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Scalable Direct-Writing of Lanthanide-Doped KMnF3 Perovskite Nanowires into Aligned Arrays with Polarized Upconversion Emission Shuo Shi, Ling-Dong Sun, Ying-Xian Xue, Hao Dong, Ke Wu, Shi-Chen Guo, Botao Wu, and Chun-Hua Yan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b00396 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018

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Scalable Direct-Writing of Lanthanide-Doped KMnF3 Perovskite Nanowires into Aligned Arrays with Polarized Upconversion Emission Shuo Shi,a Ling-Dong Sun,*,a Ying-Xian Xue,b,c Hao Dong,a Ke Wu,a Shi-Chen Guo,a Bo-Tao Wu*,b and Chun-Hua Yan*,a a

Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth

Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China b

State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai

200062, China c

Department of Basic Courses, Shanxi Institute of Energy, Jinzhong 030600, China

KEYWORDS: nanowires, oriented attachment, nanogel, assembly, polarized upconversion emission


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ABSTRACT

One-dimensional nano/micro-structured semiconductor and lanthanide materials are attractive for polarized light emission studies. Upconversion emission from single nanorod or anisotropic nanoparticle has also been discussed with a degree of polarization. However, microscale arrays of nanoparticles, especially well-aligned one-dimensional nanostructures as well as their upconversion polarization characters have not been investigated yet. Herein, we present a novel and facile paradigm for preparing highly-aligned arrays of lanthanide-doped KMnF3 (KMnF3:Ln) perovskite nanowires, which are good candidates for polarized upconversion emission studies. These perovskite nanowires, with a width of ten nanometers and length of a few micrometers, are formed through oriented attachment of KMnF3:Ln nanocubes along the [001] direction. By employing KMnF3:Ln nanowire gel as nanoink, a direct-writing method is developed to get diverse types of aligned patterns from nanoscale to wafer scale. Upconversion emissions from the highly-aligned nanowire arrays are polarized along the array direction, with a polarization degree up to 60%. Taking advantage of microscopic nanowire arrays, these polarized upconversion emissions should offer them potential applications on light or information transportation.


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Upconversion emissions from lanthanide-doped nanocrystals are featured with excellent photostability, sharp emission bands, and long emission lifetimes, etc.1±3 By rationally engineering lanthanide energy transfer pathways, the resulting upconversion emission intensity,4,5 selectivity,6,7 and lifetime8,9 can be tailored easily. This endows the upconversion nanocrystals with extensive attention in research frontiers ranging from optical devices12,13 to biological theranostics.10,11 In contrast, only few studies focus on the polarized upconversion emission.14±16 It is expected that nanocrystals with one-dimensional (anisotropic) morphology can induce polarized luminescence output by affecting light propagation.17 And unique polarized upconversion emissions have been observed from anisotropic NaYF4:Ln nanorods14,15 and nanodisks16 at single nanocrystal or a few aggregates level. These important findings offer these anisotropic nanocrystals a convenient way in applications in barcodes, light and information transportation.18 By virtue of efficient coupling of optical, electronic, and magnetic properties, large-scale ordered assembly of nanocrystals is more attractive in applications. A few types of nanocrystals, including quantum dots,19 metal nanocrystals20 and their composites21 have been used as building blocks to investigate the assembly structure correlated properties. Nonetheless, assembly of upconversion nanocrystals, in particular the anisotropic ones, has not been reported. Therefore, it is greatly desirable to construct ordered assembly of anisotropic upconversion nanocrystals, and investigate the corresponding emission polarization behavior. Perovskite structured KMnF3 has been identified as one of the good supporters for lanthanide upconversion emission.22 And Mn2+ ions in the host lattice can mediate upconversion through energy transfer with Ln3+, to get spectrally pure red emissions than mixed one. In addition, anisotropic KMnF3 nanostructures, such as nanowires23, have been synthesized with great


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potential for ordered assembly. However, KMnF3:Ln tends to grow into three-dimensional isotropic nanocubes for its cubic structure as previously reported.24 Herein, we describe the first synthesis of ultrathin KMnF3:Ln (Ln = Yb, Er, Ho, Tm) nanowires, with a diameter of 10 nm and length up to a few micrometers, which are formed through oriented attachment of KMnF3:Ln nanocubes along the [001] direction. Single-band red emission is achieved from these nanowires with the help of energy transfer via Mn2+. A facile direct-writing approach is developed to get large-scale ordered nanowire assemblies, in which KMnF3:Ln nanowires are aligned along their longitudinal direction into bundles which can be scaled up to micrometers in width. Besides well-aligned one-dimensional assemblies, crossed or bended ones can also be formed via directly writing. These aligned nanowires showed a polarized upconversion emissions, with a degree up to 60%, benefit from the emission intrinsically propagated along the nanowires. Combining the advantage of unique upconversion properties and anisotropic feature, KMnF3:Ln nanowires are expected to be a kind of novel upconversion luminescence materials. As a proof-of-concept, KMnF3:Yb,Er (Yb: 9 mol%, Er: 2 mol%) nanowires were synthesized with a modified high-temperature coprecipitation method.25 A stoichiometric amount of ammonium fluoride and potassium hydroxide was added to the solution of metal-oleate complexes at room temperature to initiate the crystal growth. Subsequently, the reaction temperature was increased to 290 qC to facilitate the growth of the nanocrystals. Transmission electron microscopy (TEM) image (Figure 1a) shows that the as-synthesized KMnF3:Yb,Er are nanowires which are homogenous in width, with a diameter of ca. 10 nm. The product can be obtained with a high purity of nanowires as revealed by low magnified TEM image shown in Figure 1b, in which the nanowires are dominated with length up to a few micrometers. Highmagnified TEM (HRTEM) image (Figure 1c) shows the lattice fringes of 0.42 nm and 0.30 nm,


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which corresponds well to the (100) and (110) planes of cubic phased KMnF3, a perovskite structure. Fast Fourier Transform (FFT) pattern (Figure 1d) further confirms the cubic structure of the nanowires which typically grow along the [001] direction. From the X-ray diffraction (XRD) pattern (Figure 1e), relative strong diffractions from (100) and (200) can be identified significantly as compared to the standard one in the inset of Figure 1e. This deviated diffraction pattern also suggests the anisotropic KMnF3 nanowires along [001], which is well consistent with that of HRTEM image. Followed the same synthetic protocol, KMnF3:Yb,Ho and KMnF3:Yb,Tm nanowires are obtained (Figure S1), and they all grow along the [001] direction. Figure 1f depicts the upconversion emission spectra of KMnF3:Yb,Ln (Ln=Er, Tm, Ho) nanowires. It is significant that almost all of the spectra are dominated with a single band emission, with a band width of a few tens of nanometers, across a wide spectral range from visible to NIR. As can be seen from the upper panel in Figure 1f, spectrally-pure red emission at 660 nm is released from KMnF3:Yb,Er nanowires, which is distinct from simultaneous green and red emissions from Yb±Er pairs.26 This phenomenon is reported as the non-radiative energy transfer between Er3+ and Mn2+ which happened from 2H11/2, 4S3/2 states of Er3+ to 4T1 state of Mn2+, followed by energy back transfer to Er3+ (4F9/2) as shown in the inset of Figure 1f. This process greatly depletes transition directly happened from 2H9/2 and 4S3/2, and only transitions from 4F9/2:4I15/2 happened to give out red emissions. As shown in the energy diagram, the introduction of Mn2+ not only modifies the transition route, but also improves the transition selectivity from both green and red to pure red one. Similarly, almost single-band emissions can also be realized from KMnF3:Yb,Ho (Figure 1f middle panel) and KMnF3:Yb,Tm (Figure 1f lower panel) nanowires as red (Ho3+: 656 nm, 5F5:5I8) and NIR (Tm3+: 790 nm, 3H4:3H6) emissions dominated the spectra, respectively.


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Figure 1. TEM images (a, b), HRTEM image (c) and corresponding FFT pattern (d), and XRD pattern (e) of as-prepared ultrathin KMnF3:Yb,Er nanowires. The standard card in (e) is cubicKMnF3 (JCPDS 82-1334). (f) Upconversion emission spectra of KMnF3:Yb,Er/Ho/Tm nanowires under 980 nm excitation with the power density of 10 W/cm2. Insets of (f) show the simplified energy diagrams of Er3+/Ho3+/Tm3+ and Mn2+. To investigate the growth mechanism of the ultrathin KMnF3:Ln nanowires, their morphology and structure evolution at different reaction stages were monitored with TEM images. As shown in Figure 2a, only irregular small (ca. 10 nm) nanocubes could be observed at 220 qC. Strikingly, elongated nanocrystals via attachment of small nanocubes could be noticed in Figure 2b. The (100) facets of two neighbouring ones tended to fuse together into short rod, a nanowire embryo, along the [001] direction. In the meanwhile, a twin boundary was found during the growth of the nanowire. As can be seen in Figure 2c, (200) facets of the two nanocubes, (200)A and (200)B, were not parallel as can be clearly identified in the FFT pattern (inset of Figure 2c). This implied the nanocubes tended to attach for the further growth. According to previous reports,27 such


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oriented attachment happened via inter-particle interaction so as to reduce the overall surface energy. As the temperature increased to 250 qC (Figure 2d), short rods, ca. 10 nm in diameter and 100 nm in length, with a higher portion could be found. The nanorods lengthened further as the temperature increased to 290 qC (Figure 2e, f), while the diameter was almost invariant. With the attachment of the nanocubes into nanowires, the numbers of the nanocubes decreased while nanowires dominated. Based on the above observations, it is reasonable to deduce that the nanowires formed via oriented attachment of small nanocubes along the [001] direction, as the diameter maintained while fusing.

Figure 2. TEM (a, b, d±f) and HRTEM (c) images of nanocrystals obtained as the temperature increased during the reaction. (a) 200 qC, (b, c) 220 qC, (d) 250 qC, (e) 270 qC, (f) 290 qC. The inset of (c) is the FFT pattern of the region marked with red dotted frame. The solvent composition, especially the molar ratio of oleic acid (OA) to oleic amine (OM), plays an important role on the morphology evolution of KMnF3:Ln nanocrystals (Figure S2). An increased ratio of OA:OM to 3:1 (Figure S2a) yielded larger cubes and shorter rods. On the contrary, a decreased OA:OM ratio (1:3, Figure S2c) induced smaller nanoparticles. Compared


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with OM, OA has a stronger interaction ability to lanthanide ions which leads to a recrystallization tendency to make larger particles as shown in Figure S2a. While, OM can modulate the coordination ability of OA to lanthanide ions. The optimized OA to OM is 1:1 as most of the OA can be protonated (Figure S2b) to have a better regulation ability for the forming of small nanocubes and further oriented attachment growth into nanowires28. Moreover, the length of the nanowires reduced significantly with the replacement of manganese oleate with manganese acetate, which confirmed that OA ions play an important role on the formation of nanowires (Figure S3). In addition to the solvent composition, the amounts of the K+ and F also have effects on the morphology of KMnF3 nanocrystals (Figures S4, S5). Only appropriate amounts of K+ and F are favored for the formation the KMnF3 nanowires. The morphology and the width of the nanowires are maintained with lanthanide doping (Figure S6), but the length is significantly shortened with the increased doping concentration. Compare with KMnF3: 9%Yb3+, 1±2% Er3+ nanowires, the length of KMnF3: 9%Yb3+, 4% Er3+ nanowires reduced to a few hundreds of nanometers. This may come from the dissimilar charged ions induced different dipole polarizability as disclosed in rare earth doping system.29 Spectral results (Figure S7) show that all of the KMnF3:Yb,Er (Yb: 9%, Er: 1 4%) nanowires display single-band red emissions, whose intensity decreases gradually with increasing Er3+ content. This should be attributed to quenching via energy transfer to surface quencher and multiphoton cross-relaxation between Er3+ ions.25 To shed more light on the effect of the nanowire on upconversion luminescence, the assynthesized ultrathin KMnF3:Yb,Er nanowires were used as building blocks for self-assembly and polarized photoluminescence studies. Inspired by the contact printing technique,30 which


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enabled the direct transfer and positioning of NWs from a donor substrate to a receiver chip by drag force, a novel and facile direct-writing assembly strategy was developed to obtain scalable highly-aligned arrays of KMnF3:Yb,Er nanowires. Via direct-writing, colloidal KMnF3:Ln nanowire was used as nanoink to be attached on the tip of a needle to draw directly on substrates (Figure 3a). In this approach, firstly, a Ti-PDGH QHHGOH ZLWK D GLDPHWHU RI

P ZDV GLSSHG LQWR

the colloidal solution of KMnF3 nanowires. With the evaporation of the solvent, the gelation proceed and the nanogel adhered physical on the tip. Next, we utilized such a pen to draw specific pattern on a silicon wafer with appropriate speed and pressure. With this process, the nanowires were deposited on the substrate and aligned along with the shear force. As shown in Figures 3b±d, S8 and S9, except with different dimensions, all of the nanowires were well aligned along [001], the long axis of the nanowires, into highly oriented arrays. This directwriting method can be achieved on various types of substrates without surface modification or post treatment, which is facile compared to contact printing technique. KMnF3 nanowires gel plays a crucial role in the formation of the highly-aligned arrays. The gelation occurs with solvent evaporation as a result of crosslinking of nanowires through van der Waals interactions between the alkyl group of the capped reagents.31 The nanogel has an adaptable adhesive ability on various substrates, which makes it easy to attach on the donor substrate (the needle) and translate to the receiver chip (typically silicon wafer). Importantly, such soft gel of KMnF3:Ln nanowires exhibits a tailored rheological response, which makes the alignment to be easily controlled in directions. On one hand, the gel can maintain high viscosity under strong shear force. On the other hand, it is stable enough with no obvious gel-sol transition.32 By controlling the writing speed, pressure and directions, the KMnF3:Ln nanowires can be deposited and aligned specifically via the shear force.


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A variety of drawing parameters, such as type, diameter of the needle tip and concentration of the KMnF3 nanowire dispersion, have crucial effects on the size, shape of the aligned arrays. By changing the diameter of the needle tip, the width of the resulting arrays of KMnF3:Ln nanowires can be tuned from ca

QP )LJXUH E WR

P )LJXUH F DQG

P )LJXUH G ZLWK D

length of tens of micrometers. The gel has a low adhesive ability on the Ti-needle, and it is inclined to form small sized assembly. In addition, needle made by silicone with low rigidity is also adapted for direct writing. Gel has a higher adhesive ability on the silicone, which is suitable to form wafer-scale nanowires film (Figure S10). Moreover, the concentration of nanowires, which can affect the viscosity of nanoink, is important in forming aligned arrays. At a low concentration (0.01 mmol/mL), the viscosity is not high enough to keep the nanowires aligned orderly (Figure S11a). With the concentration becomes higher (0.05 mmol/mL), a monolayer can be formed but the nanowires compacted loosely (Figure S11b). As the concentration further increases, multi-layered arrays (Figure S11c) and bundles (Figure S11d) are found as the nanowires compacted densely. The size of the arrays is scalable and easy to be controlled by changing the drawing parameters. The width, length and thickness of the arrays are correlated with the diameter and the moving range of the needle tip, and the concentration of nanowires, respectively. Arrays with specific width, length and thickness are achieved under appropriate conditions, as shown in Figures S12, S13. In addition, the gel is flexible in depicting complex patterns. Figure 3e shows a crossed array by repeatedly writing along different directions. It can be noticed that each array is bundle like and has a diameter of ca

P LQGLFDWLQJ D UHSURGXFLELOLW\ PHWKRG LQ JHWWLQJ QDQRZLUH DUUD\V

By using larger tips, the crossed pattern can be made in larger scale (Figure 3f), where the diameter of each bundle reaches to ca

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by controlling the writing direction facilely. From the magnified SEM image (Figure 3h) of the bending part, it is clear that the nanowires are uniformly aligned along the curvature. Moreover, the curvature radius can be tuned easily (Figure 3i). By depositing another straight array (Figure 3i), the curved assembly is not destroyed and the curvature is maintained, suggesting the robustness of the arrays obtained with direct-writing method.

Figure 3. (a) Illustration of nanowire arrays formed with direct-writing method. SEM images of the aligned arrays (b±d), crossed (e, f) and curved arrays (g±i) of KMnF3:Yb,Er nanowires formed with direct-writing method on Si substrate. With the highly-aligned arrays of KMnF3:Ln nanowires, the polarized upconversion emission was studied. A home-built laser scanning confocal optical microscope equipped with an avalanche photo diode (APD) and spectrometer was used to detect the upconversion emission,16 and a continuous-wave 980 nm laser was used as the excitation source, with an excitation density of ca. 104 W/cm2. The schematic diagram of the scanning confocal microscope system and detail optical measurement information is shown in Figure S14. As shown in Figure 4a, obvious upconversion luminescence can be detected from the disordered (1) and highly-aligned arrays (2±4) of KMnF3:Yb,Er nanowires, where the c axis of the nanowires is parallel to the horizontal plane. The polarization direction is zero degree as the excitation parallelized to the c axis of the


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nanowire array. Figure 4b displays the typical upconversion emission spectra of the array 4 in figure 4a as a function of polarized excitation angle from 0º to 360º. Three emission bands corresponding to 2H11/2:4I15/2 (525 nm), 4S3/2:4I15/2 (545 nm), and 4F9/2:4I15/2 (660 nm) transitions of Er3+ can be observed, which is different from the spectra in Figure 1f monitored at lower excitation density. This should be ascribed to the preferred energy transfer from Yb3+ to Er3+ at higher photon flux which induce largely increased occupation of the high energy levels of Er3+ as has been discussed in previous studies.31,33 Strikingly, the overall upconversion emission intensity and each transition intensity change periodically with the polarized angle of the excitation light (Figures 4b, S15, S16). This can also be clearly observed from the diagram (Figure 4c) and polar plot (Figure 4d) of overall integrated emission intensity as a function of the excitation polarization angle. Although transitions within 4f shell are strictly forbidden, they are observed for the fact that the interaction with crystal field can mix states of different parities. The electric transitions happened between even parities are sensitive to electric component of the excitation light field. And the observed visible transitions from Er3+ are all sensitive to the excitation light polarization as shown in Figure S17. With an excitation polarization angle of 0º, the UC emission is the most intensive, while the angle turning into 90º, the UC emission becomes the weakest. The polarization degree, as depicted with ! = (Imax

Imin) / (Imax + Imin),

approaches to 0.6. In addition, there is no obvious difference in emission patterns and emission selectivity as the excitation polarization changed from parallel to perpendicular configurations (Figure S17). Compared with the aligned nanowires with significant polarization behavior, no obvious polarization was observed for disordered aggregates. As shown in the red line in Figure 4d, only a polarization degree of ca. 0.05 was determined for disordered aggregates in Figure 4a.


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We also investigated the effect of the nanowire array dimension on the polarization behavior. Three aligned arrays with different sizes (Figure 4a) are studied. As the diameter of the laser spot is only ca. 500 nm, which is smaller than the width of the arrays, the difference in polarization behavior should come from the thickness of the arrays. As shown in Figure 4d, the polarization degree is reduced from 0.60, 0.44 to 0.28 as the array thickness increased from 30 QP

P WR

P )LJXUH 6 8).

Figure 4. (a) Upconversion emission image of the disordered (1) and highly-aligned arrays (2±4) of KMnF3:Yb,Er nanowires under 980 nm excitation. (b) Typical polarized excitation angle dependent upconversion emission spectra of the nanowire array. The laser spot is marked with crossed lines with a diameter of ca. 500 nm, and the excitation power density of 3.5 x 104 W/cm2. Inset represents typical polarizations of the electric field to the nanowire arrays that gives distinct spectra. Overall upconversion emission intensity (c) and corresponding polar plots (d) of Er3+ as a function of the polarized angle of the disordered and highly-aligned arrays in (a).


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In conclusion, we describe the first synthesis of ultrathin perovskite-structured KMnF3:Ln nanowires with high aspect ratio, ca. 10 nm in diameter and a few micrometers in length, and unravel the oriented attachment growth mechanism of the cubic structured nanowires. With a novel and facile assembly strategy, highly-aligned arrays of the KMnF3:Ln nanowires are obtained with direct-writing approach. The arrays can be controlled intently into patterns and curved assemblies. Moreover, the direct-writing approach is scalable. In addition, we for the first time investigated the polarized upconversion emissions of the highly-aligned arrays of KMnF3:Ln nanowires, with a polarization degree of 0.6 along the array direction. These important findings should give inspirations to the synthesis of novel one-dimensional nanostructures and the potential applications of lanthanide-doped upconversion nanomaterials for modern photonic and display devices.

ASSOCIATED CONTENT Supporting Information Experimental section; TEM and upconversion emission characterizations of KMnF3:Yb,Er nanowires; SEM, AFM and polarized upconversion emission characterizations of nanowire arrays.

AUTHOR INFORMATION Corresponding Author * Email: [email protected], [email protected], [email protected]


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Notes The authors declare no competing finical interest. ACKNOWLEDGMENT This work was supported by NSFC (Nos. 21425101, 21331001, 21590791, 21461162001, 21771005) and MOST of China (2014CB643800, 2017YFA0205101). REFERENCES (1) Auzel, F. Chem. Rev. 2004, 104, 139-173. (2) Zhou, B.; Shi, B.; Jin, D.; Liu, X. Nat. Nanotechnol. 2015, 10, 924-936. (3) Dong, H.; Du, S. R.; Zheng, X. Y.; Lyn, G. M.; Sun, L. D.; Li, L. D.; Zhang, Z.; Zhang, C.; Yan, C. H. Chem. Rev. 2015, 115, 10725-10815. (4) Wang, Y. F.; Liu, G. Y.; Sun, L. D.; Xiao, J. W.; Zhou, J. C.; Yan, C. H. ACS Nano 2013, 7, 7200-7206. (5) Chen, G. Y.; Liu, H. C.; Liang, H. J.; Somesfalean, G.; Zhang, Z. G. J. Phys. Chem. C 2008, 112, 12030-12036. (6) Dong, H.; Sun, L. D.; Yan, C. H. Nanoscale 2013, 5, 5703-5714. (7) Wei, W.; Zhang, Y.; Chen, R.; Goggi, J. L.; Ren, N.; Huang, L.; Bhakoo, K. K.; Sun, H. D.; Tan, T. T. Y. Chem. Mater. 2014, 26, 5183-5186. (8) Dong, H.; Sun, L. D.; Feng, W.; Gu, Y. Y.; Li, F. Y.; Yan, C. H. ACS Nano 2017, 11, 3289-3297. (9) Lu, Y. Q.; Zhao, J. B.; Zhang, J. B. Nat. Photonics 2014, 8, 32-36.


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(10) Eliseeva, 6 9 %•Q]OL - & * Chem. Soc. Rev. 2010, 39, 189-227. (11) Cheng, L.; Wang, C.; Liu, Z. Nanoscale 2013, 5, 23-37. (12) Hinklin, T. R.; Rand, S. C.; Laine, R. M. Adv. Mater. 2008, 20, 1270-1273. (13) Deng, R.; Qin, F.; Chen, R.; Huang, W.; Hong, M.; Liu, X. Nat. Nanotechnol. 2015, 10, 237-242. (14) Zhou, J. J.; Chen, G. X.; Wu, E.; Bi, G.; Wu, B. T.; Teng, Y.; Zhou, S. F.; Qiu, J. R. Nano Lett. 2013, 13, 2241-2246. (15) Paloma, S. R.; Lucia, L. P.; Wawrzynczyk, D.; Nyk, M.; Samoc, M.; Kar, A. K.; Mackenzie, M. D.; Paterson, L.; Jaque, D.; Patricia, H. G. Nanoscale 2016, 8, 300-308. (16) Chen, P.; Song, M.; Wu, E.; Wu, B.; Zhou, J.; Zeng, H.; Liu, X.; Qiu, J. Nanoscale 2015, 7, 6462-6466. (17) Wang, J.; Gudiksen, M. S.; Duan, X. F.; Cui, Y.; Lieber, C. M. Science 2001, 293, 14551457. (18) Lee, J.; Bisso, P. W.; Srinivas, R. L.; Kim, J. J.; Swiston, A. J.; Doyle, P. S. Nat. Mater. 2014, 13, 524-529. (19) Abecassis, B.; Tessier, M. D.; Davidson, P.; Dubertret, B. Nano Lett. 2014, 14, 710-715. (20) Kalsin, M.; Fialkowski, M.; Paszewski, M.; Smoukov, S. K.; Bishop, K. J.; Grzybowski, B. A. Science 2006, 312, 420-424. (21) Sun, Z.; Luo, Z.; Fang, J. ACS Nano 2010, 4, 1821-1828.


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