Engineering the Thermal and Energy-Storage Properties in Quantum Dots Using Dominant Faceting: The Case Study of Silicon.

The storage and release of energy is an economic cornerstone. In quantum dots (QDs), energy storage is mostly governed by their surfaces, in particular by surface chemistry and faceting. The impact of surface free energy (SFE) through surface faceting has already been studied in QDs. Here, we introduce dominant faceting representing the structural order of the surface. In particular, we propose that realistic QDs attain complicated polyhedral quasi-spherical shapes while keeping the dominance of a certain type of facet. The type of dominant facet determines the rates of surface-related processes. Therefore, by connecting dominant faceting with SFE, trends analogical to bulk material are kept despite the lack of evident microscopic shape control. To demonstrate the applicability of dominant faceting, we synthesize sets of silicon QDs with sizes around 5 nm and classify them based on increasing SFE of the corresponding analytic geometrical models, using a detailed surface chemistry analysis. Total energies released during oxidation of the synthesized QDs reach the theoretical limit, unlike in the reference, "large" (>100 nm) silicon nanoparticles, which release about 15% less energy. Next, we perform a comprehensive experimental study of dehydrogenation and thermal oxidation of the synthesized QDs in the temperature range of 25-1100 °C, identifying SFE as the key factor determining their thermal stability and surface reactivity. In particular, four distinctive stages of energy release were observed with onset temperatures ranging between 140 and 250 °C, ≈500 and 650-700 °C, respectively, for the SFE-differing samples. Finally, the thermal oxidation of the synthesized QDs is completed at lower temperatures with increasing SFE, decreasing from 1065 to 970 °C and being > 150 °C lower in QDs than in the larger reference nanoparticles. Therefore, despite a rich mixture of features, our description based on linking dominant faceting with SFE allows us to fully explain all the observed trends, demonstrating both the potential of SFE-based engineering of energy-storage properties in QDs and the prospects of silicon QDs as an energy-storage material.