PEEM imaging of the growth of organic thin films and diffusion of molecules on topological surfaces

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A combined metastable electron emission microscopy (MEEM) and photoelectron emission microscopy (PEEM) study has been performed for copper phthalocyanine (CuPc) thin films vacuum deposited on Si(100) with a lithographic SiO_2 pattern. The MEEM image of SiO_2 pattern edges changed considerably upon deposition of very small amount of CuPc on the SiO_2/Si(100) substrate, whereas no such changes could be seen in PEEM images. The different contrast features in MEEM and PEEM can be the result of different surface sensitivities of these microscopy. The MEEM contrast characteristic for the pattern edge after the CuPc deposition may originate from the difference of the molecular orientation of CuPc which cannot be observed by PEEM. Furthermore, an unusual triangle image was observed by PEEM when a square-shaped microstructure of indium metal was deposited on a thin film of perylene-3,4,9,10-tetracarboxilicdianhydride (PTCDA) grown semi-epitaxially on a cleaved MoS_2 surface. The formation of the triangle pattern demonstrates the anisotropic surface diffusion of indium atoms on the PTCDA/MoS_2 system. The three-hold diffusion directions, which give the triangle structure, correspond excellently with the three equivalent surface crystal axes of the MoS_2 substrate. When In atoms were directly deposited on MoS_2, such diffusion was not observed, suggesting that the phenomenon is related to strong chemical interaction between In atoms and oriented PTCDA molecules.