Plasmon-induced selective enhancement of green emission in

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Plasmon-induced selective enhancement of green emission in lanthanide-doped nanoparticles Weina Zhang, Juan Li, Hongxiang Lei, and Baojun Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16586 • Publication Date (Web): 16 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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ACS Applied Materials & Interfaces

Plasmon-induced selective enhancement of green emission in lanthanide-doped nanoparticles Weina Zhang1,2, Juan Li1,2, Hongxiang Lei*1, and Baojun Li*2 1

State Key Laboratory of Optoelectronic Materials and Technologies, School of

Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China 2

Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications,

Institute of Nanophotonics, Jinan University, Guangzhou 511443, China Correspondence and requests for materials should be addressed to H.L. ([email protected]) or B.L. ([email protected]).

ABSTRACT: By introducing an 18 nm-thick Au nanofilm, selective enhancement of green

emission

from

Lanthanide-doped

(β-NaYF4:

Yb3+/Er3+)

upconversion

nanoparticles (UCNPs) is demonstrated. The Au nanofilm is deposited on a microfiber surface by the sputtering method and then covered with the UCNPs. The plasma on the surface of the Au nanofilm can be excited by launching a 980 nm-wavelength laser beam into the microfiber, resulting in an enhancement of the local electric field and a strong thermal effect. A 36-fold luminescence intensity enhancement of the UCNPs at 523 nm is observed with no obvious reduction in the photostability of the UCNPs. Further, the intensity ratio of the emissions at 523 to 545 nm and that at 523 to 655 nm 1

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are enhanced with increasing pump power, which is attributed to the increasing plasmon-induced thermal effect. Therefore, the fabricated device is further demonstrated to exhibit an excellent ability in temperature sensing. By controlling the pump power and the UCNP concentration, a wide temperature range (325–811 K) and a high temperature resolution (0.035–0.046 K) are achieved in the fabricated device.

KEYWORDS: plasmon; upconversion luminescence; selective enhancement; lanthanide-doped nanoparticle; temperature sensing

INTRODUCTION Due to unique features such as high signal-to-noise ratios, low toxicity, narrow emission peaks and good photostability; Lanthanide (Ln)-doped upconversion nanoparticles (UCNPs) have been extensively applied in areas such as solar cells,1, 2 bioimaging,3, 4 all-solid-state upconversion lasers,5, 6 and three-dimensional displays.7, 8 However, the low efficiency of upconversion luminescence (UCL) greatly limits the applications for UCNPs. Moreover, owing to the multi-energy-level structure of the Ln ions, multiple peaks emerge in the emission spectra of UCNPs. Such multimodality of the Ln ions limits the further application of UCNPs in displays and upconversion lasers.

To date,

many researchers have introduced some structures such as photonic crystals,9–11 optical gratings,12 noble-metal nanoparticles and arrays,13–17 metallic shell and tip18–21 to enhance the efficiency of UCL by modifying the electromagnetic fields around the UCNPs. These methods have exhibited good performance in enhancing the efficiency of UCL, but the enhancements were generally for all or a plurality of the emission peaks and thus the multimodality of the UCNPs experienced no significant improvement. Hence, an effective approach to realize selective enhancement of the UCL for UCNPs is highly desired. Because the energy transitions involved in the upconversion process are 2


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related to the temperature,22–26 it is possible to adjust the intensity ratio of different emission peaks by the plasmon-induced thermal effect. Therefore, the selective enhancement of UCL can be achieved by introducing an appropriate plasmonic structure. Here, a microfiber coated with an Au nanofilm and then covered with UCNPs was proposed to selectively enhance the UCL. Compared with other plasmonic structures, the enormous interfaces between the Au nanofilm and UCNPs can ensure more sufficient interaction between the UCNPs and the excitaton light. Moreover, the Au nanofilm was relatively stable and able to be uniformly coated on the surface of the microfiber. Besides, the UCNPs/Au sample was assembled on a microfiber and excited by a waveguide, it was convenient and flexible to operate. In addition, the plasmon-induced selective enhancement of the UCL was demonstrated in temperature sensing with excellent performance. The proposed structure in our work will provide a potential application in temperature sensing and biological research, such as laser-induced controlled hyperthermia,23 remote release of encapsulated material27 and low concentration target molecule detection.28

EXPERIMENTS Owing to their preferable luminescence efficiency,29,

30

hexagonal (β)-phase

NaYF4:Yb3+, Er3+ nanoparticles (NaYF4/Yb/Er = 78:20:2) with amine groups (NH2) on the surface were used as the UCNPs in our experiment, provided by Nanjing Henna Biotech Pte. Ltd. Figure 1a shows the UCL spectrum of the UCNPs and the inset was a transmission electron microscope (TEM) image of the UCNPs (38 nm in major axis, 32 nm in minor axis, see Figure S1 in Supporting Information). It can be seen that, there are three major emission bands in the visible range with center wavelengths at 523, 545 3



and 655 nm, which correspond to the transitions 2H11/2→4I15/2, 4

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4

S3/2→4I15/2 and

F9/2→4I15/2, respectively. The multimodality of the UCNPs derives from the

multi-energy-level structures of the Ln ions. The upconversion mechanism for the sensitizer-emitter pair Yb3+and Er3+codoped upconversion materials is shown in Figure 1b. The incident light at 980 nm (black solid arrow) is absorbed by the transition of the 2

F7/2 to 4F5/2 in Yb3+ ions, and is followed by an energy transfer (ET, purple dashed

arrows) to multiple energy levels in the Er3+ ions. After nonradiative relaxation (dotted arrows), upconversion emissions (colored solid arrows) occur in the visible range. The cross-relaxation (wavy arrows) represents the energy transfer between the ions at different/equivalent states.

In our study, the sample consisted of a microfiber, an 18 nm-thick Au nanofilm and UCNPs (the UCNPs/Au sample for short). The fabrication process of the UCNPs/Au sample is schematically shown in Figure 2a. The microfiber was fabricated through a flame-heating technique (see Methods), and then fixed on a glass substrate for stabilization. Next, the microfibers were placed in an ETD2000/3000 sputter coater with a vacuum level, sputtering current and sputtering time set at 10−1 mbar, 15 mA and 2 min, respectively. Thus, an 18 nm-thick Au nanofilm was deposited on the microfiber surface (Supporting Information Figure S2). The sample was then removed and placed on the experiment table where, with the assistance of a syringe, a dilute aqueous solution of UCNPs (UCNPs concentration: 0.05 mg/mL) was injected onto the Au nanofilm surface. Finally, the sample was placed in air at room temperature for 10 min to allow evaporation of the aqueous deposit. Here, the UCNPs would spread uniformly on gold surface, which was attributed to the electrostatic self-assembly process.31 In this 4


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process, the surface tension of aqueous solution and the electrostatic force generated between the UCNPs and the Au nanofilm or between the UCNPs and the UCNPs would push the UCNPs into a close-packed formation, resulting in a uniform distribution of UCNPs. After evaporation of the solvent, the UCNPs and Au surface were stably coupled together by electrostatic attraction between the positively charged NH2 groups of the UCNPs and the negatively charged free electrons of the Au surface.32 Thereby the UCNPs/Au sample was fabricated successfully. Figure 2b shows a scanning electron microscopy (SEM) image of the UCNPs/Au sample, where the microfiber with the Au nanofilm and the UCNPs has a diameter of 3.0 µm. The inset in Figure 2b shows that the UCNPs were successfully deposited onto the Au nanofilm surface with a particle densities 2.50 × 1014/m2. To have more detailed characterisation for the interface between Au nanofilm and UCNPs, the HRTEM image of the UCNPs/Au sample was captured (see Figure S3 in Supporting Information). It shows that, the surface of Au nanofilm coated on the microfiber is roughness and there is no obvious gap at the interface between gold and UCNPs, which are beneficial for the selective enhancement of the UCL. The absorbance spectrum of the 18 nm-thick Au nanofilm shown in Figure 2c indicates that the 18 nm-thick Au nanofilm has a good absorbance for the 980 nm-wavelength light (0.97) while slight absorbance for the UCL emissions (0.32, 0.31, 0.42 for 523, 545, 655 nm, respectively). Figure 2d shows the schematic of the UCNPs/Au sample for selectivity enhancement the UCL. A 980 nm-wavelength laser beam was launched into the microfiber to excite the plasma in the Au nanofilm. This resulted in a large enhancement of the local electric field for the incident light and a 5



higher utilization efficiency of the pump energy for light emission, whereby the upconversion efficiency could be enhanced. Moreover, with an increasing laser power, a strong plasmon-induced thermal effect upon the UCNPs next to the Au nanofilm could also be achieved. The increasing temperature induced a thermally-driven population increase of the 2H11/2 level and thus the emission intensity at 523 nm was selectively enhanced. RESULTS AND DISCUSSIONS By launching a 980 nm-wavelength laser beam into the microfiber (with an optical power of P = 610 mW as an example), the UCNPs/Au sample was excited and green light was emitted, as shown in Figure 3a. As a comparison, a UCNPs sample was also fabricated (Figure S4 in Supporting Information) and excited under the same conditions and yellow light was emitted, as shown in Figure 3b. Further, the light emission in Figure 3a is brighter than that in Figure 3b. To explain these observations, the luminescence spectra of the UCNPs/Au and UCNPs samples were measured by focusing on the middle of the excitation segment with a sampling window of 2 × 2 µm2 (denoted as red dotted boxes in Figures 3a and 3b) via a microspectrophotometer, as shown in Figure 3c. The luminescence spectrum of the UCNP/Au sample exhibits a major emission peak at 523 nm, indicating green light; while the luminescence spectrum of the UCNPs sample exhibits three major emission peaks centered at 523, 545 and 655 nm where, because of the effect of multiple peaks, the emission color of the UCNPs sample is yellow. In addition, the UCNPs/Au sample emission (523 nm) is enhanced by 36-fold over that of the UCNPs sample (see “the calibration of the enhancement factor” in Supporting Information), which explains why the green emission in Figure 3a is brighter than the yellow emission in Figure 3b. Furthermore, the effect of Au nanofilm 6


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with different thickness on the UCL was investigated (Figure S5 in Supporting Information). The result shows that, the Au nanofilm with the thickness of 18 nm is superior for the UCL selective enhancement. Besides, to investigate the effect of the Au nanofilm on the photostability of the UCNPs, the UCNPs/Au sample was continuously excited by a 980 nm-wavelength laser at P = 610 mW for various times t (Figure S6 in Supporting Information). The UCL intensity at 523, 545 and 655 nm emissions was decreased by 2.5%, 2.3% and 3.0% after an excitation of 600 s, respectively, which demonstrate a good photostability of the UCNPs/Au sample in our work. Therefore, the plasmonic effect of the Au nanofilm does not obviously decrease the photostability of the UCNPs. In addition, the intensity ratios (IRs) of different emissions were found to change under varying pump powers, as shown in Figure 3d. For example, at P = 40 mW, the IRs of emission intensities at 523 to 655 nm (I523/I655) and 523 to 545 nm (I523/I545) were 1.99 and 0.98, respectively. With increasing pump power, however, the intensities of all emission peaks in the UCL were enhanced while the 523 nm emission increased most quickly. At P = 460 mW, the IRs of I523/I655 and I523/I545 were 22.79 and 5.34, respectively, which indicates that the green emission at 523 nm was selectively enhanced with an increase of the pump power. To quantitatively analyze the influence of the pump power on the selective enhancement of green emission, the IRs of I523/I545 and I523/I655 in the UCNPs/Au sample was measured while varying the pump power from 40 to 610 mW, as shown in Figure 3e. The IRs increased with the pump power, which indicates the improvement of the unimodality of the UCL spectrum. The selective enhancement of green emission was considered to be related to the plasmon-induced thermal effect. This is because the transitions of different energy levels are related to temperature and the local temperature will be obviously changed under different pump 7



powers, especially for the transitions of 2H11/2→4I15/2. Compared to the reported plasmonic effect-based selective enhancement of the UCL for UCNPs with an enhancement factor of 10.5 for red emission at 650 nm and an intensity ratio (green to red) of 2,33 our work exhibits a larger enhancement for green emission at 523 nm and a higher intensity ratio at 523 to 655 nm, which indicate that the unimodality of the UCNPs was improved significantly in our work. To precisely understand the enhancement of the UCL observed from the UCNPs/Au sample, the power-dependent UCL intensity was investigated. Figures 4a and 4b show the log-log plot of the 523, 545 and 655 nm emissions intensity as a function of pump power for the UCNPs/Au and UCNPs samples, respectively. For the upconversion (UC) of the UCNPs, the excitation power dependence of UC is described as Iuc∝Pn, where Iuc is the intensity of the UC emission, P is the pump power, and n is the number of photons required to populate the excited state34–37 which was expressed as the slope of the fitted line in Figure 4. As the UC of the UCNPs is a multi-photon process, the value of n is generally larger than or equal to 2 when the pump power is low. However, n values calculated by the log–log plot were generally smaller than the actual ones, which can be attributed to the energy loss during the UCL process. For the UCNPs/Au sample (Figure 4a), at log(P) < 2.37 the values of n are 2.18, 1.55 and 1.09 for the 523, 545, 655 nm emissions, respectively. These are enhanced by 1.2, 0.32 and 0.07 orders of magnitude, respectively, over the n values for the UCNPs sample shown in Figure 4b. This increase of n indicates improvement of the utilization efficiency of the pump energy. In other words, the energy loss during the UCL process was decreased by introducing the Au nanofilm, and especially for the 523 nm-wavelength emission, which is in accordance with the selective enhancement of the green emission.

8


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Notably, n distinctly decreased with increasing pump power for the UCNPs/Au samples. At 2.63 < log(P) < 2.78 in Figure 4a, for the 523, 545 and 655 nm emissions, n decreased from 2.18, 1.55, 1.09 to 0.85, 0.21 and 0.30, respectively. This decrease in n can be attributed to the occurrence of the saturation effect.34 For this effect to take place, the upconversion processes compete with the linear decay, and the upconversion rate increases with increasing pump power. When the pump power is sufficiently high, the upconversion rate will be comparable to the linear decay rate and, in this case, the saturation effect will occur and result in a decrease of the n value. Thus, for the UCNPs/Au sample, P = 610 mW (i.e., log(P) = 2.78) is a “high” pump power, further demonstrating the increased emission efficiency and the lower pump threshold facilitated by the Au nanofilm. This is attributed to the local field enhancement of the incident light induced by the Au nanofilm. Besides, the enhancement of the UCL was also related with the enhancing radiative decay rate of the fluorescent molecules.17 But, this factor is relatively weaker and can be negligible in our work, which can be demonstrated by measuring the fluorescence lifetime of the UCNPs/Au sample. As we know, the enhancing radiative decay rate of the fluorescent molecules can significantly reduce the fluorescence lifetime of the fluorescent molecules.13, 16 Figure 5 shows the luminescence decay curves for both the UCNPs/Au sample and UCNPs sample. All the decay curves can be fitted with a single exponential function of I = I0 + Aexp(−x/τ), where τ is the lifetime of Er3+ ions. Thus, the lifetime τ of Er3+ ions for different transitions can be calculated. It was found that, compared to the UCNPs sample, the lifetime of the UCNPs/Au sample changed only 0.008, 0.009, 0.002 ms for 523, 545 and 655 nm emission, respectively, indicating the slight effect of the local field enhancement on the radiative decay rate of the upconversion fluorescence molecules. Therefore, we can conclude that the enhancement of the UCL emission is mainly due to 9



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the local field enhancement of the incident light. The selective enhancement of the UCL emissions is mainly ascribed to the plasmon-induced thremal effect. As we know, the UCL emission intensity at 523 and 545 nm is proportional to the population of the corresponding energy level (2H11/2 and 4

S3/2, respectively). At a certain temperature, the relative population of the 2H11/2 and

4

S3/2 levels gets a thermal equilibrium and follows a Boltzmann distribution. Therefore,

the intensity ratio (IR) of the two green emissions from 2H11/2 (523 nm) and 4S3/2 (545 nm) can be given as follows:25

(1) where N, g, σ, and ω are the number of Er3+ ions, the degeneracy, the emission cross-section, and the angular frequency of fluorescence transitions from 2H11/2, 4S3/2 levels to the 4I15/2 level, respectively. ∆E is the energy gap between 2H11/2 and 4S3/2 levels, kB is the Boltzmann constant, and T is the absolute temperature. Since the value of gHσHωH/gSσSωS is a constant, which can be expressed as A, the equation can be rewritten as: (2) It can be seen that, the IR will increase with the increasing temperature, that is, the selective enhancement of the UCL emission can be achieved by plasmon-induced thermal effects. Based on Eq. (2), the local temperature can be calculated (∆E is 772 cm−1 and lnA is calculated as 3.2722). Since it has been demonstrated that the IR increases with an increasing pump power (Figure 3e), the relationship between the local temperature and the pump power can be obtained as shown in Figure 6. It can be seen that, the temperature increases quickly with the increasing pump power and exhibits a

10


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maximum value of 640 K at P = 610 mW. With a wide pump power range of 40 to 610 mW, obvious temperature change of 315 K (325–640 K) occurred for the UCNPs/Au sample. To exclude the influence of near-infrared (980 nm) thermal effect, an additional experiment with the UCNPs sample has been performed (Figure S7 in Supporting Information). The result shows that, the temperature for the UCNPs sample changed a little (301–314 K), which can be ignored here. Hence, the change in the temperature for UCNPs/Au sample should be attributed to the thermal effect of Au nanofilm. To further verify the selective enhancement of UCL was mainly induced by the plasmon-induced thermal effect, an additional spacer with a polymer layer (Poly(methyl methacrylate), PMMA) between the Au nanofilm and UCNPs (UCNPs/PMMA/Au sample for short) was applied to suppress the thermal effect (see Figure S8 in Supporting Information). The result shows that the UCL intensities of the UCNPs/PMMA/Au sample were enhanced by 1.70, 1.70 and 1.69 for 523, 545 and 655 nm emission, respectively. Obviously, the enhancement factors of the three wavelengths are nearly equal and there is no selective enhancement of the UCL, which indicate that, the UCL selective enhancement is attributed to the plasmon-induced thermal effect, rather than other effect. As discussed above, with increasing pump power, the increasing plasmon-induced thermal effect can enhance the IR of the 2H11/2→4I15/2 (523 nm) and 4S3/2→4I15/2 (545 nm) transition emissions, which can be used for temperature sensing and detection. Besides the pump power, the thermal effect and the local temperature are also related to the UCNP concentration, which is mainly because of the absorbance of Au nanofilm for UCL emissions (Figure 2c). Therefore, the temperature detection properties of the UCNPs/Au samples with varying UCNP concentrations were investigated. For

11



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simplicity, the concentration of 0.02, 0.05 and 0.2 mg/mL were taken as examples, as shown in Figure 7. Figure 7a shows the IR I523/I545 as a function of temperature. For the three samples, ∆T was 125 K (341–466 K), 315 K (325–640 K) and 478 K (333–811 K), respectively, indicating the ∆T increases with the increasing of the UCNPs concentration. Furthermore, for the temperature detector, the relative sensitivity and the temperature resolution are always used as figures of merit. The relative sensitivity is defined as S = (∂Q/∂T)/Q,

(3)

where Q is the thermometric parameter which corresponds to the IR. Figure 7b shows the relative sensitivity as a function of temperature for samples with UCNP concentrations of 0.02, 0.05 and 0.2 mg/mL. S decreases with increasing temperature and the maximum sensitivity (Sm) was 1.35% K−1 at 325 K, 1.0% K−1 at 341 K and 1.46% K−1 at 333 K for the samples with 0.02, 0.05 and 0.2 mg/mL UCNP concentrations, respectively. The temperature resolution can be calculated by: δT = δQ/Sa,

(4)

where δQ is the resolution of Q that can be calculated from the standard deviation of residuals in the polynomial interpolation of the experimental data points (temperature vs. IR curve), and Sa is the absolute sensitivity defined by ∂Q/∂T. For the samples with 0.02, 0.05 and 0.2 mg/mL UCNP concentrations, the temperature resolutions were calculated as 0.035–0.046, 1.3−1.6 and 2.9–3.0 K, respectively. In order to visually compare the differences in the temperature detection properties of the UCNPs/Au sample with different UCNPs concentrations (C), a summary of the pump power (P), temperature detection range (T), maximum sensitivity (Sm) and temperature resolution (δT) was shown in Figure 7c. It should be noted that, the sample with the UCNP concentration of 12


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0.02mg/mL needs a reactively high excitation power to get a fluorescence signal. The above results show that, by changing the pump power and the UCNP concentration, the UCNPs/Au samples achieved a wide temperature detection range from 325–811 K (∆T = 486 K) with a temperature resolution between 0.035–3.0 K. This conclusion could be verified by measuring the luminescence spectra dependent temperature of the UCNPs/Au samples. The temperature detection range achieved in this experiment is larger than most previously-reported results based on the NaYF4: Yb3+, Er3+ nanoparticles.23–25 In particular, for the sample with a 0.02 mg/mL UCNP concentration, the temperature resolution (0.035–0.046 K) was an order of magnitude smaller than previous results based on the Er3+ emission.22

CONCLUTION In summary, a microfiber coated with an Au nanofilm for selectively-enhanced UCL from UCNPs was demonstrated. By launching a 980 nm-wavelength laser beam into the microfiber, the green emission at 523 nm was enhanced 36-fold. Further, the unimodality of the UCL increased with increasing pump power. This selective enhancement is beneficial to the further application of the UCNPs in display and lasers. In addition, by controlling the pump power and the UCNP concentration, a wide temperature detection range of 486 K (325–811 K) and a high temperature resolution of 0.035–0.046 K were achieved. This indicates that the optical temperature sensing properties of the UCNPs can be improved by the SPR of the Au nanofilm, which is conducive to the application of UCNPs in temperature detection.

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FIGURES

Figure 1 | (a) UCL spectrum of the β-NaYF4: Yb3+, Er3+ nanoparticles. The inset is the TEM image of the UCNPs (scale bar: 40 nm). (b) Energy-level diagram and energy-transfer upconversion process for the Yb3+ and Er3+ codoped upconversion materials. It indicates upconversion absorption (solid black arrow) and emission (solid colored arrows), energy transfer (dashed black arrows), nonradiative relaxation (dotted arrows) and cross-relaxation (wavy arrows) processes.

Figure 2 | (a) Formation process of the UCNPs/Au sample. (b) SEM images of UCNPs/Au sample. The inset is an enlarged view of a selected area. (c) The absorbance spectrum of the 18 nm-thick Au nanofilm. (d) Schematic of the UCNPs/Au sample.

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Figure 3 | Optical microscope images of the (a) UCNPs/Au and (b) UCNPs samples during emission. The red arrows represent the propagation direction of the incident light and the scale bar is 3.0 µm. (c) UCL spectra of the UCNPs/Au and UCNPs samples. (d) UCL spectra of the UCNPs/Au sample under various pump powers. (e) Power-dependent IRs of the I523/I655 and I523/I545 in the UCNPs/Au samples.

Figure 4 | The log–log plot of 523, 545 and 655 nm emissions intensity as a function of pump power for the (a) UCNPs/Au and (b) UCNPs samples. 15



Figure 5 | The decay curves of 2H11/2 → 4I15/2 (523 nm) (a), 4S3/2 → 4I15/2 (540 nm) (b) and 4F9/2→ 4I15/2 (655 nm) (c) transitions of Er3+ ions in UCNPs/Au and UCNPs samples.

Figure 6 | Temperature as a function of pump power for the UCNPs/Au sample.

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Figure 7 | (a) IR and (b) relative sensitivity as a function of temperature for the UCNPs/Au samples with UCNPs concentrations of 0.02, 0.05 and 0.2 mg/mL. (c) Summary of the pump power, temperature, maximum sensitivity and temperature resolution for the UCNPs/Au samples.

METHODS Apparatus and Measurements The waveguide excitation and spectral measurement were performed by placing the microfibers fixed on a glass substrate into a microspectrophotometer (CRAIC, 20/20PV). The microfibers were excited via waveguide excitation from a continuous wave laser at a wavelength of 980 nm. The UCL was collected by a spectrophotometer and a charge-coupled device (CCD). The optical microscope images were obtained with a computer-interfaced microscope (KH-7700, Hirox Co. Ltd.) incorporating a CCD camera. The specifications of the objective in the microscope, including magnification, numerical aperture (NA), and working distance (WD) are ×100, 0.85, and 3.4 mm, respectively. The UCL lifetimes were measured with a phosphorescence lifetime 17



spectrometer (FLS980, Edinburgh) equipped with an external 980 nm laser. Transmission electron microscope (TEM, JEM-200CX) and Scanning electron microscopy (SEM, Zeiss Gemini Ultra-55) were used to observe the surface morphologies of the UCNPs and UCNPs/Au structure. Fabrication of the microfiber The microfiber was fabricated by drawing a commercial multi-mode optical fiber (connector type: FC/PC, core diameter: 62.5 µm, cladding diameter: 125 µm) via a flame-heating technique. At one end of the fiber, the buffer and polymer jacket were stripped off using a fiber stripper and then the bare fiber was heated for about 1 min to reach its melting point. Finally, the fiber was drawn slowly until the diameter was decreased to several micrometers within a length of ~10 mm. The diameter of the microfiber used in our experiment was ~3 µm because a large diameter (~ dozens of micrometers) is disadvantage to the waveguide excitation while a small diameter (~ nanometers) will induce too much loss of the incident light before reaching the operation area.

ASSOCIATED CONTENT Conflict of Interest: The authors declare no competing financial interest. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 11274395), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13042) and the Open Fund of the Guangdong Provincial Key Laboratory of Optical Fiber Sensing and Communications (Jinan University) (No. CZ156091).

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Supporting Information Available: Details of the diameter distribution of β-NaYF4: Yb3+/Er3+ UCNPs, characterization of the Au nanofilm thickness, photostability of the UCNP/Au sample and the temperature calculation. Author contributions B. L. supervised the project; W. Z. conceived and designed the study; W. Z. and J. L. performed the experiments; W. Z. and J. L. analyzed the data; W. Z., J. L., H. L. and B. L. discussed the results and wrote the paper. REFERENCES [1]

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Plasmon-induced selective enhancement of green emission in

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