Study on Upconversion and Thermal Properties of Tm³⁺/Yb³⁺ Co-Doped La₂O₃-Nb₂O₅-Ta₂O₅ Glasses

Abstract

The effect of Yb³⁺ ions on upconversion luminescence and thermal properties of Tm³⁺/Yb³⁺ co-doped La₂O₃-Nb₂O₅-Ta₂O₅ glasses has been studied. Glass transition temperature is around 740 °C, indicating high thermal stability. The effect of Yb³⁺ ions on the thermal stability is not obvious. Both the glass forming ability and the upconversion luminescence first increase and then decrease with the increase of Yb³⁺ ions. The glasses perform low glass forming ability with ΔT around 55 °C. Blue and red emissions centered around 477, 651, and 706 nm are obtained at the excitation of 976 nm laser. The upconversion luminescence mechanism is energy transfer from Yb³⁺ to Tm³⁺ mixed with two- and three- photon processes. The thermal kinetic Differential Thermal Analysis (DTA)-analysis indicates that the average activation energy first increases and then decreases with the increase of Yb³⁺ ions. This result can be introduced in order to improve upconversion luminescence of glasses by crystallization in the future. Tm³⁺/Yb³⁺ co-doped La₂O₃-Nb₂O₅-Ta₂O₅ glasses with good upconversion and thermal properties show promising applications in solid-state laser, optical temperature sensing.

1. Introduction

Upconversion luminescence from near infrared to visible wavelength in rare earth ions doped glasses has attracted much attention for the promising applications in solid-state visible laser [1], optical temperature sensing [2], optical fiber and amplifier [3], and sea water communication [4]. As one typical rare earth ion that can emit blue emissions by upconversion luminescence, Tm³⁺ ion has been paid close attention recently [5,6,7]. Nowadays, 980 nm continuous laser is usually used to be the exciting source, because this laser is commercial and cheap. However, Tm³⁺ ions cannot absorb the wavelength of 980 nm laser directly. Fortunately, Yb³⁺ ion, which is a typical sensitizer in the upconversion luminescence process, shows large absorption efficiency at the wavelength of 980 nm. Yb³⁺ ion can absorb the incident energy and transfer the energy to Tm³⁺ ion effectively. In this way, the Tm³⁺ ion can be excited from ground state to excited levels. So, Yb³⁺ is a good sensitizer for Tm³⁺. Tm³⁺/Yb³⁺ co-doped glasses have been researched fiercely for the blue upconversion luminescence and their promising applications.

To achieve good upconversion luminescence properties, glass matrix with low phonon energy, high thermal stability, good mechanical properties, and good dissolution for rare earth ions is preferred. Novel Nb₂O₅-based glasses perform low phonon energy (~735 cm⁻¹), high refractive index, good thermal properties, and high transparency, indicating that it can be a favorable candidate for strong upconversion luminescence from Tm³⁺/Yb³⁺ ion pair [8,9,10]. This new glass is prepared without adding any glass network formers. In this case, traditional crucible experiment technique cannot complete the preparation of this new glass. Therefore, containerless processing method is introduced. This method can constrain heterogeneous nucleation, obtain deep undercooling, and achieve fast solidification. This method is often used to fabricate bulk glasses with low glass forming ability and new meta-stable materials. In the previous study, the containerless processing method is successfully employed to fabricate special glass materials with high performance [11,12,13]. So it can be expected that Tm³⁺/Yb³⁺ co-doped La₂O₃-Nb₂O₅-Ta₂O₅ (LNT) glasses prepared by containerless processing would show good upconversion and thermal properties.

In this work, the aerodynamic levitation method was used to prepare new Tm³⁺/Yb³⁺ co-doped LNT bulk glasses. To obtain the glass sample with optimal properties, Tm³⁺/Yb³⁺ co-doped LNT glasses with different contents of Yb³⁺ ions were fabricated. As an important practical property, thermal stability was characterized by measuring the Differential Thermal Analysis (DTA) curves. The effect of Yb³⁺ ions on the upconversion luminescence of LNT glasses was studied by fluorescence spectra. Moreover, the emission mechanism was discussed based on the energy level of rare earth ions. DTA curves of glasses with different contents of Yb³⁺ were recorded by different heating rates. The thermal kinetic DTA-analysis was discussed in order to obtain the activation energy (E_a) by the Kissinger method. In this way, the crystallization process of glasses during the heat treatment can be revealed. This result can be used to be a reference to decide the heat treatment conditions for crystallization to optimize the upconversion luminescence.

2. Experimental

Aerodynamic levitation furnace (self-developed) with heating lasers was introduced to prepare Tm³⁺/Yb³⁺ co-doped LNT glasses with different concentrations of Yb³⁺ ions. The constitutes of Tm³⁺/Yb³⁺ co-doped LNT glasses were 0.65Nb₂O₅-(0.29 − y)La₂O₃-0.01Tm₂O₃-yYb₂O₃-0.05Ta₂O₅ (y = 0, 0.03, 0.05). High-purity Nb₂O₅ (4N), La₂O₃ (4N), Tm₂O₃ (4N), Yb₂O₃ (4N), and Ta₂O₅ (4N) powders were mixed thoroughly in ethanol in stoichiometry composition. The resulted powders were compressed and sintered to obtain dense rod-like samples. Then, the sample was levitated by O₂ in the aerodynamic levitation furnace and melted by heating laser. After stable levitation, the melt sample was quenched into a glass sphere by containerless solidification. Finally, glass spheres with a diameter of ~3 mm were successfully prepared. The preparation was described in detail in the previous study [8,14,15]. The resulted glass spheres were then polished by two sides to be 1.5 mm thickness wafers for later measurements.

To study the thermal stability and thermal kinetic analysis, DTA curves of glass spheres were recorded at heating rates of 5, 10, 15, and 20 °C/min in the air by thermal analysis equipment (NETZSCH STA 449C, Selb, Germany). Upconversion luminescence spectra of Tm³⁺/Yb³⁺ co-doped LNT glasses with different concentrations of Yb³⁺ ions were measured at the excitation of 976 nm continuous laser by a spectrofluorometer (Edinburgh instruments FLS980, Edinburgh, UK). The excitation power is set to be 220 mW. To study the upconversion luminescence process, the spectra were tested at different excitation powers of 976 nm laser. Together with energy level structure of rare earth ions, the emission mechanism was discussed.

3. Results and Discussion

To study the thermal properties of Tm³⁺/Yb³⁺ co-doped LNT glasses, glasses with different contents of Yb³⁺ ions were characterized by DTA. The results are presented in Figure 1. All of the DTA curves have a single glass transition and an exothermic peak ascribed to the crystallization. Based on the curves, we can determine the value of the glass transition temperature T_g, the onset temperature of crystallization T_o, and the peak temperature T_p. After analysis, the values of T_g and T_o can be evaluated to be around 740 and 800 °C, while typical upconversion fluoride materials ZBLAN glasses show only ~265 °C of T_g [16]. So, this glass has high thermal stability, which is favorable for applications. In Figure 1b, the effect of Yb³⁺ ions on T_g, T_o, T_p, and ΔT of glasses is performed. It can be concluded that Yb³⁺ ions show little effect on the values of T_g, T_o, and T_p. This is because La₂O₃ is substituted by Yb₂O₃ partially. Moreover, the melting point temperatures of La₂O₃ and Yb₂O₃ are similar. So, the change of T_g, T_o, and T_p is small with the increase of Yb₂O₃. Generally, the difference ΔT between T_o and T_g is often used to evaluate the glass forming ability. The increase of ΔT often results in the increase of the glass spherical size [17]. From Figure 1b, the value of ΔT first increases and then decreases with the increase of Yb³⁺ content. When y = 0.03, the glass forming ability is the largest. When y is from 0 to 0.03, Yb₂O₃ is added into the sample, which can increase the number of the composition type for the glass. This would be helpful to improve the viscosity of the melt and then increase the glass forming ability. However, when the content of Yb₂O₃ is further increased in Tm³⁺/Yb³⁺ co-doped LNT glasses, the glass forming ability is decreased. Totally, Tm³⁺/Yb³⁺ co-doped LNT glasses perform low glass forming ability. The values of ΔT are around 55 °C, which is relatively low and is difficult to form bulk glasses by conventional container methods. So, containerless processing is introduced in order to prepare this new glass. With the advantages in preparing glasses, aerodynamic levitation method is successfully used to fabricate bulk Tm³⁺/Yb³⁺ co-doped LNT glasses.

Upconversion luminescence spectra were recorded at room temperature at the excitation of 976 nm laser. The resulted emission spectra are presented in Figure 2. According to the results, it can be seen that the emission intensity of LNT glasses with y = 0 almost cannot be detected. So Tm³⁺ ions cannot absorb the incident pump power of 976 nm laser without the help of sensitizer Yb³⁺ ions. This also indicates that the glass samples are not polluted by other active rare earth ions, which can absorb 976 nm laser. In addition, the spectra of LNT glasses with different Yb³⁺ contents perform similar features, except the changes in intensities of the emission bands as Yb³⁺ doping concentration changes. From Figure 2, blue and red upconversion luminescences are obtained from Tm³⁺/Yb³⁺ co-doped LNT glasses. Three emission bands centered around 477, 651, and 706 nm are observed in the spectra, ascribing to the transitions of ¹G₄ → ³H₆, ¹G₄ → ³F₄, and ³F_2,3 → ³H₆ in Tm³⁺ ions. Blue emission is much stronger than the red emissions. As the concentration of Yb³⁺ ions increases, the intensity of all the emission bands first increases and then decreases. In the range of low concentration, Yb³⁺ ions can act an excellent sensitizer for Tm³⁺ ions to improve the upconversion luminescence. However, the emission would be quenched if increasing the Yb³⁺ concentration further. In this case, the quenching concentration of Yb³⁺ ions can be determined to be ~0.03 in Tm³⁺/Yb³⁺ co-doped LNT glasses.

The upconversion luminescence spectra of Tm³⁺/Yb³⁺ co-doped LNT glass with y = 0.03 at different pump powers were measured to study the emission process. According to the results, the relationship between the emission intensity I and the pump power P can be determined. It has been indicated that I is propositional to the n-th power of P in the form of I∞Pⁿ [18]. Here, n is the number of pump photons that are required to excite the active rare earth ions. Therefore, the relationship between emission intensity I and pump power P in the logarithm forms is linear. Moreover, the pump photon number n is the slope of the line. The dependency of emission intensity on pump power is plotted in the logarithm forms. The log-log plot is presented in Figure 3. The slopes of blue and red emission bands are 2.66, 3.20, and 1.51, respectively. So, it can be known that the dominant population approach of ¹G₄ excited state in Tm³⁺ ions is a three-photon process, while that of the ³F_2,3 excited state is a two-photon process. Tm³⁺ ions can not absorb directly the energy of incident 976 nm laser. Yb³⁺ ions play an important role as a sensitizer in the upconversion luminescence process. The pump power of 976 nm laser is absorbed by Yb³⁺ ions in the glasses. Then, Yb³⁺ ions transfer energy to Tm³⁺ ions by transiting from excited states to ground states. So, the main upconversion luminescence mechanism of Tm³⁺/Yb³⁺ co-doped LNT glasses at 976 nm excitation is Energy Transfer (ET) from Yb³⁺ ions to Tm³⁺ ions.

According to the energy level structure of rare earth ions, the upconversion luminescence mechanism is discussed in Figure 4. The population of the excited states in Tm³⁺ ions can be revealed. Yb³⁺ ions can efficiently absorb the incident photon and then be excited to ²F_5/2 level from the ²F_7/2 ground state. With the assistance of phonons, Tm³⁺ ions can be excited by energy transfer from Yb³⁺ ions. Tm³⁺ ions in ³H₆ ground state are excited to ³H₅ state by the neighboring excited Yb³⁺ ions. This ET process can be described as ³H₆(Tm³⁺) + ²F_5/2(Yb³⁺) → ³H₅(Tm³⁺) + ²F_7/2(Yb³⁺). Subsequently, Tm³⁺ ions can transit to ³F₄ state by nonradiative relaxation. The same Tm³⁺ ions in ³F₄ states can absorb another photon from neighboring excited Yb³⁺ ion by another ET to be excited to ³F_2,3 states. So, the red emission centered around 706 nm can be generated by the transition from ³F_2,3 to the ground state ³H₆ in Tm³⁺ ions. Moreover, Tm³⁺ ions in ³F_2,3 states can relax to ³H₄ state by nonradiative relaxation. ³H₄ state can be further excited to ¹G₄ state by ET process with the help of phonons in Tm³⁺ ions, which can be described as ³H₄ (Tm³⁺) + ²F_5/2(Yb³⁺) → ¹G₄(Tm³⁺) + ²F_7/2(Yb³⁺). Blue and red emissions can be emitted by the transitions of ¹G₄ → ³H₆ and ¹G₄ → ³F₄, respectively. So, the upconversion luminescence mechanism of Tm³⁺ ions in Tm³⁺/Yb³⁺ co-doped LNT glasses is ET mixed with two- and three-photon processes.

In our previous study, crystallization was often employed to optimize upconversion luminescence of containerless glasses [19,20]. Crystallization during the heat treatment is very important to improve the emission intensity. The effort would be effective if the crystallization process is controllable. So the crystallization kinetics of glasses should be studied to provide fine heat treatment methods. Furthermore, different compositions of glasses have different kinetic parameters, indicating different optical heat treatment conditions to get strong emissions. Crystallization kinetics would be helpful in determining the heat treatment conditions for different compositions of glasses. In this work, non-isothermal kinetic analyses were introduced. Tm³⁺/Yb³⁺ co-doped LNT glasses were heated at different rates of 5, 10, 15, and 20 °C/min to obtain DTA curves. Then, the Kissinger method was used based on these DTA curves to calculate the activation energy under non-isothermal conditions. The Kissinger equation is written as ln(β/T_p²) = ln[(AR)/E_a] − E_a/(RT_p) [21]. Here, E_a is the activation energy. A is the pre-exponential factor. β is the heating rate. The average activation energy of Tm³⁺/Yb³⁺ co-doped LNT glasses with different contents of Yb³⁺ ions can be obtained by the Kissinger method. The result is presented in Figure 5. With the increase of Yb³⁺ ions, the average activation energy first increases and then decreases. Activation energy can be used to evaluate the difficulty of crystallization in glasses. According to the result, the addition of Yb³⁺ ions increase the difficulty of crystallization in Tm³⁺/Yb³⁺ co-doped LNT glasses. However, with the increase of Yb³⁺ ions further, crystallization become easier. Therefore, different compositions of LNT glasses have different kinetic parameters, which need different heat treatment conditions to optimize upconversion luminescence. According to the kinetic result, it is helpful to get suitable crystallization conditions for different compositions of LNT glasses and increase the emission intensity.

4. Conclusions

Tm³⁺/Yb³⁺ co-doped LNT glasses with different Yb³⁺ contents were prepared by the aerodynamic levitation method. The upconversion luminescence and thermal properties of glasses were studied. DTA results indicate that the values of T_g, T_o, and T_p have little change with the increase of Yb³⁺ ions. Due to the similarity of La₂O₃ and Yb₂O₃ in melting temperature, Yb₂O₃ cannot change the thermal stability of glasses largely. Moreover, the values of T_g and T_o are evaluated to be around 740 and 800 °C, indicating high thermal stability of glasses. The glass forming ability first increases and then decreases with the increase of Yb³⁺ content. Tm³⁺/Yb³⁺ co-doped LNT glasses perform low glass forming ability with ΔT around 55 °C. Blue and red emissions centered around 477, 651, and 706 nm are observed in upconversion luminescence spectra at the excitation of 976 nm. The emission intensity first increases and then decreases as the Yb³⁺ concentration increases. According to the results of spectra excited at different powers, emissions around 477 and 651 nm are three-photon process, while two-photon process of the emission around 706 nm. The upconversion luminescence mechanism is discussed by ET from Yb³⁺ to Tm³⁺. The thermal kinetic DTA-analysis of Tm³⁺/Yb³⁺ co-doped LNT glasses is studied by DTA curves at different heating rates. The average activation energy, which is calculated by the Kissinger method, first increases and then decreases with the increase of Yb³⁺ ions. This result can be a reference for the heat treatment to improve upconversion luminescence of glasses in the future.