Search

Using the normal incidence x-ray standing wave technique as well as low energy electron microscopy we have investigated the structure of quasi-freestanding monolayer graphene (QFMLG) obtained by intercalation of antimony under the $\left(6\sqrt{3}\times6\sqrt{3}\right)R30^\circ$ reconstructed graphitized 6H-SiC(0001) surface, also known as zeroth-layer graphene. We found that Sb intercalation decouples the QFMLG well from the substrate. The distance from the QFMLG to the Sb layer almost equals the expected van der Waals bonding distance of C and Sb. The Sb intercalation layer itself is mono-atomic, flat, and located much closer to the substrate, at almost the distance of a covalent Sb-Si bond length. All data is consistent with Sb located on top of the uppermost Si atoms of the SiC bulk.

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.]

In the field of van der Waals heterostructures, the twist angle between stacked two-dimensional (2D) layers has been identified to be of utmost importance for the properties of the heterostructures. In this context, we previously reported the growth of a single layer of unconventionally oriented epitaxial graphene that forms in a surfactant atmosphere [F. C. Bocquet, et al., Phys. Rev. Lett. 125, 106102 (2020)]. The resulting G-R0$^\circ$ layer is aligned with the SiC lattice, and hence represents an important milestone towards high quality twisted bilayer graphene (tBLG), a frequently investigated model system in this field. Here, we focus on the surface structures obtained in the same surfactant atmosphere, but at lower preparation temperatures at which a boron nitride template layer forms on SiC(0001). In a comprehensive study based on complementary experimental and theoretical techniques, we find -- in contrast to the literature -- that this template layer is a hexagonal B$_x$N$_y$ layer, but not high-quality hBN. It is aligned with the SiC lattice and gradually replaced by low-quality graphene in the 0$^\circ$ orientation of the B$_x$N$_y$ template layer upon annealing.

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

Frontier orbitals, i.e., the highest occupied and lowest unoccupied orbitals of a molecule, generally determine molecular properties, such as chemical bonding and reactivities. Consequently, there has been a lot of interest in measuring them, despite the fact that, strictly speaking, they are not quantum-mechanical observables. Yet, with photoemission tomography a powerful technique has recently been introduced by which the electron distribution in orbitals of molecules adsorbed at surfaces can be imaged in momentum space. This has even been used for the identification of reaction intermediates in surface reactions. However, so far it has been impossible to follow an orbital's momentum-space dynamics in time, for example through an excitation process or a chemical reaction. Here, we report a key step in this direction: we combine time-resolved photoemission employing high laser harmonics and a recently developed momentum microscope to establish a tomographic, femtosecond pump-probe experiment of unoccupied molecular orbitals. Specifically, we measure the full momentum-space distribution of transiently excited electrons. Because in molecules this momentum-space distribution is closely linked to orbital shapes, our experiment offers the extraordinary possibility to observe ultrafast electron motion in time and space. This enables us to connect their excited states dynamics to specific real-space excitation pathways.

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

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.

Molecular monolayer films containing two different types of molecules (so-called heteromolecular films) are promising candidates for the controlled functionalization of metal–organic hybrid interfaces. This is particularly true for blends formed by charge donor and acceptor molecules. Here we study heteromolecular monolayer systems containing 3,4,9,10-perylene-tetra-carboxylic-dianhydride (PTCDA) as charge acceptor, and either copper(II) or tin(II) phthalocyanine (CuPc or SnPc) as charge donor, adsorbed on Ag(111). We find that both systems exhibit structural phases with identical lateral ordering (isostructural phases), which is an important prerequisite for studying the role of the central metal atom without competing effects caused by different lateral structures. Using normal incidence X-ray standing waves and photoemission tomography we find distinct differences in the (vertical) geometric and electronic structure for the heteromolecular systems under study: While the vertical structure of CuPc is es...

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.

The hexagonal boron nitride (hBN) monolayer on the Cu(111) surface has recently been considered an example of an extremely weak hBN/metal interaction, as indicated, e.g., from the presence of an only electronic Moire-like superstructure that was observed in scanning tunneling microscopy images. From these results, a large bonding distance of the hBN sheet to the topmost Cu layer can be envisaged but has not been proven so far. We report a structural analysis of the hBN/Cu(111) interface based on high resolution low energy electron diffraction and normal incidence X-ray standing wave experiments. We find that both the boron and nitrogen atoms are located at very large vertical distances of dB = 3.25 ± 0.02 A and dN = 3.22 ± 0.03 A with respect to the nominal position of the topmost Cu(111) layer. Significant vertical buckling and lateral distortions of the hBN layer can be excluded. These results demonstrate that the hBN monolayer on the Cu(111) surface is indeed well described by a rigid and geometrically...

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.

The adsorption geometry, the electronic properties, and the adsorption energy of the prototype organic molecule 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on a monolayer of hexagonal boron nitride (hBN) grown on the Cu(111) surface were determined experimentally. The perylene core is at a large height of 3.37 $\AA{}$ and only a minute downward displacement of the functional anhydride groups (0.07 \AA{}) occurs, yielding adsorption heights that agree with the sum of the involved van der Waals radii. Thus, already a single hBN layer leads to a decoupled (physisorbed) molecule, contrary to the situation on the bare Cu(111) surface.

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.

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.

The adsorption geometry, namely the height and the site, of 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) on the Cu(100) surface was determined by the normal incidence x-ray standing wave (NIXSW) technique including triangulation. The two PTCDA molecules in the superstructure unit cell, which have perpendicular azimuthal orientation, are both located at bridge sites, the long molecular axis being parallel to the bridge. Carboxylic oxygen atoms and several atoms of the carbon backbone are located close to on-top positions. The vertical distortion motif of PTCDA on Cu(100) differs from that on the three low-index Ag surfaces, because significant downward displacement of the carboxylic oxygen atoms is lacking. In particular, the carbon backbone of PTCDA adsorbs closer to the surface than extrapolated from Ag data. This suggests a relative increase of the attractive interactions between the carbon backbone of PTCDA and the Cu(100) surface versus the attractive interactions on the carboxylic oxygen atoms.

We investigate the molecular orientation of tetracene in ordered and disordered layers on Ag(111) using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Quantitative analysis of NEXAFS intensities reveals that the compact monolayer α-phase at 150 K consists of essentially flat-lying molecules with an average tilt angle α < 9° with respect to the Ag(111) surface. At room temperature the tetracene monolayer is dynamically disordered with α = 15 ± 4° (α′-phase). The increase in the average tetracene tilt angle indicates that orientational degrees of freedom perpendicular to the surface drive the order–disorder transition in the tetracene monolayer on Ag(111). We argue that the phase transition can be explained by a delicate balance between the interfacial enthalpy and the orientational entropy. Furthermore, we suggest that tetracene/Ag(111) may be a good example of Onsager’s system of nonattracting rigid anisotropic objects (so-called “hard rods”) in two dimensions. On the other hand, the com...

On the Ag(110) surface copper phthalocyanine (CuPc) orders in two structurally similar superstructures, as revealed by low-energy electron diffraction. Scanning tunneling microscopy (STM) shows that in both superstructures the molecular planes are oriented parallel to the surface and the long molecular axes, defined as diagonals of the square molecule, are rotated by $\ensuremath{\simeq}\ifmmode\pm\else\textpm\fi{}{32}^{\ensuremath{\circ}}$ away from the high-symmetry directions $[1\overline{1}0]$ and [001] of the silver surface. Similarly to many other adsorbed metal phthalocyanines, the CuPc molecules on Ag(110) appear in STM as crosslike features with twofold symmetry. Photoemission tomography based on angle-resolved photoemission spectroscopy reveals a charge transfer from the substrate into the molecule. A symmetry analysis of experimental and theoretical constant binding energy maps of the photoemission intensity in the ${k}_{x},{k}_{y}$-plane points to a preferential occupation of one of the two initially degenerate lowest unoccupied molecular orbitals (LUMOs) of ${e}_{g}$ symmetry. The occupied ${e}_{g}$ orbital is rotated by ${32}^{\ensuremath{\circ}}$ against the [001] direction of the substrate. The lifting of the degeneracy of the LUMOs and the related reduction of the symmetry of the adsorbed CuPc molecule are attributed to an anisotropy in the chemical reactivity of the Ag(110) surface.

An aromatic triphenylene molecule (2,3-diethyl-1,4-dimethoxytriphenylene; TPH) and its regioselectively Cr(CO)3-labeled complex (η6-(1,2,3,4,4a,12b)-tricarbonyl(2,3-diethyl-1,4-dimethoxytriphenylene)chromium(0); α-TPHC) were deposited on a Ag(111) surface by vapor deposition. The monolayers were investigated by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Both substances adsorb with the extended π-system parallel to the surface and form long-range ordered monolayers. The Cr(CO)3 group of the α-TPHC complex is oriented toward the vacuum. Although the footprints of both substances are similar, the additional Cr(CO)3 group on the α-TPHC leads to a lateral order (unit cell) that is significantly different from that of TPH.

Using a scanning tunneling microscope we have measured the quantum conductance through a PTCDA molecule for different configurations of the tip-molecule-surface junction. A peculiar conductance resonance arises at the Fermi level for certain tip to surface distances. We have relaxed the molecular junction coordinates and calculated transport by means of the Landauer/Keldysh approach. The zero bias transmission calculated for fixed tip positions in lateral dimensions but different tip substrate distances show a clear shift and sharpening of the molecular chemisorption level on increasing the STM-surface distance, in agreement with experiment., accepted for publication in Applied Physics A

The functionality of organic field effect transistors resides in their interfaces. Highly ordered molecular adsorbate layers provide an excellent tool to study the relevant physical properties of these interfaces. In this article we focus on molecules at the organic/metal interface. We find that their geometry and electronic structure are very responsive to influences from the substrate and the molecular environment, while interface structures evolve cooperatively in the sway of interfacial and intermolecular interactions and can thus be extremely complex. Although our results cannot be transferred directly to real devices, a number of intriguing physical effects emerge that could play an important role in (nanoscale) OFETs, e.g. an anomalously strong energy band dispersion at the organic/metal interface, a remarkable plasticity of the electronic structure of molecules in ultra-thin films, and Kondo physics in a non-magnetic molecular wire. Finally, we report the realization of a mechanically gated single-molecule transistor that allows investigating the transport physics in a molecular wire as a function of contact properties. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We investigate the surface-catalyzed dissociation of the archetypal molecular switch azobenzene on the Cu(111) surface. Based on X-ray photoelectron spectroscopy, normal incidence X-ray standing waves and density functional theory calculations a detailed picture of the coverage-induced formation of phenyl nitrene from azobenzene is presented. Furthermore, a comparison to the azobenzene/Ag(111) interface provides insight into the driving force behind the dissociation on Cu(111). The quantitative decay of azobenzene paves the way for the creation of a defect free, covalently bonded monolayer. Our work suggests a route of surface functionalization via suitable azobenzene derivatives and the on surface synthesis concept, allowing for the creation of complex immobilized molecular systems.

X-ray measurements of its vertical absorption height indicate that hydrogen-intercalated graphene on silicon carbide is effectively free-standing.

Interfaces between organic molecules and solid surfaces play a prominent role in heterogeneous catalysis, molecular sensors and switches light-emitting diodes, and photovoltaics. The properties and the ensuing function of such hybrid interfaces often depend exponentially on molecular adsorption heights and binding strengths, calling for well-established benchmarks of these two quantities. Here we present systematic measurements that enable us to quantify the interaction of benzene with the Ag(111) coinage metal substrate with unprecedented accuracy (0.02 angstrom in the vertical adsorption height and 0.05 eV in the binding strength) by means of normal-incidence x-ray standing waves and temperature-programed desorption techniques. Based on these accurate experimental benchmarks for a prototypical molecule-solid interface, we demonstrate that recently developed first-principles calculations that explicitly account for the nonlocality of electronic exchange and correlation effects are able to determine the structure and stability of benzene on the Ag(111) surface within experimental error bars. Remarkably, such precise experiments and calculations demonstrate that despite different electronic properties of copper, silver, and gold, the binding strength of benzene is equal on the (111) surface of these three coinage metals. Our results suggest the existence of universal binding energy trends for aromatic molecules on surfaces.

In investigations of the proteins which are responsible for the surface adhesion of the blue mussel Mytilus edulis, an unusually frequent appearance of the otherwise rare amino acid 3-(3,4-dihydroxyphenyl)-L-alanine (L-DOPA) has been observed. This amino acid is thought to play a major role in the mechanism of mussel adhesion. Here we report a detailed structural and spectroscopic investigation of the interface between L-DOPA and a single-crystalline Au(110) model surface, with the aim of understanding fundamentals about the surface bonding of this amino acid and its role in mussel adhesion. Molecular layers are deposited by organic molecular beam deposition (OMBD) in an ultrahigh-vacuum environment. The following experimental techniques have been applied: ex situ Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), and scanning tunneling microscopy (STM). Vibrational spectra of isolated L-DOPA molecules and the zwitterionic bulk have been calculated using density functional theory (DFT). The predicted modes are assigned to observed spectra, allowing conclusions regarding the molecule-substrate and molecule-molecule interactions at the L-DOPA/Au(110) interface. We find that zwitterionic L-DOPA forms a monochiral, one-domain commensurate monolayer on Au(110), with the catechol rings on top of [110] gold rows, oriented parallel to the surface. The (2 x 1)-Au(110) surface reconstruction is not lifted. The carboxylate group is found in a bidentate or bridging configuration, the amino group is tilted out of the surface plane, and the hydroxyl groups do not dehydrogenate on Au(110). Similar to the case for the bulk, molecules form dimers on Au(110). However, the number of hydrogen bridge bonds between L-DOPA molecules is reduced as compared to the bulk. Thicker layers which are deposited onto the commensurate interface do not order in the bulk structure. In conclusion, our study shows that the aromatic ring system of L-DOPA functions as a surface anchor. Since it is also known that the hydroxyl groups support cross-link reactions between L-DOPA residues in the mussel glue protein, we can conclude that the catechol ring supports surface adhesion of mussel proteins via two independent functions.

We report a systematic study of the interplay between molecular orientation, film morphology and luminescence properties of tetracene thin films on epitaxial alumina films on Ni 3 Al(1 1 1), employing high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and photoluminescence spectroscopy (PL). If deposited at low temperatures, tetracene forms laterally disordered and compact films in which at least the first monolayer is oriented parallel to the substrate. For thicknesses in the range of 10 A or below, these as-deposited films show no luminescence, while thicker films exhibit weak luminescence from higher layers. On annealing to 210 K, tetracene films dewet the AlO x /Ni 3 Al(1 1 1) surface and transform into an island morphology. At the same time, molecules tend to re-orient into a more upright configuration. In this island configuration, even thin films show luminescence. We can thus conclude that in spite of the insulating nature of the surface, the interaction of flat-lying tetracene molecules with AlO x /Ni 3 Al(1 1 1) is strong enough to provide at least one efficient non-radiative decay channel.

The atomic structure of the 4H-SiC(11 2 0) surface including possible phase transformations via Si deposition and annealing has been investigated using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The sample is initially prepared by hydrogen etching before loading into the ultra-high vacuum system. The sample is then out-gassed to remove oxygen from the surface. To explore the existence of ordered surface phases, Si is deposited on the sample at 850°C for 15 minutes followed by a series of sequential annealing steps. Throughout this process, the surface is monitored by LEED, AES and XPS. LEED shows that the surface continuously maintains a (1×1) periodicity. Yet, two unique and distinguishable (1×1) phases can be identified. The changes between these phases are clearly demonstrated by the LEED spot intensities. Simultaneously, the Auger and XPS data show a decrease in Si intensity.

Low-energy electron diffraction (LEED), scanning tunneling microscopy (STM) and spectroscopy (STS), and Auger electron spectroscopy (AES) were used for a study of silver interaction with the (3×3) and ° × 30 ) 3 3 ( R surface phases of clean 4H-SiC(0001). The development of the surface structure and morphology after room temperature (RT) deposition and annealing was investigated. On the (3×3) phase silver forms small clusters leaving the initial reconstruction intact. At high coverages three-dimensional (3D) growth (Vollmer-Weber mode) was found. For the ° × 30 ) 3 3 ( R phase the initial structure seems more disturbed upon Ag deposition and thermally induced diffusion. Yet, no new surface phase develops. In both cases Ag can be removed from the surface by annealing, but Ag appears to be more stable on the ° × 30 ) 3 3 ( R phase according to AES.

Commercial on-axis wafers of 4H-SiC(0001) were etched in a standard reactor for chemical vapor deposition (CVD) using molecular hydrogen flux in order to improve the structure and morphology of the surface. The substrate temperature during etching was varied from 1400 to 1600°C. Characterization of the surface morphology was performed using optical and atomic force microscopy (AFM). Low-energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS) were also used to examine the surface structure and chemical composition of the samples. The sample of best quality was obtained for an etching temperature of 1400°C. Its surface is ° × 30 ) 3 3 ( R reconstructed and covered by an ordered “silicate” layer. Increasing the substrate temperature during etching to 1500°C leads to enhanced step-bunching and the formation of macroterraces. At 1600°C distinct depressions appear on the surface, presumably from etching of structural defects such as screw dislocations. Subsequent annealing at 1000°C in ultra-high vacuum (UHV) removes the surface oxide and produces the ° × 30 ) 3 3 ( R surface phase of clean 4HSiC( 0001).

We report on a technique to characterize the deep defects in low temperature grown GaAs (LT-GaAs), based on a thin LT-GaAs layer embedded in the intrinsic zone of a pin diode. At this structure we have performed steady state and time dependent measurements of the n-channel conductance. Thus we yield information about the density, the activation energy, and the characteristic time constants of the deep defects.

Although geometric and electronic properties of any physical or chemical system are always mutually coupled by the rules of quantum mechanics, counterintuitive coincidences between the two are sometimes observed. The coadsorption of the organic molecules 3,4,9,10-perylene tetracarboxylic dianhydride and copper-II-phthalocyanine on Ag(111) represents such a case, since geometric and electronic structures appear to be decoupled: one molecule moves away from the substrate while its electronic structure indicates a stronger chemical interaction, and vice versa for the other. Our comprehensive experimental and ab-initio theoretical study reveals that, mediated by the metal surface, both species mutually amplify their charge-donating and -accepting characters, respectively. This resolves the apparent paradox, and demonstrates with exceptional clarity how geometric and electronic bonding parameters are intertwined at metal-organic interfaces.

The formation of metalorganic hybrid interfaces is determined by the fine balance between molecule-substrate and molecule-molecule interactions at the interface. Here, we report on a systematic investigation of interfaces between a metal surface and organic monolayer films that consist of two different molecular species, one donor and one acceptor of electronic charge. Our x-ray standing wave data show that in heteromolecular structures, the molecules tend to align themselves to an adsorption height between those observed in the respective homomolecular structures. We attribute this alignment effect to a substrate-mediated charge transfer between the molecules, which causes a mutual enhancement of their respective donor and acceptor characters. We argue that this effect is of general validity for $\ensuremath{\pi}$-conjugated molecules adsorbing on noble metal surfaces.

We report on a new technique to characterize the deep defects in low temperature grown GaAs (LT-GaAs), based on a thin LT-GaAs layer embedded in the intrinsic zone of a pin diode. At this structure we have performed steady state and time dependent measurements of the n-channel conductance. Thus we deduce information about the activation energy and the capture time constant of the deep defects.

The basis for a quantum-mechanical description of matter is electron wave functions. For atoms and molecules, their spatial distributions and phases are known as orbitals. Although orbitals are very powerful concepts, experimentally only the electron densities and -energy levels are directly observable. Regardless whether orbitals are observed in real space with scanning probe experiments, or in reciprocal space by photoemission, the phase information of the orbital is lost. Here, we show that the experimental momentum maps of angle-resolved photoemission from molecular orbitals can be transformed to real-space orbitals via an iterative procedure which also retrieves the lost phase information. This is demonstrated with images obtained of a number of orbitals of the molecules pentacene (C22H14) and perylene-3,4,9,10-tetracarboxylic dianhydride (C24H8O6), adsorbed on silver, which are in excellent agreement with ab initio calculations. The procedure requires no a priori knowledge of the orbitals and is shown to be simple and robust.

The normal incidence x-ray standing wave (NIXSW) technique is used to determine the adsorption geometry of submonolayer 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) adsorbed on the Ag(110) surface. An accurate analysis of both C1$s$ and O1$s$ photoemission (PE) spectra allows the respective adsorption heights of carbon and oxygen atoms in different chemical environments within PTCDA to be distinguished. Due to the intricacy of the PE fitting models, a systematic error analysis of NIXSW structural parameters was developed and employed. Based on the adsorption geometry of PTCDA on Ag(110) a bonding mechanism is discussed.

The compressed 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA) herringbone monolayer structure on Ag(110) is used as a model system to investigate the role of molecule–molecule interactions at metal–organic interfaces. By means of the orbital tomography technique, we can not only distinguish the two inequivalent molecules in the unit cell but also resolve their different energy positions for the highest occupied and the lowest unoccupied molecular orbitals. Density functional theory calculations of a freestanding PTCDA layer identify the electrostatic interaction between neighboring molecules, rather than the adsorption site, as the main reason for the molecular level splitting observed experimentally.

The trends in the bonding mechanism of 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) to the Ag(111), Ag(100), and Ag(110) surfaces were analyzed on the basis of data obtained from x-ray standing waves and dispersion-corrected density functional theory. Of importance are the attractive local O-Ag bonds on the anhydride groups. They are the shorter, the more open the surface is, and lead even to partly repulsive interactions between the perylene core and the surface. In parallel, there is an increasing charge donation from the Ag surface into the \ensuremath{\pi} system of the PTCDA. This synergism explains the out-of-plane distortion of the adsorbed PTCDA and the surface buckling.

The structure of the tetracene/Ag(111) interface in the coverage range \ensuremath{\theta} = 0 to 2.4 ML is studied with scanning tunneling microscopy (STM) at 8 K and with low energy electron diffraction (LEED) at T = 300 \dots{} 100 K. For \ensuremath{\theta} $\ensuremath{\lesssim}$ 0.01 ML, one-dimensional (1D) diffusion of single molecules along $\ensuremath{\langle}01\overline{1}\ensuremath{\rangle}$-directions is observed even at 8 K. For 0.1 ML \ensuremath{\theta} 0.5 ML molecules are homogeneously distributed over the surface forming a disordered phase (static at T = 8 K, dynamic at T = 25 K), indicating a repulsive intermolecular interaction (\ensuremath{\delta}-phase). For \ensuremath{\theta} $\ensuremath{\gtrsim}$ 0.5 ML, local ordering in the commensurate \ensuremath{\gamma}-phase is observed. Further increase of the coverage yields a compressed monolayer (ML) phase (\ensuremath{\theta} \ensuremath{\equiv} 1 ML) with point-on-line registry (\ensuremath{\alpha}-phase). The interaction between molecules has been calculated with the force-field approach to rationalize the molecular packing motifs in the various phases. Under most circumstances molecule-molecule interactions are repulsive, in agreement with experimental findings. A simulation of the adsorption up to \ensuremath{\theta} = 1 ML according to the random sequential adsorption (RSA) algorithm shows that the disorder-to-order transition from the \ensuremath{\delta}- to \ensuremath{\gamma}-phase occurs close to random close packing (RCP), \ensuremath{\theta} = 0.5--0.6 ML. Since tetracene molecules are a two-dimensional (2D) representation of Onsager's hard rod model, this suggests that this phase transition is driven both energetically and entropically. For \ensuremath{\theta} \ensuremath{\approx} 2.23 ML a metastable bilayer phase with point-on-line coincidence is observed (\ensuremath{\beta}-phase). The basic structural unit of this phase is a triplet of molecules that are tilted along the long molecular axis against each other; at least one of these molecules is tilted out of the surface plane. Within the \ensuremath{\beta}-phase a superstructure of alternating rotation domains is observed. This superstructure has a period of 7.4 nm. The molecular packing in the \ensuremath{\beta}-phase resembles the packing in the bulk crystal structure of tetracene, its formation can therefore be interpreted as incipient pseudomorphic growth of tetracene on Ag(111). However, pseudomorphic growth cannot be continued beyond the \ensuremath{\beta}-phase.

We study the interface of an organic monolayer with a metallic surface, i.e., PTCDA (3,4,9,10-perylene-tetracarboxylic-dianhydride) on Ag(110), by means of angle-resolved photoemission spectroscopy (ARPES) and ab initio electronic structure calculations. We present a tomographic method that uses the energy and momentum dependence of ARPES data to deconvolute spectra into individual orbital contributions beyond the limits of energy resolution. This provides an orbital-by-orbital characterization of large adsorbate systems without the need to invoke a sophisticated theory of photoemission, allowing us to directly estimate the effects of bonding on individual orbitals. Moreover, these experimental data serve as a most stringent test necessary for the further development of ab initio electronic structure theory.