Sequential Thermal Decomposition of the Shell of Cubic ZnS/Zn(OH)₂ Core–Shell Quantum Dots Observed With Mn²⁺ Probing Ions

Thermally induced changes in the structure and composition of the shell of tightly aggregated cubic ZnS/Zn­(OH)₂ core–shell quantum dots of 1.9 nm average core size were investigated by multifrequency electron paramagnetic resonance of Mn²⁺ probing ions. The observed three-steps temperature induced transformation of the Mn²⁺ surface centers in the 80–450 °C temperature range was attributed to the sequential decomposition by dehydration of the disordered ε-Zn­(OH)₂ shell into ZnO, with the formation of the Zn₂O­(OH)₂ and Zn₄O₃(OH)₂ intermediate nanocompounds. The presence of a 0.3 to 1.9 nm thick surface layer of disordered nanomaterial separating the cubic ZnS cores and its shrinking to a few atomic layers by mass loss after annealing up to 350 °C was observed by high resolution transmission electron microscopy. Unlike the single step dehydration around 120 °C of the bulk ε-Zn­(OH)₂, the complex decomposition of the ε-Zn­(OH)₂ shell is attributed to its nanosized, disordered structure.