Influence of BaCl2 Nanocrystal Size on the Optical Properties of Nd3+

Article pubs.acs.org/JPCC

Influence of BaCl2 Nanocrystal Size on the Optical Properties of Nd3+ in Fluorozirconate Glass U. Skrzypczak,*,† C. Pfau,† C. Bohley,† G. Seifert,†,‡ and S. Schweizer‡,§ †

Centre for Innovation Competence SiLi-nano, Martin Luther University of Halle-Wittenberg, Karl-Freiherr-von-Fritsch-Str. 3, 06120 Halle (Saale), Germany ‡ Fraunhofer Center for Silicon Photovoltaics CSP, Walter-Hülse-Str. 1, 06120 Halle (Saale), Germany § Department of Electrical Engineering, South Westphalia University of Applied Sciences, Lübecker Ring 2, 59494 Soest, Germany ABSTRACT: The optical properties of Nd3+ ions embedded in fluorochlorozirconate glasses were studied, addressing in particular the effect of BaCl2 nanocrystal growth in the matrix upon annealing. A Judd−Ofelt analysis for these glasses and for a Nd3+-doped fluorozirconate reference glass without nanocrystals yielded various radiative decay rates as a function of nanocrystal size. For the 4F3/2 to 4I9/2 transition, where multiphonon relaxation is negligible, the rates agree very well with experimental results. Further support for the validity of the Judd−Ofelt results was obtained using an alternative approach for the determination of radiative decay rates involving emission and absorption spectra and their connection through the McCumber theory and the Füchtbauer−Ladenburg equation. The obtained variations of the Judd− Ofelt intensity parameters Ωλ reflect systematic changes in the local environment of the Nd3+ ions as an effect of the BaCl2 nanocrystal growth, indicating partial inclusion of Nd3+ into the nanocrystals.

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INTRODUCTION Photonic glasses doped with rare-earth (RE) ions such as Er3+, Nd3+, Yb3+, or Eu3+ have seen widespread interest in their possible application as fiber lasers, frequency converters, or luminophores of ionizing radiation.1−4 For the best performance as an active laser medium or as a photovoltaic upconverter, these materials require optical transitions with a very effective radiative decay, i.e., only minor losses to multiphonon relaxation (MPR). For this, the ion needs to be embedded into an environment that is both stable and transparent and features only low phonon energies. Appropriate materials include fluorozirconate (FZ) glasses (maximal phonon energy of less than 580 cm−1),5−7 which have already proven to be convenient for several applications. As shown previously,8,9 a uniform growth of hexagonal BaCl2 nanocrystals inside such glasses can be induced by thermal treatment if additional chloride is introduced at the expense of fluoride. BaCl2 in hexagonal symmetry has a maximal phonon energy of about 220 cm−1 which makes MPR even less probable,5,9,10 and hence, under the influence of BaCl2, RE ions favor radiative decays.8,9,11 However, until now only speculations were possible if the strong effects of BaCl2 nanocrystals on the optical properties of Nd3+ ions in these materials are related to the location of the RE ions at the interface between nanocrystals and the glass matrix or even their possible inclusion into the nanocrystals.11,12 In addition, it is not clear whether the physical reason for the huge lifetime increase found for several transitions is solely the change in the phonon spectra or if the nanocrystals directly affect the radiative transition © 2013 American Chemical Society

probabilities. It is the goal of this work to gather reliable information on these questions. For this purpose, the absorptive and emissive properties of Nd3+-doped fluorochlorozirconate (FCZ) glasses with differently sized BaCl2 nanocrystals were studied in a broad approach combining several experimental and theoretical methods: First, the nanocrystal sizes were determined employing X-ray diffraction and the Scherrer equation. Second, the evolution of Nd3+ absorption and emission spectra of samples with different BaCl2 crystal sizes were analyzed experimentally. Next, a Judd−Ofelt13−15 analysis has been carried out in the scope of a Maxwell−Garnett effective-n medium.16 The phenomenological intensity parameters Ωλ were determined for samples with differently sized nanocrystals to find a quantitative measure for the influence of the BaCl2 on the Nd3+ ions. In addition to this particular interest, the Judd−Ofelt calculations can also be understood as a test for the applicability of this theory on composite systems such as the presented glass ceramics. The obtained radiative parameters are then compared with measured lifetimes for transitions, where additional loss mechanisms such as MPR are negligible. Finally, in an alternative theoretical approach, the McCumber theory17,18 was used to render Nd3+ emission cross-section spectra for different environments. Rescaling the experimental spectra with the Füchtbauer−Ladenburg equation, to fit the results obtained Received: December 22, 2012 Revised: March 21, 2013 Published: April 23, 2013 10630

dx.doi.org/10.1021/jp312677u | J. Phys. Chem. C 2013, 117, 10630−10635

The Journal of Physical Chemistry C

Article

where me is the electron mass, k the wavenumber, c the speed of light, and σ(k) = α(k)/NI the absorption cross section of a Nd3+ absorption band located between k1 and k2 with the ion density NI. The contributions to the theoretical oscillator strength are

by McCumber theory, provides an independent determination of the radiative decay rates. In the end, all results including a comparison with undoped FZ glass as the host matrix are discussed with a focus on the influence of growing nanocrystals on the optical properties of Nd3+.

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fed =

METHODS Glass Ceramic Synthesis. The glass ceramics used here are based on the ZBLAN composition19 with an additional doping of chloride ions at the expense of fluoride. The chemical constituents in 52ZrF4−10BaF2−10BaCl2−19NaCl−3.5LaF3− 3AlF3−0.5InF3−1KCl−1NdF3 (values in mol %) were melted in a glassy carbon crucible at 745 °C under an inert argon atmosphere. The melt was poured into a brass mold, prepared at a temperature of 200 °C, and then left to cool to room temperature. To produce BaCl2 nanocrystals inside the glass, a subsequent annealing was carried out at different temperatures between 240 and 290 °C in steps of 10 °C, for 20 min each. The initially formed hexagonal nanocrystals undergo a phase transition to orthorhombic phase nanocrystals at treatment temperatures above 270 °C.9 With rising annealing temperatures, the visual appearance changed from transparent to milky white. Optical Spectroscopy and Nanocrystal Size Determination. The optical attenuance was recorded at room temperature in the visible and near-infrared spectral range with a double-beam spectrophotometer (PerkinElmer Lambda 900). The emission spectra were measured with an excitation source consisting of an Yb:KGW femtosecond laser (Pharos, Light Conversion), which pumps a collinear optical parametric white-light continuum amplifier (Light Conversion Orpheus). This device provides excitation pulses with temporal widths below 200 fs in a wavelength range from 300 nm to 3 μm. The pulse repetition rate was set to 100 Hz by controlling the pulse picker with a specially designed frequency division system to ensure the stationary state of the Nd3+ ions prior to excitation. The excitation intensity has been kept as low as possible to avoid any nonlinear optical or saturation effects.20 Visible and near-infrared fluorescence light was captured with a PMT (Hamamatsu R943 and H10330A, respectively) attached to a spectral monochromator (Acton SP2500); the spectral resolution was 1 nm. Lifetime measurements have been carried out with a photon counting device (PicoQuant NanoHarp 250). The nanocrystal average size was determined both with Scherrer analysis on X-ray diffraction data and with an alternative approach using Rayleigh scattering of light at these crystals. A detailed description of the method and a discussion of the results have been published elsewhere.21 Judd−Ofelt Analysis for Effective-n Medium. The Judd−Ofelt theory13−15 enables calculation of electronic transition probabilities, from which the radiative decay rates together with their respective branching ratios can be determined. A Judd−Ofelt analysis has been carried out, and the intensity parameters were determined as a function of the nanocrystal size. For the calculation of the intensity parameters, the experimental oscillator strengths f meas were determined from the baseline-corrected absorption cross-section spectra according to fmeas =

4πε0 mec 2 e2 π

∫k

k2 1

σ(k)dk

8π 2meν χ Sed 3h(2J + 1) ed

4πε0 e2

(2)

from electric dipole and fmd =

4πε0 e

2

8π 2meν χ Smd 3h(2J + 1) md

(3)

from magnetic dipole transitions. Here, ν is the mean frequency of the respective transition and J the corresponding angular momentum quantum number. Sed = ∑λΩλ|⟨i∥Uλ∥j⟩|2 and Smd = [(eh)/(4πmec)]2|⟨i∥L + 2S∥j⟩|2 are the electric and magnetic dipole line strengths, where ⟨i∥Uλ∥j⟩ means the doubly reduced matrix elements of the electric dipole tensor operator and ⟨i∥L + 2S∥j⟩ are the matrix elements of the magnetic dipole operator for the transitions between the 4fstates i and j.15,22−26 The factors χed = (n2 + 2)2/(9n) and χmd = n further denote the local field correction factors according to the tight binding model.27 The effective refractive index n has been evaluated with a Maxwell−Garnett approach16 by treating the BaCl2 nanocrystals as inclusions into the glass matrix. The effective dielectric constant of the glass ceramic is then given by 15,22

εeff =

εm(εi(1 + 2δi) − εm(2δi − 2)) εm(2 + δi) + εi(1 − δi)

(4)

where εm and εi are the dielectric constants of matrix and inclusion (with data from the literature28,29) and δi is the volume fraction. Minimizing (λ) fmeas − fmd = f ed Ωλ

(5)

with the least-squares method yields the Judd−Ofelt intensity parameters Ωλ (λ = 2,4,6). Using these parameters, the radiative emission properties can be calculated,22 in particular the radiative decay rate Aji =

64π 4ν 3 (χ Sed + χmd Smd) 3hc 3(2Jj + 1) ed

(6)

and the branching ratio, βji = τjAji, of a particular transition. Groups of at least partially overlapping levels are thermally coupled when their energy difference is in the order of kBT. Several of such groups need to be considered during the analysis due to the large inhomogeneous broadening in the glass ceramics. The total common radiative lifetime of such level groups must be calculated from the Boltzmann-weighted individual rates Aj with τ = 1/(∑jnjAj), where nj =

(2Jj + 1)e−Ej ∑i (2Ji + 1)e−Ei

(7)

is the relative occupation number of state j with the energy position Ej in units of kBT.22 McCumber Analysis. The McCumber theory17,18 allows for a quantitative prognosis of the spectral shape of emissive transitions when the corresponding absorption spectrum is known, and vice versa. The central equation of the theory relates the emission cross section σji for the transition from the

(1) 10631



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excited level j to a lower level i, especially to the ground state, to the corresponding absorption cross section σij via ⎛ hν − μ ⎞ σji(ν) = σij(ν)exp⎜ − ⎟ kBT ⎠ ⎝

(8)

Here, ν is the frequency of the transition, μ the free net energy required for an excitation from a Stark level in i to a Stark level in j, and kBT the thermal energy.18 The equilibrium population densities ni and nj of the corresponding levels are Boltzmann distributed: nj/ni = exp(−μ/kBT). In the case of Nd3+, this population density ratio canunder the assumption of equally spaced Stark levelsbe written in the form J − 1/2

ni = nj

∑si= 0 J − 1/2

∑s =j 0

(

s

exp − J − 1 / 2 ΔEi i

)

⎛ ⎛ s ⎞⎞ exp⎜ −⎜ J − 1 / 2 ΔEj + E0⎟⎟ ⎠⎠ ⎝ ⎝j

Figure 1. Spectral attenuance for different annealing temperatures.

distinct spectral features belong to transitions from the Nd3+ ground state 4I9/2 to several excited 4f states. In hosts like FCZ glass, inhomogeneous line broadening is considerable, and hence several levels are overlapping. The transitions to 2L17/2 + 4 D7/2 + 2I13/2, 2L15/2 + 4D1/2 + 2I11/2 + 4D5/2 + 4D3/2, and (2P,2D)3/2 were not included in the calculations since they are strongly overlapped by the UV absorption edge of the glass matrix. The peak assignment is consistent with the usual classification found in the literature.23,32−34 The levels and level groups were abbreviated in the form nj for simplicity. Table 1 introduces the symbolic notation used. The spectra corresponding to treatment temperatures above 270 °C were not further analyzed since here scattering was already too strong to observe all Nd3+-related transmission changes in the visible range. Also it has been suspected that the nanocrystals begin to dissolve at these temperatures.8 Judd−Ofelt Analysis. A Judd−Ofelt analysis has been carried out for samples annealed up to temperatures of 270 °C as well as for the untreated sample. To control the reliability of our analysis, also a Nd3+-doped FZ glass (without any BaCl2 addition) has been included since this glass can be compared to similar results found in the literature. The Ωλ and the calculated decay rates for the FZ glass (see Table 1) are in good agreement to similar hosts described in the literature.33,35,36 Figure 2 shows the parameters Ωλ determined for the FCZ sample without nanocrystals (labeled as untreated) and for the samples with different nanocrystal sizes (labeled by annealing temperatures). Clearly, the growth of BaCl2 nanocrystals does change the RE environment significantly; in particular, Ω2 and Ω4 are strongly affected, while Ω6 changes (decreases) only slightly. Looking in more detail, annealing at 240 and 250 °C causes quite similar, moderate changes of these two parameters, whereas higher temperatures lead to more prominent changes (decrease of Ω2 and increase of Ω4). This is compatible to the velocity of nanoparticle growth concluded from previous studies. The values of Ω4 and Ω6 are considered to be connected to the rigidity of the host matrix and the distortion of the ligand field of Nd3+; Ω2 is usually interpreted as a measure of the covalency (or ionicity) of Nd−halogenide bonds, and it is also sensitive to the local symmetry around an RE ion.34,37−41 Applied to the present case, the dependence of the Ωλ on the thermal treatment step implicates that growing nanocrystals are responsible for an increase of ionicity (decrease of covalency),42 a rising degree of symmetry in the local vicinity of the RE ions, and an increase of the rigidity of the host matrix. The latter,

(9)

where ΔEk (in units of kBT) is the energy difference between the highest and lowest lying Stark level s within level k = i,j; the sum goes over all Stark levels in each multiplet.17,18,22 E0 is the separation between the lowest components of each multiplet and is also given in kBT units. The Stark level positions are estimated as described elsewhere.18 The emission cross section of an individual transition can be converted to the radiative decay rate Arad of the respective upper level with the Füchtbauer−Ladenburg equation30 A rad =

8πn2 c2

∫ν

ν2

1

ν 2σji(ν)dν

(10)

where n is again the refractive index. The validity of radiative decay rates determined in this way can be easily checked if experimental emission spectra are available: when the spectral shape of the experimental emission cross section is in agreement with the one calculated by McCumber theory, the model assumptions (single transition, broadening by Stark splitting only) can be considered as verified. In that case, rescaling of the experimental emission spectra to the McCumber result is also a reliable method to determine absolute emission cross sections from experimental data. This method is limited to sufficiently thin samples because otherwise the amount of light observed experimentally may be disturbed by effects like scattering or radiation trapping.31 It has been shown in previous studies that the latter can be responsible for a few percent error in the decay rates.30 In this study, however, thin samples (less than 1 mm thickness) have been used so that consequently these effects did not have to be considered for the analysis.

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RESULTS AND DISCUSSION Optical Spectra. Optical attenuance spectra have been measured for a series of samples with different thermal treatment temperatures (as specified above) leading to different nanocrystal sizes. Figure 1 shows two examples obtained after annealing and the spectrum of an untreated sample for comparison. The attenuance curve measured after annealing at 270 °C clearly indicates the nanocrystal growth by the significant increase of Rayleigh scattering (increasing attenuance toward lower wavelengths). Both X-ray diffraction and Rayleigh scattering analysis yielded consistently average nanocrystal sizes increasing with annealing temperature from approximately 10 nm (at 240 °C) to 45 nm (at 270 °C). The 10632



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Table 1. FCZ:Nd3+ Levels and Corresponding Calculated (Common, Boltzmann Weighted for Level Groups) Total Radiative Decay Rates Aj (in s−1) for Different Nanocrystal Sizesa 2S+1

nj n12 n11 n10 n9 n8 n7 n6 n5 n4 n3 n2 n1 a

2

2

LJ

P1/2 + D5/2 2 G9/2 + 2K15/2 + 2D3/2 + 4G11/2 4 G7/2 + 2K13/2 + 4G9/2 2 G7/2 + 4G5/2 2 H11/2 4 F9/2 4 S3/2 + 4F7/2 2 H9/2 + 4F5/2 4 F3/2 4 I15/2 4 I13/2 4 I11/2

k/cm−1

FZ

untreated

240 °C

250 °C

260 °C

270 °C

23500 21200 19100 17400 16000 14800 13500 12500 11600 6000 4000 2000

3425 1767 1923 7194 212 2105 3012 1650 2165 n/a n/a n/a

4405 2037 2242 8696 255 2242 3289 1912 2591 n/a n/a n/a

4695 1992 2165 8333 255 2304 3390 1992 2703 n/a n/a n/a

4608 1957 2088 8065 240 2232 3268 1931 2653 n/a n/a n/a

5208 2008 2119 8197 251 2283 3390 2079 2849 n/a n/a n/a

5780 2092 2188 8264 258 2421 3584 2247 3106 n/a n/a n/a

The results for the FZ glass are included for comparison.

Figure 2. Change in Ωλ with nanocrystal size/annealing temperature. The first data point represents the untreated FCZ glass, i.e., a sample without nanocrystals.

Figure 3. Emission spectra (log scale) recorded under excitation with 515 nm. The inset shows the blue shift of the intense n4 to n1 transition with growing nanoparticles.

concluded from the increase of Ω4, is expected considering the fact that the material is partially transformed into a glass with nanocrystals. The growing ionicity of the Nd3+ bonds and local symmetry around it, as concluded from the continuing decrease of Ω2 with nanocrystal size, can be taken as evidence that a part of the Nd3+ is located in the vicinity of nanocrystals or has even been included into them. Our finding is confirmed by the blue shift of the fluorescence of Nd3+, e.g., up to 5 nm for the bright n4 to n1 transition (see Figure 3), which also indicates an increase of the ionicity between Nd3+ and its halogenide bonding partner.34,41,43,44 Finally, the more or less constant value of Ω6 is also well compatible with the above conclusions because such a very weak dependence on ionicity has been reported before.37 Table 1 lists the calculated decay rates of several Nd3+ levels. The comprehensive set of data including the branching ratios and rates for each single sublevel can be accessed in a separate publication.45 Generally, these values are subject to the usual Judd−Ofelt uncertainty of 10−20%.15 As can be seen from Table 1, all transitions exhibit an increase of their radiative decay rates by around 5−30% when moving from the FZ to the FCZ host. Then, with increasing temperature in the thermal treatment step, the rates behave differently. Most of them stay almost constant, while some, in particular n12 and n4, rise significantly at the higher annealing temperatures, where also the fastest crystal growth has occurred.21

For an experimental verification of the Judd−Ofelt results, one has to look for a transition starting from a level where radiative decay is the only or, at least, clearly dominating relaxation process. In the present case, however, the radiative decay rates are quite low so that most of the listed levels are dominatingly depopulated by multiphonon relaxation. One notable exception is the level n4, which has a very large energy gap to n3 of about 5600 cm−1, so that MPR is completely negligible. This different behavior is qualitatively indicated by the relative intensities observed in the emission spectra of Figure 3: the emission bands around wavelengths of 660 and 580 nm, representing transitions starting from n10 and including overlapping, weaker emission from levels n 7 and n 9 , respectively, exhibit a strong intensity increase from untreated FCZ sample to the sample annealed at 240 °C, whereas the calculated (radiative) decay rates are fairly constant or even suggest a decrease. In the case of the mentioned transition between n4 and the ground state in FCZ glass (around 870 nm wavelength), the slight intensity increase upon annealing corresponds very well to the calculated radiative rates. Therefore, fluorescence lifetime measurements for this transition under nonresonant excitation have been performed for all samples. For the FCZ samples, decay rates in the range between 2600 and 3100 s−1 have been obtained for the different nanocrystal sizes. Decay constants of about 370 and 365 μs have been found for the untreated sample (without nanocryst10633



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als) and another one with 45 nm nanoparticles, respectively. Measurements on the FZ glass sample yielded a lifetime of approximately 450 μs, i.e., a decay rate of about 2200 s−1. These lifetimes could be determined directly from the monoexponential decay with an accuracy of 2 μs. Comparing these results with the calculated radiative rates (in Table 1), it is noticed that the experimental values match the Judd−Ofelt results almost perfectly (within less than 5% deviation), but interestingly the calculated values for samples with nanocrystals are slightly larger than the measured decay rates in each case. One possible explanation for this delayed spontaneous emission is the inverse process to MPR, i.e., phonon absorption to higher, close-by levels. Similar findings were obtained in previous work.46,47 A general discussion of this effect is not in the scope of this article. A comprehensive analysis of all radiative and MPR processes with the help of a rate equation system will be published separately. Overall, the Judd−Ofelt calculation results for the n4 level could be reasonably verified since the calculated radiative decay rates are quite close to the experimentally determined rates. As already mentioned, all other luminescent decays are strongly affected by MPR, and hence, the Judd−Ofelt results for them cannot be confirmed directly by lifetime measurements. Therefore, another approach involving the McCumber theory will be described in the next section. McCumber Analysis. The McCumber theory was used to calculate several emission cross-section spectra for samples with different nanocrystal sizes. Figure 4(a) shows the experimentally determined emission cross section for the transition from n5 to the ground state and the respective band shape predicted

by the McCumber analysis for an FCZ sample without nanocrystals; Figure 4(b) gives the analogous comparison for the sample with 45 nm nanocrystals. In both cases the experimental spectra have been rescaled to match the calculated band shapes with minimized deviation. The good agreement between predicted and measured band shapes indicates that these emission cross-section spectra can be utilized to determine, by help of eq 10, the radiative rates for level n5 independently. In this way, a decay rate of 1928 s−1 was determined for the untreated sample, while a value of 2030 s−1 was obtained for the sample with 45 nm nanocrystals, both in excellent agreement with the results from the Judd−Ofelt theory. In addition, both the theoretical and the experimental spectra of the thermally treated samples, including the one shown in Figure 4(b), clearly show features indicating a Stark splitting, which again confirms the above finding that Nd3+ ions are in close proximity to the nanocrystals or have even been included into them. The calculated emission spectra of n4 turned out to approximate the experimental band shape correctly, although the disadvantageous signal-to-noise ratio prohibited an accurate determination of the decay rate. Still, the order of magnitude matches the Judd−Ofelt result for this transition. Furthermore, the calculations for the level groups with complicated coupling, namely, n10 and n9, whose emissions also overlap, did not lead to a reasonable agreement with experimental spectra. The same was true for the remaining, weak emission bands. Nevertheless, the alternative approach of using the McCumber theory together with eq 10 confirmed another part of the Judd− Ofelt results for the radiative decay rates.

Figure 4. Example for emission cross-section spectra predicted with McCumber theory (dashed line) together with experimentally determined spectra (solid line), scaled with eq 10 for the transition from n5 to n0 for (a) the untreated FCZ sample without nanocyrstals and (b) the sample with 45 nm nanocrystals.

CONCLUSION The Judd−Ofelt theory was applied to Nd3+ ions in an FCZ environment in dependence on growing nanocrystal sizes using an effective refractive index. Starting from a glass matrix without nanocrystals, the dependence of the decay rates on the nanocrystal size confirms the respective trend given by the Ωλ behavior. Their nanocrystal size dependence consistently reflects the structural changes in the local environment of Nd3+ and the transition to a more ionic Nd−halogenide bond with growing nanocrystal size, giving rise to the assumption that part of the Nd3+ ions are being included into the nanocrystals, or they are at least located in very close proximity to them. Previous electron paramagnetic resonance investigations on europium-doped fluorobromozirconate glass ceramics 48 showed that Eu2+ is incorporated into the barium bromide nanocrystals therein. However, it is not clear if the rare-earth enters the nanocrystals during the thermal processing through diffusion or if the rare-earth itself acts as a nucleation seed for the crystallization. Elsewhere49 it was stated that the neodymium ions enter the barium chloride nanocrystals in fluorochlorozirconate glass ceramics during the thermal processing without giving any further details on the possible mechanism behind it. Several radiative decay rates determined by the Judd−Ofelt theory could be verified with independent methods: photoluminescence lifetime measurements fully confirmed the decay rates for the level n4, which is for the investigated system the only level dominated by spontaneous emission. For several other transitions, emission spectra could be predicted with the McCumber theory in good agreement with experimental data. Rescaling the emission spectra with the Füchtbauer−Ladenburg

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(23) Suzuki, T.; Kawai, H.; Nasu, H.; Mizuno, S.; Ito, H.; Hasegawa, K.; Ohishi, Y. J. Opt. Soc. Am. B 2011, 28, 2001. (24) Kaminskii, A. A.; Boulon, G.; Buoncristiani, M.; Bartolo, B. D.; Kornienko, A.; Mironov, V. Phys. Status Solidi A 1994, 141, 471. (25) Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4424. (26) Qiao, X.; Fan, X.; Wang, J.; Wang, M. J. Non-Cryst. Solids 2005, 351, 357. (27) Fowler, W. B.; Dexter, D. L. Phys. Rev. 1962, 128, 2154. (28) Li, H. H. J. Phys. Chem. Ref. Data 1980, 9, 161. (29) Wetenkamp, L.; Westendorf, T.; West, G.; Kober, A. Mater. Sci. Forum 1988, 32−33, 471. (30) Krupke, W. F. IEEE J. Quantum Elect. 1974, 10, 450. (31) Yin, H.; Deng, P.; Zhang, J.; Gan, F. Mater. Lett. 1997, 30, 29. (32) Choi, J. H.; Margaryan, A.; Margaryan, A.; Shi, F. G. J. Lumin. 2005, 114, 167. (33) Takebe, H.; Yoshino, K.; Murata, T.; Morinaga, K.; Hector, J.; Brocklesby, W. S.; Hewak, D. W.; Wang, J.; Payne, D. N. Appl. Opt. 1997, 36, 5839. (34) Takebe, H.; Nageno, Y.; Morinaga, K. J. Am. Ceram. Soc. 1995, 78, 1161. (35) Lucas, J.; Chanthanasinh, M.; Poulain, M.; Brun, P.; Weber, M. J. J. Non-Cryst. Solids 1978, 27, 273−283. (36) Reisfeld, R.; Eyal, M.; Jørgensen, C. K. J. Less-Common Met. 1986, 126, 187. (37) Nageno, Y.; Takebe, H.; Morinaga, K. J. Am. Ceram. Soc. 1993, 76, 3081. (38) Weber, M. J.; Saroyan, R. A.; Ropp, R. C. J. Non-Cryst. Solids 1981, 44, 137. (39) Jørgensen, C. K.; Reisfeld, R. J. Less-Common Met. 1983, 93, 107. (40) Reisfeld, R.; Jørgensen, C. K. Excited state phenomena in vitreous materials. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, K. A., Eyring, L., Eds.; North-Holland: Amsterdam, 1987; Vol. 9. (41) Reisfeld, R. Ann. Chim. (Paris) 1982, 7, 147. (42) Ratnakaram, Y. C.; Reddy, A. V. J. Non-Cryst. Solids 2000, 277, 142. (43) Brecher, C.; Riseberg, L. A.; Weber, M. J. Phys. Rev. B 1978, 18, 5799. (44) Weber, M. J.; Myers, J. D.; Blackburn, D. H. J. Appl. Phys. 1981, 52, 2944. (45) Skrzypczak, U.; Pfau, C.; Bohley, C.; Seifert, G.; Schweizer, S. Dataset Pap. Phys. 2013, 2013, 236421. (46) Wetenkamp, L.; West, G. F.; Többen, H. J. Non-Cryst. Solids 1992, 140, 35. (47) Martin, R. M.; Quimby, R. S. J. Opt. Soc. Am. B 2006, 23, 1770. (48) Schweizer, S.; Corradi, G.; Edgar, A.; Spaeth, J.-M. J. Phys.: Condens. Matter 2001, 13, 2331. (49) Ahrens, B.; Eisenschmidt, C.; Johnson, J. A.; Miclea, P. T.; Schweizer, S. Appl. Phys. Lett. 2008, 92, 061905.

equation provided an independent confirmation of the Judd− Ofelt results. Over all, this study in general validates the applicability of the Judd−Ofelt theory to glass ceramics. For the particular Nd3+doped FCZ material, the strong effects of BaCl2 nanoparticle growth on the optical properties of the Nd3+ ions can unambiguously be assigned to their localization in close proximity to, or even partial inclusion into, the nanocrystals.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the FhG Internal Programs under Grant No. Attract 692 034. In addition, the authors would like to thank the German Ministry of Education and Research (“Bundesministerium für Bildung und Forschung”) for the financial support within the Centre for Innovation Competence SiLi-nano (Project No. 03Z2HN11). We also thank Thomas Pfeiffer for helpful discussions.

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REFERENCES

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