This is an open access article published under an Acs AuthorChoice License, which permitscopying and redistribution of the article or any adaptations for non-commercial purposes.ACSArticleOMEGACiteThis:ACSOmega2018,3,13704137http://pubs.acs.org/journal/acsodfLarge Negative-Thermal-Quenching Effect in Phonon-lnducedLightEmissions in Mn4+-Activated Fluoride PhosphorforWarm-WhiteLight-EmittingDiodesFei Tang,+ Zhicheng Su,t!l Honggang Ye,t,I Wenpei Gao, Xiaoqing Pan, and Shije Xu*,tDepartment of Physics and Shenzhen Institute of Research and Innovation (HKU-SIRI), The University of Hong Kong, PokfulamRoad, Hong Kong, China+Department of Materials Science & Engineering, University of California-Irvine, Irvine, California 92697, United StatesABSTRACT:Currently, hunting for anti-temperature-degra-dation high-efficiency phosphors has become crucially(ne)snaoOKsignificant for fabricating high-brightness phosphor-converted.4white light-emitting diodes (pc-WLEDs).Herein, we showthat photoluminescence in a kind of full-solution-processed1K,SiF:Mn* red phosphor exhibits an extraordinarily largenegative thermal quenching property.For instance, under theexcitation of 477 nm laser light, the sample photo-luminescence intensity amazingly increases by 347-foldwhen thetemperature is increased from 4 to 477K Thetemperature-driven transition probability enhancement of theWavelength(nmphonon-induced luminescence around Mn++ ions in thephosphor is argued to be responsible for the large negative-thermal-quenching phenomenon.We also demonstrate a pcWLED with R,of 82 and correlated color temperature of 2701 Kby using theK,SiF,:Mnred phosphor + commercial yellowphosphor of YAG:Ce*+aaHNNthat does not exhibit thermal quenching even up to 200 °C.13INTRODUCTIONThe zero-thermal-quenching phenomenon is explained bySolid-state light-emitting diode (LED) lighting technologythemaspolymorphicmodificationandpossibleenergytransfertriggered by the discovery of efficient blue LEDs offers theSNONhsS IARSh aSfrom electron-hole pairs at the thermally activated defectmost promising change in lighting technology since Edisonlevels to the Eu2+ Sd-band with increasing temperature. Thiscreatedthefirstcommercializedelectricallightbulbover130important finding also inspires the exploration of convertingyears ago.2In sharp contrast to the electricai"hot" lightbulbs,phosphors with a zero-thermal-quenching and even a negative-emerging LED lightbulbs belong to so-called “cold lightthermal-quenching property for high-power LED applications.sources.Great energy saving and environmental friendship arepepeoIn recent years, Mn+-activated inorganic phosphors havethe most favored advantages of LED-based cold lightbulbs.quickly emerged as a new class of promising phosphors forAmong the already-demonstrated white light LED bulbs,2fabricating pc-WLED because of their high-efficiency narrowInGaN blue LED chip + high-efficiency luminescent phosphorred-light emissions.14-23 Among them is cubic K,SiFg:Mn*forms a mainstream module, and has been even speciallyphosphor, which can be synthesized at -16 C with a full-termed as phosphor-converted white LED (pc-WLED).Insolution chemical route,22 showing an outstanding property ofsuch a module, phosphors play a crucial role in determiningphonon-induced dominant luminescence at room temper-the performanceand qualityof pe-WLED.In turn,theature.25 In the present study,we report an observation anddevelopment of pc-WLED lightbulbs puts forward greaterargument on large negative-thermal-quenching phenomenondemands on phosphors for luminescence quantum efficiencyin the cubic K,SiFg:Mn+phosphor for temperature variationand simultaneous thermal stability.Unfortunately,almost allfrom 4to477K, when it was under the excitation of 477nmthe existing solid luminescent materials suffer from severenonintensive laser light. Such an amazing property makes thedegradation in emission intensity and even thermal quenchingphosphor be disruptively different from the most luminescencewith the rise of temperature.Such thermal deterioration ofsolids whose luminescence intensities usually weaken with theemission intensity is mainly ascribed to the rapid increase ofrise of temperature and be appealing as a kind of thermalnonradiative multiphonon transition probability with temper-energy (phonons) converting phosphor for luminescence atature. Presently, hunting for high-efficiency and anti-temper-ature-degradation phosphors has become a key challenge forfabricating high-brightness pe-WLED lightbulbs.12,13VeryReceived:June18,2018recently,Kim and his co-workers reported a zero-thermal.Acepted:October9,2018quenching blue phosphor of Nas-2Sc2(PO)3:xEu**phosphorPublished:October 19, 201813704ACsPublications@ 2018 American Chemical SocietyDOt:10.1021/acsoeoa.8b0112ACSOmeqa2018,3,13704-13710
Large Negative-Thermal-Quenching Effect in Phonon-Induced Light Emissions in Mn4+-Activated Fluoride Phosphor for Warm-White Light-Emitting Diodes Fei Tang,†,§ Zhicheng Su,†,∥ Honggang Ye,†,⊥ Wenpei Gao,‡ Xiaoqing Pan,‡ and Shijie Xu*,† † Department of Physics and Shenzhen Institute of Research and Innovation (HKU-SIRI), The University of Hong Kong, Pokfulam Road, Hong Kong, China ‡ Department of Materials Science & Engineering, University of California−Irvine, Irvine, California 92697, United States ABSTRACT: Currently, hunting for anti-temperature-degradation high-efficiency phosphors has become crucially significant for fabricating high-brightness phosphor-converted white light-emitting diodes (pc-WLEDs). Herein, we show that photoluminescence in a kind of full-solution-processed K2SiF6:Mn4+ red phosphor exhibits an extraordinarily large negative thermal quenching property. For instance, under the excitation of 477 nm laser light, the sample photoluminescence intensity amazingly increases by 347-fold when the temperature is increased from 4 to 477 K. The temperature-driven transition probability enhancement of the phonon-induced luminescence around Mn4+ ions in the phosphor is argued to be responsible for the large negative-thermal-quenching phenomenon. We also demonstrate a pcWLED with Ra of 82 and correlated color temperature of 2701 K by using the K2SiF6:Mn4+ red phosphor + commercial yellow phosphor of YAG:Ce3+. ■ INTRODUCTION Solid-state light-emitting diode (LED) lighting technology triggered by the discovery of efficient blue LEDs offers the most promising change in lighting technology since Edison created the first commercialized electrical lightbulb over 130 years ago.1,2 In sharp contrast to the electrical “hot” lightbulbs, emerging LED lightbulbs belong to so-called “cold” light sources. Great energy saving and environmental friendship are the most favored advantages of LED-based cold lightbulbs.3 Among the already-demonstrated white light LED bulbs, InGaN blue LED chip + high-efficiency luminescent phosphor forms a mainstream module, and has been even specially termed as phosphor-converted white LED (pc-WLED).4 In such a module, phosphors play a crucial role in determining the performance and quality of pc-WLED. In turn, the development of pc-WLED lightbulbs puts forward greater demands on phosphors for luminescence quantum efficiency and simultaneous thermal stability. Unfortunately, almost all the existing solid luminescent materials suffer from severe degradation in emission intensity and even thermal quenching with the rise of temperature.5−11 Such thermal deterioration of emission intensity is mainly ascribed to the rapid increase of nonradiative multiphonon transition probability with temperature. Presently, hunting for high-efficiency and anti-temperature-degradation phosphors has become a key challenge for fabricating high-brightness pc-WLED lightbulbs.12,13 Very recently, Kim and his co-workers reported a zero-thermalquenching blue phosphor of Na3−2xSc2(PO4)3:xEu2+ phosphor that does not exhibit thermal quenching even up to 200 °C.13 The zero-thermal-quenching phenomenon is explained by them as polymorphic modification and possible energy transfer from electron−hole pairs at the thermally activated defect levels to the Eu2+ 5d-band with increasing temperature. This important finding also inspires the exploration of converting phosphors with a zero-thermal-quenching and even a negativethermal-quenching property for high-power LED applications. In recent years, Mn4+-activated inorganic phosphors have quickly emerged as a new class of promising phosphors for fabricating pc-WLED because of their high-efficiency narrow red-light emissions.14−23 Among them is cubic K2SiF6:Mn4+ phosphor, which can be synthesized at −16 °C with a fullsolution chemical route,22 showing an outstanding property of phonon-induced dominant luminescence at room temperature.25 In the present study, we report an observation and argument on large negative-thermal-quenching phenomenon in the cubic K2SiF6:Mn4+ phosphor for temperature variation from 4 to 477 K, when it was under the excitation of 477 nm nonintensive laser light. Such an amazing property makes the phosphor be disruptively different from the most luminescence solids whose luminescence intensities usually weaken with the rise of temperature and be appealing as a kind of thermal energy (phonons) converting phosphor for luminescence at Received: June 18, 2018 Accepted: October 9, 2018 Published: October 19, 2018 Article Cite This: http://pubs.acs.org/journal/acsodf ACS Omega 2018, 3, 13704−13710 © 2018 American Chemical Society 13704 DOI: 10.1021/acsomega.8b01127 ACS Omega 2018, 3, 13704−13710 This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Downloaded via SHANDONG UNIV OF TECHNOLOGY on December 28, 2022 at 04:38:56 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles
AcsOmegaArticlehigh temperatures. Together with the commercial yellow(a)phosphor of YAG:Ce3+, this kind of narrow red phosphor isutilizedtoconverta 450nmInGaNbluechipintoawarmwhiteLEDwithgoodperformance.RESULTSANDDISCUSSIONAs mentioned earlier, cubic K,SiFg:Mn++ phosphor wasnessynthesized at -16 °C with a full-solution approach. The0detailed synthesis procedure has been described elsewhere.2.5For crystalline structure characterization on the synthesized2.01.5-phosphor, precise X-ray diffraction patterns and analysis can10also be referred to in ref 24. Here, we present high-resolution0.5Stokestransmission electron microscopy (HRTEM)characterizationZpLAnti-Stokes0克1.90on the local microstructures of the K,SiFg:Mntphosphor, as1.952.002.052.10shown in Figure la.By selecting an area of about 160 × 160Energy (ev)(ba3.62K200K300333363H393K423K477K5nm1nmFigure 2. (a)PL spectra of the phosphor measured at differenttemperatures under the excitation of 477nm laser light;(b)PLphotographs of the sampleat various temperatures.excitation light by the powder sample to dazzling red becauseof amazing enhancement of the red emission of the sample. Asseen from the PL spectra, instead of the magnetic-dipoletransition-induced zero-phonon line(ZPL),25 the luminescence (phonon sidebands)around the Mn*+ion by the1nm1nmelectric-dipole transitions is always dominant in the PL spectraof the phosphor.It has been identified that the vibronic statesFigure 1. (a) HRTEM image of the K,SiF:Mn** phosphor. The insetarising from the coupling ofE,excited state of Mnt+ions withshows the selected area electronic diffraction patterns. (b-d) enlargedthe three triplet odd-parity vibration modes (yyV, and v)ofHRTEM regions (IIII) in (a), respectively, and their correspondingthe host lattice are responsible for the three main emissionFFT patterns.bands including both Stokes and anti-Stokes bands.22,24,26Disappearance of the ZPL line reflects both the spin-andparity-forbidden nature of the electronic transitions betweennm2 as the diffraction region, we completed an electronEandAzelectronicstatesof Mn++ionsinacubicKSiFdiffraction experiment. The inset shows the electron diffractionpatterns composed of spotted circles, indicating the crystallinelattice. It is known that in a perfect K,SiF。 single crystal withcubic structure, a Si4+ ion is tightly coordinated with six nearestnatureof thephosphor.Inordertoobtainacloserinspectionon the lattice structures of the crystalline phosphor, threeF-ions,constituting aperfectoctahedralstructurewithlocalinversion symmetry of point group O,26,27 When Mn atomsregions [e-g., (1), (I1), and (I11)] in Figure la were enlargedare intentionally incorporated into this host matrix to replaceand re-illustrated in (b), (c), and (d), respectively. Theircorresponding fast Fourier transformation (FFT)patterns areSi, a new octahedral structure of [MnF.J2-with some latticedepicted at the bottom corners of the respective figuresdistortion but remaining the inversion symmetry forms.The new octahedral structure of [MnF.j2-may belong to aSeveral lattice plane spacings and lattice structural angles were-32 In the host crystal, [MnF.]2- can actdetermined, as shown in Figure lb-d. In spite of the limitedpointgroupof Daas an eficient luminescence center.12,20,1Probably, anresolution, the cubic lattice structure of the phosphor can becertainly verified in the local enlarged HRTEM image, asoutstandingpropertyof this luminescencecenteristhatitsemission intensity rises so strongly with increasing themarked in Figure lc.Figure 2a shows photoluminescence(PL) spectra of thetemperature, as seen in Figure 3a where the overall integratedK,SiFg:Mnt+phosphor measured at different temperatures.APL intensity (solid circles +dashed line)versus temperature isremarkable increase of the red emission intensity withillustrated. For example, the total emission intensity amazinglytemperature may be more straightforwardly seen in photo-rises by ~347-fold when the temperature is increased from 4 tographs in Figure 2b.For the temperature increase from 4 to477KThe blue solid line represents a theoretical fitting curve477K,the color of photographs changes from bright bluewith eq 1, which is an analytical expression approximatelybased on relevant theoretical models26.3-36largely because of the Rayleigh scattering of the 477 nm13705DOt10.1021/acsonega.8b01127ACS Omega 2018, 3, 1370413710
high temperatures. Together with the commercial yellow phosphor of YAG:Ce3+, this kind of narrow red phosphor is utilized to convert a 450 nm InGaN blue chip into a warm white LED with good performance. ■ RESULTS AND DISCUSSION As mentioned earlier, cubic K2SiF6:Mn4+ phosphor was synthesized at −16 °C with a full-solution approach. The detailed synthesis procedure has been described elsewhere.22 For crystalline structure characterization on the synthesized phosphor, precise X-ray diffraction patterns and analysis can also be referred to in ref 24. Here, we present high-resolution transmission electron microscopy (HRTEM) characterization on the local microstructures of the K2SiF6:Mn4+ phosphor, as shown in Figure 1a. By selecting an area of about 160 × 160 nm2 as the diffraction region, we completed an electron diffraction experiment. The inset shows the electron diffraction patterns composed of spotted circles, indicating the crystalline nature of the phosphor. In order to obtain a closer inspection on the lattice structures of the crystalline phosphor, three regions [e.g., (I), (II), and (III)] in Figure 1a were enlarged and re-illustrated in (b), (c), and (d), respectively. Their corresponding fast Fourier transformation (FFT) patterns are depicted at the bottom corners of the respective figures. Several lattice plane spacings and lattice structural angles were determined, as shown in Figure 1b−d. In spite of the limited resolution, the cubic lattice structure of the phosphor can be certainly verified in the local enlarged HRTEM image, as marked in Figure 1c. Figure 2a shows photoluminescence (PL) spectra of the K2SiF6:Mn4+ phosphor measured at different temperatures. A remarkable increase of the red emission intensity with temperature may be more straightforwardly seen in photographs in Figure 2b. For the temperature increase from 4 to 477 K, the color of photographs changes from bright blue largely because of the Rayleigh scattering of the 477 nm excitation light by the powder sample to dazzling red because of amazing enhancement of the red emission of the sample. As seen from the PL spectra, instead of the magnetic-dipole transition-induced zero-phonon line (ZPL),25 the luminescence (phonon sidebands) around the Mn4+ ion by the electric-dipole transitions is always dominant in the PL spectra of the phosphor. It has been identified that the vibronic states arising from the coupling of 2 Eg excited state of Mn4+ ions with the three triplet odd-parity vibration modes (ν3, ν4, and ν6) of the host lattice are responsible for the three main emission bands including both Stokes and anti-Stokes bands.22,24,26 Disappearance of the ZPL line reflects both the spin- and parity-forbidden nature of the electronic transitions between 2 Eg and 4 A2g electronic states of Mn4+ ions in a cubic K2SiF6 lattice. It is known that in a perfect K2SiF6 single crystal with cubic structure, a Si4+ ion is tightly coordinated with six nearest F− ions, constituting a perfect octahedral structure with local inversion symmetry of point group Oh. 26,27 When Mn atoms are intentionally incorporated into this host matrix to replace Si, a new octahedral structure of [MnF6] 2− with some lattice distortion but remaining the inversion symmetry forms.26−29 The new octahedral structure of [MnF6] 2− may belong to a point group of D4h. 29−32 In the host crystal, [MnF6]2− can act as an efficient luminescence center.12,20,21 Probably, an outstanding property of this luminescence center is that its emission intensity rises so strongly with increasing the temperature, as seen in Figure 3a where the overall integrated PL intensity (solid circles + dashed line) versus temperature is illustrated. For example, the total emission intensity amazingly rises by ∼347-fold when the temperature is increased from 4 to 477 K. The blue solid line represents a theoretical fitting curve with eq 1, which is an analytical expression approximately based on relevant theoretical models26,33−36 Figure 1. (a) HRTEM image of the K2SiF6:Mn4+ phosphor. The inset shows the selected area electronic diffraction patterns. (b−d) enlarged HRTEM regions (I−III) in (a), respectively, and their corresponding FFT patterns. Figure 2. (a) PL spectra of the phosphor measured at different temperatures under the excitation of 477 nm laser light; (b) PL photographs of the sample at various temperatures. ACS Omega Article DOI: 10.1021/acsomega.8b01127 ACS Omega 2018, 3, 13704−13710 13705
AcsOmegaArticle o sa peaon 300Ka5-ExperimentalDataFitting Curves270KAe=477nm30.(n'e) Kaisuud2180K25120KRisingby~347times100K20+70K40K10K15-105OQ9010001002003004005000476480484468472488492496Temperature (K)Wavelength (nm)(b)Figure4.PLEspectrameasured inthe spectral range of 467-496nmT=300Katvarioustemperatures.Clearly, the PLE intensity at 477 nmMax.@97.6%100increases significantly with the rise of temperature.o(a)(b)8OK.OF40Mn*20Intermuclear Sepa3(c)1A6740SampleNo.Figure 3.(a)Total PL integrated intensities (solid circles + solid line)vs temperature;(b)PL QYsof the sample measured at roomOtemperature.V3CCV4Veexp(-△E/kgT)P(T) = C_(d)exp(h,/kgT) - 1(1)100K300150K200Kwhere C is a constant, E an energy separation with respect to250Kthe ZPL, hoy an average characteristic energy of involved293Kphonons,andkgtheBoltzmann constant.In fact,1/[exp(hoy/k.T)-1lin eg1represents theBose-Einstein distribution of-200the involved phonons.-300In order to understand the remarkable enhancement of thePL intensity in the interested temperature range, we measured2800300032003400360038004000MagneticField (Gauss)the PL excitation (PLE) spectra of the sample at differenttemperatures, as shown in Figure 4. It is obvious that the PLEFigure 5. (a) Schematic diagram of a face-centered cubic structureintensity at 477 nm increases substantially with increasing thecontaining an octahedral luminescence center of Mn*, Red-lighttemperature.The consistent enhancement in both PL and PLEemission occurs around Mntions under the excitation of a 477 nmsignals unambiguously indicates the occurrence of a negative-blue laser beam. (b)Configurational coordinate diagram for radiativetransitions around Mn4+ ions in a K,SiFhost crystal. (c) Threethermal-quenching effect in the studied K,SiFg:Mnt+ phos-dominant phonon modes engaged in the electron-phonon couplingphor. The major mechanism of the negative-thermal-andvibronictransitions.(d)EPR spectraofK,SiF:Mn*measured atquenching effect in the studied K,SiFg:Mn*+ phosphor withvarioustemperatures.cubic structure shall be the nature of phonon-induced radiativetransitions at Mntt ions which strictly occupy the inversioncenter with surrounding six nearest F- ions, as shown in Figuretransitions, some phonons with particular symmetries (ie.,Sa. Such central inversion symmetry directly leads to amodes)play an essential role.On thebasis of availabletheoretical studies,2633-36forbidden pure"E→*Azg transition of Mn*+ ions accordingtowederived an approximatedthe Laporte rule. This rule states that the d-d electronicexpression for the transition probability of vibronic transitions,transitions in atoms in a centrosymmetric environment areas formulated by egl.This expression tells us that theelectric-dipole-forbidden, which applies to octahedral coordi-transition probability depends distinctively on temperature,nation compounds of the transition metals.37 A zero orthat is, roughly exponentially grows with temperature andextremelyweakZPL line is indeed observed in theexperiment.hence thermal energy (e.g,kgT). As shown in Figure 3a,On the contrary,the phonon-assisted Eg-→Azg transitionsremarkable enhancement is indeed observed for the total PLintensity of the K,SiFg:Mnt+ phosphor as the temperature(vibronic transitions)may become dominant.In such vibronic13706DOt:10.1021/acsomega.8b01127ACS Omega 2018, 3, 1370413710
ω = −Δ ℏ − P T C E kT k T ( ) exp( / ) exp( / ) 1 B v B (1) where C is a constant, ΔE an energy separation with respect to the ZPL, ℏωv an average characteristic energy of involved phonons, and kB the Boltzmann constant. In fact, 1/[exp(ℏωv/ kBT) − 1]in eq 1 represents the Bose−Einstein distribution of the involved phonons. In order to understand the remarkable enhancement of the PL intensity in the interested temperature range, we measured the PL excitation (PLE) spectra of the sample at different temperatures, as shown in Figure 4. It is obvious that the PLE intensity at 477 nm increases substantially with increasing the temperature. The consistent enhancement in both PL and PLE signals unambiguously indicates the occurrence of a negativethermal-quenching effect in the studied K2SiF6:Mn4+ phosphor. The major mechanism of the negative-thermalquenching effect in the studied K2SiF6:Mn4+ phosphor with cubic structure shall be the nature of phonon-induced radiative transitions at Mn4+ ions which strictly occupy the inversion center with surrounding six nearest F− ions, as shown in Figure 5a. Such central inversion symmetry directly leads to a forbidden pure 2 Eg → 4 A2g transition of Mn4+ ions according to the Laporte rule. This rule states that the d−d electronic transitions in atoms in a centrosymmetric environment are electric-dipole-forbidden, which applies to octahedral coordination compounds of the transition metals.37 A zero or extremely weak ZPL line is indeed observed in the experiment. On the contrary, the phonon-assisted 2 Eg → 4 A2g transitions (vibronic transitions) may become dominant. In such vibronic transitions, some phonons with particular symmetries (i.e., modes) play an essential role. On the basis of available theoretical studies,26,33−36 we derived an approximated expression for the transition probability of vibronic transitions, as formulated by eq 1. This expression tells us that the transition probability depends distinctively on temperature, that is, roughly exponentially grows with temperature and hence thermal energy (e.g., kBT). As shown in Figure 3a, remarkable enhancement is indeed observed for the total PL intensity of the K2SiF6:Mn4+ phosphor as the temperature Figure 3. (a) Total PL integrated intensities (solid circles + solid line) vs temperature; (b) PL QYs of the sample measured at room temperature. Figure 4. PLE spectra measured in the spectral range of 467−496 nm at various temperatures. Clearly, the PLE intensity at 477 nm increases significantly with the rise of temperature. Figure 5. (a) Schematic diagram of a face-centered cubic structure containing an octahedral luminescence center of Mn4+. Red-light emission occurs around Mn4+ ions under the excitation of a 477 nm blue laser beam. (b) Configurational coordinate diagram for radiative transitions around Mn4+ ions in a K2SiF6 host crystal. (c) Three dominant phonon modes engaged in the electron−phonon coupling and vibronic transitions. (d) EPR spectra of K2SiF6:Mn4+ measured at various temperatures. ACS Omega Article DOI: 10.1021/acsomega.8b01127 ACS Omega 2018, 3, 13704−13710 13706
AcsOmegaArticle(b)(a)oex=0.69270.6.83y=0.3072TAK160.2S80620600540660Wavelength (nm)2.9(d)(c)o(n) nsuau ns0.7X=0.475006房3y0.1977TA'KaT-1050K0.2oo380780480S680Wavelength (nm)(e)(f)(ne)au0.7x=0.47100y=0.43078Te=27011380INISNIWavelength (nm)Figure 6. (a) Room-temperature PL spectrum of the K,SiF:Mn*+ phosphor alone. (b) CIE chromaticity coordinates of the PL spectrum in (a). (c)Room-temperatureluminescence spectrum of a pe-wLED consistingof an InGaN blue LED and K,SiF:Mnt+ nanophosphor.(d)CIEchromaticity coordinates of the luminescence spectrum in (c). (e) Room-temperature luminescence spectrum of a pc-WLED made of an InGaNblue LED and the mixture of K,SiFg:Mn*+ red phosphor + commercial YAG:Ce' yellow phosphor. (f) CIE chromaticity coordinates of theluminescence spectrum in (e). Note that the driving electric current of the pc-WLEDs was 60 mA.increases. Moreover, good agreement between theory andvibronic states, a widely adopted picture, namely configura-tionalcoordinatemodel, isusuallyusedtodiscussthemoexperiment is achieved. In fact, temperature-induced enhance-ment of vibronic transitions has been repeatedly observed inschematically shown in Figure Sb. In the figure, the dashedthe luminescence of analogous Cr3+ ions in garnet crystal,38vertical line represents a pure electronic transition which mayphosphor39 and transparent ceramics.25produce the so-called ZPL line. In the studied case, such aWe randomly selected eight K,SiF,:Mn++ specimens and"pure" electronic transition is a forbidden transition processmeasured theirPL quantumyields (QYs)at roomtemper-becauseof both spin and parity transition rules.The bold redature. The results are illustrated in Figure 3b. It can be seenarrow denotes the Stokes radiative transition processthat all eight specimens haveQYvaluesabove80%, and theaccompanying the phonon emission, whereas the solid bluebest one even higher than 97%.arrow stands for the anti-Stokes radiative transition in whichphonon absorption is involved. For the [MnF.]?- octahedralAs argued earlier and schematically illustrated in Figure Sa, acentral Mn++ ion tightly binds with six nearest F- ions to formconfiguration, there are totally six main vibration modes,an octahedral luminescence center [MnFJ2- in the cubicamong which only three engage in the vibronic transitions.K,SiFg:Mnt* phosphor. Under the excitation of an appropriateFigure Sc schematically illustrates the three vibration modes,light, for example, 477 nm blue laser, such a luminescencethat is, V,V4, and V.From the measured PL spectra,center can become excited via absorbing the incident photonscharacteristic energies of phonons V6 Va, and V may beand then emit red light by releasing the absorbed energy. Fordetermined to be about 28.5, 41.4, and 78.7 meV, respectively.For the electronic part of the vibronic states,3d3 electrons ofthe absorptive and emissive optical transitions between the13707DOt:10.1021/acsomega.8b01127ACS Omega 2018, 3, 1370413710
increases. Moreover, good agreement between theory and experiment is achieved. In fact, temperature-induced enhancement of vibronic transitions has been repeatedly observed in the luminescence of analogous Cr3+ ions in garnet crystal,38 phosphor39 and transparent ceramics.25 We randomly selected eight K2SiF6:Mn4+ specimens and measured their PL quantum yields (QYs) at room temperature. The results are illustrated in Figure 3b. It can be seen that all eight specimens have QY values above 80%, and the best one even higher than 97%. As argued earlier and schematically illustrated in Figure 5a, a central Mn4+ ion tightly binds with six nearest F− ions to form an octahedral luminescence center [MnF6] 2− in the cubic K2SiF6:Mn4+ phosphor. Under the excitation of an appropriate light, for example, 477 nm blue laser, such a luminescence center can become excited via absorbing the incident photons and then emit red light by releasing the absorbed energy. For the absorptive and emissive optical transitions between the vibronic states, a widely adopted picture, namely configurational coordinate model, is usually used to discuss them,40,41 as schematically shown in Figure 5b. In the figure, the dashed vertical line represents a pure electronic transition which may produce the so-called ZPL line. In the studied case, such a “pure” electronic transition is a forbidden transition process because of both spin and parity transition rules. The bold red arrow denotes the Stokes radiative transition process accompanying the phonon emission, whereas the solid blue arrow stands for the anti-Stokes radiative transition in which phonon absorption is involved. For the [MnF6] 2− octahedral configuration, there are totally six main vibration modes, among which only three engage in the vibronic transitions. Figure 5c schematically illustrates the three vibration modes, that is, ν3, ν4, and ν6. From the measured PL spectra, characteristic energies of phonons ν6, ν4, and ν3 may be determined to be about 28.5, 41.4, and 78.7 meV, respectively. For the electronic part of the vibronic states, 3d3 electrons of Figure 6. (a) Room-temperature PL spectrum of the K2SiF6:Mn4+ phosphor alone. (b) CIE chromaticity coordinates of the PL spectrum in (a). (c) Room-temperature luminescence spectrum of a pc-WLED consisting of an InGaN blue LED and K2SiF6:Mn4+ nanophosphor. (d) CIE chromaticity coordinates of the luminescence spectrum in (c). (e) Room-temperature luminescence spectrum of a pc-WLED made of an InGaN blue LED and the mixture of K2SiF6:Mn4+ red phosphor + commercial YAG:Ce3+ yellow phosphor. (f) CIE chromaticity coordinates of the luminescence spectrum in (e). Note that the driving electric current of the pc-WLEDs was 60 mA. ACS Omega Article DOI: 10.1021/acsomega.8b01127 ACS Omega 2018, 3, 13704−13710 13707
AcsOmegaArticleMntt ions play a key role. To get more useful information onInGaN/GaN disk-in-nanowire WLED with chromaticity3d3 electrons of Mn+ ions in the studied phosphor, electroncoordinates of x=0.29and y=0.37and a CCTof 5500-paramagnetic resonance (EPR)measurements were carried out6500K at an injection current of 50A/cm2,45whereas Philip etal.reported an InGaN/AIGaNnanowire WLED with a highatvarioustemperaturesunderafixed microwavefrequencyofCRI of ~98.46More recently and interestingly,Philip et al.9.414× 10°Hz.The experimental EPR data are illustrated inFigure Sd. From the figure, it can be seen that six main EPRfabricated high-brightness phosphor-free III-nitride nanowiresignatures can be well detected, having g factors of 1.8-2.2.LEDs on Cu with highly stable white-light emission and highThese six fine EPR structures exactly reflect the hyperfineCRI of ~95.47 These studies may open another technicalinteraction of 3d’electrons of the Mn ion with the surroundingapproach for fabricating phosphor-free WLEDs with highmagnetic nucleus.2 From the measured EPR data, that is, theperformance and some advantages such as more efficientspacing of two adjacent EPR structures, we may determine thethermal management and enhanced light-extraction efficiency.hyperfinecoupling constant.The obtained value of ~80 Gaussis somewhat smaller thanthat of theCaAli2Og:Mn*+(~100CONCLUSIONSGauss)phosphor probably because of the difference in localIn summary, the K,SiFg:Mn+ phosphor synthesized at lowenvironment between the twodifferentcompounds.temperature with a full-solution route is examined withInterestingly, the EPR signal intensity is observed to declineemphasis on its variable-temperature PL property. Thegradually with the rise of temperature. It is known that thephosphor possesses good cubic crystalline structure evidencedEPR signal is proportional to the population differenceby precise HRTEM images and the corresponding FFTbetween spin states of the defect.t3 The EPR signal declinepatterns. More intriguingly, it exhibits an extraordinarywith temperature may be mainly attributed to the decrease ofthermally driven enhancement of emission intensity in apopulation difference between the Zeeman states of Mn* ionswide temperature range from 4 to 477Kunder the excitationbecause of the thermal distribution at higher spin states.of a 477 nm weak laser.A significant increase of transitionIn the remainder of this paper,we examine the deviceprobability of the dominant vibronic transitions around Mn*performance of the K,SiFg:Mn*phosphor in pe-WLEDs.ions with temperature is argued to be responsible for theRoom-temperature luminescence spectra of the phosphorobserved negative thermal quenching. By combining this kindalone and the two pc-WLEDs were measured, as shown inof phosphor with commercial YAG:Ce3+yellowphosphor andFigure 6. For the K,SiFg:Mn+ phosphor powder, the450 nm blue InGaN LED chip, we demonstrate a pc-WLEDInternational Commission on Ilumination(CIE)chromaticitywith good luminous properties.coordinates of the luminescence spectrum are calculated to be0.6927 and 0.3072, located at the edge of the red region andEXPERIMENTAL DETAILSmarked by an open circle, as shown in Figure 6b. These resultssuggest that the red phosphors synthesized in this study areSynthesis of the K,SiFg:Mn4+ Phosphor.Thevery suitable for the use of high-quality back-light display withK,SiFg:Mn+phosphor was synthesized at a lowtemperaturewide color gamut.By combining it with a blue InGaN LEDof--16Cwithatwo-stepwet-chemicalmethod.Inthefirstchip (luminescence peaking at 450 nm), a pink-light LEDstep, the K,MnF powder was prepared. For the preparation ofdevice can be formed, as shown in the inset figure of Figure 6c.the K,MnFpowder, the KHF,powder was dissolved in HFThe luminescence spectrum of this pc-WLED has coordinatesolution first and then via strong stirring operation to form apositions at 0.4750 and 0.1977, and a relatively low correlateduniform solution.By adding some amount of KMnO4 powdercolortemperature(CCT)of10sKWithfurtheradditionofinto the solution, one obtained a black mixture solution.commercialYAG:Ce3++yellow phosphor, themeasuredFollowed by the addition of H,O, solution, a brown-yellowluminescence spectrum of the new pc-WLED showsfull-solution was achieved with some precipitant generated. Aftercolor emission behavior in the entire visible region, as seen inthree times of vacuum filtration and washing process, a dryingFigure 6e. The inset figure shows a photograph of the as-process was carried out at 80 C for one night before thefabricated pc-WLED device. Chromaticity coordinatesbrown K,MnFpowder was obtained. In the second step, the(0.4710, 0.4307) of the device are marked by an open circleobtained K,MnF, powder was dissolved in the HF solutionin CIE color spaces in Figure6fCompared with theand gradually formed as a uniform golden-yellow solutioncommercial pc-WLED with low color render index [colorduring a vigorous stirring process.After the addition of KHF2rendering index (CRI) or R, 5000)powder, a continuous stirring operation was carried out until aonly using YAG:Ce3+ yellow phosphor, we demonstrate auniform solution was achieved. H,SiF。solution was thenwarm-white LED with excellent parameters, that is, R of 82added dropwise into the above solution,and agolden-yellowand CCT of 2701 K by adding the K,SiFg:Mnt phosphor asprecipitant was generated in the solution. Following threethe red component phosphor. The obtained luminoustimes of vacuum filtration and washing process, a dryingefficiencyof this pc-WLED is 124 lm/W under 60 mA drivingoperation was carried out at 80 °C for one night, and a finalcurrent. The National Television System Committee (NTSC)target product was prepared.value of thepc-WLED made of K,SiF:Mn*++YAG:Ce3+wasHRTEM Characterization. Precise structural character-measured to be about 45.34%. It is obvious that both theization of the sample was carried out with HRTEM using a Jeolluminous efficiency and NTSC value of the demonstrated pc-JEM-ARM3ooF TEM microscope.ThemicroscopewasWLEDare not comparableto those of the latest commercialoperated at 30okVwith aberration correctionforHRTEMproducts. Further process optimization in the fabrication ofimaging.As the specimen consisting of fluorine was sensitive toPc-WLED shall help improve the performance of the devices.Ithigh-energy electron beam bombardment, HRTEM image-is of interest to note other demonstrated WLEDs, especiallytaking was done at a low electron dose <30 e/A2-s. ImagesonesmadeofII-nitridenanowires on Si45,46andeven onthewere acquired with Gatan OneView camera running at 25Cu substrate.47 For instance, Guo et al. demonstrated an(fps). Under such conditions, 25 frames were aligned using13708DOt10.1021/acsormega.8b01127ACS Omega 2018, 3, 1370413710
Mn4+ ions play a key role. To get more useful information on 3d3 electrons of Mn4+ ions in the studied phosphor, electron paramagnetic resonance (EPR) measurements were carried out at various temperatures under a fixed microwave frequency of 9.414 × 109 Hz. The experimental EPR data are illustrated in Figure 5d. From the figure, it can be seen that six main EPR signatures can be well detected, having g factors of 1.8−2.2. These six fine EPR structures exactly reflect the hyperfine interaction of 3d3 electrons of the Mn ion with the surrounding magnetic nucleus.42 From the measured EPR data, that is, the spacing of two adjacent EPR structures, we may determine the hyperfine coupling constant. The obtained value of ∼80 Gauss is somewhat smaller than that of the CaAl12O9:Mn4+ (∼100 Gauss) phosphor probably because of the difference in local environment between the two different compounds.43 Interestingly, the EPR signal intensity is observed to decline gradually with the rise of temperature. It is known that the EPR signal is proportional to the population difference between spin states of the defect.43 The EPR signal decline with temperature may be mainly attributed to the decrease of population difference between the Zeeman states of Mn4+ ions because of the thermal distribution at higher spin states. In the remainder of this paper, we examine the device performance of the K2SiF6:Mn4+ phosphor in pc-WLEDs. Room-temperature luminescence spectra of the phosphor alone and the two pc-WLEDs were measured, as shown in Figure 6. For the K2SiF6:Mn4+ phosphor powder, the International Commission on Illumination (CIE) chromaticity coordinates of the luminescence spectrum are calculated to be 0.6927 and 0.3072, located at the edge of the red region and marked by an open circle, as shown in Figure 6b. These results suggest that the red phosphors synthesized in this study are very suitable for the use of high-quality back-light display with wide color gamut. By combining it with a blue InGaN LED chip (luminescence peaking at 450 nm), a pink-light LED device can be formed, as shown in the inset figure of Figure 6c. The luminescence spectrum of this pc-WLED has coordinate positions at 0.4750 and 0.1977, and a relatively low correlated color temperature (CCT) of 1050 K. With further addition of commercial YAG:Ce3+ yellow phosphor, the measured luminescence spectrum of the new pc-WLED shows fullcolor emission behavior in the entire visible region, as seen in Figure 6e. The inset figure shows a photograph of the asfabricated pc-WLED device. Chromaticity coordinates (0.4710, 0.4307) of the device are marked by an open circle in CIE color spaces in Figure 6f. Compared with the commercial pc-WLED with low color render index [color rendering index (CRI) or Ra 5000) only using YAG:Ce3+ yellow phosphor, we demonstrate a warm-white LED with excellent parameters, that is, Ra of 82 and CCT of 2701 K by adding the K2SiF6:Mn4+ phosphor as the red component phosphor. The obtained luminous efficiency of this pc-WLED is 124 lm/W under 60 mA driving current. The National Television System Committee (NTSC) value of the pc-WLED made of K2SiF6:Mn4+ + YAG:Ce3+ was measured to be about 45.34%. It is obvious that both the luminous efficiency and NTSC value of the demonstrated pcWLED are not comparable to those of the latest commercial products.44 Further process optimization in the fabrication of pc-WLED shall help improve the performance of the devices. It is of interest to note other demonstrated WLEDs, especially ones made of III-nitride nanowires on Si45,46 and even on the Cu substrate.47 For instance, Guo et al. demonstrated an InGaN/GaN disk-in-nanowire WLED with chromaticity coordinates of x = 0.29 and y = 0.37 and a CCT of 5500− 6500 K at an injection current of 50 A/cm2 , 45 whereas Philip et al. reported an InGaN/AlGaN nanowire WLED with a high CRI of ∼98.46 More recently and interestingly, Philip et al. fabricated high-brightness phosphor-free III-nitride nanowire LEDs on Cu with highly stable white-light emission and high CRI of ∼95.47 These studies may open another technical approach for fabricating phosphor-free WLEDs with high performance and some advantages such as more efficient thermal management and enhanced light-extraction efficiency. ■ CONCLUSIONS In summary, the K2SiF6:Mn4+ phosphor synthesized at low temperature with a full-solution route is examined with emphasis on its variable-temperature PL property. The phosphor possesses good cubic crystalline structure evidenced by precise HRTEM images and the corresponding FFT patterns. More intriguingly, it exhibits an extraordinary thermally driven enhancement of emission intensity in a wide temperature range from 4 to 477 K under the excitation of a 477 nm weak laser. A significant increase of transition probability of the dominant vibronic transitions around Mn4+ ions with temperature is argued to be responsible for the observed negative thermal quenching. By combining this kind of phosphor with commercial YAG:Ce3+ yellow phosphor and 450 nm blue InGaN LED chip, we demonstrate a pc-WLED with good luminous properties. ■ EXPERIMENTAL DETAILS Synthesis of the K2SiF6:Mn4+ Phosphor. The K2SiF6:Mn4+ phosphor was synthesized at a low temperature of −16 °C with a two-step wet-chemical method. In the first step, the K2MnF6 powder was prepared. For the preparation of the K2MnF6 powder, the KHF2 powder was dissolved in HF solution first and then via strong stirring operation to form a uniform solution. By adding some amount of KMnO4 powder into the solution, one obtained a black mixture solution. Followed by the addition of H2O2 solution, a brown-yellow solution was achieved with some precipitant generated. After three times of vacuum filtration and washing process, a drying process was carried out at 80 °C for one night before the brown K2MnF6 powder was obtained. In the second step, the obtained K2MnF6 powder was dissolved in the HF solution and gradually formed as a uniform golden-yellow solution during a vigorous stirring process. After the addition of KHF2 powder, a continuous stirring operation was carried out until a uniform solution was achieved. H2SiF6 solution was then added dropwise into the above solution, and a golden-yellow precipitant was generated in the solution. Following three times of vacuum filtration and washing process, a drying operation was carried out at 80 °C for one night, and a final target product was prepared. HRTEM Characterization. Precise structural characterization of the sample was carried out with HRTEM using a Jeol JEM-ARM300F TEM microscope. The microscope was operated at 300 kV with aberration correction for HRTEM imaging. As the specimen consisting of fluorine was sensitive to high-energy electron beam bombardment, HRTEM imagetaking was done at a low electron dose <30 e/Å2 ·s. Images were acquired with Gatan OneView camera running at 25 (fps). Under such conditions, 25 frames were aligned using ACS Omega Article DOI: 10.1021/acsomega.8b01127 ACS Omega 2018, 3, 13704−13710 13708
AcsOmegaArticlelpresent address: Laboratory of Solid State Physics andcross correlation and integrated within 1 s to obtain a finalimage with the size of 4k × 4k.Magnetism,Department of Physics and Astronomy,KUHigh-Resolution PL Spectral Measurements.High-Leuven, Celestijnenlaan 200D, 3001Leuven, BelgiumPresent address:Department of Applied Physics, Xi'anresolution PL spectra were recorded on a home-assembled PLJiaotongUniversity,Xi'an710049,Chinasetup using the 477 nm line of an Ar-Kr ion mixed gas laser(Coherent Innova-7o)as the excitation source. In the variable-AuthorContributionstemperature PL measurements, the phosphor powder wasS.J.X. conceived the project of phonon-induced luminescencemountedwithsilverpainton thecoldfingerof aclosed cyclein solids and supervised the study. F.T. synthesized thecryostat, providing a varying temperature range of 4-300 K.phosphors and performed the PL experiments.Z.C.S.andFor the PL measurements at temperatures higher than 300 K,H.G.Y.participated in the PL measurements.W.P.G. andthe phosphor sample was directly heated by a heater in theX.QP.did HRTEM measurements on the sample.F.T. andatmospheric environment. The luminescence signal of theS.J.X, completed the data analysis and wrote the article. All thesample was dispersed by a monochromator with focal length ofauthorsdiscussed and reviewed thearticle.0.75 m (Spex 750M)and detected with a photomultiplier tubeNotes(Hamamatsu R928).A lock-in amplifier (Stanford ResearchThe authors declare no competing financial interest.SR830)together with a standard optical chopper wasACKNOWLEDGMENTSemployed to enhance the signal-to-noise ratio. Finally,adata-acquire module was used to convert the detector electricalThis work was supported by the Shenzhen Municipal Sciencesignal into digital data that can be accepted by a computer.Theand TechnologyCouncil (grant no.Innovationcontrol and data-taking software was encoded by us withJCYJ20170818141709893),NationalNatural ScienceFounda-LabVIEWtion of China (grant no.11374247),HongKongRGC-GRFTestingEquipmentandMethodofQuantumYields.Grant(grantno.HKU705812P),HKU SRTonNewPL QYs of the phosphors were measured on an EdinburghMaterials, as well as in part by HK-UGC AoE Grants (grantFLS920-s fluorescence spectrometer + integrating sphereno.AoE/P-03/08).We are grateful to Dr.W.Guo for hissystem at room temperature.professional assistance in the EPR measurements. S.J.X. andEPR Spectral Characterization. EPR spectra wereF.T. are indebted to Prof. Y. G. Cao for his kind support to therecorded on a Bruker-Biospin (E500) spectrometer with thechemical solution synthesis of fluoride phosphors.W.P.G.wasspectral resolution of 1 kHz. Sample temperature wassupported by the National Science Foundation with the grantcontrolled by using a liquid nitrogen variable-temperaturenumberCBET-1159240(X.QP.)and the School of Engineer-control system.EPR spectra were measured via applying aing at the University of California, Irvine. Electron microscopymicrowave radiation of9.41× 10°Hz and scanning an externalexperiments were carried out in Irvine Materials Researchmagneticfield.Institute (IMRI)on the Jeol JEM2100Foperated at 200kVLED-DevicePerformanceCharacterization.Pc-WLEDandJEMARM300CFTEMat300kV.deviceswerepreparedwitha450nmInGaNbluechip+a售REFERENCES商coating layer of the mixed phosphor of red K,SiFg:Mn++ andyellow YAG:Ce3+powders.Two kinds of phosphors were(1) Nakamura, S. The roles of structural imperfections in InGaN-completely mixed with each other in silicone, and the obtainedbased blue light-emitting diodes and laser diodes. Science 1998,281956-961.mixture was then coated on the surface of the LED chips for(2) Pust, P,; Schmidt, P.J; Schnick, W.A revolution in lighting. Nat.producing pc-WLEDs.Device performance was evaluated byMater. 2015, 14, 454-458.using an Edinburgh FLS920-s fluorescence spectrometer+(3)Nakamura, S., Pearton,S., Fasol, G.TheBlue Laser Diode:Theintegrating sphere system. The prepared pe-WLEDs wereComplete Story; Springer-Verlag: Berlin Heideberg,2000.operated under adriving current of 60mA at room(4) Reineke, S. Complementary LED technologies: Nat. Mater.temperature. Spectral intensity distribution of the devices2015,14,459-462wasusedtocalculatetheirCCTandCRI.Luminousefficiency(s) Okamoto, K., Niki, L; Shvartser, A, Narukawa, Y., Mukai, T.,was determined by calculatingtheratio of the measuredScherer, A. Surface-plasmon-enhanced light emitters based on InGaNquantum wells.Nat. Mater.2004, 3, 601-605.luminous flux output and the input electrical power of the pc-(6) Wiersma, D. S., Cavalieri, S.A temperature-tunable randomWLEDs.laser.Nature2001,414,708-709.(7) Green, M. A., Zhao, J; Wang, A, Reece, P. J; Gal, M. EfficientAUTHORINFORMATIONsiliconlight-emittingdiodes.Nature2001,412,805-808(8) Suyver, J. F.; Wuister, S. F., Kelly, J. J., Meijerink, A SynthesisCorrespondingAuthorand Photoluminescence of Nanocrystalline ZnS:Mn2+.Nano Lett.*E-mail: sjxu@hku.hk (SJ.X.).2001,1,429-433.(9) Bachmann, V.; Ronda, C., Meijerink, A. TemperatureORCIDQuenching of Yellow Ce3+Luminescence in YAG:Ce.Chem.Mater.Wenpei Gao: 0000-0002-2776-26762009,21,2077-2084Xiaoqing Pan:0000-0002-0965-8568(1o) Cao, X. A; LeBoeuf, S. F.; Rowland, L. B., Yan, C. H, Liu, H.Shijie Xu: 0000-0001-6522-5778Temperature-dependent emission intensity and energy shift inInGaN/GaN multiple-quantum-well light-emitting diodes. Appl.Present AddressesPhys. Lett. 2003, 82, 3614-3616.SPresent address: Jiangsu Key Laboratory of Advanced Laser(11) Wei, Z. F., Xu, S. J; Duan, R. F.; Li, Q; Wang, J; Zeng, Y. P.;Materials and Devices, School of Physics and ElectronicLiu, H. C.Thermal quenching of luminescence from buried andEngineering, Jiangsu Normal University, Xuzhou 221116,surface InGaAs self-assembled quantum dots with high sheet density.ChinaJ. Appl. Phys. 2005, 98, 084305.13709DOt:10.1021/acsomega.8b01127ACS Omega 2018, 3, 1370413710
cross correlation and integrated within 1 s to obtain a final image with the size of 4k × 4k. High-Resolution PL Spectral Measurements. Highresolution PL spectra were recorded on a home-assembled PL setup using the 477 nm line of an Ar−Kr ion mixed gas laser (Coherent Innova-70) as the excitation source. In the variabletemperature PL measurements, the phosphor powder was mounted with silver paint on the cold finger of a closed cycle cryostat, providing a varying temperature range of 4−300 K. For the PL measurements at temperatures higher than 300 K, the phosphor sample was directly heated by a heater in the atmospheric environment. The luminescence signal of the sample was dispersed by a monochromator with focal length of 0.75 m (Spex 750M) and detected with a photomultiplier tube (Hamamatsu R928). A lock-in amplifier (Stanford Research SR830) together with a standard optical chopper was employed to enhance the signal-to-noise ratio. Finally, a data-acquire module was used to convert the detector electrical signal into digital data that can be accepted by a computer. The control and data-taking software was encoded by us with LabVIEW. Testing Equipment and Method of Quantum Yields. PL QYs of the phosphors were measured on an Edinburgh FLS920-s fluorescence spectrometer + integrating sphere system at room temperature. EPR Spectral Characterization. EPR spectra were recorded on a Bruker-Biospin (E500) spectrometer with the spectral resolution of 1 kHz. Sample temperature was controlled by using a liquid nitrogen variable-temperature control system. EPR spectra were measured via applying a microwave radiation of 9.41 × 109 Hz and scanning an external magnetic field. LED-Device Performance Characterization. Pc-WLED devices were prepared with a 450 nm InGaN blue chip + a coating layer of the mixed phosphor of red K2SiF6:Mn4+ and yellow YAG:Ce3+ powders. Two kinds of phosphors were completely mixed with each other in silicone, and the obtained mixture was then coated on the surface of the LED chips for producing pc-WLEDs. Device performance was evaluated by using an Edinburgh FLS920-s fluorescence spectrometer + integrating sphere system. The prepared pc-WLEDs were operated under a driving current of 60 mA at room temperature. Spectral intensity distribution of the devices was used to calculate their CCT and CRI. Luminous efficiency was determined by calculating the ratio of the measured luminous flux output and the input electrical power of the pcWLEDs. ■ AUTHOR INFORMATION Corresponding Author *E-mail: sjxu@hku.hk (S.J.X.). ORCID Wenpei Gao: 0000-0002-2776-2676 Xiaoqing Pan: 0000-0002-0965-8568 Shijie Xu: 0000-0001-6522-5778 Present Addresses § Present address: Jiangsu Key Laboratory of Advanced Laser Materials and Devices, School of Physics and Electronic Engineering, Jiangsu Normal University, Xuzhou 221116, China ∥ Present address: Laboratory of Solid State Physics and Magnetism, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium ⊥Present address: Department of Applied Physics, Xi’an Jiaotong University, Xi’an 710049, China Author Contributions S.J.X. conceived the project of phonon-induced luminescence in solids and supervised the study. F.T. synthesized the phosphors and performed the PL experiments. Z.C.S. and H.G.Y. participated in the PL measurements. W.P.G. and X.Q.P. did HRTEM measurements on the sample. F.T. and S.J.X. completed the data analysis and wrote the article. All the authors discussed and reviewed the article. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the Shenzhen Municipal Science and Technology Innovation Council (grant no. JCYJ20170818141709893), National Natural Science Foundation of China (grant no. 11374247), Hong Kong RGC-GRF Grant (grant no. HKU 705812P), HKU SRT on New Materials, as well as in part by HK-UGC AoE Grants (grant no. AoE/P-03/08). We are grateful to Dr. W. Guo for his professional assistance in the EPR measurements. S.J.X. and F.T. are indebted to Prof. Y. G. Cao for his kind support to the chemical solution synthesis of fluoride phosphors. W.P.G. was supported by the National Science Foundation with the grant number CBET-1159240 (X.Q.P.) and the School of Engineering at the University of California, Irvine. Electron microscopy experiments were carried out in Irvine Materials Research Institute (IMRI) on the Jeol JEM 2100F operated at 200 kV and JEM ARM300CF TEM at 300 kV. ■ REFERENCES (1) Nakamura, S. 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