New Nanoimaging Method Finds App in Plasmonics

Theoretical calculations to understand and predict the optical properties of plasmonic nanomaterials exist but haven’t been verified in the lab, until now.

Plasmonic nanomaterials contain specially engineered conducting nanoscale structures that enhance the interaction between light and an adjacent material; the shape and size of such nanostructures can be adjusted to tune these interactions for applications such as invisibility cloaking, photovoltaics and sensors.

Researchers from NIST and the University of Maryland have now shown how to make nanoscale measurements of critical properties of plasmonic nanomaterials in a way that does not affect how the structure functions. They turned to photothermal induced resonance (PTIR), a chemically specific materials-analysis technique first demonstrated at the University of Paris-Sud that can image the response of plasmonic nanomaterials excited by IR light with nanoscale resolution.

Researchers from NIST and the University of Maryland have shown how to make nanoscale measurements of critical properties of plasmonic nanomaterials without affecting the material’s function. IR laser light (purple) from below a sample (blue) excites ring-shaped nanoscale plasmonic resonator structures (gold). Hot spots (white) form in the rings’ gaps. In these hot spots, IR absorption is enhanced, allowing for more sensitive chemical recognition. A scanning AFM tip detects the expansion of the underlying material in response to absorption of IR light. Courtesy of NIST.

Every material has a unique IR spectrum that acts like a chemical fingerprint, and as the laser scans across its surface, the sample at each point absorbs IR light at wavelengths that are determined by the chemical composition at that spot. The absorbed light heats the material, causing it to expand ever so slightly, which can be detected by an atomic force microscope (AFM).

Repeatedly scanning the sample at different wavelengths reveals the sample’s underlying chemical composition with a resolution determined by the AFM tip size and the sample’s thermomechanical properties.

The investigators used the PTIR method to image the absorbed energy in ring-shaped plasmonic resonators, which focus incoming IR light within the rings’ gaps, creating hot spots where the light absorption is enhanced. This enables more sensitive chemical identification. The absorption in the hot spots was quantified and showed that, for the samples under investigation, it is approximately 30 times greater than areas away from the resonators.

“We want to maximize the sensitivity of these resonator arrays and study their properties,” said lead researcher Andrea Centrone. “In order to do that, we needed an experimental technique that we could use to verify theory and to understand the influence of nanofabrication defects that are typically found in real samples. Our technique has the advantage of being extremely sensitive spatially and chemically, and the results are straightforward to interpret.”

The investigators also demonstrated that plasmonic materials can be used to increase the sensitivity of IR and PTIR spectroscopy for chemical analysis by enhancing the local light intensity, and thereby, the spectroscopic signal.

The research was published in Nano Letters (doi: 10.1021/nl401284m).

For more information, visit: www.nist.gov