Photoconductivity, High-resolution AFM, and Scanning Conductance Microscopy of Porphyrin Nanorods

We have shown$^{1}$ that the diacid form of the porphyrin tetrakis(4-sulfonatophenyl) porphine (TPPS$_{4})$ self assembles into nanorods with well-defined height and width. Upon illumination, their conductivity grows over hundreds of seconds. They also produce a zero-bias photocurrent with trainable polarity.$^{2}$ We present measurements as a function of illumination wavelength and intensity, which support a model of charge hopping along paths of previously photoionized porphyrin molecules. We also give results from Scanning Conductance Microscopy experiments; these are designed to clarify the role of the contacts in the DC measurements. Our high-resolution AFM images support the model of a hollow tube$^{3}$, which collapses on contact with the substrate. $^{1}$A.D. Schwab \textit{et al.}, J. Phys. Chem. B \textbf{107}, 11339 (2003). $^{2}$A.D. Schwab \textit{et al.}, Nano Letters \textbf{4}, 1261 (2004). $^{3}$S.C.M. Gandini, E.L. Gelamo, R. Itri, and M. Tabak, Biophys. J. \textbf{85}, 1259 (2003).