Gregor Witte

Growth of aromatic molecules on solid substrates for applications in organic electronics

The growth of molecular adlayers on solid substrates is reviewed with aspecial emphasis on molecules of relevance for organic electronics. In particular,we will consider planar molecules with extended π-systems, namely acenes (anthracene, tetracene, pentacene), perylene, coronenes, diindenoperylene, 3,4,9,10-perylene-tetracarboxylicacid-dianhydride, poly-phenylenes, oligothiophenes, and phthalocyanines. Special consideration is given to the importance of the formation of ordered molecular overlayers, which are compared with the structure of the corresponding bulk crystals. Whenever possible, aspects relevant for device fabrication (morphology of deposited films, mobilities of charge carriers) will be addressed.

Vacuum level alignment at organic/metal junctions: “Cushion” effect and the interface dipole

The electronic level alignment of various organic molecules on metal surfaces has been determined by a combined experimental and theoretical effort. Using ab initio electronic structure calculations, it is demonstrated that the commonly observed interface dipole is largely due to a quantum-mechanical phenomenon resulting from exchange repulsion. Surprisingly, this physical effect, also referred to as Pauli repulsion dominates even in the case of aromatic molecules on Cu and Au surfaces, i.e., on interfaces that are of key importance in molecular electronics.

Improved morphology and charge carrier injection in pentacene field-effect transistors with thiol-treated electrodes

The influence of chemical surface modifications of gold electrodes on the morphology and the electrical properties has been studied for pentacene based thin-film transistors with channel lengths of L⩽4μm. Self-assembled monolayers (SAMs) of various aliphatic and aromatic organothiols have been used to selectively modify the metallic source and drain electrodes and are further compared with reference samples with untreated electrodes. For all SAM-treated devices a reduced roughness of the pentacene film is observed which is accompanied by a reduction of the threshold voltage from about VT=2V for untreated transistors to −0.9V for transistors with SAM modified electrodes. Using aliphatic SAMs a poor on/off ratio of about 102 was obtained which is attributed to their low conductivity. In contrast, the on/off ratio is enhanced by four orders of magnitude if the surface is modified by an aromatic SAM. In this case a subthreshold swing as low as 0.55V/decade is achieved which corresponds to a trap density reduc...

Epitaxial growth of pentacene films on Cu(110)

The molecular structure of thin pentacene (C(22)H(14)) films grown on a Cu(110) surface has been studied by means of He atom scattering, low energy electron diffraction, thermal desorption spectroscopy, x-ray photoelectron spectroscopy, and x-ray absorption spectroscopy. Depending on the actual film thickness three different crystalline phases have been identified which reveal a characteristic reorientation of the molecular plane relative to the substrate surface. In the monolayer regime the molecules form a highly ordered commensurate (6.5x2) structure with a planar adsorption geometry. For thin multilayers (thickness <2 nm) a second phase is observed which is characterized by a lateral ((-0.65 5.69) ( 1.90 1.37)) structure and a tilting of the molecular plane of about 28 degrees around their long axis which remains parallel to the surface. Finally, when exceeding a thickness of about 2 nm subsequent growth proceeds with an upright molecular orientation and leads to the formation of crystalline films which are epitaxially oriented with respect to the substrate. The present study thus demonstrates that also on metal substrates highly ordered pentacene films with an upright orientation of the molecular planes can be grown. Photoelectron spectroscopy data indicate further that thick films do not grow in a layer-by-layer mode but reveal a significant degree of roughness.

Determination of site specific adsorption energies of CO on copper

The binding energies of (isolated) CO molecules adsorbed at several atomic sites (terrace, step, kink) on a number of differently oriented copper surfaces have been measured by thermal desorption spectroscopy (TDS). In addition to the three low-indexed Cu surfaces several regular stepped and kinked single crystal surfaces have been employed. Using LEED measurements together with available data in the literature allowed identification of the various different CO adlayers and to assign the different TDS binding energies to the different adsorbate sites. For the close-packed surfaces binding energies between 47 kJ/mol (Cu(111)) and 51 kJ/mol (Cu(100)) were observed, which increased to 58 kJ/mol for CO molecules bound to step edges. Unexpectedly, for kink sites the same binding energy (to within 1 kJ/mol) as for step edges was observed. Moreover, a very similar binding energy of 58 kJ/mol was also measured for random defect sites on sputtered and on poly-crystalline substrates.

Adsorption of acenes on flat and vicinal Cu(111) surfaces: Step induced formation of lateral order

The adsorption of benzene, naphthalene, and anthracene on Cu(111) and its vicinal Cu(221) and Cu(443) surfaces has been studied by means of thermal desorption spectra (TDS) and low energy electron diffraction. For each of the studied acenes (C4n+2H2n+4, n=1,2,3) two distinct binding states were observed in the TDS corresponding to adsorption on terraces and at step edges. The binding energies of both states are found to increase linearly with the number of aromatic rings n. In contrast to the Cu(111) surface where these acenes form disordered adlayers only, ordered films of naphthalene and anthracene with an alignment of the molecular axis along the steps could be prepared on the vicinal surfaces.

Importance of dewetting in organic molecular-beam deposition: Pentacene on gold

Organic molecular-beam deposition of pentacene on gold substrates has been investigated using a multitechnique approach. The morphology of the organic thin films depends strongly on the substrate temperature. Pronounced dewetting and island formation are observed at room temperature. Whereas pentacene molecules adopt a planar monolayer structure, they continue to grow in an upright orientation in multilayer films as inferred from x-ray absorption spectroscopy and atomic force microscopy. These results are in pronounced contrast to a recent scanning tunneling microscopy (STM) study by Kang and Zhu [Appl. Phys. Lett. 82, 3248 (2003)] and indicate fundamental problems in the interpretation of STM measurements for organic thin films.

Epitaxial growth of π-stacked perfluoropentacene on graphene-coated quartz.

Chemical-vapor-deposited large-area graphene is employed as the coating of transparent substrates for the growth of the prototypical organic n-type semiconductor perfluoropentacene (PFP). The graphene coating is found to cause face-on growth of PFP in a yet unknown substrate-mediated polymorph, which is solved by combining grazing-incidence X-ray diffraction with theoretical structure modeling. In contrast to the otherwise common herringbone arrangement of PFP in single crystals and “standing” films, we report a π-stacked arrangement of coplanar molecules in “flat-lying” films, which exhibit an exceedingly low π-stacking distance of only 3.07 Å, giving rise to significant electronic band dispersion along the π-stacking direction, as evidenced by ultraviolet photoelectron spectroscopy. Our study underlines the high potential of graphene for use as a transparent electrode in (opto-)electronic applications, where optimized vertical transport through flat-lying conjugated organic molecules is desired.

Selenium as a Key Element for Highly Ordered Aromatic Self-Assembled Monolayers†

Driven by research in the emerging field of organic electronics, self-assembled monolayers (SAMs) of aromatic molecules have attracted pronounced interest because they allow detailed studies of charge-transport processes across metal–organic interfaces, or have been used as contact primers to improve the properties of metal electrodes. To make the step from being merely model systems to use in applications requires not only local order in these systems, but also the possibility to obtain surface layers with longrange order by a rational approach. In particular, heterogeneities, which might dominate the electronic behavior, must be avoided. Therefore, a precise control of the molecular ordering and the defect density in these ultrathin organic layers is of key importance and requires a detailed microscopic understanding of both the molecule–substrate interactions and the intermolecular interactions, with the latter being particular strong for SAMs with aromatic backbones. Among the aromatic SAMs, oligophenylenethiols in particular have been studied extensively because they are commonly considered as a model system. It has been demonstrated that their long-range ordering can be significantly improved by adding a flexible aliphatic spacer chain between the anchoring unit and the aromatic moiety. Since the insertion of such insulating spacers significantly hampers the charge transport between the aromatic moiety and ametal substrate, this strategy, however, is not a suitable approach for applications in organic or molecular electronics. Moreover, it has been shown that the inter-ring torsion between the individual phenyl units provides an additional conformational degree of freedom which also effects the packing density in such films. On the other hand, in view of the successful use of acenes, such as pentacene, for electronic-device applications, acenethiols constitute a promising class of SAMs to be used as injection layers. While the band gap DE (or HOMO–LUMO separation) of acenes decreases with increasing number of aromatic rings their solubility also decreases significantly which limits the number of available and usable acenethiols. In case of anthracene-2-thiol (AntS) the band gap amounts to DE= 3.45 eV which is already distinctly smaller than the corresponding band gap for oligophenylene-based SAMs (DE= 4.3–4.5 eV, see also the Supporting Information). Note, that the band gap for oligophenylene-based SAMs is almost independent of the number of phenyl units because they are separated by C C single bonds, a feature which means that these molecules are only partly conjugated. Our previous study has shown that although AntS SAMs have excellent electronic properties, their structural quality is limited to only short-range ordering. The reason for the lack of long-range order could be traced back to a misfit between the arrangement of aromatic backbones, which have a tendency to try and pack in a similar fashion to that in the bulk anthracene, and to the restrictions on intermolecular distances in a SAM imposed by the supporting Au(111) lattice. The resulting stress in the SAM is released by introducing a high density of defects, such as domain boundaries, stacking faults, or point defects, which hampers a long-range ordering. Herein, we have analyzed a different approach to improve the structural quality of fully conjugated aromatic SAMs without introducing any spacer groups or other changes in the molecular structure. We demonstrate that replacing the sulfur anchor atom by selenium can distinctly reduce the stress in aromatic SAMs, leading to a significant improvement of structural quality. In addition, the selenium anchoring is expected to provide better electronic coupling to gold than that obtained with sulfur. While the usability of selenolates as alternative anchoring units for SAMs has been demonstrated before their ability to improve the film ordering as well as the underlying mechanism had not been identified. To avoid the formation of insoluble aromatic diselenides and a possible oxidation of selenium, an acetate protection group was used to enable the preparation of monolayer films by immersion (for synthesis and experimental details see the Supporting Information). Note, that the stabilization of an intermediate, flat-lying phase that was observed for acetateprotected aliphatic organothiols was not observed for the present systems, most likely a result of the reduced flexibility of the aromatic (acene) backbone. Figure 1a,c) shows typical STM micrographs recorded for a monolayer of anthracene-2selenolate (AntSe) demonstrating the presence of long-range ordered SAMs. The data reveal characteristic stripes of closely packed molecules along the h112̄i azimuth directions of the Au(111) substrate. This arrangement is rather robust and is not disturbed by point defects thus leading to extended domains with lateral dimensions of more than 50 nm. A [*] A. Bashir, D. K fer, Prof. C. W ll, Priv.-Doz. Dr. G. Witte Lehrstuhl f)r Physikalische Chemie I, Ruhr-Universit t Bochum 44780 Bochum (Germany) Fax: (+49)234-321-4182 E-mail: witte@pc.ruhr-uni-bochum.de Homepage: http://www.pc.rub.de

Cell-Imprinted Substrates Direct the Fate of Stem Cells

Smart nanoenvironments were obtained by cell-imprinted substrates based on mature and dedifferentiated chondrocytes as templates. Rabbit adipose derived mesenchymal stem cells (ADSCs) seeded on these cell-imprinted substrates were driven to adopt the specific shape (as determined in terms of cell morphology) and molecular characteristics (as determined in terms of gene expression) of the cell types which had been used as template for the cell-imprinting. This method might pave the way for a reliable, efficient, and cheap way of controlling stem cell differentiation. Data also suggest that besides residual cellular fragments, which are presented on the template surface, the imprinted topography of the templates plays a role in the differentiation of the stem cells.