Surface charge measurements and (dis)charging dynamics of organic semiconductors in various media using optical tweezers

An exciting application of optical tweezers is the measurement of the surface charge on a trapped particle, as well as its time evolution with a single charge resolution. We report on an optical tweezer-based method to measure the effective surface charge on an organic semiconductor film at microscopic scales, which offers opportunities for investigations of ion and electron transfer between organic molecules and surrounding medium. Effective charge densities of 13±5 elementary charges per μm 2 were observed in anthradithiophene-coated silica microspheres suspended in water, with a more than an order of magnitude reduction in charge densities upon replacing water with the 50% wt/wt glycerol/water mixture.