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

Quenching of the red Mn4+ luminescence in Mn4+-doped fluoride LED phosphors 4+ Red-emitting Mn -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 4+ 4+ quenching in Mn -doped fluorides. Furthermore, concentration quenching of the Mn luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching 4+ 4+ in Mn -doped fluorides by measuring luminescence spectra and decay curves of K TiF :Mn between 4 and 600 K 2 6 4+ 4+ and for Mn concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K TiF :Mn and other 2 6 4+ 4 Mn -doped phosphors show that quenching occurs through thermally activated crossover between the T excited 4 4+ state and A ground state. The quenching temperature can be optimized by designing host lattices in which Mn has a high T state energy. Concentration-dependent studies reveal that concentration quenching effects are limited 4+ 4+ 4+ in K TiF :Mn up to 5% Mn . This is important, as high Mn concentrations are required for sufficient absorption of 2 6 4+ 4+ blue LED light in the parity-forbidden Mn d–d transitions. At even higher Mn concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization 4+ of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn phosphors. The 4+ present systematic study provides detailed insights into temperature and concentration quenching of Mn emission 4+ and can be used to realize superior narrow-band red Mn phosphors for w-LEDs. 2+ Introduction their use also has a serious drawback. The Eu emission White light-emitting diodes (w-LEDs) are the next- band is broad and extends into the deep red spectral generation light sources for display and illumination sys- region (λ > 650 nm) where the eye sensitivity is low. This tems because of their small size, high luminous efficacy, causes the luminous efficacy of the w-LED to drop 1–5 and long operation lifetime . Conventional w-LEDs are (reduced lumen/W output). A worldwide search is composed of blue-emitting (In,Ga)N LEDs and green/ therefore aimed at finding efficient narrow-band red- yellow-emitting and orange/red-emitting phosphors that emitting phosphors that can be excited by blue light. In 5–7 4+ convert part of the blue LED emission . Both phosphors this search, Mn -doped fluoride phosphors, such as 4+ 4+ are necessary to generate warm white light with a high K SiF :Mn and K TiF :Mn , have recently attracted 2 6 2 6 9–13 color rendering index (CRI > 85). The typical red phos- considerable attention . Under blue light excitation, 2+ 4+ phors in w-LEDs are Eu -doped nitrides (e.g., CaAlSiN : Mn -doped fluorides show narrow red line emission 2+ 4,8 13–16 Eu ) . These phosphors exhibit high photo- (λ ~ 630 nm) with high luminescence QEs . Fur- max luminescence (PL) quantum efficiencies (QEs > 90%), but thermore, they are prepared through low-cost, simple 11,17 wet-chemical synthesis at room temperature . These 4+ aspects make Mn -doped fluorides very promising red- Correspondence: Tim Senden (t.senden@uu.nl) emitting phosphors for developing energy-efficient high Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, color-rendering w-LED systems . Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands © The Author(s) 2018 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to theCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Senden et al. Light: Science & Applications (2018) 7:8 Page 2 of 13 understanding of the thermal quenching behavior is Energy migration Quenching site 4+ essential for developing Mn -doped fluoride phosphors 4+ Mn ion with superior quenching temperatures, and thereby improving their potential for application in w-LEDs. Besides thermal quenching, concentration quenching is Excitation 4+ an issue for the application of Mn -doped fluorides in w- 4+ LEDs. As the Mn d–d transitions are parity-forbidden, 4+ high Mn doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light . At high dopant concentrations, energy migration among Energy 4+ 26,28 transfer to the Mn ions can result in concentration quenching , 4+ quencher as is illustrated in Fig. 1. If the distance between the Mn ions is small, excitation energy may efficiently migrate 4+ from one Mn ion to another until it reaches a quenching site (defect or impurity ion), where the exci- Host cation Radiative emission tation energy is lost non-radiatively (as heat). Studies on 4+ 4 Fig. 1 Concentration quenching for Mn in crystals. At high Mn 4+ + 4+ concentration quenching in Mn -doped fluorides are doping concentrations the Mn ions (orange) are in close 4+ limited. Several works have compared the luminescence proximity in the crystal lattice. If the Mn ions are close together, 4+ 4+ energy transfer between Mn ions (dark blue) causes the excitation properties of fluoride phosphors with varying Mn to migrate through the crystal. Eventually, it may reach a quenching concentrations, but do not measure the actual site such as a vacancy or impurity (dashed circle), where the excitation 4+ Mn concentration in the phosphors by elemental ana- energy is lost as heat. This process competes with radiative emission 29–33 4+ lysis . Determining the Mn concentration is crucial, (red) and reduces the luminescence efficiency 4+ as often only a fraction of the Mn ions is incorporated 19,34 during the synthesis . Reports that do perform ele- 4+ 4+ The application of Mn -doped fluoride phosphors in mental analysis study only a small range of Mn doping w-LEDs may, however, be hampered by thermal concentrations and do not provide insight into the role of 4+ 4+ 13,35,36 quenching of the Mn luminescence. Thermal quench- concentration quenching in Mn doped fluorides . ing of the phosphor luminescence is a serious issue, as it An in-depth investigation of concentration quenching in 4+ affects both the efficacy and color stability of the w-LED. Mn -doped fluorides is thus lacking, despite it being very 4+ In high-power w-LEDs, the temperature of the on-chip important for the application of Mn -doped fluorides in phosphor layer easily reaches 450 K. At these elevated w-LEDs. 4+ temperatures, thermal quenching occurs for Mn -doped In this work, we systematically investigate concentration 4+ fluorides. The luminescence quenching temperature T , quenching and thermal quenching in Mn -doped the temperature at which the emission intensity is fluorides. The quenching is studied by measuring lumi- reduced to half of its maximum, is typically between 400 nescence spectra and decay curves in the temperature 15,18,19 4+ and 500 K . Although the temperature dependence range of 4 to 600 K for K TiF :Mn phosphors with 2 6 4+ of the emission intensity has been measured for many Mn concentrations ranging from 0.01 to 15.7 mol% 4+ 4+ Mn -doped fluorides, the understanding of the thermal (actual Mn concentration). The temperature-dependent 4+ quenching behavior is still limited. Most studies do not luminescence measurements of K TiF :Mn and other 2 6 4+ 4+ explain which process quenches the Mn lumines- Mn -doped phosphors demonstrate that thermal 13,20–23 cence . Moreover, the few reports that do propose a quenching occurs because of thermally activated cross- 15 4 4 quenching mechanism disagree. Paulusz states that the over from the T excited state to the A ground state. 2 2 4+ luminescence of Mn -doped fluorides is quenched by This insight into the quenching mechanism shows that 4+ 4 4+ thermally activated crossing of the Mn T excited state the Mn quenching temperature can be raised by finding 4 24 4+ 4 and A ground state. In contrast, Dorenbos finds a fluoride hosts that have an increased Mn T level 2 2 relation between the quenching temperature and the energy. Concentration studies show that the lumines- − 4+ 4+ energy of the F → Mn charge-transfer (CT) state and cence QE of K TiF :Mn is high, ~80%, for doping 2 6 4+ therefore suggests that quenching involves crossover concentrations up to 5 mol% Mn . Concentration 4 4+ between the CT state and A ground state. This CT state quenching is limited for these relatively high Mn crossover mechanism was also used by Blasse and our dopant concentrations. At even higher doping con- 4+ 4+ group to explain thermal quenching in Mn -doped centrations of >10 mol%, the QE of K TiF :Mn falls 2 6 25–27 oxides . Finally, other reports claim that the quench- below 60%. Luminescence decay curves indicate ing temperature increases if the radius of the cation that the drop in QE can be attributed to an increased 4+ 11,18 substituted by Mn becomes smaller . A better probability for direct energy transfer to quenching sites Senden et al. Light: Science & Applications (2018) 7:8 Page 3 of 13 2+ 3+ (e.g., defects, impurity ions, Mn , and Mn ), the con- Hamamatsu R928 photomultiplier tube (PMT) with a 4+ centration of which increases with the Mn concentra- grating blazed at 500 nm for detection of emission. For PL tion. The present results provide an improved decay measurements, excitation was done with a tunable understanding of thermal quenching and concentration optical parametric oscillator (OPO) Opotek Opolette HE 4+ quenching in Mn -doped solids and can be used to 355II laser (pulse width 10 ns, repetition rate 10 Hz) and 4+ develop superior Mn -doped fluoride phosphors for emission was detected with a Hamamatsu H74220–60 PMT. The PL decay curves between 300 and 600 K were w-LEDs. recorded on a different setup, which had an Ekspla NT Materials and methods 342B OPO laser (pulse width 5 ns, repetition rate 10 Hz) 4+ Synthesis and characterization of K TiF :Mn phosphors as excitation source and a 0.55 m Triax 550 mono- 2 6 4+ The K TiF :Mn (x%) phosphors were synthesized chromator combined with a Hamamatsu H74220–60 2 6 according to the method of Zhu et al. For the synthesis PMT for detection of emission. All PL decay curves were 4+ of K TiF :Mn (0.8%), 0.0488 g of K MnF (prepared obtained by multi-channel scaling (MCS) with a Pico- 2 6 2 6 37,38 4+ following refs. ) was dissolved in 2.5 mL of a 40 wt% Quant TimeHarp 260 computer card. The K TiF :Mn 2 6 HF solution (Fluka, 40 wt% HF in water). Next, the phosphors were cooled down to 4 K with an Oxford obtained yellow-brown solution was mixed with 4.5730 g Instruments liquid helium flow cryostat. For PL mea- of K TiF (Sigma-Aldrich, p.a.) and then stirred for 1 h at surements between 300 and 600 K samples were heated in 2 6 4+ room temperature to form K TiF :Mn crystals. The a Linkam THMS600 temperature controlled stage. The 2 6 4+ K TiF :Mn phosphor was isolated by decanting the HF PL quantum efficiencies of the phosphors were deter- 2 6 solution, washing twice with 15 mL of ethanol and then mined with a calibrated home-built setup, which con- drying the phosphor for 7 h at 75 °C. The other K TiF : sisted of a 65 W Xe lamp, excitation monochromator, 2 6 4+ Mn (x%) phosphors were prepared following the integrating sphere (Labsphere) and CCD camera (Avantes same procedure but using other amounts of K MnF AvaSpec-2048). 2 6 4+ and K TiF as to obtain different Mn doping 2 6 concentrations. Results and discussion 4+ Powder X-ray diffraction (see Supplementary Figure S1) Luminescence of K TiF :Mn 2 6 4+ confirms that the K TiF :Mn (x%) phosphors exhibit For our quenching studies, we examine the lumines- 2 6 4+ the hexagonal crystal structure of K TiF up to the highest cence of K TiF :Mn phosphors with a wide range of 2 6 2 6 4+ 4+ doping concentration of 15.7% Mn . Furthermore, no Mn doping concentrations. A photographic image of 4+ impurities of K MnF or other crystal phases are observed the K TiF :Mn (x%) phosphors is displayed in Fig. 2a. 2 6 2 6 4+ in the diffraction patterns. Scanning electron microscopy The Mn doping concentrations x (molar percentages 4+ 4+ (SEM) images show that most K TiF :Mn phosphor with respect to Ti ) were determined by inductively 2 6 particles are irregularly shaped and have sizes ranging coupled plasma optical emission spectroscopy (ICP-OES). 4+ from 1 to 200 µm (see Supplementary Figure S2a). Some The body color of K TiF :Mn becomes more yellow 2 6 4+ particles have a hexagonal shape, in agreement with the with increasing Mn concentration as a result of hexagonal crystal structure of K TiF (see Supplementary enhanced absorption in the blue. All of the investigated 2 6 4+ 4+ Figure S2b). Energy-dispersive X-ray (EDX) spectra K TiF :Mn phosphors exhibit bright red Mn lumi- 2 6 (see Supplementary Figure S2c) confirm that the phos- nescence under UV photoexcitation. phor particles consist of potassium, titanium, fluorine, Figure 2b depicts the Tanabe–Sugano energy level 4+ 3 and manganese ions. The manganese dopant concentra- diagram of Mn (3d electron configuration) in an 4+ 39,40 3 tions in the K TiF :Mn phosphors were determined octahedral crystal field . The diagram gives the d 2 6 with inductively coupled plasma optical emission spec- energy levels as a function of the crystal field splitting Δ . 4+ troscopy (ICP-OES). The ICP-OES measurements were Due to its high effective positive charge, Mn experi- performed on a Perkin-Elmer Optima 8300DV spectro- ences a strong crystal field and therefore the E state is the meter (λ = 257.61 and 259.37 nm). For the ICP-OES lowest energy excited state. Hence, the emission spectrum em 4+ 4+ analyses, the K TiF :Mn phosphors were dissolved in of K TiF :Mn (0.8%) is dominated by narrow red 2 6 2 6 2 4 aqua regia. emission lines due to spin- and parity-forbidden E→ A transitions, as can be seen in Fig. 2c. The other K TiF : 2 6 4+ Optical spectroscopy Mn (x%) phosphors exhibit similar emission spectra. As 2 4 PL measurements were performed on an Edinburgh the potential energy curves of the E and A states are at 2 4 Instruments FLS920 fluorescence spectrometer, except the same equilibrium position, the E→ A emission is for the PL decay measurements between 300 and 600 K characterized by narrow zero-phonon and vibronic (see below). For recording excitation and emission spec- emission lines. The potential energy curves of the E and tra, we used a 450 W Xe lamp as excitation source and a A states are at the same equilibrium position because 2 Senden et al. Light: Science & Applications (2018) 7:8 Page 4 of 13 a c 4+ v Mn concentration (x %) 1.0 2 4 0.8% Mn E A 0.01 0.1 0.8 1.3 3.8 5.4 9.4 15.7 ZPL 0.8 Anti-Stokes Stokes 0.6 0.4 0.2 0.0 2 4 2 A T A 550 600 650 700 2 1 1 Wavelength (nm) 50 T 4 d 2 4 4 2 1.0 A T 2 2 0.8% Mn 4 4 0.8 30 A T 2 1 T 0.6 1 4 2 E 20 2 G 0.4 4 2 A T 2 1 0.2 0.0 F 4 300 400 500 600 0 10 20 30 40 Wavelength (nm) Δ /B 4+ 4+ 4+ Fig. 2 Mn luminescence of K TiF :Mn .a Photographic image of K TiF :Mn (x%) phosphors with x = 0.01, 0.1, 0.8, 1.3, 3.8, 5.4, 9.4, and 15.7. 2 6 2 6 4+ The phosphors have a white to yellow body color under ambient light (top) and show red Mn luminescence under 365 nm UV illumination 3 4 4 4 4 (bottom). b Tanabe−Sugano energy level diagram of the d electron configuration in an octahedral crystal field. The A → T , A → T , and 2 1 2 2 2 4 4+ E→ A transitions of Mn are indicated by the purple, blue and red arrows, respectively. Note that the excitation transitions are displaced for 4+ clarity. For a specific coordination all transitions take place around the same crystal field Δ . c Emission spectrum of K TiF :Mn (0.8%) upon O 2 6 4+ 4+ excitation with blue light (λ = 450 nm). d Excitation spectrum of the red Mn luminescence (λ = 630 nm) from K TiF :Mn (0.8%). Spectra are exc em 2 6 recorded at ambient temperature 2 4 the E and A states originate from the same t electron Figure 2d displays the excitation spectrum of the red 2g 41 4+ 4+ configuration . Mn luminescence from K TiF :Mn . The two broad 2 6 2 4 4 4 The E→ A emission spectrum consists of a weak excitation bands correspond to spin-allowed A → T 2 2 1 4 4 zero-phonon line (ZPL) at ~622 nm and more intense and A → T transitions (violet and blue arrows in 2 2 anti-Stokes and Stokes vibronic emissions (labeled ν , ν , Fig. 2b). In addition, some weak peaks are visible around 3 4 4 2 and ν ) on the high and low energy sides of the ZPL, 600 nm. These peaks are assigned to A → E and 6 2 13,15 4+ 4 2 4 2 2 respectively . The ZPL is very weak because Mn is A → T transitions. The A → T , E transitions are 2 1 2 1 located on a site with inversion symmetry in K TiF :Mn spin-forbidden and therefore low in intensity compared to 2 6 + 4 4 4 . Due to the inversion symmetry, there are no odd-parity the spin-allowed A → T , T transitions. 2 1 2 crystal field components to admix opposite parity states 4 2 2 4 4+ into the A and E states and, as a result, the E→ A Temperature dependence of the Mn luminescence 2 2 2 4 4+ transition is electric dipole forbidden. The E→ A To study the thermal quenching of the Mn emission, 4+ transition can become partly allowed, however, by cou- we measure the PL intensity and Mn emission lifetime 4+ pling with asymmetric vibrations that induce odd-parity of K TiF :Mn (0.01%) as a function of temperature 2 6 4+ crystal field components. The most intense lines in Fig. 2c between 4 and 600 K. We use a very low Mn doping 2 4 4+ are assigned to E→ A transitions coupling with the concentration of 0.01%, as for higher Mn concentra- asymmetric ν , ν , and ν vibrational modes (phonons) of tions reabsorption of emission and energy transfer 3 4 6 4+ the MnF group. Thermal population of phonons at between Mn ions can occur. These processes will 4+ room temperature allows coupling with ν , ν , and ν influence (the temperature dependence of) the Mn 3 4 6 2 6 phonon modes in the E excited state (giving rise to the luminescence spectra and decay curves . As a result, with 4+ anti-Stokes lines), while transitions to these phonon a high concentration of Mn ions, the observations may modes in the A ground state can occur at all tempera- not reflect the intrinsic thermal quenching properties of 4+ tures (Stokes lines). Mn . E /B Normalized intensity Normalized intensity Senden et al. Light: Science & Applications (2018) 7:8 Page 5 of 13 ac 6 K 2 4 4.0 E A  = 631 nm 2 4 K em 126 K 102 K 3.0 175 K 175 K 227 K 2.0 294 K 292 K –1 1.0 373 K 423 K 0.0 448 K 4.0 303 K 473 K 373 K −2 3.0 448 K 2.0 473 K 498 K 1.0 573 K −3 0.0 580 600 620 640 660 680 0 20406080 Wavelength (nm) Delay time (ms) bd 1.2 1.0 0.8 T = 457 K 0.6 T = 462 K 0.4 ½ 4 0.2 0.0 0 0 100 200 300 400 500 0 100 200 300 400 500 Temperature (K) Temperature (K) 4+ 4+ 4+ Fig. 3 Temperature dependence of the Mn luminescence from K TiF :Mn (0.01%). a Emission spectra (λ = 450 nm) of K TiF :Mn 2 6 exc 2 6 4+ (0.01%) at various temperatures between 0 and 600 K. b Integrated PL intensity of K TiF :Mn (0.01%) as a function of temperature. The integrated 2 6 PL intensity I is scaled to the integrated PL intensity at room temperature I . The red and green lines represent fits to Eqs. 6 and 7, respectively. c PL PL RT 4+ 4+ decay curves of the Mn emission from K TiF :Mn (0.01%) at various temperatures between 0 and 600 K (λ = 450 nm and λ = 631 nm). 2 6 exc em 4+ 4+ d Temperature dependence of the Mn emission lifetime for K TiF :Mn (0.01%). The red and green lines represent fits to Eqs. 4 and 8, respectively. 2 6 The cyan line gives the fit for Eq. 4 (red line) divided by two 4+ Figure 3a shows emission spectra of K TiF :Mn decay times. Figure 3c shows a selection of PL decay 2 6 4+ (0.01%) at various temperatures between 4 and 600 K. At curves of K TiF :Mn (0.01%) measured between 4 and 2 6 4+ 2 4 4+ 4 K the Mn E→ A emission spectrum consists of 600 K. The decay of the Mn emission is single expo- zero-phonon and Stokes vibronic lines. Upon raising the nential and becomes faster with increasing temperature. temperature, phonon modes are thermally populated and The PL decay time is on the order of milliseconds, which 2 4 anti-Stokes emission lines appear (solid arrow in Fig. 3a). is expected as the transition between the E and A states 4+ With the appearance of anti-Stokes lines, the relative is both parity- and spin-forbidden. In Fig. 3d, the Mn intensity of the Stokes emission decreases between 4 and emission lifetime (determined from single exponential 300 K. Above 400 K the intensities of both the anti-Stokes fitting) is plotted as a function of temperature. The life- and Stokes emission lines begin to decrease (dashed arrow time shows a steady decrease, starting above 50 K. The in Fig. 3a), which indicates the onset of non-radiative decrease levels off between 300 and 400 K but then shows transitions from the E excited state. The luminescence is a rapid decrease above 400 K. quenched at 600 K. From the measurements, we obtain The temperature dependences observed in Fig. 3b and d the temperature dependence of the integrated PL inten- are quite exceptional. For most luminescent materials, the sity (I ) relative to the integrated PL intensity at room PL intensity and lifetime are relatively constant with PL temperature (I ) (Fig. 3b). The PL intensity of K TiF : temperature and both begin to decrease once thermal RT 2 6 4+ 6,42,43 4+ Mn (0.01%) gradually increases between 4 and 350 K quenching sets in . The PL intensity of K TiF :Mn , 2 6 but then rapidly drops due to the onset of non-radiative however, rises by 40% between 4 and 350 K while the transitions (luminescence quenching). lifetime decreases before thermal quenching takes place. An alternative method to determine the luminescence To understand this peculiar temperature dependence, we quenching temperature is by measuring luminescence first discuss how the radiative decay rate of the E state Integrated I (T )/I PL intensity (10 counts) PL RT Normalized intensity Lifetime (ms) Senden et al. Light: Science & Applications (2018) 7:8 Page 6 of 13 2 4 changes with temperature. The E→ A emission of with vibrations (for more details on the vibronic structure 4+ 4 4 15,16,44 K TiF :Mn mainly consists of anti-Stokes and Stokes of the A → T excitation band, see refs. ). As a 2 6 2 2 vibronic emissions (Fig. 2c). Their transition probabilities result, the PL intensity I will scale with temperature PL 20,41,45 increase with phonon population. The population of as : phonon modes is given by the phonon occupation num- hv ber n, which increases with temperature according to : I ðÞ T ¼ IðÞ 0 coth ð6Þ PL 2k T n ¼ ð1Þ expðÞ hv=k T 1 with I(0) being the PL intensity at T = 0 K. The results in where k is the Boltzmann constant and hν is the energy Fig. 3b show that the increase in PL intensity between 4 2 4 of the phonon coupling to the E→ A transition. The and 350 K follows the temperature dependence given by transition probabilities P of the anti-Stokes and Stokes Eq. 6. This confirms that the higher PL intensity at 350 K vibronics scale with n by: is due to a stronger absorption of excitation light. An increase in PL intensity between 4 and 350 K due to Anti  Stokes : P ðÞ T ¼ P ðÞ 0 ½ n ð2Þ R R 4+ enhanced absorption is observed for all investigated Mn Stokes : P ðÞ T ¼ P ðÞ 0½ n þ 1 ð3Þ R R doping concentrations (see Supplementary Information). Although the temperature dependence of the PL intensity where P (0) is the transition probability at T = 0 K. As the follows Eq. 6, there is deviation between the fit of Eq. 6 radiative lifetime τ is proportional to 1/[P (anti- R R and the measured data (see red line in Fig. 3b). The model Stokes) + P (Stokes)], it follows from Eqs. 1–3 that: of Eq. 6 is simple and does not take into account the shift τ ðÞ 0 4 4 τ ðÞ T ¼ ð4Þ and broadening of the A → T absorption band with 2 2 cothðhv=2k TÞ temperature. Both these effects also influence the tem- perature dependence of the PL intensity, and this can Here, τ (0) is the radiative lifetime at T = 0 K. In Fig. 3d, explain the deviation between the model and the experi- Eq. 4 (red line) has been plotted for τ (0) = 12.3 ms and −1 mental data. Including the effect of a shift and broadening hν = 216 cm (phonon energy of the intense ν mode 4 4 of the A → T band on the absorption strength is emission). Equation 4 accurately describes the measured 2 2 4+ complex and will not aid a more accurate determination temperature dependence of the Mn emission lifetime of T . up to 375 K, confirming that the decay of the E state is ½ mainly radiative up to this temperature. The radiative 4+ 4+ Above 400 K the PL intensity of K TiF :Mn (0.01%) 2 6 lifetime of the Mn emission shortens with temperature begins to decrease due to the onset of non-radiative due to thermal population of odd-parity vibrational transitions (Fig. 3a, b). The non-radiative decay prob- modes at higher temperatures. ability rapidly increases with temperature above 400 K and as a result the luminescence is quenched, with no Next, we investigate the increase in PL intensity emission intensity remaining at 600 K. The quenching between 4 and 350 K. The PL intensity I equals the PL 4+ temperature T is determined to be 462 K. The Mn product of the PL QE and number of absorbed photons ½ emission lifetime also rapidly decreases once thermal (as I scales with the number of absorbed photons, the PL 4+ quenching sets in (Fig. 3d). Above 400 K the Mn excitation wavelength can have a large influence on the emission lifetime is shorter than the radiative lifetime τ temperature dependence observed for I ; see Supple- R PL 4+ predicted by Eq. 4 (red line). The lifetime shortens mentary Information). The PL QE η of K TiF :Mn can 2 6 because of an additional thermally activated non-radiative be expressed as: contribution to the decay of the E state. From the tem- η ¼ ð5Þ perature dependence of the lifetime, T can be deter- γ þ γ R NR mined by locating the temperature at which the lifetime where γ and γ are the radiative and non-radiative has decreased to half of its radiative lifetime value. To R NR decay rates of the emitting E state, respectively. The estimate T , we divide the value from the fit of Eq. 4 for τ ½ R results in Fig. 3d show that the decay of the E state is by a factor of 2 (Fig. 3d, cyan line). The cyan line crosses mainly radiative up to 375 K, so we can assume that γ is the data points at 457 K. This value for T is very close to NR ½ negligible between 0 and 350 K. The value for η is the T of 462 K obtained from the PL intensity therefore approximated as a constant close to unity measurements. 4 4 between 0 and 350 K. On the other hand, the A → T Thermal quenching can be described as a thermally 2 2 absorption will change with temperature. Like the activated process with an activation energy ΔE. The 2 4 4 4 E→ A transition, the A → T transition is electric activation energy is obtained by fitting a modified 2 2 2 dipole (parity) forbidden and gains intensity by coupling Arrhenius equation to the temperature dependence of the Senden et al. Light: Science & Applications (2018) 7:8 Page 7 of 13 43,46 −1 PL intensity I between 350 and 600 K : quenching process is ~8000 cm . The rate constants A PL and 1/τ should be approximately equal to the vibra- NR IðÞ 0 I ðÞ T ¼ ð7Þ PL tional frequencies of the MnF group. The ν vibrational 1 þ A ´ expðÞ ΔE=k T B 12 −1 mode has a frequency of 6.5 × 10 s , close to the rate constants found by fitting the data to Eqs. 7 and 8. The In Eq. 7, I(0) is the maximum PL intensity, k is the variation in activation energy values and prefactors can be Boltzmann constant and A is a rate constant for the explained by the fact that thermal quenching is not a thermal quenching process. The best fit to Eq. 7 (green −1 simple thermally activated process. Struck and Fonger line in Fig. 3b) gives an activation energy ΔE of 9143 cm 12 have shown that the temperature dependence of a non- and a rate constant A of 2.5 × 10 . We can also determine 4+ radiative process is accurately described by considering ΔE by fitting the temperature dependence of the Mn 47 ground and excited state vibrational wave function over- emission lifetime τ(T) to the following expression : 46,48 lap . According to the Struck–Fonger model, the non- τ ðÞ T radiative process occurs through tunneling (crossover) τðÞ T ¼ ð8Þ τ ðÞ T from a vibrational level of the excited state to a high 1 þ expðÞ ΔE=k T NR vibrational level of the ground state. The tunneling rate, i.e., the non-radiative decay rate, depends on the wave Here, 1/τ is the non-radiative decay rate and τ (T)is NR R function overlap of the vibrational levels involved. The the radiative lifetime as described by Eq. 4 with τ (0) = −1 4+ tunneling rate will be faster for a larger overlap between 12.3 ms and hν = 216 cm .We fit Eq. 8 to the Mn the wave functions and when the vibrational levels are in emission lifetimes (green line in Fig. 3d) and find an −1 resonance. For the present discussion, analysis of the data activation energy ΔE of 7100 cm and a prefactor 1/τ NR 12 −1 using complex models such as the Struck–Fonger model of 1.5 × 10 s . On the basis of the two similar values for is not relevant, but it is important to realize that the ΔE, we conclude that the activation energy of the thermal ac 40 CT T 500 ΔE R R 20,750 21,250 21,750 22,250 4 4 –1 A T energy (cm ) 2 2 b d 40 Fluorides CT Oxides ΔE 0 100 R R 17,000 21,000 25,000 4 4 –1 A T energy (cm ) 2 2 4+ Fig. 4 Thermal quenching in Mn -doped fluorides. a, b Configuration coordinate diagrams showing luminescence quenching due to a − 4+ 4+ 4 thermally activated crossover via the F → Mn charge-transfer (CT) state and b thermally activated crossover via the Mn T excited state. c 4+ 4 4 Quenching temperature T of Mn -doped fluoride phosphors as a function of the A → T transition energy. The red dashed line is a linear fitto ½ 2 2 4+ 4+ 4 4 the data points. d Quenching temperature T of Mn -doped fluorides (blue dots) and Mn -doped oxides (red dots) as a function of the A → T ½ 2 2 transition energy 4 –1 4 –1 Energy (10 cm ) Energy (10 cm ) Quenching temperature T (K) Quenching temperature T (K) ½ Senden et al. Light: Science & Applications (2018) 7:8 Page 8 of 13 4 4 Struck–Fonger model gives a more correct description of crossing of the T and A states. The offset of the CT 2 2 the actual quenching process. state is typically larger than the offset of the T state. Note that the diagrams in Fig. 4a and b are schematic 4+ Thermal quenching in Mn -doped fluorides configuration coordinate diagrams to illustrate the dif- 4+ To obtain insight into the thermal quenching of Mn ferent quenching mechanisms. luminescence, we will discuss four possible quenching In Fig. 4a, the CT state has a larger offset ΔR than the 4 4 processes: (1) multi-phonon relaxation, (2) thermally T state, which causes the CT parabola to cross the A 2 2 activated photoionization, (3) thermally activated cross- parabola at lower energies than the T parabola. − 4+ over via the F → Mn charge-transfer (CT) state, and Thermal activation over the energy barrier ΔE will allow 4+ 4 2 (4) thermally activated crossover via the Mn T excited crossover from the E state into the CT state followed by state. non-radiative relaxation to the ground state via the In the configurational coordinate diagram, the parabolas crossing of the CT and A parabolas. Alternatively, 4+ 2 4 4+ of the Mn E and A states do not cross and lumi- thermal quenching of the Mn luminescence may 2 4 nescence quenching by crossover from the E to the A be due to the mechanism depicted in Fig. 4b. Here, the CT states is not possible (Fig. 4a). The A ground state may state has a smaller offset ΔR compared to that shown in however be reached by multi-phonon relaxation. In Mn Fig. 4a, and its potential curve is therefore at higher + −1 4 -doped fluorides more than 30 phonons of ~500 cm energies. In addition, the T state has a slightly larger 2 4 4 are needed to bridge the energy gap between the E and offset. As a result, the crossing of the T and A para- 2 2 4 49 A states . For such high numbers of phonons (p > 30), bolas is now at a lower energy and non-radiative relaxa- 4 4 it is unrealistic that non-radiative multi-phonon relaxa- tion will proceed via the crossing of the T and A 2 2 tion is responsible for thermal quenching (see Supple- parabolas. mentary Information for a more detailed discussion). The activation energies ΔE in the configuration coor- −1 Alternatively, the thermal quenching can be due to ther- dinate diagrams are ~8000 cm , similar to the ΔE values mally activated photoionization of an electron from the obtained from the temperature-dependent measurements. 4+ 2 Mn E state to the fluoride host conduction band. This indicates that both mechanisms in Fig. 4a, b can 4+ Thermally activated photoionization typically quenches explain the thermal quenching of Mn luminescence. To the emission from a luminescent center if the emitting determine which of these two mechanisms is responsible 26,50 state is close in energy to the host conduction band . for the luminescence quenching, we compare the 4+ quenching temperature T In density functional theory (DFT) calculations, large of K TiF :Mn to the T of ½ 2 6 ½ 4+ band gaps of around 8 eV have been found for fluoride other Mn -doped materials. A relation between the 51,52 hosts like K SiF and K TiF . It is therefore expected quenching temperature and the energy of either the CT or 2 6 2 6 4+ 2 4 that the Mn E state is well below the host conduction T state in a variety of hosts will give insight. If band levels. Based on this, we conclude that thermal quenching occurs by crossover from the CT state 4+ 4 4+ quenching in Mn -doped fluorides is not caused by to the A state, T will be higher for Mn -doped solids 2 ½ 4+ thermally activated photoionization. However, more evi- with higher CT transition energies. In K TiF :Mn and 2 6 4+ − 4+ dence is necessary to exclude this quenching mechanism. other Mn -doped fluorides the F → Mn CT transi- 4+ −113,15 4+ Photoconductivity measurements on Mn phosphors at tion is at ~40,000 cm .Mn -doped oxides have 2− 4+ elevated temperatures need to be performed to provide lower O → Mn CT transition energies of −1 convincing evidence for a possible role of photoionization 30,000–35,000 cm and are therefore expected to have 4+ in the thermal quenching of Mn emission. lower T values than fluorides if quenching occurs by the 4+ 26,27,53,54 4+ Thermal quenching in Mn -doped fluorides has been mechanism in Fig. 4a . Some Mn -doped oxides, suggested to occur by thermally activated crossover via however, have much higher quenching temperatures than 4+ 4 − 4+ 4+ 4+ the Mn T state or the F → Mn charge-transfer Mn -doped fluorides. For example, Mg GeO :Mn , 2 4 6 15,24,26 4+ 4+ (CT) state . Both these states are displaced relative Mg Ge O F :Mn , and Mg As O :Mn have a T 28 7.5 38 10 6 2 11 ½ 4 55–57 4+ 4 to the potential curve of the A ground state (Fig. 4a, b). of ~700 K , while K TiF :Mn and other Mn 2 2 6 4 4 + Hence, the T and CT state parabolas cross the A -doped fluorides have a T of 400–500 K (see also 2 2 ½ ground state parabola. The difference between the Tables 1 and 2). No correlation is found between the Mn potential curve equilibrium positions is given by the offset luminescence quenching temperature and the energy of 4 2 ΔR = R ′ − R . By using the energies of the A → E, the CT transition (see Supplementary Information for an 0 0 2 4 4 4 4+ A → T and A → CT transitions in K TiF :Mn overview and a plot of quenching temperatures and CT 2 2 2 2 6 (Fig. 2d and ref. ) and assuming specific offsets ΔR for energies). From this we conclude that thermal quenching 4 4+ the T and CT states, we can construct the diagrams in in Mn -doped fluorides is not caused by thermally − 4+ Fig. 4a and b, where non-radiative relaxation occurs either activated crossover from the F → Mn CT state to the 4 4 via (a) the crossing of the CT and A states or (b) the A ground state. 2 2 Senden et al. Light: Science & Applications (2018) 7:8 Page 9 of 13 4+ 4 4 Table 1 Quenching temperature T (K) and A → T Alternatively, thermal quenching of the Mn lumi- ½ 2 2 −1 4+ energy (cm ) for Mn -doped fluoride materials nescence can be caused by thermally activated crossover 4+ 4 via the Mn T excited state (Fig. 4b). To investigate the 4 4 −1 Host lattice A → T energy (cm ) T (K) References 2 2 ½ validity of this mechanism, we compare the T and 4 4 4+ A → T transition energies for K TiF :Mn and a 2 2 2 6 K TiF 21,459 462 This work 2 6 4+ variety of other Mn -doped fluorides. From the litera- K SiF 22,099 518 This work 2 6 4+ ture and measurements on Mn luminescence we have K SiF 22,120 490 15 2 6 collected quenching temperatures and luminescence K GeF 21,280 470 15 spectra, preferably for systems with low doping con- 2 6 4+ centrations. Figures 2d and 3b show that K TiF :Mn 2 6 K TiF 21,190 450 15 2 6 4 4 −1 has a A → T energy of 21,459 cm (maximum of the 2 2 K TiF 21,368 478 13 2 6 4+ excitation band) and a T of 462 K. For K SiF :Mn ,we ½ 2 6 4 4 −1 Na SiF 21,739 488 21 2 6 measured a A → T energy of 22,099 cm and a T of 2 2 ½ 4+ Rb SiF 21,739 480 18 518 K (Supplementary Figure S6, K SiF :Mn BR301-C 2 6 2 6 commercial phosphor from Mitsubishi Chemical, Japan). Rb TiF 21,186 450 18 2 6 In Fig. 4c we plot the quenching temperature T against Rb GeF 21,739 513 60 2 6 4 4 4+ 4+ the A → T energy for K TiF :Mn ,K SiF :Mn and 2 2 2 6 2 6 4+ Cs GeF 21,277 420 22 2 6 many other Mn -doped fluoride phosphors reported in Cs SiF 21,368 430 22 the literature (displayed data also listed in Table 1). The 2 6 data show that the T increases with the energy of the T ½ 2 Cs HfF 20,964 403 44 2 6 state. The clear trend shows that the thermal quenching BaSiF 21,322 430 23 4+ in Mn -doped fluorides is due to thermally activated 4 4 BaSnF 21,008 400 45 crossover from the T excited state to the A ground 2 2 BaTiF 21,142 425 61 state. Further confirmation for this quenching mechanism 4+ is provided by Mn spectra measured at elevated tem- peratures (see Supplementary Information). Supplemen- 4+ tary Figure S7 shows emission spectra of K SiF :Mn at 2 6 4 4 4 4 Table 2 Quenching temperature T (K) and A → T ½ 2 2 T = 573 and 673 K. At 573 K a broad T → A emission 2 2 −1 4+ energy (cm ) for Mn -doped oxide materials band is observed, which is almost completely quenched at 4 4 4 4 −1 673 K. The initial rise of the T → A emission at ele- 2 2 Host lattice A → T energy (cm ) T (K) References 2 2 ½ vated temperatures confirms thermal population of the Mg GeO 23,697 730 55 T level, which eventually leads to thermal quenching of 4 6 4+ all Mn emission via this state. Mg Ge O F 23,923 700 26,55,56 28 7.5 38 10 To investigate whether thermally activated crossing via K Ge O 21,739 373 62 2 4 9 the T state is also responsible for temperature 4+ K Ge O (site 1) 19,231 160 63 2 4 9 quenching in Mn -doped oxides, we extend the K Ge4O (site 2) 21,700 379 63 data set of Fig. 4c with quenching temperatures reported 2 9 4+ for Mn -doped oxides. Figure 4d shows the quenching Rb Ge O (site 1) 19,231 162 63 2 4 9 4 4 temperature T as a function of the A → T energy for ½ 2 2 Rb Ge O (site 2) 20,850 346 63 2 4 9 4+ the Mn -doped fluorides and oxides listed in Tables 1 Y Mg Ge O 23,753 850 64 2 3 3 12 and 2. The results show that T increases with the 4 4 La GaGe O 21,413 420 65 energy of the A → T transition. This indicates that the 3 5 16 2 2 4+ Mn emission in fluorides and oxides are both La ZnTiO 19,608 230 66 2 6 quenched due to thermally activated crossover from the La MgTiO 20,000 250 66 2 6 4 T excited state, and not the CT state as previously 24–27 CaZrO 18,500 300 25,26 suggested in some reports . The present results and 4+ Mg As O 23,810 680 57 analysis provide strong evidence that in many Mn 6 2 11 phosphors the thermal quenching mechanism involves Y Al O 20,619 300 67 3 5 12 thermally activated crossover via the T excited state. A Y Al O 20,833 300 68 3 5 12 contribution from other mechanisms cannot be ruled out Sr Al O 22,222 423 69 4 14 25 and further research, for example, photoconductivity SrLaAlO 19,231 300 53 measurements and high pressure studies, can give addi- tional information on the role of alternative quenching LiGa O 20,000 350 70 5 8 mechanisms. Senden et al. Light: Science & Applications (2018) 7:8 Page 10 of 13 a c 10 6.2 1.0 Quantum efficiency 6.0 0.8 –1 5.8 0 2.5 5 4+ 0.6 Mn (x %) Delay time (ms) 0.01% 0.1% 5.6 0.4 0.8% –2 1.3% 3.8% 5.4 0.2 5.4% Emission lifetime 9.4% 15.7% –3 10 5.2 0.0 0 4 8 12 16 020 40 4+ Mn concentration (%) Delay time (ms) b d e 0 0 0 10 10 10 4+ 4+ 4+ 0.8% Mn 15.7% Mn 15.7% Mn T = 298 K T = 298 K T = 4 K –1 –1 –1 10 10 10 –2 –2 –2 10 10 10 = 5.6 ms  = 5.4 ms  = 10.6 ms fit fit fit 020 40 020 40 04 20 0 60 80 Delay time (ms) Delay time (ms) Delay time (ms) 4+ 4+ Fig. 5 Luminescence decay and quantum efficiency of K TiF :Mn as a function of the Mn doping concentration. a Room-temperature PL 2 6 4+ 4+ decay curves of the Mn emission from K TiF :Mn (x%) for 0.01% (pink), 0.1% (blue), 0.8% (green), 1.3% (orange), 3.8% (purple), 5.4% (cyan), 9.4% 2 6 4+ 4+ (yellow), and 15.7% (red) Mn (λ = 450 nm and λ = 631 nm). b PL decay curve of K TiF :Mn (0.8%) at T = 298 K. The decay time exc em 2 6 4+ corresponding to the mono-exponential fit (red line) is 5.6 ms. The bottom panel shows the fit residuals. c Mn emission lifetime (blue squares) and 4+ 4+ 4+ PL quantum efficiency (red dots) of K TiF :Mn with different Mn doping concentrations. d, e PL decay curves of K TiF :Mn (15.7%) at d T = 298 2 6 2 6 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 4+ As quenching occurs by thermally activated crossover that there is a variation in ΔR for Mn -doped fluorides. via the T excited state, the quenching temperature T of The variation in ΔR is small, however, compared to the 2 ½ 4+ 4 the Mn luminescence is controlled by the energy of the differences in the T energy, and no correlation is 4+ 4 Mn T state (the dependence of T on the energy of observed between the spectral width and quenching 2 ½ 4 4 the T state is shown in Fig. 4c,d). In addition, the T of temperatures. This indicates that the T level energy has 2 ½ 2 4+ the Mn luminescence depends on the offset ΔR the largest influence on the quenching temperature of 4 4 4+ between the T and A states, as ΔR also determines Mn -doped fluorides. 2 2 4 4 where the T and A states cross in the configuration Finally, in view of applications, it is interesting to see 2 2 coordinate diagram (Fig. 4a,b). The horizontal displace- how we can control the T level energy (and thereby T ) 2 ½ ment of the T parabola will influence the quenching through the choice of the host lattice. The energy of the 4+ 4 temperature. A variation in ΔR can explain the spread Mn T state depends on the crystal field splitting Δ 2 O observed in the data of Fig. 4c and d. To investigate the (Fig. 2b), where Δ is typically larger for shorter Mn–F 4+ 44,58 4+ variation in the offset ΔR for Mn -doped fluorides, we distances . For Mn -doped fluorides the lumines- 4 4 compare the bandwidth of the A → T excitation band cence quenching temperature can therefore be raised by 2 2 4+ 4+ 4+ 4+ − in K TiF :Mn ,K SiF :Mn and Cs HfF :Mn selecting host lattices with short M –F distances 2 6 2 6 2 6 (see Supplementary Figure S9). The width of the (see Supplementary Figure S10a). This is consistent with 4 4 4+ A → T excitation band is controlled by the displace- findings that T increases if the radius of the M host 2 2 ½ ment of the T state and therefore gives a good indication cation decreases, as expected based on crystal field the- 4 4 11,18 4+ of ΔR. Comparison of the A → T bandwidths shows ory . If, however, T is plotted against the M -ligand 2 2 ½ Normalized intensity Normalized intensity Residuals Normalized intensity Residuals Lifetime (ms) Normalized intensity Residuals Quantum efficiency Senden et al. Light: Science & Applications (2018) 7:8 Page 11 of 13 4+ 4+ 4+ distance for both Mn -doped fluorides and Mn -doped transfer for Mn ions close to a quencher. In case of oxides (see Supplementary Figure S10b), no correlation energy migration, a faster decay is also expected for longer 4+ between T and the M -ligand distance is found. This times after the excitation pulse. As this is not observed, 4 4+ shows that the crystal field splitting and T energy give a the contribution of energy migration via many Mn ions better indication of the quenching temperature for Mn to quenching sites seems to be small. -doped phosphors. To further investigate the role of energy migration in 4+ the concentration quenching of the Mn emission, we 4+ Concentration quenching measure a PL decay curve of K TiF :Mn (15.7%) at T = 2 6 In addition to insight into thermal quenching, con- 4 K, which is displayed in Fig. 5e. At T = 4 K energy 4+ 4+ centration quenching in Mn -doped fluorides is impor- migration among the Mn ions (blue arrows in Fig. 1) tant for application in w-LEDs. The weak parity-forbidden will be hampered, as there is almost no spectral overlap 4 4 4+ 2 4 4 2 A → T absorption requires that commercial phos- between the Mn E→ A emission and A → E 2 2 2 2 4+ phors have high Mn concentrations. If there is effective excitation lines (see Supplementary Figure S11). Hence, at concentration quenching, the PL decay time and QE will 4 K non-radiative decay due to energy migration to 4+ 4+ decrease when the Mn doping concentration is quenching sites will be suppressed. The Mn decay 26,28 raised . We therefore investigate concentration dynamics in Fig. 5e, however, show that the non-radiative 4+ quenching in K TiF :Mn by measuring the PL decay decay is not suppressed at 4 K. The deviation from single 2 6 4+ 4+ times and QEs of K TiF :Mn phosphors with Mn exponential behavior is similar to that at 300 K. There is 2 6 4+ concentrations ranging from 0.01 to 15.7% Mn . an initial faster decay (single-step energy transfer to Figure 5a presents room-temperature PL decay curves quenching sites) followed by an exponential decay with a 4+ 4+ 4+ of the Mn emission from K TiF :Mn with increasing decay time very close to that measured for Mn at low 2 6 4+ Mn doping concentration x. It can be seen that the PL doping concentrations. This suggests that the decrease in 4+ 4+ decay becomes slightly faster as the Mn concentration QE at higher Mn concentrations is not due to energy increases. We analyze the decay dynamics by single migration. The absence of strong concentration quench- exponential fitting of the PL decay curves. The fit for ing by energy migration is confirmed by the thermal 4+ 4+ K TiF :Mn (0.8%) is shown in Fig. 5b. The fit residuals quenching behavior measured for the different Mn 2 6 (bottom panel) are random and the PL decay thus concentrations. In Supplementary Figure S4, it can be resembles a single exponential. This indicates that the seen that the luminescence quenching temperature is decay of the approximately the same for doping concentrations of E state is mainly radiative. Consequently, the 4+ 4+ K TiF :Mn (0.8%) phosphor has a very high QE of 90%. 0.01% and 15.7% Mn , which shows that effects due to 2 6 Figure 5c gives an overview of the fitted decay times (blue thermally activated energy migration (i.e., concentration 4+ squares) and QEs (red dots) of K TiF :Mn with differ- quenching) are weak. Hence, we conclude that the non- 2 6 4+ 4+ ent Mn concentrations. The emission lifetime barely radiative decay at high Mn concentrations is not caused 4+ shortens if the Mn concentration is increased (5.7 ms by energy migration. Inefficient energy migration can be 4+ 4+ for 0.01% Mn to 5.4 ms for 15.7% Mn ). This suggests understood based on the strongly forbidden character of 2 4 4+ 4+ that energy migration to quenching sites is inefficient in the E→ A transition. This allows only Mn –Mn 4+ K TiF :Mn . To verify this, we look at the QE values energy transfer via short range exchange interaction 2 6 4+ obtained for the K TiF :Mn (x%) phosphors. The QE (see Supplementary Information for details). 2 6 4+ remains above 80% for Mn doping concentrations of 5% We instead assign the non-radiative decay to direct 4+ or less, which shows that concentration quenching is transfer of excitation energy from Mn ions to quench- 4+ indeed limited up to a concentration of 5% Mn ions. ers (green arrow in Fig. 1). This process can occur at all 4+ This result is important for applications in w-LEDs, as temperatures and becomes more efficient at higher Mn 4+ 4+ these high Mn doping concentrations (e.g., 5 mol%) are dopant concentrations. With an increasing Mn dopant required for sufficient absorption of the blue LED light in concentration, the stress on the K TiF lattice grows and 2 6 the parity-forbidden d–d transitions . as a result more crystal defects (i.e., quenchers) may be 4+ 2+ For higher Mn concentrations (x > 10%), non- formed. In addition, Mn in different valence states (Mn 2 3+ 4+ radiative decay from the E excited state becomes stron- and Mn ) may be incorporated at higher Mn con- 4+ 4+ ger, however, and as a result the QE of K TiF :Mn falls centrations. Even if a very small fraction of Mn ions has 2 6 below 60% (Fig. 5c). The non-radiative decay is also visible a different valence state than 4+, effective quenching can 4+ in the PL decay curve of K TiF :Mn (15.7%), shown in occur via metal-to-metal charge-transfer states or direct 2 6 Fig. 5d. The decay is multi-exponential, which proves that energy transfer. Consequently, the probability for energy 4+ 2 with 15.7% Mn the E state decays both radiatively and transfer to quenchers increases, resulting in faster initial 4+ 4+ non-radiatively. The faster initial decay indicates that PL decay and lower QEs for K TiF :Mn at high Mn 2 6 there is enhanced quenching by single-step energy dopant concentrations. Optimized synthesis procedures Senden et al. Light: Science & Applications (2018) 7:8 Page 12 of 13 to reduce quenchers (defects and impurity ions) are thus Conflict of interest 4+ The authors declare that they have no conflict of interest. crucial for obtaining highly luminescent Mn -doped fluoride phosphors (see also recent work of Garcia- Supplementary information is available for this paper at https://doi.org/ Santamaria et al. on concentration quenching in K SiF : 2 6 10.1038/s41377-018-0013-1. 4+ Mn ). Received: 12 October 2017 Revised: 21 February 2018 Accepted: 7 March 2018 Accepted article preview online: 13 March 2018 Conclusions 4+ Narrow-band red-emitting Mn phosphors form an important new class of materials for LED lighting and displays. For these applications, it is important to under- References stand and control the luminescence efficiency. We have 1. The Economist. 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Quenching of the red Mn4+ luminescence in Mn4+-doped fluoride LED phosphors

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Abstract

4+ Red-emitting Mn -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 4+ 4+ quenching in Mn -doped fluorides. Furthermore, concentration quenching of the Mn luminescence can be an issue but detailed investigations are lacking. In this work, we study thermal quenching and concentration quenching 4+ 4+ in Mn -doped fluorides by measuring luminescence spectra and decay curves of K TiF :Mn between 4 and 600 K 2 6 4+ 4+ and for Mn concentrations from 0.01% to 15.7%. Temperature-dependent measurements on K TiF :Mn and other 2 6 4+ 4 Mn -doped phosphors show that quenching occurs through thermally activated crossover between the T excited 4 4+ state and A ground state. The quenching temperature can be optimized by designing host lattices in which Mn has a high T state energy. Concentration-dependent studies reveal that concentration quenching effects are limited 4+ 4+ 4+ in K TiF :Mn up to 5% Mn . This is important, as high Mn concentrations are required for sufficient absorption of 2 6 4+ 4+ blue LED light in the parity-forbidden Mn d–d transitions. At even higher Mn concentrations (>10%), the quantum efficiency decreases, mostly due to direct energy transfer to quenching sites (defects and impurity ions). Optimization 4+ of the synthesis to reduce quenchers is crucial for developing more efficient highly absorbing Mn phosphors. The 4+ present systematic study provides detailed insights into temperature and concentration quenching of Mn emission 4+ and can be used to realize superior narrow-band red Mn phosphors for w-LEDs. 2+ Introduction their use also has a serious drawback. The Eu emission White light-emitting diodes (w-LEDs) are the next- band is broad and extends into the deep red spectral generation light sources for display and illumination sys- region (λ > 650 nm) where the eye sensitivity is low. This tems because of their small size, high luminous efficacy, causes the luminous efficacy of the w-LED to drop 1–5 and long operation lifetime . Conventional w-LEDs are (reduced lumen/W output). A worldwide search is composed of blue-emitting (In,Ga)N LEDs and green/ therefore aimed at finding efficient narrow-band red- yellow-emitting and orange/red-emitting phosphors that emitting phosphors that can be excited by blue light. In 5–7 4+ convert part of the blue LED emission . Both phosphors this search, Mn -doped fluoride phosphors, such as 4+ 4+ are necessary to generate warm white light with a high K SiF :Mn and K TiF :Mn , have recently attracted 2 6 2 6 9–13 color rendering index (CRI > 85). The typical red phos- considerable attention . Under blue light excitation, 2+ 4+ phors in w-LEDs are Eu -doped nitrides (e.g., CaAlSiN : Mn -doped fluorides show narrow red line emission 2+ 4,8 13–16 Eu ) . These phosphors exhibit high photo- (λ ~ 630 nm) with high luminescence QEs . Fur- max luminescence (PL) quantum efficiencies (QEs > 90%), but thermore, they are prepared through low-cost, simple 11,17 wet-chemical synthesis at room temperature . These 4+ aspects make Mn -doped fluorides very promising red- Correspondence: Tim Senden (t.senden@uu.nl) emitting phosphors for developing energy-efficient high Condensed Matter and Interfaces, Debye Institute for Nanomaterials Science, color-rendering w-LED systems . Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands Soft Condensed Matter, Debye Institute for Nanomaterials Science, Utrecht University, P.O. Box 80000, 3508 TA Utrecht, The Netherlands © The Author(s) 2018 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to theCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. 1234567890():,; 1234567890():,; 1234567890():,; 1234567890():,; Senden et al. Light: Science & Applications (2018) 7:8 Page 2 of 13 understanding of the thermal quenching behavior is Energy migration Quenching site 4+ essential for developing Mn -doped fluoride phosphors 4+ Mn ion with superior quenching temperatures, and thereby improving their potential for application in w-LEDs. Besides thermal quenching, concentration quenching is Excitation 4+ an issue for the application of Mn -doped fluorides in w- 4+ LEDs. As the Mn d–d transitions are parity-forbidden, 4+ high Mn doping concentrations (e.g., 5 mol%) are required for sufficient absorption of the blue LED light . At high dopant concentrations, energy migration among Energy 4+ 26,28 transfer to the Mn ions can result in concentration quenching , 4+ quencher as is illustrated in Fig. 1. If the distance between the Mn ions is small, excitation energy may efficiently migrate 4+ from one Mn ion to another until it reaches a quenching site (defect or impurity ion), where the exci- Host cation Radiative emission tation energy is lost non-radiatively (as heat). Studies on 4+ 4 Fig. 1 Concentration quenching for Mn in crystals. At high Mn 4+ + 4+ concentration quenching in Mn -doped fluorides are doping concentrations the Mn ions (orange) are in close 4+ limited. Several works have compared the luminescence proximity in the crystal lattice. If the Mn ions are close together, 4+ 4+ energy transfer between Mn ions (dark blue) causes the excitation properties of fluoride phosphors with varying Mn to migrate through the crystal. Eventually, it may reach a quenching concentrations, but do not measure the actual site such as a vacancy or impurity (dashed circle), where the excitation 4+ Mn concentration in the phosphors by elemental ana- energy is lost as heat. This process competes with radiative emission 29–33 4+ lysis . Determining the Mn concentration is crucial, (red) and reduces the luminescence efficiency 4+ as often only a fraction of the Mn ions is incorporated 19,34 during the synthesis . Reports that do perform ele- 4+ 4+ The application of Mn -doped fluoride phosphors in mental analysis study only a small range of Mn doping w-LEDs may, however, be hampered by thermal concentrations and do not provide insight into the role of 4+ 4+ 13,35,36 quenching of the Mn luminescence. Thermal quench- concentration quenching in Mn doped fluorides . ing of the phosphor luminescence is a serious issue, as it An in-depth investigation of concentration quenching in 4+ affects both the efficacy and color stability of the w-LED. Mn -doped fluorides is thus lacking, despite it being very 4+ In high-power w-LEDs, the temperature of the on-chip important for the application of Mn -doped fluorides in phosphor layer easily reaches 450 K. At these elevated w-LEDs. 4+ temperatures, thermal quenching occurs for Mn -doped In this work, we systematically investigate concentration 4+ fluorides. The luminescence quenching temperature T , quenching and thermal quenching in Mn -doped the temperature at which the emission intensity is fluorides. The quenching is studied by measuring lumi- reduced to half of its maximum, is typically between 400 nescence spectra and decay curves in the temperature 15,18,19 4+ and 500 K . Although the temperature dependence range of 4 to 600 K for K TiF :Mn phosphors with 2 6 4+ of the emission intensity has been measured for many Mn concentrations ranging from 0.01 to 15.7 mol% 4+ 4+ Mn -doped fluorides, the understanding of the thermal (actual Mn concentration). The temperature-dependent 4+ quenching behavior is still limited. Most studies do not luminescence measurements of K TiF :Mn and other 2 6 4+ 4+ explain which process quenches the Mn lumines- Mn -doped phosphors demonstrate that thermal 13,20–23 cence . Moreover, the few reports that do propose a quenching occurs because of thermally activated cross- 15 4 4 quenching mechanism disagree. Paulusz states that the over from the T excited state to the A ground state. 2 2 4+ luminescence of Mn -doped fluorides is quenched by This insight into the quenching mechanism shows that 4+ 4 4+ thermally activated crossing of the Mn T excited state the Mn quenching temperature can be raised by finding 4 24 4+ 4 and A ground state. In contrast, Dorenbos finds a fluoride hosts that have an increased Mn T level 2 2 relation between the quenching temperature and the energy. Concentration studies show that the lumines- − 4+ 4+ energy of the F → Mn charge-transfer (CT) state and cence QE of K TiF :Mn is high, ~80%, for doping 2 6 4+ therefore suggests that quenching involves crossover concentrations up to 5 mol% Mn . Concentration 4 4+ between the CT state and A ground state. This CT state quenching is limited for these relatively high Mn crossover mechanism was also used by Blasse and our dopant concentrations. At even higher doping con- 4+ 4+ group to explain thermal quenching in Mn -doped centrations of >10 mol%, the QE of K TiF :Mn falls 2 6 25–27 oxides . Finally, other reports claim that the quench- below 60%. Luminescence decay curves indicate ing temperature increases if the radius of the cation that the drop in QE can be attributed to an increased 4+ 11,18 substituted by Mn becomes smaller . A better probability for direct energy transfer to quenching sites Senden et al. Light: Science & Applications (2018) 7:8 Page 3 of 13 2+ 3+ (e.g., defects, impurity ions, Mn , and Mn ), the con- Hamamatsu R928 photomultiplier tube (PMT) with a 4+ centration of which increases with the Mn concentra- grating blazed at 500 nm for detection of emission. For PL tion. The present results provide an improved decay measurements, excitation was done with a tunable understanding of thermal quenching and concentration optical parametric oscillator (OPO) Opotek Opolette HE 4+ quenching in Mn -doped solids and can be used to 355II laser (pulse width 10 ns, repetition rate 10 Hz) and 4+ develop superior Mn -doped fluoride phosphors for emission was detected with a Hamamatsu H74220–60 PMT. The PL decay curves between 300 and 600 K were w-LEDs. recorded on a different setup, which had an Ekspla NT Materials and methods 342B OPO laser (pulse width 5 ns, repetition rate 10 Hz) 4+ Synthesis and characterization of K TiF :Mn phosphors as excitation source and a 0.55 m Triax 550 mono- 2 6 4+ The K TiF :Mn (x%) phosphors were synthesized chromator combined with a Hamamatsu H74220–60 2 6 according to the method of Zhu et al. For the synthesis PMT for detection of emission. All PL decay curves were 4+ of K TiF :Mn (0.8%), 0.0488 g of K MnF (prepared obtained by multi-channel scaling (MCS) with a Pico- 2 6 2 6 37,38 4+ following refs. ) was dissolved in 2.5 mL of a 40 wt% Quant TimeHarp 260 computer card. The K TiF :Mn 2 6 HF solution (Fluka, 40 wt% HF in water). Next, the phosphors were cooled down to 4 K with an Oxford obtained yellow-brown solution was mixed with 4.5730 g Instruments liquid helium flow cryostat. For PL mea- of K TiF (Sigma-Aldrich, p.a.) and then stirred for 1 h at surements between 300 and 600 K samples were heated in 2 6 4+ room temperature to form K TiF :Mn crystals. The a Linkam THMS600 temperature controlled stage. The 2 6 4+ K TiF :Mn phosphor was isolated by decanting the HF PL quantum efficiencies of the phosphors were deter- 2 6 solution, washing twice with 15 mL of ethanol and then mined with a calibrated home-built setup, which con- drying the phosphor for 7 h at 75 °C. The other K TiF : sisted of a 65 W Xe lamp, excitation monochromator, 2 6 4+ Mn (x%) phosphors were prepared following the integrating sphere (Labsphere) and CCD camera (Avantes same procedure but using other amounts of K MnF AvaSpec-2048). 2 6 4+ and K TiF as to obtain different Mn doping 2 6 concentrations. Results and discussion 4+ Powder X-ray diffraction (see Supplementary Figure S1) Luminescence of K TiF :Mn 2 6 4+ confirms that the K TiF :Mn (x%) phosphors exhibit For our quenching studies, we examine the lumines- 2 6 4+ the hexagonal crystal structure of K TiF up to the highest cence of K TiF :Mn phosphors with a wide range of 2 6 2 6 4+ 4+ doping concentration of 15.7% Mn . Furthermore, no Mn doping concentrations. A photographic image of 4+ impurities of K MnF or other crystal phases are observed the K TiF :Mn (x%) phosphors is displayed in Fig. 2a. 2 6 2 6 4+ in the diffraction patterns. Scanning electron microscopy The Mn doping concentrations x (molar percentages 4+ 4+ (SEM) images show that most K TiF :Mn phosphor with respect to Ti ) were determined by inductively 2 6 particles are irregularly shaped and have sizes ranging coupled plasma optical emission spectroscopy (ICP-OES). 4+ from 1 to 200 µm (see Supplementary Figure S2a). Some The body color of K TiF :Mn becomes more yellow 2 6 4+ particles have a hexagonal shape, in agreement with the with increasing Mn concentration as a result of hexagonal crystal structure of K TiF (see Supplementary enhanced absorption in the blue. All of the investigated 2 6 4+ 4+ Figure S2b). Energy-dispersive X-ray (EDX) spectra K TiF :Mn phosphors exhibit bright red Mn lumi- 2 6 (see Supplementary Figure S2c) confirm that the phos- nescence under UV photoexcitation. phor particles consist of potassium, titanium, fluorine, Figure 2b depicts the Tanabe–Sugano energy level 4+ 3 and manganese ions. The manganese dopant concentra- diagram of Mn (3d electron configuration) in an 4+ 39,40 3 tions in the K TiF :Mn phosphors were determined octahedral crystal field . The diagram gives the d 2 6 with inductively coupled plasma optical emission spec- energy levels as a function of the crystal field splitting Δ . 4+ troscopy (ICP-OES). The ICP-OES measurements were Due to its high effective positive charge, Mn experi- performed on a Perkin-Elmer Optima 8300DV spectro- ences a strong crystal field and therefore the E state is the meter (λ = 257.61 and 259.37 nm). For the ICP-OES lowest energy excited state. Hence, the emission spectrum em 4+ 4+ analyses, the K TiF :Mn phosphors were dissolved in of K TiF :Mn (0.8%) is dominated by narrow red 2 6 2 6 2 4 aqua regia. emission lines due to spin- and parity-forbidden E→ A transitions, as can be seen in Fig. 2c. The other K TiF : 2 6 4+ Optical spectroscopy Mn (x%) phosphors exhibit similar emission spectra. As 2 4 PL measurements were performed on an Edinburgh the potential energy curves of the E and A states are at 2 4 Instruments FLS920 fluorescence spectrometer, except the same equilibrium position, the E→ A emission is for the PL decay measurements between 300 and 600 K characterized by narrow zero-phonon and vibronic (see below). For recording excitation and emission spec- emission lines. The potential energy curves of the E and tra, we used a 450 W Xe lamp as excitation source and a A states are at the same equilibrium position because 2 Senden et al. Light: Science & Applications (2018) 7:8 Page 4 of 13 a c 4+ v Mn concentration (x %) 1.0 2 4 0.8% Mn E A 0.01 0.1 0.8 1.3 3.8 5.4 9.4 15.7 ZPL 0.8 Anti-Stokes Stokes 0.6 0.4 0.2 0.0 2 4 2 A T A 550 600 650 700 2 1 1 Wavelength (nm) 50 T 4 d 2 4 4 2 1.0 A T 2 2 0.8% Mn 4 4 0.8 30 A T 2 1 T 0.6 1 4 2 E 20 2 G 0.4 4 2 A T 2 1 0.2 0.0 F 4 300 400 500 600 0 10 20 30 40 Wavelength (nm) Δ /B 4+ 4+ 4+ Fig. 2 Mn luminescence of K TiF :Mn .a Photographic image of K TiF :Mn (x%) phosphors with x = 0.01, 0.1, 0.8, 1.3, 3.8, 5.4, 9.4, and 15.7. 2 6 2 6 4+ The phosphors have a white to yellow body color under ambient light (top) and show red Mn luminescence under 365 nm UV illumination 3 4 4 4 4 (bottom). b Tanabe−Sugano energy level diagram of the d electron configuration in an octahedral crystal field. The A → T , A → T , and 2 1 2 2 2 4 4+ E→ A transitions of Mn are indicated by the purple, blue and red arrows, respectively. Note that the excitation transitions are displaced for 4+ clarity. For a specific coordination all transitions take place around the same crystal field Δ . c Emission spectrum of K TiF :Mn (0.8%) upon O 2 6 4+ 4+ excitation with blue light (λ = 450 nm). d Excitation spectrum of the red Mn luminescence (λ = 630 nm) from K TiF :Mn (0.8%). Spectra are exc em 2 6 recorded at ambient temperature 2 4 the E and A states originate from the same t electron Figure 2d displays the excitation spectrum of the red 2g 41 4+ 4+ configuration . Mn luminescence from K TiF :Mn . The two broad 2 6 2 4 4 4 The E→ A emission spectrum consists of a weak excitation bands correspond to spin-allowed A → T 2 2 1 4 4 zero-phonon line (ZPL) at ~622 nm and more intense and A → T transitions (violet and blue arrows in 2 2 anti-Stokes and Stokes vibronic emissions (labeled ν , ν , Fig. 2b). In addition, some weak peaks are visible around 3 4 4 2 and ν ) on the high and low energy sides of the ZPL, 600 nm. These peaks are assigned to A → E and 6 2 13,15 4+ 4 2 4 2 2 respectively . The ZPL is very weak because Mn is A → T transitions. The A → T , E transitions are 2 1 2 1 located on a site with inversion symmetry in K TiF :Mn spin-forbidden and therefore low in intensity compared to 2 6 + 4 4 4 . Due to the inversion symmetry, there are no odd-parity the spin-allowed A → T , T transitions. 2 1 2 crystal field components to admix opposite parity states 4 2 2 4 4+ into the A and E states and, as a result, the E→ A Temperature dependence of the Mn luminescence 2 2 2 4 4+ transition is electric dipole forbidden. The E→ A To study the thermal quenching of the Mn emission, 4+ transition can become partly allowed, however, by cou- we measure the PL intensity and Mn emission lifetime 4+ pling with asymmetric vibrations that induce odd-parity of K TiF :Mn (0.01%) as a function of temperature 2 6 4+ crystal field components. The most intense lines in Fig. 2c between 4 and 600 K. We use a very low Mn doping 2 4 4+ are assigned to E→ A transitions coupling with the concentration of 0.01%, as for higher Mn concentra- asymmetric ν , ν , and ν vibrational modes (phonons) of tions reabsorption of emission and energy transfer 3 4 6 4+ the MnF group. Thermal population of phonons at between Mn ions can occur. These processes will 4+ room temperature allows coupling with ν , ν , and ν influence (the temperature dependence of) the Mn 3 4 6 2 6 phonon modes in the E excited state (giving rise to the luminescence spectra and decay curves . As a result, with 4+ anti-Stokes lines), while transitions to these phonon a high concentration of Mn ions, the observations may modes in the A ground state can occur at all tempera- not reflect the intrinsic thermal quenching properties of 4+ tures (Stokes lines). Mn . E /B Normalized intensity Normalized intensity Senden et al. Light: Science & Applications (2018) 7:8 Page 5 of 13 ac 6 K 2 4 4.0 E A  = 631 nm 2 4 K em 126 K 102 K 3.0 175 K 175 K 227 K 2.0 294 K 292 K –1 1.0 373 K 423 K 0.0 448 K 4.0 303 K 473 K 373 K −2 3.0 448 K 2.0 473 K 498 K 1.0 573 K −3 0.0 580 600 620 640 660 680 0 20406080 Wavelength (nm) Delay time (ms) bd 1.2 1.0 0.8 T = 457 K 0.6 T = 462 K 0.4 ½ 4 0.2 0.0 0 0 100 200 300 400 500 0 100 200 300 400 500 Temperature (K) Temperature (K) 4+ 4+ 4+ Fig. 3 Temperature dependence of the Mn luminescence from K TiF :Mn (0.01%). a Emission spectra (λ = 450 nm) of K TiF :Mn 2 6 exc 2 6 4+ (0.01%) at various temperatures between 0 and 600 K. b Integrated PL intensity of K TiF :Mn (0.01%) as a function of temperature. The integrated 2 6 PL intensity I is scaled to the integrated PL intensity at room temperature I . The red and green lines represent fits to Eqs. 6 and 7, respectively. c PL PL RT 4+ 4+ decay curves of the Mn emission from K TiF :Mn (0.01%) at various temperatures between 0 and 600 K (λ = 450 nm and λ = 631 nm). 2 6 exc em 4+ 4+ d Temperature dependence of the Mn emission lifetime for K TiF :Mn (0.01%). The red and green lines represent fits to Eqs. 4 and 8, respectively. 2 6 The cyan line gives the fit for Eq. 4 (red line) divided by two 4+ Figure 3a shows emission spectra of K TiF :Mn decay times. Figure 3c shows a selection of PL decay 2 6 4+ (0.01%) at various temperatures between 4 and 600 K. At curves of K TiF :Mn (0.01%) measured between 4 and 2 6 4+ 2 4 4+ 4 K the Mn E→ A emission spectrum consists of 600 K. The decay of the Mn emission is single expo- zero-phonon and Stokes vibronic lines. Upon raising the nential and becomes faster with increasing temperature. temperature, phonon modes are thermally populated and The PL decay time is on the order of milliseconds, which 2 4 anti-Stokes emission lines appear (solid arrow in Fig. 3a). is expected as the transition between the E and A states 4+ With the appearance of anti-Stokes lines, the relative is both parity- and spin-forbidden. In Fig. 3d, the Mn intensity of the Stokes emission decreases between 4 and emission lifetime (determined from single exponential 300 K. Above 400 K the intensities of both the anti-Stokes fitting) is plotted as a function of temperature. The life- and Stokes emission lines begin to decrease (dashed arrow time shows a steady decrease, starting above 50 K. The in Fig. 3a), which indicates the onset of non-radiative decrease levels off between 300 and 400 K but then shows transitions from the E excited state. The luminescence is a rapid decrease above 400 K. quenched at 600 K. From the measurements, we obtain The temperature dependences observed in Fig. 3b and d the temperature dependence of the integrated PL inten- are quite exceptional. For most luminescent materials, the sity (I ) relative to the integrated PL intensity at room PL intensity and lifetime are relatively constant with PL temperature (I ) (Fig. 3b). The PL intensity of K TiF : temperature and both begin to decrease once thermal RT 2 6 4+ 6,42,43 4+ Mn (0.01%) gradually increases between 4 and 350 K quenching sets in . The PL intensity of K TiF :Mn , 2 6 but then rapidly drops due to the onset of non-radiative however, rises by 40% between 4 and 350 K while the transitions (luminescence quenching). lifetime decreases before thermal quenching takes place. An alternative method to determine the luminescence To understand this peculiar temperature dependence, we quenching temperature is by measuring luminescence first discuss how the radiative decay rate of the E state Integrated I (T )/I PL intensity (10 counts) PL RT Normalized intensity Lifetime (ms) Senden et al. Light: Science & Applications (2018) 7:8 Page 6 of 13 2 4 changes with temperature. The E→ A emission of with vibrations (for more details on the vibronic structure 4+ 4 4 15,16,44 K TiF :Mn mainly consists of anti-Stokes and Stokes of the A → T excitation band, see refs. ). As a 2 6 2 2 vibronic emissions (Fig. 2c). Their transition probabilities result, the PL intensity I will scale with temperature PL 20,41,45 increase with phonon population. The population of as : phonon modes is given by the phonon occupation num- hv ber n, which increases with temperature according to : I ðÞ T ¼ IðÞ 0 coth ð6Þ PL 2k T n ¼ ð1Þ expðÞ hv=k T 1 with I(0) being the PL intensity at T = 0 K. The results in where k is the Boltzmann constant and hν is the energy Fig. 3b show that the increase in PL intensity between 4 2 4 of the phonon coupling to the E→ A transition. The and 350 K follows the temperature dependence given by transition probabilities P of the anti-Stokes and Stokes Eq. 6. This confirms that the higher PL intensity at 350 K vibronics scale with n by: is due to a stronger absorption of excitation light. An increase in PL intensity between 4 and 350 K due to Anti  Stokes : P ðÞ T ¼ P ðÞ 0 ½ n ð2Þ R R 4+ enhanced absorption is observed for all investigated Mn Stokes : P ðÞ T ¼ P ðÞ 0½ n þ 1 ð3Þ R R doping concentrations (see Supplementary Information). Although the temperature dependence of the PL intensity where P (0) is the transition probability at T = 0 K. As the follows Eq. 6, there is deviation between the fit of Eq. 6 radiative lifetime τ is proportional to 1/[P (anti- R R and the measured data (see red line in Fig. 3b). The model Stokes) + P (Stokes)], it follows from Eqs. 1–3 that: of Eq. 6 is simple and does not take into account the shift τ ðÞ 0 4 4 τ ðÞ T ¼ ð4Þ and broadening of the A → T absorption band with 2 2 cothðhv=2k TÞ temperature. Both these effects also influence the tem- perature dependence of the PL intensity, and this can Here, τ (0) is the radiative lifetime at T = 0 K. In Fig. 3d, explain the deviation between the model and the experi- Eq. 4 (red line) has been plotted for τ (0) = 12.3 ms and −1 mental data. Including the effect of a shift and broadening hν = 216 cm (phonon energy of the intense ν mode 4 4 of the A → T band on the absorption strength is emission). Equation 4 accurately describes the measured 2 2 4+ complex and will not aid a more accurate determination temperature dependence of the Mn emission lifetime of T . up to 375 K, confirming that the decay of the E state is ½ mainly radiative up to this temperature. The radiative 4+ 4+ Above 400 K the PL intensity of K TiF :Mn (0.01%) 2 6 lifetime of the Mn emission shortens with temperature begins to decrease due to the onset of non-radiative due to thermal population of odd-parity vibrational transitions (Fig. 3a, b). The non-radiative decay prob- modes at higher temperatures. ability rapidly increases with temperature above 400 K and as a result the luminescence is quenched, with no Next, we investigate the increase in PL intensity emission intensity remaining at 600 K. The quenching between 4 and 350 K. The PL intensity I equals the PL 4+ temperature T is determined to be 462 K. The Mn product of the PL QE and number of absorbed photons ½ emission lifetime also rapidly decreases once thermal (as I scales with the number of absorbed photons, the PL 4+ quenching sets in (Fig. 3d). Above 400 K the Mn excitation wavelength can have a large influence on the emission lifetime is shorter than the radiative lifetime τ temperature dependence observed for I ; see Supple- R PL 4+ predicted by Eq. 4 (red line). The lifetime shortens mentary Information). The PL QE η of K TiF :Mn can 2 6 because of an additional thermally activated non-radiative be expressed as: contribution to the decay of the E state. From the tem- η ¼ ð5Þ perature dependence of the lifetime, T can be deter- γ þ γ R NR mined by locating the temperature at which the lifetime where γ and γ are the radiative and non-radiative has decreased to half of its radiative lifetime value. To R NR decay rates of the emitting E state, respectively. The estimate T , we divide the value from the fit of Eq. 4 for τ ½ R results in Fig. 3d show that the decay of the E state is by a factor of 2 (Fig. 3d, cyan line). The cyan line crosses mainly radiative up to 375 K, so we can assume that γ is the data points at 457 K. This value for T is very close to NR ½ negligible between 0 and 350 K. The value for η is the T of 462 K obtained from the PL intensity therefore approximated as a constant close to unity measurements. 4 4 between 0 and 350 K. On the other hand, the A → T Thermal quenching can be described as a thermally 2 2 absorption will change with temperature. Like the activated process with an activation energy ΔE. The 2 4 4 4 E→ A transition, the A → T transition is electric activation energy is obtained by fitting a modified 2 2 2 dipole (parity) forbidden and gains intensity by coupling Arrhenius equation to the temperature dependence of the Senden et al. Light: Science & Applications (2018) 7:8 Page 7 of 13 43,46 −1 PL intensity I between 350 and 600 K : quenching process is ~8000 cm . The rate constants A PL and 1/τ should be approximately equal to the vibra- NR IðÞ 0 I ðÞ T ¼ ð7Þ PL tional frequencies of the MnF group. The ν vibrational 1 þ A ´ expðÞ ΔE=k T B 12 −1 mode has a frequency of 6.5 × 10 s , close to the rate constants found by fitting the data to Eqs. 7 and 8. The In Eq. 7, I(0) is the maximum PL intensity, k is the variation in activation energy values and prefactors can be Boltzmann constant and A is a rate constant for the explained by the fact that thermal quenching is not a thermal quenching process. The best fit to Eq. 7 (green −1 simple thermally activated process. Struck and Fonger line in Fig. 3b) gives an activation energy ΔE of 9143 cm 12 have shown that the temperature dependence of a non- and a rate constant A of 2.5 × 10 . We can also determine 4+ radiative process is accurately described by considering ΔE by fitting the temperature dependence of the Mn 47 ground and excited state vibrational wave function over- emission lifetime τ(T) to the following expression : 46,48 lap . According to the Struck–Fonger model, the non- τ ðÞ T radiative process occurs through tunneling (crossover) τðÞ T ¼ ð8Þ τ ðÞ T from a vibrational level of the excited state to a high 1 þ expðÞ ΔE=k T NR vibrational level of the ground state. The tunneling rate, i.e., the non-radiative decay rate, depends on the wave Here, 1/τ is the non-radiative decay rate and τ (T)is NR R function overlap of the vibrational levels involved. The the radiative lifetime as described by Eq. 4 with τ (0) = −1 4+ tunneling rate will be faster for a larger overlap between 12.3 ms and hν = 216 cm .We fit Eq. 8 to the Mn the wave functions and when the vibrational levels are in emission lifetimes (green line in Fig. 3d) and find an −1 resonance. For the present discussion, analysis of the data activation energy ΔE of 7100 cm and a prefactor 1/τ NR 12 −1 using complex models such as the Struck–Fonger model of 1.5 × 10 s . On the basis of the two similar values for is not relevant, but it is important to realize that the ΔE, we conclude that the activation energy of the thermal ac 40 CT T 500 ΔE R R 20,750 21,250 21,750 22,250 4 4 –1 A T energy (cm ) 2 2 b d 40 Fluorides CT Oxides ΔE 0 100 R R 17,000 21,000 25,000 4 4 –1 A T energy (cm ) 2 2 4+ Fig. 4 Thermal quenching in Mn -doped fluorides. a, b Configuration coordinate diagrams showing luminescence quenching due to a − 4+ 4+ 4 thermally activated crossover via the F → Mn charge-transfer (CT) state and b thermally activated crossover via the Mn T excited state. c 4+ 4 4 Quenching temperature T of Mn -doped fluoride phosphors as a function of the A → T transition energy. The red dashed line is a linear fitto ½ 2 2 4+ 4+ 4 4 the data points. d Quenching temperature T of Mn -doped fluorides (blue dots) and Mn -doped oxides (red dots) as a function of the A → T ½ 2 2 transition energy 4 –1 4 –1 Energy (10 cm ) Energy (10 cm ) Quenching temperature T (K) Quenching temperature T (K) ½ Senden et al. Light: Science & Applications (2018) 7:8 Page 8 of 13 4 4 Struck–Fonger model gives a more correct description of crossing of the T and A states. The offset of the CT 2 2 the actual quenching process. state is typically larger than the offset of the T state. Note that the diagrams in Fig. 4a and b are schematic 4+ Thermal quenching in Mn -doped fluorides configuration coordinate diagrams to illustrate the dif- 4+ To obtain insight into the thermal quenching of Mn ferent quenching mechanisms. luminescence, we will discuss four possible quenching In Fig. 4a, the CT state has a larger offset ΔR than the 4 4 processes: (1) multi-phonon relaxation, (2) thermally T state, which causes the CT parabola to cross the A 2 2 activated photoionization, (3) thermally activated cross- parabola at lower energies than the T parabola. − 4+ over via the F → Mn charge-transfer (CT) state, and Thermal activation over the energy barrier ΔE will allow 4+ 4 2 (4) thermally activated crossover via the Mn T excited crossover from the E state into the CT state followed by state. non-radiative relaxation to the ground state via the In the configurational coordinate diagram, the parabolas crossing of the CT and A parabolas. Alternatively, 4+ 2 4 4+ of the Mn E and A states do not cross and lumi- thermal quenching of the Mn luminescence may 2 4 nescence quenching by crossover from the E to the A be due to the mechanism depicted in Fig. 4b. Here, the CT states is not possible (Fig. 4a). The A ground state may state has a smaller offset ΔR compared to that shown in however be reached by multi-phonon relaxation. In Mn Fig. 4a, and its potential curve is therefore at higher + −1 4 -doped fluorides more than 30 phonons of ~500 cm energies. In addition, the T state has a slightly larger 2 4 4 are needed to bridge the energy gap between the E and offset. As a result, the crossing of the T and A para- 2 2 4 49 A states . For such high numbers of phonons (p > 30), bolas is now at a lower energy and non-radiative relaxa- 4 4 it is unrealistic that non-radiative multi-phonon relaxa- tion will proceed via the crossing of the T and A 2 2 tion is responsible for thermal quenching (see Supple- parabolas. mentary Information for a more detailed discussion). The activation energies ΔE in the configuration coor- −1 Alternatively, the thermal quenching can be due to ther- dinate diagrams are ~8000 cm , similar to the ΔE values mally activated photoionization of an electron from the obtained from the temperature-dependent measurements. 4+ 2 Mn E state to the fluoride host conduction band. This indicates that both mechanisms in Fig. 4a, b can 4+ Thermally activated photoionization typically quenches explain the thermal quenching of Mn luminescence. To the emission from a luminescent center if the emitting determine which of these two mechanisms is responsible 26,50 state is close in energy to the host conduction band . for the luminescence quenching, we compare the 4+ quenching temperature T In density functional theory (DFT) calculations, large of K TiF :Mn to the T of ½ 2 6 ½ 4+ band gaps of around 8 eV have been found for fluoride other Mn -doped materials. A relation between the 51,52 hosts like K SiF and K TiF . It is therefore expected quenching temperature and the energy of either the CT or 2 6 2 6 4+ 2 4 that the Mn E state is well below the host conduction T state in a variety of hosts will give insight. If band levels. Based on this, we conclude that thermal quenching occurs by crossover from the CT state 4+ 4 4+ quenching in Mn -doped fluorides is not caused by to the A state, T will be higher for Mn -doped solids 2 ½ 4+ thermally activated photoionization. However, more evi- with higher CT transition energies. In K TiF :Mn and 2 6 4+ − 4+ dence is necessary to exclude this quenching mechanism. other Mn -doped fluorides the F → Mn CT transi- 4+ −113,15 4+ Photoconductivity measurements on Mn phosphors at tion is at ~40,000 cm .Mn -doped oxides have 2− 4+ elevated temperatures need to be performed to provide lower O → Mn CT transition energies of −1 convincing evidence for a possible role of photoionization 30,000–35,000 cm and are therefore expected to have 4+ in the thermal quenching of Mn emission. lower T values than fluorides if quenching occurs by the 4+ 26,27,53,54 4+ Thermal quenching in Mn -doped fluorides has been mechanism in Fig. 4a . Some Mn -doped oxides, suggested to occur by thermally activated crossover via however, have much higher quenching temperatures than 4+ 4 − 4+ 4+ 4+ the Mn T state or the F → Mn charge-transfer Mn -doped fluorides. For example, Mg GeO :Mn , 2 4 6 15,24,26 4+ 4+ (CT) state . Both these states are displaced relative Mg Ge O F :Mn , and Mg As O :Mn have a T 28 7.5 38 10 6 2 11 ½ 4 55–57 4+ 4 to the potential curve of the A ground state (Fig. 4a, b). of ~700 K , while K TiF :Mn and other Mn 2 2 6 4 4 + Hence, the T and CT state parabolas cross the A -doped fluorides have a T of 400–500 K (see also 2 2 ½ ground state parabola. The difference between the Tables 1 and 2). No correlation is found between the Mn potential curve equilibrium positions is given by the offset luminescence quenching temperature and the energy of 4 2 ΔR = R ′ − R . By using the energies of the A → E, the CT transition (see Supplementary Information for an 0 0 2 4 4 4 4+ A → T and A → CT transitions in K TiF :Mn overview and a plot of quenching temperatures and CT 2 2 2 2 6 (Fig. 2d and ref. ) and assuming specific offsets ΔR for energies). From this we conclude that thermal quenching 4 4+ the T and CT states, we can construct the diagrams in in Mn -doped fluorides is not caused by thermally − 4+ Fig. 4a and b, where non-radiative relaxation occurs either activated crossover from the F → Mn CT state to the 4 4 via (a) the crossing of the CT and A states or (b) the A ground state. 2 2 Senden et al. Light: Science & Applications (2018) 7:8 Page 9 of 13 4+ 4 4 Table 1 Quenching temperature T (K) and A → T Alternatively, thermal quenching of the Mn lumi- ½ 2 2 −1 4+ energy (cm ) for Mn -doped fluoride materials nescence can be caused by thermally activated crossover 4+ 4 via the Mn T excited state (Fig. 4b). To investigate the 4 4 −1 Host lattice A → T energy (cm ) T (K) References 2 2 ½ validity of this mechanism, we compare the T and 4 4 4+ A → T transition energies for K TiF :Mn and a 2 2 2 6 K TiF 21,459 462 This work 2 6 4+ variety of other Mn -doped fluorides. From the litera- K SiF 22,099 518 This work 2 6 4+ ture and measurements on Mn luminescence we have K SiF 22,120 490 15 2 6 collected quenching temperatures and luminescence K GeF 21,280 470 15 spectra, preferably for systems with low doping con- 2 6 4+ centrations. Figures 2d and 3b show that K TiF :Mn 2 6 K TiF 21,190 450 15 2 6 4 4 −1 has a A → T energy of 21,459 cm (maximum of the 2 2 K TiF 21,368 478 13 2 6 4+ excitation band) and a T of 462 K. For K SiF :Mn ,we ½ 2 6 4 4 −1 Na SiF 21,739 488 21 2 6 measured a A → T energy of 22,099 cm and a T of 2 2 ½ 4+ Rb SiF 21,739 480 18 518 K (Supplementary Figure S6, K SiF :Mn BR301-C 2 6 2 6 commercial phosphor from Mitsubishi Chemical, Japan). Rb TiF 21,186 450 18 2 6 In Fig. 4c we plot the quenching temperature T against Rb GeF 21,739 513 60 2 6 4 4 4+ 4+ the A → T energy for K TiF :Mn ,K SiF :Mn and 2 2 2 6 2 6 4+ Cs GeF 21,277 420 22 2 6 many other Mn -doped fluoride phosphors reported in Cs SiF 21,368 430 22 the literature (displayed data also listed in Table 1). The 2 6 data show that the T increases with the energy of the T ½ 2 Cs HfF 20,964 403 44 2 6 state. The clear trend shows that the thermal quenching BaSiF 21,322 430 23 4+ in Mn -doped fluorides is due to thermally activated 4 4 BaSnF 21,008 400 45 crossover from the T excited state to the A ground 2 2 BaTiF 21,142 425 61 state. Further confirmation for this quenching mechanism 4+ is provided by Mn spectra measured at elevated tem- peratures (see Supplementary Information). Supplemen- 4+ tary Figure S7 shows emission spectra of K SiF :Mn at 2 6 4 4 4 4 Table 2 Quenching temperature T (K) and A → T ½ 2 2 T = 573 and 673 K. At 573 K a broad T → A emission 2 2 −1 4+ energy (cm ) for Mn -doped oxide materials band is observed, which is almost completely quenched at 4 4 4 4 −1 673 K. The initial rise of the T → A emission at ele- 2 2 Host lattice A → T energy (cm ) T (K) References 2 2 ½ vated temperatures confirms thermal population of the Mg GeO 23,697 730 55 T level, which eventually leads to thermal quenching of 4 6 4+ all Mn emission via this state. Mg Ge O F 23,923 700 26,55,56 28 7.5 38 10 To investigate whether thermally activated crossing via K Ge O 21,739 373 62 2 4 9 the T state is also responsible for temperature 4+ K Ge O (site 1) 19,231 160 63 2 4 9 quenching in Mn -doped oxides, we extend the K Ge4O (site 2) 21,700 379 63 data set of Fig. 4c with quenching temperatures reported 2 9 4+ for Mn -doped oxides. Figure 4d shows the quenching Rb Ge O (site 1) 19,231 162 63 2 4 9 4 4 temperature T as a function of the A → T energy for ½ 2 2 Rb Ge O (site 2) 20,850 346 63 2 4 9 4+ the Mn -doped fluorides and oxides listed in Tables 1 Y Mg Ge O 23,753 850 64 2 3 3 12 and 2. The results show that T increases with the 4 4 La GaGe O 21,413 420 65 energy of the A → T transition. This indicates that the 3 5 16 2 2 4+ Mn emission in fluorides and oxides are both La ZnTiO 19,608 230 66 2 6 quenched due to thermally activated crossover from the La MgTiO 20,000 250 66 2 6 4 T excited state, and not the CT state as previously 24–27 CaZrO 18,500 300 25,26 suggested in some reports . The present results and 4+ Mg As O 23,810 680 57 analysis provide strong evidence that in many Mn 6 2 11 phosphors the thermal quenching mechanism involves Y Al O 20,619 300 67 3 5 12 thermally activated crossover via the T excited state. A Y Al O 20,833 300 68 3 5 12 contribution from other mechanisms cannot be ruled out Sr Al O 22,222 423 69 4 14 25 and further research, for example, photoconductivity SrLaAlO 19,231 300 53 measurements and high pressure studies, can give addi- tional information on the role of alternative quenching LiGa O 20,000 350 70 5 8 mechanisms. Senden et al. Light: Science & Applications (2018) 7:8 Page 10 of 13 a c 10 6.2 1.0 Quantum efficiency 6.0 0.8 –1 5.8 0 2.5 5 4+ 0.6 Mn (x %) Delay time (ms) 0.01% 0.1% 5.6 0.4 0.8% –2 1.3% 3.8% 5.4 0.2 5.4% Emission lifetime 9.4% 15.7% –3 10 5.2 0.0 0 4 8 12 16 020 40 4+ Mn concentration (%) Delay time (ms) b d e 0 0 0 10 10 10 4+ 4+ 4+ 0.8% Mn 15.7% Mn 15.7% Mn T = 298 K T = 298 K T = 4 K –1 –1 –1 10 10 10 –2 –2 –2 10 10 10 = 5.6 ms  = 5.4 ms  = 10.6 ms fit fit fit 020 40 020 40 04 20 0 60 80 Delay time (ms) Delay time (ms) Delay time (ms) 4+ 4+ Fig. 5 Luminescence decay and quantum efficiency of K TiF :Mn as a function of the Mn doping concentration. a Room-temperature PL 2 6 4+ 4+ decay curves of the Mn emission from K TiF :Mn (x%) for 0.01% (pink), 0.1% (blue), 0.8% (green), 1.3% (orange), 3.8% (purple), 5.4% (cyan), 9.4% 2 6 4+ 4+ (yellow), and 15.7% (red) Mn (λ = 450 nm and λ = 631 nm). b PL decay curve of K TiF :Mn (0.8%) at T = 298 K. The decay time exc em 2 6 4+ corresponding to the mono-exponential fit (red line) is 5.6 ms. The bottom panel shows the fit residuals. c Mn emission lifetime (blue squares) and 4+ 4+ 4+ PL quantum efficiency (red dots) of K TiF :Mn with different Mn doping concentrations. d, e PL decay curves of K TiF :Mn (15.7%) at d T = 298 2 6 2 6 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 4+ As quenching occurs by thermally activated crossover that there is a variation in ΔR for Mn -doped fluorides. via the T excited state, the quenching temperature T of The variation in ΔR is small, however, compared to the 2 ½ 4+ 4 the Mn luminescence is controlled by the energy of the differences in the T energy, and no correlation is 4+ 4 Mn T state (the dependence of T on the energy of observed between the spectral width and quenching 2 ½ 4 4 the T state is shown in Fig. 4c,d). In addition, the T of temperatures. This indicates that the T level energy has 2 ½ 2 4+ the Mn luminescence depends on the offset ΔR the largest influence on the quenching temperature of 4 4 4+ between the T and A states, as ΔR also determines Mn -doped fluorides. 2 2 4 4 where the T and A states cross in the configuration Finally, in view of applications, it is interesting to see 2 2 coordinate diagram (Fig. 4a,b). The horizontal displace- how we can control the T level energy (and thereby T ) 2 ½ ment of the T parabola will influence the quenching through the choice of the host lattice. The energy of the 4+ 4 temperature. A variation in ΔR can explain the spread Mn T state depends on the crystal field splitting Δ 2 O observed in the data of Fig. 4c and d. To investigate the (Fig. 2b), where Δ is typically larger for shorter Mn–F 4+ 44,58 4+ variation in the offset ΔR for Mn -doped fluorides, we distances . For Mn -doped fluorides the lumines- 4 4 compare the bandwidth of the A → T excitation band cence quenching temperature can therefore be raised by 2 2 4+ 4+ 4+ 4+ − in K TiF :Mn ,K SiF :Mn and Cs HfF :Mn selecting host lattices with short M –F distances 2 6 2 6 2 6 (see Supplementary Figure S9). The width of the (see Supplementary Figure S10a). This is consistent with 4 4 4+ A → T excitation band is controlled by the displace- findings that T increases if the radius of the M host 2 2 ½ ment of the T state and therefore gives a good indication cation decreases, as expected based on crystal field the- 4 4 11,18 4+ of ΔR. Comparison of the A → T bandwidths shows ory . If, however, T is plotted against the M -ligand 2 2 ½ Normalized intensity Normalized intensity Residuals Normalized intensity Residuals Lifetime (ms) Normalized intensity Residuals Quantum efficiency Senden et al. Light: Science & Applications (2018) 7:8 Page 11 of 13 4+ 4+ 4+ distance for both Mn -doped fluorides and Mn -doped transfer for Mn ions close to a quencher. In case of oxides (see Supplementary Figure S10b), no correlation energy migration, a faster decay is also expected for longer 4+ between T and the M -ligand distance is found. This times after the excitation pulse. As this is not observed, 4 4+ shows that the crystal field splitting and T energy give a the contribution of energy migration via many Mn ions better indication of the quenching temperature for Mn to quenching sites seems to be small. -doped phosphors. To further investigate the role of energy migration in 4+ the concentration quenching of the Mn emission, we 4+ Concentration quenching measure a PL decay curve of K TiF :Mn (15.7%) at T = 2 6 In addition to insight into thermal quenching, con- 4 K, which is displayed in Fig. 5e. At T = 4 K energy 4+ 4+ centration quenching in Mn -doped fluorides is impor- migration among the Mn ions (blue arrows in Fig. 1) tant for application in w-LEDs. The weak parity-forbidden will be hampered, as there is almost no spectral overlap 4 4 4+ 2 4 4 2 A → T absorption requires that commercial phos- between the Mn E→ A emission and A → E 2 2 2 2 4+ phors have high Mn concentrations. If there is effective excitation lines (see Supplementary Figure S11). Hence, at concentration quenching, the PL decay time and QE will 4 K non-radiative decay due to energy migration to 4+ 4+ decrease when the Mn doping concentration is quenching sites will be suppressed. The Mn decay 26,28 raised . We therefore investigate concentration dynamics in Fig. 5e, however, show that the non-radiative 4+ quenching in K TiF :Mn by measuring the PL decay decay is not suppressed at 4 K. The deviation from single 2 6 4+ 4+ times and QEs of K TiF :Mn phosphors with Mn exponential behavior is similar to that at 300 K. There is 2 6 4+ concentrations ranging from 0.01 to 15.7% Mn . an initial faster decay (single-step energy transfer to Figure 5a presents room-temperature PL decay curves quenching sites) followed by an exponential decay with a 4+ 4+ 4+ of the Mn emission from K TiF :Mn with increasing decay time very close to that measured for Mn at low 2 6 4+ Mn doping concentration x. It can be seen that the PL doping concentrations. This suggests that the decrease in 4+ 4+ decay becomes slightly faster as the Mn concentration QE at higher Mn concentrations is not due to energy increases. We analyze the decay dynamics by single migration. The absence of strong concentration quench- exponential fitting of the PL decay curves. The fit for ing by energy migration is confirmed by the thermal 4+ 4+ K TiF :Mn (0.8%) is shown in Fig. 5b. The fit residuals quenching behavior measured for the different Mn 2 6 (bottom panel) are random and the PL decay thus concentrations. In Supplementary Figure S4, it can be resembles a single exponential. This indicates that the seen that the luminescence quenching temperature is decay of the approximately the same for doping concentrations of E state is mainly radiative. Consequently, the 4+ 4+ K TiF :Mn (0.8%) phosphor has a very high QE of 90%. 0.01% and 15.7% Mn , which shows that effects due to 2 6 Figure 5c gives an overview of the fitted decay times (blue thermally activated energy migration (i.e., concentration 4+ squares) and QEs (red dots) of K TiF :Mn with differ- quenching) are weak. Hence, we conclude that the non- 2 6 4+ 4+ ent Mn concentrations. The emission lifetime barely radiative decay at high Mn concentrations is not caused 4+ shortens if the Mn concentration is increased (5.7 ms by energy migration. Inefficient energy migration can be 4+ 4+ for 0.01% Mn to 5.4 ms for 15.7% Mn ). This suggests understood based on the strongly forbidden character of 2 4 4+ 4+ that energy migration to quenching sites is inefficient in the E→ A transition. This allows only Mn –Mn 4+ K TiF :Mn . To verify this, we look at the QE values energy transfer via short range exchange interaction 2 6 4+ obtained for the K TiF :Mn (x%) phosphors. The QE (see Supplementary Information for details). 2 6 4+ remains above 80% for Mn doping concentrations of 5% We instead assign the non-radiative decay to direct 4+ or less, which shows that concentration quenching is transfer of excitation energy from Mn ions to quench- 4+ indeed limited up to a concentration of 5% Mn ions. ers (green arrow in Fig. 1). This process can occur at all 4+ This result is important for applications in w-LEDs, as temperatures and becomes more efficient at higher Mn 4+ 4+ these high Mn doping concentrations (e.g., 5 mol%) are dopant concentrations. With an increasing Mn dopant required for sufficient absorption of the blue LED light in concentration, the stress on the K TiF lattice grows and 2 6 the parity-forbidden d–d transitions . as a result more crystal defects (i.e., quenchers) may be 4+ 2+ For higher Mn concentrations (x > 10%), non- formed. In addition, Mn in different valence states (Mn 2 3+ 4+ radiative decay from the E excited state becomes stron- and Mn ) may be incorporated at higher Mn con- 4+ 4+ ger, however, and as a result the QE of K TiF :Mn falls centrations. Even if a very small fraction of Mn ions has 2 6 below 60% (Fig. 5c). The non-radiative decay is also visible a different valence state than 4+, effective quenching can 4+ in the PL decay curve of K TiF :Mn (15.7%), shown in occur via metal-to-metal charge-transfer states or direct 2 6 Fig. 5d. The decay is multi-exponential, which proves that energy transfer. Consequently, the probability for energy 4+ 2 with 15.7% Mn the E state decays both radiatively and transfer to quenchers increases, resulting in faster initial 4+ 4+ non-radiatively. The faster initial decay indicates that PL decay and lower QEs for K TiF :Mn at high Mn 2 6 there is enhanced quenching by single-step energy dopant concentrations. Optimized synthesis procedures Senden et al. Light: Science & Applications (2018) 7:8 Page 12 of 13 to reduce quenchers (defects and impurity ions) are thus Conflict of interest 4+ The authors declare that they have no conflict of interest. crucial for obtaining highly luminescent Mn -doped fluoride phosphors (see also recent work of Garcia- Supplementary information is available for this paper at https://doi.org/ Santamaria et al. on concentration quenching in K SiF : 2 6 10.1038/s41377-018-0013-1. 4+ Mn ). Received: 12 October 2017 Revised: 21 February 2018 Accepted: 7 March 2018 Accepted article preview online: 13 March 2018 Conclusions 4+ Narrow-band red-emitting Mn phosphors form an important new class of materials for LED lighting and displays. For these applications, it is important to under- References stand and control the luminescence efficiency. We have 1. The Economist. Charge of the LED brigade: a global switch to LEDs will 4+ change the lighting business. 20 Aug (2011). therefore investigated quenching of the Mn lumines- 4+ 2. Krames,M.R.etal. Status andfutureof high-power light-emitting diodes for cence in Mn -doped fluorides by measuring the PL solid-state lighting. J. Disp. Technol. 3,160–175 (2007). 4+ intensity and luminescence lifetimes of K TiF :Mn 2 6 3. Harbers, G., Bierhuizen, S. J. & Krames, M. R. Performance of high power light 4+ emitting diodes in display illumination applications. J. Disp. Technol. 3,98–109 between 4 and 600 K and for Mn concentrations from (2007). 0.01 to 15.7%. Temperature-dependent measurements of 4. Setlur,A.A.Phosphors forLED-based solid-state lighting. Electrochem. Soc. 4+ 4+ the Mn emission intensity and lifetime for K TiF :Mn 2 6 Interface 18,32–36 (2009). 4+ 5. Smet,P.F., Parmentier, A. B.& Poelman, D. 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Light: Science & ApplicationsSpringer Journals

Published: May 23, 2018

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