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Charge-transfer processes at molecule-metal interfaces play a key role in tuning the charge injection properties in organic-based devices and thus, ultimately, the device performance. Here, the metal's work function and the adsorbate's electron affinity are the key factors that govern the electron transfer at the organic/metal interface. In our combined experimental and theoretical work, we demonstrate that the adsorbate's orientation may also be decisive for the charge transfer. By thermal cycloreversion of diheptacene isomers, we manage to produce highly oriented monolayers of the rodlike, electron-acceptor molecule heptacene on a Cu(110) surface with molecules oriented either along or perpendicular to the close-packed metal rows. This is confirmed by scanning tunneling microscopy (STM) images as well as by angle-resolved ultraviolet photoemission spectroscopy (ARUPS). By utilizing photoemission tomography momentum maps, we show that the lowest unoccupied molecular orbital (LUMO) is fully occupied and also, the LUMO + 1 gets significantly filled when heptacene is oriented along the Cu rows. Conversely, for perpendicularly aligned heptacene, the molecular energy levels are shifted significantly toward the Fermi energy, preventing charge transfer to the LUMO + 1. These findings are fully confirmed by our density functional calculations and demonstrate the possibility to tune the charge transfer and level alignment at organic-metal interfaces through the adjustable molecular alignment.

Metalation and self‐metalation reactions of porphyrins on oxide surfaces have recently gained interest. The mechanism of porphyrin self‐metalation on oxides is, however, far from being understood. Herein, we show by a combination of results obtained with scanning tunneling microscopy, photoemission spectroscopy, and DFT computations, that the self‐metalation of 2H‐tetraphenylporphyrin on the surface of ultrathin MgO(001) films is promoted by charge transfer. By tuning the work function of the MgO(001)/Ag(001) substrate, we are able to control the charge and the metalation state of the porphyrin molecules on the surface., The self‐metalation of 2H‐tetraphenylporphyrin on the surface of MgO is believed to be feasible only at defect sites. It is shown that this reaction is promoted by charge transfer and does not necessarily require the presence of defects. Using ultrathin MgO films as substrate and tuning its work function allows the charge and metalation state of the adsorbed molecules to be controlled.

Science advances 8(29), eabn0819 (2022). doi:10.1126/sciadv.abn0819, Published by American Association for the Advancement of Science, Washington, DC [u.a.]

ACS nano 16(10), 17435-17443 (2022). doi:10.1021/acsnano.2c08631, Published by American Chemical Society, Washington, DC

ACS nano 14(11), 15766-15775 (2020). doi:10.1021/acsnano.0c06798, Published by American Chemical Society, Washington, DC

In this work we investigate the formation of crystalline organic thin films for p-sexiphenyl (6P) on Ag(001) with a combination of STM, LEED, TPD, SFM, UPS, XRD and DFT. For weakly interacting van-der-Waals-dominated systems of small organic molecules on single crystalline metals we demonstrate the complex epitaxial evolution from sub-monolayer to bulk structure. In the delicate interplay between molecule-substrate and intermolecular interactions we identify the oligo-phenyl's flexibility of central importance for the film growth. We observe a change from a planar to a twisted molecular conformation and, accompanying, a molecular realignment during the formation of a dense monolayer as the result of enhanced 2-dimensional intermolecular interaction. A second molecular realignment occurs collectively for whole domains with the formation of 3-dimensional bilayer structures. Finally the bilayer acts as a template for the epitaxial growth of the molecular thin film, determining the azimuthal orientation and a single contact plane for the organic crystallites.

We report a method to achieve physical and electronic decoupling of organic molecules from a metal surface. Oxygen adsorbed on the Cu(100) surface immobilizes the surface electrons in the Cu-O covalent bonds. This results in electronic surface hardening and prevents charge transfer from the metal into perylene-tetracarboxylic dianhydride molecules subsequently deposited on this surface.

A novel multiresponsive hydrogel has been synthesized by initiated chemical vapor deposition (iCVD). Hydrogels are known for their dynamic swelling response to aqueous environments. A chemical functionalization of the hydrogel surface was performed to add other stimuli-responsive functionalities and obtain a smart material that responds to two stimuli: light irradiation and exposure to aqueous environment. Modifying the hydrogel surface with solution-based methods is often problematic because of the damages caused by the permeation of solvents in the hydrogel. This issue is completely bypassed by the use of solvent-free techniques. Cross-linked polymers of 2-hydroxyethyl methacrylate (HEMA) were functionalized with azobenzene groups, as confirmed by IR spectroscopy and X-ray photoelectron spectroscopy (XPS). Through photoisomerization of the azobenzene, the polarity within the hydrogel is modified and as a consequence the affinity to water. Light irradiation modifies the degree of swelling within thin hydrogel films from 13% before exposure to UV light to 25% after exposure. The possibility of controlling the degree and rate of swelling by light irradiation was never reported before on these time scales and can have exceptional implications for light-induced drug delivery or light-controlled microfluidic systems. The light-responsive hydrogels showed also biocompatibility, which makes them suitable for a great variety of applications as biomaterials.

Beilstein journal of nanotechnology 11, 1492-1503 (2020). doi:10.3762/bjnano.11.132, Published by Beilstein-Institut zur Förderung der Chemischen Wissenschaften, Frankfurt, M.

The electronic and geometric structures of tetracene films on Ag(110) and Cu(110) have been studied with photoemission tomography and compared to that of pentacene. Despite similar energy level alignment of the two oligoacenes on these surfaces revealed by conventional ultraviolet photoelectron spectroscopy, the momentum-space resolved photoemission tomography reveals a significant difference in both structural and electronic properties of tetracene and pentacene films. Particularly, the saturated monolayer of tetracene on Ag(110) is found to consist of two molecular species that, despite having the same orientation, are electronically very different-while one molecule remains neutral, another is charged because of electron donation from the substrate.

Irreproducibility is a serious issue in thin film organic photovoltaic (OPV) devices, as smallest local inhomogeneities can change the entire behaviour of identically built devices without showing obvious failure. Inhomogeneities can occur at various steps of device preparation and appear in all layers with different length scales and impact. The hole-transport interlayer (HTL) in OPV devices blocks unwanted electron diffusion to the anode and corrects energetic mismatch between oxide electrode and organic semiconductor. Most commonly used is commercial ink based on poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT PSS) colloidal particles. However, exactly these are suspected to cause microscopic inhomogeneities, causing known irreproducibility of device characteristics. Considering PEDOT:PSS acidity-caused electrode corrosion, it is questionable how much impact colloids have on device homogeneity. In this report, we give proof that a colloidal HTL does not necessarily cause device inhomogeneity and decreased efficiency, by comparing OPV devices with different HTLs, namely from commercial PEDOT:PSS ink and from MoO3, obtained from two liquid precursors, leading to quasi-continuous or colloidal layers. With a combination of X-ray diffraction, atomic force and Kelvin probe microscopy, photoelectron and ambient air photoemission spectroscopy, we discuss the layers properties from nano- to macroscale and demonstrate their impact upon implementation into OPV devices, via spatially-resolved characterization.

Die winkelaufgeloste Photoelektronen-Spektroskopie, Photoelektronen-Tomographie genannt, erlaubt die Rekonstruktion von Molekulorbitalen in drei Dimensionen. Dazu werden auf einer Metalloberflache angeordnete Molekule mit extremem ultravioletten Licht bestrahlt und die Winkel- und Energieverteilung der uber den photoelektrischen Effekt herausgelosten Elektronen gemessen. Die Ergebnisse sind ein weiterer Beleg fur das Konzept der Molekulorbitale, denen der Orbitaltheoretiker Kenichi Fukui 1977 eine “irgendwie unwirkliche Natur” zuschrieb. Anders als zum Beispiel Rastersonden-Methoden funktioniert die Photoelektronen-Tomographie auch bei Zimmertemperatur. Sie kann zudem Orbitale organischer Molekule auf reaktiven Substraten abbilden. In Zukunft konnte sie auch 3D-Bilder von dynamischen Veranderungen in Orbitalen, zum Beispiel wahrend chemischen Reaktionen, liefern.

The most active and efficient catalysts for the electrochemical hydrogen evolution reaction (HER) rely on platinum, a fact that increases the cost of producing hydrogen and thereby limits the widespread adoption of this fuel. Here, a metal-free organic electrocatalyst that mimics the platinum surface by implementing a high work function and incorporating hydrogen-affine hydrogen bonds is introduced. These motifs, inspired from enzymology, are deployed here as selective reaction centres. It is shown that the keto-amine hydrogen-bond motif enhances the rate-determining step in proton reduction to molecular hydrogen. The keto-amine-functionalized polymers reported herein evolve hydrogen at an overpotential of 190 mV. They share certain key properties with platinum: a similar work function and excellent electrochemical stability and chemical robustness. These properties allow the demonstration of one week of continuous HER operation without notable degradation nor delamination from the carrier electrode. Scaled continuous-flow electrolysis is reported and 1 L net molecular hydrogen is produced within less than 9 h using 2.3 mg of polymer electrocatalyst.

What do energy level alignments at metal-organic interfaces reveal about the metal-molecule bonding strength? Is it permissible to take vertical adsorption heights as indicators of bonding strengths? In this paper we analyse 3,4,9,10-perylene-tetracarboxylic acid dianhydride (PTCDA) on the three canonical low index Ag surfaces to provide exemplary answers to these questions. Specifically, we employ angular resolved photoemission spectroscopy for a systematic study of the energy level alignments of the two uppermost frontier states in ordered monolayer phases of PTCDA. Data are analysed using the orbital tomography approach. This allows the unambiguous identification of the orbital character of these states, and also the discrimination between inequivalent species. Combining this experimental information with DFT calculations and the generic Newns-Anderson chemisorption model, we analyse the alignments of highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) with respect to the vacuum levels of bare and molecule-covered surfaces. This reveals clear differences between the two frontier states. In particular, on all surfaces the LUMO is subject to considerable bond stabilization through the interaction between the molecular π-electron system and the metal, as a consequence of which it also becomes occupied. Moreover, we observe a larger bond stabilization for the more open surfaces. Most importantly, our analysis shows that both the orbital binding energies of the LUMO and the overall adsorption heights of the molecule are linked to the strength of the chemical interaction between the molecular π-electron system and the metal, in the sense that stronger bonding leads to shorter adsorption heights and larger orbital binding energies.

In this paper the influence of charged species on the sheet strength of viscose fibres was investigated. Four samples of chemical modified viscose fibres, as well as a reference fibre were studied. The swelling of these viscose fibres and the breaking length of hand sheets have been determined. Comparing the results, the influence of both, swelling and surface charge on the bonding force, is evident. The allocation of the charges, induced by cationic starch and Carboxmethylcellulose, has been analyzed by Titration, attenuated total reflection spectroscopy (ATR) and X-ray photoelectron spectroscopy (XPS). Titration was used to make a first estimation of the charge distribution within the fibre. Using ATR and XPS, more detailed information about the surface charge has been achieved. All measurement methods showed a significant amount of charge on the fibre surface.

Photoemission tomography (PT), the analysis of the photoemission intensity distribution within the plane wave final-state approximation, is being established as a useful tool for extracting the electronic and geometric structure of weakly interacting organic overlayers. Here we present a simple method for extending PT, which until now has been based on the calculations of isolated molecules. By including the substrate and a damped plane-wave final state, we are able to simulate the photoemission intensity distribution of two-dimensional molecular overlayers with both strong intermolecular and molecule-substrate interactions, here demonstrated for the model system 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) on Cu(100). It is shown that the interaction and hybridization of the lowest unoccupied molecular orbital of PTCDA with substrate states leads to its occupation and the formation of a strongly dispersing intermolecular band, whose experimental magnitude of 1.1 eV and $k$-space periodicity is well reproduced theoretically.

This paper investigates the effects of cesium (Cs) deposited on pentacene (5A) and sexiphenyl (6P) monolayers on the Ag(110) substrate. The process of doping and the energy level alignment are studied quantitatively and contrasted. While ultimately for both molecules lowest unoccupied molecular orbital (LUMO) filling on charge transfer upon Cs dosing is observed, the doping processes are tellingly different. In the case of 5A, hybrid molecule-substrate states and doping states coexist at lowest Cs exposures, while for 6P doping states appear only after Cs has completely decoupled the monolayer from the substrate. With the support of density functional theory calculations, this different behavior is rationalized by the local character of electrostatic potential changes induced by dopants in relation to the spatial extent of the molecules. This also has severe effects on the energy level alignment, which for most dopant/molecule systems cannot be considered continuous but discrete.

The buried interface between a molecular thin film and the metal substrate is generally not accessible to the photoemission experiment. With the example of a sexiphenyl ($6\mathrm{P}$) bilayer on Cu we show that photoemission tomography can be used to study the electronic level alignment and geometric structure, where it was possible to assign the observed orbital emissions to the individual layers. We further study the Cs doping of this bilayer. Initial Cs exposure leads to a doping of only the first interface layer, leaving the second layer unaffected except for a large energy shift. This result shows that it is in principle possible to chemically modify just the interface, which is important to issues like tuning of the energy level alignment and charge transfer to the interface layer. Upon saturating the film with Cs, photoemission tomography shows a complete doping ($6{p}^{4\ensuremath{-}}$) of the bilayer, with the molecular geometry changing such that the spectra become dominated by $\ensuremath{\sigma}$-orbital emissions.

The interactions of glycine (Gly) with amorphous solid water (ASW) nanolayers (≤ 100 ML), vapor-deposited on single crystalline AlO x surfaces at 100 K, have been investigated by near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the oxygen K -edge, temperature-programmed thermal desorption (TPD), X-ray photoelectron spectroscopy (XPS), and temperature-dependent work function measurements. Gly-on-ASW, ASW-on-Gly, and Gly on top of ASW-on-Gly ultrathin films have been fabricated. In contrast to the uniform ASW films grown directly on the hydrophilic AlO x , water molecules adsorb on the hydrophobic Gly films in the form of 3D ASW clusters. This leads to significant differences in the NEXAFS and work function data obtained from ASW-on-AlO x and ASW-on-Gly films, respectively. Furthermore, these structural differences influence the chemical state of Gly molecules (neutral vs. zwitterionic) adsorbed on top of ASW films. N1s XPS measurements revealed an increased amount of neutral Gly molecules in the film top-deposited on the ASW-on-Gly structure in comparison to the neutral Gly in the films directly condensed on AlO x or grown on the ASW substrate. H 2 O TPD spectra demonstrate that the crystallization and desorption processes of ASW are affected in a different way by the Gly layers, top-deposited on to ASW-on-AlO x and ASW-on-Gly films. At the same time, Gly adlayers sink into the ASW film during crystallization/desorption of the latter and land softly on the alumina surface in the form of zwitterionic clusters.

Charge carrier injection barriers at interfaces are crucial for the performance of organic electronic devices. In this respect, tuning the electronic interface potential or, in case of the metallic...

Here the device performance is shown to be directly related to the measured electronic level alignment at the dielectric interface in C60 organic field effect transistors (OFETs). We compare C60 on two dielectrics: Al2O3 and a divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB)–Al2O3 bilayer. The improved transistor performance and the lowering of the threshold voltage by 0.8 V due to the BCB interlayer are reflected in the ultra-violet photoemission data. These show a 0.8 eV difference in the work function and a concomitant shift in the C60 valence band. By following the work function the improvement is shown to be due to the dipole at the organic BCB–C60 interface.

The photolithographic modification of thin functional silane layers provides a versatile and powerful means of fabricating functionalized patterned surfaces which can be applied for tuning inorganic surface properties and for modern immobilisation techniques. In this contribution we present the synthesis of a new functional trichloro organosilane bearing photoreactive aryl ester groups and its application in thin silane layers on silicon oxide surfaces. Whereas the trichlorosilyl group acts as anchoring unit to the inorganic surface, the aryl ester group undergoes the photo-Fries rearrangement to yield hydroxyketones upon irradiation with UV-light of 254 nm which leads to a change in chemical reactivity of the surface. By a subsequent reaction with perfluorobutyryl chloride, the photogenerated hydroxy groups yield the corresponding perfluorinated ester compound, which allows further tuning of surface properties. The layer formation as well as the photoreaction and post-modification reaction was monitored by FTIR spectroscopy and X-ray photoelectron spectroscopy (XPS). The thickness of the obtained thin layers was determined by X-ray reflectivity (XRR). Photopatterned surfaces were produced using a contact mask during illumination followed by the post-modification reaction. Friction force microscopy (FFM) revealed the contrast between modified and unmodified regions of the patterned samples.

Electron Comings and Goings Advances in computational chemistry have enabled remarkably detailed calculations of electrons' spatial arrangements in complex molecules. Experimental confirmation of these orbital geometries remains challenging, however, particularly for molecules bound in thin layers to underlying surfaces—a morphology of increasing interest in organic semiconductor development. Puschnig et al. (p. 702 , published online 10 September) mapped the trajectories of photoelectrons emitted from polycyclic aromatic molecules deposited on copper and extracted spatial profiles of the orbitals from which these electrons emerged. The method of analysis takes advantage of the straightforward Fourier relation between position and momentum and complements orbital imaging techniques based on scanning tunneling microscopy.

Recently, there have been reports of the valence band photoemission of pentacene films grown on various substrates with particular emphasis on the highest occupied molecular orbital (HOMO) and its dispersion. In various works, evidence for HOMO band dispersion as high as 0.5 eV, even for polycrystalline films, has been presented. In apparent contradiction to these results, we have previously reported a band dispersion of only 50 meV, measured on a well characterised film with a single polymorph and single crystalline orientation, 5A(0 2 2). Here, we first present the two-dimensional momentum distribution of the HOMO of a 5A(0 2 2) film. Then the development of the valence band spectra for films grown at room temperature and low temperature are compared, and we show that morphological aspects can lead to the apparent observation of high HOMO dispersion. Finally, with the aid of the two-dimensional momentum distribution of the HOMO, we show that a reasonably large dispersion (0.25 eV) does indeed exist in 5A(0 2 2).

Here we report the electronic π-band structure of sexithiophene obtained from 6T(010) oriented films. The angle-resolved valence band photoemission results taken parallel and perpendicular to the molecular axis are compared to those of sexiphenyl and interpreted in terms of intra- and inter-molecular band dispersion. We show that the strong photoemission intensity variations with emission angle parallel to the molecular axis are well reproduced by the Fourier transforms of the molecular orbitals of the isolated molecules. These results imply that ARUPS can yield quite detailed information about molecular geometry, both in terms of molecular orientation and internal structure.

Sexithiophene (6T) films have been grown by molecular beam deposition on two different substrates: a clean Cu(1 1 0) single crystal surface and a 1:1 Cu(1 1 0)/Cu(1 1 0)-(2×1)O stripe surface. Both substrates serve as atomically ordered two-fold symmetric templates, which induce epitaxial growth of the 6T crystallites due to surface corrugations. On both surfaces, 6T crystallizes in its low-temperature phase with (0 1 0) being the main crystallographic orientation of the 6T crystals relative to the substrate surface. Two different epitaxial growth behaviors are observed: while on the oxygen-passivated surface the long molecular axes of the 6T molecules are aligned exactly along the oxygen rows, small tilt angles relative to the densely packed copper rows are found on the clean copper surface. Classical misfit calculations seem to be insufficient to describe the observed epitaxial crystallization of sexithiophene. On the striped surface clean copper- and oxygen-passivated copper regions are offered on the same surface. The dominant crystalline fraction grows in an orientation also found on the fully reconstructed Cu(1 1 0)-(2×1)O surface where it grows exclusively in this orientation. A model that explains these experimental findings is discussed and it turns out that the sticking anisotropy is the dominating effect.

The adsorption of α-sexithiophene (6T) on Cu(1 1 0), Cu(1 1 0)–(2 × 1)O and the mesoscopically patterned Cu–O striped surface have been studied by STM (scanning tunnelling microscopy), XPS (X-ray photoelectron spectroscopy) and NEXAFS (near edge X-ray absorption fine structure). The molecular resolution of the STM allowed to determine the orientation and local order of the molecules in the submonolayer and monolayer regime. It is shown that the 6T molecules align with their long molecular axis along the densely packed copper rows on Cu(1 1 0) and along the Cu–O rows on the Cu(1 1 0)–(2 × 1)O surface. On the striped phase with alternating copper and Cu–O regions the molecules adsorb first on the Cu regions and after complete filling of these regions, on the Cu–O. The orientation is the same on both areas as on the respective pristine surfaces with the only exception that the molecules reorient by 90° if the width of the copper regions is smaller than the molecular length. The NEXAFS measurements allowed for a determination of the adsorption geometry of the molecules: while 6T lies flat on the surface on clean copper, the molecular planes are inclined with an angle as high as 39° with respect to the substrate on (2 × 1)O. For the latter, this inclination angle is 4° higher than in the bulk crystal structure of 6T observed for thicker films to release stress and allow commensurability with the substrate lattice, while for the former it is a result of the aromatic system bonding to the Cu(1 1 0) surface, as confirmed by XPS.

The structure of a 30 nm thick pentacene film grown by molecular beam deposition on an oxygen passivated Cu(110) single crystal has been investigated ex-situ by X-ray diffraction methods. It is shown that pentacene crystallizes in two known bulk polymorphs with four unique crystal orientations. In all four cases, a principal pentacene direction is aligned along the surface corrugation provided by the Cu(110)–(2 × 1)O reconstruction. Since overlayer and substrate form incommensurate lattices, the results cannot be understood by classical criteria of organic epitaxy and the importance of one dimensional alignment is discussed.

Highlights • Orbital tomography within the plane wave final state approximation. • One electron orbital predictions versus angle resolved photoemission experiment. • Geometric and electronic structure of organic thin films elucidated by ARUPS. • Influence of molecular conformation and orientation on ARUPS. • Retrieval of sexiphenyl and pentacene orbitals in real space., The frontier orbitals of molecules are the prime determinants of their chemical, optical and electronic properties. Arguably, the most direct method of addressing the (filled) frontier orbitals is ultra-violet photoemission spectroscopy (UPS). Although UPS is a mature technique from the early 1970s on, the angular distribution of the photoemitted electrons was thought to be too complex to be analysed quantitatively. Recently angle resolved UPS (ARUPS) work on conjugated molecules both, in ordered thick films and chemisorbed monolayers, has shown that the angular (momentum) distribution of the photocurrent from orbital emissions can be simply understood. The approach, based on the assumption of a plane wave final state is becoming known as orbital tomography. Here we will demonstrate, with selected examples of pentacene (5A) and sexiphenyl (6P), the potential of orbital tomography. First it will be shown how the full angular distribution of the photocurrent (momentum map) from a specific orbital is related to the real space orbital by a Fourier transform. Examples of the reconstruction of 5A orbitals will be given and the procedure for recovering the lost phase information will be outlined. We then move to examples of sexiphenyl where we interrogate the original band maps of thick sexiphenyl in the light of our understanding of orbital tomography that has developed since then. With comparison to theoretical simulations of the molecular band maps, the molecular conformation and orientation will be concluded. New results for the sexiphenyl monolayer on Al(1 1 0) will then be presented. From the band maps it will be concluded that the molecule is planarised and adopts a tilted geometry. Finally the momentum maps down to HOMO-11 will be analysed and real space orbitals reconstructed.

Here the morphology, molecular orientation and electronic structure of in situ prepared para-sexiphenyl (6P) and α-sexithiophene (6T) films studied with atomic force microscopy, near edge X-ray absorption fine structure spectroscopy and valence band photoemission are presented. Attention is given to the differences between different organic crystallite orientations and the pitfalls in the interpretation of area averaging surface sensitive techniques that can arise from inhomogeneities in the films, which commonly occur even on single crystal inorganic substrates. The growth of organic–organic heterostructures is then considered for sexithiophene films grown on homogeneous upright (6P(0 0 1)) and lying (6P(2 0 3)) crystalline films. In both cases, the orientation of the substrate molecules is imposed on the molecules of the second species and thick films of upright-on-upright or lying-on-lying could be produced. The organic substrates are thus shown to be excellent templates for further organic film growth that do not require the stringent UHV conditions of inorganic templates.

The electronic level alignment of the organic semiconductor para-sexiphenyl on a nanoscopically patterned substrate was investigated with ultraviolet photoemission spectroscopy, work function measurements and scanning tunnelling microscopy. The results show that for increasing coverage on inhomogeneous surfaces, shifts in electronic level alignment occur, which are due to the change from local to average band alignment. The Cu(1 1 0)–(2 × 1)O stripe phase, used as a model substrate consists of alternating stripes of bare and oxygen passivated copper, with stripe widths of ∼35 A comparable to the sexiphenyl molecular length. In the first molecular layer the electronic bands are clearly aligned to the local surface potential of the specific stripe, resulting in the superposition of two photoemission spectra offset by 1 eV. Beyond two monolayers the valence band spectra clearly indicate a single electronic level alignment, which is determined by the average interface dipole.

With angle-resolved photoemission experiments and ab initio electronic structure calculations, the pentacene monolayers on Ag(110) and Cu(110) are compared and contrasted, allowing the molecular orientation to be determined and an unambiguous assignment of emissions to specific orbitals to be made. On Ag(110), the orbitals remain essentially isolated-molecule-like, while strong substrate-enhanced dispersion and orbital modification are observed upon adsorption on Cu(110). We show how the photoemission intensity of extended systems can be simulated and that it behaves essentially like that of the isolated molecule modulated by the band dispersion due to intermolecular interactions.

The molecular/crystal orientation and morphology of active molecular structures is a key determinant for the function of nanoscaled organic devices, yet little is known regarding the processes that govern thin film growth. Here, we show that either sticking or diffusion anisotropy can control the growth depending on preparation conditions. This is illustrated by an investigation into the growth of sexiphenyl (6P) on the anisotropic TiO 2 (1 1 0)-(1 × 1) single crystal surface. The great differences in crystallite orientation and morphology observed are explained by the domination of the growth kinetics by either sticking or diffusion anisotropy depending on growth temperature.

A para-sexiphenyl monolayer of near up-right standing molecules (nominal thickness of 30 A) is investigated in - situ by X-ray diffraction using synchrotron radiation and ex - situ by atomic force microscopy. A terrace like morphology is observed, the step height between the terraces is approximately one molecular length. The monolayer terraces, larger than 20 μm in size, are extended along the [0 0 1] direction of the TiO 2 (1 1 0) substrate i.e. along the Ti–O rows of the reconstructed substrate surface. The structure of the monolayer and its epitaxial relationship to the substrate is determined by grazing incidence X-ray diffraction. Extremely sharp diffraction peaks reveal high crystalline order within the monolayer, which was found to have the bulk structure of sexiphenyl. The monolayer terraces are epitaxially oriented with the (0 0 1) plane parallel to the substrate surface (out-of-plane order). Four epitaxial relationships are observed. This in-plane alignment is determined by the arrangement of the terminal phenyl rings of the sexiphenyl molecules parallel to the oxygen rows of the substrate.

Highly crystalline organic heteroepitaxial layers with controlled molecular orientations and morphologies are one of the keys for optimum organic device performance. With studies of molecular orientation, structure, and morphology, we have investigated the ability of oriented organic films to act as substrate templates for the growth of a second organic layer. Depending on the molecular orientation in the sexiphenyl substrate, crystalline sexithiophene nanostructures of either pyramidal or needlelike morphology, with either near vertical or parallel molecular orientations, respectively, grow.

Phosphorescent materials represent an attractive route to high-efficiency organic LEDs, since their maximum theoretical quantum efficiency is 100%, compared to the 25% of fluorescent materials. We have studied the electronic structure of three phosphorescent iridium complexes, the red-emitter Iridium bis (2-(2′-benzothienyl) pyridinato-N, C3′) (acetylacetonate), the blue emitter, Iridium bis(2-(4,6- difluorophenyl) pyridinato-N,C2′) picolinate, and the green emitter Iridium tris (2-(4-totyl)pyridinato N,C2) using electron spectroscopies. The materials were evaporated in-situ onto a clean gold substrate in ulta-high vacuum. The valence band electronic structures were investigated using resonant photoemission, with the unoccupied states determined using X-ray absorption. The data suggests a hybridisation of the Ir 5d states with the π orbitals of the ligand. NEXAFS spectra are shown to be highly sensitive to the local environment.

The understanding of fundamental growth processes is critical for the fabrication of organic semiconductor thin films to be used in optoelectronic devices. Atomic-force microscopy is a vital tool to quantitatively analyze the evolving crystalline nanostructures and their spontaneous ordering. Here, para-sexiphenyl – a semiconducting light emitting oligomer – has been grown epitaxially by different high vacuum and ultra high vacuum methods onto crystalline substrates like mica(001), TiO2(110), or Al(111). Depending on the actual growth parameters, characteristically shaped crystallites composed of lying molecules or terraced mounds of upright standing molecules are formed. With ongoing deposition, quasi 1D arrays of crystallites can evolve due to a rearrangement process of the earlier formed crystallites. For highly anisotropic surfaces, like mica(001) and TiO2(110), these para-sexiphenyl structures are correlated to the surface geometry of the substrate. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The adsorption of p-sexiphenyl on the oxygen passivated Ni(1 1 0)(2 × 1)-O reconstructed surface has been investigated by STM. The sub-molecular resolution obtained allowed the individual phenyl rings to be observed along with their orientation and registry on the surface. The molecules adsorb predominantly in a planar geometry within the surface reconstruction in a manner which maximises the number of four-fold hollow sites for the individual rings on the underlying Ni(1 1 0). Local compression of the Ni–O rows leads to the formation of adsorption channels which result in the preferential arrangement of the molecules in quasi-periodic 1-dimensional molecular strings.

Submonolayer to multilayer coverages of bithiophene on the clean Cu(1 1 0) and the Cu(1 1 0)–p(2 × 1)O reconstructed surface have been investigated comparatively with thermal desorption spectroscopy, angle resolved ultraviolet photoemission spectroscopy (ARUPS) and near edge X-ray absorption fine structure spectroscopy. On the clean Cu(1 1 0) substrate the bithiophene monolayer is strongly bound via the π-orbitals and flat lying. After the completion of the wetting layer, further growth occurs in islands with no evidence for a preferred molecular orientation or order. In contrast, on the oxygen reconstructed surface the wetting layer formed has a weaker interaction with the substrate and in this monolayer the molecular plane is tilted with respect to the surface. For the multilayer film on the Cu(1 1 0)–p(2 × 1)O reconstructed surface the ARUPS displays a distinct azimuthal anisotropy. This is suggested to arise from a template effect of the reconstruction.

Bithiophene, used as a model for the technologically important longer oligothiophenes, was adsorbed on the highly corrugated (4×1)S reconstructed Ni(1 1 0) surface. These studies have been performed to test the ability of controlling the electronic and geometric structure of the organic/inorganic interface by substrate patterning. Low temperature STM clearly confirms the ability of the above template to align the molecules along the raised ridges of the substrate. At high coverages strings of molecules are evident. The periodicity within the strings is generally forced by the substrate and the molecules are closely packed. Occasionally less dense one-dimensional rows are observed with a periodicity resulting from molecular self assembly. At low coverages the molecules are well resolved under bias conditions where tunnelling via the frontier orbitals (highest occupied molecular orbital or lowest unoccupied molecular orbital) should dominate. However, the molecular STM images are not a simple reproduction of the frontier orbitals.

The interfaces of In2O3, a model for indium–tin–oxide (ITO), with benzene, thiophene, and benzaldehyde, models for technologically important organic molecules, are studied using angle resolved ultraviolet photoemission and work function measurements. Band alignment diagrams for hypothetical Al/organic/ITO devices have been drawn, using values determined from this work and previously published studies of these molecules on Al(111). The similarity between the bonding of benzene and thiophene on Al(111) and In2O3, i.e., largely electrostatic, leads to near identical alignment at both metal and oxide interfaces. This indicates that clean Al and ITO will make a very poor electron/hole injecting pair. We suggest that the apparent efficiency of Al as an electron injecting contact in real devices is due to the presence of oxygen at the Al/organic interface. For benzaldehyde the interaction with In2O3 is largely electrostatic, in contrast to the covalent bonds formed on Al(111). This leads to very different alignment at the Al and oxide interfaces, showing the importance of the particular organic–inorganic interaction in determining band alignment.