Direct Identification of Surface Defects and their Influence on the

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Direct Identification of Surface Defects and their Influence on the Optical Characteristics of Upconversion Nanoparticles Wenjuan Bian, Yue Lin, Ting Wang, Xue Yu, Jianbei Qiu, Meng Zhou, Hongmei Luo, Siu Fung Yu, and Xuhui Xu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Nano

Direct Identification of Surface Defects and their Influence on

the

Optical

Characteristics

of

Upconversion

Nanoparticles Wenjuan Biana,d,†, Yue Linb†, Ting Wangc, Xue Yua,d*, Jianbei Qiua*, Meng Zhoud, Hongmei Luod, Siu Fung Yu c*, Xuhui Xua,c* a

Faculty of Materials Science and Engineering, Kunming University of Science and Technology, Kunming, Yunnan Province 650093, P. R. China

b

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui Province 230026, P. R. China

c

Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China.

d

Department of Chemical and Materials Engineering, New Mexico State University, Las Cruces, NM, 88003, United States.

Corresponding Authors: *Email: [email protected]; *Email: [email protected]; *Email: [email protected]; *Email: [email protected]. ABSTRACT: Core-shell structure is an obvious concept to suppress surface-related deactivations in lanthanide-doped upconversion nanoparticles (UCNPs). However, no direct observation on the atomic-scale surface restoration, which can improve the upconversion photoluminescence, has been reported. Here, we use the aberration-corrected high angle annular dark field scanning transmission electron microscopy to study the surface condition of KLu2F7:Yb3+,Er3+ bare core UCNPs. Due to the very thin and uniform thickness of the UCNPs, we observe unambiguously that the recovery from surface defects enhances upconversion photoluminescence. Furthermore, the realization of dominant green lasing emission under pulsed laser excitation confirms the high crystallinity of the UCNPs. KEYWORDS: upconversion, surface defect, surface restoration, energy back transfer, lasing

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Upconversion nanoparticles (UCNPs) with core-shell configuration, which offers a significant enhancement of photoluminescence efficiency, are promised for optical imagingguided bioimaging, therapeutics, anti-counterfeiting, and solar cells applications.1-10 This is because the shell coating eliminates quenching sites and isolates core from the surrounding deactivators (e.g. ligands, solvents) so that an effective suppression of the surface-related deactivations can be realized.11-15 Furthermore, surface trapping of dopant ions, which induces quenching of excitation energy, can be suppressed by the core-shell configuration.1620

Recent investigation has shown that the formation of NaGdF4 shell on NaGdF4:Yb/Tm

core UCNPs demonstrates a more than 450-fold enhancement of upconversion photoluminescence intensity which is an indirect proof of the elimination of core’s surface defects.21 However, there is no direct observation showing that the restoration of surfacerelated defects indeed contributes to the improvement of upconversion efficiency. Here, we propose the use of a wet chemical annealing process to restore the lanthanide-doped KLu2F7 bare core UCNPs from surface defects (i.e. disorder, vacancy and interstitial defects). As the UCNPs have a uniform thickness of a few atomic layers, the application of aberrationcorrected high angle annular dark field scanning transmission electron microscopy (HAADFSTEM) can recognize defects at an atomic scale. By restoring the surface defects through thermal annealing, the corresponding upconversion photoluminescence intensity is enhanced by an order of magnitude. This verifies that the deleterious defects are the surface luminescence quenching centers of the UCNPs. Dominant green lasing emission, which has decay lifetime three times shorter than that of the red light, is also observed under pulsed laser excitation. This is a strong suggestion that the KLu2F7:38%Yb3+,2%Er3+ UCNPs can restore high crystallinity to suppress non-radiative recombination at the short transient time. Hence, we verify unambiguously that surface restoration indeed leads to the enhancement of upconversion photoluminescence. 2


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RESULTS AND DISCUSSIONS

Figure 1. (a) Schematic diagram of the wet

chemical annealing process of

KLu2F7:38%Yb3+,2%Er3+ UCNPs. TEM images of the as-synthesized UCNPs (b) before and (c) after thermal annealing at 240 oC. (d) HRTEM image, (e) FFT profile and (f) particle size distribution profile of the as-growth UCNPs. (g) HRTEM image, (h) FFT profile and (i) particle size distribution profile of the post-growth annealed UCNPs.

Figure 1a presents the fabrication methods of the KLu 2F7:38%Yb3+,2%Er3+ UCNPs with a high doping concentration of Yb3+. In the fabrication process, the as-growth UCNPs were cooled down and washed for few times before performing post-growth annealing to avoid Ostwald ripening (Figure S1). Furthermore, as K, Lu and F ions are removed from the annealing process and the annealing temperature is lower than the formation temperature of the shell structure, core-shell configuration of the UCNPs is guaranteed not to be formed. Transmission electron microscopy (TEM) images of the regular hexagon UCNPs before and after annealing at 240 oC are shown in Figures 1b and 1c respectively. This annealing temperature was selected to optimize the photoluminescent efficiency of the UCNPs (Figure S2). Furthermore, the high-resolution TEM (HRTEM) images clearly show the lattice fringes

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at the interior of the UCNPs (Figures 1d and 1g). However, the edge of the as-growth UCNPs has an epitaxial amorphous phase but that of the post-growth annealed UCNPs has a welldefined edge. Lattice fringes and fast Fourier transform (FFT) reflections with a d-spacing attributed to the KLu2 F7 crystal configuration are also observed (Figures 1e and 1h). The lattice fringes display an interplanar spacing of 0.660 nm corresponding to the (011) plane, explicitly demonstrating an interior well-crystallized KLu2 F7 UCNPs. The UCNPs show a surprisingly monodisperse size distribution of around 30 nm without aggregation before and after annealing (Figures 1f and 1i). In addition, size growth of the UCNPs after annealing is not detected. It reveals that the wet chemical annealing process not leads to the growth of crystal size but improve the crystallinity at the edge of the UCNPs (Figure S3). Figures 2a and 2b show the HAADF-STEM images of KLu2F7:38%Yb3+,2%Er3+ UCNPs before and after annealing respectively. The magnified image reveals that the interior of the UCNPs before annealing has a good crystallinity. The atomic-level structure of KLu2F7 with overlap-atom position of Lu showing a higher contrast but that with mono-atom position of Lu showing a weaker contrast along the a-axis projection. The position distinction between overlap-atom position and mono-atom position columns can be clearly observed (Figure S4a). The magnified HAADF-STEM images exhibit an overlay of the orthorhombic (011) atomic model with the experimental atom positions, of which agrees well with each other (Figure S4b). All the atom columns are clearly resolved, and the contrast is homogeneous across the entire image. This verifies that Yb3+ and Er3+ ions substitute the crystal sites of Lu3+ successfully in an atom-scale, instead of entering the interstitial sites in the matrix.

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Figure 2. HAADF-STEM images of KLu2 F7:38%Yb3+,2%Er3+ UCNPs (a) before and (b) after annealing at 240oC. Intensity profiles recorded by scanning along the directions (c) orange and (d) green arrows of the UCNPs as shown in figures (a) and (b) respectively. Enlarged crystal edge structure images (e) before and (f) after annealing.

The fundamental behavior of the edge structure is characterized by the peak-to-valley intensities. Figures 2c and 2d show the intensity profile before and after annealing respectively extracted from the experiment (i.e. see the arrow E → I). The relaxed intensity profile of the amorphous edge shows that the uncompleted crystallization process is observed for the as-grown sample. This is because the uncompleted growth of UCNPs (Figure 2e) leads to the lattice disorder, vacancy, or interstitial form the surface defect states. On the other hand, the unrelaxed crystal edges of the (011) planes are well-crystallized with the regular structure. The magnified atomic-scale image (Figure 2f) shows no missing or exceeding of Lu ions at the outmost layer of the UCNPs. Hence, it is verified that the amorphous crystal edges in Figure S5 can be reconstructed by the wet chemical annealing.

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Furthermore, the clear atomic image shows uniform thickness of the UCNPs has ten atomic layers (Figure S6). This 2-dimensional sheet structure ensures the crystal growth is constrained in the plane perpendicular to [100] lattice orientation so that any interstitials and stacking faults will only appear near the edge of the UCNPs.

Figure 3. (a) Upconversion photoluminescence spectra of the KLu 2 F7:38%Yb3+,2%Er3+ UCNPs. Decay curves of (b) 4S3/2 4I15/2 transition (@543 nm), (c) 4F9/24I15/2 transition (@668 nm), and (d) 2F5/22F7/2 transition (@980 nm) of the UCNPs. (e) Simplified energy transfer diagram of the UCNPs under 980 nm continuous wavelength (CW) laser excitation.

From

Figure

3a,

it

is

noted that

after

thermal annealing

at

240

℃, the

KLu2F7:38%Yb3+,2%Er3+ UCNPs exhibit a simultaneous enhancement of green and red emission intensities by more than 10 times than that of the as-growth UCNPs. The decay curves of the 4S3/24I15/2 transition (@543 nm) of the UCNPs before and after thermal annealing are also shown in Figure 3b. It is noted that the decay times have been increased by ~100% after the application of thermal annealing. Increases of 4F9/24I15/2 (@668 nm) and 6


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4

F5/24F7/2 (@980 nm) decay lifetime by ~50% and ~90% respectively are also observed

(Figure 3c and 3d) after thermal annealing. Figure 3e describes the green and red radiative recombination processes of the KLu2 F7:38%Yb3+,2%Er3+ UCNPs. The dominant red emission can be explained by the enhancement of (i) energy back transfer (EBT) process with the influence of high doping concentration of Yb3+ and (ii) cross relaxation (CR) process (Figures S7-S9).22,23 This is because the EBT and CR processes reduce the population at the 4

F7/2 state which supplies carriers to realize radiative recombination between ( 2H11/2, 4S3/2)

and 4I15/2 states (i.e. green emission). The realization of high-intensity red emission (i.e. enhancement of EBT and CR processes) and prolonged carrier lifetimes indicate that the removal of amorphous surface and the improvement of surface crystallinity by thermal annealing.

Figure 4. (a) Proposed energy transfer process of KLu2F7:Yb3+,Er3+ UCNPs under 980 nm short

and

long

pulsed

laser

excitation.

Upconversion

emission

spectra

of

KLu2F7:38%Yb3+,2%Er3+ UCNPs (b) before and (c) after annealing obtained through the

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excitation of 980 nm laser diode with different laser pulse excitation with pulsewidth from CW to 6 ns.

Figure 4a explains how the KLu2F7:38%Yb3+,2%Er3+ UCNPs response to the influence of laser pulse excitation. The key factors of red emission from KLu2F7:Yb3+,Er3+ UCNPs with high concentration of Yb are the EBT process (step ③) together with the decay of 4

I11/2→4I13/2 of Er ions (step ④) as shown under the long pulse operation, see right-hand-side

of Figure 4a. On the other hand, the green emission rate from the 2H11/2, 4S3/2 states are fast due to a direct two-step population procedure and a non-radiative relaxation process with small energy gap. This can be validated by the time-resolved photoluminescence studies under 980 nm short-pulse excitation, see left-hand-side of Figure 4a. The EBT and the nonrelaxation process of step ④ can be suppressed by the short excitation pulses, which contributes to the decrease of the R:G emission intensity ratio. It is noted that EBT (step 3) and non-radiative relaxation (4I11/24I13/2, step 4) processes are suppressed under short-pulse (< 100 ns) excitation (Figure S10). This is because 2H11/2 and 4S3/2 are populated (step 6) before the 4F7/2  4I11/2 transition can take place under short-pulse excitation (Figures S11, S12). For the UCNPs have high crystallinity, the decay lifetime 2H11/2 and 4S3/2  4I15/2 increases causing further increase of the green emission, but the red emission is reduced. Hence, high crystallinity of the KLu2 F7:38%Yb3+,2%Er3+ UCNPs will favor green emission (i.e. short decay lifetime) under laser pulse excitation with pulsewidth close to that of the green decay lifetime. Figures 4b and 4c plot the upconversion emission spectra of KLu2F7:38%Yb3+,2%Er3+ UCNPs under 980 nm pulsed laser excitation (i.e. pulsewidth: CW, 5ms, 0.5ms, 50μs, and 6ns) before and after thermal annealing respectively (Figures S13). It is observed that the enhancement of green emission and simultaneously suppression of red emission under the laser pulse excitation with pulsewidth of 6 ns. This verifies that the

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suppression of non-radiative recombination at the short transient time and high-crystallinity of UCNPs should obtain after thermal annealing.

Figure 5. (a) Lasing spectra of a microcavity using KLu2F7:38%Yb3+,2%Er3+ UCNPs as the gain medium under 980 nm laser pulse excitation at room temperature. The inset shows the photo of the laser microcavity. (b) Output power and emission linewidth versus excitation power. (c) Measured values of Pth and ∆λ versus D for the green emission based on the lasing microcavity.

Figure 5a plots the emission spectra of a microcavity (with diameter D = 31 μm) using KLu2F7:38%Yb3+,2%Er3+ UCNPs as the gain medium (i.e. a mixture of UCNPs and silica resin) versus 980 nm laser pulse (6 ns) excitation at room temperature. A broad emission band with linewidth equal to ~16 nm is observed at low excitation power. However, narrow peaks, which have a linewidth of less than 0.5 nm, emerge from the emission spectra for the excitation power, P, larger than a threshold value Pth (=23.25 kW/cm2). In addition, the linewidth of the envelope of the emission spectra reduces from ~16 to ~5.5 nm with the increase of P and the output intensity of the green lasing emission increases linearly with P > Pth. The observation of a kink in the light-light curve and the narrow of the envelope of the emission spectra at P = Pth (Figure 5b) prove that the KLu2F7:38%Yb3+,2%Er3+ UCNPs have

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high optical gain to support green lasing emission. Figure 5c shows the measured Pth and ∆λ of microcavities with a different value of D. It is expected that Pth decreases with the increase of D as long laser cavity allows better amplification of resonant modes. If the side of the microcavity supports resonant optical feedback, mode spacing between the adjacent narrow peaks, ∆λ, observed from the spectra can be expressed as: ∆λ=λ 02 /πDneff

(1)

where λ0 (=547 nm) is the center peak wavelength and neff is the effective refractive index of the microcavity. By using (1) to fit the measured data of Δλ and D, it is deduced that neff is ~1.58 which is close to the refractive index of the silica resin. Hence, this confirms that the microcavities support whispering gallery lasing modes. On the other hand, red emission is suppressed for P larger than Pth so that its emission spectra are not shown in the figure. Realization of green lasing emission indicates that high crystallinity UCNPs are formed through surface restoration.

CONCLUSION The clustering structure of thin KLu2F7:38%Yb3+,2%Er3+ UCNPs was directly observed at an atomic scale by using the HAADF-STEM. Thanks to the proposed wet chemical heattreatment approach and the arrangement of the lanthanum ions in the structure, the activators located at the crystal sites of Lu3+ have been recovered without bringing undesired defects are unambiguously demonstrated. Under 980 nm CW laser excitation, the improvement of upconversion emission intensity by an order of magnitude is observed. This indicates that the influence of surface quenching appears before thermal annealing, has been suppressed. On the other hand, under 980 nm pulsed laser (6 ns) excitation, the green lasing emission is dominant from the lasing spectra. The fast-transient response to the 2H11/2 and 4S3/2 states and

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the suppression of EBT process further verify that the high crystallinity of the UCNPs can be restored after the thermal annealing.

EXPERIMENTAL SECTION Synthesis of KLu2F7:Yb3+,Er3+ UCNPs. (x=18, 28, 38, 48, 58, 78, 98) UCNPs were synthesized using a co-precipitation method. Set KLu2 F7:38%Yb3+,2%Er3+ as an example. For the synthesis of RECl3 (RE=Lu, Yb, Er) solution, high purity Lu2O3 (0.6 mmol, 99.99%, Aladdin), Yb2O3 (0.38 mmol, 99.99%, Aladdin) and Er2O3 (0.02 mmol, 99.99%, Aladdin) were added to concentrated hydrochloric acid in a 100 mL beaker and then heated to 200 ℃. The obtained RECl3 in 1mL DI water was magnetically mixed with OA (Oleic Acid, 8 mL, 85%，Aladdin) and ODE (1-Octadecene, 12 mL, >90%, Aladdin) in a 100 mL three-neck round-bottom flask. The mixture was degassed under an N2 flow and heated to 150℃ for 1 h to form a clear solution and remove water in the flask, and then cooled to room temperature. Methanol solution (9 mL, 99.9%, Aladdin) containing NH4F (3 mmol, 98%, Aladdin) and KOH (2 mmol, 95%, Aladdin) was added into the OA and ODE solution and heated to 50 ℃ for 30 min. After that, the solution was slowly heated to 110℃ and kept for 30 min to remove methanol and the remaining water completely. Then the reaction mixture was quickly heated to 290 ℃ and aged for 1h. After the solution was left to cool down to room temperature, ethanol was added to precipitate the UCNPs. The product was washed with cyclohexane and ethanol for three times, before the final KLu2 F7:38%Yb3+,2%Er3+ UCNPs were re-dispersed in 5mL cyclohexane in preparation for their further use. In the washing process, the volume ratio of cyclohexane and ethanol for the first two times is set 4:10 and the final one is 10:4. Samples of KLu2F7 with different concentration (28%, 38%, 48%, 58%, 78% and 98%) Yb3+

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and 2% Er3+ were synthesized by a similar procedure except different moles of Lu 2O3 and Yb2O3 involved in the reaction. Wet-chemical heat treatment of KLu2F7:Yb3+,Er3+ UCNPs. 8 mL OA and 12 mL ODE were mixed in a three-neck round-bottom flask, and then 0.5 mmol as-prepared bare core UCNPs were added into the solution under an N2 flow without the addition of K, Lu and F ions. The mixed solution was heated to 80 ℃ to evaporate cyclohexane with stirring. After 30 min, the mixture was heated to the heat-treatment temperature of 240 ℃ for 1.5 h, respectively. Finally, the samples were cooled down to room temperature. The annealed UCNPs were washed by a solution with a volume ratio of cyclohexane:ethanol equal to 4:10 eight times and subsequently washed by another solution with a volume ratio of cyclohexane:ethanol equal to 10:4. For the optical testing, the concentrations of the KLu2F7:38%Yb3+,2%Er3+ UCNPs was fixed to 0.03 mg/mL. Synthesis of microcavities. The microcavity was fabricated by coating a small drop of a mixture of UCNPs and silica resin on a bare optical fiber. The coated optical fiber was then hung in a vertical orientation to form an elliptical microcavity. By increasing the amount of the mixture, we can control the diameter of the elliptical microcavity. HAADF-STEM Characterization. HAADF-STEM images were recorded by a JEOL JEM-ARM200F TEM/STEM with a spherical aberration corrector. The operation of HAADF HR-TEM imaging can be explained as follows. 1) Before performing the HAADF HR-TEM, we make sure that the UCNPs are cleaned. 2) When we perform HAADF HR-TEM, we reduce the bombardment of the electron beam to the UCNPs by choosing a super short dwell time (i.e. 3 μs, it must be noted that the typical dwell time is > 10 μs). Then, a minimum spot size (i.e. 1 in the 10 levels of electron beam spots of ARM200F) of the electron beam is selected to minimize the damage to the UCNPs. Finally, we further optimize the magnification of the microscope such that the atoms

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of the UCNPs can still be clearly observed and simultaneously the damage of the UCNPs is minimized. If the electron beam energy is too high, the surface damage to the UCNPs is unavoidable (see Figure S14, Supporting Information). We find that 10 M is the best enlargement factor, which can help us to observe a clear image of atoms without damaging them. 3) Select a suitable geometry of the UCNPs for the HAADF HR-TEM imaging is also important to capture a clear image on an atomic scale. We select an UCNP with a uniform thickness so that this allows us to easily control the energy of the electron beam. General Characterization. X-Ray powder diffraction (XRD) was performed using a D8 Focus diffractometer (Bruker) with Cu-Kα radiation (λ=0.15405 nm) in the 2θ range from 10°to 80°. The particle morphology and thickness were studied with field TEM and highresolution field transmission electron microscope (HRTEM), carried out using U.S. FEI Tecnai G2 F20 operating at 200kV. The upconversion photoluminescence spectra of the samples under the excitation of 980 nm near-infrared resource with different pulsed width were recorded by HITACHIU-F-7000 spectrophotometer at a different temperature. The decay curves were measured on an Edinburgh instruments FLS980 spectrometer. The emission and excitation spectra of the sample were recorded on a HITACHI F-7000 fluorescence spectrophotometer using a static 150 W Xe lamp as the excitation source. For the laser measurement, a Continuum Panther EX optical parametric oscillator and a 355 nm frequency-tripled Continuum Powerlite DLS 9010 Q-switched Nd: YAG laser were used to generate 980 nm pump pulses of width and repetition rate of ≈6 ns and 10 Hz, respectively. [6]

ASSOCIATED CONTENT Supporting Information. More TEM and HAADF-STEM images, XRD patterns, up- and down-conversion spectra, crystal structure, R:G ratio and color coordinate images and decay files (Figure S1-S13) (PDF).

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The Supporting Information is available free of charge via the ACS Publications website at http://pubs.acs.org.

Author Contributions †

Wenjuan Bian and Yue Lin contributed equally to this work.

ACKNOWLEDGMENT This work was financially supported by the National Nature Science Foundation of China (61565009, 61775187, 11664022 and 11404314), the Foundation of Natural Science of Yunnan Province (2016FB088), the Young Talents Support Program of Faculty of Materials Science and Engineering, Kunming University of Science and Technology (14078342), Anhui Provincial Natural Science Foundation (1708085MA06) and the Fundamental Research Funds for the Central Universities (WK2340000055). This work (XHX) was partly supported by the Hong Kong Polytechnic research grant no. 1-ZVGH. We also acknowledge the partial support from National Science Foundation with DMR-1449035.

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Using

Mesoporous

Silica-Coated

NaYF4:Yb,Er/Tm

Upconversion

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ACS Nano

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Abstract Graphic - TOC

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Direct Identification of Surface Defects and their Influence on the

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