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Size dependent effect of electron-hole recombination of CdS quantum dots on emission of Dy3+ ions in boro-silicate glasses through energy transfer
Accepted Manuscript Size dependent effect of electron-hole recombination of CdS quantum dots on 3+ emission of Dy ions in boro-silicate glasses through energy transfer S.R. Munishwar, P.P. Pawar, S. Ughade, R.S. Gedam PII:
S0925-8388(17)32520-3
DOI:
10.1016/j.jallcom.2017.07.146
Reference:
JALCOM 42562
To appear in:
Journal of Alloys and Compounds
Received Date: 24 February 2017 Revised Date:
0925-8388 0925-8388
Accepted Date: 14 July 2017
Please cite this article as: S.R. Munishwar, P.P. Pawar, S. Ughade, R.S. Gedam, Size dependent effect 3+ of electron-hole recombination of CdS quantum dots on emission of Dy ions in boro-silicate glasses through energy transfer, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.146. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Size dependent effect of electron-hole recombination of CdS quantum dots on emission of Dy3+ ions in boro-silicate glasses through energy transfer S.R. Munishwar, P.P. Pawar, S. Ughade and R. S. Gedam* Department of Applied Physics, Visvesvaraya National Institute of Technology,
*Corresponding Author Dr. R. S. Gedam Department of Applied physics, Visvesvaraya National Institute of Technology,
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Nagpur-440 010,
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Nagpur-440 010, India
India Email: [email protected]
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Tel.: +91 712 2801172; Fax: +91 712 2801325.
Abstract
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The glass system SiO2-B2O3-ZnO-Na2O-K2O-CdS with addition of Dy3+ ions was synthesized by using conventional melt-quench method. The CdS quantum dots in a glass matrix was studied using optimized heat treatment schedule. The growth of quantum dots was confirmed by optical absorption (UV-Vis), X-ray diffraction (XRD) and High resolution transmission electron microscopy (HRTEM).
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The optical absorption and luminescence were studied for these glasses. The photoluminescence spectra shows energy transfer between CdS quantum dots and Dy3+ ions. Because of quantum confinement, CdS QDs emits light as a result of electron hole recombination and some energy is
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transferred to nearest energy level of the Dy3+ ions to give combined emission due to recombination of electron-hole and electronic transitions (i.e. 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 ) of Dy3+ ions. Keywords: PL, QDs, XRD, HRTEM. Introduction In recent years synthesis of isolated nanoparticles, clusters or doped particles in collides, polymers and glass matrix has been investigated. Nanoparticles with large surface area to volume ratio changes their optical properties by splitting continuous band structure into series of discrete states, increases surface states and takes part in photoluminescence process [1-3]. Glass matrix is useful host material for rare earth material as well as other impurities like semiconductor materials
ACCEPTED MANUSCRIPT because of its high transparency, compositional variety, and easy mass production. Transparent glasses doped with nanoparticles promise for potential application in optical devices [4]. Growth of nanoparticles in glass matrix provides stability to nanoparticles by avoiding them to agglomerate at room temperature[5]. Among all glasses, boro-silicate glasses are excellent host matrices in which B2O3 acts as an excellent glass former, flux material and expand glass structure at low temperature[6] which support the growth of quantum dots in glass matrix. The choice of minimum adequate
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temperature and sufficient annealing duration offer favorable conditions for growth of nanoparticles in glass matrix. CdS quantum dots in glass matrix attracts more attention due to potential application in future optoelectronic devices due to tunable electronic band gap, direct band gap, high absorption coefficient, good emission efficiency, high thermal stability and easy to synthesis[2, 7, 8]. Recently,
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we have observed that optical properties of glasses can be tuned by controlled growth of CdS QDs in boro-silicate glass system [9]. The addition of rare-earth (RE) oxides in the glasses containing semiconductor quantum dots increases interest in spectroscopic properties because of energy transfer
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from QDs to RE3+ ions [10-12]. In rare earth, emission transitions occur as 4f-4f or 5d-4f and because of this characteristic they serve as an excellent activator[13]. However, direct and indirect band gap semiconductors can be good sensitizing centers due to efficient band to band absorption and their excitation cross sections are very high [14]. Since QDs of semiconductors like CdS, CdSe, ZnO etc. exhibit intrinsic electric dipole moment the energy transfer interactions are possible between QDs of these semiconductors and rare earth ions [11]. Among all rare earth ions, Dy3+ ion is known for blue
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(4F9/2→6H15/2) and yellow (4F9/2→6H13/2) emissions which is due to 5d–4f transition and the 5d state can easily be affected by the glass matrix environment and on combination of both emissions it can give white light [15, 16]. The growth of QDs in a glass matrix doped with rare earth ions changes their optical properties due to combined effect on electron-hole recombination and occurred electronic
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transitions in energy levels of QDs and rare earth ions respectively [7]. In the present work, we have added Dy2O3 in our recently reported CdS containing boro-silicate glass and their optical properties have been studied.
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Experimental
The glass system 41.93 SiO2: 12.66 B2O3: 15.89 Na2O: 6.52 K2O: 22.50 ZnO: 0.5 Dy2O3
(mole %) with 3 wt% CdS were prepared using a conventional melt quenching technique. All the chemicals were weighed using Shimadzu analytical balance and then mixed in agate mortar and pestle up to 8 hours in order to enhance the homogeneity. After mixing thoroughly, the mixture was kept in a platinum crucible and melted in a furnace at 1200oC for 2 hours. The glasses were quenched at room temperature in aluminium mould to get transparent glass. To remove the thermal stresses, quenched glasses were annealed at
300 oC for 4 hours and allowed to cool in the furnace up to room
temperature. The differential thermal analysis (DTA) measurement of glass sample was carried out using Hitachi TG/DTA7200. The sample in powder form was put in platinum pan and heated up to
ACCEPTED MANUSCRIPT 700 °C with heating rate of 10 oC/min in nitrogen flow. These glass samples were heat treated with optimized single step heat treatment schedule at 500 oC for 10, 35 and 60 hrs. The glass samples heat treated at different annealing time are given in Table 1. Growth of quantum dots were confirmed by X-ray diffraction studies using PAnalytical X’Pert Pro. Fourier Transform Infrared (FT-IR) was carried out by using Thermo Scientific Nicolet iS5 Spectrometer with iD5 ATR Accessory. Morphology and elemental analysis of the samples were characterized by field emission-gun scanning
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electron microscopy (FEG-SEM) and energy dispersive X-ray analysis (EDAX) (JEOL JSM-7600F). The field-emission transmission electron microscope (HR-TEM, JEOL/JEM 2100F) was used for determining the size of quantum dots (QDs) and lattice fringe observation. Optical absorption and PL spectra were recorded by JASCO V-670 Spectrophotometer and JASCO Spectroflurometer FP-8200
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respectively. Results and Discussion:
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Synthesized glass samples were characterized with Differential thermal analysis (DTA) to determine the thermal behavior of glass sample. The DTA curve of glass sample GR1shows endothermic (Tg) and exothermic (Tc) curves at 485 oC and 583 oC respectively. The single step heat treatment was optimized and glass sample was heat treated at
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for different time duration to favor the growth of CdS QDs via Ostwald ripening.
Fig. 1: DTA curve of glass sample.
500 oC
ACCEPTED MANUSCRIPT Inset of Fig.2 shows images of as made glass and glass samples heat treated for several hours doped with Dy2O3. It is observed from the images that, as made glass sample (GR1) is clear and transparent and the glass samples heat treated at 500oC for different time duration (i.e. GR2, GR3, GR4 for 10, 35 and 60 hours respectively) turned dark yellowish with increase in annealing time.
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Change in color of glasses with increase in annealing time are due growth of quantum dots in glasses.
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Fig. 2: X-ray diffraction (XRD) pattern of glass samples
Fig. 2 shows XRD plots of glass samples containing Dy2O3.The XRD pattern for GR1 does not show any diffraction peak except halo shape around 20o to 40o because of their amorphous nature.
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However, CdS phases found to be developed with less intensity in heat treated glasses as amorphous nature dominates the crystalline phases of CdS. XRD results of these samples shows [100], [101], [110] and [103] phases of CdS at 2θ positions around 24.80, 28.96o, 43.71o and 47.97o respectively which are well matched with reference code 01-075-1545. It is also observed that full-width halfmaximum (FWHM) decreases and peak intensity increases with increase in heating duration due to the improved crystallinity and grain growth. The crystallite size of the grown crystals as an effect of heating duration is calculated by using Scherer equation and depicted in Table1.
ACCEPTED MANUSCRIPT Table 1: Experimental and calculated crystallite size of CdS QDs embedded in glass matrix from XRD pattern and band gap Sample
Heat treatment duration
Optical Band Gap opt
Crystallite size from Band
from XRD (nm)
( Eg ) eV
gap (nm)
GR1
As made glass
--
3.59
1.27
GR2
500 oC for 10 Hrs
2.92
2.78
GR3
500 oC for 35 Hrs
3.22
2.69
GR4
500 oC for 60 Hrs
5.65
2.44
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Name
Crystallite size
2.05 2.30
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4.70
Fig 3: FTIR spectra of glasses showing structural changes with heat treatment.
ACCEPTED MANUSCRIPT Among all glasses borate glasses have good ability of structural modifications. The addition of alkali oxides (i.e. Na2O and K2O) in borate glasses leads to change in glass network as well as increase in free charge carriers as a function of temperature[6, 17]. The glass network containing different size interstices can easily accommodate different size of alkali ions (mixed alkali) than alkali ion of same size (single alkali). The alkali ions in glass sits in more favorable positions therefore more energy required for their movement[18]. In mixed alkali glasses, larger alkali ion (Na+) jump into
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vacant position by taking elevated energy from thermal vibration during the heat treatment[19]. The heat treatment at elevated temperatures above Tg offers softness as well as thermal vibrations in the glass matrix. The softness and thermal vibration provide favorable condition for the migration of alkali ions which is responsible for compositional changes through ion exchange process. This
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observed change in the glass structure is confirmed by FTIR spectra of glasses as shown in Fig.3.
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FTIR spectra shows appearance bands A, B, C, D, E and F because of bond formation between two or more chemical compounds which is because of structural changes occur in glass matrix during heat treatment. It is observed that band A situated at 715 cm-1 which arises due to bending of B-O-B linkage[20] and it shifts towards lower energy value 684 cm-1 with increase in heating duration which indicate vibration of BO4 units [20]. The shifting of the B-O-B bond toward band of BO4 unit is possible because of interaction of alkali oxides present in the interstitial position
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with borate group. Due to thermal vibrations produced by heat treatment, Na2O may get attracted to boron site and establish the neutral behavior by formation of four coordinated boron (BO4-) unit and Na+ ion while ZnO does not show any effect[17, 20]. The possible effect of heat treatment by which
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Na2O is added to boron site is as:
ACCEPTED MANUSCRIPT The most prominent band C present at 922 cm-1 (i.e. GR1 in Figure 3) is attributed to Si-O-B stretching vibrations due to the presence of B2O3-SiO2 binary system[21, 22]. However, this band moves toward higher energy i.e. 1072 cm-1 with decrease in intensity peak when glasses were heat treated more than 10 hours. The shifting of the band C toward higher energy (i.e. ≈1070 cm-1 ) value can be due to formation of Si-O stretching modes associated with mainly SiO4 tetrahedra involving
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are converted to four coordinated boron sites as follows:
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bridging oxygens[21]. When B2O3 is added to alkali silicate sites, the non–bridging oxygens (NBO)
In the present study two sites are eliminated and two tetrahedral boron sites are created for addition of each B2O3. Boron has +3 charge and shares 4 oxygen ions in tetrahedral coordination. Na+ ion previously associated with the non-bridging oxygen site make bond with BO4- site to maintain electro
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neutrality[23]. Na+ ions can make bonding with two distinct sites as shown in Fig. 4
(a)
(b)
Fig 4: Possible sodium sites in borosilicate glass after heat treatment (a) tetrahedral boron site (b) non-bridging oxygen site. Here Na+ will be less tightly bound to a BO4– site as compared to NBO. The added K2O increases the diffusion ability of Na+ ion with initial B2O3 by increasing activation energy in the glass matrix. The
ACCEPTED MANUSCRIPT magnitude of increase in energy depend on the difference in vibrational frequencies between the Na+ on the BO4- and Si-O sites and on the total alkali content (i. e. average Na+ - Na+) separation distance. It is observed that, heating effect is responsible for the change in ratio of B2O3 to SiO2 in glass matrix. Therefore, band C got shifted to 1097 cm-1 which indicate formation of BO4 tetrahedra for glass GR4 [23]. These conversion of groups is possible because BO3- unit is stable at higher temperature and BO4- unit is stable at lower temperature[6]. Thus, the glass samples GR3 and GR4 shows additional
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band B at 875 cm-1 due to stretching vibrations of BO4- units. The band D at 1230 cm-1 attributed to asymmetric stretching vibrations of B-O bonds and the band E at 1379 cm-1 show asymmetric stretching modes of borate triangles BO3 and BO2O-. These bands also moves to longer energy side with increase in annealing time as shown in Figure 3. Such spectral shifts must be due to the treatment
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involving ion exchange between Na+ ions in the glass’s near surface[21]. Depending upon the changing nature of bonding to materials as an effect of heat treatment, some amount of moisture has been absorbed on glass surface and appear in the form of hydroxyl group around 3600 cm-1 in FTIR
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spectra. However, Dy3+ ions takes the interstitial position of glass structure together with alkali ions
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and gives absorption or emission when energy supplied by glass matrix.
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°C for 60h
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Fig. 5: (a), (b) FEG-SEM images of glass sample and (c) EDAX spectra of glasses heat treated at 500
The FEG-SEM images (Fig. 5) of glass GR4 shows clusters of nanoparticles agglomerated
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because of heat treatment offered to the glass matrix for the formation of CdS QDs via Ostwald ripening. The annealing provide thermal vibrations to glassy matrix and provide softness to the glass network therefore the extra grown impurities can easily occupy the interstitial positions. Fig. 5c shows EDAX pattern which gives the information of the chemical composition present in the given heat treated glass matrix. This pattern shows the presence of CdS and Dy3+ ions in glass matrix along with the different chemical compounds.
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Fig. 6: HRTEM images showing (a) distribution of CdS QDs in glass matrix (b) size distribution of
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CdS QDs (c) lattice fringes and (d) SAED pattern for glass sample GR4
HRTEM images (Fig. 6) of glass sample GR4 shows morphology and nanostructure of CdS
QDs present in the glass matrix. Fig. 6 a is an image captured at 50 nm scale, reveals the presence of agglomerated CdS QDs which are nearly spherical in shape. The particle size distribution (Fig. 6b) is relatively narrow with average diameter around 5.9 nm which is quite comparable with results calculated from XRD pattern (Table1). The appearance of unidirectional lattice fringes (Fig. 6c) indicates grown crystallinity of CdS QDs and the spacing between two consecutive fringes is 0.246 nm corresponds to (1 0 2) plane of hexagonal CdS. The selected area electron diffraction (SAED) pattern (Fig 6d) consists of four concentric rings specify the amorphous nature of the glass sample.
ACCEPTED MANUSCRIPT These rings corresponds to (101), (102), (110) and (211) lattice planes represents hexagonal phase of CdS material. Thus, the results of HRTEM are comparable with XRD results which confirm the
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formation of hexagonal phases of CdS QDs.
Fig. 7: Optical absorption spectra showing decrease in band gap with increase in annealing time and
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absorption bands for Dy3+ ions in boro-silicate glasses
Fig. 7 shows absorption spectra of glass samples GR1, GR2, GR3 and GR4 which confirm
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the growth of CdS QDs along with presence Dy3+ ions in the glass matrix. This spectra attest that, optical energy band gap decreases with increase in annealing time. The blue energy shift in optical energy band gap confirm the increase in particle size of CdS QDs with annealing time. Thermal annealing reinforce the diffusion of Cd2+ and S2- ions liberated during fusion of CdS and promotes nucleation and growth of CdS QDs[24]. Increase in size of semiconductor quantum dots (QDs) leads to quantum confinement and because of this they absorb as well as emit different amount of energies. The effective band gap energies of these glasses were calculated from absorption spectra using the relation [9]
α hν = A (hν − Eg opt )n
(3)
ACCEPTED MANUSCRIPT where Egopt is the optical band gap energy, A is constant which is different for different transitions, n=1/2
denotes direct allowed and n=2 denotes indirect allowed transition. The size of QDs were
calculated using Brus’s equation [25, 26] :
E opt = E g (bulk) + g
h 2Π 2 2 mo r 2
1 1 1.8e 2 + * − mh * 4 Π εε o r me
(4)
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where, Egopt is the optical band gap energy, Eg is bulk energy band gap of CdS (2.42 eV), ε is dielectric coefficient for CdS (5.7), εo is permittivity of free space, me* is effective mass of electron (0.19), mh* is effective mass of hole (0.8), mo is mass of electron and r is radius of CdS QDs. The particle sizes of CdS nanoparticles grown in glass matrix with annealing time were obtained from
glasses increases with increase in annealing time.
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Brus’s equation are depicted in Table 1. This Table reveals that, the size of the CdS Quantum dots in
Along with blue energy shift in optical band gap, absorption spectra (Fig. 7) shows electronic
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transitions attributed to 6H15/2→4I13/2 (388 nm), 6H15/2→4G11/2 (428 nm), 6H15/2→4I15/2 (449 nm) , 6
H15/2→4F9/2 (480 nm), 6H15/2→6F5/2 (791 nm), 6H15/2→6F7/2 (885 nm), 6H15/2→6F9/2, 6H7/2 (1078 nm),
6
H15/2→6F11/2, 6H9/2 (1258 nm), 6H15/2→6H11/2 (1670 nm) etc. confirm the presence of Dy3+ ions in
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glass matrix.
Fig. 8: Excitation spectra for boro-silicate glasses
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Fig. 8 shows excitation spectra of glass samples GR1, GR2, GR3 and GR4 at 575 nm is monitored which asseverate the presence of Dy3+ in glass matrix. The excitation spectra consists of well resolved peaks in range of 300 nm to 500 nm situated at 324 nm, 349 nm, 363 nm, 388 nm, 424 nm, 452 nm and 471 nm which corresponds to 6H15/2→6P3/2, 6H15/2→6P7/2, 6H15/2→6P5/2, 6H15/2→4I13/2, H15/2→4G11/2, 6H15/2→4I15/2 and 6H15/2→4F9/2 electronic transitions of Dy3+ ions respectively. It is
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observed that fluorescence properties of glasses changes with the increasing size of CdS QDs. The excitation intensity got strongly enhanced for the glass sample GR2 in which size of CdS QDs are small as compared to the other glass samples. Smaller QDs of CdS absorb large photon energy as compare to the other QDs which is sufficient to excite the Dy3+ ions when this energy transferred to
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them. The excitation spectra (Fig. 8) shows well separated excitation peaks for GR1 in which CdS QDs are absent and excitation intensity of glass sample further increases when size of CdS QDs is comparatively smaller (i.e. GR2) and difference between energy levels of electronic states of CdS
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QDs and Dy 3+ ions becomes small as compared to other glass samples. Thus, band gap engineering of CdS QDs helps for excitation of Dy3+ ions due to their broader and stronger absorption[12, 27]. However, absorption of photon energy decreases with increasing size of CdS QDs results poor correlation between CdS QDs and Dy3+ ions. The peaks of Dy3+ ions in excitation spectra looks to be suppressed when band gap of glass samples decreases with increase in 3+
insufficient amount of energy was transferred to Dy
quantum dot size because
ions by CdS QDs. This dependence of
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fluorescence intensity as a function of CdS QDs size suggest the energy transfer from the CdS QDs to Dy3+ ions in the nanocrystals of glass matrix.
There may be two ways to excite Dy3+ ions one is direct and another is indirect excitation. In
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direct excitation energy is directly supplied to the Dy3+ ions to give emission. However, in indirect excitation the energy has been transferred from valance band to conduction band of host crystal, followed by energy transfer from the host crystal to Dy3+ ions. When size of CdS QDs increases
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absorption of photons decreases which results insufficient energy transfer to the Dy3+ ions therefore the CdS QDs with decreased band gap shows upshift from the base position with suppressed excitation peaks of Dy3+ ions.
ACCEPTED MANUSCRIPT Because of the energy transfer the excitation spectra shows upshift from the base position and excitation peaks of Dy3+ got suppressed due to insufficient energy transfer from CdS QDs to Dy3+
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ions.
Fig 9: Photoluminescence spectra and CIE chromaticity diagram for glass samples excited at 345 nm.
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The emission spectra of all glass samples were studied using 345 nm as an excitation wavelength. Shorter excitation wavelength is chosen to record emission spectra because multiple
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electronic states present at higher energy levels[28]. The emission spectra (Fig.9) shows peak at 413 nm for all glass samples which is due to compositional impurities or lattice defects present in glass matrix [29, 30]. Along with the emission peak at 413 nm, glass samples shows emission peaks at 482 nm and 575 nm which arise due to electronic transitions 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 of Dy3+ respectively.
The spectra (Fig. 9) shows the PL intensity of glass samples depend on size of CdS QDs which transfer their energy to Dy3+ ions in glass matrix during recombination. This energy transfer could be understood on the basis of dipole-dipole interactions between them [11, 31]. Relatively large radius and charge mismatching between the RE3+ and divalent Cd2+ ions are responsible for the unsuccessful incorporation of RE3+ into CdS, hence the inefficient energy transfer is often observed
ACCEPTED MANUSCRIPT [13]. As a consequence of this, PL spectra indicates the decrease in energy transfer from CdS QDs to Dy3+ ions with increase in heat treatment[12]. When energy is supplied to glass sample, the excited electrons comes to their ground state via radiative as well as non-radiative transitions through several intermediate steps from conduction band (C. B.) to valance band (V. B.) of CdS QDs. In glass sample GR1, the excited electrons are captured
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by the lattice impurities or trap states present in the glass matrix to give emission of 3.0 eV (i.e. 413 nm) and some amount of energy is directly transferred to energy level 4F9/2 of Dy3+ ions to give emission for electronic transitions 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 at positions 482 nm and 575 nm respectively. However, the band positions of glass sample GR1 is very widely spaced therefore no
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band edge emission is possible.
After heat treatment, size of CdS QDs increases due to nucleation and their growth. These grown QDs are responsible for formation of new trap or recombination centers and it gives broad
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luminescence band which shows dependency on CdS QDs size. For smaller size of CdS QDs, recombination energy is high therefore we found that the intensity and FWHM of luminescence peak is more and it decreases with increase in size of CdS QDs.
In earlier study [9] emission peak was observed due to band edge transitions of CdS QDs at 436 nm and around 556.5 nm (2.23 eV) as a result of newly formed recombination centers of CdS QDs for glass sample heat treated at 500 oC for 10 hours (G2) and shows red shift in peak position
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and decrease in luminescence intensity with increase in annealing time up to 80 hours [9, 32]. In glass sample GR2, excited electrons are first captured by lattice defects present at 413 nm (3.0 eV) then they are trapped into newly formed recombination centers which appears due to growth CdS QDs which are available around 556.5 nm (2.23 eV) and then transferred to the nearest available energy
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level 4F9/2 of Dy3+ ions and then to the ground state.
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Fig. 10: Spectral overlap of emission and absorption spectra of glass sample GR2.
Fig. 10 shows spectral overlap between excitation and emission spectra of glass samples GR2.
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The two spectra meets around 457 nm (2.71 eV) which is very close to the band gap of GR2 (2.78 eV) measured from Uv-Vis spectra. This indicate that some amount of band edge energy was transferred to energy level 4F9/2 to give radiative emission for transitions
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F9/2 → 6H15/2 and 4F9/2 → 6H13/2 of
Dy3+ respectively and broadness and intensity of these emission peaks increases due to transfer of
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energy from newly formed recombination centers of CdS QDs to Dy3+ions.
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Fig. 11: Energy level diagram showing energy transfer from CdS QDs to Dy3+ ions in glass sample
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GR2.
The energy level diagram (Fig.11) shows comparative study of emission spectra of glass samples G1, GR1, G2, and GR2 respectively. The spectra of glass samples G1 and G2 were taken
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from our earlier article [9] to show the energy transfer from CdS QDs to Dy3+ ions. Thus, the overall study infer that two strong luminescence bands of Dy3+ ions which are present in the visible range
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including blue 4F9/2 → 6H15/2 and yellow 4F9/2 → 6H13/2 at positions 482 nm and 575 nm were easily affected by the external crystal fields[33]. The combination of these two colors provides a mechanism producing white light which can be modified with controlled growth of CdS QDs. The CIE chromaticity coordinates (i.e. inset of Fig.9) were measured for all glass
samples using emission spectra under the excitation of 345 nm and depicted in Table 2. The color chromaticity values depicted in Table 2 indicates shifting of color coordinates towards white region with addition of Dy3+ ions in the glass matrix compared to glass samples containing CdS only [9]. Among all glass samples GR2 have high emission intensity (Fig. 9) with color coordinates (x=0.34, y=0.43). It is also observed that emission of glasses containing CdS QDs shift towards white region with the addition of Dy3+ ions. Thus, optical properties (PL) of Dy3+ ions in glass matrix can be tuned with resonance of quantum confinement effect of CdS QDs with Dy3+ ions [34].
ACCEPTED MANUSCRIPT Table 2: CIE chromaticity co-ordinates (x, y) showing variation in emitted color by glass samples as a function of particle size Sample name
Color Coordinates y
GR1
0.33
0.39
GR2
0.34
0.43
GR3
0.38
0.47
GR4
0.39
0.45
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Conclusion
The CdS QDs were grown in a glass matrix containing Dy3+ ions using controlled single step heat treatment. The growth of CdS quantum dots in glass system were confirmed by XRD and HRTEM. The structural changes offered by heat treatment were confirmed by FTIR study. The average size of these grown CdS QDs in glass matrix is found to be 2-8 nm. PL results shows energy
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transfer between CdS QDs and Dy3+ ions because of dipole-dipole interaction. The direct and indirect energy transfer from band edge and trap states are responsible for increase in intensity as well as FWHM of the photoluminescence peak when quantum confinement is more. The energy transfer from CdS QDs to Dy3+ ions decreases with increase in heat treatment. The PL intensity of Dy3+ ions can be
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controlled by varying size of CdS QDs in glass matrix therefore these glasses can be used as a potential material for solid state lightening.
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Acknowledgements
We are very much thankful to Department of Science and Technology (DST) for financial support for scientific research.
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[9] S. Munishwar, P. Pawar, R. Gedam, Influence Of Electron-Hole Recombination On Optical Properties Of Boro-silicate Glasses Containing CdS Quantum Dots, Journal of Luminescence, (2016). [10] R. Reisfeld, M. Gaft, T. Saridarov, G. Panczer, M. Zelner, Nanoparticles of cadmium sulfide with europium and terbium in zirconia films having intensified luminescence, Materials Letters, 45 (2000) 154-156.
[11] N. Dantas, E. Serqueira, V. Anjos, M. Bell, Control of growth and the processes of energy
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transfer from CdSe quantum dots for Nd3+ ions in a vitreous system: Thermal annealing time, Applied Physics Letters, 101 (2012) 121903.
[12] C. Dey, M. Goswami, B. Karmakar, White light emitting Ho3+-doped CdS nanocrystal ingrained glass nanocomposites, Applied Physics Letters, 106 (2015) 083106.
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[13] L. Saravanan, A. Pandurangan, R. Jayavel, Synthesis and luminescence enhancement of Cerium doped CdS nanoparticles, Materials Letters, 66 (2012) 343-345. [14] D. Chen, Y. Wang, E. Ma, F. Bao, Y. Yu, Luminescence of an Er
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containing CdS quantum dots, Scripta materialia, 55 (2006) 891-894. [15] P. Pawar, S. Munishwar, R. Gedam, Physical and optical properties of Dy
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/Pr
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Co-doped
lithium borate glasses for W-LED, Journal of Alloys and Compounds, 660 (2016) 347-355. [16] D. Ramteke, R. Gedam, Spectroscopic Properties of Dysprosium Oxide Containing Lithium Borate Glasses, Spectroscopy Letters, 48 (2015) 417-421. [17] G. Padmaja, P. Kistaiah, Infrared and Raman spectroscopic studies on alkali borate glasses: evidence of mixed alkali effect, The Journal of Physical Chemistry A, 113 (2009) 2397-2404. [18] D.E. Day, Mixed alkali glasses—their properties and uses, Journal of Non-Crystalline Solids, 21 (1976) 343-372. [19] Z. Boksay, Effect of mixing mobile ions in glasses on transport processes, Journal of NonCrystalline Solids, 123 (1990) 324-327.
ACCEPTED MANUSCRIPT [20] C. Gautam, A.K. Yadav, A.K. Singh, A review on infrared spectroscopy of borate glasses with effects of different additives, ISRN ceramics, 2012 (2012). [21] N. Sharaf, R. Condrate, A. Ahmed, FTIR spectral/structural investigation of the ion exchange/thermal treatment of silver ions into a silicate glass, Materials letters, 11 (1991) 115-118. [22] A. Tenney, J. Wong, Vibrational spectra of vapor-deposited binary borosilicate glasses, Journal of Chemical Physics, 56 (1972) 5516-5523.
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[23] P. Bray, L. Pye, V. Frechette, N. Kreidl, Borate Glasses, Structure, Properties, Applications, Materials Science Research, (1978) 321.
[24] N. Dantas, E. Serqueira, A. Carmo, M. Bell, V. Anjos, G. Marques, Energy transfer between CdS nanocrystals and neodymium ions embedded in vitreous substrates, Optics letters, 35 (2010) 1329-
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dots, Journal of Materials Chemistry, 22 (2012) 14674-14681.
[27] A. Nag, S. Sapra, S. Chakraborty, S. Basu, D. Sarma, Synthesis of CdSe nanocrystals in a noncoordinating solvent: Effect of reaction temperature on size and optical properties, Journal of
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nanoscience and nanotechnology, 7 (2007) 1965-1968.
[28] W.C. Chan, D.J. Maxwell, X. Gao, R.E. Bailey, M. Han, S. Nie, Luminescent quantum dots for multiplexed biological detection and imaging, Current opinion in biotechnology, 13 (2002) 40-46. [29] J. Kumar, A. Verma, P. Pandey, P. Bhatnagar, P. Mathur, W. Liu, S. Tang, Study of optical
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Physical review B, 53 (1996) 1463.
[31] T. Nann, J. Schneider, Origin of permanent electric dipole moments in wurtzite nanocrystals, Chemical physics letters, 384 (2004) 150-152. [32] A. Verma, S. Nagpal, P.K. Pandey, P. Bhatnagar, P. Mathur, Trap elimination and reduction of size dispersion due to aging in CdS x Se1− x quantum dots, Journal of Nanoparticle Research, 9 (2007) 1125-1131. [33] S. Chemingui, M. Ferhi, K. Horchani-Naifer, M. Férid, Synthesis and luminescence characteristics of Dy 3+ doped KLa (PO 3) 4, Journal of Luminescence, 166 (2015) 82-87. [34] P. Babu, C. Jayasankar, Spectroscopic properties of Dy fluoroborate glasses, Optical materials, 15 (2000) 65-79.
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ions in lithium borate and lithium
ACCEPTED MANUSCRIPT Highlights • Glasses were prepared by conventional melt quench technique. • Glasses were heat treated by optimized single stage heat treatment schedule • Growth of CdS QDs was confirmed by XRD and HRTEM.
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• Optical properties were studied by UV-Vis and Photoluminescence.
• The effect of quantum confinement of CdS quantum dots on optical properties of
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Dy3+ ions has been studied.
S0925-8388(17)32520-3
DOI:
10.1016/j.jallcom.2017.07.146
Reference:
JALCOM 42562
To appear in:
Journal of Alloys and Compounds
Received Date: 24 February 2017 Revised Date:
0925-8388 0925-8388
Accepted Date: 14 July 2017
Please cite this article as: S.R. Munishwar, P.P. Pawar, S. Ughade, R.S. Gedam, Size dependent effect 3+ of electron-hole recombination of CdS quantum dots on emission of Dy ions in boro-silicate glasses through energy transfer, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.07.146. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Size dependent effect of electron-hole recombination of CdS quantum dots on emission of Dy3+ ions in boro-silicate glasses through energy transfer S.R. Munishwar, P.P. Pawar, S. Ughade and R. S. Gedam* Department of Applied Physics, Visvesvaraya National Institute of Technology,
*Corresponding Author Dr. R. S. Gedam Department of Applied physics, Visvesvaraya National Institute of Technology,
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Nagpur-440 010,
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Nagpur-440 010, India
India Email: [email protected]
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Tel.: +91 712 2801172; Fax: +91 712 2801325.
Abstract
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The glass system SiO2-B2O3-ZnO-Na2O-K2O-CdS with addition of Dy3+ ions was synthesized by using conventional melt-quench method. The CdS quantum dots in a glass matrix was studied using optimized heat treatment schedule. The growth of quantum dots was confirmed by optical absorption (UV-Vis), X-ray diffraction (XRD) and High resolution transmission electron microscopy (HRTEM).
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The optical absorption and luminescence were studied for these glasses. The photoluminescence spectra shows energy transfer between CdS quantum dots and Dy3+ ions. Because of quantum confinement, CdS QDs emits light as a result of electron hole recombination and some energy is
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transferred to nearest energy level of the Dy3+ ions to give combined emission due to recombination of electron-hole and electronic transitions (i.e. 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 ) of Dy3+ ions. Keywords: PL, QDs, XRD, HRTEM. Introduction In recent years synthesis of isolated nanoparticles, clusters or doped particles in collides, polymers and glass matrix has been investigated. Nanoparticles with large surface area to volume ratio changes their optical properties by splitting continuous band structure into series of discrete states, increases surface states and takes part in photoluminescence process [1-3]. Glass matrix is useful host material for rare earth material as well as other impurities like semiconductor materials
ACCEPTED MANUSCRIPT because of its high transparency, compositional variety, and easy mass production. Transparent glasses doped with nanoparticles promise for potential application in optical devices [4]. Growth of nanoparticles in glass matrix provides stability to nanoparticles by avoiding them to agglomerate at room temperature[5]. Among all glasses, boro-silicate glasses are excellent host matrices in which B2O3 acts as an excellent glass former, flux material and expand glass structure at low temperature[6] which support the growth of quantum dots in glass matrix. The choice of minimum adequate
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temperature and sufficient annealing duration offer favorable conditions for growth of nanoparticles in glass matrix. CdS quantum dots in glass matrix attracts more attention due to potential application in future optoelectronic devices due to tunable electronic band gap, direct band gap, high absorption coefficient, good emission efficiency, high thermal stability and easy to synthesis[2, 7, 8]. Recently,
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we have observed that optical properties of glasses can be tuned by controlled growth of CdS QDs in boro-silicate glass system [9]. The addition of rare-earth (RE) oxides in the glasses containing semiconductor quantum dots increases interest in spectroscopic properties because of energy transfer
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from QDs to RE3+ ions [10-12]. In rare earth, emission transitions occur as 4f-4f or 5d-4f and because of this characteristic they serve as an excellent activator[13]. However, direct and indirect band gap semiconductors can be good sensitizing centers due to efficient band to band absorption and their excitation cross sections are very high [14]. Since QDs of semiconductors like CdS, CdSe, ZnO etc. exhibit intrinsic electric dipole moment the energy transfer interactions are possible between QDs of these semiconductors and rare earth ions [11]. Among all rare earth ions, Dy3+ ion is known for blue
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(4F9/2→6H15/2) and yellow (4F9/2→6H13/2) emissions which is due to 5d–4f transition and the 5d state can easily be affected by the glass matrix environment and on combination of both emissions it can give white light [15, 16]. The growth of QDs in a glass matrix doped with rare earth ions changes their optical properties due to combined effect on electron-hole recombination and occurred electronic
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transitions in energy levels of QDs and rare earth ions respectively [7]. In the present work, we have added Dy2O3 in our recently reported CdS containing boro-silicate glass and their optical properties have been studied.
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Experimental
The glass system 41.93 SiO2: 12.66 B2O3: 15.89 Na2O: 6.52 K2O: 22.50 ZnO: 0.5 Dy2O3
(mole %) with 3 wt% CdS were prepared using a conventional melt quenching technique. All the chemicals were weighed using Shimadzu analytical balance and then mixed in agate mortar and pestle up to 8 hours in order to enhance the homogeneity. After mixing thoroughly, the mixture was kept in a platinum crucible and melted in a furnace at 1200oC for 2 hours. The glasses were quenched at room temperature in aluminium mould to get transparent glass. To remove the thermal stresses, quenched glasses were annealed at
300 oC for 4 hours and allowed to cool in the furnace up to room
temperature. The differential thermal analysis (DTA) measurement of glass sample was carried out using Hitachi TG/DTA7200. The sample in powder form was put in platinum pan and heated up to
ACCEPTED MANUSCRIPT 700 °C with heating rate of 10 oC/min in nitrogen flow. These glass samples were heat treated with optimized single step heat treatment schedule at 500 oC for 10, 35 and 60 hrs. The glass samples heat treated at different annealing time are given in Table 1. Growth of quantum dots were confirmed by X-ray diffraction studies using PAnalytical X’Pert Pro. Fourier Transform Infrared (FT-IR) was carried out by using Thermo Scientific Nicolet iS5 Spectrometer with iD5 ATR Accessory. Morphology and elemental analysis of the samples were characterized by field emission-gun scanning
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electron microscopy (FEG-SEM) and energy dispersive X-ray analysis (EDAX) (JEOL JSM-7600F). The field-emission transmission electron microscope (HR-TEM, JEOL/JEM 2100F) was used for determining the size of quantum dots (QDs) and lattice fringe observation. Optical absorption and PL spectra were recorded by JASCO V-670 Spectrophotometer and JASCO Spectroflurometer FP-8200
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respectively. Results and Discussion:
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Synthesized glass samples were characterized with Differential thermal analysis (DTA) to determine the thermal behavior of glass sample. The DTA curve of glass sample GR1shows endothermic (Tg) and exothermic (Tc) curves at 485 oC and 583 oC respectively. The single step heat treatment was optimized and glass sample was heat treated at
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for different time duration to favor the growth of CdS QDs via Ostwald ripening.
Fig. 1: DTA curve of glass sample.
500 oC
ACCEPTED MANUSCRIPT Inset of Fig.2 shows images of as made glass and glass samples heat treated for several hours doped with Dy2O3. It is observed from the images that, as made glass sample (GR1) is clear and transparent and the glass samples heat treated at 500oC for different time duration (i.e. GR2, GR3, GR4 for 10, 35 and 60 hours respectively) turned dark yellowish with increase in annealing time.
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Change in color of glasses with increase in annealing time are due growth of quantum dots in glasses.
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Fig. 2: X-ray diffraction (XRD) pattern of glass samples
Fig. 2 shows XRD plots of glass samples containing Dy2O3.The XRD pattern for GR1 does not show any diffraction peak except halo shape around 20o to 40o because of their amorphous nature.
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However, CdS phases found to be developed with less intensity in heat treated glasses as amorphous nature dominates the crystalline phases of CdS. XRD results of these samples shows [100], [101], [110] and [103] phases of CdS at 2θ positions around 24.80, 28.96o, 43.71o and 47.97o respectively which are well matched with reference code 01-075-1545. It is also observed that full-width halfmaximum (FWHM) decreases and peak intensity increases with increase in heating duration due to the improved crystallinity and grain growth. The crystallite size of the grown crystals as an effect of heating duration is calculated by using Scherer equation and depicted in Table1.
ACCEPTED MANUSCRIPT Table 1: Experimental and calculated crystallite size of CdS QDs embedded in glass matrix from XRD pattern and band gap Sample
Heat treatment duration
Optical Band Gap opt
Crystallite size from Band
from XRD (nm)
( Eg ) eV
gap (nm)
GR1
As made glass
--
3.59
1.27
GR2
500 oC for 10 Hrs
2.92
2.78
GR3
500 oC for 35 Hrs
3.22
2.69
GR4
500 oC for 60 Hrs
5.65
2.44
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Name
Crystallite size
2.05 2.30
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4.70
Fig 3: FTIR spectra of glasses showing structural changes with heat treatment.
ACCEPTED MANUSCRIPT Among all glasses borate glasses have good ability of structural modifications. The addition of alkali oxides (i.e. Na2O and K2O) in borate glasses leads to change in glass network as well as increase in free charge carriers as a function of temperature[6, 17]. The glass network containing different size interstices can easily accommodate different size of alkali ions (mixed alkali) than alkali ion of same size (single alkali). The alkali ions in glass sits in more favorable positions therefore more energy required for their movement[18]. In mixed alkali glasses, larger alkali ion (Na+) jump into
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vacant position by taking elevated energy from thermal vibration during the heat treatment[19]. The heat treatment at elevated temperatures above Tg offers softness as well as thermal vibrations in the glass matrix. The softness and thermal vibration provide favorable condition for the migration of alkali ions which is responsible for compositional changes through ion exchange process. This
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observed change in the glass structure is confirmed by FTIR spectra of glasses as shown in Fig.3.
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FTIR spectra shows appearance bands A, B, C, D, E and F because of bond formation between two or more chemical compounds which is because of structural changes occur in glass matrix during heat treatment. It is observed that band A situated at 715 cm-1 which arises due to bending of B-O-B linkage[20] and it shifts towards lower energy value 684 cm-1 with increase in heating duration which indicate vibration of BO4 units [20]. The shifting of the B-O-B bond toward band of BO4 unit is possible because of interaction of alkali oxides present in the interstitial position
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with borate group. Due to thermal vibrations produced by heat treatment, Na2O may get attracted to boron site and establish the neutral behavior by formation of four coordinated boron (BO4-) unit and Na+ ion while ZnO does not show any effect[17, 20]. The possible effect of heat treatment by which
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Na2O is added to boron site is as:
ACCEPTED MANUSCRIPT The most prominent band C present at 922 cm-1 (i.e. GR1 in Figure 3) is attributed to Si-O-B stretching vibrations due to the presence of B2O3-SiO2 binary system[21, 22]. However, this band moves toward higher energy i.e. 1072 cm-1 with decrease in intensity peak when glasses were heat treated more than 10 hours. The shifting of the band C toward higher energy (i.e. ≈1070 cm-1 ) value can be due to formation of Si-O stretching modes associated with mainly SiO4 tetrahedra involving
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are converted to four coordinated boron sites as follows:
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bridging oxygens[21]. When B2O3 is added to alkali silicate sites, the non–bridging oxygens (NBO)
In the present study two sites are eliminated and two tetrahedral boron sites are created for addition of each B2O3. Boron has +3 charge and shares 4 oxygen ions in tetrahedral coordination. Na+ ion previously associated with the non-bridging oxygen site make bond with BO4- site to maintain electro
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neutrality[23]. Na+ ions can make bonding with two distinct sites as shown in Fig. 4
(a)
(b)
Fig 4: Possible sodium sites in borosilicate glass after heat treatment (a) tetrahedral boron site (b) non-bridging oxygen site. Here Na+ will be less tightly bound to a BO4– site as compared to NBO. The added K2O increases the diffusion ability of Na+ ion with initial B2O3 by increasing activation energy in the glass matrix. The
ACCEPTED MANUSCRIPT magnitude of increase in energy depend on the difference in vibrational frequencies between the Na+ on the BO4- and Si-O sites and on the total alkali content (i. e. average Na+ - Na+) separation distance. It is observed that, heating effect is responsible for the change in ratio of B2O3 to SiO2 in glass matrix. Therefore, band C got shifted to 1097 cm-1 which indicate formation of BO4 tetrahedra for glass GR4 [23]. These conversion of groups is possible because BO3- unit is stable at higher temperature and BO4- unit is stable at lower temperature[6]. Thus, the glass samples GR3 and GR4 shows additional
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band B at 875 cm-1 due to stretching vibrations of BO4- units. The band D at 1230 cm-1 attributed to asymmetric stretching vibrations of B-O bonds and the band E at 1379 cm-1 show asymmetric stretching modes of borate triangles BO3 and BO2O-. These bands also moves to longer energy side with increase in annealing time as shown in Figure 3. Such spectral shifts must be due to the treatment
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involving ion exchange between Na+ ions in the glass’s near surface[21]. Depending upon the changing nature of bonding to materials as an effect of heat treatment, some amount of moisture has been absorbed on glass surface and appear in the form of hydroxyl group around 3600 cm-1 in FTIR
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spectra. However, Dy3+ ions takes the interstitial position of glass structure together with alkali ions
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and gives absorption or emission when energy supplied by glass matrix.
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°C for 60h
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Fig. 5: (a), (b) FEG-SEM images of glass sample and (c) EDAX spectra of glasses heat treated at 500
The FEG-SEM images (Fig. 5) of glass GR4 shows clusters of nanoparticles agglomerated
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because of heat treatment offered to the glass matrix for the formation of CdS QDs via Ostwald ripening. The annealing provide thermal vibrations to glassy matrix and provide softness to the glass network therefore the extra grown impurities can easily occupy the interstitial positions. Fig. 5c shows EDAX pattern which gives the information of the chemical composition present in the given heat treated glass matrix. This pattern shows the presence of CdS and Dy3+ ions in glass matrix along with the different chemical compounds.
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Fig. 6: HRTEM images showing (a) distribution of CdS QDs in glass matrix (b) size distribution of
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CdS QDs (c) lattice fringes and (d) SAED pattern for glass sample GR4
HRTEM images (Fig. 6) of glass sample GR4 shows morphology and nanostructure of CdS
QDs present in the glass matrix. Fig. 6 a is an image captured at 50 nm scale, reveals the presence of agglomerated CdS QDs which are nearly spherical in shape. The particle size distribution (Fig. 6b) is relatively narrow with average diameter around 5.9 nm which is quite comparable with results calculated from XRD pattern (Table1). The appearance of unidirectional lattice fringes (Fig. 6c) indicates grown crystallinity of CdS QDs and the spacing between two consecutive fringes is 0.246 nm corresponds to (1 0 2) plane of hexagonal CdS. The selected area electron diffraction (SAED) pattern (Fig 6d) consists of four concentric rings specify the amorphous nature of the glass sample.
ACCEPTED MANUSCRIPT These rings corresponds to (101), (102), (110) and (211) lattice planes represents hexagonal phase of CdS material. Thus, the results of HRTEM are comparable with XRD results which confirm the
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formation of hexagonal phases of CdS QDs.
Fig. 7: Optical absorption spectra showing decrease in band gap with increase in annealing time and
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absorption bands for Dy3+ ions in boro-silicate glasses
Fig. 7 shows absorption spectra of glass samples GR1, GR2, GR3 and GR4 which confirm
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the growth of CdS QDs along with presence Dy3+ ions in the glass matrix. This spectra attest that, optical energy band gap decreases with increase in annealing time. The blue energy shift in optical energy band gap confirm the increase in particle size of CdS QDs with annealing time. Thermal annealing reinforce the diffusion of Cd2+ and S2- ions liberated during fusion of CdS and promotes nucleation and growth of CdS QDs[24]. Increase in size of semiconductor quantum dots (QDs) leads to quantum confinement and because of this they absorb as well as emit different amount of energies. The effective band gap energies of these glasses were calculated from absorption spectra using the relation [9]
α hν = A (hν − Eg opt )n
(3)
ACCEPTED MANUSCRIPT where Egopt is the optical band gap energy, A is constant which is different for different transitions, n=1/2
denotes direct allowed and n=2 denotes indirect allowed transition. The size of QDs were
calculated using Brus’s equation [25, 26] :
E opt = E g (bulk) + g
h 2Π 2 2 mo r 2
1 1 1.8e 2 + * − mh * 4 Π εε o r me
(4)
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where, Egopt is the optical band gap energy, Eg is bulk energy band gap of CdS (2.42 eV), ε is dielectric coefficient for CdS (5.7), εo is permittivity of free space, me* is effective mass of electron (0.19), mh* is effective mass of hole (0.8), mo is mass of electron and r is radius of CdS QDs. The particle sizes of CdS nanoparticles grown in glass matrix with annealing time were obtained from
glasses increases with increase in annealing time.
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Brus’s equation are depicted in Table 1. This Table reveals that, the size of the CdS Quantum dots in
Along with blue energy shift in optical band gap, absorption spectra (Fig. 7) shows electronic
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transitions attributed to 6H15/2→4I13/2 (388 nm), 6H15/2→4G11/2 (428 nm), 6H15/2→4I15/2 (449 nm) , 6
H15/2→4F9/2 (480 nm), 6H15/2→6F5/2 (791 nm), 6H15/2→6F7/2 (885 nm), 6H15/2→6F9/2, 6H7/2 (1078 nm),
6
H15/2→6F11/2, 6H9/2 (1258 nm), 6H15/2→6H11/2 (1670 nm) etc. confirm the presence of Dy3+ ions in
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glass matrix.
Fig. 8: Excitation spectra for boro-silicate glasses
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Fig. 8 shows excitation spectra of glass samples GR1, GR2, GR3 and GR4 at 575 nm is monitored which asseverate the presence of Dy3+ in glass matrix. The excitation spectra consists of well resolved peaks in range of 300 nm to 500 nm situated at 324 nm, 349 nm, 363 nm, 388 nm, 424 nm, 452 nm and 471 nm which corresponds to 6H15/2→6P3/2, 6H15/2→6P7/2, 6H15/2→6P5/2, 6H15/2→4I13/2, H15/2→4G11/2, 6H15/2→4I15/2 and 6H15/2→4F9/2 electronic transitions of Dy3+ ions respectively. It is
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6
observed that fluorescence properties of glasses changes with the increasing size of CdS QDs. The excitation intensity got strongly enhanced for the glass sample GR2 in which size of CdS QDs are small as compared to the other glass samples. Smaller QDs of CdS absorb large photon energy as compare to the other QDs which is sufficient to excite the Dy3+ ions when this energy transferred to
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them. The excitation spectra (Fig. 8) shows well separated excitation peaks for GR1 in which CdS QDs are absent and excitation intensity of glass sample further increases when size of CdS QDs is comparatively smaller (i.e. GR2) and difference between energy levels of electronic states of CdS
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QDs and Dy 3+ ions becomes small as compared to other glass samples. Thus, band gap engineering of CdS QDs helps for excitation of Dy3+ ions due to their broader and stronger absorption[12, 27]. However, absorption of photon energy decreases with increasing size of CdS QDs results poor correlation between CdS QDs and Dy3+ ions. The peaks of Dy3+ ions in excitation spectra looks to be suppressed when band gap of glass samples decreases with increase in 3+
insufficient amount of energy was transferred to Dy
quantum dot size because
ions by CdS QDs. This dependence of
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fluorescence intensity as a function of CdS QDs size suggest the energy transfer from the CdS QDs to Dy3+ ions in the nanocrystals of glass matrix.
There may be two ways to excite Dy3+ ions one is direct and another is indirect excitation. In
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direct excitation energy is directly supplied to the Dy3+ ions to give emission. However, in indirect excitation the energy has been transferred from valance band to conduction band of host crystal, followed by energy transfer from the host crystal to Dy3+ ions. When size of CdS QDs increases
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absorption of photons decreases which results insufficient energy transfer to the Dy3+ ions therefore the CdS QDs with decreased band gap shows upshift from the base position with suppressed excitation peaks of Dy3+ ions.
ACCEPTED MANUSCRIPT Because of the energy transfer the excitation spectra shows upshift from the base position and excitation peaks of Dy3+ got suppressed due to insufficient energy transfer from CdS QDs to Dy3+
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ions.
Fig 9: Photoluminescence spectra and CIE chromaticity diagram for glass samples excited at 345 nm.
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The emission spectra of all glass samples were studied using 345 nm as an excitation wavelength. Shorter excitation wavelength is chosen to record emission spectra because multiple
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electronic states present at higher energy levels[28]. The emission spectra (Fig.9) shows peak at 413 nm for all glass samples which is due to compositional impurities or lattice defects present in glass matrix [29, 30]. Along with the emission peak at 413 nm, glass samples shows emission peaks at 482 nm and 575 nm which arise due to electronic transitions 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 of Dy3+ respectively.
The spectra (Fig. 9) shows the PL intensity of glass samples depend on size of CdS QDs which transfer their energy to Dy3+ ions in glass matrix during recombination. This energy transfer could be understood on the basis of dipole-dipole interactions between them [11, 31]. Relatively large radius and charge mismatching between the RE3+ and divalent Cd2+ ions are responsible for the unsuccessful incorporation of RE3+ into CdS, hence the inefficient energy transfer is often observed
ACCEPTED MANUSCRIPT [13]. As a consequence of this, PL spectra indicates the decrease in energy transfer from CdS QDs to Dy3+ ions with increase in heat treatment[12]. When energy is supplied to glass sample, the excited electrons comes to their ground state via radiative as well as non-radiative transitions through several intermediate steps from conduction band (C. B.) to valance band (V. B.) of CdS QDs. In glass sample GR1, the excited electrons are captured
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by the lattice impurities or trap states present in the glass matrix to give emission of 3.0 eV (i.e. 413 nm) and some amount of energy is directly transferred to energy level 4F9/2 of Dy3+ ions to give emission for electronic transitions 4F9/2 → 6H15/2 and 4F9/2 → 6H13/2 at positions 482 nm and 575 nm respectively. However, the band positions of glass sample GR1 is very widely spaced therefore no
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band edge emission is possible.
After heat treatment, size of CdS QDs increases due to nucleation and their growth. These grown QDs are responsible for formation of new trap or recombination centers and it gives broad
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luminescence band which shows dependency on CdS QDs size. For smaller size of CdS QDs, recombination energy is high therefore we found that the intensity and FWHM of luminescence peak is more and it decreases with increase in size of CdS QDs.
In earlier study [9] emission peak was observed due to band edge transitions of CdS QDs at 436 nm and around 556.5 nm (2.23 eV) as a result of newly formed recombination centers of CdS QDs for glass sample heat treated at 500 oC for 10 hours (G2) and shows red shift in peak position
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and decrease in luminescence intensity with increase in annealing time up to 80 hours [9, 32]. In glass sample GR2, excited electrons are first captured by lattice defects present at 413 nm (3.0 eV) then they are trapped into newly formed recombination centers which appears due to growth CdS QDs which are available around 556.5 nm (2.23 eV) and then transferred to the nearest available energy
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level 4F9/2 of Dy3+ ions and then to the ground state.
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Fig. 10: Spectral overlap of emission and absorption spectra of glass sample GR2.
Fig. 10 shows spectral overlap between excitation and emission spectra of glass samples GR2.
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The two spectra meets around 457 nm (2.71 eV) which is very close to the band gap of GR2 (2.78 eV) measured from Uv-Vis spectra. This indicate that some amount of band edge energy was transferred to energy level 4F9/2 to give radiative emission for transitions
4
F9/2 → 6H15/2 and 4F9/2 → 6H13/2 of
Dy3+ respectively and broadness and intensity of these emission peaks increases due to transfer of
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energy from newly formed recombination centers of CdS QDs to Dy3+ions.
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Fig. 11: Energy level diagram showing energy transfer from CdS QDs to Dy3+ ions in glass sample
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GR2.
The energy level diagram (Fig.11) shows comparative study of emission spectra of glass samples G1, GR1, G2, and GR2 respectively. The spectra of glass samples G1 and G2 were taken
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from our earlier article [9] to show the energy transfer from CdS QDs to Dy3+ ions. Thus, the overall study infer that two strong luminescence bands of Dy3+ ions which are present in the visible range
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including blue 4F9/2 → 6H15/2 and yellow 4F9/2 → 6H13/2 at positions 482 nm and 575 nm were easily affected by the external crystal fields[33]. The combination of these two colors provides a mechanism producing white light which can be modified with controlled growth of CdS QDs. The CIE chromaticity coordinates (i.e. inset of Fig.9) were measured for all glass
samples using emission spectra under the excitation of 345 nm and depicted in Table 2. The color chromaticity values depicted in Table 2 indicates shifting of color coordinates towards white region with addition of Dy3+ ions in the glass matrix compared to glass samples containing CdS only [9]. Among all glass samples GR2 have high emission intensity (Fig. 9) with color coordinates (x=0.34, y=0.43). It is also observed that emission of glasses containing CdS QDs shift towards white region with the addition of Dy3+ ions. Thus, optical properties (PL) of Dy3+ ions in glass matrix can be tuned with resonance of quantum confinement effect of CdS QDs with Dy3+ ions [34].
ACCEPTED MANUSCRIPT Table 2: CIE chromaticity co-ordinates (x, y) showing variation in emitted color by glass samples as a function of particle size Sample name
Color Coordinates y
GR1
0.33
0.39
GR2
0.34
0.43
GR3
0.38
0.47
GR4
0.39
0.45
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x
Conclusion
The CdS QDs were grown in a glass matrix containing Dy3+ ions using controlled single step heat treatment. The growth of CdS quantum dots in glass system were confirmed by XRD and HRTEM. The structural changes offered by heat treatment were confirmed by FTIR study. The average size of these grown CdS QDs in glass matrix is found to be 2-8 nm. PL results shows energy
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transfer between CdS QDs and Dy3+ ions because of dipole-dipole interaction. The direct and indirect energy transfer from band edge and trap states are responsible for increase in intensity as well as FWHM of the photoluminescence peak when quantum confinement is more. The energy transfer from CdS QDs to Dy3+ ions decreases with increase in heat treatment. The PL intensity of Dy3+ ions can be
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controlled by varying size of CdS QDs in glass matrix therefore these glasses can be used as a potential material for solid state lightening.
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Acknowledgements
We are very much thankful to Department of Science and Technology (DST) for financial support for scientific research.
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3+
ions in lithium borate and lithium
ACCEPTED MANUSCRIPT Highlights • Glasses were prepared by conventional melt quench technique. • Glasses were heat treated by optimized single stage heat treatment schedule • Growth of CdS QDs was confirmed by XRD and HRTEM.
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• Optical properties were studied by UV-Vis and Photoluminescence.
• The effect of quantum confinement of CdS quantum dots on optical properties of
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Dy3+ ions has been studied.