Localized surface plasmon resonance (LSPR) has garnered interest in a variety of fields recently, such as photocatalysis, photovoltaics, biophotonics, spectroscopy, sensing, and wave-guiding. LSPR is correlated with the density of free charge carriers in nanoparticle materials, so metals tend to have the highest LSPR frequencies, with some absorbing within the visible spectrum. Cost and production concerns motivate the search for alternative plasmonic materials, like Group IV transition metal-nitrides.1 We present a novel technique for the synthesis of plasmonic zirconium nitride (ZrN) nanoparticles using a scalable non-thermal plasma process. The system employs a solid zirconium tetrachloride precursor, which is heated to increase its vapor pressure. It is then mixed in the vapor phase with ammonia and passed through a non-thermal plasma reactor to dissociate the precursors and form ZrN. The synthesized particles exhibit a plasmonic absorption peak well within the visible spectrum, tunable from ~530 nm to ~650 nm. From XRD and TEM we infer the production of crystalline ZrN particles with a cubic rock salt structure and a tunable size distribution below 10 nm. Due to the tendency of this material to oxidize, we developed a modular non-thermal plasma system that coats the particles with amorphous silicon nitride in flight. This coating acts as an oxygen diffusion barrier when the material is exposed to atmosphere and yields blue-shifted and increased-intensity absorption. We observe different oxidation behavior than plasmonic titanium nitride (TiN) nanoparticles made using a variation of the same non-thermal plasma technique.2 In TiN nanoparticles, oxidation causes a red-shift and intensity reduction in the plasmonic absorption spectrum.2 In ZrN, oxidation does not result in a significant red-shifting of the absorption peak, though the intensity is reduced. We present additional data from density-functional theory calculations of the effect of oxidation on plasmonic resonance in each material, providing theoretical support for our observation of the variant effects of oxidation in the two similar materials.
1 Boltasseva, A. & Atwater, H.A. Science 331, (2011), 290-291.
2 Alvarez Barragan, A., Ilawe, N.V., Zhong, L., Wong, B.M., & Mangolini, L. J. Phys. Chem. C 121(4), (2017), 2316-2322.