Far-field nanoscale infrared spectroscopy of vibrational fingerprints of molecules with graphene plasmons

TL;DR

This paper introduces a graphene plasmonic structure enabling highly sensitive, tunable, far-field infrared nanospectroscopy for molecular fingerprinting at the sub-monolayer level, overcoming previous limitations in light-matter interaction.

Contribution

The authors develop a novel graphene-based plasmonic platform that achieves in situ tunability and high sensitivity for nanoscale molecular vibrational detection in the infrared fingerprint region.

Findings

Achieved sub-monolayer detection sensitivity.

Enabled in situ electrical tunability of graphene plasmons.

Demonstrated detection of both in-plane and out-of-plane vibrational modes.

Abstract

Infrared spectroscopy, especially for molecular vibrations in the fingerprint region between 600 and 1500 cm-1, is a powerful characterization method for bulk materials. However, molecular fingerprinting at the nanoscale level still remains a significant challenge, due to weak light-matter interaction between micron-wavelengthed infrared light and nano-sized molecules. Here, we demonstrate molecular fingerprinting at the nanoscale level using our specially designed graphene plasmonic structure on CaF2 nanofilm. This structure not only avoids the plasmon-phonon hybridization, but also provides in situ electrically-tunable graphene plasmon covering the entire infrared fingerprint region, which was previously unattainable. In addition, undisturbed and highly-confined graphene plasmon offers simultaneous detection of in-plane and out-of-plane vibrational modes with ultrahigh detection sensitivity down to the sub-monolayer level, significantly pushing the current detection limit of far-field mid-infrared spectroscopy. Our results provide a platform, fulfilling the long-awaited expectation of high sensitivity and selectivity far-field fingerprint detection of nano-scale molecules for numerous applications.