Both ac impedance and dc polarization have been measured to investigate the switching property of a Cu2S gapless type atomic switch with two-probe cells composed of asymmetric configuration of ion blocking and reversible electrodes in addition to symmetric one with reversible electrodes. Strong nonlinear p-type conductivity in current–voltage (I–V) characteristics and a marked change of overall ac impedance with the variation of applied dc bias voltage observed are attributed to the nonstoichiometry induced carrier modification in a Cu2S thin film modulated by the Cu vacancy migration under the presence of an electrical potential field.