Ground and excited states of the mercaptoacetic acid-capped silicon quantum dots (SiQDs) were studied using density functional theory and the configuration interaction singles methods, respectively, in conjunction with a 6-31G* basis set. The adsorption of mercaptoacetic acid reduces the optical gap of the Si5 and Si10QDs and results in a red shift of the emission spectra. The optical properties of the SiQDs show size dependence due to the quantum confinement effect. The increase in the number of adsorbed mercaptoacetic acid molecules only makes a small change in the emission spectra of SiQDs. Because of the small changes in the electronic and optical structure of the SiQDs that are capped with mercaptoacetic acid, this molecule offers a good protection of SiQDs
