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

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.

Figure 1. Graphene plasmon enhanced molecular fingerprint sensor.

(a) A schematic of the sensor. The graphene nanoribbon structure was designed on a CaF₂ dielectric substrate (300 nm thick). The graphene plasmon resonance excited by the incident infrared beam (the red shaded pillar) can be tuned in situ by electrostatic doping through the gate voltage (V_g). (b) A scanning electron microscope image of the graphene nanoribbon pattern. Ribbon width (W): 80 nm; width-to-pitch ratio: 1:3. Scale bar, 1 μm. (c) The transfer curve (green line) of our graphene/CaF₂ fingerprint sensor. The gate voltage that corresponds to the charge neutral point (CNP, V_CNP, marked as dash line) is ∼5 V.

Figure 2. Optical properties of CaF₂ nanofilm substrate.

(a) A comparison of the infrared absorption spectra of various dielectric substrates (SiO₂, h-BN and CaF₂). (b) The broadband tunable intrinsic graphene plasmon in the graphene/CaF₂ fingerprint sensor via the effective gate voltage (Δ_CNP=V_g−V_CNP). The red shaded region indicates the molecular fingerprint region.

Figure 3. Highly sensitive detection of molecular vibrational fingerprints.

(a) A comparison of the sensing results for an 8-nm-thick PEO film with (red curve) and without (black curve) graphene plasmon enhancement. The corresponding Fermi level is ∼0.2 eV. The red vertical lines indicate various PEO molecular vibrational modes. (b) The list of PEO vibrational modes in the molecular fingerprint region and their positions in a. The green, blue and red backgrounds represent the C–C, C–O–C, and methylene groups, respectively. The prefixes r, υ, ω and t indicate rocking, stretching, wagging and twisting modes, respectively. The suffixes s and a imply symmetric and anti-symmetric modes, respectively, with respect to the two-fold axis perpendicular to the helix axis and passing through the oxygen atom or center of the C–C bond. The + and − signs denote the phase relationship for the potential energy distribution of the coupled coordinates.

Figure 4. Highly selective detection of molecular vibrational fingerprints.

(a) The plasmon-induced vibrational mode response of the PEO molecules extracted from the extinction spectra of the graphene plasmonic resonance peak at different effective gate voltages. The change of the center frequency of each plasmon peaks is indicated by the red arrow. The Lorentz line shapes (the thin solid curves) are used to fit the peaks induced by different vibrational modes of PEO molecules. The shaded areas with different colours indicate the superposition of the fitted peak areas. (b) The enhancement factor of typical vibrational modes as a function of the distance between the mode (υ_Mode) and graphene plasmon resonance peak (υ_Res). The error bars in the plots are standard deviation from large numbers of measurements. (c) Enlarged extinction spectra for the PEO ultra-thin films with and without plasmon enhancement in the range 1,045–1,200 cm⁻¹. The pristine infrared absorption spectrum of the 8-nm PEO film without plasmon enhancement is shown as the black line at the bottom. The vertical lines indicate the positions of modes E, F and G.

Figure 5. Simultaneous detection of in-plane and out-of-plane vibrational fingerprints.

(a) The extinction spectra (coloured lines) of the graphene plasmon sensor covered with an h-BN monolayer. The infrared extinction spectrum (grey line) of monolayer BN is obtained with incident light normal to the h-BN basal plane. The vertical lines indicate the positions of the optical phonon modes of h-BN monolayer. Inset: the out-of-plane (the transverse optical phonon mode at ∼820 cm⁻¹, o-TO) and in-plane (the longitudinal optical phonon mode at ∼1,370 cm⁻¹, LO) modes in h-BN. (b) A schematic diagram of the interaction between the electric field of the graphene plasmon and monolayer h-BN structure vibrations. The red and green colours in the upper figure indicate the snapshot of positive and negative charge distribution of graphene plasmon. The black arrows represent graphene plasmon. The lower part shows the side view of the electric field intensity distribution calculated from 100 nm wide graphene nanoribbons with E_F=0.3 eV, obtained from a finite element electromagnetic simulation. White arrows indicate the relative direction of the distribution of electric field of graphene plasmon and the response of molecular vibrations to the plasmonic electric field is illustrated by dipoles. The colour bar indicates the field confinement of graphene plasmon, while E₀ is the electric field intensity of incident light. Scale bar, 20 nm.

Figure 6. Sub-monolayer polymer molecules detection.

(a) Near-field intensity confinement and near-field enhancement as a function of the distance d from the graphene nanoribbons. The shaded regions correspond to d <0.5 and 8 nm, respectively. E₀ is the electric field intensity of incident light. (b) The sub-monolayer residual PMMA polymer extinction spectra enhanced by the graphene plasmon sensor at different effective gate voltages for molecular vibrational fingerprinting. An absorption spectrum of a 300-nm PMMA film (the extinction strength is decreased 10 times) is provided to indicate the PMMA vibrational fingerprint modes. Vertical lines indicate four strong vibrational fingerprints of PMMA.