Integrated Strategy for High Luminescence Intensity of Upconversion

Article pubs.acs.org/journal/apchd5

Integrated Strategy for High Luminescence Intensity of Upconversion Nanocrystals Yue Yang,† Yingbin Zhu,‡ Jiajia Zhou,*,§,∥ Fan Wang,⊥,# and Jianrong Qiu*,∥ †

School of Materials Science and Engineering, §State Key Lab of Modern Optical Instrumentation, and ∥College of Optical Science and Engineering, Zhejiang University, Hangzhou, 310027, China ‡ SUTD-MIT International Design Centre, Singapore University of Technology and Design, 487372, Singapore ⊥ Institute for Biomedical Materials and Devices, Faculty of Science, and #ARC Research Hub for Integrated Device for End-user Analysis at Low-levels (IDEAL), Faculty of Science, University of Technology, Sydney, New South Wales 2007, Australia S Supporting Information *

ABSTRACT: The growing applications of upconversion nanocrystals in bioimaging, therapeutics, and photonics have given rise to a demand of high quality nanocrystals with desirable luminescence intensity. Although the design of optimal nanocrystals such as core−shell nanostructures has improved the intensity, the internal links between dopant concentration balance, epitaxial growth protection, and shell thickness effect encounter a compromised situation that lacks of integrated consideration and comprehensive assessment. Here we propose an integrated strategy based on a core−shell design for the enhancement of upconversion luminescence intensity. Epitaxial protection can enable higher activator accommodation capacity in limited spatial scale, which leads to an Er3+ concentration threshold improvement in β-NaYF4 core−shell nanocrystals from 2 to 6 mol %. We further perform a comprehensive assessment of the nanocrystals with convincing performance improvement in ensemble spectroscopic intensity, upconversion quantum yield, and single nanocrystal intensity. Our findings provide improved understanding of electronic behaviors in multiphoton upconversion and opportunities for diverse applications requiring high quality upconversion nanocrystals. KEYWORDS: upconversion nanocrystals, core−shell, quantum yield, single nanocrystal

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level, and more importantly, it may lead to side effects of damage in cell or in vivo applications. Despite diverse potential applications, UCNCs still suffer from low intensity of upconversion luminescence and quantum efficiency. On the one hand, researchers take advantage of dyesensitization to enhance UCNCs, which can alleviate weak and narrow near-infrared absorption of lanthanide ions.16,25,26 On the other hand, upconversion luminescence intensity of nanocrystals is also strongly affected by the surface ligand and crystal defect, which increase dark emitter number through nonradiative relaxation. This issue has been well addressed by core−shell structure design through crystalline epitaxial growth of an inert layer.27−30 Johnson et al.29 reported to choose NaLuF4 as an inert shell grown on monodoped NaErF4 with varying Er3+ dopant concentration, while Fischer et al.30 grew NaLuF4 on NaYF4: Yb, Er and investigated how shell thickness affected the surface quenching process on the optical properties of core−shell nanoparticles in which work the authors did timedependent luminescence, excitation spectra, and quantum yield to systematically understand the dependencies between shell

uminescence intensity of lanthanides doped upconversion nanocrystals (UCNCs) not only serve as an essential prerequisite to reveal physical science of lanthanides photonics in single nanoparticle sensitivity, but also an appraising parameter to translate the nanocrystals into applications including 3D display, bioimaging, sensing, and anticounterfeiting.1−19 Supposing that the ideal luminescence intensity can be achieved by the product of quantum yield and absorption capability, the luminescence intensity of UCNCs is in limitation of the congenital deficiency that low upconversion efficiency nonlinearly depends on the pumping power though we might use Yb3+ sensitizer to enhance the absorbance. With this in mind, concentration balance of doped lanthanides in nanoscale space is significantly important because the energy transfer efficiency relies on the ion distance and the emitting photon flux is determined by the effective emitter number. Previous reports reveal that the optimal concentrations of activators Er3+ and Tm3+ ions are ∼2 mol % with the assistance of 18−20 mol % Yb3+ sensitizers in general utilization.20−24 This concentration balance could be broken in single molecule level with extreme harsh pumping condition and so as to promote the activator concentration thresholds up to 20 and 8 mol % in βNaYF4 for much higher upconversion intensity, respectively.1,2 However, such strong irradiation is hard to satisfy in ensemble © 2017 American Chemical Society

Received: February 9, 2017 Published: July 6, 2017 1930

DOI: 10.1021/acsphotonics.7b00123 ACS Photonics 2017, 4, 1930−1936

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Figure 1. Upconversion spectroscopic analysis at room temperature. (a) Emission spectra of the nanocrystals with different shell layer thickness. (b) Intensity ratio of the core−shell nanocrystals to the core C1 at the wavelength according to 2H11/2, 4S3/2, 4F9/2 → 4I15/2 transitions. (c) Slopes n of 4 G11/2, 2H9/2, 2H11/2, 4S3/2, 4F9/2 → 4I15/2 transitions vary with shell thickness, which are calculated from the power-intensity double-log plots. (d) Local details of the emission spectra by enlarging the wavelength range from 385 to 520 nm. (e) Energy level diagram showing the energy transfer population process from Yb3+ to Er3+ in β-NaYF4. Upconversion emission spectra were measured with pressed powders, under excitation power density of 5.8 W/cm2. The laser focal area, irradiance, powder pressing density, and so on are kept the same for different samples during the measurement.

are synthesized by coprecipitation way and labeled “C1”, while the core−shell particles that regrew on top of the synthesized cores are marked as “C1@S1”, “C1@S2”, “C1@S3”, and “C1@ S4” in accordance with the shell thickness increasing (see Supporting Information, Figures S1 and S2). Comparative analysis of a series of nanocrystals with different shell thickness in upconversion spectroscopy at room temperature is performed. As shown in Figure 1a, the emission intensity dramatically increases after epitaxial protecting from inert shell, but an optimal shell thickness is observed at C1@S2. Accordingly, quantitative enhancement factors show that shell protection can enhance the red upconversion intensity approaching 150 times (Figure 1b) in maximum under the measured condition we perform the as-shown spectrum. Besides, it is obvious that the red emissions have much larger enhancement than the green emissions. By having the consensus that the shell isolation inhibits the surface quenching through quasi multiphonon relaxation, it is particularly critical to generate low red-to-green ratio by facilitating elimination of nonradiative loss from 2H11/2/4S3/2 to 4F9/2 level and 4I11/2 to 4 I13/2 level (Figure 1e).28,31,32 To understand this abnormal phenomenon, we measure the power dependent intensity variation and calculate the slopes n according to power law I ∝ Pn to clear the population dynamics of excited states (Figure S3). From Figure 1c, an interesting observation is that the n values of red peak (4F9/2 → 4I15/2) deviate gradually from the green case (2H11/2/4S3/2 → 4I15/2) to a consistency of the slopes belonging to the transitions from higher excited states

thickness and surface quenching. They both observed a consistent increase in upconversion emission intensity with increasing shell thickness, which prove that epitaxial growth of an inert layer to form core−shell structures can effectively suppress surface and concentration quenching. It is fantastic that the luminescence intensity enhancement is up to several orders of magnitude with the protection of shell layer, though the inner transition dynamics have not been studied elaborately. Herein, we conceive that an integrated strategy of concentration-balanced construction, epitaxial protection, and spatial size control should be built in nanocrystal design for upconversion emission enhancement by considering the following possibilities: First, inner crystal defect of the initial core particle might be partially eliminated as well as the ion redistribution during secondary growth; Second, isolating the surface dopants from surface quenchers will activate more lanthanides ions to be bright emitters; Third, nonradiative transitions contribute to the situation that the excited electrons release in a concentrated wavelength range for bight emission rather than blindly upward hopping with transition loss for worthless multiphoton upconversion; Fourth, large size can cause optical loss. This suggests that a new balance has to be established in core−shell nanocrystals for high upconversion luminescence intensity. As a proof-of-concept experiment, we chose β-NaYF4 core−shell nanocrystals to study the accommodation capacity of Er3+ ions with constant 20 mol % Yb3+ ions. The core particles with ∼17 nm average size in diameter 1931

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(4G11/2/2H9/2 → 4I15/2) along with the shell thickness increasing. This implies that 4F9/2 level tends to be populated through 4G11/2/2H9/2 as bridges (e.g., 4G11/2 + 4I15/2 → 4F9/2 + 4 I11/2 cross relaxation) but not 2H11/2/4S3/2 in the core−shell structures.33 In addition, the perfect slope coherence in nanocrystals C1@S3 and C1@S4 compared to the other nanocrystals indicates the fully shielding of nonradiative interference from surface quenchers, while the relative lower values are caused by further upward multiphoton transitions. This could be approved by the identified 4, 3-photon involved transitions 2P3/2 → 4I11,13/2, 4G11/2 → 4I13/2 and 4F3,5,7/2 → 4 I15/2, which hardly appear in the case of the absence of thick shell (Figure 1d). Furthermore, low temperature spectroscopy, in which the surface ligands are expected to keep at the deactive state, allows us to obtain more straightforward information associated with the intrinsic crystal feature. We here select C1 and C1@S2 for comparison, where the former holds surface ions cleating with molecular ligand directly and the latter has an inert layer for isolation. Accordingly, C1 exhibits larger intensity increase (∼5×) with temperature decrease, while C1@S2 only has about 2-fold enhancement (Figure 2a,b). Note that the maximum intensity appears at 80 K/50 K rather than 13 K, considering that low-frequency lattice vibration favors the energy transfer upconversion from Yb3+ to Er3+. And the different turning points in C1 and C1@S2 might be related, with the differences in the ligand−ion coupling strength leading

to the quenching or reduction of emission due to interactions of ions with surface ligands that characterize high vibration energy and phonon density-of-states associated with crystal size.34,35 Fine tuning data with 10 K step in temperature changing are shown in Figures S4 and S6. Besides, what’s interesting here is the sharp increase of three-photon related transitions occur at the expense of 2H11/2 → 4I15/2 transition emission in C1@S2, accompanying by the increasing red-togreen ratio. This suggests that the multiphoton upward hopping tendency in core−shell nanocrystals is not only affected by the surface state, but also an inner structure dominated behavior. Therefore, we believe that more activators could be accommodated in core−shell nanocrystals to push the energy store in intermediate levels for much brighter green and red emissions, rather than loss in the appearance of miscellaneous peaks. Before implementing the program to promote activator threshold, we try to figure out how the thickness of shell layer affect the performance of the core−shell nanocrystals. Transient decay curves of the green and red emissions at 520, 540, and 654 nm under excitation at 980 nm are given in Figure 3a. It is logical to see lifetime extension takes place as long as the shell growth layer by layer.28,36 Also, we get the increasing upconversion quantum yields with shell thickness increasing, and the maximum quantum yield 2.584% is achieved from C1@S4 in the case that C1 only has 0.07% under excitation of 103 W/cm2 980 nm laser (Figure 3b). On an intuitive level, we may think the upconversion emission has a positive correlation with the quantum yield. But here quantum efficiency increases with the shell thickness for C1−C1@S4, while upconversion luminescence intensity does not show the same tendency as C1@S3 and C1@S4 shows lower intensity than C1@S2 according to the emission spectra (Figure 1a). These two sustained positive effects with shell thickness run counter to the brightness quenching at C1@S3 and C1@S4. The result indicates that a higher QY does not necessarily correlates with stronger UC emission. This may be caused by negativity association with the particle morphology to the absorption capability of the excitation light. Epitaxial growth of inert shell can spatially isolate the core, passivate surface defect and reduce quenching effect. Therefore, the core−shell structures show an enhanced luminescent property than core only. But the optimal doping concentration in core−shell structures depends on a balance between concentration quenching and shell thickness. Considering the negative effect of the thick shell, we designed the nanocrystals with a moderate thickness S2, and tuned the doping concentration of Er3+ from 2 to 8 mol %, which are labeled as C1@S2, C2@S2, C3@S2, and C4@S2 accordingly. It is normal to find that the concentration quenching appears with too much emitters in the limited host space, as shown in Figure 4a. However, the core−shell structures in Figure 4b prove that they enable the emitter numbers up to 3 times but without luminescence quenching compared to the core only case. Figure 4c clearly demonstrates that the emission intensity in the ∼17 nm cores gradually decreases for higher Er3+ concentration, in which C4 (8 mol %) exhibits only about one-sixth of that in C1 (2 mol %). However, it is totally different that higher Er3+ concentration produces higher intensity in core−shell structures, especially C3@S2 (6 mol %) owns the maximum upconversion luminescence intensity that is 3.9× compared to C1@S2 (2 mol %). It is more convincing that the upconversion quantum yield, shows same tendency, as shown in Figure 4d.

Figure 2. Low temperature upconversion spectroscopy. (a) Upconversion emission spectra of C1 at 13, 80, and 290 K. (b) Upconversion emission spectra of C1@S2 at 13, 50, and 290 K. 1932

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Figure 3. Shell thickness assessment for high brightness. (a) Decay curves of Er3+: 2H11/2 → 4I15/2 (520 nm), 4S3/2 → 4I15/2 (540 nm), 4F9/2 → 4I15/2 (654 nm) transitions under excitation at 980 nm. (b) Upconversion quantum yields of C1, C1@S1, C1@S2, C1@S3, and C1@S4 under the excitation power density of 103 W/cm2.

Figure 4. High luminescence intensity achievement via integrated strategy. (a) Er3+ concentration dependent upconversion emission spectra of the cores C1, C2, C3, and C4. Inset TEM shows the uniform morphology and size. (b) Er3+ concentration dependent upconversion emission spectra of the core−shell nanocrystals C1@S2, C2@S2, C3@S2, and C4@S2. Inset TEM shows the uniform morphology and size. (c) Quantitative intensity variations with the increasing of Er3+ concentration, in which the intensity values obtained from green peaks at 541 nm and followed by a normalization, that is, all divided by minimum (C4). (d) Upconversion quantum yields of C1, C2, C3, C4, C1@S2, C2@S2, C3@S2, and C4@S2 under the excitation power density of 103 W/cm2. (e, f) Confocal scanning of single core−shell nanocrystals C1@S2, C2@S2, C3@S2, and C4@S2, respectively. The scanning ranges are 6 μm × 6 μm, and power density is 1 × 105 W/cm2. (i) Quantitative comparison of the single nanocrystal luminescence intensity between C1@S2, C2@S2, C3@S2, and C4@S2.

nanocrystals that observed by confocal microscopy. Multizone scanning was employed to obtain statistical intensity of each sample. As is shown in Figure 4i, high Er3+ concentration in core−shell structure is benefit for high luminescence intensity, and the saturation threshold is also found to be 6 mol % (C3@ S2, 927 counts), which has ∼2.1× count stronger than that of 2 mol % case (C1@S2, 449 counts). This observation holds consistency to the ensemble upconversion quantum yield evaluation. In summary, our experimental work performed on β-NaYF4 core−shell nanocrystals indicates an integrated strategy to

C3@S2 exhibits the maximum value of 2.86%, which equals to ∼1.7× the quantum yield from C1@S2 (1.7%). Quantum efficiency primarily depends on two factors in this work, namely the suppression of surface quenching by the shell and concentration quenching that occurs at higher Er doping. Furthermore, we assessed the upconversion luminescence intensity in single nanocrystal level, which is expected to present accurate information compromised in ensemble measurement and to develop the applicability of these nanocrystals in single particle sensitivity.37−39 The uniform fluorescent images with separate bright spots prove the single 1933

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diffractometer (Japan) with a slit of 0.02° at a scanning speed of 5 min−1 using Cu Kα radiation (λ = 1.5406 Å). TEM. Transmission electron microscopy (TEM) measurements were carried out partly on JEM-2100F (Japan) operating at a voltage of 200 kV and partly on FEG-TEM (Tecnai G2 F30 S-Twin, Philips-FEI, Netherlands) operating at an acceleration voltage of 160 kV. Ensemble Spectroscopy. Upconversion emission spectra at both room temperature and low temperatures were recorded with an FLS-920 spectrometer (Edinburgh Instruments). A continuous-wave 980 nm diode laser was employed as the excitation source. ICP-AES. Inductive coupled plasma atomic emission spectroscopy (ICP-AES) is measured by an Agilent 5110 ICP-OES. Quantum Yield. The upconversion quantum yield was measured with a Quantaurus-QY Plus UV-NIR absolute PL quantum yield spectrometer C13534 (Hamamatsu photonics K.K., Japan) with samples in powder form. The system consists of the Quantaurus Plus (C13534) with addition of supplementary units, including an excitation laser unit (L13668−980) and a filter unit (A13687−02). The excitation light source (980 nm) is used for upconversion measurement and it includes an optical system for laser mount. Dimmer filter (980 nm) is used and sensitivity calibration data is included. For the calibration of the system, three standard light sources (Xe lamp, halid lamp and tungsten lamp) are used to calibrate the detector and integrating sphere. The laser is corrected by the calibrated detector. Optical attenuators are used to control the output power of the laser depending on the requirement. The quantum efficiency is calculated based on the equation.41

enhance upconversion luminescence intensity. The optimal concentration of Er3+ has been improved to 6 mol % with the energy transfer sensitization from 20 mol % Yb3+ in the prerequisite that crystal epitaxial growth was employed to form core−shell structure. The as-synthesized heavily doped nanocrystals exhibit 3.9× the intensity enhancement in ensemble spectroscopy compared to the established optimum condition (20 mol % Yb3+-2 mol % Er3+ doped β-NaYF4 core−shell nanocrystals). Comparatively, we also assessed the nanocrystals in upconversion quantum yield and single nanocrystals brightness, which show consistent ∼2.1× superiority of our design with quantum yield of 2.86%. This investigation of upconversion luminescence intensity improvement through spectroscopic dynamic analysis enables an improved understanding of inter/intraions interactions for multiphoton upconversion in core−shell nanostructures. This study also raises the possibility of constructing high-quality UCNCs with exceptional optical features via the breaking of concentration bottleneck and so as to update existing models for a broad range of applications.

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EXPERIMENTAL SECTION Nanocrystal Synthesis. Materials. Y(OOCCH3)3·4H2O (99.99%), Yb(OOCCH3)3·xH2O (99.9%), Er(OOCCH3)3· 4H2O (99.9%) were bought from Alfa Aesar. NaOH (97%) was bought from Aladdin. NH4F (≥99.99%), 1-octadecene (ODE, 90%), and oleic acid (OA, 90%) were bought from Sigma-Aldrich. All materials were used without further purification. Synthesis of Core Nanocrystals. Typically, 4 mL of water solution of Re(OOCCH3)3 (0.2 M, Re = Y, Yb, Er) was added to a 50 mL three-neck flask containing 6 mL of OA and 14 mL of ODE. The mixture was heated to 150 °C for 30 min under vigorous magnetic stirring and then cooled down to 50 °C naturally. Afterward, a mixture of NaOH (2 mmol) and NH4F (3.2 mmol) in methanol was added and stirred for 30 min. The resultant solution was heated at 100 °C for 30 min to remove methanol. After purging with argon, the solution was heated to 290 °C and kept for 1.5 h. After cooling down to room temperature, the obtained nanocrystals were precipitated by addition of ethanol, collected by centrifugation, washed with ethanol several times, and finally redispersed in cyclohexane to be used for shell coating.40 Synthesis of Core−Shell Nanocrystals. The shell precursor was first prepared by mixing 4 mL of the water solution of Y(OOCCH3)3 (0.2M) with 6 mL of OA and 14 mL of ODE in a 50 mL flask and then heated to 150 °C for 30 min. After cooling down to 50 °C, 10 mL of methanol containing NaOH (2 mmol) and NH4F (3.2 mmol) was added and stirred for 30 min. The resultant solution was heated at 150 °C for 30 min to remove methanol. To grow the shell layer, the prepared core nanocrystals were mixed with 3 mL of OA acid and 7 mL of ODE and heated to 290 °C under argon. Once the temperature of solution reached 290 °C, shell precursor was injected several times, with 0.3 mL of shell precursor being injected and temperature being kept for 5 min for every injection. The resulting nanocrystals were collected with the same method of core nanocrystals. Core−shell nanocrystals with different shell thickness can be prepared by controlling the amount of injected shell precursor. The amount of core nanocrystals and shell precursor are used as Table S1 shows. Characterization. XRD. Powder X-ray diffraction (XRD) analysis was carried out on a RIGAKU D/MAX 2550/PC

ΦPL

∫ N (Em) = = N (Abs) ∫

λ sample ref [I (λ) − Iem (λ)]dλ hc em λ ref [I (λ) − Iexsample(λ)]dλ hc ex

where N(Abs) is the number of photons absorbed by a sample, N(Em) is the number of photons emitted from a sample, λ is the wavelength, h is Planck’s constant, c is the velocity of light, and Iref Isample ex ex are the integrated intensities of the excitation light and Iref with and without a sample, respectively, and Isample em em are the photoluminescence intensities with and without a sample, respectively. Single Nanoparticle Spectroscopy. A laser scanning confocal microscope was home-built for the intensity measurement of single upconversion nanocrystals. The excitation source is a 976 nm single mode polarized laser (229 mW), which is focused onto the sample through a 100× objective lens (NA 1.4). The emitting light from sample is collected by the same objective lens, which was refocused into an optical fiber having a core size matching with system Airy disk. A single Photon Counting Avalanche Diode (SPAD) detector is connected to the collection optical fiber to detect the emission intensity. The scanning is achieved by moving the 3D piezo stage. The monodispersed single UCNCs samples were prepared using the following steps:37 a. Put a drop of 50 μL poly-L-lysine solution (0.1% w/v in H2O) on a cleaned cover-glass, leave it for 30 min before rinsing with water, and then dry at room temperature. b. Prepare 0.1 mg/mL UCNCs dispersed in cyclohexane, then place a drop of 20 μL UCNCs dispersion onto the cover-glass, and then carefully rinse it using cyclohexane and let it dry naturally. 1934

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c. Make a drop of 20 μL embedding media on a glass slide, place the cover-glass onto the glass slide with embedding media, squeeze out the air bubbles, and then dry the sample at 60 °C.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b00123. Tables S1 and S2 and Figures S1−S9 (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jiajia Zhou: 0000-0001-9016-2799 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from National Natural Science Foundation of China (Nos. 11404311 and 51472091) and China Postdoctoral Science Foundation (No.2016M601934).

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REFERENCES

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DOI: 10.1021/acsphotonics.7b00123 ACS Photonics 2017, 4, 1930−1936