Nanoscale Physics

Paramagnetic properties of ensembles of silver and copper nanoparticles
Shanina B.D.1, Krasnovyd S.V.1, Konchits A.A.1, Rieznichenko L.S.2
1 Department of Optics and Spectroscopy, Institute of Semiconductor Physics of the NAS of Ukraine. Prospect Nauki, 41, Kyiv-03028, Ukraine. 
E-mail: sergkrasnovyd88@gmail.com
2 Department of colloidal technology of natural systems, F.D. Ovcharenko Institute of Biocolloidal Chemistry of the NAS of Ukraine. Acad. Vernadsky Boulevard, 42, Kyiv, 03142, Ukraine.

Ensembles of Au, Ag, Cu, and other nanoparticles (NPs) are widely used in biology and medicine for the preparation of antibacterial drugs, targeted drug delivery, as contrast agents in MRI, etc. The magnetic properties of such NPs are important for actual applications and are the subject of numerous studies [1,2]. In this work, the dependence of the magnetic properties of lyophilized ensembles of Ag and Cu NPs on the size of NPs, their aggregation, and oxidation processes has been systematically studied based on Kawabata theory [2] for the first time.
It is known when the size of the NP L< 100 nm, the orbital motion of electrons begins to quantize. Distance between the orbital energy levels is equal to ħor= 1/DF, where DF= (3/4) Ne/F is the electron state density at the Fermi energy level, Ne is a number of electrons in a particle. This value increases with the particle size decreases. Under quantization conditions, the shape of the EPR lines and the spectrum as a whole are formed under the action of two conditions: (1/or) <<1, Z/or<<1, where Z is the Zeeman frequency,  is the spin relaxation time. As a result, the EPR linewidth H and the shift of the resonance field (g-factor) vary, forming a complex multicomponent EPR spectrum.
Comparison of theory with experiment showed that, in our experiments, silver nanoparticles with sizes of 5, 9, and 13 nm are stable. In the ensemble of Cu NPs during storage in air, in addition to the EPR spectrum Cu2+ from NPs with 7 nm in size, a new intense spectrum is formed due to nonstoichiometric oxidation of NPs.

1.	Marenco A. J., Pedersen D. B., Trudel S. Inducing Ferromagnetic Behaviour in Cu Nanoparticles and Thin Films via Non-Stoichiometric Oxidation // J. Phys. Chem. C.-2016.-120, P. 7388-7396.
2.	Kawabata A. Electronic Properties of Fine Metallic Particles. III. ESR Absorption Line Shape // J. Phys. Soc Jap.-1970. - 29, N 4.-P. 902-911.