The upconversion luminescence properties of the Yb³⁺–Ho³⁺ system in nanocrystalline Y₂O₂S

Abstract

Near-spherical Y₂O₂S:Yb³⁺,Ho³⁺ nanocrystals (NCs) with an average particle size of 40 nm were synthesized by the coprecipitation method followed by a solid–gas sulfuration technique. The effects of the Ho³⁺ ion doping concentration on the upconversion luminescence (UCL) property of the NCs was studied through the UCL spectra. Results show that the UCL intensity of Y₂O₂S:Yb³⁺,Ho³⁺ NCs markedly changes with Ho³⁺ ion concentration, and that the Ho³⁺ ion concentration quench is observed at 0.25 mol%. This value is only half as much as that in micron Y₂O₂S prepared by a solid state reaction, which can be attributed to the distinct diffusion mechanism of activator ions in the coprecipitation process. In addition, strong red emissions can be observed in Y₂O₂S:Yb³⁺,Ho³⁺ NCs throughout all Ho³⁺ doping concentrations used. However, the Ho³⁺ in micron Y₂O₂S usually exhibits weak red UCL. Infrared spectra confirm that this result is related to the large vibrational quanta produced by OH⁻ and CO₃ ²⁻ groups adsorbed onto the surface of NCs. These large vibrational quanta can remarkably increase the probability of ⁵S₂–⁵F₅ and ⁵I₆–⁵I₇ multiphonon relaxation, leading to the enhancement of red emissions arising from ⁵F₅ → ⁵I₈ transitions. The UCL mechanism of the Yb³⁺–Ho³⁺ system in nano- and microsized Y₂O₂S is also discussed.

Introduction

For decades, nanosized materials have attracted much attention because of their unique and improved optical, electrical, and structural properties compared with conventional bulk materials. Since upconversion luminescence (UCL) was first observed in nanosized Y₂O₃:Er³⁺ by Capobianco et al. [1], UCL from near-infrared (NIR) to visible light in nanocrystals (NCs) has elicited increasing interest because of their wide potential applications in various fields, such as infrared detection, molecule recognition, and three-dimensional displays [2–4]. In recent years, the intense demand for biological fluorescence labels with higher luminescent efficiencies and stabilities have resulted in the widespread application of nanocrystalline upconversion phosphors in the field of biomedicine [5–8].

UCL efficiency depends mainly on the host materials. However, biomedical research requires that the host materials not only possess high luminescence efficiency and excellent chemical stability, but also security and innocuity. Y₂O₂S has been reported as one of the most efficient upconversion hosts because of its advantages, which include a low phonon energy, low symmetry, favorable chemical stability, and nontoxicity; thus, it has attracted considerable attention [9–11]. Currently, studies on nanocrystalline Y₂O₂S UCL materials focus mainly on the Yb³⁺, Er³⁺ co-doped system [9, 10], and studies on Yb³⁺, Ho³⁺ co-doped Y₂O₂S NCs are thus far rare. Our previous study demonstrated that the UCL efficiency of the Y₂O₂S:Yb³⁺,Ho³⁺ system is higher than that of Y₂O₂S:Yb³⁺,Er³⁺ [12], suggesting that Y₂O₂S:Yb³⁺,Ho³⁺ NCs have the potential to be applied as biological fluorescence labels with high sensitivity. Unfortunately, although Hirai et al. [10] and Xing et al. [11] successfully synthesized Y₂O₂S:Yb³⁺,Ho³⁺ NCs by liquid membrane emulsion and homogeneous precipitation methods, respectively, neither of them studied the UCL mechanism of the Yb³⁺–Ho³⁺ system in detail.

In this paper, Yb³⁺, Ho³⁺ co-doped Y₂O₂S NCs were prepared by a simple coprecipitation method followed by a solid–gas sulfuration technique, and the special UCL properties and transition mechanism of the Yb³⁺–Ho³⁺ system in Y₂O₂S NCs were studied extensively.

Experimental section

The Y₂O₂S:Yb³⁺,Ho³⁺ NCs were synthesized by the coprecipitation method followed by a solid–gas sulfuration technique. First, Y(NO₃)₃·5H₂O, Ho(NO₃)₃·5H₂O, and Yb(NO₃)₃·5H₂O were weighed to stoichiometric ratios and dissolved in 100 mL of distilled water to form aqueous 0.1 M rare earth-nitrate solutions. The rare earth doping concentrations were 6 mol% for the Yb³⁺ ion and 0.1, 0.25, 0.5, 0.75, and 1.0 mol% for the Ho³⁺ ion. A 0.1 M Na₂CO₃ aqueous solution and PEG4000 were then selected as the precipitating agent and surfactant, respectively. To prepare the Y₂O₃:Yb³⁺,Ho³⁺ precursor, the aqueous nitrate solution was added to 150 mL of the 0.1 M Na₂CO₃ solution containing 0.15 g PEG4000 under vigorous stirring. Stirring was continued for 30 min. The resultant precipitates were separated centrifugally at 3000 r/min, and washed twice with distilled water and absolute alcohol. Finally, the obtained precipitates were dried at 60 °C for 12 h and calcined at 600 °C for 1 h to produce the Y₂O₃:Yb³⁺,Ho³⁺ white powders.

The sulfuration of oxides was performed via a solid–gas reaction. The white powders produced were placed into a quartz tube and calcined at 700 and 800 °C. During the calcination, sulfur powders were heated at 400 °C to produce sulfur vapor, which was brought into the tube by an N₂ gas flow for reaction with the Y₂O₃ powder. After the reaction was maintained for 1 h, calcination was terminated and only the N₂ gas flow was supplied. When the samples had been cooled to room temperature, off-white Y₂O₂S:Yb³⁺,Ho³⁺ NCs doped with different Ho³⁺ contents were obtained.

A Shimadzu X-ray diffractometer-6000 using Cu Kα radiation was employed to analyze the crystal size and phase of the samples. The tube voltage used was 40 kV, the tube electric current was 30 mA, and the step velocity was 4°/min. The morphology of the samples was determined by transmission electron microscopy (TEM, Tecnai G² 20), and the infrared spectrum was obtained using the potassium bromide technique on a Fourier transform infrared spectrometer (Magna 2IRTM 550, USA) in the wavenumber range of 4000–400 cm⁻¹. A Hitachi F-4500 fluorescence spectrophotometer equipped with a 980 nm laser diode was used to obtain the UCL spectra of the samples; the emission slit was 1.0 nm and the excitation powder was 117 mW. For intensity emission comparisons, all UCL spectra were corrected using a xenon lamp as reference. Dark spectra obtained under the same conditions were also subtracted from all UCL spectra. However, the emission spectra were not corrected for the wavelength dependence of the detection system sensitivity and the refractive index of air (vacuum correction).

Results and discussion

Figure 1 presents the XRD spectra of the Y₂O₃:6%Yb³⁺,0.25%Ho³⁺ precursor material and the samples sulfurized at different temperatures. The Y₂O₃ precursor obtained crystallizes well and shows a pure cubic phase structure (JCPDS Card No. 74-1828). The considerably broadened diffraction peaks indicate that nanocrystalline Y₂O₃ particles were obtained. When the precursor is sulfurized at 700 °C for 1 h, the intensities of Y₂O₃ diffraction peaks decrease gradually, and some weak diffraction peaks for hexagonal Y₂O₂S structure emerge (JCPDS Card No. 24-1424). After the sulfurized temperature is increased to 800 °C, all diffraction peaks of Y₂O₃ disappear, and only strong Y₂O₂S diffraction peaks remain. This result shows that a temperature higher than 800 °C is beneficial for obtaining pure Y₂O₂S NCs. However, considering that higher temperatures may result in grain growth and particle aggregation, the sulfuration temperature used in this experiment was defined to be 800 °C. According to the Scherrer Formula: $D = {\frac{K\lambda }{\beta \cos \theta }}$ (where D is the crystal size of the particle, λ is the radiation wavelength of Cu Kα (0.154 nm), and β and θ are the half peak breadth and Bragg diffraction angle of the diffraction peak, respectively), the mean crystal size of the Y₂O₂S NCs sulfurized at 800 °C is 22.33 nm. Figure 2 shows the TEM image of Y₂O₂S:6%Yb³⁺,0.25%Ho³⁺ NCs sulfurized at 800 °C. The crystals grow relatively well and exhibit a near-spherical shape. The gain size of the sample averages 40 nm and distributes in the range of 25–50 nm. These results are slightly larger than those obtained from XRD measurements, and can be attributed to the slight aggregation of the primary crystals.

Fig. 1

XRD spectra of Y₂O₃:6%Yb³⁺,0.25%Ho³⁺ precursor and the samples sulfurized at different temperatures

Fig. 2

The TEM image of the Y₂O₂S:6%Yb³⁺,0.25%Ho³⁺ NCs sulfurized at 800 °C

To study the effects of Ho³⁺ ion content on the UCL properties of Y₂O₂S:Yb³⁺,Ho³⁺ NCs, the UCL spectra of Y₂O₂S:Yb³⁺,Ho³⁺ NCs doped with different concentrations of Ho³⁺ ions (Yb³⁺ ion content = 6 mol%) excited at 980 nm LD were obtained, as shown in Fig. 3. The inset in the figure shows the plots of green and red UCL intensities versus the Ho³⁺ ion concentrations used. Note that the peak positions of Ho³⁺ characteristic emissions in Y₂O₂S NCs are consistent with those in bulk Y₂O₂S [12]. The strong green emissions located at 545 and 548 nm arise from ⁵F₄, ⁵S₂ → ⁵I₈ transitions. The red emissions observed between 650 and 663 nm correspond to the ⁵F₅ → ⁵I₈ transition. The weak NIR emissions located around 759 nm are assigned to ⁵F₄, ⁵S₂ → ⁵I₇ transitions.

Fig. 3

The UCL spectra of Y₂O₂S:Yb³⁺,Ho³⁺ NCs doped with different concentrations of Ho³⁺ ion (Yb³⁺ content = 6 mol%) excited at 980 nm LD. Inset: The plots of green and red UCL intensities versus the Ho³⁺ concentrations

Figure 3 shows that the variation of Ho³⁺ ion concentrations does not affect the peak positions of Ho³⁺ characteristic emissions. Remarkable influence on the green and red UCL intensities, however, is found. The green UCL intensity improves considerably with increasing Ho³⁺ concentration, whereas the red UCL intensity exhibits weak enhancement. Both intensities attain maximum levels at an Ho³⁺ content of 0.25 mol%. When the Ho³⁺ content is greater than 0.25 mol%, concentration quenching takes place, and the UCL intensities of the green and red emissions begin to gradually decrease. According to the UCL measurement results, the optimal doping concentration of Ho³⁺ ions for green UCL in Y₂O₂S NCs is 0.25 mol%. This value is only half as much as that in micron Y₂O₂S (0.5 mol%) prepared by solid reaction processes [12], and can be ascribed to the distinct diffusion mechanism of activator ions in the coprecipitation process.

During the coprecipitation process, activator ions uniformly distribute into the precursor material produced by the precipitation reaction. While crystallization caused by calcination occurs, activator ions can directly occupy adjacent Y³⁺ lattices, so that the loss of activator ions is very limited. For the solid reaction, however, activator ions have to diffuse gradually from the surface to the interior of the particles. To accelerate this diffusion, some fusing agent is required to form a mass of grain boundary phases between particles and cause the activator ions to diffuse rapidly into the crystal lattices by a grain boundary diffusion mechanism [13]. According to the EDAX quantitative analysis results, most of the activator ions exist in these grain boundary phases, and the amount of ions which have been doped into the lattices and can operate on upconversion emission does not exceed half of the total doping concentration. Thus, the doping concentration of the activator in materials prepared by solid reactions is usually much higher than the actual requirement.

The red UCL of Ho³⁺ ions cannot be effectively suppressed in Y₂O₂S NCs throughout all the Ho³⁺ doping concentrations used in this experiment, especially since Ho³⁺ ions in micron Y₂O₂S typically exhibit considerably weak red UCL [12]. The relatively strong red UCL observed in Y₂O₂S NCs can be ascribed to the high surface activity of the NCs. The amount of NC surface atoms can highly increase with decreases in crystal size. The deficient coordination and high surface energy cause these surface atoms to possess high activity and show weak stability. To decrease surface energy, a sizeable amount of H₂O and CO₂ are absorbed onto the NC particle surfaces to form chemisorbed OH⁻ and CO₃ ²⁻. These findings are demonstrated by the IR spectrum of Y₂O₂S:6%Yb³⁺,0.25%Ho³⁺ NCs sulfurized at 800 °C (Fig. 4). The broad absorption bands around 3450 and 1660 cm⁻¹ can be assigned to the stretching and bending vibrations of O–H; the bands around 1530 and 1390 cm⁻¹ result from C–O asymmetrical stretching vibrations; the peak that appears at 1140 cm⁻¹ can be assigned to C–O symmetric stretching vibrations; the peaks at 993 and 725 cm⁻¹ correspond to C–O and O–H deformation vibrations; and the strong peak at ~ 500 cm⁻¹ can be assigned to the vibrations of Y–O and Y–S. These large vibrational quanta (3450–1140 cm⁻¹) can considerably influence the transition mechanism of the Yb³⁺–Ho³⁺ system in Y₂O₂S NCs. To further understand this mechanism, the UCL intensities of Y₂O₂S:6%Yb³⁺,0.25%Ho³⁺ NCs excited by 980 LD with different powers were measured.

Fig. 4

IR spectrum of Y₂O₂S: 6%Yb³⁺,0.25%Ho³⁺ NCs sulfurized at 800 °C for 1 h

The UCL intensity (I _UCL ) is proportional to some power n of the incident excitation power (I _P ) so that

$$ I_{UCL} \propto I_{P}^{n} $$

where the superscript n = 1, 2, 3… represents the number of pump photons required to populate the emitting states. The integrated intensities calculated from the area under the green, red, and NIR emissions in Y₂O₂S:6%Yb³⁺,0.25%Ho³⁺ NCs as a function of the pump intensity are shown in Fig. 5. The n values are calculated to be 2.03, 2.07, and 1.82 for the green, red, and NIR emission bands, respectively. These indicate that two IR photon absorptions are involved in the emission processes. According to the abovementioned results, the transition mechanism of the Yb³⁺–Ho³⁺ system in Y₂O₂S NCs can be built (Fig. 6).

Fig. 5

The integrated intensities of Ho³⁺ ions green, red, NIR emission in Y₂O₂S:6%Yb³⁺,0.25%Ho³⁺ NCs as a function of the pump intensity

Fig. 6

The transition mechanism of Yb³⁺–Ho³⁺ system in Y₂O₂S NCs

Under 980 nm LD excitation, the Ho³⁺ ions in the ground ⁵I₈ state are excited to ⁵F₄, ⁵S₂ states via two successive energy transfers from the Yb³⁺ ions in the ²F_5/2 state. Transitions from the ⁵F₄, ⁵S₂ states to the ⁵I₈ and ⁵I₇ states then yield green and NIR emissions, respectively. These transition processes are the same as those in micron Y₂O₂S. However, the red UCL observed from 650–663 nm shows a large difference in the transition mechanisms of the system under study and micron Y₂O₂S. The energy gaps (ΔE) of ⁵S₂-⁵F₅ and ⁵I₆-⁵I₇ are 2660 and 3220 cm⁻¹, respectively. According to the energy gap law, efficient multiphonon relaxation can take place only when ΔE is equal to or less than 4–5 times the highest energy phonons in the system. For micron Y₂O₂S co-doped with Ho³⁺ and Yb³⁺ ions, its only intrinsic phonon energy (about 520 cm⁻¹) can hardly bridge the large energy difference between the levels mentioned above, and no other phonons are available for its use. Thus, the ⁵F₅ red emission state of Ho³⁺ in micron Y₂O₂S is populated mainly by infrequent ⁵F₄/⁵S₂ → ⁵I₇ radiative transitions and the subsequent ⁵I₇ → ⁵F₅ excited-state absorption process (Route 1). Therefore, only weak red UCL can be observed in micron Y₂O₂S.

For the Y₂O₂S NCs, however, as mentioned previously, the OH⁻ and CO₃ ²⁻ groups chemisorbed onto the surface of the NCs can produce large vibrational quanta of 1500 and 3350 cm⁻¹. These quanta can easily bridge the energy gaps of ⁵F₄/⁵S₂-⁵F₅ and ⁵I₆-⁵I₇, and accordingly increase the probability of ⁵F₄/⁵S₂ → ⁵F₅ and ⁵I₆ → ⁵I₇ multiphonon relaxation. Thus, except for Route 1, the population route of the ⁵F₅ state in the NCs should also include the process of ⁵F₄/⁵S₂ → ⁵F₅ multiphonon relaxation (Route 2) and the process of ⁵I₆ → ⁵I₇ multiphonon relaxation, followed by ⁵I₇ → ⁵F₅ excited-state absorption (Route 3). Routes 2 and 3 can increase the population of ⁵F₅ red emission states to a large extent; hence, stronger red emissions can be observed in nanosized Y₂O₂S. Similar phenomena can also be observed in other nanomaterials doped with rare earth metals [3, 14]. Vetrone et al.’s report revealed that vibrations from OH⁻ and CO₃ ²⁻ groups can be still observed even after heating NCs to 1000 °C for 65 h [3].

The prepared Y₂O₂S:Yb³⁺,Ho³⁺ NCs exhibit bright green emission in the daytime that is visible to the naked eye at excitation powers as low as 25 mW (I_G/I_R = 2.53). Because previous studies on green UCL in NCs focused mainly on the Yb³⁺, Er³⁺ co-doped system, for purposes of comparison, Y₂O₂S:Yb³⁺, Er³⁺ NCs were also prepared using the procedure described in reference literature [15]. To obtain approximate I_G/I_R values, the doping concentrations of both Yb³⁺ and Er³⁺ions selected for this paper were 0.5 mol% (I_G/I_R = 2.09; the higher the Yb³⁺ or Er³⁺ ion content, the lower the I_G/I_R value is [15]). Figure 7 shows that in equivalent excitation conditions, the relative UCL intensities of Y₂O₂S:Yb³⁺,Ho³⁺ and Y₂O₂S:Yb³⁺, Er³⁺ NCs can be directly evaluated according to their UCL spectra. The green UCL intensities of the Yb³⁺–Ho³⁺ system are all three times higher than those of the Yb³⁺-Er³⁺ system throughout all the powers measured (inset, Fig. 7).

Fig. 7

The comparison of UCL intensities for Y₂O₂S:Yb³⁺,Ho³⁺ and Y₂O₂S:Yb³⁺, Er³⁺ NCs under 980 nm infrared excitation. Inset: The plots of green UCL intensities of Y₂O₂S:Yb³⁺,Ho³⁺ and Y₂O₂S:Yb³⁺, Er³⁺ NCs versus the excitation powers

Conclusion

Near-spherical Y₂O₂S:Yb³⁺,Ho³⁺ NCs with hexagonal structure were synthesized using the coprecipitation method followed by a solid–gas sulfuration technique. The crystal size of the NCs average 40 nm and are distributed from 25 to 50 nm. Because the coprecipitation method can ensure the uniform distribution of activator ions in the precursor material, the activator ions can directly occupy the adjacent Y³⁺ lattices during the crystallization process, thereby effectively decreasing the loss of Ho³⁺ ions. Consequently, the optimal doping concentrations of Ho³⁺ ions in Y₂O₂S NCs is 0.25 mol%—only half as much as that in micron Y₂O₂S. The UCL spectra indicate that the Ho³⁺ content can considerably influence the UCL efficiency of Y₂O₂S NCs. The large vibrational quanta produced by OH⁻ and CO₃ ²⁻ groups adsorbed onto the surface of the NCs can remarkably increase the probability of ⁵S₂–⁵F₅ and ⁵I₆–⁵I₇ multiphonon relaxation. These relaxation processes greatly enhance the population of ⁵F₅ red emission levels, thereby producing stronger red emissions from nanosized Y₂O₂S than from micro Y₂O₂S. These results are of importance in research studies that aim to determine nanosized upconversion biological fluorescence labels with high sensitivity.