Nonthermal Plasma-Synthesized Phosphorus–Boron co-Doped Si Nanocrystals: A New Approach to Nontoxic NIR-Emitters

We report on the successful creation of nonthermal plasma-synthesized phorphorus and boron co-doped Si nanocrystals (PB:Si NCs) with diameters (DNC) ranging from 2.9 to 7.3 nm. Peak photoluminescence (PL) emission energies for all PB:Si NC diameters are ca. 400–500 meV lower than the excitonic emission values in intrinsic Si NCs, which can be attributed to prevalent donor–acceptor (D–A) transitions within the co-doped system. This D–A transition model is further evidenced by PL lifetimes within the range of 40–80 μs, faster than what is observed for intrinsic Si NCs. By reducing the level of confinement within PB:Si NCs (i.e., DNC > 4 nm), we are able to red-shift the near-infrared (NIR)-emitting D–A transitions to below the band gap of bulk Si (1.12 eV). We quantify the PL quantum yield (PLQY) for a range of DNC and show that the plasma method can achieve reasonably high PLQY values (12% for DNC = 2.9 nm), even without any optimization of the synthesis or surface chemistry. We posit that perfect charge compensation cannot explain these results and propose a model in which dominant n-type doping accounts for the observations. Ultimately, these results demonstrate that nonthermal plasma synthesis is a viable pathway for preparing PB:Si NCs featuring NIR sub-band gap D–A transitions with relatively high quantum yields. More generally, this study provides insight into how doping affects energy and charge transfer within quantum-confined systems.