Colloidal Crystals with Tunable Colors and Their Use as Photonic

result, the photonic papers could be fabricated to appear as colorless while the written patterns .... components: the base material (part A) and the ...
0 downloads 0 Views 439KB Size
Download PDF
Langmuir 2003, 19, 9653-9660

9653

Colloidal Crystals with Tunable Colors and Their Use as Photonic Papers Hiroshi Fudouzi† and Younan Xia* Department of Chemistry, University of Washington, Seattle, Washington 98195 Received May 27, 2003. In Final Form: August 20, 2003 This article describes the fabrication and characterization of colloidal crystals whose stop bands could be varied through the application of a liquid. Such a colloidal crystal was generated by infiltrating the voids within an opaline lattice of polystyrene beads with a liquid prepolymer to poly(dimethylsiloxane), followed by thermal curing. When a liquid (e.g., a silicone fluid, hexane, or octane) capable of swelling the elastomer matrix was applied to the surface of this crystal, the lattice constant and thus the wavelength of Braggdiffracted light was increased. For instance, the color of light diffracted from a colloidal crystal made of 175 nm polystyrene beads could be varied from violet to green, orange, and red simply by swelling it with different solvents. On the basis of this mechanism, we further demonstrated a photonic paper/ink system where color patterns could be conveniently generated on the surface of a thin film of colloidal crystal by writing with a Pilot pen, by screen printing, or by microcontact printing with an elastomer stamp. To fully illustrate the potential of this paper/ink system, we have demonstrated the fabrication of photonic papers as large as 75 cm2 in area, supported on both rigid substrates and flexible Mylar films. By judicially choosing the diameter of the polystyrene beads, it was also possible to adjust the color initially displayed by a photonic paper to any wavelength within the spectral region from ultraviolet to near-infrared. As a result, the photonic papers could be fabricated to appear as colorless while the written patterns displaying a shiny color, or vice versa.

Introduction Colloidal crystals are long-range ordered lattices assembled from spherical colloids such as polymer latexes and silica spheres.1 By organizing spherical colloids into a crystalline lattice, it is possible to obtain interesting functionality not only from the constituent material of the colloidal particles but also from the periodic structure associated with a crystalline lattice. The beautiful, iridescent color of an opal (natural or synthetic), for example, originates from the Bragg diffraction of a crystalline lattice assembled from silica colloids that display no color by themselves.2 A similar mechanism has also been used by a variety of insects such as butterflies and beetles to decorate their skins with shiny colors (i.e., the so-called structural colors) without involving the use of any conventional pigments.3 In recent years, colloidal crystals have been actively explored as a unique system for generating photonic band gaps that could be used to control the propagation of electromagnetic waves in all three dimensions of space.4 They have also been demonstrated for use as functional elements in fabricating diffractive optical devices. In general, the wavelength of light diffracted from a three-dimensional (3D) colloidal crystal is determined by the Bragg equation:5 † On leave from National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047, Japan. * Corresponding author. E-mail: [email protected].

(1) Recent reviews on colloidal arrays: (a) Gast, A. P.; Russel, W. B. Phys. Today 1998 (Dec), 24. (b) Grier, D. G., Ed.; From Dynamics to Devices: Directed Self-Assembly of Colloidal Materials, a special issue in MRS Bull. 1998, 23, 21. (c) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (d) Dinsmore, A. D.; Crocker, J. C.; Yodh, A. G. Curr. Opin. Colloid Interface Sci. 1998, 3, 5. (e) Velev, O. D.; Lenhoff, A. M. Curr. Opin. Colloid Interface Sci. 2000, 5, 56. (2) Sanders, J. V. Acta Crystallogr. 1968, A24, 427. (3) (a) Sinivasarao, M. Chem. Rev. 1999, 99, 1935. (b) Ghiradella, H. Appl. Opt. 1991, 30, 3492.

mλ ) 2dhkl(na2 - sin2 θ)1/2

(1)

where m is the diffraction order (e.g., the first or second order), λ is the wavelength of the diffracted light (or the so-called stop band), na is the mean refractive index of the crystalline lattice, dhkl is the interplanar spacing along the [hkl] direction, and θ is the angle between the incident light and the normal to the (hkl) planes. Equation 1 suggests that the wavelength (and thus the color) of light diffracted from a colloidal crystal is directly proportional to the lattice constant. Any variation in the lattice constant might lead to an observable shift in the stop band position and thus the color displayed by the surface of this crystal. In this regard, colloidal crystals may represent a class of ideal candidates for fabricating optical sensors that can be used to monitor, measure, and display environmental variations in terms of color changes (which can even be easily visualized by the naked eye). As a matter of fact, such a strategy has been employed by fishes (such as the (4) See, for example: (a) Xia, Y., Ed.; Photonic Crystals, a special issue in Adv. Mater. 2001, 13, 369. (b) Polman, A., Wiltzius, P., Ed.; Materials Science Aspects of Photonic Crystals, a special issue in MRS Bull. 2001, 26, 608. (c) Stein, A.; Schroden, R. C. Curr. Opin. Solid State Mater. Sci. 2001, 5, 553. (d) 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 (London) 2000, 405, 437. (e) Vlasov, Y. A.; Bo, X. Z.; Sturm, J. C.; Norris, D. J. Nature (London) 2001, 414, 289. (f) Bertone, J. F.; Jiang, P.; Hwang, K. S.; Mittleman, D. M.; Colvin, V. L. Phys. Rev. Lett. 1999, 83, 300. (g) Wang, D.; Caruso, R. A.; Caruso, F. Chem. Mater. 2001, 13, 364. (h) Lee, W.; Prunziski, S. A.; Braun, P. V. Adv. Mater. 2002, 14, 271. (i) Ozin, G. A.; Yang, S. M. Adv. Funct. Mater. 2001, 11, 95. (j) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (k) Rugge, A.; Tolbert, S. H. Langmuir 2002, 18, 7057. (5) (a) Carlson, R. J.; Asher, S. A. Appl. Spectrosc. 1984, 38, 297. (b) Yoshino, K.; Shimoda, Y.; Kawagishi, Y.; Nakayama, K.; Ozaki, M. Appl. Phys. Lett. 1999, 75, 932. (c) Richel, A.; Johnson, N. P.; McComb, D. W. Appl. Phys. Lett. 2000, 77, 1062. (d) Mayoral, R.; Requena, J.; Moya, J. S.; Lopez, C.; Cintas, A.; Miguez, H.; Meseguer, F.; Vazquez, L.; Holgado, M.; Blanco, A. Adv. Mater. 1997, 9, 257.

10.1021/la034918q CCC: $25.00 © 2003 American Chemical Society Published on Web 09/30/2003

9654

Langmuir, Vol. 19, No. 23, 2003

blue damselfish) to reconfigure their skin colors in response to environmental changes.6 There have been a number of demonstrations with regard to the use of a colloidal crystal as the sensing device. By embedding colloidal crystals in appropriate polymer hydrogels, Asher et al. have demonstrated the fabrication of temperature-, pH-, and ion-responsive optical sensors.7 The hydrogel colloidal crystals developed by Hu et al. and Lyon et al. have enabled them to tune the color of diffracted light either by varying temperature or by applying an electric field.8 Ford et al., Foulger et al., Tsutsui et al., and Yamanaka et al. have demonstrated that colloidal crystals embedded in thin films of appropriate polymer matrices could serve as mechanical sensors to provide a platform for in situ monitoring of strains caused by uniaxial stretching or compressing.9 In addition to opals, inverse opals fabricated by templating against opals have also been examined by Braun et al. for measuring pH changes.10 In all of these demonstrations, the lattice constant and thus the color exhibited by the colloidal crystal, as determined by the Bragg equation (1), varied in response to the environmental change(s). In a number of related studies, change in refractive index was also demonstrated as a means to detect variations in the environment. For example, Stein et al. have demonstrated the use of ceramic inverse opals (fabricated by templating against the lattice of an opal) in detecting organic solvents through the change in refractive index.11 Sato et al. and Caruso et al. have recently illustrated that the color change of a colloidal crystal could be adopted to detect the binding events of a biological species.12 For most of these demonstrations (in particular, those based on changes in lattice constants), the variation in color could be readily picked up by the naked eye. Here we would like to describe another application based on the similar mechanism, in which colloidal crystals whose colors are tunable by swelling with a liquid can serve as photonic papers for writing or printing colorful patterns without using conventional organic or inorganic pigments.13 The concept of this new paper/ink system has already been reported in a communication.13 Basically, the “paper” is a colloidal crystal of polymer beads embedded in an elastomer matrix made of poly(dimethylsiloxane) (or PDMS), and the “ink” is a liquid (e.g., a silicone fluid or any other organic solvent) capable of swelling the elastomeric matrix. As the elastomer is swollen by the ink molecules, the lattice constants (and thus the color of Bragg-diffracted light) are changed. When the colors of these two states are sufficiently different to be distinguishable by the naked eye, the contrast can be exploited to write and display color letters and patterns with certain (6) (a) Kasukawa, H.; Oshima, N.; Fujii, R. Zool. Sci. 1987, 4, 243. (b) Fujii, R.; Kasukawa, H.; Miyaji, K.; Oshima, N. Zool. Sci. 1989, 6, 477. (7) See, for example: Asher, S. A.; Holtz, J. H.; Weissman, J. M.; Pan G. MRS Bull. 1998, 23, 44. (8) (a) Hu, Z. B.; Lu, X. H.; Gao, J. Adv. Mater. 2001, 13, 1708. (b) Bebord, J. D.; Eustis, S.; Debord, S. B.; Lofye, M. T.; Lyon, L. A. Adv. Mater. 2002, 14, 658. (9) (a) Jethmalani, J. M.; Ford, W. T. Langmuir 1997, 13, 3338. (b) Foulger, S. H.; Jiang, P.; Ying, Y. R.; Lattam, A. C.; Smith, D. W.; Ballato, J. Adv. Mater. 2001, 13, 1898. (c) Sumioka, K.; Kayashima, H.; Tsutsui, T. Adv. Mater. 2002, 14, 1284. (d) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (10) Lee, Y. J.; Braun, P. V. Adv. Mater. 2003, 7-8, 563. (11) (a) Blanford, C. F.; Schroden, R. C.; Al-Daous, M.; Stein, A. Adv. Mater. 2001, 13, 26. (b) Ozaki, M.; Shimoda, Y.; Kasano, M.; Yoshino, K. Adv. Mater. 2002, 14, 514. (12) (a) Gu, Z. Z.; Horie, R.; Kubo, S.; Yamada, Y.; Fijishima, A.; Sato, O. Angew. Chem., Int. Ed. 2002, 41, 1154. (b) Cassagneau, T.; Caruso, F. Adv. Mater. 2002, 14, 1629. (13) Fudouzi, H.; Xia, Y. Adv. Mater. 2003, 15, 892.

Fudouzi and Xia

spatial resolutions. As the ink molecules are evaporating, the PDMS matrix will gradually shrink back to its original state, and thus the color patterns will be automatically erased. In this article, we will address a number of issues related to this paper/ink system: for example, the reversible tuning of color between UV and visible (or visible and near-IR) states; the duration times of various inks; control over the initial color displayed by a photonic paper, as well as the spatial resolutions of patterns that could be conveniently generated on the surface of such a paper using various methods. Experimental Section Chemicals and Materials. Monodispersed polystyrene (PS) beads were obtained as aqueous dispersions from Polysciences (Warrington, PA), Seradyn (Indianapolis, IN), or Duke Scientific (Palo Alto, CA), and they were used directly or diluted with ultrapure water for crystallization. Silicone fluids with viscosities in the range 0.65-50 cSt were purchased from Gelest (DMS-T and DMS-V series, Morristown, PA). The poly(dimethylsiloxane) (PDMS) elastomer was obtained from Dow Corning (Sylgard 184, Midland, MI). It was supplied as a kit containing two separate components: the base material (part A) and the curing agent (part B). The base and curing agent are usually mixed in a 10:1 ratio (by weight) and then thermally cross-linked. In our experiments, part A was diluted with silicone fluid DMS-T00 at a 1:1 ratio (by weight) to reduce the viscosity of this elastomer precursor. Ethanol was obtained from AAPER Alcohol (Shelbyville, KY). Organic solvents such as pentane, hexane, cyclohexane, 1-propanol, 2-propanol, 1-butanol, 1-pentanol, and 1-hexanol were obtained from Fisher Scientific (Houston, TX). Octane and iso-octane were purchased from Aldrich (Milwaukee, WI). All chemicals were used as received without further purification. Substrates. Glass slides and Si wafers were used as rigid substrates and Mylar films as flexible substrates. The glass slides were purchased from VWR Scientific (Cat. #48300-025, Westchester, PA) or Corning Glass (Micro Slides No. 2947, Corning, NY). They were cut into squares of 25 mm × 25 mm or 50 mm × 50 mm in area and sequentially cleaned under sonication by sequentially immersing in detergent water, acetone, and 2-propanol. Polished Si(100) wafers (test grade, phosphorus-doped) were obtained from Silicon Sense (Nashua, NH). Mylar films of 20 µm in thickness were obtained from Fralock of Lockwood Industries (Canoga, CA). The films were cut into squares of 50 mm × 50 mm in area. The surfaces of all substrates were made hydrophilic by briefly treating with oxygen plasma (Harrick Scientific, PDC-001, Ossining, NY). Before use, the substrates were rinsed with ultrapure, deionized water (E-Pure, 18 MΩ, Barnstead, Dubuque, IA), followed by drying in a stream of nitrogen gas. Fabrication of Photonic Papers. The first step involved the fabrication of 3D colloidal crystals as thin slabs by drying aqueous dispersions of PS beads on glass substrates. In a typical procedure, ∼0.5 mL of the commercial PS dispersion was placed on a piece of glass slide (25 mm × 25 mm) to form a thin layer of liquid film. The surface of this liquid film was then completely covered with a thin layer of silicone liquid (DMS-T11, 10 cSt). Finally, this sample was placed on a bench (under the ambient conditions of a laboratory) to let water slowly evaporate by diffusing through the skin of liquid silicone. The PS beads were driven into a long-range ordered, opaline lattice by the attractive capillary forces generated during water evaporation. After crystallization, the skin of liquid silicone was carefully removed from the surface of the colloidal crystal using Kim-Wipe, and the voids among PS beads were completely filled with the premixed elastomer of PDMS through capillary action. The elastomer was then cured at room temperature for one night, followed by additional hardening at 55 °C for 6 h. For photonic papers to be used with nonpolar organic solvents, cross-linked PS beads (or silica spheres) had to be used. Otherwise, the colloids might be partially dissolved when the ink molecules were applied to the surfaces of these papers. When silicone fluids or polar organic solvents were used as the inks, it was not necessary to use cross-

Colloidal Crystals with Tunable Colors

Langmuir, Vol. 19, No. 23, 2003 9655

Figure 1. (A) A schematic illustration of the process in which the lattice constant (and thus the wavelength of light diffracted from the surface) is increased by swelling the poly(dimethylsiloxane) (PDMS) matrix with an appropriate liquid. The PDMS matrix will shrink back to its original state once the liquid has completely evaporated. (B) The top-view SEM image of a photonic paper (made of 175 nm PS beads in a PDMS matrix) showing the (111) plane oriented parallel to the substrate. (C) The cross-sectional SEM image of this paper showing the planes perpendicular to the substrate. The coexistence of more than two sets of (111) plane indicates that this crystalline lattice had a cubic-close-packed structure. linked PS beads to fabricate the photonic papers because PS was barely soluble in these solvents. Instrumentation. The photographs were taken using a digital camera (Nikon, Coolpix 885, Tokyo, Japan). The optical micrographs were captured using an optical microscope (DMLM, Leica, Germany) equipped with a CCD camera. The scanning electron microscopy (SEM) images were recorded using a field emission microscope (FEI, Siron XL30, Hillsboro, OR), with an accelerating voltage of 5 kV. All samples for SEM studies were coated with thin layers of gold (∼25 nm thick) before imaging. The transmission spectra of colloidal crystal films were recorded on a UV-vis spectrometer (Hewlett-Packard, model 8452A, Palo Alto, CA; the beam spot was 5 mm2 in area). The reflection spectra of colloidal crystals were taken using a fiber-optic UV-vis spectrometer (Ocean Optics, ST2000, Dunedin, FL; the beam spot was 0.04 mm2 in area). The incident light was aligned perpendicular to the (111) planes of the colloidal crystal for all optical measurements. When the incident angle was varied, the position of the stop band (and thus the color of diffracted light) would be changed according to the Bragg equation.

Results and Discussion Operation Mechanism for the Photonic Paper. Figure 1A schematically illustrates how a photonic paper/ ink system works. The paper is an opaline lattice of monodispersed PS beads whose void spaces are infiltrated with an elastomer such as PDMS. The lattice constant of this composite structure is tunable between d1 and d2 due to the swelling and shrinking of PDMS matrix by a liquid (i.e., the ink). According to the Bragg equation, the color of light diffracted by this crystalline lattice will be reversibly changed between two wavelengths. If the colors of these two wavelengths are sufficiently different to be distinguishable by the naked eye, one can use their contrast to write and display colored letters and patterns by applying the ink to localized regions. Once the ink has completely evaporated, the PDMS matrix will shrink back to its original state, and the color patterns will be automatically erased. For PDMS matrix, the ink can be a silicone fluid or any organic solvent capable of swelling

this elastomer.14 Figure 1B,C shows SEM images of the top surface and cross section of a photonic paper. These two images suggest that the opaline lattice of this photonic paper exhibit a cubic-close-packed structure, with one set of the (111) planes oriented parallel to the surface of the supporting substrate. Because of a relatively low contrast between the refractive indices of PS and PDMS (1.592/ 1.430) and the spherical symmetry of the lattice points, this opaline lattice only exhibited a stop band. The crosssectional SEM image also implies that the voids among PS beads has been completely filled with the PDMS elastomer. Color Change and Its Reversibility. Figure 2 shows the photographs of a typical example of photonic paper, together with its optical properties. The paper was fabricated on a 4 in. Si wafer by embedding an opaline lattice of 202 nm PS beads in the PDMS elastomer matrix. Parts A and B of Figure 2 show photographs of this paper before and after it had been swollen with 2-propanol. The color diffracted from the pristine surface of this paper at normal incidence was green (Figure 2A). When 2-propanol was applied to the surface of this paper, its color changed from green to red (Figure 2B) due to the swelling of PDMS network, as illustrated in Figure 1A. It is worthy of noting that the crystallization method described in this article was able to routinely generate photonic papers with a uniform color over areas as large as ∼75 cm2. In fact, the surface of this photonic paper was sufficiently uniform and smooth that the reflected light also clearly displayed an image of the camera used to shoot the photographs. After the 2-propanol had completely evaporated, the photonic paper went back to its original green color with full recovery (as the PDMS matrix shrank to its original state). (14) (a) Mathison, D. E.; Yates, B.; Darby, M. I. J. Mater. Sci. 1991, 26, 6. (b) Campbell, D. J.; Beckman, K. J.; Calderon, C. E.; Doolan, P. W.; Ottosen, R. M.; Ellis, A. B.; Lisensky, G. C. J. Chem. Educ. 1999, 76, 537.

9656

Langmuir, Vol. 19, No. 23, 2003

Fudouzi and Xia

Figure 2. (A, B) Photographs of a photonic paper before and after its color had been changed by covering the surface with 2-propanol liquid. The paper was supported on a 4 in. silicon wafer and composed of 202 nm PS beads in a PDMS matrix. (C) Reflectance spectra taken from the surface (at normal incidence) of this paper before and after its PDMS matrix had been swollen with 2-propanol liquid. (D) A plot showing the variation of the reflectance intensity measured at 550 nm with time. The plateaus observed in the periods of 20-110 and 190-250 s were caused by the excess amounts of 2-propanol deposited on the surface of this photonic paper.

Figure 2C shows the reflectance spectra taken from this photonic paper, with the incident light being aligned along the normal to the (111) planes of the colloidal crystal. The stop band of this paper was initially located at 545 nm (green color) and shifted to 604 nm (red color) after it had been swollen with 2-propanol liquid. As 2-propanol was evaporating, the PDMS matrix gradually shrank back to its original state, and the diffraction peak was able to fully recover its initial position at 545 nm. Such a color change was reversible, accompanying the swelling and shrinking of the PDMS matrix of a photonic paper. Figure 2D shows the intensity of reflected light measured at 545 nm as a function of time. The intensity dropped quickly during the swelling of the photonic paper, and the writing of a color pattern could be completed essentially within a few seconds. This time scale was mainly determined by the diffusion of the solvent molecules into the elastomer matrix and the expansion dynamics of the PDMS network. On the other hand, the time required for erasing a colored pattern was largely controlled by the amount of ink deposited on the surface of this photonic paper and the evaporation rate of the ink under ambient conditions. For 2-propanol, the intensity at 545 nm could be completely recovered within a period of 10 s, after the excess amount of solvent had already evaporated. Our preliminary results indicate that such a reversible change between colors could be repeated for more than 20 times without observing any deterioration in the quality of displayed color.

Pattern Formation on Photonic Papers. We have demonstrated a number of methods for writing colorful patterns on the surface of a photonic paper. Figure 3 shows four typical examples, with silicone fluid DMS-T05 (5 cSt) serving as the ink. The photonic papers were made of 202 nm PS beads, and they initially exhibited a green color when viewed at normal incidence. When the ink was applied to their surfaces, the regions swollen by the ink changed color locally from green to red. Figure 3A shows an array of squares patterned on the surface of a photonic paper supported on a 4 in. Si wafer by screen printing. Figure 3B,C shows a number of alphabetic letters printed using a conventional rubber stamp. In a typical procedure, the surface of a stamp was inked with the silicone fluid by Q-tip wiping and then brought into contact with the surface of a photonic paper for a few seconds. Only silicone fluid on the raised regions of the stamp was transferred to the surface of a photonic paper. The photonic paper was fabricated on top of a rigid Si wafer (Figure 3B) and a flexible Mylar film (20 µm in thickness, Figure 3C). Note that the crystallized PS beads had a good adhesion to the Mylar film. Such a flexible photonic paper could be folded or rolled up without peeling off the colloidal crystal from the Mylar film. In other demonstrations, letters of smaller sizes were also printed on the surface of a photonic paper by delivering the silicone ink with a microfabricated PDMS stamp. Like in microcontact printing, such a PDMS stamp could be fabricated by casting against a master whose

Colloidal Crystals with Tunable Colors

Langmuir, Vol. 19, No. 23, 2003 9657

Figure 3. Patterns generated on the surfaces of photonic papers with silicone fluid DMS-T05 as the ink. The photonic papers were assembled from 202 nm PS beads in a PDMS matrix. (A) An array of square pads screen-printed on a rigid photonic paper (supported on a 4 in. Si wafer). (B) Letters printed on the surface of a rigid photonic paper on a Si wafer using a rubber stamp. (C) Letters printed on the surface of a flexible photonic paper supported on a Mylar film (20 µm in thickness). (D) Reflective optical micrograph of a Greek character generated by microcontact printing with a PDMS stamp on a rigid photonic paper supported on a glass substrate.

surface had been patterned with appropriate relief structures.15 Figure 3D shows the optical micrograph of a Greek character, “ω”, which had an edge resolution better than ∼50 µm. Such a good resolution should make the present paper/ink system particularly promising for color printing and displaying with reasonably high resolutions. Because of the relatively low evaporating rate of this silicone fluid, the printed patterns could last for several days when left in an ordinary laboratory. If necessary, the patterns could be quickly erased by immersing the paper in a volatile solvent such as 2-propanol, followed by drying in air. The original green color of the photonic paper could be recovered without observing any change in uniformity for its appearance. Tuning of Color through the Use of Different Inks. Any solvent (e.g., a silicone fluid, an alcohol, hexane, or toluene) capable of swelling the PDMS matrix can serve as the ink to write colorful patterns on the photonic paper. The color displayed by the pattern depends on the magnitude of swelling, which is, in turn, determined by the strength of interaction between the ink molecules and the PDMS network.16 As a result, different inks usually led to the appearance of different colors when they were applied to the same photonic paper. Figure 4A shows the change in stop band position for a photonic paper when it was immersed in two different silicone inks: DMS-T15 (50 cSt) and DMS-T00 (0.65 cSt). Two major changes were observed for the stop band associated with this photonic paper: its position was red-shifted toward longer wave(15) For a review on microcontact printing: Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (16) Bueche, A. M. J. Polym. Sci. 1955, 15, 97.

lengths (due to change in the lattice constant), and its attenuation was slightly increased (as a result of change in the mean refractive index). The transmission spectrum (Figure 4A) of this paper made of 202 nm PS beads displayed a stop band at 555 nm, which was shifted to 570 and 700 nm when the surface of this paper was derivatized with silicone fluid DMS-T15 and DMS-T00, respectively. Figure 4B shows the stop band position, λmin, of this photonic paper as a function of the molecular weights of various silicone fluids, from DMS-T00 (0.65 cSt) to DMST15 (50 cSt). For this particular sample, its stop band position could be conveniently changed to cover the spectral region from 560 to 700 nm by using different silicone fluids as the inks. In accordance, the color displayed by this photonic paper (at normal incidence) could be swept from green to red. This demonstration suggests that it would be possible to achieve full-color writing or printing on the same surface by employing silicone fluids of different molecular weights as the inks. There are also many other common organic solvents that could serve as inks for photonic papers based on the PDMS system.14,16 In general, nonpolar solvents interact with the PDMS network more strongly than polar ones. Figure 5A compares the transmission spectra of a photonic paper (made of 175 nm PS beads) before and after it had been swollen with 2-propanol and hexane. For this photonic paper, the PS beads had also been crystallized into a cubic-close-packed lattice, with its (111) planes oriented parallel to the surface of the supporting substrate. The stop band position of this photonic paper in air was located at 450 nm (or blue in color at normal incidence). When 2-propanol or hexane was applied, the stop band

9658

Langmuir, Vol. 19, No. 23, 2003

Figure 4. Use of various silicone fluids as the inks to tune the stop band position. (A) UV-vis transmission spectra (at normal incidence) taken from a photonic paper made of 202 nm PS beads. The original spectrum is plotted as the dashed line, and the spectra were plotted as solid lines when this paper was swollen with silicone fluid DMS-T00 (0.65 cSt) and DMS-T15 (50 cSt). (B) Plot of the stop band (or stop band) position, λmin, as a function of the molecular weight associated with the silicone fluids whose viscosities varied in the range from 0.65 to 50 cSt. This plot indicates that λmin is inversely proportional to the log of the molecular weight of the silicone fluids.

position was shifted to 540 nm (green color) and 720 nm (red color). Figure 5B,C plots the stop band position, λmin, of this photonic paper after it had been swollen with a range of different alcohols (polar solvents) and alkanes (nonpolar solvents). In Figure 5D, the magnitude of shift in stop band position (∆λmin, relative to the sample surrounded by air) is plotted as a function of the molecular weight for these solvents, circle for polar solvent and triangle for nonpolar solvent. In general, the shift caused by a polar solvent was less than 100 nm, while that caused by a nonpolar solvent was in the range 200-300 nm. We believe that the availability of diversified inks to tune the colors of a photonic paper in the entire spectral range of visible light should make this paper/ink system particularly useful for displaying letters and patterns with full colors. Duration Time of the Written Pattern. For the purpose of recording letters or patterns, one has to consider the period of time that an ink will last. This is particularly important for the present ink/paper system because the printed pattern will start to change its color as soon as the ink molecules start to evaporate. The duration time of a written pattern can be conveniently defined and determined by following the change in transmission spectra after the surface of a photonic paper has been covered with the ink. Figure 6A shows the transmission spectra taken from a photonic paper (made of 202 nm PS embedded in PDMS matrix) after its surface had been derivatized with silicone fluid DMS-T01.5 (1.5 cSt). Note that the stop band position, λmin, associated with the swollen paper (at 715 nm) was gradually shifted toward 570 nm as the sample was dried due to the evaporation

Fudouzi and Xia

of ink molecules. Figure 6B shows the stop band position as a function of time, relative to the point where we started to observe positional change for the stop band. The duration time of this ink was defined as the period between the points where we started to observe a shift in the stop band until where the shift just stopped. This definition eliminates the errors that might be introduced when different amounts of inks were applied to the surface of a photonic paper. Figure 6C shows the relationship between the molecular weights of silicone fluids and their duration times. As the molecular weight of silicone fluid was increased, the evaporation rate was reduced, and thus the duration time of the ink was monotonically increased. By choosing liquids with different vapor pressures (or boiling points) as inks, we could easily control the period of time during which a color pattern was supposed to last. Our preliminary results indicated that this period could be stretched to cover time scales ranging from a few seconds to hundreds of hours. Even with silicone fluids containing oligomers of various molecular weights, we could tune the duration time from a few hours to several months. If necessary, the color pattern could also be permanently fixed by thermally grafting the ink molecules (e.g., silicone oligomers terminated in vinyl groups) to the PDMS backbones through an additional curing process (see Figure 7C and the text for more detailed description). Tuning the Initial Color Displayed by a Photonic Paper. The initial color of a photonic paper is mainly determined by the center-to-center distance between the PS beads embedded in the PDMS matrix, as described by the Bragg equation (1). There were two different ways to tune this distance and thus the initial color of a photonic paper. One way was to change the diameter of PS beads, and the other one was to increase the separation between the PS beads by consecutively swelling the PDMS matrix with a silicone fluid containing vinyl-terminated oligomers, followed by thermal-induced grafting. Figure 7A illustrates how the initial color of a photonic paper could be tuned by controlling the diameter of the PS beads. Here the UV-vis transmission spectra were taken from photonic papers assembled from PS beads of various sizes: (i) 172, (ii) 221, and (iii) 250 nm. When these papers were swollen with 2-propanol, their colors changed from blue to green for sample i, from green to red for sample ii, and from red to colorless (with the new stop band positioned in the near-IR region) for sample iii. After the solvent had completely evaporated in air, these photonic papers returned to their initial colors. Figure 7B shows a linear relationship between the stop band position, λmin, of the photonic paper and the diameter of the PS beads. This plot implies that one can continuously tune the stop band position to cover the entire visible region from 400 to 800 nm (or from violet to red color) simply by using PS beads with diameters varied in the range from ∼150 to ∼300 nm. Figure 7C demonstrates the ability to tune the stop band position of a photonic paper using the second method. In this case, the photonic paper was fabricated by embedding an opaline lattice of 202 nm PS beads in the PDMS matrix (corresponding to sample 1 in Figure 7C). The composite film was then swollen with DMS-V00 (0.7 cSt), followed by thermal grafting of vinyl-terminated oligomers in this silicone to the PDMS backbone (corresponding to sample 2 in Figure 7C). The stop band position was shifted from 557 to 586 nm due to the increase in center-to-center distance between the PS beads. This swelling and curing cycle could be repeated for a number of times, with the stop band position being monotonically

Colloidal Crystals with Tunable Colors

Langmuir, Vol. 19, No. 23, 2003 9659

Figure 5. Tuning of stop band position in the visible light region (400-750 nm) by using various organic solvents as the inks. (A) The UV-vis transmission spectra taken from a photonic paper (made of 175 nm PS beads) before and after it had been swollen with 2-propanol and hexanes. The colors of these films corresponded to violet (initial), green (with 2-propanol), and red (with hexane). (B, C) Plots showing the stop band positions, λmin, of this photonic paper after the PDMS matrix had been swollen with various polar (B) and nonpolar (C) solvents. (D) Dependence of the shift in stop band position (∆λmin, relative to the sample measured with the paper surrounded by air) on the molecular weights of various solvents. All measurements were based on photonic papers made of 175 nm PS beads, and the color displayed by the pristine surfaces of these papers at normal incidence was violet.

Figure 6. (A) UV-vis transmission spectra of a photonic paper after its surface had been covered with silicone fluid DMST01.5 (1.5 cSt) and then allowed to dry under the ambient condition of a wet chemical laboratory. As the sample was dried, the stop band was blue-shifted due to evaporation of the silicone fluid. (B) Plot of stop band position, λmin, as a function of time for the silicone fluid (i.e., ink). The duration time of this ink is defined as the period between the starting and ending points of the peak shift. (C) Dependence of duration time on molecular weight for a number of silicone fluids. Note that no change in peak position could be detected over a period of 1 week for silicone fluids higher than DMS-T05 (5 cSt). The photonic papers used in all these measurements were made of 202 nm PS beads.

increased from 560 to 650 nm. As shown by our SEM studies reported in a previous publication, the separation between PS beads could be increased by as much as 11.6% (e.g., from 202 to 230 nm) as the PDMS matrix was swollen

Figure 7. Tuning color initially displayed by a photonic paper: (A) Photonic papers of blue, green, and red in color that were fabricated from PS beads of 172, 221, and 250 nm in diameter, respectively. When covered with 2-propanol, their colors turned into green, red, and invisible (in the near-IR region). (B) Linear plot of the stop band, λmin, as a function of the diameter of PS beads. Note that the mean diameter of PS beads in each sample had to be determined from analysis of Bragg equation (see details in ref 5c), and it might be different from the value provided by the vendor. (C) Variation of the initial color of a photonic paper (made of 202 nm PS beads) by increasing the separation between lattice points. Here the spacing between PS beads was increased as the PDMS matrix was swollen with vinylterminated silicone fluid (DMS-V00), followed by thermal grafting. The dependence of λmin as a function of the number of rounds that DMS-V00 was added to the original PDMS matrix via swelling and grafting.

9660

Langmuir, Vol. 19, No. 23, 2003

Fudouzi and Xia

region around 590 nm. Curve ii shows the transmission spectrum of this paper after its surface had been decorated with silicone fluid DMS-T00 (0.65 cSt). As the PDMS matrix was swollen, the stop band of this photonic paper was relocated to 775 nm, which is essentially on the borderline between the visible and near-IR regions. This paper also displayed a second-order Bragg diffraction peak at ∼390 nm originated from its (111) planes, which was located in the UV region. As a result, the initial color of this photonic paper was red, and it would change to a transparent, colorless state after it had been swollen with the silicone fluid. In reality, both schemes could be employed to write and display messages with high contrasts because either the pattern or the background will be colorless. Conclusion

Figure 8. (A) UV-vis transmission spectra taken from a paper made of 155 nm PS beads before (curve i) and after (curve ii) it had been swollen with iso-octane. The film changed color from colorless (in the UV region) to green. (B) UV-vis transmission spectra taken from a photonic paper before (curve i) and after (curve ii) it had been swollen with DMS-T00. The film changed color from orange to colorless (in the near-IR region). The paper was assembled from 202 nm PS beads, and its PDMS matrix had been swollen and grafted with the vinylterminated silicone fluid for three times as demonstrated in Figure 7C.

with the silicone fluid.13 This result was consistent with the shift in λmin derived from the spectroscopic measurements. Note that this second method also enables the permanent writing or recording of colored patterns on the surface of a photonic paper. Switching between Visible and Invisible States. All above demonstrations are based on color switching within the visible region, and the performance of writing strongly depends on the contrast between the two visible colors. By selecting PS beads with appropriate sizes and solvents with appropriate properties, it was also possible to fabricate photonic papers whose colors could be switched between an invisible state (i.e., with the stop band located in the region either below 400 nm or above 780 nm) and a visible one. For these systems, the photonic papers would appear colorless while the recorded patterns would be brightly colored, or vice versa. Figure 8A shows the transmission spectra of a photonic paper made of 155 nm PS beads. As shown by curve i, this paper exhibited a stop band in the UV region with its stop band position, λmin, located at ∼380 nm. After this paper had been swollen with octane, the stop band was shifted toward the visible region (see curve ii, ∼540 nm, green color). In accordance, the colorless paper changed its appearance to green color after it has been swollen with iso-octane ink. Figure 8B shows the transmission spectra of another photonic paper that was made of 202 nm PS beads embedded in PDMS matrix (see Figure 7C, after the swelling and curing for three times). This photonic paper (see curve i) exhibited a stop band in the visible

We have demonstrated the concept of a photonic paper/ ink system by embedding a colloidal crystal of polymer beads in an elastomeric matrix and by judicially selecting a liquid capable of swelling the elastomer. This system was further combined with conventional tools to demonstrate its potential in writing and printing color letters and patterns with the use of colorless materials only. Depending on how the rubber stamp was fabricated, colored patterns with an edge resolution as high as 50 µm could be routinely achieved. Depending on the vapor pressure of the ink liquid, the colored patterns could be programmed to last for intervals of time ranging from a few seconds to hundreds of hours. Once the ink molecules had completely evaporated, the color patterns would be automatically erased. For each paper, the writing and erasing could be repeated for more than 20 times without observing any deterioration in quality. If necessary, the color pattern could also be permanently fixed by thermally grafting the vinyl-terminated ink molecules to the PDMS matrix of a photonic paper. As we have already demonstrated, the photonic paper could be routinely fabricated with a uniform color over a relatively large area: for example, on a 4 in. Si wafer. The photonic paper could also be fabricated on top of a Mylar film (∼20 µm thick) that was sufficiently flexible to be bent or rolled up without fracturing the colloidal crystal. We believe that this system may provide an alternative route to the realization of reusable papers or recording media where no pigment will be required for displaying letters or patterns in full colors. Acknowledgment. This work was supported in part by an AFOSR-MURI grant (on smart skin materials) awarded to the UW and a Fellowship from the David and Lucile Packard Foundation. Y.X. is an Alfred P. Sloan Research Fellow (2000) and a Camille Dreyfus Teacher Scholar (2002). H.F. thanks National Institute for Materials Science and the Ministry of Education, Culture, Sports, Science, and Technology of the Japanese Government for the financial support to study at an abroad university or research institute. LA034918Q