Dynamics of ternary Cu-Fe-S2 nanoparticles stabilized by organic ligands as seen by M\"ossbauer spectroscopy

TL;DR

This study uses Mössbauer spectroscopy to investigate the magnetic and dynamic behavior of Cu-Fe-S2 nanoparticles stabilized by organic ligands, revealing size-dependent relaxation and particle motion characteristics at various temperatures.

Contribution

It provides detailed insights into the magnetic relaxation and dynamic properties of ligand-stabilized Cu-Fe-S2 nanoparticles across different sizes and temperatures, using Mössbauer spectroscopy.

Findings

Magnetic hyperfine interaction relaxes below liquid nitrogen temperature.

Particle dynamics described by a two-level environment with temperature-dependent softening.

Long-range diffusion and thermal oscillations observed without significant particle rotation.

Abstract

Transmission M\"ossbauer spectra for 14.41-keV transition in 57Fe were obtained for chalcopyrite-type (Cu-Fe-S2) ternary nanocrystals stabilized by long aliphatic chain ligands. These systems could be considered as isolated hard nano-objects dispersed in soft network of organic ligands. The main attention was paid to the behavior of the particles whose average size was varied in a controllable manner from 3 to 20 nm. It was found that the magnetic hyperfine interaction was relaxed to null to the temperatures well below liquid nitrogen temperature due to the isolation of particles. The fast dynamics could be described by two level environment. Deeper level (atomic) was practically the same as for bulk material except Debye temperature, but the higher level (particle motion) was described by the classical harmonic oscillator with the spring constant dramatically softening with increasing temperature. Such behavior led to fast decrease of the recoilless fraction with increasing temperature. Practically no spectra could be obtained above 240 K. The second order Doppler shift (SOD) had additional contribution at elevated temperatures due to the induced internal oscillations within particle by surrounding thermal bath. Slow dynamics was characterized by the thermally driven overdamped harmonic oscillator motions. In addition, the long range like diffusion of particles was seen. No significant rotation of particles was found within accessible temperature range.