Au co-doped antimony oxide glass nanocomposites

Journal of Non-Crystalline Solids 463 (2017) 40–49

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Tuneable and Au-enhanced yellow emission in Dy3 +/Au co-doped antimony oxide glass nanocomposites Nilanjana Shasmal a,b, Basudeb Karmakar a,⁎ a b

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 Academy of Scientific and Innovative Research, Chennai 600113, India

a r t i c l e

i n f o

Article history: Received 9 January 2017 Received in revised form 24 February 2017 Accepted 25 February 2017 Available online xxxx Keywords: Amorphous materials Nanogold SPR band Metal enhanced photoluminescence Optical spectroscopy Luminescence

a b s t r a c t Dy3+/Au co-doped antimony oxide glass nanocomposites were prepared in a glass matrix, having molar composition of 15K2O-15B2O3-60Sb2O3-10ZnO by single-step melt-quenching technique. Au nanoparticles (NPs) are generated in-situ by thermochemical reduction of Sb3+. X-ray Diffraction (XRD) and Transmission Electron Microscopy confirm the presence of Au NPs in the samples. UV–Visible absorption spectra show the surface plasmon resonance (SPR) band of Au around 620 nm. When excited at 447 nm, all the nanocomposites exhibit visible yellow emissions yielding maximum with the sample containing 0.006 wt% Au. This sample is heat treated at 300 °C for different durations. XRD reveals that crystallization of the glassy matrix initiates after 5 h heat treatment. The SPR band of Au gets progressively widespread with increasing heat treatment duration due to rapid rate of Au NP generation at elevated temperature. Variation of photoluminescence (PL) intensity with heat treatment duration exhibits two maxima. The first one is due to attainment of critical size of NPs, for which emission becomes maximum. The second one is the effect of matrix crystallization, which reduces the phonon energy of the system and enhances PL intensity by reducing non-radiative loss. This enhancement and tuning of photoluminescence is very promising for various photonic applications. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Rare-earth (RE) elements are of significant interest in several hightech application areas, the major ones concerning magnetic and optical devices. The unique photoluminescence properties of RE ions can be utilized to develop novel and advanced lasers and optical amplifiers. The 4f–4f electronic transitions of rare-earth ions play an important role in the optical applications [1–5]. Glasses doped with rare-earth ions have been extensively investigated due to their laser action in visible and near infrared (NIR) regions [6]. In general, host glasses with low phonon energy provide less non-radiative relaxation rates and high quantum efficiencies. The introduction of heavy metal compounds such as lead oxide, lead fluoride, bismuth oxide, in conventional glasses like silicate, borate, leads to more efficient laser systems as their presence improves the effective fluorescence [7]. The addition of heavy metal oxides (HMO) decreases the host phonon energy and thereby suppresses the non-radiative losses [8,9]. Though different spectroscopic characterization have been extensively studied by the modification of chemical composition to improve the performance of laser hosts, still there is a demand for new host materials with even higher efficiency [6].

⁎ Corresponding author. E-mail address: [email protected] (B. Karmakar).

http://dx.doi.org/10.1016/j.jnoncrysol.2017.02.019 0022-3093/© 2017 Elsevier B.V. All rights reserved.

Among the heavy metal oxide (HMO) glasses, antimony oxide systems remain very less explored. The main reason is the difficulty to obtain vitreous Sb2O3 due to its low field strength (0.73), which makes it a poor glass former [Field strength=Zc/a2, where Zc = valence of the cation and a = inter-ionic distance in Å. For glass formers field strength of the glass forming cation should be high (N 1.5)] [10]. Antimony oxidebased glasses are recommended for technological applications such as optical recording media and nonlinear optical devices due to their photosensitivity, non-centro-symmetric structure, and fast response times which are the consequences of the highly polarizable Sb3+ 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 [10,11]. Earlier reports on preparation of high Sb2O3 containing glasses show they are yielded in tiny pieces or pulverized form [10,12]. Recently Monolithic antimony glasses and nanoglass-ceramics in the ternary K2O–B2O3–Sb2O3 system (KBS) and quaternary K2O–B2O3–Sb2O3–ZnO system (KBSZ) was first reported by Som et al. [11,13]. Sm3 +, Er3 +, Nd3 + doped KBS glasses have been widely studied, which reveals their remarkable upconversion luminescence properties suitable for application in various photonic devices. Sb2O3 being a heavy metal oxide, Sb2O3 glasses have lower phonon

N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

energy (around 600 cm−1) which would increase the upconversion and quantum efficiency of luminescent RE ions [14–16]. But, the most interesting aspect of antimony oxide-based glass–ceramics over conventional systems is that Sb2O3 is a mild reducing agent (reduction potential, Eo = 0.649 V) [17]. This mild reduction property enables in-situ reduction of Au3+ (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 [18]. The authors selected the antimony oxide glass host to study the optical properties of trivalent rare-earths to meet the needs of present photonic devices because of the reasons stated above. Among the RE ions, trivalent dysprosium (Dy3 +)-doped glasses have been considered as promising materials for two-color phosphors and white light emission. In the visible region, the Dy3+ ion emit intense yellow (570–600 nm) and blue (470–500 nm) luminescence corresponding to the 4F9/2 → 6H13/2 and 4F9/2 → 6H15/2 transitions, respectively along with a feeble-red luminescence (650–670 nm) due to the 4F9/2 → 6H11/2 transition. Thus, the Dy3+-doped several glasses have been studied to obtain two primary color luminescent materials as well as white light-emitting material. The intensities of hypersensitive (4F9/2 → 6H13/2) and non-hypersensitive (4F9/2 → 6H15/2) transitions depend on the coordination environment of the host material and the yellow-to-blue luminescence intensity ratio (Y/B) can be modulated by varying the composition of host matrix as well as heat treatment [6]. The luminescence properties of high silicate glass doped with Dy3+-ion in various concentrations were studied [19]. The optical properties of Dy3+ doped in the PbO-PbF2 oxyfluoride glass matrix were reported [20]. Study of optical properties of antimony-stabilized sulfide glasses doped with Dy3 + revealed that the 1.3 μm emission of Dy3 + could be observed because of the phonon energy of these glasses [21]. Spectral study of Dy3 +-doped borate glasses modified with lithium oxide and fluoride are comparable to oxide, fluoride and sulfide glass hosts [22]. Absorption and photoluminescence spectra of Dy3+ doped alkali-fluoroborate glasses of the composition B2O3-AlF3-MF-DyF3 (M = Li, Na, K) have been reported [23]. Photoluminescence study of Dy3+-doped SiO2–Al2O3–BaF2–GdF3 glasses, a bright fluorescent yellow emission at 575 nm and blue emission at 484 nm have been observed [24]. The optical properties of Dy3 + ions in B2O3–PbO–Al2O3–WO3 glasses have been examined [25]. Optical properties of Dy3 +-doped phosphate and fluorophosphates glasses have been investigated [26]. Spectral investigations on Dy3+-doped transparent oxyfluoride glasses and nanocrystalline glass ceramics revealed that the samples emit intense white light when populating the 4F9/2 level with a 451 nm laser light [27]. Calcium fluoroborate glasses doped with different concentrations of trivalent dysprosium ions were prepared and investigated [28]. Optical properties of Dy3 +-doped sodium–aluminum–phosphate glasses were reported [29]. Spectroscopic and photoluminescence properties of Dy3 +-doped lead tungsten tellurite glasses were also studied [30]. Optical properties of various other glasses like lead fluorophosphates [31], lead telluroborate [6], oxyfluoride silicate glasses [32] were studied after Dy3 +-doping. Dy3 +-doped antimony oxide based glass has not been reported so far. Metal–dielectric (glass) nanocomposites represent a class of materials having both scientific and technological interests over several years due to their extraordinary 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 [33–36]. Surface plasmon resonance is an exclusive 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 [37]. SPR is intimately dependent on the nanoparticle's size, shape, refractive index of the dielectric environment and other proximal nanoparticles (NPs) [38–40]. Specific tuning of the SPR peak across a wide spectroscopic range can be achieved by varying any of the parameters. Besides absorption and

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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 [18,41]. Recently it has been reported that enhancement and tuning of photoluminescence properties can be achieved by thermal treatment in Pr3+/Au co-doped antimony oxide glass nanocomposites [41]. Therefore it will be worthwhile to investigate the optical properties of Dy3+ and Au co-doped antimony glass, as it has not been reported so far and it has enormous possibility in photonic applications. In this paper we have reported for the first time the study of Au:Dy3+ co-embedded hybrid antimony glass nanocomposites. The nanocomposites are characterized by Differential scanning calorimetry (DSC), X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), High resolution TEM and selected area electron diffraction (SAED) analysis, UV–visible absorption spectroscopy and photoluminescence spectroscopy. Considerable enhancements have been found in down-converted 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 behaviors 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 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 Dy2O3 (99.9%, Alfa Aesar) and Au0 co-doped 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.

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

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

Composition (wt%) Dy2O3

Au

– 0.3 – 0.3 0.3 0.3 0.3

– – 0.004 0.004 0.006 0.008 0.010

Color

Form

Yellow Yellow Bluish green Yellow Yellow Yellow Yellow

Transparent monolith Transparent monolith Transparent monolith Transparent monolith Transparent monolith Transparent monolith Transparent monolith

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N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

2.2. Characterizations Differential scanning calorimetric experiment was performed by a differential scanning calorimeter (NETZSCH Model STA 449 Jupiter F3, NETZSCH-Gerätebau GmbH, Selb, Germany) taking powdered sample within the temperature range of 30°–600 °C in nitrogen atmosphere at the heating rate of 10 K/min. 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 Ni-filtered CuKα radiation with the X'celerator, with a step size of 0.05°(2θ) and a step time of 0.5 s, from 10° to 80°. The equipment has a resolution of ± 1 cm− 1. Transmission electron microscopy (TEM) images and selected area electron-diffraction (SAED) patterns of the powdered glass–ceramics were obtained using an FEI (Model Tecnai G230ST; FEI Company, Hillsboro, OR) instrument. Samples for TEM measurement were prepared by dispersing finely powdered glass–ceramics in acetone, followed by an ultrasonic agitation, and then its deposition on the carbon-coated copper grid. The averaged data of 256 scans were taken for analysis. Field-emission scanning electron microscopy (FESEM) of high-resolution (Gemini Zeiss SupraTM 35 VP model of Carl Zeiss Microimaging GmbH, Berlin, Germany) was employed to examine the microstructure of the heat-treated glass– ceramics after etching in HF solution. The UV–Vis 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. Photoluminescence spectrophotometer (model: custom made Quantum-Master: Enhanced NIR of Photon Technologies International, Canada) was employed for recording of excitation spectra. 3. Results and discussion 3.1. Differential scanning calorimetry Differential scanning calorimetry (DSC) was performed to determine the Tg and thermal behavior of the glass taking finely grinded powder of the composite KBD-4 and heating it at the rate of 10 K/min from 30 to 900 °C. The DSC thermogram has been shown in Fig. 1(a). Tg was found at about 290 °C. The onset of crystallization temperature (To) was found to be around 445 °C. 3.2. Thermochemical reduction reactions

(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 Eo values do not vary so much compared to room temperature ones. 5þ

3þ

=Sb ; Eo ¼ 0:649 V

ð1Þ

Au3þ =Au0 ; Eo ¼ 1:498 V

ð2Þ

Dy3þ =Dy0 ; Eo ¼ −2:29 V

ð3Þ

Zn2þ =Zn0 ; Eo ¼ −0:761 V

ð4Þ

Sb

Thus, Sb3+ is expected to reduce Au3+ to Au0, while it itself is oxidized to Sb5+. Besides Sb3+ has an inherent tendency to get oxidized to Sb5+. Hence, the overall reaction 3Sb

3þ

5þ

þ 2Au3þ →3Sb

þ 2Au0

has an E0 = 0.849 V which implies a spontaneous reduction reaction having a free energy (ΔG value) around −608 kJ. The thermochemical reactions 3Sb

3þ

3þ

Sb

5þ

þ 2Dy3þ →3Sb 5þ

þ Zn2þ →Sb

þ 2Dy0 ;

þ Zn0

ð6Þ ð7Þ

would have E0 values −2.939 V and −1.41 V respectively (ΔG is positive) manifesting that these reactions are non-spontaneous and thermodynamically not feasible. Besides 3 + being the highest and the most stable oxidation state of Dy and Dy3+/Dy2+ has E0 = − 2.15 V, Dy3+ would have the least tendency to undergo reduction to lower oxidation states (Dy0 or Dy2+). Thus, Dy3+ does not reduce whereas ZnO participates in glass–ceramic network formation [17,18]. 3.3. X-ray diffraction Fig. 2 shows the X-ray diffractrograms of the as-prepared (AP) samples. KBD-1 and KBD-2 exhibits fully amorphous nature having no prominent peak. For the rest samples, peaks have been found and identified with corresponding JCPDS file number. The XRD pattern of pure

A probable mechanism of selective chemical reduction of Au3+ to Au0 by Sb3+ can be explained by considering the reduction potentials

Fig. 1. Representative DSC thermogram of sample KBD-4.

ð5Þ

Fig. 2. XRD patterns of as prepared samples.

N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

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3.4. Transmission electron microscopy Fig. 3 is a representative TEM micrograph of as-prepared nanocomposite KBD-5; 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 the NPs is mostly in the range 2–10 nm, but a few NPs are found to be relatively larger (~ 20–30 nm). As the XRD result confirms, the sample contains no crystalline phase other than Au, these nanoparticles visible in TEM micrograph 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. The inter-plane distance has been measured and compared with d-spacing values of JCPDS file of Au. The measured d-value and corresponding assignment of the plane have been mentioned in the figure. Fig. 4(b) shows the SAED pattern of the same sample. Multiple rings imply the 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.

Fig. 3. TEM image of KBD-5.

3.5. UV–Vis absorption spectra

Au has also been shown for comparison. From the figure it is quite clear that the peaks are getting sharper as we go from KBD-3 to KBD-7. KBD-7 has the most prominent set of peaks at 2θ = 20.0°, 28.43°, 33.4°, 34.9°, 44.7° and 50.56°. They are due to the diffractions from (222), (220), (311), (222), (331) and (422) planes of cubic crystalline phases of Au (JCPDS file no. 021095). The difference between the diffraction patterns of KBD-1, KBD-2 and the other samples rises due to their difference in composition. KBD-1 and KBD-2 do not contain any nanometal while from KBD-3 to KBD-7, they contain gradually increasing amount of nano Au (0.004 to 0.01 wt%). That is why from KBD-3 to KBD-7, the peaks are getting sharper. The average crystallite diameter (d) was calculated using Scherrer's formula from the XRD peaks. d ¼ 0:9 λ=FWHM cosθ ðpeakÞ

ð8Þ

where λ is the wavelength of X-ray radiation (CuKα = 1.5406 Å), FWHM is the full width at half maximum at 2θ. The crystallite size of Au NPs, as calculated from the peaks of (200) and (311) planes, varies in the range 5 to 15 nm from KBD-3 to KBD-7. The size of NPs increases from KBD-3 to KBD-7 due to the increased concentration of Au.

Fig. 5 shows the UV–Vis absorption spectra of the as prepared samples. The UV–vis–NIR absorption spectrum of the undoped glass (KBD1) shows absence of any features indicating the base glass matrix is transparent in the spectral region of interest to this study. Samples containing Au, i.e. KBD-3 to KBD-7, show an absorption band around 620 nm. Coinage metal (e.g. Ag, Au, Cu) 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 [35]. It is known that higher refractive index (RI) of matrix shifts the SPR band towards longer wavelength [39]. Our KBSZ antimony glass have a RI around 1.915, [11] 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 Au0–Au0 inter-particle distance with increasing concentration of Au0 [42]. Their experimental observations

Fig. 4. (a) HRTEM image and (b) SAED pattern of sample KBD-5.

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N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

Fig. 6. Excitation Spectra of the samples under 576 nm excitation.

Fig. 5. UV–Vis absorption spectra of as prepared samples.

corroborate well with the predictions of Maxwell– Garnett (MG) theory [18,43]. 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 [37]. 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 [44]. The prominent characteristic absorption bands of Dy3+ ion generally arise at the wavelengths 250 nm, 323 nm, 364 nm, 386 nm (within the range 250–400 nm) which appears due to the spectral transitions from 6H15/2 level to various levels like 4L19/2, 4M15/2, 6P7/2, 4I11/2, 4I13/2, 4 F7/2 etc. But here the cut-off wavelength of the base glass is high, around 400 nm. At this wavelength the absorption of the glass becomes so high that the absorption peaks arising from Dy3+ ions cannot be resolved. That is why the absorption bands for the RE ion is not found in the absorption spectra of the nanocomposites.

broad band in the region 230–340 nm is attributed to the host absorption band (HAB), because the charge transfer band (CTB) of Dy3+–O2– has been located below 220 nm [19]. It is a well-known fact that the wavelength corresponding to the prominent excitation band can give intense emission. In the present investigation, the excitation band centered at 453 nm is found to be most intense. Thus, the luminescence spectra should be carried out by exciting the samples with 453 nm wavelength in order to get maximum emission intensity. Due to the unavailability of 453 nm excitation source, the emission spectra have been recorded using 447 nm (which is very much close to 453 nm) laser diode. 3.7. Photoluminescence spectra The glasses KBD-2 and KBD-4 to KBD-7 have photoluminescent properties. They exhibit strong yellow emission when excited with 447 nm diode laser source. Fig. 7 is the chromaticity diagram corresponding to the light emitted from sample KBD-2 when excited at

3.6. Excitation spectra To analyze the luminescence properties as a function of Dy3 + ion concentration, the excitation spectrum was recorded in the spectral region 200–550 nm for all the samples by monitoring the emission at 576 nm and is shown in Fig. 6. The excitation bands centered at 392, 428, 453 and 472 nm are attributed to the 6H15/2 → 4I13/2, 6H15/2 → 4 G11/2, 6H15/2 → 4I15/2 and 6H15/2 → 4F9/2 transitions, respectively. The

Fig. 7. Chromaticity diagram corresponding to the light emitted from sample KBD-2 when excited at 447 nm with a diode laser source. The point “P” represents chromaticity coordinates (x = 0.470, y = 0.483) of the emitted light. The insets show the photoluminescence photograph of KBD-2, when excited at 447 nm.

N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

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Fig. 8. Chromaticity diagrams corresponding to the light emitted from samples (a) KBD-4, (b) KBD-5, (c) KBD-6 and (d) KBD-7 when excited at 447 nm with a diode laser source. The points “Q”, “R”, “S” and “T” represent chromaticity coordinates of the emitted light from the samples KBD-4, KBD-5, KBD-6 and KBD-7 respectively.

447 nm with a diode laser source. The point “P” represents chromaticity coordinates (x = 0.407, y = 0.483) of the emitted light. The inset shows the visible yellow emission obtained from sample KBD-2, when excited at 447 nm. Fig. 8 shows the chromaticity diagrams corresponding to the light emitted from samples KBD-4, KBD-5, KBD-6 and KBD-7 excited by the same laser source. The chromaticity coordinates of all the luminescent samples are given in Table 2. From the coordinates it is observed that from KBD-2 to KBD-7 there is a slight alteration of the position. The color of the emitted light does not change due to the slight variation of chromaticity coordinates. Both the values of x and y coordinates increase from KBD-2 to KBD-7. All the luminescent samples have the same concentration of Dy and different concentrations of Au. Therefore the variation in chromaticity coordinates arises from the difference of Au concentration. The concentration of nanometal has a significant and considerable influence on the

Table 2 Chromaticity coordinates of the photoluminescent samples. Sample ID

X-coordinate

Y-coordinate

KBD-2 KBD-4 KBD-5 KBD-6 KBD-7

0.470 0.444 0.446 0.457 0.471

0.483 0.468 0.473 0.492 0.497

emission intensity of the samples. Slight difference in the intensity distribution of the emission bands leads to change in the chromaticity coordinates. Fig. 9 shows the PL spectra of the samples KBD-2 to KBD-7. They exhibit prominent emission bands at 484, 576, 664 and 754 nm. The band at 576 nm is the major one, which is in the yellow region of the visible spectrum. Fig. 10 is the partial energy level diagram of Dy3 + ion and Au NPs showing the energy transfer mechanisms for the major bands in the downconversion spectra. The bands at 484, 576, 664 and 754 nm correspond to the transitions 4F9/2 → 6H15/2, 4F9/2 → 6H13/2, 4 F9/2 → 6H11/2, and 4F9/2 → 6H9/2, 6F11/2 respectively. The inset of Fig. 9 shows the variation of PL intensity of the samples with respect to Au concentration, at 576 nm band. The plot clearly shows that there is a considerable enhancement in PL intensity from KBD-2 to KBD-4. This enhancement is attributed to the presence of Au NPs. KBD-2 and KBD-4 has the exactly same amount of RE ions and has been prepared at similar conditions. So the difference in PL intensity can only arise from the NM content of KBD-4. Such PL intensification of the Dy3+ ion emission is primarily due to the local field enhancement (LFE) around the RE ions sites induced by SPR of Au NPs [29]. The plasmonic Au NPs concentrates the incident electromagnetic field creating an additional field in the sub wavelength structures around the Dy3+ ion sites and subsequently increase their rate of excitation (by “Lightning Rod Effect”). The secondary reason for enhanced luminescence is that when the Dy3 + is present in close vicinity to the metal surface,

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N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

effect decreases. As result the PL intensity increases gradually until the size of the NPs is 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 [41,46,47]. 3.8. Heat treatment

Fig. 9. Photoluminescence spectra of as prepared samples, excited at 447 nm diode laser. Inset shows the variation of PL intensity with Au concentration.

the weak photoluminescence emissions from the Au NPs is added as a second channel of excitation energy, that is, energy transfer (ET) from Au0 to Dy3+ [18,37,41]. Thus, the Au NPs increases the photonic density around the Dy3+ ions situated in near vicinity and thereby increase the number of photons captured by the Dy3+ ions. As a result of superior excitation rate the population of the excited state of the Dy3+ ions increases and subsequently the rate of radiative decay decreases. From KBD-4 onwards the PL intensity gradually increases up to KBD5, then decreases for KBD-6 and KBD-7. This can be explained by the effect of NPs on PL emission properties of RE ions. With increasing concentration of Au from KBD-4 to KBD-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 [41,44,45]. 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

From the emission spectra of the nanocomposites it is obtained that the nanocomposite KBD-5 having Au concentration 0.006 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 AuDy3+ co-doped glasses, KBD-5 was taken as the representative of the nanocomposites for having the highest emission. Heat treatment has been performed on KBD-5 along with KBD-1, KBD-2, and KBD-3 for comparison. The temperature for heat treatment was decided as 300 °C based on the result of dilatometric and DSC thermograms. The samples were heat treated for different durations- 15 min, 30 min, 45 min, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h and 10 h. Almost all the heat treated samples were found to be transparent. The heat treated samples were characterized by XRD, FESEM, UV–Vis absorption and PL spectra. 3.8.1. XRD Fig. 11 shows the X-ray diffraction patterns of KBD-5 heat treated at 300 °C for different durations. The XRD patterns are almost unchanged from as prepared (AP) up to 3 h 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 6 h the patterns exhibit a massive change showing huge peaks at 2θ = 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. 11). This result indicates that 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 3 h, the matrix remains almost amorphous, but after prolonged exposure to heat the matrix gets well crystallized. 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 nanoparticles. Size of the crystallites has been calculated using Scherrer's formula as mentioned in Eq. (8). The size of Au NPs has been found to increase with increasing heat treatment duration. The size of Au NPs in the

Fig. 10. Partial energy level diagram of Dy3+ ion showing transitions of major emission bands and local field enhancement (LFE) by Au NPs.

N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

Fig. 11. XRD patterns of sample KBD-5 heat treated at 300 °C for different durations.

heat treated samples ranges between 15 and 35 nm. The crystallite size of Sb2O3 nanocrystals has also been calculated. Their size varies in the range 50–150 nm. Crystallite size increases with increasing heat treatment duration. 3.8.2. Field emission scanning electron microscopy Fig. 12 represents a FESEM micrograph of sample KBD-5 heat treated at 300 °C for 6 h. The micrograph has been taken after etching the sample by dipping it into 0.2 vol% HF solution for 30 min, followed by prolonged drying at 120 °C for 24 h to remove the moisture. In the image we see several disc-like structures embedded into the glass matrix. The side view of the disks resembles tiny rod-like structure. But if it is observed closely, spherical disks are clearly visible in between the nano-rods. So we can say that nano-disc shaped crystals have been generated into the glass with heat treatment. As confirmed from XRD results, the samples, which are heat treated for N5 h, contain prominent crystalline part. The sharp peaks in the XRD pattern confirms the generation of Sb2O3 nanocrystalline phase, as well as Au NPs. The diameters of the disks are in the range 100–200 nm. It is in good agreement with the particle size calculated from the XRD patterns. This size range also does

Fig. 12. FESEM image of KBD-5 after heat treatment at 300 °C for 6 h.

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Fig. 13. UV–Vis absorption spectra of samples KBD-5 heat treated at 300 °C for different durations, showing the intensification of plasmon band of Au.

not interfere with the visible range of light, as the cut-off wavelength of the glass is around 400 nm. That is why the samples are transparent even after heat treatment of 8 h. At 10 h it becomes a little cloudy. 3.8.3. UV–Vis absorption Fig. 13 represents the UV–Vis absorption spectra of KBD-5 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. 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 Au3+ to Au0 as per the redox reaction mentioned in Eq. (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. 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 [44,48]. 3.8.4. Photoluminescence spectra Fig. 14 shows the variation of PL intensity of KBD-5 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 576 nm wavelength) as a function of heat treatment duration, it will give a plot like Fig. 15. From this figure an interesting trend is noticed showing two maxima one at 1 h and another is at 6 h 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.

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N. Shasmal, B. Karmakar Journal of Non-Crystalline Solids 463 (2017) 40–49

the size factor of NPs up to this particular limit. That is why the PL intensity reaches at its maximum at 6 h 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 [41,45]. 4. Conclusions Antimony oxide borate zinc oxide (KBSZ) glass having base glass composition 15K2O-15B2O3-60Sb2O3-10ZnO was prepared and doped with Dy3+. 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.

Fig. 14. Photoluminescence spectra of sample KBD-5 heat treated at 300 °C for different durations, (excited at 447 nm).The inset shows the magnified view of the band at λ = 576 nm.

From beginning to 1 h heat treatment duration, the PL intensity shows a sharp gradual increase followed by a gradual 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 1 h 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 [41,45]. The further increase after 5 h is due to the effect of matrix crystallization. As confirmed from XRD patterns, the glass matrix starts to crystallize after 5 h 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 Sb2O3 nanocrystals within its matrix has phonon energy as low as 600 cm−1. Low phonon systems enhances the emission intensity of the RE ions by lowering the non-radiative loss. This lowering of phonon energy overweighs

• DSC reveals that the nanocomposite (KBD-4) has Tg around 290 °C. • The electrochemical reduction reaction, used here for in-situ generation of Au NPs, is a spontaneous reaction having negative ΔG° value. • XRD shows that the nanocomposites KBD-3 to KBD-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. • UV–Vis absorption spectra of the as-prepared samples show characteristic SPR band of Au at around 620 nm. • When excited at 447 nm, all the Dy3+-containing samples give prominent emission bands at 484, 576, 664 and 754 nm. The band at 576 nm is the major one, which is in the yellow region of the visible spectrum. • Among the nanocomposites prepared, KBD-5 (containing 0.006 wt% Au) gives the maximum PL intensity. KBD-5 is heat treated at 300 °C at different durations. The heat treated samples were characterized by XRD, UV–Vis absorption and PL spectra. • XRD shows that the sample gets crystallized at around 5 h heat treatment duration. Sharp peaks of Sb2O3 appear along with the existing peaks of Au. • UV–Vis 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 1 h and 6 h 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 non-radiative 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 (CSIR), 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

Fig. 15. Variation of PL intensity of sample KBD-5 as a function of heat treatment duration.

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