Making a Different Point for Better Images

Hank Hogan

Takashi Kodama and Hiroyuki Ohtani at Tokyo Institute of Technology made a point of improving microscopy imaging by developing a novel probe for apertureless near-field scanning optical microscopy.

Standard tips used in the technique are made by sputtering silver onto an atomic force microscope probe. The researchers built upon that by immobilizing silver nanoparticles to a standard tip.

A schematic illustrates an apertureless near-field optical microscope setup that includes silver nanoparticles immobilized upon the probe tip (inset). Reused with permission of the American Institute of Physics.

Kodama, now at Stanford University in California, said that their approach provides several advantages. One, there is no need to polarize the electric field along the tip axis and, two, there is no signal falloff from tip rounding. In addition, the aggregation of metal nanoparticles helps enhance the optical signal by boosting the electric field more than 10 billion times. The tip induces a huge electric field enhancement, he said.

To construct the tip, the investigators chemically modified the surface of a commercial atomic force microscope probe, then immersed it in a solution containing polyallylamine, which enabled attachment of the silver nanoparticles.

They used the tips for surface-enhanced Raman scattering imaging, bringing them close to a sample. They did so in a homemade system consisting of an inverted confocal laser scanning microscope from Tokyo Instruments Inc. in Yokohama, Japan, with an argon-ion laser operating at 488 nm, made by Nippon Laser and Electronics Laboratory of Nagoya, as an illumination source. They acquired the Raman spectra using a CCD camera from Andor Technology plc of Belfast, UK, which they equipped with a spectrograph.

Researchers used an atomic force microscope probe coated with silver nanoparticles to obtain surface-enhanced Raman spectra from onion-shaped carbon structures. Lines (a) represent data acquired from various points on the sample surface, as indicated in the surface-enhanced Raman scattering image (b). “Without tip” represents the carbon onion’s spectrum as acquired with the probe tip withdrawn from the sample surface. Reused with permission of the American Institute of Physics.

In a demonstration, the scientists performed surface-enhanced Raman scattering measurements on a carbon onion — a structure composed of concentric carbon rings. They imaged the objects well below the diffraction limit and spotted a signal resulting from dangling carbon bonds at 1320 cm^–1, as well as another signal at 1595 cm^–1 that corresponds to a carbon vibration mode.

The tips do have some disadvantages. For one, some are noisy, which could be because of the polyallylamine. Also, they are hard to use on small samples because of geometrical considerations arising from the localized nature of the signal improvement. “In order to enhance the Raman spectrum of the sample on the substrate, the height of the sample should be larger than the radius of the metal nanoparticles,” Kodama said.

Plans call for the fabrication of small metallic nanostructures on the tip surface to deal with the second problem. As for the first, the researchers are working on ways to reduce or remove the noise.

Applied Physics Letters, Nov. 27, 2006, 223107.