shown in Figure 4a, green and red

energy [cm−1] 15292 15349 15480 18216 18308 19009 19021 19041 19085 19203 19263 20348 20442 20523 20599 22048 22079 22195 22409 22595 −

level of 4G11/2 of Er3+ ions (∼26 400 cm−1),31 which facilitates the nonradiative energy transfer (ET) from the In2O3 host to Er3+ ions. Upon excitation above the bandgap, an electron−hole pair is formed in the In2O3 host. The recombination of the electron−hole pair will resonantly transfer its energy to the 4 G11/2 state of Er3+, followed by nonradiative relaxation to 4I11/2 and 4I13/2 states of Er3+, and then the characteristic NIR luminescence of Er3+ is observed. A similar ET mechanism has been proposed for the In2O3:Eu3+ samples.27 Differently, the bandgap energy of In2O3 NPs (28 818 cm−1) is coincidently located at an energy range between ∼28 000 (2G7/2) and 31 500 cm−1 (2P3/2) where no energy level of Er3+ exists.31 As no bridge states such as defect states or other Ln3+ ions exist,32−34 the hostto-Er3+ ET in In2O3:Er3+ NPs is inefficient in view of a large energy mismatch between the conduction band of In2O3 NPs and the energy levels of Er3+. PL decays from 4I11/2 and 4I13/2 of Er3+ (0.6 atom %) in In2O3 NPs were measured upon excitation at 519.2 nm at RT and 10 K, respectively. The decay curves by monitoring the 4I11/2 → 4I15/2 transition at 1038.6 nm fit well to single exponential form, and the PL lifetimes were determined to be 0.48 and 0.65 ms at RT and 10 K, respectively (Figure 2c). It can be seen that the PL decay from 4I11/2 is sensitive to temperature, and the lifetime decreases markedly at RT, which is due to the nonradiative multiphonon relaxation from 4I11/2 to the low-lying 4I13/2. The energy gap between 4I11/2 and 4I13/2 is ∼3000 cm−1, thus multiphonon relaxation can be accomplished with the assistance of five lattice phonons of the In2O3 host (the maximum phonon energy of ∼600 cm−135). By contrast, there is a much larger energy gap between 4I13/2 and 4I15/2 (∼6500 cm−1), and thus the PL decay for the 4I13/2 → 4I15/2 transition of Er3+ is less sensitive to temperature (Figure 2d). By fitting the decay curves with single exponential function, the PL lifetimes of 4I13/2 were determined to be 3.88 and 4.31 ms at RT and 10 K, respectively, which is significantly longer than that observed in indium tin oxide nanopowders (3.3 ms at RT).25 The rise times of 25 μs and

Figure 4. UCL spectra and decay curves of Er3+-doped In2O3 NPs upon excitation at 980 nm: (a) UCL spectra for Er3+ at various concentrations; (b) TRPL spectra for the UCL of the 4F9/2 → 4I15/2 transition of Er3+ (4.4 atom %); (c) ln−ln plots of the UC emission intensity versus NIR excitation power for the transitions from 2H11/2, 4S3/2, and 4F9/2 to 4I15/2 of Er3+; (d) UCL decays from 4S3/2 and 4F9/2 of Er3+ at the concentration of 0.6 and 4.4 atom %, respectively.

emissions centered at 554 and 662 nm were observed upon NIR excitation at 980 nm, which are assigned to the transitions of 2 H11/2/4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. As the Er3+ concentration increases from 0.6 to 4.4 atom %, the UC intensity of the red emission relative to the green emission is significantly enhanced. For comparison, the DS luminescence of Er3+ was also measured upon excitation from 4I15/2 to 4F7/2 at 489.2 nm. The emission spectra are dominated by the green emission of 4S3/2 → 4 I15/2, and the enhancement of red emission is negligibly small with increasing concentration of Er3+ from 0.6 to 4.4 atom % (Figure S2a, Supporting Information), which thus excludes the possible contribution from the cross relaxations between 4F7/2 → 4 F9/2 and 4F9/2 ← 4I11/2 transitions.37 Note that the nonradiative relaxation from 4F7/2 to thermally coupled 2H11/2 and 4S3/2 is instantaneous for Er3+ in In2O3 NPs. As a result, no emission from 4F7/2 was observed in the UCL spectrum. As will be discussed in Section 3.5, the UCL lifetime due to the contribution from ETU for 4F9/2 of Er3+(4.4 atom %) reaches about 0.5 ms in In2O3 NPs, which is sufficiently long compared to that of short-lived background luminescence from biological samples. The TRPL spectra for 4F9/2 of Er3+ (4.4 atom %) in D



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excluded since it requires the participation of energy migrators like Gd3+ in the host. According to the UC spectra analyzed above and energy level diagrams of Er3+, the possible UC mechanisms are schematically illustrated in Figure 5. First, the ground state of Er3+ ions absorbs a photon of 980 nm and is excited to the 4I11/2 intermediate state via the GSA process. Then, the 4F7/2 state can be populated via either the ESA process with immediate absorption of another pump photon or the ETU process by two neighboring Er3+ ions excited at the 4I11/2 state (ETU1). Following the nonradiative relaxation to the 4S3/2 and 4 F9/2 levels, the characteristic green and red emissions of Er3+ are produced. If only ESA and ETU1 processes were involved, the excited states of 2H11/2, 4S3/2, and 4F9/2 all would be populated directly from the nonradiative relaxation of 4F7/2, thus there would be no change of the relative intensity for green and red UC emissions in In2O3:Er3+ NPs. Specifically, at high concentration of Er3+ (e.g., 4.4 atom %), another ETU process (ETU2), which directly populates the red emitting state of 4F9/2 while bypassing the levels of 2H11/2 and 4S3/2, is proposed to be responsible for the enhancement of red UC emission of Er3+. The 4I13/2 state is populated via efficient multiphonon relaxation from the excited 4 I11/2, which will subsequently be excited to 4F9/2 via ETU2 by the neighboring Er3+ ion excited at the 4I11/2 state. The transition probability of ETU2 increases with the Er3+ concentration, thus enhanced UC red emission of 4F9/2 → 4I15/2 was observed in high-doping NPs. 3.4. Upconversion Luminescence Dynamics. The time evolution method is an important tool for identifying the operative UC processes.44−49 As an ESA process occurs instantaneously after the pulse excitation (pulse duration, ≤5 ns), the luminescence decay due to GSA/ESA upon NIR excitation displays a similar signature as that of DS luminescence after direct excitation. In contrast, the UCL lifetime for the ETU process may be significantly prolonged by the long-lived intermediate states such as 4I11/2 and 4I13/2 levels of Er3+. To further distinguish the ESA and ETU processes, the UC and DS luminescence decays from 4S3/2 and 4F9/2 levels of Er3+ (0.6 atom %) and Er3+ (4.4 atom %) were measured upon excitation at 980 and 519.2 nm, respectively. Upon direct excitation from 4I15/2 to 2 H11/2 at 519.2 nm, the PL decays can be fitted by a single exponential function, and the DS luminescence lifetimes of 4S3/2 and 4F9/2 were determined to be 21 and 31 μs for Er3+ (0.6 atom %) and 16 and 15 μs for Er3+ (4.4 atom %), respectively (Figure S2b,c, Supporting Information, and Table 2). However, upon NIR excitation at 980 nm, the UC decays from 4S3/2 and 4F9/2 of Er3+ exhibit obviously multiexponential nature, with a fast decay at initial time and a rather long lifetime component in the tail (Figure 4d). Further analysis reveals that the short lifetime component of the UC decay from 4S3/2 is in good agreement with the DS luminescence decay time upon excitation at 519.2 nm,

In2O3 NPs were measured upon laser excitation at 980 nm by setting different delay times ranging from 0 to 1.22 ms (Figure 4b). The UC emission peaks remain essentially unchanged in terms of line shapes and positions as the delay time increases, further confirming that the UC emissions of Er3+ are originated from the single site of Er3+ in In2O3 NPs. This TRPL technique, coupled with the NIR excitation at 980 nm, may provide higher detection sensitivity than the steady-state detection because the short-lived background luminescence can be effectively suppressed. To better understand the UC mechanisms that are responsible for populating 2H11/2/4S3/2 and 4F9/2 upon NIR excitation at 980 nm, the UCL intensities I of green (2H11/2, 4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) emissions were measured as a function of the pump power density P (Figure 5c). The dependence of the UC

Figure 5. Proposed UC processes for the UCL in In2O3:Er3+ NPs. The dash-dotted, dashed, dotted, and full arrows represent photon excitation, multiphonon relaxation, energy transfer, and emission processes, respectively.

emission intensity on the pump power density follows the relationship of I ∝ Pn, where n is the number of pumping photons required to excite the Er3+ ions from the ground state to the excited state. The resulting values of the pumping photon number n were determined to be 1.72 for 2H11/2 → 4I15/2, 1.77 for 4 S3/2 → 4I15/2, and 1.97 for 4F9/2 → 4I15/2, respectively, indicative of two-photon pumping processes for the population of 2H11/2, 4 S3/2, or 4F9/2. Generally, there are four processes that might be involved in the UCL of lanthanide ions, namely, ground state absorption/excited state absorption (GSA/ESA), ETU, photon avalanche (PA), and energy migration upconversion (EMU).26,38−43 The PA process characterized by a large threshold pump power can be ruled out as relatively small pump power was adopted and no threshold behavior was observed in this work. Similarly, the EMU process can also be

Table 2. DS Luminescence Lifetimes (Μs), Calculated UCL Lifetimes (μs), and Percentage Contributions (%) from Different ESA and ETU Processes to the UC Emissions of Er3+-Doped In2O3 NPs DS

UC Luminescence ESA

4

S3/2

4

F9/2

0.6 atom % 4.4 atom % 0.6 atom % 4.4 atom %

ETU1

ETU2

lifetime

percentage

lifetime

percentage

lifetime

percentage

lifetime

21 16 35 15

73 29 69 20

21 16 38 19

27 71 12 23

252 271 239 264

19 57

443 511

E



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which implies that this fast decay component may arise from the sequential GSA/ESA process. Meanwhile, the long lifetime component is about 1 order of magnitude larger than the DS luminescence lifetime, which may originate from a cooperative ETU1 process with the interaction of two neighboring Er3+ ions excited at 4I11/2 levels. Likewise, the short and long lifetime components in the UC decay from 4F9/2 can be assigned to ESA and ETU (both ETU1 and ETU2) processes, respectively. Thus, by comparing the UC and DS luminescence decays, the respective contribution of GSA/ESA or ETU processes to the UC decay can be qualitatively distinguished for 4S3/2 and 4F9/2 levels of Er3+. Additionally, the UC lifetime lengthening of 4F9/2 caused by the ETU process is more striking than that of 4S3/2 as the concentration of Er3+ increases from 0.6 to 4.4 atom %, implying more contribution from the ETU process to 4F9/2 at high dopant concentration, which is consistent with the red emission enhancement observed in the UCL spectra. As will be theoretically analyzed for ESA, ETU1, and ETU2 processes in Section 3.5, such significant UCL lifetime lengthening of 4S3/2 and 4F9/2 reflects higher efficiency of ETU processes at higher concentration of Er3+. 3.5. Theoretical Modeling of Upconversion Luminescence Decays. The rate equation analysis provides a clear physical interpretation to further identify the contribution from various UC processes.50−53 Taking into account the five-level system schematically depicted in Figure 5, the rate equations that include all the UC processes described above are given by dN1 N = W21N2 − C13(c)N1N2 − 1 dt τ1

σ24σ02N0P(t )2 + C13(c)N1N2 + C24(c)N22 ≪

dN1 N = W21N2 − 1 dt τ1

(7)

dN2 N = σ02N0P(t ) − 2 dt τ2

(8)

Given N0(t) = n0, P(t) = P0 as initial values, the steady state population of Ni(0) is given by N1(0) = n0σ02P0W21τ1τ2 N2(0) = n0σ02P0τ2

(9) (10)

N3(0) = (n0σ02P0)2 C13(c)W21τ22τ1τ3 + (n0σ02P0)2 C24(c)W43 τ22τ3τ4 + (n0σ24σ02P0 2)W43τ3τ4

N4(0) = (n0σ02P0)2 C24(c)τ22τ4 + n0σ24σ02P0 2τ4

(11) (12)

Supposing that the pumping laser is turned off from t = 0 (P(t) = 0 for t ≥ 0) and the time interval of laser pulses is sufficiently long relative to relaxation times of the levels Ni, the time-dependent population Ni(t) can be expressed as follows according to eqs 3, 4, 7, and 8 ⎛ exp( −t /τ ) − exp(−t /τ ) 2 1 N1(t ) = (n0σ02P0)W21 × τ2⎜ −1 −1 τ1 − τ2 ⎝ exp( −t /τ1) ⎞ ⎟ τ1−1 ⎠

(13)

N2(t ) = (n0σ02P0) × τ2 exp( −t /τ2)

(14)

(2)

+

dN3 N = W43N4 + C13(c)N1N2 − 3 dt τ3

(3)

dN4 N = C24(c)N22 + σ24σ02N0P(t )2 − 4 dt τ4

⎛ exp( −2t /τ ) − exp(−t /τ ) 2 3 N3(t ) = A1 × τ22⎜ −1 −1 −1 −1 ⎝ (τ3 − 2τ2 )(τ1 − τ2 )

(4) 4

4

where Ni (i = 0, 1, 2, 3, 4) are the populations of I15/2, I13/2, 4 I11/2, 4F9/2, and 4S3/2 of Er3+, respectively; Wij and Cij(c) are the nonradiative relaxation rate and the concentration c dependent energy transfer coefficient from level i to j of Er3+, respectively; τi is the DS luminescence lifetime experimentally determined for level i upon direct excitation; σij is the absorption cross section between levels i and j; and P(t) is the pump power. In writing these equations, it is reasonable to assume that (1) 4S3/2 are directly populated via ESA and ETU1 processes in view of the much faster nonradiative relaxation from 4F7/2 to 2H11/2 and 4S3/2 and (2) the nonradiative relaxation from the upper levels to 4I13/2 and 4I11/2 can be neglected in view of the perpetual pulsed laser excitation at 980 nm that results in efficient population of 4I11/2 and 4I13/2 levels. To simplify the rate equation analysis, it is further assumed that the depopulations from the intermediate 4 I13/2 and 4I11/2 states are not significantly influenced by ESA and ETU processes upon laser excitation at 980 nm, that is N C13(c)N1N2 ≪ 1 τ1

(6)

Thus, eqs 1 and 2 can be approximated as

(1)

dN2 = σ02N0P(t ) − σ24σ02N0P(t )2 − C13(c)N1N2 dt N − 2C24(c)N22 − 2 τ2

N2 τ2

−

exp(−t /τ1)exp( −t /τ2) − exp(−t /τ3)

τ2τ1−1(τ1−1 − τ2−1)(τ3−1 − τ1−1 − τ2−1) exp(−t /τ3) ⎞ ⎟ + A 2 × τ22 + τ1−1τ3−1 ⎠ ⎛ exp( −2t /τ ) − exp(−t /τ ) 2 3 ⎜ −1 −1 −1 −1 ⎝ (τ3 − 2τ2 )(τ4 − 2τ2 )

−

2 exp( −t /τ4) − 2 exp( −t /τ3)

τ2τ4−1(τ4−1 − 2τ2−1)(τ3−1 − τ4−1) exp(−t /τ3) ⎞ ⎟ + A3 + τ3−1τ4−1 ⎠ ⎛ exp(−t /τ ) − exp(−t /τ ) exp(−t /τ3) ⎞ 4 3 ⎟ × τ4⎜ + −1 −1 τ3 − τ4 τ3−1 ⎠ ⎝

(5)

(15) F


The Journal of Physical Chemistry C N4(t ) = B1 × +

Article

⎛

lifetime. Moreover, the UC decay time component of 4F9/2 due exclusively to the ETU2 process was found to be as long as 0.511 (or 0.443) ms for the Er3+ concentration of 4.4 atom % (or 0.6 atom %), thus the overall decay curve obviously deviates from single exponential form owing to various contributions from GSA/ESA and ETU processes that exhibit different decay times. The contribution from the ETU1 process to the UCL of 4S3/2 increases from 27% to 71% as the concentration of Er3+ increases from 0.6 to 4.4 atom %, verifying the increase of the ETU probability of Er3+ at high doping concentration. Similarly, the contribution from the ETU2 process to the population of 4F9/2 reaches as high as 57% at the Er3+ concentration of 4.4 atom %, thus the UC decay from 4F9/2 is significantly prolonged, consistent with the much longer UC decay time component from the ETU2 process (∼0.5 ms). Therefore, the UC mechanisms that include GSA/ESA and ETU processes, albeit complicated, can be clearly distinguished from each other based on the above rate equation analysis of the population on the emitting states.

exp( −2t /τ2) − exp( −t /τ4) τ22⎜ τ4−1 − 2τ2−1 ⎝

exp(−t /τ4) ⎞ ⎟ + B2 × τ4 exp(−t /τ4) τ4−1 ⎠

(16)

In eqs 15 and 16, A1, A2, and A3 denote (n0σ02P0) C13(c)W21, (n0σ02P0)2C24(c)W43, and (n0σ24σ02P02)W43, which are the weight coefficients of ETU2, ETU1, and ESA processes for the population of 4F9/2; B1 and B2 represent (n0σ02P0)2C24(c) and n0σ24σ02P02, which are the weight coefficients of ETU1 and ESA processes for the population of 4S3/2, respectively. A series of simulations for N3(t) and N4(t) are shown in Figure 6. By varying 2

4. CONCLUSIONS In summary, dual-mode NIR-to-NIR DS and NIR-to-visible UC luminescence were achieved in Er3+-doped In2O3 nanophosphors synthesized via a facile solvothermal method. Particularly, intense NIR emissions at 960−1650 nm of Er3+ were observed upon NIR excitation at 808 nm. On the basis of high-resolution site-selective spectroscopy, 41 CF levels below 23 000 cm−1 have been identified for Er3+ at a single lattice site of C2 in In2O3 NPs. The intricate UC mechanisms that involve GSA/ESA and ETU processes have been clearly distinguished from each other based on UC/DS luminescence, PL dynamics, and rate equations analysis. The UC decay times of 4F9/2 (or 4S3/2) due to ETU were found significantly prolonged by the long-lived intermediate states of 4I11/2 and 4I13/2, which are about 1 order of magnitude larger than the DS luminescence lifetimes. Furthermore, the contribution from the ETU process also enhanced the UCL intensity of 4F9/2 (or 4S3/2) as a result of the increase of the ETU probability of Er3+ at high doping concentration. A fundamental understanding of dual-mode luminescence upon NIR excitation may open up new avenues for Ln3+-doped inorganic nanoprobes in versatile bioapplications such as time-resolved biodetection and multimodal bioimaging.

Figure 6. Theoretically simulated UC decays (normalized to 1) for (a) N3(t) and (b) N4(t) with varying contributions from different UC processes, by setting appropriate values of A1, A2, A3, B1, and B2 parameters.

the coefficients of A1, A2, A3, B1, and B2, the simulated decay curves for N3(t) and N4(t) can be approximated as tri- and biexponential decays, respectively. By utilizing the corresponding DS luminescence lifetime of Er3+ at RT as parameters, the rate equations N3(t) and N4(t) were fitted to the UC decay curves for 4 F9/2 and 4S3/2 of Er3+ (Figure 4d). The excellent agreement between the simulated decays and the observed ones confirms the validity of the rate equation analysis we employed in theoretical modeling of the UCL decays. The percentage contribution from each ESA or ETU process to the UC emissions can thus be theoretically estimated from the fitting. The UC decay time for each UC process was obtained by fitting the simulated UC decay of the corresponding UC component in eqs 15 and 16. Representative simulated UC decays of ETU1 and ETU2 processes by monitoring the 4F9/2 level of Er3+ (0.6 atom %) exhibit nearly single exponential forms (Figure S3). The UC decay time components and percentage contributions from different sources of ESA and ETU processes to the UC emissions are compared with those for DS luminescence lifetime in Table 2. It turns out that the UC decay time components of 4S3/2 (or 4 F9/2) due to the GSA/ESA process are essentially the same as the DS luminescence lifetime, that is, 21 (or 38) μs for Er3+ (0.6 atom %) and 16 (or 19) μs for Er3+ (4.4 atom %). In sharp contrast, the UC decay time components of 4S3/2 (or 4F9/2) owing to the ETU1 process were found prolonged to ∼0.25 (or ∼0.24) ms, which are insensitive to the Er3+ concentration and about 1 order of magnitude larger than the DS luminescence

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ASSOCIATED CONTENT

S Supporting Information *

NIR DS emission and excitation spectra of the submicrometer counterparts at 10 K; visible DS luminescence spectra and PL decays from 4S3/2 and 4F9/2 of Er3+ in In2O3 NPs at RT; simulated UC decay curves arising from ETU1 and ETU2 processes by monitoring the 4F9/2 level of Er3+ (0.6 atom %) in In2O3 NPs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-591-87642575. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Knowledge Innovation Program of CAS for Key Topics (No. KJCX2-YW-358), Scientific EquipG



Article

(32) Liu, Y. S.; Luo, W. Q.; Zhu, H. M.; Chen, X. Y. J. Lumin. 2011, 131, 415−422. (33) Luo, W. Q.; Li, R. F.; Liu, G. K.; Antonio, M. R.; Chen, X. Y. J. Phys. Chem. C 2008, 112, 10370−10377. (34) Liu, Y. S.; Luo, W. Q.; Li, R. F.; Liu, G. K.; Antonio, M. R.; Chen, X. Y. J. Phys. Chem. C 2008, 112, 686−694. (35) Chen, C. L.; Chen, D. R.; Jiao, X. L.; Chen, S. H. J. Phys. Chem. C 2007, 111, 18039−18043. (36) Bogdanov, V. K.; Gibbs, W. E. K.; Booth, D. J.; Javorniczky, J. S.; Newman, P. J.; MacFarlane, D. R. Opt. Commun. 1996, 132, 73−76. (37) Vetrone, F.; Boyer, J. C.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. J. Appl. Phys. 2004, 96, 661−667. (38) Bloembergen, N. Phys. Rev. Lett. 1959, 2, 84−85. (39) Chivian, J. S.; Case, W. E.; Eden, D. D. Appl. Phys. Lett. 1979, 35, 124−125. (40) Joubert, M. F.; Guy, S.; Jacquier, B. Phys. Rev. B 1993, 48, 10031− 10037. (41) Guo, H.; Dong, N.; Yin, M.; Zhang, W. P.; Lou, L. R.; Xia, S. D. J. Phys. Chem. B 2004, 108, 19205−19209. (42) Wang, F.; Deng, R. R.; Wang, J.; Wang, Q. X.; Han, Y.; Zhu, H. M.; Chen, X. Y.; Liu, X. G. Nat. Mater. 2011, 10, 968−973. (43) Su, Q. Q.; Han, S. Y.; Xie, X. J.; Zhu, H. M.; Chen, H. Y.; Chen, C. K.; Liu, R. S.; Chen, X. Y.; Wang, F.; Liu, X. G. J. Am. Chem. Soc. 2012, 134, 20849−20857. (44) Balda, R.; Garcia-Adeva, A. J.; Voda, M.; Fernandez, J. Phys. Rev. B 2004, 69, 205203−205211. (45) Zalucha, D. J.; Sell, J. A.; Fong, F. K. J. Chem. Phys. 1974, 60, 1660−1667. (46) Golding, P. S.; Jackson, S. D.; King, T. A.; Pollnau, M. Phys. Rev. B 2000, 62, 856−864. (47) Martin-Rodriguez, R.; Valiente, R.; Polizzi, S.; Bettinelli, M.; Speghini, A.; Piccinelli, F. J. Phys. Chem. C 2009, 113, 12195−12200. (48) Riedener, T.; Egger, P.; Hulliger, J.; Gudel, H. U. Phys. Rev. B 1997, 56, 1800−1808. (49) Francini, R.; Pietrantoni, S.; Zambelli, M.; Speghini, A.; Bettinelli, M. J. Alloy. Compd. 2004, 380, 34−38. (50) Tsuda, M.; Soga, K.; Inoue, H.; Inoue, S.; Makishima, A. J. Appl. Phys. 1999, 85, 29−37. (51) Salley, G. M.; Valiente, R.; Gudel, H. U. J. Phys.: Condens. Matter 2002, 14, 5461−5475. (52) Cantelar, E.; Cusso, F. Appl. Phys. B: Laser Opt. 1999, 69, 29−33. (53) Liang, H. J.; Chen, G. Y.; Li, L.; Liu, Y.; Qin, F.; Zhang, Z. G. Opt. Commun. 2009, 282, 3028−3031.

ment Development Project of CAS (No. YZ201210), the NSFC (Nos. 11104266 and 11004191), the 863 program (No. 2011AA03A407), and special project of national major scientific equipment development of MOST of China (No. 2012YQ120060) and the Key Project of Science and Technology Foundation of Fujian Province (No. 2011HZ0001-2).

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REFERENCES

(1) Wang, F.; Liu, X. G. Chem. Soc. Rev. 2009, 38, 976−989. (2) Zhou, J.; Liu, Z.; Li, F. Y. Chem. Soc. Rev. 2012, 41, 1323−1349. (3) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y. H.; Wang, J.; Xu, J.; Chen, H. Y.; Zhang, C.; Hong, M. H.; Liu, X. G. Nature 2010, 463, 1061−1065. (4) Wang, J. W.; Tanner, P. A. J. Am. Chem. Soc. 2010, 132, 947−949. (5) Wang, G. F.; Peng, Q.; Li, Y. D. Acc. Chem. Res. 2011, 44, 322−332. (6) Zeng, S.; Tsang, M.-K.; Chan, C.-F.; Wong, K.-L.; Hao, J. Biomaterials 2012, 33, 9232−9238. (7) Yang, Y.; Shao, Q.; Deng, R.; Wang, C.; Teng, X.; Cheng, K.; Cheng, Z.; Huang, L.; Liu, Z.; Liu, X.; Xing, B. Angew. Chem., Int. Ed. 2012, 51, 3125−3129. (8) Wang, Z.-L.; Hao, J.; Chan, H. L. W.; Wong, W.-T.; Wong, K.-L. Small 2012, 8, 1863−1868. (9) Teng, X.; Zhu, Y.; Wei, W.; Wang, S.; Huang, J.; Naccache, R.; Hu, W.; Tok, A. I. Y.; Han, Y.; Zhang, Q.; Fan, Q.; Huang, W.; Capobianco, J. A.; Huang, L. J. Am. Chem. Soc. 2012, 134, 8340−8343. (10) Kang, X.; Cheng, Z.; Yang, D.; Ma, P. a.; Shang, M.; Peng, C.; Dai, Y.; Lin, J. Adv. Funct. Mater. 2012, 22, 1470−1481. (11) Dai, Y.; Yang, D.; Ma, P. a.; Kang, X.; Zhang, X.; Li, C.; Hou, Z.; Cheng, Z.; Lin, J. Biomaterials 2012, 33, 8704−8713. (12) Dai, Y.; Ma, P. a.; Cheng, Z.; Kang, X.; Zhang, X.; Hou, Z.; Li, C.; Yang, D.; Zhai, X.; Lin, J. ACS Nano 2012, 6, 3327−3338. (13) Tanner, P. A. Chem. Soc. Rev. 2013, DOI: 10.1039/C3CS60033E. (14) Zhou, J.; Sun, Y.; Du, X.; Xiong, L.; Hu, H.; Li, F. Biomaterials 2010, 31, 3287−3295. (15) Chen, G.; Ohulchanskyy, T. Y.; Law, W. C.; Agren, H.; Prasad, P. N. Nanoscale 2011, 3, 2003−2008. (16) Wong, H.-T.; Vetrone, F.; Naccache, R.; Chan, H. L. W.; Hao, J.; Capobianco, J. A. J. Mater. Chem. 2011, 21, 16589−16596. (17) Chen, G.; Shen, J.; Ohulchanskyy, T. Y.; Patel, N. J.; Kutikov, A.; Li, Z.; Song, J.; Pandey, R. K.; Agren, H.; Prasad, P. N.; Han, G. ACS Nano 2012, 6, 8280−8287. (18) Zeng, S.; Xiao, J.; Yang, Q.; Hao, J. J. Mater. Chem. 2012, 22, 9870−9874. (19) Chen, G.; Ohulchanskyy, T. Y.; Liu, S.; Law, W.-C.; Wu, F.; Swihart, M. T.; Agren, H.; Prasad, P. N. ACS Nano 2012, 6, 2969−2977. (20) Chen, D. Q.; Yu, Y. L.; Huang, F.; Lin, H.; Huang, P.; Yang, A. P.; Wang, Z. X.; Wang, Y. S. J. Mater. Chem. 2012, 22, 2632−2640. (21) Yu, X. F.; Chen, L. D.; Li, M.; Xie, M. Y.; Zhou, L.; Li, Y.; Wang, Q. Q. Adv. Mater. 2008, 20, 4118−4123. (22) Choi, Y. G.; Yu, S. M.; Chung, W. J. Chem. Phys. Lett. 2008, 461, 290−293. (23) Podhorodecki, A.; Kudrawiec, R.; Misiewicz, J.; Gaponenko, N. V.; Tsyrkunov, D. A. Opt. Mater. 2006, 28, 685−687. (24) Kim, H. K.; Li, C. C.; Nykolak, G.; Becker, P. C. J. Appl. Phys. 1994, 76, 8209−8211. (25) Kim, H. K.; Li, C. C.; Barrios, P. J. J. Vac. Sci. Technol. A 1994, 12, 3152−3156. (26) Vetrone, F.; Boyer, J. C.; Capobianco, J. A.; Speghini, A.; Bettinelli, M. Chem. Mater. 2003, 15, 2737−2743. (27) Xiao, Q. B.; Liu, Y. S.; Liu, L. Q.; Li, R. F.; Luo, W. Q.; Chen, X. Y. J. Phys. Chem. C 2010, 114, 9314−9321. (28) Kortum, G. Reflectance Spectroscopy; Springer-Verlag: New York, 1969. (29) Cao, G.; Rabenberg, L. K.; Nunn, C. M.; Mallouk, T. E. Chem. Mater. 1991, 3, 149−156. (30) Saini, S.; Chen, K.; Duan, X. M.; Michel, J.; Kimerling, L. C.; Lipson, M. J. Electron. Mater. 2004, 33, 809−814. (31) Chen, X. Y.; Ma, E.; Liu, G. K. J. Phys. Chem. C 2007, 111, 10404− 10411. H


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