STM observation shows that long chain alkanes increase the adsorption stability of organic molecules. In this work, the interactions of long chain alkanes adsorbed on graphite surface were simulated by molecular mechanics and the STM imaging mechanism of alkylated phthalocyanine and porphyrin was studied by semiempirical quantum mechanics method of ZINDO/1 and AM1. The calculated results illustrated that the interactions between adsorbed molecule and substrate enhanced the stability and immobility of adsorbate, and the interactions between adsorbed molecules determined the form of two- dimensional structure. Compared the electron density contour of the frontier molecular orbits, it was indicated that the STM image contrast of alkylated Copper (Ⅱ) phthalocyanine and porphyrin was attributed to the electronic effects of the core of phthalocyanine and porphyrin and topographic effects of alkyl.

Key words: ALKANE, PORPHYRIN, ZINC PHTHALOCYANINE