When light hits a semiconductor, it can knock out an electron, leaving behind a hole. The electron and hole attract each other and form a pair — an exciton. This pair can re-emit light, but not immediately: first, it exchanges energy with vibrations of the crystal lattice — phonons. Such processes leave a trace in the spectrum as sidebands.

The authors calculated these processes for two-dimensional hexagonal silicon carbide (hSiC)—a material one atom thick. They compared it with hexagonal boron nitride (hBN), which is already well studied. It turned out that in hSiC the sidebands are almost as bright, although the distance between the main peak and the sidebands is smaller, and fewer sidebands are visible. The main role is played by high-energy optical phonons — vibrations where neighboring atoms move in opposite phases.

Such a short lifetime means the exciton gives up its energy to the lattice very quickly. This is important for optoelectronics: the material can switch rapidly, and it emits in the near ultraviolet. The calculations are first-principles, with no fitting parameters, and provide a reliable picture of light–matter interaction in two-dimensional crystals.