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

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.


Supplementary Figures
Supplementary Figure 1. Absorption spectrum of a 300 nm CaF 2 film on silicon. It is highly transparent in the range from 675 to 4000 cm -1 , with no distinctive resonant mode in this spectrum. 800 1000 1200 1400 1600 800 1000 1200 1400 1600 Wavenumber (cm -1 ) substrate (c) and SiO 2 substrate (d). Graphene plasmon couples strongly with the two surface optical (SO) phonons of SiO 2 at 806 cm -1 ( sp1 ) and 1168 cm -1 ( sp2 ), indicated by vertical dashed lines in (b) and (d). This strong plasmon-phonon coupling induced electromagnetic filed cancel between plasmon and phonons, and shaped the wide plasmon resonance peaks (as shown in b and d) into well separated three sharp peaks, which cannot even cover half of the fingerprint region (red shadow). While on our CaF 2 substrate, the graphene plasmon resonance can be continuously tuned electrically in a wide spectrum range without suffering any hybridization effects from phonons. In addition, Graphene plasmon frequency changes with ribbon width accordingly following the scaling behavior of W -1/2 . [1][2][3] By varying the width of graphene nanoribbons, graphene plasmon resonance frequency can be regulated to cover the whole fingerprint region, and even the mid-and far-infrared region, which broadens the detectable spectrum range of the graphene plasmon-based IR sensor to a very large extent. In all figures, red shadow indicates the vibrational fingerprint region. Figure 7. Extinction spectra of graphene nanoribbon arrays after coating with 8 nm PEO film on CaF 2 substrate (a, c) and SiO 2 substrate (b, d). By varying the Fermi level (a, b) and the width (c, d) of graphene nanoribbons, graphene plasmon resonance frequency can be regulated to cover the whole fingerprint region. As shown, all the vibrational modes of the analyte 8 nm PEO film in the fingerprint region can be enhanced and detected by the undisturbed graphene plasmon on our CaF 2 substrate (a, c). However, the phonon-induced transparency in the hybrid peaks of graphene plasmon on the SiO 2 substrate (b, d) are very weak. It is because that the strong coupling between the graphene plasmons and substrate phonons confines the electromagnetic energy between the graphene and substrate and results in very low near-field enhancement on the top graphene surface for sensor applications. Especially in the yellow shadow regions which marks the anticrossing 1200 1400 1600 1800 1000 800

Supplementary
Wavenumber (cm -1 )   Supplementary Figure 11. The recycling of our graphene/CaF 2 sensors. When the 8 nm PEO was washed away, the graphene/CaF 2 plasmonic sensor maintained its property and had the same ability to detect the analyte. After several cycles, there is nearly no performance degradation for the sensors. It shows that our device indeed can be used repeatedly.

Supplementary Note 1. The calculation of graphene Fermi levels
The calculation of carrier densities and Fermi levels of graphene on CaF 2 is as follows. From the determined transfer characteristics, capacitance and thickness of the dielectric, we calculated the carrier density of graphene using a standard parallel plate capacitor model. For CaF 2 thin film, using a relative dielectric constant of 6.8 and a thickness of 300 nm, the capacitance was calculated as 0.021 μFcm -2 . The dependence of the carrier density on the gate voltage satisfies the equation:

Supplementary Note 2. Method to extract PEO molecule vibrational signals from plasmon extinction spectrum and calculations of enhancement Factor
The extinction spectra of our experiments ( Ex PM , Supplementary Figure 10a This phonon-induced transparency has been understood as a special case of coherent destructive interference between phonon resonances and the plasmon polaritons. [4,5] Therefore, we can obtain the absorption spectrum (Ex M , Supplementary Figure 10d The summation over a lattice vector R l translates into the following integral, The vanishing  iT (r) indicates the i-TO phonon has no effective force on the plasmon, where a non-zero  indicates coupling between the plasmon and phonon.
Further, we also carried out additional simplified simulation to quantify this coupling phenomenon between the out-plane phonon and the graphene plasmon by the Finite Difference Time Domain (FDTD) method (as discussed in references [9][10]). The simulation results are shown in Supplementary Figure 15. We also present our experimental results in Supplementary Figure 15 for comparison. It shows that the simulation results roughly agree with experimental result. Worth noting that more accurate simulation deserves further investigation.