Mechanism of the order-disorder transition in ultra-small metal nanoparticles

This paper reviews advances in physical modelling and experimental studies of order-disorder transition in ultra-small metal nanoparticles. The interrelation of temperature hysteresis and order-disorder transition in nanoparticles is analysed in detail. The recently discovered dynamic structure of ultra-small nanoparticles is also discussed in this paper. In addition, the mechanism of the order-disorder transition in the ultra-small metal nanoparticles has been established. The transition occurs due to the vacancy formation in nanoparticles. It implies that the transition occurs in a nanoparticle if the vacancy spanning cluster is formed in it. The position and dimensions of the spanning cluster change due to the exchange of energy with the environment or with the external sources. This can cause the continuous change in the structure of nanoparticles due to their ultra-small size. The calculation of the nanoparticle critical sizes at which the order-disorder transition occurs was performed. The critical diameters of the studied nanoparticles lay in the interval 1-6 nm and depend on the transition temperature. The critical diameters decrease with increasing the transition temperature. It was found that the transition of Au nanoparticles from a metallic state to non-metallic state was due to the formation of vacancies in them. All model predictions have been compared with experimental observations and reported data.