Transformation of the short-range order upon melting of CuHal nanocrystals in glasses

Abstract

The reduced density of states at the edge of interband transitions in CuCl and CuBr nanocrystals is described by a step at T → 0. As the temperature increases, the step is smeared toward the band gap and the long-wavelength absorption tail agrees with the Urbach rule. A change in the phase state of CuCl or CuBr nanoparticles (the crystal-melt transition) is accompanied by an insignificant change in the band gap, and the long-wavelength absorption tail for the interband transitions considerably extends deeper into the band gap as compared to that for crystals at the same temperature. The change observed in the band gap upon melting is explained by the transformation of short-range order in CuCl and CuBr melts. An increase in thermal smearing of the long-wavelength absorption tail for nanomelts is associated with the severalfold increase in the amplitude of thermal fluctuations of the band gap, possibly, due to the disappearance of the long-range order in the melts. A lower melting temperature of large (>30 nm) CuCl and CuBr particles as compared to the melting temperature of bulk samples indicates the presence of the second component NaCl or NaBr. Consequently, the CuCl-NaCl and CuBr-NaBr eutectic nanosystems undergo melting and crystallization. A small shift in the band gap observed in this case upon melting confirms the conclusion drawn by Zalkin that eutectic melts have a microheterogeneous structure.