Controllable Assembly of Diverse Rare-Earth Nanocrystals via the

Jul 16, 2009 - in the steady increase of the surface pressure, whereas the affinity begins to manifest itself at the convex turning point. As the spre...
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Controllable Assembly of Diverse Rare-Earth Nanocrystals via the Langmuir-Blodgett Technique and the Underlying Size- and Symmetry-Dependent Assembly Kinetics Huan-Ping Zhou, Chao Zhang, and Chun-Hua Yan* 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, Peking University, Beijing 100871, China Received May 27, 2009. Revised Manuscript Received June 30, 2009 The Langmuir-Blodgett (LB) technique provides a facile and robust method for the formation of large-area films of various nanoparticles (NPs), including 24.9 nm NaYF4:Yb,Er nanospheres, 12.0 nm LiYF4 nanopolyhedra, 14.11.8 nm triagonal-shaped LaF3, 12.6 nm square CaF2, 9.52.0 nm hexagonal EuF3, and so forth. The assembly patterns of the deposited films were studied in accordance with the π-A isotherms. Combined with the TEM observations, several representative stages of assembly process can be distinguished. The scrutiny of the self-assembly process by means of their π-A isotherms elucidates that the concentration, size, and symmetry of nanoparticles play crucial roles in this process. The concept of “effective concentration”, which is defined as the amount of nanoblocks in the “gas phase” rather than the actual number of nanoparticles at the air-water interface, was first proposed as a control parameter to elucidate the possible assembly kinetics. The similarly shaped 12.0 nm LiYF4 and the 24.9 nm NaYF4:Yb,Er were selected as the size-dependent examples. The smaller nanoparticles show a strong tendency of congregation to lower the surface energy. Three representative samples, namely, 24.9 nm NaYF4:Yb,Er nanospheres (Oh), 14.11.8 nm oblate triagonal LaF3 nanosheets (D3h), and 41.3 nm  24.6 nm NaYF4 rods (D6h), were selected as the shape-dependent samples, which showed that the assembly patterns were contributed by the stability arising from the geometry of the nanoparticles, the tendency of aggregation of nanoparticles, and the probable rotation energy during the compression. More importantly, guided by the above assembly kinetics, for the 9.52.0 nm hexagonal EuF3, we can effectively acquire the desirable assembly pattern.

1. Introduction In recent years, the great success in the fabrication of nanocrystalline films and the significant triumph of the nanosuperstructure creation have provided prospectives in the field of nanomaterials with superior performance for the applications in advanced magnetic recording media, light-emitting devices, biological tags, catalysts, and solar cells.1-4 The colloidal nanocrystals, owing to their small size and high surface-to-volume ratio, as well as assembling propensity, exhibit distinct size and shape tunable physical and chemical properties. Recognized as “artificial atoms” accordingly, these nanocrystals are accepted and put forth as the building blocks in the construction of nanodevices.5-12 Therefore, it is imperative to assemble different colloidal *Corresponding author. Fax: þ86-10-6275-4179. E-mail: [email protected]. cn. (1) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (2) Coe, S.; Woo, W. K.; Bawendi, M.; Bulovic, V. Nature 2002, 420, 800–803. (3) Nam, J. M.; Thaxton, C. S.; Mirkin, C. A. Science 2003, 301, 1884–1886. (4) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425–2427. (5) Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190–195. (6) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821–823. (7) Hines, M. A.; Guyot-Sionnest, P. J. Phys. Chem. 1996, 100, 468–471. (8) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. Rev. Mater. Sci. 2000, 30, 545–610. (9) Rogach, A. L.; Talapin, D. V.; Shevchenko, E. V.; Kornowski, A.; Weller, H. Adv. Funct. Mater. 2002, 12, 653–664. (10) Wang, Z. L. Adv. Mater. 1998, 10, 13–30. (11) Doty, R. C.; Yu, H.; Shih, C. K.; Korgel, B. A. J. Phys. Chem. B 2001, 105, 8291–8296. (12) Talapin, D. V.; Shevchenko, E. V.; Murray, C. B.; Kornowski, A.; Forster, S.; Weller, H. J. Am. Chem. Soc. 2002, 126, 12984–12988.

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nanoparticles systematically into ordered superlattices, to raise the possibility of combining the properties of individual components and their collective properties arising from the interactions between the nanocrystals. To achieve such a target, several “bottom-up” assembly techniques, such as evaporation-induced self-assembly (EISA),13-15 molecular cross-linking,16-18 template-patterning,19-21 and Langmuir-Blodgett (LB) technique,22-27 have been established, in which building blocks such as atoms, molecules, or nanoparticles of diverse sizes and shapes are organized into applicable forms like superlattices, films, and devices. For example, O’Brien et al. (13) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335–1338. (14) Shevchenko, E. V.; Talapin, D. V.; O’Brien, S.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 8741–8747. (15) Shevchenko, E. V.; Talapin, D. V.; Murray, C. B.; O’Brien, S. J. Am. Chem. Soc. 2006, 128, 3620–3637. (16) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690–1693. (17) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795– 797. (18) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (19) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389, 585–587. (20) Hoogenboom, J. P.; Retif, C.; de Bres, E.; de Boer, M. V.; van LangenSuurling, A. K.; Romijn, J.; van Blaaderen, A. Nano Lett. 2004, 4, 205–208. (21) Chowdhury, D.; Maoz, R.; Sagiv, J. Nano Lett. 2007, 7, 1770–1778. (22) Yang, P. D. Nature 2003, 425, 243–244. (23) Tao, A. R.; Huang, J. X.; Yang, P. D. Acc. Chem. Res. 2008, 41, 1662–1673. (24) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978–1981. (25) Kim, F.; Kwan, S.; Akana, J.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 4360–4361. (26) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255–1259. (27) Aleksandrovic, V.; Greshnykh, D.; Randjelovic, I.; Fromsdorf, A.; Kornowski, A.; Roth, S. V.; Klinke, C.; Weller, H. ACS Nano 2008, 2, 1123–1130.

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constituted the superlattices with AB, AB2, AB3, AB4, AB5, AB6, and AB13 stoichiometries and obtained their arrangements of cubic, hexagonal, tetragonal, and orthorhombic symmetries by the EISA method.15 Meanwhile, the LB technique, originally developed for preparing monolayers of fatty acids and many other amphiphilic molecules, has also proved to be a most promising way in creating nanocrystal-based (nanodots, nanowires, and nanorods) monolayers over macroscopic dimensions, of designable array, and of tunable properties.22-27 Recently, high-quality rare-earth functional nanocrystals have drawn great attention due to their unique physical and chemical properties and potential applications in the fields of luminescence devices, biochemical probes, medical diagnostics, and so forth.28-37 By thermolysis of rare-earth complexes (including acetylacetonates, trifluoroacetates) in high-boiling-point solvents, our group has demonstrated a general synthetic route of dispersible rare-earth oxides, fluoride nanocrystals, and sodium rare-earth fluoride nanocrystals with self-assembly capability.33-37 However, previous reports on the self-assembly of these nanoparticles primarily utilized the EISA method, and large-scale assembly of colloidal nanoparticles for practical applications can be achieved only in a trial-and-error fashion by tuning the polarity of the dispersant.33-37 Therefore, developing more rational and practical techniques toward general control over the assembly processes of high-quality rare-earth nanoparticles still remains a great challenge. In this article, the LB technique is employed as a powerful tool for the fabrication of the nanocrystal assemblies on a large scale with the as-obtained NPs, which are capped by oleic acid and thereby show fine dispersibility in apolar solvents (such as cyclohexane and chloroform) and poor dispersibility in polar solvents (such as water and ethanol). On the basis of the previous work in our group,33-37 we now concentrate on fabricating LB films of various nanocrystals with different sizes (9.5 to 24.9 nm) and shapes (spheres, triangles, hexagons, cubes, and rods), so as to systematically investigate the influence of these parameters on the assembly behaviors. The monolayers at the air-water interface at different compression stages are carefully transferred onto continuous carbon-coated copper grids or silicon wafers with the Langmuir-Schaeffer horizontal liftoff method, and the π-A isotherms are accordingly monitored. With TEM and SEM characterizations, the morphologies of monolayers at different surface pressures are investigated, and the optimized parameters for fabricating macroscopic continuous monolayers are determined. Assisted with the π-A isotherms, we can distinguish different compression stages and obtain various assembly patterns. We further put forward the concept of “effective concentration”, which is defined as the amount of the nanoblocks (including both individual nanoparticles and islands) rather than the actual number of nanoparticles at the air-water interface, to describe the assembly behaviors in a more convincing way. Then, we study the correlation between surface pressure (28) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121–124. (29) Cao, Y. C. J. Am. Chem. Soc. 2004, 126, 7456–7457. (30) Yu, T.; Joo, J.; Park, Y. I.; Hyeon, T. J. Am. Chem. Soc. 2006, 128, 1786– 1787. (31) Stouwdam, J. W.; van Veggel, F. C. J. M. Nano Lett. 2002, 2, 733–737. (32) Kompe, K.; Borchert, H.; Storz, J.; Lobo, A.; Adam, S.; Moller, T.; Haase, M. Angew. Chem., Int. Ed. 2003, 42, 5513–5516. (33) Si, R.; Zhang, Y. W.; You, L. P.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 3256–3260. (34) Zhang, Y. W.; Sun, X.; Si, R.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2005, 127, 3260–3261. (35) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426–6436. (36) Si, R.; Zhang, Y. W.; Zhou, H. P.; Sun, L. D.; Yan, C. H. Chem. Mater. 2007, 19, 18–27. (37) Sun, X.; Zhang, Y. W.; Du, Y. P.; Yan, Z. G.; Si, R.; You, L. P.; Yan, C. H. Chem.;Eur. J. 2007, 13, 2320–2332.

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and film morphology, aiming at proposing a kinetic mechanism to interpret the assembly behaviors. This underlying assembly kinetics may serve as a guidance principle for the construction of particular or more complicated assemblies consisting of binary or multiple components with interesting material properties.

2. Experimental Section 2.1. Chemicals. Oleic acid (OA; 90%, Alpha), oleylamine (OM; >80%, Acros), 1-octadecene (ODE; >90%, Acros), M(CF3COO) (M = Na, Li; >97%, Acros), absolute ethanol, hexane, and chloroform were used as received. RE(CF3COO)3 precursors were prepared by the literature method,38 using rareearth oxides (>99.95%) and trifluoroacetic acid (99%, Acros) as the starting materials. 2.2. Nanocrystal Synthesis. All the nanocrystals were synthesized using standard oxygen-free procedures. The synthetic procedure for the rare-earth nanocrystals (LaF3, EuF3, NdF3, NaYF4:Yb,Er, LiYF4, CaF2) was the same as the literature method.34,35,37,39 2.3. Fabrication of LB Films. The as-prepared nanocrystals were washed for several precipitation/dissolution cycles in ethanol/chloroform, respectively (10 mL of rare-earth nanoparticles dispersion was precipitated by adding 30 mL of ethanol, then isolated and redispersed in 1 mL of chloroform with sonication), to remove the impurities and excess OA, OM, and ODE. Monolayers of nanocrystals were formed by placing drops of rare-earth nanoparticle chloroform solution onto the ultrapure water subphase of a LB trough (Nima Technology, M611) at room temperature. The surface pressure was monitored with a Wilhelmy plate. After the chloroform was evaporated, the resulting surface layer was compressed by moving the mobile barrier such that the film surface area decreased at a rate of 15 cm2/min. At different compression stages, the rare-earth particle layers at the air-water interface were carefully transferred onto continuous carbon coated copper grids or Si wafers by the Langmuir-Schaeffer horizontal liftoff method. 2.4. Characterization. Samples for transmission electron microscopy (TEM) analysis were prepared by drying the amorphous carbon-coated copper grids. Particle sizes and shapes were examined by a TEM (200CX, JEOL, Japan) operated at 160 kV. High-resolution TEM (HRTEM) characterization was performed with a Philips Tecnai F30 FEG-TEM operated at 300 kV. SEM images were obtained with a DB-235 focused ion beam (FIB) system.

3. Results and Discussion 3.1. LB Monolayers of Rare-Earth Compounds. The LB technique acts as a robust and versatile method, facilitating the formation of ordered 2D arrays with nanoscale building blocks (such as quantum dots, nanorods, and nanowires) on different substrates.22-27 Recently, this method has been successfully employed to fabricate dense LB monolayers of cobalt-platinum nanoparticles for electrical property studies27 and the avidincoated quantum dot films for the research of fluorescence resonance energy transfer process.40 In this work, we prepared monolayer films of shape and size-tunable monodisperse rareearth nanocrystals on carbon coated copper grids or Si wafers with a LB trough. In particular, assisted by the π-A isotherms, we carefully described the assembly patterns in terms of the effects of crystallite shape and size on film formation. (38) Roberts, J. E. J. Am. Chem. Soc. 1961, 83, 1087–1088. (39) Du, Y. P.; Sun, X.; Zhang, Y. W.; Yan, Z. G.; Sun, L. D.; Yan, C. H. Cryst. Growth Des. 2009, 9, 2013–2019. (40) Gole, A.; Jana, N. R.; Selvan, S. T.; Ying, J. Y. Langmuir 2008, 24, 8181– 8186.

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Figure 1. TEM images of LB-fabricated monolayers of the nanocrystals, formed under optimized surface pressures: (a) NaYF4:Yb,Er, 43.1 mN/m; (b) LiYF4, 33.2 mN/m; (c) LaF3, 26.5 mN/m; (d) CaF2, 34.5 mN/m; (e) EuF3, 23.0 mN/m; and (f) SEM images of the LB monolayer of NaYF4:Yb,Er, 43.1 mN/m.

To acquire the ordered assembly of particles, a series of parameters were controlled precisely, such as the washing times, the solution concentration, and the spreading amount. After the rare-earth nanocrystals were spread on the water surface and compressed by the mobile barrier, the pressure steadily increased due to the reduced available surface area. On the basis of surface pressure versus surface area isotherms recorded, an optimized pressure was selected in each system for the film deposition on a wafer. At the selected surface pressure, nanoparticles were closely packed, while further compression would probably result in the collapse of the monolayer. TEM images of well-ordered nanoparticle films deposited onto carbon coated copper grids are shown in Figure 1. The 24.9 nm NaYF4:Yb,Er nanoparticles as shown in Figure 1a display a longrange ordered texture, which is similar to that of the LB film of 12.0 nm LiYF4 nanoparticles (Figure 1b). However, the 14.11.8 nm triagonal-shaped LaF3 (Figure 1c), 12.6 nm square CaF2 (Figure 1d), and 9.5  2.0 nm hexagonal EuF3 (Figure 1e) constitute continuous monolayers of nanoparticles without long-range periodicity, which are different from that of the 12916 DOI: 10.1021/la9018986

spherical NaYF4:Yb,Er or LiYF4 nanoparticles. The different patterns of these films may be ascribed to the different symmetries; that is, high symmetry of the nanoparticles is in favor of the formation of long-range ordered structures, while the less ordered textures would be obtained from the nanocrystals with the low symmetries, which will be discussed in the following paragraphs. In addition, SEM is also employed to confirm that the asfabricated films are continuous over large scales. For example, a SEM image revealing the morphology of a monolayer covering an area of 33 μm comprised of NaYF4:Yb,Er nanoparticles is shown in Figure 1f. 3.2. Formation Process of LB Films. Herein, we take the 24.9 nm NaYF4:Yb,Er nanospheres as an example to elucidate the formation process of LB films. Figure 2 shows surface pressure versus area (π-A) isotherm of the chosen nanocrystals (ca. 0.04 mg/mL, 400 μL) and the TEM images of LB films formed at several representative stages during the compression process. At low surface pressure (11.9 mN/m), the nanoparticles are packed into islands connected randomly with plenty of void observable (Figure 2b). The nanoparticles in each individual Langmuir 2009, 25(22), 12914–12925

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Figure 2. (a) Plot of the surface pressure as a function of surface area of NaYF4:Yb,Er LB films. (b-f) TEM images of the LB films of NaYF4:Yb,Er nanocrystals, formed under different surface pressures: (b) 11.9 mN/m; (c) 18.1 mN/m; (d) 24.8 mN/m; (e) 31.5 mN/m; and (f) 40.7 mN/m.

island are highly ordered, forming a hexagonal close-packed (hcp) structure. This ordered structure is very different from that of the LB films made of polydisperse particles, most probably due to the narrow size distribution of the nanoparticles.41 Upon compression, these islands gradually merge into larger ones with less void (Figure 2c-e), where a number of defects are found inside the larger islands, indicating that the larger individual islands are built up by the primeval islands. A continuous monolayer is formed with long-range ordered structures as seen in Figure 2f when the surface pressure reaches 40.7 mN/m. The morphology of the long-range ordered film may provide evidence for what Heath et al. have reported, that the ratio of the size of the passivating ligand and the particle plays important roles in determining the 2D structure.42 Unlike the LB films constructed by the PVP-capped noble metal, the shorter chain of oleic acid (compared with the PVP) and the (41) Sastry, M.; Mayya, K. S.; Patil, V.; Paranjape, D. V.; Hegde, S. G. J. Phys. Chem. B 1997, 101, 4954–4958. (42) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189– 197.

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larger particles (24.9 nm) (compared with 10 nm Pt) provide the possibility for the formation of the ordered NaYF4:Yb,Er structure.43 This assembly process was monitored in situ via the π-A isotherm by a Wilhelmy plate (Figure 2a). The presented π-A isotherm is constituted of two stages during the entire compression. The first stage is a steady pressure increase in the surface area ranging from 250 to 80 cm2, and the second is a steep jump after 80 cm2. According to the TEM images, the convex section in the isotherm (from 250 to 80 cm2) indicates the mergence of the islands with space release. Generally speaking, the more space that is released in the island mergence, the more obvious the convex section is, which will be discussed in the following paragraph. When the surface area is less than 80 cm2, there is not sufficient space to be released, resulting in the sudden increase of surface pressure, hence, forming a compact monolayer, which according to LB theories (43) Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.; Yang, P. D. J. Phys. Chem. B 2005, 109, 188–193.

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Figure 3. (a) Plots of surface pressure as a function of surface area for NaYF4:Yb,Er LB films, on condition of different spreading amounts of NaYF4:Yb,Er, 100, 200, 400, 600, and 800 μL, respectively. (b-e) TEM images of the LB films of NaYF4 nanocrystals, formed under different surface pressures and spreading amounts: (b) 0 mN/m, 100 μL; (c) 0 mN/m, 200 μL; (d) 16.6 mN/m, 600 μL; and (e) 17.0 mN/m, 800 μL.

can be regarded as the 2D “solid stage” of the nanoparticles arrangement.44 3.3. Prerequisite for Investigation of the Assembly Kinetics of the Diverse Nanoparticles;Availability of “Effective Concentration” as a Control Parameter. The LB technique is a room temperature deposition process, which was initially used to fabricate monolayer and multilayer films of amphiphilic organic materials.45 The typical description of the compression process is the π-A isotherm which is presented in the normal way (i.e., surface pressure versus area per molecule), because these molecules possess detailed information about the precise molecular weight. By manipulating hydrophobic nanocrystals on the nanometer scale, intriguing superlattice architectures can be constructed through the aggregation of the (44) Lee, S. Y.; Kang, D. Y.; Kim, T. W. Proceedings of the 5th International Conference on Properties and Applications of Dielectric Materials; 1997, 1, 654657. (45) Ikeura, Y.; Kurihara, K.; Kunitake, T. J. Am. Chem. Soc. 1991, 113, 7342– 7350.

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nanocrystals. The compression process of each nanoparticle species, regardless of their morphologies and sizes, can be recorded by the π-A isotherms, which provide the reliable and real-time information. Combined with TEM characterization, it is expected that the π-A isotherms (surface pressure versus surface area) would unveil the possible rules for the assembly process. However, the precise mass and concentration of the nanoparticles are not readily available, hence the π-A isotherm combined with its TEM images are not sufficient for investigating exactly the difference between the special nanocrystals, for the lack of a direct relationship among different π-A isotherms. In this work, the “effective concentration” rather than the exact concentration of the nanocrystals is proposed and acts as the control parameter to compare the assembly processes for the rare-earth compound nanocrystals with different morphologies and sizes. 3.3.1. Assembly Process of the NPs with Varied Spreading Amounts. In the current investigation, 24.9 nm NaYF4:Yb,Er is selected as the representative sample, and the corresponding π-A isotherms are shown in Figure 3, with different spreading Langmuir 2009, 25(22), 12914–12925

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amounts, 100, 200, 400, 600, and 800 μL under the same concentration (ca. 0.04 mg/mL). With decreasing the surface area, the surface pressure steadily increases and finally bears an obvious rise. The slope of these isotherms becomes steeper with the spreading amount increasing. Specifically, the surface pressures of the 100 and 200 μL nanoparticles burst out at around 225 and 120 cm2 with the concomitant moderate climb, respectively, and increase to 32.0 and 33.8 mN/m in turn after the final jump. For the 400, 600, and 800 μL counterparts, the initial surface pressure reaches 11.9, 16.6, and 17.0 mN/m instead of the 0 outset, and 40.7, 46.7, and 50.0 mN/m finally, showing similar profiles as those of the 100 and 200 μL nanoparticles but with a more rapid increase of the surface value at the final stages. The TEM images of NaYF4:Yb,Er nanoparticles with the spreading amounts of 100, 200, 400, 600, and 800 μL, which were taken at the representative points, including the initial stages, the turning points, and the final stages, are shown in Figure 3 and Figures S1-S4 in Supporting Information. The surface coverage increases with decreasing the surface area, and the film of the nanocrystals consequently becomes more compact, finally resulting in the continuous monolayer (400 μL spreading amount) (Figure 2f) or even multilayer (600 and 800 μL spreading amount) (Figures S3d and S4d in Supporting Information). When the spreading amount of nanoparticles is low (100 and 200 μL), the nanoparticles self-assemble into islands with a hexagonal closepacked (hcp) structure (Figure 3b,c), which is similar to that of the 400 μL spreading amount mentioned above. The profiles of their isotherms are indeed dependent on their assembly behavior, and therein the interactions of the islands, induced mainly by the steric hindrance and hydrophobic affinity, are responsible for the variation of their slopes. The domination of the repulsion results in the steady increase of the surface pressure, whereas the affinity begins to manifest itself at the convex turning point. As the spreading amount is increased to 600 and 800 μL, despite the formation of the stripes at the initial stages instead of islands (Figure 3d,e), the assembly processes are analogous to those aforementioned, that the interconnection between the stripes are accompanied by closer nanoparticles packing. These TEM images (Figures S1-S4 in Supporting Information) taken at such stages, consistent with the discussion in part 3.1, indicate that the convex stage of the plot implies the space release resulting from the formation of larger islands, while a concave stage suggests that a large number of islands were crowded together jostling in a limited area along with the rapid increase of the surface pressure. The particles formed different patterns at various stages with different spreading amounts, even capable of forming some peculiar nanostructures varied from circles or networks to stripes by raising the NP concentration. This result shows the similarity with the work reported by Heath that metal nanoparticles (4-6 nm in diameter) form circles and linear chains at the airwater interface.46 3.3.2. Origination of and Availability of “Effective Concentration”. 3.3.2.1. Mathematical Analysis of π-A Isotherms. From the discussion mentioned above, it is evident that for a given system with fixed concentration, the spreading amount influences the assembly pattern. To elucidate the interaction of the nanoparticles of different spreading amounts, we divide the abscissa of the π-A isotherms of NaYF4:Yb,Er nanoparticles, namely, the surface area, by a number proportional to spreading amount, that is, the surface area of the 100, 200, 400, 600, and 800 μL isotherms will be divided by 1, 2, 4, 6, and 8, respectively (46) Gelbart, W. G.; Sear, R. P.; Heath, J. R.; Chaney, S. Faraday Discuss. 1999, 112, 299–307.

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Figure 4. Plots of surface pressure as a function of surface area under a series of spreading amounts of NaYF4:Yb,Er, 100, 200, 400, 600, and 800 μL, and the abscissa was divided by a value proportional to the spreading amount.

(Figure 4). Then, the number of the particles per unit surface area would hypothetically become the same, and the obtained π-A isotherms are supposed to superimpose on each other. However, the obtained isotherms show some differences. Within the regions of relative low surface pressures, these isotherms are well resolved. Specifically, with respect to a same surface area, the more solution is spread, the lower the surface pressure is. Along with the increase in surface pressure, these curves gradually converge, exhibiting a similar slope at the final stage. Thus, the factors which dominate the initial assembly patterns or processes become the linchpins to explain the difference of the π-A isotherms and the possible assembly kinetics. 3.3.2.2. “Gas-Liquid” Critical Area. To gain further insight into the π-A isotherm, it is necessary to compare the profiles of the as-divided plots to verify the assumption described in the former paragraph. The variation of the slope of the π-A isotherm can be described by dπ/dA. As will be discussed later, the dπ/dA has a nonzero value when the surface pressure bursts out from 0. With regard to the plots of 100 and 200 μL nanoparticles, the initial surface pressure is zero, implying the “gas-phase” where there is no interaction between the nanoparticles, reminiscent of the unrestricted organic molecules demonstrated by the LB technique.44 According to this model, the nanoparticles, as well as some islands (they can be called as nanoblocks, collectively), are assumed to move freely at this stage. The bursting-out areas (the area at which the surface pressure burst out from 0, that is, the dπ/dA reaches a nonzero value), are distinct for different spreading amounts, for example, 120 and 112 cm2 for 100 and 200 μL nanoparticles, respectively (Figure S5 in Supporting Information). We therefore infer in terms of the conventional LB model that the monolayer is in “liquid-phase” instead of “gasphase” as soon as the surface pressure starts to increase, and that accordingly the “bursting-out area” is equivalent to the critical point of the “phase-transition” between “gas” and “liquid”.44 However, for those π-A isotherms with high spreading amounts (higher than 400 μL), their surface pressures begin at nonzero values limited by our apparatus, and there is no “bursting-out” critical area observed. Therefore, the hypothetical “gas-liquid” critical area is postulated as the horizontal intercept obtained by extrapolation of the isotherms along their initial slopes. 3.3.2.3. “Effective Concentration”. We find that the sizes of the initial islands are somewhat different for the two spreading conditions (100 and 200 μL nanoparticles); the more spreading amount there is, the larger the islands grow (Figure 3b,c). The decrease of the “gas-liquid” critical area upon increased spreading amount is considered as the result of a greater degree of DOI: 10.1021/la9018986

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Figure 5. Plots of surface pressure as a function of trough area for a series of spreading amounts, 100, 200, 400, 600, and 800 μL, and the abscissa was divided by a value proportional to the spreading amounts: (a) LiYF4; (b) LaF3; (c) NaYF4:Yb,Er rods; and (d) EuF3.

particle aggregation, yielding larger islands, and whereby more space is released to retard the “phase-transition”. As a result, the absolute number of nanoparticles is not the most important determinant of the “phase-transition” point. We therefore employ the “effective concentration” instead of the absolute concentration to describe the assembly behaviors of the nanoblocks. It is defined as the amount of nanoblocks in the “gas phase” of the monolayer, which can be evaluated by measuring its bursting-out area. 3.3.2.4. Availability of “Effective Concentration”. By virtue of the above assumption, it is expected that the “effective concentration” can be fixed by ensuring the trough area at the same value when the surface pressure starts to increase, that is, the same “gas-liquid” critical area, to make a valid comparison of the different assembly patterns for various nanoparticles. In the present case, by carefully adjusting their original concentration, we can make sure that, for various species, including LiYF4, LaF3, EuF3, and NaYF4:Yb,Er rods, with a 100 μL spreading amount, the values of “gas-liquid” critical area are kept at 120 cm2, so as to ensure equal “effective concentration”. Under the fixed condition, we obtain the π-A isotherms of the as-selected species (Figure 5). It can be found that despite the chemical species of nanocrystals, the π-A isotherms have the similar tendency as the spreading amount is varied. That is, when the spreading amount increases, the “gas-liquid” critical area decreases. The decrease of the “gas-liquid” critical area, which is considered as the result of a greater degree of particle aggregation, shows the possible evidence for the availability of “effective concentration”. The profiles of these π-A isotherms are carefully examined in terms of their similarity and difference, in the hope of conjecturing the assembly kinetics for various nanocrystals. 3.4. Underlying Size- and Symmetry-Dependent Assembly Kinetics of the Diverse Nanoparticles. It is well-known that the LB technique is a robust and versatile method facilitating 12920 DOI: 10.1021/la9018986

the formation of long-range ordered 2-D arrays on different substrates with nanoscale building blocks.22-27 However, reports about the assembly kinetics based on comparison of the profiles of π-A isotherms with different building blocks remain limited. It is thus very important to describe the assembly processes of comparable species, which contributes to elucidating the general rules of 2D array behavior and explaining the morphologies of the 2D films assembled by other building blocks with certain size and shape. In the present work, we compare the assembly processes of nanocrystals with diverse sizes and shapes, wherein, we selected the 24.9 nm NaYF4:Yb,Er nanospheres (Oh) and 12.0 nm LiYF4 nanopolyhedra (Oh) to study size-dependent assembly behaviors; and NaYF4:Yb,Er nanospheres (Oh), triagonal-oblate LaF3 (D3h) nanosheets, as well as prolate NaYF4:Yb,Er (D6h) nanorods to investigate the shape-dependent assembly behaviors. 3.4.1. Assembly Behaviors of Diverse Species. With the spreading amount of nanoparticles set at 400 μL, the assembly behavior of each species is described, aided by the π-A isotherm combined with a series of TEM images taken at every representative stage during the assembly process. 3.4.1.1. LiYF4. The π-A isotherm of LiYF4 nanopolyhedra, similar with those of the NaYF4:Yb,Er nanospheres, exhibits three assembly stages with surface pressure increasing (Figure 6). The profiles of their isotherms arise from the balance of the steric hindrance and hydrophobic affinity of the nanoblocks, as the above discussion. However, at the first stage, the assembly behavior of the 12.0 nm LiYF4 nanopolyhedra, unlike the 24.9 nm NaYF4:Yb,Er nanoparticles, is inclined to assemble partially at random instead of forming isolated hexagonal closepacked (hcp) structure. 3.4.1.2. LaF3. The unique π-A isotherm, as well as the TEM images of triagonal and oblate shaped LaF3 (Figure 7), indicates that the particle assembly at the interface undergoes three stages: (i) local ordered aggregation and isolated islands formation, (ii) compact Langmuir 2009, 25(22), 12914–12925

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Figure 6. (a) Plot of the surface pressure as a function of trough area of LiYF4 LB films. (b-d) TEM images of the LB films of LiYF4 nanocrystals, formed under different surface pressure: (b) 7.5 mN/m; (c) 24.8 mN/m; (d) 36.4 mN/m.

Figure 7. (a) Plot of the surface pressure as a function of trough area of LaF3 LB films. (b-d) TEM images of the LB films of LaF3 nanocrystals, formed under different surface pressures: (b) 5.8 mN/m; (c) 20.1 mN/m; (d) 37.1 mN/m.

aggregation and primary islands merging into ordered monolayer on a large scale, and (iii) collapse of monolayer and multilayers observed. In combination with the TEM images, we confirm the pressure range between the convex (A = 170 cm2) and concave Langmuir 2009, 25(22), 12914–12925

(A=65 cm2) turning points of its isotherm as the optimal pressure range for the formation of the monolayer. Before the convex one no compact film can be found, and multilayers appear beyond the concave inflection. Moreover, it turns out that LaF3 nanosheets DOI: 10.1021/la9018986

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Figure 8. (a) Plot of the surface pressure as a function of trough area of NaYF4:Yb,Er nanorod LB films. (b-d) TEM images of the LB films of NaYF4:Yb,Er rods nanocrystals, formed under different surface pressures: (b) 6.8 mN/m; (c) 25.9 mN/m; and (d) 56.8 mN/m.

Figure 9. Plots of surface pressure as a function of trough area for the nanocrystals (a) NaYF4 and LiYF4 with the spreading amounts of 100, 400, and 800 μL; (b) NaYF4:Yb,Er, LaF3, NaYF4:Yb,Er rods with the spreading amount of 400 uL.

are inclined to arrange side by side under low surface pressure but to erect as face to face upon the appearance of multilayers. 3.4.1.3. NaYF4:Yb,Er Rods. From nanodots to other nanostructures, now the LB technique has proved equally powerful for the assembly of nanorods and nanowires with much larger aspect ratios: Yang et al. have reported a system of BaCrO4,25 Whang et al. of silicon nanowires,26 and Tao et al. of silver nanowires.47 However, the NaYF4:Yb,Er rods are of aspect ratios about 1.7:1, smaller than that of the nanorods and nanowires mentioned above, and thus show some peculiar characteristics in both the π-A isotherm and the TEM images. Figure 8 shows that these prolate particle assemblies at the interface experiences three special stages: (i) local aggregation and packing into isolated islands, (ii) islands mergence and multilayers and voids coexistent (47) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R. R.; Sun, Y. G.; Xia, Y. N.; Yang, P. D. Nano Lett. 2003, 3, 1229–1233.

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in the intermediate stage, and (iii) compact aggregation and producing a continuous mutlilayer at the interface. The turning points of these prolate rods π-A isotherm, alternating in concave (A=110 cm2) and convex (A=70 cm2), evince a distinct behavior in contrast to the oblate LaF3 nanosheets. The π-A isotherm of nanorods shows a reversed variation tendency of the turning points, probably resulting from the lower symmetry of the rods. In addition, unlike the nanoparticles discussed in the former paragraph, it is difficult to acquire a well-arranged LB monolayer for the rodlike NaYF4:Yb,Er. The experimental and theoretical investigations of nonspherical nanoparticles at the interface have demonstrated the increased instability of prolate nanoparticles over spherical and oblate particles, which is assigned to the line tension for the nanoparticle.48,49 The more oblate the (48) Faraudo, J.; Bresme, F. J. Chem. Phys. 2003, 118, 6518–6528. (49) Bresme, F.; Oettel, M. J. Phys.: Condens. Matter 2007, 19, 1–33.

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nanoparticles are, the smaller the line tension they suffer, and the more stable they are. In this scenario, the high line tension of prolate nanoparticles like the NaYF4:Yb,Er rods results in the instability of the nanoparticles at the interface and thus prevents the formation of continuous monolayer. Hence, it is difficult to find a critical point for the monolayer-to-multilayer transition. 3.4.2. Size and Shape Dependence of the Self-Assembly Behavior. To our knowledge, surface chemistry or surface charge for different nanocrystals undoubtedly plays important roles in the assembly process. To investigate the size and shape effects for the assembly process, we select the nanocrystals prepared under almost the same synthetic conditions and post-treatment to minimize the effects arising from surface chemistry of nanocrystals. Therefore, we consider that the as-selective nanocrystals have similar surface conditions, and thus the differences in their assembly behaviors would stem from the effects of their sizes and shapes. 3.4.2.1. Effect of Size. In this section, to illustrate the effects of size upon the self-assembly behavior, we take the 12.0 nm LiYF4 nanopolyhedra and the 24.9 nm NaYF4:Yb,Er nanospheres as the size dependent examples, for they are of the same symmetry (Oh). Three paramount factors, the “gas-liquid” critical area, the slope of each stage, the variation tendency of turning points, of their π-A isotherm series with 100, 200, 400, 600, and 800 μL spreading amounts are tested in turn for an explicit and feasible comparison. Surface Energy. Figure 9a illustrates the π-A isotherms series for LiYF4 and NaYF4:Yb,Er nanoparticles at the same “effective concentration”, respectively. As mentioned above, these two species possess similar assembly processes. However, it is found that when the nanoparticle size shrinks from 24.9 to 12.0 nm, the “gas-liquid” critical area undergoes more dramatic decline as spreading amount increases. This can be attributed to the stronger inclination of aggregation of smaller nanoparticles to attenuate the surface energy (Scheme 1). From the TEM images (Figure 2 and 6), at the initial compression stage, the islands of LiYF4 are somewhat larger than those of the NaYF4:Yb,Er nanoparticles, which agrees well with our hypothesis. This aggregation tendency of LiYF4 nanoparticles, in fact, remains throughout the entire assembly process. As a result, the slopes of the LiYF4 π-A isotherms at the second stage are lower than those of NaYF4:Yb,Er, also implying more space being released. Furthermore, it is noteworthy that these plots exhibit no distinct variation tendency with respect to the particle size, suggesting that nanopaticles with similar symmetry display analogous behavior in the whole assembly process. 3.4.2.2. Effect of Shape. Three representative samples, including NaYF4:Yb,Er nanospheres (Oh), oblate triagonal LaF3 nanosheets (D3h), and prolate NaYF4:Yb,Er rods (D6h), can be therefore picked out for investigation of the shape effect (Figure 9b). The π-A isotherms for different nanoparticles, divided into three stages as discussed above, show distinctive profiles, respectively, which are attributed to, presumably, the effect of shape rather than size. The “gas-liquid” critical area decreased in the following order: the 24.9 nm NaYF4:Yb,Er nanospheres, the 41.3 nm  24.6 nm NaYF4:Yb,Er rods, and finally, the 14.1 1.8 nm LaF3 nanosheets (Figure 9b). That is, from the large particles to the smaller ones, and from the sphere particles to the nonspherical ones. This can be considered as the result of particle aggregation, which not only depends on the sizes of the nanoparticles but also on their shapes. This is possibly because the nanoparticles of either oblate or prolate shape are more likely to aggregate than the spherical (or quasi-spherical) ones, in particular under low surface pressure and spreading Langmuir 2009, 25(22), 12914–12925

Article Scheme 1. Schematic Illustration of the Surface Energy of Nanoparticles with Different Sizes

Scheme 2. Schematic Illustration of π-A Isotherms for Three Types of Nanocrystals, Including Spherical, Oblate, and Prolate Nonspherical Particles

Scheme 3. Schematic Illustration of Rotation Potential for Nanocrystals with Different Symmetries

Scheme 4. Schematic Illustration of Stability and Line Tension of Different Nanocrystals: LaF3 Nanosheets, NaYF4:Yb,Er Spheres, and NaYF4:Yb,Er Rods

amount. Also, the profile of π-A isotherms and morphology of the LB film under higher surface pressure and spreading amount show shape dependent behavior, as discussed above. Rotation Energy. Three samples (NaYF4:Yb,Er nanospheres, NaYF4:Yb,Er nanorods, and the LaF3 nanosheets) with a 400 μL spreading amount were selected as examples (Scheme 2). DOI: 10.1021/la9018986

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Figure 10. (a) Plot of the surface pressure as a function of trough area of EuF3 LB films. (b-d) TEM images of the LB films of EuF3 nanocrystals, formed under different surface pressures: (b) 9.3 mN/m; (c) 22.3 mN/m; and (d) 45.5 mN/m.

The π-A isotherm of LaF3 (D3h) nanosheets shows two turning points and a plateau between them. The π-A isotherm of NaYF4: Yb,Er rods (D6h) displays an obvious concave turning point before the convex one, between which there is a steep jump. The profiles of these π-A isotherms are the characteristic of the nanoparticles with lower symmetry, while for the higher symmetry, such as NaYF4:Yb,Er nanospheres, the π-A isotherm was almost linear without obvious turning points. As mentioned above, the convex turning point evinces the formation of a large scale monolayer, along with the space release, while the appearance of the concave turning point can be attributed to the exhaustion of such released space, with the monolayer collapsing and folding into multilayers. Therefore, the assembly process of nanospheres resulting in linear profile of its π-A isotherm, probably can be attributed to inadequate space released to counteract the ascending surface pressure prior to the formation of the compact layer. The fundamental reasons for the different assembly behaviors were considered to be the great energy lowering when the smaller nonspherical particles aggregate, including both size-related surface energy (Scheme 1) and shape-related rotation energy (Scheme 3). As shown by Scheme 3, the shaperelated rotation energy can be explained by the tendency of formation of islands. We all know that the rotation and reversal for a spherical nanoparticle shows little restriction, for there is nearly no energy difference in all rotation directions, while for a nonspherical nanoparticle, the rotation and reversal needs to overcome certain rotation energy barriers. Therefore, the formation of islands effaces the possibility of rotation and reversal of nonspherical particles, while the highly symmetrical sphere particles would have less tendency for the formation of large islands. Therefore, the shape-related energy lowering during particle aggregation further affects the profile of the π-A isotherm. Line Tension. Although LaF3 nanosheets (D3h) and NaYF4: Yb,Er rods (D6h) exhibit similar symmetry, the π-A isotherms 12924 DOI: 10.1021/la9018986

and the corresponding TEM images indicate distinct assembly behaviors, which is indeed attributed to the particle geometry rather than merely the symmetry. For example, under a proper pressure, the oblate LaF3 nanosheets are apt to aggregate rapidly into a continent monolayer from the islands, severely tamping down the energy of individual nanoparticles and causing an obvious convex turning point in the π-A isotherm, whereas the case is reversed with respect to the prolate NaYF4:Yb,Er rods. Initial aggregation would help to lower the energy; however, when the aggregation exceeds a certain threshold, the energy seems increased with further aggregation. This is probably owing to the strengthened line tension of the elongated nanoparticles, as reported by Oettel et al.48,49 They assert that when particle size is in a certain range, oblate nanoparticles are the most stable at the air-water interface, the spherical ones are metastable, and the prolate particles would be unstable and detached from the interface (Scheme 4), which is consistent with the stability sequence predicted by their thermodynamic model. This theory explains that the present prolate NaYF4:Yb,Er rods become unstable with the surface pressure increasing, detaching from the water subphase, and puckering into multilayers. In addition, the profile of the π-A isotherm of these prolate nanoparticles indicating a concave turning point and a steep jump can be ascribed to the reluctance of the prolate rods of forming large monolayer islands and to the greater repulsion among the islands produced with the reduced available surface area. Therefore, we suggest that the differences in assembly behaviors would be attributed to the combined influence of various factors, including size-related surface energy, shape-related rotation energy, and line tension. Generally, the size has a clear impact on the slope of the π-A isotherm: nanoparticles with smaller size show a strong inclination of aggregation to attenuate the surface energy, resulting in a milder slope of the π-A isotherm. The symmetry of nanoparticles influences the slope and length of Langmuir 2009, 25(22), 12914–12925

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the plateau, seemingly, the higher the symmetry is, the more obscure the plateau and the turning points are. We therefore assume that the decrease of rotation energy upon aggregation becomes less pronounced when the nanoparticle is of higher symmetry. Furthermore, the shape-related line tension of nanoparticles at the water surface would have an effect on the variation tendency of the π-A isotherm and the corresponding formed films. The larger the line tension is, the more unstable the particles become, and the more difficult it is to obtain well-ordered monolayers. 3.4.3. Above Assembly Kinetics as a Guidance: Taking EuF3 as an Example. The hexagonal EuF3 nanosheets show similar morphology but a bit higher symmetry with respect to the triagonal LaF3. Therefore, the representative π-A isotherm of the entire assembly process is expected to show much similarity to that of LaF3, with a much milder slope. And there may probably be two turning points and a plateau between them. The convex turning point implies the formation of a large-scale monolayer, along with space release, while the appearance of the concave turning point can be attributed to the exhaustion of such released space, whereafter the monolayer collapses and folds into multilayers. The assembly pattern of 400 μL of EuF3, described by the π-A isotherm (Figure 10), is basically consistent with our prediction. Its assembly process resembles that of LaF3 nanosheets, with two turning points, convex (A = 185 cm2) and concave (A=150 cm2), though not so clear as those of LaF3. With this guidance, we can acquire a large-scale continuous EuF3 monolayer (A = 185-150 cm2) and multilayer (A < 150 cm2), confirmed by the TEM images (Figure 10b-d). Also TEM images clearly show that the monolayer arises from the incorporation of some large-sized islands. Therefore, the whole assembly process could be divided into three stages analogous to that of LaF3 nanosheets.

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4. Conclusion In summary, the LB technique provides a facile albeit robust method for 2D nanoscale manipulation of various NPs (including 24.9 nm NaYF4:Yb,Er, 14.11.8 nm triagonal-shaped LaF3, 12.6 nm square CaF2, 12.5  2.0 nm hexagonal EuF3, etc.). In the investigation of the self-assembly behaviors of NPs through the LB technique, several assembly stages can be identified in the π-A isotherms. The concentration, size, and symmetry of NPs crucially affect the self-assembly processes. Then, by virtue of the proposed concept of “effective concentration”, we study the correlation between surface pressure and film morphology and postulate a possible kinetic mechanism to interpret the assembly behaviors of diverse nanocrystals. The presented comparison results among the various building blocks, reinforces the perception on the stability of nanoparticles at the interface, and provides a possible guidance for predicting the assembly process of nanoparticles. With the robust LB technique, assisted by the above comprehension, the fabrication of monolayer, multilayer, or some exotic patterns for specific nanoparticles will become rational and facile. Furthermore, it can be envisaged that under carefully adjusted conditions, the composition of the film can be varied, with controlled interlayer spaces and continuously variable thickness. With our expectation, the highquality rare-earth based functional films could serve as the basis of prominent devices with wide current and future interests. Acknowledgment. This work is supported by NSFC (Nos. 20671005 and 20921091) and MOST (2006CB601104). Supporting Information Available: TEM images for the NaYF4:Yb,Er nanocrystal pattern at different compression stages with changeable spreading amount. This material is available free of charge via the Internet at http://pubs.acs.org.

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