Strain-engineered silicon nanocrystals (SiNCs) have recently been shown to possess direct bandgap. Here, we report the observation of a rich structure in the single-nanocrystal photoluminescence spectra of strain-engineered direct-bandgap SiNCs in the temperature range of 9–300 K. The relationship between individual types of spectra is discussed, and the numerical modeling of spectral diffusion of the experimentally acquired spectra reveals a common origin for most types. The intrinsic spectral shape is shown to be a structure that contains three peaks, approximately 150 meV apart, each of which possesses a Si phonon substructure. Narrow spectral lines, reaching ≤1 meV at 20 K, are detected. The observed temperature dependence of the spectral structure can be assigned to the radiative recombination of positively charged trions, in contrast to several previous reports linking a very similar shape to phonons in the surface capping layers. Our result serves as strong additional support for the direct-bandgap nature of the investigated SiNCs.
Direct-band silicon nanocrystals coated with methyl groups - CH3: Scheme of the formation of a trione emission line by radiant recombination of an electron with one of two holes occupying the surface near the top of the valence band (left panel), part of the recombination energy is passed to the second hole. Diagram of phonon replicas of each line by passing a part of the electron-hole energy to the oscillations of the crystal lattice of the nanocrystal (middle panel) and the experimentally obtained spectrum (right panel).