Colloidal Crystals with Tunable Optical Properties - American

Chapter 25

Photonic Papers: Colloidal Crystals with Tunable Optical Properties

Downloaded by OHIO STATE UNIV LIBRARIES on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch025

Hiroshi Fudouzi, Yu Lu, and Younan Xia* Department of Chemistry, University of Washington, Seattle, WA 98195 *Corresponding author: [email protected] (email)

This paper demonstrates the fabrication of colloidal crystals with tunable optical properties and their utilization as photonic papers for displaying colored letters and patterns. In a typical procedure, monodispersed spherical colloids were assembled into a three-dimensional crystal, followed by infiltration and curing of an elastomer such as poly(dimethylsiloxane). When an ink (i.e., any liquid capable of swelling the elastomer) was applied to the surface of this crystal, the lattice constant (and thus the color of Bragg-diffracted light) was changed. If the difference between the colors of the initial and final states is sufficiently large to be distinguishable by the naked eye, this system can be used to write and print colored patterns with an edge resolution as high as ~50 μm.

© 2005 American Chemical Society In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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330 Colloidal crystals are long-range ordered lattices assembledfromspherical colloids such as polymer latexes and silica beads (I). The ability to organize colloidal particles into crystalline lattices has enabled us to obtain interesting (and often useful) functionality not onlyfromthe constituent material of the colloidal particles, but alsofromthe periodic structure intrinsic to a crystalline lattice. The beautiful, iridescent color of an opal, for example, originates from the optical diffraction of a crystalline lattice made of silica colloids that display no color by themselves (2). In recent years, colloidal crystals have been actively explored as photonic band gap (PBG) materials to control the propagation of electromagnetic waves in the three-dimensional space (3). They have also been demonstrated for use as functional elements in fabricating diffractive optical devices. The wavelength of light diffracted from the surface of a colloidal crystal is determined by the Bragg equation (4): 2

2

m^ = 2.d (n - sin 0) hkl

l/2

(1)

a

where m is the diffraction order (e.g., the 1st or 2nd order), λ is the wavelength of the diffracted light (or the so-called stop band), n is the mean refractive index of the crystalline lattice, d is the interplanar spacing along the [hki] direction, and θ is the angle between the incident light and the normal to the (hki) planes. The average refractive index is related to the filling ratio,/ of spherical colloids in the crystalline lattice by the following equation: a

hkl

n ^ f ^ + O-tyno

(2)

where it\ and n are the refractive indices of the colloids and the surrounding medium, respectively. Equation (1) suggests that the wavelength of light Braggdiffractedfromthe surface of a colloidal crystal is dependent upon the angle (Θ) between the incident light and the normal to the (hki) planes, average refractive index of the crystal (na), and the lattice constants. In particular, any variation in the lattice constants will lead to an observable shift in die stop band position, and thus the color displayed by the crystal. In this regard, a colloidal crystal is able to serve as an optical sensor to monitor, measure, and display environmental variation in terms of change in color that can be easily and clearly visulized by the naked eye. By embedding colloidal crystals in appropriate polymer hydrogels, Asher et al have demonstrated the fabrication of temperature-, pH-, and ion-responsive optical sensors (5). The hydrogel colloidal crystals developed by Hu et al and Lyon et al have enabled them to tune the color of diffracted light by varying temperature or by applying an electric field (6). Stein et al have explored the use of a ceramic inverse opal (fabricated by replica molding against die lattice of a colloidal crystal) in detecting various organic solvents due to the changes in mean refactive index (7). Sato et al and Caruso et al recendy illustrated that the reversible color tuning of a colloidal crystal could be adopted to detect the 0

In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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binding events of a biological species (8). Ford et al, Foulger et al, and Tsutsui et al demonstrated that colloidal crystals embedded in thin films of appropriate polymers could serve as mechanical sensors to monitor in situ strains caused by uniaxial stretching or compressing (9). In all of these demonstrations, the lattice constants (and thus the color displayed by the colloidal crystal as a result of Bragg diffraction) changed in response to the environmental change(s). In some cases, the variation in color could be readily picked up by the naked eye. Here we describe another application based on the same mechanism (10), in which colloidal crystals with tunable colors were exploited for use as photonic papers.

PDMS swollen with the ink Figure 1. Schematic illustration of the mechanism by which the color diffracted from the crystalline lattice of a photonic paper is reversibly changed: the lattice constant (and thus the wavelength of diffracted light) is changed by swelling the PDMS matrix with an "ink . The PDMS matrix shrinks back to its original state once the ink molecules have completely evaporated. 11

Figure 1 illustrates how this new type of paper allows for color writing with a colorless ink. The paper is typically a crystalline lattice of polystyrene (PS) beads whose voids are completely filled with poly(dimethylsiloxane) (PDMS). The ink is a silicone fluid or any other liquid capable of swelling PDMS (11). As the ink is applied to the surface of the paper, the position of stop band will be shifted too a new wavelength, and thus the color displayed by this crystalline lattice will change. If the colors of these two states are sufficiently different to be distinguishable by the naked eye, one can use their contrast to achieve color writing with materials that are colorless by themselves. As the ink evaporates,


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the PDMS will shrink back to its original state and the colored patterns will be automatically erased. The ability to write and erase colored patterns reversibly represents probably the most attractive feature associated with this new type paper. Our preliminary results indicate that such a reversible color change could be repeated for more than 10 times without observing any deterioration in the quality of color displayed by the colloidal crystal. If, necessary, the color could also be fixed by cross-linking the ink molecules to the PDMS network.

Figure 2. SEM images of a photonic paper (made of202-nm PS beads) before and after its PDMS matrix had been swollen with a siliconefluidthat contained vinyl-terminated siloxane oligomers. The swollen sample was fixed by thermally cross-linking the silicone oligomers with the PDMS network. Figure 2A shows an SEM image of the (111) plane of a photonic paper, with 202-nm PS beads being arranged in an ABC stacking of the cubic-close-packed (ccp) lattice. The voids of this crystalline lattice (-26% by volume) had been infiltrated with PDMS. The (111) planes of this lattice were oriented parallel to the solid support, which was also consistent with optical diffraction studies. The PS beads were in physical contact within the (111) plane. Figure 2B shows an SEM image of die same colloidal crystal after it had been swollen with a vinylterminated silicone fluid that was subsequently cross-linked with the PDMS backbone. In this case, the PS beads were further separated from each other within the (111) plane, and the center-to-center distance between PS beads had been increased from 206 to 230 nm (or by 11.6%) as a result of the swelling of the PDMS matrix. Plate 1 shows the transmission spectra of a photonic paper assembled from 175-nm PS beads, before (a) and after (b-e) it has been swollen with silicone fluids having different molecular weights (and viscosities). The incident light was aligned perpendicular to the (111) plane of this colloidal crystal for all measurements. The magnitude of swelling is mainly determined by the strength of interactions between the PDMS network and the silicone fluid, and in this case, by the molecular weight of silicone oligomers contained in the fluid (12).



333 As the molecular weight of silicone oligomers was decreased, we observed two major changes for the stop band associated with the crystalline lattice: its midgap position was continuously shifted towards longer wavelengths; and its attenuation was gradually increased. For this particular paper made of 175-nm PS beads, its stop band could be changed to cover the spectral region from 450 to 580 nm. Accordingly, the color diffracted from the surface of this photonic paper could be easily tuned from blue through green to orange. The ink could be applied to the surface of a photonic paper using a number of ways to generate colored patterns. For example, we could directly spot the ink on the surface of a paper using a Pilot pen. Plate 2A shows a photograph of two letters (in green color) that were written with spotted dots of octane. The colloidal crystal was assembled from 175-nm PS beads, followed by infiltration with PDMS elastomer. The color displayed by the pristine surface of this paper was violet at normal incidence, while the regions that were covered and then swollen by octane diffracted green light. As octane started to evaporate, these letters would disappear within a period of 5 min. In another set of experiments, we also generated letters and other test patterns on the surface of a paper by delivering the ink with an elastomeric PDMS stamp widely used in microcontact printing (13). In a typical procedure, die surface of a PDMS stamp was inked with a silicone fluid by wiping with a Q-tip and then brought into contact with the surface of a paper for a few seconds. Only silicone fluid on the raised regions of the stamp could be transferred onto die surface of the paper, just as one would have experienced with the microcontact printing process. Plate 2B shows a photograph of letters printed using a rubber stamp. The paper was crystallized from 202-nm PS beads, and it exhibited a green color when viewed at normal incidence. The regions swollen by the silicone fluid (T05) displayed a red color. It is worth mentioning that these pattern could have an edge resolution as high as -50 μηι, a feature that will hold die promise for color writing/printing at reasonably high resolution. In addition, the crystallization method described here was able to routinely generate photonic papers with a uniform color over areas as large as 25 cm , and it was also possible to fabricate the photonic papers on transparency films instead of rigid substrates. In addtion to color tuning within the visible region (where the performance of writing strongly depends on the contrast between die two visible colors), it is also possible to fabricate photonic papers whose colors can be switched between an invisible state (e.g., with the stop band located in the region either below 400 nm or above 780 nm) and a visible one by selecting colloids with appropriate sizes and solvents with appropriate properties. For such systems, the photonic papers would appear colorless while the recorded patterns would be brightly colored, or vice versa. Figure 3A shows the transmission spectra of a photonic paper made of 155-nm PS beads. As shown by curve-/, this paper exhibited a stop band in the U V region with its stop band position located at -380 nm. After 2



334

Wavelength (nm)

Plate 1. UV-Vis transmission spectra taken from a photonic paper (assembled from 175-nm PS beads) before (curve, a) and after (curves, b-e) it had been swollen with silicone liquids of different molecular weights (and viscosities): b) T12 (M ~2000, 20 cSt), c) Til (M =1250, 10 cSt), d) T05 (M =770, 5 cSt), ande) TOO (M =162, 0.65 cSt). (See Page 7 of color insert.) w

w

w

w

Plate 2. (A) A photograph of two dotted letters written on a photonic paper by delivering octane droplets to its surface using a Pilot pen. (B) A photograph of letters formed on the surface of a photonic paper by stamping with a silicone fluid (Til, M =1250). This paper was assembledfrom PS beads of202 nm in diameter, and it exhibited a green color when viewed at normal incidence. Note that the pattern shown in (B) had an edge resolution better than 50 pm. (See Page 8 of color insert.) w



335 this paper had been swollen with octane, the stop band was shifted to the visible region (-540 nm, green color, see curve-κ)· Accordingly, the paper changed its appearance from colorless to green after it has been swollen with the octane ink. Figure 3B shows the transmission spectra of another photonic paper that was made of 202-nm PS beads embedded in PDMS matrix. This photonic paper, see curve-κϊ, exhibited a stop band in the visible region around 590 nm. The curveIV shows the transmission spectrum of this paper after its surface had been coated 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 at the crossline between the visible and near IR regions. This paper also displayed a second-order Bragg diffraction peak at -390 nm from its (111) planes, which was located in the U V 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, these two new systems could lead to the writing and display of messages with high contrasts because either the pattern or the background will be colorless.

Wavelength (nm)

Wavelength (nm)

Figure 3. (A) UV-visible transmission spectra takenfroma paper made of 155nm PS beads before (curv-i) and after (curve-ii) it had been swollen with isopropanol. Thefilmchanged colorfromcolorless (in the UV region) to green. (B) UV-visible transmission spectra takenfroma photonic paper before (curveHi) and after (curve-iv) it had been swollen with iso-propanol Thefilmchanged colorfromorange to colorless (within the near IR region). The paper was assembledfrom202-nm PS beads, and its PDMS matrix had been swollen and grafted with the vinyl-terminated silicone fluid before it was swollen with the iso-propanol ink. In summary, we have demonstrated a photonic paper system by embedding colloidal crystals of polymer beads in an elastomeric matrix and by judicially selecting liquids capable of swelling the matrix. This new paper system could be combined with conventional tools to generate colored letters and patterns by using colorless materials only. This system may provide an alternative route to


336 the realization of reusable papers (14) or recording media where no pigment will be required for color displaying at relatively high edge resolutions. As we have demonstrated, colored patterns with an edge resolution of ~50 μπι could be routinely generated by printing the ink with a rubber stamp.


Acknowledgment This work was supported in part by an AFOSR-MURI grant 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. is grateful to 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. Y.L. thanks the Center for Nanotechnology at the UW for a fellowship award.

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337 Hiltner, P. Α.; Krieger, I. M . J. Phys. Chem. 1969, 73, 2386. d) Goodwin, J. W.; Ottewill, R. H.; Parentich, A. J. Phys. Chem. 1980, 84, 1580. (5) a) Weissman, J. M . ; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959. b) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. c) Reese, C. E.; Baltusavich, M . E.; Keim, J. P.; Asher, S. A. Anal. Chem. 2001, 73, 5038. (6) a) Hu, Ζ. 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. (7) Blanford, C. F.; Schroden, R. C.; Al-Daous, M . ; Stein, A. Adv. Mater. 2001, 13, 26. (8) a) Cassagneau, T.; Caruso, F. Adv. Mater. 2002, 14, 1629. b) Gu, Z.-Z.; Horie, R.; Kubo, S.; Yamada, Y.; Fijishima, Α.; Sato, O. Angew. Chem. Int. Ed. 2002, 41, 1154. (9) a) Jethmalani, J. M.; Ford, W. T. Langmuir 1997, 13, 3338. b) Foulger, S. H.; Jiang, P.; Lattam, A. C.; Smith, D. W.; Ballato, J. Langmuir 2001, 17, 6023. c) Foulger, S. H.; Jiang, P.; Ying, Y. R.; Lattam, A. C., Smith, D. W.; Ballato, J. Adv. Mater. 2001, 13, 1898. d) Sumioka, K.; Kayashima, H.; Tsutsui, T. Adv. Mater. 2002, 14, 1284. (10) Fudouzi, F.; Xia, Y. Adv. Mater. 2003, 15, 892. (11) Siloxane Polymers; Clarson, S. J.; Semlyen, J. Α., Eds.; PTR Prentice Hall: Englewood, NJ, 1993;p64. (12) a) Bueche, A. M . J. Poly. Sci. 1955, 15,97. b) Mathison, D. E.; Yates, B.; Darby, M . I. J. Mater. Sci. 1991, 26, 6. c) 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. (13) Xia, Y.; Whitesides, G. M . Angew. Chem. Int. Ed. 1998, 37, 550. (14) Comiskey, B.; Albert, J. D.; Yoshizawa, H.; Jacobson, J. Nature 1998, 394, 253. b) Tamaoki, N . Adv. Mater. 2001, 13, 1135. c) Brehmer, M . ; Lub, J.; van de Witte, P. Adv. Mater. 1998,10,1438.


Colloidal Crystals with Tunable Optical Properties - American

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