Posted in | Microscopy | Nanoanalysis

New Technique for Improving the Resolution of Terahertz Emission Spectroscopy

A new way to launch a powerful form of spectroscopy into the nano-world has been demonstrated by Brown University researchers. This technique has been used for studying an extensive range of materials.

Researchers have improved the resolution of terahertz spectroscopy by 1,000 times, making the technique useful at the nanoscale.Mittleman Lab / Brown University

Laser terahertz emission microscopy (LTEM) is an expanding means of characterizing the performance of integrated circuits, solar cells and various other materials and systems. Laser pulses illuminating a sample material result in the emission of terahertz radiation, which carries vital information about the sample’s electrical properties.

“This is a well-known tool for studying essentially any material that absorbs light, but it’s never been possible to use it at the nanoscale,” said Daniel Mittleman, a professor in Brown’s School of Engineering and corresponding author of a paper describing the work.

Our work has improved the resolution of the technique so it can be used to characterize individual nanostructures.

Daniel Mittleman, a professor in Brown’s School of Engineering

LTEM measurements are typically performed with resolution of a few tens of microns, however, this new technique allows measurements down to a resolution of 20 nm, roughly 1,000 times the resolution earlier possible using standard LTEM techniques.

The research, featured in the journal ACS Photonics, was headed by Pernille Klarskov, a postdoctoral researcher in Mittleman’s lab, along with Hyewon Kim and Vicki Colvin from Brown’s Department of Chemistry.

For their research, the team adapted for terahertz radiation a technique earlier used for improving the resolution of infrared microscopes. This technique uses a metal pin, tapered down to a sharpened tip just a few tens of nanometers across, that hovers just above a sample that is to be imaged. After illuminating the sample, a small portion of the light is captured directly beneath the tip, which enables imaging resolution approximately equal to the size of the tip. It is possible to develop ultra-high resolution images of an entire sample by moving the tip around.

Klarskov succeeded in showing that the same technique could also be employed for increasing the resolution of terahertz emission. For their study, Klarskov and her colleagues were able to image a separate gold nanorod with 20-nm resolution using terahertz emission.

The researchers are certain their new technique could be extensively used in characterizing the electrical properties of materials in exceptional detail.

“Terahertz emission has been used to study lots of different materials — semiconductors, superconductors, wide-band-gap insulators, integrated circuits and others,” Mittleman said.

Being able to do this down to the level of individual nanostructures is a big deal.

Daniel Mittleman, a professor in Brown’s School of Engineering

According to Mittleman, one example of a research area that could obtain benefits from the technique refers to the characterization of perovskite solar cells, a developing solar technology studied widely by Mittleman’s colleagues at Brown.

“One of the issues with perovskites is that they’re made of multi-crystalline grains, and the grain boundaries are what limits the transport of charge across a cell,” Mittleman said. “With the resolution we can achieve, we can map out each grain to see if different arrangements or orientations have an influence on charge mobility, which could help in optimizing the cells.”

That is one example of where this could be useful, Mittleman said, but it is definitely not limited to that.

“This could have fairly broad applications,” he noted.

The National Science Foundation, the Danish Council for Independent Research and Honeywell Federal Manufacturing & Technologies supported the research.

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