Self-organization of porphyrin structures on Si

The system produced by deposition of aqueous solution (10− 3-10− 6 M) of mesotetra(4-sulfonatophenyl)porphine (TPPS4) on Si was studied. The morphology of self-organized structures of TPPS4 was investigated by atomic force microscopy (AFM). The rod- and fiber-like structures of 250-1000 nm width and 30-200 nm height have been found on glass substrates and crystalline Si (c-Si). The aggregation state of TPPS4 was determined by analysis of optical absorption of aqueous solutions in a visible spectral range and spectroscopic ellipsometry of TPPS/Si samples in the region 1-5 eV. The deposition of aqueous solution of metal-free TPPS4 and Fe-TPPS4 was examined. The difference in the adsorption of TPPS4 on c-Si, porous Si and glass substrates was discussed. The self-ordered ring effect caused inhomogeneous distribution of Fe-TPPS4 on the c-Si surface. From optical studies, it was found that in TPPS/Si samples formed from aqueous solution of TPPS4 of high acidity the optical response of J-aggregates was dominant. In contrast, the fine structure in the optical spectra of the samples Fe- TPPS4/Si corresponded to the excitation of monomers in aqueous solution independently on the pH-value of deposited aqueous solutions. The dependence of spectra for TPPS/Si samples on substrate type, orientation of c-Si, and acidity of aqueous solution was discussed in order to clarify the mechanism of porphyrin adsorption on Si.