Refractory Plasmonics of Reactively Sputtered Hafnium Nitride Nanoparticles: Pushing Limits

High-temperature plasmonics deals with optically active nanostructures that can withstand high temperatures. A conventional approach relying on standalone noble metal nanoparticles fails to deliver refractory plasmonic nanomaterials, and an alternative route envisions metal nitrides. The main challenge remains the development of advanced synthesis techniques and the insight into thermal stability under real-life application conditions. Here, hafnium nitride nanoparticles (HfN NPs) can be produced by gas aggregation using reactive magnetron sputtering, a technique with a small environmental footprint are shown. As-deposited NPs are of 10 nm mean size and consist of stoichiometric, crystalline fcc HfN. They are characterized by optical absorption below 500 nm caused by interband transitions and in the red/near-infrared (NIR) region due to intraband transitions and localized surface plasmon resonance (LSPR). The optical response can be engineered by tuning the NP composition as predicted by finite-difference time-domain (FDTD) calculations. Going beyond the state-of-the-art, the HfN NP thermal stability is focued under ultrahigh vacuum (UHV) and in air. During UHV annealing to 850 degrees C, the NPs retain their morphology, chemical and optical properties, which makes them attractive in space mission and other applications. During air annealing to 800 degrees C, HfN NPs remain stable until 250 degrees C, which sets a limit for air-mediated use. Hafnium nitride nanoparticles (NPs) are synthesized via reactive magnetron sputtering, with optical response modulation by adjusting the chemical composition. The NP refractory characteristics are examined under ultrahigh vacuum and in air using in situ techniques, tracking thermally-induced changes in NP chemistry, microstructure, and optical properties. The HfN NPs remain stable up to 850 degrees C in vacuum and 250 degrees C in air. image