High Energy Limit of the Size-Tunable Photoluminescence of Hydrogen-Terminated Porous Silicon Nanostructures in HF

The photoluminescence (PL) of various porous silicon (PSi) layers was studied during chemical dissolution in HF. The relative PL quantum efficiency of some layers was also monitored. Typically, the PL increased, reached a maximum and then dropped down to complete extinction, accompanied with a PL blueshift. During PL fall, both the PL intensity and layer quantum efficiency fell sharply, accompanied by a decrease in full width at half maximum and a slowing blueshift. In the final stage, the PL intensity decreased without any further blueshift, the saturated PL peak wavelength being similar to 515 nm (similar to 2.4 eV) for most layers, identifying a high energy limit for the achievable PL of hydrogen-terminated Si nanostructures. Our results show that sudden catastrophic mechanical failure of nanostructure cannot explain the sharp PL drop and saturation of PL blueshift. Rather, they support the idea of a critical size (similar to 1.5-2 nm) below which the PL quantum efficiency vanishes. The possible reasons were discussed, privileging the emergence of structural non-radiative defects below a certain size, though the decreasing intrinsic quantum efficiency of Si nanocrystals with decreasing size could also play an important role. Maximum PL intensity was generally obtained for a peak wavelength of similar to 565 nm (similar to 2.2 eV).