Novel Technique Stamps Out Nanoprobes

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2017-05-31

One of the cornerstones of the U.S. National Nanotechnology Initiative was the establishment of nanotechnology research facilities that would be available to outside users

One of the five user facilities operated by the U.S. Department of

Energy (DOE) is located at Berkeley National Lab and is known as The Molecular Foundry.

In a visit to The Molecular Foundry, we had the opportunity to discuss the state of nanofabrication with Stefano Cabrini,

who is the director of its nanofabrication facility. “The mission of

The Molecular Foundry is dedicated to nanoscience in general, but it is a

user facility,” Cabrini told IEEE Spectrum. “So we are hosting

here people from all over the world, from companies and from academia,

from Europe, from Asia, from everywhere. They come here to work with us,

to use or our instruments and facilities, but also to have access to

our expertise.”

The goal of the research facility is to split the research 50-50

between outside users and the resident experts at the lab. However,

Cabrini acknowledges that the boundaries quickly become blurry,

especially when pursuing the best science.

While nanofabrication is Cabrini’s main bailiwick, much of his work

of late has been focused on nanophotonics, especially in the emerging

fields of metamaterials and plasmonics

Plasmonics exploits the waves of electrons (plasmons) that are

created on the surface of a metal when it is struck by photons.

Metamaterials are artificially structured materials fabricated by

assembling different objects in place of the atoms and molecules that

make up a conventional material. The resulting material has very

different electromagnetic properties than those found in naturally

occurring or chemically synthesized materials. Both plasmonics and

metamaterials are often used in antenna technologies for essentially

squeezing down the size of light waves.

Back in 2012, Cabrini and his colleagues described, in the journal Science, a

device that operates like a transducer for far-field light to

near-field light. The near-field light overcomes the diffraction limit

of light and provides a much higher resolution than far-field light in

microscopy.

Prior to this work, electron microscopy was the way to collect

information about nanomaterials, but the data it collects about the

nanomaterials is only at the sub-atomic level. But information about the

nanomaterial’s chemical makeup requires analysis at the larger

molecular level. While optical or vibrational spectroscopy typically

provides chemical information on materials on the macroscale, the

problem with these light-based techniques is that the light comes up

against its diffraction limit when it tries to focus in on the

nanoscale. You simply cannot focus a light down to a spot smaller than

half its wavelength.

By using plasmonics, the researchers were able to exploit the surface

plasmons that are created on a metal surface when light hits it. When

the plasmons on two surfaces are separated by a small gap, it’s possible

to collect and amplify the optical field in the gap, making a stronger

signal for scientists to measure.

“We saw that with plasmonics it was possible to focus light onto a

very small spot—a few nanometers—and normally you can’t squeeze the

light in that small a dimension,” said Cabrini.  

When the light is squeezed down to these very small spots, it begins

to interact with the matter it is focused on. Once you have collected

the photons that are either scattered or emitted because of this

interaction with the near-field light and the material, it is possible

to turn it back into far-field light.

The key to making all this work was the design and fabrication of the

near-field probe. Cabrini and his colleagues were able to fabricate a

tapered, four-sided tip on the end of an optical fiber. The researchers

dubbed the resulting tip “campanile” after the tower that resembles a

church campinile on the UC Berkeley campus. Two of the campanile's

gold-coated sides 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.

Research has continued at the Foundry with this device and has been led by Keiko Munechika, the manager for nanofabrication at aBeam Technologies, a small company that is working on next-generation nanofabrication.

In the latest research, described in the journal Scientific Reports,

Munechika and her colleagues have developed a simplified and robust

method for fabricating the campanile near-field probe. The

nanoimprinting technique operates much like a stamp used on sealing wax.

There is a mold with the desired shape that is pressed down into a

polymer that sits on the optical fiber. Once the polymer cools, the tip

of the optical fiber is in the shape of the near field probe .

“The original version of the campanile was made one-by-one, by

curving out an optical fiber into a campanile structure consisting of

micro and nanometer features using a focused ion beam,” explained

Munechika, in an email interview with IEEE Spectrum. “It

requires both time and expertise (and a lot of patience). The throughput

was only a few campanile probes per month. Many researchers have been

interested in using the probes but there was no way to scale up the

fabrication. So, we developed a new way to fabricate campanile probes

using nanoimprint lithography. Now it’s possible to make several probes

per day.”

The probes will be used for nano-spectroscopic imaging of various

materials including new-generation photovoltaic materials to learn

optoelectronic processes at the nanometer scale resolution. “We are

hoping that many new discoveries will be made, now that these probes

will be more accessible to researchers,” said Munechika.

This latest research highlights what Cabrini believes is one of the

foundations of The Molecular Foundry’s mission: that technologies be

developed for widespread use.

Cabrini adds: “Our aim here is not just to make the smallest things

while maintaining the same quality, but to transfer these devices to the

widest spectrum of applications so that everyone can use them.”

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