Monolithic Photonic Crystals Created by Partial Coalescence of Core

Feb 12, 2014 - Monolithic Photonic Crystals Created by Partial Coalescence of Core–Shell Particles. Joon-Seok Lee†‡, Che Ho Lim†‡, Seung-Man...
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Monolithic Photonic Crystals Created by Partial Coalescence of Core−Shell Particles Joon-Seok Lee,†,‡ Che Ho Lim,†,‡ Seung-Man Yang,†,‡,§ and Shin-Hyun Kim*,† †

Department of Chemical and Biomolecular Engineering and ‡National Creative Research Initiative Center for Integrated Optofluidic Systems, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea S Supporting Information *

ABSTRACT: Colloidal crystals and their derivatives have been intensively studied and developed during the past two decades due to their unique photonic band gap properties. However, complex fabrication procedures and low mechanical stability severely limit their practical uses. Here, we report stable photonic structures created by using colloidal building blocks composed of an inorganic core and an organic shell. The core−shell particles are convectively assembled into an opal structure, which is then subjected to thermal annealing. During the heat treatment, the inorganic cores, which are insensitive to heat, retain their regular arrangement in a facecentered cubic lattice, while the organic shells are partially fused with their neighbors; this forms a monolithic structure with high mechanical stability. The interparticle distance and therefore stop band position are precisely controlled by the annealing time; the distance decreases and the stop band blue shifts during the annealing. The composite films can be further treated to give a high contrast in the refractive index. The inorganic cores are selectively removed from the composite by wet etching, thereby providing an organic film containing regular arrays of air cavities. The high refractive index contrast of the porous structure gives rise to pronounced structural colors and high reflectivity at the stop band position. inversion16−18 have been developed. The opal structures composed of latex particles can be strengthened by thermal annealing above their glass transition temperature (Tg), where individual particles in the fcc lattice are physically fused with their neighbors.13−15 However, the interparticle fusion frequently decreases reflectivity at the stop band, and delicate control over the degree of fusion is required to avoid formation of a continuous polymer film without periodicity. Use of polymeric core−shell particles partially solves the problems.19−25 Under hot compression, the shells with low Tg melt and form a continuous phase, while cores with high Tg remain intact and form regular arrays by shear force. This provides a simple method for preparation of the continuous composite film. Especially for elastic shells, the composite becomes stretchable up to 500% after fusion, thereby enabling the strain-induced color tuning.25 However, refractive index contrast is relatively low, and the crystallinity of the core particles is limited. Inverse opals can be prepared by inverting the opal structure with matrix materials.16−18 The continuous matrix with air cavities embedded is structurally stable, and optical properties can be controlled by adjusting the refractive index of the matrix materials. Moreover, annealing of the opal

1. INTRODUCTION Photonic crystals which have a periodic modulation of refractive index in half the wavelength scale exhibit photonic band gap properties. Light with a specific wavelength or frequency whose energy is in the band gap is prohibited in the structure due to its low photon density of states.1 This exceptional property of the photonic band gap has provided new opportunities in a wide range of novel photonic applications, including waveguides, lasers, displays, and microsensors.2−6 In addition, the photonic devices can be designed to be adaptive by employing responsive materials and structures in photonic crystals.7 To create such periodic nanostructures, various types of building blocks, including colloids, block copolymers, and cholesteric liquid crystals, have been used for self-assembly. Among them, colloidal crystallization has been considered as the most promising approach because of simple and cheap fabrication procedures; monodisperse colloidal particles can be synthesized in bulk, and their crystallization into a face-centered cubic (fcc) structure, an opal structure, can be simply achieved by convective assembly.8−12 However, the opal structures composed of colloidal arrays lack mechanical stability due to their low physical or chemical connections between colloids. Moreover, the position of the stop band is directly determined by the particle size without any tunability over photonic properties. To overcome these limitations, two distinct approaches of interparticle fusion13−15 and material © 2014 American Chemical Society

Received: December 22, 2013 Revised: February 8, 2014 Published: February 12, 2014 2369

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control the thickness of the shell, the amount of emulsion was adjusted while the composition of the emulsion was maintained; the emulsions with 2- and 4-fold volume result in shell thicknesses of 22 and 25 nm, respectively. The final suspension contained both SiO2@PMMA core−shell particles and small PMMA free particles; approximately 2% of MMA injected into the reactor forms the shell of particles. To separate the core−shell particles from the mixture, the suspension was subjected to centrifugation and washing several times. 2.2. Fabrication of a Porous Photonic Film. Colloidal crystals were deposited onto a silicon wafer by convective assembly.28 The silicon substrate was pretreated with piranha solution, a mixture of sulfuric acid and hydrogen peroxide in a volume ratio of 3:1, to remove organic impurities and render the surface hydrophilic. The substrate was dipped into a 5% (w/w) aqueous suspension of SiO2@PMMA particles with an angle of 65°. The suspension was maintained at 65 °C for 24 h without any disturbance. As water evaporates, the suspension meniscus lowers, thereby depositing colloidal crystals on the surface of the substrate. The colloidal crystals formed on the substrate were subjected to thermal annealing in a convection oven to induce coalescence of PMMA shells. The annealing temperature was maintained at 123 °C while the annealing time was adjusted from 0 to 3 h to control the degree of coalescence. The annealed structures were further treated with 2% (w/w) hydrofluoric acid (HF; 48%, Sigma-Aldrich) solution for 10 s to etch out SiO2 cores and washed with distilled water several times. 2.3. Characterization. Colloidal crystals and porous films were observed by optical microscopy (Nikon, L150). The images were captured by a camera (Nikon, DS-5M), and the reflection spectra were measured by a fiber-coupled optical spectrometer (Ocean Optics, USB4000) mounted onto an optical microscopy with a 10× lens. SiO2@PMMA core−shell particles and their assemblies were observed by scanning electron microscopy (SEM; Hitachi, S-4800) and scanning transmission electron microscopy (STEM; Hitachi, HD2300A). For SEM observation, the samples were coated with OsO4 to render them conductive. The glass transition temperature of the PMMA shells was measured by differential scanning calorimetry (DSC; Q-100, TA Instrument).

templates before material deposition provides additional controllability over the structural configuration and band gap properties.26,27 However, this approach involves multiple complicated fabrication steps. In particular, material deposition in the interstices between colloids requires a delicate control of material flow to fill the tortuous space homogeneously. Therefore, there is still intense demand for simple methods which provide high controllability over photonic properties, high structural stability, and easy processing. In this paper, we report a novel strategy for fabrication of photonic crystals by hybridizing interparticle fusion and material inversion. Using core−shell particles composed of an inorganic SiO2 core and a poly(methyl methacrylate) (PMMA) shell, opal structures are prepared by convective assembly and are then subjected to thermal annealing to induce intershell coalescence. Near the glass transition temperature of PMMA, the shells are partially coalesced with their neighbors, while the SiO2 cores remain intact, thereby yielding a monolithic PMMA structure containing crystals of SiO2 cores embedded. In addition, the degree of coalescence which determines the stop band position can be precisely controlled by the annealing time. The PMMA shell is designed to be thin enough to prevent complete coalescence, which results in small air cavities in the interstices even for a long-term annealing. Moreover, selective removal of SiO2 cores from annealed colloidal crystals produces inverse opal-like porous structures without any material deposition step which exhibit high reflectivity at the stop band due to enhanced index contrast. This simple hybrid approach enables us to simultaneously achieve easy processing, high mechanical stability, and tunability of photonic properties.

2. EXPERIMENTAL SECTION 2.1. Preparation of SiO2@PMMA Core−Shell Particles. Monodisperse SiO2@PMMA core−shell particles were synthesized by emulsion polymerization which is slightly modified from that in a previous report.28 Monodisperse SiO2 cores with a diameter of 229 nm were prepared by a combination of the two-phase method for the synthesis of seed nanoparticles and the Stöber method for the growth of the nanoparticles.29 The SiO2 particles were pretreated with [3(methacryloyloxy)propyl]trimethoxysilane (MPTMS; 98%, SigmaAldrich) before emulsion polymerization to promote the formation of a covalent bond between the SiO2 and PMMA. For this, 1.2 g of SiO2 particles were dispersed in 400 mL of ethanol, and 1 mL of NH4OH, 6 mL of MPTMS, and 10 mL of H2O were injected into the SiO2 suspension. The mixture was vigorously stirred for 2 days at room temperature. The SiO2 particles were washed with ethanol several times. The MPTMS-treated SiO2 particles were then subjected to emulsion polymerization. Typically, 90 mg of MPTMS-treated SiO2 particles were dispersed in 90 mL of H2O containing 0.6 g of poly(vinylpyrrolidone) (PVP; Mw = 55 000, Sigma-Aldrich). After the mixture was stirred for 12 h in a round flask, 3 mL of an aqueous solution of potassium persulfate (KPS; 99.99%, Sigma-Aldrich) at a concentration of 20 mg/mL was injected into the mixture as the initiator. The temperature of the mixture was raised to 60 °C and maintained for 30 min under a nitrogen atmosphere. An emulsion was independently prepared by ultrasonication of 1 mL of methyl methacrylate (MMA; 99%, Sigma Aldrich) in 10 mL of H2O containing 20 mg of sodium dodecyl sulfate (SDS; 99%, Sigma Aldrich) and 15 mg of potassium hydroxide (KOH; 85%, Junsei) as emulsion stabilizers. The emulsion was injected into the SiO2 suspension in a volumetric flow rate of 24 mL/h using a syringe pump. After the injection, the suspension was kept at 70 °C for 30 min under continuous stirring. To increase conversion of MMA, 3 mL of initiator solution, a 20 mg/mL aqueous solution of KPS, was additionally injected, and the suspension was allowed to react for 1.5 h. These procedures result in a 15 nm thick PMMA shell. To

3. RESULTS AND DISCUSSION 3.1. Convective Assembly of Core−Shell Particles. We synthesize highly monodisperse SiO2 particles with a diameter of 229 nm and use them as seeds for PMMA shell formation. The shell is grown on the surface of SiO2 cores, the thickness, δ, of which is controlled by the amount of MMA monomers: Three distinct thicknesses of the shells are prepared. Using the SiO2 particles and three different core−shell particles, we prepare opal films on a silicon substrate by convective assembly. The particles form an fcc structure whose (111) planes are aligned along the substrate. The opal film composed of SiO2 particles exhibits a cyan reflection color as shown in Figure 1a. For core−shell particles, the thickness of the PMMA shell determines the reflection colors of opal films as shown in Figure 1b−d, where reflection colors are yellow, orange, and red, respectively. Convective assembly makes inevitable cracks in the films of colloidal crystals as shown in Figure 1b. TEM images of an individual core−shell particle in the insets clearly show the thickness of the PMMA shell. The stop band positions of the opal structures can be measured from reflectance spectra as shown in Figure 1e. An increase of the shell thickness provides a reflectance peak at longer wavelength; the peaks are located at 503.24, 574.36, 605.91, and 621.75 nm for the colloidal crystals in parts a−d, respectively, of Figure 1. The peak position follows Bragg’s law for stacked (111) planes of the fcc lattice which has (111) plane spacing d and effective refractive index neff: 2370

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respectively, of Figure 1; these thicknesses are consistent with TEM observations. 3.2. Partial Coalescence of Core−Shell Particles. The colloidal crystals composed of core−shell particles are subjected to thermal annealing to partially fuse PMMA shells and make physical connections between particles. To determine the annealing temperature, Tg of the PMMA shells is measured by DSC as shown in Figure S1 of the Supporting Information; the glass transition starts at 116.52 °C and ends at 128.52 °C, and Tg is evaluated as 125.90 °C. We select an annealing temperature of 123 °C, which induces slow fusion and therefore enables control of the degree of fusion with the annealing time. Upon annealing, the PMMA shells begin to coalesce with their neighbors and form bridges between particles as schematically illustrated in Figure 2a. This leads to a decrease in the interparticle distance as the molten PMMA fills the interstices of particles in the fcc lattice. This compression is spontaneously driven by minimization of the surface energy at the annealing temperature. For a long-term annealing, a thin layer of PMMA remains on the surface of SiO2 particles even in the case where the interstices are not fully occupied by the molten PMMA. This is attributed to formation of an immobile layer of PMMA on the surface of the SiO2 cores.30,31 When the PMMA shells are grown on the surface of the SiO2 core, pretreatment with MPTMS is used to promote the formation of a covalent bond between SiO2 and PMMA. Therefore, the PMMA chains are chemically attached on the SiO2 surface; this makes the layers immobile. This is confirmed by SEM characterization and reflectance spectra of annealed colloidal crystals. When the colloidal crystals composed of the core−shell particles with δ = 25.3 nm are annealed, the particles fuse and the void fraction decreases as shown in Figure 2b−d, where the annealing time is controlled from 0 to 30 and 120 min. Although the thickness of the PMMA shells, δ = 25.3 nm, is enough to fill all the interstices between SiO2 cores if all PMMA becomes mobile, a long-term annealing leaves a small air void. To characterize evolution of the structure and optical properties during the annealing, we measure the reflectance spectra of colloidal crystals which have different annealing times as shown in Figure 2e. In the course of the thermal annealing, the reflectance peak blue shifts from 621.75 to 610.73 nm as denoted with triangles in Figure 2f. The reflectivity at the stop band deceases as the shell fuses. We attribute this to size reduction of air pockets during the fusion. The size reduction of the scatters, which is smaller than the wavelength of light, leads to a decrease of the scattering intensity, thereby resulting in dwindling of reflectivity in the course of annealing.13,14 We observe insignificant deterioration of the degree of particle ordering during annealing; this is important for production of high-quality porous photonic crystals through selective removal of silica particles after annealing. The colloidal crystals composed of the core−shell particles with shell thicknesses of 15.0 and 21.9 nm also exhibit blue shifts of the peak as denoted with squares and circles; the corresponding reflectance spectra are shown in Figure S2 of the Supporting Information. These blue shifts are consistent with previous reports on thermal annealing of colloidal crystals composed of poly(styrene) latex or SiO2 particles.13−15 The shift of the stop band can provide a change in the particle volume fraction during the annealing. The interparticle distance, D(t), can be expressed in terms of the particle volume fraction, ϕp(t), by volume conservation:

Figure 1. (a−d) Optical microscopy images of colloidal crystals composed of (a) SiO2 particles and (b−d) SiO2@PMMA core−shell particles, where bare SiO2 particles with a diameter of 229 nm were used in (a) and the same particles were employed to deposit PMMA shells and used in (b)−(d). The thicknesses of the PMMA shells were controlled to be (b) 15 nm, (c) 21.9 nm, and (d) 25.3 nm. The insets show transmission electron microscopy images of the corresponding spheres, where all four images were taken at the same magnification. (e) Reflectance spectra of the four colloidal crystals shown in (a)−(d).

λ = 2dneff =

8 D(ϕpn p2 + (1 − ϕp)nm 2)1/2 3

(1)

where ϕp is the volume fraction of particles and np and nm are the refractive indices of the particle and matrix, respectively. For SiO2 opals, refractive indices of the particle and matrix are those of SiO2 (np = 1.45) and air (nm = 1), respectively, and the value D = 229 nm provides a stop band position of 504 nm, which is consistent with the reflectance peak position. For the opals composed of core−shell particles, the refractive index of the particle can be estimated as the volume-weighted average of the refractive index of the core, ncore, and that of the shell, nshell: ⎫1/2 ⎧⎛ D ⎞3 ⎛ ⎛ Dcore ⎞3⎞ core 2 2 ⎟ n ⎜ ⎟ ⎟n ⎬ n p = ⎨⎜ core + ⎜1 − ⎝ D ⎠ ⎠ shell ⎭ ⎝ ⎩⎝ D ⎠ ⎪

⎪

⎪

⎪

(2)

where Dcore is the diameter of the core (Dcore = D − 2δ). The refractive indices of the core and shell are those of SiO2 (ncore = 1.45) and PMMA (nshell = 1.49). By using these equations, we can estimate the thickness of the PMMA shell from the peak position in Figure 1e. This provides thicknesses of 15.0, 21.9, and 25.3 nm for the core−shell particles used in parts b−d, 2371

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Figure 2. (a) Schematic illustration of a partial fusion of the SiO2@PMMA core−shell particles in their crystal structures upon thermal treatment. (b−d) A series of scanning electron microscopy images of colloidal crystals composed of SiO2@PMMA particles, where the crystals were annealed for (b) 0 min, (c) 30 min, and (d) 120 min at 123 °C. Core−shell particles with a core diameter of 229 nm and shell thickness of 25.3 nm were used. (e) Reflectance spectra of colloidal crystals which were annealed for different times from 0 to 180 min. The same core−shell particles in (b)−(d) were used. The peak positions were blue-shifted in the course of the annealing. (f) Annealing time dependence of the reflectance peak position. Three different particles were used, the core diameters of which are all the same, 229 nm, while the shell thicknesses are different: 15.0 nm (■), 21.9 nm (●), and 25.3 nm (▲).

⎛ ϕ ⎞1/3 p0 ⎟ D(t ) = D0⎜⎜ ⎟ ϕ ( ⎝ p t) ⎠

thickness; a thicker shell gives a lower void fraction. The colloidal crystals composed of core−shell particles with δ = 15.0, 21.9 and 25.3 nm have ϕair = 0.152, 0.123, and 0.096 for 3 h of annealing, respectively, and the values remain unchanged for a longer annealing time. In addition, the interparticle distance can be calculated using eq 4. The distance steeply decreases initially and converges to a finite value as shown in Figure 3b; the colloidal crystals composed of core−shell particles with δ = 15.0, 21.9 and 25.3 nm have D(t) = 247.5, 257.8, and 261.6 nm for 3 h of annealing, respectively. From these final interparticle distances, we can estimate the thicknesses of immobile PMMA shells, δim, for all three particles as δim = (D(∞) − Dcore)/2; the values of δim are 9.25, 14.4, and 16.3 nm for the particles with δ = 15.0, 21.9, and 25.3 nm, respectively. During the annealing process, the colloidal crystals become mechanically stable due to formation of physical connections; the PMMA shells fuse with their neighbors and the bottom layer of colloidal crystals fuse with the substrate silicon wafer, thereby making a monolithic structure. The size of the single monolithic structure ranges from 50 μm × 50 μm to 1000 μm × 1000 μm, which originates from the convective assembly

(3)

where D0 and ϕp0 are the interparticle distance and particle volume fraction before annealing, respectively. Therefore, the stop band position can be expressed in terms of ϕp(t): λ(t ) = 2d(t ) neff (t ) =

⎛ ⎞1/3 8 ⎜ ϕpo ⎟ D0 (ϕp(t )n p2 + (1 − ϕp(t ))nm 2)1/2 3 ⎜⎝ ϕp(t ) ⎟⎠ (4)

where ϕp(t) is larger than ϕp0 = 0.74. Although the effective refractive index increases as the volume fraction increases, the influence of the decrease in interparticle distance dominates, thereby leading to a blue shift of the stop band position. From the blue shift of reflectance peaks in Figure 2f, we can calculate the evolution of the void fraction, ϕair(t) =1 − ϕp(t), using eq 4. The void fraction decreases from 0.26 to a finite value as the annealing time increases for all three core−shell particles, as shown in Figure 3a. The final void fraction depends on the shell 2372

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retardation of evolution in the colloidal crystals composed of the core−shell particles is attributed to chemical bonding between PMMA and SiO2. More importantly, distinct SiO2 cores of the core−shell particles enable maintenance of periodic structures even after complete fusion. 3.3. Selective Removal of SiO2 Cores. The optical property of annealed colloidal crystals can be enhanced by selective removal of SiO2 cores as schematically illustrated in Figure 4a. To do this, we prepare the colloidal crystals with core−shell particles with δ = 25.3 nm and anneal them at 123 °C for 60 min; this results in red reflection colors as shown in Figure 4b and reflectivity of 23% at a wavelength of 614.34 nm as shown in Figure 4f. The annealed film is treated with diluted HF solution to etch out SiO2 particles from the PMMA matrix. The resultant film exhibits a pronounced blue color as shown in

Figure 3. (a, b) Time dependences of the (a) air fraction and (b) interparticle distance in colloidal crystals, where three different crystals were annealed at 123 °C. The crystals were composed of SiO2@ PMMA core−shell particles with all the same SiO2 cores with a diameter of 229 nm and three different shells with thicknesses of 15 nm (■), 21.9 nm (●), and 25.3 nm (▲). Both the air fraction and interparticle distance are calculated from the reflectance peak position using Bragg’s law.

step; there is no crack formation during annealing. To prove the enhancement of mechanical stability, we press the annealed and unannealed films with a rectangular indenter with dimensions of 1 mm × 0.2 mm at a pressure of 1.76 × 106 Pa and observe them with an optical microscopy. For thermally annealed colloidal crystals composed of SiO2@PMMA core− shell particles, the film maintains the structure and color after indentation, although small scars are generated as shown in Figure S3a,b of the Supporting Information. By contrast, the film prepared by convective assembly of monodisperse SiO2 particles is broken and destroyed during indentation, as shown in Figure S3c,d. The evolution of colloidal crystals composed of SiO2@ PMMA core−shell particles during the thermal annealing is very different from that of colloidal crystals composed of PMMA particles. When the colloidal crystals composed of PMMA particles are annealed at 123 °C, the particles completely coalesce and form a bulk film as shown in Figure S4 of the Supporting Information. Therefore, the initial periodic structure disappears, and therefore, its stop band is also closed. In addition, the structure rapidly evolves, which limits controllability of the degree of coalescence. The

Figure 4. (a) Schematic illustration of fabrication of porous photonic crystals through removal of SiO2 cores from annealed colloidal crystals. (b, c) Optical microscopy images of (b) annealed colloidal crystals and (c) porous photonic crystals. (d, e) SEM images of fractures of porous photonic crystals. Small air cavities are observed in the interstices between large cavities as indicated with arrows in (e). (f) Reflectance spectra of the colloidal crystals, annealed crystals, and porous crystals. 2373

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2006-0050630) and an International Collaboration grant (Sunjin-2010-002) from the Ministry of Trade, Industry, and Energy (MI, Korea). We acknowledge Dr. T. Ding at Nanyang Technological University for useful discussion on the synthesis of core−shell particles.

Figure 4c. In addition, the reflectivity at the stop band position of 520.86 nm is doubled as shown in Figure 4f. An increase of refractive index contrast gives high reflectivity, and a decrease of the effective refractive index induces a blue shift of the peak position. Formation of air cavities in the place of SiO2 cores can be confirmed with SEM characterization. To observe the internal structure, we make a fracture in the film as shown in Figure 4d and Figure S5a,b of the Supporting Information. The fracture shows arrays of both spherical PMMA shells and air cavities on the surface, which proves that the PMMA shells are partially fused and the structure becomes monolithic. The magnified image of the air cavity array shows small air pockets in the interstices between large air cavities as indicated with arrows in Figure 4e; these air pockets are additional evidence of incomplete infiltration during thermal annealing. Air cavities are also confirmed by TEM images in Figure S5c,d. The annealing improves the mechanical stability of the structures through formation of physical connections between PMMA shells. This is important to maintain the periodic structures during the etching step. The colloidal crystals which are subjected to HF treatment without the annealing are distorted due to low mechanical stability as shown in Figure S6 in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Optical microscopy, SEM, and TEM images of colloidal crystals, DSC data of core−shell particles, and reflectance spectra of annealed colloidal crystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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4. CONCLUSIONS We have demonstrated a novel method to make colloidal and porous photonic crystals using particles composed of an inorganic core and a polymeric shell. The core−shell particles in the fcc lattice are partially coalesced with their neighbors by thermal annealing, thereby providing enhanced mechanical stability. In addition, the degree of coalescence is precisely controlled by the annealing time, which enables the control of the void fraction and optical properties. The enhanced stability by the partial fusion enables the selective removal of the inorganic core by wet etching, thereby making porous photonic crystals with high index contrast; this is otherwise difficult to achieve without thermal annealing. Therefore, this approach does not involve any complicated process of material deposition and provides a means to create mechanically stable photonic crystals with high index contrast. Moreover, the inorganic core can be potentially replaced with high-index particles such as titania and zirconia; this will provide high optical performance even without selective removal of the cores. This hybrid approach of interparticle fusion and material inversion based on particles with an inorganic core and a polymeric shell will provide new opportunities for implementation of various practical photonic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. § Professor Yang passed away unexpectedly on Sept 26, 2013. We dedicate this work as a memorial to him. 2374

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dx.doi.org/10.1021/la4049117 | Langmuir 2014, 30, 2369−2375