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 (D_NC) 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., D_NC > 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 D_NC and show that the plasma method can achieve reasonably high PLQY values (12% for D_NC = 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.