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ACCENT JOURNAL OF ECONOMICS ECOLOGY & ENGINEERING

Peer Reviewed and Refereed Journal IMPACT FACTOR: 2.104 (INTERNATIONAL JOURNAL) UGC APPROVED NO. 48767, (ISSN NO. 2456-1037)

Vol. 03, Issue 02,February 2018 Available Online: www.ajeee.co.in/index.php/AJEEE

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OPTICAL AND LUMINESCENCE STUDY OF RARE EARTHDOPED LEAD BORATE GLASSES FOR SOLID STATE LIGHTING APPLICATIONS

Priyanka Srivastava

K. N. Govt. P. G. College, Gyanpur, Bhadohi, INDIA

Abstract - This paper presents the luminescence properties of rare earth ion (Pr³⁺⁾ in borate based glasses on addition of lead oxide. The concentration of lead oxide is varied from 0 to 30 mol%. Glasses were prepared by conventional melt quenching technique. The amorphous character of these glasses has been confirmed through XRD measurements.

Refractive index and density of all glass samples were measured. With the help of refractive index and density all relevant physical parameters were calculated. Luminescence spectra of the glasses were recorded using 476 nm line of Ar⁺ion laser. The addition of lead oxide has significant effect on luminescence spectra of Pr³⁺ions in borate based glasses. It is observed that the intensity of fluorescence peaks enhances with addition of lead oxide concentration. From the emission spectra effective band width (Δλeff) and stimulated emission cross section (σ) were calculated. The attractive luminescence features of proposed glass system with 30% of lead oxide suggests that it is promising luminescent material towards solid state lighting applications.

Keywords: Luminescence, refractive index, glasses, band width.

1. INTRODUCTION

Glasses and crystals doped with rare earth ions have found potential applications in laser materials, optical fibers, fluorescent devices, detectors and optical waveguides^1–4. The physical and optical properties of these rare earth ions are strongly affected by chemical and structural composition of the glassy or crystalline host⁵. RE-doped glasses are the favorable materials for the optoelectronic applications because they hold advantages like fluorescence over UV-Vis-IR spectral regions, longer lifetimes and higher quantum efficiency.

Quantum efficiency of luminescent levels of rare earth ions can be enhance by selecting appropriate host material and by adjusting local environment surrounding them. Such modifications are frequently achieved by network formers and network modifiers. Among different glass formers borate(B2O3) glasses are one of the best and most well-known glass formers due to their unique and attractive features such as ease of preparation, low melting point, high transparency and chemical stability, good rare earth ion solubility and most importantly it is inexpensive^6-7. Due to these properties B2O3 is suitable host material to prepare fluorescent devices doped with rare earth ions. Since the glasses with B2O3 possess high phonon energy which in turn increases the non radiative decay process and reduces strongly the quantum efficiency. The addition of heavy metal oxides such as

PbO and Bi2O3 in borate glasses reduces its phonon energy^8-9 thus enhances radiative emission.

Recently, Praseodymium ions doped in crystals and glasses have been widely investigated because these exhibit very rich emission spectra extending from UV to the infrared region^10-11. Praseodymium in glass matrices shows efficient luminescence only when it is triply ionized. The observed luminescence spectra of Pr³⁺ in the visible and near infrared regions arise mainly from the excited states ³P0,1,2 and ¹D2, which combine through electric dipole transitions to ground state ³H4 or other low lying excited states. The transition ³P0, 1,2 → ³H4 exhibits a short decay time of the order of 3μs¹².

In the present paper Pr³⁺ doped borate based glasses were prepared with lead oxide varied from 0 to 30 mol%. The other modifiers taken are lithium carbonate and calcium carbonate. All physical properties of glasses were calculated. The fluorescence spectra of these glasses were recorded and analyzed.

2. MATERIALS AND METHOD 2.1 Glass Preparation

The molar compositions of praseodymium doped borate based glasses are as follows:

(the numbers in front of the chemical symbols are the mole percentage of the compound used in the preparation of the glass)

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2 1) 69.75 H3BO3 +20Li 2CO3 +(10-x) CaCO3

+x PbO +0.25Pr2O3

x = 0, 10

2) 69.75 H3BO3 +(20 –y) Li 2CO3+(10+y) PbO +0.25Pr2O3

y =10, 20

All compounds are thoroughly crushed in an agate mortar in order to mix chemicals homogeneously. The mixture is then put in a platinum crucible and melted in an electric furnace at 1100⁰ C for one hour.

The melt is then air quenched by pouring it on a rectangular iron cast kept at 500⁰ C. It was then slowly cooled to room temperature to get a properly annealed glass. The glasses were polished carefully for spectroscopic measurements.

2.2 Physical and Spectroscopic Measurement

The refractive index of each glass was measured using Brewster angle method at

650 nm. The densities of the glasses were measured through the Archimedes principle using xylene as the immersing liquid. For fluorescence measurement 476.5 nm line of Coherent 10 watt Ar+

Laser with power 500mw and band width 0.01nm was used as excitation source.

XRD of prepared glasses were recorded by X-ray diffractometer (Philips PW 1710) to check their lack of crystalline character.

The fluorescence signal is dispersed by Spex, 0.5m monochromator and detected by a R928 photomultiplier. All the measurements were done at room temperature.

3. RESULTS AND DISCUSSION 3.1 Physical Properties

Various physical properties of the Pr³⁺ion doped without lead oxide and with 30mol% of lead oxide are calculated by using relevant equations^13-14and obtained results are given in Table 1.

Table: 1.Physical Properties of 0.25 mol% of Pr³⁺ doped glasses Properties Without PbO With 30%PbO Density (gm/cm³)

Refractive Index

Average molecular weight(gm/mol) Molar volume (cm³/mol)

Ion concentration(ion/cm³) Electronic polarizability(cm³) Molar refractivity (cm³) Ionic radius(A⁰) Field strength(cm^-2) Reflection losses

2.12 1.26 70.28 25.61 0.36×10²² 1.22×10^-24 7.78 1.08 0.60×10¹⁶

17%

3.41 1.68 81.05 41.00 0.63×10²² 1.33×10^-24

8.98 2.17 0.83×10¹⁶

25%

3.2 X-Ray Diffraction Spectral Analysis The XRD spectrum of glasses shows a broad hump which is the characteristic of an amorphous material. Hence the prepared glasses are of amorphous in nature.

3.3 Fluorescence Spectra and Radiative Properties

The fluorescence spectra in the 540- 750nm range due to Pr³⁺ ions in the glasses with varying concentration of lead oxide are shown in fig-1. The assignments of the peaks to specific transitions are made on the basis of the known energy levels of Pr³⁺ as reported by Dieke¹⁵ and earlier workers^16-17. All the fluorescence spectra were recorded in the same experimental conditions i.e. laser power, PMT voltage, recorder speed, sample position and temperature etc were kept unchanged. The same scale has also been used for the recordings. It is found that

the addition of PbO has a significant effect on the fluorescence spectrum. The first and most interesting observation is that the intensity of the fluorescence peak at 614.5nm corresponding to ¹D2→³H4

transition increases with increasing PbO concentration and is a maximum for 30%

PbO. Another important observation is that two new fluorescence peaks at 577.8nm and at 588.7nm are seen in addition to the other well-known peaks when the concentration of lead oxide is greater than 10%. The peak at 713.0nm is observed only when PbO concentration is 30%.

The fluorescence measurement was made using the 476 nm line of Ar⁺ laser as the exciting line. The energy for this radiation is 20986cm^-1 and is considerably higher (~by more than 500cm^-1) than the energy required for excitation of the ³P0 level (20474cm^-1) of Pr³⁺ but is less than the excitation energy

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3 for the ³P1 level (21066cm^-1). This can be seen clearly in the energy level diagram (fig.2). The most intense fluorescence peak at 614.5 nm (16268cm^-1) in all the spectra is ascribed to the ¹D2→³H4

transition. The peak observed at 655.9nm is ascribed to ³P0→³F2 transition. The peaks at 577.8nm (17303cm^-1) and 588.7nm (16981cm^-1) which appear only when there is an appreciable amount of PbO (more than 10%) cannot be ascribed to transitions from the ³P0 or ¹D2 levels (which can be directly populated by the incident radiation) to any known low lying levels but match in energy with the transitions ¹I6→³H6 and ³P1→³H6

respectively. The ³P1 level may be populated even due to thermalization (since E (³P1) – E (³H4) is only ~ 80cm^-1 more than the incident photon energy).

But for the level ¹I6, this energy difference [E (¹I6) – E (³H4) is 21395cm^-1] is 409cm^-1 more than the available photon energy.

Moreover, these peaks are seen only when lead oxide is present in appreciable concentration so we assume that the phonon energy available in PbO (~450cm^-

1)^18-19 is involved in these transitions.

It can be seen from the fluorescence spectrum that the intensity of the line due to ¹D2→³H4 transition shows a maximum when PbO concentration is 30%. The increased intensity of this peak is almost five times the corresponding intensity in absence of PbO. In fact in the glass with 30% PbO the intensity of all observed fluorescence peaks is seen to be much larger and in addition a peak at 713 nm is seen in the glass with 30% PbO which can be ascribed to ³P0→³F3 transition. The enhancement of the fluorescence intensity is probably to be attributed to the structural change in the glasses following the addition of lead oxide. It has already been noted that conversion of BO3 to BO4

is maximum at 40% of PbO, so it seems that existence of BO4 is more suitable for fluorescence. The phonon energy for BO4

is less than the corresponding value for BO3 and this might be the reason for the enhanced fluorescence as the reduced phonon frequency decreases the rate of non-radiative relaxation.

Fig.1. Fluorescence spectra of Pr³⁺

doped glasses with different concentration of PbO

Fig. 2. Energy level diagram of Pr³⁺

For laser applications the values of the stimulated emission cross section are of great interest. The stimulated emission cross-section (σ) for the ¹D2→³H4

transition corresponding to various glass compositions has been calculated and is listed in Table-2. For calculating the stimulated emission cross-section we first calculate the effective bandwidth using

the formula²⁰

( ) ) (

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0

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 

  ^d

where ∫ I(λ) dλ represents the effective area of the peak and I(λ0) is the intensity of the peak at λ0.

Table.2 Effective line width and stimulated emission cross-section for ¹D2

→³H4 transition

% lead oxide Δλ (nm) σ10^-20 cm² 0%

10%

20%

30%

23.5 23.5 21.8 20.4

0.837 0.685 0.726 0.770

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4 It is also seen from table.2 that the value of σ decreases with increase in PbO concentration, contrary to our experimental observation (increase in luminescent intensity with PbO). This might be a reflection of the inadequacy of the Judd-Ofelt theory to explain the spectral properties of Pr³⁺.

4. CONCLUSION

In the present work Pr³⁺ doped borate glasses of good optical quality with Li2CO3

and CaCO3 and PbO as modifier were prepared with melt quenching technique.

The luminescence and lasing properties of all the glasses were investigated. It is observed that with addition of lead in borate glass the fluorescence intensity of all transition of Pr³⁺ ion enhances, which suggests that PbO is better modifier and this glass with 30% of Pbo can be used as luminescent material towards solid state lighting applications.

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View of OPTICAL AND LUMINESCENCE STUDY OF RARE EARTH DOPED LEAD BORATE GLASSES FOR SOLID STATE LIGHTING APPLICATIONS

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