Versatile Ca4F2Si2O7 Host from Defect-Induced Host Emission to

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Versatile CaFSiO Host From Defect-Induced Host Emission to WhiteEmitting Ce Doped CaFSiO Phosphor for Near-UV Solid-State Lighting 3+

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Paulraj Arunkumar, Yoon Hwa Kim, and Won Bin Im J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10375 • Publication Date (Web): 12 Feb 2016 Downloaded from http://pubs.acs.org on February 15, 2016

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The Journal of Physical Chemistry

Versatile Ca4F2Si2O7 Host From Defect-Induced Host Emission to WhiteEmitting Ce3+ Doped Ca4F2Si2O7 Phosphor for Near-UV Solid-State Lighting

Paulraj Arunkumar, Yoon Hwa Kim, and Won Bin Im* School of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju, 61186, Republic of Korea.

*To whom correspondence should be addressed. Tel : +82-62-530-1715 Fax : +82-62-530-1699 E-mail : [email protected]

1 ACS Paragon Plus Environment


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Abstract White-emitting Ca4-xF2Si2O7:xCe3+ phosphor and violet-emitting oxyfluoride Ca4F2Si2O7 host were synthesized by solid-state reaction. Ca4-xF2Si2O7:xCe3+ has strong absorption in the nearUV region (370 nm) and shows a broad emission in the range of 390 – 600 nm centered at 475 nm. Under 315 nm excitation, a narrow blue emission was observed. The Ca4F2Si2O7 host synthesized under the same reduction condition exhibited violet emission, due to the formation of anion-deficient nonstoichiometric Ca4F2-δSi2O7+δ/2 species. The broad emission of Ca4F2Si2O7:xCe3+ phosphor is attributed to Ce3+ occupying two crystallographic calcium sites available in the host. A white LED device was fabricated using Ca4F2Si2O7:Ce3+ without any additive phosphor, displaying excellent CIE chromaticity (0.29, 0.35) close to white emission with a color rendering index of 97. These exceptional optical properties of Ca4F2Si2O7:Ce3+ suggest the promising application of the single activator phosphor that could produce white light under near-UV based LEDs.

Keywords: Defect emission, Ca4F2Si2O7, white-LED, and single-component phosphor


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1. Introduction White light-emitting diodes (WLEDs) are the next generation lighting technology surpassing the conventional incandescent and fluorescent lamps due to their high energy efficiency, long life, and environmental benignity.1 White light is produced in the LED industry by the combination of yellow-emitting Y3Al5O12:Ce3+ (YAG:Ce3+) phosphor with blue LED; however, the demerits of white light, including low color rendering index (CRI, 75), and high correlated color temperature (CCT, 7756 K) due to a deficiency in the red component in the visible spectrum restricts its major applications.2 Hence, the WLED fabricated using near-UV light (350 - 400 nm) coupled with multi-color emitting phosphors has received considerable attention, despite its poor luminous efficiency associated with the reabsorption process. Therefore, WLED using near-UV LED chips with single-component phosphor is considered to be a promising approach due to its high color rendering index, low color aberration, low cost, tunability of color coordinates, and CCT.3 A single-component white-emitting phosphor can be produced either by co-doping a sensitizer and an activator or using two different activators in the same crystalline host matrix using the principle of energy transfer from the sensitizer to the activator or activator to activator.4-5 For the single-component phosphor, broad-band activators, namely Eu2+ and Ce3+, are the most appropriate activators that can cover the partial or full region of the visible spectrum for WLEDs due to the d-f transitions.6-7 Moreover, the emissions arising from d-f transitions are wavelength tunable because they are sensitive to the crystal field splitting and nephelauxetic effects from the phosphor host. The most exploited and efficient yellow-emitting YAG:Ce3+ phosphor developed by Blasse and Bril,8 has initiated the exploration of new phosphor host materials including oxide,



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oxyfluoride, nitride, and oxynitrides. Among these, oxyfluoride hosts have received greater attention attributed to their high quantum efficiency (Sr2.875-xBaxCe0.025AlO4F, quantum efficiency~100 %) surpassing the efficient commercial YAG:Ce3+ (~97 %), due to excellent structural rigidity, moisture stability, and high anisotropic crystal field.9-12 The use of oxyfluoride host as a good host material has attracted increasing attention primarily due to its stable crystal structure and the presence of fluorine having strong electronegativity that can produce tunable luminescence. Oxyfluoride hosts have recently been reported to show defect-induced emission when exposed to a reducing atmosphere due to the anion-deficient nonstoichiometry.13 These hosts acts as a self-activating emissive host even under near-UV excitation, with potential use in WLEDs.13 Ca4F2Si2O7 (CFSO) materials have recently been explored as a phosphor host due to their large indirect optical band gap (~5.0 eV), which is a favorable property for luminescence materials to accommodate the excited states of activator ions within the band gap.14 CFSO, a component of the cuspidine family crystallizing into a monoclinic structure with the space group P21/c, has recently been investigated as a phosphor host for WLEDs. Color tunable CFSO:Eu2+, Mn2+ phosphors, were fabricated with blue-emitting Eu2+ and red-emitting Mn2+ activators under 400 nm LED chip with color coordinates of (0.347, 0.338) for warm white LEDs.15 Whiteemitting CFSO:Ce3+, Mn2+ phosphor, with blue emission from Ce3+ and red emission from Mn2+ were reported with color coordinates of (0.29, 0.39) under 365 nm LED chip.16 These reports show significant deviation of color coordinates from the ideal white color coordinate (0.33, 0.33) even in the presence of dual activators. The luminescent properties of the CFSO host under a reducing atmosphere were not investigated. Therefore, we present the structure and luminescence properties of a single-component white-emitting CFSO:Ce3+ and defect-induced


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emission of a CFSO host. In addition, neutron diffraction refinements were conducted and a discussion on the crystallographic site occupation and decay time are provided. CFSO:Ce3+ exhibit white emission under 370 nm with color coordinates of (0.29, 0.35), while an intense blue emission was observed for CFSO:Ce3+ and the CFSO host upon host sensitization at 315 nm. The luminescence properties and energy transfer from the host to the Ce3+ activator were investigated. Prototype LEDs were fabricated with the single-component CFSO:Ce3+ for their practical application in WLED based solid-state lighting.

2. Experimental 2.1. Materials and synthesis Ca4-xCexF2Si2O7 with x = 0 − 0.5, where the Ce3+ ions substituted the Ca sites, were synthesized through solid-state reaction. Typically, stoichiometric amounts of precursors without further purification, namely CaCO3 (Aldrich, 99.99 %), CaF2 (Aldrich, 99.99 %), SiO2 (Aldrich, 99.9 %), and CeO2 (Aldrich, 99.99 %) were employed. The precursors were mixed in an agate mortar for about 30 min by adding an appropriate amount of acetone followed by drying. The powder mixtures were then annealed in a tubular furnace at 1350°C/4 h in a 25 % H2/75 % N2 gas mixture to produce the final material.

2.2. Characterization The X-ray diffraction (XRD) patterns were obtained using a Philips Xpert diffractometer in the angle range of 10° ≤ 2θ ≤ 110° with CuKα radiation (λ = 1.5405 Å). Neutron powder diffraction (NPD) data were collected at room temperature using a high-resolution powder diffractometer (Hanaro Center of the Korea Atomic Energy Research Institute). NPD data were



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collected over an angle range of 0° ≤ 2θ ≤ 160° at a wavelength of 1.8348 Å (neutron), with 4 h of collection time. The General Structure Analysis System (GSAS) program was used to conduct the structure refinements.17 The room temperature photoluminescence (PL) spectra were recorded using a Hitachi F-4500 spectrophotometer over a wavelength range of 200 – 700 nm using a 150 W xenon lamp. The thermal quenching characteristics were measured using a Hitachi F-4500 fluorescence spectrometer connected to an integrated heater, a temperature controller, and a thermal sensor. Diffuse reflectance spectral measurement was carried on with a Hitachi U-4100-Vis/NIR spectrophotometer. Thermoluminescent (TL) measurements were recorded using thermally stimulated luminescence with a RISO TL/OL reader equipped with a 90

Sr/90Y beta-ray source at a dose rate of 3.6 Gy/minute. The temperature range was kept at 25 -

300˚C for all the samples at a heating rate of 5K s-1.

2.3. White LED fabrication Prototype WLED devices were fabricated with Ca3.8Ce0.2F2Si2O7 phosphor using transparent silicone resin (Dow Corning, OE-6630) placed on a near-UV LED chip (λmax = 370 nm). The device was then encapsulated in a phosphor/silicone mixture, the mixture was placed directly on the header, and was then cured at 150°C for 1 h. After the packaging was completed, the WLED devices with phosphor were measured in an integrating sphere under a direct current forward bias.

3. Results and discussion 3.1. Phase identification


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The composition, phase purity, and crystal structure of the as-prepared materials, namely the Ca4F2Si2O7 (CFSO) host and Ca3.8Ce0.2F2Si2O7 (hereafter the specific Ca3.8Ce0.2F2Si2O7 composition is denoted as CFSO:Ce3+) were identified using X-ray and neutron diffraction patterns. Fig. 1 illustrates the crystal structure of CFSO (Figs. 1a, 1b) and their representative powder X-ray diffraction pattern (Fig. 1c). The synthesized phosphors were single phased and are in good agreement with the monoclinic CFSO phase with ICDD no. 41-1474. The CFSO structure represents the diorthosilicate type containing the single [Si2O7] group in combination with the (CaOn) chains of tilleyite ribbons that are condensed to build up a tessellation of fourcolumn-wide polyhedral walls, developing along the [001] plane. Each wall is connected by sharing the corners of the other four walls and with six disilicate groups, whereas each Si2O7 group links three walls. Four independent Ca cation polyhedral sites are available with the coordination number ranging from 6 to 8 in a unit cell of CFSO. Fig. 1b shows the presence of four different crystallographic Ca environments: Ca(1) atom with eight-coordination by five O atoms with an average Ca(1)–O distance of 2.5357 Å and three F atoms with an average Ca(1)–F distance of 2.4141 Å, Ca(2) atom surrounded by four O and three F atoms at average distances of 2.4949 and 2.3300 Å respectively, Ca(3) atom is seven-coordinated by six O atoms at an average Ca(3)–O distance of 2.4506 Å and one F atom at a Ca(3)–F distance of 2.3753 Å, and Ca(4) atoms six-coordinated by five O atoms at an average Ca(4)–O distance of 2.3680 Å and one F atom at an average Ca(4)–F distance of 2.2912 Å. The ionic radii for the six-, seven-, and eightcoordinated Ca2+ ions are 1.00, 1.06, and 1.12 Å, respectively which may be occupied by the Ce3+ ions with similar ionic radii of 1.01, 1.07, and 1.14 Å, respectively for the same coordination. Since the cations have similar effective ionic radii with specific coordination, the



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Ce3+ ions could probably occupy any of the four available Ca crystallographic sites in the CFSO host. Fig. 2 shows the observed (dots) and calculated (blue line) neutron diffraction profiles and their difference (bottom) for the Rietveld refinement of CFSO:Ce3+ at room temperature with λ = 1.8348 Å. This result reveals that Ce3+ is doped into the CFSO host, and exists as a single phase. CFSO crystallized into a monoclinic structure with space group P21/c(14). The refined lattice parameters were determined and converged with profile R-factors, Rp = 4.07 % and Rwp = 5.72 % and the goodness-of-fit parameter χ2 = 1.91, which is shown in Table 1. The amount of impurities in the CFSO:Ce3+ phase is negligible and the final refined Wyckoff positions of all the atoms are listed in Table 2.

3.2. Optical properties Fig. 3a exhibits the PL excitation and emission spectrum of CFSO:Ce3+. The excitation spectrum clearly shows a broad absorption band extending from 250 to 400 nm with the maxima at 315 nm, which was attributed to the electronic transition from the 4f ground state to the 5d excited state of the Ce3+ ion as reported in the literature.16 The emission spectrum of CFSO:Ce3+ exhibits a broad band extending from 340 to 650 nm, peaking at 420 nm under λex = 315 nm, which according to the reported literature, is due to the transition from the 5d to the 4f level.16 The origin of the broad and symmetric emission bands of CFSO:Ce3+ may be attributed to the electronic transitions of Ce3+ ions occupying four crystallographically different Ca2+ sites in the host. A similar broad emission band for the CFSO host with Eu2+ and Ce3+ activators were reported and assigned due to the distribution of the activator in the four different crystallographic Ca2+ sites of this cuspidine host.15-16 When the CFSO:Ce3+ phosphor was excited at 365 nm, the


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emission peak shifted to a higher wavelength with a maxima at 475 nm and broader emission peaks covering the whole visible spectrum, and was found to be very close to the ideal white emission. However, the intensity of this emission peak is half that of the emission intensity at λex = 315 nm. The excitation spectrum at 475 nm shows the extended excitation band shifted to a higher wavelength from 280 to 450 nm with a large absorption area in the range of from 350 to 420 nm. The origin of the wavelength dependent emission of the CFSO:Ce3+ phosphor can be understood by exploring the luminescence property of the CFSO host. The PL intensity of the Ce3+ as a function of its doping concentration (x) in the CFSO:xCe3+ phosphor is illustrated in Fig. 3b. The optimal Ce3+ doping concentration in the CFSO host was found to be x = 0.2, with the highest emission intensity. The emission intensity decreased above this concentration due to the concentration quenching effect. The concentration quenching arises from the spontaneous energy transfer from one activator to another leading to non-radiative transitions. The concentration quenching characteristics of the Ca4-xCexF2Si2O7 (measured under 365 nm excitation) can be further understood by evaluating the critical distance (Rc) of the Ce3+ activator ion. The critical distance was calculated by employing the Blasse’s concentration quenching method.18 The critical distance between Ce3+ ions for efficient energy transfer in the CFSO host was calculated using the unit cell volume (V) and the number of Ce3+ sites per unit cell (N) together with the critical concentration (Xc) by employing equation (1).

≈ 2 [

]

(1)

From the known structural parameters of the CFSO:Ce3+ phosphor, cell volume V = 820 Å3, N = 8 (4 (Z) × 2 Ce3+ sites) and Xc = 0.2, and the critical radius was found to be ~ 9.9 Å. The 9 ACS Paragon Plus Environment


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mechanism of concentration quenching (under 365 nm excitation) can be explained further by the type of interaction between activators/sensitizers which can be calculated from equation (2).19

= [1 + )⁄ ]

(2)

where x is the activator concentration; I/x is the emission intensity (I) per activator concentration (x); k and β are constants for the same excitation conditions; and θ is a function of multipolemultipole interaction where values of 6, 8, and 10 are assumed for the dipole-dipole, dipolequadrapole, and quadrapole-quadrapole interactions, respectively. Fig. 3c depicts a plot of log (I/x) vs log(x) and the slope of the linear fit was found to be -1.20 for CFSO:Ce3+. The θ value of 3.60 (3 × slope) reveals a weaker dipole-dipole interaction in the mechanism by which the concentration quenching of CFSO:Ce3+ occurs.

3.3 Defect emission of CFSO host Recent findings on the defect-induced emission of the oxyfluoride host, synthesized under reducing atmosphere, which was attributed to the anion-deficient nonstoichiometry, have instigated a probe on the luminescent properties of the CFSO host.13 To understand the defectinduced emission of the CFSO host, the samples were synthesized under air, 5 % H2/95 % N2, and 25 % H2/75 % N2 reducing atmosphere at 1350˚C and their luminescent properties were examined. The PL emission and excitation spectra of the CFSO host synthesized under 5 % H2/95 % N2 and 25 % H2/75 % N2 reducing atmosphere are presented in Figs. 4a and 4b, respectively. No emissions were observed for the CFSO host synthesized under air (not shown). On the other hand, a broad violet emission peaking at 390 nm under λex = 315 nm was observed


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when the samples were exposed to the reducing atmosphere (5 % H2 and 25 % H2) during synthesis. The defect structures are tunable by controlling the amount of reducing gas atmosphere used during synthesis which influences the defect-induced emission. The PL intensity of the violet emission band increases from 5 % H2/95 % N2 (Fig. 4a) to 25 % H2/75 % N2 (Fig. 4b) reducing atmosphere due to the formation of a greater number of defects in the structure under the higher reducing atmospheric condition. The excitation spectrum reveals a characteristic absorption band of the host at 315 nm, which was also observed in the excitation spectrum of the CFSO:Ce3+ phosphor (Fig. 3a). The nature of the host lattice emission band is attributed to the formation of defect structures due to the anion-deficient nonstoichiometric species in the host with the plausible general formula of Ca4F2-δSi2O7+δ/2, on exposure to the reducing atmosphere. In the fluoride host, a broad host lattice emission band is often observed, which is assigned to the defect related trapped exciton emission.20 From these results it could be understood that under reducing conditions, defects responsible for converting the expected stoichiometric Ca4F2Si2O7 into nonstoichiometric self-activating Ca4F2-δSi2O7+δ/2 phosphors were generated. These defects were not substantiated by the diffraction results, indicating the formation of undetectable low concentration of the defect formed in the sublattice. The defect structure formed is reproducible under repeated experimental conditions. The generation of the defect emission is caused by the electron-hole pair recombination within specific anionic sites preferably in fluorine sites in the lattice without activators. Defectinduced emissions from the wide band gap CFSO (this work) and other reported fluoride hosts, namely Sr3-xAxAlO4-αF1-δ (A = Ca, Ba; blue emission, 495 nm)13 and Ca5.45Li3.55(SiO4)3O0.45F1.55 (blue emission, 445 nm)21 without activators, are attributed to the formation of anion-deficient nonstoichiometric species that form a metastable defect electronic level below the conduction



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band. The presence of the Ca-F bond in the CFSO host formed in the four crystallographically distinct Ca sites can cause the formation of F-centers. Any possibility of the presence of a trace CaF2 impurity (in the CFSO host) under high temperature and reducing atmosphere could create F-centers and cause host emission. CaF2 nanocrystals were reported to show blue emission at 396 nm and 425 nm under 265 and 350 nm excitation, respectively, under gamma irradiation because the formation of surface defects and F-centers causes trapping of electrons at vacant fluorine sites.22 Although the diffraction results (X-ray and neutron) and Rietveld refinements of the CFSO host and CFSO:Ce3+ confirmed no trace of impurity phase in the sample, an attempt to fire CaF2 at 1350°C under 25 % H2/75 % N2 atmosphere to create the defects showed no emission (not shown). This demonstrates that the defect structures were created in the CFSO host on the fluorine site of the Ca-F bond rather than any undetectable trace of CaF2 impurities. The diffuse reflectance results of the CFSO host synthesized in air, 25 % H2 (reducing atmosphere), and the CFSO:Ce3+ host are shown in Fig. 4c. The host absorption band in the range of 200 to 250 nm remains the same for the CFSO host synthesized under air and 25 % H2. The extra band at ~ 315 nm for the CFSO host synthesized under 25 % H2 is assigned to the defectinduced absorption band. The CFSO:Ce3+ phosphor exhibited broad absorption ranging from 250 to 400 nm with two characteristic bands at 315 and 365 nm attributed to the defect-induced and Ce3+ d-f absorption band, respectively, which are in agreement with the corresponding excitation band (Fig. 3a). The asymmetric nature and broadness of the CFSO:Ce3+ emission indicate the presence of multiple emission bands due to the existence of four different crystallographic Ca sites for Ce3+. The Ca ions form four polyhedral units, with oxygen and fluorine atoms with eight (CaF5O3), six, (CaF5O), and two (CaF4O3 and CaFO6) seven-coordination sites which provide four sites for


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Ce3+ to accommodate. The luminescent property depends on the crystallographic orientation and the local environment of the Ce3+ ion due to its exposure to the different crystal field effect on the 5d state resulting in the broadening of the emission band in the CFSO host. The PL spectrum of the CFSO:Ce3+ phosphor at λex = 315 and 365 nm was deconvoluted using Gaussian profiles into five and four Gaussian profiles as depicted in Figs. 5a and 5b, respectively. Under 315 nm excitation, an emission band was fitted by a sum of five Gaussian peaks in which four are attributed to the Ce3+ band centered at 423 (23644 cm-1) and 460 nm (21704 cm-1), and 509 (19625 cm-1) and 569 nm (17575 cm-1) with an energy difference of 1940 and 2050 cm-1, respectively, and another Gaussian peak is assigned to the host emission band at ~388 nm. The energy difference was found to be in good agreement with the theoretical energy value (~2000 cm-1), due to the ground state splitting between the 2F7/2 and 2F5/2 levels of Ce3+ ions.23 This confirms that the double dual band occurs because the Ce3+ ion occupies two different Ca2+ sites, as assumed in Rietveld refinement, and with the similar atomic radii. From the deconvoluted peak positions, it may well be assumed that the Ce3+ ions occupy two Ca sites. The emission peaks under 365 nm excitation disclosed a large red shift which was fitted with four Gaussian profiles at 425, 464 nm and 481, 533 nm that are assigned to Ce3+ at two different Ca2+ sites. To understand the origin of the observed emission center, the van Uitert equation was employed to quantitatively analyze the site occupancy of Ce3+ in the Ca sites. An empirical formula was employed using the lower d-band edge of the free Ce3+ ion, that is strongly influenced by the local environment on the emission band of Ce3+; the equation can be given as

('∗(∗)* ∗ $ × 10 +, ]

= [1 − ( )


(3)


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where E represents the energy position of the d-band edge of Ce3+ in cm-1; Q is the energy position of the lower d-band edge for the free activator ions (50000 cm-1 for Ce3+); V* is the valence of the activator ion (V* = 3 for Ce3+ ion); n is the number of anions in the immediate shell around the Eu2+ ion; r is the effective radius of the host cation replaced by the Ce3+ ion (in Å); and Ea is the average electron affinity of the surrounding anions which are dependent on the polyanionic complex. The Ea for the oxygen and fluorine atoms used for the calculation are 1.32 and 3.28 eV, respectively. The calculated energy position of Ce3+ for the emission peak at ~420 nm (λex = 315 and 365 nm) can be assigned to CaF3O5 (eight coordinated site, ionic radii of Ca2+CN = 8 = 1.12 Å) and the lower energy emission peaks (509 and 481 nm at λex = 315 and 365 nm, respectively) are assigned to the CaF3O4 (seven coordinated site, ionic radii of Ca2+CN=7 = 1.06 Å). A large difference between the calculated and observed energy position of Ce3+ for the seven coordinated Ca site (CaF3O4) under 315 nm excitation was observed. This difference in the energy position may be due to several factors: primarily the influence of the host emission on the emission band of CFSO:Ce3+, the overlapping of the host emission with the excitation band of the Ce3+, crystal disorder, lack of charge neutrality (Ce3+ at Ca2+ site), and the charge compensation influenced by the local environment of the Ce3+ ion which alters the position of the 4f-5d transition. However, these effects are not precisely identified by the Van-Uitert fundamental empirical formula. Moreover, Dorenbos24-25 reported that as the size of the coordinating polyhedron increased, the crystal field splitting of the 5d levels reduced and shorter the emission wavelength. Hence, the assigning of a shorter emission wavelength (~420 nm) with larger ionic radii of 1.12 Å to the eight-coordinated Ca2+ site agrees well with the Dorenbos et al. report. It is found that the Ce3+ ions occupy the eight- (CaF3O5) and seven-coordinated (CaF3O4) Ca sites with three-coordinated fluorine.


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The influence of the host emission on the CFSO:Ce3+ emission band is clearly observed by the significant spectral overlap between the emission band of the CFSO host and the excitation band of CFSO:Ce3+ synthesized under 25 % H2 reducing atmosphere under 315 nm excitation (as seen in Figs. 4b and 3a, respectively). Therefore, the energy transfer efficiency (ηT) from the CFSO host to the Ce3+ ions can be calculated from equation (4)

-. = 1 − / /0

(4)

where Iso is the emission intensity of the CFSO host (in the absence of activator) and Is is the emission intensity of the host in the presence of an activator (CFSO:Ce3+ phosphor). The ηT value is determined to be as high as 68 % for the CFSO:Ce3+ phosphor synthesized under 25 % H2/75 % N2 reducing gas atmosphere (CFSO host also prepared under the same synthetic condition). To further confirm the defect-induced CFSO host emission, thermoluminescent (TL) curves of the CFSO host synthesized under 25 % H2 and CFSO:Ce3+ phosphor are shown in Figs. 6a and 6b, respectively. A strong TL curve at 150°C and two shoulders at ~217 and 275°C, respectively, were observed, revealing the presence of several traps or defect levels in the CFSO host. CFSO:Ce3+ phosphor showed a prominent TL curve at 150°C, similar to the CSFO host, though with a lower relative intensity than the CFSO host. Also, a weak TL curve at ~217 was observed, which is buried inside the broad TL curve. The trap depths of these defects were calculated by approximate equation, ET(eV) = TM(K)/500, where ET is the thermo-active energy of the trap depths and TM is the temperature at which the TL glow curve is the maximum.26 Three traps were observed for the CFSO host with activation energies of 0.84, 0.98, and 1.09 eV at 150, 217, and 275˚C, respectively, in which the first value constitutes shallow traps, while the 15 ACS Paragon Plus Environment


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last two values constitute deeper traps. These results demonstrate the presence of defect levels in the CFSO host, which are responsible for the defect-induced host emission under 315 nm excitation. The fluorescence decay curves of the CFSO host and Ce3+ emission in the CFSO host under 280 and 350 nm excitation are depicted in Fig. 6c. The decay curves of the CFSO host under 280 nm excitation fit well with the first-order exponential decay mode by equation (5)

(1) = 0 + 2 34(5⁄6)

(5)

where I and Io are the luminescence intensity at time t and 0; A is a constant; t is time; and τ is the decay time for an exponential component. The measured decay curves were fitted based on equation 5 and the lifetime value determined was 41 ns. However, the CFSO:Ce3+ under 280 nm was fitted with third-order exponentials, and the calculated life times are 53, 21, and 0.8 ns, which can be assigned to the host emission and the two types of Ce3+ emission centers in the Ca2+ sites in the CFSO host, respectively, which agree with the results of the deconvoluted emission peak of CFSO:Ce3+ under 315 nm excitation (Fig. 5a). The decay curves of the CFSO:Ce3+ phosphor under 350 nm excitation were fitted with second-order exponential, the lifetime values of which are 56 and 17 ns, which can be assigned to the presence of the two types of Ce3+ emission centers in the Ca2+ sites when excited at 350 nm that are in accordance with the earlier deconvoluted results of CFSO:Ce3+ under 365 nm excitation (Fig. 5b). It is worthy to note that no feature of host emission was observed under 350 nm excitation. Figure 7 illustrates the schematic energy level diagram of the CFSO host emission, and white-emitting Ce3+ emission under 365 nm excitation. The excitation above the bandgap of the CFSO host (Eg ≈ 5 ev) leads to the transition of electrons from the valence band (VB) to the


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conduction band (CB), leaving holes in VB. These electrons are subsequently trapped from the CB by the intermediate defect traps which lie very closely beneath the CB with trap depths of 0.84, 0.98, and 1.10 eV, where the first trap is heavily populated and consequently the electrons are transferred to the deeper traps. The electrons at these deep traps then recombine with the holes at the energy level which lies directly above the VB and generates a violet emission at ~390 nm under host (315 nm) excitation. The defect levels are formed in the CFSO host by the anion deficient nonstoichiometry in the CFSO host due to fluorine vacancy. On the other hand, in the CFSO:Ce3+ phosphor under 365 nm excitation, Ce3+ is excited at the 5d state, and recombines subsequently to form white-emission with a peak maxima at 475 nm.

3.4 Thermal quenching properties of CFSO:Ce3+ phosphor The thermal quenching properties of CFSO:Ce3+ were studied in the temperature range from ambient temperature to 200°C and are depicted in Fig. 8. Generally, phosphor suffers from a decrease in PL efficiency with an increase in the operating temperature of LED due to the increase in non-radiative transition. As the temperature increases, CFSO:Ce3+ shows a gradual decrease in the luminescence along with the increase in broadness of the emission band and the blue shift of the emission band. The CFSO:Ce3+ exhibits 96, 78, 57, and 40 % luminescence with the ambient temperature intensity at 50, 100, 150, and 200 ºC, respectively. The rapid quenching at >150ºC is due to the temperature dependent phonon interaction. At high temperature, the electrons at the lowest excited state can be easily activated and released through the crossing point between the excited and ground states in the configurational coordinate diagram.27-28



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3.5 Fabrication of prototype WLED The feasibility of the CFSO:Ce3+ phosphor for practical WLED lighting was monitored using an InGanN LED source (λmax = 370 nm) without any additive phosphors. The electroluminescent (EL) spectra and CIE chromaticity coordinates of the prototype WLED were measured for the fabricated LED with CFSO:Ce3+ phosphor under different forward bias currents (20 to 100 mA), as shown in Fig. 9. The emission from the InGaN LED chip used for WLED fabrication was observed at ∼ 370 nm and LED chips were operated in the voltage range of 3.0 to 3.6 V. The EL spectra for the fabricated WLED using CFSO:Ce3+ alone without any additive phosphor exhibiting almost ideal white CIE color coordinates (0.29, 0.35) were obtained under 40 mA at a correlated color temperature of 7692 K, with color rendering index Ra = 97. The optical parameters for the WLED prototype of CFSO:Ce3+ without any phosphor additives are shown in Table 3. The performance of WLED relies on the operating current, emission wavelength maxima, and ratio of epoxy resin to phosphor powder, and it is anticipated that the scope of optimization may result in improving the optical properties of CSFO:Ce3+ phosphor. The ideal white emission from the fabricated LED shows the promising role of the CFSO:Ce3+ phosphor as a single-component white-emitting phosphor for the WLED based solid-state lighting.

4. Conclusion A single-component white-emitting Ca3.8Ce0.2F2Si2O7 phosphor was synthesized, showing a broad emission in the visible region that can be excited under near-UV radiation (365 nm). The presence of Ce3+ ions in two different sites of Ca2+ sites (eight- and sevencoordination) in the host and the contribution of a dual band for each Ce3+ site led to the superposition of emission spectra that cover the entire visible spectrum. For the first time, we


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report the defect-induced broad blue emission of a CFSO host at ~390 nm, due to the selfactivation of an oxyfluoride host lattice by the anion-deficient nonstoichiometric species, creating defects under appropriate reduction synthesis condition. The energy migration mechanism of the Ce3+ ions was demonstrated to be a weak dipole-dipole interaction. At a 315 nm excitation, energy transfer from the host to the Ce3+ activator with an efficiency of 68 % was observed. The fabricated WLED showed close to ideal white color coordinates with (0.29, 0.35) and color rendering index of 97. The luminescent properties of the CFSO:Ce3+ phosphor indicate a promising single-component white-emitting phosphor for near-UV based solid-state lighting applications.

Acknowledgements This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology. This work was also supported by the Strategic Key-Material Development under the supports of the Ministry of Knowledge Economy (MKE, Korea). The authors also gratefully acknowledge the financial support from the Joint Research Project.



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Figure captions: Fig. 1 (a) Unit cell representation of the Ca4F2Si2O7 with red, white, green, and blue spheres represent Ca, O, F, and Si atoms, respectively. (b) Illustration of Ca sites containing four different crystallographic sites (Ca(1), Ca(2), Ca(3), and Ca(4)) with eight-, seven-, seven-, and six-coordinations, respectively, and (c) X-ray diffraction profile of CFSO host and Ca3.8Ce0.2F2Si2O7 phosphor compared with CFSO (PDF – 41-1474). Fig. 2 Rietveld refinement of neutron diffraction profile of Ca3.8Ce0.2F2Si2O7 with data (dots) and fit (lines) profile along with profile difference. Fig. 3 PL excitation and emission spectra of (a) CFSO:Ce3+, Ce3+ concentration dependent emission intensity of Ca4-xCexF2Si2O7 under (b) λex = 315 and (c) 365 nm, (d) plot showing log (I/x) vs log (x) in Ca4-xCexF2Si2O7 under λex = 365 nm. Fig. 4 PL excitation and emission spectra of CFSO host synthesized under (a) 5 % H2 and (b) 25 % H2 and (c) diffused reflectance spectra of CFSO:Ce3+, CFSO host under air, and 25 % H2. Inset containing the diffused reflectance of CFSO host under air and 25 % H2 in the wavelength range of 275 to 370 nm. Fig. 5 Deconvoluted PL emission spectra of CFSO:Ce3+ under (a) λex = 315 nm and (b) λex = 365 nm with the inset containing eight- (CaF3O5) and seven- (CaF3O4) coordinated Ca sites where Ca, F, and O atoms are represented as red, green, and white spheres, respectively. Fig. 6 Thermoluminescence curves of (a) CFSO host – 25 % H2 and (b) CFSO:Ce3+ and (c) decay curves of CFSO host – 25 % H2 under λex = 280 nm and CFSO:Ce3+ under λex = 280 and 350 nm. Fig. 7 Schematic energy level diagram of CFSO host emission, CFSO:Ce3+ emission under λex = 365 nm, where VB and CB are the valence and conduction bands of the CFSO host, respectively. Fig. 8 (a) Temperature dependent PL emission and (b) temperature-dependent normalized emission intensity of CFSO:Ce3+ in the temperature range from 25 to 200°C under 365 nm excitation.



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Fig. 9 (a) EL spectra of CFSO:Ce3+ under 370 nm LED chip with the inset containing fabricated LED chip (i) without and (ii) with applied current of 120 mA and (b) CIE chromaticity coordinates of the device under different forward bias currents. The Planckian locus line and the points corresponding to the color temperatures of 3500 and 6500 K are indicated.


Page 25 of 37

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Table 1. Rietveld refinement parameters of Ca3.8Ce0.2F2Si2O7 obtained from neutron diffraction data at room temperature. The numbers in parentheses are the estimated standard deviations of the last significant figure. Formula

Ca3.8Ce0.2F2Si2O7

T/ºC

25

symmetry

Monoclinic P21/c

space group a/Å

7.5389(8)

b/Å

10.5423(1)

c/Å

10.9147(6)

α, γ/deg

90.00

β/deg

70.38(2)

volume/Å3

817.1(5)

Z

4

Rp

4.07 %

Rwp

5.72 %

χ2

1.91



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Table 2. Refined structural parameters of Ca3.8Ce0.2F2Si2O7 obtained from neutron diffraction data at room temperature. The numbers in parentheses are the estimated standard deviations of the last significant figure. g

100 × Uiso/Å2

0.1719(3) 0.1342(6) 0.4166(9)

1.0

1.52(9)

4e

0.6567(8) 0.1356(3) 0.4211(4)

1.0

1.70(5)

Ca3

4e

0.9715(3) 0.4031(1) 0.3004(7)

1.0

0.74(3)

Ca4

4e

0.4729(1) 0.4133(6) 0.3142(3)

1.0

0.27(5)

Si1

4e

0.2720(7) 0.1933(1) 0.1311(3)

1.0

1.04(6)

Si2

4e

0.8470(3) 0.1916(7) 0.1223(8)

1.0

1.01(2)

O1

4e

0.0648(7) 0.2243(5) 0.1212(8)

1.0

2.25(6)

O2

4e

0.2839(4) 0.0485(5) 0.1671(1)

1.0

1.75(2)

O3

4e

0.8025(4) 0.0443(5) 0.1591(1)

1.0

0.77(9)

O4

4e

0.2721(1) 0.2832(9) 0.2546(2)

1.0

0.67(1)

O5

4e

0.7165(9) 0.2741(3) 0.2415(7)

1.0

1.95(2)

O6

4e

0.4032(4) 0.2373(7) 0.9909(1)

1.0

1.18(3)

O7

4e

0.8640(7) 0.2310(7) 0.9795(6)

1.0

1.59(1)

F1

4e

0.5729(3) 0.4995(8) 0.1010(5)

1.0

0.95(2)

F2

4e

0.0727(6) 0.5006(5) 0.1036(1)

1.0

2.22(7)

atom

site

Ca1

4e

Ca2

x

y

z


Page 27 of 37

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Table 3. Optical parameters for fabricated Ca3.8Ce0.2F2Si2O7 based WLED prototype under InGaN LED (λmax = 370 nm). current (mA)

CIE x

CIE y

CCT

Ra

40

0.290

0.352

7692

97

60

0.279

0.365

8141

90

80

0.270

0.367

8333

85

100

0.267

0.368

8485

83

120

0.267

0.358

8560

81



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Figures:

Fig. 1 (Arunkumar et al.)


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Versatile Ca4F2Si2O7 Host from Defect-Induced Host Emission to

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