Silicon nanocrystals (SiNCs) with bright bandgap photoluminescence (PL) are of current interest for a range of potential applications, from solar windows to biomedical contrast agents. Here, we use the liquid precursor cyclohexasilane (Si6H12) for the plasma synthesis of colloidal SiNCs with exemplary core emission. Through size separation executed in an oxygen-shielded environment, we achieve PL quantum yields (QYs) approaching 70% while exposing intrinsic constraints on efficient core emission from smaller SiNCs. Time-resolved PL spectra of these fractions in response to femtosecond pulsed excitation reveal a zero-phonon radiative channel that anticorrelates with QY, which we model using advanced computational methods applied to a 2 nm SiNC. Our results offer additional insight into the photophysical interplay of the nanocrystal surface, quasi-direct recombination, and efficient SiNC core PL.

Efficient photoluminescence (PL) from silicon nanocrystal (SiNC) composites has important implications for emerging solar-collection technologies, yet a detailed understanding of the landscape of PL relaxation in vitrified colloidal SiNCs is still materializing. Here, we explore details of PL relaxation in photo-polymerized off-stoichiometric polymer/nanocrystal hybrids. Specifically, thiol-ene polymer/nanocrystal composites were synthesized from a tetra-functional thiol, tri-functional allyl, and a family of dodecyl-passivated colloidal SiNCs with peak PL wavelength spanning 700−950 nm. We find time-and air-stable emission from dilute composites with up to 70% quantum yield, and we investigate PL relaxation in the parameter space of nanocrystal size and temperature, focusing on changes in the partitioning of multimodal decay upon going from the colloid to composite. In light of previous work, our results reveal similarities between the impacts of crosslinking and cooling to cryogenic temperatures, both of which are characterized by a relative reduction in the availability of phonons.