Luminance Conversion Property of Er and Yb Doped KZnF₃ Nanocrystal Synthesized by Hydrothermal Method

Abstract

In order to make full use of exposure energy, one feasible way is to modify the luminance of crystal by rare earth doping technique. KZnF₃:Er³⁺ and KZnF₃:Er³⁺/Yb³⁺ nanocrystals of uniform cuboid perovskite type morphology, with average diameter of 130 nm, has been synthesized by hydrothermal method. When Yb³⁺ ions were codoped with Er³⁺, absorption peak at 970 nm has been heightened and widened, and the photon absorption cross section increased. The common xenon lamp exposure cannot initiate obvious nonlinear phenomenon of the doped Er³⁺ and Yb³⁺, and exposing at 245 nm only excites the fluorescence around 395 nm. Contrarily, under high power IR exposure at 980 nm, obvious upconversion photoluminescence (PL) has been observed due to the two-photon process. The PL mechanism of the doped Er³⁺ ion in KZnF3:Er³⁺/Yb³⁺ nanocrystals is confirmed. Furthermore, Yb³⁺ codoped as sensitizer has modified the PL intensity of Er³⁺ from green light range to red range, and the primary channel is changed from ⁴S_3/2(Er³⁺) → ⁴I_15/2(Er³⁺) of only Er³⁺ doped KZnF₃ nanocrystal to ⁴F_9/2(Er³⁺) → ⁴I_15/2(Er³⁺) of Er³⁺/Yb³⁺ codoped sample. With exposure energy increasing, such primary transition channel after two-photon excitation is unchanged.

1. Introduction

Since display issues have been receiving extensive attention, the photoluminescence (PL) technology based on inorganic, organic, or microcrystalline properties has developed rapidly [1]. For such a purpose, the rare earth doping technique has been investigated during recent years and was applied in many areas such as phosphors, display monitors, X-ray imaging, lasers, amplifiers for fiber-optic communication, and biological fluorescence labeling [2–7]. Among these techniques, an important research topic has attracted much attention which makes full use of exposure energy, especially converting near-infrared (NIR) irradiation into visible light, due to the easy acquirement of low-cost and high-power NIR laser diodes nowadays. Erbium cation (Er³⁺) doped materials have been well studied, which can convert NIR exposure energy for different usages [8]. Obregóna and Colón reported that Er³⁺ doping has enhanced the photocatalytic activity of TiO₂ under sun-like excitation, acting in different energy conversion mechanisms under UV or NIR excitation, and verified the upconversion contribution of Er³⁺ ion [9]. Xin et al. doped NaGdF₄:Er³⁺ crystal in glass matrix and tuned the upconversion emission intensity by changing the Er³⁺ concentration, which has potential usage for the silicon solar cells [10]. It has been verified and accepted that the upconversion process is nonlinear, in which a photon at shorter wavelength is emitted after continuously absorbing more than one longer wavelength photon by long-lived intermediate energy state of lanthanide rare earth ions (Tm³⁺, Ho³⁺, and Er³⁺), usually as two-photon or even three-photon excitation reactions [11]. However, restricted by the dipole-forbidden intra-4f transition, the photon absorption cross section of Er³⁺ is relatively small.

To enhance the upconversion efficiency, auxiliary methods have been put forward. Aisaka et al. present that upconversion PL obtains more benefits from the metal-enhanced fluorescence technique than the downconversion PL, due to strong enhancement of upconversion photoluminescence in visible range by placing Er³⁺ near rough Ag islands in Al₂O₃ films [12]. Zhang et al. also find such enhancement in Au 3-dimensional (3D) nanostructures. Surface plasmon resonances have improved the photon absorption efficiency [13]. In such a meaning, sensitizers can also achieve improvement of photon absorption cross section. For example, the ytterbium ions (Yb³⁺) can collect the pumping light and transfer the energy to Er³⁺ ions to participate in further upconversion [14, 15].

Except for the above stated photon absorption aspect, the substrate, in which Er³⁺ and other ions are doped, also has nonignorable influence on the upconversion efficiency. Oxide or fluoride thin films have been usually selected as substrates. The phonon energy is lower in fluoride film than oxide one, which can decrease the nonradiative loss in multiphonon relaxation and then obtain a high quantum efficiency of the desired luminescence [16–18]. Fluoride film can be synthesized by solid phase reaction or sol-gel process, but the former has oxidation problem, and the latter is difficult to perform.

In this paper, the KZnF₃ fluoride nanocrystals, doped by Er³⁺ and sensitizer Yb³⁺, are synthesized by hydrothermal method. In order to clarify the modification of upconversion mechanism, the Yb³⁺ is set at 2% mole ratio, with the Er³⁺ at 0.5%. The nanocrystal diameter is 130 nm. Such fluoride substrate for upconversion was not reported thoroughly. The luminance excited at 980 nm has been observed with blue shift compared to the exciting wavelength. Upconversion has been achieved. In order to explore the luminance conversion effects, the morphology of KZnF₃ nanocrystal, the absorption, reflection, and FT-IR spectra and the PL spectra are characterized. Meanwhile, the mechanism of luminance conversion is discussed in this paper.

2. Experimental

2.1. Preparation of Er and Yb Doped KZnF₃ Nanocrystal

Rare earth oxides Er₂O₃ (99.9%, Guangfu) and hydrous chloride YbCl₃·6H₂O (99.99%, Alfa Aesar) are selected as the doping ingredients without further purification. K₂CO₃ (99.9%), ZnF₂ (99.5%), and NH₄HF₂ (99.9%) are used to synthesize substrate. The Er³⁺ doping concentration of 0.5% mole ratio and the Yb³⁺ doping as ()K₂CO₃·YbCl₃ (, 2%) are mixed in 15 mL H₂O. The pH is adjusted at 3.

After the preparation procedure, the mixture is sealed in reactor. Then, hydrothermal method is put forward. Since crystallization can be generated in high-temperature aqueous solutions under high vapor pressures, the reaction is operated at 180°C for 30 h. The resultants are filtrated and preserved in room temperature for 10 days. Finally, the XRD, SEM, and spectra technique are employed.

2.2. Characterization of Er and Yb Doped KZnF₃ Nanocrystal

The crystallinity of the prepared nanocrystal is analyzed by XRD (XRD6000, Shimadzu, Japan) with Cu K_α radiation ( nm) at 40 kV and 25 mA, under scanning rate of 10°/min. The nanocrystal morphology is observed by SEM (FEI-quanta-200F, FEI, Netherlands) and Particle Size Analyzers (Zetasizer NANO ZS90, Malvern, United Kingdom). The absorption/reflection spectra, FT-IR spectra, and common fluorescence spectra are detected by UV-visible spectrometer (U-4100, Hitachi, Japan), FT-IR spectrophotometer (Tensor27, Bruker, Germany), and fluorescence spectrometer (F-4600, Hitachi, Japan), respectively. To investigate the luminance conversion property, photoluminescence (PL) emission spectra are measured using photoluminescence spectrophotometer (Fluorolog-3-TAU, Horiba Jobin Yvon, France), with the exposure source irradiated at 980 nm. All analyses were done at room temperature.

3. Results and Discussion

3.1. XRD Analysis

Phase identification of nanocrystals is performed via X-ray diffraction shown in Figure 1. X-ray diffraction pattern of the nanocrystals is matched well with the standard pattern of PDF#06-0439, which suggested that the material has crystallized as KZnF₃ structure.

Figure 1 

X-ray diffraction pattern of the Er³⁺, Er³⁺/Yb³⁺ doped nanocrystals and the standard KZnF₃ XRD card.

There is a minute location increase of the diffraction peaks compared to the standard KZnF₃ pattern, and it can be ascribed that the diameters of the doped Er³⁺ and Yb³⁺ ions are different from the substituted ions in KZnF₃ crystal lattice. In addition, some additional weak peaks, such as the peaks appearing at 19.0 degrees in only Er³⁺ doped material and 13.6, 15.5, 19.1, 28.1, and 47.8 degrees of Er³⁺/Yb³⁺ doped nanocrystals, might belong to the incompletely reacted fluoride raw materials, unwanted oxide, or oxy-fluoride impurities.

3.2. SEM and Diameter Distribution Analysis

The SEM graph is shown in Figure 2. The nanocrystal has cuboid perovskite type structure with smooth surface. Such structure can be used as optical substrate, since many ions possessing different valence states can be doped into its crystal lattice.

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(b)

(c)

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Figure 2 

SEM graph of the synthesized nanocrystals.

The size statistics are presented in Figure 3. The average diameter of the nanocrystals is about 130 nm and has a narrow distribution, which is an advantage to realize homogeneous optical property.

Figure 3 

Diameter distribution of the synthesized nanocrystals.

3.3. UV-Vis and FT-IR Analysis

To clarify the luminance conversion mechanism, the raw materials of Er₂O₃ and YbCl₃ are investigated in Figures 4(a) and 4(b) by reflection and absorption spectra. Two reagents have absorption peak around 974 and 970 nm, respectively. The molecular Er₂O₃ has more absorption bands present in Figure 4(a) than the YbCl₃ in Figure 4(b). And the ZnF₂ only has long wavelength absorption band.

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(b)

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(d)

Figure 4 

(a) Reflection spectra of Er₂O₃ and ZnF₂; (b) absorption spectra of YbCl₃; (c) reflection and (d) absorption spectra of Er³⁺ doped, Er³⁺/Yb³⁺ codoped nanocrystals.

After KZnF₃ crystallization, narrow absorption peak around 970 nm appeared for only Er³⁺ doped material, but the absorption intensity is relatively low. When 2% Yb³⁺ ions were codoped with Er³⁺, the absorption peak has been heightened and the absorption band has been widened, as shown in Figures 4(c) and 4(d) in reflection and absorption spectra. This suggests that the Yb³⁺ can increase the photon absorption efficiency based on its higher photon absorption cross section.

The FT-IR spectra are shown in Figure 5. The spectra peak positions are similar between the only Er³⁺ doped and the Er³⁺/Yb³⁺ codoped samples. But the highest peak is different. The Er³⁺/Yb³⁺ codoped nanocrystal has the highest absorbance at 748 cm⁻¹, whereas the highest absorbance of only Er³⁺ doped sample is around 420 cm⁻¹.

Figure 5 

FT-IR spectra of Er³⁺ and Er³⁺/Yb³⁺ doped nanocrystals.

This result qualitatively indicates that Yb³⁺ ions can easily form chemical bonds when doped into the KZnF₃ nanocrystal lattice.

3.4. Luminance Spectra Analysis with Common Exposure Source

With common xenon lamp as exposure source, monitored at 490 nm, the excitation spectra have one peak at 245 nm in Figure 6(a), which is related to the transition channel of Er³⁺:

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Figure 6 

(a) Excitation and (b) emission spectra of the nanocrystal exposed to common xenon lamp.

Further exposed to irradiation of 245 nm, the emitted fluorescence is similar between the only Er³⁺ doped and Er³⁺/Yb³⁺ codoped nanocrystals in Figure 6(b). The emission peak appears at 395 nm with a band from 350 to 550 nm.

The exposure energy of common xenon lamp is very low, and no nonlinear phenomenon appeared, even if the UV excitation at 245 nm has been employed.

3.5. PL Spectra Analysis under 980 nm Laser Exposure

When exposed to 980 nm laser with its energy adjusted in the scale of 50–200 mW, the emitted luminance can be directly observed by naked eyes. Fluorescence of the prepared KZnF₃:Er³⁺ and KZnF₃:Er³⁺/Yb³⁺ nanocrystals in the wavelength region of 350–750 nm is shown in Figures 7(a) and 7(b), respectively. The luminance has blue shifted compared to the exposing wavelength of 980 nm. PL upconversion has been achieved for both only Er³⁺ doped and Er³⁺/Yb³⁺ codoped KZnF₃ nanocrystals.

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(b)

Figure 7 

PL spectra of (a) Er³⁺ and (b) Er³⁺/Yb³⁺ codoped nanocrystals when exposed to 980 nm laser.

Several similar distinct emission bands are observed for the two kinds of samples, centered around 653, 539, 528, 484, and 407 nm. These emissions are ascribed to the doped Er³⁺ ions. The relationship between PL luminance peak and transition channel of Er³⁺ is shown in Table 1.

Table 1 

Relationship between PL peak and transition channel.

The differences of the only Er³⁺ doped and Er³⁺/Yb³⁺ codoped nanocrystals are laid on the PL intensity. The blue PL at 407 and 377 nm are relatively low for only Er³⁺ doped nanocrystal, and such emission peaks even disappear for Er³⁺/Yb³⁺ codoped nanocrystal. In green and red PL band, higher green luminance at 539 nm than red light at 653 nm appears for the only Er³⁺ doped material in Figure 7(a), and the red luminance at 654 nm has been remarkably enhanced and becomes higher than the green luminance around 539 nm after 0.5% Er³⁺ and 2% Yb³⁺ codoped, which is shown in Figure 7(b). This indicates that red light irradiation process of Er³⁺ ions has become the primary transition channel due to the Yb³⁺ codoping with Er³⁺ in KZnF₃ substrate.

The relative energy level state of Er³⁺ and Yb³⁺ is shown in Figure 8. According to Figure 8, the kind of emitting photon is determined by the occupied high energy level including ⁴F_7/2(Er³⁺), ²H_11/2(Er³⁺), ⁴S_3/2(Er³⁺), and ⁴F_9/2(Er³⁺), in which the ²H_11/2(Er³⁺) and ⁴S_3/2(Er³⁺) can be generated by nonradiative relaxation from ⁴F_7/2(Er³⁺), and the ⁴F_9/2(Er³⁺) is generated by energy relaxation from ⁴S_3/2(Er³⁺), or relaxation from ⁴I_11/2(Er³⁺) to ⁴I_13/2(Er³⁺) and further access to ⁴F_9/2(Er³⁺) by obtaining another 980 nm photon energy.

Figure 8 

Schematic diagram illustrating the relative energy level of Er³⁺ and Yb³⁺.

Consequently, the spontaneous emission occurs to emit one green photon or one red photon based on Mechanism 1 in Table 2. The multistep relaxation to ⁴F_9/2(Er³⁺) has low quantum efficiency, and the red light of only Er³⁺ doped sample has relatively lower intensity compared to green light emission as seen in Figure 7(a). The major PL transition channel is after two-photon-exciting the only Er³⁺ doped sample.

Table 2 

Sequential two-photon exciting upconversion transition channels for Er³⁺ doped or Er³⁺/Yb³⁺ codoped nanocrystals.

In Er³⁺/Yb³⁺ codoped nanocrystal, except that all the reactions in Mechanism 1 are taking place, the Yb³⁺ has also acted as sensitizer, which is shown in Mechanism 2. Yb³⁺ ion in ²F_5/2(Yb³⁺) state can nonradiatively transfer the first absorbed 980 nm photon energy (10.204 × 10³ cm⁻¹) and excited Er³⁺ from ground state ⁴I_15/2(Er³⁺) to ⁴I_11/2(Er³⁺), for which ²F_5/2(Yb³⁺) and ⁴I_11/2(Er³⁺) are at resonance level. Meanwhile, the Yb³⁺ ion returns to its ground state ²F_7/2(Yb³⁺). Then, another energy transfer process can take place after ²F_7/2(Yb³⁺) absorbs the second 980 nm photon, and the energy is transferred to excite Er³⁺ from ⁴I_11/2(Er³⁺) to ⁴F_7/2(Er³⁺) in order to further emit green and red upconversion photon according to the first part of Mechanism 2.

After Yb³⁺ was codoped with Er³⁺, the enhanced red photon emitting from ⁴F_9/2(Er³⁺) to ⁴I_15/2(Er³⁺) can be ascribed to another procedure shown in the last part of Mechanism 2. The excited ⁴I_11/2(Er³⁺) ions relax to metastable state ⁴I_13/2(Er³⁺), and then the transferred energy from ²F_5/2(Yb³⁺) excites ⁴I_13/2(Er³⁺) to ⁴F_9/2(Er³⁺) more efficiently. Therefore, remarkable enhancement of red emission appears in Figure 7(b), and the Yb³⁺ codoping with Er³⁺ has modified the transition channel of Er³⁺, with the primary transition channel changing from to . At the same time, the green emission channel has been suppressed in a given extent.

Also, it should be noticed that such transition channel after two-photon excitation does not change with exposure energy increasing in Figure 7, since the PL spectra curves under different exposure energies are similar.

Based on the above statement, it is obvious that the energy transfer from Yb³⁺ has modified PL emission of Er³⁺ from green light band to red band. And the energy transfer efficiency is depending on three factors. One is the matching energy levels between the two different kinds of ions; another is that the sensitizer has relatively big absorption cross section; the last is that Er³⁺ and Yb³⁺ ions must be near enough in crystal lattice.

4. Conclusion

Codoping Yb³⁺ with Er³⁺ in KZnF₃ cuboid perovskite type nanocrystal has been synthesized using hydrothermal method in this paper. Conclusions are presented as follows.

(1) After two-photon (980 nm) energy acquisition of the doped Er³⁺ ion, red emissions were derived from transition channel ⁴F_9/2 → ⁴I_15/2, green from ⁴S_3/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2, and ⁴F_7/2 → ⁴I_15/2, and violet from ²H_9/2 → ⁴I_15/2. And the major transition channel is ⁴S_3/2(Er³⁺) → ⁴I_15/2(Er³⁺) of the only Er³⁺ doped KZnF₃ sample.

(2) Modification of upconversion photoluminescence (PL) of Er³⁺, changed from green light band to red band, has been achieved by Yb³⁺ codoped with Er³⁺. The red photon emitting has been remarkably intensified compared to only Er³⁺ doped KZnF₃ nanocrystal. The major transition channel has changed to ⁴F_9/2(Er³⁺) → ⁴I_15/2(Er³⁺) of the Er³⁺/Yb³⁺ codoped sample.

(3) The mechanism of the modification has been discussed in which the excited ⁴I_11/2(Er³⁺) state relaxes to metastable state ⁴I_13/2(Er³⁺), and then the transferred energy from ²F_5/2(Yb³⁺) efficiently excites ⁴I_13/2(Er³⁺) to ⁴F_9/2(Er³⁺). In such two-photon upconversion process, the Yb³⁺ has acted as sensitizer, not only for its higher absorption section than Er³⁺, but also for its high energy transfer efficiency.

Such upconversion nanocrystals can be applied as luminescent probes in biological labeling and imaging technology. Results obtained in this paper can be used for rare earth doping for spectrum conversion, in order to improve the display efficiency.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work is financially supported by the Specialized Research Fund for the Doctoral Program of Higher Education 20101301120002, International Foundation for Science C4690, and the Natural Science Foundation of Hebei Province F2011201103.