Optical Carbon Dioxide Detection in the Visible Down to the Single Digit ppm Range Using Plasmonic Perfect Absorbers

To tackle climate change and reduce CO₂ emissions, it is important to measure CO₂ output precisely. Even though there are many different techniques, no simple and cheap optical method in the visible is available. This work studies plasmonically enhanced optical carbon dioxide sensors in the visible wavelength range. The sensor samples are based on an inert plasmonic perfect absorber, which can be easily and cheaply fabricated by colloidal etching lithography. A CO₂-sensitive polyethylenimine (PEI) layer is then spin-coated on top to complete the samples. The samples are examined continuously by microspectroscopy during different CO₂ exposures to track spectral changes, particularly the position of the resonance centroid wavelength. The samples exhibit a resonance shift of up to 7 nm, depending on the CO₂ concentration and the temperature. The temperature influences the rise time as well as the sensitive concentration range. The concentration dependence of the resonance shift overall follows the shape of a Langmuir isotherm, which includes a nearly linear relation at lower concentrations and elevated temperatures and a saturating behavior at higher concentrations and lower temperatures. The results indicate that a sensitivity in the full range from 100 vol % to below 1 ppm can be achieved. The samples degenerate in a dry inert atmosphere in a matter of days but are useable over multiple weeks when exposed to humidity and CO₂. The PEI reacts very selectively to CO₂, showing no response to CO, NH₃, NO₂, CH₄, H₂, and only a very small response to O₂. Overall, polyethylenimine is very promising as a CO₂-sensitive material for many practical sensing applications over a wide range of concentrations. An adjustment of the temperature is mandatory to control the sensitivity and response time.