Borate Glasses for Luminescence Applications – Potential Materials for White LEDs and Laser Sources

The development of borate based glasses for luminescence applications are reviewed. The present work covers various aspects including the search and synthesis of new materials, the formation of glasses, the structural and optical properties of absorption, excitation and luminescence on the prepared glasses, Information on the Judd-Ofelt intensity parameters as calculated from absorption spectra and the studies on the emission properties. CIE colour coordinates are helpful to analyze the potentiality of the glasses for white light emission. We aim to provide a comprehensive review of borate glass materials for LED and screen display applications etc.


Introduction
Light is a form of energy. To produce light, another form of energy must be supplied. There are two common ways to produce light, incandescence and luminescence. Incandescence is generating light from heat energy. The phenomena which involve absorption of light energy and subsequent emission of light are come under the term 'luminescence'. Luminescence is the emission of light by a substance. It can be caused by electrical energy, chemical reactions, and subatomic motions. The term Luminescence was introduced in 1888 by Eilhard Wiedemann, in Latin 'lumen' means 'light'. The materials exhibiting this phenomenon are known as 'luminescent Materials' or 'phosphors' meaning 'light bearer' in Greek. The term phosphor was introduced in 17 th century by an Italian alchemist Vincentinus Casaiarolo of Balogona.
Phosphors are luminescent materials that emit light when excited by radiation, and are usually microcrystalline powders or thin-films designed to provide visible color emission. In an important group of luminescent materials, material is crashed: Ex: Diamond.

Crystalo-luminescence
Crystallo-luminescence is a type of Luminescence generated during crystallization and it is used to determine the critical size of the crystal nucleus. There is a theory that the light from Crystallo-luminescence emerges through the micro-fracture of growing crystallites. This effectively classifies Crystallo-luminescence as a type of Tribo-luminescence and a subtype of Electroluminescence.

Mechano-luminescence
A result of a mechanical action by a solid: Ex: Sensing and imaging devices.

Fracto-luminescence
Generated when bonds in certain crystals are broken by fractures: Ex: Quartz.

Piezo-luminescence
Produced by the action of pressure on certain solids: Ex: Actuating element in vehicles.

Radio-luminescence
A result of bombardment by ionizing radiation such as beta particles: Ex: Radio-luminescent paint is used for clock hands and instrument dial, enabling them to be read in dark.

Thermo-luminescence
The re-emission of absorbed energy when a substance is heated: Ex: Dating some archeological artifacts.

Cryo-luminescence
The emission of light when an object is cooled: Ex: Ratio metric thermometer, solar cell.

Seismo-luminescence
Seismoluminescence is a reference to Earthquake Lights, rare glowing phenomena that are occasionally seen before or during seismic event. Ex: Lightning 1.1.14. Photoluminescence Photoluminescence (PL) processes can be classified by various parameters such as the energy of the exciting photon with respect to the emission. Optical excitation by absorption of a photon leads to a major class of technically important luminescent species which fluorescence or phosphorescence. PL is simple, versatile, and non-destructive in nature. PL investigations can be used to characterize a variety of material parameters. PL spectroscopy provides electrical (as opposed to mechanical) characterization, and it is a selective and extremely sensitive probe of discrete electronic states. Features of the emission spectrum can be used to identify surface, interface, and impurity levels and to gauge alloy disorder and interface roughness. The advantages of PL analysis are the simplicity of optical measurements and the power to probe fundamental electronic properties [4].

Fluorescence
In general, fluorescence is a "fast" process, and photoluminescence in which the emitted photons are of lower energy than those absorbed. Afterglow is observed independent of the excitation intensity and of temperature, with lifetime less than 10 -8 seconds. Ex: Fluorescent lamps.

Phosphorescence
In general, phosphorescence is a "slow" process, and fluorescence slightly delayed after initial absorption of radiation. Afterglow is observed dependent of the excitation intensity and strongly temperature dependent, with life time more than 10 -8 seconds. Ex: Light sources, multi colour displays.   (1) shows a typical spectrum of the excitation and emission of a fluorochrome. These spectra are generated by an instrument called a spectrofluorometer. The colour spectra are described quantitatively by the wavelength of emitted light. The colours of the visible spectrum can be broken up into the approximate wavelength ranges as shown in table 1 [3]. On the short-wavelength end of the visible spectrum the near-ultraviolet (near-UV) band from 320 to 400 nm and on the long-wavelength end is the near infrared (near-IR) band from 750 to approximately 2500 nm were tabulated in Table 1. The broad band of light from 320 to 2500 nm marks the limits of transparency of prepared glasses and this is the band most often used in fluorescence microscopy [4].

Experimental Methods
The luminescent materials are synthesized by solid state reaction technique at high temperatures in ambient conditions. All the precursor chemicals taken in appropriate proportions are crushed in an agate mortar until a homogeneous mixture was obtained. The homogeneous mixture is taken is a suitable crucible like platinum or silica or aluminium. The crucible is then heated at high temperature around (1200 • C) in an electric furnace for a desired period, and cooled rapidly to obtain homogeneously mixed melt. Such melt is then quenched suddenly on a pre-heated brass mould and pressed quickly by another brass mould. The glass samples thus prepared were annealed for around 3 to 4 hours for the removal of thermal stress acquired by glass matrix during quenching.

Common analytical Techniques
The prepared and well polished glasses are subjected to physical properties analysis, structural analysis and optical analysis. The following properties are studied.

Density
The densities of the prepared glasses are measured at room temperature with the help of immersion in a liquid by using the Archimedes principle. The examples of liquids water, xylene and monobromonaphthalene etc.

Refractive Index
Refractive indices for the prepared glasses were measured by using Brewster's law with some sources. Ex: laser (He-Ne) 650 nm, sodium vapour lamp (589 nm). Refractive index is a critical parameter in the control of mode profile and can affect the performance of an optical amplifier. The higher refractive indices observed for these glasses make them as promising material for optical fiber with more numerical aperture values.
The following terms are some physical properties to study glassy material for effective luminescence applications.
Where M is the average molecular weight of the glass and ρ is the density.
Inter atomic distance (r i ): Polaran radius (r p ): r p ( ) Field strength (F): Where Z is the charge of the ion; Z=3e; e is the electron charge, 0 ε is the permittivity of free space p r is the distance between two points.

Numerical Aperture (NA)
When the glass under investigation is used as the core of the optical fiber; ∆ = 2 n NA (8) Where ∆ is the fractional RI change:

Powder X-Ray Diffraction Analysis
Powder X-Ray Diffraction method is a standard technique to confirm the amorphous nature of the glassy materials. The spectrum is usually recorded using an X-Ray diffractometer instrument and this spectrum does not show any sharp peaks but gives broad hump (noise) which is the characteristic feature of a glassy material that confirms its amorphous nature.

FTIR Analysis
Fourier transform infrared Spectroscopy (FTIR) is one of the most useful analytical methods to identify a functional group of chemical a composition. One advantage of IR over other analytical techniques is that spectra can be collected on small amounts of sample using KBr pellet technique.

Raman Analysis
Raman spectroscopy provides information about molecular vibrations that can be used for sample identification and quantization.

Absorption Spectral Analysis
The absorption spectra for the prepared glasses are recorded in the 400-1000 nm wavelength range on a UV-Vis-NIR spectrophotometer and this instrument is equipped with a photomultiplier tube and a detector to acquire data in UV-Vis NIR region with a resolution 05 . 0 ≤ nm. This method is very useful for the investigation of induced transitions and it gives information regarding band structure and energy gap of amorphous materials.
Nephelauxetic ratio (β R ) and bonding parameter (δ): The Nephelauxetic ratio (β R ) is calculated by using the following expression Where β R is equal to the ratio between the observed wavenumber (in cm -1 ) of a particular transition of the RE ion under investigation and the wavenumber (in cm -1 ) for the corresponding transition of aquo-ion [5]. From the average values of β (referred as ) β is used to estimate bonding parameter, δ using the relation

Bandgap and Urbach's Energy Analysis
Photons with energy greater than optical bandgap energy will get absorbed by the material and based on this principle the electronic band structure of an amorphous material can be studied through the absorption spectra. The bandgap in electronic band structure of solids generally refers to the energy difference between the top of the valence band and bottom of the conduction band in insulators and semiconductors. The bandgap is an important factor which determines the electrical conductivity of the solid material. Optical bandgap studies have proven to be an effective tool for investigating the electronic band structure of amorphous materials. The direct and indirect bandgap energy values can be determined through the optical absorption spectra at the fundamental absorption edge of the material. The absorption co-efficient , near the absorption edge is calculated using the expression given below (13) Where d is the thickness of the sample, I 0 and I t are the intensities of the incident and transmitted radiations respectively.
Urbach's energy expression given by Davis and Mott [6].
The absorption coefficient as a function of photon energy ( ) for the direct and indirect allowed transitions can be calculated, where B is the band tailing parameter, E opt is the optical bandgap energy, is the incident photon energy and r is the index number to determine the type of optical transition. The materials with larger Urbach energy would have greater tendency to convert weak bonds into defects. The decreasing Urbach's energy values indicate the presence of minimum number of defects in the prepared glasses. The Urbach's energy values vary inversely with optical bandgap values. Therefore, the lower Urbach energies indicate the lower extension of width of the tail of localized states into the forbidden band gap.

Oscillator Strength and Judd-Ofelt Analysis
The most difficult problem in rare earth spectroscopy is the measurement of the intensity of absorption band expressed in terms of oscillator strength, which gives the intensity of absorption band corresponding to a particular transition. The experimental oscillator strength (ƒ exp ) of electric dipole transition is defined as (15) Where ) (ν ε is the molar extinction coefficient corresponding to the energy (ν cm -1 ) and ν d is the half bandwidth of the absorption band. According to JO theory, the oscillator strength (ƒ cal ) of an electric dipole-dipole absorption transition from initial state depends on three Ω λ (λ = 2, 4 and 6) as given by (16) Where m is the mass of the electron, λ is the mean wavelength of the transition, n is the refractive index, Ω λ the JO parameters and the doubly reduced matrix elements of unit tensor operator which are independent of the host matrix. These reduced matrix elements were taken from the reference [6] for the JO analysis. The oscillator strengths for the various transitions are calculated using the Equations. 15 and 16. A least square fitting approach is used for Eq. 16 to determine JO parameters, which gives the best fit between experimental and calculated oscillator strengths. The Judd-Ofelt intensity parameters Ω λ (λ = 2, 4 and 6) measured through least squaring fitting method. Ω 2 parameter is sensitive to both asymmetry and covalency at the rare earth sites and large value indicates a strong covalency of the metal-ligand bond in glasses. Ω 4 parameter seems to be mainly dependent on the long range effects and related to the bulk properties of the glasses. Ω 6 parameter indicates the rigidity of the medium in which the lanthanide ions are embedded. Although, the intensity parameters, Ω 4, Ω 6 need not depend on the local crystal field, they play a crucial role in defining the radiative properties such as transition probability, branching ratio and stimulated emission cross section of active ions. The ratio (Ω 4 / Ω 6 ) is known as the spectroscopic quality factor (χ).

Emission Spectral Analysis
The Judd-Ofelt parameters evaluated from least square fitting method are used to predict the radiative properties such as radiative transition probability (A), total transition probability (A T ), radiative lifetime ( R τ ), branching ratio (β R ) and stimulated emission cross section (σ se ) [7].
The radiative transition probability A can be expressed from initial a J andfinal ' J a as, Where λ 0 is the peak wavelength of emission band, S ed and S md are the electric and magnetic dipole line strength respectively.
The sum of radiative transition probability from a particular energy level to all the lower levels gives total radiative transition probability A T .
The radiative lifetime ( R τ ) of an excited state is determined using the formula The branching ratio (β R ) is another important parameter in understanding the optical potentiality of the prepared glass material which characterizes the possibility of attaining stimulated emission for a specific transition which can be determined by Where I is the fluorescence intensity and I max is the intensity at band maximum.

Decay Times and Quantum Efficiencies
The effective decay time is evaluated using the following expression The luminescence quantum efficiency (η) is defined as the ratio of the number of photons emitted to the number of photons absorbed. In the case of RE 3+ ion incorporated systems, it is equal to the ratio of the measured lifetime to the predicted lifetime for the corresponding levels and is given by

Colour Coordinates
The colour of any light source can be described just by three variables x(λ),y(λ) (colour matching functions), which are dimensionless quantities [8]. The chromaticity coordinates, x and y are calculated from the tristimulus values according to the equations: Where X,Y and Z are the tristimulus values, which gives the stimulation (i.e. power) for each of the three primary red, green and blue colours needed to match the colour P(λ) (Power spectral density). The locus of all the monochromatic colour coordinates makes the perimeter of CIE (Commission International de I'Eclairage) 1931 chromaticity diagram. All the mono chromatic wavelengths will lie within the area of chromaticity diagram.

Family of Borate Glasses Glass
It is an amorphous, hard, brittle, super-cooled liquid of infinite viscosity. This non-crystalline solid material is often transparent and has wide spread practical technological & decorative usage in things such as window panes, tableware and opto-electronics. The inorganic material of glass has low melting point, it can absorb, reflect, and transmit the light.

Borates
The inorganic oxide glasses like phosphate (PO 4 4-), borate (BO 3 3-), tellurite (TeO 3 2-), germanate (GeO 4 4-), venadate (VO 4 3-) and slicate (SiO 4 4-). It is well known that boron tri oxide (B 2 O 3 ) is one of the good glass formers and can form glass independently with good transparency, high chemical durability, thermal stability and good rare earth ion solubility [9]. However a glass with boron tri oxide (B 2 O 3 ) alone will possess high phonon energies ( ̴ 1300 cm -1 ) which cannot suppress the non-radiative transitions and hence rare earth ion emissions are strongly reduced. Addition of heavy metal oxides to B 2 O 3 makes them more stable, moisture resistant, minimize the phonon energies and can give intense fluorescence than the silicates and phosphates [9]. The borate glass environment possess advantageous photonic properties like good transparency, high density, high mechanical stability, good infrared transmission, optimum bandwidth, corrosion resistance is also incost. Borate glasses are commonly used for its wide range of photonic applications such as optical amplifiers, lasers, and have large non-linear refractive index and non-linear absorption coefficients and are promising materials for fiber telecommunication.

Rare Earths/Lanthanides
Rare earths are a series of chemical elements found in the earth's crust that are vital to many modern technologies, including consumer electronics, computers and networks, communications, clean energy, optical amplifier, advanced transportation, health care, environmental mitigation, national defence, and many others.
Lanthanides are the series of elements from lanthanum (La, Z = 57) to lutetium (Lu, Z = 71). The 'rare earth' is the traditional name for the group of elements consisting of the lanthanides together with scandium (Sc) and yttrium (Y).Rare earth-doped glasses are considered to be very important class of photonic materials due to their several inherent advantages over crystalline materials. Doping of glasses with terbium (Tb), europium (Eu), samarium (Sm) or dysprosium (Dy) leads to light emission in the visible range. The beauty of this family of elements is that although the members are very similar from a chemical point of view, each of them has their own specific physical properties including colour, luminescent behavior, nuclear and magnetic properties. In our modern world, lanthanides are becoming very important, especially where light emission is involved. Their very high colour purity is a major advantage of the lanthanides. The trivalent lanthanide ions are known for their sharp electronic transitions in the ultraviolet, visible and near-infrared spectral regions. Excitation and emission spectra of RE ions in glasses are due to 4f electronic transitions, which are sensitive to the local symmetry, structure of the environment and on phonon energies of the host glass. In order to optimise the efficiency of rare earth doped glasses, the local environment around RE ions has to be changed by changing the glass composition or by changing the concentration of RE ions in a host glass. The ligand fields produced by a host matrix will have considerable influence on the absorption, emission and quantum efficiency of the doped ions. In general a host glass with low phonon energies decreases non radiative relaxation rates and gives high quantum efficiencies [10].

Pr 3+ and Er 3+ Ions Doped in Lithium Cesium Borate Glasses
The mixed alkali borate glasses were prepared in different mol concentration using melt quenching technique by Y.C. Ratnakaram et al [11]. Pr 3+ doped glass fibers are currently viewed as very promising for communication systems based on 1.3 μm radiation and integrated optical lasers and amplifiers [12]. They stated among different rare earth ions, erbium is the best suitable ion for up-conversion process because of its energy level structure. The up-conversion process efficiency increases with decrease in non-radiative loss due to multiphonon relaxations. The Judd-Ofelt intensity parameters ( ) , , 6 4 2 Ω Ω Ω depend on the glass composition and 2 Ω parameter increases when second alkali is added to the glass matrix indicating an increase in covalency with addition of second alkali to the binary borate glass.
2 Ω parameter is high at x = 24 mol% and low at x = 8 mol% indicating strong and weak covalencies at these compositions in lithium cesium glass matrix. The order of magnitude of Judd-Ofelt parameters is

Dy 3+ Ions Doped Strontium Lithium Bismuth Borate Glass (SLBiB)
Different concentrations of dysprosium doped (SLBiB) glasses were prepared by melt quenching technique by Y.C. Ratnakaram et al [13]. Figure (2) shows the XRD analysis of the prepared glass and which reveals its amorphous nature.
This type of X-ray diffraction pattern is observable for all glass samples. Raman spectroscopy reveals the coexistence of trigonal BO 3 and tetrahedral BO 4 units. From the absorption spectra, JO intensity parameters and radiative properties are calculated and the hypersensitivity transition,

Dy 3+ Ions Doped Zinc Alumino Bismuth Borate Glasses (ZnAlBiB)
Good optical quality dysprosium doped in zinc alumino bismuth borate (ZnAlBiB) glasses was synthesized in different molar concentrations using melt quenching technique by A. Srinivasa Rao et al [14]. Dy 3+ is capable of emitting several interesting wavelengths in its f-f transitions that are having potential applications in diversified fields. Among the rare earth ions, Dy 3+ is one of the important rare earth ions for the preparation of phosphor material which plays a major role in the preparation of white lighting emitting phosphors. Dy 3+ ions have two (yellow and blue) characteristic intense emission bands in the visible region. Physical properties like density (1.80) and refractive index (4.02) and few other parameters were calculated. The XRD analysis shows formation of non crystalline solids and from absorption spectra, the three JO parameters Ω λ ( λ=2,4 and 6) are measured. The intensity of Dy 3+ emission spectra increases from 0.5 to 1 mol% and beyond 1 mol%, concentration quenching is observed. The branching ratio and stimulated emission cross-section for two strong transitions such as

Nd 3+ Ions Doped Zinc Alumino Bismuth Borate Glasses (ZnAlBiB)
Neodymium doped with zinc alumino bismuth borate glasses were prepared in different molar concentrations using conventional melt quenching technique by A. Srinivasa Rao et al [15]. The XRD spectral measurements confirm the glassy nature of the prepared glasses. Physical, absorption and emission properties reveal the lasing potentialities of these glasses. It is also observed that the Ω 2 , Ω 4 and Ω 6 are taking maximum values for 1 mol% of Nd 3+ ion concentration indicating that glass is more symmetric, more covalent and more rigid when compared with other concentrations. Figure (4) shows the emission spectra of the prepared glasses and three prominent transitions The intensity of Nd 3+ emission spectra increases with increasing concentrations of Nd 3+ up to 1 mol% and beyond 1 mol% concentration quenching is observed. Figure (5) shows the energy level diagram of Nd 3+ ion. High stimulated emission cross section and branching ratios of these glasses have potential for infrared lasers.

Nd 3+ Ions Doped Lithium Fluoro Borate Glasses (LFB)
Different modifier oxides based neodymium doped (LFB) glasses were synthesized by melt quenching technique by Y.C. Ratnakaram et al [16]. The spectra exhibits a broad scattering which represents a typical long range structural disorder and confirms the amorphous nature of the glass.

Eu 3+ Ions Doped Telluro Fluoro Borate Glasses (TFBO)
The glasses of europium doped telluro fluoro borate (TFBO) were prepared in different molar concentrations using melt quenching technique by K.Marimuthu et al [17]. The optical properties of absorption, excitation and luminescence were studied. Physical properties like density (2.97), refractive index (1.61) and some other parameters were calculated. Nephelauxetic ratio (β R ) and bonding parameters (δ) were calculated from absorption spectra and reveals that nature of bonding between the Eu 3+ ion and its surrounding ligands. The optical band gap and Urbach energy also calculated from absorption spectra to study the electronic band structure and it's observed that, the band gap values vary inversely with the Urbach energy values. Judd-Ofelt intensity parameters were calculated from the emission spectra provides information about all the radiative properties. From emission spectra two emission

Ho 3+ Ion Doped Lithium Fluoro Borate Glasses (LFB)
Holmium ions have wide transparency in the ultraviolet to infrared region. For this purpose they prepared holmium doped (LFB) glasses using conventional melt quenching technique by Y.C. Ratnakaram et al [18]. Structural analysis of XRD, SEM reveals glassy nature for these glasses. FT-IR shows good bond formation and JO intensity parameters were calculated and used explain radiative properties. Figure  (8) shows the Raman spectra of the prepared glasses with different modifier oxides. Raman band observed in the low frequency (185 cm -1 ) is associated to the symmetric M-O stretching in the MO 4 pyramid configuration (where M= Mg, Ca, Cd, Pb). The band in (452 cm -1 ) could be attributed B-O-B bending vibrations. The strong band at (674 cm -1 ) and (1334 cm -1 ) are attributed to the meta borate groups containing non-bridging oxygen and stretching vibrations of B-Oin different borate groups. Finally cadmium glass matrix has larger stimulated emission cross section and branching ratio which indicates that this is a good lasing material at this wavelength and highly useful for laser excitation.

Sm 3+ Ions Doped Zinc Alumino Bismuth Borate Glasses (ZnAlBiB)
Luminescent material of zinc alumino bismuth borate Glasses was prepared in different mol concentration using the melt quenching technique by A. Srinivasa Rao et al [19]. The grown glasses were subjected to X-ray diffraction analysis and it shows amorphous structure. The density (3.54), refractive index (1.81) and some other physical parameters are calculated and reported. The fluorescence intensity of Sm 3+ ion depends on the concentration of RE ions as well as on the glass network. The covalent character of Sm 3+ ions with its surrounding ligands has been confirmed through optical spectra. The absorption and emission spectra of these glasses were recorded and analyzed using Judd-Ofelt theory. The reasonably small rms deviations observed between the measured and calculated oscillator. From this evaluated JO parameters, the radiative properties such as transition probability, radiative lifetimes, branching ratios and integrated absorption cross sections were calculated. The emission spectra recorded for all the glasses Glasses (BTFB) The glasses of erbium doped barium tellurofluoroborate were prepared in different molar concentrations using conventional melt quenching technique by K.Marimuthu et al [20]. The density (5.21), refractive index (1.71) and few other physical parameters like polaran radius, field strength, molar refractivity, dielectric constant, reflection losses and electronic polarizability were calculated. The X-ray diffraction analysis reveals that the prepared glasses are in amorphous nature. The infrared spectroscopy is shows that particular functional groups are present. The absorption intensity is found to increase as the erbium ion concentration in the glass matrix increases. The Nephelauxetic ratio (β) and bonding parameter (δ) studies indicate that the ionicity of the Er-O bond decreases as the concentration of RE ion in the host matrix increases. The product of FWHM and stimulated emission cross section of 0.5 mol% of the glass has the maximum value of 134.87. Finally these glasses are suggested to be a promising candidate for the fabrication of optical devices and photonic applications.

Conclusions
The present review work presents a consolidated overview on parameters related to glasses for optical applications, mainly for realizing white light, and an analysis of potential borate glasses till now.