Chapter 3. Characterization and. photoluminescence. properties of undoped and. rare earth (Eu 3+, Dy 3+ ) doped CaSiO 3

Transcription

1 70 Chapter 3 Characterization and photoluminescence properties of undoped and rare earth (Eu 3+, Dy 3+ ) doped CaSiO 3

2 71 Chapter 3.1: Characterization and photoluminescence studies of undoped CaSiO3 Table of Contents Introduction Synthesis of undoped CaSiO3 nanophosphor Results and discussion Powder X-ray diffraction Scanning electron microscopy Fourier transform infrared spectroscopy UV-Visible absorption Photoluminescence Conclusions 84

3 Introduction The silicates of calcium are known for their thermal stability, high temperature strength, low thermal expansion, creep residence and chemical inertness [118]. Silicate phosphors are used for a fluorescent, a cathode-ray tube, a luminous body, a vacuum ultraviolet excitation light emitting element etc. Specifically, for a vacuum ultraviolet excitation light emitting element such as PDP, an improvement of the brightness of the element has been highly desired, and therefore, an improvement of the brightness of the silicate phosphor has been required, hence are used in Calcium Silicate (CaSiO3), otherwise known as wollastonite has a good matrix of luminescent material [119]. Calcium Silicate acquires a higher luminous efficiency when it is doped with rare earth activated ions. The PL behavior of undoped CaSiO3 was investigated by a very few research groups. Several varied methods have been followed to prepare CaSiO3.The silicate is synthesised by sol-gel [120], hydrothermal [121], reverse micells [122] and colloidal method. It is also prepared by colloidal emulsion method [123]. Solid state reaction method has been followed by many researchers to prepare undoped CaSiO3 and Calcium Silicate doped with rare earth and transition metal. However, the focus has always been on increasing the luminescence properties of the CaSiO3. The different methods followed to synthesise CaSiO3 have their own disadvantages. Sol-gel method requires expensive ingredients while the

4 73 output is very low in the case of hydrothermal method. High sintering temperature (in range of 1400 C), presence of more impurities and non-uniform mixing are the demerits of solid state synthesis. The final product in all the cases does not suit the needs of phosphorous industry. An efficient method avoiding these demerits was evolved for synthesis of CaSiO3. The process is less expensive, works at low temperatures, safe and energy-conservative. The synthesis also allows addition of dopant even at the ppm level Synthesis of undoped CaSiO3 nanophosphors The ingredients used in the synthesis of CaSiO3 are analar grade calcium nitrate (Ca(NO3)2. 6H2O; Merck), silica fumes (SiO2, 99.9% surface area 200 m 2 /g and Diformyl hydrazine (C2H4N2O2; DFH) fuel. The fuel was synthesised in the Laboratory by inducing a reaction of formic acid and hydrazine. The process is more fully described in literature [124]. A cylindrical petridish of about 300 ml capacity was used to dissolve the ingredients in minimum quantity of doubly distilled water. A magnetic stirrer was used to disperse the heterogeneous mixture for about five minutes. Later, the petridish containing the mixture was placed over a muffle furnace heated to a temperature of 500 C. Gaseous products such as oxides of carbon and nitrogen are released as the mixture undergoes dehydration and ignition. Once ignited, the

5 74 combustion propagates on its own without the need of any external heat. The silicate in a foamy form was obtained finally. The flow chart of combustion synthesis is shown in Fig Assuming total combustion of the redox mixture, the theoretical equation for synthesis of CaSiO3 could be written as: 4Ca(NO3)2 (aq) + 4SiO2 (aq) + 5C2H4N2O2 (aq) 4CaSiO3 (s) + 9N2 (g) + 10H2O (g) +10CO2 (g) (3.2) Calcium nitrate Ca(NO 3 ) 2 Diformyl hydrazine (Fuel) + Silica fumes Aqueous redox mixture Combustion Final product Fig. 3.1 Flow chart for solution combustion method for preparation of CaSiO3.

6 Results and discussion Powder X-ray diffraction Philips X-ray Diffractometer with a Cu Kα radiation (λ=1.5405ǻ) was used to study the X-Ray diffraction of the sample. The silicate formed and calcined at 700 C, 950 C, 1100 C and 1200 C for three hours [shown in fig 3.2 (a-d)]. The XRD pattern shows that the formed powder is amorphous, but not formed the phase. When the sample calcined at 700 C, the sample shows smaller amount of crystallinity. As the temperature increased to 950 C, the sample indexed the β- CaSiO3 and α phase at 1200 C. All the diffraction peaks are consistent with the standard PDF database (JCPDS file No ). No prominent impurity peaks are detected. If the calcination temperature was 1100 C sample shows the mixed phase. The high intensity characteristic peaks indicate the good crystalline nature of the product [ ]. Fig. 3.2 PXRD patterns of CaSiO3 (a) as- formed (b) calcined at 950 C for 3h (c) calcined at 1100 C for 3h (d) calcined at 1200 C for 3h

7 76 Using Debye-Scherer s formula (Ref. Eq. 2.3, chapter-2), the average crystallite size (D) of CaSiO3 was estimated from the full width at half maximum (FWHM) of the diffraction peak of the powders. The crystallite size was found to be in the range of nm. Fig. 3.3 Williamson Hall plots of CaSiO3 (a) calcined at 950 C for 3h (b) calcined at 1200 C for 3h Williamson-Hall (W-H) formula (Ref. Eq. 2.4, chapter-2) was used to calculate the strain present in CaSiO3 nanophosphors. The strain (ε) is represented by the slope of the line; intercept (0.9λ/D) on Y -axis indicates crystallite size (D). The W-H plots of calcined samples of the salt at 950 C and 1200 C are shown in fig 3.3 and table 3.1 shows the corresponding parameters.

8 77 Temperatur e(calcined 3h) Crystallite size (nm) Scherer s W-H method method (d) (D) Strain (x10-4 ) 950 C C Table 3.1 Estimated parameters of CaSiO3 nanoparticles at different calcinations temperatures. The strain observed to be more in the 1200 C calcined sample when compared to 950 C calcined sample. This is because, the crystallinity increases particle size are also increases with heat treatment. Fig 3.4 represents the variation of crystallite size and strain at different calcination temperatures. Fig.3.4 Variation of crystallite size and strain of CaSiO3 (a) calcined at 950 C for 3h (b) calcined at 1200 C for 3h (c) strain of calcined at 950 C and 1200 C.

9 Scanning electron microscopy Fig 3.5 SEM micrograph of CaSiO3 (a) calcined at 950 C for 3h (b) calcined at 1200 C for 3h Scanning electron microscopy (SEM) was used to study the morphology of the sample. Fig 3.5 (a, b) shows the SEM micrographs of the as-formed and calcined at 950 C for 3h, 1200 C for 3h CaSiO3 prepared by solution combustion method. More pores and voids shown at 950 C for 3h could be attributed to huge amount of gases liberated from the reaction mixture during combustion. The calcined sample at 1200 C for 3h shows more agglomeration and the phenomenon could be on account of minimising their surface energy. It is well-established that the morphological characteristics of the sample hinges on the heat and gases produced during the complex decomposition under combustion method. The heat released during

10 79 the process is crucial for crystalline growth while large amount of gases facilitate preparation of fine particles Fourier Transform Infrared Spectroscopy FTIR spectroscopy studies confirmed the formation and purity of the products. The result of the studies is indicated in Fig. 3.6 (a, b). The structures of the samples are explained on the basis of the FTIR spectra. The presence of β-casio3 is established by the IR peak at 431, 621, 989 cm -1 and the existence of CaO in the structure is confirmed by the peak at 466, 963, 1487 cm -1 [128]. The absorption band occurs at 700 cm -1 is due to O-Si-O vibration and absorption at m -1 is due to Si-O vibration. The absorption peak at 3418 cm 1 is attributed to H2O, absorbed by nanocrystalline materials from the environment on account of their high surface-to-volume ratio. The Si-O asymmetric stretching is indicated by the bands in the range of cm 1

11 80 Fig 3.6 FTIR spectra of CaSiO3 (a) calcined at 950 C for 3h (b) calcined at 1200 C for 3h UV-Visible absorption and band gap measurements The absorption characteristics of CaSiO3 nano particles was studied with UV-Vis absorption spectroscopy. The absorption spectrum of the nanopowder at room temperature is shown in fig 3.7. An intense absorption band at nm which corresponding to oxygen to silicon (O -Si) ligand-to- metal charge-transfer (LMCT) in the SiO 3 2- group. The broad bands in the range nm are explained as due to intra configurational 4f - 4f transitions from the ground 7 F0 level corresponding to excitation spectra. These results are good agreement with the reported literature [129].

12 81 Absorbance (a.u) (b) (a) Wavelength (nm) Fig 3.7 UV Absorption spectra of CaSiO3 (a) Calcined at 950 o C 3h (b) Calcined 1200 o C 3h The energy gap (Eg) of pure CaSiO3 estimated using Wood and Tauc relation (Ref. Eq. 2.5 section-2) by plotting ( αhυ) 2 versus (αhυ) shown in Fig 3.8. The samples display an indirect allowed electronic transition and hence the standard was arrived with k = 2, according to literature [130]. The linear region of the curve or tail [(αhυ) 1/k = 0] in the UV Visible absorbance spectra was extrapolated to arrive at Eg values. Experimental conditions (calcination temperature and processing time) and preparation methods determine Eg values. To be specific, the formation of structural defects are favored or inhibited by these crucial factors. The structural defects have a control over the extent of structural order/disorder of the sample. This, in turn, will decide the number of intermediary energy levels within the band gap.

13 82 Fig 3.8 A plot of (αe) n Vs energy for CaSiO3 (a) Calcined at 950 C 3h (b) Calcined 1200 C 3h The changes in lattice or variations on the structural order-disorder degree brought about a significant change in the optical absorption measurements. The values for optical energy band gap for undoped sample varies between ev, these values are correlated with those mentioned in the literature Photoluminescence The PL spectra of CaSiO3 nanopowders calcined at 950 C and 1200 C is shown in Fig 3.9. The spectra consisting of emission peaks at 369, 418, 484, 528 nm. The emission peak at 369 nm is due to charge transfer between ligand (O 2 2- ) to metal (Si). But, photoluminescence experiments at room temperature showed the emission band at ~370 nm in all products at excitation wavelength of 237 nm, the emission

14 83 peaks at 418, 484 and 528 nm are due to defects, the peak intensity of the samples increases with calcination temperature due to heat treatment of the sample and emission spectra as evidenced by results arrived by Carnall et.al. [131]. 369nm Excitation -237 nm Intensity(a.u) 418nm 484nm 528nm (b) (a) Wavelength(nm) Fig 3.9 PL Emission spectra of CaSiO3 (a) Calcined 950 C 3h (b) Calcined 1200 C 3h The peak intensity of PL in amorphous sample has been lower than the crystalline samples because the presence of nonradiative centres in the disordered structure. Moreover, the deficiency of oxygen during calcination it affects PL intensity. Because the nonradioative centers in the disorder structure are more in sample (a) when compared to the sample (b). The steady state luminescence of CaSiO3 has been discussed and the results are in line with those of Min`ko et al.

15 Conclusions In conclusion, CaSiO3 phosphor has been synthesized successfully by solution combustion method using diformylhydrazine as fuel. The method followed has several benefits such as low cost, energy efficiency, high production volume and above all, easy method of preparation. PL of the CaSiO3 phosphor was observed and analyzed. Experiment results showed that the calcinations temperature and defects in the CaSiO3 may affect the luminescence intensity of the sample.

16 Synthesis, Characterization and Photoluminescence properties of CaSiO3: Eu 3+ red phosphor Table of Contents Introduction Synthesis of Eu 3+ doped CaSiO3 nanophosphor Results and discussion Powder X-ray diffraction Scanning electron microscopy Fourier transform infrared spectroscopy UV-Visible absorption Photoluminescence Conclusions 101

17 Introduction Phosphors have acquired a place of prominence in modern technology mainly on account of their ability to convert electromagnetic radiation into light [132]. On account of their high efficiency emission performance, rare earth ions are used as activators in different host matrices. In addition to the common properties of rare earth elements, Europium, a special element in the lanthanides, displays the property of valence fluctuation with divalent or trivalent valence state. Europium also exhibits different luminescence characteristics due to different valences [133]. The emission spectrum of Eu 3+ ions (electronic configuration 4f 6 ) shows emission lines extending from visible region to the near infrared. Eu 3+ is an ideal element for experimental probe of the crystalline structure due to its relatively simple energy level structure, with its 5 D0 7 FJ range facilitating ascertainment of microscopic symmetry around the site [134]. Eu 3+ ions are extensively used in electroluminescence panels (EL), plasma display panels (PDP), higher efficiency fluorescent lamps etc since they are important emitters in the red region of the visible spectrum [135, 136]. In the present study, low temperature solution combustion route has been adopted to synthesize Eu 3+ doped CaSiO3 (1-5 mol %) nanophosphor. X-ray diffraction, scanning electron microscopy, UV-Vis and Fourier transform infrared (FTIR) spectroscopy are used the study

18 87 the structural properties while photospectrometre has been used to study the photoluminescent properties of the synthesized nanophosphor powder Synthesis of Eu 3+ doped CaSiO3 nanophosphor Calcium nitrate (Ca(NO3)2.6H2O), silica fumes (SiO2), europium oxide (Eu2O3) and DFH were used as the starting materials. In actual synthesis, europium oxide is transformed into europium nitrate. A cylindrical petridish of 300 ml capacity was used to form a solution of calcium nitrate and DFH in a minimum quantity of water. Fumed silica was added to the solution and the mixture was dispersed well for 5 to 10 minutes using a magnetic stirrer. The dish was then transferred into a furnace maintained at 500 C. Initially, the redox mixture melted and underwent dehydration followed by decomposition with the release of large amount of gases. The frothed mixture swells and the foam formed is ruptured with a flame resulting in incandescence. The foam further swells to the whole of the container during incandescence. For the preparation of CaSiO3:Eu 3+ the required mole ratio of Ca(NO3)2.6H2O:SiO2:DFH becomes 1:1:1.25 respectively Results and Discussion Powder X-ray diffraction studies

19 88 PXRD patterns of (1-5 mol %) Eu 3+ doped CaSiO3 phosphors are shown in Fig The samples are calcined at 950 C for 3hrs. All the samples display similar diffraction patterns and the XRD patterns readily index to β-casio3 triclinic phase with space group P-1(2) [137]. The diffraction peaks are in consonance with the JCPDS card No and tally with literature [138]. It is found that there were no significant changes in PXRD profiles with the increase in Eu 3+ content. Fig 3.10 PXRD patterns Eu 3+ doped (a) 1 mol% (b) 2 mol% (c) 3 mol% (d) 4 mol% (e) 5 mol% CaSiO3 phosphor The average crystallite size (D) of CaSiO 3: Eu 3+ phosphor was estimated from the full width at half maximum (FWHM) of the diffraction peaks by using Scherrer s formula (Ref Eqn 2.3, chapter-2). The average crystallite size of the Eu 3+ doped CaSiO3 phosphor is calculated by choosing strongest diffraction peaks (320), (202), (401). The crystallite size is found to be in the range nm.

20 89 It is well-established fact that the morphological characteristics of the products derived from combustion are heavily dependent on the heat and gas generated during the reaction. The heat released during the process contributes for crystalline growth whereas huge amount of gases released during combustion are conducive for formation of fine particles. The strain component for the samples are playing important role in crystal size hence to find strain we plot W-H plots for of the Eu 3+ doped CaSiO3 phosphor samples are shown in Fig The crystallite size of the sample determined from W H formula (Ref. Eqn 2.4, chapter-2) is slightly higher than that calculated using Scherrer s formula. The minor variation in the values is on account of the fact that the strain component is assumed to be zero in Scherrer s formula and the broadening of diffraction peak is attributed to reduction in crystallite size. The values of crystallite size (D) calculated from W-H plots and Scherer`s formula with the matching strains for various Eu 3+ doping concentration in CaSiO3, are shown in Table 3.2. (e) C o s ( d) ( c) ( b) ( a) o o o o S in Fig 3.11 WH plots of Eu 3+ doped (a) 1mol % (b) 2 mol % (c) 3 mol % (d) 4 mol % (e) 5 mol % CaSiO3 phosphor

21 90 Sample(Calcined at Crystallite size (nm) Strain (x10-4 ) 950 C, 3h) Scherer s method W-H method (D) CaSiO3:Eu 3+ 1mol % CaSiO3:Eu 3+ 2 mol % CaSiO3:Eu 3+ 3 mol % CaSiO3:Eu 3+ 4 mol % CaSiO3:Eu 3+ 5 mol % Table 3.2. Estimated values of Crystallite size (D) by W-H plots, Scherer`s formula and strain for different Eu 3+ (1 5 mol %) doping concentration in CaSiO Scanning electron microscopy Fig 3.12 (a -d) depict the SEM micrographs of un-doped and Eu 3+ doped CaSiO3 phosphors. The micrographs show irregularly shaped (most of them angular) crystallites to be having several voids and pores and the structure is attributed to escaping of gases during the combustion. The crystallites have no uniform shape and size. This is attributed to non uniformity in distribution of temperature and mass flow in the combustion flame. Combustion synthesised powders are characterised by typical porous network. The highly friable nature of porous powders facilitate easy grinding for obtaining finer particles. Pores are formed

22 91 simultaneously with small particles near the pores during the escape of gases. [22]. Fig SEM micrographs of (a) undoped (b)1 mol % Eu (c) 3 mol % Eu (d) 5 mol % Eu doped CaSiO3 phosphor Fourier transform infrared spectroscopy The structures of the samples were interpreted by recording the characteristic FTIR spectra of CaSiO3:Eu 3+ phosphors (Fig 3.13). IR

23 92 peaks at 465, 504, 691 and 964 cm -1 are due to β-casio3 [125, 128]. The presence of CaO in the structure is confirmed by the peak at 1460 cm -1. O Si O vibration is attributed to the absorption band at 750 cm -1 while absorption at is due to Si O vibration. (e) 80 (d) (c) Transm ittance (% ) (b) (a) Wavenumber (cm -1 ) Fig FTIR spectra of Eu 3+ doped (a) 1 mol % Eu ( b) 2 mol % Eu (c) 3 mol% Eu (d) 4 mol% Eu (e) 5 mol% Eu CaSiO3 phosphor UV-Visible absorption spectrum and optical energy gap The absorption bands of UV-Vis spectrum of un-doped and Eu 3+ doped phosphor recorded with UV-Vis diffuse reflection spectroscopy are shown in Fig An intense absorption band observed at nm corresponds to oxygen to silicon (O Si) ligand-to-metal

24 93 charge-transfer (LMCT) in the SiO 3 2 group. The bands in the range nm are due to the intra configurational 4f 4f transitions from the ground 7 F0 level, corresponds to the excitation spectra. The results conform to the reported literature [140]. The optical energy gap Eg of un-doped and Eu 3+ doped (1-5 mol %) CaSiO3 were calculated using Wood and Tauc relation [Ref. Eqn 2.4, chapter-2]. The absorption coefficients α was calculated from optical absorption spectra. The values of the optical band gaps of the undoped and Eu 3+ doped CaSiO3 are obtained by plotting (αe) n versus E in the high absorption range. The linear region of the plots was extrapolated to (αe ) n = 0 as shown in fig Fig UV-Vis absorption spectra of (a) undoped CaSiO3 (b) Eu 3+ (5 mol %) doped CaSiO3 Phosphor.

25 94 Fig The interpretation of the present data showed that the plots of (αe) n against hυ give linear relations which are the most fitted for above equation with n = 2 for both un-doped and Eu doped samples. The optical energy band gap for un-doped sample is found to be 5.51 V, for 3mol% and 5mol% the band gap is found to be 5.68 ev and 5.72 ev respectively. These values correspond to those reported in literature [141]. The band gap is less in un-doped sample compared to the doped one. 2.0x x10 9 ( h ) 2 1.0x10 9 (a) (b) 5.0x (c) Energy (ev) Fig Optical band gap of (a) Undoped CaSiO3 (b) 3 mol % Eu 3+ doped CaSiO3 (c) 5 mol % Eu 3+ doped CaSiO Photoluminescence (PL) studies Photoluminescence spectroscopy is an important tool to study the optical properties of phosphor materials. The room temperature

26 95 excitation PL spectrum of CaSiO3:Eu 3+ phosphor is shown in Figure The excitation spectrum shows a charge transfer band (CTB) of Eu 3+ O 2 band in the nearer ultraviolet region (250 nm) [142]. Efficiency of the energy process from the CTB to the Eu 3+ emitting level is the principal factor affecting the intensity of the charge transfer band. It is reported by Meng et al [143] that with increase in calcination temperature, the interaction between O 2 and Eu 3+ ions in the silicate host becomes stronger thus facilitating the electron transfer from O 2 to Eu 3+. According to Fan et al [144] the excitation peak at 394 nm is making stronger with the weak interactions between Eu 3+ and O 2. In the present study, the weak excitation peak at 393 nm suggests a stronger interaction between O 2 and Eu 3+ indicating an increase in the efficiency of the energy transfer process from charge transfer band (250 nm) to Eu 3+ emitting levels. The other observed peaks at 330 nm, 365 nm, 435 nm and 393 nm are assigned to the transitions from ( 7 F0 5D4), ( 7 F0 5 L7), ( 7 F0 5 D3) and ( 7 F0 5 L6) of Eu 3+ ion respectively. The prepared phosphor powder under UV light and absence of light is shown in Fig 3.17a and b respectively. The photoluminescence emission spectra of CaSiO3:Eu 3+ (1-5 mol %) phosphors excited at 254 nm are shown in Figure The strong and sharp peaks are attributed to 5 D0 7 FJ emission transitions, indicating the existence of Eu 3+ ions in CaSiO3 matrix. The spectra consists of well-resolved features at 581, 593, 614, 654 and 724 nm which can be explained as

27 96 due to 5 D0 7 FJ (J = 0, 1, 2, 3 and 4) [ ] transitions of Eu 3+ ions namely the 5 D0 7 F0 (581 nm), 5 D0 7 F1 (593 nm), 5 D0 7 F2 (614 nm), 5D0 7 F3 (654 nm), and 5 D0 7 F4 (724 nm), respectively Excitation spectrum 500 Intensity (a.u.) w avelength (nm) Fig Excitation spectra of 2 mol % Eu 3+ doped CaSiO3 phosphor The first transition is strongly forbidden but is observed with appreciable intensity. The 5 D0 7 F1 transition is allowed as magnetic dipole transition. This is the only transition when Eu3+ is located at a site coinciding with a centre of symmetry.

28 97 Fig CaSiO3: Eu 3+ (4 mol %) phosphor (a) Without UV light (b) Under UV light PL intensity (a.u.) b a 581 nm 593 nm 614 nm d PLIntensity c Eu mol % e 724 nm 654 nm W avelength (nm) Fig PL emission spectra of Eu 3+ (a)1mol % (b) 2mol % (c) 3mol % (d) 4mol % (e) 5mol % doped CaSiO3 phosphor

29 98 The 5 D0 7 F2 transition is induced when Eu 3+ is situated at a site which lacks the inversion symmetry and the process is allowed as forced electric dipole transition. This shift is much stronger compared to the transition to 7 F1 state. In addition, all the lines representing these transitions split into a number of components as decided by the local symmetry. The 593 nm line corresponding to 5 D0 7 F1 transition is split into two components 585 nm and 589 nm whereas no appreciable splitting is observed for 614 nm line. The intensity of 614 nm peak is twice more than that of 593 nm peak. The energy level diagram of Eu 3+ ions and possible pathways involved in the process are depicted in fig Eu 3+ ion is raised to 5 L6 level from the ground state when the sample is excited by 254 nm wavelength. Eu 3+ ion decays step wise from 5 L6 to 5 D0 level giving small quanta of energy to the lattice, during emission. The ion decays nonradiatively between 5 L6 and 5 D0 state. The stepwise decay process stops at 5 D0 7 FJ (J = 0, 1, 2, 3, 4) due to the large separation, and returns to ground state by giving emission in the orange and red regions. Eu 3+ impurity will give both magnetic and electric dipole transitions if the ion does not occupy centre of symmetry of the crystal lattice. Only magnetic dipole transition is allowed when the rare earth impurity ion is located at the centre of symmetry in the relevant crystal lattice. However, in the present case, emission lines indicating both magnetic and electric dipole transitions 5D0 7 F1 and 5 D0 7 F2 respectively are observed. Lack of centre of

30 99 symmetry is a cause for the forced electric dipole transitions. Magnetic dipole transition 5 D0 7 F1 is responsible for the emission in the vicinity of 600 nm. The emission in the range nm is due to the electric dipole transition of 5 D0 7 F2 which is induced by the lack of inversion symmetry at the Eu 3+ site, and is much stronger than that of the 7 F1 state. The 5 D0 7 F2 transition is dependent upon the local symmetry, whereas 5 D0 7 F1 emission may be related to the local symmetry due to insensitivity to the site symmetry. Since 5 D0 7 F2 transition is much stronger than 5 D0 7 F1 transition, the Eu 3+ ion in the CaSiO3 prepared by combustion synthesis is situated at the low symmetry sites. In doped CaSiO3:Eu 3+ phosphor, the Eu 3+ ion enters into Ca 2+ lattice site. The ionic radii of Eu 3+ and Ca 2+ are nm and nm, respectively. Since ionic radius of Ca 2+ is smaller than Eu 3+, CaSiO3 host could accommodate only small percentage of impurity ions. Doping of trivalent Eu 3+ cations results in charge imbalance in the host lattice. The imbalance may trap emitted light, decreasing the intensity. The doping concentration of Eu 3+ in CaSiO3 phosphor casts an effect on the relative intensity of the emission lines of Eu 3+. The transition 5 D0 7 F2 (614 nm) shows an enhanced emission with the increase in Eu 3+ concentration; the transition first reaching to a maximum value at x = 4 mol% and then decreasing with an increase of Eu 3+ content (inset of Fig. 3.18) due to concentration quenching. The

31 100 concentration quenching of Eu 3+ luminescence might be due to the cross-relaxation. Fig The energy level of Eu 3+ ion diagram showing the states involved in the luminescence process and the transition probabilities Conclusions A novel, low-temperature initiated, self-propagating and gas producing solution process has been successfully employed to synthesize Red phosphors of CaSiO3:Eu 3+ (1-5 mol %). The study of PXRD patterns reveal that β-casio3 phase can be obtained at C. The crystallites, as shown by SEM micrograhs, are irregular shape, mostly angular, and also contained several voids and pores. The UV- Vis absorption of undoped and doped phosphor show an intense

32 101 absorption band at nm corresponding to oxygen to silicon (O Si) ligand-to-metal charge-transfer (LMCT) in the SiO3 2 group. The PL emission spectra of CaSiO3:Eu 3+ (1-5mol %) phosphor excited at 254 nm shows strong and sharp peaks which are attributed to 5D0 7 FJ emission transitions. The spectra consist of well resolved features at 581, 593, 614, 654 and 724 nm which can be assigned to 5 D0 7 F0 (581 nm), 5 D0 7 F1 (593 nm), 5 D0 7 F2 (614 nm), 5 D0 7 F3 (654 nm), and 5 D0 7 F4 (724 nm), respectively. The optical band gap value is less in undoped sample when compared to Eu 3+ doped CaSiO3. The applied combustion process is simple and fast, the starting materials are easily available and economic. Further the excellent properties of this phosphor suggest that it can be used for display applications.

33 Synthesis and luminescencent properties of CaSiO3:Dy 3+ nanophosphor Table of Contents Introduction Synthesis of Dy 3+ doped CaSiO3 nanophosphor Results and discussion Powder X-ray diffraction Scanning electron microscopy Transmission electron microscopy Fourier transform infrared spectroscopy UV-Visible absorption Photoluminescence Conclusions 121

34 Introduction The novel luminescent properties [ ] of nanophosphors particularly those with three-dimensional (3D) microfabrications has triggered extensive research on the synthesis and characterization of nanophosphors in recent years. Of these, the synthesis and characterization of nanophosphors doped with rare earth (RE) elements is of particular interest, the focus being on its luminescence properties and their variation with reduced dimensionality. The energy efficiency, greater radiation stability, colour rendering index and improved lumen output render RE ions effective luminescent centres in host lattices [ ]. Dy 3+ ions have been investigated extensively since they give out two typical emission bands in the blue (480 nm) and yellow (570 nm) regions, the quintessential requirement for full colour displays [ ]. Moreover, the crystal field and radial integral of 4f and 5d electrons greatly influence the emission probability of electric-dipole transitions. Thus, the study of luminescence properties of Dy 3+ ion in different host lattices proves itself to be an interesting investigation. Silicates are considered one of the best host materials for luminescent centres due to their excellent chemical and thermal stability. When mixed with RE ions or transition metal ions, silicates become phosphor powders that produce excellent luminescence in the blue, green and red regions of the spectrum. Furthermore, considerable attention has been paid to silicate hosts because of their stable crystal structure, high physical and chemical stability, good

35 104 formability, long persistence time, relatively easy preparation, multicolour phosphorescence and resistance to acid, alkali and oxygen [158, 159]. Phosphors are generally prepared by the traditional method involving solid-state reaction. The product can contain several impurity phases on account of insufficient mixing and low reactivity of raw materials. Moreover, the size of phosphor particles synthesized by solid-state reaction is large, usually in the micrometer range [160]. Combustion synthesis has several advantages over solid-state reaction method. Simple experimental set-up, quick synthesis with very short time between the preparation of reactants and availability of the product, low cost and energy conservation are some of the several merits of combustion method. A study of the literature discloses that there are no earlier reports of preparation of Dy 3+ activated CaSiO3 nanophosphors through solution-combustion route. This chapter presents a study of luminescence properties of different CaSiO3: Dy 3+ nanophosphors of different compositions prepared by the combustion process Synthesis of Dy 3+ doped CaSiO3 nanophosphor Stoichiometric quantities of analytical grade calcium nitrate (Ca(NO3)2.6H2O), silica fumes (SiO2), dysprosium oxide (Dy2O3) and fuel (diformylhydrazine, DFH) were used as the starting materials for the synthesis. Dysprosium oxide is converted into dysprosium nitrate in

36 105 the actual process. Calcium nitrate and DFH were dissolved in minimum quantity of water in a 300-ml capacity cylindrical petridish. Fumed silica was added to this and the mixture was dispersed for 5 to 10 minutes with magnetic stirrer. The heterogeneous reaction mixture was heated over a muffle furnace heated to a temperature of 500 C). In the earlier stage, the mixture boils and thermally dehydrates forming foam. The foam ultimately ruptures with a flame resulting in incandescence. During the process, the foam swells to the entire capacity of the container. The whole combustion was over within five minutes. The flow chart for synthesis of CaSiO3:Dy 3+ phosphors prepared by solution combustion method is given in fig Results and discussion Powder X-ray diffraction The powder X-ray diffraction (PXRD) profiles of Dy 3+ - doped CaSiO3 (1-5 mol. %) phosphors, calcined at 950 C for 3h are shown in fig The patterns for 1-4 mol % Dy- doped samples readily index to the β-casio3 phase. The peak positions conform to the literature values. However, when doped with 5mol. % Dy, the sample exhibited γ- CaSiO3 phase (JCPDF ) along with small traces of the orthorhombic CaSiO3 phase.

37 106 Calcium Nitrate + Dysprosium nitrate + Diformyl hydrazine+ silica fumes Preheated muffle furnace Combustion Final Product CaSiO 3 :Dy 3+ Fig Flow chart for synthesis of CaSiO3: Dy 3+ phosphor All the diffraction peaks tallied well the standard values from the joint committee on powder diffraction standards (JCPDF card No ) and indexed to the monoclinic phase [161, 162]. Scherrer s formula [Ref. Eqn 2.3, chapter-2] was used to estimate the average particle size (D) of CaSiO3: Dy 3+ phosphors from the full width at half

38 107 maximum (FWHM) of the diffraction peak of the powders. The size was found to be in the range nm. It was observed that different impurities affect the size of CaSiO3: Dy 3+ nanoparticles with the same preparation condition. The analysis described by W-H method (Ref. Eq. 2.4, chapter 2) was followed to estimate the average crystallite size by powder X-ray diffraction line broadening. Fig 3.22 shows the W-H plots of Dy doped CaSiO3. A comparative study indicates that the grain size determined from W-H method is slightly higher than that calculated from Scherer formula. The minor variation is because under the Scherer s method, the strain component is assumed to be zero and observed broadening of diffraction peaks is considered to be the result of reducing grain size only. It is well established that the morphological characteristics of the powder prepared through combustion method strongly depend on the amount of heat and the gases generated during the complex decomposition. The heat released during the process is a crucial factor for crystalline growth and huge amount of gases relased facilitate formation of fine particles.

39 108 (e) Intensity (a.u.) (d) (c) (b) (a) (deg) Fig PXRD Patterns of Dy 3+ doped (a) 1 mol % (b) 2 mol % (c) 3 mol % (d) 4 mol % (e) 5 mol % CaSiO3 nano phosphor (e) (d) cos (c) (b) (a) sin Fig W-H Plots of Dy 3+ doped (a) 1 mol % (b) 2 mol % (c) 3 mol % (d) 4 mol % (e) 5 mol % CaSiO3 nano phosphor

40 109 The incorporation of suitable alkali atoms in to the lattice of CaSiO3: Dy 3+ improves the crystalline and better particle formation. However, with inverse of concentration, the excess atoms are aggregated on the grain boundaries due to low solubility of alkali atoms in CaSiO3: Dy 3+ Such surface states would affect the surface kinetics and affect the morphology as seen in CaSiO3: Dy 3+ samples with higher concentration. Sample(Calcined at 950 C, 3h) Crystallite size (nm) Scherer s W-H method method Strain (x10-4 ) CaSiO3: Dy 3+ 1 mol % CaSiO3: Dy 3+ 2 mol % CaSiO3: Dy 3+ 3 mol % CaSiO3: Dy 3+ 4 mol % CaSiO3: Dy 3+ 5 mol % Table The values of D obtained from W-H plots and Scherer`s formula and the respective strains for different Dy (1-5 mol%) concentration in CaSiO3

41 Scanning electron microscopy (SEM) The powders also show a porous structure. The system undergoes violent combustion since the redox reaction mixture with nitrate salts is strongly exothermic. The pores formed are sparse and irregular owing to the uncontrolled dynamics of the process. However, the porous structure is distributed non-uniformly in the matrix. (a) (b) (c) (d)

42 111 (e) (f) Fig SEM micrographs of (a) Undoped and Dy 3+ doped (b) 1 mol % (c) 2 mol % (d) 3 mol % (e) 4 mol % (f) 5 mol % CaSiO3. Non-uniform distribution of temperature and mass flow in the combustion flame are believed to be responsible for the non-uniformity of shape. Small particles and pores are the characteristics of the phosphors obtained from the combustion process [163]. The phosphors becomes more porous with an increase of of Dy dopant, as shown in figures 3.22 (b f) Transmission electron microscopy (TEM) The TEM image of 2 mol % Dy 3+ - doped CaSiO3 heat treated at 950 0C for 3 h at fig 3.24 shows that the particles are irregular in shape and size. The fact is further confirmed from SEM micrographs. The mean particle size is observed to be 50 nm. The mean particle size calculated from Scherrer`s formula and the one observed in TEM image is almost the same. Bright spots showed in the corresponding selected area electron diffraction (SAED) (inset in Fig.3.24) study indicate single

43 112 crystal nature and high degree of crystallanity of the synthesized product. Fig TEM image of 2 mol% Dy 3+ doped CaSiO Fourier transform (FTIR) infrared spectroscopy The FTIR spectra of CaSiO3:Dy 3+ phosphors are shown in Figure The analysis confirmed that the IR peaks at 465, 504, 691 and 964cm -1 are due to β-casio3 [164, 165]. The existence of Cao in the structure it established by the peak at 1460 cm -1. O-Si-O vibrations are responsible for the absorption band at 750 cm -1 and the absorption at cm -1 is from Si-O vibration.

44 113 Fig FTIR spectra of CaSiO3: Dy 3+ (a) 1mol % (b) 2mol % (c) 3 mol% (d) 4 mol% (e) 5 mol% UV-Visible absorption spectrum and optical energy gap Diffuse reflection spectroscopy was adopted to measure the UV- Visible absorption spectra of Dy 3+ doped (1-5 mol %) CaSiO3 phosphor (Fig 3.26). The spectra were deconvoluted and displayed an absorption band at ~ nm, corresponding to oxygen to O-Si ligand-to-metal charge-transfer (LMCT) in the SiO 3 2- group. The broad band at ~ nm is explained by the intra-configurational 4f - 4f transitions from the ground 7 F0 level, corresponding to the excitation spectra. The results are in conformity with those reported in the literature [166]. Fig 3.26 (a -e) shows the optical energy gaps, Eg, of undoped and Dy 3+

45 114 doped (1-5 mol. %) CaSiO3 estimated using Wood and Tauc relation [167, 168]. The absorption coefficients were calculated from the optical absorption spectra. A graph of (αhν) n versus hν in the highabsorption range followed by extrapolation of the linear region of the plots to (αhν) n =0 was plotted to obtain the values of the optical band gaps of the undoped and Dy 3+ doped CaSiO nm Absorbance (a.u.) nm (d) (b) (a) (c) wavelength (nm) (e) Fig UV-Vis absorption spectra Dy 3+ doped CaSiO3 (a) 1mol % (b) 2 mol % (c) 3 mol% (d) 4 mol% (e) 5 mol% An analysis of the data available showed that the plots for both undoped and Dy- doped samples give linear relationships satisfying the above equation with n = 2. The optical energy band gap for undoped

46 115 sample is found to be 5.45 ev and it varies between 5.49 and 5.65 ev for Dy 3+ - doped (1-5 mol. %) phosphors (Fig 3.27). The values tally with those reported in literature [169]. The optical band gap is found to be lower in the undoped sample compared to Dy- doped CaSiO3. Fig Optical energy gap of (a) 1mol % (b) 2 mol % (c)3 mol % (d) 4 mol % (e) 5 mol % of CaSiO3: Dy Photoluminescence studies (PL) PL spectroscopy study is an important tool to understand the optical properties of the nanophosphor material. Fig 3.28 shows excitation spectrum (in the range nm) of 2 mol % of CaSiO3: Dy 3+ nanophosphor, measured with the emission wavelength of fixed at 577 nm corresponding to the electronic transition 4 F9/2 6 H13/2. The spectrum is characterized by sharp and broad excitation bands at 258 nm, assigned to the host absorption band (HAB). It also displays broad excitation bands at nm regions. Clearly, the later arise from f-

47 116 f transitions of Dy 3+ within its 4f 9 configuration, which are assigned to the electronic transitions ( 6 H15/2 5 P3/2) at 320 nm, ( 6 H15/2 5 P7/2) at 351 nm, ( 6 H15/2 5 P5/2) at 366 nm, ( 6 H15/2 4 I13/2) at 378 nm. CTB Intensity (a.u.) 6 H 15/2 6 P 3/ H 15/2 5 P 6/2 6 H 15/2 4 I 13/2 378 emi = 574nm Wavelength (nm) Fig Excitation spectrum of the CaSiO3: Dy 3+ (2 mol %) nanophosphor. Fig 3.29 shows the concentration effect (1-5 mol. %) on the emission of CaSiO3: Dy 3+ nanophosphors excited at 351nm. The photoluminescence (PL) spectra comprises three main groups of lines in the blue ( nm) and yellow ( nm) regions. Apart from this, some weak lines are observed in red (677nm) region. The blue, yellow and red emissions are attributed to the electronic transitions 4F9/2 6 H15/2, 4 F9/2 6 H13/2 and 4 F9/2 6 H11/2 respectively.

48 117 The blue emission ( 4 F9/2 6 H15/2) pertains to magnetic dipole transition while yellow ( 4F9/2 6 H13/2) emission represents hypersensitive (forced electric dipole) transition with the selecti on rule J =2 [53-55]. The transitions of excitation and emission spectra have been assigned based on the earlier results of Carnall et al [170]. 1mol% Dy 2mol% Dy 3mol% Dy 4mol% Dy 5mol% Dy 4 F9/2 6 H13/2 exi =351 nm F9/2 H15/2 F9/2 H11/2 PL Intensity (a.u) Wavelength (nm) Dy conc. (mol %) Fig PL emission spectra of Dy 3+ doped (a) 1 mol % (b) 2 mol % (c) 3 mol % (d) 4 mol % (e) 5 mol % CaSiO3 nanophosphor The crystal-field splitting components of Dy 3+ correlate well with the Kramer s doublets, (2J+1)/2, where J is the angular momentum of the electrons. The correlation indicates that Dy 3+ ions are well substituted into Ca 3+ sites, since the ionic radius of Dy 3+ (0.103 nm) is less than Ca 3+ (0.106 nm). The intensity of yellow emission is greater than that of the blue emission in the case of CaSiO3: Dy 3+

49 118 nanophosphors. The difference could be explained thus: The hypersensitive (forced electric dipole) transition is known be to strongly influence by external environment whereas the magnetic dipole transition is not sensitive to crystal field strength around Dy 3+ ions. The yellow emission is often dominant in the emission spectrum when Dy 3+ is located at a low-symmetry local site (without inversion symmetry). The blue emission is dominant in the emission spectrum and stronger than the yellow emission when Dy 3+ is at a highsymmetry local site (with inversion symmetry ) [171]. The PL intensity goes upto 2 mol % of Dy 3+ and later the intensity of the emission decreases with further increase of Dy 3+ dopant concentration. The behaviour can be explained by concentration quenching. Two factors enhance the concentration quenching - (i) the excitation migration due to resonance between the activators enhances with the increase in doping concentration, and thus the excitation energy reaches quenching centres, and (ii) the activators are paired or coagulated and changed to quenching centre [172]. In addition, it is found that the yellow to blue (Y/B) ratio depends on the concentration of Dy 3+. Thus, the ratio increases when the concentration increases to 2 mol%, and Y/B decreases as the concentration increases above 2 mol%. The phenomena can be understood if one considers the J = 2 transition probability changes with polarity of the neighbouring ions.

50 o C 1050 o 4 6 C F9/2 H13/ o C 4 F9/2 6 H15/2 exi = 351 nm 4 F9/2 6 H11/2 Intensity (a.u) Wavelength (nm) Temperature ( o C) Fig PL emission spectra of heat-treated (950 C, 1050 C, 1150 C for 3h) CaSiO3: Dy 3+ (2 mol %) phosphor. The study of emission intensity of Dy 3+ with respect to annealing temperature is depicted in figure The intensity of the yellow emission goes up as the temperature rises from C. The behaviour could be explained as the improved crystallization of the products which reduces defect concentration and a more uniform distribution of Dy 3+ in host lattices. The energy level scheme adopted to explain the mechanisms involved in the emission processes of CaSiO3:Dy 3+ nanophosphors [173, 174] is shown in fig 3.31.

51 H A B 5 P 3/2 5 P 7/2 4 I 13/2 Energy (x 10 3 cm -1 ) 258 nm 320 nm 351 nm 378 nm 482 nm 577 nm 677 nm 4 F 9/2 6 H 11/2 6 H 13/2 0 6 H 15/2 Fig The energy level scheme of CaSiO3; Dy 3+ phosphors Conclusions Combustion method has been followed for the first time to prepare 1-5 mol% Dy 3+ doped CaSiO3 nanophosphors using DFH as fuel. In this process, the reaction temperature for the formation of CaSiO3 is found to be lower compared to other methods and this is one of the important result of the present work. The PL of the Dy 3+ doped CaSiO3 phosphor has been observed and analyzed. The emission peaks at 483, 573 and 610 nm corresponds to emission of Dy 3+ and can be attributed

52 121 to the transitions of 4F9/2 6 H15/2, 4 F9/2 6 H13/2 and 4 F9/2 6 H11/2 respectively and dominated by the Dy 3+ 4 F9/2 6 H13/2 hyperfine transition. The emission pattern has been explained in terms of an energy level scheme. The results disclose that heat treatment and concentration of Dy 3+ in the CaSiO3 host may affect the luminescence intensity. Optimum luminescence conditions can be obtained with 2 mol% concentration of Dy 3+. The optical energy band gap of undoped phosphor is found to be lower than that of the Dy 3+ doped nanophosphors.