Article pubs.acs.org/IC
Enhanced Luminescence Performances of Tunable Lu3−xYxAl5O12:Mn4+ Red Phosphor by Ions of Rn+ (Li+, Na+, Ca2+, Mg2+, Sr2+, Sc3+) Jiaqi Long, Yuzhen Wang, Ran Ma, Chaoyang Ma, Xuanyi Yuan, Zicheng Wen, Miaomiao Du, and Yongge Cao* Beijing Key Laboratory of Optoelectronic Functional Materials & Micro-Nano Devices, Department of Physics, Renmin University of China, Beijing 100872, China ABSTRACT: A novel red-emitting Lu3Al5O12:Mn4+ (LuAG:Mn4+) phosphor was synthesized by a solid-state reaction. The emission-band position is shifted to red region by gradually increasing the amount of substitution of Lu3+ for Y3+, eventually yielding fully Y 3Al 5O12:Mn4+ (YAG:Mn4+). The compared structural and optical properties of the phosphors were investigated. From the experimentally measured spectroscopic data, crystal field parameter Dq and Racah parameters B and C are calculated to be 2127, 1464, and 3620 cm−1 in LuAG:Mn4+, respectively. In YAG:Mn4+, Dq, B, and C are calculated to be 2082, 1524, and 3740 cm−1, respectively. Impressively, Ca2+/Li+/Mg2+/Sr2+/Sc3+/Na+ dopants were found to be beneficial for enhancing Mn4+ luminescence, and the related mechanisms were systematically discussed.
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different ions. The crystal structure and fluorescence properties are characterized by X-ray diffractometer (XRD) and Hitachi F7000 spectro-photometer, respectively.
INTRODUCTION Red-emitting phosphors play a key role in solid-state lighting for illumination applications.1−4 The commercial white lightemitting diodes (W-LEDs) are phosphor-converted LEDs (pcLEDs) made of a blue InGaN chip and a yellow phosphor, Y3Al5O12:Ce3+ (YAG:Ce3+).5 However, this strategy suffers from major shortcomings such as cold white light and a low color-rendering index (CRI), which results from the lack of red spectral contribution. Therefore, red phosphors with preferable luminescent performance and strong blue absorption are highly demanded. Compared with the traditional commercial red phosphors, for example, CaAlSiN3:Eu2+6 and Sr2Si5N8:Eu2+,7 which are usually accompanied by a broad emission band, Mn4+-activated oxide and fluoride phosphors exhibit sharp emission spectrum in the red region. Moreover, the preparation of nitride phosphors requires harsh conditions, high temperature and pressure, which to some extent, increase the cost and difficulty of preparation. Meanwhile, rare-earth (RE) elements are too expensive to make the phosphors cost-efficient. Recently, Mn4+doped fluoride red phosphors have received much attention8,9 because of simple and efficient fabrication process, which can be prepared at room temperature. However, hydrofluoric acid was still corrosive as a starting material. Mn4+-doped oxide phosphors were prepared using conventional oxides and carbonates as starting materials.10−13 This article has three purposes. The first one is to explore a novel red-emitting Lu3Al5O12:Mn4+ (LuAG:Mn4+) phosphor by conventional solid-state reaction method. The second one is to modulate the emission spectrum wavelength of LuAG:Mn4+ by doping Y and compare the crystal field strength by calculation. The last one is to optimize luminous intensity by doping © 2017 American Chemical Society
1. EXPERIMENTAL SECTION 1.1. Materials and Synthesis. The Lu 3−x Y x Al 5 O 12 :Mn 4+ phosphors were synthesized by a high-temperature solid-state reaction. The constituent oxides or carbonates Y2O3 (99.9%), Lu2O3 (99.9%), Al2O3 (99.9%), and MnCO3 (99.99%) were employed as the raw materials. Individual batches of 10 g were weighted according to the designed stoichiometry and mixed homogeneously with the same mass of absolute ethyl alcohol as the dispersant. After planetary ball-milling process, the obtained homogeneous slurry was placed in a Petri dish and dried in an oven. Then, the dried mixtures were put into a crucible with a lid and heated in a tubular furnace at 1500 °C for 5 h in air. When cooled to room temperature, the prepared phosphors were crushed and ground for subsequent measurements. 1.2. Characterization. The composition and crystal structure of the samples were analyzed by powder XRD using a Rigaku XRD (Tokyo, Japan) with a graphite monochromator using Cu Kα radiation (λ = 1.540 56 Å), operating at 40 kV and 40 mA. The investigation range was 10−70°. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the phosphors were measured using a Hitachi F-7000 spectro-photometer (Tokyo, Japan) with a 150 W Xe lamp. The diffuse reflection (DR) spectrum was analyzed by PerkinElmer Lambda 950 UV/vis/NIR spectrometer (USA). The particle size was measured by Jnwinner Winner 2000 laser particle size analyzer (Jinan, China). Measurements were performed at room temperature unless otherwise specified. Received: November 2, 2016 Published: March 6, 2017 3269
DOI: 10.1021/acs.inorgchem.6b02647 Inorg. Chem. 2017, 56, 3269−3275
Article
Inorganic Chemistry
2. RESULTS AND DISCUSSION 2.1. Phase Identification and Crystal Structure. Figure 1a shows the XRD patterns of Lu3−xYxAl5O12:Mn4+ (x = 0, 0.6,
ionic radius of Y (1.019 Å, eight-coordinated) is larger than that of Lu (0.977 Å, eight-coordinated), the peak of XRD has a linear shift as the amount of Y replaced by Lu increases, which is shown in Figure 1b. As shown in Figure 1c, the lattice parameter (a) linearly increases with the increase of the Y ion ion, which leads to the lattice parameter (a) increasing from 11.9 to12 Å. This indicates that the Y ions have been successfully incorporated into the structure of Lu3Al5O12 and replaced the Lu ions. The crystal structure information on the host materials is closely related with their luminescence properties. The crystal phase of both Lu3Al5O12 and Y3Al5O12 belong to cubic system with the space group of Ia3̅d (230), and the cell parameters are a = b = c = 11.906 Å for the former and a = b = c = 12.008 Å for the latter, respectively. Figure 2a,b represents the crystal structure of Lu/YAG of the 1 × 1 × 1 unit cell, and LuAG is isostructural with YAG. Lu3+/Y3+ coordinates with eight O2− ions to form polyhedral shape, and Al3+ has two kinds of lattice sites in Lu/YAG, coordinated with six O2− ions to form AlO6 octahedral shape and four O2− ions to form AlO4 tetrahedral shape, respectively. The AlO6 octahedral and AlO4 tetrahedral shapes connect with each other by means of sharing the same O2− ion. The effective ion radii (IR) of Y3+ = 1.019 Å, IR of Lu3+ = 0.977 Å, IR of Al3+ = 0.535 Å, and IR of Mn4+ = 0.53 Å are known. The bond lengths between Mn and ligand O are 1.864 and 1.921 Å in LuAG and YAG, respectively. As the atom number per cell keeps constant, the increase of lattice parameter means the lengthening of distance between atoms. This weakens the Mn interaction between atoms, so Mn4+ will experience a weaker crystal field as the content of Y increases. 2.2. Photoluminescence Properties. Figure 3a,c shows the photoluminescence emission (PL) and excitation (PLE) spectra of the Lu3−xYxAl5O12:Mn4+ (x = 0, 0.6, 1.2, 1.6, 2.2, 3.0) samples measured with 310 nm ultraviolet (UV) light and 670 nm red light, respectively. In the PL spectrum, two sharp peaks around 642 and 670 nm were obtained due to a spin- and
Figure 1. (a) XRD patterns of Lu3−xYxAl5O12:Mn4+ (x = 0, 0.6, 1.2, 1.6, 2.2, 3.0) samples; bars represent standard Lu3Al5O12 (JCPDS Nos. 88−2047) and Y3Al5O12 (JCPDS Nos. 73−1368) crystal data; blue and green small triangles represent YBO3 and LuBO3, respectively. (b) The enlarged XRD patterns of Lu3−xYxAl5O12:Mn4+ (x = 0−3) samples in the range of 33.0−33.8°. (c) Variation of lattice parameter (a) of Lu3−xYxAl5O12:Mn4+ (x = 0−3) samples showing a linear increase following Veǵard’s law dependent on increasing Y content (x).
1.2, 1.6, 2.2, 3.0) phosphors. The XRD patterns of samples Lu3Al5O12:Mn4+ and Y3Al5O12:Mn4+ match well with JCPDF cards numbered 73−1368 and 88−2047, respectively. Since the
Figure 2. Unit cell representation of the crystal structure of Lu3Al5O12 (a) and Y3Al5O12 (b), respectively. Aluminum site of Lu3Al5O12 (c) and Y3Al5O12 (d) is depicted with six-coordination with oxygen atoms. 3270
DOI: 10.1021/acs.inorgchem.6b02647 Inorg. Chem. 2017, 56, 3269−3275
Article
Inorganic Chemistry
Figure 3. (a) PL (λex = 310 nm) spectra of Lu3−xYxAl5O12:Mn4+ (x = 0, 0.6, 1.2, 1.6, 2.2, 3.0) sample. (b) The enlarged PL spectra of Lu3−xYxAl5O12:Mn4+ (x = 0−3) samples in the range of 662−678 nm (c) PLE (λem = 670 nm) spectra of Lu3−xYxAl5O12:Mn4+ (x = 0, 0.6, 1.2, 1.6, 2.2, 3.0) sample. (d) The enlarged PLE spectra of Lu3−xYxAl5O12:Mn4+ (x = 0−3) samples in the range of 290−350 nm.
parity-forbidden transition of 2E → 4A2. The red shift (∼4 nm) of the emission spectrum as a whole occurs with Y replacing Lu in LuAG, which is shown in Figure 3b. Unlike the PL bands, two broad bands are observed, which are attributed to the 4A2 → 4T1 (∼310 nm) and 4A2 → 4T2 (∼455 nm) transitions in Mn4+ ions. The relatively large spectral bandwidths of the PLE bands are attributed to a strong electron−phonon interaction. The enlarged PLE spectra of Lu3−xYxAl5O12:Mn4+ (x = 0−3) samples are shown in Figure 3d. The red shift (∼4 nm) of the excitation spectrum as a whole also occurs with Y replacing Lu in LuAG. The range of red shift wavelength is not large, which is mainly contributed to the little difference occurred between 4 A2 and 2E levels with varying octahedral crystal field.14 To understand the effect of the crystal field strength on red luminescence of the Mn4+-doped compounds, we calculated the crystal field strength (Dq) and the Racah parameters (B and C) by analyzing the PLE and PL spectra of Lu/YAG:Mn4+. The energy levels of 3d3 ions depend on the parameters Dq, B, and C, where Dq is a parameter that characterizes the strength of the octahedral crystal field, while B and C are Racah parameters. Take LuAG:Mn4+, for example, the energy corresponding to the peak of 4A2 → 4T2 is ∼21276 cm−1, which is calculated from the PLE multipeak fitting analysis of the bands as shown in Figure 4. Thus, Dq is calculated to be ∼2127 cm−1. Instead of using the zero phonon lines (ZPL), the energy corresponding to the wavelength of the emission band was used for the calculations, as ZPLs are not distinguishable in the room-temperature PL. We determined the values of these parameters from observed peak energies of 4T2, 4T1, and 2Eg states. The values of Dq, B, and C can be calculated based on experimentally determined energy levels using the following equations:9
Dq =
Figure 4. PL spectra of the LuAG/YAG:Mn4+ fitted with Gaussian curves assuming emission from 4A2 → 4T1. Fitted Gaussian peaks are shown in solid red, green, and blue lines.
Dq 15(x − 8) = 2 B (x − 10x)
E(4 T2g − 4A 2g) 10
(1)
(2)
where the parameter x is defined as 3271
DOI: 10.1021/acs.inorgchem.6b02647 Inorg. Chem. 2017, 56, 3269−3275
Article
Inorganic Chemistry x=
E(4A 2g →4 T1g ) − E(4A 2g → 4 T2g ) Dq
(3)
E(2 E g − 4A 2g)/B = 3C /B + 9 − 9B /Dq 4
4
(4) 2
From Figure 4, the energy levels of T2, T1, and E in the LuAG host were determined at 21 276, 33 222, and 14 970 cm−1, respectively. The energy levels of 4T2, 4T1, and 2E in the YAG host were determined at 20 825, 32 858, and 14 897 cm−1, respectively. From eqs 1−4, the crystal field parameters of Dq, B, and C in the LuAG were calculated to be 2127, 1464, and 3620 cm−1, respectively. The crystal field parameters of Dq, B, and C in the YAG were calculated to be 2082, 1524, and 3740 cm −1 , respectively. Once these parameters have been determined, the energies of all other states such as 2T1, 2A1, and 4T1 can be theoretically predicted by E(2 T1 − 2 E) = 66B2 /(10Dq)
(5)
E(2A1 − 4A 2) = 10Dq + 4B + 3C
(6)
Figure 5. Tanable−Sugano energy-level diagram for Mn4+ in the octahedral site of YAG and LuAG host, wherein the ground electronic state (4A2) and the lowest excited state (2E) come from the t23 configuration, whereas the 4T1 and 4T2 levels arise from the t22e electronic orbital.
E(4 T1(P) − 4A 2) = 15Dq + 7.5B − 0.5 2
than that of 4A2 and 2E (or 2T1). Larger displacement implies stronger electron−phonon interaction and thus a larger spectral bandwidth of the transition. Additionally, according to the spin selection rule of ΔS = 0, the transitions between the 4T1, 4T2, and ground-state 4A2 levels are spin-allowed. Therefore, intense excitation or absorption bands with relatively large bandwidths are expected between these levels. Upon excitation to the 4T1 or 4T2 level, the excited ions usually relax nonradiatively to 2E, followed by the spin-forbidden 2 E → 4 A 2 transition characterized by sharp emission lines. As shown in Figure 5, the corresponding Dq/B of YAG and LuAG was marked as blue and red, respectively. The stronger crystal field strength due to shorter band length between Mn4+ and ligand O2− in LuAG makes the energy-level splitting more, compared with YAG. The digital images of the Lu3−xYxAl5O12:Mn4+ (x = 0, 0.6, 1.2, 1.6, 2.2, 3.0) phosphors under sunlight and 365 nm UV−blue light are shown in Figure 6.
2 1/2
(100Dq − 180DqB + 225B )
(7)
The crystal field parameters and the energies states in LuAG and YAG crystal lattices are summarized in the Table 1. Using Table 1. Crystal Field Parameters and Energy States in LuAG and YAG Crystal Lattices Dq B C Dq/B 4 A2→4T2 4 A2→2T2 4 A2→4T1 2 E→4A2 2 E→2T1 4 A2→2A1 4 A2→4T1(P)
LuAG (cm−1)
YAG (cm−1)
2127 1464 3620 1.45 21 276 27 855 33 222 14 970 11 946 37 993 33 222
2082 1524 3740 1.37 20 825 29 968 32 858 14 897 7363 38 142 32 858
the obtained Dq and B values, Dq/B value can be calculated to be ∼1.45 and ∼1.37 for LuAG:Mn4+ and YAG:Mn4+, respectively. The increased lattice parameter weakens the Mn ions interaction between Mn ions and ligand O ions, so Mn4+ will experience a weaker crystal field, as the content of Y increases. From the calculated results, YAG has weaker crystal field strength than LuAG, which is consistent with the above conclusions. In general, Mn4+ luminescence is highly related to the crystal field strength and site symmetry of the host. Tanabe−Sugano energy-level diagram (Figure 5) illustrates the dependence of energy levels of Mn4+ on the octahedral. The 2E, 2T1, 2T2, and 4 A2 levels are derived from the t23 electronic orbital, whereas the 4 T1 and 4T2 levels are derived from another t22e orbital. As such, a large lateral displacement can be observed between the parabolas of ground-state 4A2 and 4T1 (or 4T2) and a small displacement between the parabolas of 4A2 and 2E (or 2T1 and 2 T2). We also noticed that larger lateral displacement can be observed between the parabolas of ground-state 4A2 and 2T2
Figure 6. Digital images of the as-prepared Lu3−xYxAl5O12:Mn4+ (x = 0, 0.6, 1.2, 1.6, 2.2, 3.0) phosphors under sunlight (upper) and 365 nm UV−blue light (lower).
The Figure 7 shows the PL spectra of Lu3Al5O12:yMn4+ (y = 0.05%, 0.1%, 0.5%, 0.8%, 1%) under excitation at 365 nm UV− blue light. With the increase of Mn4+ doping concentration, the luminescence intensity increases and then decreases, and at y = 0.1%, the intensity reaches the maximum, as shown in the insets (upper). Concentration quenching mechanism leads to the inability to increase the emission intensity by simply increasing 3272
DOI: 10.1021/acs.inorgchem.6b02647 Inorg. Chem. 2017, 56, 3269−3275
Article
Inorganic Chemistry
Figure 7. PL spectra of Lu3Al5O12:yMn4+ under excitation at 310 nm UV−blue light with y = 0.05%, 0.1%, 0.5%, 0.8%, 1%.
Figure 8. Representative diffuse reflection spectra of Lu3Al5O12:yMn4+ (y = 0.05%, 0.1%, 0.5%, 0.8%, 1%).
Mn4+ ions. The inset (lower) shows digital images of the asprepared Lu3Al5O12:yMn4+phosphors under 365 nm UV−blue light and sunlight. The main matrix is titanate and aluminate in Mn4+-doped red phosphor, which is found until now. The best doping concentration of Mn4+ in most of titanate or aluminate host is close to 0.1%. Different hosts, doping concentration, sintering temperature, and PL and PLE information about Mn4+-doping red phosphor are summarized in Table 2. The diffuse reflection (DR) spectra of LuAG doping with various Mn4+ contents are shown in Figure 8. One can see two typical strong spin-allowed Mn4+: 4A2 → 4T1 and Mn4+: 4A2 → 4 T2 transitions peaking at ∼320 nm (32 258 cm−1) and ∼480 nm (20 833 cm−1) and a weak spin-forbidden Mn4+: 4A2 → 2T2 one locating at ∼350 nm (28 571 cm−1). The other is between 240 and 280 nm, which is due to the existence of a charge transfer band (CTB) of Mn4+−O2−.15 To increase the luminous intensity of the phosphor, other ions are codoped with Mn4+. Figure 9a exhibits the PL spectra of LuAG:Mn4+ without any dopant and doped by various ions Ln (Ln = Ca2+, Li+, Mg2+, Sr2+, Sc3+, Na+, Ti4+, Si4+) ions. All the doping concentrations of Ca2+, Li+, Mg2+, Sr2+, Sc3+, Na+, Ti4+, Si4+ ions are the same as 2 mol %. The particle size of all
samples is measured to eliminate the influence of particle size on PL emission intensity. As shown in Figure 9b, the average particle sizes of all samples were measured to be ∼4 μm, indicating the influence of particle size on PL emission intensity was the same. Various ion doping influences the intensity of the emission rather than the position of the emission band. Impressively, luminescence intensity of LuAG:0.1%Mn4+ increased by 2 times when Ca2+ ions were codoped, as shown in Figure 9b. The luminescence is enhanced to a certain extent when the charge number of the doped ions is less than or equal to 3+, while the ions doped with a charge of 4+ will greatly weaken the luminescence intensity. However, the origin of this phenomenon is not clearly understood at present. One possible reason is that the substitution of Al3+ for Ln4+(Ti4+, Si4+) results in a positive local charge, which is competitive with the substitution of Al3+ by Mn4+. Otherwise, substitution of Al3+ or Lu3+ with a smaller charge than 3+ results in negative local charge, thus allowing more Mn4+ to replace Al3+, which shows a charge compensation mechanism. The ionic radius of Ti4+(0.605 Å), Si4+(0.4 Å), Mg2+(0.72 Å), Li+(0.76 Å), and Sc3+(0.74 Å) is similar to that of Al3+(0.535 Å), so Al3+ is supposed to be replaced by Ti4+, Si4+, Mg2+, or Li+, as shown in Figure 10. The ionic radius of Na+(1.02 Å), Ca2+(1 Å), and
Table 2. Synthetic and Optical Information of Mn4+ Ion in Some Different Host Lattices
titanates
aluminates
hosts
concentration
T (°C)
Li2MgTi3O8 Gd2ZnTiO6 Li2TiO3 MgTiO3 Mg2TiO4 LiGaTiO4 SrMgAl10O17 CaMg2Al16O27 Sr2MgAl22O36 CaAl12O19 SrAl12O19 Sr4Al14O25 Ca14Al10Zn6O35 Ca14Zn6Ga10O35 CaYAlO4 SrAl4O7 CaAl2O4 YbAlO3 Y3Al5O12 Lu3Al5O12
0.1% 0.2% 1%
