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The β-NaLuF4: Yb3+/Ho3+ nanocrystals have been synthesized via a solvothermal method, which were protected in a SiO2 capillary tube. Upon 980 nm continuous laser excitation, the micro-tube waveguide characteristic was studied. Additionally, the upconversion emissions from the 5F4, 5S2→5I8 (~545 nm), 5F5→5I8 (~650 nm) and 5F4, 5S2→5I7 (~750 nm) transitions of Ho3+ ions in the micro-tube were obtained and studied as a function of temperature in the range of 300~500 K. The maximum sensitivities derived from the fluorescence intensity ratio technique are 1.53% K−1 (I750/I650) and 0.09% K−1 (I545/I650) at 300 K. The result demonstrated that the near infrared-red absolute sensitivity is more sensitive as a thermometer than the green-red at the whole temperature range. The device is a promising candidate for high resolution luminescent thermometers operating under temperature of 300~500 K.
© 2017 Optical Society of America
1. Introduction
Optical upconversion (UC) is a process that converts near-infrared photons to short-wavelength emission. UC materials based on rare earth ions have been the subject of research interest in over past several years due to the special optical property. It has many applications, including full-color display [1–3], biological label [4, 5], active laser materials [6–10], optical temperature sensing [11–19]and so on. Temperature is an essential parameter in various areas. An accurate temperature sensing in hostile environment becomes necessary. Noncontact temperature sensing technique based on fluorescence intensity ratio (FIR) method could be employed under harsh environment to deliver higher sensitivities and accurate measurements has been a concern in the current research because of its flexibility of utilization [20–22]. For the situation described above, rare earth doped luminescent material is an important and attractive candidate for temperature dependent optical property.
In recent years, RE ions have attracted considerable attention because of the 4f-4f intracongurational transition. It is noteworthy that Ho3+ has ample energy levels in the spectroscopic range from ultraviolet to near-infrared [23]. The Ho3+ doped luminescent material was proved to be another potential application for FIR-based temperature sensors. The optical temperature sensor base on 5F4/5S2→5I8, 5I7, 5F4, 5S2→5I8, 5F3, 3K8→5I8, 5G6/5F1, 5F2,3/3K8→5I8 transition were widely studied with a maximum sensitivity of 0.0053K−1 (93 K) (SA = 182.3/T2) [24], 0.0098 K−1 (130 K) (SA = 255/T2) [25], 0. 0066 K−1 (353 K) (SA = 648.5/T2) [15], 0.005 K−1 (923 K) (SA = 1890/T2) [26], respectively, However, the 5F5,5F4/5S2→5I8, 5I7 and 5F5,5F4/5S2→5I8 transition as temperature sensor are scarcely observed. Therefore, it is of great scientific interest and importance to obtain the sensitivity at the temperature range 300K to 500K. Moreover, the RE ions doped nanocrystals are unstable at high temperature, Wang et.al achieved the stability by chemical synthesis method, such as SiO2 coated fluoride nanocrystals to form a core-shell structure [27], while we adopt a simple physic method to achieve its stability.
In the present work the β-NaLuF4: Yb3+/Ho3+ nanocrystals have been synthesized by using a solvothermal method. The powder was incorporated within a SiO2 capillary tube, then the SiO2 capillary tube was drawn into several micrometers in diameter and hundred micrometers in length. Subsequently, the UC emission spectra of the capillary tube waveguide and transmission characteristic were obtained under 980 nm laser pumping. Afterward, the thermal behaviors were derived from the capillary in the temperature region between 300 K and 500 K and the temperature relative sensitivity based on FIR technique was determined. The maximum sensitivity of the temperature sensor was recorded as 1.53% K−1 at 300 K.
2. Experiment
2.1 Synthesis
The rare earth nano-crystals of β-NaLuF4: Yb3+, Ho3+ were synthesized by using the solvothermal method [28]. NaOH (27 mmol) was added into a mixed solvent composed of oleic acid (OA) (18.7 mmol) and ethanol (72 mmol) at room temperature, resulting in a white viscous solution. Afterwards, 2.4 mmol of NaF was dissolved in the mixed solution. After vigorous stirring for 30 min, Ln(NO3)3 (Lu/Yb/Ho = 78:20:2, 0.8 mmol in total) was introduced into the above solution under vigorous stirring. After stirring for 1 h, the mixed solution was transferred into a 50 ml Teflon vessel, sealed in an autoclave and heated to 180 °C for 24 h. After reaction, the autoclave was cooled down to room temperature naturally, the precipitates were washed several times with deionized water and ethanol in sequence, accompanied with centrifugal separation and then dried in a vacuum at 80 °C for 12 h in air. The final powder was further processed and characterized.
In order to obtain the micro-tube, firstly, the coat layer of silica capillary tube (inner diameter is 100µm ± 2µm, outside diameter is 167 ± 2µm, Polymicro Technologies, L. L. C.) was removed. A fiber probe with approximately 5 µm tip diameter was fabricated by a flame-heated drawing method using the single-mode fiber with a diameter of 125µm. The fiber probe was fixed on the resolution of a 0.1 μm three-dimensional adjustment stage to push the RE nanocrystals into the micro-tube. The powder of nano-crystals were filled into SiO2 capillary tube through fiber probe, as shown in Fig. 1(a-d). Secondly, the capillary tube was heated by a Bunsen burner and stretched. The micro-tube was as an optical probe for temperature sensor.
2.2 Characterization
The optical characterization and spectral measurement of rare earth nanocrystalline in micro-tube were carried out under an optical microscope, as schematically illustrated in Fig. 2. For the excitation of the micro-tube (supported on MgF2 substrate to reduce loss), 980 nm laser was launched into micro-tube of leftmost endface through a standard single-mode fiber tip coupled by non-contact method, as shown in the insert. The standard single-mode fiber was fixed on the 3D adjustment with the resolution of 0.1 μm. The photoluminescence (PL) signals were collected by a 20 × objective (NA = 0.45) and directed through 980nm filter and beam splitter to a CCD camera and a spectrograph for imaging and spectral measurement, respectively.
The crystal structures of the powders were characterized by an X-ray diffraction (XRD) using Cu Kα radiation with 2θ from 10° to 70°. Scanning electron microscope (SEM, SU8000) was carried out to characterize the morphology.
3. Results and discussion
The crystal structure of the as-prepared sample was identified by XRD, as shown in Fig. 3(a). All the peaks located at 2θ values of 10–70° were corresponded to the characteristic diffractions of hexagonal phase NaLuF4 (JCPDS card 27-0726). No impurities was detected, which revealed that pure β-NaLuF4 have been fabricated. A typical peg-top-like was observed, as shown in insert of Fig. 3(a). The synthetic β-NaLuF4:Yb3+/Ho3+ nanocrystals consists of uniform hexagon rod 300-400 nm in diameter and 500-700 nm in length, and concave edge parts on the tops/bottoms sides. Figure 3(b) shows a micro-tube after stretched with approximately 5 μm outer diameter, wall thickness of about 100 nm. The image depicts the micro-tube with a uniform radius and smooth surface. It is indicated that the rare-earth nanocrystals are not on the outer surface of the micro-tube.
Fig. 3 The characteristic of images. (a) XRD and SEM (insert image) of β-NaLuF4: Yb3+/Ho3+; (b) SEM of micro-tube after stretched.
The heat effect caused by the excitation power should be excluded in our experimental process [29]. Figure 4 shows UC emission spectra under different pump power densities at temperature of 350 K. From the normalized spectra, the power density of 73.5mW/mm2 at the emission band of 5F5→5I8 transitions is slightly higher than the others. However, there are no obvious difference at 62.5 mW/mm2 and 27.5 mW/mm2. It is indicated that the heating effect can be neglected when the power density is not larger than 62.5 mW/mm2.
The Fig. 5(a)-5(d) shows the dark-field photoluminescence (PL) microscope image of a micro-tube (diameter of ~5 μm and length of ~100 μm). Two bright spots exist in both ends of micro-tube, which is a typical characteristic of optical waveguide [30]. To quantitatively analyze the guiding performance, the micro-tube was excited at a difference position along the tube and measured the output spot normalized PL intensity. The spot images are converted from RGB to gray styles in Adobe Photoshop, as shown in insert Fig. 5, whose gray values are calculated by Matlab to characterize the corresponding intensity. Due to the small fluctuations in excitation power, the normalization was considered in the measured intensities at the end-faces against those measured at each excitation location on micro-tube body. The experiment data (Fig. 5(e)) were fitted well by a first-order exponential decay function, defined as Iendface/I0 = exp(-αx), where Iendface is the luminescent intensity measured at the endface, I0 is the normalized intensity at the excitation spot, x is the propagation distance and α is a fitting parameter. With the increase of the propagation distance, the intensity of the output light decreases exponentially. The result demonstrated that the micro-tube is a typical feature of active waveguides [31]. As shown in Fig. 5(f), the excitation spectrum of micro-tube reveals the some peaks at 485, 545, 650, and 750 nm corresponding to the transitions of Ho3+ ion from 5F3, 5F4/5S2 and 5F5 to the ground level 5I8, and 5F4/5S2 to 5I7.
Fig. 5 a-d) PL image of a single micro-tube. PL image were collected upon excitation at different positions; e) Normalized dependence of the endface out-couple emission intensity on excitation location. The line is an exponential fit to the data yielding the loss coefficient for the waveguide; f) The PL emission spectra of single micro-tube.
In order to study the characteristics for temperature sensing of the Ho3+/Yb3+ codoped in micro-tube, the UC emissions were measured from 300 to 500 K as a function of temperature ranging, as shown in Fig. 6. The pumping power of 980 nm laser is 60 mW. The position for peak is not changed with the increase of temperature, but the peak intensity decreases markedly. It was reported that the relative population of the thermally coupled levels (TCL) accord with a Boltzman type population distribution [32], the emission intensity ratio (FIR) from the TCL of active ions is expressed as
where a is a constant, b is a modify factor, ΔE is the energy gap separating the two excited states, k is the Boltzmann constant, and T is the absolute temperature. The near infrared-red FIR1 (I750/I650) and green-red FIR2 (I545/I650) were researched as a function of temperature. The FIR1 and FIR2 at the range of 300~500 K were calculated and plotted in Fig. 7(a)-7(b)). The experiment data are fitting by Eq. (1). It can be observed that the fittings agree well with the experimental data. The fitting coefficient, a, b and ΔE, were (0.0014, 0.496 and 1353.1) for FIR1 and (0.0657, 1.039 and 812.6) for FIR2, close to the value calculated from the emission spectra (1450 cm−1, 930 cm−1). These results confirmed that the 5F5, 5F4/5S2→5I8, 5I7 and 5F5, 5F4/5S2→5I8 states are TCL. This clearly demonstrates that the temperature can be accurately measured from FIR1 and FIR2 at the temperature range from 300 K to 500 K.Fig. 6 UC spectra in the range from 500 nm to 750 nm at various temperatures located 300 K to 500 K.
Fig. 7 The fluorescence intensity ratio vs. temperature ranges from 300 K to 500 K. (a) I750/I650. (b) I545/I650.
The variation of sensitivity with temperature is an important parameter for optical temperature sensor. Then, the absolute sensitivity Sa has been expressed as [21, 33]:
where the subscripts i stand for 1 (I750/I650) or 2 (I545/I650). With the increase of temperature, the Sai keep decreasing and reach maximum value about 1.53% K−1 (Sa1) and 0.9% K−1 (Sa2) at 300 K, respectively, as shown in Fig. 8. Notably, Sa1 is approximately 2 times of Sa2. The term Sai is dependent on ΔEfit. If ΔEfit agrees well with the experimental energy difference ΔEfit, the values of Sai is accurate. The error δ is expressed as [17]. The value of δ is 6.6% and 12.6%, respectively. The result show the value of Sai is relatively accurate. It is demonstrated that the near infrared-red absolute sensitivity are more sensitive for use as a thermometer at room temperature than the green-red.4. Conclusion
In summary, The Yb3+/Ho3+ co-doped β-NaLuF4 nanocrystals were synthesized by the solvothermal method and the SiO2 capillary tube containing nanocrystals of few micrometers in diameter was prepared by flame-heated drawing method. The effectively UC emission spectra of the capillary tube waveguide and high sensitivity were obtained through the utilization of different thermal behaviors of Ho3+ ions at the temperature range of 300 K~500 K. The maximum sensitivity of the temperature sensor was recorded as 1.53%K−1 at 300 K, illustrating that the device can achieve higher sensitivity and accuracy, as well as it could easy be used as a temperature sensor. Moreover, the micro-tube will make it possible to excite this micro-system in flowing gas or fluid. Our work represents developing a new generation of micro-light emitting device in future integrated photonic platforms.
Funding
National Key Laboratory of Tunable Laser Technology; National Natural Science Foundation of China (NSFC) (No. 61078006 and No. 61275066); and National Key Technology Research and Development Program of the Ministry of Science and Technology of China (No.2012BAF14B11).
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