Abstract
Infrared spectroscopy, especially for molecular vibrations in the fingerprint region between 600 and 1,500 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 molecular 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 spectroscopies. 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.
Highlights
Infrared spectroscopy, especially for molecular vibrations in the fingerprint region between 600 and 1,500 cm À 1, is a powerful characterization method for bulk materials
In the molecular fingerprint region[4,5,6] from 600 to 1,500 cm À 1
High sensitivity and selectivity molecular detection at the nanoscale level in the fingerprint region is in great demand for various applications
Summary
Especially for molecular vibrations in the fingerprint region between 600 and 1,500 cm À 1, is a powerful characterization method for bulk materials. In the technologically important MIR fingerprint region, graphene plasmon applications are severely restrained by strong coupling between the graphene plasmons and substrate phonons[46,47,48,49,50,51,52,53,54], which strongly confines the electromagnetic energy between graphene and substrate This results in low near-field enhancement on the graphene top surface and a limited spectral tunability, which are not suitable for sensing applications (for example, molecule fingerprint sensing). We demonstrate a hybrid graphene plasmonic structure on a CaF2 nanofilm (graphene/CaF2), in which there is no substrate phonon effect in the MIR region This eliminates the strong plasmon–phonon coupling issue in conventional graphene plasmonic structures, and enables the first electrically tunable plasmon that covers the entire molecular fingerprint region for molecular detection with extremely high sensitivity down to submonolayer level. Our results provide a route for highly sensitive and selective identification of molecular structure fingerprints for diverse applications in anti-terrorism, food and healthcare and biosensing
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