Nanometer-sized forms of silicon exhibit greatly enhanced optical efficiencies relative to the bulk crystalline phase, creating opportunities for non-toxic, fluorescent bio-labels, efficient light-emitting diodes, and devices utilizing Si for both optical and electronic operations. The photoluminescence (PL) spectrum of quantum-confined silicon nanocrystals is comprised of a rapidly decaying, high-energy feature (2 - 3 eV) as well as a long-lived, lower-energy feature (1 - 2 eV). Despite numerous studies, disagreement exists regarding the origin of both the fast and slow decay processes. Here, we focus on the dynamics of the high-energy feature, which has previously been attributed to "hot" phononless emission from crystalline Si enabled by the quantization of electronic states.
In particular, we quantify the decay time and spectral profiles of high-energy PL from multiple sizes of plasma-synthesized Si nanocrystals. We also perform such measurements for a series of increasingly disordered Si nanoparticles, ranging from fully crystalline to fully amoprhous. Comparison of PL dynamics between samples leads us to suggest that the observed dynamics proceed too slowly to originate from intraband carrier thermalization, and instead suggest that ultrafast, high-energy PL is associated with a ubiquitous amorphous layer. We support our conclusions with high-resolution TEM analysis, Raman spectroscopy, and molecular dynamics simulations.