Higher Intrinsic Quantum Yield of Eu3+-Doped Zn2sio4 Obtained by Sol-Gel Process, Calculated by Judd - Ofelt Theory

Adriana Souza de Oliveira1, Juliana Maria Martins Buarque1, Luiz Fernando Cappa de Oliveira2, Clebio Soares Nascimento Jr.3, Marco Antonio Schiavon1, Helliomar P. Barbosa4, Jose Henrique Faleiro4 and Jefferson Luis Ferrari1,2*

1Grupo de Pesquisa em Quimica de Materiais – (GPQM), Departamento de Ciencias Naturais (DCNAT), Universidade Federal de São João del-Rei (UFSJ), Campus Dom Bosco (CDB), Praca Dom Helvecio, 74, 36301-160, Sao Joao del-Rei, MG, Brazil
2Núcleo de Espectroscopia e Estrutura Molecular (NEEM), Universidade Federal de Juiz de Fora (UFJF), Juiz de Fora, Minas Gerais, José Lourenço Kelmer, s/n – Martelos, Brazil, 36036-330
3Universidade Federal de São Joao del-Rei (UFSJ), Campus Dom Bosco (CDB), Praca Dom Helvecio, 74, 36301-160, São João del-Rei, MG, Brazil

4Desenvolvimento de Materiais Inorgânicos com Terras Raras (DeMITeR), Laboratorio de Materiais Fotoluminescentes (LAMAF), Instituto de Quimica – (IQ), Universidade Federal de Uberlândia – (UFU), Av. Joao Naves de Ávila, 2121 – Bairro Santa Mônica, CEP: 38400-902, Uberlândia, MG, Brazil

Corresponding author: Prof. Jefferson Luis Ferrari, Grupo de Desenvolvimento de Materiais Inorgânicos com Terras Raras (DeMITeR),Laboratório de Materiais Fotoluminescentes (LAMAF – Bloco 3Z), Universidade Federal de Uberlândia Instituto de Química – Bloco 1D, Av. João Naves de Avila, 2121 – Santa Mônica, Uberlândia – MG, Brazi E-mail: jeffersonferrari@gmail.com or jeffersonferrari@ufu.br

Citation: Luis Ferrari J, Adriana Souza de O, Juliana Maria MB, Luiz Fernando Cappa de CO, et al. (2020) Higher Intrinsic Quantum Yield of Eu3+-Doped Zn2sio4 Obtained by Sol-Gel Process, Calculated by Judd - Ofelt Theory. Int J Mater Res Sci Tech 1(1):1-14.

https://dx.doi.org/10.47890/IJMRST/2020/JLFerrari/14205431

Received Date: May 15, 2020; Accepted Date: May 21, 2020; Published Date: May 28, 2020

Abstract

The sol-gel process allows the obtaining of Eu3+-doped Zn2SiO4with 1 mol% with different molar ratios between Zn2+ and Si4+. The materials reported here were obtained with a resistive furnace at 600, 900, 1000 and 1100°C degree, during 2 h. All the materials exhibited an intense photoluminescence emission with bands assigned to the Eu3+ transitions. Among the materials studied, the samples containing 70 and 80% of Si4+, exhibited higher Eu3+ lifetimes values. The predominance of the quartz phase decreases micro deformations and the location of the Eu3+ ions in higher symmetry sites, reducing the intensity of the photoluminescent materials. The increasing of annealing temperature contributes to the detrimental lifetime and luminescence materials due to the location of the Eu3+ in the site of high symmetry. Through Judd-Ofelt theory was possible to obtain higher values of intrinsic quantum yield (QLn Ln) between 32 and 66% dependent on the temperature of heat-treatment, in comparison with the same material reported in the literature.

Keywords: Orthosilicates; Photoluminescence; Rare Earth; Europium;

Introduction

Among the many classes of materials that currently contribute to the improvement of the worldwide quality of people’s lives, those with photoluminescent properties are among the most sought after and researched [1]. Currently, there is interest in the development and improvement of photoluminescent materials with higher efficiency on its properties and also high stability in chemical characteristics [2–7]. The luminescent properties mainly of inorganic materials are very attractive, due to their potential applications in optoelectronics [8], photonics, optics [4,9–15], as light-emitting diodes (LEDs) [16], sensors, scintillators[6, 7, 17], solar cells [7, 18–21], lasers and others [4,22–29]. Typically these materials can be obtained with a composition in which comprise a host matrix and an activator Rare Earth (RE3+) ion emitter [5,31–33].

RE3+ doped different kinds of materials are very attractive for applications in photoluminescence due to their intrinsic spectroscopic characteristics, that are dependent on the electronic configuration and consequently of f-f intraconfigurational transitions [34, 35]. Due to the lanthanide contraction properties, the f orbitals are located internally, making these orbitals be weakly affected by environment or crystal field, resulting in thin spectral lines and monochromatic spectra characteristics [32, 36, 37].

Among the most promising luminescent materials, zinc orthosilicate, Zn2 SiO4 , shows interesting properties like high photoluminescent efficiency, transparency from UV to the visible range, high refractive index (~ 1.7), high-energy of band gap (5.5 eV) and crystalline structure with high thermal and mechanical stability, making it a suitable host matrix for RE3+ [38, 39]. Dependent on the characteristics of the energy level of RE3+ dopant ion, these materials can be an emitter of electromagnetic radiation in a wide range from ultraviolet to medium infrared region. Emitter materials in the blue, green and red region can be obtained easily when the Ce3+, Tb3+, and Eu3+, are present in the host matrix [40]. RE3+-doped Zn2 SiO4 has applications in many optoelectronic devices, such as cathode ray tubes, light emitting diodes, coverings, lamps, flat screen monitors, amplifiers, lasers, sensors and radiation detectors for medical imaging systems [31,37,41 and 42].

Especially in search of materials that can emit in the red region of the electromagnetic spectrum, the Eu3+ it is widely used due mainly to its emission attributed to the transition 5D0 →7F2 of f-f intraconfigurational transition. This emission is widely studied in the exact literature as it contributes to the use of energy in the region of approximately 612 nm [43]. The Eu3+ ion doped Zn2 SiO4 host lattice and due to the intraconfigurational transitions 4f-4f can show narrow bands in the visible spectrum, which can improve the luminescence efficiency of these phosphors [44]. In addition, these matrices can act as an excellent sensitizer transferring part of their energy to the emitting 5 D1 , 5 D0 levels of the Eu3+ ion. Moreover, the principal emitting 5 D0 level is non-degenerate and exhibits a long phosphorescence decay time (around milliseconds).

Many kinds of the experimental procedure for obtaining of Zn2 SiO4 are reported, that include the conventional solid-state reaction, solution-based syntheses like sol-gel process, coprecipitation, hydrothermal method, and others. Among them, the solid-state reaction is the most used to produce silicate due to its simplicity of operation and suitability for industrial applications. However, this procedure is not feasible due to the high temperatures of calcination, long reaction time, and difficulty in controlling and agglomerating particles [45, 46]. The sol-gel process is quite feasible because the reactions of precursor’s solutions are conducted at low temperatures of reaction, resulting in products with good crystallinity and small diameter of particles. Another very important point that needs to be taken into account is the homogeneity of the components in the precursor solution that improve the high quality in the material final composition [47, 48].

Among many ways of RE3+ doped zinc orthosilicates applications, such as those previously reported, White Light Emitting Diodes (WLED’s) are receiving great attention because of their luminescent efficiency, low power consumption, the long service life of the devices [49, 50]. Luminescent materials for WLED’s applications normally combine material that emits blue light (In ,Ga)N and another that emits yellow light, usually cerium doped yttrium aluminate (Y3 Al5 O12), (YAG: Ce) [44, 51]. These WLED’s emit a bluish cold light and are normally used in lighting systems that require a long lifetime of use, being applied in car lights, traffic lights, external lighting, among others. If, on the one hand, these WLED’s have high luminous efficiency, they still have drawbacks to color reproduction, specifically the weakness of the red-light component due to thermal and suppression of luminescence. Therefore, it is necessary to develop warm WLED’s and more efficient color reproduction, which can be used in more sophisticated applications such as indoor lighting. Devices made by combining blue LEDs with phosphors that emit green and red and LED light with red, green, and blue phosphors are among the alternative methods of generating white light. These WLED’s support multiple broadband emitters, improving color reproduction, which makes the light source more similar to sunlight, needed for more specific applications [52, 53].

To contribute with the understanding of better photoluminescence properties in the materials obtained, the JuddOfelt theory is an interesting procedure to obtain information about radiative transition rates, nonradiative rates, and emission intrinsic quantum yield when it comes based on RE3+ doped materials [54– 56].

Considering all these needs and applications, this study aims to apply the sol-gel process for preparing of Eu3+-doped zinc silicate with low dopant concentration (1 mol%). Besides, the influence of the heat-treatment temperature and the ratios between the amount of silicon (Si) and zinc (Zn) used in the precursor solution was verified. As a base of the obtained materials, its photoluminescent properties and structural characteristics of the obtained materials were evaluated to design a possible application in photonic materials.

Experimental

Sample Preparation

The syntheses were performed by sol-gel process with the molar ratios of Si:Zn of 90:10 (9SZ), 80:20 (8SZ), 70:30 (7SZ), 60:40 (6SZ) and 50:50 (5SZ), with Si+Zn = 0.445 mol L-1 were based on the work reported by Ferrari, J.L. and [14,57,58]. The tetraethoxysilane, TEOS (98% - Sigma Aldrich), and europium oxide, Eu2 O3 (Sigma Aldrich – 99.99%), were used as silicon and rare-earth precursors, respectively. The doping was performed at 1 mol% of Eu3+, calculated concerning the total number of moles of Zn+Si. The stock solution of Eu3+ 0.01 mol L-1 was prepared and standardized with EDTA 0.01 mol L-1 to ensure effective doping concentration in each material obtained. In this point it is important to report that de RE2 O3 behaves as a primary standard. Initially, two solutions were prepared separately. The first one containing TEOS, anhydrous ethanol and hydrochloric acid (0.27 mol L-1), with the molar ratio between TEOS:HCl equal to 1:0.007, respectively. The second one is composed of a mixture of Zn2+ and Eu3+ in an anhydrous alcohol solution. Both solutions were mixed, resulting in 20 mL of the final solution, and kept under stirring for 15 min, at room temperature, for homogenization of the components to obtain the sol. The sol was kept in an oven at 60 °C for 24 h to obtain the xerogel. The xerogels obtained were grounded in an agate mortar and then were submitted to the heat-treated in an oven at 600, 900, 1000 and 1100°C for 2 h with a heating rate of 10°C min-1. The xerogels obtained at 60°C were analyzed by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) using a Shimadzu thermal analyzer, model DTG-60H with a heating rate of 10 °C min-1 under air atmosphere from room temperature up to 1100°C. The FT-IR spectra of the materials after heat-treatment at different temperatures, as well as their precursors, were obtained using a Perkin-Elmer Spectrum GX spectrometer, in the region of 4000-400 cm-1. The samples in powder form were prepared in the form of pellets, diluted in spectroscopy degree KBr under 10 tons of pressure. In order to obtain information about the crystalline structure, the materials were analyzed by X-ray diffraction (XRD), with a Shimadzu diffractometer, CrKα (λ = 2.2909 Å) radiation, in the 2θ range of 10-70° with a scanning rate of 0.02° s-1. The positions and intensity of the peaks were compared with the standard JCPDS card to identify the crystalline phase formed. Through the analysis of data from XRD patterns, the crystallite size of the samples was calculated using Debye–Scherrer’s Equation (1) [59–63].


k . λ Δβcoscosθ                 (1) MathType@MTEF@5@5@+= feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qadaWcaaWdaeaapeGaam4AaiaabccacaGGUaGaaeiiaiabeU7aSbWd aeaapeGaeuiLdqKaeqOSdiMaci4yaiaac+gacaGGZbGaci4yaiaac+ gacaGGZbGaeqiUdehaaiaabccacaqGGaGaaeiiaiaabccacaqGGaGa aeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabccaca qGGaGaaeiiaiaabccacaqGOaGaaeymaiaabMcaaaa@519C@

in which D is the nanocrystallite size (nm), K is the shaper factor (0.9) (in which the use this value is considering the spherical shape of the particles), λ is the wavelength of CrKα radiation, Δβ is the correction of full width at half maximum (FWHM) of the most intense diffraction peak and θ is the diffraction angle. To correct the FWHM, Si with particle size >20 mm was used as a pattern. The calculation of lattice strain (micro deformations) was achieved with the classic Williamson-Hall method (W-H) [64].


ΔK 2 = 0.9 D WH 2 +4 ε 2 K 2                  (2) MathType@MTEF@5@5@+= feaahqart1ev3aqatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq=Jc9 vqaqpepm0xbba9pwe9Q8fs0=yqaqpepae9pg0FirpepeKkFr0xfr=x fr=xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaaeaaaaaaaaa8 qadaqadaWdaeaapeGaeuiLdqKaam4saaGaayjkaiaawMcaa8aadaah aaWcbeqaa8qacaaIYaaaaOGaeyypa0ZaaeWaa8aabaWdbmaalaaapa qaa8qacaaIWaGaaiOlaiaaiMdaa8aabaWdbiaadseapaWaaSbaaSqa a8qacaWGxbGaeyOeI0IaamisaaWdaeqaaaaaaOWdbiaawIcacaGLPa aapaWaaWbaaSqabeaapeGaaGOmaaaakiabgUcaRiaaisdacqaH1oqz paWaaWbaaSqabeaapeGaaGOmaaaakiaadUeapaWaaWbaaSqabeaape GaaGOmaaaak8aacaqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabcca caqGGaGaaeiiaiaabccacaqGGaGaaeiiaiaabccacaqGGaGaaeiiai aabccacaqGGaGaaeiiaiaabIcacaqGYaGaaeykaaaa@583D@

Wherein ∆K=2 cosθ Δβ/λ; Δβ is the correction of FWHM; θ is the Bragg angle; λ is the wavelength of the incident radiation CrKα (2.2909 Å); K = 2 senθ/λ and DW-H is the size of crystallite; ε is the lattice strain (microdeformation). Using the data of the diffraction peaks to build a linear curve with(∆K)2 as a function of K2 whose slope gives the value of the strain of the crystal lattice. The photoluminescence emission spectra, between 550 and 750 nm, were obtained at room temperature using a spectrofluorometer SPEX Fluorolog F2121/Jobin-Yvon-HORIBA with excitation fixed at 255, 394 and 463 nm. The excitation and emission slits used were 4 and 2 nm, respectively. The filter cut-off bellow 490 nm was used in the entrance of emission monochromator. The study of the emission decay curves as a function of time to obtain the values of the lifetime of the excited state of the Eu3+ was conducted with the same equipment, at room temperature. The samples were excited using a pulsed lamp in the same wavelengths of the excitation reported before, while the wavelength of emission was fixed at 612 nm. The slits used in this experiment were the same used to obtain the emission spectra. The morphology of the obtained materials were analyzed by Scanning Electron Microscopy (SEM), using a Hitachi TM 3000 bench microscope with 1000X of magnification and a voltage of 15 eV.

Results and Discussion

The profile of the thermal decomposition of xerogels may indicates the temperatures used in the heat-treatment of precursors for obtaining the desired material. The concomitant analysis of TG and DTA curves permits the identification of the temperatures associated with the crystallization process of the zinc silicate. Figure 1 shows the TG and DTA analysis of the synthesized xerogels. The steps of TG and DTA curves of the sample with the highest percentage of zinc, Figure 1(A), 50 mol% of zinc, are very well defined when compared to the curves obtained from the other xerogel. DTA curves exhibit an intense endothermic peak around 104°C in which may be associated with the evaporation of adsorbed water and organic compounds from solutions precursors. There are consecutive events associated with the loss of mass between 182 and 550°C that may be associated with the decomposition of organic groups from TEOS used as a precursor. The presence of the events related to the burning and decomposition of organic compounds is important once the conversion of precursors in Zn2 SiO4 can be induced during the removal of these species. DTA curves suggest a slow crystallization of the samples, because a well-defined exothermic peak is not observed in temperatures above 600°C. The exothermic broad peaks positioned between 808 and 900°C are attributed to the crystallization of the silicate phase and phase transition from β-Zn2 SiO4 to α- Zn2 SiO4 . The total mass loss, shown in Table 1 changes significantly as a function of xerogel samples composition. Xerogels containing higher amounts of silicon in the composition, Figure 1 (B) (90% of Si), presents less mass loss due to the formation of a dense SiO2 network structure.

Table 1: Loss of mass of the samples during thermogravimetric analysis.

Composition

Mass loss/%

5SZ

62

6SZ

58

7SZ

41

8SZ

39

9SZ

33

Figure 1: TG and DTA curves of xerogel samples for (A) 5SZ and (B) 9SZ.

Analyses of FT-IR (Figure 2) were performed to monitor the structural changes as a function of heat-treatments and different compositions of materials. The wide band located at ~3510 cm-1 is attributed to O-H stretching vibrations present in silanol groups physically adsorbed on the surface of the particles. The bands centered at 1609 cm-1 is associated to the vibrational mode assigned to H-O-H angular deformation of water molecules present in the materials. This water molecule localized on the particle probably was adsorbed during the cooling process or from the precursors used in the synthesis process. During the sol-gel process, water molecules are formed as a product of the main reaction.

Both bands are absent or have a considerably diminished intensity as a function of heat-treatment temperature. The elimination of O-H groups is convenient because this drastically influences the optical properties of the materials, compromising their photoluminescence efficiency. The spectra present as an absorption band at 1100 cm-1 with a shoulder at 1225 cm-1 assigned to the asymmetric stretching mode of Si-O-Si, typical of SiO2 , suggesting the formation of a SiO2 network in the samples. The samples not heat-treated exhibit vibration modes at 952 cm-1 associated with the Si-O stretching in Si-OH groups and Si-Opositioned on the surface. The bands at 472 cm-1 and 805 cm-1 are associated to the bending and stretching, respectively, of Si-O-Si present in SiO2[65].

Structural changes after heat-treatment were observed based on the appearance of absorption bands between 400 and 1350 cm-1, indicating the formation of Zn2SiO4 phase. The characteristic bands of willemite (highlighted in Figure 2) occur at 978, 929 and 897 (ν3, SiO4), 865 (ν1, SiO4 ) and 461 (ν4, SiO4 ), and 615 (ν3, ZnO4 ) and 574 (ν1, ZnO4 ); in which ν1 refers to fully symmetrical stretching, ν3 asymmetric stretching and ν4 asymmetric deformation[66]. Representative bands of willemite are very well defined in the spectra of the samples containing higher amounts of zinc. 7SZ and 9SZ samples exhibit absorption band around 790 cm-1 indicating the presence of SiO2 . For composition 7SZ, this band is less evident, however, for the composition 9SZ, the band is quite significant, showing higher intensity compared to the absorption bands between 980-897 cm-1 assigned to Zn2 SiO4 . It is considered that the stretching Si-O-Zn, of Zn2 SiO4 , which occurs at 900-950 cm-1 was overlaid by the strong and intense absorption SiO4 modes. However, the formation of Zn-Si-O- bonding is evidenced not only by the appearance of bands assigned to the willemite, but also by the shift of the positions of bands assigned to the ZnO and SiO2 .

The structure of willemite consists of silicon and oxygen tetrahedron, SiO4 4-, in which atoms are covalently bonded. There is an ionic interaction between the zinc atoms and the oxygen of the tetrahedron. There is, therefore, connection between complex anions since they are surrounded by Zn2+. The distance of Si-O bonds is smaller than the distances between the Zn-O. Thus, the changes in wavenumbers depicted in the spectra are assigned to the strengthening of the Zn-O bond and reduction of the average energy on bonding of Si-O due to the formation of Si-O-Zn in the materials.

Figure 2: FTIR spectra of the samples (A) 5SZ, (B)7SZ and (C) 9SZ.

The diffractograms of the samples are depicted in Figure 3. The diffraction peaks of the samples 5SZ agree with the JCPDS 37- 1485 card confirming the formation of the willemite phase, with a trigonal system. In accordance with other compositions, the 8SZ 9SZ sample shows the presence of a quartz phase, in accordance with JCPDS 46-1045, trigonal system, and space group R3H (148). It is possible to verify that the silica formation as well as the silicate phase formation are directly dependent on the amount of Si4+ and Zn2+.

The graphs of crystallite size (CS) and lattice strain (LS) results as a function of the amount of silicon used were plotted and are depicted in Figure 4. It is observed that the crystallite size (Figure 4A) becomes larger with the increase in temperature of the heattreatment. Furthermore, when annealed at 900°C, a large crystallite size distribution is observed for the different compositions of materials. Differently from the samples heat-treated at 1000 and 1100 °C, which exhibit greater uniformity of CS. In general, it is possible to notice a decrease in the LS (Figure 4B), by increasing of the amount of silicon to a percentage of 80 mol%; for samples 9SZ the LS returns to being expressive.

The emission spectra of samples excited at 255, 394 and 463 nm are shown in Figures 5, 6 and 7, respectively. All the emission spectra show bands characteristic of Eu3+ emission, assigned to the intraconfigurational f-f transitions, originated from 5D0→7F2 levels, wherein J = 0, 1, 2, 3 and 4, positioned at 578, 591, 612, 652 and 701 nm, respectively. The band centered at 614 nm, corresponding to the 5D0→7F2 transition that occurs in the red region, exhibits greater intensity for all samples. This transition is forbidden by parity but becomes allowed when the Eu3+ is positioned in the site without an inversion symmetry center. Thus, this transition occurs through forced dipole-electric and is hypersensitive to the local environment in which the ion is positioned. The transition 5 D0 →7 F1 , in turn, takes place via magnetic dipole and is independent of the local symmetry in which the RE3+ is located. In general, the broad emission band profile in the spectra is observed for all samples under excitation in all wavelengths. However, the profiles of emission bands in the spectra of samples heat-treated at 1100°C, are thinner in comparison to another one. As discussed by XRD analysis, the sample heat-treated at 1100°C presents the zinc silicate phase in greater proportion. In this sense, this crystalline phase promotes the presence of the sites with higher symmetry contributing to the formation of thinner band shape for the emission bands spectra of the samples heat-treated at 1100°C. In the emission bands spectra of the samples 9SZ heat-treated at 900, 1000 and 1100°C under excitation at 255 nm it is observed the presence of a broad band below 600 nm. Another point observed is that the intensity of the emission bands assigned to the 5D0 ®7FJ with J = 0, 3 and 4 for samples 9SZ heat-treated at 900, 1000 and 1100°C under excitation at 255 nm, are almost not present. The absence of these bands can be attributed to the presence of defects in SiO2 amorphous phase acting as a deactivator of the excited state.

Figure 3: Diffractograms of the: (A) 5SZ, (B)6SZ, (C) 7SZ, (D)8SZ and (E) 9SZ of the samples heat-treated at different temperatures obtained by the sol-gel process.

Figure 4: (A) Crystallites size of the samples calculated by Scherrer’s equation and (B) lattice strain values calculated by Williamson-Hall method, both obtained as a function of percentage of Si and heat-treatment temperature.

Figure 5: Emission spectra of the samples heat-treated at (A) 600, (B) 900 and (C) 1000 and 1100 °C under excitation at 255 nm.

Figure 6: Emission spectra of the samples heat-treated at (A) 600, (B) 900 and (C) 1000 and 1100°C under excitation at 394 nm.

Figure 7 and 8 shows the ratio graphics depending on the amount of silicon in the samples. The ratio between the intensities of these transitions, 5 D07 F2 / 5 D07 F1 , is fundamental to investigate the vicinity of Eu3+, acting, therefore, as a structural probe of local symmetry. The higher the ratio values between both transitions corresponds to the site of symmetry with a high magnitude of electric dipole.

Figure 7: Emission spectra of the samples heat-treated at (A) 600, (B) 900 and (C) 1000 and 1100°C under excitation at 463 nm

Figure 8: Relation of the intensities between transitions 5 D07 F2 and 5 D07 F1 depending on amount of silicon in the samples, when excited at (A) 255, (B) 394 and (C) 463 nm.

All materials exhibit higher values of R, indicating that the Eu3+ is inserted in non-centrosymmetric sites. Moreover, there is a wide range of values of ratio (2.5 to 4.0) for the materials synthesized, suggesting a significant change in the symmetry sites occupied by the Eu3+. The highest R values (~ 4.0), obtained for the 5SZ and 7SZ samples, both treated at 900°C, depict the low symmetry in Eu3+ ions site are located which explains the high intensity of the 0-2 transition regarding the 0-1 transition due to the high magnitude of electric dipole present in the environment of the ion. The samples of the photoluminescence intensity are observed to be strongly correlated to the heat-treatment temperature of the samples as well as the concentration of system components.

The lifetimes of the values for all materials are shown in Table 2. In general, the lifetime values decrease as the heat-treatment temperature increases. It may be related to the insertion of the Eu3+ in sites with higher symmetry. A representative curve of lifetime for all samples is depicted in Figure 9 and is adequately described by monoexponential decays.

Table 2: Values of the lifetimes of the materials heat-treated at different temperatures.

 

λexc/nm

 

T/°C

Lifetime/ms

5SZ

6SZ

7SZ

8SZ

9SZ

 

255

900

1.616

1.578

1.675

1.807

1.796

1000

1.406

1.406

1.654

1.596

1.508

1100

1.155

1.198

1.416

1.274

0.864

 

394

900

1.704

1.614

1.709

1.832

1.725

1000

1.466

1.404

1.692

1.624

1.542

1100

1.173

1.311

1.444

1.301

1.059

 

463

900

1.599

1.513

1.641

1.711

1.599

1000

1.444

1.314

1.58

1.55

1.222

1100

1.104

1.209

1.279

1.182

0.863

Figure 9: Decay curve for 5D0→7F2 transition of sample 8SZ under excitation at 255 nm in which is representative for all samples obtained in this work.

To understanding better the photoluminescent properties of the RE3+ doped materials the intensity of f-f transition is fundamental. Then, based on results obtained in this work, the Judd-Ofelt theory [54–56] was applied to obtain information about of two empirical parameters Ω2 (corresponding to the 5 D07 F2 transition is mainly influenced by small angular changes in coordination geometry of the Eu3 + ion) and Ω4 (corresponding to the 5 D07 F4 is greatly influenced by the distance of the bond formed between the Eu3+ ion and the ligand) [67–69]. In the Table 3 are exhibited the Judd-Ofelt parameters obtained in this work. In accordance with the values obtained, the intrinsic quantum yield is higher in comparison with the same material obtained by polymer assisted by sol-gel method [70]. In comparison with this work reported literature, the process of synthesis used in our work showing better and easier to obtain the same material.

The Figure 10 shows the SEM images of the samples. The size and shape of the particles does not show significant difference. The bigger particles observed can be related with the sintering process that occurs during the heat-treatment process.

Figure 10: Scanning Electron Microscopy of the materials obtained.

Table 3: Judd-Ofelt parameters Ω2, Ω4 Arad (radiative rate), Anrad (nonradiative rate), Atotal (Total radiative and nonradiative rate), t (lifetime) and QLnLn (intrinsic quantum yield).

2

4

Arad

Anrad

Atotal

t

QLnLn

Eu3+

 

Heat-treatment temperature

(10−20 cm2)

(10−20 cm2)

(s−1)

(s−1)

(s−1)

(ms)

(%)

Sample

 

5

1

342

237

580

1.725

59

9SZ

 

900 ºC

 

6

1

357

188

546

1.832

65

8SZ

6

1

386

199

585

1.709

66

7SZ

6

1

383

237

620

1.614

62

6SZ

5

1

342

245

587

1.704

58

5SZ

5

1

327

322

649

1.542

50

9SZ

 

1000 ºC

6

1

353

262

616

1.624

57

8SZ

5

1

329

262

591

1.692

56

7SZ

5

1

320

393

712

1.404

45

6SZ

5

0.8

302

380

682

1.466

44

5SZ

5

1

398

646

944

1.059

32

9SZ

 

1100 ºC

4

1

316

453

769

1.301

41

8SZ

5

1

322

371

693

1.444

46

7SZ

5

1

304

459

763

1.311

40

6SZ

4

0.9

296

556

853

1.173

35

5SZ

Conclusions

The luminescence Eu3+-doped Zn2 SiO4 material obtained by sol-gel process were successfully prepared. The structural characteristics of the study reported a majority of silicate phase for all samples except for the materials containing a higher percentage of silicon, in which the composition was formed basically of quartz. Among the materials studied, those containing intermediate amounts of silicon, 7SZ samples, and 8SZ, exhibited higher lifetimes values. The predominance of the quartz phase decreased the presence of micro deformations induced in the crystal lattice and the location of the Eu3+ in higher symmetry sites, reducing the intensity of the photoluminescent materials. Increasing the heat-treatment temperature provoked the decrease of lifetime and luminescence intensity in materials. This behavior may be associated with the high crystallinity of the samples, promoting the position of Eu3+ in sites with higher symmetry sites. The values of the intensity parameters Ω2 greater than those of Ω4 suggest a hypersensitive behavior of the transition 5 D07 F2 of the Eu3+ ion. Comparing the intensity parameters Ω2 of the doped materials, they are similar, suggesting that there is no change in the hypersensitive character of the transition 5 D07 F2 . The intrinsic quantum yield of the material obtained here showed the highest values in comparison with the same material reported in the literature however, obtained by another way. The particles do not show significant difference as a function of heat-treatment temperature. Thus, our synthesis is remarkably interesting to obtain the material with higher intrinsic quantum yield values. In this way, the obtained materials show to be promising applications in the field of photonics as in solid state lighting.

Acknowledgements

The authors would like to acknowledge FAPEMIG, FINEP, and CNPq. This work is a collaboration research project of members of the Rede Mineira de Química (RQ-MG) supported by FAPEMIG (Project: CEX - RED-00010-14). This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001

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