Habit and structure of Cd(S, Se) quantum dots in silicate glasses: High-resolution transmission electron microscopy

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Abstract
Cd(S, Se) nanocrystals were developed in silicate glass by heat treatments and were characterized by high-resolution transmission electron microscopy. They are crystallized in the hexagonal structure of wurtzite type. Their average size increases with increasing heat-treatment time from 5 to 13 nm at 700°C and from 5 to 10 nm at 675°C. The crystal morphology depends on the size. Below a critical size sc, the shape is close to either a sphere or a hexagonal prism with nearly equivalent dimensions in every direction. Above sc, the crystallites definitely adopt a prismatic habit, growing longer in the direction of the c axis. The critical size depends on the temperature of the heat treatment: At 700°C, s c is about 10nm and, at 675°C, it is around 6–7 nm. The basal faces of the prism present an asymmetry, indicating different growth rates in opposite directions of the c axis, in relation to the polarity of the wurtzite structure.