1. Introduction

Ce³⁺ ions are widely used as a luminescence center for various materials, such as phosphate, silicate and crystal materials. They can be used as phosphors, scintillators, light-emitting diodes (LEDs) and detectors for ionizing radiation, gamma radiation, and brachytherapy for its allowed 5d-4f transitions [1,2]. Moreover, the main application of the Ce-doped materials is as a scintillator for radiation detection. In the traditional radiation detection, scintillators are often bulk glasses. They are very difficult to realize the remote radiation measurement [3,4]. In addition, the polymethylmethacrylate (PMMA) plastic optical fibers are also used as a real-time gamma detector [5]. However, plastics fibers are easy to age and have large transmission losses [6]. Recently, Ce-doped fiber materials are becoming a promising and attractive luminescence material because of its potential applications for remote and real-time radiation measurement. C. N. Liu et al. fabricates a Ce-doped YAG/silica fiber based on rod-in-tube technique for high-resolution optical coherence tomography source [7]. M. Alshourbagy et al. prepares a Ce³⁺ doped YAlO₃ crystal fiber by μ-pulling down technique [8]. However, homogenous and uniform single crystals are hard to obtain. Moreover, the coupling efficiency between single crystal fibers and commercial fibers is still low. Compared to these materials, Ce-doped silica fiber materials show unique advantages, such as radiotolerance, electromagnetic insensitiveness, low transmission losses and low coupling loss with the commercial single mode fibers (SMF) [9]. A. V. Kir’yanov et al. proposes a CeO₂/Au-codoped alumino-phosphosilicate fiber and utilizes Ce⁴⁺ ions’ properties to diminish photo-darkening in doped fibers [10]. S. Girard et al. fabricates Ce-doped silica rods with ~1 mm size diameters for proton bean monitoring [11]. In this work, we prepare the Ce-doped silica fiber with size matching the SMF. Moreover, most reports on Ce doped materials including SiO₂ are investigated in the experiments. In addition, Ce³⁺ ions are sensitive to external environment because of their exposed 5d orbital shell. Different optical properties of Ce³⁺ will be reflected in different substrate materials. In recent years, the development of density functional theory (DFT) provides a possibility to evaluate theoretically optical properties of the rare earth doped materials [12,13]. However, for the Ce-doped silica fiber materials, it is still unclear for understanding the fluorescence properties at the microstructural level.

In this study, a Ce-doped silica fiber with Ge dopants is prepared using powder-in-tube (PIT) technique via sol-gel method. Both excitation and emission spectra of proposed fiber material are measured and analyzed. A local microstructure model of the Ce-doped silica fiber is developed and its structure characteristics, energy level parameters, absorption and emission spectra are evaluated. In addition, an energy level diagram of the doped silica fiber material is constructed.

2. Experimental section

2.1 Materials and preparation

The Ce-doped silica material is prepared by sol-gel technique. The source materials are mainly silicon tetraethoxide (TEOS), distilled water, hydrochloric acid (as a catalyst), cerium nitrate (Ce(NO₃)₃) and ethanol (EtOH). In addition, in order to improve the refractive index of the doped optical fiber, GeO₂ units are also introduced during the preparation process (the following Ce-doped fiber and powder samples all contain the Ge dopants). The precursor for Ge dopants is the germanium tetraethoxide (TEOG). The synthesis process is described as follows: First, the TEOS and EtOH were mixed, and subsequently, stirred for approximately 30 min. Second, the cerium nitrate was dissolved in the distilled water, and then mixed with TEOS and EtOH with a volume ratio of 1:2:3. The TEOG solution was also added slowly to the silica sol. Then a few drops of the hydrochloric acid were added for adjusting the PH to approximately 2. Finally, the mixed solution was stirred at room temperature for 2h, and the resulting homogeneous sol was dried at 60°C for 2-3 days. The gel-to-glass transition is processed by slowly heating treatment in the tube furnace. The detailed thermal treatment process contains two steps: First, the xerogel samples are heated from room temperature to 450°C and maintained for 2 h at 150, 250, 350 and 450 °C, respectively. Second, the xerogel samples are heated up to 500, 700 and 1100 °C, successively. Thereafter, the doped samples are cooled to room temperature. Finally, the xerogel powder is filled into a quartz tube and drawn into silica optical fiber in the fiber drawing tower by powder-in-tube (PIT) method [14]. The element compositions of the Ce-doped optical fiber are analyzed by a Scanning Electron Microscope (JSM-7500F field emission SEM, Japan) combining with Energy Dispersive Spectrometer (SEM-EDS, MX80-EDS, OXFORD, England). The atomic concentration of the Ce ions in the doped fiber core is approximately 0.16 at%, as shown in Fig. 1.

 

Fig. 1 The EDS spectrum line and concentration of different elements in the doped fiber core.

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2.2 Fluorescence properties

First, the excitation and emission spectra of 0.1% Ce(NO₃)₃ solution are measured, as shown in Fig. 2. The results show excitation and emission peaks at 300 nm and 360 nm, respectively. Furthermore, Fig. 3 shows the excitation and emission spectra of the doped fiber sample. The broad band around 430 nm is assigned to the allowed transition from the lowest 5d energy level of Ce³⁺ ions to the 4f level, which is split by spin-orbit interaction into two sublevels (²F_J = 5/2, 7/2) [15]. The analysis on the broad band with the Gaussian functions is also shown in this pattern. Two Gaussian curves with the maximum at 430 nm and 470 nm could suitably fit the emission spectrum and no extra constrains are imposed on the fitting parameters. Moreover, their energetic difference is approximately 2360 cm⁻¹, which is consistent with the typical L-S splitting of Ce³⁺ ions (2200 cm⁻¹).

 

Fig. 2 Excitation and emission spectra of 0.1% Ce(NO₃)₃ solution.

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Fig. 3 Excitation and emission spectra of the Ce-doped silica fiber sample. The blue dash lines show a fit carried out with two Gaussian functions.

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For the difference between the two emission spectra, the intuitive explanation is the two physical forms of Ce³⁺ ions: liquid and solid phase. During the drawing process (above 2000°C), the temperature plays a major role in the process of the wavelength shift. Generally, the role of temperature, on the one hand, is to remove organic matter and OH groups, on the other hand, is to format infinite framework of SiO₄ tetrahedra and Si-O ring structures. For observing the shift of the emission peak, different temperatures at 100, 500, 700 and 1100°C, respectively are adopt to treat the Ce-doped powder sample. Figure 4 shows the emission spectra of the doped powder sample excited at 300 nm after heat-treated at 100, 500, 700 and 1100 °C, respectively. There exist two obvious luminescence bands at 365 and 430 nm, respectively. When the heat-treated temperature is low, such as 100°C, a strong emission peak is found around 365 nm, which is similar to the situation of the Ce³⁺ solution. With the increasing of temperature, more than 500°C, the emission band red shifts and there appears an emission peak at 430 nm. Interestingly, the shift of the emission peak comes from the influence of heat-treated temperature. Furthermore, the increasing of the heat-treated temperature will increase the thermal movement of the Ce³⁺ ions, which will provide Ce³⁺ ions sufficient energy to interact with amorphous silica network. Moreover, the fluorescence properties of Ce³⁺ ions result from the allowed 5d-4f transitions, because their 5d and 6s orbitals lack electrons, resulting in an empty 4f orbital in the outer shell. This implies that the excited states of Ce³⁺ ions are easily affected by outer matrix environment. Then, the external energy from silica substrate makes 5d energy level of Ce³⁺ ions to drift downward, and a longer emission wavelength will appear. In addition, the intensity of emission peak at 430 nm decreases because a portion of Ce³⁺ ions are oxidized and converted into Ce⁴⁺ ions.

 

Fig. 4 Emission spectra excited at 300 nm of the Ce-doped powder sample with different heat-treated temperature.

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3. Theoretical calculation and discussion

3.1 Local structural models

For amorphous silica fiber materials, there are mainly a large number of ring units, such as three-membered-rings (3MRs), 4MRs, 5MRs, 6MRs, and hybrid rings structural units [16]. A 3MR microstructure model is suitable and widely accepted for doped silica structure [17–19]. In this study, the 3MR structural model is employed to characterize the amorphous silica network and quantized structure parameters and energy level properties are investigated using the DFT theory. The calculations are carried out using the Gaussian-09 program. The ground-state molecular structures are first optimized using DFT with Becke-3-Lee-Yang-Parr (B3LYP) hybrid function [20,21]. For the oxygen, hydrogen and silicon elements, 6-31 + G** basis sets are used. For the cerium element, 11 valence electron relativistic effective core potentials (RECPs) for trivalent state is employed [22,23]. After optimal structure parameters are obtained, the properties of absorption and emission spectra are evaluated using the time-dependent density functional theory (TD-DFT) [24]. Moreover, after every geometric optimization process, the harmonic vibration frequencies are calculated.

When the Ce atoms are doped into silica network, they replace the site of the original silicon atoms or they jointly form a binary system. Moreover, considering the large atomic size of the rare earth ions, they do not easily embed into silica network, and they prefer to act as network modifiers coordinating with non-bridging oxygen ions, and not with the bridging oxygen ions [25]. In this section, three possible Ce-doped silica structural models are developed to visualize the ring structure, as shown in Fig. 5. Model (a) is a closed bonding structure including Ce doped in the outer silica ring and Model (b) is an open bonding structure including Ce doped outside silica ring. Model (c) is a closed bonding structure including Ce embedded into silica ring. After the ground state structures of the minimum energy are obtained, the binding energy (eV) is calculated using

 

Fig. 5 Local microstructure models of Ce-doped 3MR.

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The binding energies of Models (a), (b) and (c) are 111.7538, 119.9404 and 90.5778 eV, respectively. It is clear that the binding energy of Model (c) is the lowest, which indicates that the molecular system is unstable when Ce³⁺ ion is embedded into silica rings. In addition, the difference in the binding energies between Modes (a) and (b) are relatively small. However, the following results of excitation and emission spectra based on Model (b) are not in good agreement with experimental results or literature reports. Hence, the following discussion is based on Model (a). After Model (a) is optimized, the average bond length of Si-O is found to be 1.6627$\stackrel{\text{°}}{\text{A}}$_, and the average bond angles of O-Si-O and Si-O-Si are 105.7347° and 134.2653°, respectively.

3.2 Frontier molecular orbital

The frontier molecular orbital (FMO) contains two parts: the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). The FMO is close to absorption and emission properties and provides a reasonable qualitative indication of the excitation states properties. Figure 6 shows the FMO energy eigenvalues and electron density distributions of the doped material. It is clear that O^2- ions in the silica matrix act as an electron provider, whereas the Ce³⁺ ions act as an electron acceptor. In addition, due to only a single electron in the 4f orbital of the Ce³⁺ ion, the multiplicity is two for the Ce-3MR model. The spin-down state is close to the spin-up state. Hence, only one sketch diagram of spin-up state is showed. For 3MR model, the FMO energies, from HOMO_-3 to LUMO₊₃, are −9.0136, −8.4789, −8.4753, −8.3493, 0.3380, 1.2234, 1.2251 and 2.2398 eV, respectively. The orbital energy difference between HOMO and LUMO is approximately 8.687 eV (optical gap, typical experimental value for amorphous SiO₂ is 8.9 eV [26]). Furthermore, when a Ce³⁺ ion is attached to silica ring, the optical gap reduces to 5.203 eV. The FMO energies of spin-up state, from HOMO_-3 to LUMO₊₃, are −8.6193, −8.4598, −8.4525, −8.2862, −3.0834, −2.1748, −2.1138 and −1.7456 eV, respectively. The small energy difference between the E_HOMO (3MR) and E_HOMO (Ce-3MR) indicates that it is relatively easy to make the active electrons transition from the silica matrix to Ce³⁺ ions. Moreover, the E_LUMO (3MR) > E_LUMO (Ce-3MR), implies that the lowest 5d level of Ce³⁺ ions is located below the host conduction band of silica matrix.

 

Fig. 6 Energy eigenvalues and electron density distributions of the HOMO and LUMO (left) 3MR structure model (right) spin-up state of Ce-3MR structure model.

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3.3 Calculated absorption and emission spectra

The absorption and emission spectra of the Ce-3MR structural model are evaluated using TD-DFT theory. Table 1 lists the electronic transitions, oscillator strengths and configurations. The lower oscillator strengths (f<0.0005) are omitted. There are several excitation energy levels of 2.7827, 2.9413, 3.6844 and 4.2674 eV, corresponding to the oscillator strengths (f) 0.0036, 0.0047, 0.1129 and 0.0006, respectively.

Table 1. Excited States Parameters of Ce-3MR Local Structure Model

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Figure 7(a) shows the visual absorption spectrum, which is calculated theoretically and a strong excited state at 336 nm. Moreover, an excitation peak at 320 nm is found from experiment. The peak at 336 nm is mainly attributed to the electronic transitions from excited states S₀-S₃ and S₀-S₄. A significant positive relationship between absorption intensity and oscillator strength is observed. The calculation results show that the transition states of doped materials are S₁ and S₂ state, and the corresponding energy values are 2.7827 and 2.9413 eV. Subsequently, the corresponding excited state is optimized and the emission properties are obtained, as shown in Fig. 7(b). Major excitation energy is found at 2.5478 eV and the corresponding oscillator strength is 0.0548. The emission peak is at 486 nm, which matches with the previous experimental result—a broad emission band is observed in the range of 420-470 nm. In addition, based on the relationship between the fluorescence characteristics and the oscillator strength, the radiative lifetime τ for spontaneous emission is calculated using the Einstein transition probabilities as expressed in the following Eq. (2) (in au) [27]:

 

Fig. 7 Calculated spectra of Ce-3MR local structure model (a) absorption (b) emission.

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Here, c is the velocity of light, E_Flu is the transition energy, and f is the oscillator strength. For the calculation, E_Flu is 2.5478 eV and f is 0.0548. The radiative lifetime τ for the Ce-3MR model is 64.82 ns at 486 nm. Moreover, studies [28,29] also reported that τ value is between 30 and 80 ns.

The experimental results above show that for Ce³⁺ ions, without the influence of silica substrate, a strong emission band is found at 360 nm. However, if the Ce³⁺ ions interacted with silica matrix, the emission peak red shifts to 430 nm. The change in the fluorescence spectra may result from the effect of silica network structure, which leads to location movement of the lowest 5d energy level of the Ce³⁺ ions. M. Raukas et al. arrived at a similar opinion that there exists a relationship between the location of the lowest 5d level of Ce³⁺ ions and host conduction band (CB) [30]. When the lowest 5d level of Ce³⁺ ions is mixed with CB of silica matrix, the Ce³⁺ ions will donate their 4f electrons to the silica matrix through resonant energy transfer of the 5d level with the host CB without light yield. When the lowest 5d level of Ce³⁺ ions locates below the bottom of the CB, the luminescence characteristics of Ce³⁺ ions will clearly reflect consequently. The calculated results show that E_LUMO (3MR) > E_LUMO (Ce-3MR), means that the lowest 5d level of Ce³⁺ ions is located below the CB of silica matrix, and optical gap of the system reduces from 8.687 eV to 5.203 eV, as shown in Fig. 8 (a). Moreover, there exists an absorption edge red shift phenomenon, from 360 to 430 nm in the experiment. Further, quantitative optical properties of the Ce-doped silica fiber are calculated using DFT and TD-DFT theory. The results shows several clear oscillator strengths (f), which are 0.0036, 0.0047, 0.1129 and 0.0006, corresponding to transition wavelengths of 445, 421, 336, and 290 nm, respectively. The maximum oscillator strength is 0.1129, and its absorption peak is at 336 nm, corresponding to an absorption band around 320 nm in the experiment. Furthermore, the excited state of doped material at 421 nm is optimized and the excited properties are evaluated. The calculated emission peak is approximately at 486 nm and the corresponding radiative lifetime τ is approximately 64.82 ns. Correspondingly, it is closed to the broad emission at 430 nm in the experiment. These results show that a transition state near 421 nm exists between the 4f ground state and the excited state at 336 nm, as shown in Fig. 8 (b). The electrons of the excited state at 336 nm will first transfer to transition state near 421 nm without radiation. Subsequently, the electrons of transition state will transfer to ground state with optical transitions, corresponding to a broad band emission between 420 and 490 nm. Moreover, the red lines represent the possible emission process from the transition state to the ground state, and the dashed lines represent non-radiative transitions process. In addition, the radiative lifetime τ calculated also reveals theoretically that the Ce-doped fiber is possible as a sensitive scintillator material.

 

Fig. 8 Energy level diagrams of Ce-3MR local structure model.

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4. Conclusion

In this study, a Ce-doped silica fiber with Ge dopants is prepared by the PIT technique via sol-gel method. The absorption and emission spectra of the proposed optical fiber are measured and analyzed. The excitation peak is at 320 nm, and the emission spectrum is a typical broad emission, which can be ascribed to two contribution bands at 420 and 470 nm. Moreover, a Ce-3MR local structure model is developed and its structure parameters, absorption and emission spectra are calculated using Gaussian-09 software via DFT and TD-DFT theory. The calculated absorption peak is at 336 nm and the emission peak is at 486 nm. In addition, an energy level diagram is then constructed for the doped fiber materials. This study provides a microstructural understanding for the Ce-doped silica fiber materials and a realizable theoretical method that investigates the optical properties of those rare earth doped silica fiber materials.