1. Introduction

The fabrication of new luminescent materials that exhibit wide emission is highly desirable for applications in fields as diverse as solid-state lighting, telecommunications, and optical tomography [1, 2]. In particular, there has been a special interest in preparation of broadband near-infrared (NIR) luminescent materials through exploration of novel emission centers such as transition metal and main group ions (Ni and Bi active centers), because the electronic transitions in their outermost d or p orbitals are strongly influenced by their ligands and thus their emission bands potentially present wide-ranging characteristics [3–7]. However, the synthesis of broadband luminescent materials featuring flat emission remains a formidable challenge, as the radiative state density of an active center generally presents a discontinuous energy distribution.

In principle, a single matrix co-doped with multiple luminescent ions may display a flat emission profile. Unfortunately, co-doping also introduces the complication of unwanted energy transfer between different active centers. This phenomenon may lead to serious energy cross-relaxation that induces luminescence quenching [8]. Although several attempts have been made to hinder unintended energy transfer [9], an effective method for providing broad and flat emission has not been conclusively established. Herein, we report an effective way to broaden the bandwidth of near-infrared emission from Ni/Bi co-doped 58SiO₂–21ZnO–13Al₂O₃–5TiO₂–3Ga₂O₃ glass through nanocrystallization. We show that the energy transfer between Ni and Bi active centers can be efficaciously suppressed by mutual isolation through selective incorporation of Ni active centers into ZnAl₂O₄ nanocrystals. Importantly, the obtained nanocrystallized glasses present much broader and flatter NIR emission and can serve as ideal gain materials for fiber amplifiers for broadband optical signal amplification without complex structures.

2. Experimental

Glass samples with matrix composition 58SiO₂–21ZnO–13Al₂O₃–5TiO₂–3Ga₂O₃ (ZAS) and doped with 0.1 mol% NiO (marked as Ni-glass), 0.5 mol % Bi₂O₃ (marked as Bi-glass) and co-doped with 0.1 mol% NiO and 0.5 mol% Bi₂O₃ (marked as Bi-Ni-glass) were prepared by conventional melt-quenching. Analytical reagent grade ZnO, Al₂O₃, SiO₂, TiO₂, Ga₂O₃, Bi₂O₃ and 99.99% purity NiO were used as raw materials. The batches were mixed thoroughly and melted in a corundum crucible in air atmosphere at 1600 °C for 1.5 h. Then the melts were poured onto a stainless steel plate. We examined the crystallization behavior of the prepared glass samples by differential thermal analysis (DTA) at a heating rate of 10 °C/min. The glass transition temperature (T_g) and crystallization temperature (T_x) were estimated to be around 669 and 800 °C, respectively. Based on this result, we determined the following heat-treatment procedures for crystallization: 680 °C for 5 h (for nucleation) and 820 °C for 12 h (for crystal growth). The nanocrystallized glass samples were marked as Ni-GC, Bi-GC and Bi-Ni-GC. The crystalline phase in the nanocrystallized glass samples was identified by X-ray diffraction (XRD) using Cu/K_α1 radiation. Transmission electron microscopy (TEM) was performed using a JEOL 2010F operating in TEM mode. Absorption spectra were recorded by a double-beam spectrophotometer (JASCO FP-6500). The infrared luminescent spectra were measured using a ZOLIX SBP300 spectrophotometer with InGaAs as a detector in the 1000-1800 nm wavelength region. The fluorescence decay curves were recorded by a FLS920 fluorescence spectrophotometer (Edinburgh Instrument Ltd, UK). All the measurements were carried out at room temperature.

3. Results and discussion

Figure 1 shows the XRD patterns and TEM images of the as-made and nanocrystallized samples. The large halo in XRD patterns of the as-made samples indicates their amorphous nature. After heat-treatment, diffraction peaks can be clearly observed in all of the nanocrystallized samples, and they agree well with those of ZnAl₂O₄ spinel (ICCD Card File No. 5669). By using the Scherrer Eq., the average crystallite size of the precipitated nanocrystals was calculated from the strongest XRD peak at 36 ^o. All of the three samples exhibited a similar crystallite size, of around 6 nm. The TEM image (inset of Fig. 1(a)) shows a homogeneous distribution of nanocrystals in the glassy phase, ideal for fabrication of highly transparent composites because of the suppression of Rayleigh scattering.

 

Fig. 1 (a) XRD patterns of glass samples doped with Ni²⁺, Bi⁺, and Ni²⁺/Bi⁺ (curves a, b, and c), and nanocrystallized glass doped with Ni²⁺, Bi⁺, and Ni²⁺/Bi⁺ (curves d, e, and f). The insert shows a TEM image of the nanocrystallized glass. (b) HRTEM images of Ni²⁺/Bi⁺-codoped nanocrystallized glass and Ni/Bi concentrations in the glass-rich region and crystal-rich regions marked with A and B.

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To investigate the dopant distribution in the nanocrystallized glass, high-magnification TEM and micro-EDS measurements were carried out, with the results shown in Fig. 1(b). Interestingly, Ni signal could only be observed in crystalline regions (e.g. region A) and was almost absent in glassy regions (e.g. region B). Moreover, a relatively high concentration of Bi (0.81 at.%) was detected in the glassy phase compared with the crystalline phase (0.40 at.%). Due to the spatial limitations, the EDS measurement might not accurately give the real concentration of Ni and Bi, but it presents clear evidence for the inhomogeneous distribution of Ni and Bi in nanocrystallized glass.

To gain better knowledge about the dopant distribution, spectroscopic measurement and analysis were performed. Since the optical properties of transition metal and main group ions are very sensitive to their ligand environments, the optical absorption spectra can be used to distinguish the variation of ligand ðelds of transition metal and main group ions. Figure 2 displays the absorption spectra of as-made and nanocrystallized glasses doped with Ni, Bi and Bi/Ni. The absorption spectrum of the Ni-doped glass consists of three main absorption bands with maxima at 1744, 872 and 439 nm. These absorption bands can be attributed, respectively, to the transitions of ³E’(F)→³E”(F), ³E’(F)→³A’₂(F) and ³E’(F)→³A’₂(P) of the trigonal bipyramidal ðve coordinated Ni centers [10]. After heat-treatment, the shape of the absorption spectrum completely changed compared with that of the as-made Ni-doped glass. Absorption bands of the Ni-doped nanocrystallized glass centered at 1007 and 600 nm could be attributed, respectively, to the ³A₂(F)→³T₂(F) and ³A₂(F)→³T₁(F) transitions of Ni active centers in octahedral sites [9]. The complete disappearance of the characteristic bands of trigonal bipyramidal ðve-coordinated Ni firmly demonstrates that most of the Ni²⁺ ions have been incorporated into the nanocrystals [9, 10]. The nanocrystalline phase of ZnAl₂O₄ belongs to the spinel family with Zn²⁺ and Al³⁺ cations occupying one-eighth of the interstitial tetrahedral sites and half of the octahedral sites, respectively. Therefore, it can be supposed that Ni²⁺ ions selectively substitute for the Al³⁺ positions in octahedral sites after nanocrystallization of glass. For Bi-doped samples, two absorption bands can be observed around 500 and 700 nm which have been ascribed to the electron transitions of Bi active centers (Bi⁺, most probably) [5]. Unlike the Ni-doped samples, no obvious peak shift can be observed in the absorption spectrum of the Bi-doped samples after heat-treatment. The slight absorption enhancement for Bi-doped nanocrystallized glass is probably due to the light scattering of nanocrystals. For Bi/Ni codoped samples, the absorption spectra can be well deconvoluted into the absorption bands of Ni and Bi singly doped samples. Therefore, we may conclude that Bi active centers are still located in glass phase after heat treatment.

 

Fig. 2 Absorption spectra of the Ni²⁺, and Bi⁺ singly doped and Ni²⁺/Bi⁺ co-doped as-prepared and nanocrystallizated glasses. The inset shows photographs of Ni²⁺/Bi⁺ co-doped as-prepared glass (left) and nanocrystallized glass (right).

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Figure 3(a) shows the NIR luminescence spectra of as-made and nanocrystallized glasses excited at 980 nm. Ni singly doped glass shows no emission in the NIR region, and the Bi singly doped and Bi/Ni co-doped glasses present a single-band NIR emission with the central wavelength at 1100 nm. After heat treatment, the single-band luminescence feature of Bi-doped glass remains similar to the as-prepared sample, while another single-band NIR emission with the central wavelength at 1270 nm was detected in Ni singly doped nanocrystallized glass. The 1100 nm emission band of the Bi-doped sample and 1270 nm band of the Ni-doped sample can be attributed to the downward ³T₂(F)→³A₂(F) transition of octahedral Ni²⁺ and ³P₁→³P₀ transition of Bi⁺, respectively [5, 7, 11]. Significantly, the co-doped nanocrystallized glass shows an ultra-broadband emission centered at 1160 nm with a full width at half maximum (FWHM) of 350 nm, which is larger than that of the Ni singly doped sample and twice the width of Bi singly doped one (~160 nm). The observed unusual super-broadband NIR emission in Bi/Ni co-doped nanocrystallized glass is largely originated from the overlapping fluorescence of Bi and Ni active centers. This indicates that the selective incorporation of Ni²⁺ ions into nanocrystals during nanocrystallization may lead to an effective isolation of Ni from Bi active centers, resulting in a significant suppression of energy transfer between them.

 

Fig. 3 (a) Emission spectra of the Bi⁺, and Ni²⁺ singly doped and Ni²⁺/Bi⁺ co-doped nanocrystallized glasses under excitation at 980 nm. (b) Decay lifetime curve of the 1125 nm emission of Bi⁺, and Ni²⁺ singly doped and Ni²⁺/Bi⁺ co-doped nanocrystallized glasses with length at 980 nm.

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To clarify the detailed energy transfer process, we further examined the dynamic decay process of NIR emission. Figure 3(b) presents the measured decay profiles by monitoring the luminescence of Bi active centers in different samples. Significantly, the nanocrystallization of Bi/Ni codoped glass leads to a notable extension of NIR luminescence decay lifetime (τ), which increases from 386 to 476 μs. Considered together, this phenomenon and the observed broadened luminescence in the nanocyrstallized sample firmly demonstrate the suppressed energy transfer due to the spatial segregation of Ni and Bi active centers. A relatively high decay rate of Bi/Ni codoped nanocrystallized glass compared with Bi singly doped samples (~541 μs) is mostly associated with the Bi/Ni interactions at the crystal-glass phase boundaries. The energy transfer efficiency can be estimated using the Reisfeld Eq.: [12]

To provide an in-depth view of the results, schematic diagrams of the in situ nanocrystallization of the glass phase are shown in Fig. 4 along with partial energy level diagrams of Ni and Bi active centers summarizing the excitation, energy transfer, and radiative processes. According to the Förster model [13], the energy-transfer rate can be determined by the relation:

 

Fig. 4 A schematic illustration of the spatial distribution of Ni²⁺ and Bi⁺ and proposed mechanisms of energy transfer and radiative/irradiative electron transitions in co-doped and nanocrystallized glasses. The dashed, dotted, dashed-dotted, and full arrows represent photo excitation, irradiative relaxation, energy transfer, and emission, respectively.

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The successful suppression of energy transfer inside co-doped glass through nanocrystallization is highly attractive for improving the performance of glassy materials and even construction of novel functional light sources. For example, for valuation of gain media for tunable lasers and fiber amplifiers, stimulated-emission cross section (σ_em), bandwidth, and decay lifetime are exceptionally important parameters. The larger the value of σ_em × FWHM, the better the gain bandwidth. Moreover, the amplification gain is proportional to the value of σ_em × τ, and the laser oscillation threshold is inversely proportional [15]. In our research, the values of FWHM and τ in Bi/Ni nanocrystallized glass were simultaneously enhanced compared with those of co-doped glass, indicating its promising application as an excellent gain medium for tunable lasers and fiber amplifiers.