We investigate the effect of a surface defect, such as losing ligands and external charge, on methyl passivated silicon quantum dots (QDs). We use Si29(CH3)36 as the QD model with a diameter of ~1.4 nm to perform DFT and  linear response TDDFT calculations.  Our calculations show that removal of few ligands from the QD surface leads to the trap states within the band gap of the QD. These trap states are mostly not optically allowed having dark or semi-dark character of optical transitions. Adding +2 and -2 charge to the QD also results in the mid-gap states. However, these mid-gap states are optically available, resulting in strongly red shifted lowest-energy absorption band. Such optically bright character of mid-gap states originates from a strong delocalization of charge density over the entire QD, due to strong confinement of small QDs. A loss of one or few ligands from the charged QD's surface, however, localizes the charge, leading to the optically dark and semi-dark lower energy transitions, which likely quench photoluminescence of Si QDs.

Effect of surface defects on electronic and optical properties of silicon quantum dots

248th National Meeting (2014)

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