Au co-doped antimony oxide glass nanocomposites by thermal treatment

Journal of Alloys and Compounds 688 (2016) 313e322

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Enhancement and tuning of photoluminescence properties in Pr3þ/Au co-doped antimony oxide glass nanocomposites by thermal treatment Nilanjana Shasmal a, b, Basudeb Karmakar a, * a CSIR-Network Institute for Solar Energy, Glass Science and Technology Section, CSIR-Central Glass and Ceramic Research Institute, 196 Raja S C Mullick Road, Kolkata 700032, India b Academy of Scientific and Innovative Research, Chennai 600113, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 May 2016 Received in revised form 15 July 2016 Accepted 20 July 2016 Available online 21 July 2016

Pr3þ/Au3þ co-doped antimony oxide glass nanocomposites has been synthesized in a dielectric glass matrix having composition 15K2Oe15B2O3e60Sb2O3e10ZnO (KBSZ) by single-step melt-quenching technique. Au nanoparticles (NPs) are generated in-situ by thermochemical reduction by Sb3þ. The Tg has been found to be 290e295  C as obtained by dilatometry and differential scanning calorimetry. X-ray diffraction (XRD) patterns show the presence of nanocrystalline phase of Au in the as-prepared samples. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns also confirm the presence of Au NPs. UVeVis absorption spectra shows both the characteristic absorption bands of Pr3þ, around 445 nm and Au surface plasmon resonance (SPR), around 620 nm. When excited with 447 nm diode laser, all the nanocomposites show strong orange emissions. The sample containing 0.008 wt% Au gives the maximum emission. This sample is heat treated at 300  C for different durations. XRD of heat treated samples reveals that crystallization of the glassy matrix initiates at around 480 min heat treatment. The UVeVis absorption spectra of the heat treated samples show increased absorbance of the SPR band of Au around 620 nm. The SPR band of Au gets more and more widespread with increasing heat treatment duration which is due to rapid rate of Au NP generation by thermochemical reduction at elevated temperature. Variation of PL intensity with heat treatment duration exhibits two maxima (at 45 and 480 min durations). The first one is due to the attainment of the critical size of Au NPs, for which the emission becomes maximum. And the second one is the effect of matrix crystallization, which lowers down the phonon energy of the system and enhances PL intensity by reducing the nonradiative loss. This enhancement and tuning of photoluminescence is enormously useful for various photonic applications. © 2016 Elsevier B.V. All rights reserved.

Keywords: Amorphous materials Rapid-solidification quenching Optical properties Transmission electron microscopy Optical spectroscopy Luminescence

1. Introduction Rare-earth (RE) doped glasses have turned out to be an important class of photonic materials on account of their nonlinear optical (NLO) properties, laser amplification, and frequency up and down conversion properties which are significantly important to develop short wavelength (visible) lasers, color displays, remote sensing, optical communication, bar-code reading, laser printing, etc [1e3]. The spectroscopic properties of the RE ions are influenced by the glass matrix environment [1]. The rich multicolored emissions from RE ions, attributed to the intrashell parity forbidden

* Corresponding author. E-mail address: [email protected] (B. Karmakar). http://dx.doi.org/10.1016/j.jallcom.2016.07.220 0925-8388/© 2016 Elsevier B.V. All rights reserved.

f-f transitions, are typically sharp, photostable, and long lived [1]. The excitations and emissions of them are due to the transitions from the 4fn electronic states of trivalent RE ions, which are highly sensitive to the symmetry, structure of the local environment and phonon energy of the host matrix. Consequently, there has been an exponential interest to enhance the efficiencies of this group of solid-state visible laser materials. Recently, coupling RE ions with metal nanoclusters has been developed as a valuable strategy to improve the luminescence efficiency of RE ions. These materials with enhanced luminescence properties, particularly nano metal enhanced upconversion and downconversion, are promising for plasmon controlled nano-photonic technologies [3e5]. Trivalent praseodymium ion (Pr3þ) is very attractive for visible solid-state lasers because it generates several transitions in the red, orange, green, and blue spectral regions. Recently, many research

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groups have studied visible lasers made from different crystals or glasses with Pr3þ ion because crystals like fluoride show lower phonon energy than oxide ones; therefore, luminescent intensity is higher without multiphonon relaxation. A requirement for an efficient amplifier is that the glass host is a material with low phonon energy to reduce the non-radiative multiphonon relaxation rate [6e8]. The synthesis and optical properties of Pr-doped Ge-Ga-S glasses were reported [9]. Germanium sulfide glass was investigated as host glass material for Pr3þ-doped fiber amplifiers [7]. Room-temperature visible luminescence (600 nm) from Pr3þ ions embedded in germanium sulfide glass hostsis reported along with edge luminescence from these hosts. The important role of energy transfer processes between the host and Pr ions is demonstrated experimentally through the dependencies of photoluminescence on excitation wavelength, temperature, and polarization [10]. Emission and excitation spectra are presented for Pr3þ(0.01%) doped oxyfluoride glass host containing LaF3 nanocrystals. Prominent emission bands were found around 600 nm. Two types of Pr3þ ions, those in LaF3 nanocrystals and those in the glass host, are characterized by spectroscopic and dynamical studies [11]. Oxyfluoride glasses with composition 50GeO2e(50x) PbOexPbF2 (x  15 mol%) containing Pr were prepared and studied by Klimesz et al. [12]. Pr3þ-doped fluoride nanocrystals-based oxyfluoride glass ceramics has been reported [13]. Orange-to-blue frequency upconversion was found in Pr3þ doped chalcogenide glass (Ga10Ge25S65) doped with Ag2S and heat treated under different conditions to nucleate silver nanoparticles. The enhancement observed in the upconverted emission at 494 nm, due to the3P0/3H4 transition of the Pr3þ ion, is attributed to the large local field acting on the emitting ions due to the presence of the metallic NPs [14]. The photoluminescence properties of Pr3þ doped phosphate glass having composition P2O5eCaOeSrOeBaO have been investigated at room temperature [15]. Luminescence properties of Pr3þ-doped transparent oxyfluoride glasseceramics containing BaYF5 nanocrystals were investigated by Gu et al. [16]. Pr-doped borophosphate glass was also investigated. The results indicate that the Pr-doped glass can be used as an amplification medium for tunable lasers and broadband optical amplifiers for wavelength division multiplexing transmission system applications [17]. Pr3þ doped TeO2eBaF2eNaF glasses were prepared which are potential candidates for laser applications [18]. Luminescence properties of Pr3þ doped Li2O-MO-B2O3 glasses were studied [19]. Luminescences properties of Pr3þ ions in germinate glasses modified by BaF2 were investigated. Several luminescence bands originating to transitions from the 3P0 state to the lower lying states of Pr3þ were registered under 450 nm excitation [20]. Considering the above facts, it seems to be very interesting to investigate the visible photoluminescence of Pr3þ ions in glasses. Metaledielectric (glass) nanocomposites represent a class of materials having both scientific and technological interests over several years owing to their unusual optical and electrical properties. They are widely used for real applications, such as fluorescence (photonic) and non-linear optical devices. Noble metal nanoparticles (Au or Ag) are associated with the unique surface plasmon resonance (SPR) phenomenon [21e24]. Surface plasmon resonance is a unique optical phenomenon occurring at the surfaces of metal particles in the nanometer size regime. It is the collective oscillation of the outer conduction electrons upon electromagnetic excitation [25]. SPR is intimately dependent on the nanoparticle's size, shape, refractive index of the dielectric environment and other proximal nanoparticles (NPs) [26e28]. Precise tuning of the SPR peak across a wide spectroscopic range can be accomplished by varying any of the aforesaid parameters. Besides absorption and scattering, SPR also generates intense local electric fields around the metal nanostructures. Rare-earth (RE) ions when present in close vicinity of

such plasmonics nanoparticles experiences enhanced excitation rate leading to an enormous increase in their fluorescence intensity [29]. Among the heavy metal oxide (HMO) glasses, antimonyoxide systems remain very less explored; this is because of difficulties in obtaining vitreous Sb2O3 due to its low field strength (0.73), which makes it a poor glass former [30]. Again, the volatility of the melts, the intense crystallization while cooling the melts, and the difficulty in preparing monolithic glass with a very high Sb2O3content (above 50 mol%), which is essential for practical applications, have limited the study of antimony systems, particularly in the areas of optics and photonics [31]. Antimony oxide-based glasses are suggested for technological applications such as optical recording media and nonlinear optical devices due to their photosensitivity, non centro-symmetric structure, and fast response times as a consequence of the high polarizable Sb3þ ions with a strongly localized, stereo chemically active, lone pair of electrons (5s2). Sb2O3-containing glasses are expected to possess a combination of attractive properties like high density, large transmission window (UV to IR), low phonon energy, low melting and glass transition temperature, and high linear thermal expansion, matching even those of certain metals and alloys [30,31]. Earlier reports on preparation of high Sb2O3 containing glasses show they are yielded in tiny pieces or pulverized form [30,32]. Recently Monolithic antimony glasses and nano glass-ceramics in the ternary K2OeB2O3eSb2O3 system (KBS) and quaternary K2OeB2O3eSb2O3eZnO system (KBSZ) was first reported by Som et al. [31,33]. Sm3þ, Er3þ, Nd3þ doped KBS glasses have been widely studied, which reveals their remarkable upconversion luminescence properties suitable for application in various photonic devices. Sb2O3 being a heavymetal oxide, Sb2O3 glasses have lower phonon energy (around 600 cm1) which would increase the upconversion and quantum efficiency of luminescent RE ions [34e36]. But, the most interesting aspect of antimony oxide-based glasseceramics over conventional systems is that Sb2O3 is a mild reducing agent (reduction potential, E ¼ 0.649 V) [37]. This mild reduction property enables in-situ reduction of Au3þ (HAuCl4. xH2O) to Au0 than RE ion in a single-step during the melting process thereby providing for a simple, low cost method for the preparation of bulk nanophotonic materials [29]. Pr3þ and Au co-doped antimony glass has not been reported so far though it has enormous possibility in photonic applications. We are the first to demonstrate here: (i) the Au-embedded antimony glass (mol%) 15K2Oe15B2O3e60Sb2O3e10ZnO (KBSZ) nanocomposites and (ii) Au:Pr3þ co-embedded hybrid antimony glass nanocomposites. The nanocomposites are characterized by Differential scanning calorimetry (DSC), dilatometry, X-ray diffraction (XRD) analysis, transmission electronmicroscopy (TEM), High resolution TEM and selected area electron diffraction (SAED) analysis, UVevisible absorption spectroscopy and photoluminescence spectroscopy. Considerable enhancements have been found in downconverted frequency when excited by 447 nm laser diode source. The critical concentration of Au for which the emission reaches the maximum value has been found out. The effect of thermal treatment on the optical properties has also been studied. The emission intensities can be tuned by the duration of heat treatment. Transformation of glass matrix into glass-ceramics has a tremendous impact on the photoluminescence behaviours of the nanocomposites. That has been established by experimental evidences. 2. Experimental 2.1. Glass preparation Antimony oxide borate zinc oxide (KBSZ) glass having base glass composition 15K2O-15B2O3-60Sb2O3-10ZnO was prepared by

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conventional melt quenching technique. The raw materials were potassium metaborate, KBO2. xH2O (15.7% H2O, Johnson Matthey), antimony (III) oxide, Sb2O3 (GR, 99%, LobaChemie) and zinc oxide, ZnO (GR, 99%, Fluka). They were used directly without any further purification. 20 g of glasses were prepared by melting the well mixed batches of calculated composition in a high purity silica crucible at 900  C for 10 min with intermittent stirring for 0.5 min in air in a raising hearth electric furnace. All the molten samples were cast into a carbon plate in air and annealed at 260  C for 3 h to remove thermal stresses followed by slow cooling down to room temperature. The Au0-doped nanocomposites (NCs) were prepared in a similar technique using respective dopant concentrations (in excess) as shown in Table 1. The precursor material for Au used here is chloroauric acid, HAuCl4. xH2O (49% Au, Loba Chemie). To study the photonic application, four Pr2O3 (99.9%, Alfa Aesar) and Au0 codoped NCs were synthesized similarly. The melting and annealing times were kept constant for all the samples. Samples of about 2.0 ± 0.01 mm thickness for optical measurements were prepared by cutting, grinding and polishing with cerium oxide.

315

3. Results and discussion 3.1. Differential scanning calorimetry and dilatometry Differential scanning calorimetry (DSC) was performed to determine the Tg and thermal behaviour of the glass taking finely grinded powder of the composite KB-4 and heating it at the rate of 10 K/min from 30-900  C. The DSC thermogram has been shown in Fig. 1(a). Tg was found at about 290  C. Glass transition temperature (Tg) and dilatometric softening point (Td) of the nanocomposite KB-4 were measured by dilatometric measurement taking a cylindrical sample of approximately 25 mm length and 5 mm diameter and heating it at a rate of 4 K/ min up to the temperature where the glass softens. Fig. 1(b) shows the dilatometric curve of the glass. Tg and Td was found at around 295 and 315  C respectively. The Tg determined by dilatometry and DSC are quite close. The small difference arises due to the difference in heating rate and physical state of the sample. 3.2. Thermochemical reduction reaction

2.2. Characterizations The X-ray diffraction (XRD) patterns of the bulk samples were recorded in an X'pert Pro MPD diffractometer (PANalytical, Almelo, the Netherlands), operating at 40 kV and 30 mA, using Nifiltered CuKa radiation with the X'celerator, with a step size of 0.05 (2q) and a step time of 0.5s, from 10 to 80 .Transmission electron microscopy (TEM) images and selected area electrondiffraction (SAED) patterns of the powdered glass-ceramics were obtained using an FEI (Model Tec-nai G230ST; FEI Company, Hillsboro, OR) instrument. Samples for TEM measurement were prepared by dispersing finely powdered sample in acetone, followed by an ultrasonic agitation, and then its deposition on the carbon-coated copper grid. The UVeVis absorption spectra and the PL spectra of the all the samples were studied using polished 2 mm thick samples with the help of fiber optic spectrometer (AvaSpec-3648-USB2, Avantes) and diode lasers source of wavelength 447 nm. Differential scanning calorimetric experiment was performed by a differential scanning calorimeter (NETZSCH Model €tebau GmbH, Selb, Germany) STA 449 Jupiter F3, NETZSCH-Gera taking powdered sample within the temperature range of 30o600  C in nitrogen atmosphere at the heating rate of 10 K/min. The coefficient of thermal expansion (CTE, a), glass transition temperature (Tg), and dilatometric deformation temperature (Td) was measured using a horizontal vitreous silica dilatometer (DIL 402C, Netzch-Ger€ atebau GmbH, Bavaria, Germany) with a heating rate of 4 K/min taking a cylindrical sample of approximately 25 mm length and 5 mm diameter and heating it at a rate of 4 K/min up to the temperature where the glass softens, after calibration with a standard alumina supplied with the instrument by the manufacturer.

KB-1 KB-2 KB-3 KB-4 KB-5 KB-6 KB-7

Composition (wt %) Pr2O3

Au

e 0.3 e 0.3 0.3 0.3 0.3

e e 0.004 0.004 0.006 0.008 0.010

Color

Form

Yellow Yellow Bluish Green Green Green Green Green

Transparent Transparent Transparent Transparent Transparent Transparent Transparent

Sb5þ/Sb3þ, E ¼ 0.649 V

(1)

Au3þ/Au0, E ¼ 1.498 V

(2)

Pr3þ/Pr0, E ¼ 2.353 V

(3)

Zn2þ/Zn0, E ¼ 0.761 V

(4)

Thus, Sb3þ is expected to reduce Au3þ to Au0, while it itself is oxidized to Sb5þ. Besides Sb3þ has an inherent tendency to get oxidized to Sb5þ. Hence, the overall reaction 3Sb3þ þ 2Au3þ / 3Sb5þ þ 2Au0

(5)

has an E0 ¼ 1.05 V which implies a spontaneous reduction reaction having a free energy (DG value) around 608 kJ. The thermochemical reactions 3Sb3þ þ 2Pr3þ / 3Sb5þ þ 2Pr0,

(6)

Sb3þ þ Zn2þ / Sb5þ þ Zn0

(7)

would have E0 values 6.65 and 1.41 V respectively (DG is positive) manifesting that these reactions are non-spontaneous and thermodynamically not feasible. Besides 3þ being the highest and the most stable oxidation state of Pr and Pr3þ/Pr2þ has E0 ¼ 3.1 V, Pr3þ would have the least tendency to undergo reduction to lower oxidation states (Pr0 or Pr2þ). Thus,Pr3þ does not reduce whereas ZnO participates in glasseceramic network formation [29,37].

Table 1 Composition and some physical properties of the nanocomposites. Sample ID

A probable mechanism of selective chemical reduction of Au3þ to Au0 by Sb3þ can be explained by considering the reduction potentials (Eo) of the respective redox systems. As the standard potential values for antimony glasses at high temperature are unavailable in literature, so we have used here the room temperature standard potential for simples systems at equilibrium with air, although the high temperature E values do not vary so much compared to room temperature ones.

monolith monolith monolith monolith monolith monolith monolith

3.3. X-ray diffraction Fig. 2 shows the X-ray diffractrograms of the as-prepared (AP) samples. KB-1 and KB-2 exhibits fully amorphous nature having no prominent peak. For the rest samples, peaks have been found and

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Fig. 1. (a) Differential scanning calorimetry and (b) dilatometric thermogram of glass sample KB-4.

identified with corresponding JCPDS file number. The XRD pattern of pure Au has also been shown for comparison. From the figure it is quite clear that the peaks are getting sharper as we go from KB-3 to KB-7. KB-7 has the most prominent set of peaks at 2q ¼ 20.0 , 33.4 , 34.9 and 44.7. They are due to the diffractions from (222), (311), (222) and (331) planes of cubic crystalline phases of Au (JCPDS file no. 021095). The difference between the diffraction patterns of KB1, KB-2 and the other samples rises due to their difference in composition. KB-1 and KB-2 do not contain any nanometal while from KB-3 to KB-7, they contain gradually increasing amount of nano Au (0.004e0.01 wt%). That is why from KB-3 to KB-7, the peaks are getting sharper. The average crystallite diameter (d) was calculated using Scherrer's formula from the XRD peaks.

d ¼ 0:9l = FWHM cosq ðpeakÞ

(8)

where l is the wavelength of X-ray radiation (CuKa ¼ 1.5406 Å), FWHM is the full width at half maximum at 2q. The crystallite size of Au NPs, as calculated from the peaks of (200) and (311) planes,

Fig. 2. X-ray diffraction pattern of as-prepared glass samples.

varies in the range 5e15 nm from KB-3 to KB-7. The Size of NPs increases from KB-3 to KB-7 due to the increased concentration of Au. 3.4. Transmission electron microscopy Fig. 3 is a representative TEM micrograph of an as-prepared nanocomposite KB-6, it shows the magnified view of the nanoparticles embedded in the glass matrix. In the figure, several tiny particles are clearly visible, the size of which is in the range 3e10 nm. As confirmed from the XRD result, the sample contains nanocrystalline phase of Au, these nanoparticles are definitely Au NPs distributed almost homogeneously within the glass matrix. The most of the NPs are not perfectly spherical in shape. They are mostly elliptical. Fig. 4(a) shows a high resolution TEM (HRTEM) image of the same sample. Several planes having different spacings and orientations indicate the polycrystalline nature of the crystalline phases. The inter-plane distances have been measured and compared with d-spacing values of JCPDS files mentioned in XRD analysis. The measured d-values and corresponding assignment of peaks have been mentioned in the figure. Fig. 4(b) shows the SAED pattern of the same sample. Multiple rings imply the

Fig. 3. TEM micrograph of sample KB-6.

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Fig. 4. (a) HRTEM image and (b) SAED pattern of glass sample KB-6.

polycrystalline nature of the samples. The planes have been identified which are responsible for the diffraction pattern. The (hkl) index and corresponding JCPDS file numbers have also been mentioned in the figure. Therefore both HRTEM and SAED pattern supports the XRD results and confirms the presence of Au nanocrystals in the glass matrix of the nanocomposites. 3.5. UVeVis absorption Fig. 5 shows the UVeVis absorption spectra of the as prepared samples. From the figure it is clear that all the samples are transparent in the visible and NIR range. The samples containing Pr3þ, i.e. KB-2, KB-4, KB-5, KB-6 and KB-7, show absorption bands at around 445 nm and 481 nm which are characteristic absorption bands for Pr3þ, they appear due to the spectral transition from 3H4 to 3P1and 3H4 to 3P0 level of Pr3þ ion, respectively [20]. Samples containing Au, i.e. KB-3 to KB-7, show an absorption band around 620 nm. Samples containing both Pr3þ and Au, i.e. KB-4 to KB-7, show all the bands (at 445, 481 and 620 nm). KB-1 has neither Pr3þ nor Au in its composition. That is why it does not show any band. Coinage metal (e.g. Ag, Au, Cu etc) NPs exhibit a potential resonant absorption and scattering of the electromagnetic light wave in the visible region which is the result of the collective

coherent oscillation of the conduction electron gas of metal NPs. This is called local surface plasmon resonance (LSPR). The absorption spectra of the Au0-doped samples display well defined LSPR bands (Lorentzian curves) characteristic of nano sized Au0. Sodalime silicate glasses with a RI around 1.5 features plasmon peak of Au0 nanoparticles around 525 nm [38]. It is known that higher refractive index (RI) of matrix shifts the SPR band towards longer wavelength [27]. Our KBSZ antimony glass have a RI around 1.915 [31], hence the Au0 plasmon peak has been shifted to around 620 nm. The SPR band gradually broadened and their tails have extended up to 1100 nm. These broadening and tailing of the SPR band are due to decrease in Au0eAu0 inter-particle distance with increasing concentration of Au0 [39]. Their experimental observations corroborate well with the predictions of Maxwelle Garnett (MG) theory [29,40]. This is known that peak position as well as the width of absorption band depends upon the size and shape of ingrained particles. With increasing heat treatment time/doping level, two different phenomena can take place: enlargement of existing particles (spherical as well as non spherical) and/or, formation of new particles (spherical as well as non spherical) of different size. Now, both of these mechanisms may give rise to the appearance of several peaks (which mainly represent non-spherical particles, on both sides of the spherical peak ~620 nm). Hence, the overall absorption band, which can just be thought as the superposition of all these peaks, can broaden asymmetrically with increasing doping level [25]. Due to increasing doping level, different NPs tend to aggregate. When such an aggregate is irradiated by electromagnetic light wave, the inter particle interaction is considerable, as the NPs in the aggregate can undergo relative motion to attain new equilibrium states. This may lead to appearance of multiple peaks in the absorption spectra shifted on both sides from the single particle SPR peak. Higher the doping concentration greater would be the number of NP members in those aggregates, and thus, an increase in intensity of those multiple peaks takes place in an asymmetric manner, owing to the asymmetric broadening of the overall absorption spectrum [41]. 3.6. Photoluminescence (PL) spectra

Fig. 5. UVeVis absorption spectra of the as prepared samples showing the characteristic absorption band of Pr3þ and SPR band for nano Au.

The glasses KB-2 and KB-4 to KB-7 have photoluminescent properties. They exhibit emission in visible range when excited with 447 nm diode laser source. Fig. 6(a) and (b) show the color coordinates of the light emitted by KB-2 and KB-6 respectively,

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when excited at 447 nm. From the figure it is clear that KB-2 gives orange and KB-6 gives red emission having chromaticity coordinates (x ¼ 0.602, y ¼ 0.357) and (x ¼ 0.527, y ¼ 0.301) respectively. The visual appearance is shown in the insets of the figures, which exactly match with their corresponding color coordinates. The difference in emitted color of KB-2 and the other samples can be explained by the difference in the ratio of intensity of the major bands. Fig. 7 shows the PL spectra of the mentioned samples. They exhibit prominent emission bands at 488, 530, 604, 650, 705 and 733 nm. The band at 604 nm is the major one, which is in the orange region of the visible spectrum. Magnified views of the major bands have been shown in the insets of the figure. Here the bands at 488 and 604 nm are the most prominent emission bands. Therefore it can be assumed that intensity ratio of these two bands decides the color of emission. Table 2 shows the intensities of these two bands (I488 and I604) for all the photoluminescent samples. The ratios of intensity (I604/I488) of KB-4 to KB-7 are very close, almost in similar range, while that of KB-2 is nearly twice than the others. That is why KB-2 emits visibly different color (orange) from the rest samples (pink). This difference in intensity of emission bands, which gives rise to difference in emitted colors, is due to the presence of Au NPs. Presence of elliptical NPs distributed among the glass matrix significantly influences the nature and intensity of the emitted light from the samples. It not only enhances the intensity of the emission bands, but also alters their intensity distribution. The color coordinates of P and Q in Fig. 6 are the visible evidence of this phenomenon. Fig. 8 is the partial energy level diagram of Pr3þ ion and Au NPs showing the energy transfermechanisms for the major bands in the downconversion spectra. The bands at 488, 530, 604, 650, 705 and 733 nm corresponds to the transitions 3P1 / 3H4, 3P1 / 3H5, 3 P0 / 3H6, 3P0 / 3F2, 3P0 / 3F3, and 3P0 / 3F4 respectively. It is clear from the insets of Fig. 7 that there is a considerable enhancement in PL intensity from KB-2 to other nanocomposites and the extent of enhancement varies with the amount of Au present in the sample. Fig. 9 shows the variation of PL intensity of the samples with respect to Au concentration, at different band positions. The plot clearly shows that there is a considerable enhancement in PL intensity from KB-2 to KB-4. This enhancement is attributed to the presence of Au NPs. KB-2 and KB-4 has the exactly same amount of RE ions and has been prepared at similar

Fig. 7. Photoluminescence spectra (excited at 447 nm) of samples KB-2, KB-4, KB-5, KB-6 and KB-7. The insets show the magnified views of the major emission bands.

Table 2 Intensity ratio of the major emission bands of the photoluminescent samples. Sample

I488

I604

I604/I488

KB-2 KB-4 KB-5 KB-6 KB-7

8497 21195 25715 28620 23132

31812 41823 51628 53383 42969

3.74 1.97 2.01 1.87 1.86

conditions. So the difference in PL intensity can only arise from the NM content of KB-4. Such PL intensification of the Pr3þ ion emission is primarily due to the local field enhancement (LFE) around the RE ions sites induced by SPR of Au NPs [30]. The plasmonic Au NPs concentrates the incident electromagnetic field creating an additional field in the sub wavelength structures around the Pr3þ ion sites and subsequently increase their rate of excitation (by ‘‘Lightning Rod Effect’’). The secondary reason for enhanced luminescence is that when the Pr3þ is present in close vicinity to the metal surface, the weak photoluminescence emissions from the Au NPs is

Fig. 6. Chromaticity diagram corresponding to the light emitted from sample (a) KB-2 and (b) KB-6 when excited at 447 nm with a diode laser source. The point “P” and “Q” represents chromaticity coordinates (x ¼ 0.602, y ¼ 0.357) and (x ¼ 0.527, y ¼ 0.301) respectively of the emitted light. The insets show the photoluminescence photographs of (a) KB-2 and (b) KB-6respectively, when excited at 447 nm (See Table 1 for composition).

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lower than the value of the mean free path. When the size of the NPs exceeds the value of mean free path, decrease in the PL intensity is observed due to radiative damping effect [43,44]. 3.7. Heat treatment

Fig. 8. Partial energy level diagram of Pr3þ ion showing transitions of major emission bands and local field enhancement (LFE) by Au NPs.

Fig. 9. Variation of PL intensity as a function of Au concentration, for different PL bands.

added as a second channel of excitation energy, that is, energy transfer (ET) from Au0 to Pr3þ [25,29]. Thus, the Au NPs increases the photonic density around the Pr3þ ions situated in near vicinity and thereby increase the number of photons captured by the Pr3þ ions. As a result of superior excitation rate the population of the excited state of the Pr3þ ions increases and subsequently the rate of radiative decay increases. From KB-4 onwards the PL intensity gradually increases up to KB-6, then decreases in KB-7. This can be explained by the effect of NPs on PL emission properties of RE ions. With increasing concentration of Au from KB-4 to KB-7, both the size and number of NPs increase. The strong local electric field in the Au NPs arising from the SPR excitations which leads to the enhancement of the electric fields of the incoming (exciting) and outgoing (emitting) photons by the excited NPs, is responsible for this increment in PL intensity [5,41]. But PL intensity cannot increase continuously with increasing size of the NPs. Beyond certain concentration of Au, PL intensity gradually decreases. When the size of NPs are too small as compared to the mean free path of the conduction electrons inside the particles, the collision of the conduction electrons with the surface of the NPs causes damping of the SPR excitations and PL intensity remains low. With increasing concentration the particle size increases and consequently the damping effect decreases. As result the PL intensity increases gradually until the size of the NPs is

From the emission spectra of the nanocomposites it is obtained that the nanocomposite KB-6 having Au concentration 0.008 wt% has the highest PL intensity among all the samples, when excited at 447 nm. Now in order to investigate the effect of heat treatment on the Au- Pr3þ co-doped glasses, KB-6 was taken as the representative of the nanocomposites for having the highest emission. Heat treatment has been performed on KB-6 along with KB-1, KB-2, and KB-3 for comparison. The temperature for heat treatment was decided as 300  C based on the results of dilatometric and DSC thermograms. The samples were heat treated for different durations- 15 min, 30 min, 45 min, 60 min, 90 min, 150 min, 210 min, 270 min, 330 min, 480 min, 660 min and 840min. The heat treated samples were characterized by XRD, UVeVis absorption and PL spectra. 3.7.1. XRD Fig. 10 shows the X-ray diffraction patterns of KB-6 heat treated at 300  C for different durations. The XRD patterns are almost unchanged from as prepared (AP) up to 270 min of heat treatment. Up to that limit the sample only shows the peaks for Au nanometal as assigned in Fig. 2. No other distinguishable peaks arise. But from 480 min the patterns exhibit a massive change showing huge peaks at 2q ¼ 13.7, 25.6 , 27.5 , 28.3 , 33.9 , 36.6 , 46.0 , 47.0 positions and the already existing peaks for nano Au particles also get intensified. The new peaks belong to crystallized phase of Sb2O3, which is present as the major component in the nanocomposite. All the major peaks have been assigned with their corresponding JCPDS file numbers (Fig. 10). At 840 min all the peaks get sharper and more prominent. This result indicates that the initially the composite has glassy matrix with Au nanoparticles embedded within it, with increasing heat treatment at temperature near the glass transition temperature of the glass, the glass matrix begins to crystallize. Up to 270 min, the matrix remains almost amorphous, but after prolonged exposure to heat the matrix gets well crystallized and with further increase the extent of crystallization gradually increases. Not only the glass matrix, but also the peaks of Nano Au also become sharper with increasing heat treatment duration. This indicates increase of both size and numbers of Au

Fig. 10. XRD patterns of sample KB-6 heat treated at 300  C for different durations.

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nanoparticles. Size of the crystallites has been calculated using Scherrer's formula as mentioned in equation (8). The size of Au NPs has been found to increase with increasing heat treatment duration. The size of Au NPs in the heat treated samples ranges between 20 and 35 nm. The crystallite size of Sb2O3nanocrystals has also been calculated. Their size varies in the range 30e80 nm. Crystallite size increases with increasing heat treatment duration. 3.7.2. UVeVis absorption spectra Fig. 11 (a) and (b) are the UVeVis absorption spectra of KB-3 and KB-6 respectively heat treated at 300  C for different durations. From the figures it is clear that the intensity of the plasmon band of Au at around 620 nm gradually increases with increasing heat treatment durations for both the glasses. This happens due to the generation of new Au NPs as well as the increase in size of the already generated NPs. With increasing heat treatment duration the extent of reduction of Au3þ to Au0as per the redox reaction mentioned in Equation (5) gets amplified producing more numbers of Au NPs. The newly generated NPs get themselves spread over the glassy matrix and act as potential centre for both scattering and surface plasmon resonance. Increased number of nanoparticles causing SPR automatically increases the absorbance bandwidth at around the characteristic SPR band position of Au. It is also evident from Fig. 11(b) that with progressive heat treatment the absorbance also increases for the 445 nm and 481 nm bands, which are the characteristic absorption bands for Pr3þ originating from the spectral transition from 3H4 to 3P1 level and 3H4 to 3P0 level of Pr3þ ion respectively. This is again the effect of the in-situ generation of Au NPs into the glass matrix. As the number of NPs increases the extent of absorption by the RE ions increases due to the LSPR effect exerted by the nanometals. This enhanced absorption leads to enhancement of the emission intensity of the heat treated nanocomposites which will be reflected in their photoluminescence spectra. Here also we see asymmetric broadening of absorption spectra for both KB-3 and KB-6. This is due to the changed size distribution and aggregation of Au NPs with increased heat treatment duration [41,42].

3.7.3. Photoluminescence spectra Fig. 12 shows the variation of PL intensity of KB-6 with heat treatment at 300  C for different durations, excited by 447 nm laser diode. The heat treated samples exhibit the emission bands at the same positions as the as-prepared sample giving the same emitted color also, but the intensity varies with duration of heat treatment. If we plot the intensity of the most prominent band (i.e. at 604 nm wavelength) as a function of heat treatment duration, it will give a plot like Fig. 13. From this figure an interesting trend is noticed showing two maxima one at 45 min and another is at 480 min duration. The second one is even more intense than the former one. This peculiar variation can be explained considering two influencing factors, first, the generation of Au NPs and second, crystallization of the glass matrix. From beginning to 45 min heat treatment duration, the PL intensity shows a sharp gradual increase followed by a gradual

Fig. 12. Photoluminescence spectra of sample KB-6 heat treated at 300  C for different durations, (excited at 447 nm). The inset shows the magnified view of the band at l ¼ 604 nm.

Fig. 11. UVeVis absorption spectra of samples (a) KB-3, (b) KB-6 heat treated at 300  C for different durations, showing the intensification of plasmon band of Au.

N. Shasmal, B. Karmakar / Journal of Alloys and Compounds 688 (2016) 313e322

Fig. 13. Variation of PL intensity of sample KB-6 as a function of heat treatment duration.

decrease up to certain duration. The decisive factor of this variation is the in-situ generation of Au NPs through thermochemical reduction at elevated temperature. With increasing duration both the number and size of the Au NPs gradually increase leading to increased emission intensities. During this period new NPs are generated and they get distributed all over the glass matrix. The newly generated NPs enhance the absorbance and hence lead to increased emission. The increase in size of the NPs also broadens the absorption band resulting in greater PL intensity. At 45 min duration the number, size and distribution of NPs achieves an optimum level where the NPs reach the critical size for which it exhibits the maximum PL intensity. After that the size of NPs surpasses the critical size and PL intensity decreases due to radiative damping effect. The further increase after 330 min is due to the effect of matrix crystallization. As confirmed from XRD patterns, the glass matrix starts to crystallize at 480 min heat treatment duration. Initially very small crystallites are formed having size in the range of nanometers. Formation of these nanocrystals lowers down the phonon energy of the glass system. From the peak assignment of the XRD plot we get sharp peaks of Sb2O3 in major amount. The glass containing these Sb2O3nanocrystals within its matrix has phonon energy as low as 600 cm1. Low phonon systems enhances the emission intensity of the RE ions by lowering the non-radiative loss. This lowering of phonon energy overweighs the size factor of NPs up to this particular limit. That is why the PL intensity reaches at its maximum at 480 min heat treatment duration. After that the size factor of NPs again predominates over the crystallization effect because due to the prolonged heat treatment duration the size of NPs becomes too big to enhance the emission intensity. As result it exhibits a slow rate of decrease harmonizing both the influencing factors [5]. 4. Conclusions Antimony oxide borate zinc oxide (KBSZ) glass having base glass composition 15K2O-15B2O3-60Sb2O3-10ZnO was prepared and doped with Pr3þ. Au NPs have been generated into the glass by electrochemical reduction method. The glasses were heat treated at 300  C for different durations. The as-prepared and heat treated samples were characterized. The following points can be concluded from this study.

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 Both DSC and dilatometric study shows the nanocomposite (KB4) has Tg around 290 -295  C.  The electrochemical reduction reaction, used here for in-situ generation of Au NPs, is a spontaneous reaction having negative DG value.  XRD shows that the nanocomposites KB-3 to KB-7 has Au NPs embedded in their glassy matrix.  TEM, HRTEM and SAED confirm the presence of Au NPs in the nanocomposites, which supports the XRD results.  UVeVis absorption spectra of the as-prepared samples show the characteristic absorption bands for Pr3þas well as the characteristic SPR band of Au at around 620 nm.  When excited at 447 nm, all the Pr3þ-containing samples give prominent emission bands at 448, 530, 604, 650, 705 and 733 nm. The band at 604 nm is the major one, which is in the orange region of the visible spectrum.  Among the nanocomposites prepared, KB-6 (containing 0.008 wt% Au) gives the maximum PL intensity. KB-6 is heat treated at 300  C at different durations. The heat treated samples were characterized by XRD, UVeVis absorption and PL spectra.  XRD shows that the sample gets crystallized at around 480 min heat treatment duration. Sharp peaks of Sb2O3 appear along with the existing peaks of Au.  UVeVis absorption spectra show the enhanced absorbance of the heat treated samples. The SPR band of Au gets more and more widespread with increasing heat treatment duration which is due to rapid rate of Au NP generation by thermochemical reduction at elevated temperature.  Variation of PL intensity with heat treatment duration exhibits two maxima (at 45 min and 480 min durations). The first one is due to the attainment of the critical size of Au NPs, for which the emission becomes maximum. And the second one is the effect of matrix crystallization, which lowers down the phonon energy of the system and enhances PL intensity by reducing the nonradiative loss. Acknowledgements The authors are thankful to Dr. K Muraleedharan, Director of the institute and Dr. Ranjan Sen, Head, Glass Division for their encouragement and support. The authors acknowledge the technical supports provided by the X-ray and electron microscopy section of the institute. NS would like to express her sincere gratitude for the financial support of the Council of Scientific and Industrial Research, New Delhi in the form of SRF under sanction number 31/15(128)/2015-EMR-1. Partial financial support under CSIR-TAPSUN Project NWP0055 is also gratefully acknowledged. References [1] K. P atek, Glass Lasers, Butterworth, London, 1970. [2] M. Fukushima, N. Managaki, M. Fujii, H. Yanagi, S. Hayashi, Enhancement of 1.54-mm emission from Er-doped sol-gel SiO2 films by Au nanoparticles doping, J. Appl. Phys. 98 (2005) 0243161e0243164. [3] T. Som, B. Karmakar, Nanosilver enhanced upconversion fluorescence of erbium ions in Er3þ: Ag-antimony glass nanocomposites, J. Appl. Phys. 105 (2009) 0131021e0131028. [4] P.N. Prasad, Nanophotonics, Wiley, New Jersey, 2004, pp. 129e151. [5] N. Shasmal, K. Pradeep, B. Karmakar, Enhanced photoluminescence up and downconversions of Sm3þ ions by Ag nanoparticles in chloroborosilicate glass nanocomposites, RSC Adv. 5 (2015) 81123e81133. [6] Y. Fujimoto, O. Ishii, M. Yamazaki, Multi-colour laser oscillation in Pr3þ-doped fluoro-aluminate glass fibre pumped by 442.6 nm GaN-semiconductor laser, Electron. Lett. 45 (2009) 1301e1302. [7] D.R. Simons, A.J. Faber, H. de Waal, GeSx glass for Pr3þ doped fiber amplifiers at 1.3 mm, J. Non-Cryst. Solids 185 (1995) 283e288. [8] Y. Ohishi, A. Mori, T. Kanamori, K. Fujiura, S. Sudo, Fabrication of

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