Tip-and plasmon-enhanced infrared nanoscopy for ultrasensitive molecular characterizations

TL;DR

This paper introduces a combined tip and plasmon-enhanced infrared nanoscopy technique that significantly improves the sensitivity of molecular vibrational spectroscopy at the nanoscale, enabling detection of molecules as small as 1-2 nm.

Contribution

It develops a quantitative model for s-SNOM signals and demonstrates the use of plasmonic resonances in graphene nanodisks to enhance IR vibrational signals of nanoscale molecules.

Findings

Effective excitation of gate-tunable plasmonic resonances in graphene nanodisks.

Strong electric field enhancement enables detection of molecules 1-2 nm in size.

Potential for ultra-sensitive IR biosensing applications.

Abstract

We propose a novel method for ultra-sensitive infrared (IR) vibrational spectroscopy of molecules with nanoscale footprints by combining the tip enhancement of the scattering-type scanning near-field optical microscope (s-SNOM) and the plasmon enhancement of the breathing-mode (BM) plasmon resonances of graphene nanodisks (GNDs). To demonstrate that, we developed a quantitative model that is capable of computing accurately the s-SNOM signals of nanoscale samples. With our modeling, we show that the s-SNOM tip can effectively excite gate-tunable BM plasmonic resonances in GNDs with strong field enhancement and sensitive dependence on the size of GND. Moreover, we demonstrate that the intense electric field of tip-excited plasmonic BMs can strongly enhance the IR vibrational modes of molecules. As a result, IR vibrational signatures of individual molecular particles with sizes down to 1-2 nm can be readily observable by s-SNOM. Our study sheds light on future ultra-sensitive IR biosensing that takes advantage of both the tip and plasmon enhancement.