Size effects in the exciton energy and first-order phase transitions in CuCl nanocrystals in glass

Abstract

The absorption spectra and the melting and crystallization kinetics of CuCl nanocrystals in glass are investigated in the range of particle radii 1–30 nm. Three discontinuities are found on the curves representing the size dependence of the melting point T _m(R) and the crystallization point T _c(R). As the particle radius gradually decreases from 30 nm in the range R⩽12.4 nm there is a sudden 60° drop in the temperature T _c in connection with the radius of the critical CuCl nucleus in the melt. A 30° drop in T _m is observed at R=2.1 nm, and a second drop of 16° in the temperature T _c is observed for CuCl particles of radius 1.8 nm. The last two drops are associated with changes in the equilibrium shape of the nanoparticles. In the range of smaller particles, R⩽1.34 nm the T _c(R) curve is observed to merge with the T _m(R) curve, owing to the disappearance of the work of formation of the crystal surface during crystallization of the melt as a result of the zero surface tension of CuCl particles of radii commensurate with the thickness of the effective surface layer. An increase in the size shift of the exciton energy is observed in this same range of CuCl particle radii (1–1.8 nm). The size dependence of the melting and crystallization temperatures of the nanoparticles is attributed to variation of the free energy in the surface layer of a particle.