Submolecular-scale Investigations on metal-phthalocyanine monolayers by frequency modulation atomic force microscopy

Copper-phthalocyanine (CuPc) monolayers and cobalt-phthalocyanine monolayers deposited on Au(111) surfaces were investigated by frequency modulation atomic force microscopy (FM-AFM). Submolecular-resolution topographic images were successfully obtained for both samples. Despite the similar molecular geometry of the two molecules, they showed clearly different contrasts in the topographic images. The origin of the contrast is discussed in terms of the relationship of the molecular orbitals and the chemical interaction between the tip and the molecules. In addition, a molecular-resolution surface potential (SP) image was obtained on CuPc monolayers using Kelvin probe force microscopy (KFM) utilizing FM-AFM. The molecular-scale SP contrast was explained by the electric dipole moment at the organic/metal interface. This result suggested the possibility of the detection of the single molecular dipole moment by KFM.