Effect of symmetry breaking on the optical absorption of semiconductor nanoparticles

The shape of semiconductor nanoparticles (NPs) can have an important influence on their optical absorption spectra with enhanced absorption resulting from breaking their symmetries. To illustrate this effect, we present broad energy optical excitation spectra of ∼2 nm Si and PbSe nanoparticles, obtained from extensive timedependent density functional theory calculations. We considered both highly symmetric spherical shapes and low-symmetry rodlike and disklike shapes. The low-symmetry shapes exhibit an increase in absorption at low and relatively high energies compared to the absorption of spherical NPs of similar volume, independent of their chemical compositions and surface structures. Our results elucidate the mechanism of enhanced multiexciton generation in semiconductor NPs, which is important in the quest to improve their photovoltaic applications.