1. Introduction

Recently, WLEDs have been paid increasing attention due to the advantages over traditional incandescent or fluorescent lightings such as high luminous efficiency, long lifetime, energy saving and therefore environmental benefits etc, and for these, they have been promoted in the widespread applications for instance in domestic and commercial lighting, automobiles, communications, imaging, agriculture, and medicine [1–3]. Currently for fabrication of pc-WLEDs, the mainstream technology is to combine a single LED chip with one or more conversion phosphors [2–6]. For the purpose, desirable phosphors are highly needed to efficiently convert the near ultraviolet (typically 380 nm to 420 nm) or blue emission (typically 450 nm to 480 nm) from (In,Ga)N LED chip to visible lights. Among three primary color phosphors, searching a new red phosphor is one of the big challenges in the field of luminescent materials particularly when a white lighting is preferred in some occasions with a warm perception, a high color rendering ability and full color gamut [2–6]. It, thus, has triggered a series of findings on new red phosphors (e.g. activated by rare earth, transition metal, divalent bismuth, or oxygen related defects etc as well as nanostructured composite [7–16]. This has led to the discovery of the outstanding Eu²⁺ doped oxynitrides and nitrides phosphors [2,3,9,17].

The general requirements of potential red phosphor for instance for a blue chip scheme are: (1) significant absorption in blue but not in green; (2) high quantum efficiency of the red emission; (3) good resistance of the emission to thermal impact; (4) good tolerance to ambient; and (5) compatibility with present and future environmental regulations, for instance, free of toxic species, green compositions. Some of them cannot be met simultaneously by previous approaches. For an instance, Eu²⁺-doped red nitride phosphors M₂Si₅N₈:Eu²⁺ (M = Ca, Sr, Ba) can absorb blue and green lights [2,3,6,17]. Thus, it is unavoidable to reabsorb the co-existent green emission of WLEDs.

Recently Mn⁴⁺ doped phosphors were recognized as one of the promising alternatives, because Mn⁴⁺ usually exhibits broad and strong absorption between 300 and 480 nm and emits light between 600 nm to 760 nm once it is introduced into an octahedral site [18–22]. The luminescent properties remain quite similar from host to host because the lowest excited state energy, ²E(t₂³), barely changes upon variation of the crystal field. Lately Park et al reported CaAl₄O₇:Mn⁴⁺ as a new good candidate of red phosphor for WLEDs using UV chips [21]. This argument, however, needs reconsideration not only because a confirmation has to be done on whether CaAl₄O₇:Mn⁴⁺ is a red phosphor, but also because the properties of the phosphor are not well known as we check it over with all the requirements listed above. A single phase of CaAl₄O₇:Mn was not actually synthesized with boron acid H₃BO₃ as flux though the phase dominated over the other phases (α-Al₂O₃ and CaAl₁₂O₁₉) in some cases [21]. The authors commented that the contribution of minor phase could be negligible to the red luminescence of the sample (see Fig. 1 of [21]). The comment, however, is questionable in respect of the facts: (1) the red luminescence of Mn⁴⁺ in Al₂O₃ and CaAl₁₂O₁₉ has been confirmed undoubtedly, and particularly the luminescence of CaAl₁₂O₁₉:Mn⁴⁺ is very efficient [18,19]; (2) no red luminescence was observed from Mn doped sample where CaAl₄O₇ and Al₂O₃ coexisted [21]. This is one of the impetuses to the present study. Another motivation is to understand the capacity of the title compound to resist temperature quenching if it was a red phosphor. It is inspired by the fact that the temperature of the layer capped on top of a blue diode can reach 150-200°C due to the heat rouse from the p-n junction and the phosphor layer as the device works [2,3,16,22].

 

Fig. 1 (a) XRD patterns of CaAl₄O₇: 0.1%Mn prepared at different temperature in air with 2%H₃BO₃ as flux and reference pattern of CaAl₄O₇ (ICSD #34487) at the bottom. (b) XRD pattern (-o-) of the sample of CaAl₄O₇: 0.1%Mn prepared at 1400°C_, Rietveld refining results (−), Bragg reflections (|) and the profile difference between experimental and calculated values (−). Inset shows the lattice cell of the compound viewed along c; blue ball: Ca, green ball: Al, red ball: O; purple polyhedra: CaO₇, dark cyan polyhedra: AlO₄. (c) Exemplary excitation and emission spectra of CaAl₄O₇: 0.1%Mn⁴⁺ prepared at 1400 (curves 1 and 2) and 1600 °C (curves 3 and 4). Calcium sites which could stabilize and accommodate Mn⁴⁺: (d) ideal CaO₆ octahedron in the compound of CaO, where Ca-O bond length is 2.407(6)Å; (e) CaO₇ polyhedron in CaAl₄O₇, where one Ca-O(1) bond is 2.488(9)Å, two Ca-O(2) bond are 2.367(6)Å, two Ca-O(2) bond are 2.925(6)Å and two Ca-O(3) bond are 2.310(2)Å.

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We have shown, here, it is possible to synthesize the pure phase of CaAl₄O₇:Mn even when H₃BO₃ is used as flux. We have confirmed the red luminescence from it due to the Mn⁴⁺ substitution for calcium sites. This is rather different from what Park et al [21] reported on as well as the spectroscopic properties. Meanwhile, we have inspected the temperature dependent luminescence in a wide range from 10K to 500K, and discussed the mechanism for the severe thermal quenching.

2. Experimental procedure

The samples were synthesized by typical high temperature solid state reaction with analytical reagents CaCO₃, Al(OH)₃, MnCO₃ and H₃BO₃ as raw materials. Individual 10-gram batches were weighed according to CaAl₄O₇: 0.1%Mn, where 2mol% extra H₃BO₃ was added as flux. The samples were mixed homogeneously in an agate mortar, and moved to alumina crucibles for subsequent processes. They were heated up to 700°C with a rate of 1.2°C/min and held for 5h to thermally decompose the starting reagents, and then cooled down to room temperature naturally. They, after intermediate grinding to improve the mixing homogeneity, were heated at a rate of 2°C/min to 1300, 1400, 1500 and 1600°C, respectively, and sintered at each temperature for 5h, and finally cooled down to room temperature. All the operations were performed in air.

X-ray diffraction (XRD) patterns of the samples were recorded by a Rigaku D/max-IIIA X-ray diffractometer (40 kV, 1.2° min⁻¹, 40 mA, Cu-Kα1, λ = 1.5405 Å). Static excitation and emission spectra, dynamic emission decay spectra, the emission spectra at 10 to 500K were measured by an Edinburgh FLS 920 instrument equipped with a red-sensitive photomultiplier (Hamamatsu R928P) and a closed cycle helium cryostat. Excitation curves were corrected over the lamp intensity with a silicon photodiode, and the emission curves were corrected by the PMT spectral response. Measurements were performed at room temperature unless otherwise specified.

3. Results and discussion

When prepared at 1200°C, Park et al found the phases of α-Al₂O₃ and CaAl₄O₇ coexisted in Mn doped sample [21]. This changed as the preparation temperature rose up to 1300, 1400 or 1500 °C. The secondary minor phase became into CaAl₁₂O₁₉ instead of α-Al₂O₃. The scenario is very different in our case. Figure 1(a) depicts XRD pattern of CaAl₄O₇: 0.1%Mn prepared at different temperature in air. Comparison to reference pattern of CaAl₄O₇ (ICSD #34487) reveals the nature of pure phase in all the samples. For achieving crystallographic data for the following discussion, we analyzed for example the XRD pattern of the sample CaAl₄O₇: 0.1%Mn prepared at 1400°C with the classic Rietveld refining technique. The refining starts with the crystallographic data of CaAl₄O₇ (ICSD #34487) and it converges with residual values of R_p = 10.50%, R_wp = 14.10%, and R_exp = 7.62%. The structural refining results are depicted in Fig. 1(b) along with the different profile between experimental and calculated values. The comparison confirms that the sample can be indexed in a monoclinic C12/c1 space group. The refining calculation produced lattice parameters of a = 12.893(9)Å, b = 8.888(8)Å, c = 5.446(9)Å, β = 106.91(3) and cell volume V = 597.2707 ų, well consistent with reference data of CaAl₄O₇ (ICSD #34487).

As illuminated with either UV or Blue lights, all the samples exhibit typical Mn⁴⁺ red luminescence. And this, however, differs from what Park et al reported [21]. They observed that the excitation spectrum consisted of three peaks at 335, 380 and 467nm, the emission spectrum was peaked at 656nm and the two UV excitation peaks exhibited comparable intensity particularly for their sample prepared at 1300 °C. This was very similar to CaAl₁₂O₁₉:Mn⁴⁺ as noticed by Park et al [21], and it was possibly due to the large amount of CaAl₁₂O₁₉ phase in the samples. The blue excitation peak showed fine structure similar to xenon lamp, possibly due to improper calibration [21]. Figure 1(c) exemplarily illustrates excitation and emission spectra of our samples prepared at different temperatures. The excitation spectrum comprises also three peaks of Mn⁴⁺ but at ~330, ~390, and ~464nm. The intensity of ~390nm shoulder peak is roughly half of the strongest peak at ~330nm, and the blue peak after well calibrated does not appear structured. The emission spectrum peaks at ~655nm upon 330nm excitation. It is noticed that the preparation temperature has influences on emission intensity and lifetime but not the spectral shape (see Fig. 1(c)). For instance, as the temperature rises up from 1300 to 1600°C. the lifetime prolongs from 642 to 963μs.

Park et al reported red luminescent Mn⁴⁺ was substituted for tetrahedral Al³⁺ sites in the sample CaAl₄O₇: Mn⁴⁺ [21]. This contradicts what have been reported so far, and only Mn⁴⁺ in octahedral crystal field can emit red light [18–20, 22]. In the compound as revealed by the refining, there are one type of seven-coordinate calcium sites Ca(1), two types of tetrahedral aluminum sites Al(1) and Al(2), and four types of oxygen sites O(1), O(2), O(3) and O(4) (shown as inset of Fig. 1(b)). So clearly, even if Mn⁴⁺ could substitute for either Al(1) or Al(2) in view of size match, the tetrahedral Mn⁴⁺ ions cannot red luminesce. The rest host cation sites that Mn⁴⁺ ions possibly reside will be Ca(1). Actually reports have confirmed that octahedral Mn⁴⁺ ions can disolvedly substitute for the Mg or Ca sites in MgO and CaO which are fully built up by MgO₆ and CaO₆ polyhedra, respectively [18]. For ideal CaO₆ polyhedra as depicted in Fig. 1(d), length of each Ca-O bond is 2.407(6)Å, and one Ca site and four O sites share equatorial plane. The polyhedron of CaO₇ in CaAl₄O₇ could be regarded as a distorted version of CaO₆ polyhedron (see Fig. 1(e)) once the two O(2) atoms which connect to Ca in the longest bond were treated as an imaginary atom entity. When Mn⁴⁺ replaces Ca(1), local field perhaps is distorted due to increased Coulomb interaction, and it turns into typical octahedral field eventually as evidenced by Fig. 1(c) and Fig. 2.

 

Fig. 2 (a) Excitation spectra (λ_em = 652nm), (b) emission spectra (λ_ex = 325nm) and (c) decay curves (λ_em = 652nm, λ_ex = 325nm) of CaAl₄O₇: 0.1%Mn⁴⁺ at different temperatures as indicated. (d) The temperature dependence of the integrated emission intensities (light green ellipse pattern: “red star” for λ_ex = 325nm; “blue ball” for λ_ex = 470nm) and lifetimes (light blue ellipse pattern: “red star” for λ_ex = 325nm and λ_em = 652nm; “blue ball” for λ_ex = 470nm and λ_em = 652nm). The green line through the data points are fits to τ(T) = τ₀/(1 + τ₀Ce^-Ea/kT) with τ₀ = 1435μs, C = 1.47, and E_a = 0.196eV. The goodness of fitting is 96.0%.

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The luminescent properties of the phosphor of CaAl₄O₇: Mn⁴⁺ show clear temperature dependence (Fig. 2). At 10K, the excitation spectrum does not appear fine structured, and it comprises three peaks at 325nm, 381nm and 452nm, due to the transitions of ⁴A₂→⁴T₁ (partly Mn⁴⁺-O^2- charge transfer transition), ⁴A₂→²T₂, ⁴A₂→⁴T₂, respectively [18–20,22]. These peaks redshift obviously as the temperature increases (Fig. 2(a)). For instance, the strongest peak lies at ~325nm at 10K, and it goes to ~330nm, ~336nm, ~342nm at 300K, 400K, and 500K, respectively.

The emission spectra show also redshift as Fig. 2(b) depicts. At low temperature, e.g. 10K, anti-Stokes feature disappears, and two sharp peaks were observed at 15649cm⁻¹ and 15337cm⁻¹ along with Stokes sidebands. The two peaks show different red shifts as the temperature rises up. The former peak shift is 48cm⁻¹ and 97cm⁻¹, and the shift of the latter is 70cm⁻¹ and 116cm⁻¹ when the temperature increases from 10K to 300K and 450K, respectively. This implies that the two peaks are possible associated with different Mn⁴⁺ emission centers. According to the criterion developed by Brik and Srivastava [18], the low energy peak of 15337cm⁻¹ should be due to a Mn⁴⁺ site with higher covalency.

Regardless of consequent charge compensation and distortion after substitution of Mn⁴⁺ for Ca(1) site in CaAl₄O₇: Mn⁴⁺, the symmetry of the Mn⁴⁺ site is C₂. At the site without inversion symmetry, electric dipole character can be introduced into the forbidden transition of ²E→⁴A₂ by mixing 3d³ with odd parity configurations via crystal field. Therefore, zero phonon line of the transition is expected strong at the site of C₂. Indeed, it is the case in CaAl₄O₇: Mn⁴⁺ where the peak at 15337cm⁻¹ (652nm) dominates the spectrum at 10K. At higher temperature, vibronic anti-Stokes and particularly Stokes sidebands are evidently enhanced, which reflects the phonon-involved nature of these transitions (see Fig. 2(b)).

Emission decay behavior depends strongly on temperature (Fig. 2(c)). At lower temperature, decay curve fits well single exponential decay equation, and the fitting deteriorates as the temperature increases. The lifetime reduces significantly from 1578μs@10K to 13μs@500K for instance upon excitation of 325nm. Figure 2(d) summarizes the dependences of emission lifetime and intensity on temperature. And they show similar trend for excitation schemes of either UV or blue. The temperature at which the intensity decreases to half of the value at 10K is about 200K. At 500K, only very weak luminescence of Mn⁴⁺ was recorded. Significant lifetime shortening gives evidence for both cases of excitation that there is the intrinsic severe quenching in CaAl₄O₇: Mn⁴⁺. The decrease in lifetime is accompanied by the decrease of emission intensity.

One of the mechanisms for Mn⁴⁺ luminescence quenching might be quenching by thermally activated crossover from the first excited state of ²E to the ground state ⁴A₂. For evaluating activation energy (Ea) for thermal quenching, we fit the temperature dependence of emission lifetime rather than intensity to a modified Arrhenius equation [6]: τ(T) = τ₀/(1 + τ₀Ce^-Ea/kT), where τ(T) is the emission decay time at temperature T (K), τ₀ is the emission decay time at 0K, C is a rate constant for the process. This is because emission intensity when compared to lifetime is very susceptible to various factors. Best fitting leads to Ea = 0.196eV. It is much lower than 0.6eV of SrSi₂O₂N₂:Eu²⁺, which can well explain why luminescence quenching occurs in CaAl₄O₇: Mn⁴⁺ but not SrSi₂O₂N₂:Eu²⁺ at 500K [6].

When inspecting the excitation spectra at 450 and 500K, we noticed there appears a sharp peak at ~406nm and it is intensified as temperature increases. The peak comes from the transition of ⁶A₁(S)→⁴E(G), ⁴A₁(G) of a typical octahedral Mn²⁺ site [7,23]. The trace evidence reveals the thermal induced reduction happens in situ from Mn⁴⁺ to Mn²⁺. This means that valence change might be one of the causes for luminescence quenching of Mn⁴⁺. And it could lead to permanent thermal degradation of the phosphor. In addition, negative defects, which are possible created for charge compensation due to aliovalent substitution of Mn⁴⁺ for host cations, might also play as luminescence quencher.

4. Conclusions

Indeed, CaAl₄O₇: Mn⁴⁺ is a red phosphor and it can be synthesized in pure phase in presence of boric acid as flux. The red luminescence comes from a distorted octahedral Mn⁴⁺ sites due to substitution for Ca(1) sites in the compound. And it, however, suffers from strong thermal quenching, revealed by the low quenching temperature and Ea. The dominating quenching mechanism may be the thermally activated crossover from the first excited state to the ground state and the in situ reduction of Mn⁴⁺ to Mn²⁺ at higher temperature. This work reveals to us that CaAl₄O₇: Mn⁴⁺ would be less practically valuable in the application of pc-WLEDs if it was not functionally engineered in surface, size, morphology etc. Meanwhile, it implies a general strategy to explore a new Mn⁴⁺ phosphor with better performances in future, that is, the host compound should better comprise regular octahedral sites that Mn⁴⁺ could occupy.