Letter pubs.acs.org/NanoLett
Facet-Selective Growth of Organic Heterostructured Architectures via Sequential Crystallization of Structurally Complementary π‑Conjugated Molecules Yilong Lei,†,‡ Yanqiu Sun,§ Liangsheng Liao,*,§ Shuit-Tong Lee,*,§ and Wai-Yeung Wong*,†,‡,∥ †
Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, P. R. China ‡ HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen 518057, P. R. China § Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of Functional Nano and Soft Materials (FUNSOM), Soochow University, Suzhou, Jiangsu 215123, P. R. China ∥ Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Hong Kong, P. R. China S Supporting Information *
ABSTRACT: In contrast to those for their polymeric counterparts, the controlled construction of organic heterostructured architectures derived from π-conjugated organic molecules has been rare and remains a great challenge. Herein, we develop a simple single-step solution strategy for the realization of organic heterostructures comprising coronene and perylene. Under a sequential crystallization process, an efficient doping step for coronene and perylene domains enables their perfect lattice matching, which facilitates facet-selective epitaxial growth of perylene domains on both the tips and the side surfaces of the preformed seed microwires by manipulating the growth pathways of the two pairs of materials. The present synthetic route provides a promising platform to investigate the detailed formation mechanism of complex organic heterostructures with specific topological configurations, further directing the construction of more functional heterostructured materials. KEYWORDS: π-conjugated organic molecules, organic heterostructured architectures, sequential crystallization, facet-selective growth, structural matching
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multiblock architectures with spatially distributed segments made of block copolymers and π-conjugated polymers. In contrast to those for their polymeric counterparts, integrating different π-conjugated molecular pairs into heterostructured architectures is extremely difficult, although expectedly the latter case possesses more advantages given the diversity of πconjugated small molecules available and their wide-ranging functional properties. To achieve this, a possible solution is to join two structural compatible organic species together, where one component serves as a seed crystal and a second material is grown on certain facets of the starting crystal.17−21 Although this so-called two-step seeded growth method has achieved some successes, it is still hard to achieve the desired location of the growing component on the existing seeds owing to the dynamic nature of molecular self-assembly as well as complex epitaxial relationships of material combinations. Thereby, a present key issue in organic heterostructured systems that
rganic heterojunctions, via joining together two dissimilar semiconductor materials, represent an effective tool to improving the efficiency of organic solar cells (OSCs).1−3 Nevertheless, the heterojunctions are commonly formed by simply physical mixing of electron donor/acceptor pairs, thus giving rise to the occurrence of phase separation.4,5 To meet the practical technological requirements, organic nanoheterojunctions made of two or more distinct nanostructured domains may be designed through the creation of proper interfaces between diverse semiconductor components. For instance, nanotubular segmented heterojunctions were achieved by stepwise coassembly of graphite-like semiconducting πconjugated polymer pairs.6 As a consequence, two tubular segments can electronically communicate with one another, thereby generating highly efficient excitation energy transfer and charge transport across the heterojunction interface. Besides the promising electronic properties, these hybrid labon-a-particle systems also exhibit a variety of potential applications in photonic fields, such as fluorescent barcodes7−9 and optical routers.10,11 As exemplified by previous studies,6−9,12−16 many efforts have been directed toward the fabrication of supramolecular © XXXX American Chemical Society
Received: September 8, 2016 Revised: December 9, 2016 Published: December 27, 2016 A
DOI: 10.1021/acs.nanolett.6b03778 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters needs to be addressed is how to select appropriate π-conjugated molecule pairs and arrange them into desired topological configurations in a predictable manner. In view of the small differences between structurally complementary materials, another great challenge is how to efficiently identify the composition information on each domain of organic heterostructures. The solution to these problems will greatly enable us to gain an understanding of the detailed growth mechanism, further directing the development of complex functional organic heterostructures. As the typical examples of π-conjugated molecules, coronene and perylene can self-assemble to one-dimensional (1D) nanowires and two-dimensional (2D) nanosheets, respectively.22−25 Both of them have centrosymmetric planar structures and high structural compatibility, which prompts us to consider whether they can be integrated together to form organic heterojunctions with desired spatial distribution. We herein report facet-selective epitaxial growth of two types of microscale organic heterojunctions using doped coronene microwires as seeds. We also demonstrate an efficient doping process for coronene and perylene domains due to their small structural mismatch, hence favoring the secondary growth of perylene crystals on the seed wires. During a sequential crystallization process, if perylene has enough time to seek a topological configuration that allows minimizing the overall surface and interfacial energy of the system, selective growth of perylene particles containing coronene at the ends of the seed wires can be achieved. If not, doped perylene domains will exclusively deposit on the bodies of the seeds instead of the seed tips. Unexpectedly, polymorph-selective growth of perylene particles in the β-phase crystals was also realized. The ability to arrange coronene and perylene into specific topological configurations provides an intriguing approach to explore the detailed growth mechanism, which would open a door in the understanding of how to control organic heterostructures made of π-conjugated molecules in a rational manner. We selected coronene and perylene (Figure 1a), as the model materials to fabricate organic heterostructures by a single-step liquid-phase route. First, single-crystalline 1D coronene microwires and 2D perylene nanosheets (Figure 1b and 1c) were achieved by simply injecting coronene and perylene solutions in THF into an ethanol/H2O mixture at 70 °C, respectively. Surprisingly, upon drying a drop of hot saturated solution of coronene and perylene in THF on a plasma-modified quartz substrate, selective growth of cubeshaped crystals on the tips of an organic microwire (Figure 1d and 1e) was obtained readily, which appeared as dumbbell-like structures. These microdumbbells reveal that each individual wire with ribbon-like structure has an average thickness of 2 μm and a length of tens of micrometers. Meanwhile, one can observe that a few particles at the tips have not fused into the complete cube-shaped crystals (solid boxes). In view of the typical assembly features of coronene and perylene, we preliminarily conclude that cube-shaped domains are derived from perylene, whereas the wire is due to coronene. The single typical microdumbbell presented in Figure 1f−h further shows a partial encapsulation of one wire tip by a cube-shaped crystal (dashed boxes), suggesting that the microwire was first formed and then served as a seed to induce the secondary growth of perylene domains. Furthermore, the optical microscopy results of the microdumbbells captured by the real-time video (Supporting Information (SI), Videos S1 and S2) clearly
Figure 1. (a) Schematic showing the molecular structures of coronene and perylene. (b,c,d,e) Typical SEM images display coronene microwires (b), perylene nanosheets (c), and microdumbbells containing 1D seed wires and cube-shaped tips (d,e). A few particles at the tips have not evolved into the whole cube-shaped crystals (solid boxes). (f,g,h) Individual typical SEM images show the partial encapsulation of a wire tip by a cube-shaped particle (dashed boxes). All scale bars are 10 μm.
demonstrated that sequential crystallization of two constituent domains is involved in the single-step assembly route, similar to the traditional two-step seeded growth process.6,18−21 Specifically, the cube-shaped particles would first grow on the tip edges of a seed wire and then evolved into a whole cube by an Oswald ripening process, leading to the formation of microdumbbells. Apart from the dumbbell-like microstructures, we wonder whether perylene domains can be grown exclusively on other facets of the seed wires. To realize this purpose, a simple solvent exchange process was also designed to manipulate the coassembly of coronene and perylene. Typically, 1 mL of a stock solution of coronene and perylene in THF was injected rapidly into 5 mL of an ethanol/H2O mixture at room temperature, and a colorless suspension was formed within a few seconds. Surprisingly, cube-shaped particles tightly adhere to a group of specific side surfaces of 1D microwire instead of the wire tips (Figure 2), also indicating the microwires serve as an epitaxial seed crystal. To precisely achieve the amount of cube-shaped particles, the supersaturation needs to be controlled by systematically adjusting the volume ratio of ethanol to water (ethanol/H2O, v/v). Figure 2a displays that a large amount of 1D microwires were formed and few cubeshaped crystals were observed on the side surfaces of the preformed microwires at v/v = 4:1. Also, it is well-known that coronene and perylene are slightly soluble in ethanol and nearly insoluble in water. Both of them would reach supersaturation instantly and then undergo nucleation and growth when the THF solution was mixed with an 4:1 ethanol/H2O mixture. However, due to the higher solubility of perylene than B
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control experiments were examined by tracing the growth processes of the two types of organic heterostructures. It can be expected that the dumbbell-like microstructures would be formed if the above-mentioned solvent exchange process is performed at high temperature followed by cooling of the solution to a low temperature. Typically, 1 mL of a stock solution of coronene and perylene in THF was rapidly mixed with 5 mL of an 7:3 (v/v) ethanol/H2O mixture at 70 °C, and the mixture was allowed to stand for 5 min. Subsequently, the hot colloidal suspension was placed in an ice−water bath and kept for 3 h. Interestingly, only smooth 1D microwires with a rectangle cross section were achieved before cooling, whereas mostly dumbbell-like structures were formed after cooling (SI S1). Our present results also clearly demonstrate that the preformed 1D microwires act as seeds and then cube-shaped perylene domains are grown on the tips of the existing seed wires via sequential crystallization. Naturally, we can speculate that selective growth of perylene domains on the side surfaces of the seed wires would become feasible if perylene can reach supersaturation instantly during the crystallization process. Based on this viewpoint, a common two-step seeded growth method was also employed by injecting a stock solution of perylene in THF to the colloid solution of coronene microwires. That is, as-prepared pure coronene microwires (Figure 1b) were applied as seeds to afford the nucleation and growth of perylene domains, i.e., coronene/ perylene heterojunctions. As expected, irregular block particles were grown on the side surfaces of coronene microwires, which were demonstrated by SEM and fluorescence microscopy results (SI S2). Obviously, pure coronene microwires emit blue-green light under excitation by UV light, which turns into green emission when excited by blue light (SI S3a and S3b). Meanwhile, pure perylene nanosheets exhibit yellow emission when excited by UV light and become stronger when the excitation wavelength is changed to blue light (SI S3c and S3d). By means of the typical emission features of pure coronene or perylene, we attempt to determine the composition of each domain of the coronene/perylene heterojunctions (SI S2c and S2d). Specifically, 1D microwires emit blue-green light, whereas irregular block particles exhibit yellow-green light when excited by UV light. The former becomes nearly nonemissive, whereas the latter exhibits stronger fluorescence upon irradiation by blue light. For the coronene/perylene heterojunctions formed
Figure 2. (a,b,c,d) SEM images of side surface growth of cube-shaped particles on seed microwires formed by adding a stock solution of coronene and perylene in THF into 5 mL of ethanol/H2O mixtures at v/v = 4:1 (a), 3:1 (b), 7:3 (c), and 13:7 (d), respectively.
coronene in the present solvent system, only a small amount of perylene molecules can assemble into cube-shaped crystals. Upon decreasing v/v from 4:1 to 13:7, the crystallization of coronene and perylene in the THF/ethanol/H2O mixtures would become faster and faster, and most of the constituent molecules would participate in the crystallization process, thus generating higher density of cube-shaped domains deposited on the side surfaces of the seed wires, as shown in Figure 2b−d. As a consequence, the coronene and perylene domains obtained at v/v below 4:1 have smaller sizes as well as weaker crystallinity features, similar to the case of naphthalene−TCNB microtubes formed in the acetonitrile/ethanol/H2O mixture.26 In brief, epitaxial growth of cube-shaped particles on both the tips and specific side surfaces of the seed wires can be realized readily via a single-step assembly route. Also, it is well-known that the topological selectivity of inorganic nanoheterostructures can be elaborately determined by the control of growth pathways of two dissimilar materials during the stepwise assembly process.27−30 By analyzing the above results, we inferred that the main differences between tip and side surface growth of perylene domains are the relative crystallization rates of perylene to coronene. To verify the hypothesis, a series of
Figure 3. (a,b,c) Fluorescence microscopy images of tip growth of cube-shaped particles on seed wires upon excitation by nonfocused UV (a), blue (b), and green light (c), respectively. (d,e,f) Fluorescence microscopy images of side surface growth of cube-shaped particles on seed wires when excited by nonfocused UV (d), blue (e), and green light (f), respectively. The scale bar corresponds to 10 μm. C
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Figure 4. (a) Fluorescence spectra of pure coronene microwires (black curve), square perylene nanosheets (red curve), microdumbbells (blue curve), and perylene growth on the side surfaces of seed wires (cyan curve) spin-coated on a quartz substrate. The inset shown in panel a exhibits the photographs of pure coronene microwire and perylene nanosheet films when excited by UV light (365 nm). (b) Bright-field optical image of a single typical microdumbbell. The scale bar is 10 μm. (c) Microarea PL spectra of main body of the seed wire (black curve), whole microdumbbell (red curve), and end tip (blue curve) under excitation by a 408 nm laser and same end tip (cyan curve) when excited by a 532 nm laser.
of the peaks of the two types of heterostructures via a singlestep assembly route would become weaker, also suggesting the occurrence of an efficient doping process between coronene and perylene. As shown in Figure 4a, photoluminescence (PL) measurements were performed to study the effect of perylene domains on the optical properties of the seed wires. The spectrum (black curve) from pure coronene microwire film spin-coated on a quartz substrate exhibits multiple emission bands at 475, 484, 502, 512, and 524 nm, corresponding to the typical blue-green light of coronene.22,23 In the previous report,25,33 β-phase perylene crystals exhibited three emission peaks at 522, 564, and 627 nm. Meanwhile, the perylene molecule exhibits its absorption at around 400−450 nm in diluted solution.25,33 Hence, energy transfer from coronene to perylene cannot occur because there is no overlap of the PL spectrum of greenemitting β-phase perylene crystals with the absorption spectrum of coronene. The spectrum (orange curve) from square α-phase perylene sheet film shows an emission band at 560 nm, as reported previously.24,25 Besides a shoulder peak at 478 nm, the PL spectrum (blue curve) of the microdumbbells shows an emission band at 517 nm, and that (cyan curve) of perylene growth on the side surfaces of the seed wires consists of a weak shoulder peak at 475 nm and another peak at 525 nm. Thus, we can infer that the two shoulder peaks of the two types of organic heterostructures are derived from coronene, while the main peaks are hard to distinguish owing to their spectral overlap between coronene and β-phase perylene in the green light region. For comparison, the PL spectrum of the coronene/perylene heterojunctions via a two-step seeded growth route was also recorded, as shown in SI S6. Similarly, the peaks from perylene cannot also be clearly assigned although pure coronene seed wires were used as seeds. Actually, the doping and heterogeneous growth processes are both involved in the two types of organic heterostructures. To separate these two processes, we also attempted to perform a control experiment by introducing perylene as a dopant into coronene host. Typically, 1 mL of a stock solution of coronene in THF containing 10% m/m (molar ratio) perylene was rapidly mixed with 5 mL of ethanol/H2O mixtures at v/v = 7:3, 13:7, and 3:2, respectively. The emission spectra of doped coronene wires dispersed in ethanol/H2O mixtures (SI S7) show that there is a larger full-width at the half-maximum (fwhm) when compared to that of pure coronene wires. In addition, the two shoulder peaks at 482 and 510 nm originated from coronene would disappear after doping. Hence, it can be further concluded that perylene molecules take part in the
by a two-step seeded growth method, it is apparent that the seed wires are composed of coronene and the block particles are originated from perylene. Likewise, we also expect to identify the composition of each domain of the two types of organic heterostructures via a single-step assembly route. The fluorescence microscopy images shown in Figure 3 reveal the luminescent features of different domains of the two types of heterojunctions comprising tip and side surface growth of cube-shaped crystals. Obviously, upon excitation by UV and blue light, the seed wires and end tips of the microdumbbells almost emit green light (Figure 3a and b). Yet, the former nearly becomes nonemissive, whereas the latter exhibits strong red light when excited by green light (Figure 3c). Similar to those of the microdumbbells, the emission of each domain was also examined when cubeshaped particles were grown on the side surfaces of the seed wires, as shown in Figure 3d−f. We can infer from the above results that an efficient doping process for coronene and perylene domains may be involved in the present coassembly system. That is, coronene microwires contain a dopant of perylene, while coronene was also incorporated into cubeshaped perylene domains regardless of the types of organic heterostructures. To further determine the crystal structure of each domain of the organic heterostructures via a single-step assembly route, XRD patterns of pure coronene wires, square perylene sheets, and heterostructures including tip and side surface growth of cube-shaped domains were also performed (SI S4). Specifically, the peaks of square perylene sheets are almost consistent with those of α-phase perylene, except for a weak peak at 9° assigned to β-phase perylene, indicating a small amount of rhombic perylene sheets as byproduct were obtained. Also, we can see that the two types of heterostructures have similar peaks whether or not perylene domains are grown on the tips or side surfaces of the seeds. The peak at 9.3° is originated from coronene, whereas the weak peak at 9° is attributed to β-phase perylene, suggesting that unstable monomeric structure of perylene31 can be exclusively obtained in the presence of coronene during sequential crystallization. Yet, α-phase perylene rather than β-phase perylene was commonly achieved because it has a more stable crystal form based on the previous reports.31,32 Thus, polymorph-selective growth of perylene in the β-phase crystals was determined depending on the constituent of the seed. Similarly, the XRD pattern of the coronene/perylene heterojunctions (SI S5, blue curve) via a two-step seeded growth route also shows that β-phase perylene crystals can be achieved upon employing coronene as a seed. In contrast to those of pure coronene wires or perylene sheets, all D
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Nano Letters coronene host in the present work. However, it is still hard to determine the actual composition information on each domain. Microarea PL spectral characterization of the organic heterostructures via a single-step assembly route was also measured through near-field scanning optical microscopy (NSOM). As a representative example, the body and the tip of a single typical microdumbbell (Figure 4b) was excited by two different laser wavelengths and their corresponding microarea PL spectra were collected, as shown in Figure 4c. Specifically, the PL spectrum (black curve) of the body (dashed box) excited by a 408 nm laser exhibits a broad peak at 525 nm with a shoulder at 480 nm, similar to those of the whole microdumbbell (red curve). Moreover, the emission tail extends almost beyond 700 nm. When compared to those of pure coronene wires, these two peaks of the body are clearly originated from coronene domain doped with perylene. Meanwhile, the PL spectrum (blue curve) of the tip (solid box) displays an emission band at 603 nm with a weak shoulder at 525 nm upon excitation with a 408 nm laser, while only a single peak at 598 nm (cyan curve) would appear when the wavelength of the laser is changed to 532 nm. When compared to those of pure α- or β-phase perylene, the emission bands are believed to be derived from β-phase perylene domain doped with coronene, which is consistent with fluorescence microscopy observation (Figure 3a−c). That is, an efficient doping process for coronene and perylene domains would occur, similar to the case of tetracene-doped anthracene.34 All compositional analyses further demonstrate that the microwire in the middle body is perylene-doped coronene, while the microparticles at the tips are perylene containing coronene. To gain an in-depth understanding of how to arrange πconjugated molecule pairs into desired topological configurations, it would be necessary to illustrate the mechanism for selective growth of cube-shaped perylene domains on the tips and side surfaces of the seed wires. As we know, coronene has a monoclinic structure with lattice parameters (a = 10.122 Å, b = 4.694 Å, c = 15.718 Å, β = 106.02°), whereas perylene crystallizes in two monoclinic phases, i.e., α- (a = 10.24 Å, b = 10.79 Å, c = 11.13 Å, β = 100.92°) and metastable β-phase (a = 9.76 Å, b = 5.84 Å, c = 10.61 Å, β = 96.77°). The selected-area electron diffraction (SAED) patterns shown in Figure 5a and b exhibit that the top/bottom surfaces of a single coronene nanowire are bound by (10̅ 1)/(101)̅ planes, whereas those of a square perylene nanosheet are covered by (100)/(1̅00) planes. On the basis of the crystal structures of bulk crystals for coronene, α-, and β-phase perylene, we further demonstrate that the lattice distance of di01,coronene = 9.453 Å (Figure 5c) is ̅ similar to that of d100,β‑perylene = 9.695 Å (β-phase perylene, Figure 5e) and their structural mismatch f is 2.5%. Meanwhile, a larger structural mismatch between coronene and α-phase perylene (d100,α‑perylene = 10.054 Å, Figure 5d) is also clearly verified (f = 5.98%). Actually, the highly similar molecular structures of coronene and perylene would also facilitate the doping process between them during the crystallization process of each domain, which enables their better structural matching. As a consequence, the preferential deposition of β-phase perylene on coronene would become feasible due to wellmatched interfaces between them, as reported previously.6,18 By analyzing the growth pathways of the organic heterostructures comprising tip and side surface growth of perylene domains, it can be demonstrated that 1D wires made of perylene-doped coronene would be first formed and serve as seeds to afford the growth of β-phase perylene crystals, when
Figure 5. (a) TEM image of a single coronene nanowire and its corresponding SAED pattern (inset). (b) TEM image of a single square perylene nanosheet and its corresponding SAED pattern (inset). (c,d,e) The molecular arrangements of bulk crystals for coronene (c) along [1̅01] direction, α- (d), and β-phase (e) perylene along [100] direction, respectively. (f,g) Schematic representation of growth mechanism of perylene domains deposited on the tips (f) and side surfaces (g) of the doped coronene microwires.
coronene and perylene in THF was injected into ethanol/H2O mixtures. During this process, perylene would not have enough time to seek a topological configuration with minimized overall surface and interfacial energy, thereby driving the epitaxial growth of a large amount of β-phase perylene particles containing coronene on the (1̅01)/(101̅) planes of 1D seed wires with larger surface to volume (S/V) ratio (Figure 5g).27−29 For the latter, coronene wires doped with perylene would also be first formed and act as seeds upon drying saturated coronene and perylene in THF at 70 °C on a quartz substrate. As a result, β-phase perylene particles containing coronene would have enough time to epitaxially grow on the (1̅01)/(101̅) planes near the end tips of 1D seed wires and then evolved into the complete cube-shaped crystals by an Ostwald ripening process, hence giving rise to the selective growth of perylene domains on the unstable or higher-energy end facets, i.e., (010) planes (Figure 5f).27−29 That is, facetselective epitaxial growth of perylene domains on the tips or side surfaces of the doped coronene wires depends on the growth pathways of perylene, which may be governed by thermodynamics or kinetics control.21,35 By means of a twostep seeded growth method, pure coronene microwires can also serve as seeds to assist the side surface growth of perylene domains, as shown in SI S2. Nevertheless, the growing domains, i.e., perylene particles would have weaker crystallinity due to the lack of a doping process between them, when compared to those of cube-shaped domains through a singleE
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step solution strategy. Similarly, we also attempted to explore the possibility of epitaxial growth of coronene domains on the perylene sheets. Specifically, the preformed α-phase square perylene sheets were employed as seeds to induce the nucleation and growth of coronene domains, i.e., perylene/ coronene heterojunctions. It is obvious that coronene wires cannot selectively grow on the specific facets of perylene sheets in a controlled manner (SI S8) owing to the undesired structural matching (f = 5.98%) between coronene and α-phase perylene. Interestingly, two other material combinations, such as coronene and benzo[b]perylene as well as perylene and benzo[b]perylene, would form two-component mixed crystals instead of organic heterostructures with specific spatial distribution, in which 1D ribbons and square 2D sheets were obtained, respectively (SI S9 and S10). Hence, phase separation will occur if the structural mismatch of two constituent materials is large, whereas two-component mixed crystals will form if they have almost no structural mismatch, which can be used as a design principle to direct the fabrication of organic heterostructures by a judicious selection of materials combinations with appropriate lattice mismatch. In summary, we introduce a simple and convenient singlestep solution assembly strategy to construct organic heterostructures with two topological configurations during the stepwise assembly process. A doping step between two πconjugated molecule pairs enables their perfect lattice matching, which provides a possibility for the desired location of one organic crystal on the performed seed of another organic crystal. Importantly, we can determine whether or not the secondary crystal is grown on the tips and side surfaces of the seed by the control of growth pathways of the constitute materials. The present rational control of topological selectivity for organic heterostructures represents a promising platform for understanding the heterogeneous growth mechanism of two or more π-conjugated molecules, which can be further applied to the fabrication of miniaturized optoelectronic devices, such as p−n junctions for solar cells and field-effect transistors and multicolor optical barcodes.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS W.-Y.W. thanks the Hong Kong Research Grants Council (HKBU 12302114), National Natural Science Foundation of China (Project No. 51573151), Areas of Excellence Scheme of HKSAR (AoE/P-03/08), Science, Technology, and Innovation Committee of Shenzhen Municipality (JCYJ20140419130507116), Hong Kong Baptist University (FRG2/13-14/083), and the Hong Kong Polytechnic University for the financial support.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b03778. Experimental methods and supporting figures (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*L.S.L. E-mail:
[email protected]. *S.-T.L. E-mail:
[email protected]. *W.-Y.W. E-mail:
[email protected]; wai-yeung.wong@ polyu.edu.hk. ORCID
Liangsheng Liao: 0000-0002-2352-9666 Wai-Yeung Wong: 0000-0002-9949-7525 Author Contributions
Y.L.L. and Y.Q.S. contributed equally to this work. W.-Y.W., S.T.L., and L.S.L. directed the experiments. Y.L.L. and Y.Q.S. designed and performed all of the synthetic work and data analysis. W.-Y.W., S.-T.L., and Y.L.L. prepared the manuscript and all authors have agreed to the content of the manuscript. F
DOI: 10.1021/acs.nanolett.6b03778 Nano Lett. XXXX, XXX, XXX−XXX
Letter
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DOI: 10.1021/acs.nanolett.6b03778 Nano Lett. XXXX, XXX, XXX−XXX
