1. Introduction

Over the past decades, white light-emitting diodes (W-LEDs), the next generation of solid-state light source, have attracted significant attention not only due to their long lifetime, environment benefits and low energy consumption, but also their wide applications for displays and lighting, such as devices indicators, backlights, automobile headlights and general illumination, etc [1–6].

Recently, W-LEDs fabricated by ultraviolet (UV) LED chip coupled with tri-color phosphors have been investigated extensively due to the remarkable development of UV diodes. However, the efficiency of W-LEDs developed by this method is low since the strong re-absorption of blue light by green and red phosphors [3]. Therefore, it is urgent and interesting 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 [7–11]. Besides, compared to conventional phosphors used for W-LEDs, luminescent glasses not only present excellent optical properties, better mechanical properties, lower production cost and simpler manufacturing procedures, but also show an epoxy-resin free assembly process [12–14].

Consequently, white light has been generated by doping multi-activators such as Ce³⁺/Mn²⁺ [9–11], Eu²⁺/Mn²⁺ [7, 8], Ce³⁺/Sm³⁺ [15] and Tb³⁺/Sm³⁺ [16] in various powder phosphors and glasses. More interesting, the luminescent properties and ET process of noble metal species and rare earth ions (REI) co-doped glasses such as (Cu⁺)₂/Eu³⁺ [14], Ag/Eu³⁺ [13, 17], Au/Eu³⁺ [17], Ag/Sm³⁺ [17, 18] and Au/Sm³⁺ [17] co-doped systems have become an attractive subject to achieve tunable luminescence. But, to the best of our knowledge, no report has been carried out on Cu⁺, Sm³⁺ co-doped glasses.

Fluorescence 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. Generally, the s excited state splits into singlet (¹T_2g, ¹E_g) and triplet (³T_2g, ³E_g) parts, and after that the triplets may be split further by spin-orbit interactions [14, 19–21]. Thus Cu⁺ ions exhibit broad excitation at UV band and wide emission at visible (VIS) region, which are very sensitive to host compositions. Accordingly, for Cu⁺, Sm³⁺ co-doped glasses, the ET process from Cu⁺ to Sm³⁺ can occur, and consequently the luminescence may be tuned.

In this manuscript, borosilicate glasses were selected as host materials [22]. The luminescent properties of Cu⁺, Sm³⁺ single- and co-doped borosilicate glasses and ET process from Cu⁺ exhibit to Sm³⁺ were investigated systematically. Adjustable emissions from blue white through pure white to yellow white were obtained.

2. Experimental

The nominal host composition (in mol%) is 50SiO₂-14B₂O₃-10Al₂O₃-3Na₂O-8MgO-8ZnO-3.5ZrO₂-3.5Gd₂O₃. Samples G-host (the host), GSm2 (doped with 2 SmO_3/2), GCux (xCuO), GCu2Smy (2CuO and ySmO_3/2) were prepared by melt-quenching method. Raw materials were first mixed homogeneously in an agate mortar by adding ethanol and dried in an oven at 120 °C for 20 min, then melted in a covered corundum crucible at 1400 °C for 1 h in air atmosphere. The melts were 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 glass samples were sliced and polished with thickness of 2 mm for optical measurements.

Absorption spectra were measured by a U-3900 UV-VIS spectrophotometer (Hitachi). 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, and external quantum efficiency was obtained using a BaSO₄-coated integration sphere. Decay curves were performed on a Cary Eclipse spectrofluorometer (Varian Instruments). All the experiments were carried out at room temperature.

3. Results and discussion

Figure 1 reveals transmission spectra of G-host, GSm2 and GCu2Smy (y = 0, 1, 2, 4, 6) samples in UV-VIS regions. All samples are highly transparent (about 82%). The sharp absorption bands located at 359, 373, 402 and 470 nm in GSm2 sample can be assigned to transitions from ⁶H_5/2 ground state to ⁴H_7/2, ⁶P_7/2, ⁴F_7/2 and ⁴I_11/2 excited states of Sm³⁺ ions, respectively [23]. Also, the absorption at 402 and 470 nm in GCu2Smy samples becomes more distinct with the increase of Sm³⁺ content. Significantly, compared to G-host and GSm2 samples, GCu2 and GCu2Smy samples present an obvious absorption from 250 to 380 nm, which can be generally attributed to Cu⁺ corresponding to the transition from ground d state to excited s states [14]. Besides, there is no obvious absorption band from 510 to 800 nm, which confirms that there is no Cu nanoparticles [24] or Cu²⁺ [25] in glasses.

 

Fig. 1 Transmission spectra of G-host, GSm2 and GCu2Smy (y = 0, 1, 2, 4, 6) samples.

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Excitation and emission spectra of GCu2 sample are given in Fig. 2(a) and 2(b), respectively. In excitation spectra (λ_em = 540 nm), a broad excitation band ranging from 250 to 360 nm with a maximum at about 330 nm is observed, arising from d→s transition of Cu⁺, which is consistent with the absorption band of Cu⁺ in Fig. 1. The high excitation intensity at 300-360 nm suggests that Cu-doped borosilicate glasses may be good candidates for UV-pumped W-LEDs. Meanwhile, under the excitation of 330 nm, GCu2 sample presents a green-yellow emitting band centered at 540 nm, corresponding to transition from s→d transition of Cu⁺ [14].

 

Fig. 2 (a) Excitation spectra, (b) emission spectra and (c) decay curves of GCu2 sample.

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More interesting, monitored at 440 nm emission, the excitation peaks experience a blue shift from 330 to 280 nm. And the continuous shortening of the excitation wavelength results in a successive blue shift of the respective emission spectra.

In our previous work, blue shift of excitation and emission bands with decreasing Cu⁺ content was detected and this phenomenon was caused by the energy level structure of Cu⁺ changed with distances of Cu⁺, namely, the change of crystal field strength. With decreasing Cu⁺ content, the Cu⁺-Cu⁺ distance is increased and the interaction of Cu⁺-Cu⁺ is weakened. That is, the ligand field strength surrounding Cu⁺ is weakened, making s states more away form d ground state. Thus, extending of Cu⁺-Cu⁺ distance would produce blue shift [14, 19]. A similar situation was also reported in Ag species doped oxyfluoride glasses and the change of Ag₄²⁺ energy configuration with Ag-Ag distances was accounted for their phenomenon [26, 27]. In the present work, with the decrease of Cu⁺ content, blue shift of emission band excited by the same wavelength is also observed.

Figure 3 shows (a) excitation (λ_em = 540 nm) and (b) emission (λ_ex = 330 nm) spectra of GCux samples (x = 0.5, 1, 2, 3). With decreasing Cu⁺ content, excitation peaks in Fig. 3(a) experience a blue shift from 330 to 310 nm. And emission peaks in Fig. 3(b) show a blue shift from 560 to 460 nm also.

 

Fig. 3 (a) Excitation spectra and (b) emission spectra of GCuy (y = 0.5, 1, 2, 3) samples.

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Therefore, it is reasonable to suppose that 330 excitation and related 540 nm emission are due to Cu⁺ with short distance, while 280 nm excitation and related 440 nm emission are owing to Cu⁺ with long distance. In other words, long wavelength emission is attributed to Cu⁺ with strong ligand field, while short wavelength emission is assigned to Cu⁺ with weak ligand field. And the successive blue shift of emission spectra with shortening of excitation wavelength is owing to the different distribution crystal field for Cu⁺ ions. The different crystal field may relate to the lattice positions (mostly substitute Na⁺ positions due to their similar ionic radii and ionic valence) and (or) interstitials that Cu⁺ located. For samples with low Cu⁺ content, Cu⁺ ions prefer locate in weak ligand field and will result in the blue shift of excitation and emission bands.

To further testify this point, decay curves of GCu2 sample were measured and are shown in Fig. 2(c). Non-exponential luminescence decay curves are obtained. Hence, the lifetimes are characterized by average lifetime ($\overline{\tau }$), and can be derived by [28],

As mentioned above, the luminescent color can be tuned by different excitation wavelength. Figure 4(a) and Table 1 shows CIE chromaticity coordinate (x, y) of emissions of GCu2 sample for excitation wavelength from 280 to 330 nm. A 10 nm variation in excitation wavelength leads to a significant change in CIE color coordinate, and the emission shifts from blue through green white to green-yellow region. Figure 4(b) gives blue, green white and green-yellow luminescent photos of GCu2 sample taken under 280, 310 and 330 nm excitations in dark, respectively. In order to obtain perfect white-light emission, Cu⁺, Sm³⁺ co-doped samples were elaborated and investigated.

 

Fig. 4 (a) The CIE chromaticity diagram corresponding to emission of GCu2 and GCu2Sm2 samples for excitation wavelengths from 280 to 330 nm with 10 nm steps, photos of (b) GCu2 and (c) GCu2Sm2 samples excited by 280, 310 and 330 nm light, respectively.

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Table 1. Chromaticity coordinates (x, y) of GCu2 and GCu2Sm2 samples under UV excitation (λ_ex = 280, 290, 300, 310, 320 and 330 nm) and GCu2Smy (y = 1, 4, 6) samples under 330 excitation.

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Figure 5(a) presents excitation spectra (λ_em = 600 nm) of GSm2 and emission spectra (λ_ex = 280 nm and λ_ex = 330 nm) of GCu2 samples. It can be clearly seen that the excitation spectra of GSm2 sample consist of several sharp peaks at 359, 373, 402 and 470 nm, which are the characteristic f-f transitions of Sm³⁺. The excitation bands are in agreement with the absorption bands of Sm³⁺ ions in Fig. 1. The peaks at 402 and 470 nm are assigned to ⁶H_5/2→⁴F_7/2 and ⁶H_5/2→⁴I_11/2 transitions, respectively.

 

Fig. 5 (a) Spectral overlap between excitation of GSm2 and emission of GCu2 samples. (b) Emission spectra (λ_ex = 330) of G-host, GSm2, and GCu2Smy samples (y = 0, 1, 2, 4, 6). (c) Excitation spectra of GSm2, GCu2 and GCu2Sm2 samples. (d) Energy-level diagram of Cu+ and Sm3+ ions and possible ET process.

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Significantly, GSm2 sample shows an absorption band ranging from 360 to 500 nm, while GCu2 sample exhibits broad emission at 360-720 nm. It means that there is a strong overlap between emissions of Cu⁺ with excitation of Sm³⁺ in the range of 360-500 nm. Therefore, it is expected that ET process can occur from Cu⁺ to Sm³⁺.

Figure 5(b) depicts emission spectra of G-host, GSm2 and GCu2Smy samples (y = 0, 1, 2, 4, 6). Excited by 330 nm light, the optimal excitation wavelength for Cu⁺ but not for Sm³⁺, almost no emission is detected for host glass G-host and Sm³⁺ single-doped GSm2 samples, while emission spectra of GCu2Smy samples not only include green-yellow emission from Cu⁺ but also contain strong orange emission from Sm³⁺ with the characteristic peaks at 563 (⁴G_5/2→⁶H_5/2), 600 nm (⁴G_5/2→⁶H_7/2) and 645 nm (⁴G_5/2→⁶H_9/2) [23]. With increasing Sm³⁺ content, emission intensity of Cu⁺ decreases, while emission intensity of Sm³⁺ increases at first, reaches a maximum at y = 2, and then remarkably decreases when Sm³⁺ content is further increased due to concentration quenching. Especially, compared with GSm2, enhancement of Sm³⁺ emission by 22 times is observed in GCu2Sm2 sample. Furthermore, the external quantum efficiencies measured are about 18.6, 16.9, 17.9, 13.6 and 10.1% for GCu2Smy samples with y = 0, 1, 2, 4 and 6, respectively. And quantum efficiency may be improved by using raw materials with spectral purity, optimizing glasses composition and heat-treatment process, et, al.

Figure 5(c) gives excitation spectra of GCu2, GSm2 and GCu2Sm2 samples. Excitation spectra of Cu⁺, Sm³⁺ co-doped GCu2Sm2 sample monitored Sm³⁺ emission (λ_em = 600 nm) ranging from 250 to 360 nm is almost similiar with excitation spectra of GCu2 sample monitored at 540 nm emission of Cu⁺. All above phenomena indicate that high light output of Sm³⁺ emission actually comes from ET process from Cu⁺ to Sm³⁺. ET efficiency ($\eta$) can be estimated by the following formula [10],

To further validate the ET process from Cu⁺ to Sm³⁺, decay curves of emission of Cu⁺ were measured (Figure not shown herein). All curves are non-exponential and the average lifetimes calculated from Eq. (1) are 91, 86, 80, 75 and 71 μs for GCu2Smy samples with y = 0, 1, 2, 4 and 6, respectively. The increase of Sm³⁺ doping leads to faster decay, which can be attributed to ET process from Cu⁺ to neighboring Sm³⁺. The luminescent spectra, ET efficiency, and decay times of Cu⁺ demonstrate the presence of ET process from Cu⁺ to Sm³⁺ and that the ET process is very efficient.

Though the ET efficiencies of GCu2Sm4 and GCu2Sm6 samples are higher than 33.3%, their emission intensity in Fig. 5(b) and external quantum efficiency are low, indicating serious concentration quenching occurs for glasses doped with high Sm³⁺ content. To verify this phenomenon, decay curves of Sm³⁺ emission were measured with excitation at 470 nm and monitored at 600 nm (Figure not shown here). The average lifetimes calculated are about 2.2, 2.1, 1.7 and 1.5 ms for GCu2Smy samples with y = 1, 2, 4 and 6, respectively, implying the optimum content of Sm³⁺ is y = 1 or 2. This result is in accordance with that of external quantum efficiency.

The energy level diagram of Cu⁺ [14] and Sm³⁺ ions [23] and possible ET process are schematically presented in Fig. 5(d). Here Cu⁺ (S-S) stands for Cu⁺ with short distance and in strong ligand field, while Cu⁺ (L-W) stands for Cu⁺ with long distance and in weak ligand field. Cu⁺ (S-S) and Cu⁺ (L-W) model may explain the blue shift of emission spectra with the shortening of the excitation wavelength and the blue shift of excitation and emission bands with decreasing Cu⁺ content logically. For Cu⁺, Sm³⁺ co-doped samples, because the excited state of Cu⁺ and ⁴I_11/2 and other levels of Sm³⁺ ions are energetically close to each other, ET process from Cu⁺ to Sm³⁺ ions can be easily proceed. An excited Cu⁺ relaxes from excited state to ground state nonradiatively and transfers the excitation energy to a neighboring Sm³⁺, promoting it from ⁶H_5/2 ground state to ⁴I_11/2 and other levels [Fig. 5(d), ET]. Sm³⁺ ions in the populated levels undergo multi-phonon relaxation to luminescent ⁴G_5/2 level and then radiatively relax to ⁶H_J/2 (J = 5, 7, 9) levels, resulting in characteristic emissions of Sm³⁺.

It is worthwhile to notice emission of Cu⁺ located at green-yellow region and that of Sm³⁺ ions at orange region under 330 nm excitation. Combining them, tunable white-light emission can be achieved. Hence, the chromaticity coordinates of emissions of GCu2Smy (y = 0, 1, 2, 4, 6) samples excited by 330 nm light are shown in Table 1. More interesting, according to continuous emission bands of Cu⁺ with different distances, and efficient ET from Cu⁺ to Sm³⁺, a perfect and continuous different white-light emission bands via altering excitation wavelength can be obtained in GCu2Smy samples. Emission spectra of GCu2Sm2 sample excited by 280 to 330 nm light with 10 nm steps are given in Fig. 6 Their luminescent colors are characterized by CIE chromaticity diagram and are displayed in Fig. 4(a) and Table 1. It is apparent that the chromaticity coordinates of emissions of GCu2Sm2 sample gradually move from blue white to pure white (x = 0.30, y = 0.33) and into yellowish white region with increasing excitation wavelength. Figure 4(c) gives blue white, pure white and yellowish white luminescent photos of GCu2Sm2 sample taken under 280, 310 and 330 nm excitations in dark, respectively, further indicating that Cu⁺, Sm³⁺ co-doped borosilicate glasses may be used as converting phosphors for UV LED chips to generate W-LEDs.

 

Fig. 6 Emission spectra of GCu2Sm2 sample for excitation wavelengths from 280 to 330 nm with 10 nm steps.

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4. Conclusions

Cu⁺, Sm³⁺ co-doped borosilicate glasses and ET from Cu⁺ to Sm³⁺ were first investigated systematically. The broad luminescence from Cu⁺ ranging from 380 to 720 nm was observed and the peaks shift to blue with shortening of excitation wavelength was caused by the different distribution of Cu⁺ with different strength of crystal field. Combined broad emission of Cu⁺ and sharp orange emission of Sm³⁺, varied hues from blue white to pure white (x = 0.30, y = 0.33) and eventually to yellow white can be generated due to efficient ET from Cu⁺ to Sm³⁺ by changing the excitation wavelength in Cu⁺, Sm³⁺ co-doped glasses. 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.