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Abstract
Multifunctional materials that integrate optical and electric properties into a single composite have tremendous research value and application prospects for future optoelectronic devices. Meanwhile, the enhancement of luminescent performances of active materials through modulation of the microstructure has triggered the development of high-performance photonic devices. In this work, the nanocrystal embedded composite was fabricated to investigate their luminescent properties with and without compositional change. The results show that both the introduction of an impurity and the application of an electric field results in the enhancement of photoluminescence in this hybrid system. The observed phenomena can be ascribed to the modification of the environment around Er3+ by different approaches. The samples prepared in this work have proven to possess luminescent and electric properties simultaneously, and the light amplification caused by the polarizing process offers a novel approach in glass systems to enhance infrared photoluminescence without compositional change, which will make this kind of material more competitive in the optoelectronics field.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Advanced optical materials doped with luminescent active ions have gradually paved the path of ample optic fields such as solid state lighting, telecommunication and biological imaging [1, 2]. Among them, several lanthanide ions that emit near infrared (NIR) fluorescence have been extremely appealing due to their value in biological and optical telecommunications [3–6]. For instance, the Er3+ ions can provide 1.5 μm emission because of the 4I13/2→4I15/2 transition and the light sources operating around 1.5 μm region can be used in the areas of optical amplifiers, up-conversion solar cells and optical addressing [7, 8].
The spectroscopic features are fundamental for the dopants-matrix system, which are sensitive to the chemical state and the surrounding ligand field around the doping center [9]. Hence, the past few decades have witnessed the tuning of luminescent properties through the modification of the chemical state by additional doping or compositional change, which is easy to realize and less dependent to the structure of doping matrix. By contrast, the efforts on the tuning of ligand field without changing the components are little made, which is essential for the reversible and in situ research. In this case, the structure of the doping matrix is of highly importance so as to respond sensitively to the external environment such as the electric field and elevated temperature. Meanwhile, the unique structure of matrix will provide undesired but valuable property of the material, which enlarges their application and meet the demand as being multifunctional [3, 10–15]. For instance, the ferroelectric phases with oxygen octahedrons are sensitive to external electric field and are easy to realize structural evolution, thus the ligand field around doping ions could be tailored and results in the tuning of spectroscopic properties. And the light sources with tunable fluorescence in the ferroelectric system widen their applications in fields of biological labeling and 3D display [16–20].
Therefore, the Er3+ doped composite enriched with ferroelectric nano-particles provide us a possibility to fabricate new kinds of devices such as optical resonators that work at the third telecommunication window wavelength range (~1.5 μm) [21] or the range finding machines that require mass storage [22]. Meanwhile, as mentioned above, due to the special oxygen octahedrons structure of ferroelectric phases, these material will be potential to combine multiple variables such as electric field, polarization degree and temperature in a single compound [4, 23, 24], which is beneficial for fabricating new devices that can be better applied in the electric-dependent or temperature-dependent optical field.
According to the previous work, it is found that the NIR emission intensity in the rare earth ion doped ferroelectric glass ceramics is relatively low due to the concentration quenching effect resulted from the clustering of RE in the ferroelectric phase [25–27], which limits their future applications in this essential infrared optical window. To date, there are numerous research about the influence of chemical doping on the infrared fluorescence, but the attempt to realize tunable luminescence without compositional change has seldom been approached. Therefore, in order to make this lanthanide-ferroelectric material more competitive in optoelectronic field, the approaches to modify the luminescent properties with and without compositional change are tried and compared in this work. As for the chemical method, there is validate method in other systems to enhance emission intensity by co-doping materials with impurities such as alkali metal ions (Li+) [28–30], which is of researching value because its influences on the ferroelectric properties are also worth to be discovered. And as for the strategy without modulating component, applying with different electric field is obtained so as to realize different extent of polarization resulted from the moving of electric dipole moments.
In this paper, a series of Er3+ doped ferroelectric composites addition with different concentration of Li+ ions are synthesized. The effects of Li+ on the structure, luminescent properties as well as ferroelectric properties are discussed, and the polarization dependent luminescent property is also studied in this paper.
2. Experimental details
The glass samples of the following system: 99(SrO-BaO-Nb2O5-B2O3)-1Er2O3-xLi2O (x = 0, 1, 2, 3, 4, 5) were prepared by the traditional melting method and all the raw materials are of high purity. For each batch, about 10 g of the well-mixed raw materials were placed in a covered alumina crucible and melted in air atmosphere at 1400 °C for 45 min. And then they were moved to an electric furnace to relinquish in inner stress at 450 °C for 2 h after casting onto the cold stainless steel plate. Whereafter, the SBN glass ceramics were obtained after crystallizing at 710 °C for 3 h. They were polished into desired size to make optical tests. Then the samples were painted with silver paste on both sides to measure ferroelectric properties.
The NETZSCH DTA 404 PC differential scanning calorimeter was applied to get characteristic temperatures (Tg, Tx and Tp), which was measured to 1000 °C at a heating of 10 °C/min. The crystal phase of the glass ceramics were investigated by X-ray powder Diffract (XRD, D2 PHASER, BRUKER) using Cu Kα radiation. The PL spectra was tested using a Triax 320 type spectrometer by pumping the samples with 980 nm laser diode (LD). Polarization-electric field (P–E) curves were measured under the frequency of 10 Hz using ferroelectric testing system (Radiant Precision Premier II Technology) in a silicon oil bath to avoid electrical discharges. The measurements of dielectric constant and dielectric loss were performed using a precision multifunction LCR meter (Model HP4292A, Agilent) at elevated temperatures at 1 MHz. All the measurements were carried out at room temperature.
3. Results and discussion
The phase characters of the prepared samples are analyzed through XRD features. Figure 1(a) presents the XRD patterns of the prepared samples crystallized at 710°C for 3 hours, which is applied to investigate the structure evolution of the samples after heat treatment process.
Fig. 1 (a) X-ray diffraction patterns of SBNL samples crystallized at 710°C. (b) The magnified Ba0.27Sr0.75Nb2O5.78 XRD peak around 32°. (c) The magnified LiNbO3 XRD peak around 17°. (d) TEM micrograph of SBNL-3. (e) High resolution transmission electron microscope (HRTEM) image of SBNL-3. (f) 3D cell model of SBN from top view (c axis).
As shown in the figure, all the peaks from 20° to 60° match well with the JCPDS file card: 31-0166 (Ba0.27Sr0.75Nb2O5.78) without any impurity phase. Therefore, the samples are named as SBNL0, SBNL1, SBNL2, SBNL3, SBNL4, SBNL5 hereafter for convenience. Meanwhile, the second phase relating to LiNbO3 (abbreviated as LNO hereafter) is emerging after the doping of Li+ ions as shown in the curves of SBNL1-5 samples. It’s worth noting that the peak positions of both SBN and LNO shift gradually towards smaller angle after Li+ doping and their enlarged drawings are shown in Fig. 1(b) and (c), which indicates the increasing of the crystallite volume resulted from the incorporation of Li+ and Er3+ ions. In order to further analyze the growth of the nano-crystals, the Scherrer formula [31] is applied to calculate their average diameter:
where λ is the wavelength of the X-ray radiation, β is the full width at half maximum (FWHM) of the most intense peak. The calculated average diameters are about 25~30 nm as shown in Table 1. Meanwhile, according to Fig. 1(b), the peak position of SBNL0 sample shows a bit shift towards bigger angle compared with the JCPDS file card, which reveals the decreasing of SBN volume and can be ascribed to the substitution of Er3+ ions (0.89 Å) for Sr2+ ions (1.18 Å) or Ba2+ ions (1.35 Å). And in terms of the smaller radius of Li+ (0.76 Å) compared with Sr2+ and Ba2+, the incorporated Li+ ions are of little probability to replace any of them or the average diameter of SBN would decrease, which is totally different with the calculated result. Therefore, the Li+ ions are mainly embedding into the interstitial lattice site thereby causing the volume augment.
Table 1. Calculated average diameters of the crystallites
In consideration of a better visualization of the microstructure of SBNL sample, the TEM (Transmission electron microscopy) analysis of SBNL-3 sample is performed and the graph is shown in Fig. 1(d). The distribution of SBN crystallites in the glass matrix is clearly depicted and the crystal size is approximately ~28 nm, which is in line with the XRD result. A high resolution transmission electron microscope (HRTEM) image with the d-spacing structure is shown in Fig. 1(e). The calculated d-spacing value is 0.32 nm, which is consistent with the (211) plane of the tetragonal Ba0.27Sr0.75Nb2O5.78 nano-crystals (d(211) = 3.22 Å). To acquire a better understanding of the inner structure of SBN lattice, the 3D cell model is obtained from the top view (c axis) and exhibited in Fig. 1(f). The positions of Sr and Ba are clearly illustrated inside the 3D cell model and their connections with oxygen ions (SrO6 and BaO6 octahedron) are also depicted.
Normally, Er3+ doped photonic materials are capable of emitting up-conversion visible and down-conversion NIR luminescence bands, which are essential for biomedicine, optical communication and optoelectronic devices. Figure 2(a) and (b) present the up-conversion and NIR fluorescence spectra of Er3+ doped SBNL samples pumped by 980 nm LD. It’s obviously shown in Fig. 2(a) that there are two main emission peaks centered around 545 nm and 660 nm, corresponding to the transition of Er3+: 4H11/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 respectively. And the emission peak in Fig. 2(b) around 1.5 μm is originated from the 4I13/2→4I15/2 transition of Er3+. Meanwhile, with the co-doping of Li+ ions, the emission intensity of both visible and NIR wavelength range increases gradually till 3mol% and then decreases. This phenomenon indicates that the photoluminescence can be significantly improved by introducing impurity, which will be discussed later in detail. Furthermore, in an attempt to find the rough relationship between the time-resolved photoluminescence and environmental change around doping center, the fluorescence decay curves of Er3+: 4I13/2→4I15/2 (1.5 μm) for the SBNL0-5 samples are tested and shown in Fig. 2(c). All the curves of these samples can be well fitted to a single exponential function [32]:
where I(t) and I(0) are the luminescence intensities at time t and 0. A1 is a constant; t is the time, and τ1 is the decay times for the exponential components. And their fitting curves are depicted inside Fig. 2(c). The lifetimes of 4I13/2 state of Er3+ ions for SBNL0-5 samples are calculated to be 3.81, 3.79, 4.12, 4.50, 4.03, 3.94 ms, respectively. It’s clearly observed that the lifetimes of Er3+: 4I13/2 state of the SBNL1-5 samples are longer than that of SBNL0 as a whole, and the longest lifetime are obtained in the SBNL3 sample (doping with 3 mol% Li+ ions), whose variation trend is in great agreement with the results of emission intensity. Figure 2(d) presents the energy transfer process of Er3+ under 980 nm pumping, and the corresponding transitions are labeled.Fig. 2 (a) Upconversion emission spectra of Er3+. (b) Infrared emission spectra of Er3+ (1.55 μm). (c) The measured infrared decay curves of Er3+: 4I13/2→4I15/2. Inset depicts its fitted decay curves. (d) The energy transfer diagram of Er3+ ions.
As is clearly exhibited in Fig. 2(a) and (b), the luminescence is most significantly tuned by 3 mol% doping of Li+ ions in both visible and NIR wavelength range, which can be well explained according to the change of environmental symmetry around Er3+. Compared with the SBNL0 sample, the symmetry of the crystal field around Er3+ ions in SBNL3 is decreasing when the doping amount of Li+ ions is small (3 mol%) because their incorporation into the SBN lattice and the oxygen octahedron structure tailors the local ligand field. The approximately structural evolution diagrams are shown in Fig. 3(a)-(c) for a better vision of the structure, which is obtained based on the cell model of SBN as illustrated in Fig. 1(f), and the Nb atoms are not depicted inside Fig. 3 because their locations are overlapping with O atoms and they don’t participate in the oxygen octahedron structure.
Fig. 3 (a)-(c) Approximately top view projection drawings of SBN obtained from SBN model cell. (a) Unit cell structure of SBN. (b) Unit cell structure of SBN after the substitution of Er3+ for Sr2+. (c) Unit cell structure of SBN after the embedding of Li+.
Furthermore, according to the law of electric charge conservation, there will be quantities of electron holes emerging in the SBN lattice (around the Er3+ ions), which also proves the changing of ligand field. The defect reaction equation can be expressed as:
Meanwhile, in another aspect, when the amount of Li+ ions increases from 1 to 3 mol%, more of them will enter into the SBN lattice and diffuse around Er3+ ions. Although the inner strong Er-O bonds can’t be broken by the Li+ ions, they will still be slightly adjusted and distorted so that the asymmetry of the crystal field around Er3+ ions is increasing [30]. The increment of environmental asymmetry results in the increasing of radiative transition probability [33, 34], therefore the emission intensity is rising. Reversely, the emission intensity drops when the concentration of Li+ ions is more than 3 mol%, which can be ascribed to the excessive distortion of the optically active center [35]. It can also be ascribed that the embedding of Li+ ions on the interstitial lattice site would induce the fast energy transfer process from host to Er3+ ion, which leads to an increasing in the hole concentration and thereby decreasing the competitive absorption [34]. It’s obvious that the tunable emission intensities of the SBNL3 sample in the red light range and NIR range are 4 times and 1.4 times larger than SBNL0 sample, respectively. By contrary, the enhancement on the green light emission is less obvious according to Fig. 2(a), especially the emission band peaked at 530 nm (Er3+: 4H11/2→4I15/2), which may be owing to the fact that the amplification effect of Li+ ions on the emission intensity is wavelength dependent. Besides, the remarkable Stark splits in all the emission bands are also due to the complicated crystal field around Er3+ ions especially after the emerging of LNO crystal phase. Unexpectedly, the shape of the red emission band transforms gradually with the increasing amount of Li+ ions in SBNL0-2 samples and keeps the same in the SBNL2-5 samples according to the red light emission spectra from 640 to 700 nm in the Fig. 2(a), which is not observed in the other two wavelength range (green light and NIR). This can be ascribed to the fact that the influence of second phase (LNO) on the emission intensity is wavelength dependent, which gives us a possibility to modify the fluorescence of red light by inducing second phase for specific applications. Meanwhile, the prolonged lifetime of SBNL-3 as shown in Fig. 2(c) is also due to the tailoring of ligand field around Er3+ ions after the co-doping of Li+ ions as discussed above.
To further investigate the polarization dependent luminescent properties so as to make this material more competitive in electro-optical industry, the emission spectra of SBNL0 sample is tested before and after applying with different electric field and is shown in Fig. 4(a), as the polarization extent (remanent polarization) varies with the electric field before electric breakdown. It’s obvious that the emission intensity shows a positive correlation along with the applying electric field and the emission intensity of the SBNL0 sample after polarized with 200 kV/cm is 5 times larger than the un-polarized one, which reveals that the luminescence can be modified by external electric field in this system and the reason can also be ascribed to the changing of environmental symmetry around Er3+ before and after the polarization process. The polarization dependent NIR decay properties of SBNL0 sample are also analyzed and exhibited in Fig. 4(b), and inset shows their fitting curves. The calculated lifetimes are 3.81, 3.75, 3.72 ms, respectively. Contrast to the variation trend of emission intensity, the lifetime decreases slightly after polarization and keeps almost the same with the increasing remanent polarization (3.75 ms under 60 kV/cm and 3.72 ms under 200 kV/cm), which reveals that the polarization process has little impact on the rate of spontaneous emission from 4I13/2→4I15/2 of Er3+.
Fig. 4 (a) The measured infrared emission spectra of SBNL0 sample applied with different electric field. (b) The measured infrared decay curves of SBNL3 (Er3+: 4I13/2→4I15/2) under different electric field. Inset depicts its fitted decay curves.
Figure 5(a) and (b) are the brief schematic diagrams of the excitation process with and without applying with electric field. After applying with electric field along the direction of spontaneous polarization, there will be a certain amount of remnant polarization as illustrated in Fig. 5(b), which induced the asymmetry increment around Er3+. In principle, the increasing asymmetry around lanthanide ions promotes more uneven crystal-field components into 4f orbital hence increasing the electric dipole (ed) transition probabilities of the dopant ions [23]. The Judd–Ofelt (J–O) theory [36, 37] can be used here as an assistant explanation as listed below:
where ed is the electron dipole, e is electron charge, h is plank constant λ is the wavelength of the transition, n is the refractive index at the wavelength of the transition, Aed is the spontaneous emission probability of ed transition between initial J manifold [S, L]J and a final J manifold [S’, L’]J’. Sed is the ed line strength, which can be expressed by another Equation:where three terms〈‖U(t)‖〉are reduced matrix elements of the unit tensor operators, Ωt (t = 2, 4, 6) are J-O intensity parameters relating to different properties of rare earth ion, in which Ω2 represents for the asymmetry of the lanthanide ion sites [38], and the larger Ω2 value stands for the higher asymmetry. Thus the increasing asymmetry resulted from the polarization process will lead to a higher Sed value, as well as the spontaneous emission probability. Therefore, the symmetry reduction caused by the polarization effect enhances the emission intensity as depicted in Fig. 4(a) and they are positively related (double after 60 kV/cm and 5 times after 200 kV/cm).Fig. 5 (a)-(b) Approximately top view projection drawings of SBN obtained from SBN model cell. (a) Unit cell structure of SBN0before polarization. (b) Unit cell structure of SBN0 after polarization.
The remanent polarization dependent luminescence as discussed above offers a new approach in glass system to enhance photoluminescence without the tuning of composition or the changing of the doping concentration of rare earth ions. Furthermore, this enhancing effect can be permanent once after polarizing because the remanent polarization is very steady and non-volatile without external electric field.
Excellent ferroelectric properties are also essential in this photoelectric system, which is briefly studied here. The polarization-electric field (P-E) hysteresis loops of the SBNL samples are shown in Fig. 6(a) and its first quarter part is shown in Fig. 6(b). It’s seen that the P-E loops first increase till 3 mol% doping and then decrease. The temperature dependent dielectric constant and dielectric loss of the SBNL samples are depicted in Fig. 6(c). Both the dielectric constant and dielectric loss curves show a slight decrease trend with the increasing temperature, and the similar result was also found in a former work [39]. The Li+ concentration dependent dielectric constant and dielectric loss of the SBNL samples measured at room temperature is shown in Fig. 6(d). The variation of the dielectric constant is due to the change of SBNL structure, which is the same as the results obtained in the P-E loops. The increase in Fig. 6(a) is due to the volume augment after doping small amount of Li+ ions, and the decrease after that can be associated with the excessive distortion of Sr-O or Ba-O bonds caused by the further doping of Li+ ions as discussed above. According to its first quarter P-E loops, the remanent polarization also increases at first and the decreases, showing the deflection ability of the electric dipole moments after the structural disorder resulted from the doping of different concentration of Li+ ions. Meanwhile, due to the existence of dense glass phase, the samples possess excellent electrical resistance so that they can be tested over 200 kV/cm without electric breakdown.
Fig. 6 (a) The whole and (b) The first quarter of Polarization-electric field (P-E) hysteresis loops of the SBNL samples. (c) Temperature dependent dielectric constant and dielectric loss of the SBNL samples. (d) Li+ concentration dependent dielectric constant and dielectric loss of the SBNL samples at room temperature.
All these ferroelectric results reveal that the prepared samples are multifunctional as they possess luminescent and ferroelectric properties simultaneously, which provides a better choice for fabricating novel optical functional devices.
4. Conclusion
In summary, a series of Er3+: ferroelectric composite codoping with different concentration of Li+ ions were prepared by the melting and heat treatment method. The new optical composite containing ferroelectric nanoparticles has been proved by XRD, TEM, photoluminescence, and electric results. The infrared photoluminescence is greatly enhanced both after the modulation of impurity Li+ ions (3 mol%) and the polarization by electric field (200 kV/cm) in this Er3+: Ba0.27Sr0.75Nb2O5.78 hybrid system. The enhancement of photoluminescence originated from the reduction of symmetry around Er3+ ions is explained through the dynamic structural evolution diagrams and theoretical equations. The dielectric constant increases from 84 to 142 after doping with 3 mol% of Li+, meanwhile the dielectric loss keeps almost unchanged. These results reveal that the prepared composites are multifunctional as they possess luminescent and electric properties simultaneously, which will make this kind of material extremely promising in the optoelectronic field such as novel luminescent devices that are sensitive to the external electric field. Further study about the in situ relationship between electric field and photoluminescence in this system and the electroluminescence in the ferroelectric field are of researching value.
Funding
National Natural Science Foundation of China (No 61705214, 51272243 and 51472225); Zhejiang Provincial Natural Science Foundation of China (No LD18F050001).
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