Spectroscopic analysis of Eu³⁺ doped silica–titania–polydimethylsiloxane hybrid ORMOSILs

Abstract

Eu³⁺ doped silica–titania–polydimethylsiloxane hybrid ORMOSILs were synthesized via a non-hydrolytic sol–gel route. The structural and thermal analyses of the samples confirmed that the matrix structure remains unaffected by doping with different concentrations of Eu³⁺ ions. Photoluminescence (PL) studies performed at 394 nm on Eu³⁺ doped ORMOSILs imply that they emit broad blue host emission and the characteristic Eu³⁺ red emissions simultaneously. Also, the samples were excited at the charge transfer (CT) band and this confirmed the existence of an energy transfer path from the host to the Eu³⁺ ions via Ti⁴⁺–O²⁻–Eu³⁺ bonds. The phonon energy of the host matrix was estimated by phonon sideband (PSB) analysis and the results were substantiated by Raman analysis. Judd–Ofelt (JO) parameters were also evaluated which give details about the local surroundings of the Eu³⁺ ions in the system and these parameters were further used for predicting the radiative properties of ⁵D₀ → ⁷F_1,2,4 transitions of Eu³⁺ ions. Furthermore, the quantum efficiency and CIE co-ordinates were evaluated and it was found that Eu³⁺ doped silica–titania–polydimethylsiloxane ORMOSIL has an intense pinkish red emission with a quantum efficiency of 30.7%.

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1. Introduction

Research has been progressing for decades on the development of luminescent materials and their applications in fabricating optical devices. Materials doped with rare-earths (REs) have drawn considerable attention due to their excellent fluorescent properties. Cerium, samarium, europium, gadolinium, terbium, dysprosium, erbium, and ytterbium are some of the rare-earths commonly used for luminescence applications. But the intensity of emission by these rare-earths is greatly influenced by the site and its neighbourhood occupied by the RE ions in the host matrix.¹ Europium (Eu³⁺) ions emit radiation in the red region resulting from the ⁵D₀ → ⁷F₂ hypersensitive transition and its intensity is highly influenced by the symmetry of the positions occupied by Eu³⁺ ions in the host matrix. Consequently, the Eu³⁺ ions are effectively used to explore the symmetry and bonding nature around these dopant ions in the system.² Hence new host matrices are developed to incorporate these RE ions and the study of the performance of the RE ions in these new systems is a fascinating field of research in material science. Thus a new family of organically modified silicate is introduced as host matrix in the present work.

The electrons that are excited to the ⁵D_1,2,3 levels of the Eu³⁺ ions de-excites non-radiatively to the ⁵D₀ level by multiphonon relaxation. Studies show that the multiphonon decay rate increases with increase in phonon energy and electron–phonon coupling strength.^3,4 Hence it is also important to estimate the phonon energy associated with the present glass system.

In many recent works, silica is effectively used as the host matrix to incorporate the rare-earth ions so that these ions can be protected from the environmental effects and can enhance emission intensity.⁵ Also, the binary matrices such as TiO₂–SiO₂ and TiO₂–ZrO₂ doped with RE ions were widely examined during the last decades as they find applications in lasers, lighting and displays, up-conversion and white light emission.^6–9 However, the surface defects and lattice distortions in these host matrices adversely influence the luminescence properties of the RE ions which leads to quenching of luminescence.⁴ Hence the decay life time and luminescence intensity of these host matrices have to be improved. To achieve this goal, organics are incorporated into the inorganic matrix which leads to the formation of new Organically Modified Silicates (ORMOSILs) and Organically Modified Ceramers (ORMOCERs) with improved mechanical, thermal and optical properties.¹⁰ In the light of the above investigations, the present work aims at the preparation and characterisation of polydimethylsiloxane modified silica–titania (SiO₂–TiO₂–PDMS) ORMOSIL matrix doped with Eu³⁺ ions. Here, PDMS is selected as the organic polymer for modifying the silica–titania inorganic matrix because both PDMS and SiO₂ have similar –Si–O–Si– chemical bond and hence forms covalently bonded network.^11,12 Here non-hydrolytic route of sol–gel method is adopted for the sample preparation since the organic polymer PDMS is insoluble in water and TiO₂ precipitates in the hydrolytic process.¹³

Interestingly, the silica–titania–polydimethylsiloxane (SiO₂–TiO₂–PDMS) host matrix has a broad blue emission peaking at 470 nm when excited with UV light.¹⁴ Thus the photoluminescence emission of the Eu³⁺ doped SiO₂–TiO₂–PDMS hybrid matrix is the resultant of the blue emission of the host matrix and the red emission of the Eu³⁺ ions which finds application in WLEDs, novel luminescent phosphors, displays etc.

2. Experimental details

The inorganic precursors for synthesising the samples were tetraethyl orthosilicate/TEOS (Merck) and titanium(IV) isopropoxide/TIP (Aldrich) which contains SiO₂ and TiO₂ respectively. The organic polymer for modifying the inorganic was OH-terminated polydimethylsiloxane/PDMS (MW = 4200) (Alfa Aesar). Here, ethanol (Merck) was the solvent and hydrochloric acid (HCl) was the catalyst. The source of Eu³⁺ ions for doping was europium(III) nitrate hexahydrate (Alfa Aesar). The molar ratio of (SiO₂ + PDMS)/TiO₂ is taken as 3 : 1. The SiO₂ and PDMS were taken together since the organic polymer PDMS has the same –Si–O–Si– bonding as that of SiO₂.

Mixtures 1 and 2 were prepared during synthesis. Mixture 1 contains 5.2 mL of TEOS, 0.1 mL of PDMS and 5.8 mL of ethanol and the molar ratio of EtOH/TEOS is kept as 4. Mixture 2 consists of 1.9 mL of TIP and 1.8 mL of ethanol where the molar ratio of EtOH/TIP is kept as 5. After stirring these mixtures separately for 30 minutes, they were added together and the stirring was further continued for another one hour. Europium(III) nitrate hexahydrate dissolved in ethanol was then added to the resultant SiO₂–TiO₂–PDMS mixture in various concentrations ranging from 1 to 4 wt% and hydrochloric acid was also added in very small quantity. This mixture is vigorously stirred to obtain a clear and homogenous sol which is transferred to polypropylene petridish and covered with para film. These dishes were kept without any disturbance until transparent glass samples were formed. All the synthesis procedures were carried out at room temperature and normal atmospheric conditions. In this discussion undoped ORMOSIL host matrix SiO₂–TiO₂–PDMS is denoted as Eu0 and the Eu³⁺ doped samples were denoted as Eu1, Eu2, Eu3 and Eu4 for 1, 2, 3 and 4 wt% doping respectively.

X-Ray powder diffraction analysis and Fourier transform infra-red characterization were carried out for the analysis of the sample structure using PANalytical X'Pert PRO and Perkin Elmer Spectrum 400 respectively. The thermal stability of the samples was studied using PerkinElmer STA 8000 thermo gravimetric analyser. Also, Flurolog-3 Spectrofluorometer (Horiba Scientific) was used for photoluminescence characterisations and Fluoromax-4, TCSPC Fluorescence Lifetime Measurement System (Horiba Scientific) for lifetime measurements. The Raman spectra of the samples were also recorded for analysing the phonon side band (PSB) using WITec Alpha 300 RA Confocal Raman Spectrometer.

3. Results and discussions

3.1 XRD analysis

X-Ray diffraction analyses were carried out on the pure host matrix Eu0 and samples doped with varying Eu³⁺ concentrations. All the samples show identical diffraction pattern and hence the patterns of representative samples Eu0, Eu2 and Eu4 are depicted in Fig. S1.† The X-ray diffraction pattern has a broad hump between 15° and 30° which is a characteristic of the SiO₂ component of the host matrix.^15,16 Here, the peaks of TiO₂ are not distinctly visible as the amorphous nature of SiO₂ is prominent. The diffraction patterns of all the samples also show similar behaviour which confirms that the structure of the host matrix remains unaltered even after Eu³⁺ doping.

3.2 FTIR analysis

The FTIR spectroscopy can be adopted as an effective tool for analysing the chemical and structural changes of the host matrix with Eu³⁺ doping. The FTIR spectra of the pure (Eu0) and Eu³⁺ doped samples (Eu2 and Eu4) were recorded from 400 cm⁻¹ to 4000 cm⁻¹ and are shown in Fig. 1.

Fig. 1 FTIR spectra of pure and Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL samples (inset figures (A and B) shows the peaks in the range 750 cm⁻¹ to 975 cm⁻¹ and 1240 cm⁻¹ to 1275 cm⁻¹ respectively).

The FTIR spectra of pure and Eu³⁺ doped samples show identical nature in respect of peak positions and intensity. The small peak at 849 cm⁻¹ in the spectra establishes that copolymerisation reaction has occurred between Si–OH groups of hydrolysed TEOS and PDMS molecules. Also, the peak located at 945 cm⁻¹ is ascribed to the Si–O–Ti bond formed during condensation reaction. The asymmetric vibration of Si–O–Si bonds results in the absorption band at 1066 cm⁻¹.¹⁷ Also, the band at 803 cm⁻¹ is assigned to the net effect of Si–C stretching and CH₃ rocking in Si–CH₃ and that at 1267 cm⁻¹ is due to the symmetric deformation of CH₃ in Si–CH₃ group.^18,19 Mackenzie et al. have suggested in the report on SiO₂–PDMS ORMOSIL that the –CH₃ groups in Si–CH₃ performs in a way similar to the non-bonding oxygen in silicate glasses.²⁰ Thus it can be concluded that the FTIR spectroscopy confirms the presence of oxygen related defects in the host matrix which plays an important role in the luminescence properties of the SiO₂–TiO₂–PDMS ORMOSIL host matrix.¹⁴ Absorption bands were also present at 1648 cm⁻¹ and 3300 cm⁻¹ which occurs by the stretching and bending vibrations of surface adsorbed water and OH groups respectively.²¹ The similarity in the FTIR spectra of both pure and Eu³⁺ doped samples further confirms that the matrix structure remains unchanged by doping with different concentrations of Eu³⁺ ions.

3.3 TG analysis

Thermo gravimetric (TG) analysis was done in the nitrogen atmosphere up to 700 °C to study the thermal stability of pure and Eu³⁺ doped samples and the curves are illustrated in Fig. S2.†

The TG curves depicts that a weight loss of only 15% occurred on heating up to 150 °C for all the samples. This is due to the evaporation of adsorbed water and trapped solvents in the matrix, whose presence is confirmed in FTIR analysis.²¹ The curves become almost straight in the range 150–700 °C and is indicating that the samples are thermally stable up to 700 °C. The similar nature of TG curves for the pure and doped samples confirms that the matrix structure remains unaffected by Eu³⁺ doping.

3.4 Photoluminescence analysis

The photoluminescence (PL) analysis is used as an efficient tool for investigating the energy level transitions in Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSILs. The excitation spectra of the samples were recorded fixing λ_em at 613 nm which is the prominent emission wavelength of Eu³⁺ ions. The excitation spectra of all the samples are depicted in Fig. S3† and as a representative case, the spectrum of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL (Eu2) is shown in Fig. 2.

Fig. 2 Excitation spectrum of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL sample.

The excitation spectrum of Eu2 sample consists of a broad band peaking at 276 nm which is due to the overlapping of the O–Eu charge transfer (CT) band and the SiO⁻–Ti ligand-to-metal electron transfer [LMET] band. This indicates an energy transfer from the O–Ti charge transfer band to the Eu³⁺ ions in the SiO₂–TiO₂–PDMS system.^22,23 The lines at 360, 374, 380, 394, 414, 464 and 524 nm are assigned to the electronic transitions of Eu³⁺ ions from the ground ⁷F₀ level to ⁵D₄, ⁵G₄, ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂ and ⁵D₁ levels respectively. Also, there is a peak at 534 nm which corresponds to the ⁷F₁ → ⁵D₁ energy level transition of Eu³⁺ ions.² The peak at 450 nm is the phonon sideband of the host matrix which will be discussed in the forthcoming section. The prominent excitation transition at 394 nm and the charge transfer band were chosen for the investigation of the photoluminescence characteristics of Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSILs.

The emission spectra of all the samples were obtained upon excitation at 394 nm and are shown in Fig. 3. Here, the spectra consist of a broad emission peaking at 450 nm which is assigned to the oxygen related defects in the host matrix.^21,24 The authors reported in their previous work that the pure SiO₂–TiO₂–PDMS host matrix has a weak excitation around 276 nm and a strong excitation around 396 nm, monitoring the emission at 470 nm.¹⁴ Hence, when the Eu³⁺ doped system is excited at 394 nm, both the host matrix and the Eu³⁺ ions were excited giving host emission along with the characteristic emissions of Eu³⁺ ions as shown in the spectra. It is observed that the host emission intensity increases with increase in Eu³⁺ doping concentration. As the Eu³⁺ ions were embedded in the host matrix, its lattice gets distorted due to the difference in the radius of Eu³⁺ ions (0.098 nm) and Ti⁴⁺ ions (0.061 nm). Also, a charge imbalance exists between Eu³⁺ and Ti⁴⁺ ions which leads to the formation of oxygen vacancies.^6,25 Thus as the Eu³⁺ ion concentration increases, these defects also increases which in turn enhances the host emission.

Fig. 3 Emission spectra of Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSILs excited at 394 nm.

The line emissions in the spectra correspond to ⁵D₀ → ⁷F₀ (577 nm), ⁵D₀ → ⁷F₁ (590 nm), ⁵D₀ → ⁷F₂ (613 nm), ⁵D₀ → ⁷F₃ (651 nm) and ⁵D₀ → ⁷F₄ (698 nm) transitions of Eu³⁺ ions.^2,6 Though the electrons were excited to the higher energy levels of Eu³⁺ ions, only radiative transitions from ⁵D₀ level to the lower f levels were observed in the emission spectra. This indicates the existence of non-radiative transition from the higher energy levels to ⁵D₀ level. Even though ⁵D₀ → ⁷F₀ and ⁵D₀ → ⁷F₃ transitions are present in the spectra, they are forbidden according to the Judd–Ofelt (JO) theory. The presence of these transitions were due to J-mixing which is enhanced by strong crystal-field effects of the host matrix.² Hence it can be confirmed that the Eu³⁺ ions have asymmetric vicinity in the matrix. Furthermore, the ⁵D₀ → ⁷F₀ transition is narrow which confirms that the Eu³⁺ ions occupy asymmetric sites in the host matrix which is a characteristic of amorphous hybrid samples.²⁶

The ⁵D₀ → ⁷F₁ transition is a magnetic dipole transition while the ⁵D₀ → ⁷F₂ transition is an electric dipole transition. Hence the ⁵D₀ → ⁷F₁ transition does not depend on local environment of the Eu³⁺ ions while the ⁵D₀ → ⁷F₂ transition is affected by the local surroundings of the Eu³⁺ ions and the nature of the ligands in the host matrix. Thus the ⁵D₀ → ⁷F₂ transition is a hypersensitive transition which can be used as a probe to measure the asymmetry of Eu³⁺ occupied site. On exciting the samples at 394 nm, the peak corresponding to ⁵D₀ → ⁷F₂ transition at 613 nm is very strong compared to ⁵D₀ → ⁷F₁ transition at 590 nm. As the Eu³⁺ ions occupy asymmetric sites in the host matrix the electric dipole transition dominates the magnetic dipole transition.²⁷ Hence it can be concluded from the photoluminescence studies that the Eu³⁺ ions occupy asymmetric sites in the SiO₂–TiO₂–PDMS ORMOSIL host matrix. The luminescence intensity is found to increase with an increase in the Eu³⁺ ion concentration up to 2 wt%. The glass exhibits luminescence quenching above this concentration and hence 2 wt% Eu³⁺ is the optimum doping concentration in the present system. As the number of Eu³⁺ activator ions increase, the interionic distance decreases which results in a fast energy transfer between the ions. Thus a part of the excitation energy is non-radiatively dissipated resulting in the quenching of luminescence.²⁸

The emission spectra were also recorded on exciting the Eu³⁺ doped samples in the CT band (276 nm) and are shown in Fig. 4. The spectra shows similar nature as that of the directly excited samples which clarifies that the Eu³⁺ ions in the same site are excited in both cases. The Eu³⁺ activator ions are not directly excited in the CT band but the high emission intensity of the Eu³⁺ ions may be due to the efficient energy transfer from the host matrix to the Eu³⁺ activator ions via Ti⁴⁺–O²⁻–Eu³⁺ bond.²⁹ When the samples are excited at 276 nm, the electrons in the O 2p states are excited to the Ti 3d states of the host matrix which is transferred to the 4f state of the Eu³⁺ ions.²⁵ Thus the Eu³⁺ activator ions are efficiently sensitized by the host. Since the excited electrons in the host matrix are transferred to the Eu³⁺ ions, the host emission intensity is not prominent in the spectra. The magnified image in the inset of Fig. 4 shows that though the host emission intensity is feeble, the trend shown is same as that of the directly excited samples. This confirms that the Eu³⁺ insertion in to the host matrix generates defects which in turn increases the host emission. The variation in the photoluminescence intensity of ⁵D₀ → ⁷F₂ transition with Eu³⁺ concentration in both the excitation wavelengths are also depicted in Fig. 5.

Fig. 4 Emission spectra of Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSILs excited at 276 nm (inset shows the magnified image of the host emission in the range 400–550 nm).

Fig. 5 Variation of PL intensity of 613 nm emission with Eu³⁺ concentration.

The luminescence life time of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL (Eu2) was evaluated by monitoring the decay profile of 613 nm emission of the sample on exciting with 394 nm and the luminescence time spectrum is shown in Fig. 6.

Fig. 6 Luminescence decay time profile of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL sample.

The average fluorescence lifetime (τ) of the energy level can be obtained from the formula, , where I(t) represents the luminescence intensity at time t.³⁰ The average lifetime of Eu2 ORMOSIL glass was found to be 0.90 ms. A relatively longer lifetime of the sample shows that the Eu³⁺ ions are evenly embedded in the host matrix so that ion clustering is reduced.²⁹ The nature of the decay curve also indicates that the Eu³⁺ ions are present in an asymmetric environment in the host matrix.³¹ A comparative study with the lifetimes of other Eu³⁺ doped samples are tabulated in Table 1.

Table 1 Comparison of average life time of Eu³⁺ ions in similar host matrices

Sample	Average lifetime, τ (ms)	Reference
SiO2–TiO2–PDMS–Eu^3+	0.900	Present work
EuTTA–PMMA	0.364	32
TPBFEu16	1.2002	3
SiO2–PMMA–Eu^3+	0.275	26
TiO2–Eu^3+	0.550	33

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The asymmetric ratio (R) is another effective tool to analyse the symmetry of site occupied by Eu³⁺ ions in the host matrix. It is the ratio between the integrated emission intensity of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ energy level transitions.⁶ As discussed earlier, ⁵D₀ → ⁷F₂ transition is due to the electric dipole mechanism and ⁵D₀ → ⁷F₁ transition due to magnetic dipole mechanism which is not affected by the local surroundings of the ions. Hence the intensity of ⁵D₀ → ⁷F₁ transition is taken as the reference. Thus the R values can be used to understand the asymmetry in the neighbourhood of Eu³⁺ ions in the host matrix and also the degree of covalency between Eu³⁺ and O²⁻ ions.^1,3 The R values obtained in the present study are 3.35, 3.44, 3.37 and 2.5 for Eu1, Eu2, Eu3 and Eu4 samples respectively. The larger values of R indicates that the asymmetry and covalency effect around the Eu³⁺ ion site is higher in the present system. Julian et al. have reported that the R value is in between 3 and 6 for Eu³⁺ doped silica glasses, polymers or crystals.³⁴ Also as the Eu³⁺ ion concentration increases, the R value reaches a maximum for Eu2 sample and decreases above that concentration. This confirms the cluster forming tendency of Eu³⁺ ions at higher concentration which leads to luminescence quenching.³⁵

3.5 Phonon sideband analysis

The Eu³⁺ ions doped in the SiO₂–TiO₂–PDMS host matrix are excited to ⁵D_1,2,3,4, ⁵L₆, ⁵G₂ energy levels. But the phonon energy of the sample hinders the radiative emission from these higher energy levels to the ground state. Hence, all these excited electrons of Eu³⁺ ions can populate the lowest excited ⁵D₀ energy level after non-radiative decay. From this ⁵D₀ level, the excited electrons return to the ⁷F_J (J = 1, 2, 3, 4) ground state emitting its characteristic radiation. The excitation spectrum of Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL contains a peak in the higher energy side of the pure electronic band (PEB) transition, ⁷F₀ → ⁵D₂ and this peak is assigned as the phonon sideband (PSB) as shown in Fig. 7. The PSB analysis is carried out to study the multiphonon relaxation in Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL host matrix.

Fig. 7 Phonon sideband spectrum of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL with ⁷F₀ → ⁵D₂ excited state transition.

The PSB includes simultaneous vibrational and electronic transitions and its presence in the excitation spectrum is an indication of the coupling of excited electrons with the excess energy of vibrational modes around the Eu³⁺ ions. The difference in energy between the PEB and PSB band corresponds to the phonon energy, ħω and here it is estimated to be 670 cm⁻¹. The coupling of a particular vibrational mode to an electronic transition is termed as electron-phonon coupling strength, g and is estimated to be 0.2984 for Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL. Similar high values of g was also reported for Eu³⁺ doped ZrO₂–PEG composites³⁵ and for Eu³⁺ doped SiO₂–TiO₂–ZrO₂ glasses.⁴ Also, L. Skuja has reported the significance of electron-phonon coupling constant (g) in optical transition. That is if the g value is below ∼4, the optical absorption or luminescence spectra contains zero-phonon and vibronic lines even at low temperatures.³⁶ The relative non-radiative decay rate W_mp/W₀ were also calculated for the ⁵D₁, ⁵D₂ and ⁵D₃ levels of the sample and are tabulated in Table 2. The equations and methods of PSB analysis have been published previously^3,37 and are included in Section S1 of the ESI.†

Table 2 Phonon energy (ħω), electron-phonon coupling constant and non-radiative decay rate for the ⁵D₁, ⁵D₂ and ⁵D₃ levels of the 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL (Eu2)

ħω (cm^−1)	g	^7F0 → ^5D1		^7F0 → ^5D2		^7F0 → ^5D3
		α (×10^−3 cm)	Wmp/W0	α (×10^−3 cm)	Wmp/W0	α (×10^−3 cm)	Wmp/W0
670	0.2984	1.667	0.0541	2.169	0.0044	2.35	0.0014

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The observed PSB band for the Eu³⁺ ions in the SiO₂–TiO₂–PDMS ORMOSIL glass is correlated with its Raman spectra which is shown in Fig. 8.

Fig. 8 Raman spectrum of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL sample.

The assignments of chemical bonds in the host matrix that produces the vibrational frequencies in the Raman spectrum are given in Table 3.

Table 3 Chemical bond assignments of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL

Vibrational frequency (cm^−1)			Chemical bond assignments (cm^−1)	Reference
1	2	Total
45	97	142	Symmetric stretching vibrations of oxygen atoms in O–Ti–O bond	38
672	—	672	Stretching vibration of Ti–O bond of TiO6 octahedral units	39 and 40

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Thus it is evident from Raman spectrum that the most energetic vibrational mode which substantially contributes to the appearance of phonon sideband is at 672 cm⁻¹ and is correlated with the stretching vibration of Ti–O bond of the host matrix. Furthermore, the presence of Ti–O bond in the Eu2 sample is confirmed in FTIR analysis and also the possibility of charge transfer from Ti–O bond to Eu³⁺ ions is confirmed. Marchese et al. have also reported similar photoluminescence emissions in grafted Ti-MCM41 silicas.⁴¹ Thus the features of the phonon side band of Eu2 glass is complementary with its Raman spectrum.

3.6 Judd–Ofelt (JO) analysis

The chemical surroundings of the site occupied by the Eu³⁺ ions in the host matrix has a strong effect on its emission intensity. Judd–Ofelt (JO) theory is a very efficient tool to study the variation in emission intensity of Eu³⁺ ions with its varying chemical environment in the host medium.^4,27 Usually, the JO parameters are determined from the absorption spectrum using the least square fit method. However, in several cases of Eu³⁺ ions, there are difficulties to derive the JO parameters due to (i) thermal corrections⁴² (ii) zero magnitude of U⁽⁴⁾ matrix element²⁸ (iii) poor intensity of ⁷F₀ → ⁵D₂ level transition.⁴³ Hence as an alternative method the JO intensity parameters can be calculated from the emission spectrum based on the procedure reported by Peng and Izumitani.⁴⁴ The ⁵D₀ → ⁷F₁ transition is the magnetic dipole transition and hence its intensity is independent of the host matrix, whereas the ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₄ and ⁵D₀ → ⁷F₆ transitions are electric dipole transitions and depends upon Ω₂, Ω₄ and Ω₆ values respectively. Among these electric dipole transitions, ⁵D₀ → ⁷F₆ transition is weak and falls in the infrared region (∼800 nm). Hence this transition is not observed properly due to instrumental limitations. Thus the Ω₆ intensity parameter is taken as zero for determining the radiative parameters of Eu³⁺ ion.³ The following equation is used for calculating the radiative intensity parameters^45,46
where ν₁ and ν_J are the wavenumbers in cm⁻¹ for the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F_J (J = 2, 4, 6) transitions respectively. S_md gives the magnetic dipole line strength of the ⁵D₀ → ⁷F₁ transition of Eu³⁺ ions and n is the refractive index of the glass and for Eu2 glass its value is 1.562. The calculated JO parameters of Eu2 sample are listed in Table 4 and are compared with previous works.

Table 4 Comparison of JO intensity parameters of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL with other glass hosts

Sample	Ω2 (×10^−20)	Ω4 (×10^−20)	Reference
SiO2–TiO2–PDMS–Eu^3+ (Eu2)	6.03	5.18	Present work
SiO2–PMMA–Eu^3+	11.0	—	26
TPBFEu16	3.91	1.54	3
ME17.5	5.32	1.38	4
U(2000)100 Eu(CF3SO3)3	9.6	8.1	8

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3.7 Radiative parameters

Radiative parameters such as total radiative transition probability (A_T), radiative life time (τ_R) and branching ratios (β_R) for the ⁵D₀ excited state of Eu2 sample have been evaluated from the JO intensity parameters^3,4 and the equations used for calculation are given in Section S2 of the ESI.† The results obtained are tabulated in Table 5. The branching ratio β_R represents the amplification of emission and it is considered that the energy level transition having β_R value greater than 0.50 is good for optical amplification.³ Here, the transition ⁵D₀ → ⁷F₂ (red emission) of the Eu2 sample has relatively high transition probability and branching ratio which confirms that the sample provides good optical amplification to the radiation emitted by the Eu³⁺ ions.

Table 5 Calculated radiative parameters of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL^a

Sl no.	^5D0 →	Energy (cm^−1)	Sed (×10^−22)	Smd (×10^−22)	Aed (s^−1)	Amd (s^−1)	A (s^−1)	βR
a Total radiative transition probability, AT = 341.37 s^−1; radiative lifetime, τR = 2.929 ms.
1	^7F4	14 463	1.41	0.00	105.74	0.00	105.74	0.3098
2	^7F3	15 433	0.00	0.00	0.00	0.00	0.00	0.0000
3	^7F2	16 283	1.70	0.00	181.37	0.00	181.37	0.5313
4	^7F1	16 943	0.00	0.40	0.00	54.26	54.26	0.1590
5	^7F0	17 320	0.00	0.00	0.00	0.00	0.00	0.0000

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The experimental lifetime (τ) of Eu2 sample obtained by monitoring the 613 nm emission (λ_ex = 394 nm) is 0.90 ms and the total calculated radiative life time (τ_R) is 2.929 ms. The difference in the lifetimes of the Eu2 sample may be due to the non-radiative decays which is not considered while calculating total radiative lifetime. The ratio between the experimental lifetime and calculated lifetime of the particular excited level gives the luminescence quantum efficiency (η).² Thus the quantum efficiency of the Eu2 sample is estimated to be 30.7% with a relative accuracy of 86.7%. The quantum efficiency of the Eu2 sample is also compared with various similar host matrices and are summerized in Table 6.

Table 6 Comparative study of the quantum efficiency of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL with similar host matrices

Sample	Quantum efficiency (%)	Reference
SiO2–TiO2–PDMS–Eu^3+ (Eu2)	30.7	Present work
SiO2–PMMA–Eu^3+	18.6	26
SiO2–TiO2 (6/4)-Eu^3+	9.1	47
SiO2–TiO2 (2/8)-Eu^3+	8.4	47
TPBFEu16	53.4	3

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Also, the effective band-width (Δλ_eff), experimental branching ratio (β_exp), peak stimulated emission cross-sections (σ_e) and optical gain (ΔG) are estimated from the emission spectrum of Eu2 sample for the ⁵D₀ → ⁷F_1,2,4 transitions and are represented in Table 7. The branching ratio gives information about the possibility of attaining stimulated emission from any particular transition. Also, the value of the stimulated emission cross-section signifies the gain provided to the emission of the doped rare-earth by the host material. The gain-bandwidth (σ_e × Δλ_eff) and optical gain, ΔG (σ_e × τ_R) are also critical parameters which predicts the optical amplification of the host medium in which the rare-earth ions are embedded.⁴⁸

Table 7 Effective band width, stimulated emission cross-section, optical gain and experimental branching ratios of the ⁵D₀ → ⁷F_1,2,4 transitions of 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL sample

Sl no.	^5D0 →	Δλeff (nm)	σe (×10^−22) cm^2	Gain-bandwidth (×10^−28) cm^3	ΔG (×10^−25) cm^2 s	βexp
1	^7F4	13.463	3.190	4.300	9.350	0.3000
2	^7F2	9.450	4.760	4.490	13.929	0.5380
3	^7F1	8.645	1.320	1.140	3.858	0.1610

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It is clear from Table 7 that the experimental branching ratio agrees well with the calculated values. Among the transitions, ⁵D₀ → ⁷F₂ has a relatively higher branching ratio of 53.8% and an emission cross-section of 4.76 × 10⁻²² cm², which proposes that the Eu2 sample can be used for photonic applications. Also, its gain-bandwidth and optical gain values show that the sample can be employed as a good optical amplifier.

3.8 Colorimetric analysis

The color luminescence emission intensities of the samples has been evaluated with Commission International d' Eclairage (CIE) 1931 system and also the dominant emission wavelength of the sample has been identified for evaluating its performance. The CIE co-ordinates calculated are (0.142, 0.192) and (0.411, 0.220) respectively for Eu0 and Eu2 samples and are marked in Fig. 9.

Fig. 9 CIE chromaticity diagram of pure (Eu0) and 2 wt% Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSIL (Eu2) samples.

4 Conclusion

XRD, FTIR and TG analyses of Eu³⁺ doped SiO₂–TiO₂–PDMS ORMOSILs confirmed that their structure remain unaltered with Eu³⁺ doping and these samples have appreciable thermal stability. The photoluminescence (PL) analysis by both direct excitation and CT band exhibits similar pattern and SiO₂–TiO₂–PDMS ORMOSIL doped with 2 wt% Eu³⁺ (Eu2) has maximum PL intensity and quenches above that concentration. The JO intensity parameters were determined for Eu2 sample and it was found that Ω₂ > Ω₄ which is an indication of the asymmetry in sites occupied by the Eu³⁺ ions in the host matrix. The phonon energy (ħω) associated with the non-radiative decay of the host matrix, obtained by Raman analysis (672 cm⁻¹) agrees well with that determined by PSB analysis (670 cm⁻¹). The radiative parameters of ⁵D₀ → ⁷F_1,2,4 transitions of the Eu2 sample were predicted using JO intensity parameters and the values imply that the Eu2 sample can be used for optical amplification applications. The decay lifetime of the Eu2 sample has been obtained as 0.9 ms which is fairly greater than that of various similar host matrices. The resultant pinkish red emission of the sample having a quantum efficiency of 30.7% confirmed that it can be effectively used in the WLED (RGB) fabrication.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful to the University Grants Commission, Govt. of India and to the Department of Science and Technology, Govt. of India for the support in establishing experimental facilities at the School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, India through SAP-DRS programme No. F.530/12/DRS/2009 (SAP-1) and DST-PURSE programme No.SR/417&418/2017 respectively.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra03073b

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