Influence of Ce3+ Concentration on the Thermal Stability and Charge

Article pubs.acs.org/JPCC

Influence of Ce3+ Concentration on the Thermal Stability and ChargeTrapping Dynamics in the Green Emitting Phosphor CaSc2O4:Ce3+ Suchinder K. Sharma,† Marco Bettinelli,‡ Irene Carrasco,‡ and Maths Karlsson*,† †

Department of Chemistry and Chemical Engineering, Chalmers University of Technology, 412 96 Göteborg, Sweden Luminescent Materials Laboratory, Department of Biotechnology, University of Verona and INSTM, UdR Verona, Strada Le Grazie, 15371 34 Verona, Italy

‡

ABSTRACT: The influence of the Ce3+ concentration on the excitation and emission characteristics, thermal stability, and charge-trapping−detrapping dynamics, of the green-emitting phosphor Ce3+ doped calcium scandium oxide (CaSc2O4) with very dilute Ce3+ substitutions (0.5, 1.0, and 1.5%), has been investigated using optical spectroscopy techniques. The diffuse reflectance and excitation spectra are found to exhibit a nonsystematic behavior with varying Ce3+ concentration, mainly linked to spectral band-overlap, whereas the emission spectra display only minor changes with varying Ce3+ concentration, suggesting that the local structural coordination of the Ce3+ dopants remains the same for different Ce3+ dopant levels. The major impact of Ce3+ concentration is seen on the thermal quenching temperature, which is found to be as high as T50% ≈ 600 K for the most dilute Ce3+ doping (0.5%), followed by T50% ≈ 530 K for 1.0% doping and T50% ≈ 500 K for 1.5% doping, respectively. The materials are found to display a red-shift of the emitted light from 518 to 535 nm with increasing temperature from T = 80 K to T = 800 K, for all Ce3+ dopant levels. Thermoluminescence glow curves provide evidence for five charge-trapping defects, which are found to exhibit different charge-trapping dynamics for excitation into different 5d levels. It is argued that the three deeper traps can be filled by athermal tunneling of charges from the Ce3+ 5d1 level, while the two shallower traps can only be filled when the charges move through the conduction band of the material.

1. INTRODUCTION Phosphor-converted white-light emitting diodes (pc-WLEDs) offer extraordinary energy savings compared to traditional light sources due to their high efficiency, tunable color, and long lifetimes.1−5 One of the currently most important phosphors is yttrium aluminium garnet (Y3Al5O12, YAG), which when substituted with a few percent of Ce3+ (YAG:Ce3+) gives rise to green-yellow emission in the 500−700 nm range upon excitation with a blue light source. The combination of the two colors (blue-light and YAG:Ce3+ emission) gives rise to a whitelight emission spectrum, suitable for lighting, high-tech displays, and electronic devices. However, due to weak red emission in the spectrum of YAG:Ce3+, the emitted white light is perceived as “cold” and “too blue” for high-quality illumination. Another problem affecting YAG:Ce3+ is its relatively low thermal stability: about 20% of the room temperature intensity is lost at T = 500 K and more than 50% at T = 700 K, depending on Ce3+ dopant concentration and excitation wavelength.6,7 Since in pc-WLEDs, the heat created at the p−n junction of the underlying LED transfers easily to the phosphor layer, the phosphor reaches a temperature of ca. 450 K, thus leading to significant efficiency losses.6,8 This becomes even more evident when one takes the fast development of high-power LEDs into account, because by increasing the power consumption the heat created by the LED will increase as well, as the temperature of the phosphor layer will reach temperatures well above the current value of 450 K in commercially available products.6−8 © XXXX American Chemical Society

To resolve these problems, the development and use of red and green phosphors, with strong resistance to thermal quenching of emitted light, is needed instead of a single yellow phosphor. A novel and technologically promising green phosphor is calcium scandium oxide, CaSc2O4 (CSO), which when doped with a small amount of Ce3+ to substitute for Ca2+ (CSO:Ce3+), shows a broad-band absorption spectrum in the range 360−500 nm, which overlaps with the emission from a blue LED, and green-light emission in the range 450−700 nm due to electronic Ce3+ 5d → 4f transitions.9−12 The CSO crystal structure may be described in terms of a three-dimensional network of edge- and corner-sharing ScO6 octahedra with an overall orthorhombic symmetry (space group Pnma).13 There is one Ca, two Sc, and four different O sites (Figure 1). The Ce3+ dopant is expected to substitute for Ca2+, due to their similar ionic radii in an 8-fold coordinated site, cf. 1.143 Å (Ce3+) vs 1.120 Å (Ca2+).14 In more detail, results from electron paramagnetic resonance measurements have revealed that the Ca2+ site is split into one relatively symmetric site, that accounts for approximately 91% of the Ce3+ ions in CSO:Ce3+, and a more distorted environment around the Ce3+ dopants, which accounts for the remaining 9%.15 The fraction of distorted sites is found to increase with increasing Ce3+ Received: August 18, 2017 Revised: September 22, 2017

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DOI: 10.1021/acs.jpcc.7b08263 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

in photon counting mode for the detection of the emitted signal. Variable temperature emission spectra and luminescence decay curves were measured at Chalmers University of Technology, Göteborg, Sweden, over the temperature range T = 80−800 K. The temperature was controlled using a Linkam THMS 600 heating stage, with the samples prepared as discshaped pellets of 5 mm in diameter by uniaxial pressing of powder mixtures of CsI and phosphor in a 5:1 ratio. The excitation was performed using a 454 nm laser (DeltaDiode DD-450L) connected to a DD-C1 ps diode controller from Horiba Scientific. The decay curves were detected using a H10721-20 photosensor module from Hamamatsu containing a PMT and a high-voltage power supply circuit with a spectral response in the 230−920 nm wavelength range. The decay curves were monitored on an Agilent DSO-X 2022A oscilloscope. The emission spectra were recorded on an Ocean Optics USB2000+ UV−vis spectrometer. The TL studies were performed at Chalmers University of Technology. The excitation for charge storage was achieved using 254 nm (Philips 6W tube), 340 nm (M340D3 LED from ThorLabs), and 454 nm laser (Deltadiode DD-450L from Horiba Scientific) sources. The TL signal was monitored using an Agilent DSO-X 2022A oscilloscope and detected using a H10721-20 photosensor module from Hamamatsu. The measurements were performed at a linear heating rate of 1 K/s in the temperature range 290−470 K using a Linkam THMS 600 heating stage.

Figure 1. Schematic diagram of the crystal structure of CSO:Ce3+. The small blue spheres are oxygen atoms, the gray spheres are Sc atoms, the light green spheres are Ca atoms, and the dark green sphere is a Ce atom.

concentration and correlates with a strong decrease in quantum efficiency (QE) of the material; QE = 82% for 0.5% Ce3+, QE = 63% for 1.0% doping, and QE = 36% for 1.5% doping.15 The decrease in QE as a function of increasing Ce3+ concentration is possibly related to an increasing concentration of Sc/Ca vacancies and/or interstitial oxygen atoms,16 which are formed as a charge-compensating effect and which are likely to act as charge-trapping defects and thus to quench luminescence. However, the role of Ce3+ concentration on the thermal stability, and on the color of the emitted light, and the nature of charge-trapping−detrapping dynamics are not known. Toward this end, we present a systematic analysis of the evolution of the excitation and emission characteristics, thermal stability, and charge-trapping dynamics of CSO:Ce3+ with very dilute Ce3+ substitutions, i.e. 0.5%, 1%, and 1.5%. By studying these properties, we aim to provide detailed insight into the structure−property correlations and hence the origins of the performance of CSO:Ce3+. The methods used are diffuse reflectance spectroscopy, photoluminescence (PL), and thermoluminescence (TL) techniques.

3. RESULTS 3.1. Diffuse Reflectance Spectra. In Figure 2 the diffuse reflectance spectra are shown for the four samples, CSO, CSO

2. EXPERIMENTAL DETAILS The samples, CSO, and CSO:Ce3+ with 0.5, 1.0, and 1.5% Ce3+ doping, hereafter labeled as CSO, CSO (0.5), CSO (1.0), and CSO (1.5), were prepared by conventional solid state synthesis. Details of the materials synthesis and the crystallographic parameters of the samples are described in ref 15. Diffuse reflectance spectra of the samples were measured over the wavelength range 220−700 nm at the University of Bologna, Italy, using a PerkinElmer Lambda 45 double beam spectrophotometer equipped with a RSA-PE-20 integrating accessory from Labsphere. The samples were measured as solid mixtures in a matrix of ground NaCl. A measurement of a pure sample of NaCl was used as reference spectrum. The reflectivity data for each sample was corrected for the weight% of each sample in NaCl. Room temperature PL spectra were measured at the University of Verona, Italy, using a Fluorolog 3 spectrofluorometer (Horiba-Jobin-Yvon), equipped with a Xe lamp, a double excitation monochromator, a single emission monochromator (mod. HR320), and a photomultiplier tube (PMT)

Figure 2. Diffuse reflectance spectra for CSO, CSO (0.5), CSO (1.0), and CSO (1.5). The arrow indicates a ≈7 nm shift of the maximum absorption for the 4f → 5d1 transition for CSO (1.0) and CSO (1.5) as compared to CSO (0.5).

(0.5), CSO (1.0), and CSO (1.5), over the wavelength range 220 to 700 nm. For the undoped sample (CSO), the reflectance is, apart from the slight absorption at the shortest wavelengths (≲260 nm), approximately 90% and essentially wavelength independent. This suggests that CSO may be used as a transparent material in the UV−vis range for optical applications, but of more relevance here it also means that any features in the reflectivity data for the Ce3+ doped samples (CSO:Ce3+) may be directly linked to the dopants. B


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The Journal of Physical Chemistry C CSO:Ce3+ exhibits an absorption band in the range 400−500 nm, assigned as electronic Ce3+ 4f → 5d1 transitions, and a broad, asymmetric, absorption band between 220 and 400 nm, assigned as, somewhat spectrally overlapping, electronic Ce3+ 4f → 5d2 and Ce3+ 4f → 5d3 transitions.10−12 As can be seen, the intensity of the Ce3+ 4f → 5d1 transition increases systematically with increasing Ce3+ concentration, showing that the level of Ce3+ doping does not lead to a saturation of the absorption of light in this spectral range. The maximum absorption intensity for the 4f → 5d1 transition exhibits a shift of approximately 7 nm toward shorter wavelengths for the two highly doped samples, CSO (1.0) and CSO (1.5), as compared to the weakly doped one, CSO (0.5). In regard to the features at shorter wavelengths, 200−400 nm, the Ce3+ concentration dependence is less clear due to the overlapping nature of the bands, especially for the two more highly doped materials, CSO (1.0) and CSO (1.5). For CSO (0.5), the bands are more distinguished and located at around 225 and 320 nm, respectively. More detailed information about the Ce3+ 4f → 5d absorption bands is obtained from PL spectroscopy. 3.2. Photoluminescence Spectra. Figure 3(a,b) shows the room temperature PL excitation and emission spectra as normalized to the maximum intensity for all Ce3+ doped samples. The excitation spectra [Figure 3(a)] were measured at fixed emission wavelengths of 518 and 567 nm, respectively, and possess three maxima at approximately 445 nm (4f → 5d1

transitions), 325 nm (4f → 5d2 transitions), and 285 nm (4f → 5d3 transitions), respectively.9−12 The position and width of each excitation band do not change significantly as a function of the Ce3+ concentration or by fixed emission wavelengths. However, for the 325 and 285 nm bands, related to the 4f → 5d2 and 4f → 5d3 transitions, respectively, the largest excitation intensity is observed for CSO (1.0), with a slightly lower intensity for the other compositions. The somewhat nonsystematic dependence of the excitation spectra as a function of Ce3+ dopant level is in line with the diffuse reflectance spectra shown in Figure 2. The PL emission spectra were measured by fixing the excitation wavelength at 325 nm (corresponding to 4f → 5d2 transitions) and 445 nm (corresponding to 4f → 5d 1 transitions) and were overall similar to each other. Figure 3(b) shows, as an example, the PL emission spectra as measured at room temperature for excitation at 445 nm, for the three samples. The two maxima, at 518 and 567 nm, respectively, as marked with vertical lines, are assigned as electronic Ce3+ 5d1 → 2F7/2 and 5d1 → 2F5/2 transitions, respectively. At variance with the excitation spectra, the emission spectra measured at room temperature show only minor differences for different Ce3+ dopant levels. With respect to Ce (0.5), the emission spectra for Ce (1.0) and Ce (1.5) are slightly narrowed (by less than 3 nm) on the long-wavelength and short-wavelength side, respectively. A much larger effect of the Ce3+ concentration is observed on the thermal stability of the samples. To investigate the thermal stability, especially in regard to the thermal quenching of Ce3+ luminescence, both the integrated emission intensity and the lifetime of the Ce3+ emission were measured. Figure 4 shows the emission spectra for excitation at 454 nm for all compositions over the temperature range T = 100 K to T = 800 K. A general feature is the systematic decrease in emission intensity as a function of increasing temperature for all samples. Figure 4(d) compares the temperature dependence of the normalized emission intensity

Figure 3. (a) Normalized PL excitation spectra measured by fixing the emission wavelength at 518 and 567 nm, respectively. The insets show a close-up of the wavelength region 275−350 nm. (b) Normalized PL emission spectra measured for excitation at 445 nm. All measurements were performed at room temperature.

Figure 4. Temperature dependence of the PL emission spectra for (a) CSO (0.5), (b) CSO (1.0), and (c) CSO (1.5), measured upon excitation at 454 nm. (d) Temperature dependence of the emission intensity, as integrated over the wavelength region 475−700 nm. T95% and T50% are indicated in the figure. C


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The Journal of Physical Chemistry C as integrated over the wavelength region 460−660 nm, i.e. the 5d1 − 4f band, for all samples, which allows for a quantitative comparison of the thermal stability for the three samples. The onset temperature of thermal quenching, which is defined here as the temperature at which the PL intensity becomes 95% of that at low temperature (here T = 80 K), is T95% ≈ 120 K for all compositions. The thermal quenching temperature, which is defined here as the temperature at which the PL intensity becomes 50% of the low-temperature value, is T50% ≈ 520 K for CSO (0.5) and T50% ≈ 460 K for CSO (1.0) and CSO (1.5), respectively. Also of relevance is the general shift of the emission maximum toward longer wavelengths (red-shift) upon increasing temperature, from around 518 nm at T = 100 K to 535 nm at T = 800 K. The magnitude of the red-shift is virtually independent of Ce3+ concentration, which is further evident from the CIE 1931 diagram as shown in Figure 5. The color

Figure 6. Temperature dependence of the luminescence decay curves for (a) CSO (0.5), (b) CSO (1.0), and (c) CSO (1.5). Lines are single exponential fits. (d) Luminescence decay times determined from the single-exponential fits in (a−c) as a function of temperature.

temperatures are at T50% ≈ 600 K for CSO (0.5), T50% ≈ 530 K for CSO (1.0), and T50% ≈ 500 K for CSO (1.5); see Table 1. The lines in Figure 6(d) are fits to a single-barrier quenching model according to the relationship 1 τ (T ) = Γν + Γ0 exp(−E /kBT ) (1)

Figure 5. CIE 1931 color coordinate diagram for CSO:Ce3+ for excitation at 454 nm at T = 100 K and T = 600 K. Note the red-shift for all samples upon increasing temperature (close-up). The T = 100 K data is shown in black color, and the T = 600 K data in red color.

from which we have extracted the activation energy, E, for the temperature quenching for each sample; Γν is the radiative rate, Γ0 is the attempt rate of nonradiative processes, and kB is the Boltzmann constant.18 The activation energies are in the range 0.16−0.25 eV (Table 1) and decrease with increasing Ce3+ concentration and represent “apparent” values reflecting the energy needed to bring the 5d1 electrons into charge-trapping defects (“killer centers”), either via the conduction band (CB) or through, so-called, athermal tunneling of charges. In order to interpret in more detail these experimental findings, especially in regard to the position of Ce3+ dopants with respect to the valence and CB of CSO:Ce3+, we have constructed the vacuum referred binding energy (VRBE) diagram of Ce3+ and Ce2+. For comparison, we have also constructed the VRBE diagram for all other lanthanides in their +2 and +3 oxidation states. The construction of the VRBE diagram is based on the chemical shift model as discussed by Dorenbos.19 3.4. Vacuum Referred Binding Energy Diagram. Sufficient information can be found on the way to build the VRBE diagram for different hosts,19−27 and this will not be discussed in detail here. The model provides electronic structure with absolute binding energies relative to the energies of the electron at rest, in vacuum. For CSO, the parameters used to construct the VRBE diagram were the exciton creation energy, Eex = 6.264 eV,28 the charge transfer energy, ECT (O2− →Eu3+) = 4.51 eV,29 the exchange energy, Eexch = 0.859 eV,30,31 and the red-shift, D(3+,CSO) = 3.33 eV (as obtained from the PL spectra in the present work, cf. Figure 3). Based on these values, we calculated the Coulomb repulsion energy, U (6, CSO) = 6.46 eV, the electron binding energy in the ground state of Ln2+, E4f (7, 2+, CSO) = −3.81 eV, the electron binding energy in the excited state of Ln2+, E5d (7, 2+, CSO) = −1.49

coordinates (x and y) at T = 100 K for the three samples are shown in Table 1. The color coordinates for CSO (0.5) are slightly lower than for CSO (1.0) and CSO (1.5), the latter two being essentially the same. Table 1. Quenching Temperature (as Determined from Luminescence Lifetime Data), T50%, Activation Energy, E, and Color Coordinates (x and y) at T = 100 K, for CSO (0.5), CSO (1.0), and CSO (1.5) Sample

T50% (K)

E (eV)

x

y

CSO (0.5) CSO (1.0) CSO (1.5)

600 530 500

0.25 ± 0.08 0.18 ± 0.03 0.16 ± 0.05

0.3297 0.3367 0.3353

0.5758 0.5815 0.5832

3.3. Luminescence Lifetime Measurements. Figure 6(a−c) shows the luminescence decay curves upon excitation at 454 nm for the three Ce3+ doped samples. All decay curves can be adequately approximated by single exponential fits (solid lines in the figures). Figure 6(d) compares the luminescence decay time, τ(T), over the temperature range T = 80−600 K, as extracted from the single exponential fits. At the lowest temperature (T = 80 K), the luminescence decay time is approximately 36 ns for CSO (0.5), 33 ns for CSO (1.0), and 29 ns for CSO (1.5), and thus decreases with increasing Ce3+ concentration. The onset of temperature quenching is at T95% ≈ 370 K for CSO (0.5), T95% ≈ 190 K for CSO (1.0), and T95% ≈ 170 K for CSO (1.5). The corresponding quenching D


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Considering first the most weakly doped material, CSO (0.5), and the glow curves measured upon excitation at the two shorter wavelengths, 254 and 340 nm, we observe five glow curve maxima, at approximately Tm = 317, 348, 361, 394, and 414 K, respectively, where Tm is referred to as the temperature maximum of a glow curve. The glow curve maxima correspond to different types of charge-trapping defects, here labeled as Trap1, Trap2, Trap3, Trap4, and Trap5. Regarding the effect of excitation time, we observe a saturation of the intensity of the peaks corresponding to Trap1 and Trap2 for excitation at 254 nm for longer than 2 and 5 min, respectively. It follows that the capability of defects to store charges in Trap1 and Trap2 does not increase further with increasing charging time. In comparison, a trend of increasing intensity as a function of increasing charging time is seen for all other traps and excitation wavelengths, showing that the population of the charges in the five traps increases. Further, the positions of the glow curve maxima do not shift significantly with excitation time, suggesting that recombination processes dominate over retrapping processes. This implies that the trap depth, Ea, may be determined from the relationship Ea = 25kBTm.33 By doing so, the trap depths have been determined to be 0.68, 0.75, 0.78, 0.85, and 0.89 eV, respectively. For excitation at the longer wavelength, 454 nm, the two shallower traps (Trap1 and Trap2) cannot be observed, whereas the positions of the remaining peaks, Trap3, Trap4, and Trap5, are at Tm = 364 K, 388 K, and 408 K, respectively. The TL glow curves for the higher Ce3+ dopant levels, CSO (1.0) and CSO (1.5), are generally similar to the glow curves for Ce (0.5); however, some important differences can be discerned. In particular, we observe that none of the five traps saturates for the different excitation wavelengths and charging times. Moreover, we observe that the TL glow curves for CSO (1.5) are somewhat less distinct, especially for excitation at 254 and 340 nm. The generally broad nature of the TL glow curves reflects a quasi-continuous distribution of traps, meaning there are small variations in the local structure around each trap throughout the CSO lattice. This is in agreement with the locally disordered nature of the CSO lattice, as found by the analysis of neutron total scattering and X-ray absorption spectroscopy data.15 Different excitation sources had different power, and their relative TL intensities could not be compared.

eV, the top of the valence band, EV = −8.32 eV, the electron binding energy in the exciton state, Ex = −2.05 eV, and the bottom of the CB, EC = −1.55 eV. The results are summarized in the VRBE diagram shown in Figure 7. The obtained energies

Figure 7. VRBE diagram of CSO doped with different lanthanide ions in the divalent and trivalent state. The connected blue line indicates the ground state (4f) for all lanthanides in their +3 state, whereas the connected orange line represents the ground state for all lanthanides in their +2 state. The connected violet and red lines represent the lowest 5d states for all lanthanides in their +3 and +2 state, respectively. The green arrow represents the charge transfer energy ECT (O2− → Eu3+). The top of the valence band (EV), the electron binding energy in the exciton state (Ex), and the bottom of the CB (EC), as well as the highspin (HS) and low-spin (LS) states, are also indicated. The HS and LS states represent spin-forbidden and spin-allowed transitions for the 4f−5d1 transition, respectively.

of the Ce3+ 5d1,2,3 levels are located at −2.23, −1.21, and −0.67 eV, respectively. The higher Ce3+ 5d4,5 levels, which are expected at approximately −0.06 ± 0.02 eV (5d4) and 0.90 ± 0.11 eV (5d5), respectively,32 could not be observed in the PL spectra due to instrumental limitations. Figure 7 shows that the 5d states of all lanthanides in their +3 state are below the CB and hence may be efficiently doped in CSO to synthesize and optimize new phosphors, whereas, in contrast, the 5d states of lanthanides in their +2 state are within the CB and it will be impossible to stabilize lanthanides in their divalent state in CSO. As for Ce3+, the VRBE diagram predicts that the energy separation between the top of the valence band and the lowest 4f level of Ce3+ is 3.29 eV, whereas the Ce3+ 4f → 5d1 and 4f → 5d2 absorption transitions are found at 2.73 eV (445 nm) and 3.81 eV (325 nm), in agreement with the PL spectra in Figure 3. Crucially, the 5d1 level of Ce3+ is approximately 0.66 eV below the CB, whereas the higher 5d levels are located within the CB. It follows that one can expect different charge-trapping dynamics for excitation at 445 nm as compared with excitation at 325 nm or with an even shorter wavelength. In order to obtain information about chargetrapping−detrapping dynamics, as well as the number and activation energy (trap depth) of the traps present, the samples were further investigated using TL techniques. 3.5. Thermoluminescence Glow Curves. The TL glow curves were measured as a function of different excitation wavelengths and charging times. Prior to the measurements, which were performed by detecting the total number of emitted photons upon increasing temperature from T = 290 K to T = 470 K with a linear heating rate of 1 K/s, the samples had been charged at 273 K for different times (1, 2, 5, and 10 min). Figure 8 shows the TL glow curves for all Ce3+ doped samples.

4. DISCUSSION The diffuse reflectance, PL, and TL measurements provide new insights into the influence of Ce3+ concentration on the thermal stability and charge-trapping dynamics in CSO:Ce3+, as well as into the potential of this material to be applied in efficient, high-power, pc-WLEDs. Most important is the observed strong Ce3+ concentration dependence of the thermal stability of CSO:Ce3+ as investigated from both the temperature dependence of the integrated emission intensity and luminescence lifetime, respectively. At first sight, the clearly different quenching temperatures as determined from the two sets of measurements may be confusing, considering that both are expected to be affected in a similar manner by thermally induced nonradiative relaxation processes. However, because the complexity of the various factors that can contribute to the temperature dependence of the luminescence intensity (e.g., temperature dependence of the absorption strength and temperature dependence of energy migration and reabsorption), an absolute, quantitative analysis of the temperature quenching is E


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Figure 8. TL glow curves for CSO (0.5), CSO (1.0), and CSO (1.5), as recorded after excitation at (a) 254 nm, (b) 340 nm, and (c) 454 nm.

difficult from emission intensity data alone.6 In comparison, the luminescence lifetime data is expected to provide a better, more accurate determination of the quenching temperature of CSO:Ce3+. For an allowed transition, the radiative lifetime usually does not change strongly with temperature,34,35 wherefore the nearly constant luminescence decay time up to a temperature of around T = 350 K for the most dilute material, CSO (0.5), suggests that the luminescence lifetime measurements indeed can be used to accurately determine the quenching temperature for CSO (0.5). The relatively high quenching temperature of CSO (0.5), T50% = 600 K, shows it has a high thermal stability, which is even comparable to that of YAG:Ce3+ for high Ce3+ dopant levels (T50% ≈ 750 K for 1% Ce3+ doping, and T50% ≈ 650 K for 3% Ce3+ doping), as can be estimated from the luminescence lifetime data reported in ref 6. For CSO (1.0) and CSO (1.5), quenching sets in around T95% ≈ 190 K and T95% ≈ 170 K, respectively, with the corresponding quenching temperatures at T50% ≈ 530 K for CSO (1.0) and T50% ≈ 500 K for CSO (1.5). As for YAG:Ce3+,6 the significant reduction in quenching temperature with increasing Ce3+ dopant concentration can be most likely explained by thermally activated concentration quenching. This implies that the increase in temperature facilitates the transfer of energy between neighboring Ce3+ ions due to an increased overlap between the excitation and emission bands. The energy transfer is expected to occur through electric−dipole interaction between the Ce3+ ions and the migration of energy increases the probability of trapping the

energy at a defect located close to a Ce3+ dopant. The difference in luminescence decay time at T = 80 K among CSO (0.5) (36 ns), CSO (1.0) (33 ns), and CSO (1.5) (29 ns) [Figure 6(d)] suggests that thermally activated concentration quenching is relevant already at this low temperature. The strong Ce3+ concentration dependence of the thermal quenching of CSO:Ce3+ is in agreement with the reported trend in QE, which evolves from QE = 82% for CSO (0.5), to QE = 63% for CSO (1.0), and QE = 36% for CSO (1.5),15 hence as a result of an increasing portion of the incoming light absorbed/trapped by defects as the Ce3+ concentration is increased. Further information about the thermal quenching of CSO:Ce3+ comes from the PL emission and diffuse reflectance spectra as shown in Figure 3(b) and Figure 2, respectively. On the one hand, the similarity among PL emission spectra upon varying Ce3+ concentration suggests that the local structural coordination of the Ce3+ dopants remains essentially the same for different Ce3+ dopant concentrations. On the other hand, the 7 nm shift in the diffuse reflectance spectra for CSO (1.0) and CSO (1.5), as compared to CSO (0.5), indicates a change in the extent of overlap between the 5d1 and 5d2 bands and/or a redistribution between different Ce3+ sites, possibly related to the increase of local structural distortions around the Ce3+ ions with increasing Ce3+ dopant concentration.15 However, the shift of 7 nm may not be reflected in the PL excitation/ emission spectra as the PL was monitored by fixing emission at only one wavelength, which may respond to only one single type of Ca2+ site as occupied by the Ce3+ ions. Therefore, the F


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The Journal of Physical Chemistry C substitution of Ce3+ for Ca2+ does not induce significant local structural distortions that lead to a change in the bonding characteristics of the Ce3+ dopants, which would be manifested as an alteration of the color of the emitted light. This implies that there must be no significant differences in the position of the 5d1 level with respect to both the 4f level and bandgap of CSO:Ce3+ for the different Ce3+ dopant levels. In effect, this means that the reduction of the quenching temperature with increasing Ce3+ concentration is dictated neither by thermal ionization of 5d electrons into the CB of CSO:Ce3+ nor by 5d− 4f cross-relaxation mechanisms.6 Rather, our results suggest that the lowering of the quenching temperature for higher Ce3+ dopant levels is the result of an increase of thermally activated energy migration processes, which increases the probability of athermal tunneling36 of charges to defects (“killer centers”). In this context, it is of relevance that the activation energies (0.16−0.25 eV), as extracted from the temperature dependence of the luminescence lifetime data, are lower than the 0.66 eV as predicted by the VRBE diagram. A similar observation has, however, been made for YAG:Ce3+, for which TL excitation experiments, photoconductivity, and temperature dependent emission spectra underestimated the activation energy by ca. 0.5 eV as compared to VRBE diagram analysis.18 The observed differences may be attributed to a series of factors, including, e.g., the temperature dependence of the oscillator strength of the absorption (excitation) transitions, as well as the contribution of additional nonradiative relaxation mechanisms. A second, important, feature of CSO:Ce3+ is the significant red-shift upon temperature increase for all materials. The origin of the red-shift is unclear but may be due to an increase of the crystal field acting on the Ce3+ ions upon increasing temperature, e.g., through the pronunciation of vibrationally induced tetragonal distortions of the CeO8 moieties as seen for garnet type phosphors containing such moieties.17 The red-shift may also be explained by thermally activated energy migration and energy transfer to Ce3+ ions emitting at longer wavelength. By bringing together the results from our optical spectroscopy studies, we reach a plausible mechanism of charge storage (trapping) and release (detrapping), which is depicted in the VRBE diagram in Figure 9. Upon excitation at 454 nm (2.73 eV), electrons are excited into the 5d1 level, which may move via athermal tunneling36 from the 5d1 level into defect levels at 0.78, 0.85, and 0.89 eV. Upon excitation at 340 nm (3.64 eV) and 254 nm (4.88 eV), electrons are excited into the CB, where they get delocalized followed by trapping at all five defect levels. The stored charges in all traps can be detrapped at elevated

temperature to yield green luminescence. The possibility for retrapping seems low, as the TL glow curve maxima do not shift significantly with an increase in the exposure time for any of the three excitation wavelengths. Similarly, the probability for the formation of new charge-trapping defects for the higher Ce3+ dopant levels seems low because the shape of the glow curves is essentially the same for the different samples. This new insight motivates efforts to determine the nature of chargetrapping defects present, in combination with further PL, TL, and luminescence lifetime studies, as a route to elucidate the origins of the macroscopic luminescent performance of CSO:Ce3+ as well as which local structural properties accommodate the highest QE and thermal stability.

5. CONCLUSIONS Our combined diffuse reflectance, PL, TL, luminescence lifetime, and VRBE diagram study of the novel and technologically promising green-emitting phosphor CSO:Ce3+ with very dilute Ce3+ substitutions, i.e. 0.5%, 1%, and 1.5%, reveals a strong influence of the Ce3+ concentration on the thermal stability of CSO:Ce3+. The major impact of Ce3+ concentration is seen on the thermal quenching temperature, which is found to be as high as T50% ≈ 600 K for the most dilute Ce3+ doping (0.5%), which is even comparable with the technologically very important phosphor YAG:Ce3+. For higher Ce3+ concentrations, the quenching temperature decreases as a result of thermally activated concentration quenching. TL glow curves provide evidence for five charge-trapping defects, which are found to exhibit different charge-trapping dynamics for excitation into different 5d levels, for all Ce3+ concentrations as investigated here. It is argued that the three deeper traps (at 0.78, 0.85, and 0.89 eV) can be filled by athermal tunneling of charges from the Ce3+ 5d1 level, while the shallower traps (at 0.68 and 0.75 eV) can be only filled when the charges move through the conduction band of CSO:Ce3+. Besides being of considerable fundamental interest, this new insight provides design criteria that are expected to be useful for the discovery, optimization, and development of novel and efficient phosphors for solid-state white-lighting applications.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Suchinder K. Sharma: 0000-0002-8351-4597 Irene Carrasco: 0000-0002-7854-4214 Maths Karlsson: 0000-0002-2914-6332 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Swedish Research Council Formas (Grant No. 2013-1723) and the European Commission for funding through the Marie Curie Initial Training network LUMINET (Grant No. 316906) is gratefully acknowledged. Furthermore, we thank the group of Prof. Ram Seshadri at the University of California in Santa Barbara, U.S., for providing the samples as studied in this work. We also thank Marianna Marchini, Department of Chemistry, University of Bologna, Italy, for the measurement of diffuse reflectance spectra.

Figure 9. Proposed scheme of charge-trapping−detrapping pathways in CSO:Ce3+. G


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Influence of Ce3+ Concentration on the Thermal Stability and Charge

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