Asymmetric Aluminum Antennas for Self-Calibrating Surface-Enhanced Infrared Absorption Spectroscopy

While there has been a tremendous increase of recent interest in noble metal-based antennas as substrates for surface-enhanced infrared absorption spectroscopy, more abundant and manufacturable metals may offer similar or additional opportunities for this mid-infrared sensing modality. Here we examine the feasibility of aluminum antennas for SEIRA, by designing and fabricating asymmetric aluminum cross antennas with nanometer-scale gaps. The asymmetric cross design enables the simultaneous detection of multiple infrared vibrational resonances over a broad region of the mid-infrared spectrum. The presence of the Al2O3 amorphous surface oxide layer not only passivates the metal antenna structures but also enables a very straightforward covalent binding chemistry for analyte molecules to the antenna through multiple approaches, in this case by the use of carboxylic acid functional groups. The aluminum–oxygen stretching mode of the oxide can be used as a self-calibration standard to quantify the number of analyte molecules on the antenna surface.