Realization and characterization of SEIRA gas sensors

Surface-Enhanced Infrared Absorption spectroscopy (SEIRA) methane sensors enable the detection of low gas concentrations under ambient conditions, making them suitable for applications in medical diagnostics, industrial process control, and safety monitoring in the food and energy industries. This research work focuses on the development and characterization of a SEIRA-based methane sensor. The physical phenomena of SEIRA can be used to enhance the Infra-Red (IR) absorption band of methane at 3016 cm^−1, achieved through the excitation of surface plasmon polaritons in a gold microstructured surface combined with a microporous storage layer that locally increases methane concentration near the metal. It is researched how to obtain a homogeneous microporous layer on the gold surface by realizing a Self Assembled Monolayer (SAM) with molecules presenting thiol groups, such as thiophenol or 7-mercapto-4-methylcoumarin. Different microporous materials, including Polymer of Intrinsic Microporosity (PIM), Porous Aromatic Framework (PAF), and Metal-Organic Framework (MOF), are deposited under various conditions and characterized to optimize the deposition process. The microstructure geometry is designed and validated through COMSOL Multiphysics simulations to maximize plasmonic enhancement. Fabrication is performed on a sapphire substrate using direct laser writing, followed by electron beam evaporation of 10 nm of titanium / 100 nm of gold and metal lift-off. The process required extensive optimization to reliably obtain 1.86 μm square features with a pitch of 3.64 μm over 1 cm^2 areas. The final SEIRA device is obtained by drop-casting PIM-1 or PAF-1 solutions onto the microstructured surface. The on-chip SEIRA performance is evaluated through Fourier Transform Infra-Red spectroscopy (FTIR) transmission measurements under low-pressure methane exposure (33 mbar). Throughout the optimization steps, multiple complementary fabrication and characterization techniques were employed: Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), both reflective and transmissive FTIR analysis, and optical microscopy.