Ionization-induced optical heterogeneity and ion-like direct emission in 1-nm silicon nanoparticle grains: Prospect for fast optical modulation

Silicon, a highly symmetric and homogeneous material, does not exhibit fast optical modulation. Recent classical electrodynamics simulations, however, demonstrated transient optical heterogeneity in silicon nanostructures, in which a high-density of excitonic electron–hole pair plasma and charge is created. The phenomenon, however, requires a specific particle size (∼100 nm diameter) and a high-density (1023/cc) plasma. We examine here the quantum aspect of the heterogeneity in 1-nm Si nanoparticles. Due to the small number of atoms, 1 nm nanoparticles are amenable to the Hartree–Fock first principle atomistic quantum theory simulations procedure, while single ionization events are sufficient to provide high charge density (2 × 1021/cc). The simulations show that the charge distribution in singly charged 1-nm particles is nonlinear and heterogeneous, accompanied with structural distortion that produces an electric dipole moment. Electronically, the simulations show that the single charge induces stationary Coulomb states that riddle the bandgap of the neutral particle, with dipole-allowed transitions, effectively inducing partial conducting-like behavior. Optically, when the charge is produced by ionizing UV radiation, the ionized particle survives and exhibits both extended (wide-band) as well as atomic- or ion-like sharp emission, in agreement with infrared polarimetry and spectroscopy observations in the solar coronal holes, as well as under synchrotron irradiation. Not only do ionized Si nanoparticles (charged nanosilicon grains) afford fast optical modulations, but they may also prove pivotal for understanding features of interstellar medium, observed throughout the Milky Way and other galaxies, including spectroscopic and material composition, as well as neutral hydrogen abundancy.