Structure and Photoluminescence Properties of Sm³⁺ Ion-Doped YInGe₂O₇ Phosphor

Abstract

A new phosphor, Sm³⁺ ion-doped YInGe₂O₇, was synthesized using a planetary ball mill solid state reaction. The XRD patterns show that all of the peaks can be attributed to the monoclinic YInGe₂O₇ crystal structure when the Sm³⁺ ion concentration is increased up to 20 mol %. Under an excitation wavelength of 404 nm, the Sm³⁺ intra-4f transition appears in the emission spectrum including two stronger emission peaks located at 560–570 nm and 598 nm correspond to the ⁴G_5/2 → ⁶H_5/2 and ⁴G_5/2 → ⁶H_7/2 transitions, respectively, and another weak emission peak located at 645 nm is due to the ⁴G_5/2 → ⁶H_9/2 transition. The decay time decreases from 4.5 to 0.8 ms as Sm³⁺ ion concentrations increase from 1 to 20 mol %, and the decay mechanism of the ⁴G_5/2 → ⁶H_7/2 transition is a single decay component between Sm³⁺ ions only. The concentration quenching effect occurs when the Sm³⁺ ion concentration is higher than 3 mol %. The CIE color coordinate of Y_0.97Sm_0.03InGe₂O₇ phosphor is at x = 0.457 and y = 0.407, which is located in the orange-yellow light region.

1. Introduction

Over the last decade, luminescent properties of inorganic phosphors have been extensively investigated to make flat panel displays such as field emission displays (FEDs), plasma display panels (PDPs), and thin film electro-luminescent devices (TFEL). Many efforts have been made to discover oxide based phosphors to improve luminescent performance—including color purity, emission intensity, and quantum efficiency—because of the higher chemical stability of oxide phosphors relative to that of sulfide phosphors. Many studies have been conducted for developing new oxide phosphors in powder form suitable for full-color emissive displays [1,2,3,4,5,6,7].

Yttrium indium germanate (YInGe₂O₇) has the thortveitite structure with symmetry described by the space group C2/m (No. 12). This crystallizes in the monoclinic system, with cell parameters a = 6.8286 Å, b = 8.8836 Å, c = 4.9045 Å. The In³⁺ and Y³⁺ cations occupy the same octahedral site forming a hexagonal arrangement on the a–b planes [8]. In their turn, the hexagonal arrangements of InO₆/YO₆ octahedral layers are held together by sheets of isolated diorthogroups composed of a double tetrahedral sharing a common vertex. It is suggested that the YInGe₂O₇ possesses excellent optical properties. Previous studies indicated that YInGe₂O₇ doped with different rare earth ions can emit different color tones under different excitation wavelengths, and could be an excellent candidate for a phosphor host. For example, red light emission for Eu³⁺ ion-doped, green light for Tb³⁺ ion, blue light for Tm³⁺ ion, and near white light for Dy³⁺ ion-doped [9,10,11,12].

Rare earth ions have been extensively employed as activators for various phosphors. Sm³⁺ ion is widely used as an activator of reddish orange emission due to its ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2, 9/2, 11/2) transitions, which is the most suitable source for lighting and display from a practical viewpoint [13,14,15,16]. Based on the previous information, the YInGe₂O₇ is chosen to be the host, and references concerning the role of the rare-earth Sm³⁺ ion-doped in YInGe₂O₇ have not yet been studied. In this investigation, YInGe₂O₇ samples doped with different Sm³⁺ concentration phosphors have been prepared by a planetary ball mill solid state reaction process at 1300 °C in air, and the characterization and luminescent properties have also been examined.

2. Experimental Procedure

2.1. Preparation of Samples

The Sm³⁺ ion-doped YInGe₂O₇ phosphor was prepared using a planetary ball mill solid state reaction with Y₂O₃, In₂O₃, GeO₂, and Sm₂O₃ powders. The starting materials (99.99% purity) were purchased from the Sigma-Aldrich Chemical Company (St. Louis, MO, USA) and Alfa Aesar (Ward Hill, MA, USA). The materials were weighed according to the stoichiometric ratios and mixed by a planetary ball mill (Microwolf PA02, Torrey Hills Technologies, San Diego, CA, USA) at a rotational speed of 150 rpm for 15 min with zirconia balls (Φ = 5 mm, 80 balls) in a polyethylene jar. After mechanically mixing, the mixtures were calcined at 1300 °C in air for 10 h.

2.2. Characterization

The structure for Sm³⁺ ion-doped YInGe₂O₇ phosphors were identified by X-ray powder diffractometry (XRD, Rigaku Dmax-33 X-ray diffractometer, Tokyo, Japan) with Cu-Kα radiation with a source voltage of 30 kV and a current of 20 mA. The surface morphology was examined using high-resolution scanning electron microscope (HR-SEM S-4200, Hitachi, Tokyo, Japan). Optical absorption spectra for phosphor powders were placed in a holder, and measured using an ultraviolet-visible (UV–Vis) spectrophotometer (U-3010, Hitachi, Tokyo, Japan) in the range of 200–700 nm at room temperature. Both excitation and luminescence spectra of the phosphors were recorded on a Hitachi F-4500 fluorescence spectrophotometer using a 150 W xenon arc lamp as the excitation source at room temperature.

3. Results and Discussion

3.1. Structure

Figure 1a shows the XRD patterns of YInGe₂O₇ doped with different Sm³⁺ ion concentrations. All of the diffraction peaks are attributed to the monoclinic YInGe₂O₇ phase. These results demonstrate that the Sm³⁺ ion (r = 0.964 Å) incorporated into the Y_1-xSm_xInGe₂O₇ monoclinic structure and replaced the Y³⁺ ion (r = 0.90 Å) to form the solid solution [17]. The full width of half maximum (fwhm) of these peaks seems to increase and the crystallinity of YInGe₂O₇:Sm³⁺ becomes worse with the increase in Sm³⁺ ion concentrations. In addition, the diffraction peaks seem to shift to a lower diffraction angle as the Sm³⁺ ion concentration increases. Figure 1b shows the diffraction peaks of the (201) shift to a lower diffraction angle as the Sm³⁺ ion concentration increases. The shift in the position of the peak is Δθ = 0.08° between the Sm³⁺ ion concentrations of Sm³⁺ = 1 mol % and 20 mol %. These results indicate that the lattice constant for the Sm³⁺ ion-doped phosphor is larger than that for the Sm³⁺ free YInGe₂O₇ phosphor. Because the Sm³⁺ ion has a larger radius than the Y³⁺ ion, lattice distorts and intra-stress occurs when the concentration of Sm³⁺ ions increases sufficiently to allow the substitution of Y³⁺ ions into the YInGe₂O₇ crystal.

3.2. Microstructures

The size and shape of phosphor particles affects the emission intensity and the efficiency of a device. The size of phosphor particles should be as homogeneous as possible without any aggregates or agglomerates, and the surface should also be as smooth as possible and have a high crystallization degree to improve efficiency [17]. Figure 2 shows the SEM micrograph of YInGe₂O₇ powders doped with different Sm³⁺ ion concentrations calcined at 1300 °C for 10 h in air. The results show that the crystallized particles aggregated and were irregular. The powder micrographs seem not to obviously vary as the Sm³⁺ ion concentration increases, which indicates that there are no influences on the surface morphology of the powders.

3.3. Optical Properties

Figure 3 show the UV-visible absorption spectrum (Figure 3a), and excitation spectrum (λ_em = 598 nm) (Figure 3b) for YInGe₂O₇ doped with 3 mol % Sm³⁺ ion calcined at 1300 °C for 10 h in air. Figure 3c is the energy level diagram of Sm³⁺ ion. According to the previous study, there are two major absorption bands appearing in the absorption spectrum for pure YInGe₂O₇. The first absorption band between 200 and 280 nm can be assigned to the charge transfer between the In³⁺ and O²⁻ ions of the InO₆ anionic group in the host lattice [17,18]. Another absorption band between 280 and 450 nm is caused by the oxygen deficient center of the GeO₄ anion [19]. For YInGe₂O₇:3 mol % Sm³⁺, the compounds exhibited an absorption peak between 230 to 290 nm due to the charge transfer state (CTS) of the Sm³⁺-O²⁻ ions [20,21], which overlaps with the absorption band of the host lattice and causes the charge transfer state phenomenon to be undetectable in this study.

A series of sharp absorption bands present between 310 and 540 nm, centered at 332, 345, 361, 376, 391, 404, 418, 440, 464, 474, 501, and 526 nm for each absorption peak labeled from 3–14 in the absorption spectrum, respectively, which correspond to the typical f–f transition of Sm³⁺ ions [21,22,23,24]. The strongest peak is located at 404 nm, which can be assigned to the ⁶H_5/2 → ⁴K_11/2 transition [24]. The excitation spectrum (λ_em = 598 nm) for YInGe₂O₇ doped with 3 mol % Sm³⁺ ions calcined at 1300 °C for 10 h in air is shown in Figure 3b. As can be seen, there are two parts for excitation behavior: (1) A weak broad band between 235 and 285 nm is the charge transfer state (CTS) bands due to the samarium–oxygen interactions [20,21]; and (2) A series of excitation peaks in the range from 292 to 490 nm are due to the typical intra-4f transitions of the Sm³⁺ ions that appear at 345, 361, 376, 391, 404, 418, 440, 464, 474, 501, and 526 nm (labeled 4–14 in the excitation spectrum), which were attributed to (⁶H_5/2 → ³H_7/2), (⁶H_5/2 → ⁴F_9/2), (⁶H_5/2 → ⁴D_5/2), (⁶H_5/2 → ⁶P_7/2), (⁶H_5/2 → ⁴K_11/2), (⁶H_5/2 → ⁶P_5/2 + ⁴M_19/2), (⁶H_5/2 → ⁴G_9/2 + ⁴I_15/2), (⁶H_5/2 → ⁴F_5/2 + ⁴I_13/2), (⁶H_5/2 → ⁴I_11/2 + ⁴M_15/2), (⁶H_5/2 → ⁴G_7/2), and (⁶H_5/2 → ⁴F_3/2) transitions [21,22,23,24]. The strongest excitation peak is located at 404 nm, and it is in good accordance with the absorption analysis results.

Figure 4 shows the photoluminescence emission spectra for YInGe₂O₇ doped with different concentrations of Sm³⁺ ions under an excitation wavelength of 404 nm. There are two stronger emission peaks located at 560–570 nm, and 598 nm correspond to the Sm³⁺ intra-4f transition from the excited levels to lower levels, the ⁴G_5/2 → ⁶H_5/2 and ⁴G_5/2 → ⁶H_7/2 transitions, respectively [18,19]. Another weak emission peak located at 645 nm is due to the ⁴G_5/2 → ⁶H_9/2 transition [18]. Due to the absence of a center of symmetry, the 4f orbitals mix with the opposite parity orbitals resulting in the appearance of electric-dipole transitions (⁴G_5/2 → ⁶H_9/2) [19], and their intensity is hyper-sensitive to the variation of the local structure environment of the Sm³⁺ ions. While the ⁴G_5/2 → ⁶H_7/2 emission is a magnetic-dipole allowed transition, its intensity hardly changes with the local structure symmetry of the Sm³⁺ ions [25]. The higher intensity for the ⁴G_5/2 → ⁶H_7/2 transition appears in this study that means the Sm³⁺ ion occupied a symmetry lattice.

In general, fluorescence describes the spontaneous emission of a photon from a molecule or atom after excitation of the electronic system by absorption of light. The emitted photon usually has less energy compared to the excitation photon and consequently has a longer wavelength (Stokes shift) which leads a blue excitation may cause green emission. If the two atoms are close very much, the energy can ‘hop’ directly from the ‘donor’ to the ‘acceptor’, and does not involve the emission and re-absorption of a light. This direct exchange of energy is called Förster Resonance Energy Transfer (FRET) [26]. If FRET occurs, the collected emission is normally not only green but red photons will also be emitted if the sample is illuminated with blue light. Therefore, the green and red light emissions are observed in the emission spectrum for YinGe₂O₇:Sm³⁺ phosphor under an excitation wavelength of 404 nm in this study.

According to the studies [8], the In³⁺ and Y³⁺ cations occupy the same octahedral site forming a hexagonal arrangement on the a–b planes of the YInGe₂O₇ structure. In their turn, the hexagonal arrangements of InO₆/YO₆ octahedral layers are held together by sheets of isolated diorthogroups composed of a double tetrahedron sharing a common vertex, and every two GeO₅ hexahedrons form the Ge₂O₉ structure via oxygen ion sharing which causes the YO₆ octahedral layer to become disordered and introduce the non-symmetric center of lattice due to internal stress. Therefore, when a Sm³⁺ ion substitutes the Y³⁺ ion lattice position, and it is guessed that the Sm³⁺ ion occupies such a non-symmetric center to cause the intensity of ⁴G_5/2 → ⁶H_9/2 transition higher than that of ⁴G_5/2 → ⁶H_7/2 transition. However, it is not true in this study, and it might be that the radius of Sm³⁺ ion is larger than the Y³⁺ ion, and leads the Sm³⁺ ion lattice position to become more symmetrical.

Figure 5 shows the relationships between e emission intensity of the ⁴G_5/2 → ⁶H_7/2 transition of Sm³⁺ ion in YInGe₂O₇ plotted versus the Sm³⁺ ion concentrations under an excitation of 404 nm. The emission intensity of ⁴G_5/2 → ⁶H_7/2 transition increases with increasing Sm³⁺ ion concentration in the lower Sm³⁺ ion contents region until the saturated PL intensity is reached and then decreases, which indicated that the concentration quenching is active when the Sm³⁺ ion concentrations is higher than 3 mol.%. In addition, the quantum yield (1 mol %:5.5, 3 mol %:10.5, 5 mol %:9.2, 10 mol % 4.7, 20 mol %:1.03) also appears the same tendency for various Sm³⁺ ion-doped phosphor. This is due to the distance between the Sm³⁺ ions becoming shorter, changing the cross-relaxation mechanism to active [17]. The following cross-relaxation may occur

⁴G_5/2 (Sm I) + ⁶H_5/2 (Sm II) → ⁶F_9/2 (Sm I) + ⁶F_9/2 (Sm II)

(1)

Figure 6 shows the decay curve and decay time of YInGe₂O₇ doped with different Sm³⁺ ion concentrations under an excitation of 404 nm with signals detected at 598 nm. As can be seen, the decay time decreases from 4.5 to 0.8 ms as Sm³⁺ concentration increases from 1 to 20 mol %. At low Sm³⁺ ion concentrations (1 and 3 mol %), the decay rate is almost the same, but decreases rapidly when the Sm³⁺ ion concentrations are higher than 3 mol %. This is caused by the effect of the energy exchange between the Sm³⁺ ions as the distance between the Sm³⁺ ions decreased with increasing Sm³⁺ ion concentrations, enhancing the energy depletion rate and causing the decay time to decrease. Moreover, all of the decay curves were attributed to a single exponential behavior, indicating that the decay mechanism of the ⁴G_5/2 → ⁶H_7/2 transition is a single decay component between Sm³⁺ ions only.

In YInGe₂O₇:Sm³⁺ system, different concentrations of Sm³⁺ ion did not change the shape of curves but did change the intensities of the emission spectra. Figure 7 shows the CIE color coordinates of YInGe₂O₇ doped with 3 mol % Sm³⁺ ions. The color coordinates of the emission were x = 0.457 and y = 0.407, which is located in the orange-yellow region.

4. Conclusions

A new phosphor, Sm³⁺ ion-doped YInGe₂O₇, was synthesized and its luminescence properties were investigated. The XRD patterns show that all of the peaks can be attributed to the monoclinic YInGe₂O₇ crystal structure when the Sm³⁺ ion concentration is increased up to 20 mol %. Under an excitation of 404 nm, there are two stronger emission peaks located at 560–570 nm, and 598 nm corresponding to the Sm³⁺ intra-4f transition for the ⁴G_5/2 → ⁶H_5/2 and ⁴G_5/2 → ⁶H_7/2 transitions, respectively. Another weak emission peak located at 645 nm is due to the ⁴G_5/2 → ⁶H_9/2 transition. The concentration quenching effect occurs when Sm³⁺ ions rise above 3 mol % with a CIE color coordinate of x = 0.457 and y = 0.407, which is located in the orange-yellow light region. The decay curve results indicate that the decay time decreases from 4.5 to 0.8 ms as Sm³⁺ concentration increases from 1 to 20 mol %, and the decay mechanism of the ⁴G_5/2 → ⁶H_7/2 transition is a single decay component between Sm³⁺ ions only.