Research Article www.acsami.org
Photomanipulated Architecture and Patterning of Azopolymer Array Xueli Kong,† Xiaofan Wang,† Tianchan Luo,‡ Yuan Yao,*,† Lei Li,‡ and Shaoliang Lin*,† †
Shanghai Key Laboratory of Advanced Polymeric Materials, Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China ‡ College of Materials, Xiamen University, Xiamen 621005, China S Supporting Information *
ABSTRACT: Here reported is the approach to prepare the tunable 3D architecture and patterning through photoinduced orientation of azopolymer. The hemispherical PAzoMA array can be transformed into spindlelike, flat ellipsoidlike, thick spindlelike, near-hexagon, near-quadrangle, and near-rhombus arrays while being exposed to linearly polarized light (LPL). The size and alignment of the arrays can be precisely controlled by manipulating the irradiation time. Furthermore, complex 3D architectures of the PAzoMA array are readily fabricated through secondary irradiation along different direction. This technique is promising for functionalized surfaces and photonic devices. KEYWORDS: surface patterning, azobenzene, photomanipulation, micro array, 3D architecture
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INTRODUCTION Morphology on solid surface with periodic micro- and nanoscaled patterns has attracted significant attention for its extraordinary microfluidic, wetting, photonic, electronic, and biorelated characters.1−3 These micro- and nanopatterned topographies with spatial periodicity enable materials unique acoustic, electronic, optical, mechanical, and biological properties.4,5 Generally, photolithography, printing, materials deposition, and colloidal lithography (CL) have been adopted to fabricate ordered patterns on solid surfaces,6−8 in which CL is extremely promising because of its simplicity, regular periodicity, large area available, and low cost.9 Through the packing of self-assembled colloidal crystals, two-dimensional (2D) ordered hexagonal-close-packed (HCP) arrays could be easily formed. However, multiple nonspherical arrays are still difficult to fabricate through the simple self-assembly process because the HCP array is the most thermodynamically stable arrangement of the spherical colloids. To overcome this problem, the microsphere arrays are transferred onto the surface of poly(dimethylsiloxane) (PDMS) stamps through soft lithography.10−14 The flexible PDMS stamps act as new templates for further deformation, and nonspherical arrays can be obtained and adjusted by mechanical stretching of the PDMS replica molds.15,16 Unfortunately, the © XXXX American Chemical Society
natural stretching force distribution makes it difficult to ensure the uniformity and repeatability of the whole pattern. Therefore, it is still a challenge to find a precisely controllable and economically feasible CL-based technology to manufacture ordered nonspherical molds or templates with large-area regularity. Photoresponsible azopolymer17 is a promising candidate for the fabrication of nonspherical array because the directional mass-migration effect upon polarized light in both microscopic18,19 and macroscopic20−22 scales that referred to as photoinduced orientation. This is a consequence of the repeat trans−cis−trans photoisomerization cycles of the azobenzene chromophores, which takes preferential orientation along the direction perpendicular to the light polarization upon exposure to a linearly polarized beam.23−26 For example, azobenzenecontaining materials have been adopted to fabricate some sophisticated micro/nano architectures through directional photofluidization lithography (DPL).27−33 Photorotational micropillar arrays could also be obtained by introducing passive polymer into azo-polyelectrolyte architecture.34 Recently, our Received: March 26, 2017 Accepted: May 15, 2017 Published: May 15, 2017 A
DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Scheme 1. Schematic Illustration of the Preparation of PAzoMA Hemispherical Pattern and Photo-Manipulated PAzoMA Patterns under Irradiation ‘a’ and Vertical Secondary Irradiation ‘b’ of LPL
Figure 1. SEM images of (a) PS hcp array; (b) PS ncp array by RIE of a; (c) PAzoMA hemispherical array originated from CL and PDMS soft lithography of b. (d) Topographic image of the PAzoMA hemispherical array. (e, f) SEM images of PAzoMA spindle-like arrays after irradiation of LPL for 5 min along different polarization directions.
Furthermore, secondary irradiation along different direction was introduced to further tune the complex 3D morphologies of the arrays. The photomanipulated approach will hopefully act as a flexible technique to diversify the 3D architectures for controllable wettability, anisotropic wetting and superhydrophobicity33,40 for water-harvesting,41 and antifog.42 Moreover, the patterns originated from multiple irradiation schemes will present diverse optical properties for photonic devices.43−46
group reported the photomanipulation of ordered porous architectures from azobenzene-containing polymer through breath figure arrays.35,36 The original round pores were converted to rectangular, rhombic, and parallelogram-shaped pores under the irradiation of linearly polarized light (LPL) in a short time. Moreover, micelles containing azobenzene37 and azobenzene polymeric compounds (namely azopolymers) colloid spheres38,39 were also reported significantly elongated along the polarized direction under LPL irradiation. Herein, we report the photomanipulation of complex threedimensional (3D) architecture and patterning of large-area ordered azopolymer array (Scheme 1) by LPL. The pristine pattern was made of polystyrene (PS) colloidal spheres through solvent evaporation. Then reactive ion etching (RIE) technique was adopted to transform the HCP array into a non-closepacked (NCP) one with more deformation space.9 After duplicated twice through soft lithography, the large-area ordered azopolymer array was obtained. The array is apt to transform into different shapes, sizes and alignments by simply manipulating the irradiation time and polarization direction.
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EXPERIMENTAL SECTION
Reagents and Materials. The 2.00 μm PS colloidal microspheres dispersion (2.50 wt % in water) was purchased from Alfa. PDMS (Sylgard 184) elastomer was obtained from Dow Corning Co. Ltd.. Azobenzene homopolymer poly[6-(4-methoxy-4′-oxy-azobenzene) hexyl methacrylate] (PAzoMA; Mn = 2.60 × 104, Mw/Mn = 1.23, from gel permeation chromatography (GPC)) was synthesized through atom transfer radical polymerization (ATRP) in anisole as reported earlier.38 Silicon substrates were cleaned with acetone, ethanol, and ultrapure water (UPW, 18.25 MΩ·cm) in sequence in an ultrasonic bath, and then treated by immersing in piranha solution (98% H2SO4/30%H2O2, 3:1 in volume) for 1 h to obtain hydrophilic B
DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 2. Schematic and SEM images of the photomanipulated patterns for different irradiation times. (a) SEM of original PAzoMA pattern. (b) Original unit array and (c) its photoinduced deformation along the V direction (perpendicular to the side of the unit hexagon, which is drawn by the white dashed lines; red two-way arrow represents the polarized direction). Deformed PAzoMA patterns after irradiation along the V direction for (d) 5, (e) 10, (f) 30, (g) 60, (h) 150, and (i) 300 min, respectively. mold was separated from the PAzoMA film to obtain the replicated 2D NCP hexagonal pattern. Photomanipulation Technique. The 450 nm wavelength LED lamp (Uvata UP114) with tunable intensity was set vertical to the PAzoMA sample as the light source for photomanipulation.36,37 The polarization direction could be manipulated using polarizer (PL-FL061, Φ25 mm) between the laser and the sample. The irradiation area is ca.1 cm2 and the overall intensity of LPL was 613 mW cm−2. Characterization. The morphology of the samples were recorded by field emission scanning electron microscope (SEM, S-4800, HITACHI) with an accelerating voltage of 15 kV after sputtered with a thin layer of Au to improve conductivity. Topographic images were obtained with multimode atomic force microscopy (AFM, PARK/XE-100) using a silicon tip with a spring constant of 20 N m−1 of noncontact mode under ambient condition. Contact angles of distilled water on the prepared samples were measured by pendent drop method using a contact angle analyzer (Powereach JC200D3) as follows: a 2.50 μL water droplet was gently placed onto the samples, and the contact angles were measured by the h/2 method after 20 s placement. Each contact angle value available here is the average of nine data sampling from three samples, and three different positions were chosen to measure on each samples.
surfaces. The substrates were rinsed with UPW, followed by dried with nitrogen gas before use. Other reagents and solvents (A.R. grade) were purchased from Sinopharm Chemical Reagent and used as received. Preparation of 2D NCP PS Colloidal Array. As shown in Scheme 1, two-dimensional PS colloidal crystal monolayer (CCM) film was prepared on cleaned substrate by the solvent evaporation method first.47−49 Briefly, 10 μL dispersion containing 2.50 wt % PS microsphere suspension and ethanol (1:1, v/v) was dropped onto the top surface of a piece of silicon substrate (1 × 1 cm). The PS suspension spread freely and evaporated with the help of ethanol rapid volatile, resulting in a colorful CCM film after natural drying at room temperature. The obtained film was then treated with RIE (using O2 plasma) for 1 h to modify the PS HCP CCM array into NCP array as the master pattern for soft lithography. Preparation of PDMS Replica Mold. The mixture of PDMS prepolymer and curing agent at a 10:1 weight ratio was treated under vacuum for 1 h to remove the entrained gas bubbles. Then the mixture was poured onto the master pattern with a thickness of ca. 2 mm. After cured at 65 °C under vacuum for 6 h, the PDMS stamp was carefully peeled off, and then cleaned with ethanol before dried in air. The obtained PDMS replica mold was used as a secondary template. Preparation of 2D NCP PAzoMA Array. A certain amount of PAzoMA was dissolved in chloroform at a concentration of 15 mg mL−1. The solution was cast onto a silicon substrate by spin-coating at 6000 rpm for 30 s, resulting in a PAzoMA layer with the thickness of ca. 700 nm. The prepared PDMS mold was then placed onto the PAzoMA film under pressure at 125 °C for 2 h. Finally, The PDMS
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RESULTS AND DISCUSSION Fabrication of PAzoMA Ordered Arrays. The SEM image of Figure 1a showed the morphology of PS spheres array on treated substrate. It was a HCP array consist of monolayer C
DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces PS colloidal spheres with a diameter of 2.00 μm. Then the NCP array consisted of PS hemispheres with a diameter of 0.90 μm was obtained by etching, as shown in Figure 1b. The PAzoMA array (see Figure 1c) perfectly duplicated the morphology of PS through soft lithography. Consequently, the periodicity of the PAzoMA hemispherical array was 2.00 μm, and each protrusion was 0.90 μm in diameter (Figure 1c) and 632 nm in height (Figure 1d). The uniform, large-area ordered NCP PAzoMA hemispherical arrays were applied in the subsequent manipulations. Photomanipulation of Patterning through Successive Irradiation Time. Figure 1e, f showed the morphology of the PAzoMA arrays after irradiated by LPL for 5 min. A large-area uniform pattern with identical spindle-like protrusions was obtained from the initial hemispherical protrusions array. To analyze the photoinduced deformation process quantitatively, we describe each protrusion with its dimension along the LPL direction as L and vertical to the LPL direction as D, respectively. The corresponding aspect ratio A (A = L/D) was also utilized to measure the elongation and expansion. On the basis of the original NCP array (Figure 2a), a hexagonal cell (white dashed line in Figure 2b, c) was illustrated as the minimum repeating unit of the PAzoMA array, which connected the centers of six surrounding protrusions and integrated them with the central one. Two polarization directions were defined here as the V direction (perpendicular to the side of cell, Figure 2c) and the S direction (parallel to the side of cell). The irradiation time played a crucial role in the shape variation of each protrusion on the micropattern. As shown in Figure 2, the hemispherical PAzoMA array was illuminated by 450 nm LPL for 5−300 min. Every protrusion elongated along the polarization direction and accumulated its elongation with the increasing of illumination time during the beginning stage (irradiated for 5, 10, and 30 min). At this stage, the protrusion changed from hemisphere (Figure 2a) to spindlelike (Figure 2d−f) with the elongation along the polarization direction and the shrinkage vertical to the polarization direction. This deformation was attributed to that LPL triggered a diffusive effect so that a certain amount of PAzoMA vigorously grew along the polarization direction and simultaneously shrank along the orthogonal direction through mass-migration.50 Different from the gradual elongation along polarization direction, the original protrusion shrank obviously vertical to the polarization direction in the beginning 5 min of irradiation and nearly maintained the dimension until 30 min. When illuminated for a longer time, the protrusion preferred to expand rather than elongate, and the shrinkage was apparently recovered because of the slow collapse of the whole protrusion. As shown in Figure 2g−i, flat, ellipsoidlike protrusions were obtained during the next stage (irradiation time of 60, 150, and 300 min) and in the end, every protrusion connected with the adjacent others. The specific discussion on the deformation of single PAzoMA protrusion was simply based on the data of V direction because the original protrusions were isotropous. The variation of L, D, and A values with the irradiation time was shown in Figure 3a, b. In the beginning stage, the length of L increased to 1.26, 1.52, and 1.79 μm with the irradiation time of 5, 10, and 30 min, respectively, whereas the length of D sharply decreased from 0.90 to 0.38 μm in the initial 5 min, and then kept almost constant at ca. 0.38−0.41 μm. As the result, the A value rapidly grew from 1.00 to 3.32 in 5 min (Figure 3b). The
Figure 3. Relationship between (a) the length of L and D; (b) aspect ratio (A = L/D); (c) the height of the protrusion on pattern and the irradiation time for the two types of irradiation direction (the S direction and the V direction).
value grew then slower in the next 25 min to the maximum (ca. 4.37). When the value of L achieved a certain extent, 1.79 μm in 30 min here, the growth along the polarization direction reached the balance. Unlike the slow growth of L length (from 1.79 μm of 30 min to 1.81 μm of 60 min), the length of D increased beyond the original diameter (0.90 μm) and reached 1.23 μm, approximately triple its minimum (5 min, ca. 0.38 μm). In consequence, the A value underwent a fast decrease from its maximum 4.37 at 30 min to 1.47 at 60 min. While the array was irradiated for 150 and 300 min, both the length of L D
DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 4. Schematic and SEM images of the photomanipulated patterns for different irradiation times. (a) SEM of original PAzoMA pattern. (b) Original array and (c) its photoinduced deformation along the S direction (parallel to the side of the unit hexagon, which is drawn by the white dashed lines. The red two-way arrow represents the polarized direction.). Deformed PAzoMA patterns after irradiation along the S direction for (d) 5, (e) 10, (f) 30, (g) 60, (h) 150, and (i) 300 min, respectively.
and D grew slowly, as same as A value. As shown in Figure 3a, the length of L increased with the irradiation time at a progressively slower pace, but the length of D rapidly zoomed out at first, and followed by keeping invariant for a period, then slowed down the pace after a sudden increase. The relevant value of A first gradually increased as same as L, and achieved its maximum at 30 min, then remained constant after a sharp reduction. The photoinduced orientation of azobenzene moieties caused the deformation of each PAzoMA protrusion on the micropattern when exposure to the LPL. The Weigert effect51−54 elucidates that the quantity of azobenzene moieties whose transition moments vertical to the polarization direction of LPL gradually accumulates, leading to the light-selective alignment. In addition, the strong light-driven mass migration of azobenzene moieties under the LPL led to a decrease in the height of protrusions, which resulted from the diffusion away from the volume initially occupied in their as-fabricated state. As shown in Figure 3c, the height of PAzoMA protrusion decreased with negative exponential trend over time. The lightdriven orientation and softening of azobenzene moieties gave rise to the spindlelike and the flat ellipsoidlike protrusions. The results indicated that each PAzoMA protrusion of the micropattern underwent the process of elongation, expansion, and ultimately flattening as the PAzoMA microparticles.38 The
topographic images in three-dimensional (3D) scale (Figure S1) would further corroborate the explanation. Photomanipulation Patterning by Tuning the Polarization Direction. The effect of polarization direction of LPL on the deformation of PAzoMA array was also studied. The deformation of the pattern along S direction (see Figure 4) showed obvious difference with the pattern along V direction (see Figure 2). Although the shape changing of each protrusion did not show distinct with the polarization direction, it brought about different alignment changing for the entire pattern morphology. For instance, comparing with a shorter time of 150 min to observe the interconnection of protrusions under the irradiation of S direction LPL (Figure 4h), it took almost 300 min under V direction LPL irradiation (Figure 2). It is because that the distance between two adjacent protrusion centers is 2.00 μm along S direction and 3.46 μm along V direction, respectively, and therefore a protrusion would take fewer time to interconnect with the adjacent ones (P1−P4, see Figure 2c) along S direction. These results enabled artificially design and fabrication a series of PAzoMA patterns with desirable shape, size and alignment by precisely modulating the irradiation time and polarization orientation. The SEM images of patterns under polarization orientations other than the S and V direction were shown in Figure S2. A LPL with the polarization 15° included to the S direction was adopted. The E
DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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azobenzene moieties continued to migrate along the secondary polarization direction to form the new morphology (as schemed in Figure 6c). Considering the pristine anisotropic spindle-like PAzoMA protrusion before secondary irradiation, the reshaped architecture possessed part of the features of spindle-like and came into different complex 3D morphology depending on the secondary irradiation direction and time. The secondary deformation could be measured by the length along the initial polarization (B1B2 in Figure 6c) and the secondary polarization (C1C2 in Figure 6c) directions, respectively. According to the SEM images (Figure 5 and Figure 6d−g), the length along the initial polarization was measured as B1B230° = 1.23 μm, B1B245° = 1.22 μm, B1B260° = 1.19 μm, and B1B290° = 1.29 μm, respectively, after 5 min of secondary polarization illumination with different shift angles. The values were significantly decreased comparing with the length of spindlelike protrusion without secondary irradiation (L10 min = 1.52 μm), indicating the shrinkage along initial polarization direction during the secondary illumination due to the migration of azobenzene moieties. In addition, the growth along secondary polarization direction was obvious resulting from the value of C1C2 (C1C230° = 1.14 μm, C1C245° = 1.06 μm, C1C260° = 1.02 μm, C1C290° = 1.12 μm, see Figures 5 and 6d−g). The images of AFM (Figure 6h−k) confirmed the growth along the secondary polarization direction. Compared with the initial morphology (Figure S1f), the enlargement along the secondary polarization direction became gradually obvious with the shift angle increasing from 30 to 90°. A distinct orthorhombic structure of protrusions could be identified under the 90° secondary polarization for 5 min (Figure 6k). Influence of Irradiation Intensity and Circular Polarized Light (CPL). For further investigating the photoresponsibility, PAzoMA arrays were irradiated with weaker light intensity (280 mW cm−2) for 10 min, 30 min, 60 and 150 min, respectively (See Figure S5). The results indicated PAzoMA protrusions also underwent the process of elongation and expansion with reduced deformation rate under lower light intensity. For instance, the aspect ratio (A) of one protrusion is ca. 3.41 after irradiated for 30 min with the light intensity of 280 mW cm−2, much lower than that of 631 mW cm−2 light intensity (ca. 4.37) because of reduced light intensity, i.e. lower energy adsorption per azobenzene unit in same time. While CPL was adopted to manipulate the array, the PAzoMA protrusion migrated along all directions and covered gradually larger area with the irradiation time from 10 to 720 min (see Figure S6). Different from LPL, CPL does not feature a linear polarized direction because of the clockwise rotating electric field vector.23,28,55 In consequence, CPL gave rise to uniform photofluidization to PAzoMA array toward the radial direction and resulted in isotropic expansion of PAzoMA protrusion in the horizontal plane.56,57 Potential Applications. Determinate and sophisticated 3D PAzoMA architectures obtained through photomanipulation processes diversify the patterns in micro/nanotechnology. The complex 3D architecture patterns will bring more possibility in utilization. For instance, other functional polymer arrays with complex 3D architecture could also be available by using the deformed PAzoMA patterns as the templates. These 3D patterns of multiple materials were expected to be applied as superhydrophobicity surface,33 surface-enhanced Raman scattering (SERS)28 and photonic devices.44,58,59 Moreover, the deformed patterns brought anisotropic wetting on the surface.23 As shown in Figure S7, the initial CA was 105° and the surface
patterns obtained here were distinct to those prepared under the S or V direction LPL beams. The result confirmed that the protrusions would elongate along the polarization direction first, and followed the same rule during the variation of the protrusion morphology. Finally, the protrusions interconnected with the nearest neighbors along the expansion direction of themselves. Secondary Polarization Illumination. PAzoMA patterns could be reshaped by irradiating with another polarization direction targeting on the established array. In this work, thick spindlelike PAzoMA array was further manipulated by rotating the polarization direction by 30° for another 5 min after irradiated by LPL for 10 min. Figure 5 recorded the
Figure 5. SEM images of the reshaped patterns under initial (white solid arrow) and different secondary (white dashed arrow) polarization illumination. The patterns were reshaped by rotating the polarization direction by (a) 30, (b) 45, (c) 60, and (d) 90° for another 5 min after irradiation of LPL for 10 min along the V direction.
morphology of PAzoMA patterns after the initial (white solid arrow, V direction) and secondary (white dashed arrow) polarization illumination. By tuning the secondary rotation angle of LPL to 45, 60, and 90°, respectively, the near-hexagon array, near-quadrangle array and near-rhombus array were obtained. Different initial polarization direction endowed the intricate arrays with diverse alignments. Here, the SEM images of reshaped patterns with S direction as the initial polarization direction were presented in Figure S3. These consequences confirmed the photomanipulation strategy is reliable to tune the shape and alignment of the PAzoMA pattern. The morphology changing of one protrusion under secondary polarization illumination were recorded by SEM and AFM (see Figure 6). At first, the isotropic PAzoMA hemisphere (Figure 6a) migrated along the polarization direction of LPL, leading to the anisotropic spindlelike protrusion (Figure 6b). When the polarization direction shifted to another direction, the azobenzene moieties were gradually shifting perpendicular to the secondary polarization direction according to the Weigert effect.51−54 Therefore, all the F
DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 6. SEM images, 3D AFM images, and schematic of protrusions before and after different irradiation methods: SEM images of one PAzoMA protrusion: (a) Before irradiation, (b) after irradiation of LPL for 10 min and after secondary irradiation for another 5 min based on irradiation for 10 min by rotating the polarization angle by (d) 30, (e) 45, (f) 60, and (g) 90°. (c) Schematic of the secondary deformation. 3D AFM images of reshaped arrays after rotating the polarization angle (h) 30, (i) 45, (j) 60, and (k) 90°, the secondary irradiation of 5 min was based on the initial irradiation of LPL for 10 min along the S direction. The dashed arrows show the secondary polarization direction, whereas the solid arrows represent the original polarization direction.
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featured hydrophobic. With the irradiation of LPL, the CA value parallel to the polarized direction decreases with the increasing of LPL irradiation time as the result of continually reduced surface roughness by photofluidization. The maximum cumulative decline was ca. 54° after irradiated for 300 min and the surface turned to hydrophilic. On the other hand, the CA value vertical to the polarized direction decreased slower at the beginning, and then kept ca. 14−16° higher than that of parallel to the polarized direction after 60 min of irradiation, indicating the roughness along polarization direction was a bit lower than the orthogonal direction due to the anisotropic photofluidization.
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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/acsami.7b04273. SEM images; 2D AFM images of reshaped arrays; relationship between the diameter of one PAzoMA protrusion and CPL irradiation time; contact angle (CA) of PAzoMA array before and after LPL irradiation for different time parallel and vertical to the polarized direction, respectively, and the relationship between CA and irradiation time (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (S. L.). *E-mail:
[email protected] (Y. Y.).
CONCLUSION
A photomanipulating technique was employed to successfully fabricate the micropatterns ranging from spindlelike, flat ellipsoidlike, thick spindlelike, near-hexagon, and near-quadrangle to near-rhombus arrays. Tuning the irradiation time, polarization orientation, and secondary irradiation allowed the patterns variable in shape, size and alignment. This approach should be highlighted in the preparation of patterns with largearea ordered complex morphology and arrangement, and is considered promising in constructing phototunable functional surface and photonic devices.
ORCID
Lei Li: 0000-0003-2732-9116 Shaoliang Lin: 0000-0003-3374-9934 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (51622301, 51573046, and 51573088). Support from Projects of Shanghai Municipality (14SG29) and G
DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Fundamental Research Funds for the Central Universities (B14018 and WD1616010) is also appreciated.
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DOI: 10.1021/acsami.7b04273 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
