1. Introduction

During the past decades, white light-emitting diodes (W-LEDs) have attracted considerable attention as light source to replace fluorescent and incandescent lamps due to their vital economic and technological advantages in power efficiency and environmental friendly character [1–7]. More recently a new approach to obtain white light, the combination of ultraviolet (UV) LED chip with red, green and blue phosphors, has aroused scientists’ great interest because it can offer tunable color temperature and high color-rendering index [8]. Specifically, several significant works have been reported in advancing the deep UV and mid UV emitter technology by using high Al-content AlGaN [9] and AlInN [10] alloys, which enable the practical implementation of UV-excitation white LEDs. Thus, it is urgent to develop novel single-phased multi-activators co-doped systems capable of emitting white light under UV chip excitation, which are based on the luminescence and energy transfer (ET) between multi-activators [8,17–21].

On the other hand, compared with conventional powder phosphors used for W-LEDs, transparent luminescent glasses show better mechanical properties, lower production cost, simpler manufacture procedure, excellent optical properties, and epoxy resin free in assembly process. Consequently, the investigations on application of glasses in W-LEDs have been carried out extensively [7,11–19]. Highly transparent sodium silicate glasses were suggested to be excellent host glasses for rare earth ions (REI) to design and develop highly efficient luminescent materials because of their good solubility for REI without forming ion clusters and wide range for chemical modifications [12].

Luminescence of Cu⁺ comes from excited s state to ground d state (d⁹s→d¹⁰) transition, which is strictly forbidden for free ions but partially allowed in solids by electronic coupling with lattice vibrations of odd parity [12,20]. Cu⁺ ions exhibit broad excitation at UV band and wide emission at visible (VIS) region, which are very sensitive to their environmental conditions, such as covalence, coordination number, and site symmetry. Our previous work showed that Cu⁺ doped sodium silicate glasses can emit yellowish green light under the excitation of UV light [12].

In this paper, the luminescent properties of Cu⁺, Sm³⁺ single- and co-doped sodium silicate glasses and ET process from Cu⁺ to Sm³⁺ were investigated systematically. Adjustable emissions from yellowish green to orange and white were obtained.

2. Experimental

The nominal host composition is 73.2SiO₂-15.3Na₂CO₃-6MgO-2ZnO-3.5CaCO₃ (in wt%). Samples G-host (the host), G-2Cu (host doped with 2CuO only), G-1Sm (doped with 1Sm₂O₃ only), G-2CuxSm (host doped with 2CuO and xSm₂O₃, x = 0, 0.5, 1, 1.5, 3, 5) and G-2Cu1SmyCe (host doped with 2CuO, 1Sm₂O₃ and yCeO₂, y = 1, 2, 3) were prepared by melt-quenching method. Raw materials were first mixed and ground thoroughly in an agate mortar by adding ethanol for 30 minutes and dried in an oven at 100 °C for 20 minutes. Then the mixtures were melted in a covered corundum crucible at 1500 °C for 1 h at air atmosphere. The melt was poured onto a 300 °C preheated stainless steel mold and pressed by another plate, then cooled down to room temperature to form glasses. Finally, all glasses were sliced and polished with thickness of 1.5 mm for optical measurements.

Absorption spectra were measured by a U-3900 UV-VIS spectrophotometer (Hitachi). Excitation and emission spectra were recorded by FLS920 spectrofluorometer (Edinburgh Instruments) by using a continuous wave 450 W Xe lamp as excitation source. Decay curves were measured by using a microsecond flashlamp (μF900) as excitation source. All the measurements were carried out at room temperature.

3. Results and discussion

Figure 1 reveals transmission spectra of G-host, G-1Sm and G-2CuxSm (x = 0, 0.5, 1, 1.5, 3, 5) samples in UV-VIS region, and the gray dash-dotted line is the difference spectra of G-host and G-2Cu samples. All samples are highly transparent (above 85%) and colorless. Compared with G-host and G-1Sm samples, G-2CuxSm samples present a distinct absorption band range from 250 to 360 nm centered at 320 nm, which can be generally attributed to the transition of Cu⁺ from ground d state to excited s state [12]. For Sm³⁺ doped G-1Sm and G-2CuxSm samples, characteristic absorption from f-f transitions of Sm³⁺ can be observed. The dominant absorption peaks locate at 401 nm (⁶H_5/2→⁴F_7/2) and 471 nm (⁶H_5/2→⁴I_11/2) [21,22]. And the absorption peaks become more distinct with the increase of Sm³⁺ concentration. Besides, no other obvious absorption band from 510 to 800 nm can be observed, indicating the absence of Cu²⁺ or Cu nanoparticles in glasses [12,23].

 

Fig. 1 Transmission spectra of G-host, G-1Sm and G-2CuxSm (x = 0, 0.5, 1, 1.5, 3, 5) samples in UV-VIS regions, and the gray dash-dotted line is the difference spectra of G-host and G-2Cu samples.

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Excitation and emission spectra of G-2Cu sample are given in Fig. 2.In excitation spectra (λ_em = 532 nm), a broad excitation band ranging from 250 to 360 nm with a maximum at about 320 nm arising from d→s transition of Cu⁺ is detected [12], which is consistent with the absorption band of Cu⁺ in Fig. 1. The high excitation intensity at 300-360 nm suggests that Cu-doped sodium silicate glasses may be good candidates for UV-pumped W-LEDs. Meanwhile, under the excitation of 320 nm, G-2Cu sample presents a yellowish green band centered at 532 nm, corresponding to s→d transition of Cu⁺ [12].

 

Fig. 2 Excitation (λ_em = 532 nm) and emission (λ_ex = 320 nm) spectra of G-2Cu sample, and excitation (λ_em = 601 nm) and emission spectra (λ_ex = 401 nm) of G-1Sm sample.

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Excitation (λ_em = 601 nm) and emission spectra (λ_ex = 401 nm) of G-1Sm sample are plotted in Fig. 2 as well. It can be clearly seen that the excitation spectra of G-1Sm sample consist of several sharp peaks at 343, 360, 373, 401 and 471 nm, which are the characteristic f-f transitions of Sm³⁺ [21]. Excitation bands at 401 nm (⁶H_5/2→⁴F_7/2) and 471 nm (⁶H_5/2→⁴I_11/2) are dominant, in agreement with the absorption bands of Sm³⁺ ions in Fig. 1. The emission spectra are composed of four manifolds with maxima at 563, 601, 650 and 707 nm and are assigned to the ⁴G_5/2→⁶H_J (J = 5/2, 7/2, 9/2, 11/2) transitions of Sm³⁺, respectively [21]. Interestingly, G-2Cu sample exhibits a broad emission band at 400-750 nm, while G-Sm1 sample shows an absorption band ranging from 400 to 500 nm. It means that there is a big overlap between emission of Cu⁺ and excitation of Sm³⁺ in the range of 400-500 nm. Thus, it is reasonable to suppose that ET process can occur from Cu⁺ to Sm³⁺ in sodium silicate glasses [24].

Figure 3(a) and Fig. 3(b) reveal emission spectra (λ_ex = 320 nm) and excitation spectra (λ_em = 601 nm) of G-2Cu, G-1Sm and G-2CuxSm (x = 0.5, 1, 1.5, 3, 5) samples, respectively. As shown in Fig. 3(a), excited by 320 nm light, the optimal excitation wavelength for Cu⁺ but not for Sm³⁺, almost no emission can be detected in Sm³⁺ single doped G-1Sm sample. While emission spectra of G-2CuxSm samples not only include yellowish green emission from Cu⁺ but also contain orange emission from Sm³⁺ with the characteristic peaks at 563, 601, 650 and 707 nm. With increasing Sm³⁺ content, emission intensity of Cu⁺ decreases, while that of Sm³⁺ increases firstly and reaches a maximum at x = 1 and then remarkably decreases when Sm³⁺ content is further increased due to concentration quenching. Excitation spectra of Cu⁺, Sm³⁺ co-doped G-2CuxSm samples monitored Sm³⁺ emission (λ_em = 601 nm) ranging from 250 to 360 nm given in Fig. 3(b) are almost similar with excitation spectrum of G-2Cu sample monitored at 601 nm emission of Cu⁺. All above phenomena indicate that high light output of Sm³⁺ emission actually comes from ET process from Cu⁺ to Sm³⁺.

 

Fig. 3 (a) Emission (λ_ex = 320 nm) and (b) excitation (λ_em = 601 nm) spectra of G-1Sm and G-2CuxSm (x = 0, 0.5, 1, 1.5, 3, 5); (c) Decay curves of Cu⁺ emission (λ_ex = 320 nm, λ_em = 532 nm) in G-2CuxSm (x = 0, 1, 3, 5); (d) Energy level diagram of Cu⁺ and Sm³⁺ in sodium silicate glasses and possible energy transfer process from Cu⁺ to Sm³⁺.

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To further prove the ET process from Cu⁺ to Sm³⁺, decay curves of Cu⁺ emission were measured (λ_ex = 320 nm, λ_em = 532 nm) and are given in Fig. 3(c). Since all curves are non-exponential, the decay processes of these samples are characterized by average lifetime (${\overline{\tau }}_x$), which can be evaluated by [14]

ET efficiency (${\eta }_{ET}$) can be estimated by the following formula [1,25],

The energy-level diagram of Cu⁺ and Sm³⁺ and possible ET process are schematically plotted in Fig. 3(d). Under 320 nm excitation, Cu⁺ ions are excited from ground d state to excited s state. The excited Cu⁺ ions at high phonon energy level non-radiatively relax to low phonon energy level (that is Stokes shift). They can partly radiate back to ground state with emitting 532 nm yellowish green light. As the excited state of Cu⁺ is energetically close to the ⁴I_11/2 and other levels of Sm³⁺, the ET process from Cu⁺ to Sm³⁺ can easily proceed. An excited Cu⁺ ion relaxes from excited state to ground state non-radiatively and transfers the excitation energy to a neighboring Sm³⁺, promoting it from ⁶H_5/2 ground state to ⁴I_11/2 level and other levels [Fig. 3(d), ET]. After then, Sm³⁺ ions in the populated levels undergo multi-phonon relaxation to luminescent ⁴G_5/2 level and radiatively relax to ⁶H_J/2 (J = 5, 7, 9, 11) levels, resulting in characteristic orange emissions of Sm³⁺.

Emission of Cu⁺ ions locates at yellowish green region and that of Sm³⁺ ions is at orange region. Combining both of them, white light emission may be achieved. Hence, luminescent colors of G-2CuxSm (x = 0, 0.5, 1, 1.5, 3, 5) samples excited by 320 nm light are characterized by Commission International de I’Eclairage (CIE) chromaticity diagram and displayed in Fig. 4.It is clear that the chromaticity coordinates of G-2CuxSm samples gradually move from yellowish green to orange region with increasing x.

 

Fig. 4 CIE chromaticity diagrams corresponding to emission light of G-2CuxSm (x = 0, 0.5, 1, 1.5, 3, 5) and G-2Cu1SmyCe (y = 1, 2, 3) samples (λ_ex = 320 nm).

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In order to obtain perfect white-light emission, G-2Cu1SmyCe (y = 1, 2, 3) co-doped with Ce³⁺ ions were elaborated and their CIE chromaticity diagram is also displayed in Fig. 4. The CIE chromaticity coordinate (X, Y) of emission of G-2CuxSm (x = 0, 0.5, 1, 1.5, 3, 5) and G-2Cu1SmyCe (y = 1, 2, 3) samples (λ_ex = 320 nm) are listed in Table 1. A perfect white light emission can be obtained for G-2Cu1Sm1Ce sample and the CIE coordinates (X = 0.320, Y = 0.321) is very close to the standard equal energy white light illuminate (X = 0.333, Y = 0.333). The inset of Fig. 4 gives yellowish green, orange and perfect white luminescent photos of G-2Cu, G-2Cu1Sm and G-2Cu1Sm1Ce samples taken under 320 nm excitation in dark, respectively, further indicating that Cu⁺ and Sm³⁺ co-doped sodium silicate glasses could be used as converting phosphors for UV LED chips to generate W-LEDs.

Table 1. Chromaticity coordinates (X, Y) corresponding to emission light of G-2CuxSm (x = 0, 0.5, 1, 1.5, 3, 5) and G-2Cu1SmyCe (y = 1, 2, 3) samples (λ_ex = 320 nm).

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4. Conclusion

Efficient energy transfer from Cu⁺ to Sm³⁺ in Cu⁺,Sm³⁺ co-doped highly transparent (above 85%) sodium silicate glasses was investigated systemically. Excited by 320 nm UV light, varied hues from yellowish green to orange were obtained by combining the emission of Cu⁺ and Sm³⁺ ions. In addition, a perfect white emission with CIE coordinates (X = 0.320, Y = 0.321) was realized in Cu⁺,Sm³⁺,Ce³⁺ co-doped samples. Our results may extend the understanding of interactions between Cu⁺ and REI, and Cu⁺,Sm³⁺ co-doped glasses may provide a new platform to design and fabricate novel luminescent materials for W-LEDs in the future.