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Enhanced green photoluminescence of erbium doped Zn2SiO4 glass-ceramics as phosphor in optoelectronic devices
Journal of Alloys and Compounds 783 (2019) 441e447
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced green photoluminescence of erbium doped Zn2SiO4 glassceramics as phosphor in optoelectronic devices Nuraidayani Effendy a, Sidek Hj Ab Aziz a, *, Halimah Mohamed Kamari a, Khamirul Amin Matori a, b, Mohd Hafiz Mohd Zaid a, b a
Department of Physic, Faculty of Science, Universiti Putra Malaysia, UPM, 43400 Serdang, Selangor Malaysia Material Synthesis and Characterization Laboratory, Institute of Advanced Technology, University Putra Malaysia, UPM, 43400 Serdang, Selangor, Malaysia
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2018 Received in revised form 24 December 2018 Accepted 30 December 2018 Available online 31 December 2018
This paper reported the fabrication of Er3þ-doped zinc silicate (Zn2SiO4:xEr3þ) glass-ceramics using recycle waste glass bottles. The glass-ceramics samples were synthesized with different percentage doping of Er2O3 using conventional melting and quenching method. The structural of a-Zn2SiO4:Er3þ crystal phase and microstructure growth were analysed via X-ray diffraction and field emission scanning electron microscopy, respectively. Fourier transform infrared spectroscopy has confirmed the network structure of a-Zn2SiO4:Er3þ crystal phase. The three excited transition states from UVevisible absorption spectra were enhanced significantly at the states 4F7/2, 2H11/2, 4F9/2. The wide-ranging green emission at around 553e559 nm under excitation 385 nm from a-Zn2SiO4:Er3þ crystal exhibit the energy transition of Er3þ ions from 4S3/2 / 4I15/2. Such luminescence of a-Zn2SiO4:Er3þ based glass-ceramics are probably to have a huge prospective application for glass phosphor devices. © 2019 Elsevier B.V. All rights reserved.
Keywords: Zinc silicate Waste glass Glass-ceramic Emission Phosphor
1. Introduction Research in rare earth ions doped glass-ceramics materials have attracted significant attention due to their vast capabilities for designing new materials in optoelectronic industries and laser technology [1]. Rare earth ions consist of very strong intensities and sharp spectral characteristic in 4f transitions [2]. Through in modern technology todays, rare earth ions have been listed as one of the superior candidates for luminescence activator in phosphor materials [3]. From previous studies, it was found the rare earth ions become the most needed materials to enhance excellent luminescence. It is because the phosphor applicability since the huge values of excitation and emission bands rises up in energy level transitions. The rare earth ions, especially the most significant Erbium trioxide, Er3þ is one of the best examples rare earth ions that having optically active ions for luminescence activities. Er3þ ions have two transitions: 4I15/2 / 4I9/2 (800 nm) and 4I15/2 / 4I11/2 (980 nm in near infrared spectral region which easily and efficiently excited
* Corresponding author. E-mail addresses: [email protected] (N. Effendy), [email protected]. my (S.H.A. Aziz), [email protected] (H.M. Kamari), [email protected] (K.A. Matori), [email protected] (M.H.M. Zaid). https://doi.org/10.1016/j.jallcom.2018.12.362 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and yielding the multiple emission such as blue, green and red upconversion [4,5]. Besides, the trivalent erbium cation are attractive in high phosphor materials because of its high chemical and thermal stability assimilated into the crystalline phase, extraordinary mechanical strength and highly transparent in the range of visible light spectra [6]. Zinc silicate (Zn2SiO4) or so-called willemite has been acknowledged as a perfect glass-ceramic host for various rare earth ions for efficient luminescence and electronic application [7e9]. Zinc silicate having the phenakite structure and rigid lattice with non-centrosymmetric cation sites [10] is widely being used as the main host materials to attain excellent optical properties. The excellent properties of Zn2SiO4 for example chemical stability, transparency in visible range and ultraviolet, will supporting to improve the optical characteristic that fit the standard for phosphor material in various applications, include televisions, fluorescent lamp, and other lighting or displays devices [11e15]. Additionally, willemite exists in three different phases which are a-Zn2SiO4, bZn2SiO4 and g-Zn2SiO4. The a-Zn2SiO4 is most common practical and stable crystalline phase. The b-Zn2SiO4 and g-Zn2SiO4 is thermodynamically metastable phases will transform to a- Zn2SiO4 at high temperature [16]. Doping of Zn2SiO4 or willemite is no longer new among the researcher [17e20]. Nevertheless, there are limited and no advance
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studies concerning on the fabrication and synthesis of aZn2SiO4:Er3þ glass-ceramic from waste material. In this current work, an efficient research on Er2O3 doped into waste material (soda lime silica (SLS) glass waste as a source of silica) derived zinc silicate-based glass-ceramics will be accomplished to study the effect of erbium oxide on the physical, structural and optical properties of the glass-ceramics.
400e1400 cm1 using Attenuated Total Reflectance (ATR) mode. The UVevisible Spectrophotometer (Lambda 35, Perkin Elmer) was used to analyse the absorption spectra and to measure the optical band gap of the glass-ceramics sample in the wavelength range 250e800 nm at room temperature. Emission spectra photoluminescence with excitation at 385 nm at room temperature were recorded by using Perkin Elmer LS 55 Fluorescence spectrometer.
2. Methodology
3. Results and discussion
2.1. Sample preparation
3.1. Physical parameter
x(Er2O3)100-x[50(ZnO)50(SLS)] glass samples were fabricated with different concentration of erbium oxide; where x ¼ 0e5 wt% using conventional melting and quenching method. Approximate 50 g of mixture from SLS waste glass powder, pureness of ZnO (Alfa Aesar, 99.99%) and Er2O3 (Alfa Aesar, 99.99%) were weighed and mixed homogenously. The homogenous mixture was transferred into an alumina crucible and melted at 1400 C in the electrical furnace with 10 C min1 heating rate for 2 hours. Subsequently, the molten mixture was quench into cold water to gain the glass fritz. The glass fritz was in transparent formed and cooled in room temperature. Then, the glass frit was crushed and sieved into the very fine glass powder with average particles size about 63 mm. In order to form a bulk pellet for physical measurement, the obtained glass powder was pressed using 13 mm diameter mould with a load of 5 tons. This process also uses polyvinyl alcohol (PVA) as the binder to reinforce the body of the bulk pallet. After that, the pellets have been heat treated at 900 C for 2 hours in order to produce aZn2SiO4:Er3þ based glass-ceramics.
Density of the zinc silicate glass-ceramics samples with different Er2O3 concentration are plotted in Fig. 1. As presented in Fig. 1, the
2.2. Characterization Density of the prepared glass-ceramics sample was calculated using Archimedes principle and water as the buoyancy liquid. The compact crystal structure of glass-ceramics samples was analysed via X-ray diffraction (XRD) pattern using a Philips X'pert PRO PW 3040 MPD. The surface morphology of glass-ceramics sample which has been coated with gold (Au) was observed using field emission scanning electron microscopy (FESEM) using FEI NOVA NanoSEM 230 Series under high vacuum. The functional group or chemical bonding of glass-ceramics samples was verified via Perkin Elmer Spectrum 100 Series spectrophotometer within the range of
Fig. 1. Zn2SiO4:Er3þ density with variations of Er2O3 concentration after heat treatment process.
Fig. 2. Zn2SiO4:Er3þ linear shrinkage with variations of Er2O3 concentration after heat treatment process.
Fig. 3. Zn2SiO4:Er3þ crystal phase with variations of Er2O3 concentration after heat treatment process.
N. Effendy et al. / Journal of Alloys and Compounds 783 (2019) 441e447
density of Zn2SiO4:Er3þ glass-ceramics samples increased from 3.379 to 3.864 gcm3 with an increase in Er2O3 percentage doping concentration after treated at 900 C for 2 hours. The increasing value of density is associated to the appearance of relatively high molecular weight of Er2O3. It is mainly due to the heavier atomic mass of erbium (167.259 a.m.u) substitute in the zinc silicate glassceramics samples. In general, the linear shrinkage is closely related to the material density. In this study, the linear shrinkage Zn2SiO4:Er3þ sample is increased with respect to increase of Er2O3 content heat treated at 900 C for 2 hours as shown in Fig. 2. The increment in linear shrinkage of Zn2SiO4:Er3þ samples is affected on a decrease in pore size of samples resulting in increasing densification [21]. The purpose of heat treatment process is to transform the porous into dense material [22].
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3.2. Crystal structures Fig. 3 presented the XRD pattern for Zn2SiO4:Er3þ glassceramics samples treated in the temperature at 900 C for 2 hours. The XRD results indicates that, the XRD diffraction peaks in Fig. 3 match very well and fit with the single a-Zn2SiO4 phases with JCPDS no. of (37e1485). The diffraction pattern of a-Zn2SiO4 crystal phase have twelve major diffraction peak at 2q ¼ 22.07, 25.54 , 31.53 , 34.00 , 38.82 , 45.01, 46.97, 48.93 , 57.60 , 59.52 , 65.63 and 68.67 with the corresponding planes (3 0 0), (2 2 0), (1 1 3), (4 1 0), (2 2 3), (6 0 0), (5 2 0), (3 3 3), (7 1 0), (0 0 6), (7 1 3), and (6 3 3), respectively. As can be seen in Fig. 3, the intensity of diffraction peak of a-Zn2SiO4:Er3þ increased with the increasing of Er2O3 content from 1 to 5 wt% in the glass-ceramics samples. The occurrence of this might be due to the completely incorporates of the
Fig. 4. Zn2SiO4:Er3þ microstructure with variations of Er2O3 concentration after heat treatment process.
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Er3þ cations into the Zn2SiO4 lattice after heat treatment process [23]. It can be concluded that a-Zn2SiO4:Er3þ stable crystal phase obtained after heat treated in the temperature at 900 C for 2 hours. The morphology of Er3þ-doped zinc silicate glass-ceramics samples sintered in the temperature at 900 C for 2 hours with different weight percentage of Er2O3 concentration are shown in Fig. 4. It can be seen that there are no obvious changes occurs in the microstructure of the glass-ceramics samples with the different Er2O3 content. The surface morphology of the Zn2SiO4:Er3þ glassceramics samples looked to have a same and homogenous distribution and the microstructure of the samples become granular which are composed of hexagonal and rectangular shape after the heat treatment process. FTIR spectra for the Zn2SiO4:Er3þ glass-ceramics samples treated in the temperature at 900 C for 2 hours with different Er2O3 concentration are measured at the frequency 400-1400 cm1 as shown in Fig. 5. The series of all detected infrared absorption bands are summarized in Table 1. It is deduced that the FTIR bands for Er3þ-doped zinc silicate glass-ceramics positioned at 471 cm1, 585 cm1, 701 cm1, and 892 cm1. The bands observed at 701 cm1 could be assigned as the totally symmetric stretching vibration of SiO4 group groups [24e26]. The band positioned at 892 cm1 corresponded to the SiO asymmetric stretching vibration of SiO4 group. The peak located at 585 cm1 due to the ZnO asymmetric stretching vibration in ZnO4 groups [27e30]. The peak at lower wavelength of 471 cm1 was assigned to the symmetric stretching of ZnO4 groups. As a whole, the existence vibrations associated with groups SiO4 and ZnO4 would propose the establishment phase of Zn2SiO4 [31]. 3.3. Optical properties The Zn2SiO4:Er3þ absorption spectra glass-ceramics samples with different weight percentage Er2O3 treated in temperature at 900 C for 2 hours, range from 250 to 800 nm at room temperature is presented in Fig. 6. Below 400 nm, UV region shows the broad absorption band and these inhomogeneous absorption spectra is
Table 1 Zn2SiO4:Er3þ infrared vibration band with variations of Er2O3 concentration after heat treatment process. Sample
Assignment of vibrational mode
400e700 460 550e600 700e750 890e980
ZnO4 symmetric stretching vibration SiO4 asymmetric deformation ZnO4 asymmetric stretching vibration SiO4 totally symmetric stretching vibration SiO4 asymmetric stretching vibration
caused of the Er3þ ions are surrounded by a crystalline environment [32]. As the Er2O3 content increases, some absorption bands that corresponding to the transitions initially from the ground state to the higher levels due to the characteristic of Er3þ ions are clearly observed. Result showed the optical transitions of Er3þ ions from 4 I15/2 state to 4F7/2, 2H11/2 and 4F9/2 state are located at the peaks of 492, 524 and 650 nm, respectively. It can also be seen that the intensity of the absorption spectra increases with the increasing of Er2O3 concentration. From the fundamental of absorption edge, Er3þ-doped zinc silicate glassceramics samples increased due to the transition of Er2O3. Optical band gap of glass-ceramics samples is agreed to be assumed as direct forbidden transition given by:
ðahvÞ2=3 ¼ b hv Eopt
(1)
where a is the optical absorption coefficient obtained from UVeVisible measurement, b is the constant independent of photon energy (hv) and Eopt is the optical band gap energy. The optical band gap of Er3þ-doped zinc silicate glass-ceramics samples can be determined by extrapolating of the linear parts of (ahv)2/3 and photon energy (hv) curve to (ahv)1/n ¼ 0 as shown in Fig. 7 and have been tabulated in Table 2. Based on the result from Table 2, the obtained value of optical band gap from direct forbidden of Er3þdoped zinc silicate glass-ceramics samples are decreased with different weight percentage sintered in the temperature at 900 C
Fig. 5. Zn2SiO4:Er3þ infrared spectra with variations of Er2O3 concentration after heat treatment process.
N. Effendy et al. / Journal of Alloys and Compounds 783 (2019) 441e447
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Fig. 6. Zn2SiO4:Er3þ absorption spectra with variations of Er2O3 concentration after heat treatment process.
Fig. 7. Zn2SiO4:Er3þ plot for (ahv)2/3 vs. hv with variations of Er2O3 concentration after heat treatment process.
Table 2 Zn2SiO4:Er3þ optical band gap with variations of Er2O3 concentration after heat treatment process. Sample 0 wt% 1 wt% 2 wt% 3 wt% 4 wt% 5 wt%
Er2O3 Er2O3 Er2O3 Er2O3 Er2O3 Er2O3
Optical band gap (eV) 3.325 3.204 3.236 3.152 3.051 2.982
for 2 hours. The decrease of optical band gap can be ascribed by the dopant defect in the bands [33]. The Er3þ defects will result in the absorption of incident photons. The optical band gap is affected due to the strong internal forces by the electron from the valence band to the conduction band which means this electron need greater energy resulted from the Er3þ transitions [33,34]. 3.4. Photoluminescence properties Fig. 8 shows the room temperature of emission spectra for the Er3þ-doped zinc silicate glass-ceramics samples with different weight percentage heat treated in the temperature at 900 C for 2
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photoluminescence were gives a green emission luminescence at 559 nm due to Er3þ ions transition under 385 nm excitation. Acknowledgement The financial support from Ministry of Science, Technology and Innovation, Malaysia and Universiti Putra Malaysia (UPM), each under the Inisiatif Putra Berimpak and Inisiatif Putra Muda (IPM) are gratefully acknowledged. References
Fig. 8. Zn2SiO4:Er3þ emission spectra with variations of Er2O3 concentration after heat treatment process.
hours. The excitation peak around 385 nm is ascribed to the excitation of the Er3þ charge transfer transition from its ground state 4 I15/2 to the conduction band. The strong green emission spectra are observed shifted in the spectral regions range from 553 to 559 nm correspond to the 4S3/2 / 4I15/2 transitions, respectively. The most significant changes that can be observed as a result of the increasing in weight percentage of Er2O3 is by comparing the emission intensities. The intensity of the emission spectral for Er3þdoped zinc silicate glass-ceramics samples increased with the increasing of weight percentage Er2O3. This result matched well with the XRD patterns in which it is revealed that at 5 wt % of Er2O3, the crystalline particles should be more participated in the based glass and more Er3þ ions dissolved in the crystalline phase that responsible for the increase in luminescence. In the crystalline phase glass-ceramics, the emission spectrum shows a very thin line. The changes in the spectrum shape is caused by the crystalline phase presence in the glass matrix which leads to the modification of an environment of the Er3þ ions in sintered glass. When the heat treatment temperature increases, the crystalline particles could be more participated in the glass samples. Hence, the broadening of optical spectrum is due to the Er3þ ions has differences in the occupation sites. From other researches, the band of emission in the glass-ceramics is generally narrower due to the lack of disorder inhomogeneous broadening [35]. Therefore, the appearance of the crystallites may be related to the changes in the emission spectrum shape. Generally, this behaviour is related to the Stark effect, which tends to increase the levels of 4S3/2 and 4I15/2 involved in the transition is usually more significant in the crystalline compounds. 4. Conclusion Conventional melt and quenching technique were used to prepare Er3þ-doped zinc silicate (Zn2SiO4:xEr3þ) glass-ceramics samples with different percentage doping of Er2O3. The results were indicated that the density of Zn2SiO4:Er3þ glass-ceramics samples increased with increasing Er2O3 percentage content after treated at 900 C for 2 hours. The crystal phase, microstructure and network structure of a-Zn2SiO4:Er3þ were analysed via XRD, FESEM and FTIR measurements, respectively. The energy band gap of sintered Zn2SiO4:Er3þ glass-ceramics samples at 900 C for 2 hours decreased with increasing Er2O3 percentage content. Results from
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced green photoluminescence of erbium doped Zn2SiO4 glassceramics as phosphor in optoelectronic devices Nuraidayani Effendy a, Sidek Hj Ab Aziz a, *, Halimah Mohamed Kamari a, Khamirul Amin Matori a, b, Mohd Hafiz Mohd Zaid a, b a
Department of Physic, Faculty of Science, Universiti Putra Malaysia, UPM, 43400 Serdang, Selangor Malaysia Material Synthesis and Characterization Laboratory, Institute of Advanced Technology, University Putra Malaysia, UPM, 43400 Serdang, Selangor, Malaysia
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 August 2018 Received in revised form 24 December 2018 Accepted 30 December 2018 Available online 31 December 2018
This paper reported the fabrication of Er3þ-doped zinc silicate (Zn2SiO4:xEr3þ) glass-ceramics using recycle waste glass bottles. The glass-ceramics samples were synthesized with different percentage doping of Er2O3 using conventional melting and quenching method. The structural of a-Zn2SiO4:Er3þ crystal phase and microstructure growth were analysed via X-ray diffraction and field emission scanning electron microscopy, respectively. Fourier transform infrared spectroscopy has confirmed the network structure of a-Zn2SiO4:Er3þ crystal phase. The three excited transition states from UVevisible absorption spectra were enhanced significantly at the states 4F7/2, 2H11/2, 4F9/2. The wide-ranging green emission at around 553e559 nm under excitation 385 nm from a-Zn2SiO4:Er3þ crystal exhibit the energy transition of Er3þ ions from 4S3/2 / 4I15/2. Such luminescence of a-Zn2SiO4:Er3þ based glass-ceramics are probably to have a huge prospective application for glass phosphor devices. © 2019 Elsevier B.V. All rights reserved.
Keywords: Zinc silicate Waste glass Glass-ceramic Emission Phosphor
1. Introduction Research in rare earth ions doped glass-ceramics materials have attracted significant attention due to their vast capabilities for designing new materials in optoelectronic industries and laser technology [1]. Rare earth ions consist of very strong intensities and sharp spectral characteristic in 4f transitions [2]. Through in modern technology todays, rare earth ions have been listed as one of the superior candidates for luminescence activator in phosphor materials [3]. From previous studies, it was found the rare earth ions become the most needed materials to enhance excellent luminescence. It is because the phosphor applicability since the huge values of excitation and emission bands rises up in energy level transitions. The rare earth ions, especially the most significant Erbium trioxide, Er3þ is one of the best examples rare earth ions that having optically active ions for luminescence activities. Er3þ ions have two transitions: 4I15/2 / 4I9/2 (800 nm) and 4I15/2 / 4I11/2 (980 nm in near infrared spectral region which easily and efficiently excited
* Corresponding author. E-mail addresses: [email protected] (N. Effendy), [email protected]. my (S.H.A. Aziz), [email protected] (H.M. Kamari), [email protected] (K.A. Matori), [email protected] (M.H.M. Zaid). https://doi.org/10.1016/j.jallcom.2018.12.362 0925-8388/© 2019 Elsevier B.V. All rights reserved.
and yielding the multiple emission such as blue, green and red upconversion [4,5]. Besides, the trivalent erbium cation are attractive in high phosphor materials because of its high chemical and thermal stability assimilated into the crystalline phase, extraordinary mechanical strength and highly transparent in the range of visible light spectra [6]. Zinc silicate (Zn2SiO4) or so-called willemite has been acknowledged as a perfect glass-ceramic host for various rare earth ions for efficient luminescence and electronic application [7e9]. Zinc silicate having the phenakite structure and rigid lattice with non-centrosymmetric cation sites [10] is widely being used as the main host materials to attain excellent optical properties. The excellent properties of Zn2SiO4 for example chemical stability, transparency in visible range and ultraviolet, will supporting to improve the optical characteristic that fit the standard for phosphor material in various applications, include televisions, fluorescent lamp, and other lighting or displays devices [11e15]. Additionally, willemite exists in three different phases which are a-Zn2SiO4, bZn2SiO4 and g-Zn2SiO4. The a-Zn2SiO4 is most common practical and stable crystalline phase. The b-Zn2SiO4 and g-Zn2SiO4 is thermodynamically metastable phases will transform to a- Zn2SiO4 at high temperature [16]. Doping of Zn2SiO4 or willemite is no longer new among the researcher [17e20]. Nevertheless, there are limited and no advance
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studies concerning on the fabrication and synthesis of aZn2SiO4:Er3þ glass-ceramic from waste material. In this current work, an efficient research on Er2O3 doped into waste material (soda lime silica (SLS) glass waste as a source of silica) derived zinc silicate-based glass-ceramics will be accomplished to study the effect of erbium oxide on the physical, structural and optical properties of the glass-ceramics.
400e1400 cm1 using Attenuated Total Reflectance (ATR) mode. The UVevisible Spectrophotometer (Lambda 35, Perkin Elmer) was used to analyse the absorption spectra and to measure the optical band gap of the glass-ceramics sample in the wavelength range 250e800 nm at room temperature. Emission spectra photoluminescence with excitation at 385 nm at room temperature were recorded by using Perkin Elmer LS 55 Fluorescence spectrometer.
2. Methodology
3. Results and discussion
2.1. Sample preparation
3.1. Physical parameter
x(Er2O3)100-x[50(ZnO)50(SLS)] glass samples were fabricated with different concentration of erbium oxide; where x ¼ 0e5 wt% using conventional melting and quenching method. Approximate 50 g of mixture from SLS waste glass powder, pureness of ZnO (Alfa Aesar, 99.99%) and Er2O3 (Alfa Aesar, 99.99%) were weighed and mixed homogenously. The homogenous mixture was transferred into an alumina crucible and melted at 1400 C in the electrical furnace with 10 C min1 heating rate for 2 hours. Subsequently, the molten mixture was quench into cold water to gain the glass fritz. The glass fritz was in transparent formed and cooled in room temperature. Then, the glass frit was crushed and sieved into the very fine glass powder with average particles size about 63 mm. In order to form a bulk pellet for physical measurement, the obtained glass powder was pressed using 13 mm diameter mould with a load of 5 tons. This process also uses polyvinyl alcohol (PVA) as the binder to reinforce the body of the bulk pallet. After that, the pellets have been heat treated at 900 C for 2 hours in order to produce aZn2SiO4:Er3þ based glass-ceramics.
Density of the zinc silicate glass-ceramics samples with different Er2O3 concentration are plotted in Fig. 1. As presented in Fig. 1, the
2.2. Characterization Density of the prepared glass-ceramics sample was calculated using Archimedes principle and water as the buoyancy liquid. The compact crystal structure of glass-ceramics samples was analysed via X-ray diffraction (XRD) pattern using a Philips X'pert PRO PW 3040 MPD. The surface morphology of glass-ceramics sample which has been coated with gold (Au) was observed using field emission scanning electron microscopy (FESEM) using FEI NOVA NanoSEM 230 Series under high vacuum. The functional group or chemical bonding of glass-ceramics samples was verified via Perkin Elmer Spectrum 100 Series spectrophotometer within the range of
Fig. 1. Zn2SiO4:Er3þ density with variations of Er2O3 concentration after heat treatment process.
Fig. 2. Zn2SiO4:Er3þ linear shrinkage with variations of Er2O3 concentration after heat treatment process.
Fig. 3. Zn2SiO4:Er3þ crystal phase with variations of Er2O3 concentration after heat treatment process.
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density of Zn2SiO4:Er3þ glass-ceramics samples increased from 3.379 to 3.864 gcm3 with an increase in Er2O3 percentage doping concentration after treated at 900 C for 2 hours. The increasing value of density is associated to the appearance of relatively high molecular weight of Er2O3. It is mainly due to the heavier atomic mass of erbium (167.259 a.m.u) substitute in the zinc silicate glassceramics samples. In general, the linear shrinkage is closely related to the material density. In this study, the linear shrinkage Zn2SiO4:Er3þ sample is increased with respect to increase of Er2O3 content heat treated at 900 C for 2 hours as shown in Fig. 2. The increment in linear shrinkage of Zn2SiO4:Er3þ samples is affected on a decrease in pore size of samples resulting in increasing densification [21]. The purpose of heat treatment process is to transform the porous into dense material [22].
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3.2. Crystal structures Fig. 3 presented the XRD pattern for Zn2SiO4:Er3þ glassceramics samples treated in the temperature at 900 C for 2 hours. The XRD results indicates that, the XRD diffraction peaks in Fig. 3 match very well and fit with the single a-Zn2SiO4 phases with JCPDS no. of (37e1485). The diffraction pattern of a-Zn2SiO4 crystal phase have twelve major diffraction peak at 2q ¼ 22.07, 25.54 , 31.53 , 34.00 , 38.82 , 45.01, 46.97, 48.93 , 57.60 , 59.52 , 65.63 and 68.67 with the corresponding planes (3 0 0), (2 2 0), (1 1 3), (4 1 0), (2 2 3), (6 0 0), (5 2 0), (3 3 3), (7 1 0), (0 0 6), (7 1 3), and (6 3 3), respectively. As can be seen in Fig. 3, the intensity of diffraction peak of a-Zn2SiO4:Er3þ increased with the increasing of Er2O3 content from 1 to 5 wt% in the glass-ceramics samples. The occurrence of this might be due to the completely incorporates of the
Fig. 4. Zn2SiO4:Er3þ microstructure with variations of Er2O3 concentration after heat treatment process.
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Er3þ cations into the Zn2SiO4 lattice after heat treatment process [23]. It can be concluded that a-Zn2SiO4:Er3þ stable crystal phase obtained after heat treated in the temperature at 900 C for 2 hours. The morphology of Er3þ-doped zinc silicate glass-ceramics samples sintered in the temperature at 900 C for 2 hours with different weight percentage of Er2O3 concentration are shown in Fig. 4. It can be seen that there are no obvious changes occurs in the microstructure of the glass-ceramics samples with the different Er2O3 content. The surface morphology of the Zn2SiO4:Er3þ glassceramics samples looked to have a same and homogenous distribution and the microstructure of the samples become granular which are composed of hexagonal and rectangular shape after the heat treatment process. FTIR spectra for the Zn2SiO4:Er3þ glass-ceramics samples treated in the temperature at 900 C for 2 hours with different Er2O3 concentration are measured at the frequency 400-1400 cm1 as shown in Fig. 5. The series of all detected infrared absorption bands are summarized in Table 1. It is deduced that the FTIR bands for Er3þ-doped zinc silicate glass-ceramics positioned at 471 cm1, 585 cm1, 701 cm1, and 892 cm1. The bands observed at 701 cm1 could be assigned as the totally symmetric stretching vibration of SiO4 group groups [24e26]. The band positioned at 892 cm1 corresponded to the SiO asymmetric stretching vibration of SiO4 group. The peak located at 585 cm1 due to the ZnO asymmetric stretching vibration in ZnO4 groups [27e30]. The peak at lower wavelength of 471 cm1 was assigned to the symmetric stretching of ZnO4 groups. As a whole, the existence vibrations associated with groups SiO4 and ZnO4 would propose the establishment phase of Zn2SiO4 [31]. 3.3. Optical properties The Zn2SiO4:Er3þ absorption spectra glass-ceramics samples with different weight percentage Er2O3 treated in temperature at 900 C for 2 hours, range from 250 to 800 nm at room temperature is presented in Fig. 6. Below 400 nm, UV region shows the broad absorption band and these inhomogeneous absorption spectra is
Table 1 Zn2SiO4:Er3þ infrared vibration band with variations of Er2O3 concentration after heat treatment process. Sample
Assignment of vibrational mode
400e700 460 550e600 700e750 890e980
ZnO4 symmetric stretching vibration SiO4 asymmetric deformation ZnO4 asymmetric stretching vibration SiO4 totally symmetric stretching vibration SiO4 asymmetric stretching vibration
caused of the Er3þ ions are surrounded by a crystalline environment [32]. As the Er2O3 content increases, some absorption bands that corresponding to the transitions initially from the ground state to the higher levels due to the characteristic of Er3þ ions are clearly observed. Result showed the optical transitions of Er3þ ions from 4 I15/2 state to 4F7/2, 2H11/2 and 4F9/2 state are located at the peaks of 492, 524 and 650 nm, respectively. It can also be seen that the intensity of the absorption spectra increases with the increasing of Er2O3 concentration. From the fundamental of absorption edge, Er3þ-doped zinc silicate glassceramics samples increased due to the transition of Er2O3. Optical band gap of glass-ceramics samples is agreed to be assumed as direct forbidden transition given by:
ðahvÞ2=3 ¼ b hv Eopt
(1)
where a is the optical absorption coefficient obtained from UVeVisible measurement, b is the constant independent of photon energy (hv) and Eopt is the optical band gap energy. The optical band gap of Er3þ-doped zinc silicate glass-ceramics samples can be determined by extrapolating of the linear parts of (ahv)2/3 and photon energy (hv) curve to (ahv)1/n ¼ 0 as shown in Fig. 7 and have been tabulated in Table 2. Based on the result from Table 2, the obtained value of optical band gap from direct forbidden of Er3þdoped zinc silicate glass-ceramics samples are decreased with different weight percentage sintered in the temperature at 900 C
Fig. 5. Zn2SiO4:Er3þ infrared spectra with variations of Er2O3 concentration after heat treatment process.
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Fig. 6. Zn2SiO4:Er3þ absorption spectra with variations of Er2O3 concentration after heat treatment process.
Fig. 7. Zn2SiO4:Er3þ plot for (ahv)2/3 vs. hv with variations of Er2O3 concentration after heat treatment process.
Table 2 Zn2SiO4:Er3þ optical band gap with variations of Er2O3 concentration after heat treatment process. Sample 0 wt% 1 wt% 2 wt% 3 wt% 4 wt% 5 wt%
Er2O3 Er2O3 Er2O3 Er2O3 Er2O3 Er2O3
Optical band gap (eV) 3.325 3.204 3.236 3.152 3.051 2.982
for 2 hours. The decrease of optical band gap can be ascribed by the dopant defect in the bands [33]. The Er3þ defects will result in the absorption of incident photons. The optical band gap is affected due to the strong internal forces by the electron from the valence band to the conduction band which means this electron need greater energy resulted from the Er3þ transitions [33,34]. 3.4. Photoluminescence properties Fig. 8 shows the room temperature of emission spectra for the Er3þ-doped zinc silicate glass-ceramics samples with different weight percentage heat treated in the temperature at 900 C for 2
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photoluminescence were gives a green emission luminescence at 559 nm due to Er3þ ions transition under 385 nm excitation. Acknowledgement The financial support from Ministry of Science, Technology and Innovation, Malaysia and Universiti Putra Malaysia (UPM), each under the Inisiatif Putra Berimpak and Inisiatif Putra Muda (IPM) are gratefully acknowledged. References
Fig. 8. Zn2SiO4:Er3þ emission spectra with variations of Er2O3 concentration after heat treatment process.
hours. The excitation peak around 385 nm is ascribed to the excitation of the Er3þ charge transfer transition from its ground state 4 I15/2 to the conduction band. The strong green emission spectra are observed shifted in the spectral regions range from 553 to 559 nm correspond to the 4S3/2 / 4I15/2 transitions, respectively. The most significant changes that can be observed as a result of the increasing in weight percentage of Er2O3 is by comparing the emission intensities. The intensity of the emission spectral for Er3þdoped zinc silicate glass-ceramics samples increased with the increasing of weight percentage Er2O3. This result matched well with the XRD patterns in which it is revealed that at 5 wt % of Er2O3, the crystalline particles should be more participated in the based glass and more Er3þ ions dissolved in the crystalline phase that responsible for the increase in luminescence. In the crystalline phase glass-ceramics, the emission spectrum shows a very thin line. The changes in the spectrum shape is caused by the crystalline phase presence in the glass matrix which leads to the modification of an environment of the Er3þ ions in sintered glass. When the heat treatment temperature increases, the crystalline particles could be more participated in the glass samples. Hence, the broadening of optical spectrum is due to the Er3þ ions has differences in the occupation sites. From other researches, the band of emission in the glass-ceramics is generally narrower due to the lack of disorder inhomogeneous broadening [35]. Therefore, the appearance of the crystallites may be related to the changes in the emission spectrum shape. Generally, this behaviour is related to the Stark effect, which tends to increase the levels of 4S3/2 and 4I15/2 involved in the transition is usually more significant in the crystalline compounds. 4. Conclusion Conventional melt and quenching technique were used to prepare Er3þ-doped zinc silicate (Zn2SiO4:xEr3þ) glass-ceramics samples with different percentage doping of Er2O3. The results were indicated that the density of Zn2SiO4:Er3þ glass-ceramics samples increased with increasing Er2O3 percentage content after treated at 900 C for 2 hours. The crystal phase, microstructure and network structure of a-Zn2SiO4:Er3þ were analysed via XRD, FESEM and FTIR measurements, respectively. The energy band gap of sintered Zn2SiO4:Er3þ glass-ceramics samples at 900 C for 2 hours decreased with increasing Er2O3 percentage content. Results from
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