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 [15]. 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+ [79]. 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 4G5/2-6HJ ( = 5/2, 7/2, 9/2, 11/2) transitions, which is the most suitable source for lighting and display from a practical viewpoint [1012]. CaIn2O4 is a semiconductor with band gap (Eg) 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 [1416]. 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 [1416]. 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 [1316]. 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.

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  Å3, the obtained value is 30.37 Å. Therefore, the energy transfer in the present case also occurs by electric multipole-multipole interaction.

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 6H5/2 ground state to the corresponding excited state of Sm3+. Among these sharp lines, the strongest peak centered at 407 nm is assigned to 6H5/2-4K11/2. 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.

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].

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.

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.


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).