SCIENCE & TECHNOLOGY C0L0RFUL Dalai (from left), Moore, Stiegman, and grad student Chris Ramsey examine a piece of their photomagnetic material.
CLEAR & PRESENT SUPERPARAMAGNET New approach traps nanoparticle magnets in transparent porous glass ELIZABETH Κ. WILSON, C&EN WEST COAST NEWS BUREAU
A
LBERT E. STIEGMAN HAS A
knack for designing useful ma terials in the guise ofpretty col ored glasses. Over the years, the Florida State University (FSU) chemistry professor has fabricated porous silica gels thatfluoresceon their own or that turn vibrant oranges, yellows, and greens when they're exposed to pollutants. His latestfindmaybe the most provoca tive yet: a beautiful purple glass impreg nated with nanoscale-sized magnets. Not only is the bulk material transparent and superparamagnetic—that is, each nanopar ticle behaves like a single independent magnetic domain—but its magnetic prop erties can be tuned by doping it with am monia and exposing it to light [Angew. Chem. Int. Ed., 42,2741 (2003)1. It's a powerful collection of properties that has a number of researchers interest ed in the material's potential use in devices such as magnetic switches. The nanoparticles are made of cobaltiron analogs of the complex Prussian blue: C^IFeiCN)^· Scientists havefocusedin tensely on this class ofcompounds because they make promising molecular magnets. Unlike their metal and metal-oxide coun terparts, molecular magnets can be de signed with precisely tailored properties. HTTP://WWW.CEN-ONLINE.ORG
Several years agp, Kazuhito Hashimoto, chemistry professor at the University of Tokyo, increased interest in the Prussian blue compounds when he demonstrated that cobalt-iron analogs dramatically in crease their magnetic moments when they're exposed to light. Heating the ma-
nm particles of the same Prussian blue analogs Stiegman used, in water-in-oil microemulsions [Nana Lett., 2,225 (2002)}. Stiegman and his colleagues, including graduate student Joshua G. Moore, FSU chemistry professor Naresh S. Dalai, and researcher Eric J. Lochner at FSLPs Cen ter for Materials & Research Technology, merely intended to create a material with photo-optical properties when they added Prussian blue analogs to the sol-gel. They expected the complexes to pre cipitate readily into macroscopic clusters as the gel hardened. But to their surprise, under extremely dilute conditions, they were able to trap 8- to 10-nm particles. "It was a race between whether the gel would gelfirstor the Prussian blue would fall out," Stiegman says. "We hit a sweet spot where we arrested the precipitation at the nanoparticle stage. I didn't believe it at first.'' Shining light on the material increases its magnetization; and doping the materi al with ammonia changes its reaction to the light, allowing the researchers to finetune the photomagnetic effect. Kim R. Dunbar, chemistry professor at Texas A&M University, who studies mo lecular magnetics, notes that, "this is the first example of someone making a nanostructured material out of a Prussian blue [analog].'' There's a good deal of potential in a
TRAPPED Nanoparticles of Prussian blue analogs sequestered in porous glass produce a material both superparamagnetic and tunable. terial or exposing it to higher energy light erases the effect. Although many of these complexes lose their magnet ic properties at room tem perature, in recent years researchers have cre ated some Prussian blue-based magnets that survive above 100 °C. However, the complexes in the new work still require temperatures of about 14K(-259 °C ) to perform. So far, most Prussian blue materials have taken the form ofbulk solids or thin films. In one exception, the group of chemistry professor Stephen Mann at the Universi ty of Bristol, in England, created 12- to 20-
magnetic substance that can be molded, says Geoffrey F. Strouse, inorganic chemistry pro fessor at the University ofCal ifornia, Santa Barbara. "It's truly a neat application with real potential in the design and development of mag netic materials," continues Strouse, who will be joining FSLPs chemistry de partment nextyear. "This could give rise to some fascinating magnetic structures that can literally be cast into a device, rather than having to be formed via epitaxial-lay er or thin-film methods."» ^
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