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Quenching of the red Mn4+ luminescence in Mn4+-doped fluoride LED phosphors

Light: Science & Applicationsvolume 7, Article number: 8 (2018) | Download Citation

Abstract

Red-emitting Mn4+-doped fluorides are a promising class of materials to improve the color rendering and luminous efficacy of white light-emitting diodes (w-LEDs). For w-LEDs, the luminescence quenching temperature is very important, but surprisingly no systematic research has been conducted to understand the mechanism for thermal quenching in Mn4+-doped fluorides. Furthermore, concentration quenching of the Mn4+ luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching in Mn4+-doped fluorides by measuring luminescence spectra and decay curves of K2TiF6:Mn4+ between 4 and 600 K and for Mn4+ concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K2TiF6:Mn4+ and other Mn4+-doped phosphors show that quenching occurs through thermally activated crossover between the 4T2 excited state and 4A2 ground state. The quenching temperature can be optimized by designing host lattices in which Mn4+ has a high 4T2 state energy. Concentration-dependent studies reveal that concentration quenching effects are limited in K2TiF6:Mn4+ up to 5% Mn4+. This is important, as high Mn4+ concentrations are required for sufficient absorption of blue LED light in the parity-forbidden Mn4+ dd transitions. At even higher Mn4+ concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn4+ phosphors. The present systematic study provides detailed insights into temperature and concentration quenching of Mn4+ emission and can be used to realize superior narrow-band red Mn4+ phosphors for w-LEDs.

Introduction

White light-emitting diodes (w-LEDs) are the next-generation light sources for display and illumination systems because of their small size, high luminous efficacy, and long operation lifetime1,2,3,4,5. Conventional w-LEDs are composed of blue-emitting (In,Ga)N LEDs and green/yellow-emitting and orange/red-emitting phosphors that convert part of the blue LED emission5,6,7. Both phosphors are necessary to generate warm white light with a high color rendering index (CRI > 85). The typical red phosphors in w-LEDs are Eu2+-doped nitrides (e.g., CaAlSiN3:Eu2+)4,8. These phosphors exhibit high photoluminescence (PL) quantum efficiencies (QEs > 90%), but their use also has a serious drawback. The Eu2+ emission band is broad and extends into the deep red spectral region (λ > 650 nm) where the eye sensitivity is low. This causes the luminous efficacy of the w-LED to drop (reduced lumen/W output). A worldwide search is therefore aimed at finding efficient narrow-band red-emitting phosphors that can be excited by blue light. In this search, Mn4+-doped fluoride phosphors, such as K2SiF6:Mn4+ and K2TiF6:Mn4+, have recently attracted considerable attention9,10,11,12,13. Under blue light excitation, Mn4+-doped fluorides show narrow red line emission (λmax ~ 630 nm) with high luminescence QEs13,14,15,16. Furthermore, they are prepared through low-cost, simple wet-chemical synthesis at room temperature11,17. These aspects make Mn4+-doped fluorides very promising red-emitting phosphors for developing energy-efficient high color-rendering w-LED systems9.

The application of Mn4+-doped fluoride phosphors in w-LEDs may, however, be hampered by thermal quenching of the Mn4+ luminescence. Thermal quenching of the phosphor luminescence is a serious issue, as it affects both the efficacy and color stability of the w-LED. In high-power w-LEDs, the temperature of the on-chip phosphor layer easily reaches 450 K. At these elevated temperatures, thermal quenching occurs for Mn4+-doped fluorides. The luminescence quenching temperature T½, the temperature at which the emission intensity is reduced to half of its maximum, is typically between 400 and 500 K15,18,19. Although the temperature dependence of the emission intensity has been measured for many Mn4+-doped fluorides, the understanding of the thermal quenching behavior is still limited. Most studies do not explain which process quenches the Mn4+ luminescence13,20,21,22,23. Moreover, the few reports that do propose a quenching mechanism disagree. Paulusz15 states that the luminescence of Mn4+-doped fluorides is quenched by thermally activated crossing of the Mn4+ 4T2 excited state and 4A2 ground state. In contrast, Dorenbos24 finds a relation between the quenching temperature and the energy of the F → Mn4+ charge-transfer (CT) state and therefore suggests that quenching involves crossover between the CT state and 4A2 ground state. This CT state crossover mechanism was also used by Blasse and our group to explain thermal quenching in Mn4+-doped oxides25,26,27. Finally, other reports claim that the quenching temperature increases if the radius of the cation substituted by Mn4+ becomes smaller11,18. A better understanding of the thermal quenching behavior is essential for developing Mn4+-doped fluoride phosphors with superior quenching temperatures, and thereby improving their potential for application in w-LEDs.

Besides thermal quenching, concentration quenching is an issue for the application of Mn4+-doped fluorides in w-LEDs. As the Mn4+ dd transitions are parity-forbidden, high Mn4+ doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light12. At high dopant concentrations, energy migration among the Mn4+ ions can result in concentration quenching26,28, as is illustrated in Fig. 1. If the distance between the Mn4+ ions is small, excitation energy may efficiently migrate from one Mn4+ ion to another until it reaches a quenching site (defect or impurity ion), where the excitation energy is lost non-radiatively (as heat). Studies on concentration quenching in Mn4+-doped fluorides are limited. Several works have compared the luminescence properties of fluoride phosphors with varying Mn4+ concentrations, but do not measure the actual Mn4+ concentration in the phosphors by elemental analysis29,30,31,32,33. Determining the Mn4+ concentration is crucial, as often only a fraction of the Mn4+ ions is incorporated during the synthesis19,34. Reports that do perform elemental analysis study only a small range of Mn4+ doping concentrations and do not provide insight into the role of concentration quenching in Mn4+ doped fluorides13,35,36. An in-depth investigation of concentration quenching in Mn4+-doped fluorides is thus lacking, despite it being very important for the application of Mn4+-doped fluorides in w-LEDs.

Fig. 1: Concentration quenching for Mn4+ in crystals.
Fig. 1

At high Mn4+ doping concentrations the Mn4+ ions (orange) are in close proximity in the crystal lattice. If the Mn4+ ions are close together, energy transfer between Mn4+ ions (dark blue) causes the excitation to migrate through the crystal. Eventually, it may reach a quenching site such as a vacancy or impurity (dashed circle), where the excitation energy is lost as heat. This process competes with radiative emission (red) and reduces the luminescence efficiency

In this work, we systematically investigate concentration quenching and thermal quenching in Mn4+-doped fluorides. The quenching is studied by measuring luminescence spectra and decay curves in the temperature range of 4 to 600 K for K2TiF6:Mn4+ phosphors with Mn4+ concentrations ranging from 0.01 to 15.7 mol% (actual Mn4+ concentration). The temperature-dependent luminescence measurements of K2TiF6:Mn4+ and other Mn4+-doped phosphors demonstrate that thermal quenching occurs because of thermally activated crossover from the 4T2 excited state to the 4A2 ground state. This insight into the quenching mechanism shows that the Mn4+ quenching temperature can be raised by finding fluoride hosts that have an increased Mn4+ 4T2 level energy. Concentration studies show that the luminescence QE of K2TiF6:Mn4+ is high, ~80%, for doping concentrations up to 5 mol% Mn4+. Concentration quenching is limited for these relatively high Mn4+ dopant concentrations. At even higher doping concentrations of >10 mol%, the QE of K2TiF6:Mn4+ falls below 60%. Luminescence decay curves indicate that the drop in QE can be attributed to an increased probability for direct energy transfer to quenching sites (e.g., defects, impurity ions, Mn2+, and Mn3+), the concentration of which increases with the Mn4+ concentration. The present results provide an improved understanding of thermal quenching and concentration quenching in Mn4+-doped solids and can be used to develop superior Mn4+-doped fluoride phosphors for w-LEDs.

Materials and methods

Synthesis and characterization of K2TiF6:Mn4+ phosphors

The K2TiF6:Mn4+ (x%) phosphors were synthesized according to the method of Zhu et al.13 For the synthesis of K2TiF6:Mn4+ (0.8%), 0.0488 g of K2MnF6 (prepared following refs. 37,38) was dissolved in 2.5 mL of a 40 wt% HF solution (Fluka, 40 wt% HF in water). Next, the obtained yellow-brown solution was mixed with 4.5730 g of K2TiF6 (Sigma-Aldrich, p.a.) and then stirred for 1 h at room temperature to form K2TiF6:Mn4+ crystals. The K2TiF6:Mn4+ phosphor was isolated by decanting the HF solution, washing twice with 15 mL of ethanol and then drying the phosphor for 7 h at 75 °C. The other K2TiF6:Mn4+ (x%) phosphors were prepared following the same procedure but using other amounts of K2MnF6 and K2TiF6 as to obtain different Mn4+ doping concentrations.

Powder X-ray diffraction (see Supplementary Figure S1) confirms that the K2TiF6:Mn4+ (x%) phosphors exhibit the hexagonal crystal structure of K2TiF6 up to the highest doping concentration of 15.7% Mn4+. Furthermore, no impurities of K2MnF6 or other crystal phases are observed in the diffraction patterns. Scanning electron microscopy (SEM) images show that most K2TiF6:Mn4+ phosphor particles are irregularly shaped and have sizes ranging from 1 to 200 µm (see Supplementary Figure S2a). Some particles have a hexagonal shape, in agreement with the hexagonal crystal structure of K2TiF6 (see Supplementary Figure S2b). Energy-dispersive X-ray (EDX) spectra (see Supplementary Figure S2c) confirm that the phosphor particles consist of potassium, titanium, fluorine, and manganese ions. The manganese dopant concentrations in the K2TiF6:Mn4+ phosphors were determined with inductively coupled plasma optical emission spectroscopy (ICP-OES). The ICP-OES measurements were performed on a Perkin-Elmer Optima 8300DV spectrometer (λem = 257.61 and 259.37 nm). For the ICP-OES analyses, the K2TiF6:Mn4+ phosphors were dissolved in aqua regia.


Optical spectroscopy

PL measurements were performed on an Edinburgh Instruments FLS920 fluorescence spectrometer, except for the PL decay measurements between 300 and 600 K (see below). For recording excitation and emission spectra, we used a 450 W Xe lamp as excitation source and a Hamamatsu R928 photomultiplier tube (PMT) with a grating blazed at 500 nm for detection of emission. For PL decay measurements, excitation was done with a tunable optical parametric oscillator (OPO) Opotek Opolette HE 355II laser (pulse width 10 ns, repetition rate 10 Hz) and emission was detected with a Hamamatsu H74220–60 PMT. The PL decay curves between 300 and 600 K were recorded on a different setup, which had an Ekspla NT 342B OPO laser (pulse width 5 ns, repetition rate 10 Hz) as excitation source and a 0.55 m Triax 550 monochromator combined with a Hamamatsu H74220–60 PMT for detection of emission. All PL decay curves were obtained by multi-channel scaling (MCS) with a PicoQuant TimeHarp 260 computer card. The K2TiF6:Mn4+ phosphors were cooled down to 4 K with an Oxford Instruments liquid helium flow cryostat. For PL measurements between 300 and 600 K samples were heated in a Linkam THMS600 temperature controlled stage. The PL quantum efficiencies of the phosphors were determined with a calibrated home-built setup, which consisted of a 65 W Xe lamp, excitation monochromator, integrating sphere (Labsphere) and CCD camera (Avantes AvaSpec-2048).

Results and discussion

Luminescence of K2TiF6:Mn4+

For our quenching studies, we examine the luminescence of K2TiF6:Mn4+ phosphors with a wide range of Mn4+ doping concentrations. A photographic image of the K2TiF6:Mn4+ (x%) phosphors is displayed in Fig. 2a. The Mn4+ doping concentrations x (molar percentages with respect to Ti4+) were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES). The body color of K2TiF6:Mn4+ becomes more yellow with increasing Mn4+ concentration as a result of enhanced absorption in the blue. All of the investigated K2TiF6:Mn4+ phosphors exhibit bright red Mn4+ luminescence under UV photoexcitation.

Fig. 2: Mn4+ luminescence of K2TiF6:Mn4+.
Fig. 2

a Photographic image of K2TiF6:Mn4+ (x%) phosphors with x = 0.01, 0.1, 0.8, 1.3, 3.8, 5.4, 9.4, and 15.7. The phosphors have a white to yellow body color under ambient light (top) and show red Mn4+ luminescence under 365 nm UV illumination (bottom). b Tanabe−Sugano energy level diagram of the d3 electron configuration in an octahedral crystal field. The 4A2 → 4T1, 4A2 → 4T2, and 2E → 4A2 transitions of Mn4+ are indicated by the purple, blue and red arrows, respectively. Note that the excitation transitions are displaced for clarity. For a specific coordination all transitions take place around the same crystal field ΔO. c Emission spectrum of K2TiF6:Mn4+ (0.8%) upon excitation with blue light (λexc = 450 nm). d Excitation spectrum of the red Mn4+ luminescence (λem = 630 nm) from K2TiF6:Mn4+ (0.8%). Spectra are recorded at ambient temperature

Figure 2b depicts the Tanabe–Sugano energy level diagram of Mn4+ (3d3 electron configuration) in an octahedral crystal field39,40. The diagram gives the d3 energy levels as a function of the crystal field splitting ΔO. Due to its high effective positive charge, Mn4+ experiences a strong crystal field and therefore the 2E state is the lowest energy excited state. Hence, the emission spectrum of K2TiF6:Mn4+ (0.8%) is dominated by narrow red emission lines due to spin- and parity-forbidden 2E → 4A2 transitions, as can be seen in Fig. 2c. The other K2TiF6:Mn4+ (x%) phosphors exhibit similar emission spectra. As the potential energy curves of the 2E and 4A2 states are at the same equilibrium position, the 2E → 4A2 emission is characterized by narrow zero-phonon and vibronic emission lines. The potential energy curves of the 2E and 4A2 states are at the same equilibrium position because the 2E and 4A2 states originate from the same \(t_{{\mathrm{2g}}}^3\) electron configuration41.

The 2E → 4A2 emission spectrum consists of a weak zero-phonon line (ZPL) at ~622 nm and more intense anti-Stokes and Stokes vibronic emissions (labeled ν3, ν4, and ν6) on the high and low energy sides of the ZPL, respectively13,15. The ZPL is very weak because Mn4+ is located on a site with inversion symmetry in K2TiF6:Mn4+. Due to the inversion symmetry, there are no odd-parity crystal field components to admix opposite parity states into the 4A2 and 2E states and, as a result, the 2E → 4A2 transition is electric dipole forbidden. The 2E → 4A2 transition can become partly allowed, however, by coupling with asymmetric vibrations that induce odd-parity crystal field components. The most intense lines in Fig. 2c are assigned to 2E → 4A2 transitions coupling with the asymmetric ν3, ν4, and ν6 vibrational modes (phonons) of the \({\mathrm{MnF}}_6^{2 - }\) group. Thermal population of phonons at room temperature allows coupling with ν3, ν4, and ν6 phonon modes in the 2E excited state (giving rise to the anti-Stokes lines), while transitions to these phonon modes in the 4A2 ground state can occur at all temperatures (Stokes lines).

Figure 2d displays the excitation spectrum of the red Mn4+ luminescence from K2TiF6:Mn4+. The two broad excitation bands correspond to spin-allowed 4A2 → 4T1 and 4A2 → 4T2 transitions (violet and blue arrows in Fig. 2b). In addition, some weak peaks are visible around 600 nm. These peaks are assigned to 4A2 → 2E and 4A2 → 2T1 transitions. The 4A2 → 2T1,2E transitions are spin-forbidden and therefore low in intensity compared to the spin-allowed 4A2 → 4T1,4T2 transitions.

Temperature dependence of the Mn4+ luminescence

To study the thermal quenching of the Mn4+ emission, we measure the PL intensity and Mn4+ emission lifetime of K2TiF6:Mn4+ (0.01%) as a function of temperature between 4 and 600 K. We use a very low Mn4+ doping concentration of 0.01%, as for higher Mn4+ concentrations reabsorption of emission and energy transfer between Mn4+ ions can occur. These processes will influence (the temperature dependence of) the Mn4+ luminescence spectra and decay curves6. As a result, with a high concentration of Mn4+ ions, the observations may not reflect the intrinsic thermal quenching properties of Mn4+.

Figure 3a shows emission spectra of K2TiF6:Mn4+ (0.01%) at various temperatures between 4 and 600 K. At 4 K the Mn4+ 2E → 4A2 emission spectrum consists of zero-phonon and Stokes vibronic lines. Upon raising the temperature, phonon modes are thermally populated and anti-Stokes emission lines appear (solid arrow in Fig. 3a). With the appearance of anti-Stokes lines, the relative intensity of the Stokes emission decreases between 4 and 300 K. Above 400 K the intensities of both the anti-Stokes and Stokes emission lines begin to decrease (dashed arrow in Fig. 3a), which indicates the onset of non-radiative transitions from the 2E excited state. The luminescence is quenched at 600 K. From the measurements, we obtain the temperature dependence of the integrated PL intensity (IPL) relative to the integrated PL intensity at room temperature (IRT) (Fig. 3b). The PL intensity of K2TiF6:Mn4+ (0.01%) gradually increases between 4 and 350 K but then rapidly drops due to the onset of non-radiative transitions (luminescence quenching).

Fig. 3: Temperature dependence of the Mn4+ luminescence from K2TiF6:Mn4+ (0.01%).
Fig. 3

a Emission spectra (λexc = 450 nm) of K2TiF6:Mn4+ (0.01%) at various temperatures between 0 and 600 K. b Integrated PL intensity of K2TiF6:Mn4+ (0.01%) as a function of temperature. The integrated PL intensity IPL is scaled to the integrated PL intensity at room temperature IRT. The red and green lines represent fits to Eqs. 6 and 7, respectively. c PL decay curves of the Mn4+ emission from K2TiF6:Mn4+ (0.01%) at various temperatures between 0 and 600 K (λexc = 450 nm and λem = 631 nm). d Temperature dependence of the Mn4+ emission lifetime for K2TiF6:Mn4+ (0.01%). The red and green lines represent fits to Eqs. 4 and 8, respectively. The cyan line gives the fit for Eq. 4 (red line) divided by two

An alternative method to determine the luminescence quenching temperature is by measuring luminescence decay times. Figure 3c shows a selection of PL decay curves of K2TiF6:Mn4+ (0.01%) measured between 4 and 600 K. The decay of the Mn4+ emission is single exponential and becomes faster with increasing temperature. The PL decay time is on the order of milliseconds, which is expected as the transition between the 2E and 4A2 states is both parity- and spin-forbidden. In Fig. 3d, the Mn4+ emission lifetime (determined from single exponential fitting) is plotted as a function of temperature. The lifetime shows a steady decrease, starting above 50 K. The decrease levels off between 300 and 400 K but then shows a rapid decrease above 400 K.

The temperature dependences observed in Fig. 3b and d are quite exceptional. For most luminescent materials, the PL intensity and lifetime are relatively constant with temperature and both begin to decrease once thermal quenching sets in6,42,43. The PL intensity of K2TiF6:Mn4+, however, rises by 40% between 4 and 350 K while the lifetime decreases before thermal quenching takes place. To understand this peculiar temperature dependence, we first discuss how the radiative decay rate of the 2E state changes with temperature. The 2E → 4A2 emission of K2TiF6:Mn4+ mainly consists of anti-Stokes and Stokes vibronic emissions (Fig. 2c). Their transition probabilities increase with phonon population. The population of phonon modes is given by the phonon occupation number n, which increases with temperature according to41:

$$n = \frac{1}{{\exp \left( {hv/k_{\mathrm{B}}T} \right) - 1}}$$
(1)

where kB is the Boltzmann constant and is the energy of the phonon coupling to the 2E → 4A2 transition. The transition probabilities PR of the anti-Stokes and Stokes vibronics scale with n by:

$${\mathrm{Anti-Stokes}}:\; P_{\mathrm{R}}\left( T \right) = P_{\mathrm{R}}\left( 0 \right)\left[ n \right]$$
(2)
$${\mathrm{Stokes}}:\;P_{\mathrm{R}}\left( T \right) = P_{\mathrm{R}}\left( 0 \right)\left[ {n + 1} \right]$$
(3)

where PR(0) is the transition probability at T = 0 K. As the radiative lifetime τR is proportional to 1/[PR(anti-Stokes) + PR (Stokes)], it follows from Eqs. 1–3 that:

$$\tau _{\mathrm{R}}\left( T \right) = \frac{{\tau _{\mathrm{R}}\left( 0 \right)}}{{{\mathrm{coth}}(hv/2k_{\mathrm{B}}T)}}$$
(4)

Here, τR(0) is the radiative lifetime at T = 0 K. In Fig. 3d,Eq. 4 (red line) has been plotted for τR(0) = 12.3 ms and  = 216 cm−1 (phonon energy of the intense ν6 mode emission). Equation 4 accurately describes the measured temperature dependence of the Mn4+ emission lifetime up to 375 K, confirming that the decay of the 2E state is mainly radiative up to this temperature. The radiative lifetime of the Mn4+ emission shortens with temperature due to thermal population of odd-parity vibrational modes at higher temperatures.

Next, we investigate the increase in PL intensity between 4 and 350 K. The PL intensity IPL equals the product of the PL QE and number of absorbed photons (as IPL scales with the number of absorbed photons, the excitation wavelength can have a large influence on the temperature dependence observed for IPL; see Supplementary Information). The PL QE η of K2TiF6:Mn4+ can be expressed as:

$$\eta = \frac{{\gamma _{\mathrm{R}}}}{{\gamma _{\mathrm{R}} + \gamma _{{\mathrm{NR}}}}}$$
(5)

where γR and γNR are the radiative and non-radiative decay rates of the emitting 2E state, respectively. The results in Fig. 3d show that the decay of the 2E state is mainly radiative up to 375 K, so we can assume that γNR is negligible between 0 and 350 K. The value for η is therefore approximated as a constant close to unity between 0 and 350 K. On the other hand, the 4A2 → 4T2 absorption will change with temperature. Like the 2E → 4A2 transition, the 4A2 → 4T2 transition is electric dipole (parity) forbidden and gains intensity by coupling with vibrations (for more details on the vibronic structure of the 4A2 → 4T2 excitation band, see refs. 15,16,44). As a result, the PL intensity IPL will scale with temperature as20,41,45:

$$I_{{\mathrm{PL}}}\left( T \right) = I\left( 0 \right){\mathrm{coth}}\left( {\frac{{hv}}{{2k_{\mathrm{B}}T}}} \right)$$
(6)

with I(0) being the PL intensity at T = 0 K. The results in Fig. 3b show that the increase in PL intensity between 4 and 350 K follows the temperature dependence given by Eq. 6. This confirms that the higher PL intensity at 350 K is due to a stronger absorption of excitation light. An increase in PL intensity between 4 and 350 K due to enhanced absorption is observed for all investigated Mn4+ doping concentrations (see Supplementary Information). Although the temperature dependence of the PL intensity follows Eq. 6, there is deviation between the fit of Eq. 6 and the measured data (see red line in Fig. 3b). The model of Eq. 6 is simple and does not take into account the shift and broadening of the 4A2 → 4T2 absorption band with temperature. Both these effects also influence the temperature dependence of the PL intensity, and this can explain the deviation between the model and the experimental data. Including the effect of a shift and broadening of the 4A2 → 4T2 band on the absorption strength is complex and will not aid a more accurate determination of T½.

Above 400 K the PL intensity of K2TiF6:Mn4+ (0.01%) begins to decrease due to the onset of non-radiative transitions (Fig. 3a, b). The non-radiative decay probability rapidly increases with temperature above 400 K and as a result the luminescence is quenched, with no emission intensity remaining at 600 K. The quenching temperature T½ is determined to be 462 K. The Mn4+ emission lifetime also rapidly decreases once thermal quenching sets in (Fig. 3d). Above 400 K the Mn4+ emission lifetime is shorter than the radiative lifetime τR predicted by Eq. 4 (red line). The lifetime shortens because of an additional thermally activated non-radiative contribution to the decay of the 2E state. From the temperature dependence of the lifetime, T½ can be determined by locating the temperature at which the lifetime has decreased to half of its radiative lifetime value. To estimate T½, we divide the value from the fit of Eq. 4 for τR by a factor of 2 (Fig. 3d, cyan line). The cyan line crosses the data points at 457 K. This value for T½ is very close to the T½ of 462 K obtained from the PL intensity measurements.

Thermal quenching can be described as a thermally activated process with an activation energy ΔE. The activation energy is obtained by fitting a modified Arrhenius equation to the temperature dependence of the PL intensity IPL between 350 and 600 K43,46:

$$I_{{\mathrm{PL}}}\left( T \right) = \frac{{I\left( 0 \right)}}{{1 + A \times {\mathrm{exp}}\left( { - \Delta E/k_{\mathrm{B}}T} \right)}}$$
(7)

In Eq. 7, I(0) is the maximum PL intensity, kB is the Boltzmann constant and A is a rate constant for the thermal quenching process. The best fit to Eq. 7 (green line in Fig. 3b) gives an activation energy ∆E of 9143 cm−1 and a rate constant A of 2.5 × 1012. We can also determine ∆E by fitting the temperature dependence of the Mn4+ emission lifetime τ(T) to the following expression47:

$$\tau \left( T \right) = \frac{{\tau _{\mathrm{R}}\left( T \right)}}{{{\mathrm{1}} + \left( {\frac{{\tau _{\mathrm{R}}\left( T \right)}}{{\tau _{{\mathrm{NR}}}}}} \right){\mathrm{exp}}\left( { - \Delta E/k_{\mathrm{B}}T} \right)}}$$
(8)

Here, 1/τNR is the non-radiative decay rate and τR(T) is the radiative lifetime as described by Eq. 4 with τR(0) = 12.3 ms and  = 216 cm−1. We fit Eq. 8 to the Mn4+ emission lifetimes (green line in Fig. 3d) and find an activation energy ∆E of 7100 cm−1 and a prefactor 1/τNR of 1.5 × 1012 s−1. On the basis of the two similar values for ∆E, we conclude that the activation energy of the thermal quenching process is ~8000 cm−1. The rate constants A and 1/τNR should be approximately equal to the vibrational frequencies of the \({\mathrm{MnF}}_6^{2 - }\) group. The ν6 vibrational mode has a frequency of 6.5 × 1012 s−1, close to the rate constants found by fitting the data to Eqs. 7 and 8. The variation in activation energy values and prefactors can be explained by the fact that thermal quenching is not a simple thermally activated process. Struck and Fonger have shown that the temperature dependence of a non-radiative process is accurately described by considering ground and excited state vibrational wave function overlap46,48. According to the Struck–Fonger model, the non-radiative process occurs through tunneling (crossover) from a vibrational level of the excited state to a high vibrational level of the ground state. The tunneling rate, i.e., the non-radiative decay rate, depends on the wave function overlap of the vibrational levels involved. The tunneling rate will be faster for a larger overlap between the wave functions and when the vibrational levels are in resonance. For the present discussion, analysis of the data using complex models such as the Struck–Fonger model is not relevant, but it is important to realize that the Struck–Fonger model gives a more correct description of the actual quenching process.

Thermal quenching in Mn4+-doped fluorides

To obtain insight into the thermal quenching of Mn4+ luminescence, we will discuss four possible quenching processes: (1) multi-phonon relaxation, (2) thermally activated photoionization, (3) thermally activated crossover via the F → Mn4+ charge-transfer (CT) state, and (4) thermally activated crossover via the Mn4+ 4T2 excited state.

In the configurational coordinate diagram, the parabolas of the Mn4+ 2E and 4A2 states do not cross and luminescence quenching by crossover from the 2E to the 4A2 states is not possible (Fig. 4a). The 4A2 ground state may however be reached by multi-phonon relaxation. In Mn4+-doped fluorides more than 30 phonons of ~500 cm−1 are needed to bridge the energy gap between the 2E and 4A2 states49. For such high numbers of phonons (p > 30), it is unrealistic that non-radiative multi-phonon relaxation is responsible for thermal quenching (see Supplementary Information for a more detailed discussion). Alternatively, the thermal quenching can be due to thermally activated photoionization of an electron from the Mn4+ 2E state to the fluoride host conduction band. Thermally activated photoionization typically quenches the emission from a luminescent center if the emitting state is close in energy to the host conduction band26,50. In density functional theory (DFT) calculations, large band gaps of around 8 eV have been found for fluoride hosts like K2SiF6 and K2TiF651,52. It is therefore expected that the Mn4+ 2E state is well below the host conduction band levels. Based on this, we conclude that thermal quenching in Mn4+-doped fluorides is not caused by thermally activated photoionization. However, more evidence is necessary to exclude this quenching mechanism. Photoconductivity measurements on Mn4+ phosphors at elevated temperatures need to be performed to provide convincing evidence for a possible role of photoionization in the thermal quenching of Mn4+ emission.

Fig. 4: Thermal quenching in Mn4+-doped fluorides.
Fig. 4

a, b Configuration coordinate diagrams showing luminescence quenching due to a thermally activated crossover via the F → Mn4+ charge-transfer (CT) state and b thermally activated crossover via the Mn4+ 4T2 excited state. c Quenching temperature T½ of Mn4+-doped fluoride phosphors as a function of the 4A2 → 4T2 transition energy. The red dashed line is a linear fit to the data points. d Quenching temperature T½ of Mn4+-doped fluorides (blue dots) and Mn4+-doped oxides (red dots) as a function of the 4A2 → 4T2 transition energy

Thermal quenching in Mn4+-doped fluorides has been suggested to occur by thermally activated crossover via the Mn4+ 4T2 state or the F → Mn4+ charge-transfer (CT) state15,24,26. Both these states are displaced relative to the potential curve of the 4A2 ground state (Fig. 4a, b). Hence, the 4T2 and CT state parabolas cross the 4A2 ground state parabola. The difference between the potential curve equilibrium positions is given by the offset ∆R = R0′ − R0. By using the energies of the 4A2 → 2E, 4A2 → 4T2 and 4A2 → CT transitions in K2TiF6:Mn4+ (Fig. 2d and ref. 13) and assuming specific offsets ∆R for the 4T2 and CT states, we can construct the diagrams in Fig. 4a and b, where non-radiative relaxation occurs either via (a) the crossing of the CT and 4A2 states or (b) the crossing of the 4T2 and 4A2 states. The offset of the CT state is typically larger than the offset of the 4T2 state. Note that the diagrams in Fig. 4a and b are schematic configuration coordinate diagrams to illustrate the different quenching mechanisms.

In Fig. 4a, the CT state has a larger offset ∆R than the 4T2 state, which causes the CT parabola to cross the 4A2 parabola at lower energies than the 4T2 parabola. Thermal activation over the energy barrier ∆E will allow crossover from the 2E state into the CT state followed by non-radiative relaxation to the ground state via the crossing of the CT and 4A2 parabolas. Alternatively, thermal quenching of the Mn4+ luminescence may be due to the mechanism depicted in Fig. 4b. Here, the CT state has a smaller offset ∆R compared to that shown in Fig. 4a, and its potential curve is therefore at higher energies. In addition, the 4T2 state has a slightly larger offset. As a result, the crossing of the 4T2 and 4A2 parabolas is now at a lower energy and non-radiative relaxation will proceed via the crossing of the 4T2 and 4A2 parabolas.

The activation energies ∆E in the configuration coordinate diagrams are ~8000 cm−1, similar to the ∆E values obtained from the temperature-dependent measurements. This indicates that both mechanisms in Fig. 4a, b can explain the thermal quenching of Mn4+ luminescence. To determine which of these two mechanisms is responsible for the luminescence quenching, we compare the quenching temperature T½ of K2TiF6:Mn4+ to the T½ of other Mn4+-doped materials. A relation between the quenching temperature and the energy of either the CT or 4T2 state in a variety of hosts will give insight. If quenching occurs by crossover from the CT state to the 4A2 state, T½ will be higher for Mn4+-doped solids with higher CT transition energies. In K2TiF6:Mn4+ and other Mn4+-doped fluorides the F → Mn4+ CT transition is at ~40,000 cm−113,15. Mn4+-doped oxides have lower O2− → Mn4+ CT transition energies of 30,000–35,000 cm−1 and are therefore expected to have lower T½ values than fluorides if quenching occurs by the mechanism in Fig. 4a26,27,53,54. Some Mn4+-doped oxides, however, have much higher quenching temperatures than Mn4+-doped fluorides. For example, Mg4GeO6:Mn4+, Mg28Ge7.5O38F10:Mn4+, and Mg6As2O11:Mn4+ have a T½ of ~700 K55,56,57, while K2TiF6:Mn4+ and other Mn4+-doped fluorides have a T½ of 400–500 K (see also Tables 1 and 2). No correlation is found between the Mn4+ luminescence quenching temperature and the energy of the CT transition (see Supplementary Information for an overview and a plot of quenching temperatures and CT energies). From this we conclude that thermal quenching in Mn4+-doped fluorides is not caused by thermally activated crossover from the F → Mn4+ CT state to the 4A2 ground state.

Table 1 Quenching temperature T½ (K) and 4A2 → 4T2 energy (cm−1) for Mn4+-doped fluoride materials
Table 2 Quenching temperature T½ (K) and 4A2 → 4T2 energy (cm−1) for Mn4+-doped oxide materials

Alternatively, thermal quenching of the Mn4+ luminescence can be caused by thermally activated crossover via the Mn4+ 4T2 excited state (Fig. 4b). To investigate the validity of this mechanism, we compare the T½ and 4A2 → 4T2 transition energies for K2TiF6:Mn4+ and a variety of other Mn4+-doped fluorides. From the literature and measurements on Mn4+ luminescence we have collected quenching temperatures and luminescence spectra, preferably for systems with low doping concentrations. Figures 2d and 3b show that K2TiF6:Mn4+ has a 4A2 → 4T2 energy of 21,459 cm−1 (maximum of the excitation band) and a T½ of 462 K. For K2SiF6:Mn4+, we measured a 4A2 → 4T2 energy of 22,099 cm−1 and a T½ of 518 K (Supplementary Figure S6, K2SiF6:Mn4+ BR301-C commercial phosphor from Mitsubishi Chemical, Japan). In Fig. 4c we plot the quenching temperature T½ against the 4A2 → 4T2 energy for K2TiF6:Mn4+, K2SiF6:Mn4+ and many other Mn4+-doped fluoride phosphors reported in the literature (displayed data also listed in Table 1). The data show that the T½ increases with the energy of the 4T2 state. The clear trend shows that the thermal quenching in Mn4+-doped fluorides is due to thermally activated crossover from the 4T2 excited state to the 4A2 ground state. Further confirmation for this quenching mechanism is provided by Mn4+ spectra measured at elevated temperatures (see Supplementary Information). Supplementary Figure S7 shows emission spectra of K2SiF6:Mn4+ at T = 573 and 673 K. At 573 K a broad 4T2 → 4A2 emission band is observed, which is almost completely quenched at 673 K. The initial rise of the 4T2 → 4A2 emission at elevated temperatures confirms thermal population of the 4T2 level, which eventually leads to thermal quenching of all Mn4+ emission via this state.

To investigate whether thermally activated crossing via the 4T2 state is also responsible for temperature quenching in Mn4+-doped oxides, we extend the data set of Fig. 4c with quenching temperatures reported for Mn4+-doped oxides. Figure 4d shows the quenching temperature T½ as a function of the 4A2 → 4T2 energy for the Mn4+-doped fluorides and oxides listed in Tables 1 and 2. The results show that T½ increases with the energy of the 4A2 → 4T2 transition. This indicates that the Mn4+ emission in fluorides and oxides are both quenched due to thermally activated crossover from the 4T2 excited state, and not the CT state as previously suggested in some reports24,25,26,27. The present results and analysis provide strong evidence that in many Mn4+ phosphors the thermal quenching mechanism involves thermally activated crossover via the 4T2 excited state. A contribution from other mechanisms cannot be ruled out and further research, for example, photoconductivity measurements and high pressure studies, can give additional information on the role of alternative quenching mechanisms.

As quenching occurs by thermally activated crossover via the 4T2 excited state, the quenching temperature T½ of the Mn4+ luminescence is controlled by the energy of the Mn4+ 4T2 state (the dependence of T½ on the energy of the 4T2 state is shown in Fig. 4c,d). In addition, the T½ of the Mn4+ luminescence depends on the offset ∆R between the 4T2 and 4A2 states, as ∆R also determines where the 4T2 and 4A2 states cross in the configuration coordinate diagram (Fig. 4a,b). The horizontal displacement of the 4T2 parabola will influence the quenching temperature. A variation in ∆R can explain the spread observed in the data of Fig. 4c and d. To investigate the variation in the offset ∆R for Mn4+-doped fluorides, we compare the bandwidth of the 4A2 → 4T2 excitation band in K2TiF6:Mn4+, K2SiF6:Mn4+ and Cs2HfF6:Mn4+ (see Supplementary Figure S9). The width of the 4A2 → 4T2 excitation band is controlled by the displacement of the 4T2 state and therefore gives a good indication of ∆R. Comparison of the 4A2 → 4T2 bandwidths shows that there is a variation in ∆R for Mn4+-doped fluorides. The variation in ∆R is small, however, compared to the differences in the 4T2 energy, and no correlation is observed between the spectral width and quenching temperatures. This indicates that the 4T2 level energy has the largest influence on the quenching temperature of Mn4+-doped fluorides.

Finally, in view of applications, it is interesting to see how we can control the 4T2 level energy (and thereby T½) through the choice of the host lattice. The energy of the Mn4+ 4T2 state depends on the crystal field splitting ΔO (Fig. 2b), where ΔO is typically larger for shorter Mn–F distances44,58. For Mn4+-doped fluorides the luminescence quenching temperature can therefore be raised by selecting host lattices with short M4+–F distances (see Supplementary Figure S10a). This is consistent with findings that T½ increases if the radius of the M4+ host cation decreases, as expected based on crystal field theory11,18. If, however, T½ is plotted against the M4+-ligand distance for both Mn4+-doped fluorides and Mn4+-doped oxides (see Supplementary Figure S10b), no correlation between T½ and the M4+-ligand distance is found. This shows that the crystal field splitting and 4T2 energy give a better indication of the quenching temperature for Mn4+-doped phosphors.

Concentration quenching

In addition to insight into thermal quenching, concentration quenching in Mn4+-doped fluorides is important for application in w-LEDs. The weak parity-forbidden 4A2 → 4T2 absorption requires that commercial phosphors have high Mn4+ concentrations. If there is effective concentration quenching, the PL decay time and QE will decrease when the Mn4+ doping concentration is raised26,28. We therefore investigate concentration quenching in K2TiF6:Mn4+ by measuring the PL decay times and QEs of K2TiF6:Mn4+ phosphors with Mn4+ concentrations ranging from 0.01 to 15.7% Mn4+.

Figure 5a presents room-temperature PL decay curves of the Mn4+ emission from K2TiF6:Mn4+ with increasing Mn4+ doping concentration x. It can be seen that the PL decay becomes slightly faster as the Mn4+ concentration increases. We analyze the decay dynamics by single exponential fitting of the PL decay curves. The fit for K2TiF6:Mn4+ (0.8%) is shown in Fig. 5b. The fit residuals (bottom panel) are random and the PL decay thus resembles a single exponential. This indicates that the decay of the 2E state is mainly radiative. Consequently, the K2TiF6:Mn4+ (0.8%) phosphor has a very high QE of 90%. Figure 5c gives an overview of the fitted decay times (blue squares) and QEs (red dots) of K2TiF6:Mn4+ with different Mn4+ concentrations. The emission lifetime barely shortens if the Mn4+ concentration is increased (5.7 ms for 0.01% Mn4+ to 5.4 ms for 15.7% Mn4+). This suggests that energy migration to quenching sites is inefficient in K2TiF6:Mn4+. To verify this, we look at the QE values obtained for the K2TiF6:Mn4+ (x%) phosphors. The QE remains above 80% for Mn4+ doping concentrations of 5% or less, which shows that concentration quenching is indeed limited up to a concentration of 5% Mn4+ ions. This result is important for applications in w-LEDs, as these high Mn4+ doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light in the parity-forbidden dd transitions12.

Fig. 5: Luminescence decay and quantum efficiency of K2TiF6:Mn4+ as a function of the Mn4+ doping concentration.
Fig. 5

a Room-temperature PL decay curves of the Mn4+ emission from K2TiF6:Mn4+ (x%) for 0.01% (pink), 0.1% (blue), 0.8% (green), 1.3% (orange), 3.8% (purple), 5.4% (cyan), 9.4% (yellow), and 15.7% (red) Mn4+ (λexc = 450 nm and λem = 631 nm). b PL decay curve of K2TiF6:Mn4+ (0.8%) at T = 298 K. The decay time corresponding to the mono-exponential fit (red line) is 5.6 ms. The bottom panel shows the fit residuals. c Mn4+ emission lifetime (blue squares) and PL quantum efficiency (red dots) of K2TiF6:Mn4+ with different Mn4+ doping concentrations. d, e PL decay curves of K2TiF6:Mn4+ (15.7%) at d T = 298 K and e T = 4 K. The decay times corresponding to the mono-exponential fits (red lines) are 5.4 and 10.6 ms, respectively. The bottom panels show the fit residuals

For higher Mn4+ concentrations (x > 10%), non-radiative decay from the 2E excited state becomes stronger, however, and as a result the QE of K2TiF6:Mn4+ falls below 60% (Fig. 5c). The non-radiative decay is also visible in the PL decay curve of K2TiF6:Mn4+ (15.7%), shown in Fig. 5d. The decay is multi-exponential, which proves that with 15.7% Mn4+ the 2E state decays both radiatively and non-radiatively. The faster initial decay indicates that there is enhanced quenching by single-step energy transfer for Mn4+ ions close to a quencher. In case of energy migration, a faster decay is also expected for longer times after the excitation pulse. As this is not observed, the contribution of energy migration via many Mn4+ ions to quenching sites seems to be small.

To further investigate the role of energy migration in the concentration quenching of the Mn4+ emission, we measure a PL decay curve of K2TiF6:Mn4+ (15.7%) at T = 4 K, which is displayed in Fig. 5e. At T = 4 K energy migration among the Mn4+ ions (blue arrows in Fig. 1) will be hampered, as there is almost no spectral overlap between the Mn4+ 2E → 4A2 emission and 4A2 → 2E excitation lines (see Supplementary Figure S11). Hence, at 4 K non-radiative decay due to energy migration to quenching sites will be suppressed. The Mn4+ decay dynamics in Fig. 5e, however, show that the non-radiative decay is not suppressed at 4 K. The deviation from single exponential behavior is similar to that at 300 K. There is an initial faster decay (single-step energy transfer to quenching sites) followed by an exponential decay with a decay time very close to that measured for Mn4+ at low doping concentrations. This suggests that the decrease in QE at higher Mn4+ concentrations is not due to energy migration. The absence of strong concentration quenching by energy migration is confirmed by the thermal quenching behavior measured for the different Mn4+ concentrations. In Supplementary Figure S4, it can be seen that the luminescence quenching temperature is approximately the same for doping concentrations of 0.01% and 15.7% Mn4+, which shows that effects due to thermally activated energy migration (i.e., concentration quenching) are weak. Hence, we conclude that the non-radiative decay at high Mn4+ concentrations is not caused by energy migration. Inefficient energy migration can be understood based on the strongly forbidden character of the 2E → 4A2 transition. This allows only Mn4+–Mn4+ energy transfer via short range exchange interaction (see Supplementary Information for details).

We instead assign the non-radiative decay to direct transfer of excitation energy from Mn4+ ions to quenchers (green arrow in Fig. 1). This process can occur at all temperatures and becomes more efficient at higher Mn4+ dopant concentrations. With an increasing Mn4+ dopant concentration, the stress on the K2TiF6 lattice grows and as a result more crystal defects (i.e., quenchers) may be formed. In addition, Mn in different valence states (Mn2+ and Mn3+) may be incorporated at higher Mn4+ concentrations. Even if a very small fraction of Mn4+ ions has a different valence state than 4+, effective quenching can occur via metal-to-metal charge-transfer states or direct energy transfer. Consequently, the probability for energy transfer to quenchers increases, resulting in faster initial PL decay and lower QEs for K2TiF6:Mn4+ at high Mn4+ dopant concentrations. Optimized synthesis procedures to reduce quenchers (defects and impurity ions) are thus crucial for obtaining highly luminescent Mn4+-doped fluoride phosphors (see also recent work of Garcia-Santamaria et al.59 on concentration quenching in K2SiF6:Mn4+).

Conclusions

Narrow-band red-emitting Mn4+ phosphors form an important new class of materials for LED lighting and displays. For these applications, it is important to understand and control the luminescence efficiency. We have therefore investigated quenching of the Mn4+ luminescence in Mn4+-doped fluorides by measuring the PL intensity and luminescence lifetimes of K2TiF6:Mn4+ between 4 and 600 K and for Mn4+ concentrations from 0.01 to 15.7%. Temperature-dependent measurements of the Mn4+ emission intensity and lifetime for K2TiF6:Mn4+ and other Mn4+-doped phosphors show that thermal quenching is caused by thermally activated crossover via the Mn4+ 4T2 excited state. As a result, the quenching temperature is higher in Mn4+-doped materials with higher 4T2 state energies. These findings can be used to engineer Mn4+-doped fluoride phosphors with higher quenching temperatures for application in high-power w-LEDs.

Furthermore, quantum efficiency and luminescence decay measurements for a wide range of Mn4+ doping concentrations show that no concentration quenching occurs up to 5% Mn4+ in K2TiF6:Mn4+. This is important for the application of Mn4+-doped materials in w-LEDs, as high Mn4+ doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light in the parity-forbidden Mn4+ dd transitions. At very high Mn4+ doping concentrations (>10 mol%) the quantum efficiency of K2TiF6:Mn4+ decreases due to enhanced direct energy transfer from Mn4+ to quenching sites. Concentration quenching by Mn4+–Mn4+ energy migration is limited. To optimize the efficiency in highly doped Mn4+ phosphors, a synthesis procedure aimed at reducing quenching sites (defects, impurity ions, Mn2+, and Mn3+) will be crucial.

Additional information

Accepted article preview online: 13 March 2018

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Acknowledgements

We thank Stephan Zevenhuizen and Hans Meeldijk for performing the SEM and EDX measurements. Mart Peeters is acknowledged for measuring the PL quantum efficiencies. Suzanne Verkleij is acknowledged for taking the photographic images of the K2TiF6:Mn4+ phosphors. This work is financially supported by Technologiestichting STW, which is part of the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO).

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  1. Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA, Utrecht, The Netherlands

    • Tim Senden
    •  & Andries Meijerink
  2. Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA, Utrecht, The Netherlands

    • Relinde J.A. van Dijk-Moes

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The authors declare that they have no conflict of interest.

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Correspondence to Tim Senden.

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https://doi.org/10.1038/s41377-018-0013-1