Selective doping of Ni2+ in highly transparent glass-ceramics containing nano-spinels ZnGa2O4 and Zn1+xGa2−2xGexO4 for broadband near-infrared fiber amplifiers

Selective doping of Ni2+ in octahedral sites provided by nanocrystals embedded in glass-ceramics (GCs) is crucial to the enhancement of broadband near-infrared (NIR) emission. In this work, a NIR emission with a full-width-at-half-maximum (FWHM) of 288 nm is first reported from ZnGa2O4: Ni2+ nano-spinels embedded GCs with excellent transparency. A comparison is made of the NIR luminescence properties of Ni2+ doped GCs containing ZnGa2O4, germanium-substituted ZnGa2O4 nano-spinels (Zn1+xGa2−2xGexO4), and Zn2GeO4/Li2Ge4O9 composite nanocrystals that are free of Ga3+. The results show that ZnGa2O4: Ni2+ GCs exhibit a significantly enhanced NIR emission. The incorporation of the nucleating agent TiO2 is favored in terms of the increased luminescence intensity and prolonged lifetime. The possible causes for the enhancement effect are identified from the crystal structure/defects viewpoint. The newly developed GCs incorporate good reproducibility to allow for a tolerance of thermal treatment temperature and hence hold great potential of fiberization via the recently proposed “melt-in-tube” method. They can be considered as promising candidates for broadband fiber amplifiers.

transparent GCs and even glass-ceramic (GC) optical fibers have been produced 16 . Promising results such as the ligand-field driven wavelength tunable and broadband NIR emission of Ni 2+ have also been observed 5,17 .
The pursuit of Ni 2+ doped GCs is being driven continuously by newly invented crystals with excellent luminescence properties, for example, the germanium-substituted ZnGa 2 O 4 spinel of the general formula Zn 1+x Ga 2−2x Ge x O 4 (0 ≤ x ≤ 1) and this has attracted significant attention due to the unprecedented persistent luminescence observed in Zn 1+x Ga 2−2x Ge x O 4 :Cr 3+ 18 . Such nanocrystals can simultaneously provide tetrahedral (occupied by 4 Zn 2+ and 4 Ge 4+ ) and octahedral (occupied by 6 Ga 3+ ) sites for Mn 2+ , Co 2+ , Ni 2+ , Cr 3+ and Mn 4+ etc., and thus can be used as a multi-functional platform for diverse applications in lighting, display, telecommunication and bio-imaging etc. [19][20][21] . Additionally, transparent GCs containing Zn 1+x Ga 2−2x Ge x O 4 nanocrystals have been recently fabricated 21,22 . Selective doping of Ni 2+ in Zn 1+x Ga 2−2x Ge x O 4 nanocrystals embedded in GCs has been reported; however, the NIR luminescence properties were not clearly identified in this case 22 . To date the authors of this article are not aware of any study relating to the luminescence properties of ZnGa 2 O 4 : Ni 2+ GCs, although GCs containing ZnGa 2 O 4 : Cr 3+ nanocrystals, showing persistent luminescence and temperature sensing properties, have recently been extensively studied 12,23 .
In the work reported in this article a detailed study has been undertaken and comparison made of the NIR luminescence properties of Ni 2+ doped GCs containing ZnGa 2 O 4 and germanium-substituted ZnGa 2 O 4 (Zn 1+x Ga 2−2x Ge x O 4 ) nano-spinels. To underline the important role of 6 Ga 3+ , other Ni 2+ doped GCs containing Zn 2 GeO 4 /Li 2 Ge 4 O 9 composite nanocrystals that are free of Ga 3+ were also prepared for comparison. The synthesized ZnGa 2 O 4 : Ni 2+ GCs are highly reproducible which allows for a tolerance of thermal treatment temperature, and thus are perfectly matched for the recently proposed "melt-in-tube" method. For functional GC fibers cannot be obtained using the conventional "rod-in-tube" method 16,[24][25][26] , the 'melt-in-tube method has recently facilitated fabrication of GC fibers doped with Ni 2+ 16 , Bi 24 , Cr 3+ 25 , or quantum dots 26 which have exhibited excellent optical quality. The study described in this article is expected not only provide a candidate fiber amplifier material, but also advance understanding of the correlation between the structure of the nanocrystals and NIR luminescence of Ni 2+ , and thus will provide useful guidance for designing novel Ni 2+ doped GCs with enhanced luminescence properties such as ultra-broadband tunable NIR emission 27 . Moreover, the present work may also advance the understanding of the mechanism underlying the persistence luminescence of Cr 3+ doped spinels which is currently still open to question 18 .

Experiments
Three different types of the nominal composition (in mol. %) of Ni 2+ -doped glasses and GCs were prepared using high purity (4N) raw materials of SiO 2 , GeO 2 , Ga 2 O 3 , ZnO, Na 2 CO 3 , K 2 CO 3 , Li 2 O, ZrO 2 , TiO 2 and NiO. Transmission spectra were measured using a Perkin-Elmer Lambda 950 UV-VIS spectrophotometer in the spectral range of 200-1800 nm. Refractive indices were measured using an Abbe refractometer AR2008 (KRÜSS, Germany). Photoluminescence (PL) spectra were recorded using a Fluorolog-3-P UV-vis-NIR fluorescence spectrophotometer (JobinYvon, Longjumeau, French). The decay curves were measured using a FLS920 Fluorescence spectrometer (Edinburgh Instruments) from room temperature (300 K) down to liquid helium temperature (10 K). The samples used for the PL measurement were plane-parallel well polished plates with the identical dimension of ~10 × 10 mm 2 and thickness of 1 mm. The fluorescence was collected in the direction perpendicular to the direction of the pump beam, and the pump light was focused (to a spot of diameter ~4 mm) using a lens and incident at a 45° angle to the normal of the front surface of the sample. In the experiment, both the power of the pump light and the configuration of the light path were kept the same. Because only very thin samples were used for the measurement, reabsorption is not significant and any effects due to this can be omitted in the present study, in accordance with the work of Loiko 30 .
X-ray diffraction (XRD) patterns of all the samples were recorded under the same measurement conditions using an X-ray diffractometer (D/MAX 2550VB/PC, Rigaku Corproation, Japan) with Cu-Kα irradiation. The microstructure of the crystallized glasses was studied using a JEM-2100 high-resolution transmission electron microscope (HRTEM). Raman spectra were measured by RenishawInvia Raman microscope (Renishaw, Gloucestershire, UK) with an excitation wavelength of 515 nm.

Results and Discussion
From the XRD patterns of the crystallized glasses, the precipitation of ZnGa 2 O 4 ( Fig. 1(a)), and Zn 1+x Ga 2−2x Ge x O 4 nano-spinels ( Fig. S1(a), supporting information), as well as Zn 2 GeO 4 /Li 2 Ge 4 O 9 composite phases (Fig. S2 in the supporting information) can be discerned in accordance to the literature 21,28,29 . The formation of the target nanocrystals were also confirmed from the Raman spectra where the crystallized glasses show sharp scattering peaks well match those of the standard polycrystals ( Fig. 1(b)). According to the work of Zhuang et al. 21 , it is very difficult to determine unambiguously the exact Zn 1+x Ga 2−2x Ge x O 4 phase in GCs, owing to the undistinguishable XRD patterns between the two end-members, ZnGa 2 O 4 (x = 0) and Zn 2 GeO 4 (x = 1). By comparing the Raman spectra of the crystallized glasses with the standard Zn 1+x Ga 2−2x Ge x O 4 polycrystals synthesized in our lab by solid-state reaction (for more detail, refer to our previous work 19 ), we provide the first direct evidence for the formation of Zn 1+x Ga 2−2x Ge x O 4 with x ≥ 0.4 in GCs ( Fig. S1(b), supporting information).
The crystallinity (volume fraction of the crystalline phase) of the GCs can be estimated by the ratio of the area under the indexed diffraction peaks to that under the whole XRD patterns 14 . For ZnGa 2 O 4 and Zn 1+x Ga 2−2x Ge x O 4 GCs, the crystallinities are approximately 37% and 32%, respectively, which are close in value to each other. The total molar concentration of ZnO and Ga 2 O 3 is only 36 mol. % for the ZnGa 2 O 4 GCs, which is less than the calculated crystallinity. The reason for the discrepancy is not clear and the validity of this result has yet to be confirmed. Here, it should be noted that the presence of nucleating agent such as TiO 2 in gallium-containing GCs favors the substitution of 6 Ni 2+ (ionic radius: 0.69 Å) for 6 Ga 3+ (ionic radius: 0.62 Å) via the following substitutional mechanism: Ti 4+ + Ni 2+ → 2Ga 3+ , where Ti 4+ acts as charge compensator 31,32 . The incorporation of Ti 4+ in the precipitated nanospinels was confirmed by the TEM-EDS analysis on the selected crystallization area in the ZGO-0.15GC sample (Fig. S3, supporting information). It is possible that a certain degree of inversion may occur in realistic spinels during crystallization, i.e., a fraction of the Ni 2+ can occupy non-luminescent tetrahedral sites as found in NiAl 2 O 3 crystals 33,34 , and hence the selective doping of Ni 2+ in octahedral sites is highly desirable for enhanced NIR luminescence 33 .
The morphology, distribution and particle sizes of nanocrystals were determined from the HRTEM measurements. The precipitated nanoparticles, approximately 15 nm in diameter, are distributed uniformly in all the GCs (Fig. 1(c) and (d)). The ultra-fine particle size allows these materials to be polished as they are in the glass state and then crystallized without any significant degradation of the surface quality (shown photographically in Fig. 2(a) and Figs S4 and S5, supporting information). The crystallization process was highly reproducible as The coordination states of Ni 2+ can be approximately inferred from the color of the glasses and GCs, for example, blue, brown and yellow-green in the case of 4 Ni 2+ , 5 Ni 2+ , and 6 Ni 2+ coordination, respectively 8 . The as-made ZGO ( Fig. 2(a)) and ZGGO glasses (Fig. S4, supporting information) are light brown in appearance, suggesting 5 Ni 2+ and 4 Ni 2+ coordination states, whereas the color of the crystallized glasses becomes light green and blue, indicative of 6 Ni 2+ . Since 6 Ni 2+ possesses a larger crystal field stabilization energy (CFSE) value than that of 5 Ni 2+ , the unstable 5 Ni 2+ in glasses tends to transform into 6 Ni 2+ during crystallization of the spinel phases. The absorption related to 6 Ni 2+ (e.g., around 1160 nm due to the 3 A 2 ( 3 F) → 3 T 2 ( 3 F) transition) in GCs increases with the concentration of NiO, indicative of an efficient partition of Ni 2+ in ZnGa 2 O 4 nanocrystals, e.g. more than 90% of Ni 2+ can be successfully embedded in gallium-containing GCs 7 , whereas it is well known that substitutional doping a large fraction of TM ions into semiconductor nanocrystals is extremely difficult because of the intrinsic self-purification mechanism 35 . The absorption bands can be well fitted to the Tanabe-Sugano (TS) diagram for d 8 ions (Fig. 2(d)), with the values of Racah parameter (B) and crystal field strength (Dq) equal to 767 cm −1 and 917 cm −1 , respectively.
An inspection of the transmission spectra of the Zn 2 GeO 4 /Li 2 Ge 4 O 9 GCs (Fig. S5, supporting information) also indicates the presence of 6 Ni 2+ , which is possible via the substitution of 6 Ni 2+ for 6 Ge 4+ in Li 2 Ge 4 O 9 nanocrystals. Meanwhile, since both the valence and ionic radius of 4 Zn 2+ (0.60 Å) matches those of 4 Ni 2+ (ionic radius: 0.55 Å), the substitution of 4 Ni 2+ for 4 Zn 2+ in ZnGa 2 O 4 and Zn 2 GeO 4 nanocrystals may also occur, similar to the embedding of Ni 2+ in Zn 2 SiO 4 crystals 36 . For a detailed analysis and discussion of the absorption spectra, refer to Supporting Information (Fig. S6). The fabricated GCs with transmission larger than 80% demonstrate great potential to be drawn into fibers for use as fiber lasers and amplifiers.
The use of the nucleant TiO 2 is very important; it drastically increases both the emission intensity (Fig. S7, supporting information) and lifetime (Fig. S8, supporting information) of the GCs as compared to those free of TiO 2 but otherwise the GCs containing TiO 2 were thermally treated under identical conditions. The enhancement effect can be understood based on the substitution mechanism by which Ni 2+ substitutes for Ga 3+ favorably as mentioned above. An intense broadband NIR emission (from 1100 to 1700 nm) was recorded from ZnGa 2 O 4 : Ni 2+ and Zn 1+x Ga 2−2x Ge x O 4 : Ni 2+ GCs, but was very weak from Zn 2 GeO 4 /Li 2 Ge 4 O 9 : Ni 2+ GCs. Both the emission intensity ( Fig. 3(a)) and lifetime (Fig. 3(b)) (defined as the time taken for the emission intensity to decay to 1/e of its initial value) increase with NiO for the Zn 1+x Ga 2−2x Ge x O 4 : Ni 2+ GCs, in contrast to ZnGa 2 O 4 : Ni 2+ GCs where concentration quenching has already set in at the lowest doping level (~0.15 mol. %). However, in the cases with a fixed NiO, the ZnGa 2 O 4 : Ni 2+ GCs exhibit stronger NIR emission and longer lifetime than those of Zn 1+x Ga 2−2x Ge x O 4 : Ni 2+ GCs, e.g., a five-fold increase in the intensity and a two-fold increase in the lifetime when NiO was 0.15 mol. %. For the Zn 1+x Ga 2−2x Ge x O 4 : Ni 2+ GCs, the emission intensity appears not to saturate at 0.5 mol. %. GCs have also been fabricated doped with 0.7 mol. % NiO. However, the samples suffer from significant devitrification due to NiO-assisted growth of large sized crystals commonly found in glasses heavily doped with NiO 7 . As a result, the 0.7 mol. % doped GCs become opaque and this restricts their use for optical applications, and hence does not warrant further study.
The differences in the crystal structures between the ZGO and ZGGO GCs may account for the contrast in the luminescent property. The Zn 1+x Ga 2−2x Ge x O 4 crystals were assumed to be a solid solution between the normal ZnGa 2 O 4 and inverted Zn 2 GeO 4 spinel structures 19,37 . Pure Zn 1+x Ga 2−2x Ge x O 4 spinels can be synthesized for x ranging from 0 to 0.5 19 . Our recent study of Mn doped Zn 1+x Ga 2−2x Ge x O 4 phosphors shows that the substitution of Ge 4+ for octahedrally coordinated 6 Ga 3+ helps to separate Mn 4+ which also substitutes for 6 Ga 3+ , thus resulting in an enhanced emission of Mn 4+ 38 . It is assumed that a similar separating effect exists for Ni 2+ as well, i.e., 6 Ni 2+ ions are well separated in the ZGGO GCs, and thus the concentration quenching is postponed. As more Ni 2+ ions diffuse from the surface to the inside of the nanocrystals and/or are shielded by the nanocrystals from the outside high-phonon energy environment, the non-radiative relaxation rate is reduced, and the lifetime increases accordingly. However, in the case of ZnGa 2 O 4 GCs, Ni 2+ substituting for 6 Ga 3+ is not well separated because there is no "separating agent" akin to Ni 2+ doped Ga 2 O 3 GCs where concentration quenching already starts at 0.10 mol. % low content of Ni 2+ 7 , and this accounts for the observed decreasing lifetime in the ZGO GCs.
Previous studies have shown that the Cr 3+ -doped and germanium-substituted compounds (Zn 1+x Ga 2−2x Ge x O 4 , x ≤ 0.5) exhibit much brighter and longer persistence luminescence than pure Cr 3+ -doped ZnGa 2 O 4 spinels 37 . The 2Ga 3+ → Ge 4+ + Zn 2+ substitution induces an inversion increase in the spinel structure, that is, an increased amount of Ga 3+ now occupies the tetrahedral 4 Zn 2+ sites, forming the so-called anti-site defects Zn . According to ref. 37, the enhancement of Cr 3+ emission relies on the formation of anti-site defects, however, the presence of such defects definitely has an adverse effect on the luminescence of Ni 2+ because of the reduced proportion of 6 Ga 3+ sites. On the other hand, it is possible that the substitution of Ge 4+ for 6 Ga 3+ may generate octahedrally coordinated 6 Ge 4+ , which may in turn be substituted by Ni 2+ . However, considering the fact that only weak NIR emission was observed from the Zn 2 GeO 4 /Li 2 Ge 4 O 9 : Ni 2+ GCs, and the large mismatch in valence and ionic radii between 6 Ni 2+ and 6 Ge 4+ (ionic radius: 0.53 Å), the substitution of 6 Ni 2+ for 6 Ge 4+ is severely limited, akin to the partition of Ni 2+ in K 2 SiF 6 nanocrystals embedded GCs 17 . Moreover, no NIR emission related to the tetrahedrally coordinated 4 Ni 2+ , e.g., Ni 2+ doped Zn 2 SiO 4 or Zn 2 GeO 4 crystals, has been recorded even at cryogenic temperatures 36 . All these effects account for the observed weaker emission intensity of the ZGGO GCs than that of the ZGO GCs.
The ZnGa 2 O 4 : Ni 2+ GCs, were selected for further study of internal fluorescence quantum efficiency (η) due to the stronger NIR emission and longer lifetime of Ni 2+ . Figure 4 shows the NIR emission lifetime of Ni 2+ as a function of temperature from room temperature (300 K) down to liquid helium temperature (10 K). The sudden drop in lifetime at around 100 K indicates the occurrence of phonon-assisted non-radiative relaxation 39 . As shown in the inset, the decay curve has a strong non-exponential characteristic, implying multiple site effects of Ni 2+ and non-radiative multipolar interactions among Ni 2+ . The value of η can be calculated as η = τ 300K /τ 0K , where τ 300K (~0.17 ms) and τ 0K (~0.62 ms, obtained by linear extrapolation to 0 K) are the lifetimes at the room and absolute zero temperatures, respectively. It is about 25% for the ZnGa 2 O 4 : Ni 2+ GCs, which is less than that of ZnAl 2 O 4 : Ni 2+ (~55%) 40 , LiGa 5 O 8 : Ni 2+ (~60%) 12 and BaAl 2 Ti 6 O 16 : Ni 2+ (~65%) 39 GCs. However, it is comparable to that of Ga 2 S 3 : Cr 4+ chalcogenide GCs (~25%) 4 and even larger than that of pure Ni 2+ doped ZnGa 2 O 4 crystals (~18%) 41 .  39 GCs, and much larger than Ga 2 S 3 : Cr 4+ chalcogenide (ChG) GCs (~0.62 × 10 −24 cm 2 ·s) which are known for the difficulty of preparation 4 . Light amplification at similar O-band wavelengths can be also achieved for Pr 3+ or Dy 3+ doped fluoride and ChG glasses of very low phonon energy. In comparison, the FOM of the ZGO-0.15GC is less than that of Pr 3+ doped Ge-Ga-S ChG glass (4.79 × 10 −24 cm 2 ·s) 42 , however, it is much larger than in the case of Pr 3+ doped ZBLAN (0.38 × 10 −24 cm 2 ·s) and Dy 3+ doped Ge-Ga-S ChG glasses (1.4 × 10 −24 cm 2 ·s) 43,44 . Moreover, the present GCs are superior to rare-earth doped glasses in terms of the availability of a broad tuning range of wavelength. A comparison of the luminescent properties (λ peak , peak emission wavelength, τ 298K , decay lifetime at the room temperature, and FOM) and crystal field parameters (Dq and B) of Ni 2+ in GCs containing different spinels is shown in Table 1. The magnitude of crystal field strength Dq is a measure of the interaction of the 3d-electrons with the rest of the lattice, and the main contribution arises from the nearest neighbors. Although Ni 2+ substitutes for Ga 3+ in both ZGO and ZGGO GCs, the Dq value of the latter is slightly less than that of the former GCs, which, according to the ligand field theory, is due to the distortion of ligands inducing a weakening effect on the crystal field strength of the central ion 45 .
It is important to stress that the synthesized ZnGa 2 O 4 : Ni 2+ GCs are highly reproducible to allow for a fluctuation in the thermal treatment temperature, for example, transparent GCs with the broadband near NIR emission can be obtained at a crystallization temperature ranging from 750 to 800 °C (Fig. S9 in the supporting information). This is a very important advantage for the "melt-in-tube" method, for which the core fiber is covered with the SiO 2 cladding, and the heat transfer process during the heat treatment can be different from that of the glass sample. Because of different thermal treatment temperature, higher for the "melt-in-tube" method, GCs with required luminescent properties and transparency should be obtained in a temperature range as broad as possible.  In this respect, the studied ZGO GCs are perfectly matched to the "melt-in-tube" method, which will be the subject of our next study to succeed in making them into fibers.

Conclusion
The selective doping of Ni 2+ in ZnGa 2 O 4 and Zn 1+x Ga 2−2x Ge x O 4 nano-spinels via the controlled crystallization results in a broadband NIR emission. The use of nucleating agents such as TiO 2 promotes occupation of the octahedral Ga 3+ sites by Ni 2+ and leads to enhanced luminescence and prolonged lifetime, whereas the partition of Ge 4+ in ZnGa 2 O 4 spinels leads to a reduced NIR emission, which is assumed to be related to the formation of anti-site defects. The large mismatch of valence and ionic radii between 6 Ni 2+ and 6 Ge 4+ considerably limits the substitution of 6 Ni 2+ for 6 Ge 4+ , which also partly accounts for the comparatively weaker NIR emission from the Zn 1+x Ga 2−2x Ge x O 4 : Ni 2+ GCs. The stronger NIR emission, excellent optical quality and reproducibility, as well as a tolerance for thermal treatment temperature make ZnGa 2 O 4 : Ni 2+ nano-spinels embedded GCs highly promising candidates for broadband fiber amplifiers. Future work will focus on fabricating GC fibers by the "melt-in-tube" method.