Concentration Dependent Improved Spectroscopic Characteristics and Near White Light Emission in Boro Phosphate Glasses Doped with Holmium

Abstract

The physical, structural, thermal, and optical properties of heavy metal (Ho³⁺) oxide incorporated lithium-boro-phosphate glass prepared utilizing melt quenching process are presented in this paper as a function of dopant concentration. To support the findings of the FTIR and DSC studies, many theoretical, experimental, physical, and optical parameters were calculated. XRD and FTIR measurements revealed the prepared glasses’ amorphous nature and the presence of significant borate functional groups. The optical band gap, Urbach energy, and steepness characteristics were tested to validate the structural results. The emission spectrums were recorded in the prepared glasses for an excitation of 450 and 550 nm to find powerful emission color. The color co-ordinates (0.33, 0.41) were found to be quite comparable to white light color co-ordinates. The present glasses can, therefore, be ideal candidates for possible applications with light-emitting diodes.

1. Introduction

The use of glasses in various industries, including displays, lasers, optical fibers, upgrading converters, photonics and medicine, has been growing in everyday life in recent years [1,2,3]. Borate and phosphate are the most frequent glass-tech network formers because they offer beneficial physical qualities for the formation of glass, such as a low temperature of glass transition, a low melting point, and a high refractive index [3,4]. In addition, it has been found to boost glass chemical stability by combining both network formers. Mixed glass formers (MGFs) of alkaline and alkaline such as boro-phosphates [4,5], boro-silicates [6,7], and telluro-phosphate [8,9] are thoroughly investigated since the structure of a glass that is formerly used for host-glass is accessible and, according to the study, provides favorable places for cations. Glasses with borosilicate have specific and enhanced qualities, including capacity to transmit UV-NIR, high clarity, mechanical strength, resistance to corrosion, strong chemical stability, high transitional glass temperature and low optical losses in the near and mid-infrared range [10,11]. The main subject of spectroscopic investigations on Ho³⁺-doped ternary glasses [4,11,12] were fluorescence characteristics of near-infrared and visible wavelength regions. Due to long persistent intermediate metastable levels, the energy levels of the trivalent Ho³⁺ ion are favorable to population inversions (⁵I₆, ⁵I₇). In optical fiber communication, medicines and gas sensors [13], the 2 mg emission corresponding to the ⁵I₇→⁵I₈ transition of the ion Ho³⁺ was utilized [13]. In numerous oxide hosts Ho³⁺ can lead to fascinating light behaviors [3,14,15,16]. Ho³⁺ stages ⁵S₂ and ⁵F₅: oxides have both green and red emissions. The host, temperature and Ho³⁺ concentration are the main determinants of green and red emission intensity. Ho³⁺: oxides study is also predicated on the promise of a shortwave laser powered by a long wavelength optical pump as a solid-state longitudinal laser [17,18]. Low-phononamorphous glasses, with rare earth ions, have been said to be very promising in the development of near-infrared laser sources. The glasses exhibit better thermal stability, reduced phonon energy and a decreased OH absorption coefficient with the addition of Holmium ions into the basic composition [17,18]. This leads to a greater stimulated emission cross-section and a longer emission life when rare earth ions undergo a near-infrared laser transition [19,20]. Boro-phosphate glasses are commonly used for the purpose of solid-state lasers and optical fiber amplifier and have a low melting-point. This investigation soughtto identify the structural, thermal and optical properties of lithium boro-phosphate glasses, as well as the optimal concentration for future uses in solid-state lasers, for holmium integration and concentration.

2. Experimental

2.1. Synthesis

The traditional melt quenching procedure was used to prepare a set of glasses. The chemical compositions of primed glasses are as follows, xLi₂O-yBi₂O₃-zP₂O₅-aHo₂O₃-(1-x-y-z-a) B₂O₃; where x = 25 mol%, y = z = 15 mol%, a is varied with respect to (45 mol%) B₂O₃ in steps of 0.1, 0.3, 0.5 mol%. For the preceding set of samples, stoichiometric quantities are considered, such as lithium carbonate (Li₂CO₃), bismuth trioxide (Bi₂O₃), ammonium dihydrogen phosphide ((NH₄)H₂PO₄), holmium trioxide (Ho₂O₃)as rare earth integrated with boric acid. All these chemicals were of AR graded chemicals procured from SRL company, Bangalore, India and used without further purification. These compounds were broken until the homogenous mixture had been reached for 30 min in agate mortar. Following the blend, the porcelain sinker was transferred and held in silted oven in steps of 300 °C until it reached 1150 °C for 1 h and 30 min to make a homogenous melting. In between the brass moulds, homogenous melt was then quenched for samples called 0, 0.1, 0.3 and 0.5 mol percent of Ho³⁺ dopant concentration, which were called B0H, B01H, B03H and B05H. Care was needed to ensure that the glass samples were not broken during quenching of the hot brass moulds preheated to 200 °C. For optical investigations and powder shapes for other research, such as XRD, heat etc., other samples were split into solid pellets.

2.2. Characterization

The prepared B0H set of glass samples were initially analyzed using an X-ray diffractometer (XRD) to validate the amorphous structure using the RIGAKU, ULTIMA IV; 40 kV, 30 mA, which used Cu K radiation with a wavelength of 1.5406 Å as a source. The refractive index and density of the sample were determined using the Digital Abbe refractometer ATAGO of wavelength 589.3 nm with an accuracy of 0.001 and the Archimedes principle with an immersion liquid toluene (=0.866 g/cm³), respectively, after the amorphous structure of the B0H sample set was confirmed. The glass transition temperature of the B0H sample set was determined using DSC analysis, which was carried out using the Mettler instrument; DSC 1 with nitrogen as the atmosphere temperature ranged from RT to 500 °C, with a heating rate of 10 °C/min. Thermo Nicolet’s Avatar 370, with a resolution of 4 cm⁻¹, was utilized as a way to comprehend different modes of vibration in the B0H sample set, which was carried out utilizing the KBR pellet technique and a Fourier transform infrared spectroscope (FTIR). Absorption studies in the UV-Vis-NIR range and photoluminescence studies in the visible region were conducted using a double beam spectrophotometer, Perkin-Elmer Lambda 750, with a resolution of 1 nm, and a spectrofluorometer, Horiba Jobin Yvon fluorolog-3, both of which used a 450 W source xenon lamp. All of the aforementioned characterizations were completed at room temperature (RT).

3. Results and Discussion

3.1. XRD Analysis

Figure 1 depicts the XRD patterns of the prepared B0H set of samples. All of the XRD patterns show two big humps in the 20° to 60° range in the 2 range, with no well-crystalline peaks. After and before Ho³⁺ ion doping, the obtained B0H set of samples are non-crystalline in nature.

3.2. Physical Properties with Theoretical Discussion

In order to investigate the effect of Ho³⁺ doping on the physical and optical properties of the prepared glasses, various physical and optical parameters, such as, density, molar volume, average boron-boron separation, packing density, oxygen packing density (OPD), average coordination number, concentration of holmium ions ($C_{H{o}^{3+}}$), holmium inter-ionic distance (r_i), polaron radius (r_p), field strength (F) of holmium field, molar refractivity, polarizability, reflection loss, transmission coefficient, and dielectric constant were evaluated using the various theoretical formulae [18,21,22,23] and experimentally acquired results.

The estimated values are presented in

Table 1. Summary of various physical and optical parameters obtained from the theoretical and experimental analysis.

Parameters	B0H	B01H	B03H	B05H
ρ	4.041 (1)	3.802 (1)	3.873 (1)	3.934 (1)
t	1.58 (2)	2.03 (3)	1.48 (2)	1.62 (1)
M.W	133.444 (5)	133.760 (5)	134.392 (5)	135.148 (5)
N	1.651 (1)	1.652 (1)	1.653 (1)	1.653 (1)
Vm	33.022 (1)	35.181 (1)	34.699 (1)	34.353 (1)
dB-B	376.121 (1)	383.905 (1)	381.685 (1)	380.000 (1)
Vp	0.631 (1)	0.592 (1)	0.601 (1)	0.607 (1)
OPD	84.790 (1)	79.586 (1)	80.691 (1)	81.504 (1)
CNav	3.850 (6)	3.853 (6)	3.859 (6)	3.871 (6)
nb	0.702 (1)	0.659 (1)	0.669 (1)	0.678 (1)
$C_{H{o}^{3+}}$	-	0.351 (1)	1.067 (1)	1.800 (1)
ri	-	3.100 (5)	2.140 (5)	1.798 (5)
rp	-	12.492 (3)	8.628 (3)	7.250 (3)
F	-	1.922 (2)	4.029 (2)	5.707 (2)
Rm	12.059 (1)	12.863 (1)	12.702 (1)	12.576 (1)
αm	4.780 (1)	5.098 (1)	5.035 (1)	4.984 (1)
RL (%)	6.030 (1)	6.044 (1)	6.058 (1)	6.058 (1)
ℇ	2.725 (3)	2.729 (3)	2.732 (3)	2.732 (3)
T	0.886 (1)	0.886 (1)	0.885 (1)	0.885 (1)
ℇop	1.725 (1)	1.729 (1)	1.732 (1)	1.732 (1)
M	0.634 (1)	0.634 (1)	0.633 (1)	0.633 (1)
${\alpha }_{o^{2-}}$	1.706 (1)	1.820 (1)	1.798 (1)	1.780 (1)
Λ	0.691 (1)	0.752 (1)	0.741 (1)	0.731 (1)
Λth	0.810 (4)	0.808 (4)	0.805 (4)	0.804 (4)
Χ	1.171 (2)	1.327 (2)	1.323 (2)	1.322 (2)
χe	0.137 (2)	0.137 (2)	0.137 (2)	0.137 (2)
χ(3) × 10−14	3.555 (2)	3.582 (2)	3.610 (2)	3.610 (2)

. We observed the variation in the values of the density of the B0H set due to the incorporation of Ho³⁺ that was added to the host glass matrix to improve structure compactness, as illustrated in Table 1. Furthermore, holmium molecular weight (377.68 g/mol) differs from borate mass (69.63 g/mol); holmium M.W, in particular, is higher, which results in enhanced formation of non-bridging oxygen’s (NBOs). However, in the B0H, the volume of the molar reduces with the increased holmium concentration due to the shrinking bond length. Due to the difference in holmium ionic radii (0.91 Å) and borate (0.27 Å), a variance in the molar volume can also be expected. Further, with increase in the holmium concentration, the <d_B-B> also enhanced which supports the variation observed in molar volume. It is found that Li₂O, Bi₂O₃, B₂O₃, P₂O₅, and Ho₂O₃ have packing factors of 9.14 cm³/mol, 26.28 cm³/mol, 20.87 cm³/mol, 34.89 cm³/mol, and 24.46 cm³/mol, respectively. The packing density for the whole B0H set was determined using the packing factor, and the difference in the packing density shows that the oxide glass’ rigidity increases as the holmium concentration. The creation of non-bridging oxygen (NBO) after the addition of holmium to the B0H set is explained by OPD variations. The increase in OPD leads to a rise in glass hardness as holmium concentration increases. It is observed that the number of bonds per unit length in the B0H set also decreased as the holmium concentration increased. There is a difference in Ri and an increase in the molecular weight also with the holmium content in the B0H glasses. This has led to the development of a densely packed glass network. We would expect to increase field strength in B0H close to Ho³⁺ ions, as r_p decreases. The results were the same as the estimated values. With holmium concentrations increased greatly, the refractive index of B0H glasses suggested that optical not-linearity was improved in the produced glasses. Thus, effective polarisation and molar repressiveness decreases were found from B01H to B05H. The R_L value of B0H set signified that the low reflection loss resulted in the high transmission coefficient. The conduction width would increase as the holmium concentration in B0H rises. As a result, the metallization requirement is less than 1, implying that the B0H set of materials is insulating [23,24,25].

Electronic oxide polarizability can be estimated using experimental refractive index as follows,

$${\alpha }_{o^{2-}}=[\left(\frac{V_m}{2.52}\right)\left(\frac{n^2-1}{n^2+2}\right)-{\displaystyle \sum }{\alpha }_{cat}]/N_{o^{2-}}{\mathsf{\AA }}^3$$

(1)

where, ∑α_cat denotes molar cation polarizability and $N_{o^{2-}}$ represents number of oxide ions present in B0H set. For the present B0H set, aLi₂O-bBi₂O₃-cP₂O₅-dB₂O₃-eHo₂O₃; ∑α_cat can be estimated as a(2)α_Li + b(2)α_Bi + c(2)α_P + d(2)α_B + e(2)α_Ho and $N_{o^{2-}}$ can be estimated as a(1) + b(3) + c(5) + d(3) + e(3). Hereα_Li = 0.0029 ų, α_Bi = 1.508 ų, α_P = 0.021 ų, α_B = 0.003 ų and α_Ho = 0.91 ų. Also, a, b, c, d and e are mole fractions of respective compounds used in batch calculation.

Furthermore, the optical basicity can be calculated using the expression [23,24,25],

$$\mathsf{\Lambda }=1.67\left(1-\frac{1}{{\alpha }^{{02}^{-}}}\right)$$

(2)

The optical basicity (Λ) is derived from considering theoretical optical basicity (Λ_th),

$$\mathsf{\Lambda }={\mathsf{\Lambda }}_{\mathrm{th}}={\mathrm{X}}_1{\mathsf{\Lambda }}_1+{\mathrm{X}}_2{\mathsf{\Lambda }}_2+{ \mathrm{X}}_3{\mathsf{\Lambda }}_3+\cdots +{\mathrm{X}}_n{\mathsf{\Lambda }}_n$$

(3)

where, X_Li2O, X_Bi2O3, X_P2O5, X_B2O3 and X_Ho2O3 are equivalent mole fractions of different oxides and Λ_Li2O = 0.87, Λ_Bi2O3 = 1.19, Λ_P2O5 = 0.33, Λ_B2O3= 0.42 and Λ_Ho2O3 = 0.945 are the theoretical basicity of oxides.

Electro negativity (χ) is estimated using the relation,

$$\chi =\frac{\mathsf{\Lambda }}{0.75}+0.25$$

(4)

Electric susceptibility (χ_e) is estimated by a relation,

$${\chi }_e=\frac{n^2-1}{4\pi }$$

(5)

Using Miller’s rule, the third order nonlinear optical susceptibility can be predicted as,

χ⁽³⁾ = [χ⁽¹⁾]⁴ × 10⁻¹⁰esu

(6)

Figure 2 shows the variation of estimated various physical parameters as a function of Holmium concentration in the B0H set of samples. In the current B0H glass, the oxide ion polarizability rose with increasing holmium content, resulting in more NBO production. To assess the degree of basicity in B0H glass, the optical basicity, which measures the glass’s potential to donate the ion’s negative charge, was determined. Chemically, glasses having a high electron density would be unstable. The basic nature of the B0H glasses, therefore, means that, with a low level of basicity, the chemistry of the created glasses increases [20,21,22,23,24,25] as the amount of basicity grows with low electron donations. The theoretical optical basis was further calculated and compared to the empirically measured optical fundamental values. The non-linearity values achieved in B0H were high and, therefore, strongly applicable to non-linear optical systems.

3.3. Glass Transition Temperature

The thermal characteristics of B0H glasses were determined by using a differential scan calorimetry (DSC). DSC may be used to determine glass transition (T_g) temperatures of the B0H. Figure 3 shows B0H and B05H glass samples as a heat flow vs. temperature graph. The T_g in the plot was acquired in the DSC curve as a turning point in the transitional glass area. The T_g decreasing with the insertion of Ho³⁺ ions in the B0H set, which could be caused by the large molar holmium mass, is based on the existing B0H’s and B05H samples.

3.4. Structural Analysis from FTIR Spectroscopy

FTIR was used for structural investigation, and the stacked spectra for the B0H group of materials are shown in Figure 4. The spectra transmittance vs. wavenumber demonstrates that the majority of the vibrations were caused by borate groups. The cage vibration of metal cations such as Bi³⁺, Li⁺ [26,27] was characterized by three primary bands of 400–750 cm⁻¹, 750–1200 cm⁻¹ and 1200–1650 cm⁻¹ in the region with a weak band of less than 600 cm⁻¹. In the region of 650–750 cm⁻¹, the bending of several borate segments results in a peak. This band has a tiny inclination for moving to a higher wave number called the blue shift. A higher bond strength value reduces the length of bond [18,19,20,21,22,23,24]. In this way, Ho increased the structure groups’ compactness. The higher concentration in the B0H package caused the rise in density to vary due to the difference in atomic weights of Ho and B. The result was a change in free volume. The outcome was that the band area between 750 and 1200 cm⁻¹ was connected with BO₄ deformation and non-bridging oxygen NBO movements. A symmetrical B-O stretching of the BO₃ group was assigned to the region from 1200 to 1650 cm⁻¹. In addition, the BO₄ field was employed to approximate the four-coordinated boron atoms for the bands and BO₃ units [28]. After integration in the B0H host, the production of NBOs was seen. The BO₃ units broke up into BO₄ units that create NBOs, also called dangerous bonds. Figure 5 shows the difference between the four-coordinated boron atoms and the B0H set.

3.5. Optical Properties

For the investigation of optical characteristics, absorption and emission spectra were used. The absorption of the rare earth/dopant (here holmium), depending on the range of UV (ultraviolet) to (near infrared) nitrogen was estimated. The greatest intensity of absorption wavelength in the spectrum was seen for sample B0H spectrum; the same wavelength as excitation wavelength was reported.

3.5.1. UV-Vis-NIR Analysis with Tauc’s Plots

Figure 6 and Figure 7 display absorption spectra in the 400–2200 nm range, with transitions numbered. B0H without holmium does not show peaks in Figure 6, but the strength of the peaks grew with the increased concentration of Ho³⁺-ions in the B0H set. The ground condition (⁵I₈) of Ho³⁺-ions with a 4f transition was detected in seven transitions in the UV-Vis-NIR area, as can be seen in the standard spectra (Figure 7) of the B05H sample. Peak transitions of 417,450, 486, 538, 642, 1152 and 1948 nm are assigned to (⁵G, ³G)₅, ⁵G₆, ⁵F₃, ⁵S₂, ⁵F₅, ⁵I₆ and ⁵I_7, respectively [13,18,29,30]. Among all transitions, ⁵G₆ is a hypersensitive transition that is specific to the surrounding atmosphere of the Ho³⁺-ions; it follows the selection rules |ΔS| = 0, |ΔL| ≤ 2 and |ΔJ| ≤ 2. Electric-dipole interactions cause transitions in the 400–700 nm range, while magnetic-dipole interactions cause transitions around 1950 nm [28,29,30,31].

The absorption co-efficient was estimated using,

$$\alpha \left(h\nu \right)=\frac{2.303\times A}{t}$$

(7)

where, A is absorbance recorded for the B0H set and t is the thickness of the B0H set.

Details on electronic states may be revealed by the higher energy spectral region, referring to interband electronic transitions. Photon absorption is initially utilized to drive electrons from a full band into an empty band, and the coefficient of absorption increases afterwards. This abrupt change in α(hν), and the difference in energy is the resultant energy, is the fundamental absorption edge [32,33]. Davis and Mott suggested a Tauc’s [34,35] relation between α(hν) and the incident radiations energy (photon energy), which can be expressed as,

$$\alpha \left(h\nu \right)=\frac{\left[t_p{\left(h\nu -E_{g^{opt}}\right)}^n\right]}{h\nu }$$

(8)

where t_p is the band tailing parameter (constant), E_g^opt is the optical band gap energy, and n is depending on the form of transition (direct or indirect). The transitions with n = 2, 3, 1/2, and 1/3 will be indirect allowed, indirect forbidden, direct allowed, and direct forbidden, respectively. This n is often affected by the material’s composition, such as crystalline or amorphous. The indirect transition curve is extrapolated in the linear component using photon energy v/s (αhν)^1/2 to achieve optical energy gap ($E_{g^{opt}}$) and the estimated values are tabulated in

Table 2. Summary of Optical parameter such as Indirect band gap, Urbach energy, Steepness parameter for B0H set of samples.

Samples	Indirect Band Gap (eV) (±0.01)	Urbach Energy (eV) (±0.01)	Steepness Parameter (±0.01)
B0H	2.50	0.34	0.07
B01H	2.89	0.23	0.11
B03H	2.88	0.22	0.11
B05H	2.78	0.26	0.09

.

Localized states with expanded optical band gaps called exponential tails, also known as Urbach tails, occur in amorphous materials (disordered structure). In an absorption coefficient curve, this tail is above the optical band gap. The Urbach analytical rule [33] expresses the relationship between the absorption co-efficient and photon energy as follows:

$$\alpha \left(h\nu \right)=\alpha {\left(h\nu \right)}_{o}exp\frac{h\nu }{\mathsf{\Delta }E}$$

(9)

Here α(ν)_o is a constant and ΔE is known as Urbach energy. The straight-line equation can be obtained by adding logarithm to both sides of the above equation. As a result, ΔE can be determined by plotting the incident photon energy (hν) versus lnα(hν) and taking the inverse of the straight line’s slope. Table 2 shows the predicted values for the B0H set of samples, which corroborate the existence of indirect or phonon-assisted transformations, as well as defects caused by the presence of Ho³⁺-ions in B0H glass. ΔE stands for structural stability and the lower the ΔE, the greater the structural stability [36,37,38]. The steepness parameter (S) [39] represents the relaxation of the exciton—phonon due to the expansion of optical absorption. This can be determined using the formula:

$$S=\frac{kT}{\Delta E}$$

(10)

where, k is Boltzmann constant, T is the room temperature and ΔE is Urbach energy.

A lower S value suggests that temperature was less of a consideration. Table 2 summarizes the results. Figure 8 depicts an indirect energy (Tauc’s) and the Urbach energy plots.

3.5.2. Photoluminescence and CIE Diagram

The emission spectra in the visible field were recorded using excitation wavelengths of 450 and 550 nm. The transition states and their spectra are depicted in Figure 9. Similar transitions can be detected in emission spectra with an excitation wavelength of 450 nm, whereas spectra with an excitation wavelength of 550 nm show a significant peak around 600 nm. Because of the spectral overlap in the host, 0.3 mol% in B0H indicates a higher intensity of the ⁵F₅–⁵I₈ transition [29,30,31].

To validate the emission color acquired for the B0H collection, the CIE (Commission International de l’Eclairage) coordinates diagram was plotted (Figure 10) using the emission data supplied.

Table 3. CIE co-ordinates and CCT values.

Sample	Excitation Wavelength (nm)	(x, y) Co-Ordinates	Emission Colour	CCT (K)
B01H	450	(0.34, 0.42)	Yellowish white	5287 (cool)
B03H	450	(0.33, 0.41)	Yellowish white	5578 (cool)
B05H	450	(0.33, 0.41)	Yellowish white	5578 (cool)
B01H	550	(0.57, 0.42)	Reddish-orange	1805
B03H	550	(0.57, 0.43)	Reddish-orange	1846
B05H	550	(0.58, 0.42)	Reddish-orange	1771

illustrates the findings of applying the CIE 1931 chromaticity diagram [40] and the McCamy analytical formula for CCT (correlated color temperature) [41] under 450 and 550 nm excitation, which are corroborated by (x, y) co-ordinate values. Based on the applications [42], we will customize the host to have a desired emission color. The results show that, when the prepared B0H set of glasses was excited at 550 nm, it emitted orange red light and, when excited at 450 nm, it emitted near-white light. This shows that the produced samples could be used in near-infrared light emission applications.

4. Conclusions

Holmium-doped heavy metal oxide incorporated lithium boro-phosphate glasses were prepared using the standard melt quenching convention technique. XRD analysis confirmed the amorphous structure of the novel holmium inclusion heavy metal oxide containing lithium boro-phosphate glasses. Density increases with holmium addition reflect holmium’s larger molecular weight than borate, which is well supported by DSC and FTIR experiments. The present glasses’ third-order non-linearity values were high, indicating that they should be employed in non-linear optical systems. Absorption investigations in the UV-Vis-NIR region revealed seven transitions, including the hypersensitive transition (⁵G₆). As the holmium concentration in the glasses grew, the Urbach energy, or steepness parameter, represented glass stability. The emission profile of glasses for 450 and 550 nm excitations was evaluated in terms of emission wavelength and intensity for both concentrations of holmium-doped glasses. As a result of the spectral overlap in the host, 0.3 mol% for 550 nm excitation displays the maximum intensity in the ⁵F⁵→⁵I₈ transition. According to the CIE chromaticity diagram, the emission of all the glass samples was yellowish-white and reddish-orange for two excitations that are substantially closer to the white light emission zone. According to McCamy CCT, the current glasses are suited for solid state laser applications with 450 nm excitation. It’s also worth noting that the physical, structural, thermal, and optical properties of today’s glasses are all intrinsically correlated.