Low-energy electron transmission spectroscopy of thin films of chloroaluminum phthalocyanine on MoS2

The growth of chloroaluminum phthalocyanine (ClAlPc) thin films on MoS2 surfaces was studied by low-energy electron transmission (LEET) spectroscopy. We observed that the as-grown monolayer, prepared by vacuum deposition, consists of islands of ClAlPc multilayers and the molecules spread over the substrate surface to form a uniform monolayer by heat treatment. Furthermore, we found that for heat-treated films the vacuum level of the sample system oscillates with increasing the film thickness from 0 to 2 monolayers. For the monolayer, the change of the vacuum level with respect to the substrate Δφ (= φfilm-φsubstrate) was positive, while for the doublelayer it was negative. These results indicate that in the monolayer the molecules lie flat with the Cl atoms protruding outside the film to form an electric dipole layer directing to the substrate, and in the doublelayer the molecules in the outer layer are turned over with the Cl atoms protruding inside the film to compensate the dipole originating from the first monolayer. Furthermore the thickness independent characteristics of LEET spectra above the doublelayer suggest that thicker films consist of a stack of a double layer-like structures.