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.

Using a combination of density-gradient and analytical ultracentrifugation, we studied the photophysical profile of CsPbBr3 nanocrystal (NC) suspensions by separating them into size-resolved fractions. Ultracentrifugation drastically alters the ligand profile of the NCs, which necessitates postprocessing to restore colloidal stability and enhance quantum yield (QY). Rejuvenated fractions show a 50% increase in QY compared to no treatment and a 30% increase with respect to the parent. Our results demonstrate how the NC environment can be manipulated to improve photophysical performance, even after there has been a measurable decline in the response. Size separation reveals blue-emitting fractions, a narrowing of photoluminescence spectra in comparison to the parent, and a crossover from single- to stretched-exponential relaxation dynamics with decreasing NC size. As a function of edge length, L, our results confirm that the photoluminescence peak energy scales a L –2, in agreement with the simplest picture of quantum confinement.

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.

Silicon-carbide (SiC) nanocrystals (NCs) of controlled 2–4 nm size are produced in low-pressure nonthermal plasma from the simple alkylsilane precursor tetramethylsilane (TMS). Generating material on the slightly carbon-rich side of 50/50 Si/C, we establish a process for thermally removing residual carbon, which in turn promotes a degree of intrinsic solubility in polar solvents such as isopropanol (IPA). Using the size-dependent Tauc gap of luminescent silicon NCs (Si NCs) as a point of reference, we demonstrate quantum confinement in nanocrystalline β-SiC but without measurable luminescence. Surface-sensitive spectroscopic techniques reveal an oxide shell surrounding a nanocrystalline SiC core, where negative surface charge groups promote solubility while likely acting as efficient trap states for nonradiative recombination. An analytical model is presented that combines electrostatic repulsion with van der Waals attraction to explain experimental observations of concentration-dependent cluster formation and reversible NC aggregation. We anticipate that these materials will be of interest for use as nanofillers in polymer composites and in specialty coatings, while providing a foundation for exploring routes to band gap emission from nanocrystalline SiC.

The synthesis and photophysics (UV–vis absorption, emission, and transient absorption) of four neutral heteroleptic cyclometalated iridium­(III) complexes (Ir-1–Ir-4) incorporating thiophene/selenophene-diketopyrrolopyrrole (DPP)-substituted N-heterocyclic carbene (NHC) ancillary ligands are reported. The effects of thiophene versus selenophene substitution on DPP and bis- versus monoiridium­(III) complexation on the photophysics of these complexes were systematically investigated via spectroscopic techniques and density functional theory calculations. All complexes exhibited strong vibronically resolved absorption in the regions of 500–700 nm and fluorescence at 600–770 nm, and both are predominantly originated from the DPP-NHC ligand. Complexation induced a pronounced red shift of this low-energy absorption band and the fluorescence band with respect to their corresponding ligands due to the improved planarity and extended π-conjugation in the DPP-NHC ligand. Replacing the thiophene units by selenophenes and/or biscomplexation led to the red-shifted absorption and fluorescence spectra, accompanied by the reduced fluorescence lifetime and quantum yield and enhanced population of the triplet excited states, as reflected by the stronger triplet excited-state absorption and singlet oxygen generation.