Accepted Manuscript Optical properties of titania nanoparticles embedded Er Ofelt analysis
3+ -doped tellurite glass: Judd-
N.N. Yusof, S.K. Ghoshal, M.N. Azlan PII:
S0925-8388(17)32468-4
DOI:
10.1016/j.jallcom.2017.07.102
Reference:
JALCOM 42518
To appear in:
Journal of Alloys and Compounds
Received Date: 10 April 2017 Revised Date:
15 June 2017
Accepted Date: 8 July 2017
Please cite this article as: N.N. Yusof, S.K. Ghoshal, M.N. Azlan, Optical properties of titania 3+ nanoparticles embedded Er -doped tellurite glass: Judd-Ofelt analysis, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.102. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphical Abstract
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT Optical Properties of Titania Nanoparticles Embedded Er3+-Doped Tellurite Glass: JuddOfelt Analysis 1 N. N. Yusof, 2*S. K. Ghoshal, , 3M.N Azlan 1,2
1
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Advanced Optical Materials Research Group, Physics Department, Faculty of Science Universiti Teknologi Malaysia, 81300, Johor Bahru, Skudai, Malaysia. 3 Physics Department, Faculty of Science, Universiti Putra Malaysia, 43400 Serdang Selangor, Malaysia
[email protected], 2*
[email protected],
[email protected]
Keywords: A. Amorphous Material, Metallic Glasses, Optical Materials, Rare Earth Alloys And Compounds C. Optical Properties D. Luminescence
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Abstract: Judd–Ofelt (J–O) theory was used to analyze the optical properties of erbium (Er3+) ions doped zinc-sodium-tellurite glass system incorporated with titania (TiO2) nanoparticles (TNPs) for the realization of up-converted solid state lasers. Such glass systems were synthesized using melt quenching method to determine the influence of varying TNPs contents on the absorption and emission properties. J-O intensity parameters (Ω2, Ω4, Ω6), spectroscopic quality factor (χ=Ω4/Ω6), radiative transition probabilities, branching ratio, and radiative lifetime of various transitions involved in the Er3+ ions were calculated from the measured optical spectra. XRD pattern verified the amorphous nature of the prepared glass samples. TEM images manifested the growth of TNPs inside the glass matrix having mean size between 15 to 25 nm. UV-Vis-NIR spectra exhibited ten absorption bands centred at 407, 444, 452, 489, 522, 552, 653, 800, 976 and 1532 nm. Two surface plasmon resonance (SPR) bands of TNPs were evidenced at 552 nm and 580 nm. Luminescence spectra revealed three prominent peaks centered at 525, 545 and 660 nm, where the glass sample containing 0.2 mol% of TNPs displayed optimum intensity enhancement by a factor of 30.00, 28.57 and 19.60, respectively. This enhancement is primarily attributed to the TNPs surface plasmon enabled colossal localized electric field in the proximity of Er3+ ions and subsequent energy transfer to the Er3+ ions. Values of Ω2, Ω4, Ω6 and χ were ranged between ( 2.14 − 3.72) ×10−20 , (1.27 − 2.77 ) ×10−20 , (1.42 − 2.22) ×10−20 cm2 and (0.58-1.94), respectively. Occurrence of higher values of Ω2 and Ω6 indicated the existence of lower symmetry and stronger covalency around the Er3+ ions. Furthermore, the decrease of χ values with increasing TNPs up to 0.2 mol% approved intensified lasing transition. Achieved higher values of Ω4 and Ω6 demonstrated that the present glass composition is a prospective lasing media.
Introduction
In the recent past, to improve the spectroscopic properties of rare-earth ions (REIs) doped inorganic glass systems, metal NPs incorporation into the amorphous matrix was adopted as an ingenious strategy [1–10]. These glass systems became prospective for the development of new optical devices such as lasers, light converters, sensors, hole burning high-density memories, optical fibers, and amplifiers [11]. However, in order to achieve the desired properties of the glass suitable for optical devices, in-depth theoretical understanding on the optical enhancement mechanism due to the embedment of NPs into the REIs doped glass is greatly significant [12]. A comprehensive review of literature revealed that the past researchers included different kind of metal NPs such as gold [1], silver [3], copper [13], nickel [14], zinc [15] and iron [16] into the
ACCEPTED MANUSCRIPT REIs doped glass systems to examine their effect on the optical, structural, thermal and physical properties. In this regard, titania NPs included REIs doped tellurite glass systems are not explored so far.
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Several articles on REIs doped binary/ternary glass systems with metal NPs inclusion argued that the NPs assisted localized surface plasmon resonance (LSPR) effect is primarily responsible for the generation of strong local electric field in proximity of REI [1,3,4,6]. Subsequently, the emergence of this localized SP field was attributed to the modification of the spectroscopic properties REIs inside the glass system [1,3]. To achieve the optimum optical enhancement the selection of the host glass, optimization of appropriate concentration of both REIs and metal NPs became prerequisite. The high solubility of REIs, low phonon energy (600700 cm-1), low melting temperature, high non-linear and linear refractive index of tellurite glass system make them suitable for diverse photonic applications [17–20]. Meanwhile, Er3+ ions as doping agent into the glass reveals strong up-conversion emission intensities due its distinctive energy levels spacing and sharp spectral attributes in the 4f electronic transitions [21]. However, the Er3+ ions doping level must be below 1.0 mol% to avoid the luminescence quenching that originates from the aggregation and clustering of REIs [1]. Generally, such clustering of REIs raises the probability of the energy absorbed and subsequent reduction in the emission crosssection of REIs via the conversion into phonons [6]. To surmount this low emission cross-section related limitations, embedment of metal NPs into the RE-doped glass system was considered as synergistic approach.
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Despite extensive research on various metal NPs embedded tellurite glass system with erbium doping, studies on TNPs incorporated REIs doped glass system and the role of SPR effect on the optical and self-cleaning characteristics are still lacking [22]. Furthermore, TNPs are cost-effective, easily synthesizable and abundant compare to gold and silver NPs. This work is a major extension of our previous report on similar glass system [23], where an in-depth analysis on the optical and structural correlation of erbium-doped tellurite glass incorporated with low concentration of TNPs (0.0-0.4 mol%) is made that has not been reported earlier. We scrutinized the TNPs embedded erbium-tellurite glass luminescence properties which remained unexplored. Besides, the intensity parameters and radiative properties of the proposed glass system was evaluated using J-O theory to determine their effectiveness as up-converted lasing medium [24]. Using J-O analysis, the strength of REIs luminescence was evaluated depending on the glass environment [25,26]. J-O intensity parameters (Ω2, Ω4, Ω6) and radiative parameters including the branching ratios, emission cross-section and radiative lifetime were computed to select the best glass composition. These parameters provided significant information on the local structure and bonding in the vicinity of REIs which affected the spectroscopic quality factor [2,27]. Based on J-O analysis, it is established that the proposed glass compositions are potential for laser applications. Sample Preparation and Characterizations
Tellurite glass system with composition (69-x)TeO2-20ZnO-10Na2O-1Er2O3-(x)TiO2, where x = 0.0, 0.1, 0.2, 0.3 and 0.4 mol% were synthesized using conventional melt quenching method. Table 1 enlists the detail glass composition (in mol%) and their designation. The concentration of Er3+ was kept constant (1.0 mol%) to prevent any undesirable attenuations such as dipolar interaction and luminescence quenching. Analytical grade powders (from Sigma Adrich) of TeO2 (purity 99.5%), ZnO (purity 99.9%), Na2O (purity 80.0%), Er2O3 (purity 99.9%) and TiO2 NPs (purity 99.7%; size below 25 nm) were used as glass constituents and
ACCEPTED MANUSCRIPT thoroughly grinded. A platinum crucible containing these mixtures of glass constituents was placed in an electrical furnace and heated at 900 oC for 20 minutes. Next, the molten high viscous fluid was poured on pre-heated stainless-steel mould and further annealed at 300 oC for 3 hours to remove any thermal strain that causes glass embrittlement. Afterward, the annealed samples were cooled down to room temperature and stored inside desiccators to avoid moisture attack. Finally, the sample were cut and polished to a thickness of 2.5±0.5 mm to produce shiny and scratch free surface for further optical characterizations.
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Table 1. Glass code and its composition Glass Composition (mol %) TeO2 ZnO Na2O Er2O3 TiO2 69 20 10 1 0 68.9 20 10 1 0.1 68.8 20 10 1 0.2 68.7 20 10 1 0.3 68.6 20 10 1 0.4 70 20 10 0 0 69.6 20 10 0 0.4
Glass Code
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TZNE TZNE0.1Ti TZNE0.2Ti TZNE0.3Ti TZNE0.4Ti TZN TZN0.4Ti
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The amorphous nature of the prepared glass systems were verified using X-ray Diffractometer (MiniFlex300, Rigaku, Japan) with Cu Kα radiations (λ=1.54 Å), operated at 30 kV and 10 mA where the scanning angle 2θ was ranged from 20o to 80o. The room temperature absorption spectra in the range of 400–1600 nm were recorded using Shimadzu UV-3600PC scanning spectrophotometer (Kyoto, Japan). Transmission electron microscopic (TEM) imaging was carried out using a Hitachi H800 which operated at 250 kV. The emission spectra were measured using a Hitachi F850 Fluorescence spectrometer (Tokyo, Japan) which used pulsed Xenon lamp as excitation source. The emitted light was dispersed by Monk-Gillieson monochromators and was detected using the standard photomultiplier tube.
Theoretical Calculation
3.1
Nephelauxetic Ratio and Bonding Parameter
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3.
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The absorption spectra of the prepared glass samples were used to calculate nephelauxetic ratio (β ) via the relation:
νT νR
(1)
where ν T and ν R are the wavenumber (cm-1) of a selected transition in the system and in
( )
surrounding aqua, respectively. The average value of the nephelauxetic ratio β was used to estimate the bonding parameter (δ ) using the expression: 1 − β
δ = β
(2)
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Judd-Ofelt Parameters
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Absorption and luminescence spectral data of all glass sample containing different concentration of TNPs was used to calculate the J-O parameters. The area under the absorption bands were used to calculate oscillator strength via the relation: 2303mc 2 Pexp = ε a (v)dv (3) πe 2 N 0 ∫ where m is the mass of electron and e is the electronic charge, c is the velocity of light, N 0 is the Avogadro's constant, and ε a ( v ) is the molar extinction coefficient which was calculated from the Beer-Lambert law given by:
log 0 (I 0 I t ) C RE t
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ε a (v ) =
(4)
where log 0 ( I o / I t ) is the measured absorbance at the wave-number ν (cm-1), CRE is the
( mol/1000 cm )
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concentration of the RE ions
3
and t is the thickness of the sample (cm).
According to J-O theory [25,26], the total probability of dipole oscillator strength for a transition from the ground state (aJ ) to an excited state (bJ ' ) of Er3+ ions within 4f configuration yields:
8π 2 mcv Pcal = Ped + Pmd = (χ ed S ed + χ md S md ) 3h(2 J + 1)e 2 n 2
(5)
)
2
(6)
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χ ed
(
n n2 + 2 = 9
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where χed and χ md are Lorentz local field correction accounting for electric and magnetic dipole-dipole transition, respectively. They can be calculated via the relation:
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χ md = n 3
(7)
where n is the refractive index of the sample. The line-strength for electric (S ed ) and magnetic
dipole (S md ) transitions takes the form:
(
)
S ed aJ , bJ ' = e 2
(
)
S md aJ , bJ ' =
∑ Ω λ aJ U λ bJ '
2
(8)
λ =2,4,6
e2 aJ L + 2 S bJ ' 2 2 4m c
2
where the reduced matrix elements of
(9)
aJ U λ bJ ′
2
and
aJ L + 2S bJ ′
2
are calculated
following Carnall et al [28]. Only the transitions with selection rules ∆S = ∆L = 0, ∆J = 0, ±1 could contribute efficiently to the oscillator strength of REIs. The least squares fitting approach
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(
)
AR aJ , bJ ' = Aed + Amd =
64π 4 v 3 (χ ed S ed + χ md S md ) 3h(2 J + 1)
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1/2
(11)
(
)
AR aJ , bJ ' ∑bJ ' AR (aJ , bJ ' )
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β R (aJ , bJ ' ) =
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where Aed and Amd are the electric-dipole and magnetic-dipole contributions, respectively to the total spontaneous emission probability. The emission branching ratio of a transition is defined as: (12)
The radiative lifetime of an emitting state is related to the total spontaneous emission probabilities for all transitions via the expression:
1 ∑bJ ' A aJ , bJ '
(
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τ rad =
(13)
∆λeff =
∫ I (λ )dλ I
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where the sum is extended over all the states at energies lower than aJ . The effective luminescence bandwidth was calculated using:
(14)
σ PE =
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where I is the average intensity of the integrated area. The stimulated emission cross-section of the band are evaluated using the expression:
λ4P AR 8πcn 2 ∆λeff
(15)
where λ p is the peak wavelength of the emission transition band and ∆λeff is the full width at half maximum (FWHM) of the emission transition.
3.3
Inter-nuclear Properties
The inter nuclear properties of the sample was calculated following the relations:
ACCEPTED MANUSCRIPT N ( ions or atoms/cm 3 ) = ( Ap × N 0 × ρ / M v )
(16)
1/3 ° Inter nuclear distance, ri A = ( 1 / N 0 ) ° 1/3 Polaron radius, rp A = ( 1 / 2 )(π / 6 N 0 )
(18)
(Z / r ) 2 p
(19)
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The field strength, F =
(17)
where A p is the ion percentage of TNPs or Er2O3, ρ is the glass density in g cm-3, Mv is the average molecular weight of the glass and Z is molecular mass of the dopant [1].
4.
Results and Discussion
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Figure 1 shows the XRD pattern of the prepared glass samples. The presence of a broad hump between 25o and 35o and complete absence of any sharp crystalline peaks confirmed the amorphous nature of prepared glass system [1,29]. No crystalline peaks of TNPs as well as long range atomic order with well-defined plane were evidenced due to its small concentration in the glass [1]. TZNE0.4Ti
Intensity (a.u.)
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20
30
TZNE
40 50 60 2θ (degree)
70
80
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Figure 1. Typical XRD pattern of all prepared glass samples.
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Figure 2 shows the TEM images of the proposed glass system containing TNPs concentration of (a) 0.2 mol% and (b) 0.3 mol%, where the inset represents the TNPs mean size distribution around 14.52 and 24.92 nm, respectively after fitting. The TEM data for particle size was converted directly to cumulative number-based distributions [30]. For particles of irregular shape like ellipsoidal, it is not possible to assign definite sizes. In such cases, it is easier and most frequent to represent the particle shape as an equivalent sphere using an appropriate method [31]. Thus, the diameter of all NPs is measured at a fixed angle [32]. The most common way to represent the nearly symmetric size distribution is through a Gaussian, log-normal [33], Weibull distributions [30] etc. that allows one to characterize the mean particle size and the standard deviation. However, typically the NPs size distribution is found to be non-symmetrical. Therefore, the NPs distribution for both sample TZNE0.2Ti and TZNE0.3Ti is fitted with statistical distribution function in order to increase the reliability of the results. Three different fittings (such as Log-normal, Weibull and Gamma, specifically for continuous and asymmetrical distribution) are performed to achieve an appropriate statistical distribution for estimating the NPs mean size more accurately [34]. The appearance TNPs with different sizes was attributed to the Oswald’s ripening process wherein the small NPs gone coalescence to form bigger NPs.
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Furthermore, the observed increase in the average NPs size was ascribed to the interfacial free energy minimization via non-local diffusion [35]. However, as the Oswald ripening process continued, the thermodynamic stability was disturbed and the big particle was dispersed to small particles to retain the stability [35]. This process is called digestive ripening process and responsible for the appearance of different shape and size of NPs as observed.
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Figure 2: TEM images of the proposed glass system containing TNPs concentration of (a) 0.2 mol% and (b) 0.3 mol% (Inset: TNPs average size distribution with three different fits).
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The UV-Vis-NIR absorption spectra of all samples as depicted in Figure 3 was comprised of ten absorption bands centred at 407, 444, 452, 489, 522, 653, 800, 976 and 1532 nm which were allocated to the transition from the Er3+ ions ground state (4I15/2) to excited states of 2G9/2, 4 F3/2 , 4F5/2 , 4F7/2, 4H11/2, 4S3/2, 4F9/2, 4I9//2, 4I11/2 and 4I13/2, respectively. The occurrence of an weak absorption band at 552 nm was assigned to the transition to 4S3/2 level which overlapped with upper neighbouring level of 4H11/2 [1]. Spectral features of all samples were found to be qualitatively similar except an increase in the absorbance with the increase in TNPs contents. However, no plasmon band was manifested due to the dominance of Er3+ band intensity which overlaid with the the plasmon band. Thus, to probe the plasmon band precisely another sample devoid of Er3+ ions was prepared.
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3
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Absorbance (a.u.)
2H 11/2
4F 4F9/2 7/2 4 4I 2 9/2 I11/2
1 4F5/2 400 600
TZNE TZNE0.1Ti TZNE0.2Ti TZNE0.3Ti TZNE0.4Ti 4I 13/2
800 1000 1200 1400 1600 Wavelength (nm)
Figure 3. UV-Vis-NIR absorption spectra of synthesized glass samples.
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Figure 4(a) displays the identified surface plasmon bands of zinc-sodium tellurite glass without (lower curve) and with TNPs (upper curve). The appearance of two absorption peaks centred at 552 nm and 580 nm were assigned to the transverse and longitudinal plasmon resonance modes of TNPs, respectively [36–38]. Transverse mode referring to the polarization of incident light is perpendicular to the axial direction of the NPs. Meanwhile, for longitudinal mode, the polarization of incident light is parallel to the axial direction of the NPs [39]. Generally, the effect of plasmon excitation can be identified using far-field and near-field properties. The far-field properties are associated with the existence of optical extinction maxima at the plasmon resonance frequencies. Meanwhile, the near-field properties are related to the enhanced electric-field in the close proximity (surface) of the NPs. UV-visible-NIR spectroscopy is a convenient technique to measure the optimum plasmon frequencies (far-field effect). However, the weak absorption from multipolar excitation remains undetectable by this method [40,41]. The strong absorption of tiny metallic nanoparticles is attributed to the dipolar oscillation of the electron and higher order mode that are usually neglected [42]. Though the extinction cross section of the spherical NPs can be obtained as a series of multipolar oscillator (if boundary conditions are fulfilled), but the electric field amplitude may undergo damping that cause the broadening of the plasmon band and thus absent in the spectral range [40,43]. Since the measurement considers only the diameter of the spherical NPs which are much smaller than the wavelength of the radiation (within the quasi-static limit), thus the only lower order plasmon oscillations associated with the dipole oscillation contributes to the extinction cross section. Only few particle structure exhibits adequately resolved higher order multipoles and thus remain difficult to resolve [41]. The resonance frequency is strongly dependent on the aspect ratio of NPs (the ratio of length to width of the NPs) [36]. Nanoparticles having extremely high aspect ratio may produce well-resolved resonances [41,44]. In the present case, usually the length and width of the spherical NPs are nearly the same and thus correspond to the low aspect ratio. Meanwhile, the length and width of greatly non-spherical NPs are usually very different, thus producing high aspect ratio and well-resolved resonances.
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For nanosystem with small aspect ratio (e.g. spherical NPs), the excitations originating from the d-band strongly mixes with the excitations emerging from the cylindrical orbitals, making the longitudinal plasmon mode difficult to identify. Actually, for these systems the interband and intra-band transitions have similar energies, making their coupling more efficient. As the aspect ratio increases (especially for non-spherical NPs) the intra-band excitations become lower in energy than the inter-band excitations. Subsequently, the coupling between these excitations decreases, generating a strong and easily identifiable plasmon peak. Therefore, the longitudinal modes are likely to be associated with the non-spherical NPs where the plasmon band may shift to lower energy (approaching IR region) due to high aspect ratios [45]. Conversely, with decreasing aspect ratio, the longitudinal peak shifts towards the lower wavelength and starts to overlap with the transverse band. Consequently, the peak intensity of the longitudinal band appears similar to that of the transverse band especially for spherical TNPs. This transverse excitation usually does not shift or alter significantly with the increase of spherical TNPs diameter [45]. Therefore, non-spherical TNPs are responsible for the occurrence of two different mode such as longitudinal and transverse excitation which are less likely to happen for spherical NPs [42]. The TEM micrograph (Figure 2 (a)-(b)) clearly confirmed the existence of spherical and non-spherical TNPs which was responsible for the occurrence of such SPR modes. Figure 4 (b) shows the SPR absorption band of zinc-magnesium phosphate glass system from previous studies where TNPs concentration was varied in the range of 1.0 to 4.0 mol%. The inset of Figure 4(b) displays the TEM image of glass system containing 4.0 mol% of TNPs together with the histogram of size distributions (NPs average size ≈5.78 nm). The surface
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plasmon resonance (SPR) peaks are evidenced between 208 and 324 nm, where the peak positions are not altered significantly despite an increase in the concentration [46]. When compared with the present results, the glass system contained smaller size TNPs with all spherical in shape. The shrinkage of NPs size and the change of NPs shape was attributed to the thermal fragmentation [1,47,48]. The SPR band was also located at lower wavelength indicating higher energy compared to the present observation. The UV-region of the SPR band was attributed to the presence of small-spherical TNPs that has higher energy, yet nearly constant due to its small aspect ratio [45]. The observation of red-shifted SPR band in the present work compared to the earlier studies is mainly due to the difference of NPs concentration dependent morphology change and the refractive index of host material [3,49]. This red-shift of the SPR peak wavelength can be further explained through Mie's scattering theory [14,50–52] via the expression:
λmax 2 = (2π c ) 2 mNe 2 (ε ∞ + 2n 2 ) / ε 0
(20)
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where c is the speed of light, m is effective mass of the conduction electron, N is the free electron concentration, e is the electronic charge, ε 0 is the free space permeability and ε ∞ is the optical dielectric function of the metal. The SPR peak wavelength is directly proportional to the refractive index and dielectric function of the host material [53–55]. Thus, the SPR band extend to higher wavelength due to relatively higher refractive index (≈2.5) of tellurite glass system [1] compare to phosphate glass system (≈1.5) [54].
TZN0.4Ti TZN
552 nm
560 580 Wavelength (nm)
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540
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580 nm
(b) 218 nm 227 nm
A bsorbance (a.u.)
Absorbance (a.u.)
(a)
600
200
209 nm
322 nm 324 nm
0.0 mol% TNPs 1.0 mol% TNPs 2.0 mol% TNPs 3.0 mol% TNPs 4.0 mol% TNPs
320 nm 317 nm 208 nm
300
400 500 600 Wavelength (nm)
700
Figure 4(a) SPR band of TZN0.4Ti glass with respect to the reference TZN sample (b) SPR absorption band of zinc-phosphate glass for different TNPs concentration, adapted from [46].
Table 2 enlists the values of average nephelauxetic ratio and the bonding parameter of all glass samples. The positive or negative sign of the bonding parameter (δ ) signified the nature of covalent and ionic bonding between the Er3+ ion and the ligands in the glass network. The covalent bonding inside the glass was enhanced (positive sign) with the increase in TNPs contents which was attributed to the generation of more non-bridging oxygen (NBOs) inside the glass [56]. This observed higher covalency was supported by the J-O analysis.
ACCEPTED MANUSCRIPT Table 2: Band position compared to aquo. Energy level TZNE 4 4 6671 I 15 / 2 → I 13 / 2 4 10235 I 15 / 2 → 4 I 11 / 2
(cm-1) and bonding parameter β n and δ
(
)
TZNE0.1Ti 6675
TZNE0.2Ti 6675
TZNE0.3Ti 6675
TZNE0.4Ti 6675
Aquo [28] 6600
10235
10235
10235
10235
10250
of all glass samples as
I 15 / 2 → 4 I 9 / 2
12484
12484
12484
12484
12484
12400
4
I 15 / 2 → F9 / 2
15313
15313
15313
15313
15313
15250
4
I 15 / 2 → 4 S 3 / 2
18348
18348
18348
18348
4
I 15 / 2 → 4 H 11 / 2
19157
19157
19158
19157
4
I 15 / 2 → 4 F7 / 2
20449
20449
20449
20449
4
I 15 / 2 → 4 F5 / 2
22123
22123
22123
20876
4
I 15 / 2 → 4 F3 / 2
4
I 15 / 2 → G9 / 2
22522 24570
22522 24570
22523 24570
22123 24570
10.02 1.002
10.02 1.002
10.02 1.002
-0.0023
-0.0024
-0.0024
β
βn
δ
18350
19157
19150
20449
20450
20876
22100
22123 24570
22500 24550
9.95 0.995
9.95 0.995
-
0.0050
0.0050
-
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2
18348
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Table 3 summarizes the measured (Pexp) and calculated (Pcalc) value of the oscillator strength of all glass samples. It provided indirect information on the symmetry and bonding of REI within the matrix [57]. Transition band at 4I15/2 → 2H11/2 showed the highest oscillator strength which was attributed to the hyper sensitive transitions (HSTs) [58]. These HSTs are sensitive to the small changes in the environment around REIs and obey the selection rule of |∆J|≤ 2, |∆L|≤ 2 and ∆S = 0 [38, 39]. The observed increase in the oscillator strengths of HSTs with the increase of TNPs content indicated the presence of lower symmetry around REIs with strong covalency [1,7,61]. The achieved very small RMS value in the range of (4.83-7.22) × 10−6 confirmed the reliability of the data [8].
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Table 3: The experimental (Pexp, ×10-6) and calculated (Pcal, ×10-6) oscillator strength of the Er3+ absorption transition from the grounds state 4I15/2 to the excited states for all glass samples. Absorption Glass transition TZNE TZNE0.1Ti TZNE0.2Ti TZNE0.3Ti TZNE0.4Ti Pexp Pcalc Pexp Pcalc Pexp Pcalc Pexp Pcalc Pexp Pcalc 4 2 11.86 16.41 13.84 19.36 15.90 19.82 16.27 20.29 14.53 21.12 I15/2 → G9/2 4 5.24 4.95 6.58 5.78 5.65 5.978 5.78 6.12 7.18 6.288 I15/2 → 4F7/2 4 2 14.57 9.98 17.33 11.76 15.95 12.02 16.33 12.30 19.48 12.82 I15/2 → H11/2 4 6.01 6.11 5.62 5.84 4.92 4.87 5.04 4.98 5.63 5.87 I15/2 → 4F9/2 4 4 1.02 1.19 1.13 1.61 2.07 1.86 2.12 1.90 1.30 1.83 I15/2 → I11/2 4 4 2.26 0.08 2.72 0.08 2.85 0.06 2.92 0.06 4.00 0.08 I15/2 → I13/2 4.83 5.90 4.40 4.51 7.22 rms (×10-6) Table 4 summarizes the values of J-O intensity parameters
(Ω 2 , Ω 4 , Ω6 ) and
spectroscopic quality factors χ = (Ω 4 / Ω 6 ) for all glass samples. The values of Ω 2 , Ω 4 and Ω 6
were in the range (2.14-3.72) × 10−20 cm2, (1.27-2.77) × 10−20 cm2, (1.42-2.22) × 10−20 cm2,
ACCEPTED MANUSCRIPT respectively. Generally, Ω 2 is sensitive to the asymmetry around the REIs (short-range effects) inside the glass matrix. Conversely, the changes in Ω 4 and Ω 6 are correlated to the long-range effects of glass host glass [62–66]. In the present TNPs included tellurite glass system the value to Ω 2 was improved as compared to the reported Au and Ag NPs embedded tellurite glass
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system [7,66]. Thus, such higher Ω 2 values at elevated TNPs concentration indicated a lower symmetry around Er3+ ions and higher covalency in the glass system [64,65]. Meanwhile, higher values of Ω 6 at elevated concentration of TNPs indicated the emergence of stronger long range effects, higher rigidity and refractive index of the prepared glass system [7–9,57,59,67–71]. Alternatively, change in the J-O parameter Ω 4 has revealed an effect on some transitions of Er3+ ions together with parameter Ω 6 .
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The erbium emission intensity was characterized uniquely in terms of spectroscopic quality factor χ using the values of Ω 4 and Ω 6 parameters [72]. Earlier, it was reported that for
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larger value of χ = (Ω 4 / Ω 6 ) , the laser transition was more intense [73]. Later, Fares et al and Langar et al demonstrated that smaller quality factor indicated more intense laser transition [43, 44] . In our study, values of χ for the prepared glass samples were ranged within 1.94 to 0.58 (Table 4). These values (for TNPs incorporated glass) were being lower than Au and Ag NPs embedded erbium-doped tellurite glass system indicated a higher efficiency of laser transitions [7,63]. Thus, in the presence of TNPs as sensitizer the observed reduction in the quality factor reflected their role in modifying the absorption and fluorescence dynamics of Er3+ doped glass system [65]. The J-O parameters in the present glass system followed a trend of Ω 2 > Ω 4 > Ω 6 for glass sample without and with extremely low (0.1 mol%) contents of TNPs. However, glass system prepared with higher TNPs concentration (0.2-0.4 mol%) revealed a modified trend of Ω2 > Ω6 > Ω4 . It is worth noting that smaller Ω 4 and larger Ω 6 values are favourable for the luminescence transition [4]. Therefore, the present glass composition offered better J-O intensity parameter values than Er3+-doped tellurite glass containing Au and Ag NPs (Table 4) as reported elsewhere [7,66].
EP
Table 4: J-O intensity parameters ( Ω λ ×10-20 cm2) and spectroscopic quality factor
(χ = Ω 4 / Ω 6 ) for all prepared glass samples. Metallic Contents NPs used (mol%) 0
Ω2
Ω4
Ω6
Trends of Ω λ
χ
Ref.
2.14
2.77
1.42
1.94
Present
TZNE0.1Ti
Ti
0.1
3.08
2.15
1.88
1.14
Present
TZNE0.2Ti
Ti
0.2
3.64
1.27
2.17
Ω2 > Ω4 > Ω6 Ω2 > Ω4 > Ω6 Ω2 >Ω6 >Ω4
0.58
Present
TZNE0.3Ti
Ti
0.3
3.72
1.30
2.22
0.59
Present
TZNE0.4Ti
Ti
0.4
3.49
1.90
2.09
0.91
Present
TZNEA
Au
0.2
0.90
0.21
0.02
Ω2 >Ω6 >Ω4 Ω2 >Ω6 >Ω4 Ω2 > Ω4 > Ω6
0.85
[7]
ECYA2
Ag
0.5
1.87
2.63
1.32
Ω2 > Ω4 > Ω6
2.99
[66]
TZNE
3+
Er
AC C
Glass code
The J-O intensity parameters were further used to compute the radiative properties of ions such as transition probability Arad (electric dipole transition probability AED and
ACCEPTED MANUSCRIPT magnetic dipole transition probability AMD ), branching ratio ( β R ) and radiative lifetime ( τ rad ) [74]. Table 5 enlists the calculated values of these parameters. Values of Arad for 4I15/2 → 2H11/2, 4 I15/2 → 4S3/2 and 4I15/2 → 4F9/2 transitions were increased with the increase of TNPs contents. This indicated the better feasibility to achieve green and red emissions suitable for lasers [75]. Generally, the value of β R for a specific transition determines the probability in obtaining
β R (%) of all prepared samples.
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Table 5: Values of Arad (s-1), τ (ms) and Transition Parameter TZNE -1 4 4 A (s ) I15/2 → I13/2 657.47 1.52 τ (ms) β R (%) 100.00 4 4 A (s-1) I15/2 → I11/2 629.53 τ (ms) 1.35 β R (%) 85.00 -1 4 4 A (s ) I15/2 → I9/2 1087.27 0.75 τ (ms) β R (%) 82.00 -1 4 4 A (s ) I15/2 → F9/2 8662.75 0.11 τ (ms) β R (%) 92.00 -1 4 4 A (s ) I15/2 → S3/2 5472.81 0.12 τ (ms) β R (%) 67.00 -1 4 2 (s ) A I15/2 → H11/2 18451.55 0.05 τ (ms) β R (%) 94.00 4 4 A (s-1) I15/2 → F7/2 15636.22 0.05 τ (ms) β R (%) 74.00 4 2 I15/2 → G9/2 A (s-1) 6240.83 τ (ms) 0.07 β R (%) 43.00
SC
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stimulated emission [76,77]. In the present case, values of β R for 4I15/2 → 2H11/2, 4I15/2 → 4S3/2 and 4I15/2 → 4F9/2 transition were appeared in the range of 67-95%, demonstrating a strong possibility of attain green and red emission under 800 nm excitation. Meanwhile, the attainment of shorter lifetime in the range of (0.50-0.40), (0.12-0.08), (0.13-0.11) ms corresponding to the transitions of 2H11/2 → 4I15/2 , 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 for glass system containing different concentration of TNPs clearly indicated the emergence of strong green and red emission [78]. The lifetime has been shown to be proportional to the radiative decay rate determined by the J-O parameters [79]. Thus, shorter lifetime for these transitions was favourable for suppressing the non-radiative process and governing the strong emission of Er3+ ions inside the prepared glass system.
TZNE0.2Ti
TZNE0.3Ti
TZNE0.4Ti
792.18 1.26
888.08 1.13
907.72 1.10
916.88 1.09
100.00 840.12 1.03
100.00 999.74 0.88
100.00 1025.07 0.85
100.00 1006.69 0.87
87.00 860.40 0.85
88.00 546.94 1.09
88.00 560.80 1.07
87.00 829.05 0.83
73.00 8177.79 0.11
60.00 7010.43 0.13
60.00 7188.06 0.13
69.00 8643.45 0.11
91.00 7246.48 0.09
90.00 8600.91 0.08
90.00 8818.83 0.08
91.00 8665.88 0.08
67.00 21456.61 0.04
68.00 22571.01 0.04
68.00 23142.90 0.04
68.00 24589.38 0.04
94.00 18016.58 0.04
95.00 19183.81 0.04
95.00 19669.88 0.04
94.00 20618.31 0.04
79.00 7780.85 0.06
85.00 8840.04 0.05
85.00 9064.02 0.05
81.00 9138.41 0.05
45.00
47.00
47.00
45.00
AC C
EP
TE D
TZNE0.1Ti
ACCEPTED MANUSCRIPT The stimulated emission cross section signifies the rate of energy extraction from the lasing material [1], wherein the lasing action is considered to be probable if its value is greater than unity [3]. Wide stimulated emission cross section can be induced from narrow emission bandwidths. However, the broadening of the emission bands is decided by the variation of the local structure and coordination of Er3+ ions inside the glass networks [74–77]. For the present glass compositions, the value of stimulated emission cross section for 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 −27 2 and 4F9/2 → 4I15/2 transitions were discerned in the range of ( 22.36 − 30.88) ×10 m ,
(18.00 −18.33) ×10−27 m2 , (13.36 −17.53) ×10−27 m2 ,
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respectively as enlisted in Table 6. The stimulated emission cross section was enhanced as the concentration of TNPs in the glass was increased. The present glass system exhibited higher stimulated emission cross section compared to the Er3+ doped tellurite glass containing Au NPs [1]. Based on the observed enhancement in the stimulated emission cross section it was affirmed that our TNPs embedded Er3+ doped tellurite glass system is a prospective for host material for laser fabrication.
Table 6: The values of peak wavelength ( λ p , nm), effective bandwidth ( ∆λ eff , nm) and
524 524 524 524 524
15.51 14.23 14.21 13.78 13.54
22.36 27.82 27.59 29.13 30.88
544 544 544 544 544
18.33 18.28 18.28 18.00 18.11
9.15 11.90 14.03 14.59 14.01
656 656 656 656 656
25.36 24.95 24.95 22.71 22.71
σ PE 16.05 15.13 13.36 15.02 17.53
TE D
TNZE TZNE0.1Ti TZNE0.2Ti TZNE0.3Ti TZNE0.4Ti
M AN U
stimulated emission cross section ( σ PE ×10-27, m2) for all synthesized glass samples, 2 4 4 Sample H11/2 → 4I15/2 S3/2 → 4I15/2 F9/2 → 4I15/2 λp ∆λeff λp ∆λeff λp ∆λeff σ PE σ PE
EP
Figure 5 shows the up-conversion (UC) photoluminescence (PL) spectra of Er3+ ion in all samples under the excitation wavelength of 800 nm. The emission spectra of Er3+ ion exhibited three prominent bands centred at 525, 545 and 660 nm which were assigned to 2H11/2 → 4I15/2, 4 S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions, respectively.
8
4
Intensity (×10 a.u.)
AC C
10
6 4
4 S 3/2
TZNE TZNE0.1Ti TZNE0.2Ti TZNE0.3Ti TZNE0.4Ti
2 H 11/2
2 0 500
4 F 9/2
550 600 650 Wavelength (nm)
700
Figure 5. Emission spectra of all glass samples with 800 nm excitation. Table 7 enlists the PL intensity enhancement factors as a function of TNPs contents meanwhile Figure 6 displays the integrated PL intensity for the observed band. The enhancement factor was estimated by dividing the maximum PL intensity of each emission band (525, 545,
ACCEPTED MANUSCRIPT 660 nm) for the glass samples with varying TNPs content with the one without containing TNPs. The erbium doping in all samples was constant. The PL enhancement factor was calculated using the relation [80,81]:
η max =
I m ax TZNExTNPs I m ax TZNE
(21)
RI PT
where η max is the emission enhancement factor, I m ax TZNExTNPs is the maximum PL intensity for
SC
sample containing different TNPs concentration and I m ax TZNE is the corresponding maximum PL intensity for the sample devoid of TNPs contents. The emission bands of TZNE0.2Ti glass sample were enhanced by a factor of 30.00 (525 nm), 28.57 (545 nm) and 19.60 (660 nm) times. This enhancement in the PL intensity was attributed to SPR mediated local field of TNPs mediated that affected the Er3+ ions transitions [82]. The green band at 525 and 545 nm showed great enhancement due to closeness of such emission to the plasmon band wavelength [83]. The achieved significant enhancements in the PL intensity was ascribed to the optimum size of the NPs and their proximity to the Er3+ ions [1,3,7].
30 25
AC C
EP
Integrated PL Intensity (a.u.)
TE D
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Table 7: Glass code and maximum enhancement factor (η max ) of the three PL bands. Glass code TNPs (mol%) Enhancement factor (η max ) 525: 545 : 660 (nm) TZNE 0 1: 1: 1 TZNE0.1Ti 0.1 18.75: 21.71: 12.80 TZNE0.2Ti 0.2 30.00: 28.57: 19.60 TZNE0.3Ti 0.3 11.25: 11.14: 6.80 TZNE0.4Ti 0.4 13.13: 14.29: 9.60
525 nm 545 nm 660 nm
20 15 10 5
0 0.0
0.1 0.2 0.3 0.4 TiO2 NPs Concentration (mol %) Figure 6. TNPs concentration dependent integrated PL intensity of the glass system.
Table 8 summarizes the variations in inter-nuclear properties of respective glass samples. The calculated inter-nuclear distance and polaron radius are found to to be sensitive on TNPs concentration variation. The maximum enhancement in the PL intensity was achieved for sample containing 0.2 mol % of TNPs with separation Ti0–Ti0 of 251.60 Å and Ti0-Er of 344.56 Å. The occurrence of extremely short distance between REI and TNP was attributed to the generation of multipole interaction and hence the local field enhancement [1]. This interaction in turn
ACCEPTED MANUSCRIPT
RI PT
increased the energy transfer from excited REIs to NPs and led to the effect of luminescence quenching. Thus, the luminescence intensity of REIs was enhanced due to the presence of intensified local electric field associated with the optimal separation between these two entities [14–16]. Meanwhile, the field strength was weakened from 8.28 to 7.53 cm3 with increasing TNPs content due to the strong stretching force of the bond. This decrease in the field strength led to an enhancement in the electronic polarizability and refractive index, thereby altered the absorption and luminescence properties of the glass [84].
Table 8. Various inter-nuclear properties of the synthesized glass systems. Parameters TZNE TZNE0.1Ti TZNE0.2Ti TZNE0.3Ti TZNE0.4Ti TNPs concentration 0.00 2.15 4.11 6.02 7.91
N ×10
21
( ion/cm ) 3
Er3+ ion concentration 2.28
2.05
Polaron radius of TNPs, 0.00 r p (Å) 0.00 Ti-Ti distance, ri (Å)
755.26
624.29
312.45
251.60
221.51
202.28
Polaron radius of Er3+, 352.57 r p (Å)
359.84
365.08
367.94
369.08
142.09
145.02
147.14
148.29
149.04
352.57
348.59
344.56
337.13
330.57
8.28
7.95
7.73
7.61
7.53
( ion/cm )
Ti
Ti
Er
Er
FEr × 10 −15 ( cm 2 )
Er
TE D
Er-Er distance, ri (Å) Ti-Er distance, ri (Å)
2.01
1.98
549.63
501.90
M AN U
N ×10
3
SC
2.15
22
AC C
EP
The mechanism for PL intensity enhancement or quenching was explained using partial energy diagram of Er3+ ion (Figure 7). The PL UC emission (centred at 525, 545 and 660 nm) occurred via several processes including ground state absorption (GSA), excited state absorption (ESA), energy transfer (ET), multi-phonon relaxation process (MPR), radiative decay (R), nonradiative decay (NR) and local field effect (LFE) of TiO2 NPS. At 800 nm excitation, Er3+ ion was excited from the ground state (4I15/2) to the excited state (4I9/2) of Er3+ ion via GSA. Subsequently, NR decay take place where electron from 4I9/2 populated to transition 4I11/2 and 4 I13/2 through MR process. From these two metastable state 4I11/2 and 4I13/2, the electron then populated to higher excited states (4F7/2 and 2G9/2) by ESA. Afterward, NR decay occurred from 4 F7/2 and 2G9/2 to 2H11/2 and 4S3/2, which resulted two green emission centred at 525 and 545 nm. Meanwhile, the NR decay from 4S3/2 to 4F9/2 populated the red emission via R process at wavelength 660 nm [1,83,85]. PL intensity quenching occurred as result of ET from Er3+ ions to TNPs, causing the conversion of more energy into heat through NR decay [1,82,86]. The enhancement of luminescence was interpreted via these two competitive process: (a) the local field (LFE) assisted enhancement and (b) the energy transfer (ET) between RE ions and TNPs [1]. In the first process, the density of photons around the Er3+ ion is modified by SPR excitation from the presence of TNPs that was positioned in the proximity of RE ions. The SPR excitation of TNPs produced strong localised electric field around it which subsequently enhanced the transition yield of REIs. Conversely, the quenching effect was associated with the re-absorption by SPR due to the increase of NPs contents where plasmon absorption band extended over the Er3+ ion emission peaks. The variation in the NPs size distributed in the glass matrix strongly
ACCEPTED MANUSCRIPT affected the local field enhancements by altering the collective oscillation of electrons. In the second process, the enhancement in the PL intensity was ascribed to the energy transfer (ET) from TNPs to the REIs (TNPs → Er3+) and the PL intensity quenching acted as a reverse process (Er3+ → TNPs) [1,3]. 2
24
4 TNP
RI PT
SC
LFE
M AN U
8
GSA
12
MPR
3
ET
NR 4S R R NR 4 3/2 F R 4 9/2 I 4 9/2 I11/2
Green (525 nm) Green (545 nm) Red (660 nm)
ESA
-1
Energy (×10 cm )
16
ESA
NR
20
0
G9/2 F 4 3/2 F 4 5/2 F7/2 4 H11/2 4
3+
4
I13/2
4
I15/2
Er Ion
Figure 7. Partial energy level diagram of Er3+ ion demonstrating the mechanisms of UC PL intensity enhancement and quenching.
Conclusions
TE D
5.
AC C
EP
For the first time, we analyzed the TNPs induced modifications in the spectroscopic properties of Er3+ ions doped zinc sodium tellurite glass using J-O theory. This glass system was prepared using melt-quench method and characterized. The existence TNPs of average sizes between 10 to 26 nm in the glass matrix was manifested in the TEM images. The detection of surface plasmon absorption bands at 525 and 545 nm verified the nucleation of non-spherical TNPs in the glass network. The J-O intensity parameters and the spectroscopic quality factor for various lasing transition was found to be greatly sensitive to the concentration of TNPs incorporation in the glass. The green PL band revealed prominent intensity enhancement by a factor as much as 30.00, which was ascribed to the localised SPR effect of TNPs. It was established that by altering the size and shape of TNPs it is possible to modify of the optical properties as reflected in the evaluated J-O parameters. Excellent features of the results suggest that the newly proposed glass composition may be beneficial for development of optical devices and solid state lasers.
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Improvement of spectroscopic properties of Er3+ ions doped tellurite glass via TNPs embedment. Observation of two characteristics SPR bands of TNPs at 552 nm and 580 nm. PL intensity enhancement of the green band by a factor of 30 due to LSPR mediated effect. Large Ω4 and Ω6 values affirmed the prospect of present glass composition for photonic devices. Sensitiveness of JO intensity parameters and spectroscopic quality factors on TNPs contents.
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