Anti-Stokes Emission in Gold Nanorods

When you light up a metal nano­particle, you get light back. It’s often a different color. That’s a fact – but the why is up for debate. Now, Rice chemist Stephan Link and graduate student Yi-Yu Cai make a case that photo­luminescence, rather than Raman scattering, gives gold nano­particles their remarkable light-emitting pro­perties. The researchers say under­standing how and why nano­particles emit light is important for improving solar-cell effi­ciency and designing particles that use light to trigger or sense bio­chemical reactions.

Rice researchers argued for the dominance of photoluminescence as the source of light emitted by plasmonic metal nanoparticles in a new paper. Their techniques could be used to develop solar cells and biosensors. (Source: A.Joplin, Rice U.)

The long­standing debate, with determined scientists on either side, is about how light of one color causes some nano­particles to emit light of a different color. Cai said the debate arose out of semi­conductor research in the 1970s and was more recently extended to the field of plas­monic structures. “The Raman effect is like a ball that hits an object and bounces off,” Cai said. “But in photo­luminescence, the object absorbs the light. The energy in the particle moves around and the emission comes after­wards.”

Eight years ago, Link’s research group reported the first spectro­scopy study on lumines­cence from single plasmonic nanorods, and the new results build upon that work, showing that the glow emerges when hot carriers – the electrons and holes in con­ductive metals – are excited by energy from a continuous wave laser and recombine as they relax, with the inter­actions emitting photons. By shining specific fre­quencies of laser light onto gold nanorods, the researchers were able to sense tempera­tures they said could only come from excited electrons. That’s an indi­cation of photo­luminescence, because the Raman view assumes that phonons, not excited electrons, are responsible for light emission.

Link and Cai say the evidence appears in the effi­ciency of anti-Stokes as compared to Stokes emission. Anti-Stokes emission appears when a particle’s ener­getic output is greater than the input, while Stokes emission, the subject of an earlier paper by the lab, appears when the reverse is true. Once considered a background effect related to the pheno­menon of surface-enhanced Raman scat­tering, Stokes and anti-Stokes measure­ments turn out to be full of useful infor­mation important to researchers, Cai said.

Silver, aluminum and other metallic nano­particles are also plasmonic, and Cai expects they’ll be tested to determine their Stokes and anti-Stokes properties as well. But first, he and his colleagues will investigate how photo­luminescence decays over time. “The direction of our group moving forward is to measure the lifetime of this emission, how long it can survive after the laser is turned off,” he said. (Source: Rice U.)

Reference: Yi-Yu Cai et al.: Anti-Stokes Emission from Hot Carriers in Gold Nanorods, Nano Lett., online 18 January 2019; DOI: 10.1021/acs.nanolett.8b04359

Link: Laboratory for Nanophotonics, Rice University, Houston, USA

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