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

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 4G5/2 → 6H5/2 and 4G5/2 → 6H7/2 transitions, respectively, and another weak emission peak located at 645 nm is due to the 4G5/2 → 6H9/2 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 4G5/2 → 6H7/2 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 Y0.97Sm0.03InGe2O7 phosphor is at x = 0.457 and y = 0.407, which is located in the orange-yellow light region.


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 (YInGe 2 O 7 ) 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 In 3+ and Y 3+ cations occupy the same octahedral site forming a hexagonal arrangement on the a-b planes [8]. In their turn, the hexagonal arrangements of InO 6 /YO 6 octahedral layers are held together by sheets of isolated diorthogroups composed of a double tetrahedral sharing a common vertex. It is suggested that the YInGe 2 O 7 possesses excellent optical properties. Previous studies indicated that YInGe 2 O 7 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 Eu 3+ ion-doped, green light for Tb 3+ ion, blue light for Tm 3+ ion, and near white light for Dy 3+ ion-doped [9][10][11][12].
Rare earth ions have been extensively employed as activators for various phosphors. Sm 3+ ion is widely used as an activator of reddish orange emission due to its 4 G 5/2 → 6 H 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 YInGe 2 O 7 is chosen to be the host, and references concerning the role of the rare-earth Sm 3+ ion-doped in YInGe 2 O 7 have not yet been studied. In this investigation, YInGe 2 O 7 samples doped with different Sm 3+ 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.

Preparation of Samples
The Sm 3+ ion-doped YInGe 2 O 7 phosphor was prepared using a planetary ball mill solid state reaction with Y 2 O 3 , In 2 O 3 , GeO 2 , and Sm 2 O 3 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.

Characterization
The structure for Sm 3+ ion-doped YInGe 2 O 7 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. Figure 1a shows the XRD patterns of YInGe 2 O 7 doped with different Sm 3+ ion concentrations. All of the diffraction peaks are attributed to the monoclinic YInGe 2 O 7 phase. These results demonstrate that the Sm 3+ ion (r = 0.964 Å) incorporated into the Y 1-x Sm x InGe 2 O 7 monoclinic structure and replaced the Y 3+ 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 YInGe 2 O 7 :Sm 3+ becomes worse with the increase in Sm 3+ ion concentrations. In addition, the diffraction peaks seem to shift to a lower diffraction angle as the Sm 3+ ion concentration increases. Figure 1b shows the diffraction peaks of the (201) shift to a lower diffraction angle as the Sm 3+ ion concentration increases. The shift in the position of the peak is ∆θ = 0.08 • between the Sm 3+ ion concentrations of Sm 3+ = 1 mol % and 20 mol %. These results indicate that the lattice constant for the Sm 3+ ion-doped phosphor is larger than that for the Sm 3+ free YInGe 2 O 7 phosphor. Because the Sm 3+ ion has a larger radius than the Y 3+ ion, lattice distorts and intra-stress occurs when the concentration of Sm 3+ ions increases sufficiently to allow the substitution of Y 3+ ions into the YInGe 2 O 7 crystal.

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 YInGe 2 O 7 powders doped with different Sm 3+ 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 Sm 3+ ion concentration increases, which indicates that there are no influences on the surface morphology of the powders.

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 Sm 3+ 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 Sm 3+ ion concentration increases, which indicates that there are no influences on the surface morphology of the powders.   Figure 3c is the energy level diagram of Sm 3+ ion. According to the previous study, there are two major absorption bands appearing in the absorption spectrum for pure YInGe 2 O 7 . The first absorption band between 200 and 280 nm can be assigned to the charge transfer between the In 3+ and O 2− ions of the InO 6 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 GeO 4 anion [19]. For YInGe 2 O 7 :3 mol % Sm 3+ , the compounds exhibited an absorption peak between 230 to 290 nm due to the charge transfer state (CTS) of the Sm 3+ -O 2− 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.  Figure 3 show the UV-visible absorption spectrum (Figure 3a), and excitation spectrum (λem = 598 nm) ( Figure 3b) for YInGe2O7 doped with 3 mol % Sm 3+ ion calcined at 1300 °C for 10 h in air. Figure 3c is the energy level diagram of Sm 3+ 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 In 3+ and O 2− 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 % Sm 3+ , the compounds exhibited an absorption peak between 230 to 290 nm due to the charge transfer state (CTS) of the Sm 3+ -O 2− 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.
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  Figure 4 shows the photoluminescence emission spectra for YInGe 2 O 7 doped with different concentrations of Sm 3+ 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 Sm 3+ intra-4f transition from the excited levels to lower levels, the 4 G 5/2 → 6 H 5/2 and 4 G 5/2 → 6 H 7/2 transitions, respectively [18,19]. Another weak emission peak located at 645 nm is due to the 4 G 5/2 → 6 H 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 ( 4 G 5/2 → 6 H 9/2 ) [19], and their intensity is hyper-sensitive to the variation of the local structure environment of the Sm 3+ ions. While the 4 G 5/2 → 6 H 7/2 emission is a magnetic-dipole allowed transition, its intensity hardly changes with the local structure symmetry of the Sm 3+ ions [25]. The higher intensity for the 4 G 5/2 → 6 H 7/2 transition appears in this study that means the Sm 3+ 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 YinGe 2 O 7 :Sm 3+ phosphor under an excitation wavelength of 404 nm in this study. 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:Sm 3+ phosphor under an excitation wavelength of 404 nm in this study. According to the studies [8], the In 3+ and Y 3+ 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 Sm 3+ ion substitutes the Y 3+ ion lattice position, and it is guessed that the Sm 3+ ion occupies such a non-symmetric center to cause the intensity of 4 G5/2 → 6 H9/2 transition higher than that of 4 G5/2 → 6 H7/2 transition. However, it is not true in this study, and it might be that the radius of Sm 3+ ion is larger than the Y 3+ ion, and leads the Sm 3+ ion lattice position to become more symmetrical. Figure 5 shows the relationships between e emission intensity of the 4 G5/2 → 6 H7/2 transition of Sm 3+ ion in YInGe2O7 plotted versus the Sm 3+ ion concentrations under an excitation of 404 nm. The emission intensity of 4 G5/2 → 6 H7/2 transition increases with increasing Sm 3+ ion concentration in the lower Sm 3+ ion contents region until the saturated PL intensity is reached and then decreases, which indicated that the concentration quenching is active when the Sm 3+ 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 Sm 3+ ion-doped phosphor. This is due to the distance between the Sm 3+ ions becoming shorter, changing the cross-relaxation mechanism to active [17]. The following cross-relaxation may occur 4 G5/2 (Sm I) + 6 H5/2 (Sm II) → 6 F9/2 (Sm I) + 6 F9/2 (Sm II) (1) According to the studies [8], the In 3+ and Y 3+ cations occupy the same octahedral site forming a hexagonal arrangement on the a-b planes of the YInGe 2 O 7 structure. In their turn, the hexagonal arrangements of InO 6 /YO 6 octahedral layers are held together by sheets of isolated diorthogroups composed of a double tetrahedron sharing a common vertex, and every two GeO 5 hexahedrons form the Ge 2 O 9 structure via oxygen ion sharing which causes the YO 6 octahedral layer to become disordered and introduce the non-symmetric center of lattice due to internal stress. Therefore, when a Sm 3+ ion substitutes the Y 3+ ion lattice position, and it is guessed that the Sm 3+ ion occupies such a non-symmetric center to cause the intensity of 4 G 5/2 → 6 H 9/2 transition higher than that of 4 G 5/2 → 6 H 7/2 transition. However, it is not true in this study, and it might be that the radius of Sm 3+ ion is larger than the Y 3+ ion, and leads the Sm 3+ ion lattice position to become more symmetrical. Figure 5 shows the relationships between e emission intensity of the 4 G 5/2 → 6 H 7/2 transition of Sm 3+ ion in YInGe 2 O 7 plotted versus the Sm 3+ ion concentrations under an excitation of 404 nm. The emission intensity of 4 G 5/2 → 6 H 7/2 transition increases with increasing Sm 3+ ion concentration in the lower Sm 3+ ion contents region until the saturated PL intensity is reached and then decreases, which indicated that the concentration quenching is active when the Sm 3+ 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 Sm 3+ ion-doped phosphor. This is due to the distance between the Sm 3+ ions becoming shorter, changing the cross-relaxation mechanism to active [17]. The following cross-relaxation may occur 7 of 9 Materials 2017, 10, 779 7 of 9 Figure 5. The relationships between the intensity of the emission peak ( 4 G5/2 → 6 H7/2) and Sm 3+ ion concentrations. Figure 6 shows the decay curve and decay time of YInGe2O7 doped with different Sm 3+ 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 Sm 3+ concentration increases from 1 to 20 mol %. At low Sm 3+ ion concentrations (1 and 3 mol %), the decay rate is almost the same, but decreases rapidly when the Sm 3+ ion concentrations are higher than 3 mol %. This is caused by the effect of the energy exchange between the Sm 3+ ions as the distance between the Sm 3+ ions decreased with increasing Sm 3+ 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 4 G5/2 → 6 H7/2 transition is a single decay component between Sm 3+ ions only. In YInGe2O7:Sm 3+ system, different concentrations of Sm 3+ ion did not change the shape of curves but did change the intensities of the emission spectra. Figure 7 shows the CIE color Figure 5. The relationships between the intensity of the emission peak ( 4 G 5/2 → 6 H 7/2 ) and Sm 3+ ion concentrations. Figure 6 shows the decay curve and decay time of YInGe 2 O 7 doped with different Sm 3+ 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 Sm 3+ concentration increases from 1 to 20 mol %. At low Sm 3+ ion concentrations (1 and 3 mol %), the decay rate is almost the same, but decreases rapidly when the Sm 3+ ion concentrations are higher than 3 mol %. This is caused by the effect of the energy exchange between the Sm 3+ ions as the distance between the Sm 3+ ions decreased with increasing Sm 3+ 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 4 G 5/2 → 6 H 7/2 transition is a single decay component between Sm 3+ ions only.
Materials 2017, 10, 779 7 of 9 Figure 5. The relationships between the intensity of the emission peak ( 4 G5/2 → 6 H7/2) and Sm 3+ ion concentrations. Figure 6 shows the decay curve and decay time of YInGe2O7 doped with different Sm 3+ 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 Sm 3+ concentration increases from 1 to 20 mol %. At low Sm 3+ ion concentrations (1 and 3 mol %), the decay rate is almost the same, but decreases rapidly when the Sm 3+ ion concentrations are higher than 3 mol %. This is caused by the effect of the energy exchange between the Sm 3+ ions as the distance between the Sm 3+ ions decreased with increasing Sm 3+ 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 4 G5/2 → 6 H7/2 transition is a single decay component between Sm 3+ ions only. In YInGe2O7:Sm 3+ system, different concentrations of Sm 3+ ion did not change the shape of curves but did change the intensities of the emission spectra. Figure 7 shows the CIE color In YInGe 2 O 7 :Sm 3+ system, different concentrations of Sm 3+ 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 YInGe 2 O 7 doped with 3 mol % Sm 3+ ions. The color coordinates of the emission were x = 0.457 and y = 0.407, which is located in the orange-yellow region.
Materials 2017, 10, 779 8 of 9 coordinates of YInGe2O7 doped with 3 mol % Sm 3+ ions. The color coordinates of the emission were x = 0.457 and y = 0.407, which is located in the orange-yellow region.

Conclusions
A new phosphor, Sm 3+ 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 Sm 3+ 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 Sm 3+ intra-4f transition for the 4 G5/2 → 6 H5/2 and 4 G5/2 → 6 H7/2 transitions, respectively. Another weak emission peak located at 645 nm is due to the 4 G5/2 → 6 H9/2 transition. The concentration quenching effect occurs when Sm 3+ 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 Sm 3+ concentration increases from 1 to 20 mol %, and the decay mechanism of the 4 G5/2 → 6 H7/2 transition is a single decay component between Sm 3+ ions only.