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Abstract
Transition probability is of vital importance for luminescence process, whereas the effects of doping concentration have not been explored in the Er3+:NaGdF4. In this work, we investigate the radiative transition probabilities of Er3+ highly doped NaGdF4 sub 10 nm nanocrystals using J-O theory. It is found that the transition probabilities vary with changing Er3+ concentration, especially altering the ratio of Er3+ 2H11/2 to 4S3/2 level, which is highly useful for optical thermometers as they are thermally coupled. To validate the concentration dependent transition probabilities, significant enhancements of upconversion luminescence are achieved by epitaxial growth of the inert shell, and thermal sensing behaviors are investigated using the improved samples.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Rare earth ions, especially Er3+ has abundant energy levels and long lifetime of excited states, which makes it difficult to directly measure the important radiation rate constants of the relevant Er3+ emission states, because the total decay of these energy levels includes many non-radiative paths such as multiphonon relaxation, Er–Er cross relaxation and so on [1]. Fortunately the J-O theory, proposed in 1962, could predict the radiation parameters by investigating the absorption spectra and emission spectra of the f–f energy transitions of lanthanides [2,3]. This method is most widely used in rare earth doped transparent glasses or crystals [4–7], and less used in powder samples [1,8] due to their poor transparency and low rare earth concentration.
As one of the most efficient infrared-to-visible upconversion (UC) fluorescent host materials, NaGdF4 exhibits adequate thermal and environmental stabilities [9,10], which make β-NaGdF4 an ideal matrix candidate [11]. Additionally, Gd3+ ions possess paramagnetic properties, providing the potential of UC nanocrystals (NCs) as multifunctional markers, both optical and magnetic, in biology and medicine [12,13]. Although the structure, morphology and luminescent properties of β-NaGdF4 matrix have been sufficiently investigated, there are very few reports about the transition parameters of different concentrations of Er3+ highly doped in NaGdF4 NCs, which makes it almost impossible to directly refer to Er3+ transition parameters in NaGdF4 matrix.
In addition, the effects of doping concentration on the spectroscopic parameters, especially on radiative transition probabilities, have not been carefully investigated to date. It is well known that the spectroscopic parameters depend on the surrounding environment of rare earth ions. Although the dopants (Er3+, Tm3+, Yb3+, etc) possess high similarity with NaLnF4 host elements, it still can be taken into account that these doping ions acting as additives arouse slight structural change of the matrix. Therefore the various doping levels should alter the spectroscopic parameters. This variation might not be obvious in low doping samples. However, it should be quite evident in high doping conditions, which is a promising solution of enhancing the UC efficiency after suppression of concentration quenching [14,15].
Furthermore, the Er3+ doped UC fluorescent materials also serve as thermometers based on measuring the fluorescence intensity ratio (FIR) of the 2H11/2 and 4S3/2 levels [16]. So far, there have been many investigations with respect to Er3+ doped matrix materials, including glass ceramics, phosphors, and UC NCs [17–24]. However, due to the limitation of high concentration quenching effect, low concentration doping has been used, resulting in the lack of exploration of highly doping concentration in FIR temperature sensing till now.
Herein, we synthesized different concentrations of Er3+ highly doped in NaGdF4 ultra-small NCs, and investigated the transition probability dependence on the concentration of the doping ions. Besides, remarkable luminescence enhancements are observed in the inert coating NCs. Using the efficient core-shell (C-S) NCs, temperature sensing behavior were carefully investigated, to validate the effects of concentration on the transition properties.
2. Experimental
Different concentrations of Er3+ (20, 50 and 80 mol%) doped NaGdF4 ultra-small NCs and their C-S counterparts were prepared using a modified thermal decomposition route [25]. The absorption spectra of different samples were measured by a Shimadzu UV-2450 spectrometer. Luminescence spectra were measured by the ocean optical QE Pro spectrometer. The crystal structures of the samples were identified by powder X-ray diffraction (XRD, X’pert Pro), and the diffraction patterns were recorded in the range of 2θ from 17 to 67° with a resolution of 0.03°/step. The particle morphologies were recorded on a transmission electron microscopy (TEM, Tecnai G2).
3. Structure characterization
XRD analysis depicted in Fig. 1(a) revealed that the samples were hexagonal phase. All diffraction peaks can be indexed as the β-NaGdF4 (JCPDS No. 27-0699), and no other peaks can be seen, indicating that the synthesized samples are relatively pure and free of other impurities. It can also be seen form Fig. 1(b) that the refraction peaks move to the larger angle as increasing the doping concentration, evidencing that the bigger lattice Gd3+ (0.94 Ǻ) are substituted by smaller Er3+ (0.88 Ǻ) ions.
Fig. 1 (a) XRD patterns of the as-synthesized different Er-doped NaGdF4 core nanoparticles. The standard data for β-NaGdF4 (JCPDS No. 27-0699) is shown as a reference, (b) The peak shifts of the three most intensive refrections.
The TEM images of Er-doped NaGdF4 core and C-S NCs are shown in Figs. 2(a)-2(f). It is observed that the UC NCs are mono-dispersed in cyclohexane and the average size of the core is 8~9 nm [Figs. 2(a)-(c)]. Epitaxial growth of pure NaGdF4 shell (~3 nm) on the core enables the mean size of all C-S samples increased to ~15 nm [Figs. 2(d)-(f)], indicating the success of inert coating. Importantly, the similar sizes of core and C-S samples support us to exclude the effects of different particle sizes on the luminescence properties, and focus on the influence of doping concentration.
Fig. 2 TEM images and size-distributions of (a) 20Er core with an average size of 8.0 nm, (b) 50Er core 9.3 nm, (c) 80Er core 9.1 nm, (d) 20Er C-S 15.3 nm, (e) 50Er C-S 15.5 nm, (f) 80Er C-S 14.9 nm.
4. Optical properties
4.1 Absorption spectra
The linear absorption coefficients of X ( = 20, 50 and 80) mol% Er:NaGdF4 are presented in Fig. 3. In the spectral range of 340 nm to 850 nm, 11 absorption bands centered at 355 nm, 364 nm, 377 nm, 405 nm, 441 nm, 448 nm, 489 nm, 521 nm, 540 nm, 654 nm, and 802 nm, correspond to Er3+ ions absorption from the ground state 4I15/2 level to 4G7/2, 4G9/2, 4G11/2, 2G9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, and 4I9/2 respectively. The absorption peak positions are all substantially unchanged, whereas the intensities of the coefficients increase with Er3+ ion doping concentration, due to the stronger absorption.
Fig. 3 Linear absorption coefficients of Er:NaGdF4 in the range of 420 nm to 850 nm at room temperature. Inset shows the detailed absorptions in the range of 340 nm to 420 nm.
4.2 Concentration dependent transition probabilities
In order to investigate the effects of doping concentration on the radiative transition probabilities AJJ´, all the 11 absorption bands as shown in Fig. 3 are chosen for the calculation of transition parameters using J-O theory. The linear absorption coefficient and integral strength of the transition from ground state to excited state are obtained from measurement. The oscillator strength of the magnetic dipole of Er3+ is much smaller than the electric dipole in magnitude, therefore only electric dipoles oscillator strength is considered here. The following two formulas are used to calculate the measured oscillator strength (fmeas) and the calculated oscillator strength (fcalc) for the transition of the Er3+ ion ground state 4I15/2 (J) to each excited state J' energy level:
The measured the oscillator strength is:
The calculated oscillator strength is:
In the formula, J and J' are the energy level total angle quantum number of the initial state and the final state. c, h, e, m are the light speed in vacuum, Planck constant, electronic charge and mass, respectively. ɛ(v) is the absorption coefficient, v is the energy (cm−1) of the transition. n is the refractive index of the Er:NaGdF4, which is similar to the Er:NaYF4 matrix and can be written as: n(λ) = A + B/λ2 + C/λ4, where A = 1.464, B = 9.364 × 102 nm2, and C = 1.197 × 109 nm4 [26]. The matrix element U is not sensitive to the ion environment and can be found in the literature [27]. Ωi (i = 2, 4, and 6) is the three phenomenological J-O intensity parameters. N is the ion number density of Er3+ ions in the sample.
A measurement of the accuracy of the fit is given by the root-mean-square deviation:
where ∆f = fcalc - fmeas is the deviation, q the number of transitions selected, p the number of J-O parameters. q and p are 11 and 3 in the calculation, respectively. The deviations and the calculated J-O parameters are listed in Table 1.
Table 1. The fitting deviation and JO parameters of various Er3+ doped NaGdF4
Using the above calculated J-O parameter, the following formula can be used to calculate the spontaneous radiation probability between the corresponding energy levels:
As shown in Table 2, all the transition probabilities increase with the doping concentration, except that from 4G11/2 and 2H11/2 levels in 80Er doped sample. It is well know that the transition properties of RE ions are sensitive to the surrounding environment, herein the Er3+ ions as the additive in the NaGdF4 matrix lead to slight structural change [Fig. 1(b)], and thus alter the emission probabilities.
Table 2. Spontaneous emission probability (AJJ´) of Er3+ in Er:NaGdF4
4.3 Enhanced luminescence through inert coating
In the present work, to obtain the detectable absorption spectra, high concentrations of Er3+ are used. As can be seen in Fig. 4(a), the luminescence intensities of all samples are extremely weak due to the strong concentration quenching effect, which is the energy dissipation process aroused by efficient energy transfer of Er → Er and Er → Quenchers. The high concentration dopant and ultra-small particle size both are responsible for the efficient luminescence quenching.
Fig. 4 (a) UC spectra of Er:NaGdF4 core NCs. (b) UC spectra of the C-S NCs with ~3 nm inert coating. (c) Comparison of red and green intensity of core and C-S NCs. (d) Schematic of the pathways of Er3+ UC processes.
To utilize the UC signal of Er3+ heavily doped samples, inert coating of pure NaGdF4 (~3 nm) by epitaxial growth is carried out. The typical UC spectra of the corresponding C-S counterparts are shown in Fig. 4(b). Impressively, we observed significant luminescence enhancements in the C-S NCs compared to the corresponding core only NCs, the maximal enhanced factor reaches up to 300 [Fig. 4(c)]. The UC emission of C-S NCs increases monotonically with increasing Er3+ doping concentration, and the brightest being the most heavily doped (80 mol% Er3+) C-S NCs, which is in good agreement with that reported by Johnson et al [14]. The significant enhancement in the C-S NCs can be attributed to the surface passivation of the inert shell. The inert coating effectively prevents the luminescence quenching, due to the C-S structure isolates the activators from surface quenchers and thus suppresses the detrimental energy transfer. As a result, the non-radiative relaxation process is less efficient than the core-only sample [28]. The possible energy level transitions of the samples luminescence are shown in Fig. 4(d). The intensity ratios of red light and green light (I654/I540) gradually increase with increasing concentration, both in the core and C-S NCs, which can be ascribed to that the strong cross-relaxation between Er3+ ions under high doping conditions.
4.4 Temperature sensing through Er:NaGdF4@NaGdF4 core-shell nanocrystals
To further validate the concentration dependent transition probabilities, we use Er:NaGdF4@NaGdF4 C-S samples for temperature sensing on the basis of FIR technique. Based on the above results, it is found that the ratio of the transition probabilities between 2H11/2 and 4S3/2 (A2/A1) is closely related to the Er3+ doping concentration (the ratio decreases from 15.2 to 10.6, as shown in Table 2). It is noted that the A2/A1 value can be deduced from FIR calibration, thus the concentration dependent transition probabilities can be verified by temperature sensing behaviors. The FIR generated from transitions of 2H11/2 and 4S3/2 levels as follows:
where Ii and Ai are the fluorescence intensity and spontaneous emission probability of the 2H11/2 (i = 2) and 4S3/2 (i = 1) thermal coupled levels, respectively. C is a constant, ΔE is the energy gap, k is the Boltzmann constant, and T is the absolute temperature.Figure 5 depicts the FIR values as a function of temperature within the range of 300-400 K. The experimental data can be well fitted according to Eq. (5). It can be seen that the energy gap remains substantially unchanged whereas the constant term C varies evidently with the doping concentration, that is, it decreases from 14.32 to 8.96 with increasing Er3+ content.
The variations of A2/A1 value from Table 2 and C value from Fig. 5, with Er3+ concentration, are shown in Fig. 6. It is found that the decreasing trends of the two values are highly consistent, evidencing our hypothesis of the concentration dependent transition probabilities, as these two values are obtained from two independent methods.
Fig. 6 Comparison of A2/A1 value deduced by J-O calculation and C value obtained from FIR calibration.
Eventually, we calculate the absolute sensitivities (SA) and the relative sensitivities (SR) of the Er3+ heavily doped samples according to the expression SA = d(FIR)/dT, SR = [d(FIR)/dT]/FIR, respectively. It can be seen that both sensitivities decrease with the Er3+ concentration (Fig. 7), indicating low doping sample could provide more sensitive sensing ability. Especially, the relative sensitivity of 20Er sample reaches up to ~1.3% at 300 K, which is higher/comparable to the previous reports, as shown in Table 3, indicating the relatively high performance of our thermometer.
Table 3. Comparison of relative sensitivity in different materials
5. Conclusion
In summary, we synthesized the X ( = 20, 50 and 80 mol%) Er:NaGdF4 by high temperature thermal decomposition method. By measuring the absorption spectra in 340-850 nm range, the radiative transition probabilities are calculated using J-O theory. It is found that most radiative transition probabilities increase with Er3+ concentration. In addition, the corresponding core-shell structured samples are synthesized through hot injection technique, and significant enhancements are achieved, especially in 80% Er C-S sample the enhanced factor reaches up to 300 times. Thermal sensing behaviors are investigated using these improved samples, and further be utilized to verify the concentration dependent transition probabilities. Finally, it is found that lower dopant concentration results in higher sensing sensitivity. This work provides valuable reference of spectroscopic properties, such as spontaneous transition probabilities and fluorescence branch ratio, in efficient Er3+ doped NaGdF4 NCs, and also renews the understanding about the doping concentration dependent spectroscopic parameters.
Funding
111 project (B13015); Fundamental Research Funds for the Central Universities.
Acknowledgments
The authors thank Dr. H. Tian for the assistance of XRD measurements.
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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