Impact of annealing time on silver nanoparticles growth assisted spectral features of erbium-zinc-boro-tellurite glass

Author’s Accepted Manuscript Impact of annealing time on silver nanoparticles growth assisted spectral features of erbium-zincboro-tellurite glass Zahra Ashur Said Mahraz, M.R. Sahar, S.K. Ghoshal www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(16)30174-0 http://dx.doi.org/10.1016/j.jlumin.2016.07.051 LUMIN14145

To appear in: Journal of Luminescence Received date: 7 February 2016 Revised date: 22 June 2016 Accepted date: 27 July 2016 Cite this article as: Zahra Ashur Said Mahraz, M.R. Sahar and S.K. Ghoshal, Impact of annealing time on silver nanoparticles growth assisted spectral features of erbium-zinc-boro-tellurite glass, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2016.07.051 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 galley proof before it is published in its final citable 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.

1

Impact of annealing time on silver nanoparticles growth assisted spectral features of erbium-zinc-boro-tellurite glass

Zahra Ashur Said Mahraz*, M.R. Sahar, S.K. Ghoshal

Advanced Optical Materials Research Group, Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, Skudai 81310, Johor, Malaysia. * E-mail: [email protected], Tel: +6075534024, Fax: +6075566162

Abstract Modifying the optical response of rare earth doped inorganic glasses by embedding nanoparticles is a never-ending quest. Accurate size and shape control of metal NPs inside the glass matrix through precise heat treatment (annealing) is challenging. We report for the first time, the effects of annealing time on the optical properties of the Er3+doped zinc-boro-tellurite glasses containing silver NPs. Glasses are prepared using meltquenching method where the growth of NPs is tuned by varying heat treatment duration. Modifications in physical, optical, and structural parameters are ascribed to the alteration of non-bridging oxygen due to HT. Shrinkage of NPs sizes from 12.8 to 6.6 nm for annealing time beyond 6 hr at 410 oC is ascribed to their diffusion limited growth. Surface plasmon resonance bands at 550 and 580 nm revealed red shift. The intensity parameters related to the radiative transitions within 4fn configuration of Er3+ ion are determined and analyzed using Judd-Ofelt theory. The room temperature emission spectra under 476 nm excitation exhibited three peaks centered at 536, 550 and 630 nm corresponding to the transitions from 2H11/2, 4S3/2 and 4F9/2 excited states to 4I15/2 ground state. Luminescence intensity enhancement (by a factor as much as 4.52) is majorly

2

attributed to the local field effect of Ag NPs and quenching is due to the energy transfer from NPs to Er3+. Present glass compositions are demonstrated to be promising for the development of photonic devices. Keywords: Heat treatment; Nanoparticles; Boro-tellurite glass; Optical properties; JuddOfelt analysis. 1.

Introduction Undoubtedly, the applications of rare earth ions (REIs) doped glasses are limited

due to their miniature oscillator strength (fstrength) of the 4f electronic transitions [1]. Consequently, the emission intensity readily gets quenched due to de-excitation mediated losses among different energy levels. Lately, the coupling of REIs with plasmonic nanostructures is demonstrated to be an alternative route to augment their luminescence intensity. These efforts are in conjunction with the prospect of plasmonic nanoparticles (NPs), electro-magnetic (EM) confinement and simultaneous optical excitation in nanoscale volume. The strong optical interactions within this volume are mediated by local electric field enhancements [2]. The energy transfer (ET) between metallic NPs and REIs located in the proximity of NPs are responsible for such local field enhancement [3]. Actually, the strong localized surface plasmon resonance (SPR) effects are attributed to such enhancements. However, creation of metallic NPs of desired sizes and shapes and positioning them in the vicinity of RE ions are far from being achieved. In the past, glasses containing metallic nanostructures are developed via controlled heat treatments (HTs). Glassy state being meta-stable tends to transform continuously towards the more stable state following the mechanisms of structural relaxation and crystallizations. The structural relaxation process is driven by the

3

considerable amount of strain frozen-in during glass formation. Conversely, in the annealing process the material relaxes via atomic diffusion towards the internal equilibrium state of the liquid from which it is obtained [4]. The crystallization process which occurs via nucleation and growth depends critically on the kinetic and thermodynamic factors [5]. Interestingly, the growth and nucleation of NPs can be controlled through annealing above the glass transition temperature [6]. The properties of glasses can easily be tuned by varying thermal treatments which in turn enhances the luminescence [7]. The process of HT and NPs concentration is varied to optimize the NP growth that subsequently modified the optical properties of glasses [7]. Turba et al. [8] acknowledged the thermally induced particle growth by involving migration of adatoms (Ostwald ripening) or migration and coalescence of NPs. It is asserted that the SPR wavelength depends on the host and metal-dielectric as well as together on the dimensions and shape of NPs. The tunability of plasmon band positions at different wavelengths facilitates varieties of applications [8]. Accordingly, the electrons modify their collective oscillation with the change in shape and size of the NPs. Variation in the dielectric constant of the surrounding medium affects the oscillation frequency, where the surface is capable in accommodating electron charge density from the NPs [9]. However, the correlation between SPR and ligands fields interaction due to the growth of NPs is not yet addressed. Recently, Kassab et al. [3] studied the effect of HT on the luminescence of Eu3+doped germinate glasses containing silver or gold NPs. An enhancement ~700% with Ag and 500% with Au are observed for 3h of continuous annealing. However, prolonged HT is found to quench the luminescence intensity of Eu3+ ions. The controlled HT mediated

4

nucleation and growth of Au NPs in Er3+ doped lead–tellurite glass is reported by Ezza et al. [10], where the intensity of emission peaks for heat-treated (16 hr) samples are found to be 3.0 times larger than their singly-doped counterpart, while the visible transitions experienced a quenching for further HT up to 24 hr. Nevertheless, the optimized NPs growth inside Er3+-doped zinc boro-tellurite matrices and subsequent enhancement of optical response are not reported in the literature. The aim of the present study is to investigate the influence of HT time on the growth and nucleation of Ag NPs inside Er3+doped zinc boro-tellurite glass host responsible for enhanced absorption and emission of Er3+ ions. Also to calculate the Judd-Ofelt parameters of Er3+ ions before and after heat treatment and finally to determine the radiative properties of important excited states. 2.

Materials and methods Glass samples having compositions (60-x)TeO2-30B2O3-10ZnO-0.5Er2O3-xAgCl,

where x = 0 and 1.0 mol% are prepared using melt-quenching technique. First, 11 grams of raw materials (analytical grade purity) of tellurium dioxide (Sigma-Aldrich 99%), Boron trioxide (Sigma-Aldrich 99.999%), zinc oxide (ACROS 99%), erbium oxide (Sigma-Aldrich 99.99%), and silver chloride (HmbG 99%) are weighted and mixed. The amount used for each starting material is calculated taking into account their molecular weight and the glass composition. Homogenously mixed powders are melted at 950 oC for 20 minutes in an alumina crucible before being poured into preheated (300oC) steel mold and kept inside the furnace for 3 hr to avoid any mechanical stress. Glasses are then annealed at 410 oC for 6, 12, and 24 hr to tune the Ag NPs growth. Table 1 summarizes various sample compositions with codes and HT durations.

5

Bruker D8 Advance X-ray diffractometer (XRD) is used to verify the amorphous nature of prepared samples with Cu-Kα radiations (λ = 1.54 Å) at 40 kV and 100 mA. TEM images are obtained using Philips CM12 operated at 200 kV to confirm the existence of Ag NPs having different sizes. The room temperature absorption spectra in the wavelength range of 4001650 nm are recorded via Shimadzu UV-3101PC (UV-Vis) spectrophotometer (Kyoto, Japan). Perkin Elmer (LS55) photoluminescence (PL) spectrometer (UK) equipped with a Xenon flash lamp is used to measure the emission spectra in the range of 450–800 nm under 476 nm excitations. The glass density (ρ in g.cm-3) is determined using Archimedes method with distilled water as an immersion liquid. Density is calculated from,



Aa     0 a Aa  B w

(1)

Where Aa and Bw are the weight of the sample in the air and in distilled water, respectively with ρ0 is the density of pure water and ρa is that of air. The molar volume (Vm) in terms of the molecular weight (M) yields,

Vm 

M



(2)

By applying Tauc method, the optical band gap (Eg) is estimated using Davis Mott equation [11],

 v   B

 h  E g  r h

(3)

6

Where the index r signifies the nature electronic transitions which takes a value ½ for direct allowed transitions and 2 for indirect allowed transitions. A graph between (αhν)1/r versus photon energy (hν) called Tauc plot is generated by substituting the value of r = 1/2 and r = 2 in Eq. (3) to obtain the optical band gap for direct (Edir) and indirect (Eind) allowed transitions, respectively. The linear portion of the curves are extrapolated to (αhν)2 = 0 and (αhν)1/2 = 0 to determine the values of Edir and Eind, respectively. The value of α(ν) near the absorption band edge exhibits an exponential behavior on the photon energy hν and obeys the Urbach’s empirical relation [12],

 v    0 exp h  

(4)

Where α0 is a constant and ΔE is the Urbach energy. This exponential behavior is due to the band tails associated with the valence and conduction bands which extends into the band gap. The glass refractive index (n) is calculated from optical band gap values using, 2 g n 1  1 2 20 n +2

(5)

The experimental (fexp) [1] oscillator is calculated using,

f exp 

4.318  109   ( )d N

(6)

Where α is the absorption coefficient (cm-1), N is the number of active ions (mol. L-1). The theoretical (fcal) [13] oscillator strengths yields,

7

f cal

2  n + 2 8  2 mc ' ' ' S , LJ ; S , J J  S ed 3h e2 2 J + 1 9n



 



2

(7)

Where m is the electron mass, e is the electronic charge, c is the light velocity, n is the refractive index and λ is the wavelength. The expression for the electric dipole line strength (Sed) is given by,

2 S ed  e



t

t  2, 4, 6

 S , L J U t  S ' , L 'J '

2

(8)

The reduced matrix elements ||U(t)||2 with t= 2, 4 and 6 are invariant from host to host. Judd-Ofelt intensity parameters Ωt can be estimated by a least-square fit of experimental oscillator strengths on calculated ones. Root mean square error (RMSE) between experimental and calculated oscillator strengths is tallied by,   f exp  f cal 

2

RMSE 

(9)

 3

Where ξ is the number of transition concerned in Judd-Ofelt parameters calculation and the summation is over all transitions. The Judd-Ofelt transition probabilities A(J→Jˊ) are expressed as,





A J  J' 

64 4  n2 + 2



2

27nh2J + 1  3

S ed J  J '



(10)

The branching ratio (β) that is used to predict the relative intensities of all emission lines originating from a given excited state is defined as,



 



 J  J '  A J  J '

(11)

8

The radiative lifetime (τ) of an emitting state is related to the total spontaneous emission probabilities for all transitions via the equation,



  1  A J  J '



(12)

J

where the sum is extended over all the states at energies lower than J.

3. 3.1.

Results and discussion Physical properties Table 2 summarizes the calculated physical properties of all glass samples. The

decrease in glass density from 4.6401 to 4.6042 g.cm-3 due to the introduction of Ag NPs is ascribed to the alteration in the network structure via the generation more non-bridging oxygen atoms (NBOs) [9, 14, 15]. Interestingly, prolonged HT reduces the generation of NBOs and results an increase in the glass density [14]. However, the observed decrease in the molar volume (26.138–26.143 cm3.mol-1) with the increase of Ag NPs concentration is attributed to the reduction of total volume that contributes to the glass compactness [16]. As the glass undergoes through the heat-treatment process, the spatial distance between atoms gets decreased [17]. The decrease in molar volume signifies that the heat-treatment process has contracted the fragile network in the studied glass. The decrease in refractive index from 2.471 to 2.432 is due to the insertion of Ag NPs in the glass network that reduces the polarizability of cations. The direct (2.807–2.892 eV) and indirect (2.461–2.491 eV) optical band gap energies both are found to exhibit a shift towards lower values (for heat treated glass samples) due to the formation of NBO. In fact, the NBO which bounds an excited

9

electron less tightly than the bridging oxygen is more polarizable. Thus, the creation of more NBO is attributed to the shift of fundamental absorption edge towards higher wavelengths. This edge shift in turn is responsible for the decrease in the optical band gap energies. Furthermore, the increase in Urbach energy from 0.293 to 0.334 eV indeed supports the decrease in band gap energies for HT samples. The increase in ΔE indicates the escalation of glass disorder as a consequence of more extension of the localized states within the gap, where weak bonds turn into defects by creating large number of NBO’s and make the system more polarizable. 3.2

XRD pattern Fig. 1 shows the typical XRD pattern of glasses without and with HT for 6, 12,

and 24 hr of durations. The presence of a broad hump between 20-35o without any sharp peak confirm the amorphous nature of the heat treated glass samples. The complete absence of sharp and strongly diffracted peaks in the X-ray diffraction pattern from glass indicates that there are no well-defined planes in the structure on or around which the constituent atoms are regularly arranged. 3.3.

TEM images Fig. 2 displays the TEM images of glass samples without and with HT (6, 12, and

24 hr). The occurrence of non-spherical Ag NPs inside the glass matrix with homogenous distribution is manifested. The appearance of broad distribution in NPs size is due to their growth at different times. During melting, first Ag+ ions are formed and then reduced to Ag0 via redox reaction. HT above the glass transition temperature (Tg) facilitates Ag0 to aggregate and form NPs [18]. Average diameter of Ag NPs in the non-heat treated glass sample is found to be ~ 8.4 nm (Fig. 2(a)). TEM image (Fig. 2(b)) of the sample with 6 hr

10

of HT clearly reveals an increase in the NPs average diameter (~ 12.8 nm). Conversely, further HT for 12 and 24 hr causes a decrease in the NPs average diameter from 9.6 (Fig. 2(c)) to 6.8 nm (Fig. 2(d)), respectively. The annealing assisted NPs growth is attributed to the coarsening mechanisms including Ostwald ripening and diffusion coalescence [19]. Besides, the shrinking of NPs beyond 6 hr of HT is ascribed to the thermal fragmentation of larger particles [20]. The histograms displaying the size variations of Ag NPs for all glass samples without and with annealing are presented in Fig. 3. Fig. 4 depicts the corresponding heat treatment time dependent variation in the average size of NPs. These variations of the annealing time dependent average diameters are interpreted in terms of the crystallization process as acknowledged by Hillig and Turnbull [21]. This crystallization process proceeds via the growth of a new phase after stable nuclei formation in the parent phase. The rate of crystallization depends on total atomic numbers that move from liquid surface to crystal surface and also on the activation energy. Overall, the variation in NPs size distribution (Fig. 4) is related to the diffusion limited growth processes. The observed Ag NPs size (~8.4 ± 0.4 nm) for un-annealed sample indicates the occurrence of nucleation and growth on the surface at early stage. The size reaches a maximum of 12.8 ± 0.4 nm for 6 h of HT and steadily decreases thereafter. Further increase in HT time to 12 and 24 h causes a decrease in NPs size from 9.6 ± 0.25 to 6.8 ± 0.1 nm, respectively. This error originates from the instruments and remains within the experimental limit~ ±5%. At shorter HT duration, there are still sufficient components which can be brought to crystal growth and the free energy is greater than the temperature. However, as the time of heat treatment is increased up to 6 h, the diffusion process can no longer replenish crystal

11

growth components which finally saturate after a certain time of heat treatment with lower free energy overcoming the Brownian randomness. 3.4.

UV–Vis-NIR absorption spectra The room temperature UV-Vis-NIR absorption spectra (Fig. 5 (a)) reveal seven

bands centered at 6553, 10224, 12484, 15267, 19230, 20661 and 22522 cm–1 corresponding to the transitions from 4I15/2 to 4I13/2, 4I11/2, 4I9/2, 4F9/2, 2H11/2, 4F7/2 and 4F3/2 states of Er3+ ions. The absorption spectra of Er3+ ions exhibiting several bands in the visible region often overshadow the plasmon band of silver NPs [18]. Therefore, a new sample of composition 1.0AgCl-10ZO-30B2O3-59TeO2 without containing REI is prepared to locate the SPR band. Subsequently, this sample is subjected to identical HT for 6 hours duration to probe the SPR peak of Ag NP. The clear manifestation of intensive SPR absorption band in the visible region of the UV-Vis spectrum verifies the existence of Ag NPs in the glass matrix [22]. This absorption peaks at different wavelengths originate from the resonance effects of incident photons with the collective oscillations of conduction electrons [23, 24]. The SPR wavelength depends on the dielectric functions of host and metal as well as on the dimensions, size, and shapes of NPs. Fig. 5(b) displays the appearance of SPR bands around 550 and 580 nm for unannealed sample without containing RE. Furthermore, the red shifts of these SPR bands (Fig. 5(c)) to 580 and 630 nm after 6 hr of HT are ascribed to the increase in NPs mean diameter and morphology variations. Radiative transitions within 4fn configuration of rare earth ion are analyzed using J–O parameters [1, 13]. The values of fcal and fexp are enlisted in Table 3. The strong dependence of the oscillator strengths on Er2O3 contents implies that the non-symmetric

12

component of the electric field acting on Er3+ is relatively high. The oscillator strength provides indirect information on the symmetry and bonding of RE ion within the glass matrix. The smaller oscillator strengths indicate higher symmetry around the RE ion. The hypersensitive transition (HST) being very sensitive to small changes in the environment around lanthanide ions obeys the selection rules, |ΔJ| ≤ 2, |ΔL| ≤ 2 and ΔS = 0 [25, 26]. The values of oscillator strengths are found to decrease with the increase of HT durations. The increase in the HST oscillator strengths for 6 hr of HT is attributed to the changes in site symmetry of Er3+ ions. The generation of higher number of NBOs results as increase in the asymmetry of the bond to the neighboring network cations [27]. The J–O parameters calculated from the electric dipole contributions of the experimental oscillator strength using the least square fitting approach [39] with the matrix elements [1, 13, 28] are summarized in Table 3. The values of Ω2 and Ω6 are related to the symmetry and network covalency of the glass hosts, respectively. In fact, Ω6 decreases with the increase of covalence nature of the Er–O bond [21]. The trend for the Ω parameters in the heat treated glasses follow Ω2>Ω4>Ω6 trend. The value of Ω2 signifying the dependence of covalency between the rare earth ions and ligand an ions clearly reflects the presence of asymmetry of the local environment in the proximity of the RE ion site. The characteristic feature of Ω2 parameter is related to asymmetry of structural coordination, bonding nature and polarizability of the ligand ions or molecule, which is sensitive to the local environment of the RE ions. The higher value of Ω2 representing less ionic nature of the chemical bond with the ligands is responsible for the increase in the covalent character [29, 30]. The increasing value of Ω2 with HT duration indicates the lower symmetry in the structural coordination surrounding the Er3+ ion [31].

13

The decrease in the symmetry of RE ions sites is caused by the enhancement of the NBO ions in the glass network which in turn increase the value of Ω2 [32]. The achieved smaller Ω4 and larger Ω6 values are favorable for the luminescence transition as acknowledged previously [33]. The decrease in rigidity of the host glass with the increase of volume indicates higher degree of covalence of the RE oxygen bonds. Consequently, these compositions are potential as laser hosts. The annealing time dependent spontaneous transition probabilities, radiative lifetimes, and the branching ratios are summarized in Table 4. The radiative transition probability is observed to decrease with the increase of HT time [34]. Furthermore, a slight decrease in the effective band width implies the augmentation ligand field asymmetry. The large value of branching ratio as much as 90% for 4F9/2→4I15/2 and 4

S3/2→4I15/2 transitions suggests that efficient green and red emissions are achievable in a

tunable fashion. 3.5.

Photoluminescence spectra The emission spectra (Fig. 6(a)) of Er3+ ion reveal three significant peaks centered

at 536, 550 and 630 nm originate from excited states 2H11/2 , 4S3/2 and 4F9/2 to ground state 4

I15/2 transitions, respectively. The observed enhancement in the luminescence intensity

for the annealed (6 hr) sample is attributed to the Ago NPs SPR mediated local field effects [35]. The occurrence of Ag NPs in the vicinity of Er3+ ions radically changes the photons density of states. Consequently, the rate of radiative transitions and excited states lifetime of Er3+ ions are significantly altered [36]. This clearly demonstrates the modifications in the optical responses mediated via localized SPR effects. Conversely, further increase in HT duration beyond 6 hr results luminescence quenching. This drop in

14

the luminescence intensity is ascribed to energy transfer from Er3+ ions to Ag NPs and subsequent re-absorption by SPR. This happens because the plasmon absorption band is extended over the emission peak position of Er3+. Fig. 6(b) illustrates the annealing time dependent relative intensity (I/I0) of each band, where I and I0 are the intensities of a specific emission after annealing and in the absence of Ag NPs (normalized to 1), respectively. The appearance of larger relative intensity enhancement for the red band compared to the green one is related to the charge cloud effects of the plasmon band, which largely affects the levels lying nearer to the plasmon frequency band of Ag NPs [36, 37, 38]. The partial energy level diagram of Er3+ ions is schematically shown in Fig. 7. The partial energy levels diagram of Er3+ ion shows down-conversion (DC) emissions at 536, 550 and 632 nm through ground state absorption (GSA), cooperative energy transfers (CET), energy transfer (ET), radiative decay (R), non-radiative decay (NR) and localized surface plasmon resonance effect (LSPR) due to silver NPs. The excitation under 476 nm stimulates the ion from 4I15/2 to 4F7/2 level, where the multi-phonon nonradiative (NR) decays populate (2H11/2 + 4S3/2) and 4F9/2 excited states, where the green and the red lines are generated, respectively. The presence of the B-O-B and O-B-O groups with relatively high phonon energies favor the multi-phonon decays which facilitates the red emission of the Er3+ ions in our glass system. Moreover, 4F9/2 excited state can be populated by the cooperative energy transfers (CET) between two neighboring erbium ions in the present system.

15

4.

Conclusion The influence of annealing time on the optical properties of Er3+ doped zinc boro-

tellurite glass containing Ag NPs is inspected. The HT stimulated growth of NPs with desired size and distribution is demonstrated. Thermal annealing is found to significantly modify the physical, structural, and optical properties where growth of NPs played a decisive role. TEM images confirmed the presence of Ag NPs in the glass matrix with average diameter between 8.4–12.8 nm. Absorption spectra manifested localized SPR bands of Ag. Luminescence enhancement is attributed to the Ag NPs mediated intensified localized SPR effects. Conversely, the luminescence intensity quenching is ascribed to the energy transfer from Ag NPs to Er3+ ions. The intensity parameters corresponding to the radiative transitions within 4fn configuration of Er3+ ion are calculated using Judd– Ofelt (J–O) theory. It followed Ω2>Ω6>Ω4 trends for heat-treated glass samples. The achieved smaller Ω4 and larger Ω6 values are affirmed to be favorable for lasing transition. Branching ratio as much as 90% for 4F9/2→4I15/2 and 4S3/2→4I15/2 transitions are achieved. It is established that efficient green and red emissions are achievable in these glasses by controlling the growth of NPs through precise HT. Admirable features of the results nominate these glasses as potential candidate for display, laser and memory devices. Acknowledgements The financial support from Malaysian Ministry of Education (Vot. 4L032, 4F424, and 05H36) is gratefully acknowledged. Zahra is thankful to Ministry of Higher Education, Libya for doctoral scholarship.

16

References [1] B.R. Judd, Phys Rev. 127 (1962) 750. [2] M.R. Dousti, R.J. Amjad, R. Hosseinian, M. Salehi, M.R. Sahar, J. Lumin. 159 (2015) 100. [3] A. Awang, S.K. Ghoshal, M.R. Sahar, M.R. Dousti, R.J. Amjad, F. Nawaz, Curr. Appl. Phys. 13 (2013) 1813.

[4] W.A. Johnson, R. F. Mehl, Am Inst Min Metall Eng. 135 (1939) 416. [5] A. Langar, C. Bouzidi, H. Elhouichet, M. Férid, J. Lumin. 148 (2014) 249 [6] V.A.G. Rivera, D. Manzani, Y. Messaddeq, L.A.O. Nunes, Jr. E. Marega, J. Phys Conf Ser. 274 (2011) 012123. [7] R.S. Panikkanvalappil, P. Thalappil, Nano Res. 2 (2009) 306. [8] T. Turba, M.G. Norton, I. Niraula, D.N. McIlroy, J. Nanopart Res.11 (2009) 2137. [9] J. Fu, Y. Zhao, Nanotech. 21 (2010) 175303. [10] E.S. Sazali, M.R. Sahar, S.K. Ghoshal, R. Arifin, M.S. Rohani, R.J. Amjad, J. NonCryst Solids. 410 (2015) 174. [11] E.A. Davis, N.F. Mott, Philos Mag. 22 (1970) 903. [12] F. Urbach, Phys Rev. 92 (1953) 1324. [13] G.S. Ofelt, J. Chem Phys. 37 (1962) 511. [14] A. Awang, S.K. Ghoshal, M.R. Sahar, R. Arifin, F. Nawaz, J. Lumin. 149 (2014) 138. [15] M.K. Halimah, W.M. Daud, H.A.A. Sidek, A. W. Zaidan, A.S. Zainal, Mater Sci Pol. 28 (2010) 173.

17

[16] E.S. Sazali, M.R. Sahar, S.K. Ghoshal, R. Arifin, M.S. Rohani, A. Awang, J. Alloys Compd. 607 (2014) 85. [17] A. Agarwal, I. Pal, S. Sanghi, M.P. Aggarwal, Opt Mater. 32 (2009) 399. [18] V.A. Rivera, S.P.A. Osorio, Y. Ledemi, D. Manzani, Y. Messaddeq, L.A.O. Nunes, E. Marega Jr, Opt Exp. 18 (2010) 25321. [19] W. Ostwald, Z Phys Chem. 34 (1900) 495. [20] C.B. De Araujo, L.R.P. Kassab, R.A. Kobayashi, L.P. Naranjo, P.A. Santa Cruz, J. Appl Phys. 99 (2006) 123522. [21] W.B. Hilling, D. Turnbull, J. Chem Phys. 24 (1956) 914. [22] R.J. Amjad, M.R. Dousti, M.R. Sahar, Curr. Appl. Phys. 15 (2015) 1. [23] S.A. Kalele, N.R. Tiwari, S.W. Gosavi, S.K. Kullkarni, Nanophotonics. 1 (2007) 012501. [24] D.A. Stuart, A. Haes, A.D. MvFarland, S. Nie, R.P.V. Duyne, Proceedings of SPIE. 5327 (2004) 60. [25] R. Cases, M.A. Chamarro, J. Solid State Chem. 90 (1991) 313. [26] S.M. Nasir, H. Nur, J. Fundamental Sci. 4 (2008) 245. [27] S. Mohan, K.S. Thind, G. Sharma, Brazilian J. Phys. 37 (2007) 1306. [28] R. Reisfeld, Struct Bond. 22 (1975) 123. [29] D. Rajesh, Y.C. Ratnakaram, A. Balakrishna, J. Alloys Compd. 563 (2013) 22. [30] A.D. Sontakke, K. Annapurna, J. Mat chem phys . 137 (2013) 916. [31] P.V. Do, V.P. Tuyen, V.X. Quang, N.T. Thanh, V.T.T. Ha, N.M. Khaisukov, Y. Lee, B.T. J. Alloys Compd. 520 (2012) 262. [32] W. Xiang, X. Liang, Physica B. 403 (2008) 3271.

18

[33] D. Pankaj, J. Opto electron Adv M. 6 (2012) 427. [34] K.V. Raju, C.N. Raju, S. Sailaja, B.S. Reddy, Solid State Sci. 15 (2013) 102. [35] D.M. De Silva, L.R.P. Kassab, S.R. Luthi, C.B. De Araujo, A.S.L. Gomes, M.J.V. Bell, Appl Phys Lett. 90 (2007) 081913. [36] J.A. Jimenez, Phys Chem. 15 (2013) 17587. [37] M. Mattarelli, M. Montagna, K. Vishnubhatla, A. Chiasera, M. Ferrari, G.C. Righini, Phys Rev B. 75 (2007) 125102. [38] S.K. Singh, N.K. Giri, D.K. Rai, S.B. Rai, Sol State Sci. 12 (2010) 1480.

Tables caption Table 1 Glass codes and compositions (in mol%), HT durations (hr), NPs average diameter (d) and maximum intensity enhancement factor (η). Table 2 Density (ρ, g.cm-3), molar volume (Vm, cm3 mol-1), energy for direct (Edir, eV), and indirect (Eind, eV) band gap, Urbach energy (ΔE, eV), and refractive index (n) of the synthesized glass samples. Table 3 Experimental and calculated oscillator strengths (×10-6) of the prepared glasses, root mean square error (RMSE×10-6) of list square fitting. Table 4 Judd-Ofelt intensity parameters (×10-20 cm2) of all glass samples. Table 5 Radiative transition probability (A, s-1), branching ratio (β, %) and lifetime (τ, μs).

19

Figures Caption Fig. 1. Typical XRD patterns of glasses before and after HT. Fig. 2. TEM images of all samples displaying the presence of Ag NPs (indicated by arrowhead), heat treated at temperature 410 °C for (a) 0 hr, (b) 6 hr, (c) 12 hr, and (d) 24 hr. Fig. 3. Heat treatment duration dependent size distribution of Ag NPs (a) 0 hr, (b) 6 hr, (c) 12 hr, and (d) 24 hr. Fig. 4. Annealing time dependent growth of Ag NPs (line is a guide to the eye). Fig. 5. (a) HT time dependent absorption spectra of all samples and SPR band of Ag NPs (b) un-heat-treated (c) heat-treated for 6 hr. .

Fig. 6. Annealing time dependent (a) PL spectra (b) PL intensity for three bands.

Fig. 7. Interaction of REIs and Ag NPs involving the energy transfer process under 476 nm excitations with indicated three emissions from Er3+ ion.

Table 1 Codes

TeO2

B2O3

ZnO2

Er2O3

AgCl

ZBTEA0hr

58.5

30

10

0.5

ZBTEA6hr

58.5

30

10

ZBTEA12hr

58.5

30

ZBTEA24hr

58.5

ZBTA

59.5

 (630 nm) 1

1.0

HT (hr) -

d (nm) 8.4

0.5

1.0

6

12.8

7

10

0.5

1.0

12

9.6

2

30

10

0.5

1.0

24

6.8

3.5

30

10

0.0

0.5

0

-

-

20

Table 2 Codes ZBTEA0hr

ρ 4.6401

Vm 26.138

Edir 2.745

Eind 2.561

ΔE 0.293

n 2.471

ZBTEA6hr

4.5878

26.380

2.892

2.491

0.322

2.472

ZBTEA12hr

4.5972

26.291

2.891

2.481

0.330

2.453

ZBTEA24hr

4.6042

26.143

2.807

2.461

0.334

2.432

Table 3 Energy level I15/2 → 4 I13/2 4 I11/2 4 I9/2 4 F9/2 2 H11/2 4 F7/2 4 F3/2 RMSE

4

ZBTEA0hr

ZBTEA6hr

ZBTEA12hr

ZBTEA24hr

fexp

fexp

fexp

fexp

fcal

fcal

fcal

fcal

4.58 4.10 6.58 5.83 4.18 3.97 3.77 3.50 2.19 5.57 4.97 6.61 2.27 5.58 2.98 6.46 0.73 1.63 1.94 2.29 0.86 1.66 1.79 0.65 3.31 2.79 3.59 4.35 2.94 3.78 3.14 3.98 16.32 16.27 18.23 17.59 16.78 16.45 17.91 16.91 1.44 1.48 4.84 7.06 3.67 5.36 4.66 6.21 0.46 3.88 3.80 5.76 1.75 3.92 2.74 4.72 2.31 1.42 0.58 0.49

Table 4 ZBTEA0hr

ZBTEA6hr

ZBTEA12hr ZBTEA24hr

Ω2

1.890

4.62

2.98

5.62

Ω4

4.525

2.23

1.51

2.99

Ω6

7.419

2.74

1.74

4.89

Ω4/ Ω6

0.6099

0.813

0.867

0.611

Trends of Ωλ Ω6>Ω4> Ω2 Ω2>Ω6> Ω4 Ω2>Ω6> Ω4 Ω2>Ω6> Ω4

21

Table 5 ZBTEA0hr

ZBTEA6hr

ZBTEA12hr

ZBTEA24hr

A 2438.3

β 100

τ 2.55

A 8156.6

Β 100

τ 0.12

A 32637.7

Β 100

τ 0.14

A 2322.5

β 100

τ 3.47

2891.8

87.49

0.61

3054.5

86.80

0.48

1853.3

85.56

0.53

1044.9

88.41

1.44

278.3

65.24

1.11

665.5

65.24

0.87

238.6

65.19

1.02

232.7

65.07

2.10

6937.3

97.90

0.05

9107.3

96.56

0.03

1903.3

95.34

0.04

828.8

95.05

0.84

3060.3

91.42

0.09

9435.2

91.32

0.01

42915.0

91.00

0.03

1096.4

90.97

1.33

40116.3

65.81

0.06

43598.5

64.20

0.02

21643.2

65.02

0.04

64.16

0.35

Figures Fig. 1.

ZBTEA0hr ZBTEA6hr ZBTEA12hr ZBTEA24hr

Intensity (a.u.)

Glass Transition 4 I13/2 → 4 I15/2 4 I11/2 → 4 I15/2 4 I9/2 → 4 I15/2 4 F9/2 → 4 I15/2 4 S3/2 → 4 I15/2 2 H11/2 → 4 I15/2

10

20

30

40

50

60

2 (Degree)

70

80

90

2466.4

22

Fig. 2.

23

Fig. 3.

24

Fig. 4.

Size of Ag NPs (nm)

13 12 11 10 9 8 7 6 0

5

10

15

20

25

Heat Treatment Duration (hr)

Optical Density (a.u.)

Fig. 5.

2

4

4

ZBTEA0hr ZBTEA6hr ZBTEA12hr ZBTEA24hr

H11/2

F7/2

4

F9/2 4

I9/2

4

I11/2

(a)

4

I13/2

F3/2 600

900

1200

Wavelength (nm)

1500

1800

25

Optical Density (a.u.)

SPR=nm

540

Optical Density (a.u.)

(b)

SPR = 550 nm

555 570 585 Wavelength (nm)

SPR= 580 nm

580

(c)

SPR= 630 nm

600 620 640 660 Wavelength (nm)

680

26

Fig. 6.

Intensity (a.u)

(a) ZBTEA0hr ZBTEA6hr ZBTEA12hr ZBTEA24hr

Relative Intensity (I/I0)

550

600 650 700 Wavelength (nm)

750

800

em= 536 nm (b)

4

em= 550 nm 3

em= 632 nm

2 1 0

5 10 15 20 Annealing Time (h)

25

27

Fig. 7.