Unusual size dependence of the optical emission gap in small hydrogenated silicon nanoparticles

It is well known that the electronic and optical absorption gaps of hydrogenated silicon nanoparticles are inversely proportional to the particle size. Here, the authors show that their optical emission gaps are remarkably different and dully dependent on the size for those smaller than 1.5nm, based on their excited-state calculations of a series of nanoparticles from Si5H12 to Si199H140 using a time-dependent tight-binding density-functional method. It is revealed that this unusual size dependence is due to the strong excited-state structure relaxation in the particle core region that becomes significant when the size decreases.