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Luminescence and Luminescence Quenching of K2Bi(PO4)(MoO4):Eu3+ Phosphors with Efficiencies Close to Unity Julija Grigorjevaite and Arturas Katelnikovas* Department of Analytical and Environmental Chemistry, Vilnius University, Naugarduko 24, LT−03225 Vilnius, Lithuania S Supporting Information *

ABSTRACT: A very good light emitting diode (LED) phosphor must have strong absorption, high quantum efficiency, high color purity, and high quenching temperature. Our synthesized K2Bi(PO4)(MoO4):Eu3+ phosphors possess all of the mentioned properties. The excitation of these phosphors with the near-UV or blue radiation results in a bright red luminescence dominated by the 5D0 → 7F2 transition at ∼615 nm. Color coordinates are very stable when changing Eu3+ concentration or temperature in the range of 77−500 K. Furthermore, samples doped with 50% and 75% Eu3+ showed quantum efficiencies close to 100% which is a huge benefit for practical application. Temperature dependent luminescence measurements showed that phosphor performance increases with increasing Eu3+ concentration. K2Eu(PO4)(MoO4) sample at 400 K lost only 20% of the initial intensity at 77 K and would lose half of the intensity only at 578 K. Besides, the ceramic disks with thicknesses of 0.33 and 0.89 mm were prepared from K2Eu(PO4)(MoO4) powder, and it turned out that they efficiently converted the radiation of 375 nm LED to the red light. The conversion of 400 nm LED radiation to the red light was not complete; thus, the light sources with various tints of purple color were obtained. The combination of ceramic disks with 455 nm LED yielded the light sources with tints of blue color due to the low absorption of ceramic disk in this spectral range. In addition, these phosphors possess a very unique emission spectra; thus, they could also be applied in luminescent security pigments. KEYWORDS: red phosphor, thermal quenching, quantum efficiency, color coordinate, luminous efficacy

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Eu2+ in these materials exhibit broad band emission and, therefore, some of it lies in the deep red region (above 650 nm) and significantly decreases the luminous efficacy due to the low human eye sensitivity above this wavelength. Moreover, the emission above 650 nm is also considered as waste.20 In this case, the red line emitting phosphor (such as Eu3+) would be superior. With the constant development of LEDs, near-UV emitting semiconductor chips became more and more efficient. These have opened a new possibility for white light generation; i.e., a near-UV emitting chip can be combined with blue, green, and red phosphors to yield a white light. This approach has some advantages, since there are many more phosphors excitable with near-UV radiation when compared to the blue light. The efficient blue (BaMgAl10O17:Eu2+)21 and green (SrSi2O2N2:Eu2+, Ba2SiO4:Eu2+)22,23 phosphors already exist; thus, the main issues arise with finding efficient and low cost red emitting phosphors. In such investigations, molybdate based phosphors doped with Eu3+ ions received much attention. This is due to abnormally strong absorption of

INTRODUCTION Since the discovery of the efficient blue light emitting diode (LED) by Nakamura in 1991,1 solid state light sources based on blue LEDs became a revolution in the lighting industry. The first white LEDs combined blue light from an InGaN semiconductor chip and yellow light obtained from a partial conversion of the diodes blue light by the YAG:Ce (Y3Al5O12:Ce3+) coating. However, such a blend resulted in poor white light quality; i.e., it was a so-called cool white light with high correlated color temperatures (CCT) and low color rendering indices (CRI) due to the deficiency in the red spectral region. Later on, the attempts of shifting the YAG:Ce emission to red were performed and numerous new phosphors, such as Y3Mg2AlSi2O12:Ce3+,2 Lu2CaMg2(Si,Ge)3O12:Ce3+,3 and Mg3Y2Ge3O12:Ce3+,4 were reported. Another way to increase the intensity in the red spectral region was the application of the additional orange-red phosphor. Such investigations yielded some efficient and thermally stable nitride/oxynitride phosphors doped with Eu2+ ions, for instance, CaAlSiN 3 :Eu 2 + , 5 SrAlSiN 3 :Eu 2 + , 6 (Ca,Sr,Ba)2Si5N8:Eu2+,7−9 α-SiAlON:Eu2+,10,11 Ca15Si20O10N30:Eu2+,12 Ba3Ga3N5:Eu2+,13 Sr[Mg3SiN4]:Eu2+,14 Ba[Mg3SiN4]:Eu2+,15 (Ca,Sr,Ba)[Mg 2 Al 2 N 4 ]:Eu 2+ , 1 6 SrAlSi 4 N 7 :Eu 2 + , 1 7 Ca[LiAl3N4]:Eu2+,18 Sr[LiAl3N4]:Eu2+,19 and so on. However, © 2016 American Chemical Society

Received: September 16, 2016 Accepted: October 31, 2016 Published: October 31, 2016 31772

DOI: 10.1021/acsami.6b11766 ACS Appl. Mater. Interfaces 2016, 8, 31772−31782

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ACS Applied Materials & Interfaces Eu3+ in these lattices resulting from the admixing of charge transfer (CT) band and excited states of Eu3+ ions.24 The recent development of red phosphors based on the molybdate matrixes yielded some efficient line emitting red phosphors, such as NaxEuy(MoO4)z,25 LiEu(MoO4)2,26 Y2Mo4O15:Eu3+,24 and Li3Ba2La3(MoO4)8:Eu3+,27 and the investigation is still ongoing. The crystal structure of Na2Y(PO4) (MoO4) was first reported by Ben Amara and Dabbabi in 1987.28 Nevertheless, it took more than two decades for the first papers to appear dealing with luminescence of various modifications (Na2RE(PO4)(WO4),29 K2Y(PO4)(MoO4):Ln3+,30 etc.) of this class of compounds. In 2006, a new derivative K2Bi(PO4)(MoO4) was reported by Zatovsky et al.31 The basic luminescence properties of K2Bi(PO4)(Mo1−yWyO4):Eu3+ phosphors were investigated by He et al. in 2010;32 however, no data on absorption, luminous efficacy, color coordinates, quantum yield, thermal quenching behavior, and other important phosphor parameters were given. To this end, we thoroughly investigated K2Bi(PO4)(MoO4):Eu3+ phosphors and evaluated their potential to be applied in light emitting devices based on a near-UV emitting semiconductor chip. The presented data will include reflection, excitation and emission spectra, luminescence lifetimes, luminous efficacies, quantum efficiencies, color coordinates in both CIE 1931 and CIE 1976 color space diagrams, temperature dependent emission spectra, luminescence lifetimes, and color coordinates.

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Excitation and emission spectra were recorded on the Edinburgh Instruments FLS980 spectrometer equipped with double excitation and emission monochromators, 450 W Xe arc lamp, a cooled (−20 °C) single-photon counting photomultiplier (Hamamatsu R928), and mirror optics for powder samples. The photoluminescence emission spectra were corrected by a correction file obtained from a tungsten incandescent lamp certified by NPL (National Physics Laboratory, UK). When measuring emission spectra (λex = 394 nm), excitation and emission bandwidths were set to 0.4 and 0.15 nm, respectively. When measuring excitation spectra (λem = 615 nm), excitation and emission bandwidths were set to 0.2 and 0.3 nm, respectively. The excitation spectra were corrected by a reference detector. In both cases, step width was 0.5 nm and integration time was 0.4 s. Color coordinates in CIE 1931 and CIE 1976 color space diagrams were calculated from emission spectra employing the Edinburgh Instruments F980 software (version 1.3.1). For temperature dependent emission and photoluminescence decay measurements, a cryostat “MicrostatN” from the Oxford Instruments had been applied to the present spectrometer. Liquid nitrogen was used as a cooling agent. The measurements were performed at 77 K and at 100−500 K in 50 K intervals. Temperature stabilization time was 90 s, and temperature tolerance was set to ±5 K. During the measurements, dried nitrogen was flushed over the cryostat window to avoid the condensation of water at low temperatures on the surface of the window. The photoluminescence decay curves were measured on the FLS980 spectrometer. A Xe μ-flash lamp μF920 was used as an excitation source. Excitation wavelengths of 265, 394, and 465 nm were selected while emission was monitored at 615 nm. External quantum efficiencies (EQE) were calculated by measuring emission spectrum of the Teflon sample in a Teflon coated integration sphere. Excitation wavelengths were 265, 394, and 465 nm, and emission spectra were recorded in the ranges of 250−800, 375−800, and 445−800 nm, respectively. The excitation and emission slits were set to 1.50 and 0.35 nm, respectively. Step width was 0.5 nm, and integration time was 0.4 s. The same measurements were repeated for the phosphor samples. The EQE values were obtained by employing the following formula:33

EXPERIMENTAL SECTION

Synthesis. Herewith described K2Bi1−xEux(PO4) (MoO4) phosphor powders were prepared by conventional high temperature solidstate reaction. The stoichiometric amounts of high purity raw materials Bi2O3 (99.9% Acros Organics), Eu2O3 (99.99% Tailorlux), MoO3 (99+% Acros Organics), K2CO3 (99+% Acros Organics), and NH4H2PO4 (99% Reachem Slovakia) were weighed to the nearest 0.0001 g and thoroughly mixed in an agate mortar employing acetone as the grinding medium. The obtained blend was transferred to the porcelain crucible and annealed three times at 600 °C for 12 h in air with regrinding of the powder after each annealing. The K2Eu(PO4)(MoO4) ceramic disks with a thickness of 0.33 and 0.89 mm were prepared by applying 30 kN force (ø 8 mm disk) on the phosphor powder for 3 min. Subsequently, the obtained pellets were sintered at 600 °C for 4 h in air. Characterization Techniques. XRD patterns for the phase identification were recorded at 5° ≤ 2θ ≤ 80° using Ni-filtered Cu Kα radiation on a Rigaku MiniFlexII diffractometer. The step width and scanning speed was 0.02° and 5°/min, respectively. XRD data for Rietveld refinement were collected at 5° ≤ 2θ ≤ 100° (step width of 0.01° and integration time of 3 s) using Ni-filtered Cu Kα radiation on a Bruker D8 Advance diffractometer with Bragg− Brentano focusing geometry and a position sensitive LynxEYE detector. The Rietveld refinement was carried out on FullProf Suite Program (3.00) software. Infrared spectra were recorded in the range of 3000−400 cm−1 employing a Bruker ALPHA ATR spectrometer with 4 cm −1 resolution. The SEM images of phosphor powders and ceramic disks were taken by a FE-SEM Hitachi SU-70. The accelerating voltage was 2.0 kV. Reflection spectra were recorded on an Edinburgh Instruments FLS980 spectrometer equipped with double excitation and emission monochromators, a 450 W Xe arc lamp, a cooled (−20 °C) singlephoton counting photomultiplier (Hamamatsu R928), and an integration sphere coated with Teflon. Teflon was also used as a reflectance standard. The excitation and emission bandwidths were 3.00 and 0.15 nm, respectively. Step width was 0.5 nm, and integration time was 0.2 s.

EQE =

∫ Iem,sample− ∫ Iem,Teflon N × 100% = em × 100% Nabs ∫ Iref,Teflon − ∫ Iref,sample

(1)

Here, ∫ Iem, sample and ∫ Iem, Teflon are integrated emission intensities of the phosphor sample and Teflon, respectively. ∫ Iref, sample and ∫ Iref, Teflon are the integrated reflectance of the phosphor sample and Teflon, respectively. Nem and Nabs stand for the number of emitted and absorbed photons, respectively. The EQE measurements for each sample were repeated five times in order to get some statistical data. All measurements were performed at room temperature and ambient pressure in air unless specified otherwise.

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RESULTS AND DISCUSSION

The K2Bi(PO4)(MoO4) compound crystallizes in a body centered orthorhombic Bravais lattice with the space group Ibca (#73) (Z = 8). The unit cell contains PO4 and MoO4 tetrahedrons and BiO8 dodecahedra. Each BiO8 unit shares the vertex with two MoO4 and two PO4 tetrahedrons and the edge with two PO4 tetrahedrons and another two BiO8 octahedrons. All this forms infinite zigzag chains extending parallel to the [001] plane.34 Each of these chains are separated by two potassium layers. The unit cell of K2Bi(PO4)(MoO4) along the c-axis is shown in Figure S1. The synthesis of K2Bi(PO4)(MoO4):Eu3+ phosphors yielded single phase compounds regardless of the Eu3+ concentration (see Figure S2). This shows that a solid solution forms at any Bi/Eu ratio, which is in line with Vegard’s law and predicts that a complete solid solution forms if the size difference of cations is less than 15%.35 This is true for ionic radii of eight31773


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ACS Applied Materials & Interfaces coordinated Bi3+ (1.17 Å) and Eu3+ (1.066 Å).36 In order to evaluate the Eu3+ substitution for Bi3+ influence to the lattice parameters, a Rietveld refinement was performed. The characteristic refinement graph is depicted in Figure S3, which shows a reasonable match between measured and calculated XRD pattern. The calculated lattice parameters a, b, c, and V as a function of Eu3+ concentration are given in Figure 1. Since Eu3+ is slightly smaller than Bi3+, the increasing

The body color of undoped K2Bi(PO4)(MoO4) was pale yellowish which goes hand in hand with the reflection spectra shown in Figure 3a. The yellowish body color originates from

Figure 3. Reflection spectra of K2Bi(PO4)(MoO4) and K2Eu(PO4)(MoO4) (a) and digital photos of K2Eu(PO4)(MoO4) at daylight (b) and under 366 nm excitation (c).

the slight absence of violet-blue radiation due to absorption in the range of 360−480 nm. The absorption in the range of 250− 360 nm can be attributed to the host lattice. It is interesting to note that this band can be divided into two separate bands. The first one is at ∼310 nm and the second one at ∼340 nm. Since the conduction band of the K2Bi(PO4)(MoO4) compound is composed of unoccupied orbitals of K+, Bi3+, P5+, and Mo6+ cations, it is very likely that the first and the second bands are attributed to electron transfer from valence band to unoccupied orbitals of Bi3+ and Mo6+, respectively. This assumption is supported by very high reduction potentials of K+ and P5+ ions and relatively smaller reduction potentials of Bi3+ and Mo6+ ions. Moreover, the first band vanishes if all Bi3+ is replaced by Eu3+, once again supporting the previous statement. K2Eu(PO4)(MoO4), in turn, possesses a pale rose body (see Figure 3b) color due to absorption in the violet and blue spectral range followed by weak emission by Eu3+ ions.27 This goes hand in hand with the reflection spectrum of K2Eu(PO4)(MoO4) depicted in Figure 3a where the typical Eu3+ absorption lines originating from 7F0 → 5DJ to 7F0 → 5LJ and 7F0 → 5GJ transitions are clearly visible.37 Moreover, it is also evident that 7 F1 and 7F2 levels are also thermally populated to some extent due to the presence of 7F2 → 5D0 (ca. 615 nm), 7F1 → 5D0 (ca. 592 nm), and 7F1 → 5D1 (ca. 536 nm) transitions in reflection spectra of K2Eu(PO4)(MoO4).38 The reflectance at longer

Figure 1. Unit cell parameters of K2Bi(PO4)(MoO4):Eu3+ obtained from Rietveld refinement.

europium concentration leads to a linear (R2 = 0.999) decrease of lattice parameters b, c, and V. However, this is contrary to lattice parameter a, which tends to increase with increasing Eu3+ concentration. This demonstrates that the unit cell of K2Bi(PO4)(MoO4) lengthens along the a-axis if Bi3+ is replaced by Eu3+ ions. The calculated lattice parameters are tabulated in Table S1. One should also note, however, that the overall decrease of lattice parameters is rather small: Δa, Δb, Δc, and ΔV being 0.12%, 1.48%, 0.71%, and 2.07%, respectively. The morphological features of K2Eu(PO4)(MoO4) powders and ceramics were investigated by taking SEM images, which are given in Figure 2. The powder particles are formed from smaller crystallites that possess a rather broad size distribution and a rod-like shape. No differences in particle size and morphology were observed upon varying Eu3+ concentration. Figure 2c demonstrates that the surface of ceramic disks is formed from various size crystallites, which are well grown together and form a relatively smooth surface.

Figure 2. SEM images of K2Eu(PO4)(MoO4) powders (a, b) and ceramics (c). 31774


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ACS Applied Materials & Interfaces

Figure 4. (a) Excitation (λem = 615 nm) and (b) emission (λex = 394 nm) spectra of K2Eu(PO4)(MoO4) at 77 and 300 K. Inset shows emission (λex = 394 nm) integral intensity as a function of Eu3+ concentration.

Figure 5. Temperature dependent emission (λex = 394 nm) spectra of (a) K2Bi(PO4)(MoO4):1%Eu3+ and (b) K2Eu(PO4)(MoO4). Estimation of TQ1/2 values for the samples doped with 1%, 50%, and 100% Eu3+ from normalized emission intensity at 615 nm (c) and normalized emission integrals (d).

wavelengths for both undoped and fully Eu3+ substituted compounds is close to unity which shows a high brilliance of the samples. In order to calculate the optical band gap of synthesized materials, the absorption spectra were calculated from reflection spectra employing the Kubelka−Munk function.39 It was found that the optical band gap values are very similar regardless of Eu3+ concentration and were 314 nm (3.95 eV) and 322 nm (3.85 eV) for an undoped and fully Eu3+ substituted compound, respectively. The optical band gap

values of the compounds with the remaining Eu3+ concentrations lie within this interval. Figure 4a shows excitation spectra of the K2Eu(PO4)(MoO4) sample at 77 and 300 K for 615 nm emission. Both spectra are normalized (divided by max value) in order to get a clear picture of the change of relative line intensity. Both spectra contain a broad band in the range of 250−310 nm, and it can be attributed to the charge transfer transition from oxygen to europium. There are also several sets of lines in the range of 31775


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Such emission spectra of two representative samples doped with 1% and 100% Eu3+ are given in Figures 5a,b, respectively. In both cases, emission intensity decreases with increasing temperature; however, the decrease for 100% Eu3+ doped sample is relatively slower. This data can also be employed in calculation of the TQ1/2 values (temperature at which phosphor loses half of its efficiency). The estimation of TQ1/2 values is very important if the phosphors are to be applied in high power LEDs, which can heat up even to 150 °C. The TQ1/2 value is calculated from the Boltzmann sigmoidal fit:43

310−600 nm. These lines arise from the intraconfigurational [Xe]4f6 → [Xe]4f6 transitions of Eu3+ ions, namely, 7F0 → 5HJ (ca. 318 nm), 7F0 → 5D4 (ca. 360 nm), 7F0 → 5L7,8, 5GJ (ca. 380 nm), 7F0 → 5L6 (ca. 395 nm), 7F0 → 5D3 (ca. 415 nm), 7F0 → 5D2 (ca. 465 nm), 7F0 → 5D1 (ca. 525 nm), 7F1 → 5D1 (ca. 535 nm), 7F0 → 5D0 (ca. 580 nm), and 7F1 → 5D0 (ca. 590 nm). It is evident that the change of temperature from 77 to 300 K has an enormous effect on the relative intensity of these transitions. This is especially true for the transitions originating from the 7F1 state. At 300 K, such transitions are even more intensive than their 7F0 counterparts; however, at 77 K, transitions starting from the 7F1 state vanish due to depopulation of this state at low temperature. Furthermore, the intensive transitions at around 395 and 465 nm are very attractive for application of such phosphors as color converters in LEDs since they overlap very well with the efficient near-UV and blue emitting LEDs, respectively.24 Excitation of all samples with a UV lamp (λex = 366 nm) yielded bright red luminescence as indicated in Figure 3c. The normalized emission spectra of the K2Eu(PO4)(MoO4) sample at 77 and 300 K are depicted in Figure 4b. There are five sets of lines visible in both spectra. All these lines originate from the intraconfigurational Eu3+ transitions, i.e., 5D0 → 7F0 (ca. 580 nm), 5D0 → 7F1 (ca. 590 nm), 5D0 → 7F2 (ca. 615 nm), 5D0 → 7F3 (ca. 650 nm), and 5D0 → 7F4 (ca. 700 nm). The intensity of the 5D0 → 7F0 transition is the weakest one because J = 0 ↔ J′ = 0 transitions are always forbidden.40−42 The most intensive is an electric dipole (ED) 5D0 → 7F2 transition. The magnetic dipole (MD) 5D0 → 7F1 transition is around 2.5 times less intensive. Such a relatively small intensity difference between ED and MD transitions can be explained by a rather symmetrical dodecahedral BiO8 site, where Eu3+ ion is incorporated. The symmetric sites, especially with the inversion symmetry, are favorable to MD transitions. Besides, only minute differences in asymmetry ratio (5D0 → 7F2)/(5D0 → 7 F1) were observed upon changing Eu3+ concentration and/or temperature. For instance, the asymmetry ratio changed from 2.9 to 3.2 when Eu3+ concentration was increased from 1% to 100%. Upon changing the temperature from 77 to 500 K for 1%, 50%, and 100% Eu3+ doped samples, the asymmetry ratio changed from 2.8 to 3.1, from 3.0 to 3.2, and from 2.9 to 3.2, respectively. This suggests that neither Bi3+ replacement with Eu3+ nor increasing temperature substantially deforms the dodecahedral site, which as shall be seen later is important in color coordinate stability. Another interesting feature of the presented emission spectra is an unusually intensive and split 5 D0 → 7F4 transition. Usually, the intensity of 5D0 → 7F4 transition is very low and the peak is not split. However, the origin of such fine splitting is unknown for us, and this phenomenon requires additional investigation. Despite the unknown origin of the fine line splitting, this feature could be employed in luminescent markers where the emission spectra with distinct features are of great interest. It is also evident that the increasing temperature leads to emission line broadening which can be explained by higher lattice vibrations at elevated temperature. The inset of Figure 4b shows K2Bi(PO4)(MoO4):Eu3+ emission integral intensity as a function of Eu3+ concentration. It is obvious that emission intensity increases with increasing Eu3+ concentration and that the strongest emission is obtained when all Bi3+ is replaced with Eu3+ ions. In order to evaluate the K2Bi(PO4)(MoO4):Eu3+ phosphors performance at elevated temperature, the temperature dependent emission spectra in the range of 77−500 K were recorded.

y(x) = A 2 +

A1 − A 2 1 + e((x − x0)/dx)

(2)

Here, y(x) is the normalized emission (or emission integral) intensity at a given x (temperature in K in this case), and A1 (left horizontal asymptote) and A2 (right horizontal asymptote) are the initial and the final value, respectively. x0 (x0 = TQ1/2) is the center of the sigmoid and dx is the change in x corresponding to the most significant change in y(x) values. The fitting was performed on the normalized data (divided by the max value); therefore, the parameters A1 and A2 were set to 1 and 0, respectively. In principle, there are two sets of data that one can use for the calculation of TQ1/2 values, i.e., normalized emission of the most intensive peak or normalized emission integral. The determination of the TQ1/2 value from the first mentioned data set is depicted in Figure 5c. It is evident that the intensity of all three measured samples decrease roughly in the same manner regardless of the Eu3+ content in the phosphor. However, when the integrated emission spectra are taken, the view changes considerably (see Figure 5d). In fact, the integrated emission data is more favorable in such calculations since it also includes the line broadening at elevated temperatures. The calculated TQ1/2 values (in the range of 77−500 K) for 1%, 50%, and 100% Eu3+ doped samples are 398, 487, and 578 K, respectively. This indicates that samples doped with lower Eu3+ concentrations are more sensitive to the temperature quenching. Besides, the emission integral method is also insensitive to the emission peak position change upon increasing temperature.24 When the temperature dependent emission integral data are employed, the quenching activation energy according to the single barrier quenching model can be derived:44−46 I (T ) 1 = I0 1 + Be−EA / kT

(3)

Here, I(T) and I0 are the temperature dependent emission integral and the highest value of emission integral, respectively. EA is activation energy in eV, k is Boltzmann constant (8.617342 × 10−5 eV/K),47 and T is temperature in K. B is the quenching frequency factor and can also be expressed as Γ0/Γυ, where Γ0 is an attempt rate of the nonradiative process and Γυ is the radiative rate. Such calculations yielded activation energies of 0.058 ± 0.004, 0.074 ± 0.007, and 0.075 ± 0.008 eV for 1%, 50%, and 100% Eu3+ doped samples, respectively. Figure 6a displays photoluminescence (PL) decay curves of K2Bi(PO4)(MoO4):Eu3+ samples doped with 1%, 50%, and 100% Eu3+ when specimens were excited at 394 nm and emission was monitored at 615 nm. The decay curves are linear thus suggesting that there is only one depopulation mechanism of the emitting 5D0 state. The decay curves become less steep upon increasing the Eu3+ concentration suggesting the increase 31776


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energy migration between Eu3+ ions. The probability of such energy migration of course increases with increasing Eu3+ content in the structure; therefore, it takes longer for the luminescence to occur. However, the situation is different if samples are excited through the CT band at 265 nm. In this case, the PL lifetime values are relatively independent of the Eu3+ concentration with the exception of a sample doped with 100% Eu3+ when the PL lifetime value is virtually the same as in the case of 394 and 465 nm excitation. The calculated PL lifetime values together with standard deviations as a function of excitation wavelength and Eu3+ concentration are given in Table S2. The temperature dependent decay curves of samples doped with 1%, 50%, and 100% Eu3+ were measured in order to gain the better insight into the thermal quenching process. The representative decay curves of the K2Eu(PO4)(MoO4) specimen are shown in Figure 7a. It was found that the temperature

Figure 6. (a) Photoluminescence decay (λex = 394 nm, λem = 615 nm) curves of K2Bi(PO4)(MoO4):Eu3+ samples doped with 1%, 50%, and 100% Eu3+. (b) PL lifetimes (τ1/e) of K2Bi(PO4)(MoO4):Eu3+ as a function of Eu3+ concentration and excitation wavelength.

of photoluminescence lifetime. All decay curves were fitted by employing a single exponential decay function: I(t ) = A + Be−t/ τ

(4)

Here, I(t) is photoluminescence intensity at a given time t, A is background, B is constant, and τ is PL lifetime. The obtained lifetime values as a function of temperature and excitation wavelength (265, 394, and 465 nm) are shown in Figure 6b. First, it is necessary to mention that the calculated photoluminescence lifetimes of K2Bi(PO4)(MoO4):Eu3+ phosphors are unusually long. Typically, for the molybdate based Eu3+ doped compounds, the photoluminescence lifetime is in the range of ∼1 ms,27,44,48 whereas in our case it is around two times longer. The increased PL lifetimes might be related to the charge transfer band lying at higher energies which is in contrast with the other molybdates. The decreasing charge transfer energy is known to have a great effect on the PL lifetimes; i.e., PL lifetime increases if the energy of charge transfer band increases.49,50 It is also evident that excitation at 394 or 465 nm (7F0 → 5L6 and 7F0 → 5D2) yields virtually the same PL lifetime values. This implies that excitation to either the 5L6 or 5D2 state is followed by a fast relaxation to the emitting 5D0 state. Moreover, the PL lifetimes (when samples were excited with 394 or 465 nm radiation) increase with increasing Eu3+ concentration. This can be explained by the

Figure 7. (a) Photoluminescence decay (λex = 394 nm, λem = 615 nm) curves of K2Eu(PO4)(MoO4) at 77 and 500 K. (b) Temperature dependent PL lifetimes (τ1/e) of K2Bi(PO4)(MoO4):Eu3+ doped with 1%, 50%, and 100% Eu3+. The lines were drawn to guide the eyes.

has very little effect on the shape of decay curves; therefore, only the ones recorded at 77 and 500 K are shown. Similar changes in decay curves were also observed for the samples doped with 1% and 50% Eu3+. From every temperature dependent decay curve, the PL lifetime values were calculated and plotted in Figure 7b. The photoluminescence lifetime values for the specimens doped with 1% and 50% Eu3+ steadily 31777


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ACS Applied Materials & Interfaces decrease with increasing temperature. This shows the increasing probability of the nonradiative transitions which, in turn, decreases the lifetime of the excited 5D0 state. The situation, however, is slightly different for the K2Eu(PO4)(MoO4) sample. Even though the PL lifetime values are decreasing with increasing temperature, the observed decrease is not as steady as in the samples with lower europium content. There are two temperature ranges where the lifetime values decrease faster, i.e., 77−200 and 350−500 K, and one temperature range where lifetime values are rather constant, i.e., 200−350 K. Unfortunately, we do not have an explanation for such an odd change in lifetime values. The exact calculated PL lifetime values with the standard deviation as a function of temperature for the samples doped with 1%, 50%, and 100% Eu3+ are tabulated in Table S3. Since the overall change in PL lifetime values for all measured specimens in the temperature range of 77−500 K was only around 10%, it was not possible to apply the Boltzmann function for TQ1/2 determination. However, by comparing the temperature dependent emission integrals (see Figure 5d) with the temperature dependent PL lifetimes (see Figure 7b), one can draw a conclusion that integral intensity (which is proportional to the external quantum efficiency (EQE)) is decreasing much faster than the PL lifetimes (which are proportional to the internal quantum efficiency (IQE)). The internal quantum efficiency can be written as27,51−53 IQE =

Wr τ = Wr + Wnr τ0

Figure 8. External quantum efficiencies of K2Bi(PO4)(MoO4):Eu3+ phosphors as a function of Eu3+ concentration and excitation wavelength.

respectively. Keeping in mind that a good phosphor should also possess strong absorption, the selection of fully substituted compounds (at least in the 394 nm excitation case) would seem as a reasonable trade-off of slightly lower EQE for much stronger absorption. The relatively high standard deviations at low Eu3+ concentrations are a result of weak absorption (but not in the case of excitation over the CT band). Another important feature of the phosphor for practical application is color coordinate stability at elevated temperatures, because lots of devices, for instance high power LEDs, heat up significantly. Thus, first the color coordinates in the CIE 1931 and CIE 1976 color space diagrams as a function of Eu3+ concentration and excitation wavelength (265, 394, and 465 nm) were calculated and tabulated in Table S4. The graphical representation of the change in CIE 1931 color coordinates as a function of Eu3+ concentration (for 394 nm excitation) is given in Figure 9a. The color coordinates change very little with the increasing Eu3+ content; however, the trend is that emission becomes “more” red at high europium concentrations. Moreover, the color coordinates are located close to or on the edge of the color space diagram showing high color purity of the phosphors. The temperature dependent CIE 1931 and CIE 1976 color coordinates for 1%, 50%, and 100% doped samples under 394 nm excitation were also calculated and tabulated in Table S5. The graphical representation of the CIE 1931 color coordinate change for these specimens upon increasing temperature is shown in Figures 9b−d. Once again, the color coordinates are very close to or on the edge of the color space diagram indicating high color purity. Increasing temperature causes the “blue shift” of the color coordinates. Besides, it is also evident that the shift is linear; thus, such phosphors could also be employed in luminescence based thermometry. Tables S4 and S5 also contain the luminous efficacy (LE) data. Luminous efficacy shows how bright the radiation is perceived by the average human eye. The given numbers were calculated from emission spectra employing the following equation:55

(5)

Here, Wr and Wnr are probabilities for the radiative and nonradiative transitions, respectively, and τ and τ0 are the measured decay time and the decay time without nonradiative transitions, respectively.52 The nonradiative transition probability Wnr is commonly ruled by thermal relaxation processes (i.e., the emission of energy into the lattice vibrations).53 One can assume that at very low temperatures the nonradiative processes play a minor role; thus, IQE is close to unity. In this case, if one can measure the decay time at very low temperature, it can be considered as τ0 and the division τ/τ0 would yield an approximate value of IQE.54 The sum of Wr and Wnr is unity; thus, decreasing IQE is related to the increased probability of the nonradiative transitions. The external quantum efficiency, in turn, can be expressed as27 EQE = IQE × ηesc

(6)

Here, ηesc is the escape efficiency of photons from the phosphor particle. Thus, in our case, the faster decrease of EQE is related to the decreasing ηesc. External quantum efficiency is an important factor determining the practical value of the phosphors. The calculated external quantum efficiencies at three different excitation wavelengths (265, 394, and 465 nm) are shown in Figure 8. The excitation over the CT band leads to a steady increase of the external quantum efficiency with increasing Eu3+ concentration. The highest EQE value for 265 nm excitation was 32%. The relatively low EQE might be related to the other competing charge transfer processes as was also observed by Baur and Jüstel.54 Excitation at 394 and 465 nm gives much higher EQE values. The efficiency of 100% is reached at both excitation wavelengths for the sample doped with 50% Eu3+. Full substitution of bismuth by europium results in a decrease of EQE values to 96% and 86% for 394 and 465 nm excitation, 31778


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Figure 9. Fragments of the CIE 1931 color diagram with the color points of (a) K2Bi(PO4)(MoO4):Eu3+ as a function of Eu3+ concentration (a) and as a function of temperature of (b) 1%, (c) 50%, and (d) 100% Eu3+ doped samples. All samples were excited at 394 nm.

Figure 10. Emission spectra of (a) 375 nm LED, (b) 400 nm LED, and (c) 455 nm LED. Emission spectra of K2Eu(PO4)(MoO4) ceramic disks excited with (d) 375 nm LED, (e) 400 nm LED, and (f) 455 nm LED.

⎛ lm ⎞ ⎛ ⎞ ∫ I(λ)V (λ)dλ ⎟⎟ = 683⎜⎜ lm ⎟⎟ × LE⎜⎜ ∫ I(λ)dλ ⎝ Wopt ⎠ ⎝ Wopt ⎠

emission profile remains virtually the same under all investigated circumstances. Besides, the LE values of 200 lm/ Wopt are very high among the red emitting phosphors and are capable of even surpassing the LE values of the well-established red phosphors such as Sr2Si5N8:Eu2+ (λem = 620 nm, LE = 240 lm/Wopt), CaAlSiN3:Eu2+ (λem = 650 nm, LE = 150 lm/Wopt), and CaS:Eu2+ (λem = 650 nm, LE = 85 lm/Wopt).52 On the other hand, the 200 lm/Wopt value is smaller than some other reported molybdates doped with Eu3+ ions, for instance, Li3Ba2Eu3(MoO4)8 (LE = 312 lm/Wopt) and Y2Mo4O15:75% Eu3+ (LE = 242 lm/Wopt). This is due to the relatively intensive emission at around 700 nm of the K2Bi(PO4)(MoO4):Eu3+ phosphors. Such intensive emission at such long wavelengths

(7)

Here, I(λ) and V(λ) are the emission spectrum of the phosphor and the human eye sensitivity curve, respectively. The human eye is the most sensitive to the 555 nm radiation; thus, the highest possible LE value (683 lm/Wopt) is obtained for the monochromatic green radiation at 555 nm. The calculated LE values for the K2Bi(PO4)(MoO4):Eu3+ phosphors were around 200 lm/Wopt regardless of the Eu3+ concentration, excitation wavelength, and temperature. Such LE stability is in good agreement with the color coordinates which indicates that the 31779


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ACS Applied Materials & Interfaces results in lower overlap between the human eye sensitivity curve and emission spectra which ends up with the lower LE values. Lanthanide ions often suffer from the low absorption due to the spin and parity forbidden character of their [Xe]4fn → [Xe]4fn transitions.51−53 This is a huge drawback for practical application where strong absorption is usually one of the top requirements. This issue can be dealt with in several ways. The first one is increasing the activator concentration; however, in our case, the Eu3+ concentration cannot be further increased since the end member of the series already possess 100% Eu3+ concentration. The second way is introducing the proper sensitizer into the host lattice. In this case, sensitizer would absorb the energy and transfer it to Eu3+ ions. The third option is preparing ceramics where absorption is increased due to the increased optical pathway of the incident photons.27 In our experiments, we have chosen the third option since the Eu3+ concentration cannot be further increased as was stated above and we do not know any reasonable sensitizers that would fit to the K2Bi(PO4)(MoO4) host matrix. The ceramic disks of the K2Eu(PO4)(MoO4) sample with the thickness of 0.33 and 0.89 mm were prepared, and their optical properties were investigated. The density of the prepared ceramic disks was 3.27 g/cm3. The emission spectra of the ceramic disks were measured in the transmission mode; i.e., the ceramic disks were placed on the top of three LEDs emitting at 375, 400, and 455 nm and the respective emission spectra were recorded. The emission spectra of 375, 400, and 455 nm emitting LEDs were also measured and are shown in Figure 10a−c. The emission spectra of K2Eu(PO4)(MoO4) ceramic disks excited by the mentioned LEDs are given in Figure 10d−f. The recorded spectra reveal that the 0.33 mm thickness of the ceramic disk is not sufficient to absorb all the radiation emitted by all three used LEDs, and some portion of radiation passes through the disk. The situation changes considerably with the ceramic disk with the thickness of 0.89 mm. In this case, all the radiation emitted by 375 nm LED is absorbed and only the emission from K2Eu(PO4)(MoO4) is observed. However, such thickness was still not enough to absorb all radiation emitted by 400 nm LED. On the other hand, the unabsorbed part of the LED radiation was really small. Finally, the radiation emitted by 455 nm LED was absorbed the worst. Then again, this was anticipated since there is only one 7F0 → 5D2 absorption transition in the spectral range of 455 nm LED emission. The much stronger absorption of the 375 and 400 nm LED radiation is due to the higher abundance of absorption transitions (7F0 → 5D4, 7F0 → 5L7,8, 5 GJ, 7F0 → 5L6, 7F0 → 5D3) lying in this spectral range. It is also interesting to note that the emission originating from 5D0 → 7 F4 transition is stronger in ceramics if compared to the emission spectra of the powder samples. Figure 11 shows the color coordinates of the three LEDs and light sources obtained by combining these LEDs with the ceramic disks of both thicknesses. The exact calculated values (together with the color coordinates in the CIE 1976 color space diagram) are given in Table S6. The color coordinates of the light source obtained by combining 375 nm emitting LED with the K2Eu(PO4)(MoO4) ceramic disks are very close to the edge of the color space diagram which shows high color purity. However, this is not the case when 400 nm LED is used. It is evident that it is still possible to prepare a red light source with lower color purity by using a 0.89 mm ceramic disk, but using a 0.33 mm disk already yields color coordinates in the purplish

Figure 11. CIE 1931 color space diagram combined with the color coordinates of K2Eu(PO4)(MoO4) ceramics combined with 375, 400, and 455 nm LEDs. Stars, open, and filled symbols denote color coordinates of LEDs, LEDs with a 0.33 mm ceramic disk, and LEDs with a 0.89 mm ceramic disk, respectively.

red region. Combination of the 455 nm LED and the prepared ceramic disks yields light sources of the blue color. On the other hand, these results show that the combination of 400 nm emitting LED and the ceramic disks is the most promising one since all the colors ranging from red to purple to tints of blue could be obtained by simply varying the thickness of the ceramic disk or the intensity of LED. Table S6 also contains luminous efficacy data of the prepared light sources. The combination of 375 nm LED with 0.33 and 0.89 mm ceramic disks yields light sources with the LE values of 137 and 179 lm/ Wopt, respectively. Application of the 0.33 mm ceramic disk gives considerably lower LE values due to the presence of the unabsorbed LED radiation in the spectral range where the human eye sensitivity is low. The same applies for the light sources obtained by combining the 400 nm LED. In this case, even more radiation emitted by the LED remains unabsorbed and the LE values are 99 and 153 lm/Wopt for 0.33 and 0.89 mm disks, respectively. Finally, the lowest LE values are obtained for the light sources employing 455 LED and ceramic disks. The LE values for such light sources are 54 and 73 lm/ Wopt for thin and thick ceramic disks, respectively. Such low LE values are due to the strongest emission in the spectral ranges where the human eye sensitivity is low.

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CONCLUSIONS In summary, single phase K2Bi(PO4)(MoO4):Eu3+ phosphors were prepared by solid state reaction at relatively low temperature with Eu content up to 100%. Samples showed bright red luminescence dominated by the 5D0 → 7F2 transition at ∼615 nm under excitation of near-UV and blue radiation. The luminescence measurements revealed that prepared phosphors possess very high color purity, little thermal quenching, high color coordinate stability, large LE values, and most importantly close to 100% external quantum efficiency. All these features make K2Bi(PO4)(MoO4):Eu3+ phosphors very promising candidates for application in solid state light sources as a red-emitting component when a nearUV emitting semiconductor chip is used as a primary radiation source. Besides, the unique shape of the emission spectra enables the application of these phosphors in the luminescent security pigments. 31780


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(13) Hintze, F.; Hummel, F.; Schmidt, P. J.; Wiechert, D.; Schnick, W. Ba3Ga3N5-A Novel Host Lattice for Eu2+-Doped Luminescent Materials with Unexpected Nitridogallate Substructure. Chem. Mater. 2012, 24, 402−407. (14) Schmiechen, S.; Schneider, H.; Wagatha, P.; Hecht, C.; Schmidt, P. J.; Schnick, W. Toward New Phosphors for Application in Illumination-Grade White pc-LEDs: The Nitridomagnesosilicates Ca[Mg3SiN4]:Ce3+, Sr[Mg3SiN4]:Eu2+, and Eu[Mg3SiN4]. Chem. Mater. 2014, 26, 2712−2719. (15) Schmiechen, S.; Strobel, P.; Hecht, C.; Reith, T.; Siegert, M.; Schmidt, P. J.; Huppertz, P.; Wiechert, D.; Schnick, W. Nitridomagnesosilicate Ba[Mg3SiN4]:Eu2+ and Structure−Property Relations of Similar Narrow-Band Red Nitride Phosphors. Chem. Mater. 2015, 27, 1780−1785. (16) Pust, P.; Hintze, F.; Hecht, C.; Weiler, V.; Locher, A.; Zitnanska, D.; Harm, S.; Wiechert, D.; Schmidt, P. J.; Schnick, W. Group (III) Nitrides M[Mg2Al2N4] (M = Ca, Sr, Ba, Eu) and Ba[Mg2Ga2N4] Structural Relation and Nontypical Luminescence Properties of Eu2+ Doped Samples. Chem. Mater. 2014, 26, 6113−6119. (17) Hecht, C.; Stadler, F.; Schmidt, P. J.; der Guenne, J. S. A.; Baumann, V.; Schnick, W. SrAlSi4N7:Eu2+ - A Nitridoalumosilicate Phosphor for Warm White Light (pc)LEDs with Edge-Sharing Tetrahedra. Chem. Mater. 2009, 21, 1595−1601. (18) Pust, P.; Wochnik, A. S.; Baumann, E.; Schmidt, P. J.; Wiechert, D.; Scheu, C.; Schnick, W. Ca[LiAl3N4]:Eu2+A Narrow-Band RedEmitting Nitridolithoaluminate. Chem. Mater. 2014, 26, 3544−3549. (19) Pust, P.; Weiler, V.; Hecht, C.; Tücks, A.; Wochnik, A. S.; Henß, A.-K.; Wiechert, D.; Scheu, C.; Schmidt, P. J.; Schnick, W. NarrowBand Red-Emitting Sr[LiAl3N4]:Eu2+ as a Next-Generation LEDPhosphor Material. Nat. Mater. 2014, 13, 891−896. (20) Zukauskas, A.; Vaicekauskas, R.; Ivanauskas, F.; Vaitkevicius, H.; Shur, M. S. Spectral Optimization of Phosphor-Conversion LightEmitting Diodes for Ultimate Color Rendering. Appl. Phys. Lett. 2008, 93, 051115. (21) Bechtel, H.; Jüstel, T.; Gläser, H.; Wiechert, D. U. Phosphors for Plasma-Display Panels: Demands and Achieved Performance. J. Soc. Inf. Disp. 2002, 10, 63−67. (22) Bachmann, V.; Ronda, C.; Oeckler, O.; Schnick, W.; Meijerink, A. Color Point Tuning for (Sr,Ca,Ba)Si2O2N2:Eu2+ for White Light LEDs. Chem. Mater. 2009, 21, 316−325. (23) Lim, M. A.; Park, J. K.; Kim, C. H.; Park, H. D.; Han, M. W. Luminescence Characteristics of Green Light Emitting Ba2SiO4:Eu2+ Phosphor. J. Mater. Sci. Lett. 2003, 22, 1351−1353. (24) Janulevicius, M.; Marmokas, P.; Misevicius, M.; Grigorjevaite, J.; Mikoliunaite, L.; Sakirzanovas, S.; Katelnikovas, A. Luminescence and Luminescence Quenching of Highly Efficient Y 2 Mo 4 O 15 :Eu 3+ Phosphors and Ceramics. Sci. Rep. 2016, 6, 26098. (25) Morozov, V. A.; Lazoryak, B. I.; Shmurak, S. Z.; Kiselev, A. P.; Lebedev, O. I.; Gauquelin, N.; Verbeeck, J.; Hadermann, J.; Van Tendeloo, G. Influence of the Structure on the Properties of NaxEuy(MoO4)z Red Phosphors. Chem. Mater. 2014, 26, 3238−3248. (26) Cheng, F.; Xia, Z.; Molokeev, M. S.; Jing, X. Effects of Composition Modulation on the Luminescence Properties of Eu3+ Doped Li1‑xAgxLu(MoO4)2 Solid-Solution Phosphors. Dalton Trans. 2015, 44, 18078−18089. (27) Katelnikovas, A.; Plewa, J.; Sakirzanovas, S.; Dutczak, D.; Enseling, D.; Baur, F.; Winkler, H.; Kareiva, A.; Jüstel, T. Synthesis and Optical Properties of Li3Ba2La3(MoO4)8:Eu3+ Powders and Ceramics for pcLEDs. J. Mater. Chem. 2012, 22, 22126−22134. (28) Ben Amara, M.; Dabbabi, M. Structure du Molybdophosphate ̀ dYttrium et Sodium Na2Y(MoO4) (PO4). Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1987, 43, 616−618. (29) Daub, M.; Lehner, A. J.; Hoppe, H. A. Synthesis, Crystal Structure and Optical Properties of Na2RE(PO4) (WO4) (RE = Y, TbLu). Dalton Trans. 2012, 41, 12121−12128. (30) Han, L.; Zhao, L.; Zhang, J.; Wang, Y.; Guo, L.; Wang, Y. Structure and Luminescence Properties of the Novel Multifunctional K2Y(WO4) (PO4):Ln3+ (Ln = Tb, Eu, Yb, Er, Tm and Ho) Phosphors. RSC Adv. 2013, 3, 21824−21831.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11766. Unit cell of K2Bi(PO4)(MoO4); XRD data, IR spectra, and unit cell parameters of K2Bi(PO4)(MoO4):Eu3+; PL lifetimes, CIE color coordinates, and LE values of K2Bi(PO4)(MoO4):Eu3+; color coordinates and LE values of K2Eu(PO4)(MoO4) ceramic disks (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Tel. +370-697-23123. Author Contributions

J.G. prepared the samples and performed the measurements. A.K. initiated the research and wrote the manuscript. Both authors discussed the results and reviewed the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to D. Sakalauskas (Department of Applied Chemistry, Vilnius University) for taking SEM images. REFERENCES

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