Orientation Control of Hemispherical Janus Particles and Metal

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Orientation control of hemispherical Janus particles and metal coating on the selective surface to excite surface plasmon polaritons in the micro Kretschmann geometry So Aizawa, Keisuke Seto, and Eiji Tokunaga Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03572 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 14, 2017

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Orientation control of hemispherical Janus particles and metal coating on the selective surface to excite surface plasmon polaritons in the micro Kretschmann geometry

So Aizawa, *Keisuke Seto, and Eiji Tokunaga AUTHOR ADDRESS. Department of Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan KEYWORDS. Janus particle, orientation control, self-assemble, surface modifications, surface plasmon polaritons.

ABSTRACT Asymmetric-shaped particles (the Janus particle) are difficult to arrange with a uniform orientation on a solid substrate. This difficulty prevents further modification of the selective surface of the particles for fabrication of the Janus particle with anisotropy of the shape and surface. We successfully arranged hemispheric particles with a uniform orientation at the air/water interface. The particles were arranged on the solid substrate with the uniform

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orientation by transferring the particle film onto the substrate. This arrangement enabled fabrication of the Janus particles with anisotropy of the shape and surface by selective deposition of a film on either the equatorial plane or spherical surface. Additionally, we demonstrated the function of the microscopic Kretschmann geometry for excitation of the surface plasmon polaritons of a thin metal film on the equatorial plane of a single hemispheric particle.

INTRODUCTION

Janus particles (named after the double-faced Roman god “Janus”) are defined as particles having surfaces and/or shapes with two or more different physical and chemical properties.1 The fundamental feature of Janus particles is anisotropy due to asymmetric properties. Because of this anisotropy, various physical and chemical functions exist in one particle as follows. The particles can be given motility by imparting a propulsion force, controlling their orientation, or giving them anisotropic optical properties. Janus particles are expected to be useful not only for basic science, such as biochemistry, physics, and colloidal chemistry, but also for practical applications to chemical transportation, electronic devices, optical biosensors, and so forth. The particles of two different surfaces are applied to an optical biosensor for investigating rheology in the living cell and to a tracer in microfluidics, relying on an optical anisotropy of fluorescence or absorbance on a one side surface.2 In addition, they can be applied to chemical transportation with self-propelled force and mobility responding to the external field.3 One method to obtain the anisotropic optical response is surface modification on one side of the spherical particles with a metal coating to fabricate plasmonic Janus particles.4 The anisotropic plasmon resonance of the Janus particle induces an anisotropic photothermal


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effect, which enables manipulation of the particle,4d although anisotropic plasmon resonance is not measured for respective single Janus particles. The particles are also applied to display devices,5 such as electronic paper with the capability of alignment control. Nonspherical particles are used as the unit of unique crystal structures. The crystals function as new photonic crystals.6 The particles also express electrophoresis.7 The types of such functions and magnitude of the response depend on the shape and material distribution of the nonspherical particle. There are three approaches for fabrication of Janus particles. The first is modification of the selective surface on spherical particles of homogeneous material. In this approach, the spherical particles are arranged two-dimensionally on a substrate, and then the metal is vacuumdeposited only on one side.8 The second is anisotropic modulation of the shape or material. The methods include seed emulsion polymerization,9 precipitating a polymer in a good solvent with a poor solvent,10 and combining and shaping different materials by micro flow.5 Hemispherical,9a, 10

dimple,9b void-containing,9a,

9c

snowman-like,9d octopus ocellatus-like,9e mushrooms-like,9a

biconvex lens shaped,9a and other shaped particles have been produced. The third is a combination of these two approaches; that is, a thin film is formed only on one side of nonspherical particles. By modulating shape and surface to anisotropic materials, new and/or magnified functions are expected to be developed.

However, to fabricate a thin film selectively on one surface of such nonspherical particles, controlling the alignment of the particles is required. Due to this difficulty, the third approach has not been realized. We successfully aligned the orientation of hemispherical acrylic particles and arranged them two-dimensionally on the substrate. Furthermore, we selectively coated metals only on the equatorial plane or spherical surface of hemispherical particles. We called this particle the “metal-coated hemispherical Janus particle.” This new Janus particle has larger-


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shaped anisotropy than a spherical Janus particle, and there is a possibility that a new function has been developed. Then, to demonstrate the optical function related to surface and shape anisotropy, we excited by light the surface plasmon polaritons (SPPs) of the gold and silver thin films on the equatorial plane of single hemispherical particles. With such a structure, the hemispherical particles functioned as prisms, which matched the dispersion relation of SPPs and light in the Kretschmann geometry.11 The surface plasmon resonance in the Kretschmann geometry is applied as a probe for chemical sensing, since it is affected significantly by the indices of the mediums on the interface of the metal films.12 Chemical modification of the metal surface with an antibody for a specific sensing is also enabled.12 We confirmed that the light incident from the spherical surface excites the SPPs of the metal thin film, but that from the equatorial plane side does not. To the best of our knowledge, this is the first report of anisotropic plasmon resonance measured for single plasmonic Janus particles.

EXPERIMENTAL SECTION

1. Fabrication of hemispheric particle

Hemispheric particles were provided by Sekisui Plastics (XX - 3578 Z Lot. IG –8705; Osaka, Japan). The hemispheric particles were prepared using a seed polymerization method,9a where the monomer was adsorbed on prepared seeds to be polymerized. At first, spherical seed particles

were

prepared

via

seed

polymerization

of

isobutyl

methacrylate

with

polymethylmethacrylate (PMMA) seed particles. Then, a mixture of methyl methacrylate and poly (ethylene glycol propylene glycol) monomethacrylate was adsorbed in the seed particles and polymerized to generate the hemispheric particles. The particles were insoluble in organic


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solvents, since the material was a cross-linked polymer. The detailed synthesis methods are as follows. A mixture of 600 g water, 100 g methyl methacrylate, and 0.5 g of n-dodecyl mercaptan as a chain transfer agent was charged in a flask equipped with a stirrer and reflux condenser, and the temperature was increased to 70°C under a nitrogen purge while stirring. Then, potassium persulfate as the polymerization initiator was added to the mixture and the monomer was polymerized for eight hours to prepare an emulsion of the particle. This emulsion contained 14 wt% of 0.4-µm spherical particles (the weight-average molecular weight, 600,000). Then, 550 g water, 70 g of the aforementioned emulsion, and 100 g isobutyl methacrylate and n-dodecyl mercaptan were charged in the flask and polymerized under the same aforementioned procedure to prepare the seed particles. The emulsion contained 14 wt% of 1-µm spheric particles (the weight-average molecular weight, 610,000). A mixture of 600 g methyl methacrylate, 300 g ethylene glycol dimethacrylate, 100 g poly (ethylene glycol propylene glycol) monomethacrylate (Blemmer® 50PEP-300, Nippon Oil & Fat Corp., Tokyo, Japan), and 6 g azobisisobutyronitrile as the polymerization initiator was charged in a flask and mixed. The poly (alkylene glycol) monomethacrylate was a mixture of various chemical formulas of CH2 = C(CH3)COO[(C2H4O)m-(C3H6O)n]-H, where the average numbers of m and n were 3.5 and 2.5, respectively. The mixture was poured in 1 L water containing 10 g sodium succinate sulfonate as a surfactant, and an aqueous emulsion was prepared by mixing at 8000 rpm for 10 minutes with a TK homomixer (Primix, Mijdrecht, The Netherlands). Then, 360 g of the aforementioned emulsion of 1-µm seed particles was poured in the aqueous emulsion and mixed for three hours, and the monomers were adsorbed in the seed particles. Then, 2 kg of an aqueous solution of 40 g polyvinyl alcohol was added, and particles were polymerized for six


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hours at 60°C. The shape of the particles was verified as hemispheric with a scanning electron microscope (SEM). Particle size was 2.5 µm (sphere equivalent diameter).

2. Arrangement

Figure 1 shows the method for preparing metal-coated hemispheric Janus particles. First, 3 mg of the hemispherical particles were dispersed in 3 mL toluene (Kanto Chemical Co., Tokyo, Japan). This suspension was poured into 100 mL ion-exchanged water (Fig. 1a). Next, to release the clumping of the hemispherical particles, the suspension was sonicated by an ultrasonic cleaning machine (US-1; As One Corporation, Osaka, Japan) for 10 minutes. Toluene and water were emulsified and became cloudy (Fig. 1b). Then, toluene was left to evaporate for approximately eight hours. The hemispherical particle film spread on the water surface (Fig. 1c). Through these steps, the hemispheric particles were arranged two-dimensionally on the water surface. The particles were clumped again when left for more than one day. The film was transferred onto a hydrophilic glass slide. The hydrophilic surface was obtained as follows: the slide was immersed in a sodium hypochlorite type detergent (oxidizing agent) for one day, rinsed with ion-exchanged water, and dried. The hemispherical particle film was transferred onto the slide by vertically inserting it into the film and pulling it up (Figs. 1d, 1e). Janus particles coated with metal selectively on the equatorial plane were prepared by vacuum vapor deposition (VPC-260; Ulvac, Kanagawa, Japan) of a metal film on the transferred hemispherical particles film (Fig. 1f). Those coated selectively on the spherical side were prepared by deposition of metal on the reversed particle film. The reversed film was prepared by transferring the particles on a double-stick tape before depositing the metal and attaching the


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double-stick tape to the substrate (Figs. 1g, 1h). By choosing which side to face toward the substrate, one can select either the equatorial plane or the spherical surface to be coated. Particles stuck to the double-sided tape were removed readily with an organic solvent, such as acetone or toluene. To evaluate orientation of the particle films, silver was vapor-deposited on the asprepared particle film or reversed film. The thickness, deposition rate, and pressure were 50 nm, 0.65 nm/s, and 5.2 ×10−3 Pa, respectively. Then, the particles were treated chemically with the smoke solution. A morphological change on the silver surface was expected with this treatment.

3. Chemical composition and surface energy of hemispheric particles The particle was arranged with the aforementioned method on a cover glass, and chemical composition near the equatorial and spherical planes was evaluated with a Raman microscope (NRS3200; JASCO International Co. Ltd., Tokyo, Japan) based on Raman spectra of the test pieces of the seed and adsorbed materials. The excitation wavelength was 532 nm, and specifications of the objective lens were a magnification of ×100 and NA = 0.95. The spatial resolution along the focus direction was 0.69 µm. To investigate the driving force to determine the orientation of the particle, surface energies of the seed and adsorbed materials were evaluated with plate-shaped materials of the same compositions based on the expanded Fowkes’13 and Young’s equations. In the expanded Fowkes’ equation, the interfacial energy between substances 1 and 2 (γ 12) is assumed to be determined by the polar interaction (p) and hydrogen bonding (H), as well as dispersion force (d) as assumed in the Fowkes’ equation.14 The γ 12 is given by subtraction of the geometrical means


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of the surface energies caused by the interactions from the surface energies before the contact of the materials:

(

γ 12 = γ 1 + γ 2 − 2 γ 1d γ 2d

)

12

(

− 2 γ 1pγ 2p

)

12

(

− 2 γ 1H γ 2H

)

12

. (1)

Assuming the materials of 1 and 2 are solid (S) and liquid (L), Young’s equation is

γ S = γ SL + γ L cosθ . (2) Combining Equations 1 and 2, we obtain:

(

γ L (1 + cos θ ) = 2 γ Sd γ Ld

)

12

(

+ 2 γ Spγ Lp

)

12

(

+ 2 γ SH γ LH

)

12

. (3)

Solving the system of equations of Equation 3 with contact angles of various liquids of known

γ Ld, γ Lp, and γ LH, we obtain γ Sd, γ Sp, and γ SH. The surface energy of the solid surface is given by the sum of the energies14c:

γ S = γ Sd + γ Sp + γ SH . (4) We used diiodomethane, ethylene glycol, and water as the test liquids. The respective surface energies are shown in Table 1.13

4. Reflection spectra The film of the hemisphere particles on the water surface was transferred onto a cover glass, and the spherical surfaces were faced to the cover glass (Fig. 1f). A 50-nm thick gold thin film or 50-nm thick silver thin film was deposited on the equatorial plane of the particles. The deposition rates and pressures were 0.21 nm/s and 4.4 × 10−3 Pa, respectively, for gold and 0.3


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nm/s and 3.0 × 10−3 Pa, respectively, for silver. The reflection spectra were measured with a microscope (based on Eclipse TE300; Nikon, Tokyo, Japan). The optical system is shown in Figure 2. White light from a xenon lamp (LDLS; Energetiq Technology, Inc., Woburn, MA, USA) was collimated, and half of the collimated beam was blocked. The shape of the beam cross-section was semilunate. The s- or p-polarization was selected with the subsequent polarizer. The white light beam was guided to an objective lens (100×, NA = 1.49, TIRF; Nikon) through a strip light block on its entrance pupil. The white light was focused on the spherical plane of a single particle through the cover glass. In this configuration, we expected excitation of the SPP of the respective metal films at the metal-air interfaces. The width of the strip block was half of the pupil diameter. This block permits light to be input to around the edge of the pupil, and only a large incident angle component was irradiated on the sample. We paid attention to irradiating the center of the particle. Reflected light from the particle was collected and collimated by the objective. The collected light was reflected by a beam splitter and focused on the input optical fiber of a spectrometer (USB 2000; Ocean Optics, Amersham, United Kingdom) with an achromatic lens with f = 50 mm. The respective metal thin films on a slide glass without particle were measured as the reference spectra. We defined the relative reflection, R as follows:

R=

Reflection spectrum of metal corted hemispherical Janus particle . Refrection spectrum of metal thin film on glass slide

RESULTS AND DISCUSSION

1. Evaluation of orientation of arrayed hemispherical particles


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The results of selective deposition of the thin metal film on the equatorial plane or on the spherical surface of the hemispherical particles are as follows. Figure 3a shows an SEM (VE9800; Keyence, Osaka, Japan) image of the sample tilted at 60°. The scale bar is 1 µm. We found that the hemispherical particles were oriented with the spherical side facing the substrate side. Figure 3b is a SEM image of hemispherical particles under the reversing operation and deposition of a silver film onto them. In this sample, we found that the hemispherical particles were oriented with the equatorial plane facing the substrate side. From these Figures, it is clear that the orientation of the particles on the substrate certainly is controlled. Figure 4a shows the sample of Figure 3a chemically treated with the smoke solution. The silver vulcanizes by smoke solution, and the structure change appears only on the metal surface. In Figure 4a, the structural change is observed only on the equatorial plane, and the spherical surface is smooth. This result indicated that the silver film is formed only on the equatorial plane. Figure 4b shows the sample of Figure 3b with the smoke solution applied, and the structural change is on the spherical surface. The silver film is formed only on a spherical surface. From these results, we succeeded in fabricating thin metal films selectively on the equatorial plane or spherical surface of hemispherical particles.

2. Chemical composition and surface energy of hemispheric particles Figures 5a and 5b show Raman spectra of the adsorbed and seed materials, respectively. The spectrum of the adsorbed material (Fig. 5a) was almost the same as that of PMMA,15 since major component in the adsorbed material was cross-linked PMMA. The 813-cm−1 band in Figure 5a is assigned to the C-O-C symmetrical stretching mode. A major component in the seed material was poly (isobutyl methacrylate). In the Raman spectrum (Fig. 1b), vibrational bands of the isobutyl side chain were observed from 908 to 948 cm-1 and at 432 and 386 cm−1. The 813-


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cm−1 band of the C-O-C symmetrical stretching mode was shifted to 854 cm-1, which reflected the difference in the side-chain. The Raman spectra of the particle observed from the equatorial and spherical planes are shown in Figures 6a and 6b, respectively. The spectrum from the equatorial plane (Fig. 6a) was identical to that of the adsorbed material. Hence, the material near the equatorial plane was composed of the adsorbed material. A background like signal appeared around 400 cm−1 in the spectrum obtained from the spherical plane (Fig. 6b). This background signal was caused by an overlap of the 432- and 386-cm−1 bands of the seed material and/or scattering signal of the excitation laser due to the spherical shape. The background like signal evanesces above 700 cm−1, and hence, we safely exclude the possibility of the scattering of the excitation. A broad band was also observed around 913 cm−1 as indicated by the arrow. This broad band was assigned to an overlap of the bands of the seed material appeared from 948 to 908 cm−1 and at 854 cm−1. Therefore, we found that the material near the equatorial plane was principally composed of the adsorbed material, and the seed material was distributed mainly near the spherical plane. These result indicated that the spherical shape of the particle was formed in swelling of the seed material being suppressed of diffusion. If the seed material were diffused to homogeneously into the particle, the Raman spectra of the spherical and equatorial planes were identical to each other, and also, the shapes of the particles were formed in spherical due to isotropic mechanical property. The contact angles of the test liquids on the seed and adsorbed material are shown in Table 2, and surface energies obtained are in Table 3. A fairly large part of the surface energy of the adsorbed material was the polar component, which reflected more ether groups in the adsorbed material than in the seed material. The interfacial energies of the seed and adsorbed materials with the water were calculated to 28.6 and 33.8 mN/m, respectively, based on Equation 2.


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According to the Raman spectra, the spherical plane was composed of a mixture of the seed and adsorbed materials, and the equatorial plane was the adsorbed material. The particles were dispersed in toluene before the arrangement. Although the seed material was soluble in toluene, the Raman spectra of both planes indicated that the seed material stayed near the spherical plane. Actually, the shape of the arranged particles (Fig. 3) was not deformed. Since the radius of the particle was expanded by 2.5 times, the surface density of the seed material on the spherical plane was reduced less than 2.5−2 times after swelling. The 2.5−2 times reduced density is the maximum one under the assumption that polymer chains stay near the spherical surface. Therefore, the surface energy of the spherical plane was estimated as 29.4 × 2.5−2 + 54.4 × (1 − 2.5−2) = 50.4 mN/m at least, and the interfacial energy between the spherical plane and water was 28.6 × 2.5−2 + 33.8 × (1 − 2.5−2) = 33.0 mN/m. The surface energies of the equatorial plane and the maximum surface energies of the spherical plane were equal to those of the adsorbed material. A driving force for the orientation is explained as the minimization of the surface free energy.1 The interfacial tensions in the respective interfaces are shown in Table 3, 4. Let us assume the circumstances that the particle is placed on the interface with the orientations as in Figure 7. We assume that interfacial energy of the spherical plane ranges between those of aforementioned swelled seed material and adsorbed materials. The surface energies in the cylinder region indicated by the dotted lines are: (a) 120.3πr2 < γpe・πr2 + γpsw・2 πr2 < 122.0πr2 (b) 228.0πr2 < γpe・πr2 + γps・2 πr2 + γw・πr2 < 236.0πr2 (c) 134.6πr2 < γps・2πr2 + γpew・πr2 < 142.6πr2 (d) 207.4πr2 < γw・πr2 + γpw・2πr2 < 215.4πr2,


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where r is the radius of the particle, and γpe, γps, γpew, γpsw, and γw are the interfacial tensions of the interfaces between the equatorial plane and air, the spheric plane and air, the equatorial plane and water, the spheric plane and water, and water and air. Therefore, the surface energy is minimized in Situation (a); that is, the spherical plane is submerged, and the equatorial plane is exposed to air. The acrylic particles as prepared usually are clumped. Sonication is effective to release the clumping in the dispersion. However, The particles in water are precipitated after sonication without toluene, since the density of the main composition of PMMA is 1.2 g/cm3. We also confirmed that the pieces of seed and adsorbed materials submerged in the water. When toluene is added to the suspension, and it is sonicated, the acrylic hemispherical particles are drawn into toluene droplets. This is because the interfacial tension between the particle material and toluene is less than the critical interfacial tension of 25.9 mN/m, whereas that between the material and water is more than 28.5 mN/m. The density of toluene is 0.9 g/m3, and it rises to the water-air interface, carrying the PMMA particles when the suspension is left to stand.

3. Reflection spectra

When the metal is deposited selectively on the equatorial plane, the metal-coated particle configures the Kretschmann geometry for the metal film in which the hemispherical particles act as prisms. In the Kretschmann geometry, a prism having a high refractive index matches the dispersion relation and polarization of the SPP and p-polarized light. As a result, the SPP is excited at the crossing point (ω, k) of the SPP and light dispersion curves. When the prism is absent or the light is s-polarized, the dispersion relation or polarization between SPP and light does not match, and SPP is not excited. Thus, the SPP of the thin metal film is excited when


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the dispersion relations of SPP and light are matched by the particle. When the SPP is excited, a decrease in the reflectance is observed. Therefore, the reflection dip is expected for the metalcoated hemispherical particle with a p-polarized light incident from the spherical surface. The typical R spectra of a single gold-coated hemisphere are shown in Figure 8a. A reflectance dip appears at approximately 550 nm with p-polarized light irradiation from the spherical surface, whereas the dip is diminished with s-polarized light. The spectra of a single silver-coated hemisphere are shown in Figure 8b. In the case of the silver film, the reflectance dip appears at approximately 405 nm with p-polarized light irradiation, and the dip also is diminished with s-polarized light. From the dispersion relation of the SPPs of the gold and silver thin films, with the refractive index of acrylic of 1.49 and the film thickness of 50 nm, excitation of SPPs is known to occur in the range of 500 to 600 nm for gold and approximately 410 nm for silver.16 Therefore, the decreases in the observed reflectance are explained by the fact that SPPs of individual metals were excited. In this experiment, since it is incident at various angles, the superposition of reflection spectra at various incidence angles is obtained. Therefore, it is broader than that observed in the literature.16d

SUMMARY AND CONCLUSION

We demonstrated the orientation control of the hemisphere particles and deposition of a metal film on a selective plane to fabricate Janus particles with anisotropic shape and surface. The hemisphere particles were arranged in a film, and its equatorial planes were aligned to the air at the interface between water and air. This orientation was determined such that the surface energy was minimized. We prepared a suspension of the particles and left it to generate the film on the water surface. We dispersed the particles in toluene before suspending in water. Although


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clumping of the particles is released by sonication to form a water suspension, they are precipitated because of the higher density than that of water, so without toluene, spontaneous spreading of the particle film on the water surface does not take place. Before sonicating and suspending in water, dispersion of the particles in a “carrier medium” (toluene in this study) of smaller density and interfacial tension with particles than those of water is effective to form the particle film. The carrier medium rises to the water surface, carrying the isolated particles to form the homogeneous particle film. This film was transferred onto the glass substrate maintaining the orientation. The orientation was flipped with a sticker tape, and the particles were released readily with an organic solvent. This orientation control enabled selective deposition of the metal film. Selective deposition on the equatorial plane configures the Kretschmann geometry with a hemispherical prism. We demonstrated excitation of the surface plasmon polariton of gold and silver films in this micro Kretschmann geometry. We expect this kind of Janus particle to be applicable to chemical or magnetic sensors for microscopic regions, micro swimmers, anisotropic tracers, metamaterials, and so forth. The particles fabricated in this study will realize the microscopic Kretschmann geometry and microscopic sensing. The resonance also responds to the magnetic field,17 and the particles are applicable to a microscopic magnetic sensor. The particles also will be applicable to research for microswimmer and microfluid mechanics. They may have unique anisotropic optical responses between absorption of light incident on the spherical plane and reflection on the equatorial plane. It will induce an anisotropic photothermal effect, and the particles are driven by thermal energy.8b Additionally, the particles have potential to develop metamaterials for visible light. Fabrication of a sophisticated structure with a periodicity of the wavelength order is required for


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the metamaterials and, thus, its development for the visible wavelength is challenging.18 Two- or three-dimensional arrangements of the metal-coated Janus particles in this study of the visiblewavelength size will realize the function of the metamaterial for the visible light.


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Figure 1. Protocol to fabricate metal-coated hemispherical Janus particles. Suspension of hemispherical particles in toluene is poured on water (a). Aggregation of hemispherical particles is released by sonication (b). The film of hemispherical particles is formed after toluene is evaporated (c). The film is transferred onto the substrate by inserting it vertically and pulling it up (d, e). Metal is deposited by vacuum-deposition on as-prepared film (f). The particle film is reversed with a double-stick tape (g), and a metal film is deposited (h).


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Figure 2. Optical system for microscopic reflection spectroscopy with large angle of incidence.


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Figure 3. SEM image of silver-deposited particles. Deposition on as-prepared film (a) and reversed film (b).


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Figure 4. SEM image of vulcanized silver-deposited particles. Vulcanized sample of as-prepared film (a) and that of reversed film (b).


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Figure 5. Raman spectra of adsorbed material (a) and seed material (b). (b) Bands reflecting the difference in the side chain from PMMA are indicated by the Raman shifts in the boxes.


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Figure 6. Raman spectra of a hemisphere particle observed from the equatorial plane (a) and the spheric plane (b).


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Figure 7. Assumption of the location and orientation of the hemispherical particle to estimate surface energy for the respective situation. The dotted line indicates the cylindrical region for the estimation.


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Figure 8. Relative reflection (R) spectra of gold (a) and silver (b) films on the equatorial plane of hemispherical Janus particle with s- or p-polarized light incident from the spherical plane.


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Table 1. Interfacial tension of test liquid (mN/m, at 20°C)13 Liquid γ dL γ pL γ H L γL Diiodomethane 46.8 Ethylene glycol 30.1 Water 29.1 d p H γ L, γ L, and γ L: Surface

4.0 0 50.8 0 17.6 47.7 1.3 42.4 72.8 energy components determined by the dispersion force, polar

interaction, and hydrogen bonding, respectively.

γ L: Overall surface energy


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Table 2. Contact angle on seed and adsorbed material (°) Adsorbed Liquid Seed material material Diiodomethane 60.9 38.0 Ethylene glycol 67.7 56.3 Water 89.3 73.6


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Table 3. Surface energy of seed and adsorbed materials (mN/m) Test piece γ dS γ pS γ HS γS γ SW Seed material 25.9 2.1 1.4 29.4 28.6

Adsorbed 27.4 23.0 4.0 54.4 33.8 material γ dS, γ pS, and γ HS: Surface energy components of the solid material determined by the dispersion force, polar interaction, and hydrogen bonding, respectively.

γ S: Overall surface energy of the solid material γ SW: Interfacial energy between the solid material and water


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Table 4. Interfacial tension Interface Interfacial Tension (mN/m) Water-air 72.8 Toluene-air19 28.5 Toluene-Water20 37 Toluene-seed material Dissolve Toluene-Adsorbed < 25.9 material† † The contact angle of toluene was 0° and less than difference between the surface tensions of the adsorbed material and toluene.


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AUTHOR INFORMATION Corresponding Author *Keisuke Seto* Department of Physics, Faculty of Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan E-mail: [email protected] Funding Sources

ACKNOWLEDGMENT The hemisphere particle and cross-linked PMMA plate to test the interfacial tension is provided from Mr. Ryosuke Harada, Sekisui Plastic Co., Ltd., Nishi Tenman 2-4-4, Kita-ku, Osaka-shi, Osaka, Japan.

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Orientation Control of Hemispherical Janus Particles and Metal

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