White light-emitting Dy3+-doped transparent chloroborosilicate glass: synthesis and optical properties

ABSTRACT Dy3+-doped chloroborosilicate glasses with the composition 35.7SiO2–25.5B2O3–17BaO–3.4K2O–3.4Al2O3–15BaCl2 (mol%) were prepared using melt quenching technique. The glass transition temperature (Tg) was ~610°C. In UV–vis–NIR absorption spectra, characteristic absorption bands of Dy3+ appeared at 322, 347, 364, 388, 795 and 880 nm due to spectral transitions from the 6H15/2 level to various higher levels of Dy3+ ion. The excitation spectrum was recorded at within 200–550 nm by monitoring emissions at 576 nm. The excitation bands at 392, 428, 453 and 472 nm were attributed to the 6H15/2 → 4I13/2, 4G11/2, 4I15/2 and 4F9/2 transitions, respectively. Prominent emission bands were observed at wavelengths of 484, 576, 664 and 754 nm when excited at 447 nm. The bands correspond to the transitions 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, 4F9/2 → 6H11/2 and 4F9/2 → 6H9/2 6F11/2, respectively. The emitted light from all the samples was found to be white. Their color coordinates lie within the white range. The sample containing 0.5 wt% Dy2O3 emitted white light with the color coordinates x = 0.351, y = 0.335, which are the very closest to pure white light and whose color temperature of 4716 K is similar to daylight. Such white light-emitting transparent glasses promise to be enormously useful for various photonic applications.


Introduction
Rare earth-doped glasses are excellent luminescent materials because of the occurrence of sharp fluorescence in the ultraviolet (UV), visible and infrared (IR) regions due to the shielding effects of their outer 5s and 5p orbitals on 4f electrons. These glasses have potential applications due to their emission efficiencies of 4f → 4f and 4f → 5d electronic transitions in the RE ion. The Dy 3+ ion is one of the most studied rare earth ions. Special attention has been focused recently on the study of Dy 3+ -doped glass scintillators for radiation measurement and homeland security applications as well as for dose monitoring for Xrays and protons [1]. Dy 3+ -activated glasses and glass phosphores [2,3] have also been studied for obtaining white light as well as for blue and yellow laser emissions which find applications in the fields of biomedicine and astronomy. Dy 3+ -ions can be excited easily due to the presence of abundant 4fe4f electronic bands [4]. The emission spectrum of Dy 3+ ions consists mainly of strong bands corresponding to the 4 F 9/2 → 6 H 15/2 (blue) and 4 F 9/2 → 6 H 13/2 (yellow) transitions accompanied by a weak 4 F 9/2 → 6 H 11/2 (red) transition in the visible range. Numerous Dy 3+ -doped glass systems therefore were studied for obtaining white light through appropriate combinations of these luminescent band intensities [5] and two primary-colored luminescent materials.
The spectroscopic properties of rare earth ions depend on their 4f-4f transitions and can be controlled by changing the chemical composition of the host glass matrix.
The luminescence properties of Dy-doped high silicate glass were investigated by Nagli et al. who observed visible luminescence around 570 nm with 447 nm excitation [6]. The optical properties resulting from Dy 3+ doping in the PbO-PbF 2 oxyfluoride glass matrix were investigated by Nachimuthu et al. The first of the two most dominant emissions of Dy 3+ is the blue emission at 481 nm due to the 4 F 9/ 2 → 6 H 15/2 transition and the second appears in the yellow range at 575 nm due to the 4 F 9/2 → 6 H 13/2 transition [7]. Spectroscopic properties of Dy 3+ ions in lithium borate and lithium fluoroborate glasses were presented by Babu et al. [8]. The fluorescence properties of Dy 3+ ions in two concentrations (1.0 and 0.1 mol%) have been investigated in a variety of borate and fluoroborate glasses modified with lithium, zinc and/or lead. The fluorescence spectra emitted by the Dy 3+ ions embedded in title glasses, when excited with a 457.9-nm laser, consist of 4 F 9/ 2 → 6 H J transitions in the visible and near IR ranges of the spectrum. The blue ( 4 F 9/2 → 6 H 15/2 ) at ∼485 nm and yellow ( 4 F 9/2 → 6 H 13/2 ) at ∼575 nm emission transitions are relatively more intense than the other emission transitions ( 4 F 9/2 → 6 H 11/2,9/2,7/2 ) observed in the red and near IR regions [9]. The  [11]. Optical properties of Dy 3+ -doped phosphate and fluorophosphates glasses have been investigated. A strong yellow emission was observed from the 4 F 9/2 → 6 H 13/2 transition of Dy 3+ ions in these glasses [12]. Dy-doped oxyfluoride glasses and nanocrystalline glass ceramics were found to emit intense white light when the 4 F 9/2 level is inundated with 451 nm laser light. The chromaticity color coordinates of the visible emissions were calculated and found to be in the white light zone [13]. It was observed from the emission spectra of calcium fluoroborate glasses doped with different concentrations of Dy 3+ ions that the samples emit intense yellowish light which is close to white light [14]. The spectroscopic properties of silicate glasses doped with different concentrations of Dy 3+ ions were studied. Simulation of white light for these glasses was also performed by varying the excitation wavelength. The results showed that the white light luminescence could be tuned to various wavelength excitations, making the glass suitable for generation of white light for blue LED chips [15]. Lead tungsten tellurite glasses doped with different Dy 3+ ion concentrations also show the usual characteristic absorption and emission bands of Dy 3+ and emit white light [16]. The optical properties of Dy 3+ -doped lead phosphate glasses were also reported. The yellow-to-blue emission intensity ratios and CIE chromaticity coordinates were calculated and used to evaluate white light emissions as a function of the activator (Dy 3+ ) ion concentration [17].  [19]. The spectroscopic properties of Dy 3+ -doped Li 2 O-B 2 O 3 glasses were studied for white light-emitting material applications. The color coordinates (x, y) of the studied glass samples were found to be (0.37, 0.40), which fall in the white light range of the chromaticity diagram [20]. Dy 3+ -doped lithium borate glasses were studied for white LED (WLED) applications. The CIE chromaticity chart shows that glass containing 0.5 mol% Dy 2 O 3 has the color coordinates x = 0.33 and y = 0.37, which are nearly equal to those of pure white light. All these glasses produce emission in the white range and can thus be used for bright WLED's and modern WLED bulbs [21].
From the above discussion, it can be inferred that Dy 3+ -doped glasses offer immense promise for photonic applications, especially as white light-generating materials. It will therefore be worthwhile to dope some new glass systems with Dy 3+ and study their absorption and emission properties to find new white light-emitting materials. In response to this motivation, we investigated the synthesis and characterization of Dy 3+ ion-ingrained low-phonon chloroborosilicate glass (CBS) matrix with the composition 35.7SiO 2 -25.5B 2 O 3 -17BaO-3.4K 2 O-3.4Al 2 O 3 -15BaCl 2 (mol%). We chose this composition because it is a new, stable and transparent glass, which is also known to have low phonon energy, as reported earlier [22]. Different concentrations of Dy 3+ have been incorporated in the glass. We characterized the transparent Dy 3+ -doped glasses by density measurement, X-ray diffraction (XRD), differential scanning calorimetry (DSC), dilatometry, refractive index measurement, UV-vis-NIR absorption spectra, excitation spectra and emission spectra. Visible luminescence of a white color was observed in all the samples at 447 nm excitation. These glasses are potential materials for various photonic applications, including white light emitters, i.e. WLEDs.

Glass preparation
CBS with the composition 35.7SiO 2 -25.5B 2 O 3 -17BaO-3.4K 2 O-3.4Al 2 O 3 -15BaCl 2 (mol%) was prepared using the melt quenching technique from quartz, SiO 2 (GR, Bremthaler Quarzitwerk, Usingen, Germany), boric acid, H 3 BO 3 (GR, 99%, Loba Chemie, Mumbai, India), barium carbonate, BaCO 3 (GR, 99%, Fluka Chemie GmbH, Buchs, Switzerland), potassium carbonate, K 2 CO 3 (GR, 99%, Loba Chemie, Mumbai, India), aluminum oxide, Al 2 O 3 (GR, Aldrich Chemical Company Inc., Milwaukee 53233, USA) and barium chloride, BaCl 2 ·2H 2 O (GR, dihydrated extra pure, Loba Chemie, Mumbai, India) as the raw materials. They were used directly with no further purification. An amount of 60 g of glass was prepared by melting well-mixed batches of the calculated composition in a high-purity alumina crucible at 1250°C for 1.5 h with intermittent stirring for 0.5 min in air in a raising hearth electric furnace. The molten samples were cast into an iron plate in air and annealed at 550°C for 2 h in order to remove the residual thermal stresses, followed by a slow cooling down to room temperature. Five more glasses of the same composition were prepared with the addition of (1) 0.1, 0.3, 0.5, 0.7 and 1 wt% Dy 2 O 3 in excess (GR, 99%, Loba Chemie, Mumbai, India) using the same procedure and annealed at 550°C following the same schedule. The monolithic glasses thus obtained were cut and polished into the shapes and sizes required for the different characterizations, as described below.

Characterization
Differential scanning calorimetric experimentation was performed with a differential scanning calorimeter (NETZSCH Model STA 449 Jupiter F3, NETZSCH-Gerätebau GmbH, Selb, Germany) using powdered sample within the temperature range of 30-900°C in a nitrogen atmosphere at a heating rate of 10 K min −1 . The coefficient of thermal expansion (α), glass transition temperature (T g ) and dilatometric deformation temperature (T d ) were measured using a horizontal vitreous silica dilatometer (DIL 402C, Netzch-Gerä tebau GmbH, Bavaria, Germany) at a heating rate of 4 K min −1 by heating a cylindrical sample approximately 25 mm length and 5 mm diameter at a rate of 4 K min −1 up to the temperature at which the glass softened, after calibration with standard alumina supplied with the instrument by the manufacturer. The T g and T d values were reproducible within ±1°C for all samples. The XRD patterns of the bulk samples were recorded in an X'pert Pro MPD diffractometer (PANalytical, Almelo, the Netherlands), operating at 40 kV and 30 mA, using Ni-filtered CuKα radiation with the X'celerator, with a step size of 0.05°(2θ) and a step time of 0.5 s, at 10°-80°. The UV-vis absorption spectra and the PL spectra of all the samples were studied using polished 2-mm thick samples with the use of a fiber optic spectrometer (AvaSpec-3648-USB2, Avantes) and diode lasers with a 447-nm wavelength source. Photoluminescence spectrophotometer (model: custom made Quantum-Master: Enhanced NIR of Photon Technologies International, Canada) was employed for recording of the excitation spectra.

Results and discussion
The compositions and physical properties of the glasses with the general composition 36.75SiO 2 -26.25B 2 O 3 -20BaO-3.5K 2 O-3.5Al 2 O 3 -12.5BaCl 2 (mol%) are listed in Table 1. All the samples were obtained as colorless, transparent monoliths. Figure 1 shows photographs of the prepared samples.

Density
The densities (ρ) of the glass samples were determined by the standard Archimedes principle. The measurements were done using a single pan balance and distilled water as an immersion liquid. The densities were obtained from the following relationship: where a is the weight of the glass sample in air, b is the weight of the glass sample when suspended in distilled water (density of water, ρ x = 0.997604 g cm −3 at 25°C). Their densities are in the range of 3.5-3.6 g cm −3 . The density of the glasses did not vary much as their composition was almost the same. The base glass composition was the same for all samples, only very small variations in rare earth content that do not have visible impact on the density of the glass. A very slight variation was observed in the fourth decimal place. The measured densities are listed in Table 2.

DSC and dilatometry
DSC was performed to determine the T g and thermal behavior of the glass by heating finely ground powder of the base glass (CBS) at the rate of 10 K min −1 from 30 to 900°C. The DSC thermogram is shown in Figure 2(a). T g was determined to be about 612°C.
The glass transition temperature (T g ) and dilatometric softening point (T d ) of CBS were measured by dilatometric measurement by heating a cylindrical sample approximately 25 mm in length and 5 mm in diameter at a rate of 4 K min −1 up to the temperature at which the glass softened. Figure 2(b) shows the dilatometric curve of the glass. T g and T d were determined to be around 610 and 650°C, respectively. The T g determined by dilatometry and DSC are closely similar temperatures.

XRD
The formation of the glasses was explicitly determined by XRD analysis. Figure 3 shows the XRD patterns of all the samples. The hump between 2θ = 20°and 35°indicates amorphicity of the glasses. The presence of this hump and absence of any peaks in the samples indicates that they are amorphous in character and can be considered to be glasses.

Refractive index
The refractive index (n i ) at different wavelengths was measured for all the transparent glasses by Prism coupler using lasers of five different wavelengths.     almost identical. The n i of any particular medium is the ratio of the velocity of light through the vacuum (c) to the velocity of light through the medium (v). n i ¼ c=v 2 As the glass becomes denser, the velocity of light passing through it decreases. Hence, the higher the density of the glass, the higher the refractive index. As all the samples here are almost equally dense, the velocities of light through them are nearly equal, which resulting in almost no variation in the n i values. The slight increase in n i is due to slight variations in the density of the glasses. Table 2 lists some calculated properties of the glasses. Figure 5 shows the UV-vis absorption spectra of the samples, which reveal that all the samples are transparent in the visible and NIR ranges.  [21]. As the concentration of Dy 3+ increases from CBSD1 to CBSD5, the absorption bands become increasingly prominent and sharper. As the blank glass CBS does not contain Dy 3+ , its spectra do not exhibit any absorption bands.

Excitation spectra
To analyze the luminescence properties as functions of the Dy 3+ ion concentration, the excitation spectrum was recorded in the spectral range of 200-550 nm for all the samples by monitoring the emissions at 576 nm as shown in Figure 6. The excitation bands centered at 392, 428, 453 and 472 nm are attributable to the 6 H 15/2 → 4 I 13/2 , 6 H 15/2 → 4 G 11/2 , 6 H 15/2 → 4 I 15/2 and 6 H 15/2 → 4 F 9/2 transitions, respectively. The broad band in the range of 230-340 nm can be attributed to the host absorption band, since the charge transfer band of Dy 3+ -O 2− is located below 220 nm [23,24]. It is well known that the wavelength corresponding to the prominent excitation band can give rise to intense emissions. In the present investigation, the excitation band centered at 453 nm was found to be most intense. Thus, the luminescence spectra should be carried out by exciting the samples with a 453-nm wavelength in order to achieve the maximum emission intensity. Due to the unavailability of a 453-nm excitation source, the emission spectra were recorded using a 447-nm (extremely close to 453 nm) laser diode [24]. Figure 7 shows the PL spectra of the samples CBSD1-CBSD5. They exhibit prominent emission bands at wavelengths of 484 nm (blue), 576 nm (yellow), 664 nm (red) and 754 nm (near IR). The major band at 576 nm is in the yellow range of the visible spectrum. It is evident from the figure that the    resulting in increased inundation in this level. The excited electrons then jump to the intermediate 4 F 9/2 energy level through non-radiative transition. Radiative transitions take place from the 4 F 9/2 level to various lower energy levels resulting in visible emissions from the samples. The bands at 484, 576, 664 and 754 nm correspond to the transitions 4 F 9/2 → 6 H 15/2 (ΔJ = 3, forbidden transition), 4 F 9/2 → 6 H 13/2 (ΔJ = 2, hypersensitive electric dipole transition), 4 F 9/2 → 6 H 11/2 (ΔJ = 1, magnetic dipole transition) and 4 F 9/2 → 6 H 9/2 (ΔJ = 0, zero-zero band, forbidden transition), 6 F 11/2 , respectively [24].

Photoluminescence spectra
The light emitted from the luminescent samples is white in color. Figure 9(a) shows a chromaticity chart corresponding to the light emitted from sample CBSD3 when excited at 447 nm with a diode laser source. The point "P" represents the chromaticity coordinates (x = 0.351, y = 0.335) of the emitted light. Figure 9(b,c) are the photographs of the luminescent sample (CBSD3) when excited at 447 nm.  Both the color coordinates and the photographs clearly show that the emitted light is white. The coordinates are very close to those of pure white light (0.333, 0.333). Figure 10 shows the positions of the chromaticity coordinates of the light emitted by all the samples when excited at 447 nm. It can be seen that all the emitted light is white since all the coordinates lie within the white range of the chromaticity chart. Compared to the color coordinates of all the other glasses under investigation, those of the light emitted by CBSD3 are closest to pure white light. Table 3 presents the color coordinates of the samples and their color temperature values. From the coordinates shown in the table, it is quite clear that the coordinates of light emitted by all the glasses are very close to each other. Since the coordinates of pure white light are x = 0.333, y = 0.333, all the samples under observation emit light that is remarkably close to pure white light. The color temperature ranges from 3800 to 4700 K, which is in the "cool white" range of white light. For CBSD-3, the color temperature is 4716 K, which is comparable to daylight.
The color of the emitted light depends on the ratio of the intensities of different emission bands. In the emission spectra of Dy 3+ -doped CBS glasses, the major bands are found at 484 and 576 nm, which are in the blue and yellow ranges of the visible spectrum, respec-   tively. The intensity ratio of these two bands determines the color of the emitted light. Table 4 represents the intensity ratios of these two bands for all the Dy 3+ -doped samples. It is apparent from the table that the intensity ratios are in the range of 2.88-2.98. The ratios are very close to each other. That is why the coordinates of the emitted colors are also very close and why all of them fall within the white range. CBSD-3 which has the closest coordinates to those of pure white light has an intensity ratio of about 2.89. Based on these observations, it can be concluded that the white light emitted by Dy 3+ -doped CBS glasses under an excitation of 447 nm will be the closest to the pure white light if and only if the intensities of the two emissions, i.e. 576 and 484 nm, obey the ratio of I 576 /I 484 = 2.89.
The photoluminescence spectra have been obtained at different laser source powers. Figure  11 shows the PL spectra of sample CBSD-3 at various 447 nm laser powers. The emission intensity increases gradually with increases in power. The enhancement of intensity with the raising of pump power indicates an increase in the inundation inversion, whereas all the peak positions in the PL spectra remain the same. The inset in Figure 11 shows a plot of log of PL intensity versus a log of excitation power. Using the I versus P K relationship (where I is intensity, P is power and K is the order), the value of K is found to be around 1.06. The mechanism in this case is therefore on the order of one photon.

Conclusions
Dy 3+ -doped CBSs with the composition 35.7SiO 2 -25.5B 2 O 3 -17BaO-3.4K 2 O-3.4Al 2 O 3 -15BaCl 2 (mol%) were prepared using a melt quenching technique; Dy 3+ was doped in different concentrations (0.1, 0.3, 0.5, 0.7 and 1 wt% in excess). The transparent glasses were then characterized by several methods. The following conclusions can be drawn from the above results and discussion: • The densities of the glasses are in the range of 3.5-3.6 g cm −3 . • The glass transition temperature (T g ) is~610°C, as obtained from DSC and dilatometry. • The samples are found to be X-ray amorphous.
• The refractive indices are in the range of 1.  Figure 11. Variation of PL intensity with power of the diode laser.
• Under excitation of 447 nm, prominent emission bands were found at wavelengths of 484 nm (blue), 576 nm (yellow), 664 nm (red) and 754 nm (near IR) which correspond to the transitions 4 F 9/2 → 6 H 15/2 , 4 F 9/2 → 6 H 13/2 , 4 F 9/2 → 6 H 11/2 and 4 F 9/2 → 6 H 9/2 6 F 11/2 , respectively. The band at 576 nm is the major one. • PL intensity gradually increases with increases in the Dy 3+ concentration, with no concentration quenching occurring within the experimental range. • The emitted light from all the samples was found to be white. Their color coordinates, all lie within the white region. The sample containing 0.5 wt% Dy 2 O 3 emits white light with color coordinates x = 0.351, y = 0.335 and a color temperature of 4716 K, which appears to be the very closest to pure white light and similar to daylight. • Variation of the excitation source power revealed the process to be single photonic. • These white light-emitting transparent glasses promise to be enormously useful for various photonic applications, especially in WLEDs.