Jörg Petersen

Epitaxy and scanning tunneling microscopy image contrast of copper–phthalocyanine on graphite and MoS2

Monolayers of copper–phthalocyanine (Cu–Pc) on highly oriented pyrolytic graphite (HOPG) and MoS2 prepared by organic molecular beam epitaxy have been investigated by scanning tunneling microscopy. On both substrates there exist well defined preparation conditions leading to ordered two‐dimensional arrays of flat lying molecules. On HOPG they form a close‐packed structure with a nearly quadratic unit cell, whereas on MoS2 we found two phases, one close‐packed and one rowlike phase. This rowlike phase can be explained by a long range interaction due to an adsorbate induced superstructure of the substrate, which also can be seen in the scanning tunneling microscopy images. In images with submolecular resolution, the molecules appear different on the two substrates. On MoS2 they look like a four‐leaved clover, on graphite they show a more detailed inner structure.

Video-STM, LEED and X-ray diffraction investigations of PTCDA on graphite

Thin films of the organic molecule perylene-3,4,9,10-tetracarboxylic-dianhydride (“PTCDA”) on graphite (0001) have been investigated from the mono- to the multilayer regime with low energy electron diffraction (LEED), X-ray-diffraction in Bragg-Brentano geometry, and high resolution scanning tunneling microscopy (STM). These different methods proved epitaxial growth in a coincident superstructure and yielded congruent results concerning details of the crystallographic structure of the epilayer. In addition it was possible to resolve submolecular structures in high resolution STM images; a comparison of the 10 resolved maxima of the tunneling current with the molecular structure leads us to question the conventional model description of tunneling.

Scanning tunneling microscope investigations of lead–phthalocyanine on MoS2

Monolayers of the nonplanar organic molecule lead–phthalocyanine (PbPc) on MoS2 have been prepared by organic molecular beam epitaxy and investigated with video scanning tunneling microscopy. Three different crystallographic phases have been observed. Two of them, the close packed and the rowlike phase, have already been observed in an earlier study of the planar Cu–Pc. With PbPc we have now observed an additional phase, where three close packed rows alternate with one or two isolated single rows. In contrast to Cu–Pc, in submolecularly resolved images the PbPc appear in two different states, with either a dark or a bright center. This is attributed to the nonplanar molecular geometry, which allows the formation of two different adsorption geometries, the Pb above or below the molecular plane. In video sequences the transition from one state into the other could be observed.

A systematic STM study of planar aromatic molecules on inorganic substrates: I. Submolecular image contrast

Abstract High resolution images of planar aromatic molecules exhibit a characteristic intramolecular tunneling current distribution in the STM. In this study we have investigated the correlation of this image contrast with the specific chemical nature of the molecules. For weakly bonded molecules on graphite and MoS 2 the image contrast is dominated by the wavefunction of the specific frontier orbital being closest to the Fermi level of the substrate. For electron acceptor type molecules this is the lowest unoccupied molecular orbital (LUMO), whereas for electron donor type molecules the highest occupied molecular orbital (HOMO) has been observed. On the stronger binding Ag(111) surface we found acceptor type molecules showing an unchanged image contrast compared with graphite and donor type molecules where the image contrast changes from HOMO on graphite to LUMO on Ag(111).

STM investigations of NTCDA on weakly interacting substrates

Abstract Monolayers of the organic molecule naphthalene-1,4,5,8-tetracarboxylic-dianhydride (NTCDA) on graphite and MoS 2 have been prepared by organic molecular beam epitaxy. Two different phases have been observed: one phase where the molecules lie flat on the surface and form a coincident superstructure, and another bulk-like phase where the molecules form stacks with the molecular plane perpendicular to the substrate. The submolecular image contrast of flat-lying molecules is in agreement with the expectation derived from former results on the larger molecule PTCDA, indicating the same underlying contrast mechanism. STS measurements show a strong rectifying behaviour of the molecular layer. The normalized differential conductance shows two maxima which correspond to optical absorption measurements.

STM investigations of PTCDA and PTCDI on graphite and MoS2. A systematic study of epitaxy and STM image contrast

Monolayers of the organic molecules perylene3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and -diimide (PTCDI) on graphite and MoS 2 have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the intermolecular interaction but nearly independent of the substrate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aromatic perylene system whereas the polar oxygen and nitrogen groups are invisible. The correlation of the observed inner structure of the molecules to their molecular structure allows us to compare our results with theoretical considerations. PACS: 61.16 Di; 68.55 Bd

STM characterisation of organic molecules on H-terminated Si(111)

We present the first high resolution STM images of organic molecules on the technological important hydrogen terminated silicon surface. Ordered layers of PTCDA and PTCDI were prepared on this surface by organic molecular beam epitaxy. The submolecular contrast of these molecules on Si(111)/H obtained in the high resolution images agrees with the corresponding images on HOPG and MoS2 substrates.

STM investigations of C6Br6 on HOPG and MoS2

Monolayers of the organic molecule hexabromobenzene on HOPG and MoS2 have been imaged with scanning tunneling microscopy. The flat lying molecules form an ordered close-packed array with unit cell parameters corresponding to their van der Waals radii. In both cases the overlayers are incommensurable to the substrate. In the images with submolecular resolution, the molecules appear different on the two substrates. On HOPG the observed STM image contrast shows a detailed submolecular pattern depending on the tunneling voltage, whereas on MoS2 the molecules appear with a less pronounced inner structure. Neither on HOPG nor on MoS2 the observed submolecular contrast depends on the adsorption site.

Monolayers of tetrachloro-thioindigo and thioindigo in the STM: orientational disorder and the absence of photochromism

Abstract Monolayers of the organic dye molecules thioindigo and 4,4′,7,7′-tetrachloro-thioindigo on graphite and MoS2 have been investigated by scanning tunneling microscopy operating in air and in solution. In monolayers of tetrachloro-thioindigo, orientational disorder with two clearly discernable orientations has been observed for the first time. In contrast, the unsubstituted thioindigo forms perfectly ordered monolayers with three molecules per unit cell. Thioindigo molecules are known to show trans→cis photoisomerization in solution. In well-ordered monolayers however this trans→cis transition seems to be hindered as the STM experiments show.