1. Introduction

White light-emitting diodes (W-LEDs), the next-generation of solid-state lighting, have attracted significant attention recently due to their potential application in many fields, such as devices indicators, backlights, automobile headlights, and general illumination, etc [1–3]. At present, the common way to generate white light is the combination of blue LED chips with yellow-emitting phosphor materials. However, the difference between individual degradation rates of chips and phosphors coated on chips would cause a chromatic aberration and a poor white light performance. Thus it is a challenge to develop suitable rare earth ions (REI) doped materials capable of generating white light under ultraviolet (UV) chips excitation.

Compared to conventional phosphors used for W-LEDs, REI doped white color luminescent glasses possess some obvious advantages [4–7], such as homogeneous light-emitting, simpler manufacture procedure, lower production cost, better thermal stability and epoxy resin free in assembly process. Among the glasses systems, oxyfluoride glasses combine the advantages of high mechanical strength of oxide glasses and low phonon energy of fluoride glasses [8]. And therefore REI doped oxyfluoride white light-emitting glasses have received increasing interest recently [5,8–10].

Use of noble metals as a means for improving spectroscopic properties of REI doped glasses for functional application have attracted much attention recently [11,12]. It was first reported by Malta et al. [13] that an enhanced photoluminescence (PL) for Eu³⁺ ions doped glasses containing silver nanoparticles (NPs) occurred due to surface plasmon resonance (SPR). Sequentially many investigations were carried out on Ag-REI co-doped glasses and the PL enhancements attributing to the SPR of Ag particles were reported [14,15]. On the contrary, some researcher reported that silver just served as a sensitizer for REI in glasses, whereas no enhancement effect was ascribed to Ag NPs [16,17].

Due to the growing controversy related to metal-REI interactions, we investigate the spectroscopic properties of Eu³⁺ ions doped oxyfluoride glasses system containing silver, with the object to understand the interactions of metal and REI. A perfect white light emission was realized by combining intense red emission of Eu³⁺ ions with broad band emission of molecule-like, non-plasmonic Ag particles (ML-Ag-particles). Our research may open a new approach to extend the application of Eu³⁺ doped glasses in W-LED phosphors.

2. Experimental

Samples were prepared by melt-quenching method with the composition of 50SiO₂-20Al₂O₃-30CaF₂ (mol %). The doping species were 1 mol% EuF₃ and x wt% AgNO₃ (x=0, 5, 7) (named as G1, G2, G3, respectively). Another sample (named as G4) doping with 5 wt% AgNO₃ only was also elaborated. Raw chemicals used were first homogeneously mixed and melted at 1400 °C for 45 min in a covered corundum crucible in air. The melts were poured onto a 300 °C pre-heated stainless-steel mold, and then pressed by another plate, and cooled down to room temperature to form glasses. At last, all samples were submitted to heat treatment at 350 °C for 10 h, and then cooled to room temperature.

Microstructures of samples were observed by transmission electron microscopy (TEM) on a JEM-2010 instrument. Optical absorption spectra of samples were measured in UV-Vis range, using a Hitachi UV-3900 UV-Vis spectrophotometer. Excitation and emission spectra were recorded on an Edinburgh Instruments FS920 spectrofluorometer by using a continuous wave 450 W Xe lamp as the excitation source. All the experiments were carried out at room temperature.

3. Results and discussion

Figure 1 (a) shows the absorption spectra of G1, G2 and G3 samples in UV-Vis regions. The obvious absorption bands located at 393 nm and 464 nm can be attributed to Eu³⁺ ions absorption corresponding to the transitions from the ground state ⁷F₀ to excited states ⁵L₆ and ⁵D₂, respectively. There is no obvious absorption band from surface plasmon absorption of silver nanoparticles (usually centered around 400 + 20 nm [12]) in Fig. 1 (a), indicating that perhaps there is no silver nanoparticles in glass matrix.

 

Fig. 1 (a) Absorption spectra of G1, G2 and G3 samples, (b) TEM images of G2 sample, inset shows the selected area electron diffraction (SAED) patterns of G2 sample.

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XRD patterns (figure not show here) of all samples indicate that glasses are structurally amorphous characterized by the diffuse humps and no diffraction peaks from silver particles are observed. Figure 1(b) displays TEM image of G2 sample. Glasses are amorphous and no obvious silver nanoparticles are observed in TEM image. The selected area electron diffraction (SAED) patterns inserted further prove that there is no Ag nanoparticles exist in glass matrix.

The missing nanoparticles signals in absorption spectra, XRD patterns, TEM images and SAED patterns prove that silver in glasses is not in the form of Ag nanoparticles and indicated that silver plasmonic filed enhancement effect on luminescence will not be observed. We suppose that silver in glasses may exist in the form of very small, molecule-like, non-plasmonic Ag particles (ML-Ag-particles) [12, 18,19].

Excitation and emission spectra of G1, G2 and G3 samples are investigated and presented in Fig. 2(a) and (b) , respectively. Excitation spectra were obtained by monitoring the ⁵D₀→⁷F₂ transition of Eu³⁺ ions (613 nm). It can be clearly seen that excitation spectra of G1 sample consist of two parts: one broad band at 240 nm and several sharp peaks in 290-480 nm region. Obviously, the first one results from the well-known O²⁻-Eu³⁺ charge transfer (CT) band, and the other sharp peaks are related to f-f transitions of Eu³⁺ ions. The strongest excitation peak at 393 nm is assigned to ⁷F₀→⁵L₆ transition of Eu³⁺ ions. No apparent changes of peak positions and intensities in excitation spectra ascribing to Eu³⁺ ions were observed in G2 and G3 samples.

 

Fig. 2 (a) Excitation spectra (λ_em=613 nm), and (b) emission spectra (λ_ex=393 nm) of G1, G2 and G3 samples.

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Significantly, an additional excitation band from 325 nm to 375 nm was observed in G2 and G3 samples with AgNO₃ dopant. This excitation band is due to very small ML-Ag-particles according to literature data [12,18] and can be unambiguously distinguished from the distinct wavelengths and shapes of the well-known surface plasmon resonance absorption of silver nanoparticles (400+20 nm) [12].

In emission spectra (excited by 393 nm), the characteristic emission peaks of Eu³⁺ within the wavelength range from 580 to 720 nm were detected. The emission bands of Eu³⁺ ions can be assigned to ⁵D₀→⁷F₀ (578 nm), ⁵D₀→⁷F₁ (591 nm), ⁵D₀→⁷F₂ (613 nm), ⁵D₀→⁷F₃ (653 nm), and ⁵D₀→⁷F₄ (701 nm) transitions, respectively.

Meanwhile, emission spectra of G2 and G3 samples consist of not only sharp emission peaks from Eu³⁺ ions but also an additional broad band from 400 to 600 nm, which may be originated from emission of ML-Ag-particles [12].

In order to confirm the origin of the additional excitation (325-375 nm) and emission bands (400-600 nm), AgNO₃ single doped G4 sample was prepared. Excitation (monitored at 440 nm) and emission (excited by 352 nm) spectra of G4 sample are displayed in Fig. 3 . Obviously, excitation spectra exhibit a broad band ranges from 325 to 375 nm. And a broad band at 400-700 nm with a peak at 440nm is observed in emission spectra. The broad excitation and emission bands are consistent with the additional bands that we ascribed to ML-Ag-particles [12] of G2 and G3 samples in Fig. 2.

 

Fig. 3 (a) Excitation spectra (λ_em=440 nm) and (b) emission spectra (λ_ex=352 nm) of G4 samples.

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A pure glass without AgNO₃ and EuF₃ dopants was also prepared and there is no emission can be detected excited by 352 nm, which indicates that the broad excitation and emission bands are not caused by glass matrix. Such phenomenon further proves the broad emission originates from ML-Ag-particles.

It is worthwhile to notice that the emission band of ML-Ag-particles located at blue and green regions and the emission band of Eu³⁺ ions located at red region. Combining emissions of ML-Ag-particles and Eu³⁺ ions, white light emission can be achieved. From excitation spectra of co-doped samples (Fig. 2(a)) and excitation spectra of G4 sample (Fig. 3(a)), the light from 325 nm to 375 nm may be used as excitation source to generate white light-emitting, matching well with the emission bands of the UV LED chips.

Figure 4(a) depicts emission spectra of G1, G2 and G3 samples under 347 nm excitation. It can be clearly seen that emission spectra of G2 and G3 samples consist of two parts: one broad band at 400-700 nm and sharp emission peaks in 580-720 nm region. The broad band is due to emission of ML-Ag-particles [12] and the sharp peaks are related to ⁵D₀→⁷F_J (J=0-4) transitions of Eu³⁺ ions. It should be mentioned that G1 sample also presents a broad band emission at 400-500 nm, and it certainly does not belong to glass matrix, Eu³⁺ ions or ML-Ag-particles. This broad emission could be ascribed to 5d-4f transition of Eu²⁺ ions, because Eu³⁺ ions can be partly reduced to Eu²⁺ ions in air at high temperature in glass materials [20].

 

Fig. 4 (a) Emission spectra (λ_ex=347 nm) G1, G2 and G3 samples. The insert shows photo of samples excited by 365 nm UV lamp. (b) The CIE chromaticity diagram corresponding to emission light of G1, G2 and G3 samples under 347 nm excitation.

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Luminescence colors of samples excited at 347 nm are characterized by Commission International de I’Eclairage (CIE) chromaticity diagram and shown in Fig. 4(b). The color shifts from blue to white with increasing AgNO₃ content. Remarkably, the CIE coordinates of G3 sample (X=0.333, Y=0.307) is close to the standard equal energy white light illuminate (X=0.333, Y=0.333). The insert photo of Fig. 4(a) gives a luminescence of samples excited by 365 nm UV lamp and a perfect white light was observed in G3 sample. All the characteristics indicate that Ag and Eu³⁺ co-doped oxyfluoride glasses can act as promising white-emitting phosphors for UV LED chips.

Emission intensity of Eu³⁺ ions in Fig. 4(a) increased monotonously with the content of AgNO₃, evidencing an efficient energy transfer (ET) from ML-Ag-particles to Eu³⁺ ions. This phenomenon is also consistent with the excitation spectra of co-doped samples (Fig. 2(a)). The energy level diagram of ML-Ag-particles and Eu³⁺ ions and possible energy transfer process are schematically presented in Fig. 5 . Upon 347 nm excitation, ML-Ag-particles are excited from the ground state to the excited state. Because the excited state of ML-Ag-particles and the ⁵L₆ level of Eu³⁺ ions are energetically close to each other, energy transfer from ML-Ag-particles to Eu³⁺ ions can easily proceed. An excited ML-Ag-particles relaxes from the excited state to the ground state nonradiatively and transfers the excitation energy to a neighboring Eu³⁺ ion, promoting it from ⁷F₀ ground state to ⁵L₆ level (Fig. 5, ET). After then, Eu³⁺ ions in the populated ⁵L₆ level undergo multi-phonon relaxation to luminescent ⁵D₀ level and radiatively relax to ⁷F_J level, resulting in characteristic emissions of Eu³⁺ ions. White light emission can be achieved by combining emissions of ML-Ag-particles and Eu³⁺ ions.

 

Fig. 5 Energy level diagram of Eu³⁺ ions and ML-Ag-particles and possible energy transfer process.

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4. Conclusions

The Ag and Eu³⁺ co-doped oxyfluoride glasses were prepared by melt-quenching technique. Structural and luminescent investigations prove the broad excited band (325-375 nm) and the emission band (400-700 nm) are ascribed to very small ML-Ag-particles. The color of luminescence could be adjusted by varying the proportions of AgNO₃ and an energy transfer process from ML-Ag-particles to Eu³⁺ ions was put forward to explain the origin of white luminescence. A perfect white light emission with the CIE coordinates (X=0.333, Y=0.307) was obtained under 347 nm excitation. Our results show that these glasses may provide a new platform to design and fabricate novel luminescent materials for UV LED chips in the future.