Technique Combines Atomic Force Microscopy with Near-Field Optical Imaging

Richard Gaughan

As increasing attention is given to manipulating the structure and function of nanoscale materials, there is a simultaneously growing need for an improved understanding of nanoscale physics. Although optical microscopy has been a valuable tool for visualizing at the micron scale, extending resolution beyond the diffraction limit requires new approaches. Meanwhile, atomic force microscopy (AFM) provides information on the nanometer scale, but the information is primarily limited to topography.

Combining atomic force microscopy (AFM) and near-field scanning optical microscopy provides both topographic and fluorescence information. When a prepared AFM probe is near a fluorophore, the fluorescence is quenched. Coating the probe with 5-nm-thick nanocrystal improves the resolution. Here, a CdSe quantum dot is imaged as a bright spot on the topographic image (top) and as a dark spot in the fluorescence image (bottom).

Scientists have been combining AFM and optical methods to improve the richness of microscopy data. Now researchers at the Center for Nanoscience and Nanotechnology at the Hebrew University of Jerusalem have taken the merged techniques a step further.

The method, developed by Uri Banin and colleagues, takes advantage of the fact that fluorescence is modified when a dielectric tip is brought within tens of nanometers of a fluorophore. For the first stage in this research, a sample of quantum dots on a glass slide was illuminated with 514-nm light from an argon-ion laser and was imaged through a Zeiss Axiovert microscope. The slide was translated through the field until a dot was centered in the excitation spot. Then an AFM tip was scanned in contact mode across the glass slide while the fluorescence was simultaneously monitored.

When the AFM tip was within a few nanometers of the quantum dot, the fluorescence was quenched. By correlating the data, the fluorophore was located precisely within the topographic AFM image. Using standard silicon AFM tips or a platinum-coated one essentially produces apertureless near-field scanning optical microscopy, but by coating the AFM probe tip with quantum dot nanocrystals, the probe-sample interaction increases, and the resolution improves from 130 to about 30 nm.

Banin’s team then used the dot-coated AFM probe in tapping mode, in which the probe is oscillated vertically as it scans the surface. Banin noted that the ability to perform the optical measurement along with the tapping-mode AFM technique should be useful for the study of soft and biological samples. By time-gating the detection window, each collected photon was associated with a distance from the sample surface. A standard time-correlated single-photon-counting hardware card was combined with custom software to make the distance-correlated information directly accessible. By selecting photons only from within 10 nm of a sample surface, a 60-nm-diameter quantum dot showed up in both the fluorescence-quenching and the AFM topographical image.

Because all the collected photons are associated with a height from the surface, the technique makes it possible to perform advanced studies of the variation in photoluminescence with probe distance.

“The uniqueness of this method lies in the fact that it enables, along with high-resolution optical and topographic images, a detailed and accurate measurement of the distance-based interaction of different materials,” Banin said.

Journal of Physical Chemistry A, online May 26, 2006, doi:10.1021/jp056229o.