Article pubs.acs.org/cm
High-Quality Monolithic Graphene Films via Laterally Stitched Growth and Structural Repair of Isolated Flakes for Transparent Electronics Hongyan Sun,†,‡ Xinming Li,*,§,∥ Yuanchang Li,⊥ Guoxin Chen,† Zhiduo Liu,† Fakhr E. Alam,† Dan Dai,† Li Li,§ Li Tao,∥ Jian-Bin Xu,∥ Ying Fang,§ Xuesong Li,# Pei Zhao,@ Nan Jiang,† Ding Chen,*,‡ and Cheng-Te Lin*,†,∇ †
Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, China 315201 ‡ College of Materials Science and Engineering, Hunan University, Changsha 410082, P. R. China § National Center for Nanoscience and Technology, Beijing 100190, P. R. China ∥ Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China ⊥ Advanced Research Institute for Multidisciplinary Science, Beijing Institute of Technology, Beijing 100081, P. R. China # State Key Laboratory of Electronic Thin Films and Integrated Devices and School of Microelectronics and Solid State Electronics, University of Electronic Science and Technology of China, Chengdu, China @ Institute of Applied Mechanics and Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou 310012, P. R. China ∇ University of Chinese Academy of Sciences, 19 A Yuquan Road, Shijingshan District, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Exfoliation of graphene flakes in solution is a high-yield and low-cost synthesis method, but the quality of the obtained graphene flakes is not high, because of the presence of functional groups and structural defects. Therefore, the ability to synthesize high-quality graphene with excellent electrical properties is desirable for electronic applications. Here, we present a facile and rapid annealing approach with nickel for structural repair in isolated graphene flakes on rough insulating substrates, accompanied by lateral stitching of the isolated parts to form a continuous and monolithic film. This process involves the active carbon species being coalesced at the desaturation edge of graphene flakes. Meanwhile, the defects in graphene can be also repaired to improve its crystal quality and electrical properties. Significantly, the carrier mobility of graphene with excellent structural properties is >1000 cm2 V−1 s−1 on average, nearly 10 times higher than that of the process with copper or 100 times higher than that of graphene via mere annealing. This approach to high-quality graphene on rough insulating substrates, with transfer-free and well-adapted characteristics, is promising for electronic and optoelectronic applications. graphene via mechanical exfoliation of graphite,10 various methods of graphene synthesis have been developed, such as chemical reduction of graphene oxides,11,12 solution-exfoliated
1. INTRODUCTION Graphene, a two-dimensional material with an atomic thickness composed of hexagonally arranged carbon atoms, has attracted an enormous amount of attention because of its outstanding electrical and optical properties,1,2 which have been investigated for a wide range of applications such as sensors,3,4 optoelectronics,5,6 and energy applications.7−9 Since the successful isolation of © 2017 American Chemical Society
Received: June 7, 2017 Revised: August 29, 2017 Published: August 29, 2017 7808
DOI: 10.1021/acs.chemmater.7b02348 Chem. Mater. 2017, 29, 7808−7815
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Figure 1. (a) Schematic illustration of the transfer-free synthesis of the GF-derived graphene film on quartz. (b) Simple patterned graphene with the letter “S” on quartz through a desirable mask. Scanning electron microscopy images of (c) well-dispersed GFs and (d) GF-derived graphene.
to form a continuous and monolithic film. Importantly, the crystal quality (the degree of the defect) of GFs is greatly improved in this work, and the carrier mobility of the graphene film via annealing with Ni can reach 1164 cm2 V−1 s−1 on average, which is 10 times higher than the value determined via annealing with Cu and 100 times higher than that determined via direct annealing. Besides, this method has the following advantages: mass production of raw material, rapid growth process, and largearea transfer-free preparation on the substrates with rough features. This high-quality graphene film with low sheet resistance and high carrier mobility is promising for application in wide electronic and optoelectronic devices.
graphene flakes,13,14 and chemical vapor deposition (CVD).15−17 Nevertheless, exfoliated graphene flakes in the solution phase with low-cost but numerous defects are difficult to use in electronic devices. Removal of the functional groups and repair of the structure in graphene flakes18 to make their properties similar to those of CVD-driven graphene are highly desirable for electronic and optoelectronic devices. Chemical reduction of graphene oxide (GO) can be used to produce large quantities of reduced graphene oxide (rGO) for potential applications. To date, many reduction reagents, such as hydroquinone,19 hydrazine,20 NaBH4,21 and aluminum powder,22 have been used to reduce GO in the solution phase. However, these approaches often involve highly toxic chemicals, require long reduction times, and produce rGO with a relatively high oxygen content that gives rise to high sheet resistance. A highly efficient one-pot reduction of GO using a sodium/ ammonia solution as the reducing agent was achieved, with a record carrier mobility of 123 cm2 V−1 s−1 compared to that of the all solution reduction process.23 Recently, a simple, rapid method for reducing GO into pristine graphene using 1−2 s pulses of microwaves was reported. The mobility of rGO is approximately >1000 cm2 V−1 s−1; however, graphene flakes are isolated, and large devices are difficult to fabricate.24 Correspondingly, nickel (Ni) or copper (Cu) metal-catalyzed hightemperature processing has been used to treat carbonaceous material for the growth of large-area graphene.25 For example, few-layer graphene with a mobility of ∼420 cm2 V−1 s−1 can be directly obtained through an amorphous carbon (a-C) film after annealing.26 However, the carrier mobilities of this synthesized graphene were too low and were mainly limited by the structural defect of graphene. Hence, the improvement in crystalline quality and structural repair for high-quality graphene derived from graphene flakes are still major challenges. Here, we demonstrate a facile approach to structural repair in electrochemically exfoliated graphene flakes (GFs) based on annealing with a Ni thin film on insulating substrates with a rough surface, accompanied by the lateral stitching of the isolated parts
2. EXPERIMENTAL SECTION 2.1. Synthesis of Electrochemically Exfoliated Graphene Flakes (GFs). Natural graphite was employed as a two-anode electrode and a source of few-layer graphene for the electrochemical exfoliation method. Two electrodes were placed in an electrolyte solution, a mixture of H2SO4 and KOH. A constant current of 1 A was applied to the electrochemical system, and the direction of the current was alternately changed every 5 s from one electrode to another. Then the electrochemically exfoliated GFs were generated from the electrode surface, and the electrolyte gradually turned black. After the electrochemical exfoliation, the GFs were collected with a porous filter and washed with deionized water to remove the residual electrolyte by vacuum filtration. After drying, they were dispersed in an ethanol solution in a certain proportion. 2.2. Synthesis of the GF-Derived Graphene. To cover the substrate with a uniform carbon source film, we adopted the single-step Marangoni self-assembly. Ten milliliters of a GF/ethanol solution at a concentration of 0.02 mg/mL was injected onto the surface of 60 mL of deionized (DI) water at a speed of 1 mL/10 s by virtue of a syringe. GFs temporarily suspended in aqueous media and formed a large-area uniform film in a few seconds. This uniform GF film could be easily transferred onto the quartz by the stencil printing method and dried at 120 °C for 5 min to force the ethanol and water to evaporate. Then 300 nm thick Ni films were deposited on the GF film by a mini-type ion sputtering apparatus. For the heat treatment, the samples were placed into a standard quartz tube furnace and annealed at 950 °C for 10 min while being fed H2 (8 sccm) at a total pressure of ∼0.2 Torr. 7809
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Figure 2. (a) Schematic illustration of structural repair of GFs. (b) Representative Raman spectra of the original GFs, the processed GFs, and newborn graphene. Raman mapping of (c) ID for the original GFs, (d) the ID/IG ratio, and (e) the I2D/IG ratio for GF-derived graphene. (f) HRTEM image of representative randomly chosen edges of newborn graphene. (g and h) Representative SAED patterns of twisted BLG and AB-stacked BLG, respectively. (i) Intensity profile from the SAED patterns in panels g and h. After annealing, the samples were quickly cooled to room temperature. The Ni layer was etched by Marble’s reagent (10 g of CuSO4, 50 mL of HCl, and 50 mL of H2O) for 1−2 min, and a graphene film was obtained directly on the quartz substrate without any transfer process. Finally, the sample was washed with DI water and dried with a nitrogen gun. 2.3. Characterization. The prepared samples were systematically characterized using Raman spectra with a laser wavelength of 532 nm (Renishaw plc, Wotton-under-Edge, England), scanning electron microscopy (SEM, QUANTA FEG250), X-ray photoelectron spectroscopy (XPS, Kratos AXIS ULTR DLD), X-ray diffraction (XRD, D8 Advance, Bruker-AXS), ultraviolet−visible (UV−vis) spectroscopy (Lambda 950 PerkinElmer), contact angle measurement (CA, OCA20 Data physics), high-resolution transmission electron microscopy (HRTEM, JEOL JEM2100), and a Hall effect measurement system (Swin Hall 8800).
after the treatment process, and (3) the graphene with the lateral stitching of the isolated GFs in this work are called GF-derived graphene, processed GFs, and newborn graphene, respectively. 3.2. Crystalline Structural Repair for High-Quality Graphene. Here, the schematic of structural repair for highquality graphene through the annealing with Ni is shown in Figure 2a. Interestingly, we have found that the synthesis process of graphene derived from GFs is distinctively different from those of other carbon sources with diffusion and later segregation upon cooling as observed on CVD-grown graphene on Ni films.26 The sp2 bond is the thermodynamic stability for the driving force of the observed synthesis behavior of graphene, crystalline graphite, relative to the other solid carbon source [e.g., amorphous carbon (a-C) and nanocrystalline diamond].29 In this work, GFs preserve a large fraction of the defect-free hexagonal carbon lattice (sp2-C atoms) and have a relatively low density of oxygen-containing groups. Hence, during the initial stages of the annealing process, a large fraction of sp2-C atoms in original GFs act as preplaced “seeds” that did not dissolve into Ni, and active carbon species can dissolve into the Ni film. Subsequently, the defective structure of GFs can be repaired and laterally stitched by newborn graphene that originated from these active carbon species. To demonstrate the proposed process of GF-derived graphene, the reaction was interrupted after the 30 s annealing at 950 °C, as shown in Figure S2. We found that the original GFs with a relatively complete sp2 lattice did not disappear or dissolve into Ni, and the newborn graphene grew around the GFs and filled the gap between the isolated parts, which is consistent with the proposed process of GF-derived graphene. This crystalline quality improvement process of GF-derived graphene can be evaluated by the Raman spectra.30,31 It has been accepted that the intensity ratio of the D band (∼1350 cm−1) to G band (∼1580 cm−1) (ID/IG) is associated with the density of defects in graphene.32 As shown in Figure 2b and Figure S3, when the annealing temperature increased to 950 °C, the processed GFs and newborn graphene exhibited highly ordered graphene-like features with sharp 2D (∼2700 cm−1) and G peaks, as well as a nearly absent D peak compared to the spectrum of the
3. RESULTS AND DISCUSSION 3.1. Synthesis of GF-Derived Graphene. The dispersed GFs in this work had polygon shapes with a lateral size in the range of a few to several tens of micrometers and a thickness of ∼2.5 nm.27 The present scheme of the treatment process for graphene formation on insulating substrates is shown in Figure 1a. In short, the GF dispersion was injected into the DI water and the flakes moved from low-surface tension regions (ethanol-rich) to high-surface tension regions (DI water-rich) derived by the Marangoni effect.27 GFs would collide and bind with each other via π−π interactions.28 After being stabilized for ∼1 min, the uniform GF film could be easily transferred to the quartz. The Ni film was deposited on the GF film, and this sample underwent a rapid heating process, as depicted in Figure S1. After the annealing, the GF-derived graphene film was obtained directly on the quartz substrate without any transfer process after the Ni layer was etched. The GF film can be simply patterned on quartz substrates through a hard mask based on the stencil printing method, and patterned graphene with the letter “S” has been directly prepared on quartz, as presented in Figure 1b. Importantly, after annealing, the isolated GFs were stitched by a newborn-graphene layer to form a continuous and monolithic film, as shown in panels c and d of Figure 1. We defined the (1) the continuous and monolithic graphene films, (2) the GFs 7810
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Figure 3. Comparison of the (a) Raman spectra, (b) ID/IG and I2D/IG ratios, and (c) carrier mobility for the GFs with annealing, annealing with Cu, and annealing with Ni at 950 °C.
Figure 2h, and the line intensity of the diffraction peaks is shown in Figure 2i (bottom). We found that the ratio of the intensity of the equivalent planes [1−210] over the inner peaks from [1−100] was ∼2, which confirms the AB stacking of BLG.40,41 Besides, the HRTEM image and SAED patterns of the processed GFs are shown in Figure S8, which reflects the FLG characteristics. 3.3. Comparison of the Electrical Properties of GF-Derived Graphene Using Different Processes. To reveal the effect of Ni on the GFs in this annealing process, Raman spectra of the processed GFs under different treatment methods, such as annealing, annealing with Cu, and annealing with Ni, are compared in Figure 3a. The well-known G band and 2D band are characteristics of the sp2-hybridized C−C bonds in graphene.42 It is noted that the I2D/IG ratio has been shown to be related to the degree of recovery for sp2 carbon bonds (graphitization) in graphitic structures. It can be seen that the processed GFs via annealing with Ni result in the appearance of a sharp 2D peak, and the I2D/IG ratio increased from 0.12 to 0.59, a value higher than that determined via annealing with Cu (0.31), indicating the significant graphitization process (Figure 3b). Meanwhile, the ID/IG ratio in Figure 3b shows that the processed GFs obtained via annealing with Ni were nearly defect-free with a low ID/IG ratio (0.02), while a relatively large density of defects was observed in processed graphene obtained via annealing with Cu (0.91). Meanwhile, because of the negligible solubility of carbon in Cu, the GFs cannot be connected into a monolithic film by newborn graphene (Figure S9) in a short period of time. Besides, the I2D/IG and ID/IG ratios of the processed GFs obtained via annealing at 950 °C in the H2 atmosphere were similar to those of the original GFs. Here, although H2 reduction at 950 °C can remove most of the oxygen-containing groups for the original GFs, structural defects would still exist in GFs and H2 reduction could not efficiently improve the graphitic structure of GFs. Obviously, the crystalline quality of the GF-derived graphene film obtained via annealing with Ni is much higher than that obtained via other methods. The statistical carrier mobility of the GF-derived graphene films via various annealing processes is shown in Figure 3c. On the basis of the Van der Pauw method, the mobility of GF-derived graphene films obtained via annealing with Ni reaches 1164 cm2 V−1 s−1 on average, nearly 10 times higher than those from annealing with Cu (114 cm2 V−1 s−1) and 100 times higher than those from only annealing (13 cm2 V−1 s−1) (sample size of >5 mm × 5 mm). 3.4. Adsorption of Ni and Cu Atoms on H-Saturated Graphene Edges. In practice, the intrinsic defects in original GFs are mainly oxygen-containing functional groups. H2 reduction at 950 °C can remove most oxygen-containing functional
original GFs. In fact, the original GFs had obvious structural defects that were judged by the obvious peaks of the D band (Figure 2c and Figure S4a). After annealing, the GF-derived graphene film had a lower defect density (average ID/IG ratio of 0.5 and the newborn graphene in the space of the isolated parts exhibited a high I2D/IG ratio of >1.5 (Figure 2e). Moreover, the monolithic GF-derived graphene films and their spatial distribution of layer numbers can be confirmed by high-definition Raman spectra with the I2D/IG ratio and the peak position, full width at half-maximum (fwhm) of the 2D band, respectively (Figure 2e and Figure S4b−e). In particular, the processed GFs are Bernal (AB) stacking bilayer graphene (BLG) and few-layer graphene (FLG, ≥3 layers),33 while newborn graphene is composed mainly of twisted BLG34−36 (see the details in Figure S4). In addition, X-ray photoelectron spectroscopy (XPS) can further explain the functional group conversion trend of GFs in this process. Figure S5 shows the C 1s spectra of the original GFs decorated with oxygen-containing functional groups, and they contain a mixture of sp2-hybridized (CC, 284.6 eV) and sp3-hybridized bonds with epoxide/hydroxyl (C−O, 285.2 eV) groups on the basal plane and carbonyl (CO, 286.6 eV) and carboxyl (COOH, 288.1 eV) groups at the sheet edges.37 The oxygen-containing groups in the GFs arise from the oxidation of graphite by intercalation acid during the electrochemical exfoliation process. In contrast, the magnitudes of the peaks of the oxygen-containing group on the GF-derived graphene are dramatically decreased, showing that this process can significantly reduce the functional groups of graphene for crystalline quality improvement. Besides, the structure of the Ni was unchanged during the heating process, and no bulk crystalline nickel carbide was formed38 (Figure S6). Meanwhile, the roughness of the GF-derived graphene was significantly lower than that of the original GF film (Figure S7).39 Furthermore, HRTEM images and selected area electron diffraction (SAED) patterns can corroborate the stacking order of the GF-derived graphene. Figure 2f shows the edge of BLG of a randomly chosen newborn graphene, and the SAED result shows two sets of monolayer graphene diffraction patterns (Figure 2g). In Figure 2i (top), we learned that the BLG was composed of two layers of graphene by examining the intensity profile of the diffraction spots, and the ratio of the intensity of the equivalent planes [1−210] over the inner peaks from [1−100] was ∼0.8, which confirms the nature of twisted BLG. In contrast, only a single diffraction pattern was observed for the processed GFs in 7811
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Figure 4. (a) Representative adsorption of metal atoms at the different H-saturated graphene edges: (A) armchair, (B) transition region, and (C) zigzag. Blue, gray, and white spheres represent metal atoms (Ni or Cu), C atoms, and H atoms, respectively. (b) Corresponding binding energies.
Figure 5. SEM images of the assembled GF film with different mass concentration dispersions of (a) 0.02, (b) 0.03, (c) 0.05, (d) 0.08, and (e) 0.1 mg/mL. (f−j) SEM images of the GF-derived graphene shown in panels a−e, respectively. (k) Transmittance of GF-derived graphene corresponding to panels a−e. (l) Transmittance vs sheet resistance and (m) statistical Hall effect measurement carrier mobility results for GF-derived graphene with a different mass concentration of the GF dispersions.
groups in GFs, leading to the holes with the edges saturated with hydrogen atoms. Such defects should be the limiting factor for further improvement of the electrical properties of GFs. Nevertheless, the metal atoms may act as the catalyst via destabilizing the graphene edges, to accelerate the perturbation of edge saturation for the subsequent structural repair.43 To further illustrate this, we performed first-principles calculations on the adsorption of Ni and Cu atoms on different kinds of H-saturated graphene edges, as shown in Figure 4. This revealed that the binding energy (defined as the total energy difference between metal-adsorbed graphene and isolated graphene as well as the
metal atom) only weakly depends upon the adsorption site, and the Ni case always gives a value more or less 1.5 eV larger than that for the Cu case. This means a stronger perturbation effect of the Ni on the saturated graphene edges, which is thus beneficial for the removal of edge saturation, consequently allowing for a faster regrowth process at the edges as observed in our experiment.44 3.5. Optical and Electronic Characterizations of GF-Derived Graphene. Furthermore, thickness-tunable graphene film has attracted broad interest for its potential in optoelectronic applications. In our work, the thickness of GF-derived graphene films can be simply dictated by adjusting the concentration 7812
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Chemistry of Materials Table 1. Characterization of the GF-Derived Graphene Films for the Different Mass Concentration of GF Dispersions GF dispersion concentration (mg/mL)
transmittance (%, at 550 nm)
sheet resistance (Ω sq−1)
carrier mobility (cm2 V−1 s−1)
carrier density (cm−2)
0.02 0.03 0.05 0.08 0.1
89.2 ± 1.6 85.7 ± 1.7 80.9 ± 2.1 74.2 ± 2.5 72.2 ± 2.0
627 ± 49 531 ± 45 502 ± 40 426 ± 38 388 ± 45
1164 ± 114 938 ± 84 722 ± 65 563 ± 82 516 ± 87
(9.64 ± 1.46) × 1012 (11.2 ± 2.04) × 1012 (17.1 ± 1.68) × 1012 (22.8 ± 2.05) × 1012 (26.7 ± 2.69) × 1012
Figure 6. (a) Demonstration of the hydrophobicity and hydrophilicity of the GF-derived graphene/quartz (left) and bare quartz (right), respectively. (b) Direct preparation of the GF-derived graphene film on the rough surface of the quartz substrate. (c) Schematic illustration representing the sequential steps for transferring graphene and direct formation of the GF-derived graphene on a rough substrate. (d) Photograph of a continuous GF-derived graphene film on a rough quartz substrate.
of GF dispersions.27 For instance, 10 mL GF dispersions (0.02− 0.1 mg/mL) were injected on the surface of DI water with an area of 80 cm2, which uniquely defined various samples transferred to the substrate shown in Figure 5a−e. As the mass concentration of the GF dispersions increased, the gaps between adjacent GFs were gradually filled. After performing annealing at 950 °C with Ni (Figure 5f−j), we further characterized the optical transmittance and electrical conductivity of the GF-derived graphene films. The UV−vis spectral measurement (Figure 5k) indicates that the transmittance values of GF-derived graphene films at 550 nm incident light were 72.2−89.2% and the sheet resistances (Figure 5l) were 388−627 Ω sq−1 (Figure 5f−j). This indicates that the transmittance and resistances of the GF-derived graphene films on quartz can be dictated by adjusting the mass concentration of GF dispersions. Using Hall effect measurements, the carrier mobilities of GF-derived graphene films were approximately 1164−516 cm2 V−1 s−1 at room temperature under ambient conditions (Figure 5m and Table 1). Raman mapping can determine the layer number distributions of GF-derived graphene films (Figures S10 and S11). The twisted BLG of the newborn graphene at the gaps between GFs has the
same high quality as the graphene grown from an external carbon source.37 3.6. GF-Derived Graphene Film on a Rough Surface. Because of the high optical transmittance and carrier mobility, these high-quality GF-derived graphene films can be good candidates for transparent electrodes. The photograph in Figure 6a shows a quartz sample with and without graphene based on the simple treatment with a shadow mask. By performing water dropping tests, we can clearly observe that water aggregations can be formed on a bare quartz surface with a water contact angle of ∼26°, whereas dispersed water droplets stand up on the graphene-coated quartz with a water contact angle of ∼93°. This indicates the hydrophobic nature of the graphene surface, which is similar to the result for transferred graphene on a glass system.45 Moreover, the GF-derived graphene film can be prepared on a rough surface with a period of 2 mm × 1 mm (Figure 6b). For a traditional approach, a graphene film grown on transition metals can be transferred to the dielectric substrates by PMMA media. However, using this traditional transfer method, it was difficult for a graphene film to tightly adhere to the rough surface, as shown in the schematic diagram in Figure 6c, because 7813
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Devices of Zhejiang Province, and The Key Technology of Nuclear Energy (CAS Interdisciplinary Innovation Team, 2014).
there were many unattached suspended regions between the PMMA−graphene film and the rough surface substrate. Thus, the PMMA−graphene film indeed does not make a conformal contact following the rough surface substrate, and the unattached suspended regions would break easily after the PMMA was dissolved. In contrast, the GF-derived graphene film can be directly grown on the rough surface substrate in this work, and a continuous conductive film can be formed for transparent electrodes (Figure 6d).
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4. CONCLUSION In summary, we demonstrated a facile and rapid annealing approach with Ni for structural repair in isolated electrochemically exfoliated graphene flakes on rough insulating substrates, accompanied by the lateral stitching of the isolated parts to form a continuous and monolithic film. It is revealed that Ni, compared to Cu, is more beneficial to the structural repair and coalescence in graphene flakes. This unique process can greatly improve the crystal quality of graphene, with a carrier mobility that is increased by 2 orders of magnitude. Therefore, this work provides a promising route for obtaining high-quality graphene film for various electronic and optoelectronic devices.
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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.chemmater.7b02348. Additional SEM, Raman, XPS, XRD, roughness, and TEM characterization of a GF film and a GF-derived graphene film (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Xinming Li: 0000-0002-7844-8417 Li Tao: 0000-0002-7757-1149 Ying Fang: 0000-0003-2965-7287 Xuesong Li: 0000-0002-1157-0266 Pei Zhao: 0000-0002-9291-957X Cheng-Te Lin: 0000-0002-7090-9610 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the financial support from the National Natural Science Foundation of China (51402060, 51573201, 51501209, and 201675165), the Program for International S&T Cooperation Projects of the Ministry of Science and Technology of China (2015DFA50760), the Public Welfare Project of Zhejiang Province (2016C31026), the Science and Technology Major Project of Ningbo (2014S10001, 2016B10038, and 2016S1002), and the International S&T Cooperation Program of Ningbo (2015D10003 and 2017D10016). The authors also thank the Chinese Academy of Science for Hundred Talents Program, the Chinese Central Government for Thousand Young Talents Program and the 3315 Program of Ningbo, the open fund of the Key Laboratory of Soft Machines and Smart 7814
DOI: 10.1021/acs.chemmater.7b02348 Chem. Mater. 2017, 29, 7808−7815
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DOI: 10.1021/acs.chemmater.7b02348 Chem. Mater. 2017, 29, 7808−7815
