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

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journal contribution
posted on 24.10.2013, 00:00 by Sergiu V. Nistor, Daniela Ghica, Mariana Stefan, Leona C. Nistor
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.