A high color purity red emitting phosphor CaYAlO₄:Mn⁴⁺ for LEDs

Nowadays, phosphor-converted (pc-) white LEDs are attracting significant attention [1]–[3]. They can be classified into two approaches: blue (440 ~ 470 nm) InGaN and near (n)-UV (350 ~ 420 nm) GaN chip combined with phosphors. Recently, the commonly commercial white LED is based on the combination of blue InGaN chip and yellow YAG: Ce³⁺ phosphor. However, such white LEDs encounter low color-rendering index (Ra < 80) due to the scarcity of red emission [4]–[6].

Red emitting phosphor is one of key tricolor luminescent materials for white LEDs. Up to now, many researchers have been done on Eu³⁺[7]–[9] or Sm³⁺[10]–[12] ion doped phosphors. Unfortunately, the red phosphor doped by Eu³⁺or Sm³⁺show weak absorption peaks at about 400 nm or 460 nm because the 4f-4f absorption transitions are forbidden by the parity selection rule and its optical oscillator strength is small. Furthermore, the price of rare earth is quite high. Furthermore, some nitride based phosphors have been developed to increase the color index, or create warm-white lighting [13]–[15]. However, the synthesize conditions of nitride phosphors are usually very harsh, such as high temperature (1500–2000°C), oxygen-free environment.

Generally, Mn⁴⁺ ion with 3d³ configurations in octahedral site gives a deep red emission and has broad absorption in visible region, such as K₂SiF₆: Mn⁴⁺, K₂GeF₆: Mn⁴⁺, K₂TiF₆: Mn⁴⁺ and CaAl₁₂O₁₉: Mn⁴⁺[16]–[18]. CaYAlO₄ (CYA) crystallizes in the perovskite phase with tetragonal K₂NiF₄ structure [19]. Al³⁺ ion is octahedrally coordinated with six oxygens. Therefore, Mn⁴⁺ ion may emit red ligtht when occupied Al³⁺ ion site in CaYAlO₄. In this paper, an intense red phosphor, CaYAlO₄:Mn⁴⁺, was developed by solid state reaction. The photoluminescence excitation and emission spectra, concentration effect, thermal-dependent luminescence quenching properties and decay curves are investigated.

Syntheses: All samples CaYAlO₄:Mn⁴⁺ _x (x = 0.001, 0.005, 0.01, 0.03, 0.05) were prepared by a conventional solid-state reaction technique. The starting materials, CaCO₃ (A.R.), Al₂O₃ (A.R.), Y₂O₃ (99.99%) and MnCO₃ (A.R.) were weighed in stoichiometric amounts. Subsequently the powder mixture was thoroughly mixed in an agate mortar by grinding and was transferred into crucibles. Finally, they were sintered at 1250°C for 4 h in air.

Measurements: The phase purity of the prepared phosphors was investigated by a Rigaku D/max-IIIA X-ray Diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 30 mA. The XRD patterns were collected in range of 10° ≤ 2θ ≤ 80°.

The photoluminescence (PL), photoluminescence excitation (PLE) spectra, temperature-dependent PL spectra and the decay curves at room temperature were measured by FSP920 Time Resolved and Steady State Fluorescence Spectrometers (Edinburgh Instruments) equipped with a 450 W Xe lamp, a 100w μF920H lamp with a pulse width of 1 ~ 2 μs, a repetition rate of 50 Hz and the lifetime range of 100 μs ~ 200 s, TM300 excitation monochromator and double TM300 emission monochromators, Red sensitive PMT and R5509-72 NIR-PMT in a liquid nitrogen cooled housing (Hamamatsu Photonics K.K). The spectral resolution is about 0.05 nm in UV–VIS.

For the high temperature PL spectra in 300–500 K, the powder sample was mounted in an Optistat DNV actively cooled optical cryostat with an ITC601 temperature controller.

The powder diffuse reflection spectra (DRS) of these samples were measured on a Cary 5000 UV–vis-NIR spectrophotometer (Varian) equipped with double out-of-plane Littrow monochromator, using polyfluortetraethylene as a standard reference in the measurements.

The room temperature quantum efficiency (QE) of the sample was measured using a barium sulfate coated integrating sphere (150 mm in diameter) attached to the FSP920.

The XRD patterns of CYA: Mn⁴⁺ _x (x =0, 0.001, 0.005, 0.05) are shown in Figure 1. The results indicate that all the peaks of Mn⁴⁺ ion doped CYA can be indexed to a pure CaYAlO₄ (JCPDS 81–0742). The dopants have no obvious influence on the crystalline structure of the host. The CaYAlO₄ has tetragonal system with a space group of I4/mmm (139) and a = 3.6750(5) Å c = 12.011(2) Å c/a = 3.2683 V = 162.22(4) ų, Z = 2 [20]. There are two types of cation sites in CaYAlO₄. The Ca²⁺ and Y³⁺ ions are distributed in the nine-coordinated sites and the Al³⁺ ions occupy the six-coordinate site. It is reported that the effective ionic radius of Ca²⁺ ion (CN = 9), Y³⁺ ion (CN = 9), Al³⁺ ion (CN = 6) and Mn⁴⁺ ion (CN = 6) are 1.18 Å, 1.075 Å, 0.535 Å and 0.53 Å, respectively [21]. It is obvious that the ionic radius of Mn⁴⁺ is close to Al³⁺ and smaller than Ca²⁺ or Y³⁺, suggesting that Mn⁴⁺ ions prefer to occupy Al³⁺ site in the present host.

Figure 2 shows the powder DRS of CYA: Mn⁴⁺ _x (x = 0.001, 0.005, 0.01, 0.03, 0.05). It is clearly observed that the phosphors of CYA: Mn⁴⁺ _0.001 shows a platform of high reflection in the wavelength range of 580–1200 nm and then starts to decrease dramatically from 580 to 200 nm. As the increasing of Mn⁴⁺ concentration, two broad absorption bands appears at 200–425 nm and 425–580 nm, which is derived from the ⁴A₂ → ⁴ T₁ and ⁴ T₂ transition of Mn⁴⁺, respectively.

The PLE and PL spectra of CYA: Mn⁴⁺ _0.001 are showed in Figure 3. The PLE spectrum contains two broad bands at 250–420 nm and 420-550 nm, which can be attributed to ⁴A₂ → ⁴ T₁ and ⁴ T₂ transition of Mn⁴⁺, respectively. The PL spectra under the excitation at 335 nm, 370 nm and 460 nm exhibit a narrow band between 660 nm and 770 nm with a sharp peak at 710 nm, which is due to ⁴E → ⁴A₂ transition of Mn⁴⁺. It is to say this phosphor can be effectively excited by UV or blue LED chip and emits red light.

In order to further optimize the red emission of Mn⁴⁺ ion, the concentration dependent emission intensity of CYA: Mn⁴⁺ _x (x = 0.001, 0.005, 0.01, 0.03, 0.05) is studied. It can be seen in Figure 4 that the emission intensity of Mn⁴⁺ ion at 710 nm initially increase, then reaches a maximum at x = 0.005 and decrease due to concentration quenching. It is interesting that the chromaticity coordinates of CYA: Mn⁴⁺ _x are almostly the same with the change of Mn⁴⁺ ion dopt content. The inset of Figure 4 gives the chromaticity coordinates of CYA: Mn⁴⁺ _0.005 under 370 nm excitation. The color purity of the point in spectrum locus is 100%. So the color purity of phosphor CYA: Mn⁴⁺ is near 100%.

where E _R, E _S are the spectra of the excitation light without and with the sample in the integrating sphere, respectively, and L _S is the luminescence emission spectrum of the sample in the integrating sphere. The QE of the CYA: Mn⁴⁺ _0.005 was measured and calculated to be about 26% and 28% under 335 nm and 460 nm excitation, respectively.

The luminous efficiency of the radiation (LER) is an important parameter which shows how bright the radiation is perceived by the average human eye. Figure 5 shows the nonalized spectral eye sensitivity cures for photopic vision and spectrum of phosphor CYA: Mn⁴⁺ _0.005. It can be caculated from the emission sepctrum as: [23].

$\mathrm{LER}\left(\mathit{lm}/w\right)=683\mathit{lm}/w\cdot \frac{{\displaystyle {\int }_{380\mathit{nm}}^{780\mathit{nm}}I\left(\lambda \right)V\left(\lambda \right)\mathit{d\lambda }}}{{\displaystyle {\int }_{380\mathit{nm}}^{780\mathit{nm}}I\left(\lambda \right)\mathit{d\lambda }}}$

(2)

Where V (λ) and I (λ) are eye sensitity cure and phosphor emission spectrum respectively. The LER of the CYA: Mn⁴⁺ _0.005 is 3 lum/w which indicates the phosphor is too red for general lighting . However, it may be a promising phosphor for other artificial lighting applications, such as in plant photomorphogenesis [24].

For LEDs application, the thermal stability of phosphor is one of the important factors. Figure 6 shows the PL spectral (λ_ex = 370 nm) of CYA: Mn⁴⁺ _0.005 at the temperature range of 300–460 K. It illustrates that the position and shape of the emission spectra do not change with increasing temperature. The temperature-dependence of the integrated emission intensity for CYA: Mn⁴⁺ _0.005 is presented in the inset of Figure 6. It is clearly observed that the integrated emission intensity of CYA: Mn⁴⁺ _0.005 decreases as the temperature increases from 300 K to 460 K. The integrated emission intensity at 100 and 150°C remain about 70% and 50% when compared to room temperature. The above results mean this phoshor has a good thermal stability and is a candidate for pc-LEDs.

Figure 7 shows the decay curves of Mn^{4+ 4}E → ⁴A₂ emission excited by 335 nm. The decay behavior can be expressed asfollows: [25].

$\mathrm{I}=\mathrm{A}exp\left(-\mathrm{t}/\mathrm{t}\right)+{\mathrm{I}}_0$

(3)

where I and I₀ are emission intensity, A is constant, t is time and, τ is decay time for exponential component. For x = 0.001, 0.005, 0.01 all samples show a nearly single exponential decay behavior like CYA: Mn⁴⁺ _0.001 (the inset of Figure 7) and the life time is estimated to be 1.590 ms, 1.444 ms and 1.306 ms, respectively. When the Mn⁴⁺ concentration is further increased, the decay curves decrease more rapidly and become nonexponential. Such a fast decline of Mn^{4+ 4}E is due to the interaction or energy migration between Mn⁴⁺ ions. The same phenomenon was found in the decay curves of Mn⁴⁺ emission excited by 460 nm.

In summary, a series of CaYAlO₄: Mn⁴⁺ red phosphors with good thermal stability were investigated. Mn⁴⁺ ion gives an intense red light at 710 nm with high color purity and intense broad absorption in UV and blue range. We demonstrate that it can be a useful red phosphor for LEDs, combined with blue (440 ~ 470 nm) InGaN and near (n)-UV (350 ~ 420 nm) GaN chip.