science/technology
PHOTONIC CRYSTALS: WHOLE LOTTA HOLES
"But now if you can disturb the crystal in a way that causes the photonic band gap to be shifted to a slightly different frequency," Vos asserts, "then the photon will suddenly find itself outside of the band gap, and it can be emitted from the crystal." One way to demonstrate that effect might be to use a pulse of sound waves that causes a small change in the crystal's lattice parameters, he says. "It would be like pushing a button to cause the crystal to emit light." With semiconductors, control over electron flow is provided by a combination of dopants and applied voltages. Such controlled electron emission plays a central role in the workings of transistors and other devices. By choosing when to send current through a circuit, one chooses when to send a signal—like typing a letter on a keyboard or a number on a telephone. "Technology has been able to completely harness the flow of electrical current," Vos says, "but not the propagation of light." There are, of course, ways to redirect light using lenses, mirrors,fibers,or other devices. "But there are no ways to completely stop a light beam, do with it as you like, and then release it at will. This is the sort of thing that can be done with photonic band-gap crystals—if we can succeed in making them." Photonic band-gap applications require periodicity on a scale—the hundreds-ofnanometers size range—that is outside of molecular dimensions, except perhaps for large biomolecules, explains Sarah H. Tolbert, an assistant professor of chemistry at the University of California, Los Angeles,
Prepared by new proœdures, materials with arrays of large holes may hasten development of optical-based technologies Mitch Jacoby C&EN Chicago
I
t used to be that scientists working at making new crystalline materials concentrated onfiguringout the positions of the atoms or molecules. But recently, instead of zeroing in on matters of substance, some crystal researchers seem to be directing their attention to a whole lot of nothing. Investigators are focusing on the empty spaces in lattices and are preparing new substances composed of ordered arrays of large holes. As new synthesis procedures enlarge the holes from many angstroms to hundreds of nanometers (1 nm = 10 À), discussion of their use in catalysis, separations, host-guest systems, and the like are heard less often than the latest buzzword—photonic crystals. These substances with useful optical properties hold the promise of important technological applications. Among the most tantalizing uses are optical switching and computing. Those applications are expected of an asyet-unsynthesized photonic-band-gap crystal—the Holy Grail of holey materials. Crystals of all types can efficiently scatter some radiation, provided that the lattice dimensions are on the same order as the light's wavelength. For ordinary atomic and molecular crystals, the interaction (dirrraction) results from a match between angstrom-scale crystal spacings and X-rays' angstrom-sized wavelengths. But for light in the visible or near-infrared region to scatter coherently from a crystal, the diffracting medium must have a periodicity on the scale of hundreds of nanometers. Such photonic crystals are expected to have unique and useful properties, but preparing them long remained a formidable challenge to chemists and materials scientists. Willem Vos, a senior scientist in the physics department of the University of Amsterdam, says the recent interest in photonic materials stems in part from a 38 NOVEMBER 23, 1998 C&EN
desire to make photonic "band-gap" crystals. The term band gap is applied to this special group of photonic substances because its members are expected to function like optical analogs of semiconductors. Just as electrons with certain energies cannot move through a semiconductor because of an energy band gap, so too, light of certain frequencies cannot propagate through photonic band-gap crystals because of an optical frequency (or wavelength) band gap. In semiconductors, the gap is due to a material's electronic structure. In photonic crystals, it's due to the crystal structure, which strongly scatters certain wavelengths. By way of example, Vos explains what would happen if a molecule that ordinarily emits green light during deexcitation could be trapped—in its excited state—inside a crystal with a green photonic band gap. In principle, the molecule would remain in the excited state indefinitely because there's nowhere for the green photon to go.
Vos (far right), Wljnhoven (next to Vos) and coworkers at the University of Amsterdam's van der Waals-Zeeman Institute.
istence of the materials. And it also took time for chemists to produce quality ordered porous ceramic compositions," he says. Nature, in contrast, assembles photonic ma terials easily—albeit slowly. The rainbowlike appearance of opals aris es from their long-scale periodicity. These gems grow on a geological ΊΙΟ2 crystals shimmer like opals due to arrayof nanometer· time scale, as nanometersized holes. sized colloidal silica parti cles of equal diameter who studies structural stability of nano- slowly setde into close-packed arrays. Tak scale materials. "That's been a really hard ing a lesson from nature, scientists have now developed synthesis procedures that length scale to make," Tolbert notes. But a recent torrent of publications on yield a range of materials with opallike ordered macroporous materials shows structures. chemists are having some measure of suc cess. Tolbert adds that the current advanc Organic templates es grew out of structure-directing synthe At the University of Minnesota, Minne sis ideas developed in the early 1990s at apolis, assistant chemistry professor An Mobil to prepare large-pore molecular dreas Stein and graduate students Brian T. sieves such as MCM-41. Holland and Christopher F. Blanford have James C. Vartuli, a researcher at Mobil developed a procedure that relies on or Technology, Paulsboro, N.J., is a member ganic templates to form ordered porous of the group that synthesized those mate ceramics. Using their new procedure, the rials. "Until recendy," Vartuli says, "hun Minnesota group recendy prepared titania, dreds of papers were published on these zirconia, and alumina samples with threeordered porous materials with the theme dimensional arrays of nanometer-sized of catalysis or separations." But now other holes [Science, 281, 538 (1998)]. applications—such as photonic crystalsBy subjecting 470-nm polystyrene have emerged. "I think it took some time spheres to the Gforces of a centrifuge, the for people who are not directly involved group casts the microscopic latex balls in in catalysis or separations to realize the ex an orderly close-packed arrangement. The
packing materials in close-packed struc tures occupy only 74% of the volume, Stein points out. The remainder, more than one-fourth of the total space taken up by the structure, is divided among tiny voids between the spheres. A key step in the procedure is filling those interconnected empty spaces with a substance that solidifies, producing a skel etal network with close-packed character. Stein and coworkers do that by dripping a metal alkoxide solution on the spheres. The solution permeates the latex structure and fills its void spaces, hydrolyzing and condensing to form an organic-inorganic composite. In a final step, the Minnesota chemists heat the solids above 500 °C in air to burn out the polystyrene. The heat treatment causes some shrinkage and leaves behind ordered materials with pores in the 320360-nm size range. The heat also causes some of the wall materials (Ti0 2 and Zr0 2 ) to crystallize—meaning that the products may be ordered on a nanometer scale and an angstrom scale. "We believe the [synthesis] route is very general," Stein comments, "one that can probably be applied to many materi als." His group has begun working with other metal alkoxides—such as tungsten, iron, and antimony—and also has begun studies of hybrid organic-inorganic materi als. "There's work to be done to optimize the order," Stein acknowledges, "but in all these materials we observe that the shape of the latex sphere array becomes imprint ed in thefinalmaterial." A nearly identical procedure was devel-
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angstrom-sized reaction oped independentiy by Vos and coworkers at the Univer sites by transporting them Simple procedure yields ordered holey sity of Amsterdam. In a re to those sites via microme materials cent publication, he and ter-sized pores. postdoctoral associate Judith Inorganic templates Wijnhoven report that they Metal used polystyrene latex balls Macroporous materials alkoxide to produce T i 0 2 crystals also have been made recent and solvent with ordered spherical voids ly by procedures that closely ranging from 120 to 1,000 mimic nature's way of mak nm in size [Science, 281,802 ing opals. Last month, an in (1998)]. ternational team based at AlPreordered liedSignal in Morristown, N.J., As with the Minnesota latex spheres described a general method procedure, the Amsterdam for making ordered holey method also uses sedimenta crystals using nanometertion to order the spheres, a sized silica spheres. These titania precursor (tetragem-quality synthetic opals propoxy titane) to fill the Composite serve as templates for a fami voids, and calcination (heat ly of novel carbon materials treatment) to remove the or that possess unique and po ganic portion. The holey Calcination tentially useful properties product displays rainbowlike Vacuum [Science, 282, 897 (1998)]. qualities because the size and distances between va The work was done by cancies left by the latex are Ray H. Baughman, an Alliedin the nanometer range. Signal fellow, principal scien tists Anvar A. Zakhidov, Zafar Based on optical reflec Iqbal, and Changxing Cui, tance measurements, the visiting scientist Ilyas KhayAmsterdam group concludes rullin, and coworkers in Bra that its titania/air samples are zil and Russia. Open 3-D metal strongly photonic, because oxide network they prevent light from trav "We've developed a pro Images courtesy of the University of Minnesota eling in half of the possible cess that can be used to directions, but do not yet make materials that are show a full photonic band gap. By making tures, the investigators remove the or three-dimensionally periodic in a size holey crystals from materials with refrac ganic portion, leaving behind arrays of range that's been very difficult to access," tive indexes higher than that of Ti0 2 , Vos voids in ceramic frameworks. Baughman says, referring to the nanome and coworkers hope to produce even "By using liquid particles as tem ter-to-micrometer range. "So far, the meth stronger photonic effects. plates," Pine notes, "we are able to re od has been used to make new forms of Taking a different approach to the or- move these particles by dissolving them carbon—all of which diffract light in the dered-pores problem, David J. Pine, a pro in a solvent and then drying at room tem visible region—but it appears to be appli fessor in the chemical engineering and the perature." The shrinkage that occurs dur cable to many materials." materials departments at the University of ing the high-temperature step in other The group uses a sodium silicate route California, Santa Barbara, uses oil droplets methods typically causes crystals to to make Si0 2 spheres and then allows sizeas structure-directing elements in his mate crack into millimeter-sized or smaller selected spheres to crystallize over a 10rials syntheses. Late last year, Pine and pieces. That doesn't occur with the month period. The researchers then crepostdoctoral associate Arnout Imhof de emulsion technique, Pine scribed the procedure and showed how it asserts, noting that the may be used to prepare macroporous sam droplet method can pro ples with neatly arranged holes ranging in duce centimeter-sized size from 50 nm to several micrometers samples. [Nature, 389,948(1997)]. Though much of the in The Santa Barbara researchers use oil- terest in macroporous ma in-formamide emulsions as templates to terials is directed toward prepare holey forms of titania, zirconia, interactions with light, and silica. Employing sol-gel methods, Pine sees opportunities for the group causes the metal oxides to these materials in catalysis. condense on the exterior of tiny liquid Now he is focusing on pre droplets that are size-selected using a paring holey samples with fractionation method. Uniformity in dis pores of two different size tribution of pore sizes is key to the pro scales. The idea is to pro cedure's success, they note. By washing vide gas molecules with Mlnnesota chemists Stein (left) and Blanford use and drying the gelled composite struc- more efficient access to organic templates to form porous ceramics. 40
NOVEMBER 23, 1998 C&EN
ate a system of interconnected passageways through the silica spheres by sintering the synthetic opals at roughly 700 °C. Then they fill the void spaces with one of several types of carbon. In a final step, the team uses hydrofluoric acid to permeate the passageways and dissolve the silica balls, leaving carbon materials with ordered arrays of holes. They refer to the products as inverse opals. In one example, the AlliedSignal group used a phenolic resin to infiltrate the opal structure. That procedure resulted in porous phenolic samples or glassy carbon inverse opals—depending on heat treatment. The investigators also used various chemical-vapor-deposition methods to fill the lattice voids with carbon. With a propylene-nitrogen blend, they synthesized graphite structures. And using a methanehydrogen plasma technique, the group made diamond inverse opals. "The thing I find amazing," Baughman says, "is that in order to make these inverse opals, we have to infiltrate over millimeter thicknesses and centimeter lateral dimensions. And we do it through a periodic array of holes that are on the order of
Asher: colloidal arrays as optical devices
500 À. And in order to extract [Si02], we have to go through a different set of holes that are similar in size. And somehow both steps proceed so regularly." The group points out that one of the products, termed cubic graphite, actually
contains a new phase of carbon. The structure, which is reminiscent of carbon "onions," forms on the outer surface of the Si02 spheres and consists of graphite tiles stacked up to 10 high. Because the location and curvature of these graphite stacks are governed by the silica spheres, Baughman explains, this material has a periodicity on a scale not seen in other carbon phases. So what might one do with carbon inverse opals? For starters, they could be used as molds to template new direct opals, Zakhidov says. That would enable new photonic crystals to be prepared from metals, semiconductors, and superconductors, leading to a large variety of opallike materials with customizable electronic, magnetic, and optical properties. That is one area where the research group has recently had success, he says. Zakhidov adds that these materials might also be used as new types of membranes with extended, ordered interfaces that may be useful in catalytic and biological applications. Zakhidov stresses that a key feature of some of these novel porous structures is that they are composed of two indepen-
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NOVEMBER 23, 1998 C&EN 41
science/technology dent networks of cages. The interiors of all the spheres are interconnected and the spaces that were voids are also interconnected. But the two regions are isolated from each other. "You can fill the interior of the spheres with electron-donating polymers and the exterior with electron-accepting polymers and they will not mix," Zakhidov says, noting the similarity to doping fullerene crystals. The separation between the regions may be the key to making future device applications possible, he adds. Baughman points out that this synthesis method could also have applications in recycling colored plastics. One way to use it would be to match the thermal properties of an opalescent polymer colorant to that of a host polymer matrix. Since the coloring agent's color is derived from its crystal structure, he says, at its melting point the material will lose its color. But if the colorant's melting point is below the maximum temperature needed to process or use the plastic, and if the plastic is stable above the colorant's melting point, then the plastic can be recycled and rendered colorless. Another application uses these carbon phases as templates for electrostrictive materials. In response to an applied electrical field, these rubbery substances change dimensions—a property that can be used to control their color. Such materials hold potential as color elements for display monitors. AlliedSignal currently conducts research in that area, in collaboration with Fotios Papadimitrakopoulos, an assistant professor of chemistry at the University of Connecticut, Storrs, and Qiming M.
Zhang, an associate professor of electrical engineering at Pennsylvania State University, University Park.
Other procedures Instead of using templates to make materials adopt particular structures, University of Pittsburgh chemistry professor Sanford A. Asher relies on electrostatic repulsions between nanometer-sized colloidal particles to assemble the particles in cubic arrays. The forces arise from charged functional groups on the particles surfaces. In last month's issue of the Materials Research Society Bulletin, Asher reviewed applications of crystalline colloidal arrays in optics and chemical sensing. One device, a narrowband optical diffraction filter, is based on a cubic array of polystyrene spheres. This photonic-crystal gadget acts as a "notch filter," selectively removing light in the 488532-nm region while allowing adjacent wavelengths to pass. Raman spectroscopists, for example, could use these filters after exciting a sample to separate the signal (Ramanscattered light) from the background (elastically scattered light). Asher patented such a device in 1986.
AllledSlgnal Inverse opal team (from left) Khayrullln, Cul, Iqbal, Zakhldov, and Baughman (sitting).
42
NOVEMBER 23, 1998 C&EN
wi
Graphite Inverse opal has bright colors visible to the eye (bottom). Electron micrograph (top) reveals that this material derives Its optical properties from an array of nanometer-sized voids. (A silica sphere, visible In the left side of the micrograph, remains In the structure even after hydrofluoric acid extraction, probably because of Incomplete formation of passageways to neighboring spheres.)
Last year, Asher showed that gelled colloidal-array photonic crystals could act as chemical sensors (C&EN, Oct. 27, 1997, page 10). In that application, a molecular recognition element—a crown ether, for example—senses the presence of analytes in solution and causes the array to swell. Changes in the lattice parameters result in changes in the diffraction conditions that cause a color change that's visible to the eye. Asher and coworkers demonstrated the sensors for lead, potassium, glucose, and other analytes. Asher also investigates optically switchable photonic crystals. Using a pulse from a nanosecond laser, he heats colloidal particles that do not diffract light of a certain wavelength when cool. The rapid temperature rise causes a change in refractive index—inducing the array to quickly switch on its diffraction ability. Quite unlike templated self-assembly
processes, some researchers use a procedure of repetitive material deposition and etching to prepare ordered structures that function as photonic crystals. This past summer, researcher Shawn Y. lin and coworkers at Sandia National Laboratories, Albuquerque, N.M., made a stacked structure of micrometersized silicon rods using such methods [Nature, 394,251 (1998)]. The assembly possesses a large photonic band gap in the infrared region. The device is of interest to the military for use in enhancing infrared images. University of California, Los An- Neatly arranged stack of mlcrometer-slzed silicon geles, professor of electrical engi- can bend and direct IR light neering Eli Yablonovitch, a pioneer in the photonic band-gap field, notes that band gaps in FCC structures larger," self-assembly of face-centered-cubic (FCQ Yablonovitch says. "And that has coinstructures recently has been perfected. cided with a number of FCC self-assemOriginally, those crystals were thought to bly approaches. Thus, the recent flurry have a photonic band gap, he says. Then it of activity." was shown they didn't have one. Finally, Another pioneer in the photonic bandresearchers proved that FCC structures do gap field, University of Toronto physics have a gap—but a small one. professor Sajeev John, notes that the re"Now theoretical work is making the cently reported titania/air-crystal experi-
ments come "tantalizingly close" to providing a real photonic band gap"But the work of Vos and Baughman is really a warm-up exercise to infiltrating [these materials] with silicon or germanium," both of which have higher refractive indexes than Ti02, John notes. If that can be done, he says, it will lead to the first true large-scale, 3-D photonic band-gap material. And that development in photonics will be "comparable in importance to the fabrication of the first good semiconductors in the electronics industry. I believe that may be achieved within the next year or so." Yablonovitch also expects photonic band-gap crystals to become available soon for research, but warns that optical computers are still "really futuristic right now." The next round of results will be presented in January 1999 at a Laguna Beach, Calif., workshop on electromagnetic crystals. Information is available on the Internet at www.ee.ucla. edu/~ wecsdsap. ^
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