Far-field nanoscale infrared spectroscopy of vibrational fingerprints of molecules with graphene plasmons (1606.09327v1)

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.

• The paper introduces a graphene/CaF₂ hybrid structure that circumvents substrate limitations to boost far-field IR spectral sensitivity.
• The paper achieves over a 20-fold sensitivity increase in detecting specific polymer vibrational modes via tunable graphene plasmons.
• The paper enables simultaneous detection of in-plane and out-of-plane vibrational modes, facilitating sub-monolayer molecular analysis.

Analysis of Far-Field Nanoscale Infrared Spectroscopy Using Graphene Plasmons

The presented paper explores the challenge of molecular fingerprinting at the nanoscale using a novel approach that employs graphene plasmons on a CaF₂ nanofilm. The research seeks to address the longstanding issue of weak light-matter interaction at nanoscale dimensions, particularly in the fingerprint region of 600 to 1500 cm⁻¹, which is critical for molecular identification but often lacks the sensitivity required for trace detection. The authors introduce a specially designed graphene plasmonic structure as a solution to enhance far-field mid-infrared (MIR) spectroscopic capabilities.

Key Findings

1. Graphene/CaF₂ Hybrid Structure: The use of a graphene/CaF₂ hybrid structure effectively circumvents the limitations associated with conventional substrates such as SiO₂ and h-BN, which are plagued by strong plasmon-phonon couplings. By eliminating substrate phonon interactions, the graphene plasmons maintain their intrinsic properties, offering a highly tunable system across the entire molecular fingerprint region.
2. Optical Properties and Enhancement: The paper demonstrates substantial enhancement in the detection of molecular vibrational modes, showing more than a 20-fold increase in sensitivity for certain polymer types at the nanoscale. This enhancement is facilitated by the electrical tunability of graphene plasmons, allowing for selective amplification of specific vibrational modes by adjusting the Fermi level of graphene.
3. Simultaneous In-Plane and Out-of-Plane Detection: The approach also provides the capability to simultaneously detect in-plane and out-of-plane vibrational modes. This dual-mode detection is significant for analyzing complex molecular structures, like proteins or layered materials such as h-BN, which exhibit distinct vibrational behavior in different orientations.
4. Sub-Monolayer Detection: Detailed experiments reveal the capability of this system to detect sub-monolayer films, a critical feature for applications requiring high sensitivity, such as trace analysis in environmental and biopharmaceutical applications.

Implications and Future Directions

The findings in this paper present a substantial advancement in the field of nanophotonic sensors, particularly for applications requiring the identification and characterization of molecular species at very low concentrations. The ability to precisely tune the graphene plasmonic response enables a broader range of detectable molecular species, enhancing selectivity and specificity in spectral analysis.

The practical implications of this research are noteworthy for sectors such as safety, environmental monitoring, and healthcare, where trace detection and precise identification of molecular configurations are paramount. The theoretical advancements regarding graphene plasmon behavior in different spectral regions contribute a significant addition to the existing literature on plasmonics, opening avenues for subsequent research in plasmon-photonic hybrid systems.

Future work could build upon these findings by exploring the integration of such graphene-based plasmonic systems with existing MIR instrumentation for real-time sensing applications. Scaling these devices for broader adoption in various industrial and scientific applications will be essential. Additionally, further exploration into other 2D materials compatible with this approach could provide new insights and enhancements in nanoscale spectroscopy.

In conclusion, this work illustrates significant strides in the convergence of advanced material science and optical spectroscopy, providing a robust platform for advancing molecular detection at nano-dimensions. The delicate manipulation of graphene plasmons offers a promising path forward in the development of highly sensitive and selective optical sensing technologies.