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Abstract
Photon upconversion (UC) based on self-sensitization of lanthanide-doped nanocrystals is of great importance for biological and photonic applications. Here, we report tunable multicolor display and sensitive temperature sensing in the erbium-doped CaF2 upconversion nanocrystals (UCNCs) codoped with Tm3+ ions. Under the excitation of 980, 808 and 1532 nm lasers, the upconversion luminescence (UCL) color of these self-sensitized UCNCs can be manipulated from green to red efficiently and the red-to-green (R/G) UC intensity ratio is promoted remarkably as the doping Tm3+ ions vary from 0 to 4 mol%. Especially, we have successfully demonstrated the multicolor modulation of these UCNCs by changing the pulse width and repetition frequency under multi-wavelength excitation. The power dependence and decay lifetimes measurements of Er3+ ions under multi-wavelength excitation were carried out to clarify the UC color manipulation. In addition, the optical temperature sensing properties of CaF2:Er3+/Tm3+ (10/0.125 mol%) UCNCs are also studied thoroughly under 980, 808 and 1532 nm lasers excitation, and the results show that the UCNCs possess outstanding thermal sensitivity. The features enable these UCNCs to act as promising candidates for high-resolution biological imaging, multicolor display and nanoscale thermometer fields.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photon UC based on the lanthanide-doped nanomaterials can efficiently generate high-energy photon emission under low-energy light irradiation via the so-called “anti-Stokes process” [1–4], which has been widely studied in the frontier fields of biological imaging [5–7], anti-counterfeiting [8,9], volumetric display [10,11], super-resolution nanoscopy [12,13], temperature sensing [14,15], and waveguide-based optical devices [16,17]. The conventional strategy to achieve photon UC is using the distinct sensitizer-activator coupled scheme. Generally, the Yb3+ or Nd3+ ions (corresponding to the 980 or 808 nm laser light, respectively) act as the sensitizer absorbing the energy, and the Er3+ (Tm3+, Ho3+) ions regard as the activator emitting the UCL. Whether the Yb3+ or Nd3+ ions doped materials, their excitation lasers are all located in the short near-infrared (NIR) biological window (NIR-I) ranging from 700 to 1100 nm. Compared to the NIR-I excitation, the excitation wavelength located in NIR-II window (1100–1700 nm) has the capability of deep tissue penetration due to the decrease of auto-fluorescence and photon scattering [18]. Moreover, NIR-II excitation induced UCL provides a large anti-Stokes shift, which is beneficial to color recognition. Thus, studies on NIR-II responsive photon UC would greatly boost new findings in fundamental research and enrich biological and photonic applications. Recent studies have reported that Er3+-sensitized UC nanocrystals can be excited by ∼1532 nm wavelength for achieving intense UCL [19–23].
It is well-known that the red UC emission falls into the visible biological window (600–700 nm), which can be easily observed and will not cause adverse effects to the biological tissues. Actually, Er3+ ions could generate two strong emission bands in the visible region: green (2H11/2, 4S3/2 → 4I15/2) and red (4F9/2 → 4I15/2) UC emissions. Hence, great efforts have made to enhance the red emission (650 nm) and simultaneously suppress the green ones (525 and 545 nm) for the Er3+ doped materials from bulk, phosphors to nanomaterials under the excitation of 980 or 808 nm CW laser, such as altering doping concentration of sensitizer and activator [24,25], choosing appropriate host [26], introducing organic dye [27], and controlling the particle size or shape [28]. Additionally, the introduction of suitable rare-earth-ion (such as Tm3+) or transition-metal-ion (such as Fe3+ and Mn2+) into the Er3+-sensitized UCNCs could also efficiently facilitate the red UC emission with high purity [19–22,29–31]. In general, Er3+ ions act as self-sensitized ions, which can be excited by multiple NIR excitation sources operating at 980, 808 and 1532 nm, which corresponds to the absorption transitions of 4I15/2 → 4I11/2, 4I15/2 → 4I9/2 and 4I15/2 → 4I13/2 from Er3+, respectively. However, achieving single-band red UC emission of Er3+-sensitized UCNCs under multi-wavelength excitation, especially in one fixed UC material, is still lack of research reports until now. Moreover, the realizing of pure red emission based on the non-steady-state UC process in Er3+-sensitized UCNCs also remains poorly explored, which is vital for the application in real-time multicolor display [10,31–36]. Therefore, it is well worth exploring the pure red UC emission in Er3+-sensitized UCNCs irradiated by multi-wavelength excitation lasers (including 980, 808 and 1532 nm), which will greatly expand its further practical applications.
In addition, the development of remote, fast-responding and accurate optical temperature sensor on the basis of UCNCs is highly desirable to extend the application in nanoscale temperature sensing field, which has been demonstrated in Y2O3, NaYF4, Gd2Ce2O7, LaPO4 and Y2O2SO4 hosts [24,37–40]. As is known to all, the phonon energy and crystalline structure of host materials would have a great influence on the thermal quenching and luminescence efficiency. Thus, the thermal sensing performance is significantly affected by the host lattice. Compared to other host lattices, the CaF2 host possesses low phonon energy (∼350 cm-1) and can be easily substituted by rare-earth-ions, as well as exhibiting high thermal and chemical stability. Nevertheless, so far no comprehensive thermal sensing investigation has been reported in CaF2 UCNCs, especially lacks the systematic investigation and comparison of the optical sensing properties of Er3+-sensitized UCNCs under the multi-wavelength excitation of 980, 808 and 1532 nm lasers. Therefore, exploring the temperature sensing properties in CaF2 UCNCs will further broaden its applications in nanoscale thermometer fields.
In this work, we have demonstrated the tunable multicolor display and temperature sensing performance in the self-sensitization of erbium-doped CaF2 UCNCs. Under the excitation of 980, 808 and 1532 nm continuous-wave (CW) lasers, the R/G ratio is well tuned by adjusting the Tm3+ concentration, excitation laser pulse width and repetition frequency. The dependence of UC emission on the excitation power and time-resolved UC emission measurements were performed to illustrate the mechanism of color manipulation. In addition, we examined the thermal sensing abilities of CaF2:Er3+/Tm3+ (10/0.125 mol%) UCNCs under multi-wavelength excitation.
2. Experimental
2.1 Synthesis of CaF2:Er3+/Tm3+ UCNCs
The lanthanide-doped UCNCs were synthesized according to the similar procedure described in our previous literature [41]. Taking CaF2:Er3+/Tm3+ (10/1 mol%) as an example, 2 mmol of chloride salts (1.78 mmol CaCl2·2H2O, 0.2 mmol ErCl3·6H2O and 0.02 mmol TmCl3·6H2O) and 2 mmol EDTA were dissolved in 20 mL of deionized water under stirring for 1 hour. Sequentially, 10 mL of the NaBF4 (4 mmol) aqueous solution was added into the above prepared solutions. The mixtures were vigorously agitated for 30 minutes and then transferred into a 50 mL Teflon-lined autoclave and heated at 200°C for 30 hours. After the reaction, the autoclave slowly cooled down to room temperature. The obtained products were collected by centrifugation, washed with ethanol and deionized water for several times, and finally dispersed in cyclohexane for test.
2.2 Characterization
The crystal structures of the prepared UCNCs were characterized by a Rigaku TTR III X-ray diffractometer using Cu K radiation at 40 kV and 200 mA. The morphology of the as-obtained UCNCs was detected by scanning electron microscopy (SEM, HITACHI S4800). In the photoluminescence experiment, the UCNCs were irradiated by 808, 980 and 1532 nm diode lasers with a focus diameter of ∼1.5 mm, then the UCL generated from the UCNCs was collected by a fluorescence spectrophotometer (Zolix Omni-l3072i) coupled with a R928 photomultiplier tube (PMT). The test UCNCs were dispersed in cyclohexane with a mass concentration of 0.5 mg mL−1 by irradiating with an ultrasonic bath for 10 minutes. The temperature-dependent UCL measurements (323–573 K) were conducted with the same fluorescence spectrophotometer linking with a high-temperature instrument accessory (Shanghai Hotz instrument technology Co., Ltd).
3. Results and discussion
3.1 Structure and morphology
The typical XRD patterns of CaF2:Er3+/Tm3+ (10/x x = 0, 1, 2 and 4 mol%) UCNCs and the corresponding standard card of CaF2 (JCPDS No. 75–0097) are shown in Fig. 1(a). All the observed diffraction peaks are in good agreement with the standard card of CaF2, indicating that the pure cubic-phased products were obtained. Figure 1(b–d) present the representative morphologies of the CaF2:Er3+/Tm3+ (10/x x = 0, 1, 2 mol%) UCNCs. These as-prepared UCNCs are nanospheres with an average size of ∼500 nm, and this means that the doping of a small amount of rare-earth-ions (Er3+ or Tm3+) in CaF2 host has no impact on the size and morphology of CaF2 UCNCs. Moreover, we have further examined the chemical element composition investigation of CaF2 nanocrystals by EDS, as shown in Fig. 1(e,f). The corresponding elements are confirmed by EDS measurement for different UCNCs, which indicates that the Er3+ and Tm3+ ions have successfully entered the CaF2 host matrix.
Fig. 1. (a) XRD patterns of CaF2:Er3+/Tm3+ (10/x, x=0, 1, 2 and 4 mol%) UCNCs. Representative SEM images of as-synthesized UCNCs: (b) CaF2:Er3+ (10 mol%), (c) CaF2:Er3+/Tm3+ (10/1 mol%) and (d) CaF2:Er3+/Tm3+ (10/2 mol%). EDS analysis of the (e) CaF2:Er3+ (10 mol%) and (f) CaF2:Er3+/Tm3+ (10/4 mol%) UCNCs.
3.2 Color manipulation under multi-wavelength excitation
It is well-known that the concentration effect is a mechanistic limitation in the optimization of the UCL due to the concentration quenching in the Er3+-sensitized UCNCs. To obtain the optimal doping Er3+ concentration, we have synthesized a series of CaF2:Er3+ UCNCs with fine tuning of the Er3+ concentration from 1 to 10 mol%. The UC emission spectra of CaF2:Er3+ (x, x=1, 2, 4, 6, 8 and 10 mol%) are exhibited in Fig. S1(a–c) under 980, 808 and 1532 nm excitations (see Supplement 1). Typical green (522 and 540 nm, 2H11/2/4S3/2 → 4I15/2) and red (656 nm, 4F9/2 → 4I15/2) UC emission bands of the Er3+ were observed. Moreover, the dependence of the integral intensity of UC emissions and R/G ratios on Er3+ concentration are also demonstrated in Fig. S1(d–f). For both 980 and 808 nm excitations, the integral intensity of CaF2:Er3+ UCNCs exhibits a gradual increase with the increasing Er3+ concentration and reaches the maximum at 6 mol%. When continue to increase the doping Er3+ ions further to 10 mol%, the integral intensity shows a slight decline due to concentration quenching [42]. The R/G ratios similarly show the inversion trends that the green UC emission (522 and 540 nm) is larger than the red one (656 nm) in lowly-Er3+ doping and become opposite for highly-Er3+ doping UCNCs. However, in contrast, the R/G ratio decreases monotonically (from 4.07 to 1.52) as a function of the Er3+ concentration when the CaF2:Er3+ UCNCs are excited by 1532 nm laser. The reason is that comparing with non-radiative relaxation process from 4I9/2 to 4I11/2, the ground state absorption (GSA), excited state absorption (ESA) and energy transfer (ET) processes under 1532 nm excitation can occur easily. Similar phenomena were also reported in previous literatures [8,43]. Thus, in order to acquire intense UCL and large R/G ratio under multi-wavelength excitation, we choose the optimal Er3+ concentration at 10 mol% for further investigation.
Codoping with appropriate rare-earth-ion is an efficient approach to achieve single-band red UC emission. Although there have been some reports about the UCL in Er3+-sensitized CaF2 hosts (including bulk, nanocrystals and transparent ceramics) under NIR laser excitation until now [44–46], the tunable multicolor especially enhancing the red UC emission of Er3+ through incorporation of other rare earth ions (e.g. Tm3+) in these materials has not been investigated under multi-wavelength excitation. Here, we examined the role of Tm3+ ions as a codopant for generating the UCL under multi-wavelength excitation. Figure 2(a–c) present the UC emission spectra of CaF2:Er3+/Tm3+ (10/x, x=0, 0.125, 0.25, 0.5, 1, 2 and 4 mol%) UCNCs under 980, 808 and 1532 nm lasers excitation, respectively. It is apparent that the UC emissions of Er3+ are greatly influenced by the Tm3+ concentration. As depicted in the insets of Fig. 2(a–c), the UCL color can be tuned from green to red as the Tm3+ concentration increases from 0 to 4 mol% under the excitation of 980, 808 and 1532 nm lasers. Figure 2(d) displays the tendencies of integral green and red UC emissions and the R/G ratio of CaF2:Er3+/Tm3+ (10/x mol%) UCNCs under 980 nm excitation. The green UC emission is weaker than that of red one and decreases gradually when the doping Tm3+ ions rise from 0 to 4 mol%. On the contrary, the red UC emission firstly rises up to its maximum and then decreases monotonically as the Tm3+ concentration increases. The decrease of red UC emission stems from the increasing the non-radiative loss when the doping Tm3+ ions are relatively high. Meanwhile, the R/G ratio is promoted from 1.22 to 17.36 when the doping Tm3+ ions increase from 0 to 4 mol%. Similarly, under the 808 nm laser excitation, the same tendency of integral intensities is observed, and the R/G ratio increases from 1.40 to 9.47 as the Tm3+ concentration increases (Fig. 2(e)). As shown in Fig. 2(f), when excited by 1532 nm laser, the R/G ratio increases to the largest of 12.81 at the 2 mol% Tm3+ concentration and drops to 7.91 when the doping Tm3+ ions further grow to 4 mol%. Moreover, it can be noticed that the UCL color can all be tuned from green to red under 980, 808 and 1532 nm irradiation. These results suggest that the CaF2:Er3+/Tm3+ UCNCs can act as a potential candidate for red-emitting materials under multi-wavelength excitation and the introduction of Tm3+ ions plays an important role in achieving spectrally pure red UC emission in Er3+-sensitized UCNCs.
Fig. 2. UC emission spectra of CaF2:Er3+/Tm3+ (10/x, x=0, 0.125, 0.25, 0.5, 1, 2 and 4 mol%) UCNCs under the excitation of (a) 980, (b) 808 and (c) 1532 nm lasers. The insets show the corresponding UCL color of the UCNCs dispersed in cyclohexane. (d–f) The integral intensity of green and red UC emissions and the R/G ratio as a function of Tm3+ concentration under multi-wavelength excitation.
To clearly expound the mechanism of the UC emissions under the excitation of different wavelength lasers, the population processes and UCL transitions in CaF2:Er3+/Tm3+ UCNCs are depicted in Fig. 3. Under the excitation of 980 nm laser, for lowly Er3+ doped UCNCs, the ground state Er3+ ions can be directly populated to the 4F7/2 through GSA and ESA processes or ET processes from the neighboring Er3+ ions. Then the multi-phonon relaxation (MPR) processes lead to the population of green (2H11/2 and 4S3/2) and red (4F9/2) emitting states, which would generate the green and red UC emissions. Notably, for Tm3+-free samples, the red-emitting state 4F9/2 can also be populated by ESA process from the 4I13/2 state. When the CaF2 UCNPs are doped with high content of Er3+ ions, taking the shorter distance between Er3+ ions into account, the cross-relaxation (CR) processes among adjacent Er3+ ions may make contribute to the population of the Er3+ residing at the red-emitting state 4F9/2. The electrons in 4I11/2 state can directly lead to the population of the red-emitting state through CR processes: 4I11/2 + 4F7/2 → 24F9/2 (CR1) and 4I11/2 + 4I9/2 → 4F9/2 + 4I13/2 (CR2). In addition, the electrons in the ground state can be excited to the 4I13/2 state, which is corresponding to the third CR process: 4I15/2 + 2H11/2, 4S3/2 → 4I9/2 + 4I13/2 (CR3). The occurrence of CR1 and CR2 can populate the 4F9/2 state of Er3+ ions, while CR1 and CR3 processes would depopulate the green-emitting states (2H11/2 and 4S3/2). Besides, CR3 process would also result in the population of 4I13/2 state, which can further migrate to the 4F9/2 state through ESA process. It should be noted that the energy gap of CR1, CR2 and CR3 are 151, 742 and 377 cm-1, respectively [32]. These values are comparable with the phonon energy of the CaF2 host matrix (∼350 cm-1). Thus, by increasing the Er3+ concentration, the phonon-assisted CR processes are efficient and would lead to the enhancement of R/G ratio. Noted that these CR processes all greatly contribute to the population processes under multi-wavelength excitation.
Fig. 3. Proposed UCL mechanisms for the Er3+-sensitized CaF2 UCNCs after introducing Tm3+ ions under (a) 980, (b) 808 and (c) 1532 nm excitations. (Marks ①–③ stand for the cross-relaxation processes CR1, CR2 and CR3 in samples with high Er3+ concentration).
After introducing the Tm3+ ions into the CaF2:Er3+ host lattice, possible ET processes between Er3+ and Tm3+ ions would occur. According to the electronic structure of Tm3+ ions, the wavenumber of the 3H5 of Tm3+ locates between the 4I11/2 and 4I13/2 states of Er3+. This feature facilitates the mutual ET processes between Er3+ and Tm3+ ions. The ET process occurs between 4I11/2 state of Er3+ and 3H5 state of Tm3+, then followed by back ET process to Er3+ (4I13/2) (ET1 and ET2 in Fig. 3(a)), which give rise to the increment of the population of 4I13/2 state of Er3+. Meanwhile, the 4I11/2 state of Er3+ is directly depopulated. As a result, this leads to the population of the red-emitting state and depopulation of the green-emitting state of Er3+ ions. Thus, the ET1 and ET2 processes between Er3+ and Tm3+ ions mainly contribute to promoting the R/G ratio, leading to the UCL color changing from green to red. For 808 nm wavelength excitation (Fig. 3(b)), the remarkable increase of R/G ratio is also observed and the corresponding UCL mechanism is an analogy to that under 980 nm excitation. Under 808 nm excitation, the 2H9/2, 4F3/2 and 4F5/2 states of Er3+ ions are populated through sequentially absorbing two 808 nm laser photons, then the green-emitting states (2H11/2 and 4S3/2) are populated by MPR processes. The red-emitting states (4F9/2) can be populated by MPR, CR and ET processes. It should be mentioned that the ESA process is ineffective under 808 nm excitation. ET1 and ET2 processes between Er3+ and Tm3+ are also responsible for achieving the color manipulation. Under 1532 nm laser excitation (Fig. 3(c)), the ground state of Er3+ ions is pumped to high-lying states by successively absorbing 1532 nm photons, which causes the population of green-emitting states of 2H11/2 and 4S3/2. There are two channels for the population of red-emitting state 4F9/2: one is ESA process from 4I11/2 to 4F9/2, and the population of intermediate state 4I11/2 is originated from 4I9/2 state via MPR; the other is through the MPR process from green-emitting states. Upon introduction of Tm3+ ions, the achievement of a large R/G ratio is also ascribed to the efficient ET processes between Er3+ and Tm3+ ions.
To gain the insight to the proposed mechanism, we investigated the dependence of the green (540 nm) and red (656 nm) UC emissions on the excitation power density. Fig. S2 in Supplement 1 presents the double-logarithmic plots of both green (540 nm) and red (656 nm) UC emissions of CaF2:Er3+/Tm3+ (10/1 mol%) UCNCs versus the pump power density under 980, 808 and 1532 nm excitations. The slopes, which are all close to 2, reveal that these two UC emissions both exhibit a two-photon process under the excitation of 980 and 808 nm lasers. Notably, the UCL mechanism under 1532 nm laser excitation is also a two-photon process instead of a three-photon one. This is ascribed to the saturation of UCL when the pump power density increases to a relatively high level [47].
In addition, we examined the time-resolved profiles of the green- and red-emitting states. The decay curves of the green (540 nm) and red (656 nm) UCL transitions of Er3+ under multi-wavelength excitation are displayed in Fig. S3 in Supplement 1, and their lifetimes were also calculated by single exponential fitting. Figure 4 shows the lifetimes for the green- and red-emitting states of Er3+ ions as a function of the doping Tm3+ ions. The results show that the lifetimes for both the green- and red-emitting states drop with the increasing Tm3+ concentration under the excitation of 980, 808 and 1532 nm pulse lasers. This has been caused by the ET processes between Er3+ and Tm3+ ions [31,32].
Fig. 4. Dependence of green (540 nm) and red (656 nm) UCL lifetimes on the Tm3+ concentration in CaF2:Er3+/Tm3+ (10/x mol%) UCNCs under the excitation of (a) 980, (b) 808 and (c) 1532 nm pulse lasers.
3.3 Color manipulation via non-steady-state modulation
Besides codoping with appropriate rare-earth-ion, color manipulation can also be realized by varying the excitation of pulse width and repetition frequency through excitation modulation, which is governed by the non-steady-state UC process. Figure 5 illustrates the pulse width dependent UC spectra of CaF2:Er3+ (10 mol%) UCNCs codoped without Tm3+ and with 1% Tm3+ ions under the excitation of 980 and 808 nm lasers. For CaF2:Er3+/Tm3+ (10/1 mol%) UCNCs, the R/G ratio is remarkably promoted when the pulse width increases from 0.1 to 4 ms, whereas the R/G ratio for Tm3+-free UCNCs basically remain unchanged. Additionally, similar phenomenon is observed under the excitation of 1532 nm laser (Fig. S4 in Supplement 1). Notably, the R/G ratio of the CaF2:Er3+/Tm3+ (10/1 mol%) UCNCs will appear saturation effect when the pulse width is larger than 0.5 ms. The primary mechanism of the tunable R/G ratios under multi-wavelength excitation can all be ascribed to that the pump and ET processes possess different intrinsic rates. For Tm3+-free CaF2:Er3+ UCNCs, the CR processes (CR1, CR2 and CR3) are the main ET pathways for the population from green-emitting state to red-emitting one. The characteristic lifetimes of these processes are shorter than the pulse width of excitation laser [31]. Thus, modulating the pulse width cannot effectively change the R/G ratio. When Tm3+ ions are introduced into the CaF2:Er3+ UCNCs, there are two main slow ET processes populating the red-emitting state (4F9/2): Er3+ (4I11/2) → Tm3+ (3H5) (ET1) and Tm3+ (3H5) → Er3+ (4I13/2) (ET2). Meanwhile, the routes of populating the green-emitting states (2H11/2 and 4S3/2), such as GSA, ESA and MPR, are at a relatively fast rate. Consequently, for relatively short pulse duration, the green-emitting states can still be rapidly populated, while the red-emitting state have no enough time to be efficiently populated, which leads to the decline of R/G ratio.
Fig. 5. UCL spectra of CaF2:Er3+/Tm3+ (10/x, x=0 and 1 mol%) UCNCs under the excitation of (a,b) 980 and (d,e) 808 nm lasers operated at different excitation pulse width. (c,f) The corresponding R/G ratios as a function of pulse width under the excitation of 980 and 808 nm laser, respectively. The repetition frequency is fixed at 120 Hz.
In addition, adjusting the repetition frequency of the excitation laser source with a fixed pulse width is also an effective approach to achieve tunable R/G ratio. Thus, we further investigate the influence of repetition frequency on the R/G ratios under multi-wavelength excitation. Figure 6 shows the UC emission spectra of CaF2:Er3+/Tm3+ (10/1 mol%) UCNCs as a function of the repetition frequency under the excitation of 980 and 808 nm lasers (the pulse widths are fixed at 0.1 ms). The results reveal that the R/G ratio increases greatly as the repetition frequency adds from 120 to 1440 Hz either pumped at the 980 or 808 nm laser. The mechanism of the enhanced R/G ratio is also originated from the different population rates between green- and red-emitting states of Er3+. The short pulse width and low repetition frequency would hinder the MPR process from 4I11/2 to 4I13/2. Thus, this prevents the population of red-emitting state through ESA process. On the contrary, the high-frequency excitation would facilitate this MPR process and enhance the red UC emission. Therefore, altering the pulse width and repetition frequency under multi-wavelength excitation enables us to acquire desired UC colors through excitation technique in Er3+/Tm3+ codoped luminescent materials.
Fig. 6. The UC emission spectra of CaF2:Er3+/Tm3+ (10/1 mol%) UCNCs under (a) 980 and (b) 808 nm excitations with different repetition frequency. (c,d) The corresponding R/G ratio as a function of repetition frequency under the excitation of 980 and 808 nm laser, respectively. The pulse width is 0.1 ms.
3.4 Optical temperature sensing properties
Generally, for Er3+-doped UCNCs, the principle of optical thermometry derives from detecting the green emissions of thermally coupled states of 2H11/2 and 4S3/2. When the local environmental temperature increases, the 2H11/2 state can be populated from 4S3/2 through thermal excitation, leading to the variation of fluorescence intensity ratio (FIR). Here, we have systematically investigated the temperature sensing behaviors in CaF2:Er3+/Tm3+ (10/0.125 mol%) UCNCs. Figure 7 shows the temperature-dependent green UC emissions of Er3+ in the range of 323–573 K under the multi-wavelength excitation of 980, 808 and 1550 nm lasers. The results reveal that the 522 and 540 nm green UC emissions both exhibit a decline trend and the overall intensity also decreases gradually as the temperature increases. In principle, the population of the 2H11/2 and 4S3/2 states follow the Boltzmann distribution law, and the FIR between these two thermally coupled states is given by [48,49]:
where IH (522 nm) and IS (540 nm) represent the integrated intensities of the transitions of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 of Er3+ ions, respectively. C is the proportionality constant, $\Delta E$ is the energy gap of the two thermally coupled states, k is the Boltzmann constant and T is the absolute temperature. Figure 8(a–c) displays the calculated FIRs (I522/I540) according to the Eq. (1) under the multi-wavelength excitation. The experimental data can be well exponentially fitted and the FIRs grow with the increasing temperature from 323 to 573 K. The fitted values of $\Delta E/k$ are 944.91, 937.28 and 932.74 under the excitation of 980, 808 and 1532 nm lasers, respectively. These experimental values of $\Delta E/k$ are in good agreement with the theoretical value of 919.82, and the slight deviation is ascribed to the weak self-absorption of the fluorescence and small fluctuations of the laser power [43]. Moreover, the values of FIRs are almost equal under the three different NIR lasers excitation. The temperature sensing sensitivity is a crucial criterion for evaluating the sensing ability, and it can be determined by the following expression [49]:Fig. 7. Temperature dependent green UC emission spectra of CaF2:Er3+/Tm3+ (10/0.125 mol%) UCNCs measured at different temperatures (from 323 to 573 K) excited by (a) 980, (b) 808 and (c) 1532 nm lasers.
Fig. 8. Experimental and fitted FIR and thermal sensitivity for green UC emissions of CaF2:Er3+/Tm3+ (10/0.125 mol%) UCNCs as function of temperature under the excitation of (a,d) 980, (b,e) 808 and (c,f) 1532 nm lasers.
Based on the Eq. (2), we have calculated the corresponding sensitivity of the CaF2:Er3+/Tm3+ (10/0.125 mol%) UCNCs as a function of the temperature under the excitation of 980, 808 and 1532 nm lasers, which are shown in Fig. 8(d–f). It can be clearly found that the sensitivity curves exhibit the same trend under different wavelengths excitation. The maximum thermal sensitivities for these Er3+/Tm3+ codoped UCNCs are 0.0068 K-1 at 473 K for 980 nm, 0.0057 K-1 at 468 K for 808 nm and 0.0074 K-1 at 468 K for 1532 nm laser excitations. Table 1 shows the comparison of the maximum thermal sensitivities of the green emissions of Er3+ ions reported in a variety of host matrices [50–54]. According to the previous reports on the performance of the optical temperature sensor, the results of relatively high thermal sensitivities enable these UCNCs to act as a promising candidate for detecting the nanoscale environmental temperature under multi-wavelength excitation.
Table 1. Temperature sensing properties of green emissions generated by Er3+ doped materials including excitation wavelength, temperature range and maximum sensitivity.
4. Conclusions
In summary, we have successfully synthesized the CaF2:Er3+/Tm3+ (10/x mol%) UCNCs through a simple hydrothermal method. Intense tunable multicolor UCL were realized in Er3+-sensitized CaF2 UCNCs via introducing Tm3+ ions under the excitation of 980, 808 and 1532 nm lasers. The enhanced R/G ratio of CaF2:Er3+/Tm3+ UCNCs is mainly attributed to the ET processes from Er3+ (4I11/2) to neighboring Tm3+ (3H5) and then back to Er3+ (4I13/2). These processes would further significantly promote the proportion of red emission through ESA. The mechanism is supported by time-resolved and power dependence measurements of green and red emissions of Er3+ ions under multi-wavelength excitation. In addition, based on the non-steady-state modulation technique, R/G ratio can also be greatly tuned by varying the pulse width (0.1–4 ms) and repetition frequency (120–1440 Hz) under the 980, 808 and 1532 nm laser excitations. In addition, the optical temperature sensing properties of CaF2:Er3+/Tm3+ (10/0.125 mol%) UCNCs were evaluated based on the FIR technique under multi-wavelength excitation, and the maximum thermal sensitivity of 0.0074 K-1 at 468 K was achieved under 1532 nm laser excitation, which is a relatively high-level sensitivity. The excellent multicolor tunability and superior sensitive thermal properties under multi-wavelength excitation enable these Er3+-sensitized CaF2 UCNCs suitably to implement in biological imaging, volumetric display and thermal sensing.
Funding
Natural Science Foundation of Guangdong Province (2016A030308010).
Acknowledgements
The authors thank the Natural Science Foundation of Guangdong Province for financial support.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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