We investigated the photoluminescence properties and decay curves of SiO2 -AL2O³- Na₂O - BaO-B₂O₃-x TiO₂ glasses prepared via the conventional melt quenching method. A broad emission band 400-800 nm peaking at 500 nm was observed in emission spectrum, while a very broad excitation spectral region (from 250 nm to 430 nm) and two main absorption bands at 260 and 400 nm were found. The fluorescence lifetime was measured and all the decay curves were approximated by an exponential decay function. Electron transition occurring between surface defects, oxygen vacancies or self-trapped excitons, support the observed spectroscopic data.

Introduction

Applications of transparent glass materials cover from ordinary apparatuses (such as window glasses and glass containers) to precise optical devices (such as lens and fibers) [1-6]. For example, in photovoltaic (PV) modules for solar energy utility, a thin protective layer of superstrate glass with excellent mechanical rigidity, chemical stability, and optical transparency, is laminated on the semiconductor. The spectral transmittance is well considered to maximize the PV efficiency. In crystalline silicon PV modules, any absorption in the superstrate glass from 400 to 1000 nm will introduce additional loss to the final PV efficiency.

In fact, now, novel glasses possessing special emission band are being extensively developed to resolve this issue. Transition metal and rare earth ions are intentionally doped into glasses as active centers and endow those transparent hosts with luminescent properties at various wavelength [7-10]. Many studies have pointed out the relationships between the structure of the host glass and the properties of the doped ions. The transition metal or rare earth ions doped glasses have attracted a great deal of interest because of their well-defined and sharp energy levels and the modifications of the energy level structure.

White light glasses now have found their applications in visual display and decorative art [11-15]. Generally, white light is synthesized by the excitation of a crystalline phosphorus with an Ultra Violet (UV) In GaN LED chip. However, given that conventional LEDs are encapsulated with an epoxy resin, the increase temperature caused by the LED chip can cause resin deterioration, reducing the quality and the intensity of light emission. To replace the crystalline phosphors, one of the alternatives is the glassy systems doped transition metal or rare earth ions, due to their simple fabrication process, low production costs, high transparency and thermal stability. Here, we develop Ti⁴⁺ doped SiO₂ -AL₂O₃ -Na₂O –BaO -B₂O₃ glass which has a broad emission band 400-800 nm. Decay curves and photoluminescence were determined for the prepared glasses.

Materials and Methods

The linear accelerator

The following chemical reagents were used as starting materials: SiO₂ (99.0%), AL₂O₃ (99.9), Na₂CO₃ (99.8%), BaO (97.0%), H₃BO₃ (99.5%), TiO₂ (99.99%). In the following, sample nomenclature is xSyAzB. E.g., 30S15A20B for x = 3, y=15 and z=20. Batches of starting materials were mixed homogeneously in an agate mortar and melted in a pure alumina crucible at 1050 °C for 1h in the air, and then were quickly poured onto steel plate preheated at 400 °C, and cooled to room temperature to obtain glass. All glass samples were cut and polish in proper shape for further studies. The preparing conditions listed in Table 1 (composition, melting temperature) were determined based on the many experimental trials. The visible luminescence spectra and decay curves were measured with a high-resolution spectrofluorometer (UK, Edinburgh Instruments, and FLS980) equipped with a 500 W Xenon lamp as an excitation source, with a Hamamatsu R928P visible photomultiplier (250 nm-850 nm). The measured spectral ranges for excitation and emission were 250-430 and 400-800 nm, respectively.

Results and Discussion

The photoluminescence analysis is used for measuring of recombination of electron-hole radiative. This spectrum is used here for investigation of defects that created due to adding Titanium ion.

Figure 1 shows the emission spectrum of 30S15A20B glass excited at 254 nm. A single broad emission band was observed. Photoluminescence spectrum can be caused by surface defects, oxygen vacancy or self-trapped excitons [16-19]. In addition, surface chemistry can effect on this spectrum. The emission band at 506 nm is probably related to surface bonds or oxygen vacancy on the surface of TiO₂ crystal. Also, it is believed that 30S15A20B glass acts as electron saver for electron absorption that TiO₂ produced under light source irradiation and excited by a photon. This phenomenon resulted in decrease electron population at TiO₂ and inhibition of electron-hole (e- / h+ pair) recombination. Figure 2 shows the excitation spectrum of 30S15A20B glass. The excitation spectrum covers a very broad spectral region (from 250 nm to 430 nm) and contain two main absorption bands at 264 and 408 nm.

Figure 1: Emission spectrum of 30S15A20B glass excited at 254 nm.

Figure 2: Excitation spectrum of 30S15A20B glass monitored at 500 nm.

Figure 3 shows the emission spectrum of 30S10A25B glass excited at 254 nm. Under excitation at 254 nm, the glass exhibits the emission peaking at 510 nm in30S10A25B glass. The strong peak that its center is 510 nm is probably due to the dangling bonds or oxygen vacancies in TiO₂ crystal surface.

Figure 3: Emission spectrum of 30S10A25B glass excited at 254 nm.

Figure 4 shows the emission spectrum of 35S10A20B glass excited at 254 nm. Under excitation at 254 nm, the glass exhibits the emission peaking at 490 nm in 35S10A20B glass.

Figure 4: Emission spectrum of 35S10A20B glass excited at 254 nm.

Figure 5 shows the fluorescence decay curves of 30S15A20B glass by monitoring 506 nm emission under the excitation of 264 nm light. The fluorescence lifetime is 3.993ms. Figure 6 shows the fluorescence decay curves of 30S15A20B glass by monitoring 506 nm emission under the excitation of 408 nm light. The fluorescence lifetime is 1.649ms. The decay curves can be well fitted by the exponential function [20]:

Figure 5: Fluorescent decay curves of 30S15A20B glass. The excitation and monitoring wavelengths are 264 nm and 506 nm, respectively.

Figure 6: Fluorescent decay curves of 30S15A20B glass. The excitation and monitoring wavelengths are 408 nm and 506 nm, respectively.

I (t) =A1 exp(-t / t1)+I0 (1)

Where I(t) is relative fluorescence intensity at decay time t, A¹ and I0 are constants, and τ¹ represents the lifetime.

Conclusions

We synthesized Ti⁴⁺-doped SiO₂ -AL₂O₃ -Na₂O –BaO -B₂O₃ glasses via the conventional melt quenching method. We investigated the emission spectra and excitation spectra and decay curves of different glass composition. The 30S15A20B, 30S10A25B, 35S10A20B glasses show the broad emission band 400-800 nm peaking at 506 nm, 510 nm and 490 nm, respectively. Moreover, the excitation spectrum covers a very broad spectral region (from 250 nm to 430 nm) and contain two main absorption bands at 260 and 400 nm. The fluorescence decay curves of 30S15A20B glass can be well fitted by the exponential function. The fluorescent lifetime by using 264 nm light excitation (3.993 ms) is larger than that by using 408 nm ligh excitation (1.649 ms).

Acknowledgement

We acknowledges financial support from the National Natural Science Foundation of China (Grant Nos. 51602063, 21671045, 21471038 and 51472091). This work was also supported by the Tip‐Top Scientific and Technological Innovative Youth Talents of Guangdong Special Support Program (2016TQ03R281) and Open Fund of the State Key Laboratory of Luminescent Materials and Devices (South China University of Technology).

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