Gas-Aggregated Copper Nanoparticles with Long-term Plasmon Resonance Stability Vladimir N. Popok 1 & Sergey M. Novikov 2 & Yurij Yu. Lebedinskij 3,4 & Andrey M. Markeev 4 & Aleksandr A. Andreev 5 & Igor N. Trunkin 5 & Aleksey V. Arsenin 2 & Valentyn S. Volkov 2 Received: 1 May 2020 /Accepted: 6 September 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020 Abstract Metal nanoparticles (NPs) possessing localized surface plasmon resonance (LSPR) are of high interest for applications in optics, electronics, catalysis, and sensing. The practically important issue is the stability of the LSPR, which often limits the use of some metals due to their chemical reactivity leading to degradation of the NP functionality. In this work, copper NPs of two distinct sizes are produced by magnetron sputtering gas aggregation. This method ensures formation of the particles with high purity and monocrystallinity, enhancing the chemical inertness and providing a superior time stability of the plasmonic properties. Additionally, a simple UV-ozone treatment, which leads to the formation of an oxide shell around the copper NPs, is found to be an efficient method to prevent following gradual oxidation and assure the LSPR stability in ambient atmospheric conditions for periods over 100 days even for small (10–12 nm in diameter) NPs. The obtained results allow for significant improvement of the competitiveness of copper NPs with gold or silver nanostructures, which are traditionally used in plasmonics. Keywords Gas aggregation nanoparticle formation . Copper nanoparticles . Copper oxidation . Localized surface plasmon resonance Introduction Metal nanoparticles (NPs) in a dielectric environment are well known for the phenomenon of localized surface plasmon res- onance (LSPR) resulting in a strong enhancement of optical extinction and local electric field [1, 2]. This phenomenon is of great interest for applications in many areas, such as non- linear optics, electronics, photovoltaics, catalysis, and sensing [ 3–8]. Although many metals on the nanoscale show considerable plasmonic efficiency, only a few provide LSPR in the visible range of the spectrum. Among those, gold NPs are the most used ones due to their strong plasmonic reso- nance, high chemical stability, low toxicity, and good abilities of surface functionalization [9]. Silver nanostructures are also widely used but generally considered less attractive due to their requirements for blue-near UV light for plasmon excita- tion and lower chemical and, thus, plasmonic stability, which is often assigned to oxidation [10, 11]. Recent studies have shown that the degradation of silver plasmonic properties is also caused by reactions with sulfur, which even is present in trace amounts in ambient atmosphere [12, 13]. One more candidate with LSPR in the visible spectral range is copper. NPs of this metal can provide intense and narrow plasmonic bands at wavelengths between approximately 500 and 800 nm [1, 14]. It has been recently reported that copper can outperform gold in waveguide applications and it is com- patible with complementary metal –oxide–semiconductor technologies [15, 16]. However, copper nanostructures are even less chemically stable than those of silver. Copper is prone to relatively fast surface oxidation upon exposure to ambient atmosphere. At room temperature, the dominant product is Cu 2 O (Cu(I) oxide) with a minor or no contribution of CuO (Cu(II) oxide) [17]. In the in situ transmission electron * Vladimir N. Popok vp@mp.aau.dk 1 Department of Materials and Production, Aalborg University, 9220 Aalborg, Denmark 2 Center for Photonics and 2D Materials, Moscow Institute of Physics and Technology, Dolgoprudny, Russia 141700 3 Center of Shared Facilities in Nanotechnology, Moscow Institute of Physics and Technology, Dolgoprudny, Russia 141700 4 Institute of Laser and Plasma Technologies, National Research Nuclear University, Moscow, Russia 115409 5 National Research Center, «Kurchatov Institute», Moscow, Russia 123182 Plasmonics https://doi.org/10.1007/s11468-020-01287-4