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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 °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 °C, HfN NPs remain stable until 250 °C, which sets a limit for air‐mediated use.
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 °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 °C, HfN NPs remain stable until 250 °C, which sets a limit for air‐mediated use.
Titanium nitride is an exciting plasmonic material, with optical properties similar to gold. However, synthesizing TiN nanocrystals is highly challenging and typically requires solid‐state reactions at very high temperatures (800–1000°C). Here, the synthesis of TiN nanocrystals is achieved at temperatures as low as 350°C, in just 1 h. The strategy comprises molten salt, Mg as reductant and Ca3N2 as nitride source. This brings TiN from the realm of solid‐state chemistry into the field of solution‐based synthesis in regular, borosilicate glassware.
Oxynitrides are attracting attention as new inorganic compounds with potential applications such as high-temperature ceramics, phosphors for light-emitting diodes, photocatalysts, and electronics. In addition, new materials are being developed based on the occurrence of nanocrystallization in transition metal nitrides and oxynitrides. Crystallization of transition metal nitride (TMN) nanoparticles from amorphous TM-SiON oxynitrides has recently been reported. TMN nanoparticles have been proposed to apply for localized surface plasmon resonance (LSPR) and have the potential to exhibit performance superior to those of gold and silver nanoparticles. Their refractory nature also makes them suitable for developing new technologies, including optoelectronics, plasmonics, and metamaterials. When embedded in a silica matrix, TMN plasmonic nanoparticles are more compatible with biomedical and complementary metal-oxide semiconductor (CMOS) applications. In addition, TM oxynitrides with a perovskite structure have found applications as relaxor-type dielectric materials, as well as photocatalysts for water splitting. Polar nanoregions (PNRs) in noncentrosymmetric nanocrystallites that precipitate from the oxynitride melt are important for developing superior relaxor-type ferroelectrics. However, to control the formation of PNRs and thus improve the relaxor behavior, it is crucial to determine their local crystal structure. This perspective describes recent advantages with regard to two types of nanocrystallization in metal nitrides and oxynitrides, forming either discrete TMN nanoparticles embedded in a silica matrix or local PNRs in TM oxynitride perovskites, and discusses their future prospects.
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