Effects of O 2 , Xe, and Gating on the Photoconductivity and Persistent Photoconductivity of Porphyrin Nanorods

The photoconductivity of samples including both individual nanorods and bundles self-assembled from mesotetra(4-sulfonato-phenyl)porphine grows over several days of illumination by more than a factor of 15. It also shows a transition from an initially nonpersistent response (in which the conductivity goes to zero when illumination is removed) to a primarily persistent response (in which the conductivity decays slowly when illumination is removed). Application of a gate voltage results in an initial n-type response, most of which decays away slowly, even though the gate voltage is held constant. The addition of O-2 to the inert atmosphere surrounding the samples drastically reduces the persistent photoconductivity. When Xe is used instead of O-2, there is at most a slight reduction of conductivity. A qualitative model is presented, in which the conductivity changes are attributed to light-induced and gate-voltage-induced adsorption and desorption of O-2.