Photoluminescence Imaging of SiO₂@ Y₂O₃:Eu(III) and SiO₂@ Y₂O₃:Tb(III) Core-Shell Nanostructures

Abstract

We uniformly coated Eu(III)- and Tb(III)-doped yttrium oxide onto the surface of $SiO_2$ spheres and then characterized them by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction crystallography and UV-Visible absorption. 2D and 3D photoluminescence image map profiles were reported for the core-shell type structure. Red emission peaks of Eu(III) were observed between 580 to 730 nm and assigned to $^5D_0{\rightarrow}^7F_J$ (J = 0 - 4) transitions. The green emission peaks of Tb(III) between 450 and 650 nm were attributed to the $^5D_4{\rightarrow}^7F_J$ (J = 6, 5, 4, 3) transitions. For annealed samples, Eu(III) ions were embedded at a $C_2$ symmetry site in $Y_2O_3$, which was accompanied by an increase in luminescence intensity and redness, while Tb(III) was changed to Tb(IV), which resulted in no green emission.

Introduction

Developing color emitting phosphors is becoming increasingly important to achieve ideal colors in modern smart displays.1,2 Luminescence phosphors have also been applied to biomedical purposes.3-10 Silica spheres have been employed as phosphor support (or capping) or cell imaging materials due to their good thermal stability and biocompatibility. Zhang et al. prepared Eu@SiO2 core-shell nanoparticles for application to living HeLa cell imaging using time-resolved luminescence.3 Additionally, a Tb(III)-picolinate complex encapsulated by SiO2 was used for fluorescence biolabeling and shown to have good photostability and biocompatibility with Candida albicans cells.4 Moreover, a drug delivery vehicle was demonstrated using SiO2 hollow spheres coated with YBO3:Eu3+.7 Specifically, the fluorescence intensity of the drug carrier was used to examine the location and amount of drug released. Luminescence properties (e.g., lifetime and intensity) have been modified by employing core-shell type structures with silica.11-31 Makhinson et al. showed that the luminescence intensity of macrocyclic europium(III) chelates was enhanced dramatically when the chelates were encapsulated with silica matrix.13 For benzoate-sensitized sub-10 nm Y2O3:Eu3+ nanoparticles, Liu et al. found that the luminescence intensity was enhanced by six times when the particles were capped with SiO2 nanowires.14 Liu et al. prepared Y2O3:Eu3+@SiO2 core-shell structures and showed that their luminescence properties could be tuned by changing the thickness of the SiO2 shell.16 Yoo et al. synthesized SiO2@Y2O3:Tb3+ core-shell particles with various shell thicknesses using a heterogeneous precipitation method.32 They used sphere shape SiO2 because of many advantages including denser/uniform layer packing, uniform luminescence, and less light scattering for the displays with small pixels.21,32 Including the advantages for the displays, size-tunable spheres could also be useful for a desired size-selective biological target.

In the present study, we selected SiO2 because it shows good thermal stability and easy tailoring of its sphere size.33 Eu(III) and Tb(III) ions were chosen as activator ions because they are model activators with red and green colors associated with 5D0 → 7FJ (J = 0, 1, 2, 3, 4) and 5D4 → 7FJ (J = 6, 5, 4, 3) transitions, respectively. Yttrium oxide was used because it is known to be a good model host material for color emitting activator ions.27 For Eu(III)- and Tb(III)- doped yttrium oxide, because it is very difficult to synthesize monodisperse spherical shape particles with various sizes, as an alternative method we coated the surface of SiO2 sphere. Additionally, in the present study we introduced new 2D and 3D-photoluminesece imaging technique to fully understand photoluminescence mechanism of the core-shell structures before and after thermal treatment.

 

Experimental

SiO2 spheres were prepared by the Stöber method, which is briefly described as follows: an appropriate amount of tetraethyl orthosilicate (TEOS, 98%, Samchun Chemical Co., Korea) was added to an ethanol/water mixture solution, after which ~28% ammonia was added. Stirring the solution for 12 h yielded a milky solution, which was centrifuged, thoroughly washed with water and ethanol, and then fully dried in an oven (80 ℃). To coat the SiO2 surface, we first mixed 10 mL of 0.1 M Y(III)NO3·6H2O and 0.5 mL of 0.1 M Eu(III)NO3·6H2O (or 0.1 M Tb(III)NO3·6H2O) solution. Next, 0.25 mL of 0.1 M sodium citrate solution and 0.5 g of polyethylene glycol were added and the solution was vigorously stirred for 30 min. We then added 1 mmol SiO2 spheres and sonicated the silica-dispersed solution for 1 hour, after which the solution was stirred at 50 ℃. The final products were washed with water and ethanol and dried in an oven (80 ℃) before further characterization. Next, the morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi S-4800). Additionally, the thickness of the coated shell was examined by transmission electron microscopy (TEM, Hitachi H-7600) at 100 kV after preparing TEM specimens by dropping sample-dispersed ethanol solutions onto carbon-coated Cu grids and drying in air. X-ray diffraction (XRD) patterns were obtained using a PANalytical X’Pert Pro MPD diffractometer with Cu Kα radiation (40 kV and 30 mA) at a take-off angle of 6°. The UV-Visible absorption spectra of the powder samples were obtained using a Varian Cary 5000 UV-visible spectrophotometer. Finally, photoluminescence (PL) spectra were collected using a SCINCO FluoroMate FS-2.

 

Results and Discussion

Figure 1 shows the SEM images of as-prepared SiO2@Eu(III)-YOx and SiO2@Tb(III)-YOx, as well as the TEM image of as-prepared SiO2@Eu(III)-YOx. The amount of Eu (or Tb) ions was 5 mol % relative to yttrium. Since we chose SiO2 spheres as a support core, the spherical shape was preserved after surface coating, but the spheres were somewhat aggregated. As shown in the TEM image, the shell was fairly uniformly coated onto the sphere. The shell thickness was estimated to be 5-10 nm for the SiO2@Eu(III)-YOx sample, which was extremely thin when compared with the radius (330-360 nm) of the sphere.

Figure 2 displays the XRD patterns of as-prepared and 550 ℃-annealed SiO2@Eu(III)-YOx and SiO2@Tb(III)-YOx samples. For the as-prepared two samples, a very broad peak was found at ~22°, which showed no critical difference when compared with the XRD pattern of bare SiO2. No XRD patterns of the coated shell were observed. Upon annealing of the SiO2@Eu(III)-YOx at 550 ℃, sharper peaks were observed at 2Θ = 20.5°, 29.0°, 33.7°, 39.8°, 43.4°, 48.4°, and 57.5°. These patterns were in good agreement with those of the cubic (la-3) Y2O3 structure (JCPDS 1-083- 0927) and were assigned to the (112), (222), (004), (233), (134), (044) and (226) planes, respectively shown in Figure 2. Sharp XRD peaks were also observed for the SiO2@Tb(III)-YOx samples, but these were some what 1-803-0927 weaker than those observed for SiO2@Eu(III)-YOx due to lower crystallinity. The three major peaks were also assigned to the (222), (044) and (226) planes of the cubic Y2O3 structure.

Figure 1.SEM images of as-prepared SiO2@Eu(III)-YOx and SiO2@Tb(III)-YOx, and TEM image of as-prepared SiO2@Eu(III)-YOx.

Figure 2.Powder X-ray diffraction patterns of as-prepared and 550 ℃-annealed SiO2@Eu(III)-YOx and SiO2@Tb(III)-YOx. Standard XRD pattern of cubic Y2O3 structure (JCPDS 1-083-0927) is also displayed for comparison.

Figure 3 shows the UV-visible reflectance absorption spectra of as-prepared and 550 ℃-annealed SiO2@Eu(III)- YOx and SiO2@Tb(III)-YOx samples. The absorbance (y-axis) was converted from the diffuse reflectance data by the Kubelka-Munk method. No significant absorption of Eu(III) or Tb(III) ions was observed for the as-prepared samples due to the extremely low concentration of doped activator ions. For the as-prepared Eu(III) sample, the weak peak at 395 nm was attributed to the 5L6 ← 7F0 transition.34 Interestingly, for the annealed Tb sample, the UV-vis absorption in the visible region was significantly higher. We also found that the white as-prepared sample changed to pale brown after thermal annealing, which was attributed to a change in oxidation state from Tb(III) to Tb(IV).35-37 This appears to be related to the poor crystallinity of the annealed Tb sample relative to the annealed Eu sample, as discussed in the XRD results. For the SiO2@Tb(III)-YOx sample, the broad and strong absorption at around 400 nm has been attributed to the charge transfer (O → Tb4+) absorption of Tb(IV).35-37 Vermal et al. also found a change in oxidization state from Tb(III) to Tb(IV) in MO-Al2O3 (M=Mg, Ca, Sr, Ba) matrix after thermal treatment.35 More evidently, Zych et al. found that Tb(III) changed to Tb(IV) in La2O3, which has the same cubic crystal structure as Y2O3.36 It was reported that Tb(III) tends to oxidize to Tb(IV) in cubic crystal structure.35

Figure 3.UV-Visible diffuse reflectance absorption spectra of SiO2@Eu(III)-YOx and SiO2@Tb(III)-YOx samples before and after 550 ℃-thermal annealing.

Figure 4.Excitation and emission spectra (top left), normalized emission spectra (bottom left) at various excitation wavelengths, 2D and 3D photoluminescence imaging profiles for as-prepared SiO2@Eu(III)-YOx. Inset (bottom right) shows the CIE xyz color coordinate of the emission at an excitation of 300 nm.

Figure 4 shows the excitation/emission spectra and normalized emission spectra at various excitation wavelengths and 2D and 3D photoluminescence imaging profiles for the as-prepared SiO2@Eu(III)-YOx core-shells. Emission peaks were observed between 570 and 720 nm for the emission spectrum generated at an excitation wavelength of 300 nm and assigned to 5D0 →7FJ (J = 0-4) transitions of Eu(III) ion, 5D0 → 7F0 (579.5 nm), 5D0 → 7F1 (592.5 nm), 5D0 → 7F2 (616.2 nm), 5D0 →7F3 (650.0 and 657.2 nm), and 5D0 →7F4 (687.2 and 697.0 nm). The 5D0 →7F1 transition is magnetic dipole allowed and insensitive to the chemical environment of Eu(III). The 5D0 →7F2 transition is electric dipole allowed, but the transition is forbidden when Eu(III) has an inversion center symmetry.20,34 The I(5D0 → 7F2)/I(5D0→ 7F1) asymmetric ratio was not critically changed, although the excitation wavelength was. The luminescence profiles were very similar to those for SiO2@Eu(OH)3 core-shell microspheres reported by Ansari et al.6 These findings indicate that the site symmetry of Eu(III) for SiO2@Eu(III)-YOx is similar to that in SiO2@Eu(OH)3. Liu et al. also prepared SiO2@(Y0.95–xGdxEu0.05)2O3 core-shells to demonstrate magnetic resonance and optical imaging and reported similar emission profiles.10

For the excitation spectrum at λem = 617 nm (5D0 ←7F2 emission), various sharp peaks were observed between 250 and 550 nm. The three weak peaks between 410 and 550 nm were attributed to direct 5D3, 5D2 and 5D1 ← 7F0 excitation transitions of Eu(III), respectively.20,34 The three stronger peaks at 397, 380 and 363 nm were due to direct 5L6, 5G6 and 5D4 ← 7F0 transitions, respectively. A very broad peak at 300 nm has commonly been attributed to an Eu-O charge transfer band.10,16,21,31 For the corresponding 2D and 3D photoluminescence images, the dense region corresponds to the emission peaks. The International Commission on Illumination (CIE) xyz color coordinate indicates the redness of the as-prepared sample.

Dramatic changes in excitation/emission spectra and peak intensity were observed upon annealing of the SiO2@Eu(III)-YOx sample at 550 ℃ (Figure 5). A broad band at 258 nm was dominant in the excitation spectrum, while other f-f excitation transitions (e.g., 5L6, 5D1 and 5D2 ← 7F0) were dramatically suppressed. The broad band was commonly attributed to the charge transfer band of Eu-O. The emission spectrum at an excitation wavelength of 258 nm was well resolved and enhanced (by 1.5 ×) when compared with that of the as-prepared sample. The increase in peak intensity was mainly due to a decrease in the OH group, which commonly acts as a major luminescence quenching center.12,20,38 The decrease in the OH stretching band of the annealed sample was confirmed by FT-IR (data not shown).

The various emission peaks were assigned to 5D0 → 7FJ (J= 0-4) transitions of the Eu(III) ion, 5D0 →7F0 (581.3 nm), 5D0 →7F1 (588.3, 593.7 and 600.0 nm), 5D0 →7F2 (611.7 and 628.5 nm), 5D0 →7F3 (649.1 and 661.2 nm) and 5D0 → 7F4 (687.8, 693.8 and 708.6 nm). The hypersensitive electric dipole 5D0 →7F2 transition was much stronger than the other transition peaks. The normalized emission profiles showed no critical change with excitation wavelengths. The 2D-photoluminescence map (and the corresponding 3D-photoluminescence image) showed a critical change when compared with that of the as-prepared sample, and the most dense region was positioned at the 5D0 → 7F2 emission region and at excitations of 250-300 nm. Compared with the luminescence profiles of bulk Y2O3:Eu3+ powder sample, we observed no clear quantum confined effect for SiO2@Eu(III)-YOx sample with 5.83 nm shell thickness. The photoluminescence intensity was lower for the core-shell structure than that of bulk Y2O3:Eu3+ powder sample, plausibly due to a volume effect. We also prepared and examined photoluminescence profiles for the core-shell samples with lower (0.1-3 mol % relative to yttrium) concentrations of Eu(III) ions. Although the emission intensity was found to be lower, the photoluminescence map profiles were not significantly changed, indicating that the Eu(III) ion is located at the same symmetry site.

Figure 5.Excitation and emission spectra (top left), normalized emission spectra (bottom left) at various excitation wavelengths, 2D and 3D photoluminescence imaging profiles for 550 ℃annealed SiO2@Eu(III)-YOx. Inset (bottom right) shows the CIE xyz color coordinate of the emission at an excitation at 258 nm.

When Eu3+ is located at a symmetrical site with an inversion center, the electric dipole 5D0 →7F2 transition will be forbidden. Therefore, Eu3+ is likely positioned at the site without an inversion center. For Y2O3 with a cubic structure, Y3+ is located at either the S6 or C2 site. Since S6 has an inversion center, Eu3+ will be doped at the C2 site.21,23 For the annealed sample, the CIE xyz color coordinate shifted to a much deeper red position. Qin et al. observed a similar shift in chromaticity upon thermal annealing for SiO2@LaBO3:Eu3+ core-shell nanoparticles.11 Yoo et al. synthesized SiO2 @Y2O3:Eu3+ core-shells with various shell thicknesses and found that the (5D0→ 7F2)/I(5D0 → 7F1) asymmetric ratio decreased and the color redness deteriorated with increasing shell thickness.21

Figure 6.Excitation and emission spectra (top left), emission spectra (bottom left) at various excitation wavelengths, 2D and 3D photoluminescence imaging profiles for as-prepared SiO2@Tb(III)-YOx. Inset (bottom right) shows the CIE xyz color coordinate of the emission at an excitation at 274 nm.

Figure 6 shows the excitation/emission spectra, normalized emission spectra at various excitation wavelengths, and 2D and 3D photoluminescence imaging profiles for the asprepared SiO2@Tb(III)-YOx. At an excitation wavelength of 274 nm, an emission spectrum was collected from 440 to 720 nm and four distinctive peaks associated with the 5D4 → 7FJ (J = 6, 5, 4, 3) transitions17,34 of Tb(III) ions; 5D4 →7F6 (490 nm), 5D4 →7F5 (545 nm), 5D4 →7F4 (587 nm), and 5D4 → 7F3 (619 nm) were found. As expected, the 5D4 → 7F5 transition peak was the most intense. Ansari et al. prepared mesoporous SiO2@Tb(OH)3 core-shell nanospheres and examined their luminescence properties.17 They observed similar 5D4 →7FJ (J = 6, 5, 4, 3) transitions of Tb(III) ions, as well as a broad green emission at 577 nm from SiO2 nanoparticles.12 In the excitation spectrum, a strong peak was observed at 274 nm, which is commonly attributed to a charge transfer band. The f-f transitions (e.g., 5L10 and 5D3 →7F6) of Tb(III) ions were very weak, and the normalized emission spectra showed no significant change with excitation wavelengths. The resultant CIE xyz color coordinate was positioned in the green region.

Upon thermal annealing of the as-prepared SiO2@Tb(III)-YOx the emission spectrum changed drastically and the luminescence intensity was significantly diminished. A broad peak was observed at 470 nm, and no sharp 5D4 →7FJ transitions of Tb(III) were observed at an excitation wavelength of 379 nm. The broad peak at around 470 nm consists of several peaks, attributed to the phonon-assisted transitions of oxygen defect sites with various charge states (e.g., F0, F+ and F++ centres).39 At an excitation wavelength of 250 nm, weak 5D4 →7F5,4 transitions were observed, while the broad peak at 470 nm was dominant. These findings indicate that the Tb(III) oxidation state changed to non-fluorescent Tb(III) during thermal annealing, which was confirmed by the sample changing from white to pale brown (Figure 8) after thermal annealing. As discussed above, the UV-visible absorption band in Figure 3 was drastically enhanced in the visible region due to a charge transfer absorption of Tb(III).35-37 To further confirm the change in oxidation state, we examined the color of the bulk Tb(III)-YOx sample, and found that the color was also changed to light brown after thermal annealing. The SiO2@Eu(III)-YOx and bulk Eu(III)-YOx samples showed no change in white color after thermal annealing in Figure 8. Under UV (250 nm) irradiation, the green emission color of the SiO2@Tb(III)-YOx sample was almost quenched after thermal annealing while the red emission of SiO2@ Eu(III)-YOx sample was greatly enhanced as displayed in Figure 8. For the bulk Eu(III)-YOx sample, the red emission was much stronger.

Figure 7.Photoluminescence of 550 ℃-annealed SiO2@Tb-YOx at excitation wavelengths of 250 and 379 nm.

Figure 8.Photo images of SiO2@Tb(III)-YOx (left two) and SiO2@Eu-YOx (right: top and middle) samples before and after 550 ℃ annealing under UV (250 nm) irradiation. Photo image of bulk Eu(III)-YOx (right: bottom) sample after 550 ℃ annealing under UV irradiation. Photo images of SiO2@Tb(III)-YOx (left: middle two) and bulk Tb(III)-YOx (left: bottom two) samples before and after 550 ℃ annealing.

 

Conclusion

SiO2 spheres coated with Eu(III)- and Tb(III)-doped yttrium oxide emit red and green colors attributed to 5D0 → 7FJ (J = 0−4) and 5D4 → 7FJ (J = 6−3) transitions, respectively. Upon annealing, the Eu(III) sample showed higher crystallinity of the shell, as well as stronger emission with more redness. The Eu(III) ions appear to be doped at the C2 site without an inversion center of cubic Y2O3. The poor green emission of the annealed Tb sample was attributed to a change in oxidation state from Tb(III) to Tb(IV). The present study confirms that the core-shell structure generated using SiO2 has good luminescence properties, and the 2D and 3D photoluminescence map imaging profiles provide further new insight for designing more efficient phosphor materials for displays and biomedical applications.