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The color-tunable upconversion (UC) emission and optical temperature sensing behaviour was observed from the Er-Yb-Mo codoped Bi7Ti4NbO21 (BTN) ferroelectric oxide. By control of the Mo concentration, the ceramics are capable of generating color tunability from green to yellow, then to red. The optical temperature sensing behaviour of green and red UC emission was studied using the temperature fluorescence intensity ratio (FIR) technique at temperature region from 133 to 450K, showing a relatively high sensitivity. The experimental data fitted a linear function very well, which suggests that the oxides could be used for optical temperature sensing applications. The polarization-electric field (P-E) hysteresis loops have been investigated, indicating it maintained ferroelectric properties with doping. Based on the profiles of XRD, Rietveld refinement and the XPS analysis, the structure variety by Er-Yb-Mo codoping and mechanism responsible in color-tunable UC emission were discussed in detail.
© 2014 Optical Society of America
1. Introduction
Upconversion (UC) luminescence, which is a nonlinear optical process generating one high-energy photo by two or more low-energy photos, has attracted a great deal of attention in the fields of physics, chemistry, materials science and life science because of their potential for optical manipulation in optical devices, bio-analysis, medical therapy, display technologies, and light harvesting [1, 2]. Over the past several decades, rare earths (RE) doped UC luminescence have been intensively focused by researchers [3]. RE doped UC luminescent materials are composited of the hosts and the RE dopants [4]. In previous researches, the Er3+, Ho3+, Tm3+ ions are typically used as activator ions due to theirs abundant energy level and high luminescent quenching concentration compared to other ions [4, 5]. Additionally, Yb3+ ion acts as the sensitizer ions to enhance UC luminescence efficiency, by using its sufficient absorption cross section in the near infrared region [6]. Thus the RE ions R-Yb (R = Er, Ho, Tm) are traditionally used as the ideal pairs to design majority of UC materials [1].
The selection of the host matrix is another key factor to obtain desirable UC luminescence due to the different crystal fields caused by structural symmetry of the host materials can contribute to inner shell transition [7]. In one regard, different structural symmetry of the host materials causes different crystal fields which can further affect the inner shell transition of the doped ions [7, 8]. In the other regard, functional host lattices rationally incorporated atoms or ions of elements to yield UC emission without changing the intrinsic properties of the host is another big concern in development of multifunctional hybrid materials [9–12]. Ferroelectric oxides with optical, electrical and mechanical multifunction have great potential applications in future optoelectronic devices. Recently, ferroelectric Bi7Ti4NbO21 (BTN) oxide, Tc≈845°C, what’s more, has a high chemical and thermal stability as well as a low photo threshold energy which thought to be one of potential host matrix [13]. With these hosts, the properties of luminescence can be integrated into the ferroelectric system. So these RE doped materials in ferroelectric host will have the great potential for application use in electro-optical devices [14].
In recent years, the optical temperature sensor behaviour based on the UC emission attracted much attention for their high sensitivity and accuracy [15]. Optical thermometry of UC luminescence based on the fluorescence intensity ratio (FIR) technique, in contrast to other thermometers, possess some unique advantages, such as contactless measurement and large-scale imaging [16]. However, up to now, only the green UC emission from Er3+ ions (2H11/2 and 4S3/2 coupled levels) is widely used for temperature sensing system [17–19]. Thus, developing an alternative strategy such as using the green/red UC luminescence, is of paramount importance in order to improve the accuracy and resolution.
Based on the above mentioned issues, the Er-Yb-Mo codoped BTN ferroelectric ceramics were prepared via a conventional sintering technique and their UC emission properties and ferroelectric properties were investigated in this work. The color-tunable UC luminescence and optical temperature sensing behaviour by green/red UC luminescence were studied. Our study revealed a new approach to tune the color of UC luminescence, and by using a temperature dependent green/red UC luminescent ratio intensity we also demonstrated a novel optical thermometry technique in ferroelectrics.
2. Experiment
The Er3+, Yb3+ and Mo6+ co-doped BTN ceramics were designed according to the formula of Bi7-x-y-zErxYbyMozTi4NbO21 and synthesized by a conventional solid-state reaction technique. The starting materials were Bi2O3 (99.999%, Sinopharm Chemical Reagent Co. Ltd.), TiO2 (99.9% Kojundo Chemical Lab), Nb2O5 (99.99% Sinopharm Chemical Reagent Co. Ltd.), Er2O3 (99.99% Sinopharm Chemical Reagent Co. Ltd.), Yb2O3 (99.99% Sinopharm Chemical Reagent Co. Ltd.) and MoO3 (99.95%, Alfa Aesar). The raw materials were mixed with the addition of alcohol, dried, heated at 900°C for 7h. After calcinations, the ground powders, added with 10 wt % polyvinyl alcohol (PVA) binder, were pressed into disk-shaped pellets of 10 mm in diameter and about 1.5 mm in thickness. Finally, they were sintered at 1200°C for 2h in air.
The sample crystallization behavior was examined by XRD (XRD, RINT2000 vertical goniometer, Japan). For UC measurement, a 980nm laser diode (HJZ980-100) was used to excite the surface of ceramic samples. The X-ray Photoelectron Spectroscopy (XPS) spectrum was measured by a multi-technique electron spectrometer (Kratos AXIS Ultra, Shimadzu, Japan). The luminescences collected from the samples were directed by using a fluorescence spectrofluorometer (F-7000, Hitachi, Japan). The sample temperature was measured by a Pt-100 thermocouple located at a heating stage controlled by a TP 94 temperature controller (Linkam Scientific Instruments Ltd, Surrey, UK). After the luminescent measurement, electrodes for electrical property measurements were fabricated with gold paste. The polarization-electric field (P-E) hysteresis loops were measured with a precision ferroelectric analyzer (Premier II, Radiant Technologies Inc., Albuquerque, NM).
3. Results and discussion
3.1 XRD patterns and SEM image
Figure 1 shows the XRD patterns of the Bi7-x-y-zErxYbyMozTi4NbO21 ceramics at different doping content. The diffraction peaks of Er doped and Er-Yb co-doped BTN could be well indexed following the orthorhombic phase of BTN structure, with a space group symmetry of I2cm, indicating that the obtained samples are single phase and there is no impurity phase associated with the Er, Yb doping. The result is consistent with that reported for Pr doped BTN ceramics and Nd doped BTN ceramics [11, 20]. At the same time, the most diffraction peaks of Er-Yb-Mo co-doped BTN could be indexed following the BTN structure, meaning the main phase of BTN structure combined with a small amount of impurities. It is found that the impurity phases had formed with Mo codoping, which will be expounded in detail later. The inset of Fig. 1 shows the SEM micrographs of the ceramic samples sintered at 1200°C for 2h. It can be seen that the ceramics show flake-like grains, which is typical morphology of bismuth layered structure ferroelectrics (BLSF) [11, 18].
Fig. 1 X-ray diffraction patterns of Er, Yb, Mo doped BTN ceramics. Inset shows the SEM image of Bi7-x-y-zErxYbyMozTi4NbO21 (x = 0.04, y = 0.08, z = 0.12) ceramic.
3.2 Properties of ferroelectric
The polarization-electric field (P-E) hysteresis loops of Er-Yb-Mo codoped ceramics are shown in Fig. 2, in which there is a well-defined hysteresis loop. The vary of the remnant polarization is probably attributed to the structural distortion originated from the substitution and the cation vacancies at the BTN ceramics [21]. This result indicated that the rare earth doping, which contributing properties of luminescence, did not bring about obvious deterioration of ferroelectric properties in the ceramics. Thus the ceramics processed desirable multifunctional properties in a single entity. This result reveals the ceramics are optional multifunctional materials, lighting the way to study the interaction between the UC and electric properties [10]. The last but not least, it is believed that the ceramics have the potential for multifunctional application in electro-optical device, memory and display integrated chip.
3.3 Color-tunable upconversion emission
Figure 3 shows UC luminescent spectra and the corresponding Commission International de L’Eclairage (CIE) chromaticity diagram of Er-Yb-Mo codoped BTN with Mo concentrations ranging from 0 to 14mol% under excitation at 980nm at room temperature. Figure 3 exhibits two main emission parts: green and red bands. The green and red UC emissions were obtained originating from the 2H11/2, 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+ ion, respectively [15, 17]. The profiles of UC emission spectra in Fig. 3(a) were similar, but the intensities of the UC luminescence varied. With increasing Mo concentration, the UC intensity of green emissions decreased and the red one increased. The photographs in the inset further improved the phenomenon of color-tunable of light. Thus, the color of the UC emission is tunable by Mo doping from green to yellow and finally to the orange. As shown in Fig. 3(b), when the z≤0.12, the variety of color was regular, but the variation of the color became smaller when the z>0.12. Furthermore, it is worth to note that the UC luminescence spectra of the samples under 980nm excitation with 20mW, meaning a strong UC luminescence.
Fig. 3 The UC emission spectra (a) of the samples and the corresponding Commission International de L’ Eclairage (CIE) chromaticity diagram (b) under 980nm laser excitation with 20 mV at room temperature.
The dependence of UC luminescence intensity upon the pump power sheds further light onto the underlying mechanism [22, 23]. In the UC process, the emission intensity I is proportional to the nth of the pump power as I∝Pn, where n is the number of infrared photons required to absorb for emitting one visible photon [24]. A plot of logarithm I versus logarithm Pn yields a straight line with slope n [25]. In Fig. 4(a), the n values of Er-Yb codoped BTN ceramics for green and red were 1.72. In Fig. 4(b), the n values of Er-Yb-Mo codoped BTN ceramics are 1.45 for green emission and 1.75 for red emission, respectively. With Mo codoping, the n values of red emission increase and the n values of green emission decrease, which were good agreement with the variation of color. These results also indicated that the UC mechanism corresponding to green and red emission occurred by a typical two-photon process.
Fig. 4 The UC spectra of 4mol%Er-8mol%Yb: BTN (a) and 4mol%Er-8mol%Yb-12mol%Mo: BTN (b) ceramics under excitation with 980nm laser of various laser power at room temperature. In inset the UC intensity is log-log plotted against the laser power pump.
3.4 Optical temperature sensing behaviour
Figure 5(a) shows the UC emission spectra for Er-Yb-Mo codoped BTN at the temperature of 153, 253, 353 and 453K. With the increase of temperature, the peak positions had no change, however, the FIR of the green/red emission varied. Through the measurement of FIR from two thermally coupled level, the temperature can be measured [24, 26]. Fig. 5(b) shows a the FIR of green/red emission for the 4S3/2→4I15/2 and 4F9/2→4I15/2 transitions relative to absolute temperature in the range of 153-553K. The experimental data could be fitted by straight line with a slop of about 0.0044. Sensitivity is defined as the slope of the signal change (∆R) with T [10]. Thus, the sensor sensitivity was 0.0044K−1 in this work. Recently, most of optical temperature sensors based on the FIR technique of UC emissions in Er doped materials are dependence by measuring the florescence intensity from the green emission of 2H11/2→4I15/2 and the 4S3/2→4I15/2 transitions. So, this is the new approach, in which the temperature was measured based on FIR of red and green emission. The FIR linear to the temperature may be attributed to the much larger energy gap between the 4S3/2 and 4F9/2 states of Er3+ ions. Therefore, it can be concluded that the sensors based on Er-Yb-Mo codoped ceramics are expected a new route for temperature measurements in wide range.
Fig. 5 Temperature sensing based on the 4mol%Er-8mol%Yb-12mol%Mo: BTN. (a) UC emissions spectra at different temperatures, (b) FIR of green and red emission for the 4S3/2, 4F9/2→4I15/2 transitions relative to absolute temperature from 133 to 573K
3.5 The upconversion emission of Mo doped in B site
As we know, Aurivillius bismuth layered-structure ferroelectrics (BLSFs) are generally expressed as Bi2Am-1BmO3m + 3 = (Bi2O2)2+ (Am-1BmO3m + 1)2-, where A is a large 12-coordinate cation and B is a small 6-coordinate cation with a d0 electron configuration [27]. A site can be mono-, di-, or trivalent ions; or a mixture of them, B site represents tetra-, penta-, or hexavalent ions e.g. Ti4+, Nb5+, Ta5+ or W6+; respectively [27, 28]. Because of the small ionic radii (rMo6+ = 0.059nm) and high ion valence of Mo6+ ions, the Mo6+ ions may substitute the B sites of Ti4+ or Nb5+ ions [29]. For comparison purposes, the another Er3+, Yb3+, and Mo6+ (B site) codoped BTN ceramics were synthesized with the same method according to the formula of Bi7-x-yErxYbyMoz(Ti4Nb)1-z/5O21. Similarly, the Er-Yb-Mo codoped BTN according to the formula of Bi7-x-y-zErxYbyMozTi4NbO21 was regarded as A site. Figure 6 shows the UC emissions spectra of the samples (B site) with Mo6+ dopants, which displayed no vary of the color. This results indicated the color-tunable UC luminescence only existed in Er-Yb-Mo (A site) codoped BTN ceramics. Latter, we will discuss where the Mo ion located and the role of Mo codoping in the A site designed BTN ceramics as described in the experiment.
Fig. 6 The UC emissions spectra of the samples (B site) under 980nm LD excitation with laser power 20 mW.
3.6 X-ray diffraction analysis of Mo codoping
For understanding of the color-tunable UC processes by Mo codoing(A site), firstly, it is needed to analyse the crystallization behavior. In our previous work, the Pr3+ doped BTN ceramics had no secondary phase except for the orthorhombic phase of BTN structure [11]. Fig. 7 shows the XRD patterns of the Er-Yb-Mo codoped ceramics at different doping site. It can be observe, without Mo codoping, that all the diffraction peaks can be indexed according to the standard diffraction pattern data of orthorhombic phase BTN for Powder Diffraction Standards card (JCPDS No. 31-0202). With Mo6+ codoping (z = 0.12, A site), some diffraction peaks can be indexed following an orthorhombic phase Bi3TiNbO9 as in the JCPDS No.39-0233; some peaks can be indexed to the Bi4Ti3O12 as in the JCPDS No. 65-2527; which indicated the existence of the Bi3TiNbO9 and Bi4Ti3O12. With Mo codoping (z = 0.12, B site), some diffraction peaks can be indexed to a hexagonal phase Bi3.64Mo0.36O6.55 (JCPDS No. 43-0466), indicating the existence of the Bi3.64Mo0.36O6.55. These results indicated that Er-Yb-Mo (A site or B site) codoped BTN ceramics had formed the second phase because of the Mo ions codoping. The inset of the Fig. 7 shows the (1112) peaks shifted to higher-angle side, meaning the lattice shrank, only with the codoping of B site. This result reveals that the Mo6+/Mo5+ ions substituted the Nb5+ (rMo6+ = 0.059nm, rMo5+ = 0.061nm, and rNb5+ = 0.064nm, for a coordination number of 6) of B sites [29]. Compared with the ionic radii and ion valence, it can be concluded that the Mo5+/Mo6+ would substitute the B site of Nb5+ in the BTN ceramics (B site).
Fig. 7 XRD pattern of Er-Yb-Mo codoped BTN ceramics with x = 0.04, y = 0.08. The XRD pattern in the inset represented that main diffraction peaks of (1112) plane
To determine the structural details, Rietveld analysis has been performed [30]. The Rietveld refinement performed using the GASA program package (Larson & Von Dreele, 2000) within EXPGUI interface (Toby 2001) are shown in Fig. 8. As can be seen, Fig. 8(a) shows that the Rietveld discrepancy factors obtained are Rwp = 9.55%, Rp = 7.05%, χ2 = 4.73, indicating the well agreement between the calculated of single phase BTN and the measured pattern of undoped BTN ceramics. In Fig. 8(b), it indicated that the sample was singe phase structure with Er3+/Yb3+ ions substituting the Bi3+ of A sites. In Fig. 8(c), the results of refinement shows that the main phases composition was the orthorhombic phase BTN (Wt. = 95%), the orthorhombic phase Bi3TiNbO9 (Wt. = 4.9%) and the Bi4Ti3O12 (Wt. = 0.1%). As shown in Fig. 8(d), the quality percentage content of the cubic phase Bi3.64Mo0.36O6.55 was 8.5%, meaning the mole percentage content of Mo element was 8.17%. Thus the content with mole percentage of Mo5+ ions, which substituted the Nb5+ ions, was 3.83%. It is worth noting that the Rietveld discrepancy factors, which fited good in Fig. 8, confirmed the reliability of the refine results.
Fig. 8 The Rietveld refinement of the XRD pattern at room temperature, showing the experimental and the calculated profile.
3.7 XPS spectrum
Furthermore, in order to study the effect of the Mo ion doping (A site), an X-ray Photoelectron Spectroscopy (XPS) analysis was carried out. In the samples, the content of Er3+, Yb3+ is too small to be detected by XPS. Figure 9 exhibits the survey XPS spectrum and Nb3d photoelectron peaks of the surface with 12mol% and without Mo doping (A site), respectively. In Fig. 9(a), no peaks of Mo6+/Mo5+ ion was detected. In Fig. 9(b), the photoelectron peaks of the Nb3d showed a increase of binding energy by Mo doping. Compared with the XPS results on CaxTi1-xTiO3:Pr [31], the shift of bingding energy supposed to be the substitution of the Mo5+ ions, which is attributed to similar ions radius (rMo6+ = 0.059nm, rMo5+ = 0.061nm, and rNb5+ = 0.064nm, for a coordination number of 6). Thus, the results of XPS analysis demonstrates that the sample with x = 0.04, y = 0.08 and z = 0.12 (A site) still has a small amount of Nb5+ ions been substituted by Mo5+ or Mo6+ ions in B sites.
Fig. 9 (a) Survey XPS spectrum, (b) Nb 3d XPS spectra of Er-Yb codoped BTN and Er-Yb-Mo codoped (A site) BTN ceramics.
3.7 Possible color-tunable upconversion mechanism
For the Er-Yb codoped BTN ceramics, the photoluminescence mechanisms are similar to those of other Er-Yb codoped oxides and fluorides which has been discussed [23]. The structure analysis shows Er-Yb-Mo codoping (A site) lead to a secondary phase of orthorhombic Bi3TiNbO9. Very recently, Bao et al. found that, with increasing the content of Yb3+ ions, Er-Yb codoped Bi2Ti2O7 films can tune the UC color from the green to red [32]. In our work, the Er3+/Yb3+ ions may prefer to substitute the Bi3+ ion in Bi3TiNbO9, making the Er-Yb codoped Bi3TiNbO9 processed high content of Er3+/Yb3+ ions. At the same time, it can be concluded that the color of UC luminescence in Er-Yb codoped Bi3TiNbO9 was red because of the big content of Er-Yb codoping [33, 34]. Thus the color tunable of Er-Yb-Mo codoped BTN (A site) may ascribe to the increasing content of Bi3TiNbO9 by Mo doping. In our experiment, Mo is designed in A site, but it substituted Nb of B site partly. In addition, the possible existence of Yb3+-MoO42- dimer may imply that the populations of the 4F9/2 (Er3+) states and suggest that there was more efficient energy transfer from Yb3+ ion to Er3+ ion. [15, 21, 34] Thus, based on above results, it is believed the role of Mo codoping was the formation of the orthorhombic Bi3TiNbO9 and the Yb3+-MoO42- dimer, which leading the color-tunable.
4. Conclusions
In summary, by Er-Yb-Mo co-doping (A site), the properties of luminescence can be integrated into the Bi7-x-y-zErxYbyMozTi4NbO21 ferroelectric system, in which the emission color can be tuned by varying the Mo concentration. The optical temperature sensing behaviour of green and red UC emission was studied using the temperature fluorescence intensity ratio (FIR) technique at temperatures from 133 to 450K, showing a relatively high sensitivity. The experimental data fitted a linear function very well, which suggests that the oxides could be used for optical temperature sensing applications. Based on the profiles of XRD, Rietveld refinement and the XPS analysis, it was demonstrated that Mo5+/Mo6+ ions by Mo codoping only substituted the B sites of Nb5+ ions and also led the formation of second phase. The mechanism responsible in color-tunable UC emission was attributed to the existence of Er-Yb codoped Bi3TiNbO9 and Yb3+-MoO42- dimer by Mo codoping (A site).
Acknowledgments
This work was supported by the Natural Science Foundation of China (Nos. 51072136).
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