Seeing colour at the nanoscale

Scientists can today make and manipulate nanoscale objects with increasingly accurate control, but they are limited to black-and-white imagery.

That may all change with the introduction of a new microscopy tool from researchers at the Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab).

According to the researchers, the tool delivers chemical details with a resolution once thought impossible.

We’ve found a way to combine the advantages of scan/probe microscopy with the advantages of optical spectroscopy

The team developed the system 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,” said 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.”

For chemical information, researchers typically turn to optical or vibrational spectroscopy.

The way a material interacts with light is dictated to large part by its chemical composition, but for nanoscience the problem with doing optical spectroscopy at relevant scales is the diffraction limit, which says you can’t focus light down to a spot smaller than approximately half its wavelength, due to the wave-nature of light.

To get around the diffraction limit, scientists employ “near-field” light. Unlike the light we can see, near-field light decays exponentially away from an object, making it hard to measure, but it contains very high resolution – much higher than normal, far-field light.

Says Schuck, “The real challenge to near-field optics, and one of the big achievements in this paper, is to create a device that acts as a transducer of far-field light to near-field light. We can squeeze it down and get very enhanced local fields that can interact with matter.

“We can then collect any photons that are scattered or emitted due to this interaction, collect in the near field with all this spatial frequency information and turn it back into propagating, far-field light.”

You don’t have to be a super near-field jock anymore to get this type of data

The trick for that conversion is to 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.

Researchers have exploited these effects to make near-field probes with a variety of geometries, but the experiments typically require painstaking optical alignment, suffer from background noise, only work for narrow frequency ranges of light and are limited to very thin samples.

In this latest work, however, the Berkeley Lab researchers transcended these limitations with a cleverly designed near-field probe.

Fabricated on the end of an optical fiber, the probe has a tapered, four-sided tip. The researchers named their new tool after the campanile church tower it resembles, inspired by the landmark clock tower on the UC Berkeley campus.

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.

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.

Replacing the usual AFM tip with a campanile tip is like going from black-and-white to full colour. 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.

“That’s the beauty of these tips,” says Schuck. “You can just put them on the end of an optical fiber and then it’s just like using a regular AFM. You don’t have to be a super near-field jock anymore to get this type of data.”