A Comparison on Ce3+ Luminescence in Borate Glass and YAG

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A Comparison on Ce Luminescence in Borate Glass and YAG Ceramic – Understanding the Role of Host’s Characteristics Atul D. Sontakke, Jumpei Ueda, Jian Xu, Kazuki Asami, Misaki Katayama, Yasuhiro Inada, and Setsuhisa Tanabe J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04159 • Publication Date (Web): 14 Jul 2016 Downloaded from http://pubs.acs.org on July 17, 2016

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The Journal of Physical Chemistry

A Comparison on Ce3+ Luminescence in Borate Glass and YAG Ceramic – Understanding the Role of Host’s Characteristics Atul D. Sontakke,1,2,* Jumpei Ueda,1 Jian Xu,1 Kazuki Asami,1 Misaki Katayama,3 Yasuhiro Inada3 and Setsuhisa Tanabe1 1

Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan 2 PSL Research University, Chimie-ParisTech, CNRS, Institut de Recherche de Chimie Paris, Paris 75005, France 3 Department of Applied Chemistry, Ritsumeikan University, Kusatsu 525-8577, Japan

ABSTRACT. The inefficient luminescence performance of Ce3+ activated glasses is primarily responsible for their commercial failure compared to the Ce3+ activated crystalline materials that are widely used as phosphors and scintillators. We observed that this behavior is explicitly related to the intrinsic characteristics of the host material. Here, we present a systematic study on Ce3+ luminescence in amorphous borate glass, and make a comparison with the well-known polycrystalline Y3Al5O12:Ce3+ (YAG:Ce) phosphor. In borate glass, Ce3+ exhibits blue colored luminescence with quantum yield (QY) of about 42%, whereas the QY is more than 85% in YAG:Ce ceramic that exhibits yellow colored luminescence. This typical behavior has been discussed in terms of the site rigidity of dopant ions in the glassy and crystalline hosts, and its 1 ACS Paragon Plus Environment


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influence on the Ce3+ 5dj states’ crystal field splitting, Stokes shift and the centroid shift, as well as the probability of thermal ionization, host’s intrinsic absorption and the influence of Ce4+ impurity presence in the respective host materials. This study gives a quantitative understanding of host’s contribution on dopant’s luminescence properties, and thereby provides an optimization guideline, which is highly demanding for the design of novel luminescent materials.

INTRODUCTION Light based technologies have seen huge advancements in recent years that have influenced our lives in many ways.1 A big share goes to the phosphor converted white light emitting diodes (pc-wLEDs), which promises dramatic reduction in global energy consumption on lighting.2 YAG:Ce is widely investigated phosphor material for commercial pc-wLEDs as a yellow emitting phosphor encapsulated over the (In,Ga)N based blue LED chip.3,4 In 1967, Blasse and Bril first reported the YAG:Ce phosphor for flying spot cathode ray tubes.5 From that time, tremendous work have been done in evaluating detailed spectroscopic properties of YAG:Ce.3,610

The intense yellow colored luminescence with high luminescence quantum yield (~ 90%),

short decay lifetime (~ 60 ns) and extended thermal stability (> 500 K) are some of the key factors responsible for its wide scale acceptance as a commercial phosphor for wLEDs.3,6 With the increasing power densities of wLEDs, the thermal degradation of organic resin used for fixing the polycrystalline phosphor powder over the high power blue LED chip is becoming a serious concern.11 To counter this difficulty, the phosphor on top prototype has been proposed, where the bulk phosphor plate (single crystal, transparent ceramic or phosphor containing glass ceramic/composite) is placed above the blue LED chip.11-14 This design specifically avoids the decay in luminous efficacy and thereby enhances the life and performance of wLEDs. Along with wLEDs, high-energy radiation scintillators, lasers as well as some spectral convertors (UV 2 ACS Paragon Plus Environment

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to visible/NIR for solar energy concentration) require high quality bulk active materials. LYSO:Ce (Lu2-xYxSiO5:Ce), GAGG:Ce (Gd3Al5-xGaxO12:Ce), LuAG:Ce (Lu3Al5O12:Ce), LaCl3:Ce, LaBr3:Ce, etc., are some of the commonly used commercial Ce3+ based scintillators and laser materials.15-20 It is interesting to note that all these products are based on inorganic crystalline compounds and requires tedious growth/sintering processes (single crystal, transparent ceramics, etc.). Glasses, on the other hand can be more suitable candidates as bulk active materials for the abovementioned applications that can provide easy and economical manufacturing, huge freedom over shape, size, composition, doping levels and homogeneity, as well as low scattering and other optical losses. Particularly in case of scintillators, several Ce3+ activated glasses have been investigated for thermal neutron and high energy radiation (X-ray, γ ray, etc.) detection.21-27 However, unfortunately the light yield of glass based scintillators is proven to be significantly low compared to the crystalline scintillators.24,25,28 Similarly, the photoluminescence quantum yield (QY) in Ce3+ activated glasses is inferior compared to the crystalline phosphors such as YAG:Ce.29,30 The light yield or the luminescence QY defines the energy utilization ability of the active material, and thus plays crucial role in view of its commercial exploitation. Reisfeld and Hormadaly29 investigated the Ce3+ luminescence properties in borate glasses and suggested that the Ce3+ luminescence QY can reach to about 40 – 50%. This value is fairly inferior to Ce3+ based crystalline phosphors, which exhibit QY in the range of 80 - 90%.6 This issue of low luminescence QY, particularly for the broad band transitions (f-d, d-d, charge transfer (CT), etc.) in glasses over their crystalline counterparts was primarily attributed to the basic structural differences and the nature of dopant sites in previous studies.31 Verwey and Blasse,31,32 as well as Inbusch et al.33 argued that the loose structural nature in glassy hosts allows for stronger electron-phonon coupling (large lattice relaxation),

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which increases the probability of non-radiative relaxation through inter-configurational crossing of the potential energy curves of the excited and ground state multipletes of active ion. Prima facie, this model convincingly illustrates the relatively lower luminescence QY in amorphous hosts over the crystalline hosts, but the extent of such decrease is not clear. Other issues such as the clustering tendency of dopant ions, presence of killer (Ce4+) impurities, Ce3+-Ce4+ pairing, etc., were not considered, which have recently been found to exhibit significant influence in defining the Ce3+ luminescence in glassy hosts.24,28 In recent years, several efforts have been made to enhance the photoluminescence and radioluminescence performance of Ce3+ activated glasses. Canevali et al.28 suggested a small modification of silica glass by boron or phosphorus can improve the solubility of Ce3+ ions in the glass matrix. It avoids the Ce3+-Ce4+ pair formation as well as restricts the oxidation of Ce3+ to the Ce4+ state, and thus gives rise to an enhanced photoluminescence output. Chiodini et al.23 used high temperature rapid thermal treatment (RTT) for Ce doped silica sol-gel glasses that resulted in better radio-luminescence yield. Although the RTT slightly increased Ce4+ contents in these glasses, it effectively reduced the Ce3+-Ce4+ clustering, which is argued to be responsible for the enhancement in radio-luminescence performance. Herrmann et al.34 recently investigated the effect of optical basicity of alkaline earth modified alumino-silicate glasses on Ce3+ luminescence properties and observed that the decrease in Ce3+/Ce4+ ratio at higher basicity lead to a decrease in luminescence intensity. In our recent work on Ce3+ doped borate glasses prepared under different synthesis (reducing/oxidizing) conditions, we observed a steady improvement in Ce3+ luminescence properties with the strength of reducing action.30 Other reports also substantiate the strong influence of Ce4+ presence on Ce3+ luminescence in various glasses.35-37 4 ACS Paragon Plus Environment

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It is not the case that Ce4+ does not exist in crystalline phosphor materials. Several crystalline scintillators and phosphors reveal the presence of Ce4+ impurities.17,38-44 However, it does not necessarily affect the Ce3+ performance in those crystalline hosts, or exhibit relatively less significance, unlike in the glassy hosts that generally show strong quenching.17,30,41 It is also possible to restrict the Ce4+ contents in glasses by adopting special synthesis conditions and/or by using a suitable host glass that stabilizes trivalent dopant states.30 However, still the performance of glass based phosphors (or scintillators) is not as competitive as the crystalline phosphors (or scintillators). This issue has been undertaken in present investigation to thoroughly understand what additional factors do affect the luminescence performance of Ce3+ activated glasses. An effort has been made to correlate the observations with the intrinsic characteristics of the host material. We used borate glass as a representative amorphous host for Ce3+ ions, and the results have been compared with those of the well-known YAG:Ce polycrystalline ceramic phosphor. The photoluminescence properties, decay lifetime, thermal quenching behavior, host’s absorption and the presence of Ce4+ impurities have been investigated and discussed in view of the nature of dopant sites, electron binding energies of dopant ions relative to conduction and valence band, and other intrinsic properties of the host materials.

MATERIALS AND METHODS Ce3+: Borate glass The base compositions of borate glasses investigated in present work are (65 – x) B2O3 – x Al2O3 – 20 CaO – 15 La2O3 in mol%, where x = 0, 10 and 20. For Ce3+ doping, an equivalent fraction of La2O3 contents (0.05 - 2.5 mol%) was substituted with the respective stoichiometric amount of dopant contents. CeF3 was used as the precursor chemical for dopant ions, whereas H3BO3, Al2O3, CaCO3 and La2O3 were used as precursors for the other ingredients. All the 5 ACS Paragon Plus Environment


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chemicals were of reagent grade. The glasses were prepared by high temperature melt-quenching method. Reducing CO atmosphere was provided by putting the covered batch crucible (alumina) in carbon filled enclosing crucible.30,45 An excess 0.5 wt.% carbon powder (99.9%) was added in the precursor chemical batch for additional reducing action. The melting was carried out at 1350 °C temperature for 45 minutes, followed by rapid quenching on a warm stainless steel mold. The cast glasses were annealed at 600 °C for 1 h to relieve thermal stress and cooled slowly to the room temperature. To analyze the effect of synthesis condition, some glasses were also prepared in normal air (oxidizing) atmosphere using CeO2 precursor chemical. The glasses are identified according to their dopant concentration, composition and the synthesis condition. Unless specified, the Ce3+ doped borate glass in the text is referred to the optimum cerium concentration (0.5 mol%) doped borate glass (x = 10) prepared under reducing condition.

YAG: Ce The YAG:Ce polycrystalline ceramic used in present study was synthesized by the solid-state reaction method.46 The dopant concentration was 0.5 at.%. Reagent grade Y2O3, Al2O3 and CeO2 precursor chemicals were first mixed by ball milling (Fritsch, Premium Line P-7) using ethanol as dispersant, pulverized at 80 °C, and then the powder was pressed into 10 mm diameter × 2 mm height pellet under an uniaxial pressure of 50 MPa. The pellet was then sintered at 1600 °C for 6 h in air atmosphere to obtain single phase garnet ceramic, as was determined by the X-ray diffraction analysis (Supporting information, Fig. S1).

Spectroscopic measurements Photoluminescence (PL) and PL excitation (PLE) spectra were recorded using a Shimadzu RF-5300 spectrophotometer in the 200 – 800 nm wavelength region. For PL-PLE contour


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mapping, a Shimadzu RF-5000 spectrophotometer was used that was programmed with 5 nm excitation step size. The PL quantum yield (QY) was measured using a 10 inch integrating sphere (Labsphere Inc., LMS-100) attached with a multi-channel CCD detector (Ocean Optics Inc., USB QE Pro-65). A 372 nm LD (Nichia Co. Ltd., NDHU110APAE3) and a 450 nm blue LED were used as excitation sources. The detected signals were calibrated using a standard halogen lamp (Labsphere, SCL-600) and an auxiliary halogen lamp for absolute spectral power distribution and absorption losses, respectively. Both PL-PLE and QY measurements were performed at room temperature. The PL decay measurements were carried out at varied temperatures using a closed-cycle He cryostat (CRT-006-2600, Iwatani), a precise temperature controlled heater and a PL lifetime measurement setup (Hamamatsu-Photonics, Quantarus Tau) equipped with picosecond LEDs for excitation (temporal resolution ~ 0.5 ns). The decay curves are single exponential for low temperature measurements, but exhibit nonexponential behavior at elevated temperatures. The average decay lifetimes were obtained by fitting the decay profiles using single or multi-exponential decay functions in-built in the instrument software (Supporting information, Fig. 2). Optical absorption spectra of the transparent glass samples were recorded using a Shimadzu 3600 spectrophotometer in the wavelength range of 190 – 800 nm. Thin specimens (~ 200 µm thickness) were used for absorption measurements in order to avoid saturation in the UV region. The X-ray absorption spectroscopy (XAS) was performed at the BL9A beamline of Photon Factory (KEK, Japan). The Ce LIII X-ray absorption near edge structure (XANES) was recorded for the glass and ceramic samples in fluorescence mode. Standard Ce(NO3)3.6H2O and CeO2 samples were used for reference. For reference samples, the Ce LIII XANES were recorded in the transmission mode.



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RESULTS AND DISCUSSION Ce3+ luminescence in borate glass and YAG ceramic. Ce3+ exhibits intense and broad parity allowed 5d1 → 4f luminescence in the UV-visible spectral region. Since the luminescence arises from the non-shielded 5d electronic orbital, its properties strongly depend on the surrounding ligand field. Figure 1a, b shows the Ce3+ luminescence maps in the borate glass and YAG ceramic as a function of excitation wavelengths. In glass, the luminescence profile exhibits an asymmetric dependence on excitation wavelength. This is due to the characteristic of amorphous hosts exhibiting non-uniform dopant site distribution. In YAG, the dopant Ce3+ ion occupies only one type of site (Y3+, dodecahedral), and therefore exhibits symmetrical intensity contour with excitation wavelengths.6 This observation becomes more obvious from the Ce3+ luminescence decay lifetimes as shown in Fig. 2. The decay lifetimes exhibit large variation in the glass over the YAG ceramic for changes in excitation as well as monitoring wavelengths. This substantiates a continuous variation in the transition probability due to the site variation of Ce3+ in glass. Moreover, the luminescence peak centers in the UV-blue spectral region in the borate glass but in YAG, Ce3+ gives intense yellow colored luminescence with peak at about 530 nm.

Figure 1. PL-PLE intensity maps of Ce3+ luminescence in (a) borate glass, and (b) YAG ceramic.


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Figure 2. Ce3+ luminescence decay lifetimes as a function of monitoring wavelength in borate glass and YAG ceramic under different excitation wavelengths.

Figure 3 shows the vacuum referred electron binding energy (VRBE) of Ce3+ in the studied borate glass and YAG ceramic.47-50 The Ce3+ 5dj states in YAG experience large crystal field splitting, thereby denouncing the 5d1 state to lower energy with respect to the ground 4fj states.6,48 On the contrary, the crystal field splitting is weak in borate glass.47 In borate glass, the 5dj states are closely spaced amounting a total crystal field splitting (εcfs) of ~ 2.1 eV, whereas it is ~ 3.39 eV in YAG. This incorporates a varied energy difference between the ground state and the first excited 5d1 state of Ce3+ transition in studied hosts.

Figure 3. Ce3+ vacuum referred electron binding energy (VRBE) schemes in borate glass and YAG ceramic.



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The luminescence transition wavelength is a combined effect of the total red shift, D in the 5d1 state energy brought about due to the centroid shift (εc) and the crystal field splitting (εcfs), as well as the Stokes shift (∆S) accounting for the finite perturbations in the chemical coordination of dopant ions in the excited state configuration. The centroid shift represents the metal-ligand bonding, i.e. chemical environment around the dopant ion (Nephelauxetic effect). Among similar anionic hosts (i.e. oxides in present study), the change in the centroid shift (∆εc) may typically stretches up to a few tens of meV.51,52 This is rather insignificant in view of the wider extent of crystal field splitting of the 5dj orbitals, and therefore the red shift is primarily controlled by the crystal field effect of the host material.48 Another important issue is the Stokes shift. The large expanse of the 5dj orbitals makes the excited state electron readily interact with the ligand’s electronic field. These repulsive interactions bring a configurational rearrangement in the chemical coordination of dopant ion in the excited state configuration in order to lower the free energy, which leads to the Stokes shift in luminescence. In present case, Stokes shift has been estimated from the Ce3+ excitation (4f → 5d1) and emission (5d1 → 4f) peak maximums and is about 0.5 ± 0.1 eV in borate glass and 0.32 ± 0.05 eV in YAG ceramic. It is to be noted that the inhomogeneous site distribution of dopant ions in glass matrix incorporates relatively larger uncertainty in spectroscopic quantities such as Stokes shift (∆S), crystal field splitting (εcfs), etc. It is clear that the Ce3+ exhibits larger Stokes shift in borate glass compared to the YAG ceramic. The periodic network structure of crystalline lattice provides a rigid dopant site compared to the random glass network exhibiting large free volume.6 Such stiff dopant sites in crystalline hosts restrict the deformation in ionic arrangement on change in electronic state (4f → 5d) of valence electron, thereby causing a smaller Stokes shift. In amorphous hosts, the loose dopant sites can easily accommodate configurational distortions on electronic state change and 10 ACS Paragon Plus Environment

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gives a larger Stokes shift. This can be well understood from the configuration coordinate diagram of Ce3+ 4fj and 5d1 states in Fig. 4 approximated using experimental PL-PLE results.

Figure 4. Ce3+ 4f – 5d1 configuration coordinate diagrams revealing the configuration offset (∆r) and activation energy for inter-configurational crossover (∆E) in (a) borate glass, and (b) YAG ceramic.

From Fig. 4, it can be seen that the configurational offset (∆r) is larger in borate glass compared to the YAG ceramic. Verwey and Blasse31,32 suggested that the larger Stokes shift in glasses, that increases the configurational offset (∆r) between the bottom of excited and the ground state potential wells may reduce the activation energy (∆E) for 5d - 4f interconfigurational crossover (Fig. 4). This gives rise to an enhanced non-radiative relaxation probability of excited state population by multi-phonon assisted thermal activation (∆E) resulting in lower luminescence QY. This hypothesis was successfully discussed in explaining the CT excited Eu3+ luminescence QY in LiLaP4O12 and LaB3O6 crystals with their stoichiometric glass compositions.31,32 Accordingly, the dopant ions in glasses exhibit relatively poor QY over crystals due to the larger configurational offset between excited CT band and 4f ground state multiplets. Similar results were reported by Imbusch et al. in case of Cr3+ luminescence in several glasses.33 In case of Ce3+ too, literature reports reveal that the luminescence QY in glasses is not as competitive as many crystals.3,6,29,30 11 ACS Paragon Plus Environment


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Luminescence quantum yield (QY). In present study, the Ce3+ luminescence QY was measured using the integrating sphere method.30,53 QY is observed to be (42 ± 2)% in borate glass (λExc: 372 nm) and (86 ± 2)% in YAG:Ce (λExc: 450 nm). The systematic error in the measurement is about 5% in case of YAG ceramic, but it may be slightly higher in borate glass owing to the lower accuracy of excitation region calibration in the UV range (Supporting information, Fig. S3). The results are virtually in agreement with the above hypothesis on differences in configurational offsets. The relatively larger 5d1 – 4f configurational offset in borate glass may encourage inter-configurational crossover leading to a lower luminescence QY. However, the accuracy in defining the actual crossover energy in configuration coordinate model is not certain. The crossover point depends on the exact nature of potential curvature, which is highly complex to achieve, and needs precise knowledge of many fundamental factors.31,54 Moreover, the 4f – 5d1 energy difference is higher in borate glass due to weak crystal field effect, which can further increase the activation energy (∆E), thereby acting opposite to the configurational offset (∆r) effect. Ivanovskikh et al.9 used configuration coordinate model to illustrate the thermal quenching behavior through inter-configurational crossover in Ce3+ and Pr3+ doped YAG and LuAG. But this model fails to explain the luminescence quenching and lower QY in several other crystalline hosts like Gd3Al5O12:Ce3+ (GdAG:Ce) and Y3Al1-xGaxO12:Ce3+ (YAGG:Ce) solid solutions.46 In Y3Ga5O12:Ce3+ (YGG:Ce), the Stokes shift does not differ with the YAG:Ce and the difference between 5d1 minima to 4f-5d1 crossover, ∆E can be larger (assuming the potential curvature remains unchanged).46 This situation would prefer higher luminescence efficiency due to reduced probability of inter-configurational crossover, but the results were completely contradictory revealing very low QY. This issue was discussed by Ueda et al.46 due 12 ACS Paragon Plus Environment

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to the increased probability for thermal ionization of excited 5d1 state electrons to the conduction band, which was evident from the photocurrent excitation spectroscopy. The inclusion of Ga in YAG reduces the bandgap as well as crystal field splitting in 5dj orbitals, thereby compressing the energy difference between excited 5d1 state and the bottom of conduction band, where the later acts as a low energy recombination passage for electron to non-radiatively relax at ground state. A similar situation does exist in present borate glass, where the excited 5d1 state is in close vicinity of the conduction band (Fig. 3), favorable for thermal ionization of excited state electrons to the conduction band at ambient temperature. Thermal ionization probability. Figure 5 shows the temperature dependence of Ce3+ luminescence decay lifetime in borate glass and YAG ceramic. In glass, the decay lifetime is about 51 ns at low temperature (< 100 K), but it decreases with the increase in temperature. At room temperature, the decay lifetime reaches to about 46 ns and then quenches rapidly. On the contrary, the Ce3+ decay lifetime in YAG shows decrease at considerably higher temperature (> 550 K). The decay lifetime is about 63 ns at room temperature, increases slightly till 550 K and then quenches at higher temperature.

Figure 5. Temperature dependence of Ce3+ luminescence decay lifetime in borate glass and YAG ceramic. Solid lines represent simulations to the data points.



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The thermal quenching behavior in both the hosts follows the VRBE scheme in Figure 3. The 5d1 state in borate glass is very close to the conduction band, which suggests a low activation energy for ionization of excited state electrons. But in case of YAG, there is a wide energy difference between the excited 5d1 state and the conduction band, which is unlikely to bridge at room temperature. As mentioned in the earlier section, the thermal quenching in YAG:Ce was attributed to the inter-configurational crossover in previous studies,9 but the recent results by our group on photocurrent excitation and thermoluminescence spectroscopy showed that the thermal ionization is primarily responsible for the quenching in YAG ceramic at > 550 K temperature.55 The activation energy for thermal ionization has been derived by analyzing the temperature dependence of the decay lifetime in Figure 5 using following relation, where the non-radiative part is expressed by classical Arrhenius relation,56  − Ea  k BT

τ −1 = A0 + A1 exp 

  , 

(1)

where τ is the Ce3+ 5d1 → 4f decay lifetime, A0 and A1 are constants representing intrinsic radiative decay rate and the attempt rate for thermal ionization process, respectively, k B is the Boltzmann constant,

T is absolute temperature and E a is the activation energy for thermal

ionization. From the fitting analysis, E a is obtained to be (0.13 ± 0.01) eV and A1 is (3.90 ± 0.01) × 108 s-1 for borate glass, whereas the values are (0.75 ± 0.1) eV and (3.30 ± 0.05) × 1013 s1

in YAG ceramic, respectively. Note that the thermal activation energy is considerably higher in

YAG ceramic compared to the borate glass, which agrees well with the VRBE scheme. Moreover the attempt rate for thermal ionization in glass is fairly less than that in the YAG ceramic. A similar attempt rate is evident from the literature reports, where the glassy hosts usually exhibit the attempt rate in the order of 107 - 108 s-1, but that in crystals can reach up to 14 ACS Paragon Plus Environment

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1010 – 1015 s-1.57-58 This peculiar behavior may be related to the fundamental nature of band structure, i.e. the density of states distribution of conduction band in amorphous and crystalline hosts as well as the presence of localized electron trap centers, which needs a separate detailed investigation.59-60 According to the Boltzmann population distribution function, it is possible to quantitatively estimate the relative ionization probability of Ce3+ 5dj population in borate glass and YAG ceramic at a given temperature using following relation,

PYAG PGlass

 − E a (YAG )   A1(YAG ) exp  k B T   , =  − E a ( Glass )   A1( Glass ) exp   k BT 

(2)

where PYAG and PGlass are the relative probabilities for thermal ionization. Accordingly, the ionization probability in YAG is about 10-5 times that of in the borate glass at room temperature. This suggests a highly insignificant ionization loss of Ce3+ excitation in YAG ceramic at room temperature that is also analogous with the decay lifetime values showing no quenching in YAG ceramic, but about 10% quenching in borate glass at room temperature.

Host’s intrinsic absorption. Amorphous hosts exhibit a characteristic absorption band tailing due to its non-periodic random network structure (Urbach tail) and some localized point defects (dangling/non-bridging bonds).24,28,60,61 In Fig. 6, the optical absorption spectrum of the borate base glass is shown, revealing the exponential Urbach absorption (~ 6 - 7 eV) and the localized defects’ absorption (~ 4 - 5 eV) profiles. Both Urbach and defect absorption gives rise to an intrinsic band tailing of UV absorption edge, which extends fairly inside the band gap region and exhibits a finite absorption.62 In borate glass, this band tailing effectively overlaps the Ce3+ absorption (inset). In such situation, a fraction of excitation energy for Ce3+ luminescence may 15 ACS Paragon Plus Environment


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get absorbed by the host glass during the excitation pumping. This can be understood from the schematic representation of hosts’ band tailing and excitation pump energy shown in Fig. 7. The absorption of excitation pump energy by host material contributes to the nonradiative losses, which affects the Ce3+ luminescence properties, in particular, the luminescence QY since it is derived from the ratio of emitted photons to the absorbed photons by the material.6

Figure 6. Base glass absorption spectrum. The dotted line represents approximation at high energy region, which could not be experimentally recorded due to saturation. EU is the Urbach energy. Inset shows the absorption band tail profile (a), and Ce3+ 4f – 5dj absorption (b) in borate glass.

Figure 7. Hosts’ intrinsic absorption band tails schematic in borate glass and YAG ceramic, and their contribution in excitation pump energy absorption.


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It is reasonable that the relative contribution of host’s intrinsic absorption may decrease if the Ce3+ absorption contribution increases in the host material. This can be achieved by increasing the Ce3+ concentration or by selecting the appropriate excitation wavelength that corresponds higher absorption coefficient for Ce3+ ions. Figure 8 shows the luminescence QY for a series of dopant concentrations ranging from 0.05 to 2.5 mol% in borate glass. The QY first increased from about 35% (0.05 mol%) to 42% (0.5 mol%) in accordance with the increase in Ce3+ absorption coefficient, but then decreased for further increase in dopant concentration, which may be due to the enhanced excitation energy migration that assists in excitation trapping by killer (impurities/defects) centers. A similar increase in QY is also observed for lower excitation wavelengths, where the Ce3+ possess relatively higher absorption coefficient than the host glass (Supporting information, Fig. S4). For 0.5 mol% doped glass, the luminescence QY showed a systematic increase up to about 48%, when the excitation wavelength was reduced to 350 nm. However, the reliability of correction decreases in lower wavelengths region due to the weak signal intensity of standard halogen lamp and the poor detector response (Supporting information, Fig. S3).

Figure 8. Ce3+ luminescence QY, decay lifetime and relative intensity as a function of dopant concentration in borate glasses. Solid lines represents the variation trend of experimental data points.



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A quantitative determination of host’s intrinsic absorption contribution on measured luminescence QY has been obtained by evaluating the host corrected QY. For this, a base glass of identical dimensions was used as reference in QY experiment to nullify the host’s intrinsic absorption loss.63 The QY is obtained to be (62 ± 3)% on host correction (λExc: 372 nm), which is about 21% higher than the standard QY for studied Ce3+ doped borate glass (42 ± 2%). Unlike glass, the degree of disorderness is less in YAG ceramics because of its periodic network structure.64,65 Moreover, the larger crystal field in Ce3+ 5dj states and relatively high bandgap energy restricts the excitation energy loss in YAG ceramic (Fig. 7). Influence of Ce4+ presence. Ce4+ exhibits intense charge transfer (CT) transition with ligand anion (Ce4+ - O2- → Ce4+(-) - O-) that generally recombines through non-radiative relaxation. In the studied borate glass matrix, Ce4+ exhibits a broad CT absorption centered at about 280 nm, as observed from the absorption spectrum of air (oxidizing) atmosphere synthesized Ce - borate glass (Supporting information, Fig. S5). The CT band effectively overlaps the absorption spectrum of Ce3+ and partially overlaps the emission spectrum. This suggests that the presence of Ce4+ can be detrimental for Ce3+ luminescence. It can directly absorb the pump energy during Ce3+ excitation, and also exhibit radiative as well as non-radiative energy transfer interactions with the neighboring excited state Ce3+ ions. Figure 9a shows the normalized X-ray absorption near edge structure (XANES) of the Ce LIII absorption in doped borate glasses synthesized in oxidizing (air) and reducing (CO) atmospheres considered in the present study. Standard references for Ce3+ (Ce(NO3)3.6H2O) and Ce4+ (CeO2) are also presented for guidance. Ce3+ exhibits intense dipole allowed (2p4f15d) peak at about 5724 eV, whereas Ce4+ shows clear doublets peaking at about 5729 eV (2p4f15d1) and 5736 eV (2p4f05d1), respectively.17,40 Both the glasses show a majority contents of Ce3+ state and a 18 ACS Paragon Plus Environment

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fraction of Ce4+. The reducing atmosphere synthesized borate glass reveals a significant decrease in Ce4+ absorption intensity. In YAC:Ce, the Ce4+ presence is also evident from the XANES spectrum (Fig. 9b).

Figure 9. Ce LIII XANES spectra of (a) Ce doped glass samples and, (b) YAG:Ce ceramic. Standard reference for Ce3+ (Ce(NO3)3.6H2O) and Ce4+ (CeO2) are also included.

From the figure, the YAG ceramic and air atmosphere synthesized glass possess competitive amount of Ce4+ cations, whereas the reducing atmosphere glass shows relatively less presence of Ce4+ cations. Interestingly, the Ce3+ luminescence QY is fairly weak in glass (> 1% in air atm. glass),30 but no significant influence could be seen in YAG ceramics (QY ~ 86%) for competitive presence of Ce4+ cations. This indicates that the Ce4+ presence does not strongly affect the Ce3+ luminescence properties or the quantum yield in YAG:Ce. One reason is the non-resonance of Ce3+ and Ce4+ transitions in YAG ceramic. The Ce4+ charge transfer occurs at about 254 nm in YAG,3 but the Ce3+ absorption and emission is at far less energy owing to the wider crystal field splitting (Fig. 3). This restricts the Ce3+ → Ce4+ non-radiative energy transfer as well as the possibility of direct absorption of excitation pump energy (455 nm) by Ce4+ CT, unlike in the borate glass. Another issue is the Ce3+-Ce4+ electron transfer interaction, which is energetically favorable,56 but still does not incorporate any significant quenching, may be due to the 19 ACS Paragon Plus Environment


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preferential segregation of Ce4+ cations.66 The 10 - 15% QY loss observed in studied YAG:Ce may be related to the electron transfer interactions. It is important to note that the Ce4+ presence is more detrimental in borate glass over the YAG ceramic. In oxidizing (air) atmosphere synthesized borate glass, both direct excitation energy absorption by Ce4+ CT and the Ce3+ → Ce4+ non-radiative energy transfer as well as the electron transfer interactions (Supporting information, Fig. S6) are present.30,24 The later interactions diminish in the reducing (CO) atmosphere synthesized borate glass owing to the very small Ce4+ presence, thereby raising the luminescence QY to 42%. However, even at the trace presence, the Ce4+ ions can effectively absorb the pumping energy for Ce3+ excitation, which may bring a loss in measured luminescence QY. The rest 20 – 25% loss in Ce3+ luminescence QY, apart from about 10% thermal ionization losses and about 20% host’s intrinsic absorption losses in studied borate glass may be attributed to the losses due to Ce4+ presence. To summarize, it is clear that the basic structural characteristics of the studied borate glass and the YAG ceramic are responsible for the observed Ce3+ luminescence properties in these hosts. One significant point is the nature of the dopant site. The loose dopant site in glassy hosts assists in larger lattice relaxation leading to wider Stokes shift. Moreover, it does not significantly impose the crystal field effect, unlike in the crystalline hosts. In glasses, the crystal field splitting for Ce3+ 5dj states remains more or less same despite the glass compositions. Therefore, the change in centroid shift become more prominent in glassy hosts.36,67,68 In studied borate glass, the 5dj states shows a red shift with the substitution of borate contents by alumina (Fig. 10), which is in accordance with the Nephelauxetic effect. However, the crystal field splitting (εcfs) does not change much with compositional modification and maintains about 2.1 eV for investigated 0 – 20 mol% Al2O3 contents. In crystals, usually the crystal field effect is 20 ACS Paragon Plus Environment

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more decisive over the centroid shift due to its rigid dopant site having a definite anionic framework. In YAl1-xGaxO12, the Al substitution by Ga is similar to B substitution by Al in borate glass. Still, Ce3+ exhibits blue shift in the luminescence owing to the reduction in crystal field strength on 5dj states.46 This behavior, in fact, offers better control on Ce3+ luminescence properties in crystalline hosts for material engineering to achieve desired properties that are particularly ideal for efficient phosphors, persistence, or scintillation, etc.50 On the contrary, the control over Ce3+ luminescence in amorphous hosts is weak. The centroid shift follows the basicity and so does the bandgap and other related properties.69-71 A red shift in 5dj states with increase in glass basicity accompanies with a lowering of the bandgap, and therefore the relative energy position of 5dj states with that of the bottom of conduction band may remain similar. This suggests that the thermal ionization at room temperature shall prevail in amorphous hosts at certain finite extent. Similarly is the case for other issues like intrinsic band tail absorption and the influence of Ce4+ ions. The change in 4f – 5d1 energy will also accompany the change in bandgap that will accordingly modify the absorption band tails. The Ce4+ redox stability may increase with the increase in basicity, however on the other hand, lower basicity increases the 4f – 5d1 energy gap, allowing for more overlap of Ce3+ emission/absorption with that of the Ce4+ charge transfer state. Among all the discussed issues, Ce4+ is most detrimental for Ce3+ luminescence properties and, fortunately, it can be controlled by adopting more acidic glass as host for active ions and the use of stronger reducing conditions in the glass synthesis to shift the redox equilibrium at lower valence state.



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Figure 10. Ce3+ optical absorption spectra in studied borate glasses exhibiting different Al2O3 contents.

Conclusions The intrinsic material’s properties of borate glass and YAG ceramic hosts are shown to exhibit vital role in defining the luminescence performance of dopant Ce3+ ions. The luminescence color, spectral and decay behavior, Stokes shift, thermal quenching mechanism and luminescence QY have been discussed and successfully correlated to the respective host’s characteristics. The probability of thermal ionization, intrinsic absorption due to host material and the influence of killer (Ce4+) impurities are found responsible for the inferior QY of Ce3+ luminescence in borate glass over the YAG ceramic.

ASSOCIATED CONTENT Supporting Information. Sample images and XRD patterns, representative luminescence decay profiles, detector response curve to standard halogen lamp and its absolute spectral power in studied spectral range, optical absorption and excitation spectra of Ce3+-borate glass, Ce3+Ce4+ electron transfer interactions, and the Ce4+ charge transfer spectrum in borate glass, Figure S1 – S6. This material is available free of charge via the Internet at http://pubs.acs.org. 22 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (+33 153737928)

Notes Authors declare no competing financial interests.

ACKNOWLEDGMENT This work was carried out under the JSPS post-doctoral fellowship program (P13372). The Xray absorption spectroscopy (XAS) measurements were performed under the approval of the Photon Factory program advisory committee (No. 2014G600). Authors also thank Prof. Pieter Dorenbos (TU Delft), Prof. Bruno Viana (Chimie ParisTech-CNRS,) and Prof. Peter Tanner (Hong Kong Institute of Education) for the fruitful discussions during their stay at Kyoto University that helped in the development of this manuscript.

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Figure 1. PL-PLE intensity maps of Ce3+ luminescence in (a) borate glass, and (b) YAG ceramic. Figure 1a, b shows the Ce3+ lu 79x55mm (600 x 600 DPI)

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Figure 1. PL-PLE intensity maps of Ce3+ luminescence in (a) borate glass, and (b) YAG ceramic. Figure 1a, b shows the Ce3+ lu 79x55mm (600 x 600 DPI)


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Figure 2. Ce3+ luminescence decay lifetimes as a function of monitoring wavelength in borate glass and YAG ceramic under different excitation wavelengths. This observation becomes more 79x55mm (600 x 600 DPI)



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Figure 3. Ce3+ vacuum referred electron binding energy (VRBE) schemes in borate glass and YAG ceramic. Figure 3 shows the vacuum refe 87x67mm (600 x 600 DPI)


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Figure 4. Ce3+ 4f – 5d1 configuration coordinate diagrams revealing the configuration offset (∆r) and activation energy for inter-configurational crossover (∆E) in (a) borate glass, and (b) YAG ceramic. This can be well understood fr 79x55mm (600 x 600 DPI)



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Figure 5. Temperature dependence of Ce3+ luminescence decay lifetime in borate glass and YAG ceramic. Solid lines represent simulations to the data points. Figure 5 shows the temperature 79x55mm (600 x 600 DPI)


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Figure 6. Base glass absorption spectrum. The dotted line represents approximation at high energy region, which could not be experimentally recorded due to saturation. EU is the Urbach energy. Inset shows the absorption band tail profile (a), and Ce3+ 4f – 5d¬j¬ absorption (b) in borate glass. In Fig. 6, the optical absorpt 87x67mm (600 x 600 DPI)



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Figure 7. Hosts’ intrinsic absorption band tails schematic in borate glass and YAG ceramic, and their contribution in excitation pump energy absorption. This can be understood from th 87x67mm (600 x 600 DPI)


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Figure 8. Ce3+ luminescence QY, decay lifetime and relative intensity as a function of dopant concentration in borate glasses. Solid lines represents the variation trend of experimental data points. Figure 8 shows the luminescenc 81x62mm (600 x 600 DPI)



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Figure 9. Ce LIII XANES spectra of (a) Ce doped glass samples and, (b) YAG:Ce ceramic. Standard reference for Ce3+ (Ce(NO3)3.6H2O) and Ce4+ (CeO2) are also included. Figure 9a shows the normalized 79x55mm (600 x 600 DPI)


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Figure 10. Ce3+ optical absorption spectra in studied borate glasses exhibiting different Al2O3 contents. In studied borate glass, the 5 79x55mm (600 x 600 DPI)


A Comparison on Ce3+ Luminescence in Borate Glass and YAG

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