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Mechanisms of Energy Transfer and Enhanced Stability of Carbidonitride Phosphors for Solid State Lighting Christopher Grieco, Kurt F. Hirsekorn, Andrew T. Heitsch, Alan C. Thomas, Mark H. McAdon, Britt A. Vanchura, Michael M. Romanelli, Lora L. Brehm, Anne Leugers, Anatoliy Sokolov, and John B. Asbury ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15323 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 21, 2017

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ACS Applied Materials & Interfaces

Mechanisms of Energy Transfer and Enhanced Stability of Carbidonitride Phosphors for Solid State Lighting Christopher Grieco1, Kurt F. Hirsekorn2, Andrew T. Heitsch2, Alan C. Thomas2, Mark H. McAdon2, Britt A. Vanchura2, Michael M. Romanelli2, Lora L. Brehm2, Anne Leugers2, Anatoliy N. Sokolov2*, and John B. Asbury1* 1 2

Department of Chemistry, The Pennsylvania State University, University Park, PA 16802 USA The Dow Chemical Company, Midland, MI 48674 USA

Abstract Phosphor-converted light emitting diodes (pcLEDs) produce white light through the use of phosphors that convert blue light emitted from the LED chip into green and red wavelengths. Understanding the mechanisms of degradation of the emission spectra and quantum yields of the phosphors used in pcLEDs is of critical importance to fully realize the potential of solid state lighting as an energy efficient technology. Toward this end, time-resolved photoluminescence spectroscopy was used to identify the mechanistic origins of enhanced stability and luminescence efficiency that can be obtained from a series of carbidonitride red phosphors with varying degree of substitutional carbon. The increasing substitution of carbon and oxygen in nitrogen positions of the carbidonitride phosphor (Sr2Si5N8-[(4x/3)+z]CxO3z/2:Eu2+) systematically changed the dimensions of the crystalline lattice. These structural changes caused a red-shift and broadening of the emission spectra of the phosphors due to faster energy transfer from higher to lower energy emission sites. Surprisingly, in spite of broadening of the emission spectra, the quantum yield was maintained or increased with carbon substitution. Aging phosphors with lowered carbon content under conditions that accurately reflected thermal and optical stresses found in functioning pcLED packages led to spectral changes that were dependent on substitutional carbon content. Importantly, phosphors that contained optimal amounts of carbon and oxygen possessed luminescence spectra and quantum yields that did not undergo changes associated with aging and therefore provided a more stable color point for superior control of the emission properties of pcLED packages. These findings provide insights to guide continued development of phosphors for efficient and stable solid state lighting materials and devices. Keywords phosphor-converted LED, energy transfer mechanism, phosphor aging, phosphor stability, timeresolved photoluminescence spectroscopy

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1. Introduction Solid state lighting based on phosphor-converted LED (pcLED) packages has become an important platform for highly efficient lighting.1-4

In such pcLED packages, green and red

phosphors are deposited directly onto high intensity GaN LED chips that emit blue light at 450 nm. These phosphors absorb a portion of the blue light, and the net LED plus phosphor emission spectra can be well-matched to emission from traditional incandescent light sources.1-5 This design places significant thermal and optical stress on the phosphors, for they are in direct contact with the GaN LED chip and are subject to intense (0.1 - 1 W/mm2) optical irradiation and high temperatures (~150 oC or more).4, 6 A variety of inorganic crystalline compounds doped with rare earth ions have been explored as phosphors for lighting applications including garnets, silicates, nitrides and oxynitrides among others.3-4, 7-8 The need to maintain high luminescent efficiency at elevated temperature combined with long-term stability has been particularly challenging for red phosphors because of the need to absorb strongly at the blue wavelengths but emit at red wavelengths.4 The broad absorption spectrum, high strength and associated low thermal quenching, and tunability of Sr2Si5N8:Eu2+ phosphors have made them promising targets for commercial applications. The higher covalency of the crystalline lattice formed by the N3- ions in comparison to oxides causes the phosphor to absorb strongly at blue wavelengths but emit at red wavelengths,3 filling an important gap in phosphor emission for pc-LED applications. While advances in phosphor design have enabled increases in luminous efficacy of radiation (lumens/W), the long-term stability of emission spectra of phosphors under thermal and optical stress remains an area of active investigation.4, 9 Penetration of new phosphor materials into the pcLED market depends on meeting the initial optical performance criteria, measured as a function of total LED lumens at a specific correlated color temperature, and stringent reliability requirements as set by the LED manufacturer. Both the initial and the “aging” performance are 2 ACS Paragon Plus Environment

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critical to quality, and must be continually improved. This is particularly important for white pcLED applications where subtle changes in the intensity and spectrum of phosphor emission can degrade the color rendering index and therefore the ‘visual quality’ of white light. Phosphors have been specifically designed to achieve a maximum conversion of absorbed to emitted light (quantum efficiency) and minimize non-radiative relaxation processes.10-19 One new class of proprietary phosphors are based on the carbidonitride Sr2Si5N8-[(4x/3)+z]CxO3z/2 host crystals activated with Eu2+ ions, which form the radiative centers in the crystal lattice.11-12 This class of proprietary carbidonitride phosphors for use in pcLED packages and developed by Lightscape Materials Inc. (wholly owned by The Dow Chemical Company) was recently investigated under thermal stress.20 An earlier paper presented a systematic X-ray diffraction study of the red emitting carbidonitride phosphors. This study revealed that the carbidonitride lattice was distorted by incorporation of carbon and oxygen into the lattice leading to contraction in the (010) direction and corresponding expansion in the other two directions.20 This carbon/oxygen substitution stabilized the emission intensity at higher operational temperatures.20 This observation was significant as thermal quenching of emission has been reported as a leading issue for this phosphor family. As a potential mechanism, the incorporation of substituted carbon was attributed to reduction of thermal quenching that results from coupling of excited phonon modes of the lattice to the electronic transition of the activating Eu2+ ions.21 Understanding the mechanisms of phosphor aging, or how the initial optical properties of phosphors degrade under operation in LED packages is critical to commercial success.3-4 Being intrigued by the influence of varying carbon and oxygen amounts in carbidonitride phosphors and its impact on stabilization of the emission quantum yield at elevated temperatures,20 we therefore sought to determine the mechanism of this effect. We hypothesized that understanding the mechanism of lattice stabilization by carbon substitution would inform ongoing work in the field to improve the stability of emission properties of phosphors for pc-LED applications.



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We found that carbidonitride phosphors provide an ideal model for studying the impact of lattice expansion and contraction on phosphor emission because of the ability to systematically vary the amount of carbon/oxygen substitution in the lattice. In this paper we use time resolved photoluminescence spectroscopy (TRPL) to correlate the structural changes observed in X-ray scattering experiments to phosphor optical properties. Additionally, the influence of optical stress, or aging, of phosphor materials under realistic conditions results in a change of optical properties. TRPL is used to reveal the mechanism of degradation, reaching a critical conclusion that accumulation of lattice strain during aging is a leading cause for loss of phosphor conversion efficiency.

2. Experimental Section All measurements described in this manuscript were carried out using phosphor material dispersed within silicone films (OE-6351, Dow Corning) at 5 wt % phosphor to silicone dispersion. The accelerated aging apparatus depicted in Figure S5 was used to test the reliability performance of the thin films of the phosphor materials. The phosphors were placed onto a glass cover slip directly on the LED and were aged in ambient laboratory air. The glass cover slip (GOLD Seal Cover Glass 22mm x 22mm No. 2 – 220 µm thickness) was utilized to prevent any potential high temperature interaction between the LED silicone and the phosphor silicone. The films were left on the LED for a pre-set amount of time. Following exposure, the films could be tested for their optical properties, and then returned to the 100 W LED for continued aging. The film arrangement on the LED was changed between varying aging periods to ensure that the films were exposed to different portions of the 100 W LED to avoid systematic variations that might arise from heterogeneity of the 100 W LED array. All optical films were characterized using an LED Mimic set up represented in Figure S7. Each film was tested on both sides to improve data quality and ensure that film degradation proceeded homogeneously. Prior to the testing of each film, the LED Mimic instrument was 4 ACS Paragon Plus Environment

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calibrated for a new “Dark” reference spectrum to minimize background noise. To calibrate the lamp and detector a blue LED and a blank silicone reference were collected. Time-resolved photoluminescence spectroscopy was performed using a home-built flash photolysis set-up as described previously.22 The excitation source was a 20 Hz nitrogen laser (GL-3300, Photon Technology International) pumped dye cavity (GL-302, Photon Technology International) set to 450 nm. The laser pulse had a FWHM less than 1 nm and a pulse duration of ~1 ns.

The sample emission was dispersed using a monochromator (DK240, Spectral

Products) and focused onto a 350 MHz silicon photodiode (DET210, Thorlabs). The detected signal was further amplified using a preamplifier (HVA-200M-40-B, FEMTO) before it was digitized using a 200 MHz PC oscilloscope (Picoscope-5244A, Pico Technology). The time resolution was limited by the lowest electronic bandwidth of the oscilloscope. The instrument response function had a ~3.5 ns FWHM. The monochromator slits were set to attain an effective spectral bandwidth of 5 nm for all measurements. The illuminated area on the sample was about 0.5 cm2 and the incident energy density was ~100 μJ/cm2. The emission from the front surface of the film samples was collected at 90o relative to the excitation beam direction. The samples were approximately 45o relative to the excitation direction. Samples were measured three times at various spots on the films to ensure no spotto-spot variation, and the averages of all three scans were used to generate all spectra reported. In order to model the effect of carbon substitution on the lattice structure, three crystal structure models were developed and optimized using non-local density functional theory (DFT; generalized gradient approximation). The structure optimizations were carried out by energy minimization, including both the atomic coordinates and the unit cell parameters. The first comparative model was for a Sr2Si5N8:Eu2+ phosphor.

For the high carbon substitution

carbidonitrides, two models were selected from a separate computational study corresponding to 0.47 weight percent (wt. %) carbon (Sr2Si5O0.17N7.66C0.17). All three models are described in Section 8 of the supporting information, along with details of the density functional theory (DFT) 5 ACS Paragon Plus Environment


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computations. Crystal structure graphics were made using Mercury software (version 3.5.1). SrSr distance distributions were calculated using the CrystalMaker software package (version 9.1.1).

3. Results and Discussion Carbidonitride Sr2Si5N8-[(4x/3)+z]CxO3z/2 host crystals activated with Eu2+ ions are a promising class of thermally stable phosphors that are being developed as the red-emitter for solid state lighting applications. Carbidonitride phosphors contain carbon and oxygen that are substituted for nitrogen ions in the Sr2Si5N8 host lattice.23 The Eu2+ ions replace a fraction of the Sr2+ ions in the host crystal, forming the radiative centers in the crystalline lattice.11-12 The electronic transition responsible for the absorption and emission properties of such phosphors corresponds to a 4f 7 – 4f65d1 transition within the Eu2+ ions.24 The crystal-field splitting resulting from the interaction of the Eu2+ ion with the lattice causes the electronic transition to appear in the visible region of the electromagnetic spectrum.20

A photoluminescence spectrum of a carbidonitride phosphor

containing 0.044 wt% Eu and 0.05 wt% carbon is depicted in Figure 1. The absorption and emission spectrum of the phosphor is included as Figure S1. Previous reports demonstrated that the Sr2Si5N8 host contains two distinct sites for Sr2+ ions that differ in their coordination by nitrogen ions.24-25

The first site, termed Sr1 to maintain

consistency with the literature, is coordinated by ten nitrogen ions with an average Sr-N distance of 2.969 Å. Eu2+ ions occupying these sites give rise to the higher energy emission that is observed in the photoluminescence spectrum of the phosphor. The second site, termed Sr2, is also coordinated by ten nitrogen ions but possesses a slightly smaller average Sr-N distance of 2.928 Å. Eu2+ ions occupying these sites account for lower energy portion of the emission spectrum of Sr2Si5N8:Eu2+ phosphors.25 A schematic of the corresponding energy level diagram of a Sr2Si5N8:Eu2+ phosphor is depicted in Figure 1a.


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Figure 1. (a) Schematic energy level diagram of 2+ Sr2Si5N8:Eu activated phosphor emphasizing the presence of two distinct emissive sites in the lattice, Sr1 and Sr2. (b) Diagram of kinetic processes that occur after excitation of both types of emissive sites by 450 nm radiation. (c) Photoluminescence emission spectrum of a carbidonitride phosphor with 0.05 wt % carbon with best fit spectra corresponding to emission from the Sr1 and Sr2 sites in the lattice.

The Eu2+ ions at both Sr1 and Sr2 sites in carbidonitride phosphors can be excited at 450 nm, leading to emission at the wavelengths that are unique to each site. The sum of the emission intensity from both sites produces the observed photoluminescence spectrum as indicated in Figure 1c. However, because Eu2+ ions at Sr2 sites have lower energy ligand field states, energy transfer occurs from Eu2+ ions at Sr1 sites to Eu2+ ions at Sr2 sites.26

This energy transfer

process is labeled with the rate constant kET and will be referred to as energy transfer from Sr1 to Sr2 sites in the following discussion. The rate of energy transfer between Sr1 and Sr2 sites has a strong influence on the overall photoluminescence spectrum of the phosphors because it affects the intensity of Sr2 site emission relative to the Sr1 site. Faster energy transfer leads to more emission from the Sr2 sites and therefore more intensity on the longer wavelength side of the spectrum.



Figure 2a depicts steady-state emission spectra of the series of carbidonitride phosphors having of 0.05, 0.10, 0.20 and 0.30 wt% carbon incorporated in the lattice. The representative photoluminescence quantum yields for each sample are provided in Table S2. A representative X-ray diffraction (XRD) pattern of a carbidonitride phosphor is represented in Figure S2. A shift of some of the diffraction peaks is observed with increasing carbon content. Figures S3 and S4 highlight the changes of the lattice parameters that were obtained from analysis of the (010) and (002) diffraction peaks as discussed in an earlier publication.20 The relative % carbon was quantified by combustion infrared analysis using a LECO CS844 analyzer. It should be noted that other techniques useful for carbon analysis (EELS, Solid-state NMR) are not applicable for low

1

Greater [C] Greater Sr2 emission

PL Intensity (a.u.)

PL Intensity (arb.u.)

(a)

[C] (wt%) 0.05 0.10 0.20 0.30

0 550

600

650

700

750 750

Wavelength (nm) Wavelength (nm)

(b) 1.2 ISr2 / ISr1

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1 0.8 0.6 0.4 0

0.1

0.2

0.3

[C] (wt%) Figure 2. (a) Steady-state PL spectra of carbidonitride phosphors incorporating various amounts of carbon. (b) Plot of the intensity ratio of Sr2 versus Sr1 emission as a function of carbon concentration in the phosphor lattice. The contribution of the Sr2 emission site increases with increasing concentration of carbon.


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carbon loadings because of their limited sensitivity. Moreover, the XRD data unambiguously show that the carbon is part of the phosphor lattice and is not present as an inert phase (e.g. SiC). The emission spectra in Figure 2a have been normalized at their peak maxima to emphasize changes of their spectral shapes. The intensity of emission increases on the longer wavelength side of the spectrum in phosphors that incorporate more carbon. These emission spectra were fit using the Sr1 and Sr2 emission spectra depicted in Figure 1c (blue and red curves, respectively) to quantify the contribution that Sr2 versus Sr1 sites make to the overall emission of the phosphors. The ratios of the integrated emission spectra from Sr2 versus Sr1 sites ISr2/ISr1 in the phosphors appear in Figure 2b plotted relative to the wt% carbon. The error bars reflect the uncertainty in the best fit spectra that arise because of the overlap of the emission spectra from the two sites. The data reveal that the increase of emission on the longer wavelength side results from greater contribution of emission from Eu2+ ions at Sr2 sites in phosphors that incorporate more carbon, suggesting that the rate of energy transfer from Sr1 to Sr2 sites may increase with increased carbon substitution. A two-dimensional wavelength-time graph of TRPL spectra measured in a carbidonitride phosphor containing 0.05 wt% carbon is plotted versus the corresponding time delay in Figure 3a. Immediately following the 450 nm, 800 ps laser pulse used to excite the phosphor, emission occurs from both Sr1 and Sr2 sites in proportion to the concentration of Eu2+ ions that were initially excited at these sites. The sum of the emission spectra from both sites determines the shape of the spectrum labeled ‘5 ns’ in Figure 3b, which was measured five nanoseconds after the initial excitation of the phosphors. A dotted vertical line in Figure 3a indicates the time slice at which the spectrum was collected, which describes the initial ISr2/ISr1 ratio prior to occurrence of energy transfer. At longer time delays, energy transfer from Sr1 to Sr2 sites increases emission from Sr2 sites relative to Sr1 sites.26 Therefore, the change of the emission spectra measured at longer time delays following the initial 450 nm pulsed excitation provides a direct measure of energy transfer from Sr1 to Sr2 sites. The time-dependent shift of the maxima of the emission spectra is 9 ACS Paragon Plus Environment


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highlighted in Figure 3a by the dashed curve. A spectral slice measured 2500 ns following pulsed excitation appears in Figure 3b and is indicated by the dotted vertical line in Figure 3a. The shift to longer wavelength emission at 2500 ns time delay was due to energy transfer from Sr1 to Sr2 sites.

Figure 3. (a) Two-dimensional wavelength-time plot of TRPL spectra of a carbidonitride phosphor with 0.05 wt % carbon represented with a logarithmic time axis measured after pulsed excitation at 450 nm. The schematic diagram above emphasizes the energy transfer process that causes the spectral shift highlighted by the dashed curve. (b) Spectral slices taken from the two-dimensional data set showing the change in spectrum that results from energy transfer from Sr1 to Sr2 sites. The time delays at which the earliest and latest spectral slices were taken are indicated in the contour plot above. The data are represented as gray lines and the fits are represented as colored lines.


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We considered the possibility that the time-dependent shift of the TRPL spectra in Figure 3 might be due to a shorter excited state lifetime of Eu2+ ions at Sr1 sites in the lattice. However, prior work by Sohn et al.24 on Sr2Si5N8:Eu2+ phosphors demonstrated that the rates of excited state relaxation out of Sr1 and Sr2 sites (apart from energy transfer) are the same. From the supporting information (Section 7) we observed that this conclusion also appears valid for the carbidonitride phosphors. Figure 4 depicts a comparison of TRPL emission spectra measured five nanoseconds after 450 nm excitation of the series of carbidonitride phosphors. The spectra have been normalized to their emission maxima to facilitate comparison of their line shapes. The spectra exhibit only slight growth on the longer wavelength side with increasing carbon content, growth that is significantly smaller than the change in spectral shape observed in the steady-state emission spectra (Figure 2a). The comparison indicates that the changes of emission spectra that occur with increased carbon substitution do not result from changes in occupation of Eu2+ ions at Sr1 versus Sr2 sites, which is consistent with prior work on Sr2Si5N8:Eu2+ phosphors.24

Figure 4. Time-resolved emission spectra of carbidonitride phosphor samples incorporating different amounts of carbon measured five nanoseconds after pulsed excitation at 450 nm.

Therefore, we quantified the time-dependence of the emission maxima and the full-width at half-maximum (FWHM) of the emission spectra of each phosphor to understand the relationship between energy transfer rates and carbon content since this relationship affects the emission



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spectra and therefore the color of the phosphors when used in pc-LED applications. The analysis procedures are described in detail in the supporting information (Section 6).

The time-

dependence of emission maxima and FWHM of the TRPL spectra are plotted versus the corresponding time delays in Figure 5.

The increased shift to longer wavelength at 0.30 wt%

carbon loading indicates faster energy transfer. Similarly, the time-dependence of the FWHM demonstrates that the Sr2 sites contribute proportionately more to the total emission at longer time delays at higher carbon loading. This confirms that the rates of energy transfer from Sr1 to Sr2 sites increase in carbidonitride phosphors with increasing amounts of carbon and oxygen substitution.

Figure 5. (a) Time-dependent peak maxima obtained by analysis of the TRPL data of phosphors incorporating various amounts of carbon and oxygen. (b) Time-dependent FWHM of the emission measured in the set of phosphors. The data demonstrate that substituting carbon and oxygen in the carbidonitride lattice increases the rate of energy transfer from Sr1 to Sr2 sites in the phosphors.


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We quantified the changes of the energy transfer rates from Sr1 to Sr2 sites using the kinetic model depicted in Figure 1b, which simultaneously describes the time-dependence of the TRPL spectra and the corresponding Sr1 and Sr2 population dynamics. To describe the TRPL data, we assumed a normalized initial population of excited Eu2+ ions at Sr1 sites with concentration n, [Sr1]t=0 = n, and an initial population of excited Eu2+ ions at Sr2 sites with concentration 1-n, [Sr2]t=0 = 1–n. The parameter n is therefore a fraction describing the initial population of excited Eu2+ ions at Sr1 sites relative to those at Sr2 sites. The rates of relaxation of excited Eu2+ ions at Sr1 and Sr2 sites (radiative + non-radiative relaxation) to the ground state are described by kSr1 and kSr2, respectively. The initial ratio of excited [Sr1] and [Sr2] populations was determined by fitting the PL spectra of each sample at the earliest time point available (5 ns). The rate of energy transfer between Sr1 and Sr2 sites is KET as described above. Because the energy of Sr2 sites is lower than Sr1 sites by ~100 meV,24 we assumed the rate of energy transfer from Sr2 back to Sr1 sites was negligible. Therefore, we excluded this process from the model and considered energy transfer to be unidirectional from Sr1 to Sr2 sites as indicated in Figure 1b. Section 9 in the supporting information describes the detailed development of the kinetic model, which permitted us to fit the TRPL data and uniquely extract rate constants for energy transfer from Sr1 to Sr2 sites kET for each carbidonitride phosphor. The values of the energy transfer rate constants are tabulated in Table S4 and plotted versus the wt% carbon substituted in the corresponding phosphors in Figure 6. The error bars reflect the uncertainty limits of the nonlinear least-squares fits of the [Sr1] population decay curves represented in Figure S13. The data demonstrate that substituting 0.30 wt% carbon resulted in a nearly three-fold enhancement of the energy transfer rate constant in comparison to the phosphor with only 0.05 wt% carbon substitution prior to aging. We note that energy transfer is typically a loss mechanism in luminescent materials because excited state energy can migrate to non-emissive sites in the lattice. However, the photoluminescence quantum yields of the carbidonitride phosphors (Table S2) do not decrease with the same linear trend with increasing carbon substitution that is 13 ACS Paragon Plus Environment


observed for the energy transfer rates. This finding suggests that the increased rate of energy transfer does not decrease the initial quantum yield of carbidonitride phosphors.

1.6 1.4

kET x 105 (s-1)

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1.2 1 0.8 0.6 0

0.1

0.2

0.3

%wt Carbon Figure 6. Energy transfer rate constants kET for carbidonitride phosphors substituted with various amounts of carbon. The dashed lines are linear fits to the data that serve as guides to the eye.

Independent of the precise mechanism, the rate of energy transfer should depend sensitively on the distance between Sr1 and Sr2 sites in the carbidonitride lattice. Direct measurement of Sr1 to Sr2 site distances in the phosphors is challenging because the synthesis method makes it difficult to isolate sufficiently large single-crystals. Therefore, we constructed structure models of the Sr2Si5N8 and the carbidonitride lattice using Density Function Theory (DFT) computations for comparison to the X-ray powder diffraction measurements appearing in Figures S2, S3 and S4. Figure 7a depicts the phosphor unit cell with no carbon substitution into the lattice, and Figure 7b represents a supercell with 0.47 wt% carbon and oxygen in the lattice (corresponding to two carbon and two oxygen atoms in the supercell with 96 nitrogen sites), respectively. To verify that the structure models accurately describe the phosphors, atomic coordinates and unit cell parameters were extracted from optimized structures of both models.20

These

optimizations were performed using the PW91 DFT method and the DNP double numerical basis set as implemented in the Materials Studio DMol^3 (version 8.0) software package. 27-28 The computed lattice constants of the Sr2Si5N8 model were in close agreement with the experimentally 14 ACS Paragon Plus Environment

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measured values (a = 5.76 Å, b = 6.86 Å, and c = 9.43 Å).24-25 The average Sr-N spacings were 2.9931 and 2.9537 Å for the Sr1 and Sr2 sites, respectively. These values are close to the experimentally determined distances of 2.969 and 2.928 Å and accurately reproduce the 0.04 Å difference between the sites.24-25 The computed supercell with formula Sr2Si5O0.17N7.66C0.17 (0.47 wt% carbon) described the carbidonitride lattice using the space group Pn (7) and a 1×2×3 kpoint mesh.

A 0.023 Å (0.34%) contraction along the (010) direction was observed

computationally which agrees closely with experimental contraction for a similarly substituted carbidonitride phosphor (Figure S4).20 Details of the DFT computations and relevant parameters are provided in Supplemental Information.

Figure 7. (a) Unit cell of a Sr2Si5N8 lattice obtained from DFT computations. (b) Structure of a supercell of a carbidonitride lattice with 0.47 wt% carbon substituted in the lattice. The structure was obtained by minimizing the energy of Sr2Si5O0.17N7.66C0.17 supercell using DFT calculations.

Based on the agreement between experimental and computational results, the average Sr1Sr2 site distances were calculated for the Sr2Si5N8 and the Sr5Si5O0.17N7.66C0.17 carbidonitride lattice (0.47 wt% carbon) using procedures detailed in the supporting information (Section 11). From this analysis, we found that the carbidonitride lattice exhibited a reduction of the average Sr1-Sr2 distance by 3.9% when compared to the Sr2Si5N8 lattice. To extend this analysis of the structure models to the experimentally measured carbidonitride phosphors, we note that the lattice constants of the carbidonitrides vary linearly with increasing wt% carbon substitution as 15 ACS Paragon Plus Environment


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reported previously.20 Therefore, the average Sr1-Sr2 distances in the experimentally examined carbidonitride phosphors are expected to decrease linearly with increasing carbon substitution. As a consequence, the average Sr1-Sr2 site distance decreased by approximately 2.5% in the 0.30 wt% phosphor samples. We considered a variety of energy transfer mechanisms to determine whether changes in structure of the carbidonitride phosphors described above could quantitatively explain the observed variation of energy transfer rates and corresponding emission spectra. The analysis described above reveals that for the carbidonitride phosphors, a three-fold change in the rate of energy transfer is associated with a 2.5% change in the Sr1-Sr2 distance. Previous studies of energy transfer in activated phosphors identified dipole-dipole or higher order multi-polar energy transfer as the dominant mechanism on the basis of the variation of energy transfer rates with Eu2+ concentration.29-33

The 1/r6 distance dependence of the dipole-dipole energy transfer

mechanism34 predicts that the average Sr1-Sr2 energy transfer distance must decrease 15% to predict the same three-fold change of the measured energy transfer rate. Multi-polar energy transfer processes with higher order distance dependence such as 1/r8 and 1/r10 still predict distortions of 11% and 9% for the 0.05 wt% and 0.30 wt % carbon substituted carbidonitride phosphors, respectively.

The data therefore indicate that an energy transfer mechanism

depending more sensitively on distance is required to explain the observed change in energy transfer rates. Energy transfer via the tunneling mechanism is known to occur in other contexts for which long-range energy transfer is important.35 This mechanism predicts a much steeper exponential dependence of the energy transfer rate with distance. Following development of the model as outlined in the supporting information (Section 12), the tunneling mechanism predicts a reduction of the energy transfer distance of only 3-5% to reproduce the measured change in energy transfer rates comparing the 0.30 and 0.05 wt% carbon substituted carbidonitride phosphors for a variety of realistic estimates of electronic coupling HDA and reorganization energy  values in the model. 16 ACS Paragon Plus Environment

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This change in energy transfer distance is close to the value of 2.5% calculated from analysis of the carbidonitride supercell in Figure 7b.

We note that calculation of HDA and  for the

carbidonitride phosphors examined here would be needed to definitively establish tunneling as the mechanism of energy transfer. Never the less, the tunneling mechanism exhibits the strong distance dependence that is needed to account for the measured changes in energy transfer rates. We therefore conclude that the structural changes caused by carbon and oxygen in the carbidonitride lattice are quantitatively responsible for the faster energy transfer rates and associated spectral changes. We used the correlation of structure changes and energy transfer rates described above to elucidate the origin of the enhanced aging stability of the carbidonitride phosphors (Figure S8). Figure 8a compares the normalized steady-state emission spectrum of a pristine carbidonitride phosphor with 0.05 wt% carbon content with the emission spectrum of the same carbidonitride phosphor after accelerated aging. Here, the relative emission intensity on the longer wavelength side of the spectrum increased even while the total emission intensity decreased after aging of the phosphor. The decrease in the emission led to a significant decrease in the luminescent efficiency (Figure S8). In contrast, the carbidonitride phosphor containing 0.30 wt% carbon did not exhibit the same red-shift with accelerated aging (Figure 8b). This phosphor also exhibited the smallest decrease of luminescent efficiency with aging that was observed in the set (Figure S8). In Figure 8c, the emission spectrum of the aged carbidonitride phosphor containing 0.05 wt% carbon is compared to the emission spectrum of the pristine carbidonitride phosphor with 0.30 wt% carbon. The comparison demonstrates that aging has the same effect on the emission spectrum of the 0.05 wt% phosphor that is observed with substitution of 0.30 wt% carbon in the lattice. We therefore hypothesized that aging caused the 0.05 wt% carbon phosphor to undergo an increase of energy transfer rate from Sr1 to Sr2 sites, implying that aging caused a lattice change corresponding to the reduction of the Sr1 to Sr2 site distances. Unfortunately, direct measurement of Sr1 to Sr2 17 ACS Paragon Plus Environment


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Figure 8. (a) Comparison of emission spectra of the carbidonitride phosphor with 0.05 wt% carbon before (solid black) and after (dashed black) aging. The shaded region highlights the change of the emission spectra due to aging. (b) Comparison of emission spectra of the carbidonitride phosphor with 0.30 wt% carbon before (solid blue) and after (dashed blue) aging. (c) Comparison of spectra of the unaged 0.30 wt% carbon phosphor with the aged 0.05 wt% phosphor. Substitution of carbon in the lattice has the same effect on the spectrum as aging but without the loss of luminescence efficiency (Figure S8).

distance after aging the samples using X-ray powder diffraction is challenging because of the small crystallite sizes that are obtained from the synthesis method. Furthermore, the phosphor crystals cannot easily be separated from the polymer binder used to mimic functional pc-LED packages.


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Therefore, we used TRPL spectroscopy to measure the time-dependence of the emission spectra of the phosphors before and after aging. Correlating changes of energy transfer rates with aging provides a means to learn about the corresponding structural changes that accompany aging. Figures 9a and 9b represent the time dependence of the emission maxima and the FWHM of the spectra of carbidonitride phosphors with 0.05, 0.10, 0.20, and 0.30 wt% carbon substitution. The shaded regions highlight changes that result from aging of the phosphors. Shifts to longer wavelength and broadening of emission FWHM are indicative of faster energy transfer with aging. Notably, the 0.30 wt% carbon substituted carbidonitride phosphor underwent negligible change of energy transfer rate with aging, and the lowest decrease in emission intensity during aging. In contrast, the carbidonitride phosphor with 0.05 wt% carbon exhibited the largest change in energy transfer within the set and the most pronounced decrease of emission intensity with aging.

Figure 9. (a) The time-dependence of the photoluminescence (PL) peak positions of the phosphors measured after pulsed excitation and incorporating various amounts of carbon and oxygen. (b) The time-dependence of the fullwidth at half-maxima (FWHM) of the PL peaks of the phosphors further highlights the effects of aging.

We used the kinetic model (Equation S4) and spectral fitting procedure (see supporting information, Section 9) to fit the TRPL spectra of the aged phosphors for comparison to the corresponding phosphors before they were aged. As before, we calculated the rates of energy transfer and plotted the values of kET in Figure 10 (data labeled ‘Aged’). The values of the energy transfer rate constants are tabulated in Table S4.

The data reveal that unlike the pristine 19

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phosphors, the rate of energy transfer from Sr1 to Sr2 sites changed little with carbon content after the phosphors were aged. Energy transfer rates are associated with structural changes of the carbidonitride lattice. Therefore, the analysis of energy transfer rates reveals that the pristine 0.30 wt% phosphor possesses average Sr1-Sr2 site distances that are obtained in the other phosphors only by accelerated aging. Because the 0.30 wt% carbidonitride phosphor adopted the structure corresponding to a smaller Sr1-Sr2 site distance in initial synthesis, it did not undergo subsequent structural change during aging.

1.6

kET x 105 (s-1)

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1.4 1.2 1 0.8

Aged Pristine

0.6 0

0.1

0.2

0.3

%wt Carbon Figure 10. Energy transfer rate constants kET for carbidonitride phosphors substituted with various amounts of carbon before and after they were aged. The dashed lines are linear fits to the data that serve as guides to the eye.

Importantly, substitution of 0.30 wt% carbon in the carbidonitride lattice occurred during synthesis, at high temperature, where the lattice was able to accommodate the compressed (010) crystal axis. Although a compressed structure along the (010) direction was formed by aging phosphors with lower carbon content, these structural changes occurred at much lower temperature (only ~170 oC). At these temperatures, the crystalline lattice could not reorganize to minimize the perturbation caused by contraction along the (010) direction (and associated expansion in the other two directions).

Therefore, the accumulation of strain in the lattice

enhanced the coupling of phonon modes to the electronic transition,21 which caused the reduction of luminescent efficiency of the aged carbidonitride phosphors with lower wt% carbon substitution 20 ACS Paragon Plus Environment

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(Figure S8). However, because the 0.30 wt % carbon substituted phosphor already exhibited the contraction along the (010) direction, aging caused little distortion of the lattice and improved maintenance of color stability and photoluminescence efficiency.

4. Conclusions In this work, we explored the influence of carbon substitution and aging on carbidonitride emission characteristics. Previous measurements of carbidonitride phosphor luminescence with small amounts of substitutional carbon exhibited markedly improved luminescent maintenance at 150 o

C.20 This improvement in luminescent maintenance was attributed to contraction of the (010)

direction of the crystalline lattice that reduced the coupling of phonon modes to the electronic transition of the Eu2+ ions that activated the phosphor. The structural changes that gave rise to improved luminescent maintenance of the phosphors also changed the average distances between Sr1 and Sr2 sites in the carbon substituted lattice by about 2.5%. The increased rate of energy transfer that accompanied the addition of carbon to the lattice could be quantitatively described by long-range tunneling of the excited state energy. The faster rate of energy transfer increased the contribution of lower energy emission of the Sr2 site to the total emission, which red-shifted the spectrum of the phosphors incorporating carbon into the carbidonitride lattice. Aging of phosphors with varying carbon substitution was explored under conditions mimicking the pcLED package. The energy transfer rates of all phosphors after aging approached the faster rate measured in the 0.30 wt% carbon substituted phosphor even though these rates differed among the phosphors prior to aging. We therefore concluded that aging resulted in the same type of structural change of the lattice that was accomplished by the substitution of carbon and oxygen in the carbidonitride lattice. Because the average Sr1 and Sr2 distances in the 0.30 wt% carbon substituted phosphor had already contracted by substitution of carbon during synthesis, this phosphor exhibited negligible change in energy transfer rate and luminescence spectrum with aging. Furthermore, substitution of carbon in the lattice during synthesis occurred at sufficiently 21 ACS Paragon Plus Environment


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high temperature that the lattice was able to accommodate the contraction of the (010) direction of the crystal, thereby avoiding strain-induced quenching of phosphor emission. Importantly, our observation that lattice strain can be tuned during synthesis in a way that mimics the effects of aging opens new opportunities to utilize this approach to stabilize the emission spectra and luminescence efficiencies of a variety of red emitting phosphors. These findings and the associated mechanism of aging via accumulated lattice distortion will inform ongoing work in the field to improve the stability of emission properties of phosphors for pc-LED applications.

Author Information Corresponding Authors

A.N.S.: [email protected] J.B.A.: [email protected]

Acknowledgements This work was funded by The Dow Chemical Company funded under the University Project Initiative #225559AG. CG and JBA are grateful for financial support for construction of the timeresolved photoluminescence spectrometer used in this work from the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through Grant DE-SC0008120.

Associated Content Supporting Information Available: Synthesis methods, characterization of phosphor structures and photoluminescence properties, aging apparatus, time-resolved photoluminescence modeling


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procedures, density functional theory computational procedures, kinetic modeling, energy transfer rate analysis. This material is available free of charge via the Internet at http://pubs.acs.org.

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