Spectroscopically detected, size-dependent temperature effects in I–VII compound nanocrystals: phase transitions and shift of the exciton peak energies

The absorption spectra of CuBr, CuCl, CuI, and AgI nanocrystals (NC) embedded in a glass matrix have been investigated in a wide temperature range between T=6 and 860 K. The change of the absorption in the vicinity of the exciton resonance for CuBr and CuCl reveals a pronounced reduction of the melting temperature of NCs compared to the bulk value and shows a strong hysteresis between melting and solidification temperatures. The melting temperature and the hysteresis width depend on the sizes of NCs. For CuBr NCs a second hysteresis around the bulk melting point is observed. For large NCs the temperature dependence of the exciton peak energies is analogous to its counterpart in bulk crystals. For CuI NCs the shrinkage effect of the gap for both cubic and hexagonal modifications is observed and the temperature coefficients of the energy gaps are deduced. For AgI NCs a rather unusual temperature dependence of energy gap is observed. This dependence is nonmonotonic and shows a change of the energy gap slope from positive to negative.