1. Introduction

Upconversion luminescence materials doped with rare earth ions have attracted more and more attention due to their promising applications in lighting, color display, waveguides, communication fibers, solar cells, and high density optical storage, et al [1–7]. As one of the typical ions which can emit blue upconversion emissions, Tm³⁺ ions have been paid close attention [8, 9]. Yb³⁺ ions show large absorption coefficient at 980 nm, resulting in excellent sensitizing properties in the upconversion luminescence materials. Moreover, Yb³⁺ ions behave as very efficient sensitizers for the mostly used active rare earth ions such as Er³⁺, Tm³⁺, and Ho³⁺ [10–12]. Therefore, Tm³⁺ and Yb³⁺ can be employed as a favorable rare earth ion combination to obtain strong blue upconversion emissions.

In the previous study, the enhancement of upconversion luminescence has been mainly ascribed to controlling experimental conditions such as temperature, time, et al [13–15]. Moreover, the composition changes of hosts also play an important role in optimizing upconversion luminescence properties [16]. As the key factor, the doping ions should be paid more attention and optimized more reasonably during the design and study on upconversion luminescence materials. It is a favorable way to increase the emission intensity by changing the doping of rare earth ions. In the Tm³⁺/Yb³⁺ system, Yb³⁺ ions appear as an efficient sensitizer to increase the emission intensity of Tm³⁺ ions. Yb³⁺ can absorb the energy of 980 nm laser intensely and then behave an obviously positive action on the luminescence process. Therefore, Yb³⁺ ion is one of the mostly used sensitizers in upconversion luminescence to harvest incident photons and subsequently promote neighboring active ions to excited states. Yb³⁺ ions show high quenching concentration in various types of hosts for its simple two-level structure. Yb³⁺ and Tm³⁺ ions have similar chemical properties and ionic radius, which allows their homogenous doping easier and weak interaction with each other. Yb³⁺ ions can transfer energy from itself to active rare earth ions efficiently. Based on these favorable factors, it is fatal and urgent to study the effect of Yb³⁺ ions on the upconversion luminescence of Tm³⁺ ions. The mechanism and process of Yb³⁺ ions’ action on the photoluminescence behavior should be revealed to understand the essential of spectra results. This research can be employed as reference to optimize the upconversion luminescence of rare earth ions doped materials.

The upconversion luminescence of rare earth ions are easily quenched by the quick multiphonon relaxation which originates from the small energy difference between the emission level and the next lower-lying level when they are incorporated in conventional oxide glasses. This is owing to the high cutoff phonon energy of most oxide glasses, such as ~1100 cm⁻¹ of silica glasses [17]. Thus, it is necessary to develop novel robust oxide glasses as the host to obtain intense luminescence of Tm³⁺/Yb³⁺ system. TiO₂-based glasses exhibit low phonon energy (600-700 cm⁻¹), high thermal stability, excellent mechanical and chemical properties, high refractive index, and good transparency, indicating that it is a good candidate to be used as the host of Tm³⁺/Yb³⁺ system and then achieve efficient upconversion luminescence [18–23]. However, without adding any network former oxides, it is difficult to prepare bulk TiO₂-based glasses by traditional methods due to its low glass forming ability [24]. The containerless processing methods can keep samples levitated without contacting walls of containers by using magnetic field, electrostatic field, fast gas, et al to resist gravity. These methods can avoid melt contamination from crucible, preclude the source for heterogeneous nucleation, and obtain deep undercooling for glass forming and meta-stable phase transition [25–27]. They can be used to exploit new types of glasses which cannot be obtained by conventional techniques. Therefore, bulk TiO₂-based glasses can be successfully fabricated by aerodynamic levitation which is one of the mostly used containerless processing methods.

In the present work, La₂O₃-TiO₂-ZrO₂ (LTZ) system was selected as the host glasses which were successfully prepared by aerodynamic levitation. Tm³⁺ and Yb³⁺ were doped into the host glasses as active and sensitized ions, respectively. The Raman spectrum and refractive index were measured to reveal the maximum phonon energy and optical properties of LTZ glasses. The effect of Yb³⁺ ions on the thermal stability of LTZ glasses was studied by DTA curves. The optical absorption spectrum was recorded to confirm that LTZ glasses can be excited efficiently by 980 nm laser. The upconversion luminescence spectra of LTZ glasses incorporating different Yb³⁺ contents were performed by using 980 nm LD laser. The sensitizing effect of Yb³⁺ on the luminescence process of Tm³⁺ was analyzed based on the study of upconversion luminescence mechanism. A model was proposed to understand the action of active ions, sensitizers, and hosts on the luminescence properties.

2. Experimental

Aerodynamic levitation furnace was successfully introduced to prepare this type of glasses. The compositions of Tm³⁺/Yb³⁺ co-doped LTZ oxide glasses were 27.95LaO_1.5-0.15TmO_1.5-yYbO_1.5-55TiO₂-14ZrO₂ (y = 0.01, 0.15, 0.30, 0.50, 1.0, 1.50, 2.0). High-purity La₂O₃ (4N), TiO₂ (CP), ZrO₂ (SP), Tm₂O₃ (3N), and Yb₂O₃ (4N) powders were mixed thoroughly in ethanol in stoichiometric composition. The mixed samples were levitated by containerless processing in an aerodynamic levitation furnace and then were melted by using the CO₂ laser. In the preparation process, the raw samples were firstly put into an aluminum nozzle with smooth surfaces, and then the samples were levitated with the help of fast gas flow from the bottom of the nozzle. The samples were melted with a laser from the top of the furnace. The melt stayed at high temperature for several minutes to make itself homogeneous. Finally, the homogeneous melt was quenched into a solid spherical glass at a fast cooling rate. The detailed description of the synthesis was introduced elsewhere [28–30]. The as-prepared glass spheres have a diameter of ~3mm. Then the glass spheres were polished by two sides or one side for later measurements.

The Raman spectrum of Tm³⁺/Yb³⁺ co-doped glass was recorded by Laser Microscopic Raman Spectrometer (Thermo Scientific DXR) in full range scan at the excitation of 532 nm continuous laser. The refractive index values of the glasses were measured and fitted by spectroscopic ellipsometry (J.A. Woollam M-2000). The incident wavelength ranged from 400 nm to 1000 nm. And then the dependence of refractive index on the wavelength was obtained. DTA curves of Tm³⁺/Yb³⁺ co-doped LTZ glasses with different Yb³⁺ contents were measured by thermal analyzer (NETZSCH STA 449C) in the air at the heating rate of 10 °C/min. The optical absorption spectrum was characterized by ultraviolet spectrophotometer (Cary 5000). Tm³⁺/Yb³⁺ co-doped LTZ glasses which had been polished by two sides were heat-treated at 890 °C at the heating rate of 10 °C/min in the air. Then the samples were cooled at the condition of furnace cooling. In this way, glass-ceramics samples were prepared. Then upconversion luminescence spectra of glasses and glass-ceramics were recorded by spectrofluorometer (Fluorolog-3 and Edinburgh) with a wavelength of 980 nm continuous laser as the excitation source at the same testing conditions, respectively. During the test, the stability of laser power and the environmental temperature may have effects on the measurement and may result in errors. So the spectra are recorded at the same conditions.

3. Results and discussion

The Raman spectrum of 27.95LaO_1.5-0.15TmO_1.5-2YbO_1.5-55TiO₂-14ZrO₂ glass indicates that the maximum phonon energy is ~747 cm⁻¹, which is helpful to obtain high efficient luminescence. The refractive index of 27.95LaO_1.5-0.15TmO_1.5-2YbO_1.5- 55TiO₂-14ZrO₂ glass is presented in Fig. 1.From the result, n_d, n_f, and n_c can be evaluated to be 2.261, 2.305, and 2.244, which are obviously higher than those of traditional optical glasses including heavy-metal glasses incorporating lead. n_d is often used to characterize refractive properties. So this type of glasses shows high refractive index which can allow high radiative transition rate and large excited cross-section for rare earth ions. Abbe number (v_d) can be used to characterize dispersive ability of optical glasses. The calculation equation of Abbe number is listed as following:

 

Fig. 1 The dependence of Tm³⁺/Yb³⁺ co-doped LTZ glass’s refractive index value measured and fitted by spectroscopic ellipsometry on the incident wavelength.

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Together with the results of refractive index, v_d can be decided as 20.7. The Abbe number of commonly used optical glasses verifies from 20 to 90, indicating that this type of glasses can satisfy the requirement of Abbe number in the application. Therefore, Tm³⁺/Yb³⁺ co-doped LTZ glasses perform low phonon energy, high refractive index and Abbe number, resulting in excellent upconversion luminescence and optical properties.

To study the effect of Yb³⁺ ions on the thermal stability, DTA curves of 27.95LaO_1.5-0.15TmO_1.5-yYbO_1.5-55TiO₂-14ZrO₂ (y = 0.01, 0.15, 0.30, 0.50, 1.0, 1.50, 2.0) glasses were measured at the same testing conditions. The results are shown in Fig. 2(a), which exhibits a glass transition and a crystallization peak. The obtained DTA curves perform similar features except for the changes in positions and intensities of the glass transition and the crystallization peak as the Yb³⁺ contents change. The peak temperature of crystallization (T_p) can be acquired and the dependence of T_p on the concentration of Yb³⁺ is presented in Fig. 2(b). The T_p values of all the glasses are higher than 900 °C, indicating that the glasses show good thermal stability. As the concentration of Yb³⁺ increases, the value of T_p decreases. This reveals that Yb³⁺ ion can reduce the thermal stability of Tm³⁺/Yb³⁺ co-doped LTZ glasses.

 

Fig. 2 (a) DTA curves of Tm³⁺/Yb³⁺ co-doped LTZ glasses, (b) the dependence of peak temperature T_p on the concentration of Yb³⁺.

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The absorption spectrum has been performed to confirm that the absorption band of Tm³⁺/Yb³⁺ co-doped LTZ glasses lies around 980 nm. The resulting spectrum of 27.95LaO_1.5-0.15TmO_1.5-2YbO_1.5-55TiO₂-14ZrO₂ glass is shown in Fig. 3(a).The glass has a strong absorption band around 980 nm. Hence, Tm³⁺/Yb³⁺ co-doped LTZ glass can be excited efficiently by 980 nm laser. Furthermore, absorption peaks centered at 686 nm, 791 nm, and 1212 nm can be ascribed to the absorption of Tm³⁺ ions [31, 32]. The action of Yb³⁺ ions on the upconversion behavior has been studied by performing the emission spectra of Tm³⁺/Yb³⁺ co-doped LTZ glasses with different Yb³⁺ contents at the same testing conditions. According to the resulting upconversion luminescence spectra shown in Fig. 3(b), a blue emission band and a red emission band centered at 476 nm and 650 nm have been obtained. Moreover, the blue emission is obviously stronger than the red emission. As the concentration of Yb³⁺ increases, the upconversion luminescence increases. The glass with y = 2.0 shows the strongest emissions. The relationship between the emission intensity and the concentration of Yb³⁺ is presented in Fig. 3(c) to reveal the effect of sensitizers on the luminescence process. As the concentration of Yb³⁺ increases, besides that the emission intensity increases, the increasing slope also moves up. The emission intensity shows non-linear enhancement, which indicates that the upconversion luminescence process is being changed to be more efficient. Therefore, as Yb³⁺ ion increases, energy transfer from Yb³⁺ to Tm³⁺ become more and more smooth and energy loss is reduced greatly, resulting in stronger upconversion luminescence.

 

Fig. 3 (a) Absorption spectrum of Tm³⁺/Yb³⁺ co-doped LTZ glass, (b) upconversion luminescence spectra of 27.95LaO_1.5-0.15TmO_1.5-yYbO_1.5-55TiO₂-14ZrO₂ glasses at 980 nm excitation, the dependence of emission intensity of Tm³⁺/Yb³⁺ co-doped LTZ (c) glasses and (d) glass-ceramics on the concentration of Yb³⁺.

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In order to study the effect of Yb³⁺ ion on the upconversion process further, Tm³⁺/Yb³⁺ co-doped LTZ glasses with different Yb³⁺ contents have been heated at 890 °C to prepare glass-ceramics. And then the upconversion luminescence spectra of glass-ceramics have been performed under the excitation of 980 nm laser at the same testing conditions. Two main emission bands which can be considered to be blue and red emissions are centered at 476 nm and 650 nm. The relationship between emission intensity and Yb³⁺ concentration has been analyzed in Fig. 3(d). As the concentration of Yb³⁺ increases, the upconversion luminescence ascends and then descends. The increasing slope of upconversion luminescence increases gradually until the peak appears. It is noteworthy that the emission intensity peak of glass-ceramics is achieved at y = 0.50 while that of glasses at y = 2.0. It can be concluded that rare earth ions are enriched into crystals during the heat treatment. The crystals in the glass matrix can provide favorable local environment with dense lattice and strong partition to reduce non-radiative relaxation resulting from interaction among rare earth ions [30]. So the crystals obtained by heat treatment are helpful to improve upconversion luminescence. In this way, the concentration of Yb³⁺ is improved in the favorable local environment and even exceeds the quenching concentration which can’t be achieved in the glasses. The suitable Yb³⁺ concentration which the strongest upconversion luminescence requires can be satisfied in the crystals, leading to much more efficient energy transfer from Yb³⁺ to Tm³⁺ and less energy loss. So it can be known that crystallization of LTZ glasses increases the emission intensity through favorable local environment of crystals and enrichment of rare earth ions.

As Yb³⁺ concentration increases, the increasing slope of the upconversion luminescence of glasses and glass-ceramics can be improved gradually. To discuss this phenomenon, the local environment including one Tm³⁺ ion, several Yb³⁺ ions and enough host atoms can be regarded as an Upconversion Luminescence Unit (ULU). In this model, Tm³⁺ is the emission core, Yb³⁺ is the sensitizing shell, and the host is the protective shell. The detailed description of ULU is presented in Fig. 4.In the previous researches, it has been approved that intense upconversion luminescence can be efficiently obtained by doping low concentration of active ions like Tm³⁺, Nd³⁺, Ho³⁺ ions [18, 33, 34]. However, as the doping concentration increases, the emission intensity can be decreased obviously. This is because non-radiative relaxation can easily happen when doping high concentration of active ions. As the concentration of Tm³⁺ increases, the density of ULU increases leading to thinner protective shell. And then the interaction among rare earth ions becomes stronger, which can decrease the emission intensity. Hence, the active ions should be doped in the range of low concentration [35]. To achieve intense upconversion luminescence, the active ion should be surrounded by thick enough sensitizing shell. In another word, the concentration of Yb³⁺ should be obviously higher than that of Tm³⁺. This can improve the energy transfer from Yb³⁺ to Tm³⁺ and then greatly help the cooperative sensitization which is the dominant upconversion mechanism in Tm³⁺ ions [30, 36, 37]. Compared to base glasses, crystals in glass-ceramics can provide the shell with dense lattice, order, and strong partition for rare earth ions to reduce non-radiative relaxation, resulting in the enhancement of upconversion luminescence [30]. This is one of the reasons why glass-ceramics perform better luminescence properties than glasses. Therefore, the effects of active ions, sensitizers, and hosts on the upconversion luminescence have been discussed through ULU model which can be considered as an instruction for the optimization of upconversion luminescence in rare earth ions doped materials.

 

Fig. 4 The diagram of Upconversion Luminescence Unit.

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4. Conclusion

Bulk Tm³⁺/Yb³⁺ co-doped LTZ glasses with different Yb³⁺ contents have been successfully fabricated by containerless processing. LTZ glasses show excellent optical properties with refractive index value of n_d = 2.261 and Abbe number of v_d = 20.7, which indicates promising application in optical industry. Although Yb³⁺ can reduce T_p value slightly, all the glasses show high thermal stability. Yb³⁺ ions play an important role in the upconversion luminescence process of Tm³⁺ by improving energy transfer and reducing energy loss. As Yb³⁺ concentration increases, the enhancement difference of emission intensity between glasses and glass-ceramics reveals that rare earth ions have been enriched in crystals during heat treatment. ULU has been proposed to illuminate the effects of active ions, sensitizers, and hosts on the upconversion luminescence. Tm³⁺/Yb³⁺ co-doped LTZ glasses perform outstanding optical properties and intense photoluminescence, showing prospective applications in multifunctional devices.