Effects of O2, 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 meso-tetra(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 O2 to the inert atmosphere surrounding the samples drastically reduces the persistent photoconductivity. When Xe is used instead of O2, 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 O2.