The DTA curve (Fig. 1) of Nd³⁺/Yb³⁺: LTZ glass has a single glass transition and a single crystallization peak. The results show that the glass transition temperature T_g and the onset temperature of crystallization T_o can be determined to be 799℃ and 880℃, respectively. This indicates that the LTZ glass performs much higher thermal stability than ZBLAN glass which is a fluoride system with high upconversion luminescence efficiency^{[ 13]}. So the upconversion luminescence devices made of LTZ materials can be used in technological applications more widely. Furthermore, the difference △ T between the onset temperature of crystallization and the glass transition temperature is an important parameter to characterize the glass forming ability^{[ 14]}. △ T of the present LTZ glass can be calculated to be ~81℃, which is much lower than that of most traditional oxide glasses, such as silicate and borate glass systems^{[ 15, 16]}. This indicates that LTZ glasses show a comparatively poor glass forming ability, leading to much difficulty to obtain bulk glasses by conventional melt-quenching methods. Thus, ALM is introduced for the fabrication of LTZ glasses. Moreover, the heat treatment temperatures can be determined based on the DTA curve.

Fig. 1

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	Fig. 1 DTA curve of Nd3+/Yb3+: LTZ

To enhance the upconversion luminescence, LTZ glasses were heated at different temperatures holding for different times with a heating rate of 10 ℃/min. The upconversion luminescence spectra of the resulting samples were recorded in Fig. 2. There are five emission bands in the curves corresponding to energy transitions among energy levels in Nd³⁺ ions. One blue emission band centered at 497 nm is observed, owing to the²G_9/2→⁴I_9/2transition^{[ 17]}. Two green emissions, at 523 and 545 nm, are attributed to the transitions from Nd³⁺:⁴G_9/2 and⁴G_7/2 levels to the ground state⁴I_9/2, respectively^{[ 18, 19]}. Again two emission bands are observed at 603 and 657 nm originated from the Nd³⁺:⁴G_7/2→⁴I_11/2 and⁴G_7/2→⁴I_13/2 transitions, respectively^{[ 18, 20]}. The²G_9/2,⁴G_9/2,⁴G_7/2 states of Nd³⁺ ions in LTZ samples are populated viaa two-photon process, which has been discussed by Pan, et al^{[ 21]}. Excited-State Absorption (ESA) and Energy Transfer Upconversion (ETU) are two possible mechanisms which are responsible for the observed emission bands in Nd³⁺/Yb³⁺: LTZ samples. Nd³⁺ ions can’t absorb the 980 nm photons directly due to the nonexistence of a matching energy level. But Yb³⁺ ions, with a huge absorption coefficient (at 980 nm), can act as efficient sensitizers to transfer the absorbed energy from Yb³⁺ ions to neighboring Nd³⁺ ions. Therefore, the main upconversion luminescence mechanism can be considered as ETU.

Fig. 2

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	Fig. 2 Upconversion emission spectra of Nd3+/Yb3+: LTZ samples heat-treated (a) at different temperatures and (b) at 880℃ for different holding times under 980 nm excitation with 313.9 mW power

The possible ETU mechanisms which can explain the above upconversion emissions are shown in Fig. 3. Yb³⁺ ions are efficiently excited to²F_5/2 states at the excitation of 980 nm laser. Subsequently, ETU processes from Yb³⁺ ions to Nd³⁺ ions populate²G_9/2 levels through cooperative sensitization and Phonon Assisted ETU (PAETU)^{[ 22, 23, 24]}. Then Nd³⁺ ions in²G_9/2 states decay to⁴G_9/2and⁴G_7/2 states via nonradiative relaxation. Moreover, the⁴G_7/2 levels can be populated by ETU among neighboring Nd³⁺ ions through the following channel: (⁴F_3/2,⁴F_3/2)→(⁴G_7/2,⁴I_13/2). Finally, upconversion emissions centered at 497, 523, 545, 603 and 657 nm are obtained by corresponding transitions as indicated in Fig. 2(a).

Fig. 3

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	Fig. 3 Schematic illustration of the upconversion luminescence mechanism of Nd3+/Yb3+: LTZ

In Fig. 2(a), the emission intensity increases and then decreases as the temperature increases. In Fig. 2(b), the intensity increases and then decreases at 880℃ as the holding time prolongs. It can be indicated that the optimal heat treatment is at 880℃ for 50 min. The glass-ceramic samples heat-treated at 880℃ for 50 min perform the best luminescence property with intensity (at 545 nm) as 11 times high as that of the precursor glasses. The XRD result (Fig. 4) shows the precipitation of La₄Ti₉O₂₄ crystals in the LTZ glass matrix.

Fig. 4

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	Fig. 4 XRD pattern of the Nd3+/Yb3+: LTZ heat-treated at 880℃ for 50 min

In order to discuss the enhancement of upconversion luminescence of glass-ceramic samples compared with the precursor glasses, Nd³⁺/Yb³⁺: LTZ samples were heated at 880℃ for 40, 50 and 60 min. TEM image and the selected-area electron diffraction pattern in Fig. 5(a) show that the crystals in the glass-ceramics fabricated at 880℃ for 40 min grown incompletely and are rare in the glass matrix. According to Fig. 5(b), columnar crystals with ~100 nm diameter and various lengths are obtained after heat treatment at 880℃ for 50 min. The sharp spots of the selected-area electron diffraction pattern indicate the single-crystalline nature of the crystals with a good single crystallinity. The crystals in Fig. 5(c) are obtained after heat treatment at 880℃ for 60 min. In Fig. 5(a,c), two kinds of crystals are not the optimal environment for the rare earth ions to gain high upconversion luminescence efficiency. Therefore, heat treatment at 880℃ for 50 min gives the best result that the columnar crystals can obviously improve upconversion luminescence properties, which is in good agreement with the results of upconversion emission spectra (Fig. 2).

Fig. 5

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	Fig. 5 TEM images and SAED patterns of Nd3+/Yb3+: LTZ samples heat-treated at 880℃ for (a) 40 min; (b) 50 min; (c) 60 min

The crystals in the samples heated at 880℃ for 50 min show good single crystallinity. So the local environment of active rare earth ions has dense lattice fields and strong partitions caused by the crystals. Then the mutual nonradiative relaxation rate among Nd³⁺ ions can be intensively decreased, which results in increased upconversion emission intensity. The distributions of Nd³⁺ ions in the glass-ceramic samples have been detected by EDS. The patterns (Fig. 6) show the enrichment of active Nd³⁺ ions in crystals. It has been reported that rare earth ions can concentrate mainly in the crystals of the glass-ceramics^{[ 25, 26]}. More Nd³⁺ ions can locate in the crystals which can efficiently reduce the energy losses caused by the mutual interactions among rare earth ions. In this way, the upconversion luminescence can be improved obviously after heat treatment. Hence, crystals with a good single crystallinity are helpful for active rare earth ions to achieve high upconversion luminescence efficiency.

Fig. 6

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	Fig. 6 EDS pattern of (a) crystal and (b) glass matrix in the sample heat-treated at 880℃ for 50 min