Self-Trapped Exciton and Large Stokes Shift in Pristine and Carbon-Coated Silicon Carbide Quantum Dots

SiC quantum dots have potential applications in the fields of optoelectronics, biological imaging, etc. However, the theoretical studies of their optical spectra have been limited to the energy gap calculations, and the microscopic mechanism of the photoluminescence process has not been clearly understood. Here, we study the optical absorption and emission spectra for pristine and carbon-coated SiC quantum dots via time-dependent density functional tight-binding method. The results show that optical absorption spectra of pristine SiC quantum dots are generally governed by the quantum confinement effect, which however will break down for the carbon-coated SiC quantum dots. Large Stokes shift in the optical emission spectra for all quantum dots is found, which is attributed to the self-trapped exciton accompanying with stretched chemical bond. The location of self-trapped exciton and the type of stretched bond will be affected by the coated carbon shell, which can be utilized to tune the optical emission properties.