Abstract: An embodiment of the invention is luminescent silicon nanoparticle polymer composite that can serve as a wavelength converter or a UV absorber. The composite includes a polymer or an organosilicon compound; and within the polymer or organosilicon compound, a dispersion of luminescent silicon nanoparticles. In a preferred composite, the silicon nanoparticles have multiple Si—H termination sites, the silicon nanoparticles being linked to a C site to produce a silicon carbide bond (Si—C). In a preferred embodiment, the polymer comprises polyurethane. A composite of the invention can perform wavelength conversion. In a wavelength converted film of the invention, the silicon nanoparticles are incorporated into the polymer or organosilicon compound in a quantity sufficient for wavelength conversion but small enough to have no or an insubstantial effect on the properties of the polymer or the organosilicon compound. A white LED of the invention includes a light emitting diode having a narrow band wavelength output.