Seeing in color at the nanoscale

Berkeley Lab scientists develop a new nanotech tool to probe solar-energy conversion
December 10, 2012

An electron micrograph of the tip of a new tool that promises to revolutionize nanoscale imaging. Inset: the UC Berkeley campanile bell-tower for which it is named. (Credit: Lawrence Berkeley National Lab)

A new microscopy tool from researchers at the Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) delivers exquisite chemical details with a resolution once thought impossible.

The team developed the tool, which they call a “campanile,” to investigate solar-to-electric energy conversion at its most fundamental level, but their invention promises to reveal new worlds of data to researchers in all walks of nanoscience.

“We’ve found a way to combine the advantages of scan/probe microscopy with the advantages of optical spectroscopy,” says Alex Weber-Bargioni, a scientist at the Molecular Foundry, a DOE nanoscience center at Berkeley Lab. “Now we have a means to actually look at chemical and optical processes on the nanoscale where they are happening.”

Electromagnetic fields are enhanced in the gap as the campanile squeezes light beyond the diffraction limit, as shown in these simulations (credit: Lawrence Berkeley National Lab)

The scientists use surface plasmons — collective oscillations of electrons that can interact with photons. Plasmons on two surfaces separated by a small gap can collect and amplify the optical field in the gap, making a stronger signal for scientists to measure.

Fabricated on the end of an optical fiber, the probe has a tapered, four-sided tip. Two of the campanile’s sides are coated with gold and the two gold layers are separated by just a few nanometers at the tip. The three-dimensional taper enables the device to channel light of all wavelengths down into an enhanced field at the tip. The size of the gap determines the resolution.

Using the campanile tip, Berkeley Lab researchers take “color” images with nanoscale resolution. A photovoltaic indium-phosphide nanowire is easy to see in a black-and-white electron micrograph (left) but chemical information has low resolution in a normal confocal micrograph (right). The campanile tip reveals both shape and chemistry of a nanowire (center). (Credit: Lawrence Berkeley National Lab)

In a regular atomic force microscope (AFM), a sharp metal tip is essentially dragged across a sample to generate a topological map with sub-nanoscale resolution. The results can be exquisite but only contain spatial information and nothing about the composition or chemistry of the sample.

Replacing the usual AFM tip with a campanile tip is like going from black-and-white to full color. You can still get the spatial map but now there’s a wealth of optical data for every pixel on that map. From optical spectra, scientists can identify atom and molecule species, and extract details about electronic structure.

This research was supported by the DOE Office of Science.