1. Introduction

Rare earth (RE) ions doped upconversion materials, which possess the ability to convert a longer wavelength radiation to a shorter wavelength radiation via multi-photon process, have been extensively explored in virtue of their potential applications in optical devices, color displays, bio-imaging, etc [1–3]. As is well known, RE fluoride nanocrystals (such as NaYF₄) are considered as the most efficient host matrices for upconversion due to their low phonon energy that can minimize the non-radiative relaxation probability and then leads to efficient upconversion luminescence [1]. In recent years, RE ions doped Lu-based fluoride nanocrystals, for instance, BaLuF₅, NaLuF₄ and LiLuF₄, have aroused increasing attention due to their unique upconversion behavior and more efficient upconversion luminescence than that of Y-based counterparts [4–6].

As an important Lu-based fluoride, KLu₂F₇, with phase transformation from high-temperature monoclinic phase to low-temperature orthorhombic phase at 730 °C, has received scarce attention due to the difficulty to elaborate single phase KLu₂F₇ [7]. More recently, orthorhombic KLu₂F₇ single crystals were successfully fabricated by micro-pulling-down method and the scintillation properties of Ce³⁺ doped KLu₂F₇ single crystal were also reported [8]. Similar to other Lu-based fluorides, KLu₂F₇ may be excellent upconversion host matrix. However, there is no report on the study of upconversion properties of KLu₂F₇-based materials.

On the other hand, RE fluorides based transparent oxyfluoride glass-ceramics, novel nanostructured composites for RE ions doping, have been proven as promising luminescent materials [9–19]. They combine the best characters from both fluoride nanocrystals and oxide glasses, such as low phonon energy environment for RE ions dopant, isotropy, high chemical and mechanical stability, and ability of being processed into various shapes [15,16]. Generally, such materials are generated through controlled crystallization from precursor glasses by heat-treatment. After crystallization, RE ions will abound in the precipitated fluoride nanocrystals preferentially [9–12] and show advantageous luminescent behavior such as high quantum efficiency, long luminescence lifetime and excellent upconversion properties owing to the reduced non-radiative relaxation and enhanced energy transfer (ET) efficiency arising from shorter RE³⁺-RE³⁺ distance [13,14].

In this context, we successfully fabricate Er³⁺-doped transparent oxyfluoride glass-ceramics containing novel orthorhombic KLu₂F₇ nanocrystals and systematically investigate their upconversion behavior. Excited by near-infrared (NIR) light, the upconversion intensity in glass-ceramics is tremendously increased compared to that in precursor glasses. The different upconversion mechanisms before and after crystallization were discussed in detail. Our research may enrich the understanding of upconversion mechanisms of Er³⁺ in glass-ceramics.

2. Experimental

Glass samples with nominal composition of 54SiO₂-10K₂CO₃-15Al₂O₃-15KF·2H₂O-5LuF₃-1ErF₃ (in mol %) were prepared by melt-quenching method in air atmosphere. SiO₂, Al₂O₃, K₂CO₃, KF·2H₂O (A.R., all from Sinopharm Chemical Reagent Co., Ltd.) and high purity LuF₃, ErF₃ (99.99%, from AnSheng Inorganic Materials Co., Ltd.) were used as starting materials. The well ground stoichiometric chemicals were put into a covered alumina crucible and melted at 1550 °C for 1 h in air atmosphere. The melt was poured onto a stainless-steel plate and then pressed by another plate to form precursor glasses (labeled as PG). Subsequently, PG glasses were heat-treated for 2 h at 700 °C to form transparent glass-ceramics, which was labeled as GC700. All samples were polished optically for further characterization.

X-ray diffraction (XRD) patterns were performed on a Philips X’Pert PRO SUPER X-ray diffraction apparatus with CuKa radiation. Transmittance spectra were measured on a U-3900 ultraviolet-Visible (UV-VIS) spectrophotometer (Hitachi). The microstructure of glass-ceramics was analyzed by a JEM-2010 transmission electron microscopy (TEM) (JOEL Ltd.). Upconversion spectra were measured with a FS920 spectrofluorometer (Edinburgh Instruments) with a 980 nm laser as the excitation source. Decay curves measurements were performed on a SBP 500 monochromater coupled to a R928P photomultiplier with Tektronix TDS5052 oscilloscope. All the measurements were carried out at room temperature.

3. Results and discussion

Figure 1(a) depicts the XRD patterns of PG and GC700 samples. The standard data of JCPDS card No. 27-0459 for orthorhombic KYb₂F₇ with space group of Pna21 (33) is used as a reference because the unavailability of standard XRD patterns of orthorhombic KLu₂F₇ and the isostructural character of KLu₂F₇ to KYb₂F₇. The cell parameters of KLu₂F₇ (a = 11.702 Å, b = 13.109 Å and c = 7.711 Å) [20] are slightly smaller than those of KYb₂F₇ (a = 11.710 Å, b = 13.240 Å and c = 7.739 Å). As can be seen in Fig. 1(a), PG is amorphous while GC700 shows intense diffraction peaks. Obviously, the XRD patterns of GC700 are almost identical to that of orthorhombic KYb₂F₇ and shift slightly to larger Bragg angle. Accordingly, the observed phenomenon can be taken as a solid evidence of the formation of orthorhombic KLu₂F₇ nanocrystals in GC700. The mean crystalline size D of the precipitated KLu₂F₇ nanocrystals in GC700 estimated by Scherer’s equation [15] is about 22 nm for GC700.

 

Fig. 1 (a) XRD patterns of PG and GC700, and the reference data of JCPDS card No. 27-0459 for orthorhombic KYb₂F₇, (b) Transmittance spectra of PG and GC700, (c) TEM image of GC700, the inset is SAED patterns, (d) HRTEM image of GC700.

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The transmittance spectra of PG and GC700 in UV-VIS region are given in Fig. 1(b). Clearly, a range of absorption peaks located at 376, 488, 519 and 652 nm are observed, which are assigned to the transitions from ⁴I_5/2 ground state to ⁴G_11/2, ⁴F_7/2, ²H_11/2 and ⁴F_9/2 excited states of Er³⁺, respectively [13]. Besides, due to the small size of precipitated KLu₂F₇ nanocrystals and their narrow size distribution, the light scattering in glass-ceramics is minimized, and thus GC700 still maintains a fairly good transparency in VIS region [16].

TEM and high-resolution TEM (HRTEM) images of GC700 are recorded to investigate the microstructures of glass-ceramics. TEM bright-field micrograph given in Fig. 1(c) clearly demonstrates the homogeneously distribution of spherical nanocrystals with a narrow size distribution around 24 nm in diameter among the residual glass phase. The selected area electron diffraction (SAED) patterns [inset of Fig. 1(c)] manifest the polycrystalline diffraction feature of the glass-ceramics. HRTEM image in Fig. 1(d) clearly displays the well-defined lattice structure of precipitated fluoride nanocrystals and the associated interplanar spacing d value is estimated to be 3.184 Å, which can be indexed to (231) crystal plane of orthorhombic KLu₂F₇ (d₍₂₃₁₎ = 3.188 Å).

Figure 2(a) shows the upconversion spectra of PG and GC700 (λ_ex = 980 nm). Characteristic emission bands of Er³⁺ at 525 nm (²H_11/2→⁴I_15/2), 545 nm (⁴S_3/2→⁴I_15/2) and 660 nm (⁴F_9/2→⁴I_15/2) [21] can be observed in all samples. For PG sample, the relatively weak upconversion emission can be easily detected by naked eyes and spectrofluorometer. It is noticeable that the upconversion signal of GC700 is tremendously enhanced after crystallization. Specifically, the upconversion intensity of GC700 increases by 1340 and 680 times for green and red emission compared to that of PG, respectively. Moreover, pronounced Stark splitting in the emission bands of GC700 are observed as well. All above phenomena suggests the successful partition of Er³⁺ ions into the precipitated nanocrystals with high symmetrical environment and low phonon energy after crystallization.

 

Fig. 2 (a) Upconversion spectra of PG and GC700 (λ_ex = 980 nm), Dependence of upconversion intensity on pump power for (b) PG and (c) GC700.

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The dependence of upconversion emission intensities (I) on pump power (P) is measured to study the excitation mechanisms accounting for upconversion luminescence. It is well known that in unsaturated condition, I is proportional to the nth power of P and follows formula [21,22],

Summarized briefly, the strongly enhanced upconversion intensity and the obvious Stark splitting in GC700 all indicate the preferential incorporation of Er³⁺ ions into KLu₂F₇ nanocrystals after crystallization. As a result, the upconversion mechanisms in PG and GC700 will be different from each other. Figure 3(a) illustrates the energy level diagram of Er³⁺ and proposed upconversion mechanisms accounting for green and red emissions.

 

Fig. 3 (a) Energy level diagrams of Er³⁺ and possible upconversion mechanisms, (b) Luminescence decay curves for ⁴S_3/2→⁴I_15/2 transition (545 nm) of Er³⁺ in PG and GC700 (λ_ex = 980 nm).

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Excited by 980 nm laser, Er³⁺ ions at ⁴I_15/2 ground state are firstly pumped to ⁴I_11/2 intermediate excited state by ground state absorption (GSA). Subsequently, these ions at ⁴I_11/2 level can be populated to higher emitting levels through two most common excitation processes: (1) excited state absorption (ESA), (2) energy transfer upconversion (ETU) [21]. ESA process is a single ion process. While ET process involves two ions and therefore the rate depends strongly on the distance between the active ions [22].

For PG sample, due to the low concentration and homogeneous dispersion of Er³⁺ ions in glassy phase, the distance between Er³⁺ ions is quite long. Thus, ETU can be ruled out and ESA process is responsible for the population of ²H_11/2, ⁴S_3/2 and ⁴F_9/2 emitting levels. For green emissions from ²H_11/2 and ⁴S_3/2 emitting levels, Er³⁺ ions at ⁴I_11/2 level are excited to ⁴F_7/2 level by ESA1 process: ⁴I_11/2 + hv → ⁴F_7/2, where hv is the absorbed 980nm photon energy. After that, these ions at ⁴F_7/2 level non-radiatively relax to ²H_11/2 and ⁴S_3/2 emitting levels and further radiatively relax down to ⁴I_15/2 ground state to produce emissions at 525 and 545 nm, respectively [21]. For red emission (⁴F_9/2→⁴I_15/2), there are two possible ways to feed the ⁴F_9/2 emitting level. One is multi-phonon non-radiative relaxation from upper ⁴S_3/2 level. The other is ESA2 process: ⁴I_13/2 + hv → ⁴F_9/2. The ⁴I_13/2 intermediate excited state may be populated through a multi-phonon non-radiative process from ⁴I_11/2 level and radiative process from upper emitting levels, such as ⁴S_3/2 level. Finally, Er³⁺ ions at ⁴F_9/2 level radiatively relax to ⁴I_15/2 ground state to give red emission [21].

For GC700, owing to the incorporation of Er³⁺ ions into the precipitated fluoride nanocrystals, the Er³⁺-Er³⁺ distance is greatly shortened. In this case, the dominant upconversion mechanism is ETU, which is a more efficient way to promote those Er³⁺ ions at ⁴I_11/2 level to upper emitting levels to produce much stronger upconversion luminescence. The ²H_11/2 and ⁴S_3/2 emitting levels can be pumped by ETU1 process [21]. An excited Er³⁺ ion at ⁴I_11/2 level relaxes non-radiatively to ⁴I_15/2 level and transfers its excitation energy to a neighboring Er³⁺ ion at the same level, promoting the later to ⁴F_7/2 level: ⁴I_11/2 + ⁴I_11/2 → ⁴I_15/2 + ⁴F_7/2. While the ⁴F_9/2 red emitting level can be populated by ETU2 process: ⁴I_11/2 + ⁴I_13/2 → ⁴I_15/2 + ⁴F_9/2 [21]. What’s more, the low phonon energy of fluoride nanocrystals can effectively reduce the non-radiatively relaxation of Er³⁺ ions and further enhance the upconverson efficiency. Accordingly, efficient ETU processes and low non-radiative relaxation rate, deriving from the incorporation of Er³⁺ ions into the KLu₂F₇ fluoride nanocrystals, contribute to the extremely strong upconversion luminescence in GC700.

To further prove the partition of Er³⁺ ions into the precipitated fluoride nanocrystals, the measurement of lifetimes for upconversion emissions of Er³⁺ are performed. Figure 3(b) shows luminescence decay curves for ⁴S_3/2→⁴I_15/2 (545 nm) transition of Er³⁺ in PG and GC700 excited by a pulsed 980 nm laser. The average lifetime ($\overline{\tau }$) can be calculated by [23],

4. Conclusion

Novel KLu₂F₇:Er³⁺ transparent glass-ceramics with high transparency have been successfully fabricated by melt-quenching technique with further heat-treatment. After crystallization, tremendously enhanced upconversion emissions with obvious Stark splitting, longer luminescence lifetime and increased rise time in decay curve are observed, suggesting the incorporation of Er³⁺ ions into the precipitated fluoride nanocrystals. For up-conversion mechanisms, ESA processes are dominant in PG sample and ETU processes are dominant in GC700 sample. Our investigation demonstrates that KLu₂F₇-based transparent glass-ceramics are excellent matrices for upconversion and may find applications in upconversion detectors, optical fibers and solid-state lasers.