To study the implications of highly space-demanding organic moieties on the properties of self-assembled monolayers (SAMs), triptycyl thiolates and selenolates with and without methylene spacers on Au(111) surfaces were comprehensively studied using ultra-high vacuum infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy and thermal desorption spectroscopy. Due to packing effects, the molecules in all monolayers are substantially tilted. In the presence of a methylene spacer the tilt is slightly less pronounced. The selenolate monolayers exhibit smaller defect densities and therefore are more densely packed than their thiolate analogues. The Se-Au binding energy in the investigated SAMs was found to be higher than the S-Au binding energy.

The introduction of new substrate materials into the world of electronics has previously opened up new possibilities for novel applications and device designs. Here, the use of ion‐exchanged sodium aluminosilicate (NAS) glass is presented as a new type of substrate that is not only highly damage resistant, but also allows the fabrication of high performance organic electronic devices. The smoothness of the NAS glass surface enables favorable growth of the semiconductor layer, enabling high charge carrier mobilities for typical organic semiconductors, such as pentacene or C60, and a polymer semiconductor. No degradation of the device performance is observed as a result of ion migration into the active device region, and no compromise in substrate strength due to the processing conditions is made. This work suggests the possibility of new, highly durable electronic devices on glass in large area format.

The microstructure of pentacene films grown on the basal plane of graphite has been investigated. By combining various complementary techniques including scanning tunneling microscopy, atomic force microscopy, x-ray diffraction, thermal desorption spectroscopy, and x-ray absorption spectroscopy the molecular orientation, crystalline structure, and morphology of the films as well as their thermal stability have been characterized in detail as a function of the film thickness. Initial film growth leads to the formation of a commensurate monolayer consisting of flat-lying molecules while upon subsequent deposition epitaxially ordered (022)-oriented pentacene films are formed which adopt the Siegrist phase. The detailed analysis shows that this epitaxial growth of films with an essentially recumbent molecular orientation is brought about by a slight rotation of the molecules in the first layer around their long molecular axis upon deposition of overlying molecular layers. Such a structural modification is unusual and becomes possible by the rather weak adsorption energy on graphite. In contrast, a very different film structure including an upright orientation of molecules even in the first layer is found on nonperfect but rough graphite surfaces leading to the formation of (001) oriented films which initially reveal the thin-film phase and continue to grow in the Campbell phase of pentacene.

Self assembled monolayers (SAMs) made from an aromatic organodithiol, 2-mercaptomethylbenzenethiol (C6H4SHCH2SH) on Au(111) have been investigated in the context of a combined experimental and theoretical approach. The SAMs prepared by immersion of an Au-substrate in corresponding ethanolic solutions were characterized using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS) and thermal desorption spectroscopy (TDS). Adapted from the experimentally obtained unit cell of the SAM, density functional theory (DFT) calculations were applied to get a deeper insight into the structure of these dithiolate based SAMs. On the basis of the experimental and theoretical findings we provide a detailed structural model for this aromatic SAM consisting of the phenyl-group rigidly anchored to the substrate by two thiolate-bonds.

Evidence for a band-like, lateral transport of electrons through the cores of HBC-thiolates, forming a highly ordered self-assembled monolayer (SAM) containing a very regular array of HBC-cores, is provided based on a detailed analysis of temperature-dependent scanning tunneling microscopy (STM) data recorded for islands of aromatic SAMs immersed in an insulating matrix.

Self-assembled monolayers (SAMs) of pi-conjugated organic molecules have attracted significant interest in the field of molecular and organic electronics because of their ability to change electrode work functions combined with a considerable conductivity. Studies simultaneously addressing both their geometrical and morphological structure as well as their electronic properties are, however, scarce. Here, we provide a detailed description of layers consisting of anthracene-2-selenolate on Au(111), which display extraordinarily well and long-range ordered structures. Combining experimental data with the results of slab-type band-structure calculations, we are able to unambiguously determine the alignment of the molecules on the surface. The electronic structure of the SAMs is then determined by ultraviolet photoelectron spectroscopy (UPS) and by density functional theory (DFT) based simulations. For the SAM-induced work-function modification a particularly close agreement between the experimental value of -1.3 eV and the calculated -1.37 eV is found. This supports the notion that the currently available modeling approaches have the potential to quantitatively predict important aspects of the electronic structure of SAMs as long as truly well-ordered monolayers are investigated.

The charge transport across a pentacene/SAM interface has been studied by scanning tunnelling spectroscopy (STS) as a function of temperature and film thickness in order to obtain information on the transport mechanisms and in particular on the importance of interfacial OH-groups on n-transport in organic semiconductors. The current-voltage (I-V) characteristics of pentacene thin films deposited on a mercaptoundecanol self-assembled monolayer (SAM) on Au(111) reveal an asymmetric behaviour. At positive sample bias the onset currents shift towards higher voltages for decreasing temperatures, whereas such changes are not seen at negative bias. For lower temperatures, the variation of current onset with layer thickness is absent. These observations are explained by OH-groups at the SAM-surface effectively acting as charge traps. When electrons are caught in these traps at the organic-organic interface, charge transport is severely affected. Imaging of the SAM after loading the traps suggests that the attachment of electrons to the OH-groups exposed at the organic surface is a reversible process.

When aromatic self-assembled monolayers (SAMs) are electron-irradiated, intermolecular cross-links are formed and the SAMs transform into carbon nanosheets with molecular thickness. These nanosheets have a very high mechanical stability and can withstand temperatures above 1000 K. In this report, we investigate the electron induced cross-linking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UV photoelectron spectroscopy (UPS). The experimental data were acquired as a function of electron dose and temperature and compared with quantum chemical calculations. Details of the intermolecular cross-linking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. Our analysis shows that room-temperature electron irradiation causes a lateral cross-linking via the formation of C-C linked phenyl species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and saturates at approximately 50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete cross-linking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked molecules. At temperatures above 800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.

We have investigated a recently established strategy of modifying organic surfaces exposed by thiolate SAMs (self-assembled monolayers) deposited on Au substrates by employing so-called click chemistry. This term is used to denote a modified Huisgen 1,3-dipolar cycloaddition. We demonstrate the potential of this method by coupling ferrocene and azido acetic acid to alkyne/azide-terminated SAMs. After the surface reaction, the modified organic monolayers were analyzed using infrared spectroscopy (IR), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Under the conditions used in this study, only for the azide-terminated SAMs could successful grafting of the ferrocene be achieved whereas for the alkyne-terminated SAMs the spectroscopic studies reveal a rather low yield of the coupling reaction.

Stressabbau: Selbstorganisierte Monoschichten aromatischer Moleküle zeigen eine wesentlich bessere Ordnung, wenn als Ankergruppe Selen- statt Schwefelatome verwendet werden. Vermutlich führt eine geringere Korrugation der Au-Se-Wechselwirkung dazu, dass die Moleküle leichter Strukturen annehmen, die von den aromatischen Grundgerüsten vorgegeben werden.

A theoretical analysis of charged adsorbates on a metal surface reveals a pronounced polarity asymmetry between electropositive and electronegative species, thus reproducing a well known but so far not properly understood experimental fact. For ionic adsorbates on metal surfaces, we analyze the several, often cancelling, terms that contribute to the change of the interface dipole and, hence, to work-function changes, Deltaphi. We demonstrate that for the prototypic case of I on Cu(111) the magnitudes and the signs of these terms can be understood on the basis of their physical and chemical origins. An important consequence of their cancellation is that negatively charged adsorbates can lead to a paradoxical Deltaphi<0 rather than the expected Deltaphi>0.

The molecular packing motifs occurring in the crystal structures of pentacene and its two oxo-derivatives (6,13-pentacenequinone and 5,7,12,14-pentacenetetrone) have been analyzed. Both oxygen containing species exhibit an almost coplanar stacking while pentacene adopts a face-on-edge herringbone packing. The different packing motifs are well explained by quantum chemical ab initio calculations of the electronic structure of the molecular entities exhibiting a pronounced charge localization at the oxygen atoms of both oxo-derivatives which causes an additional electrostatic O-pi interaction favoring a planar stacking. Moreover, the polarizability of the pi-system is reduced and the molecular quadrupole moment is altered, both resulting in a decrease of the lattice energy of the oxo-species as evidenced by the sublimation energy obtained for all three species from thermal desorption measurements. This emphasizes the importance of the balance between electrostatic and van der Waals interactions for the packing in molecular crystals.

Angewandte Chemie International EditionVolume 47, Issue 28 p. 5250-5252 Communication Selenium as a Key Element for Highly Ordered Aromatic Self-Assembled Monolayers† Asif Bashir, Asif Bashir Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this authorDaniel Käfer, Daniel Käfer Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this authorJan Müller, Jan Müller Fachbereich Chemie, Philipps-Universität Marburg, 35032 Marburg (Germany)Search for more papers by this authorChristof Wöll Prof., Christof Wöll Prof. Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this authorAndreas Terfort Prof., Andreas Terfort Prof. Fachbereich Chemie, Philipps-Universität Marburg, 35032 Marburg (Germany)Search for more papers by this authorGregor Witte Priv.-Doz. Dr., Gregor Witte Priv.-Doz. Dr. witte@pc.ruhr-uni-bochum.de Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this author Asif Bashir, Asif Bashir Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this authorDaniel Käfer, Daniel Käfer Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this authorJan Müller, Jan Müller Fachbereich Chemie, Philipps-Universität Marburg, 35032 Marburg (Germany)Search for more papers by this authorChristof Wöll Prof., Christof Wöll Prof. Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this authorAndreas Terfort Prof., Andreas Terfort Prof. Fachbereich Chemie, Philipps-Universität Marburg, 35032 Marburg (Germany)Search for more papers by this authorGregor Witte Priv.-Doz. Dr., Gregor Witte Priv.-Doz. Dr. witte@pc.ruhr-uni-bochum.de Lehrstuhl für Physikalische Chemie I, Ruhr-Universität Bochum, 44780 Bochum (Germany), Fax: (+49) 234-321-4182 http://www.pc.rub.deSearch for more papers by this author First published: 20 June 2008 https://doi.org/10.1002/anie.200800883Citations: 71 † This work was funded by the DFG (focus program OFET) and supported by a grant from the Dr. Otto Röhm Gedächtnisstiftung (A.T.) and the Studienstiftung des Deutschen Volkes (D.K.). Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract Less stress: Self-assembled monolayers of aromatic molecules become significantly better ordered, if selenium atoms instead of sulfur atoms are used as anchoring groups. Presumably as a result of a lowered corrugation for the Au–Se interaction, the molecules can adapt more easily to structures governed by the carbon backbone. Citing Literature Supporting Information Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2008/z800883_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article. Volume47, Issue28June 27, 2008Pages 5250-5252 RelatedInformation

The growth and evolution of pentacene films on gold substrates have been studied. By combining complementary techniques including scanning tunneling microscopy, atomic force microscopy, scanning electron microscopy, near-edge x-ray-absorption fine structure, and x-ray diffraction, the molecular orientation, crystalline structure, and morphology of the organic films were characterized as a function of film thickness and growth parameters (temperature and rate) for different gold substrates ranging from Au(111) single crystals to polycrystalline gold. Moreover, the influence of precoating the various gold substrates with self-assembled monolayers (SAM's) of organothiols with different chemical terminations has been studied. On bare gold the growth of pentacene films is characterized by a pronounced dewetting while the molecular orientation within the resulting crystalline three-dimensional islands depends distinctly on the roughness and cleanliness of the substrate surface. After completion of the first wetting layer where molecules adopt a planar orientation parallel to the surface the molecules continue to grow in a tilted fashion: on Au(111) the long molecular axis is oriented parallel to the surface while on polycrystalline gold it is upstanding oriented and thus parallels the crystalline orientation of pentacene films grown on SiO2. On SAM pretreated gold substrates the formation of a wetting layer is effectively suppressed and pentacene grows in a quasi-layer-by-layer fashion with an upstanding orientation leading to rather smooth films. The latter growth mode is observed independently of the chemical termination of the SAM's and the roughness of the gold substrate. Possible reasons for the different growth mechanism as well as consequences for the assignment of spectroscopic data of thin pentacene film are discussed.

The structure and thermal stability of differently anchored aromatic self-assembled monolayers (SAMs) of benzenethiol (BT) and benzeneselenol (BS) on Au(111) have been studied in-depth by means of thermal desorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption spectroscopy, and scanning tunneling microscopy. In contrast to long range ordered BS-SAMs consisting of closed packed, upstanding molecules forming a ((1)(7)-(4)(4)) structure which resembles the molecular packing of benzene in the crystalline phase, BT-SAMs are characterized by a loose packing of largely tilted molecules which are arranged in a ((6)(1)(24)) structure appearing only in small domains. Thermal desorption spectroscopy measurements clearly show that BS is less strongly bound to the gold substrate and moreover reveal different desorption peaks for each SAM, thus indicating the presence of different adsorption states. Heating both SAMs causes initially a partial desorption and formation of a diluted phase where the remaining molecules adopt a nearly flat laying adsorption geometry. The diversity of both molecular films is discussed in terms of the different substrate interaction of both anchoring groups.

The formation and structure of pentacene films upon molecular beam deposition at room temperature onto a Ag(1 1 1) surface have been studied by means of SEM, XRD, TDS and NEXAFS. it is shown that multilayer films actually consist of separate crystalline islands revealing a bulk structure reported before by Siegrist et al. [T. Siegrist, C. Kloc, J.H. Schon, B. Batlogg, R.C. Haddon, S. Berg, G.A. Thomas, Angew. Chem. Int. Ed. 40 (2001) 1732]. Distinctly different pi*-resonances were observed in the C1s NEXAFS spectra of pentacene for the first monolayer and the crystalline films reflecting differences in the electronic coupling and the molecular orientation. Moreover, such characteristic pi*-signatures were used to monitor in situ the film evolution upon deposition indicating an immediate formation of crystalline islands on top of a chemisorbed wetting layer. (c) 2007 Elsevier B.V. All rights reserved.