A series of Sb3+ and Dy3+ ion doped highly transparent magnesium sodium-phosphate glasses were successfully prepared via the melt-quenching method. Their luminescent properties were systematically investigated through optical absorption, photoluminescence and decay analysis. Under deep ultraviolet excitation, through an efficient energy transfer process, the obtained co-doped glasses exhibit both a wide emission at visible band (300-700 nm) of Sb3+ ions and characteristic emission peaks of Dy3+ ions. The energy transfer efficiency, luminescent quantum yield and chromaticity coordinates were obtained. Our result shows a method for spectra modification in luminescent magnesium sodium-phosphate glasses.
© 2017 Optical Society of America
Recently, spectra modification has been investigated deeply due to their potential applications in lamp phosphors, solar cells, photosynthesis and so on [1–5]. Typically, the spectra modification relies on energy transfer (ET) and luminescence processes from luminescent centers. Rare earth (RE) and transition metal (TM) ions possess abundant excited states, which enable them to absorb and emit photons from ultraviolet (UV) to infrared region [6–11].
Dy3+ ions doped samples usually exhibit two intense emission bands at around 484 and 573 nm, corresponding to 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions, respectively. Interestingly, the 4F9/2→6H15/2 transition is a magnetic dipole one while the 4F9/2→6H13/2 transition is an electric dipole one. The intensity of electric dipole transition depends on the nature of the host greatly . White emission can be obtained in Dy3+ ions doped materials . Consequently, Dy3+ is one of the best suitable RE ions for analyzing the luminescence properties in different hosts. Since the absorption efficiency of f-f transition of Dy3+ is low due to the spin- and parity-forbidden electric dipole transition, sensitizing ions (Eu2+, Ce3+, Ag+, etc.) are often used to enhance the absorption of Dy3+ ions by means of ET .
While for sensitizer, Sb is a promising low-cost and environmentally-friendly element . Generally, Sb2O3 is used as a fining agent in glass melting . In addition, luminescence of Sb3+ ions comes from 5s5p→5s2 transition . Sb3+ ions usually present broad excitation at UV band (220-320 nm) and wide emission at visible (VIS) band (300-700 nm) that originated from 3P1→1S0 transition, which are very sensitive to surrounding environment . Many TM and RE ions, such as Mn2+, Eu3+ and so on, have characteristic excitation peaks in the region of 300-500 nm [12–15]. There exist an overlap between the excitation spectra of these ions and the emission spectra of Sb3+ ions. Therefore, Sb3+ ions could be a proper sensitizer for many activator ions .
Among the different matrix materials, luminescent phosphate based glasses present excellent optical properties, better thermal resistance, lower production cost and simpler elaboration procedures [13, 15, 16]. Hence, the phosphate glasses are ideal hosts for RE and TM ions. To our best knowledge, there is no report on ET and tunable emission based on Sb3+ and Dy3+ ions co-doped magnesium sodium-phosphate glasses.
In this work, the preparation and luminescent properties of Sb3+ and Dy3+ ions doped 60P2O5-35MgO-5Na2O glasses are presented. The ET process from Sb3+ to Dy3+ ions was investigated systematically. Results indicated that blue emission in Sb3+ doped glasses have potential application in photosynthesis.
The composition of the glass matrix is 60P2O5-35MgO-5Na2O (in mol%). Samples G-host (pure host without doping), GSx (doped with xSb2O3 only, x = 0.20, 0.35, 0.50, 1.00, 1.50, 2.00), GSDz (doped with 1Sb2O3 and zDy2O3, z = 0.25, 0.50, 1.00, 1.50, 2.00, 2.50), GD (doped with 1Dy2O3 only) were prepared by melt-quenching method. P2O5, MgO, Na2CO3, Sb2O3 (A.R., all from Sinopharm Chemical Reagent Co., Ltd., China) and high purity Dy2O3 (99.99%, from AnSheng Inorganic Materials Co., Ltd.) were used as starting materials. The weighted raw materials (15 g) were mixed thoroughly, then transferred to an alumina crucible and melted at 1200 °C for 1 h in air atmosphere. After melting, the glass melts were poured onto a preheated copper plate and pressed by another plate quickly for quenching. Finally, the glasses were sliced and polished with a thickness of 2 mm for optical measurements.
Transmission spectra were recorded by a U-3900 ultraviolet-visible (UV-VIS) spectrophotometer (Hitachi). The excitation, emission spectra and quantum yield were measured on an Edinburgh FS5 spectrofluorometer equipped with a continuous wave 150 W Xe lamp and SC-30 Integrating Sphere. Lifetime measurement was acquired on an Edinburgh FLS920 spectrofluorometer equipped with a nanosecond flashlamp (nF900) as excitation sources. All measurements were carried out at room temperature.
3. Results and discussion
Figure 1 presents the UV-VIS transmission spectra of all samples. All samples are colorless and high transparence (higher than 85%), which will benefit their practical application. There is only weak absorption band around 200-300 nm in the transmission spectra of G-host. While compared with G-host sample, all GSx samples [Fig. 1(a)] show a distinct absorption from 240 to 320 nm attributed to 1S0→ 3P1 transition of Sb3+. With increasing of Sb3+ content, red shift of absorption band can be observed.
Aside from the broadband absorption of Sb3+, GSDz samples [Fig. 1(b)] present sharp absorption peaks centered at 294, 324, 350, 364, 386, 424, 453, 750 and 800 nm, which can be assigned to 6H15/2 → 4D7/2, 6P3/2, 6P7/2, 6P5/2, 4I13/2, 4G11/2, 4I15/2, 6H1/2 and 6H3/2 transitions of Dy3+, respectively [17–19]. And it is obvious that the absorption peaks of Dy3+ become more distinct with the increase of Dy3+ concentration.
Figure 2(a) depicts excitation and emission spectra of GSx samples. It can be seen that the excitation spectra (λem = 420 nm) for all samples covers a broad UV wavelength region (225-320 nm) with a maximum at about 266 nm, which is assigned to 1S0 → 3P1 transition of Sb3+ ions . At the same time, with the increase of Sb2O3 content, the excitation peaks experience red shift from 258 to 270 nm also. Such phenomenon is similar to the red shift of absorption band in Fig. 1(a). Both phenomena can be explained as that the values for the optical band gap undergo an obvious decrease with increasing Sb3+ content [16, 20].
Upon 266 nm excitation, no luminescence can be observed in G-host notably. While GSx samples exhibit a broadband emission ranging from 300 to 700 nm, which is related to 3P1 → 1S0 transition of Sb3+ ions . With increasing Sb3+ content, the emission intensity increases rapidly at first and reaches the maximum value for GS1.00 sample, and then remarkably decreases due to concentration quenching. Interestingly, the main absorption of chlorophylls (380-480 nm) matches well with the blue emission of Sb3+ ions (marked by blue dashed square in Fig. 2(a)), which means that Sb3+ ions can convert UV light into blue light to promote the photosynthesis of plant . Thereby, the Sb3+ ions doped magnesium sodium-phosphate glasses are promising materials for greenhouse application.
Figure 2(b) shows excitation and emission spectra of GD sample. The excitation spectrum (λem = 573 nm) mainly exhibits nine bands peaking at 293, 323, 336, 349, 364, 386, 423, 451 and 476 nm, corresponding to the transitions from 6H15/2 ground level to higher energy levels of Dy3+ [17–19, 21]. Additionally, the weak broad excitation band around 250 nm comes from charge transfer band (CTB) of O2-→Dy3+ . For the emission spectra (λex = 349 nm), the sharp peaks at 484 nm, 573 nm and 662 nm are related to 4F9/2→6HJ (J = 15/2, 13/2 and 11/2) transitions of Dy3+ ions, respectively . There is no luminescence in G-host sample, too. Besides, excited by characteristic excitation peaks of Sb3+ ions (λex = 266 nm), almost no emission is detected for GD sample. Hence, 266 nm is not ideal excitation wavelength for Dy3+ ions.
By carefully comparing emission spectra of GSx samples and excitation spectrum of GD sample given in Fig. 2(a) and 2(b), respectively, it can be clearly seen that there is a strong overlap between the emission of Sb3+ ions and the excitation of Dy3+ ions in the range of 300 to 500 nm. This overlap indicates that an efficient ET process may occur from Sb3+ to Dy3+.
Figure 3(a) presents emission spectra of GS1.00 and GSDz samples. The inset is enlarged spectra of emission peaks at 573 nm. Excited by 266 nm light, the optimal excitation wavelength for Sb3+, but not for Dy3+, as expected, emission spectra of the GSDz samples not only includes blue emission from Sb3+ ions but also contains emission peaks of Dy3+ ions. With increasing Dy3+ ions doping content, the emission intensity of Sb3+ ions decreases continuously. While the intensity of Dy3+ emission increases rapidly first and reaches a maximum at z = 1.00 and then remarkably decreases when Dy3+ content is further increased due to concentration quenching. Such phenomena indicate that the quenching concentration of Dy3+ is 2 mol% (z = 1.00). In addition, the emission band of Sb3+ presents several little dips, which is due to weak radiative re-absorption process from Dy3+ ions.
By monitoring 420 nm emission of Sb3+ ions in GS1.00 and GSDz samples, the excitation intensity of Sb3+ ions decreases continually with increasing Dy3+ content (as shown in Fig. 3(b)). The excitation spectra of GSDz samples (λem = 573 nm), GS1.00 (λem = 420 nm) and GD sample (λem = 573 nm) are presented in Fig. 3(c). By contrast with the excitation spectrum of GD sample, the excitation spectra of GSDz samples present an additional excitation band located at 266 nm. Significantly, this excitation band is similar to excitation band of Sb3+ emission of GSx samples. Excitation and emission spectra in Fig. 3 indicate the high light output of Dy3+ by the excitation of UV light is due to the ET process from Sb3+ to Dy3+ (major in non-radiative ET process and minor in radiative re-absorption process).
Given this, the ET efficiency (η) from Sb3+ to Dy3+ is estimated according to the following equation [22, 23]:
The luminescent quantum yield was measured and the values are 50.5, 43.5, 37.5, 32.9, 25.9, 19.6, 14.0% for GS1.00, GSD0.25, GSD0.50, GSD1.00, GSD1.50, GSD2.00 and GSD2.50 samples, respectively.
In order to give a convincing evidence for the presence of ET from Sb3+ to Dy3+, decay curves of Sb3+ 420 nm emission in GS1.00 and GSDz samples were systemically investigated and are illustrated in Fig. 3(d). The lifetimes are characterized by average lifetime () :
The energy-level diagram of Sb3+ and Dy3+ ions, the possible ET process is schematically demonstrated in Fig. 4(a). Upon 266 nm excitation, Sb3+ is excited from 1S0 ground state to 3P1 excited state. Then, a part of Sb3+ ions will relax to ground state and then exhibit the broadband emission at 300-700 nm. At the same time, the emission of Sb3+ ions can be re-absorbed by Dy3+ ion at the ground state.
Because the excited state of Sb3+ is close to 6P7/2, 6P5/2 and other levels of Dy3+, other part of excited Sb3+ ions relax from 3P1 excited state to ground state non-radiatively and transfer the energy to neighboring Dy3+ ions, promoting them from 6H15/2 ground state to 6P7/2, 6P5/2 and other excited states. Afterwards, Dy3+ ions at 6P7/2, 6P5/2 and other levels non-radiatively relax (NR) to 4F9/2 excited state and give strong emissions at 484, 573 and 662 nm of Dy3+ (4F9/2→6HJ, J = 15/2, 13/2 and 11/2).
Finally, luminescent colors of GS1.00 and GSDz (z = 0.25, 0.50, 1.00, 1.50, 2.00 and 2.50) samples excited by 266 nm light are characterized by the Commission International Eclairage (CIE) chromaticity diagram and shown in Fig. 4(b). The inset of Fig. 4(b) gives the photos of GS1.00 and GSDz samples taken under the 266 nm excitation in dark, respectively. It is apparent that the chromaticity coordinates and emitting color of the GS1.00 and GSDz samples gradually move from blue to white with increasing z.
In summary, a series of Sb3+ and Dy3+ ions doped magnesium sodium-phosphate glasses with high transparence have been prepared at low-melting temperature. Sb3+ single doped glasses exhibit a broadband emission with luminescent quantum yield of 50.5% for GS1.00 sample, indicating that Sb3+ ions doped magnesium sodium-phosphate glasses are promising material for greenhouse application. White-light emission is obtained under deep UV (266 nm) excitation by combining the blue emission of Sb3+ and the yellow emission of Dy3+ due to efficient energy transfer process from Sb3+ to Dy3+. Our result shows a method to convert deep UV light into visible band in luminescent magnesium sodium-phosphate glasses.
NSFC (No. 11374269).
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