Bruno Gompf

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.

Dielectric properties of ultrathin metal films around the percolation threshold

Optical reflection measurements of thin Au films at and around the percolation threshold (film thickness 3\char21{}10 nm) are performed in an extremely broad spectral range up to

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

35\text{ }000\text{ }{\text{cm}}^{\ensuremath{-}1}

Kramers-Kronig-consistent optical functions of anisotropic crystals: generalized spectroscopic ellipsometry on pentacene

. Combining spectroscopic ellipsometry, Fourier-transform infrared spectroscopy, and dc measurements, the dielectric properties of the films can be described over the whole frequency range by Kramers-Kronig consistent effective dielectric functions. The optical conductivity of the films is dominated by two contributions: a Drude component starting at the percolation threshold in the low-frequency range and by plasmons in the near-infrared region, which shift down in frequency with increasing film thickness. The interplay of both components leads to a dielectric anomaly in the infrared region with a maximum of the dielectric constant at the insulator-to-metal transition. The results are compared to predictions from effective-medium approximations and percolation theory.

Scanning tunneling microscope investigations of lead–phthalocyanine on MoS2

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.

Transport properties of [2,2]-paracyclophane thin films

The Kramers-Kronig relations between the real and imaginary parts of a response function are widely used in solid-state physics to evaluate the corresponding quantity if only one component is measured. They are among the most fundamental statements since only based on the analytical behavior and causal nature of the material response [Phys. Rev. 104, 1760-1770 (1956)]. Optical losses, for instance, can be obtained from the dispersion of the dielectric constant at all wavelengths, and vice versa [Handbook of optical constants of solids, Vol. 1, p. 35]. Although the general validity was never casted into doubt, it is a longstanding problem that Kramers-Kronig relations cannot simply be applied to anisotropic crystalline materials because contributions from different directions mix in a frequency-dependent way. Here we present a general method to identify frequency-independent principal polarizability directions for which the Kramers-Kronig relations are obeyed even in materials with lowest symmetry. Using generalized spectroscopic ellipsometry on a single crystal surface of triclinic pentacene, as an example, enables us to evaluate the complex dielectric constant and to compare it with band-structure calculations along the crystallographic directions. A general recipe is provided how to proceed from a macroscopic measurement on a low symmetry crystal plane to the microscopic dielectric properties of the unit cell, along whose axes the Kramers-Kronig relations hold.

Infrared spectroscopy on the charge accumulation layer in rubrene single crystals

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

The transport properties of [2,2]-paracyclophane thin films prepared by molecular beam epitaxy were studied by space-charge-limited current measurements. This organic semiconductor, which has a three-dimensional π-system comparable to that of C60, is an interesting material for molecular electronic applications due to its remarkably high mobility and its insensitivity against photo-oxidation. The current–voltage characteristics were recorded using a coplanar electrode geometry which has the advantage, that after deposition no further processing of the organic semiconducting layer is necessary. In polycrystalline films hole mobilities up to 10−2 cm2 V−1 s−1 were observed at room temperature.The transport properties of [2,2]-paracyclophane thin films prepared by molecular beam epitaxy were studied by space-charge-limited current measurements. This organic semiconductor, which has a three-dimensional π-system comparable to that of C60, is an interesting material for molecular electronic applications due to its remarkably high mobility and its insensitivity against photo-oxidation. The current–voltage characteristics were recorded using a coplanar electrode geometry which has the advantage, that after deposition no further processing of the organic semiconducting layer is necessary. In polycrystalline films hole mobilities up to 10−2 cm2 V−1 s−1 were observed at room temperature.

STM investigations of NTCDA on weakly interacting substrates

To get access to the intrinsic properties of organic semiconductors, investigations on single crystals are essential. The authors report on far- and mid-infrared spectroscopies of the charge accumulation layer in an organic field-effect transistor fabricated on a rubrene single crystal. By charge modulation spectroscopy in the range between 70 and 4750cm−1, they were able to detect the Drude response of the accumulated charges in the channel. From this they can extract important intrinsic transport parameters such as the mobility, the plasma frequency, the effective mass, and the scattering rate.

Temperature- and frequency-dependent optical properties of ultrathin Au films

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).