Luminescence Properties of Dy 3 +-doped Lutetium-Calcium-Silico-Borate Glasses for White Light Emitting Applications

Lutetium-calcium-silico-borate glasses doped with various concentrations of dysprosium ions (Dy 3+ ) have been fabricated. The amorphous nature of glasses has been confirmed by XRD technique. The luminescence properties have been studied throughout the emission spectra, the combination of blue and yellow emissions was shown for all the glasses and white light emission could be observed. The x, y coordinates of the prepared glasses pass through the white light region in the CIE 1931 diagram and are suitable for white light emitting devices.


Introduction
Recently, the research on rare earth doped materials is significant emphasis due to their potential applications in fields of laser technology, optoelectronic devices, fiber optic amplifiers, infrared to visible up-converters and phosphors (1)(2)(3)(4) .The design and development of optical devices suitable for the above applications is required in detailed on characterization of optical absorption and luminescence properties.These properties mostly depend on nature and type of local environment around the rare earth ions (RE n+ ) and on the phonon energies of the host materials.
Glasses doped with rare earth ions are proving to be luminescence materials as they have high emission efficiencies.These emissions correspond to 4f-4f and 4f-5d electronic transitions in the RE n+ .The 4f-4f transition gives an especially sharp fluorescence pattern from the UV to the infrared region.This is due to shield effects of the outer 5s and 5p orbitals on the 4f electrons.In recent years, glasses doped with rare earth ions have drawn much attention due to their potential applications in solid state lasers, optical amplifiers and three-dimensional displays (1,(5)(6)(7) .Therefore luminescence properties of rare earth doped different glass hosts are being prepared and investigated with the purpose to know their utility for luminescence applications.Various glassy materials such as fluoride, phosphate, fluorophosphate, borate, fluroborate and tellurite have been widely investigated to understand the effect of host glass on the lasing properties of rare earth ions (8) .In general, a host glass with low phonon energies decreases non-radiative relaxation rates and gives high quantum efficiencies, which are useful to design lasers and optical fiber amplifiers (7,9) .Out of the 14 lanthanide elements, dysprosium in its trivalent state (Dy 3+ ) is an efficient emitter in the visible region and it is the only ion that emits two intense colors which on mixing in suitable proportions yields white light (5,(10)(11)(12) .Further, the 1.3 μm emission of Dy 3+ ion is useful for optical communication and up conversion emission in the blue region may be utilized for the development of blue lasers (5,11,(13)(14) .
The present work reports the systematic investigations of Dy 3+ ions doped lutetium-calcium-silico-borate glasses.The luminescence properties and CIE 1931 chromaticity coordinates have been calculated as a function of Dy 3+ ion concentration.The results obtained in the present work have been discussed.

Sample fabrication
A series of based-glass with the molar compositions of 25.0Lu 2 O 3 : 10.0CaO : 10.0SiO 2 : (55.0-x)B 2 O 3 (was represented as LuCSB glass) has been prepared with various doping concentration of xDy 2 O 3 , ranging from 0.0, 0.1, 0.3, 0.5 and 1.0 mol%.The nominal compositions of all glass samples here after were referred as LuCSB:0.0Dy,LuCSB:0.1Dy,LuCSB:0.3Dy,LuCSB:0.5Dy and LuCSB:1.0Dy.Following high purity analytical reagents: Lu 2 O 3 , CaO, SiO 2 , H 3 BO 3 and Dy 2 O 3 were taken as raw materials in glass preparation process by using melt-quenching method.Each batch of 20 g of well-mixed chemicals was taken into high alumina crucible and was melted at 1500  C in and electric furnace for 3 hours.The homogenous melts were quenched by pouring on a preheated stainless steel mold.The transparent samples were annealed at 550  C for 3 hours, subsequently cooled down to room temperature to remove the residue strain.The samples thus prepared were cut into rectangular-shaped and optically polished to obtain uniform thickness with the dimensions of 1.0 × 1.5 × 0.3 cm 3 for further investigations.

Measurements
The amorphous nature of the prepared glass samples was confirmed by XRD technique, using Shimadzu X-ray Diffractometer XRD-6100 with CuK radiation source.The absorption spectra were monitored under absorbance mode in UV-VIS and NIR regions in the wavelength of 300-1800 nm using UV-VIS-NIR spectrophotometer (UV-3600, Shimadzu).The emission photoluminescence spectra of the Dy 3+ -doped LuCSB glasses were recorded using Cary Eclipse fluorescence spectrophotometer with a xenon flash lamp.The densities were measured according to the conventional Archimedes's principle with distilled water as an immersion liquid.All the measurements were carried out at room temperature.The other physical properties such as molecular weight and molar volume were also calculated using the relevant expressions reported in our previous work (15) and the results are represented in Table 1.

XRD analysis
X-ray Diffractometer scans were taken to measure the glassy nature for all the glasses.The typical XRD profile of the LuCSB glass doped with 1.0 mol% of Dy 2 O 3 is shown in Figure 1.
From this Figure, the XRD pattern does not exhibit and detectable sharp characteristic peak, but the broad diffused scattering occurs at the lower angles which indicates the glass sample is amorphous in nature and has non-crystalline structure.Remaining the glass samples also exhibit similar in nature, so they are not shown for other glasses in Figure 1.

Absorption spectra
The optical absorption spectra of Dy 3+ -dope LuCSB glasses in the UV-VIS and NIR regions are shown in Figure 2(a) and 2(b), respectively.The absorption spectra of the all glasses were found to be alike with slightly difference in their intensity.From observation, the absorption intensity of the all bands increased with the increasing of Dy 2 O 3 concentration.The spectra consist of inhomogeneous absorption bands due to the transitions assigned from ground state, 6 H 15/2 to various excited states of Dy 3+ ions.

Luminescence spectra
The luminescence behavior of Dy 3+ -doped LuCSB glasses have been studied throughout the emission spectral measurement.The luminescence spectra have been recorded by monitoring an excitation at 388 nm as a function of various Dy 3+ ion concentrations and are shown in Figure 3.The mainly three emission luminescence bands corresponding to 4 F 9/2  6 H J (J = 15/2, 13/2 and 11/2) transitions located the same wavelength nearly at 481 nm (blue), 575 nm (yellow) and 664 nm (red) are observed, in sprite of change in concentration.From Figure 3, it is also indicates that the luminescence intensity increases along with increase in Dy 3+ ion concentration.Among the three emission bands, the band at 4 F 9/2  6 H 15/2 is magnetic dipole (MD) transition possessing the most intense, 4 F 9/2  6 H 13/2 transition possessing moderate intensity, which is related to electric dipole (ED) transition, and 4 F 9/2  6 H 11/2 transition possessing the lowest intensity (6)(7)9) .  Dy  doped LuCSB glasses (7,18) .

Color coordinates
From emission spectra in Figure 3, the Dy 3+ doped LuCSB glasses having important emissions in blue (481 nm) and yellow (575 nm) regions.The appropriate combination of blue and yellow light can generate white light emission.In order to find out color coordinates for all the glasses, the emission spectra have been characterized by the CIE 1931 chromaticity diagram (6) .The color of the luminescent source can be described by three color matching function parameters, , which are the dimensionless parameters (6,18) .For a given power spectral density P(), the degree of stimulation required to match the color of P() is express by (18) dλ Where X, Y and Z are the tristimulus values which gives the power for each of the three primary colors to match with the color of P() and from the tristimulus values the color chromaticity coordinates x and y can be calculated using the following expression (18) The estimated CIE chromaticity coordinate values (x,y) for all the glasses are presented in Table 2.All the samples possess the chromaticity coordinates lies within the white light region.From this result, it is suggested that the all glass samples can be used as white light emitting devices.

Conclusions
The Dy 3+ -dope lutetium-calcium-silico-borate based glasses have been fabricated and characterized their properties through, absorption spectra, emission spectra and CIE 1931 color chromaticity studies.The amorphous nature of the glasses was confirmed by XRD technique.With increasing of Dy 3+ ions concentration, the densities were increased due to the high relative molecular mass of Dy 2 O 3 compared to other glass constituents.While the additional of Dy 2 O 3 leads to the decrease of the molar volume due to the increase of bond strength between atoms.

Fig. 3 .
Fig. 3.The partial energy level diagram of Dy 3+ ions showing the possible emission transitions and non-radiative (NR) decay in