We have performed a detailed x-ray diffraction study of the structure of the 7 X 7 and 5 X 5 reconstructions that are observed after deposition of submonolayers of Sn on the Ge(111}surface and subsequent annealing. The structure factors for both reconstructions show considerable similarity with those of Si{111)7X7.Our analysis shows that the dimer-adatom-stacking-fault model is applicable to the Sn-induced Ge(111)7X7 and Ge(111)SXS structures. The adatoms are identified to be Sn. The structural refinement shows that the atoms in the upper five atomic layers are displaced from their ideal positions. The displacements around the adatoms are similar to the displacements of the Ge(111)&3X &3-Sn surface. Furthermore, the observed relaxations are in good agreement with the predictions of total-energy calculations. In order to perform a detailed comparison between the experimentally and theoretically determined atomic positions, a series of elastic strain calculations using a Keating model have been carried out. It is demonstrated that this model gives a good description of the atomic relaxations of the surfaces.

The electronic structure of vacuum-sublimed layered organic heterostructures of pentacene (PEN) and fullerene (C60) on conducting polymer substrates was investigated using ultraviolet photoelectron spectroscopy (UPS). The conditions at the PEN/C60interface changed from thermodynamic non-equilibrium (i.e. the onset of the PEN highest occupied molecular orbital above the substrate Fermi-energy) for thin PEN coverages on C60 to thermodynamic equilibrium for thicker PEN coverages (i.e. Fermi-level pinning of PEN). This finding is attributed to a coverage-dependent pinhole connection of PEN through the C60 layer with the substrate. The experiments demonstrate the importance of organic thin film morphology for UPS measurements to assess the energy level alignment at organic/organic heterointerfaces.

Metal work-function modification with the help of organic acceptors is an efficient tool to significantly enhance the performance of modern state-of-the-art organic molecular electronic devices. Here, the prototypical organic acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane, F4TCNQ, is characterized on Ag(111), Au(111), and Cu(111) metal surfaces by means of density-functional theory calculations. Particular attention is paid to charge-transfer processes at the metal-organic interface; a subtle balance between charge forward and backward donations in combination with a strong adsorption-induced geometry change are found to be responsible for the observed increase in the system work function. A larger effect is obtained for the metals with larger initial work function. Interestingly, this results in similar charge-injection barriers from the substrate metal into an organic semiconductor deposited on top of the F4TCNQ layer. The impact of the F4TCNQ packing density of the electronic properties of the interface is also addressed. Comparing the calculated energy-level alignments and work-function modifications to experimental data from ultraviolet photoelectron spectroscopy yields good agreement between experiments and simulations.

The impact of intramolecular polar bonds (IPBs) on the energy level alignment in layered systems of rodlike conjugated molecules standing on the substrate was investigated for pentacene (PEN) and perfluoropentacene (PFP) on SiO2 using ultraviolet photoelectron spectroscopy. A remarkably large energy offset of 1.75 eV was found between the highest occupied molecular orbital (HOMO) levels of PEN and PFP caused by IPBs at the surface of standing PFP layers. This large HOMO-level offset results in a narrow intermolecular energy gap of approximately 0.4 eV at the interface between PEN and PFP layers. However, the absence of significant spatial overlap of PEN and PFP electron wave functions across the layers suppresses interlayer optical transitions.

Photoemission measurements reveal energy level shifts toward the Fermi level when a strong electron acceptor (tetrafluoro-tetracyanoquinodimethane, F4-TCNQ) is deposited on pristine layers of 4,4′,4″-tris(N,N-diphenyl-amino)triphenylamine (TDATA) or 4,4′-bis(N-carbazolyl)biphenyl (CBP). The shifts of the TDATA and CBP energy levels toward the Fermi level of the Au substrate could, in principle, arise from p-type doping of the intrinsic organic layers. While this indeed takes place in TDATA, doping of CBP by F4-TCNQ, i.e., charge transfer complex formation, does not occur. The shifts observed in CBP arise from the diffusion of F4-TCNQ toward the Au substrate, which modifies the buried metal surface potential, leading to a realignment of the energy levels of the organic overlayer.

In this study the performance differences of layered and bulk-heterojunction based organic solar cells composed of the prototypical p- and n-type organic semiconductors pentacene (PEN) and fullerene (C60) are correlated with the physical properties of the heterostructures. The electronic structure of layered and codeposited thin PEN and C60 films on the conducting polymer substrate poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) was investigated with ultraviolet photoelectron spectroscopy. Layered structures of C60 on PEN precovered PEDOT:PSS exhibited an offset of the highest occupied molecular orbital (HOMO) levels of 1.45 eV. In contrast, codeposited films of PEN and C60 showed a reduced HOMO-level offset of 0.85 eV, which increased to 1.45 eV by precoverage of the substrate with a thin PEN layer. In this case, the PEN-HOMO level was Fermi-level pinned at 0.35 eV binding energy and charge transfer between PEN and PEDOT:PSS decreased the vacuum level by 0.75 eV. In addition, the morphology and crystal structure of the respective systems have been investigated by atomic force microscopy (AFM), x-ray diffraction (XRD) and Fourier-transform infrared spectroscopy, which indicated pronounced phase separation of PEN and C60 in the codeposited films. XRD revealed crystalline growth of PEN in all investigated cases forming crystallites that exceeded the nominal film thickness by an order of magnitude, whereas C60 was crystalline only if grown on the PEN precovered substrates. AFM investigations allowed to correlate morphology and structure revealing micro- and nanophase separation between PEN and C60.

Although an isolated individual molecule clearly has only one ionization potential, multiple values are found for molecules in ordered assemblies. Photoelectron spectroscopy of archetypical pi-conjugated organic compounds on metal substrates combined with first-principles calculations and electrostatic modelling reveal the existence of a surface dipole built into molecular layers. Conceptually different from the surface dipole at metal surfaces, its origin lies in details of the molecular electronic structure and its magnitude depends on the orientation of molecules relative to the surface of an ordered assembly. Suitable pre-patterning of substrates to induce specific molecular orientations in subsequently grown films thus permits adjusting the ionization potential of one molecular species over up to 0.6 eV via control over monolayer morphology. In addition to providing in-depth understanding of this phenomenon, our study offers design guidelines for improved organic-organic heterojunctions, hole- or electron-blocking layers and reduced barriers for charge-carrier injection in organic electronic devices.

The changes induced by exposing multi-walled carbon nanotubes (CNTs) to atomic nitrogen were analysed by high-resolution transmission electron microscopy (HRTEM), x-ray and ultraviolet photoelectron spectroscopy. It was found that the atomic nitrogen generated by a microwave plasma effectively grafts chemical groups onto the CNT surface altering the density of valence electronic states. HRTEM showed that the exposure to atomic nitrogen does not significantly damage the CNT surface.

The effect of the oxygen plasma treatment on the electronic states of multi-wall carbon nanotubes (MWCNTs) is analyzed by X-ray photoemission measurements (XPS) and UPS, both using synchrotron radiation. It is found that the plasma treatment effectively grafts oxygen at the CNT-surface. Thereafter, the interaction between evaporated Pd and pristine or oxygen plasma-treated MWCNTs is investigated. Pd is found to nucleate at defective sites, whether initially present or introduced by oxygen plasma treatment. The plasma treatment induced a uniform dispersion of Pd clusters at the CNT-surface. The absence of additional features in the Pd 3d and C 1s core levels spectra testifies that no Pd–C bond is formed. The shift of the Pd 3d core level towards high-binding energy for the smallest clusters is attributed to the Coulomb energy of the charged final state.

For the prototypical conjugated organic molecules pentacene and perfluoropentacene, we demonstrate that the surface termination of ordered organic thin films with intramolecular polar bonds (e.g., -H versus -F) can be used to tune the ionization energy. The collective electrostatics of these oriented bonds also explains the pronounced orientation dependence of the ionization energy. Furthermore, mixing of differently terminated molecules on a molecular length scale allows continuously tuning the ionization energy of thin organic films between the limiting values of the two pure materials. Our study shows that surface engineering of organic semiconductors via adjusting the polarity of intramolecular bonds represents a generally viable alternative to the surface modification of substrates to control the energetics at organic/(in)organic interfaces.

Interface energetics are of fundamental importance in organic and molecular electronics. By combining complementary experimental techniques and first-principles calculations, we resolve the complex interplay among several interfacial phenomena that collectively determine the electronic structure of the strong electron acceptor tetrafluoro-tetracyanoquinodimethane chemisorbed on copper. The combination of adsorption-induced geometric distortion of the molecules, metal-to-molecule charge transfer, and molecule-to-metal back transfer leads to a net increase of the metal work function.

Multi-wall carbon nanotubes (CNTs) were modified by an oxygen radio frequency plasma treatment, the appropriate duration of which was determined from post-treatment high-resolution transmission electron microscopy (HRTEM) images. A comparison of HRTEM results from gold-decorated pristine and plasma treated CNTs showed that the plasma treatment improves the uniformity of the distribution of surface defects. X-ray photoelectron spectroscopy confirms that oxygen atoms attach to the surface of CNTs. Changes in the C 1s photoemission signal point towards a loss of delocalization of valence electrons, confirmed by the modifications of valence band spectra, and consistent with currently accepted theory.

Using ultraviolet photoemission spectroscopy, we investigated the energy level alignment at the interfaces of typical anodes used in organic electronics, indium tin oxide (ITO) and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), with the oligomeric hole transport material N,N,N′,N′-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD), and studied the influence of electrical interface doping by the strong electron acceptor tetrafluoro tetracyanoquinodimethane (F4-TCNQ). The fundamentally different anode materials with work functions of 4.40eV (ITO) and 4.85eV (PEDOT:PSS) show different hole injection barriers, which also depend on the thickness of the F4-TCNQ interface dopant layer. PEDOT:PSS anodes exhibit a consistently lower hole injection barrier to MeO-TPD compared to ITO by 0.1eV. We attribute this low hole injection barrier to additional charge transfer reactions at the PEDOT:PSS/MeO-TPD interface. In contrast, the deposition of the electron acceptor at the interface helps significantly to lower the hole injection barrier for ITO anodes.

Evidence for charge transfer (CT) between the electron acceptor molecule octafluoroanthraquinone (FAQ) and the metal surfaces Ag(111) and polycrystalline Au is provided by ultraviolet photoelectron spectroscopy. The energy level alignment of subsequently deposited sexithienyl (6T) on FAQ-precovered metal substrates was investigated. Due to the metal work function change induced by the FAQ-metal CT, the hole injection barrier of 6T on FAQ-precovered metals could be reduced by up to 0.60 eV compared to that of 6T on pristine metal surfaces.

Photoemission studies on interfaces between an electroluminescent green polyfluorene (GPF) and two conducting polymers comprising ethylenedioxythiophene and sulfonate moieties are reported. Despite the chemical similarity of both conducting polymers, the hole injection barriers to GPF were found to differ by 0.35eV and the interface dipoles by up to −0.4eV. Polymer light emitting devices fabricated with the two conducting polymers exhibit nearly identical current-voltage characteristics, but the luminance-voltage characteristic is enhanced by a factor of 20 by the presence of near-surface poly(styrenesulfonate). We attribute the enhancement to the electron- blocking behavior of the sulfonate moieties at the GPF interface and the concomitant modification of the device built-in voltage.

Aluminum is widely used as cathode material in organic electroactive devices. In this work the interface formation between the electroluminescent material p-sexiphenyl(6P) and this metal was investigated with photoelectron spectroscopy. In strong contrast to the findings for the interaction of polyenes and oligothiophenes with Al, where covalent bonds are formed, we obtain clear evidence for physisorption of Al on 6P. This is supported by quantum chemical calculations, carried out on model systems.

The application of ele ctroactive organic materials in devices (light emitting diodes, photovoltaic cells) often requires electrodes with a low workfunction. Commonly, aluminum or alkaline earth metals are used, which usually exhibit strong interaction with the organic material, such as the formation of new covalent bonds or doping. This results in a strong modification of the electronic structure of the organic/metal interface, and in most cases does not yield the energy level alignment expected for the unreacted interface. As a n alternative to the above-mentioned metals we propose the use of samarium, with a workfunction of 2.7 eV, for the following reason: we have studied the interface formation between Sm and p -sexiphenyl (6P, which exhibits intense blue electroluminescence), with ultraviolet photoelectron spectroscopy. Sm was deposited stepwise onto thin films of 6P in ultrahigh vacuum, and the photoelectron spectra were recorded after each step. We did not find any indication for a strong interaction between the two materials. Metallic Sm is formed instantaneously and the valence electronic structure of 6P remains unchanged upon the metal deposition. The weak interaction at this interface allows one to determine the energy level alignment between a metal and an organic material in a direct manner from the photoelectron spectra, without the need for making any assumptions on the workfunction or ionization potential.

Abstract The atomic structure of the Ni(110)4 × 1-S reconstruction has been determined on the basis of surface X-ray diffraction measurements. An analysis of the in-plane diffraction data shows that the model consists of Ni rows along the [001] direction, two for every 4 × 1 unit cell, corresponding to 0.5 ML Ni coverage. The S is chemisorbed in pseudo two-fold hollow sites both on the Ni rows and in the troughs between the rows with a S coverage of 0.75 ML. Furthermore, rod-scans along fractional-order reflections reveal sub-surface relaxations. The results are in good agreement with recent STM and previous AES and radioactive tracer studies.