Gd₂O₂S:Yb,Er submicrospheres with multicolor upconversion fluorescence

Abstract

Monodisperse hexagonal-phase Gd₂O₂S:Yb³⁺,Er³⁺ submicrospheres have been successfully prepared for the first time by means of a solvothermal method. The SEM and TEM results show that the spherical size is between 210–300 nm and the submicrospheres are of polycrystalline nature. Under the excitation of 980 nm, the bright green emission of the Gd₂O₂S:Er³⁺ at about 524 and 548 nm are assigned to the ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 transitions of the Er³⁺ ions, respectively. The incorporation of Yb³⁺ ions into the Gd₂O₂S:Er³⁺ leads to the decrease in the intensity ratio of the green luminescence to the red luminescence. The corresponding emission color can be tuned from green to yellow by varying the Yb³⁺ concentration. The analysis reveals that the two-photon absorption is mainly responsible for the upconversion luminescence in Gd₂O₂S:Yb³⁺,Er³⁺ submicrospheres.

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1. Introduction

Since its discovery in the 1960s, upconversion (UC) materials containing rare earth ions have been the focus of much research.¹ Along with the progress of science and nanotechnology, UC nanocrystals have received much attention owing to their wide potential applications in such fields as infrared detection, molecule recognition, three-dimensional displays, and especially luminescent probes in biological labeling and imaging technology.² Up to now, various upconversion materials have been developed and studied, most of the work is focused on the investigation of halides, glass, and oxides.³ It is well-known that host materials with low phonon energies (ħω) show high upconversion luminescence efficiency by suppressing the nonradiative multiphonon relaxation. Usually, ħω_fluoride< ħω_sulfide < ħω_oxide, so most efficient upconversion luminescence has been realized in fluoride hosts. Many micro-/nano scale fluorides upconversion materials have been reported, such as, NaYF₄:Yb³⁺,Er³⁺,⁴ BaYF₅:Yb³⁺,Er³⁺(Tm³⁺),⁵ and LaF₃:Yb³⁺,Er³⁺(Tm³⁺,Ho³⁺).⁶ However, these materials usually present poor mechanical properties, moisture sensitivity, and non-eco-friendly. Most of the oxides are chemical stabile and nontoxic, are also widely reported as candidates of upconversion materials,⁷ but they show low upconversion luminescence efficiency. Rare-earth oxysulfides possess favorable chemical durability, and they are eco-friendly and nontoxic. Yocom et al.⁸ demonstrated that Y₂O₂S:Yb³⁺,Er³⁺ exhibited 82% lighter output to that of fluoride. Liu et al.⁹ reported the anomalous thermalization effect in Er³⁺ doped Y₂O₂S. So rare-earth oxysulfides may be candidates for upconversion materials.

The trivalent rare-earth (RE) ions, such as Er³⁺, Tm³⁺, Ho³⁺, Nd³⁺, Yb³⁺, and Pr³⁺, are suitable candidates for UC processes due to their abundant energy levels and narrow emission spectral lines. Especially the introduction of Yb³⁺ ions as a sensitizing center with a pumping wavelength of around 1 μm, for which high-power sources are commercially available, has allowed a large enhancement of the UC luminescence of the activators Tm³⁺, Er³⁺, and Ho³⁺.¹⁰

Gd₂O₂S nanocrystals doped with RE³⁺ ions have particularly attracted considerable interest in terms of high chemical durability and thermal stability. A number of different techniques have been developed to synthesize nanocrystals, such as the thermal decomposition method and conversion of the Gd(OH)₃ method.¹¹ So far, the investigations for RE³⁺-doped Gd₂O₂S nanocrystals have mostly concentrated on downconversion luminescence such as Eu³⁺- and Tb³⁺-doped Gd₂O₂S nanocrystals. But little is reported on the upconversion luminescence of Gd₂O₂S nanocrystals. On the other hand, it is well-known that the oxysulfides have low phonon energies, low symmetry, favorable chemical stability and nontoxicity.¹² Thus, Gd₂O₂S is an ideal host for the design of upconverting phosphors. In 2004, Hirai and Orikoshi¹³ reported upconverting phosphor Gd₂O₂S:Yb,Er particles synthesized by an emulsion liquid membrane method. Under 980 nm laser excitation, Gd₂O₂S:Yb,Er particles exhibited green emission. In 2007, Hu et al.¹⁴ reported an upconversion afterglow phenomenon of Er³⁺ in Gd_1.93O₂S:0.01Er,0.03Yb,0.02Ti,0.01Mg phosphor. In addition, with the codoping of Ti⁴⁺ and Mg²⁺ ions, the unusual phosphorescence was improved. Although the upconversion emission for Gd₂O₂S:Yb,Er has been reported, the effect of Yb³⁺ concentration on the upconversion properties has not been discussed in detail. In this paper, we report the preparation and UCL properties of Gd₂O₂S:Yb³⁺/Er³⁺ monodisperse submicrospheres, especially the influence of Yb³⁺ codoping on UCL process of Er³⁺ ions.

2. Experimental section

2.1 Materials

The initial chemicals, including Gd₂O₃, Yb₂O₃, and Er₂O₃ (all with purity = 99.99%, Shanghai Yuelong Non-Ferrous Metals Limited., China), HNO₃, ethylene glycol, poly(vinyl pyrrodlidone) (PVP K30, M = 40 000), thiourea, and ethanol (all with purity of A.R., Beijing Fine Chemical Company, China), were used without further purification. Rare-earth nitrate stock solutions were prepared by dissolving the corresponding metal oxide in nitric acid at elevated temperatures.

2.2 Preparation

In a typical procedure, appropriate amounts of Gd(NO₃)₃ (1 M), Yb(NO₃)₃ (0.5 M), and Er(NO₃)₃ (0.05 M) were added into 25 mL of ethylene glycol. 2.0 g PVP was added into the above solution. After vigorous stirring for 10 min, the PVP was dissolved thoroughly. Then 10 mL of ethanol solution containing 0.11 g thiourea was added dropwise into the above solution. The as-obtained solution was stirred for another 30 min. The resulting transparent feedstock was transferred to a 50 mL Teflon-lined stainless autoclave and heated at 200 °C for 24 h. After naturally cooling to room-temperature, the precursors were separated by filtration, washed with ethanol and deionized water several times, and dried in atmosphere at 60 °C over night. The final products were obtained through a heat treatment at 700 °C for 2 h under N₂/S atmosphere.

2.3 Characterization

Powder X-ray diffraction (XRD) measurements were performed on a Bruker D8 focus X-ray powder diffractometer with Cu-Kα radiation (λ = 0.15405 nm). The size and morphology of the samples were inspected using field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi, Japan). The transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were obtained by a JEOL-2010 transmission electron microscope at an accelerating voltage of 200 kV. The UC emission spectra were obtained by using a 980 nm laser diode Module (K98D08M-30W, China) as the excitation source and detected by an R955 instrument (hamamatsu) from 400 to 900 nm. All the measurements were performed at room temperature.

3. Results and discussion

3.1 Structure and morphology

Fig. 1 shows the XRD patterns of the precursor and Gd₂O₂S:Yb,Er submicrospheres. There are two broad peaks near 2θ = 28.55° and 47.26° for the precursor, indicating that the precursor is composed of amorphous gadolinium compound. After being annealed at 700 °C for 2 h in N₂/S atmosphere, all diffraction peaks can be readily indexed to the pure hexagonal phase of Gd₂O₂S (space group: P3m1(164)) according to the JCPDS file no. 26-1422. No additional peaks of other phases have been found, indicating that the Yb³⁺ and Er³⁺ ions are effectively built into the host lattice.

Fig. 1 XRD patterns of the precursor and Gd₂O₂S:Yb,Er submicrospheres.

SEM and TEM are used to characterize the morphology and crystal structure of the products. Fig. 2(a) and (b) show typical SEM images of the precursor and the final product. The precursors consist of homogeneous and monodisperse spheres with diameters 240–330 nm. After being annealed at 700 °C, the obtained Gd₂O₂S:1%Er³⁺ inherits the spherical shape from the precursor. But their average diameter is reduced to 210–300 nm because of the decomposition of the precursor. A typical TEM image and selected area electron diffraction (SAED) pattern for the Gd₂O₂S:1%Er³⁺ (Fig. 2(c) and 2(d)) clearly confirm that the diameter of the Gd₂O₂S:1%Er³⁺ submicrospheres is about 210–300 nm and the spheres actually further consist of small grains, consistent with the value shown in the SEM images. The SAED image contains partial ring and dot patterns, indicating that the Gd₂O₂S:1%Er³⁺ submicrospheres are of polycrystalline nature.

Fig. 2 SEM micrographs of well-dispersed precursor (a) and Gd₂O₂S:1%Er submicrospheres (b). TEM micrograph (c) and SAED pattern (d) of Gd₂O₂S:1%Er submicrospheres.

3.2 Upconversion luminescence properties

Fig. 3 shows the room-temperature upconversion fluorescence spectra of the Gd₂O₂S:Yb³⁺,Er³⁺ submicrospheres with different contents of Yb³⁺ ions. Under 980 nm NIR excitation, the Gd₂O₂S:1%Er³⁺ exhibits bright green emission. Two primary bands at about 524 and 548 nm are assigned to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of the Er³⁺ ions, respectively. A weak band at about 671 nm is ascribed to the ⁴F_9/2 → ⁴I_15/2 transition of the Er³⁺ ions.^15,16 However, the green emission of the Gd₂O₂S:1%Er³⁺ submicrospheres changes greatly when Yb³⁺ ions were doped into Gd₂O₂S:1%Er³⁺. As can be seen from Fig. 3, the Gd₂O₂S:20%Yb³⁺,1%Er³⁺ submicrospheres show mainly red emission at about 671 nm and weak green emissions at about 524 and 548 nm of the Er³⁺ ions, which can be assigned to the transitions of ⁴F_9/2→⁴I_15/2, ²H_11/2→⁴I_15/2, and ⁴S_3/2→⁴I_15/2, respectively. The 410 nm emission (²H_9/2→⁴I_15/2) is also measured, but the intensity is much weaker than that of green and red luminescence, as shown in the inset of Fig. 3. The concentration dependence upconversion spectra of Gd₂O₂S:xYb,1%Er are investigated and shown in Fig. 3. With the increase of Yb³⁺ concentration from 0 to 20 mol%, the green emission intensity decreases remarkably while the red emission intensity increases. Thus, it is possible to tune the emission color of the Gd₂O₂S:Yb³⁺,Er³⁺ submicrospheres by varying the Yb³⁺ concentration. The Commission Internationale de L'Eclairage (CIE) chromaticity coordinates for Gd₂O₂S:xYb³⁺,1%Er³⁺ are represented in Fig. 4. With increasing Yb³⁺ content, the emission color changes gradually from green to yellow.

Fig. 3 Room-temperature upconversion fluorescence spectra of the Gd₂O₂S:Yb,Er with different content of Yb³⁺ ions.

Fig. 4 CIE chromaticity diagram for Gd₂O₂S:xYb³⁺,1%Er³⁺ submicrospheres excited at 980 nm. A: (0.308, 0.666); B: (0.389, 0.596); C: (0.417, 0.569); D: (0.483, 0.507); and E: (0.496, 0.490).

It is well known that the UPL intensity (I_UPL) is proportional to some power n of the incident excitation power (I_P) according to I_UPL∝Iⁿ_p, where superscript n represents the number of pump photons required to populate the emitting states. The number of pumping photons (n) can be determined from the slope of the photoluminescence intensity versus the laser excitation power in a log–log plot.¹⁷ These plots are shown in Fig. 5 for the Gd₂O₂S:10%Yb³⁺,1%Er³⁺ submicrospheres. The n values of Gd₂O₂S:10%Yb³⁺,1%Er³⁺ are calculated to be 2.25 and 2.26 for the green and red emissions, respectively. The results show that two-photon process is mainly responsible for green and red upconversion for Gd₂O₂S:10%Yb³⁺,1%Er³⁺ sample. The similar phenomenon was also observed in other Yb³⁺/Er³⁺ codoped Gd-based oxide particles, where the upconverted emissions were also two-photon process.¹⁸ Therefore, the upconversion mechanism of Yb³⁺–Er³⁺ systems in Gd₂O₂S spheres can be built according to the experimental results and the references,^18,19 as shown in Fig. 6.

Fig. 5 Power dependence of UC intensity of Gd₂O₂S:10%Yb³⁺,1%Er³⁺ submicrospheres.

Fig. 6 Energy level diagrams of Yb³⁺ and Er³⁺, and possible UC processes.

In principle, three basic population mechanisms may be involved in the upconversion process, namely excited state absorption (ESA), energy transfer upconversion (ET), and photon avalanche. Since no power threshold has been observed, the photon avalanche mechanism may be neglected in our case. Therefore, upconversion luminescence of the Gd₂O₂S:Er and Gd₂O₂S:Yb,Er involve ESA and ET processes and their possible schematic diagram is shown in Fig. 6.

For Gd₂O₂S:1%Er³⁺ submicrospheres, the excitation wavelength from 980 nm LD matches the absorption transition between the ground state ⁴I_15/2 and the excited level ⁴I_11/2 (GSA). After the first-level excitation, the same wavelength laser pumps the excited ions from the ⁴I_11/2 to the ⁴F_7/2 level (ESA). Finally, radiant transitions from these levels yield the emissions at 524 and 548 nm (²H_11/2, ⁴S_3/2→⁴I_15/2) (most strong) and at 671 nm (⁴F_9/2→⁴I_15/2), respectively (Fig. 3). For Gd₂O₂S:xYb³⁺,1%Er³⁺ samples, the Er³⁺ ion can be excited from the ground-state ⁴I_15/2 to the some excited-states by general multiphoton processes through the ground-state absorption and the energy transfer from the excited Yb³⁺ ions (⁴I_15/2 (Er) + ²F_5/2 (Yb) → ⁴I_11/2 (Er) + ²F_7/2 (Yb)). The latter become dominant when the Yb³⁺ and Er³⁺ ions are codoped into the host lattices because the Yb³⁺ ions have a much larger absorption cross section as compared to that of the Er³⁺ ions around 980 nm. Therefore, the Er³⁺ ions were first mainly excited to the ⁴I_11/2 level via the energy transfer from the excited Yb³⁺ ions. Then, the electrons in the ⁴I_11/2 level were excited to the ⁴F_7/2 level by the energy transfer from the Yb³⁺ ions to the excited Er³⁺ ions or the ESA process. After that, the excited ions undergo multi-phonon relaxation to luminescent levels ²H_11/2 and ⁴S_3/2, which lead to the green ²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2 emissions. Alternatively, the electrons of some Er³⁺ ions in the excited state will nonradiatively decay to the ⁴I_13/2 state because of the short lifetime of ⁴I_11/2 state, and then jump to the ⁴F_9/2 level via the energy transfer from the excited Yb³⁺ ions. The Er³⁺ ions transit to the ground state and emit red light. If the excitation photon density is high enough, the electron at the ⁴S_3/2 level of the Er³⁺ ion can absorb another photon and transit to the ²K_15/2 level. After nonradiative decay to ⁴G_11/2 state, electrons can populate the ⁴F_9/2 state by energy transfer from the excited Er³⁺ ions to the ground-state Yb³⁺ ions through the following process:

⁴G_11/2 (Er) + ²F_7/2 (Yb) → ⁴F_9/2 (Er) + ²F_5/2 (Yb)

In addition, the electrons also can nonradiatively relax to the ²H_9/2 state, resulting in 410 nm luminescence, which should be responsible for the three-photon process. The fact that the red emission is enhanced with increasing Yb³⁺ concentration reveals that the level ⁴F_9/2 was largely populated. One of the most likely reasons is that introduction of an elevated amount of Yb³⁺ dopants into the Gd₂O₂S host lattice would decrease the interatomic distance between the Yb³⁺ ions and Er³⁺ ions and thus facilitates the back-energy-transfer process from the Er³⁺ to Yb³⁺ ions: ⁴G_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴F_9/2 (Er³⁺) + ²F_5/2 (Yb³⁺). The back-energy-transfer should subsequently suppress the population in excited levels of the ⁴S_3/2(²H_11/2), resulting in the decrease of the green-light emission (²H_11/2/⁴S_3/2 → ⁴I_15/2). Meanwhile, the back-energy-transfer directly populates the ⁴F_9/2 (Er³⁺) level, producing the enhancement of red (⁴F_9/2→⁴I_15/2) emission. Considering the back-energy-transfer and the nonradiative relaxation processes, the required photon number of upconversion red and green emissions should be more than two, which agrees well with our experimental results.

Conclusion

In summary, a general approach has been developed for the synthesis of the well-disperse hexagonal Gd₂O₂S:Yb³⁺,Er³⁺ submicrospheres. Under the excitation of 980 nm, the red emission (⁴F_9/2–⁴I_15/2) of the Gd₂O₂S:Yb³⁺/Er³⁺ increases with increasing the Yb³⁺ concentration, while that of the green emission (⁴S_3/2/²H_11/2–⁴I_15/2) decreases. The red emission enhancement is attributed to the enhanced population of the ⁴F_9/2 level via the energy transfer (⁴G_11/2 (Er³⁺) + ²F_7/2 (Yb³⁺) → ⁴F_9/2 (Er³⁺) + ²F_5/2 (Yb³⁺)), while green emission diminishment is attributed to ⁴S_3/2 + ²F_5/2 → ²K_15/2 + ²F_7/2, which depopulates the excited ⁴S_3/2 level at higher Yb concentrations. So the emission color can be tuned from green to yellow only by changing the Yb³⁺ concentration. We have also confirmed that the upconversion process in Gd₂O₂S:Yb³⁺,Er³⁺ submicrospheres results from two-photon processes. These materials may have potential applications as bio-probes and displays.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of China (Grant 20771098) and the NSFC Fund for Creative Research Group (Grant No. 20921002), and the National Basic Research Program of China (973 Program, Grant 2007CB935502).

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