Tunability and Flammability of Plasma-Synthesized Silicon Quantum Dots

Silicon is the most sustainable, influential and critical material for our technological advancement. It is the material base for modern electronics, the second most abundant element in the Earth's crust and it has very low inherent toxicity. However, in its bulk form, it is practically unable to luminesce due to the indirect nature of its bandgap. This drawback is circumvented in silicon quantum dots (SiQDs). Countless other types of semiconductor quantum dots, such as CdSe, GaN or the modern CsPbI3 perovskites, are somewhat more efficient light emitters than SiQDs, however, they almost exclusively contain toxic or scarce elements. In addition to low toxicity, there is one aspect where SiQDs outperform their rival materials even when it comes to light emission. As opposed to other semiconductor QDs which are ionic crystals, the nature of the bonding inside the SiQDs' core is covalent. The higher degree of electron sharing associated with covalent bonding makes the electronic properties of SiQDs more sensitive to small changes in surface ligands or in the crystalline core. Thus, most likely as a result of covalent bonding, SiQDs exhibit much broader tunability of their light emission than QDs based on ionic crystals. This tunability makes SiQDs unique light emitters with interesting application prospects and, moreover, SiQDs can serve as a model material for other covalently bonded QDs.<br/><br/>With silicon being a high melting-point material, SiQDs can be advantageously synthesized in non-thermal plasma. However, synthesis in plasma is not the only fabrication route. One aspect in which the synthesis in non-thermal plasma is different from the other methods, be it annealing of Si-rich silicon oxides or electrochemical etching of crystalline silicon, is that it completely bypasses the step of annealing the material to its crystallization temperature. Thus, a question arises as to how the SiQDs synthetized in non-thermal plasma compare to those fabricated by other methods. Here, we will use the spectral tunability of photoluminsecence of SiQDs as a tool to monitor their electronic properties. We will focus mainly on the comparison of SiQDs synthesized in non-thermal plasma with those fabricated by electrochemical etching, because these two fabrication routes represent the completely opposite approaches with regards to the formation of the small crystals: whereas the low-temperature plasma bypasses the crystallization temperature, SiQDs fabricated by electrochemical etching are etched down from perfect bulk crystals. Moreover, we will discuss the possibility of tuning the faceting of the SiQDs produced in plasmatic synthesis and the mechanisms responsible for the thermal stability or instability of these QDs.