The Photoluminescence Behaviors of a Novel Reddish Orange Emitting Phosphor CaIn₂O₄:Sm³⁺ Codoped with Zn²⁺ or Al³⁺ Ions

Abstract

A novel reddish orange phosphor CaIn₂O₄:Sm³⁺ codoped with Zn²⁺ or Al³⁺ ions was prepared by solid state reaction and their luminescence properties were investigated under near ultraviolet excitation. The strategy of Zn²⁺ or Al³⁺ ions codoping was used with the aim to improve the luminescence properties of CaIn₂O₄:Sm³⁺, but the concrete effects of the two ions is different. The introduction of Zn²⁺ ions can produce defects that favor charge balance in CaIn₂O₄:Sm³⁺ to facilitate its photoluminescence. The effect of Al³⁺ ions codoping can effectively transfer energy from charge-transfer absorption band to characteristic transition of Sm³⁺ ions, utilizing more energy from host absorption for the photoluminescence of Sm³⁺ ions. Based on these mechanisms, the luminescence intensity of CaIn₂O₄:0.6%Sm³⁺ was enhanced to 1.59 times and 1.51 times when codoping amount of Zn²⁺ and Al³⁺ ions reached 0.6%. However, the chromaticity coordinates of CaIn₂O₄:0.6%Sm³⁺ almost did not have any changes after Zn²⁺ ions or Al³⁺ ions codoping; those are still located at reddish orange region. The excellent luminescence properties of CaIn₂O₄:0.6%Sm³⁺,0.6%Zn²⁺ and CaIn₂O₄:0.6%Sm³⁺,0.6%Al³⁺ demonstrate that they both have potential application value as new-style reddish orange phosphors on light-emitting diode.

1. Introduction

White light-emitting diodes (WLEDs), with the advantages of long lifetime, less energy consumption, and environment friendly characteristics, are thought to be the most important solid state light sources for substituting the widely used incandescent lamps and fluorescent lamps [1–5]. But it still needs to overstep many technological obstructions. It is well known that WLEDs can be realized by combing a single near ultraviolet (NUV, 370–410 nm) light-emitting diodes (LEDs) chip with tricolor phosphors or combining a simple blue LEDs chip with yellow phosphor [6]. With the development of semiconductor technology, NUV-LEDs chips can offer a higher efficient solid state light. Thus the corresponding phosphors which can be efficiently excited by NUV urgently needed to be explored, but the red phosphor excited by NUV light is rare at present. In last decade, many scientists have been devoted to the red phosphors doped by Eu³⁺ and Eu²⁺ [7–9]. However, commercial red phosphors (e.g., Y₂O₂S:Eu³⁺, CaS:Eu²⁺, and SrS:Eu²⁺) do not have enough absorption in the NUV region and the lifetime of the LEDs is limited because of the degradation of these sulfide phosphors during LEDs operation [8].

Sm³⁺ ion is widely used as activator of reddish orange emission due to its ⁴G_5/2-⁶H_J ( = 5/2, 7/2, 9/2, 11/2) transitions, which is the most suitable source for lighting and display from a practical viewpoint [10–12]. CaIn₂O₄ is a semiconductor with band gap (E_g) as 3.9 eV, belonging to ordered CaFe₂O₄ structures with the Pca21 or Pbcm space group, and the lattice parameters a = 9.70 Å, b = 11.30 Å, and c = 3.21 Å for ( is the number of formula units per unit cell) [5, 13]. InO₆ octahedra connect to each other by sharing edges and corners to form a tunnel network structure and Ca²⁺ ions locate in the tunnels. The ionic radius of six-coordinated Sm³⁺ (0.958 Å) is similar to that of Ca²⁺ (1.14 Å) but larger than that of In³⁺ (0.94 Å), so Sm³⁺ ions more easily occupy Ca²⁺ sites in the tunnel of InO₆ octahedra. So far the luminescence properties of CaIn₂O₄:Tb³⁺, CaIn₂O₄:Pr³⁺, CaIn₂O₄:Dy³⁺, and CaIn₂O₄:Eu³⁺ under UV and VUV (vacuum ultraviolet) excitation have been investigated for the application on field emission displays (FEDs), cathode ray tube display (CRTs), and WLEDs [14–16]. To the best of our knowledge, there are few reports about the luminescence properties of CaIn₂O₄:Sm³⁺ for WLEDs application. Yan et al. reported the photoluminescence of CaIn₂O₄:Eu³⁺,Sm³⁺. The doped Sm³⁺ can sensitize the emission of Eu³⁺ and be effective to extend and strengthen the absorption of near-UV light with wavelength of 400–405 nm. The temperature quenching effect proved that CaIn₂O₄ has good stability. And doping amount variation of Sm³⁺ can tune the CIE chromaticity coordinates of CaIn₂O₄:Eu³⁺,Sm³⁺ better than conventional red phosphor Y₂O₂S:0.05Eu³⁺ under the excitation of NUV light [17]. Thus the research about the luminescence properties of CaIn₂O₄:Sm³⁺ under NUV excitation is very significant. So, in our paper, the luminescence properties of CaIn₂O₄:0.6%Sm³⁺ codoped with Zn²⁺ or Al³⁺ ions were investigated in detail for basic research and potential application in WLEDs.

2. Experimental Details

The powder samples CaIn₂O₄:Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,Al³⁺ were prepared by solid state reactions [14–16]. The purity of CaCO₃ and In₂O₃ are A.R., while Sm₂O₃, ZnO and Al₂O₃ are better than 99.99%. Stoichiometric amounts of the starting reagents were thoroughly mixed and ground together. Then the mixture was heated at 1150°C for 12 h. Finally the samples were naturally cooled to room temperature in the furnace.

The normal crystal structure was characterized using Cu Kα radiation (DX-2700 powder X-ray diffractometer) over the angular range . The UV photoluminescence (PL) and photoluminescence excitation (PLE) spectra were recorded by F-7000 fluorescence spectrophotometer with Xe lamp as the light source. The quantum efficiencies were measured by FLS920 Fluorescence Spectrometer. All above spectra were recorded at room temperature.

3. Results and Discussion

CaIn₂O₄:0.6%Sm³⁺ was characterized by the powder X-ray diffractometer and corresponding powder X-ray diffraction (XRD) patterns are demonstrated in Figure 1 as a sample. It is obvious that most diffraction peaks can be indexed with the Joint Committee on Powder Diffraction Standards (JCPDS) card number 17-0643 indicating that it is almost single-phase structure. That also corresponds to XRDs reported in [13–16]. While there is a little different peak located at 30.6°, which is attributed to the diffraction peak of In₂O₃, the little impurity cannot play an important role in their luminescence properties.

(a)

(b)

Figure 1 

XRD patterns of CaIn₂O₄:0.6%Sm³⁺ and the JCPDS card number 17-0643.

PL spectra of CaIn₂O₄:Sm³⁺ phosphors with different Sm³⁺ concentrations are shown in Figure 2. As can be seen from these figures, the emission intensity initially increases with Sm³⁺ concentration increasing and reaches a maximum at 0.6 mol% Sm³⁺ and then decreases which resulted from concentration quenching, because nonradiative energy transfer from one Sm³⁺ ion to another takes place. According to [18, 19], the probability of energy transfer among Sm³⁺ ions increases when the Sm³⁺ concentration increases. Nonradiative energy transfer from one Sm³⁺ ion to another usually may occur by exchange interaction, radiation reabsorption, or multipole-multipole interaction. For CaIn₂O₄:Sm³⁺, the concentration quenching is caused by the electric multipole-multipole interaction. We can roughly estimate the critical distance of energy transfer (). The distance at which the probability of transfer is equal to the probability of radiative emission can be made using the relation by Blasse to calculate between activator ions of the kind doped in a host lattice: where is the volume of the unit cell, the critical concentration of activator ion, and the number of formula units per unit cell [20]. For CaIn₂O₄ host, using , = 0.6 mol%, and  ų, the obtained value is 30.37 Å. Therefore, the energy transfer in the present case also occurs by electric multipole-multipole interaction.

Figure 2 

The PL spectra of CaIn₂O₄:Sm³⁺ () ( nm).

The PLE and PL spectra of CaIn₂O₄:0.6%Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,0.2%Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,0.2%Al³⁺ are compared in Figures 3 and 4, respectively. The PLE spectra are monitored by 606 nm. In PLE spectra, the excitation bands from 200 nm to 275 nm with a maximum at 225 nm belong to charge-transfer absorption band (CTB) of O²⁻ ions to Sm³⁺ ions. In the longer wavelength region, the f-f transitions within the Sm³⁺ 4f⁵ configuration can be detected, which is assigned as transition from the ⁶H_5/2 ground state to the corresponding excited state of Sm³⁺. Among these sharp lines, the strongest peak centered at 407 nm is assigned to ⁶H_5/2-⁴K_11/2. It is obviously shown that 0.2%Zn²⁺ codoping can increase the characteristic transitions and CTB of Sm³⁺ ions. In CaIn₂O₄ matrix, the ionic radius of Zn²⁺ ion is 0.88 Å, similar to that of In³⁺ ion (0.94 Å), so Zn²⁺ ion mainly occupies the In³⁺ ion sites. When Sm³⁺ ions replace Ca²⁺ ions in CaIn₂O₄, it causes the defects and defects occur, which is to the disadvantage of photoluminescence. The introduction of small amount of Zn²⁺ ions can favor the charge balance in CaIn₂O₄:Sm³⁺ and further facilitate its photoluminescence. The enhanced Sm³⁺ emission comes mainly from radiative recombination of the large amount of trapped carriers excited from the CaIn₂O₄ host. Upon photoexcitation, more oxygen holes can be trapped at the defects in CaIn₂O₄:Sm³⁺,Zn²⁺. The increased recombination probability of electrons and trapped holes would cause the highly enhanced emission intensity of Sm³⁺ ions by the resultant energy transfer to the Sm³⁺ ions nearby. It also could be assumed that the incorporation of Zn²⁺ ions creates the oxygen vacancies, which might act as the sensitizer for the energy transfer to the rare earth ion due to the strong mixing of charge transfer states resulting in the highly enhanced luminescence. However, there is one different PLE spectrum with 0.2%Al³⁺ ions codoping. When the intensities of characteristic transitions of Sm³⁺ ions increase by a little amount of Al³⁺ ions codoping, the CTB intensities decrease. In ZnNb₂O₄:Dy³⁺,Al³⁺ material, the fact that energy can be transferred from CTB to excitation states of Dy³⁺ ions by codoping with Al³⁺ ions has been reported [21]. Thus Al³⁺ ions also can help energy transfer from CTB to characteristic transitions of Sm³⁺ in CaIn₂O₄ matrix.

Figure 3 

The PLE spectra of CaIn₂O₄:0.6%Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,0.2%Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,0.2%Al³⁺ ( nm).

Figure 4 

The PL spectra of CaIn₂O₄:0.6%Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,0.2%Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,0.2%Al³⁺ ( nm).

In the corresponding PL spectra of CaIn₂O₄:0.6%Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,0.2%Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,0.2%Al³⁺, all emission peaks are obtained by 407 nm excitation and indexed in Figure 4. It is clearly shown that the little amount of Zn²⁺ or Al³⁺ ions codoping can enhance the intensity of photoluminescence, which corresponds to the PLE spectra in Figure 3. Therefore, the PL properties of the series CaIn₂O₄:0.6%Sm³⁺,Zn²⁺ and CaIn₂O₄:0.6%Sm³⁺,Al³⁺ were further investigated.

Figures 5 and 6 present the series of PL spectra of CaIn₂O₄:0.6%Sm³⁺,Zn²⁺ and CaIn₂O₄:0.6%Sm³⁺,Al³⁺ under 407 nm excitation, respectively. A little amount of Zn²⁺ and Al³⁺ codoping can facilitate the enhancement of luminescence intensity of CaIn₂O₄:0.6%Sm³⁺. When codoping amount of Zn²⁺ is 0.6%, the luminescence intensity of CaIn₂O₄:0.6%Sm³⁺ can be enhanced to 1.59 times as Zn²⁺ free sample. On the same conditions, 0.6%Al³⁺ ions codoping can make 1.51 times luminescence intensity enhancement compared to Zn²⁺ free sample. The codoping amount of 0.6% is critical doping concentration of Zn²⁺ and Al³⁺ ions. When the codoping concentration exceeded 0.6%, the PL intensity became decreasing; that was also caused by nonradiative energy transfer among Sm³⁺ ions. The corresponding quantum efficiencies of CaIn₂O₄:0.6%Sm³⁺,0.6%Zn²⁺ and CaIn₂O₄:0.6%Sm³⁺,0.6%Al³⁺ were measured as 39.67 and 37.21 under 407 excitation, respectively. The values are higher than that of commercial red phosphor of Y₂O₂S:Eu³⁺ (QE = 35%) under 317 nm excitation [22].

Figure 5 

The PL spectra of CaIn₂O₄:0.6%Sm³⁺,Gd³⁺ () ( nm).

Figure 6 

The PL spectra of CaIn₂O₄:0.6%Sm³⁺,Zn²⁺ () ( nm).

The chromaticity coordinates for CaIn₂O₄:0.6%Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,0.6%Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,0.6%Al³⁺ phosphors with 407 nm excitation were simulated and listed in Table 1. With 0.6%Zn²⁺ or 0.6%Al³⁺ ions codoping, the chromaticity color coordinates of CaIn₂O₄:0.6%Sm³⁺ have hardly any movements, which are all in reddish-orange region.

Table 1 

The chromaticity coordinates for CaIn₂O₄:0.6%Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,0.6%Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,0.6%Al³⁺ phosphors ( = 407 nm).

4. Conclusion

The series of phosphors CaIn₂O₄:Sm³⁺ codoped with Zn²⁺ or Al³⁺ ions were prepared by solid state reaction and their luminescence properties were investigated. Codoping Zn²⁺ or Al³⁺ ions can enhance the emissions of CaIn₂O₄:0.6%Sm³⁺ excited by 407 nm, but the effects are different. The introduction of Zn²⁺ ions can favor the charge balance in CaIn₂O₄:Sm³⁺ to facilitate its photoluminescence, but Al³⁺ ions can effectively help energy transferred from CTB to characteristic transition of Sm³⁺ ions, utilizing more energy from host absorption to the photoluminescence of Sm³⁺ ions. All the chromaticity coordinates of CaIn₂O₄:0.6%Sm³⁺, CaIn₂O₄:0.6%Sm³⁺,0.6%Zn²⁺, and CaIn₂O₄:0.6%Sm³⁺,0.6%Al³⁺ are in reddish orange region and very close. The excellent luminescence properties of CaIn₂O₄:0.6%Sm³⁺,0.6%Zn²⁺ and CaIn₂O₄:0.6%Sm³⁺,0.6%Al³⁺ both present potential application value for WLEDs.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This research was supported by the Fundamental Research Funds for the Central Universities (GK201402015), National Natural Science Foundation of China (21302118), and Fundamental Doctoral Fund of Ministry of Education of China (20130202120015).