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Thin Film Interference of Colloidal Thin Films Hailin Cong and Weixiao Cao* College of Chemistry and Molecular Engineering, Peking University, Beijing, 100871, People’s Republic of China Received April 6, 2004. In Final Form: June 23, 2004 A stairlike colloidal crystal thin film composed of poly(styrene-methyl methacrylate-acrylic acid) (P(StMMA-AA)) monodispersed colloids was fabricated on an inclined silicon substrate. Different bright colors were observed on the various parts of the film with different layers as white light irradiated perpendicularly on it. The relationship between the colors and layers of the film was investigated and discussed according to the principle of thin film interference. On the basis of the phenomenon of thin film interference, a one-layer colloidal film having uniform color was researched and it would display diverse colors before and after swollen by styrene (St). A circular stairlike colloidal film was achieved to mimic the colors of the peacock tail feather.
Introduction The beautiful colors of a peacock tail feather and a Morpho butterfly wing are not produced from pigments but from the order array of small dots or fine lines.1,2 The diffraction and scattering of light by microstructures on the animal’s body tell us nature got wise to photonic arrays long ago. The colors generated by the microstructures are called structural colors, which are important not only in biology but also for the understanding of photonic crystals.3 It is believed that photonic crystals could play an important role in the field of optical devices and communications.4-7 The ordered three-dimensional (3D) packing of monodispersed colloids to form colloidal or photonic crystals has been extensively investigated recently.8-10 By use of the changes of structural color generated from Bragg diffraction, a lot of functional and intelligent materials such as tunable band-gap materials, environment (pH, temperature, ionic, and stress, etc.) sensitive materials, photonic papers, and color changeable coatings have been developed.11-15 As we know, the soap bubble and oil films floating on water will display iridescent colors in sunlight caused by thin film interference.16 Can thin film interference be occurring at colloidal crystal films and causing them to show corresponding structural colors? In this article we * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Gu, Z. Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894. (2) Ball, P. Chem. Brit. 2003, 8, 23. (3) Gu, Z. Z.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2002, 41, 2067. (4) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (5) Wijnhoven, J. E. G. J.; Vos, W. L. Science 1998, 281, 802. (6) Blanco, A.; Chomski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P.; Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437. (7) Joannopoulos, J. D. Nature 2001, 414, 257. (8) Vlasov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature 2001, 414, 289 (9) Griesebock, B.; Egen, M.; Zentel, R. Chem. Mater. 2002, 14, 4023. (10) Reculusa, S.; Ravaine, S. Chem. Mater. 2003, 15, 598. (11) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (12) Foulger, S. H.; Jiang, P.; Ying, Y.; Lattam, A. C.; Smith, D. W., Jr.; Ballato, J. Adv. Mater. 2001, 13, 1898. (13) Gu, Z. Z.; Horie, R.; Kubo, S.; Yamada, Y.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2002, 41, 1153. (14) Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 9104. (15) Fudouzi, H.; Xia, Y. Adv. Mater. 2003, 15, 892. (16) Cutnell, J. D.; Johnson, K. W. Physics, 4th ed.; John Wiley & Sons: New York, 1997; p 837.
report that diverse colors are observed in a stairlike colloidal crystal thin film upon perpendicular irradiation of white light. An explanation for this unique optical phenomenon according to the principle of thin film interference has been proposed, and the application of this phenomenon in bionic and photonic materials has been discussed preliminary. Experimental Section Synthesis of Monodispersed P(St-MMA-AA) Colloidal Particle. Styrene (St), methyl methacrylate (MMA), and acrylic acid (AA) were distilled before use. Sodium dodecyl sulfate (SDS) was chemical grade reagent and purified by recrystalliztion in ethanol before use. Ammonium persulfate ((NH4)2S2O8) and ammonium bicarbonate (NH4HCO3) were chemical grade reagent and used as received. The monodispersed P(St-MMA-AA) colloids with ∼190 nm diameter were synthesized by emulsion polymerization, a typical sample was prepared as follows: A 120 mL portion of aqueous solution (A) containing (NH4)2S2O8 (0.4 g), NH4HCO3 (0.8 g), and SDS (0.08 g) in a funnel and 25 mL of monomer mixture (B) consisting of St/MMA/AA (90:5:5 v/v/v) in another funnel were added at the same time into a 250 mL flask, the mixture was stirred at 70 °C in a N2 atmosphere for 5 h to obtain a homogeneous latex with a particle diameter of ∼190 nm, and the latex particles are almost monodispersed. Monodispersed P(St-MMA-AA) colloids with ∼380 nm in diameter were synthesized with soap-free emulsion polymerization described elsewhere.17 Preparation of Colloidal Crystal and Characterization. Stairlike colloidal thin films were fabricated by depositing the colloids (∼380 nm in diameter) on an inclined silicon substrate as described previously.18 The circular colloidal film to mimic the peacock tail feather was achieved as follows: A drop of P(StMMA-AA) latex composed of monodispersed colloids (∼190 nm in diameter) with a concentration of 50 mg mL-1 was added carefully on a clean silicon substrate. After evaporation of the water, a circular colloidal thin film with the thickness increasing continuously from outside to center was obtained. A scanning electron microscope (Hitachi S-4200, Japan) was used to observe the structures and morphologies of the thin films, and a CCD microscope system (Sony XC-999P, Japan) was used to observe the colors of the film under perpendicular irradiation of white light.
Results and Discussion Stairlike Colloidal Thin Films. Figure 1a is a scanning electron microscopy (SEM) planar image of the (17) Cong, H.; Cao, W. Chem. J. Chin. Univ. 2003, 24, 1489. (18) Cong, H.; Cao, W. Langmuir 2003, 19, 8177.
10.1021/la049118+ CCC: $27.50 © 2004 American Chemical Society Published on Web 07/31/2004
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Figure 1. SEM and CCD images of the formed colloidal thin film: (a) SEM image; (b) CCD image.
Figure 2. SEM flattened images of the colloidal thin films shown in Figure 1a. (a-d) show details of the marked rectangular areas at the A, B, C, and D regions of Figure 1a, respectively.
formed colloidal thin film from ∼380 nm colloids, which can be divided into A, B, C, and D striped regions, and Figure 1b is the corresponding CCD image showing diverse colors of different regions under perpendicular irradiation of a white light. To understand the diverse colors of different regions in the film, detailed SEM images of the marked rectangular areas at the A, B, C, and D regions of Figure 1a are shown in Figure 2a-d, respectively. From Figure 2a-c, we can see that the A, B, and C regions are one-, two-, and threelayer films of colloids with hexagonal array, respectively. The square array (Figure 2d) was found in the D region, which is a transition area 18 from two layers to three layers of the film. Figure 3 shows the SEM section profiles of the colloidal thin film. Figure 3a (one layer), 3b (two layers), 3c (three layers), and 3d (two to three transition layer) are corresponding micrographs of the A, B, C, and D regions of Figure 1a. From Figures 2 and 3, we can see that the film shown in Figure 1a has a stairlike structure, and the main difference of the A, B, and C regions should lie in the thickness. Scheme 1 is an illustration of the section profiles
of the colloidal thin film with one to three layers. According to Scheme 1, the thickness of the A, B, and C regions can be calculated to be ∼380, ∼692, and ∼999 nm. Thin Film Interference Occurred in the Colloidal Films. Like the oil films floating on water showing iridescent colors due to thin film interference, we think thin film interference should also occur in the colloidal thin films and is the cause of various colors appearing at different thicknesses of the film. The lights reflected from top and bottom surface of the film will undergo thin film interference when the thickness (t) and refractive index (n) of the film satisfy eq 1 as the refractive index of the film is less than that of the substrate.16 In eq 1, λ is the wavelength of the reflected light and m is an arbitrary integer coefficient.
λ ) 2nt/(m + 0.5)
(1)
m ) 0, 1, 2, 3, ... The refractive index (n) of the colloidal thin film can be calculated according to eq 2,19 where Φ represents the (19) Schroden, R. C.; Daous, M. A.; Blanford, C. F.; Stein, A. Chem. Mater. 2002, 14, 3305.
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Figure 3. SEM section profiles of the colloidal thin film shown in Figure 1a. (a-d) show section profiles of A, B, C, and D regions of the film, respectively. Scheme 1. Schematic Illustration of the Section Profiles of the Stairlike Colloidal Thin Film (d Represents the Diameter of Colloids)
void ratio of the film and np and nair represent the refractive indices of the P(St-MMA-AA) and air, respectively. For a close packing (face-centered or hexagonal close packing) of spheres with equal diameter, the void ratio is ∼26%, which is calculated from a colloidal crystal with many layers of sphere, and the influence of the voids at the surface and bottom layers is neglected. But for the colloidal thin films with only a few layers of sphere, as shown in Scheme 1, the influence of voids at the surface and bottom layers cannot be neglected. The void ratios in this case for the one-, two-, and three-layer films can be calculated to be ∼40%, 34%, and 31%, respectively. The np is ∼1.59 due to ∼90% PSt in colloids composition (nPSt ) 1.59),17 and the nair equals 1.0. So the refractive indices of the A, B. and C regions can be calculated to be ∼1.35, 1.39, and 1.41, respectively.
n ) (1 - Φ)np + Φnair
(2)
For the A region of the film (t ) 380 nm, n ) 1.35), according to eq 1, the wavelength of the reflected light, which will be eliminated from the white light by the thin film interference, can be calculated to be 684 nm (m ) 1) and 410 nm (m ) 2). As a result the color of this region will show green, which accords well with the result of the experiment (see Figure 1b). If other values of m such as
m ) 0, 3, etc. are used, the calculated λ will be over the region of visible light that does not influence the film’s color. Bragg diffraction needs a set of lattice planes and cannot appear in one-layer colloidal films, so the green color shown in the A region is direct evidence of thin film interference occurring in the one-layer colloidal films. For the B region of the film (t ) 692 nm, n ) 1.39), the wavelength of the reflected light, eliminated from the white light by the interference, can be calculated to be 550 nm (m ) 3) and 428 nm (m ) 4). As a result the color of this region will show orange, which accords well with the result of the experiment (see Figure 1b). If other values of m such as m ) 0, 1, 2, etc. are used, the calculated λ will be over the region of visible light that does not influence the film’s color. And for a film with more than two layers, like the C region, the color becomes complicated because Bragg diffraction begins to appear in the multilayer colloidal crystal film and disturbs the structural colors of thin film interference. Application in Bionic and Photonic Materials. An interesting experiment performed in our laboratory recently that a circular colloidal thin film formed by just evaporation of the water from a drop of latex, which is composed of monodispersed colloids with the size of ∼190 nm in diameter, displays the colors similar to those of peacock tail feather (Figure 4a,b). And it has been confirmed that the circular colloidal film has the stairlike structure with the thickness increasing continuously from outside to center. As shown in Figure 4c-f, the A, B, and C regions of the circular colloidal thin film have one, two, and three layers, respectively. The colors predicted from eq 1 accord well with those shown in Figure 4a. A one-layer colloidal thin film of the ∼380 nm colloids displaying a homogeneous green color is shown in Figure 5a. We fix the one-layer colloidal film on a specimen stage of the CCD microscope, and scratch a “V” shape mark on the film using a sharp tool. We can see that the color of the film changes from green (Figure 5a) to yellow (Figure
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Figure 4. Colors and structure of the circular colloidal thin film: (a) colors of the film; (b) a beautiful peacock; (c) SEM image of the marked rectangular region of (a); (d-f) showing the details of the marked rectangular areas at the A (one-layer), B (two-layer), and C (three-layer) regions of (c), respectively.
Figure 5. CCD and SEM images of one-layer colloidal films before and after swelling by St: (a) CCD image before swelling; (b) after swelling; (c) after evaporation of St. SEM images; (d) SEM image of (c); (e) detail of the marked region of (d); (f) detail of the marked region of (e).
5b) as a drop of styrene (St) was trickled on the film (for 10 min at ∼ 25 °C), then the color will return to green (Figure 5c) about 20 min later when the styrene was all evaporated. Figure 5d-f shows SEM images of the marked film, Figure 5e and Figure 5f are the details of the marked rectangular regions in Figure 5d and Figure 5e, respectively. From the SEM images shown in Figure 5d-f, we
can confirm that the light yellow color of the “V” mark in Figure 5a-c is attributable to the reflection of the light from the silicon substrate, and the green (or yellow) color of Figure 5a and Figure 5c (or Figure 5b) is attributed to the reflection of light from the one-layer colloidal film. The arrows shown in Figure 5b indicate the width of the mark. It becomes narrow after the film is swollen by
Thin Film Interference Scheme 2. Schematic Illustration of the Variation in Thickness of One-Layer Film before and after Swelling by St (t0, the Thickness before Swelling; t1, the Thickness after Swelling)
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swelling is t0, it becomes t1 after swelling and turns back to t0 as the St was all evaporated. Correspondingly the color of the film is changed from green to yellow and recovered to green (see Figure 5a-c). It is believed that the different colors displayed by the one-layer colloidal film should be attributed to the diameter variation of the colloids, i.e., the thickness variety of the film. Conclusions Thin film interference was observed in the stairlike colloidal thin films composed of monodispersed P(St-MMAAA) colloids with ∼380 nm in diameter, and the structural colors of the one- and two-layer films caused by thin film interference were green and orange, respectively, under perpendicular irradiation of white light. As more than two layers, the film colors became complicated due to the presence of a Bragg diffraction, which would disturb the colors from thin film interference. On the basis of the principle of thin film interference, a circular stairlike colloidal thin film, composed of ∼190 nm colloids, was achieved to mimic the colors of the peacock tail feather. After the one-layer film of ∼380 nm colloids swollen by St, the thickness of the film increased, and the film’s color changes from green to yellow. The color will return to green again as the St in the film is all evaporated. The colorful display of the one-layer colloidal thin films caused by thin film interference may provide an easy way to make photonic papers.
St (compared to Figure 5a). When the St in the film is all evaporated, the width of the mark as well as the color of the film (see Figure 5c) are recovered. Scheme 2 is a schematic explanation for a one-layer colloidal thin film before and after being swollen by St. If the thickness before
Acknowledgment. The authors are grateful to the NSFC for financial support of this work (Grant No. 20274002). LA049118+
