Nanowire Probes Provide Visual Enhancement for AFM

Nanowire probe tips originally intended for atomic force microscopes in near-field scanning microwave microscopy (NSMM) applications could have another use.

A team from the National Institute of Standards and Technology’s Physical Measurement Lab (PML) has demonstrated that such probe tips, created from gallium nitride (GaN) nanowires coated with titanium and aluminum, could also be used as LEDs that can illuminate tiny sample regions.

Optical radiation can excite a sample differently than a microwave signal does during NSMM scanning. This adds a new dimension to the characterization of nanoelectronic materials and devices, according to the researchers.

A single GaN nanowire is removed from a set grown by molecular beam epitaxy. Courtesy of the National Institute of Standards and Technology.

“With the new design, the illumination will be applied directly over the probe tip at the same place on the sample that is being exposed to the microwave signal,” said Pavel Kabos, a researcher with the Advanced High-Frequency Devices Program in PML’s electromagnetics division. “That could be particularly beneficial in characterizing photovoltaic materials where you could apply a light and get the carrier concentration at the same time.”

To date, flood illumination has been used to study such materials. However, LED probe tips could allow researchers to delve deeper. Additional studies are needed, they said, to learn how the GaN nanowires could increase light output efficiency, and how topographic, microwave and optical modalities could be used to better coordinate and integrate measurements.

The nanowire probe tips have also recently been found to “substantially outperform” commercial silicon and platinum tips for measurements in applications, including microchip fabrication and nanobiotechnology.

“Our nanowire probe tips have a calibration lifetime about 10 times longer than any commercial tip,” said Kris Bertness, project leader for metrology and synthesis of 3-D nanostructures in the NIST quantum electronics and photonics division. “We see no visible wear after performing tens of scans, whereas platinum deforms, losing resolution and calibration, after five to 10 scans.”

In the study, the GaN tips were tested against those made of silicon and platinum. Each was scanned across an array of microcapacitors of different sizes. The GaN nanowires proved to be about twice as sensitive as the platinum probes, the researchers said, and about four times as sensitive as the silicon. They also demonstrated enhanced mechanical performance.

The new probes are expected to show new aspects of nanostructure composition and function. They could also have potential for biological materials applications, such as locating the attachment of chemical agents or particles that are bound to a cell, and aiding in the study of protein dynamics.

The research is published in Applied Physics Letters.

For more information, visit www.nist.gov/pml.