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RESEARCH NEWS May 2003 12 The optoelectronic properties of conjugated polymers are promising for a number of applications, but nanoscale patterning of these polymer films is essential for their use in multicolored displays, integrated electronics, or photonic structures. Researchers at the University of Cambridge and University College London, UK, and at the Instituto Superior Técnico, Portugal, have developed a lithographic technique to do just that. Using a scanning near-field optical microscope (SNOM), which the team designed and constructed, well-defined patterns can be written directly into conjugated polymer films [R. Riehn, et al., Appl. Phys. Lett. (2003) 82 (4), 526]. The method works by using thin films of a soluble poly(p-phenylene vinylene) or PPV precursor. Exposure to ultraviolet (UV) light removes a labile group, making the polymer insoluble. After the SNOM UV writing step, the nonexposed material is washed off. The patterned polymer is then converted into the fully conjugated PPV by heating it at 220°C under vacuum. Features with a minimum size of 160 nm were obtained in this way. Franco Cacialli and coworkers have demonstrated the potential of this SNOM lithography by fabricating a two-dimensional photonic crystal, including intentional defects, with a periodicity suitable for visible light applications. The researchers claim that the SNOM lithography technique can be used to fabricate arbitrary patterns in any photosensitive conjugated polymer. By modeling the near-field illumination intensity within the polymer film, the researchers were also able to explain the effect of UV intensity on feature size. Writing with polymers NANOFABRICATION Mitsuhiko Shionoya and coworkers at the University of Tokyo and the Institute for Molecular Science in Japan have used DNA with modified bases to construct discrete one-dimensional metal arrays [Science (2003) 299, 1212]. Instead of the standard bases, which pair to form the classic DNA double helix, the group synthesized DNA strands with modified metal-binding bases. The introduction of the Cu 2+ ions mediates the inter-strand base- pairing. In this way, the researchers aligned up to five Cu 2+ ions along the helix axis. The electron spins on adjacent Cu 2+ centers couple ferromagnetically, forming a magnetic chain. The researchers hope, in future, to coordinate metal ions in more complicated DNA structures. Researchers at The DuPont Company, Delaware and the Massachusetts Institute of Technology (MIT) have recently reported similar work showing the potential of biological molecules to control the structural organization of nanoelectronic components. They used phage display, a standard molecular biology technique, to select short peptide sequences that bind carbon nanotubes (CNTs) with high affinity [Nature Materials (2003) 2 (3), 196]. The group describe the peptides as ‘specific chemical handles’ that could be used to manipulate, disperse, and functionalize CNTs. Biomolecules organize nanomaterials NANOBIOTECHNOLOGY Inspiration from nature BIOMIMETICS Learning from mineralization in biological systems, Joanna Aizenberg and coworkers at Bell Laboratories and Mat-Sim Research, New Jersey, have succeeded in growing large single calcite crystals patterned at the micrometer scale [Science (2003) 299, 1205]. They believe that their scheme could provide a general materials fabrication technique. Crystal formation in biology is normally controlled by organic frameworks that direct the nucleation, crystallographic orientation, and extent of mineral deposition. In the same way, the researchers used an organically modified framework to control crystallization from an amorphous CaCO 3 phase. The framework includes specific nucleation sites and a square array of microscale posts. The posts define the patterning of the single crystal, but also act to release stress and impurities from the growing crystal phase. In an accompanying article [Science (2003) 299, 1192], Trevor Douglas of Montana State University says, “This novel model... points to a bright bio-inspired future for materials synthesis.” SEM images of micropatterned single calcite crystals in an echinoderm (top), and formed using a template (bottom). (Top image © 2003 American Association for the Advancement of Science. Bottom image: courtesy of Joanna Aizenberg.) Cu 2+ mediated DNA duplex formation. (© 2003 American Association for the Advancement of Science.)

Biomolecules organize nanomaterials: Nanobiotechnology

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RESEARCH NEWS

May 200312

The optoelectronic properties of

conjugated polymers are promising for

a number of applications, but

nanoscale patterning of these polymer

films is essential for their use in

multicolored displays, integrated

electronics, or photonic structures.

Researchers at the University of

Cambridge and University College

London, UK, and at the Instituto

Superior Técnico, Portugal, have

developed a lithographic technique to

do just that.

Using a scanning near-field optical

microscope (SNOM), which the team

designed and constructed, well-defined

patterns can be written directly into

conjugated polymer films [R. Riehn,

et al., Appl. Phys. Lett. (2003) 82 (4),

526].

The method works by using thin films

of a soluble poly(p-phenylene vinylene)

or PPV precursor. Exposure to

ultraviolet (UV) light removes a labile

group, making the polymer insoluble.

After the SNOM UV writing step, the

nonexposed material is washed off.

The patterned polymer is then

converted into the fully conjugated PPV

by heating it at 220°C under vacuum.

Features with a minimum size of 160

nm were obtained in this way.

Franco Cacialli and coworkers have

demonstrated the potential of this

SNOM lithography by fabricating a

two-dimensional photonic crystal,

including intentional defects, with a

periodicity suitable for visible light

applications. The researchers claim

that the SNOM lithography technique

can be used to fabricate arbitrary

patterns in any photosensitive

conjugated polymer.

By modeling the near-field illumination

intensity within the polymer film, the

researchers were also able to explain

the effect of UV intensity on feature

size.

Writing withpolymersNANOFABRICATION

Mitsuhiko Shionoya and coworkers at the Universityof Tokyo and the Institute for Molecular Science inJapan have used DNA with modified bases toconstruct discrete one-dimensional metal arrays[Science (2003) 299, 1212]. Instead of thestandard bases, which pair to form the classic DNAdouble helix, the group synthesized DNA strandswith modified metal-binding bases. The introductionof the Cu2+ ions mediates the inter-strand base-pairing. In this way, the researchers aligned up tofive Cu2+ ions along the helix axis. The electronspins on adjacent Cu2+ centers coupleferromagnetically, forming a magnetic chain. Theresearchers hope, in future, to coordinate metalions in more complicated DNA structures.Researchers at The DuPont Company, Delawareand the Massachusetts Institute of Technology (MIT)have recently reported similar work showing thepotential of biological molecules to control thestructural organization of nanoelectroniccomponents. They used phage display, a standard

molecular biology technique, to select short peptidesequences that bind carbon nanotubes (CNTs) withhigh affinity [Nature Materials (2003) 2 (3), 196].The group describe the peptides as ‘specificchemical handles’ that could be used to manipulate,disperse, and functionalize CNTs.

Biomolecules organize nanomaterialsNANOBIOTECHNOLOGY

Inspiration from natureBIOMIMETICS

Learning from mineralization in biologicalsystems, Joanna Aizenberg and coworkers atBell Laboratories and Mat-Sim Research, NewJersey, have succeeded in growing large singlecalcite crystals patterned at the micrometerscale [Science (2003) 229999, 1205]. Theybelieve that their scheme could provide ageneral materials fabrication technique.Crystal formation in biology is normallycontrolled by organic frameworks that directthe nucleation, crystallographic orientation,and extent of mineral deposition. In the sameway, the researchers used an organicallymodified framework to control crystallizationfrom an amorphous CaCO3 phase. Theframework includes specific nucleation sitesand a square array of microscale posts. Theposts define the patterning of the singlecrystal, but also act to release stress andimpurities from the growing crystal phase.In an accompanying article [Science (2003)229999, 1192], Trevor Douglas of Montana StateUniversity says, “This novel model... points toa bright bio-inspired future for materialssynthesis.”

SEM images of micropatterned single calcite crystals in anechinoderm (top), and formed using a template (bottom). (Top image © 2003 American Association for the Advancement ofScience. Bottom image: courtesy of Joanna Aizenberg.)

Cu2+ mediated DNA duplex formation. (© 2003 AmericanAssociation for the Advancement of Science.)