COLORFUL NANOSHAPES Silver nanoprisms, gold nanorods, and gold nanospheres scatter red, orange, and green light, respectively.

Chemistry professors Chad A. Mirkin and George C. Schatz and coworkers at Northwestern University used fluorescent light to convert surfactant-stabilized silver nanospheres into triangular nanoprisms in high yield [Science, 294, 1901 (2001)]. The silver nanoprisms scatter red light, whereas conventional silver nanospheres scatter blue light.

The team also demonstrated that the light-scattering properties of nanoparticles of gold, as well as silver, can be controlled by varying the shape and size of the particles.

"Shape control over nanoparticles is one of the great frontiers for researchers working in the area of nanoscience," Mirkin tells C&EN. "If one can control size and shape on the 1- to 100-nm length scale, one can control virtually all of the properties of a material.

"We are now very good at making spherical nanoparticles, but we have only limited understanding of how to make different particle shapes, other than nanorods, in high yield," he continues. "Most nanoparticle synthetic procedures are thermally controlled. Our work suggests we should start exploring the use of light and different surfactants to attain anisotropic particles."

The silver nanoprisms have flat tops and bottoms with edges of 90 nm and are about 15 nm thick. The structural anisotropy of the nanoprisms results in striking light-absorption, light-scattering, and other optical properties. The Northwestern team observed, for example, two distinct quadrupole plasmon resonances for the nanoprisms.

"The structures should also have all sorts of wonderful Raman-enhancing and nonlinear optical properties," Mirkin adds. "They are therefore a new Tinkertoy for nanoscientists to play with in the years to come. They make one of the most compelling arguments for nanotechnology."

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