Oxyfluoride Glass-Ceramics for Transition Metal Ion Based Photonics

Feb 12, 2016 - In order to explore new photonic materials and to utilize the above-mentioned advantages of oxyfluoride GCs, in the following an effect...
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Oxyfluoride Glass-Ceramics for Transition Metal Ion Based Photonics: Broadband Near-IR Luminescence of Nickel Ion Dopant and Nanocrystallization Mechanism Changgui Lin,*,†,‡ Legang Li,† Shixun Dai,† Chao Liu,§ Zhiyong Zhao,§ Christian Bocker,‡ and Christian Rüssel‡ †

Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang Province, Advanced Technology Research Institute, Ningbo University, Ningbo 315211, P.R. China ‡ Otto-Schott-Institut für Materialforschung, Jena University, Fraunhoferstrasse 6, 07743 Jena, Germany § State Key Laboratory of Silicate Materials for Architecture, Wuhan University of Technology, Wuhan, Hubei 430070, P.R. China ABSTRACT: Nickel ion doped transparent bulk glass-ceramics containing K2SiF6, ZnF2, and KZnF3 nanocrystals were elaborated to show the prospect that this novel group of oxyfluoride glass-ceramics is promising for transition metal ion based photonics. These new oxyfluoride glass-ceramics exhibit a new broadband luminescence of Ni2+ ions in the intriguing near-IR spectral region ranging from 1200 to 2400 nm. Ni2+ ions are incorporated into the precipitated K2SiF6, ZnF2, and KZnF3 crystals, which provide octahedral sites for Ni2+ and make it optically active. Taking into account the microstructure of phase separation observed by replica TEM, a detailed mechanism of phaseseparation-assisted nanocrystallization was described for these oxyfluoride glasses. These results allow one to establish a full physical model of the nanocrystallization mechanism. For the precipitation of fluoride crystallites, not only can the nanometric size be explained but also the formation of different crystal phases can be controlled by the initial amorphous phase separation of droplet or interpenetrating structure with a sufficiently elaborated glass design.

1. INTRODUCTION Since the first spectroscopic study of Er3+ and Yb3+ codoped transparent fluoroaluminosilicate glass-ceramics (GCs) reported by Wang and Ohwaki in 1993,1 oxyfluoride GCs have attracted considerable attention in photonics because of their unique comprehensive properties of low phonon energy of fluoride crystals and the favorable durability and mechanical properties of oxide glasses.2−4 In the past decade, much effort has been devoted to improve and develop new oxyfluoride GC materials with good transparency and various fluoride crystallites suitable for rare earth-based luminescence. According to a recent review paper published by Fedorov et al.,3 all rare earth elements have been comprehensively studied in several common types of fluoride crystals, such as MF2 (M = Ca, Sr, Ba, Pb), RF3 (R = La, Y, Cd), ARF4 (A = Li, Na, K; R = La, Y, Gd), and B2RF7 (B = Sr, Ba; R = La, Y, Gd), that precipitated in silicate glass matrices. Because of the similar radii, optically active ions tend to substitute for the alkalineearth and rare-earth ions (REIs) and are hence incorporated into the low phonon energy crystalline environment of these fluoride crystals. Consequently, REI-doped oxyfluoride GCs show intriguing optical properties for applications in telecommunications and optoelectronics, e.g., solid-state lasers, optical amplifiers, LEDs.5−8 © XXXX American Chemical Society

Such oxyfluoride GCs open a new path to develop potential luminescence and laser materials based on REIs, and great success has been achieved. However, the reported fluoride crystallites were designed and fabricated to offer different sites only for the incorporation of REIs. Up to now, spectroscopic studies of another group of important luminescent-active ions, i.e., transition-metal ions (TMIs), are still widely absent in the field of oxyfluoride GCs. In comparison to the f−f transitions of REIs, the d−d transitions of TMIs are much more sensitive to the local environment, which could result in some unique luminescence properties. In fact, some TMI-doped oxide GCs have successfully been prepared through the precipitation of various oxidic crystalline phases that could provide a suitable ligand field environment for the unique TMI-based photonics, e.g., Ni2+-, Cr3+-, and Mn2+-doped LiGa5O8 or Ga2O3 GC, respectively.9−12 Therefore, it is quite meaningful to design and fabricate new oxyfluoride GCs with fluoride crystallites suitable to incorporate TMIs, especially taking the following two reasons into consideration: Received: January 21, 2016 Revised: February 12, 2016

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DOI: 10.1021/acs.jpcc.6b00683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

fluoride crystal host for tunable lasers.22 The precipitation of these three fluoride crystal phases from oxyfluoride GCs could provide octahedral sites for the incorporation of most optically active TMIs, making them highly interesting for TMI-based photonics. In the following, we report on the elaboration of new KF− ZnF2−SiO2 oxyfluoride glass-ceramics containing ZnF2, K2SiF6, and KZnF3 nanocrystals through controlled nanocrystallization of initially phase separated glasses. Among the dozens optically active TMIs, Ni2+ was specially selected because of its broadband near-IR emissions if incorporated in octahedral coordination. Recently, it has been reported from Ni2+-doped GCs that super broadband near-IR emissions ranging from 1000 to 2400 nm can be obtained and tuned through the variation of crystal hosts from oxides to fluorides.34 It would contribute to the development of broad optical amplification for future telecommunication, such as broadband optical amplifiers in the whole telecommunication window, eye-safe radars, noninvasive medical diagnostics, atmospheric pollution monitoring, remote sensing, and effective pump resource for mid-IR optical parametric oscillators. Preliminary results of the nanocrystallization behavior in undoped KF−ZnF2−SiO2 glasses as well as the luminescence properties in 25KF−25ZnF2−50SiO2 GCs doped with 0.5 mol % Ni2+ have recently been published in brief reports.34,35 They revealed an interesting crystallization mechanism and showed a brand new near-IR emission of Ni2+ ions. In the following, efforts are devoted to complete the previous studies on controlled nanocrystallization and luminescence properties of Ni2+ ions in oxyfluoride GCs.

Despite the fact that the mechanism of controllable crystallization is still an open problem in glass science, it has recently been evidenced that controlled nanocrystallization can be achieved through self-organized processes, e.g., if an amorphous phase separation (APS) takes place and fluorides precipitate from oxyfluoride glasses.13,14 In the case of phaseseparated fluoroaluminosilicate glasses,15−21 fluorine-enriched droplet phases are formed. In a subsequent step, nucleation and crystal growth occur in the droplet phase. Since this phase is enriched in compounds that decrease the viscosity, i.e., predominantly in network modifiers and fluorine, the viscosity in the residual glassy matrix increases during the course of the phase separation process. Thus, a diffusion barrier around each droplet is formed to inhibit further crystal growth, resulting in nanometer-sized fluoride crystals. A similar mechanism also takes place, if crystallization is achieved in a homogeneous glass. Then also a diffusion barrier enriched in SiO2 is formed around the growing crystals. Fluoride crystals have been considered as preferable hosts in order to achieve favorable spectroscopic properties of TMIs.22 The low phonon energy of fluoride crystals should lead to a decrease of nonradiative transitions and to an increase in the luminescence efficiency of TMIs, akin to that of REIs. Furthermore, from the well-known spectrochemical series of Dq(F−) < Dq(O2−), a smaller crystal field splitting can be expected in fluoride host lattices in comparison to oxide materials. Therefore, different or new optical transitions between the vibronic levels could be allowed because of the small splitting of the ground state of TMIs. In order to explore new photonic materials and to utilize the above-mentioned advantages of oxyfluoride GCs, in the following an effective strategy to precipitate an important group of fluoride crystals, KZnF3, K2SiF6, and ZnF2, from oxyfluoride glasses is described. These phases are promising for the photonic applications based on TMIs. As shown in Figure 1a, KZnF3 is a typical member of the fluorine perovskites

2. EXPERIMENTAL SECTION Glass samples with nominal compositions of xKF−xZnF2− (100 − 2x)SiO2 (x = 15, 20, 22.5, and 25 mol %) doped with y mol % of NiO (y = 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, or 0.6 mol %) were prepared by melting raw materials KF (AR, Chemiewerk Nünchritz), ZnF2 (AR, Chemiewerk Nünchritz), SiO2 (Sipur A1, Schott), and NiO (AR, Aladdin). The batches were melted in a Pt−Rh crucible in a middle-frequency induction furnace. The melting temperatures ranged from 1400 to 1550 °C, depending on the respective glass composition. After the conventional procedures of melting, quenching, and annealing, transparent bulk samples were obtained and then cut into small pieces. Glass slices with a thickness of 2 mm were optically polished on both sides. The glass transition temperatures, Tg, of the glasses were determined by a dilatometer (DIL 402 PC, Netzsch) with a heating rate of 5 K/min. The GC samples were fabricated through careful thermal treatments at the temperature above Tg for different periods of time. The prepared crystallized and subsequently powdered samples were characterized by X-ray diffraction (XRD, D5000, Siemens). Transmission electron microscopy (TEM) at 200 kV (H8100, HITACHI) was used to study the microstructure of some selected samples. Bulk TEM samples were mechanically polished, dimpled, and finished by gentle ion beam milling (3 kV, 1.5 mA). Replica TEM samples were prepared by etching the fractured surface with diluted HF and coated afterward with Pt−Ir−C by slanted shadowing. Transmission and absorption spectra were recorded in a wavelength range from 200 to 2000 nm using a UV−vis−NIR spectrometer (Shimadzu UV 3102 PC). The emission was collected at the direction perpendicular to the excitation beam

Figure 1. Crystal structures of (a) cubic perovskite KZnF3, (b) cubic K2SiF6, and (c) tetragonal ZnF2.

[Pm̅ 3m space group (No. 221)] that could provide a perfectly octahedral fluorine site for the divalent 3d ions, such as Ni2+, Co2+ , and Mn2+, through the substitution of the Zn 2+ position.22 Thus, interesting and unique spectroscopic properties have been obtained and intensively studied in such TMIsdoped KZnF3 crystals.23−28 K2SiF6 is one of the hexagonal complex alkaline fluorides (Figure 1b), which is also extremely suitable for doping TMIs, especially for manganese ions. Red and yellow emissions can be obtained in the K 2 SiF 6 nanocrystals doped with tetra- or divalent manganese, respectively.29,30 ZnF2 has a rutile-type crystal structure (Figure 1c) that is formed by the stacking of ZnF6 octahedra.31 TMIs, such as Co2+ and Ni2+,32,33 can substitute for Zn2+ in the slightly distorted octahedral sites, and hence, ZnF2 is a potential B

DOI: 10.1021/acs.jpcc.6b00683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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nm, as shown in Figure 3a. This emission behavior originated from the 3T2g(F) → 3A2g(F) radiative transition of octahedral

and dispersed into a 0.25 m monochromator, and the intensities of the emission in the near-IR wavelength range were detected with an InSb detector. A digital oscilloscope (HP 54503a) was employed to record the decay curves. All the optical measurements were carried out at room temperature.

3. RESULTS AND DISCUSSION 3.1. Near-IR Emission from Ni2+ Ions in Oxyfluoride GCs. In the previous set of experiments,35 it was found that KZnF3, K2SiF6, and ZnF2 nanocrystals could be precipitated from glasses in the system xKF−xZnF2−(100 − 2x)SiO2 (with x = 15, 20, 22.5, and 25 mol %). Here Ni2+, as the first attempt with TMIs, is introduced into the oxyfluoride glasses from which different fluoride crystals (ZnF2, K2SiF6, and KZnF3) can be crystallized. Figure 2 shows transmission spectra and Figure 3. (a) Near-IR emission spectra of xNiO doped 15KF− 15ZnF2−70SiO2 samples crystallized at 530 °C for 20 h (x = 0.025, 0.05, 0.1, 0.2, 0.4, and 0.6) under excitation at 405 nm. (b) The peak intensities and peak wavelengths of the obtained emission curves as a function of Ni2+ content.

Ni2+ and showed a unique doping-dependent luminescence feature (Figure 3b). With increasing Ni2+ concentrations, the maximum intensity of the luminescence increases first and then decreases, showing a peak value at x = 0.2 mol %. Furthermore, the wavelengths attributed to the peaks shift to shorter wavelengths from 1680 to 1500 nm, suggesting that the crystal-field states of octahedral Ni2+ are varied. The results clearly show that the Ni2+-doped K2SiF6 and ZnF2 nanocrystals embedded in a glass matrix give a new broadband near-IR emission at wavelengths in the range from 1200 to 2000 nm that has not been reported previously. The spectroscopic characteristics are strongly dependent on the Ni2+ concentration, which should be correlated to the interaction between Ni2+ ions and the fluoride crystal phases. Nevertheless, the detailed mechanism is still unclear due to the perturbation of the simultaneous existence of two crystal phases. To further clarify the effect of crystal phases on the luminescence behavior of Ni2+, specific investigations were performed on the crystallization behavior and near-IR emission properties of 15KF−15ZnF2−70SiO2 samples doped with 0.1 mol % NiO. It is obvious that, as shown in Figure 4a, a drastic change occurs in the appearance and absorption characteristic of the samples, once the thermal treatments at 530 °C for different periods of time have been performed. XRD patterns in Figure 4b clearly show two appearance orders of the crystallizing phases of K2SiF6 and ZnF2, akin to that reported in the undoped samples.35 In the first 10 h, solely diffraction peaks (2θ = 18.5°, 30.6°, 37.7°, and 43.7°) ascribed to K2SiF6 are observed; their intensities gradually increase. After crystallization for 15 h, first XRD peaks due to of ZnF2 are observed. Thus, the relationship between the near-IR luminescence of Ni2+ and the crystal phases of K2SiF6 and ZnF2 can be revealed in this case. Under excitation at 405 nm, a broadband near-IR emission from 1200 to 2000 nm also can be observed in the 0.1 mol % NiO doped 15KF−15ZnF2−70SiO2 GC samples (Figure 5a). With the prolongation of the crystallization times, as illustrated in Figure 5b, the peak wavelength of the emission shifts to shorter wavelength in analogy to that observed in the samples doped with different Ni2+ concentration (Figure 3), whereas

Figure 2. Transmission spectra and photographs of 15KF−15ZnF2− 70SiO2 samples doped with x mol % NiO (x = 0.025, 0.05, 0.1, 0.2, 0.4, and 0.6): (a) as-prepared glasses and (b) glass-ceramic samples crystallized at 530 °C for 20 h.

photographs of as-prepared glasses and crystallized samples with the compositions 15KF−15ZnF2−70SiO2 doped with x mol % NiO (x = 0.025, 0.05, 0.1, 0.2, 0.4, and 0.6). In the glass samples (Figure 2a), three absorption bands centered at 440, 870, and 1910 nm are ascribed to transitions originating from 5-fold and tetrahedrally coordinated Ni2+ ions in the glass matrix.11,36 After thermal treatment at 530 °C for 20 h, obvious changes in the sample colorations (from brown-red to light green as displayed by the photographs in Figure 2b) and in the absorption characteristics are observed for all six samples. This is due to the occurrence of K2SiF6 and ZnF2 nanocrystals in the GC samples, which crystallized at the temperature supplied. This confirmed that Ni2+ was incorporated in the crystalline phases. The newly appeared bands at 400, 750, and 1190 nm are attributable to the 3A2g(F) → 3T1g(P), 3A2g(F) → 3T1g(F), and 3A2g(F) → 3T2g(F) electronic transitions of octahedral Ni2+, which was confirmed by the theoretical electronic transitions calculated on the basis of Tanabe−Sugano theory.37 The notable shift in the absorption bands observed during crystallization is due to the smaller crystal field splitting that can be expected in fluoride host lattices. Obviously, Ni2+ in the oxyfluoride glass is 4- or 5-fold coordinated and during thermal treatment changes its coordination and is then 6-fold coordinated with fluorine. These nanocrystallized samples show broadband near-IR luminescence from 1200 to 2000 nm upon excitation at 405 C

DOI: 10.1021/acs.jpcc.6b00683 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. (a) Absorption spectra and photographs and (b) XRD patterns of 0.1 mol % NiO doped 15KF−15ZnF2−70SiO2 samples heat-treated at 530 °C for different times from 0 to 70 h. The standard JCPDF card no. 851382 for K2SiF6 and no. 71-1971 for ZnF2 are also collected in part b. Figure 6. (a) Absorption spectra and photographs and (b) XRD patterns of 0.05 mol % NiO doped 25KF−25ZnF2−50SiO2 samples heat-treated at 520 °C for different times (from 0 to 20 h). The crystallized phases are indexed by comparing the diffraction peaks with the standard JCPDF card no. 89-4110 for KZnF3 and no. 71-1971 for ZnF2.

different periods of time (from 0 to 20 h). Similarly, the coloration of Ni2+ changes from brown to light green. It is due to the precipitation of KZnF3 and ZnF2, as indicated by the XRD patterns in Figure 6b, during thermal treatment. Also in this series of experiments, as a function of time, two different crystalline phases are formed. First, solely KZnF3 is formed at crystallization times of up to 10 h. At longer crystallization times, additionally ZnF2 is formed. The as-prepared sample has several low-intensity and broadened diffraction peaks at 21.6°, 30.7°, 37.5°, 44.1°, 55.2°, and 64.5°, which show only small deviations from the respective peaks of the standard KZnF3 crystal phase. It might be ascribed to the composition, which can be considered as the initial step of the nanocrystallization mechanism. It can be attributed to the small near-IR luminescence of the as-prepared sample, as displayed in Figure 7a. Further thermal treatments result in the variation of luminescence intensity (the inset of Figure 7b), which increases in the first 10 h and approaches a constant value with a fluctuation after longer times. This luminescence is ascribed to the incorporation of Ni2+ ions into the KZnF3 and ZnF2 crystals. The variation of the luminescence intensity is in line with the crystallization order of KZnF3 and ZnF2. Notably, the luminescence intensities here are much stronger than in the case where K2SiF6 and ZnF2 are crystallized, and the luminescence range from 1200 to 2400 nm is broader. Obviously, KZnF3 crystallites are more favorable for the broadband near-IR luminescence of octahedral Ni2+. In addition, the fluorescence decay curves could be recorded as shown in Figure 7b by monitoring the emission at 1695 nm, because of the much stronger luminescence in this case (dozens of times larger than that observed in K2SiF6 crystallized samples, as indicated in Figures 3a, 5a, and 7a). They are nearly single-exponential. The calculated lifetimes were 135, 683, and 1080 μs after the thermal treatments of 0, 10, and 20 h, respectively. The lifetime gets drastically longer with the

Figure 5. (a) Near-IR emission spectra of 0.1 mol % NiO doped 15KF−15ZnF2−70SiO2 samples crystallized at 530 °C for different times (0, 5, 10, 15, 30, 50, and 70 h) under excitation at 405 nm. (b) The maximal intensity and peak wavelength of the obtained emission curves as a function of the crystallization times.

the luminescence intensity increases continuously. It is noteworthy that both of them show an abrupt change between the crystallization times of 10 and 15 h, which is corresponding to the beginning of the precipitation of the second crystal phase, ZnF2. Consequently, it is believed that, in the samples with short crystallization times (