Bi 3+ effects on down-/up-conversion luminescence, temperature sensing and optical transition properties in Bi 3+ /Er 3+ co-doped YNbO 4 phosphors

A series of Bi 3+ single-doped and Bi 3+ /Er 3+ co-doped YNbO 4 phosphors with various concentrations of Bi 3+ ions were prepared by a conventional high temperature solid-state reaction method. The results of XRD and Rietveld refinement confirmed that monoclinic phase YNbO 4 samples were achieved. The down-/up-conversion luminescence of Er 3+ ions were investigated under the excitation of ultraviolet light (327 nm) and near infrared light (980 nm). Under 327 nm excitation, broad visible emission band from Bi 3+ ions and characteristic green emission peaks from Er 3+ ions were simultaneously observed, while only strong green emissions from Er 3+ ions were detected upon excitation of 980 nm. Remarkable emission enhancement was observed in down-/up-conversion luminescence processes by introducing Bi 3+ ions into Er 3+ -doped YNbO 4 phosphors. By analyzing the laser working current dependent up-conversion luminescence spectra, two-photon processes were confirmed to be responsible for both the green and the red up-conversion emissions of Er 3+ ion. The temperature sensing property of Er 3+ was studied by using the temperature dependent up-conversion luminescence spectra and it was found that the temperature sensitivity was sensitive to the doping concentration of Bi 3+ ions. By comparing the experimental values of the radiative transition rate ratio of the two green emission levels of Er 3+ ions and the theoretical values calculated by Judd-Ofelt (J-O) theory, it was concluded that energy level splitting had significant influences on the temperature sensing property of Er 3+ ions.


Introduction
With the rapid development of modern technology, rare earth (RE 3+ ) ions doped luminescent materials have evoked great attention because of their potential and practical applications in solid-state lighting, optical imaging, temperature sensors, color displays and so on [1][2][3][4]. Among numerous RE 3+ ions, Er 3+ ion is considered to be an excellent green emitting center in down-/up-conversion (DC/UC) luminescence materials thanks to its predominant ( 2 H11/2, 4 S3/2) → 4 I15/2 transitions [5]. Moreover, these two green-emitting energy levels 2 H11/2, 4 S3/2 are thermally coupled energy levels, whose populations fulfill Boltzmann's distribution law. The fluorescence intensity ratio (FIR) of the two green emissions depends on the sample temperature, Er 3+ ion has been extensively studied as an optical temperature sensing unit [6][7][8].
Compared with traditional contact temperature detection devices, this kind of optical temperature sensor based on FIR technique exhibits higher resolution and can realize non-contact and real-time temperature sensing in harsh environments [9,10].
In general, the host matrix has significant effects on the optical properties of the luminescent materials. LnNbO4 (Ln = La, Gd, Y) compounds, as a type of niobates, have been investigated as RE 3+ -doped DC/UC luminescent materials owing to their excellent chemical property, good thermal stability, and lower phonon energy than the most other oxide compounds [11][12][13]. YNbO4, as a self-activated phosphor, exhibits an efficient blue emission upon 254 nm excitation [14], white-light-emitting has been realized in a single composition YNbO4: Eu 3+ , Tb 3+ phosphor by tuning the relative doping concentrations of Eu 3+ and Tb 3+ [14].
It is well known that adding proper sensitizers in the luminescent materials can improve the optical performance of the activators [15,16]. Bi 3+ ion, as a good sensitizer, has been adopted as co-dopant in many RE 3+ -doped DC luminescent materials and significant enhancements in the emission intensities of Er 3+ , Eu 3+ and Dy 3+ ions have been achieved [17][18][19]. Additionally, remarkable enhancement of UC luminescence of RE 3+ (RE=Er, Tm, Ho) was also observed in fluoride materials via adjusting the host lattice and the local crystal field by co-doping of Bi 3+ ions [20].
Er 3+ -doped YNbO4 is a good phosphor from which the green emissions can be obtained under both DC and UC excitation conditions. In order to improve the luminescence performances of Er 3+ ions in YNbO4 samples, Bi 3+ /Er 3+ co-doped phosphors were prepared and the luminescence properties were systematically studied.
Building from the above ideas, in the present work, Bi 3+ /Er 3+ co-doped YNbO4 phosphors were synthesized using a high temperature solid-state reaction method. The influence of Bi 3+ concentration on the DC/UC luminescence properties of Er 3+ ions was studied. In the presence of Bi 3+ ions, significant enhancements in the emission intensities of Er 3+ ions in both DC and UC luminescence processes were observed.
Moreover, the temperature sensing properties of Bi 3+ /Er 3+ co-doped YNbO4 phosphors were also studied. The effect of energy level splitting on the temperature sensitivity and optical transition properties were discussed based upon the framework of Judd-Ofelt (J-O) theory.

Experimental
A series of Bi 3+ single-doped and Bi 3+ /Er 3+ co-doped YNbO4 powder phosphors were prepared via a traditional high temperature solid-state reaction method. Er 3+ concentration was fixed as 5.0 mol%, and Bi 3+ concentrations (x mol%) were designed to be 0, 0.5, 1.0, 3.0, and 5.0 mol%, respectively. In a typical synthesis procedure, the raw materials Y2O3 (99.99%), Bi2O3 (99.99%), Er2O3 (99.99%) and Nb2O5 (99.99%) were weighed on the basis of stoichiometric ratio and well ground with the help of an agate mortar and pestle. After homogeneous mixing, these mixtures were transferred into alumina crucibles and placed into a programmed muffle furnace at room temperature. The furnace was heated from room temperature to 1300 ℃ within 2 h, and these mixtures were kept at 1300 ℃ for 4 h. Lastly, the products were obtained after the furnace-cooled naturally to room temperature. X-ray diffraction (XRD) measurements were carried out by a powder diffractometer (SHIMADZU, XRD-6000, Japan) operating at 40 kV and 30 mA with CuKα1 radiation (λ = 0.15406 nm). The continuous-scanning mode with a scanning rate of 4°/min was adopted for phase identification with a 2θ range from 20° to 70°.
The DC luminescence spectra were recorded by a Hitachi F-4600 fluorescent spectrometer equipped with a built-in excitation source (150 W Xenon lamp). The UC luminescence spectra were measured by the same spectrometer assisted by an external power-controllable 980 nm fiber laser. The temperature of the samples was adjusted by a homemade temperature controlling system, DMU-TC 450, which was composed by a small stove and an intelligent digital-display-type temperature control instrument.
The controlling accuracy for temperature was about ± 0.5 ℃. The diffuse-reflection spectrum was collected by a spectrophotometer (Shimadzu, UV-3600, Japan) equipped with an integrating sphere accessory, and BaSO4 powder was used as the reference.

Crystal structure
The phase identification of the as-synthesized samples was examined by XRD measurement. Fig. 1 shows the XRD patterns of 5.0 mol% Bi 3+ single-doped and 5.0 mol% Er 3+ /x mol% Bi 3+ (x = 0, 1.0, and 5.0) co-doped YNbO4 powders as well as the standard pattern of monoclinic YNbO4 reported in JCPDS No. 83-1319. It can be seen that all the diffraction peaks of the as-synthesized samples are in accordance with the standard data. No extra peak corresponding to any other impurity is observed even in the samples with higher doping concentrations of Bi 3+ or Er 3+ ions.

<<Fig. 1 around here>>
To better describe the variation of the crystal lattice parameters of YNbO4 samples after introducing various concentrations of Er 3+ and Bi 3+ ions, Rietveld refinements were performed by using the general structure analysis system (GSAS) program [21,22]. The refinement results are depicted in Fig. 2 and the corresponding lattice parameters are listed in Table 1. As can be seen in Fig. 2 [23]. It is reported that such substitution can tailor the local crystal field around Er 3+ ions in the host lattice, which may have effects on the UC luminescence property of Er 3+ ions [24].  Fig. 3(b). It can be seen that the emission intensity increases with the augment of Bi 3+ concentration, and then starts to decline when Bi 3+ concentration reaches a maximum value at 3.0 mol% because of concentration quenching. phosphor, the excitation and emission spectra are in agreement with the above results shown in Fig. 3. As depicted in Fig. 4(b), it can be seen that the excitation spectrum of Er 3+ single-doped YNbO4 sample monitored at 555 nm (corresponding to the 4 S3/2→ 4 I15/2 transition of Er 3+ ions) consists of two parts. One is a broad excitation band centered at around 265 nm, which can be assigned to the absorption of NbO4 4- [7].

Luminescence properties of Er 3+ /Bi 3+ co-doped YNbO4 phosphors
The other part comprises of some sharp peaks located in the wavelength region from 350 nm to 600 nm, corresponding to the characteristic 4f-4f transitions from the ground state 4 I15/2 level to the excited states 4 G11/2, 2 H9/2, 4 F3/2, 4 F5/2, and 2 H11/2 levels of Er 3+ , respectively. Under 379 nm excitation, only green emissions can be detected, which are originated from the characteristic transitions ( 2 H11/2, 4 S3/2) → 4 I15/2 of Er 3+ ions. Obviously, Fig. 4(a) and (b) reveal a wide spectral overlap between the emission spectrum of Bi 3+ ion and the excitation spectrum of Er 3+ ion, which indicates that effective energy transfer from Bi 3+ to Er 3+ is expected in Bi 3+ /Er 3+ co-doped YNbO4 phosphors [27]. This energy transfer process can also be proved by the excitation and emission spectra of Bi 3+ /Er 3+ co-doped YNbO4 phosphors. As shown in Fig. 4(c), by monitoring the emission at 555 nm, the excitation spectrum of Bi 3+ /Er 3+ co-doped sample is composed of a broad excitation band of Bi 3+ ion as well as the 4f-4f transitions of Er 3+ ion. Moreover, upon the excitation of 327 nm, the emission spectrum of Bi 3+ /Er 3+ co-doped sample contains not only the broad emission band from Bi 3+ ions but also the characteristic emission peaks from Er 3+ ions. The above results indicate that the introduction of Bi 3+ ions greatly extends the excitation band range of Er 3+ -doped YNbO4 phosphors in near-ultraviolet light region, which is benefit for the application in ultra-violet excited white light emitting diodes. ions. The intensification of the intensity at low Bi 3+ doping case can be ascribed to the enhancement of the interaction between Bi 3+ and Er 3+ ions caused by the shorten distance between them as Bi 3+ concentration rising, while the attenuation of the emissions at higher Bi 3+ doping case is probably due to the concentration quenching effect, which mainly results from non-radiative energy transfer among the activators [26].

<<Fig. 4 around here>>
Chromatic property is one of the important parameters for evaluating the phosphors' performance. According to the intensity-calibrated emission spectra, the

<<Fig. 6 around here>>
To provide more in depth understanding of the UC luminescence mechanism of Er 3+ ions in Er 3+ /Bi 3+ co-doped YNbO4 phosphors, the UC emission spectra of the samples doped with various concentrations of Bi 3+ ions as a function of pumped power (P) were examined under 980 nm excitation. It is well known that for a n-photon UC luminescence process (n is the number of required photon), the UC emission intensity (Iup) is proportional to P n , which can be expressed as Iup = MP n (M is a constant) [28]. To determine the values of n, the integral intensities of the green and the red UC emissions were recorded and the dependence of them on the pumped power density are shown in Fig. 7. It can be seen that the intensities of both the green and the red UC emissions increase as the pumped power density rising. Then, the data were fitted by using the aforementioned equation and the fitting results are also shown in Fig. 7, denoted as solid lines. It is confirmed that the n values of both the green and the red UC emissions are all around 2, meaning that two-photon processes are responsible for both the green and the red UC emissions no matter what the concentration of Bi 3+ ion is. The detailed UC luminescence processes are similar with those reported results and can be referred to Ref. [29].

<<Fig. 7 around here>>
From the above-mentioned excitation and emission spectra, it has been found that Bi 3+ ion possesses a broad absorption band around 327 nm and a broad emission band extending from 350 nm to 625 nm. No absorption of Bi 3+ ion at around 980 nm was observed [30], thus Bi 3+ ion cannot absorb 980 nm excitation energy and there is no energy transfer from Bi 3+ ions to Er 3+ ions upon 980 nm excitation. Meanwhile, because no emission from Bi 3+ ion was found in the UC emission spectra, there is no energy transfer from Er 3+ ions to Bi 3+ ions either. Therefore, energy transfer is not the reason for the enhancement of UC luminescence of Er 3+ ions by introducing Bi 3+ ions.
According to the quantum selection rules, the intra-4f electronic transitions of RE 3+ ions are parity forbidden and strongly depend on the local crystal field and site symmetry, which can be partially broken when RE 3+ ions situate at low symmetry sites [17,20,31]. Because the radius of Bi 3+ ion is different from that of Y 3+ ion, the crystal field environment around Er 3+ ion may be modified when Y 3+ sites are substituting by Bi 3+ ions, which may induce the change of the radiative transition rate and affect the UC emission intensity of Er 3+ ions. Zhang et al. have theoretically illustrated that significant UC luminescence enhancement of Er 3+ ion arises from the synthesized tailoring effect induced by the Li + ions [24]. Thus, it is considered that the modification of the local crystal field environment around Er 3+ ions via Bi 3+ doping is the main reason for the enhancement of UC luminescence intensity of Er 3+ ions.
Moreover, When the doping concentration of Bi 3+ ion exceeds 3.0 mol%, the excessive Bi 3+ ions will enlarge the distance between Er 3+ ions and weaken the interaction between them and then induce the decrease of the emission intensities of Er 3+ ions.

Temperature sensing properties of YNbO4: Er 3 /Bi 3+ phosphors
To investigate the temperature dependent luminescence behavior of the prepared phosphors with different Bi 3+ concentrations, the temperature-varied emission spectra were measured in the temperature region of 303-573 K under 980 nm excitation with the pumping power density of 24.84 W/cm 2 , and the results are shown in Fig. 8. It can be seen that the intensities of both the two green emissions came from 2 H11/2→ 4 I15/2 and 4 S3/2→ 4 I15/2 transitions of Er 3+ ions decrease as an increase in sample temperature.
According to the temperature-varied UC luminescence spectra, the FIRs of the two green emissions were calculated based upon their integral intensities obtained by Gaussian fitting. As representatives, Fig. 9 shows the relationship between the FIR of the two green emissions and the sample temperature for YNbO4: Er 3+ samples doped with x mol% Bi 3+ (x = 0, 1.0, 3.0 and 5.0). It is apparent that the FIRs of all samples monotonically increase as temperature rising.
It is well known that 2 H11/2 and 4 S3/2 levels of Er 3+ ions belong to thermally coupled energy levels, whose population numbers follow Boltzmann's distribution law, thus the FIR of the two green emissions can be expressed as [32]: (1) where IH, IS, NH, NS, and AH, AS are the integral fluorescence intensities, the population numbers, and the radiative transition rate of the emissions from 2 H11/2→ 4 I15/2 and 4 S3/2→ 4 I15/2 transitions, respectively. ΔE, k, and T are the energy distance between 2 H11/2 and 4 S3/2 levels, Boltzmann's constant and absolute temperature, respectively.
All the experimental data shown in Fig. 9 are fitted by Eq. (1) and the solid and dashed lines represent the fitting results. As can be seen, the experimental data are fitted well by Eq. (1). Moreover, in the fitting processes, the values of AH/AS and ΔE/T were obtained, and the confirmed equations for all samples are also exhibited in Fig. 9.
It can be found that the values of AH/AS decline with the increase of Bi 3+ concentration.

<<Fig. 8 around here>> <<Fig. 9 around here>>
Usually, the temperature sensitivity (S(T)) can be defined as the rate of change of R per unit temperature, and S(T) of Er 3+ -based temperature sensor can be mathematically expressed as [33]: <<Fig. 10 around here>>

Judd-Ofelt analysis of YNbO4: Er 3+ /Bi 3+ phosphors
According to the results of temperature sensing property, it can be found that the concentration of Bi 3+ ion has significant effects on the value of AH/AS and the temperature sensing sensitivity of Er 3+ ions. As Bi 3+ concentration rising, both of them decrease dramatically. It is well known that J-O theory provides a helpful method to investigate the optical transition properties of RE 3+ -doped luminescent materials, including radiative transition rates, fluorescence branching ratios, quantum efficiencies and so on [34,35]. On the basis of this theory, the influence of the local matrix environment on the optical transition properties of Er 3+ ion in YNbO4: Er 3+ /Bi 3+ samples were discussed more in-depth and theoretical. The J-O parameters of Er 3+ and Sm 3+ single-doped powder samples have been calculated by J-O theory before and the detailed calculation process can be referred to Refs. [36][37][38].
To    splitting. This is also the reason why the deviation from theoretical to experimental values in Table 3

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.