Optimal Sensitizer Concentration in Single Upconversion

Publication Date (Web): April 24, 2017 ... concentration quenching effect does not limit the sensitizer concentration and NaYbF4 is the most bright ho...
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Optimal Sensitizer Concentration in Single Upconversion Nanocrystals Chenshuo Ma, Xiaoxue Xu, Fan Wang, Zhiguang Zhou, Deming Liu, Jiangbo Zhao, ming guan, Candace I Lang, and Dayong Jin Nano Lett., Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 25, 2017

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Optimal Sensitizer Concentration in Single Upconversion Nanocrystals Chenshuo Ma§, ‡, Xiaoxue Xu‡, £*, Fan Wang‡, £,*, Zhiguang Zhou£, Deming Liu£, Jiangbo Zhao₸, Ming Guan£, Candace I Lang§, and Dayong Jin‡, £, †* § Department of Engineering, Faculty of Science and Engineering, Macquarie University, Sydney, NSW, 2109, Australia;

‡Advanced Cytometry Labs, ARC Centre of Excellence for Nanoscale BioPhotonics, Macquarie University, Sydney, NSW, 2109, Australia;

£Institute for Biomedical Materials and Devices (IBMD), Faculty of Science, University of Technology Sydney, NSW, 2007, Australia;

₸Institute for Photonics and Advanced Sensing (IPAS) and School of Physical Sciences, University of Adelaide, Adelaide, SA 5005, Australia;

†ARC Research Hub for Integrated Device for End-user Analysis at Low-levels (IDEAL), Faculty of Science, University of Technology Sydney, NSW, 2007,

Australia;

ABSTRACT: Each single upconversion nanocrystal(UCNC) usually contains thousands of photon sensitizers and hundreds of photon activators to up-convert near infrared photons into visible and ultraviolet emissions. Though in principle further increasing the sensitizers’ concentration will enhance the absorption efficiency to produce brighter nanocrystals, typically 20% of Yb3+ ions has been used to avoid the so-called “concentration quenching” effect. Here we report that concentration quenching effect does not limit the sensitizer concentration and NaYbF4 is the most bright host matrix. Surface quenching and the large size of NaYbF4 nanocrystals are the only factors limiting this optimal concentration. Therefore we further designed sandwich nanostructures of NaYbF4 between a small template core to allow an epitaxial growth of the sizetunable NaYbF4 shell enclosed by an inert shell to minimize surface quenching. As a result, the suspension containing 25-nm heterogeneously synthesized UCNCs is 1.85 times brighter than the homogenously doped ACS Paragon Plus Environment

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ones, and the brightness of each single 25 nm heterogeneous UCNC is enhanced by nearly 3 times compared 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

to the NaYF4: 20% Yb3+, 4% Tm3+ UCNCs in the similar size. Particularly, the blue emission intensity of the UCNCs with the sandwich structure in the size of 13.6 nm and 25.2 nm are 1.36 times and 3.78 times higher than that of the monolithic UCNCs in the similar sizes. Maximizing the sensitizer concentration will accelerate the development of brighter and smaller UCNCs as more efficient biomolecule probes or photon energy converters. KEYWORDS: upconversion, single nanoparticle, core shell, concentration quenching, lanthanide TOC GRAPHIC:

Rare-earth doped upconversion nanocrystals (UCNCs) are typically doped with ytterbium sensitizer ions (Yb3+), which absorb infrared radiation and non-radiatively transfer their excitation to activator ions such as erbium (Er3+), thulium (Tm3+) or holmium (Ho3+)1-4. UCNCs capable of converting lower-energy infrared photons into high energy emissions, such as visible and ultraviolet light5-7, are promising for a broad range of applications spanning background-free bio-labeling and bio-sensing8, 9, light-triggered drug delivery10-13, multimodal bio-imaging14, full-color 3D display15, solar energy harvesting and light-emitting diodes (LEDs)16-19. As upconversion luminescence is a multiphoton process, one of the long pursued goals in promoting UCNCs for crystal applications is to enhance upconversion efficiency and generate stronger upconversion brightness20, 21. Thanks to decades of investigation in upconversion luminescence as well as the advancement of synthesis and instrumental characterisation at nanoscale, various enhancement strategies to amplify upconversion luminescence have been proved, including optimizing crystal hosts and dopant concentrations, coating inert/active shells around nanocrystals12,

22-26

, tuning sizes of nanocrystals5,27-29,

engineering sublattice arrangements30and coupling with plasmonic noble metal nanostructures31, 32. Very recently, high- irradiance excitation was used to alleviate concentration quenching of upconversion luminescence for orders of magnitude brightness enhancement, where the optimal activator concentrations of Tm3+ and Er3+ in NaYF4 increase from 0.5 mol% to 8 mol%, and from 2 mol% to more than 10 mol% respectively9, 33-35. However, the corresponding optimal sensitizer concentration, i.e. the typical trivalent Yb3+ ions, for the new level of activator concentration has not been updated yet. This gap is becoming pronounced when the UCNCs shrink smaller than 30 nm, where the upconversion brightness proportionally drops as the number of dopants per crystal reduces and the ratio of surface quenchers increases. ACS Paragon Plus Environment

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For bio-labeling applications, both the excitation IR wavelength and 800 nm emission wavelength of 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Tm3+ doped UCNCs are attractive as they are within a “window of optical transparency” for imaging biological tissues36. Strategies to enhance the upconversion luminescence at 800 nm are accordingly of particular interest for in vivo bio-imaging37, 38. Nanocrystals optimally range from several nanometers to tens of nanometers in size39-41. Nanocrystals greater than ten nanometres in size are not transported efficiently through sub-cellular membranes, or cannot be easily cleared from the body, and for in vitro diagnostics applications, smaller fluorescent nanocrystals would be more useful as these are expected to avoid nonspecific binding issues. However, a small crystal size also results in a brightness drop as the number of dopants per crystal reduces, and the impact of surface quenching is expected to increase with the increased surface-to-volume ratio. Consequently, a major objective is to develop brighter and smaller UCNCs. Increasing the dopant concentration would be a straight forward strategy. In this work, we find that the Yb3+ concentration is not limited by the concentration quenching effect and NaYbF4 crystal is the best host for sensitizing more excitation photons to increase the brightness of UCNCs. We carefully look into this phenomenon by systematical design, synthesis and characterization of β-NaYF4 UCNCs with an incremental high concentration of sensitizer ions. By excluding the size and crystal structure effect, we directly confirm the sensitizer effect. To produce smaller UCNCs with optimal Yb3+ concentration, we first employ a Tb3+-rich core as the template to allow epitaxial growth of the size-controllable active layer of NaYbF4. We further find the high concentration of Yb3+ ions on the relatively large surface of nanocrystals induce significant quenching. To passivate UCNCs from surface quenchers, an inert shell of NaYF4 can be employed. Such a sandwich design has been proved useful to produce a range of brighter and size-tunable UCNCs42-44. We confirm that high dopant concentration of sensitizer ions Yb3+ will definitely benefit the overall brightness of UCNCs. Both Fluorolog spectrophotometer and upconversion confocal microscopy approach are used in this work to quantitatively characterize the emission counts of UCNCs dispersions and single UCNCs, which facilitates our investigation of the underpinning physics of photon upconversion process within the sensitizer-activator network of a single nanocrystal. In our experiment, a series of NaYF4: 4% Tm3+, x% Yb3+ UCNCs (x= 0, 20, 30, 45, and 60) were synthesized under identical preparation conditions. As shown in the transmission electronic microscopy (TEM) images (Figure 1a-c and Figure S1a and b),the crystal size increases with the Yb3+ doping levels, from 19.6±1.0 nm (0% Yb3+) to 51.3±1.8 nm (60% Yb3+) which shown in Figure S1e，the crystal size distribution histograms are shown in Figure S1c and d and Figure S2a-c. The X-ray powder diffraction (XRD) patterns of all the as-synthesized UCNCs are indexed to pure hexagonal phase (NaYF4, JCPDS: 00016-0334 and NaYbF4, JCPDS: 00-027-1427 (Figure S3). The XRD peaks shift towards higher angles as a function of Yb3+ concentration, this relates to a reduction of unit-cell volume due to the substitution of Y3+ ion (r=0.880Å) by the smaller Yb3+ion (r=0.858Å). We normalized to the same Tm3+ concentration by the ACS Paragon Plus Environment

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subtractive weighting method and inductively coupled plasma mass spectrometry (ICP-MS). Figure 1j 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

shows that the luminescence intensity of the UCNCs dispersion is amplified as the Yb3+ concentration and crystal size increase, where the upconversion luminescence at 800 nm with 60% Yb3+(~51.3 nm) is more than 36 times brighter than that of conventional 20% Yb3+ (~ 24.6 nm).

Figure 1. TEM images of NaYF4: 4% Tm3+, x% Yb3+nanocrystals: (a) x=30, (b) x=45, (c) x=60; and TEM images of NaYF4: 4% Tm3+, 20% Yb3+, y% Lu3+nanocrystals: (d) y=10, (e) y=25, (f) y=40, Scale bar is 100 nm; Luminescent emission spectra of the UCNCs dispersions measured using Fluorolog Photospectrometer: (g) NaYF4: 4% Tm3+, 30% Yb3+ and NaYF4: 4% Tm3+, 20% Yb3+, 10% Lu3+, (h) NaYF4: 4% Tm3+, 45% Yb3+ and NaYF4: 4% Tm3+, 20% Yb3+, 25% Lu3+, (i) NaYF4: 4% Tm3+, 60% Yb3+ and NaYF4: 4% Tm3+, 20% Yb3+, 40% Lu3+, (j) NaYF4: 4% Tm3+, x% Yb3+ (x=30, 45, 60) and NaYF4: 4% Tm3+, 20% Yb3+, y% Lu3+ (y= 10, 25, 40); (k) the size of unit cells of NaYF4 and NaLuF4 crystals.

To shed more light on the weight contribution (crystal size vs. Yb3+ concentration) to the enhanced upconversion luminescence, we grew NaYF4: 4% Tm3+, 20% Yb3+, y% Lu3+ nanocrystals (y= 0, 10, 25, and 40) with varying sizes comparable to NaYF4: 4% Tm3+, 20~60% Yb3+ nanocrystals. To closely mimic the effect of Yb3+ dopants on NaYF4 nanocrystals, an optically inert Lu3+ ion (r=0.848Å) was selected in the ACS Paragon Plus Environment

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design, where 20% Yb3+ + 10% Lu3+, 20%Yb3+ + 25% Lu3+, 20% Yb3+ + 40% Lu3+ correspond to 30% Yb3+, 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

45% Yb3+, and 60% Yb3+, respectively. As Y3+ ions in NaYF4 nanocrystals are substituted with the smaller ionic radii dopants, such as heavy Yb3+ and Lu3+ ions, larger nanocrystals tend to be formed owing to the higher crystal growth rate (Figure 1k)18. Thus, the additional doping of Lu3+ ions in NaYF4 nanocrystals can generate different sized nanocrystals, under identical reaction conditions as doping with Yb3+ ions did. Moreover, the similarity in ionic radii between Yb3+ and Lu3+ ensures a negligible effect of local crystallization distortion on upconversion intensity. As shown in TEM images Figure 1d-f, the additional doping of Lu3+ ions at 10%, to 25%, and 40% results in the crystal size increasing from ~24.6 nm (NaYF4: 4% Tm3+, 20% Yb3+ nanocrystals) to ~28.9 nm, ~39.0 nm, and ~56.0 nm, respectively (Figure 1d-f). The crystal size distribution histograms are shown in Figure S2d-f. Interestingly, when 60% Lu3+ is used much larger nanocrystals are obtained (~148.4 nm) as shown in the Figure S4a-c. Importantly, smaller Lu3+ ions than Yb3+ ions leads to slightly larger hexagonal phase nanocrystals as indicated in TEM images and XRD analysis due to the reduced dipole polarizability of Lu3+ ions, which is convinced by the increased diffraction peak shift in XRD as a result of the decrease in unit-cell volume (Figure S5). Similarly, the doping bigger lanthanide ion (Tb3+ ions with ionic radius (r=0.923Å)) induces the formation of smaller nanocrystals. Figure S6 shows TEM images of the NaYF4: 4%Tm3+, z%Tb3+ in which Tb3+ concentration increases from 20% to 60%. Clearly the size of nanocrystals decreases from 16.4 nm, 12.7 nm, 10.6 nm and 8.9 nm. Figure S7a-d and Figure S8 further confirm the size measurement results and the XRD patterns for the NaYF4: z%Tb3+, 4%Tm3+ (z=20, 30, and 45) nanocrystals. The XRD peaks shifted to lower angle suggesting larger unit cell volume due to the substitution of larger Tb3+ ions in Y3+ ion positions. In contrast to the upconversion intensity of NaYF4: 4%Tm3+, 60%Yb3+ nanocrystals, the ~56.0 nm NaYF4: 4% Tm3+, 20% Yb3+, 40% Lu3+nanocrystals induced only 7.2 times enhancement at 800 nm (Figure S4d). Since Lu3+ ions do not participate in upconversion process, this enhancement contribution is attributed to the increased crystal size. In combination of the optical observation of NaYF4: 4% Tm3+, 60% Yb3+ (20% + 40%), the enhancement factor was more than 36 times compared to the UCNCs with a conventional doping concentration of 20% Yb3+ ions (Figure S1f). The comparison can be clearly seen in Figure 1g-i. Optimizing the sensitizer concentration plays a greater part in amplifying upconversion luminescence than increasing the crystal size. The increase of its concentration does not induce cross-relaxation or concentration quenching effect if there are no other quenchers nearby, therefore an increase in Yb3+ concentration in principle should benefit the energy sensitization by more Yb3+ ions and contribute to the overall brightness of UCNCs.

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Figure 2. Confocal microscopy quantitative measurement of whole spectrum luminescence emission of single NaYF4: x% Yb3+, 4% Tm3+ UCNCs with different Yb3+ concentration: (a) x=20, (b) x=30, (c) x=45, (d) x=60, respectively. (e) The emission intensity of single UCNCs vs. Yb3+ concentration. (f) The whole spectrum emission of NaYF4: 60% Yb3+, 4% Tm3+ UCNC characterized using the Scanning Confocal Microscopy.

The uniformity of the emission intensity for each individual nanocrystal from the same batch confirms the intensity and mono-dispersity of each synthesis of single nanocrystals. We measured the brightness of each single UCNC using a purpose-built confocal microscope (the system layout is shown in Figure S9)45, The single particle’s brightness is analyzed from confocal scanning image, where each of particles present a Gaussian spot. The maximum pixel value for each of the Gaussian spots is used to represent that particle’s brightness. Their quantitative intensity images are shown in Figure 2a-d. Figure 2e shows that intensity increases with increased Yb3+ concentration. The intensity (average photon counts) of 60% Yb3+ UCNCs is almost 4 times stronger than the 20% Yb3+ doped UCNCs (14143 ± 2448 vs. 3516 ± 276). This enhancement factor is consistent with the results under low power conditions for the UCNC suspensions. Interestingly, high-excitation irradiance, by focusing laser power into a confocal spot, can pump upconversion emissions into a higher excited state, so that the blue emission is much stronger, as the luminescent emission spectrum of NaYF4: 60% Yb3+, 4% Tm3+ UCNCs shown in Figure 2f. To develop smaller, size-tunable Yb3+-rich UCNCs, we designed a sandwich nanostructure, as illustrated in Figure 3a. We employed the 60% concentration of Tb3+ ions to create a small (8.9 nm; shown in Figure 3b) core as the template core that allows epitaxial growth of NaYbF4: 4% Tm3+. Two core-shell UCNCs in the sizes of 11.9 nm and 23.3 nm are presented in Figure 3c and e, suggesting an active upconversion shell of NaYbF4: 4%Tm3+ in different thickness of 3.0 nm and 14.4 nm formed respectively. We normalized to the same Tm3+ concentration and the luminescent spectra of the bulk dispersion samples measured using the Fluorolog spectroscopy is shown in Figure 3g suggest that both core-shell UCNCs in different sizes display very weak emission. This can be attributed to the high concentration of Yb3+ ions on the relatively large surface of the UCNCs (Figure 3c and e) could induce large surface quenching. To solve this issue, a thin ACS Paragon Plus Environment

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layer (less than 2 nm) of NaYF4 inert shell is further coated to passivate the upconversion process. TEM images of the template core - active shell - inert shell UCNCs are shown in Figure 3d and f. This strategy has significantly enhanced the brightness of 13 nm and 25nm UCNCs, by 6.3 and 345 times respectively which measuring by Fluorolog spectrofluorometer.

Figure 3. (a) Diagram of the sandwich nanostructure. TEM images of (b) template core nanocrystals of NaYF4: 60% Tb3+, 4% Tm3+ with the size of 8.9 nm,(c) template core @ active shell of NaYbF4: 4% Tm3+ with size of 11.9 nm, (d) template core @ active shell @ inert shell of NaYF4 nanostructure with the size of 13.6nm, (e) template core @ active shell with the size of 23.3 nm, (f) template core @ active shell @ inert shell with the size of 25.2nm; (g) Luminescent emission spectra of samples (c-f) measured using Fluorolog spectrofluorometer.

According to our previous report45, the 13.0 nm (Figure S10) and 24.6 nm UCNCs of NaYF4: 20% Yb3+, 4% Tm3+ were synthesized by carefully controlling the heating up rate to obtain efficient UCNCs with minimized interior defects. These two UCNCs were used here as comparison to quantify the enhancement factor of the brightness of heterogeneous Yb3+-rich UCNCs. The emission spectra using Fluorolog spectrophotometer in Figure S11 further show a similar level of brightness for both the 13.0 nm monolithic nanocrystals and 13.6 nm core-shell-shell nanocrystals. This suggests the overall volume of the active layer (1.5 nm thick NaYbF4) of the heterogeneous UCNCs is far less than the monolithic ones. When increasing the thickness of the NaYbF4 active layer (14 nm), the 25 nm heterogeneous UCNCs exhibits 1.85 times higher emission intensity than the monolithic UCNCs in the size of 24.6 nm. ACS Paragon Plus Environment

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To further quantify the upconversion brightness for microscopy applications, single UCNCs are imaged 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and shown in Figure 4a-f. Figure 4a exhibits quite low emission intensity (~354 counts) for the template core @ active shell nanostructure (11.9 nm in size), but the sandwich UCNCs (Figure 4b, 13.6 nm in size) show strong emission intensity (~1081 counts), higher than the monolith nanocrystals (Figure 4c, 13.0 nm; ~795 counts). The nanocrystals with thicker NaYbF4 shell but without inert passivation shell also exhibit relatively weak emission (Figure 4d, ~1046 counts), compared to the passivated UCNCs (Figure 4e, ~8386 counts) that are 2.4 times brighter than the monolithic UCNCs (Figure 4f, ~3516 counts), which shown in Figure 4g. Figure 4h-k shows the single emission images in blue (475 nm) of the sandwich heterogeneous UCNCs and the monolithic UCNCs. Figure 4l shows that the blue emission intensity of the sandwich UCNC nanostructures in the size of 13.6 nm and 25.2 nm are 1.36 times and 3.78 times higher than that of the monolithic UCNCs in the similar sizes, which shown in Figure 4l. These results measured for single UCNCs using the Scanning Confocal Microscopy are consistent with the measurements for the UCNCs in the suspension using Fluorolog spectrometer. To investigate whether the Tb-containing template core may quench the active NaYbF4 host, small NaYF4: 20% Gd3+, 4%Tm3+ UCNCs (13.5 nm) are first synthesized as the template core. The same thickness of active shell (NaYbF4: 4%Tm3+; 3.4 nm) and the inert shell (NaYF4; 1.6 nm thick) are used for comparison experiment. Figure S12 shows the TEM images of template core, core-active shell and template core - active shell - inert shell nanocrystals, with the sizes of 13.5 nm, 20.2 nm and 23.3 nm, respectively. Shown in Figure S13, the brightness of the single template core - active shell nanocrystals and the sandwich structured UCNCs has increased from 891 to 3730 counts. The similar sized monolithic UCNC (24.6 nm) shows emission intensity at 3516 counts. Compared to the Tb3+-doped template cores, shown in Figure S14, the emission intensity increases with the volume of the active shell, i.e. the total number of Yb3+ ions in the active shell. This experiment confirms the Tb3+-rich template core has negligible influence on the upconversion emission of the NaYbF4 active shell, as the size (volume) of the Tb3+ doped core nanocrystal (8.9 nm) is comparably small.

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Figure 4. Confocal microscopy quantitative measurement of the whole spectrum emission for single UCNCs: (a) UCNCs of template core @ active shell of NaYbF4: 4% Tm3+ with the size of 11.9 nm, (b) UCNCs of template core @ active shell @ inert shell of NaYF4 with the size of 13.6 nm, (c) monolithic UCNCs of NaYF4: 20% Yb3+, 4% Tm3+ in size of 13.0 nm, (d) UCNCs of template core @ active shell with the size of 23.2 nm, (e) UCNCs of template core @ active shell of NaYbF4: 4% Tm3+ @ inert shell of NaYF4 with the size of 25.2 nm, (f) monolithic UCNCs of NaYF4: 20% Yb3+, 4% Tm3+ in size of 24.6 nm, (g) quantitative analysis of the whole spectrum emission intensities for single UCNCs corresponding to (a-f). Confocal microscopy quantitative measurement for the blue emission of single UCNCs: (h) UCNCs of template core @ active shell of NaYbF4: 4% Tm3+@ inert shell of NaYF4 with the size of 13.6nm, (i) monolithic UCNCs of NaYF4: 20% Yb3+, 4% Tm3+in size of 13.0 nm, (j) UCNCs of template core @ active shell of NaYbF4: 4% Tm3+ @ inert shell of NaYF4 with the size of 25.2 nm, (k) monolithic UCNCs of NaYF4: 20% Yb3+, 4% Tm3+ in size of 24.6 nm, (l) quantitative analysis of the blue spectrum emission intensities for single UCNCs corresponding to (h-k).

In conclusion, we find the optimal sensitizer concentration in single UCNCs is not limited by the ‘concentration quenching’ effect as long as surface defects and interior defects can be minimized. Maximizing Yb3+ concentration to directly dope Tm3+ activator ions into the pure NaYbF4 crystal host without the inert Y3+ ions can easily improve the brightness of UCNCs. Due to the smaller ionic radius and stronger electro negativity, the Yb3+ ions, having stronger binding energy to the F- ions, result in the formation of larger nanocrystals. The high concentration of Yb3+ ions on the nanocrystal surface could significantly quench the overall brightness. To overcome both issues in development of NaYbF4 based UCNCs, a template core can be used to allow epitaxial growth of thickness-tunable active layer of NaYbF4: 4% Tm3+ crystal and an inert shell of NaYF4 can be further coated to passivate the quenchers on the surface. Future work by minimizing the size of the template core will further improve the ratio of active NaYbF4 to the overall volume of the heterogeneous nanostructure towards the brighter and smaller UCNCs. ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting information

Supporting Information Available: Detailed experimental section, size distribution histograms, X-ray diffraction patterns, Transmission electronic microscopy images, Fluorolog spectrophotometer measurement, as well as the Schematic illustration of the system setup for customized scanning confocal microscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

*Email: [email protected] *Email: [email protected] *Email: [email protected]

ACKNOWLEDGMENTS We thank the Microscope Unit at Macquarie University for the TEM characterization. This project is primarily supported by the Australian Research Council (ARC) Future Fellowship Scheme (FT 130100517; Dayong Jin), Macquarie University Research Fellowship Scheme (Xiaoxue Xu), China Scholarship Council CSC scholarships (Chenshuo Ma: No.201408530049), Australian Government Research Training Program Scholarship.

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