Luminance Conversion Property of Er and Yb Doped KZnF3 Nanocrystal Synthesized by Hydrothermal Method
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 4S3/2(Er3+) → 4I15/2(Er3+) of only Er3+ doped KZnF3 nanocrystal to 4F9/2(Er3+) → 4I15/2(Er3+) of Er3+/Yb3+ codoped sample. With exposure energy increasing, such primary transition channel after two-photon excitation is unchanged.
Since display issues have been receiving extensive attention, the photoluminescence (PL) technology based on inorganic, organic, or microcrystalline properties has developed rapidly . 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 . 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 . 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 . 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 . 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 . Zhang et al. also find such enhancement in Au 3-dimensional (3D) nanostructures. Surface plasmon resonances have improved the photon absorption efficiency . 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.
2.1. 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 Kα 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.
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.
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.
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.
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.
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+:
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.
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.
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 4F7/2(Er3+), 2H11/2(Er3+), 4S3/2(Er3+), and 4F9/2(Er3+), in which the 2H11/2(Er3+) and 4S3/2(Er3+) can be generated by nonradiative relaxation from 4F7/2(Er3+), and the 4F9/2(Er3+) is generated by energy relaxation from 4S3/2(Er3+), or relaxation from 4I11/2(Er3+) to 4I13/2(Er3+) and further access to 4F9/2(Er3+) by obtaining another 980 nm photon energy.
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 4F9/2(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.
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 2F5/2(Yb3+) state can nonradiatively transfer the first absorbed 980 nm photon energy (10.204 × 103 cm−1) and excited Er3+ from ground state 4I15/2(Er3+) to 4I11/2(Er3+), for which 2F5/2(Yb3+) and 4I11/2(Er3+) are at resonance level. Meanwhile, the Yb3+ ion returns to its ground state 2F7/2(Yb3+). Then, another energy transfer process can take place after 2F7/2(Yb3+) absorbs the second 980 nm photon, and the energy is transferred to excite Er3+ from 4I11/2(Er3+) to 4F7/2(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 4F9/2(Er3+) to 4I15/2(Er3+) can be ascribed to another procedure shown in the last part of Mechanism 2. The excited 4I11/2(Er3+) ions relax to metastable state 4I13/2(Er3+), and then the transferred energy from 2F5/2(Yb3+) excites 4I13/2(Er3+) to 4F9/2(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.
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 4F9/2 → 4I15/2, green from 4S3/2 → 4I15/2, 2H11/2 → 4I15/2, and 4F7/2 → 4I15/2, and violet from 2H9/2 → 4I15/2. And the major transition channel is 4S3/2(Er3+) → 4I15/2(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 4F9/2(Er3+) → 4I15/2(Er3+) of the Er3+/Yb3+ codoped sample.
(3) The mechanism of the modification has been discussed in which the excited 4I11/2(Er3+) state relaxes to metastable state 4I13/2(Er3+), and then the transferred energy from 2F5/2(Yb3+) efficiently excites 4I13/2(Er3+) to 4F9/2(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.
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.
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