Evaluating The Refractometric Sensing Performance of Plasmonic Titanium/Hafnium Nitride Nanohole Arrays

Noble metals, such as gold and silver, have conventionally been the preferred materials for plasmonic applications owing to their elevated electrical conductivity, surface plasmon frequencies within the visible spectrum, and chemical stability. However, they are increasingly expensive to utilize, incompatible with conventional CMOS process, difficulties in large-scale manufacturing, and have low melting temperatures at the nanoscale. In this perspective, refractory group IVB metal nitrides present a remarkable ability to modulate their plasmonic behavior within the visible to near-infrared range as alternatives to noble metals. These materials can exhibit exceptional resistance to high temperatures, mechanical robustness, compatibility with CMOS technology, chemical inertness, and outstanding compatibility with prevailing electronic platforms. Thus, these properties render group IVB metal nitrides, especially titanium nitride (TiN) and hafnium nitride (HfN), highly promising candidates for diverse plasmonic applications.<br/>In this study, we have investigated the optical properties of TiN and HfN thin films using variable angle spectroscopic ellipsometry, focusing on the Ar:N₂ ratio to improve the metallic properties of thin films. X-ray diffraction and Raman spectroscopy have been thoroughly carried out on thin TiN and HfN films to assess the stoichiometry and structure of thin films with different gas flow rates. In addition, periodic nanohole arrays of TiN and HfN (especially HfN is a hard material, called conductive ceramic) were successfully fabricated by electron beam lithography (EBL) and induced coupled plasma reactive ion etching (ICP-RIE) to evaluate the refractometric sensing of plasmonic assays. Finally, to explore the potential of plasmonic metal nitrides as alternatives to noble metals, the refractive index sensitivities of TiN and HfN nanohole arrays were investigated using a custom micro-spectrometry setup under different mediums. The refractive index sensitivities for TiN and HfN nanohole arrays achieved 180 and 631 nm/RIU, respectively. This study provides unique insights into the behaviors of TiN and HfN nanohole arrays in refractometric sensing applications, and the full potential of refractory metal nitrides in plasmonics is yet to be realized.