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We have experimentally determined the adsorption structure, charge state, and metalation state of porphin, the fundamental building block of porphyrins, on ultrathin Ag(001)-supported MgO(001) films by scanning tunneling microscopy and photoemission spectroscopy, supported by calculations based on density functional theory. By tuning the substrate work function to values below and above the critical work function for charging, we succeeded in the preparation of 2H-P monolayers which contain negatively charged and uncharged molecules. It is shown that the porphin molecules self-metalate at room temperature, forming the corresponding Mg-porphin, irrespective of their charge state. This is in contrast to self-metalation of tetraphenyl porphyrin (TPP), which occurs on planar MgO(001) only if the molecules are negatively charged. The different reactivity is explained by the reduced molecule-substrate distance of the planar porphin molecule compared to the bulkier TPP. The results of this study shed light on the mechanism of porphyrin self-metalation on oxides and highlight the role of the adsorption geometry on the chemical reactivity.

Semiconducting O-doped polycyclic aromatic hydrocarbons constitute a class of molecules whose optoelectronic properties can be tailored by acting on the π-extension of the carbon-based frameworks and on the oxygen linkages. Although much is known about their photophysical and electrochemical properties in solution, their self-assembly interfacial behavior on solid substrates has remained unexplored so far. In this paper, we have focused our attention on the on-surface self-assembly of O-doped bi-perylene derivatives. Their ability to assemble in ordered networks on Cu(111) single-crystalline surfaces allowed a combination of structural, morphological, and spectroscopic studies. In particular, the exploitation of the orbital mapping methodology based on angle-resolved photoemission spectroscopy, with the support of scanning tunneling microscopy and low-energy electron diffraction, allowed the identification of both the electronic structure of the adsorbates and their geometric arrangement. Our multi-technique experimental investigation includes the structure determination from powder X-ray diffraction data for a specific compound and demonstrates that the electronic structure of such large molecular self-assembled networks can be studied using the reconstruction methods of molecular orbitals from photoemission data even in the presence of segregated chiral domains.

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.

International audience; We report on the formation of a high-order commensurate (HOC) structure of 5,14-dihydro-5,7,12,14-tetraazapentacene (DHTAP) molecules on the highly corrugated Cu(110)–(2 × 1)O surface. Scanning tunnelling microscopy shows that the DHTAP molecules form a periodic uniaxial arrangement in which groups of seven molecules are distributed over exactly nine substrate lattice spacings along the [[1 with combining macron]10] direction. DFT-calculations reveal that this peculiar arrangement is associated with different tilting of the seven DHTAP molecules within the quasi one-dimensional HOC unit cell. The orientational degree of freedom thus adds a new parameter, which can efficiently stabilize complex molecular structures on corrugated surfaces.

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

Uncommon metal oxidation states in porphyrinoid cofactors are responsible for the activity of many enzymes. The F₄₃₀ and P450nor co-factors, with their reduced NiI- and FeIII-containing tetrapyrrolic cores, are prototypical examples of biological systems involved in methane formation and in the reduction of nitric oxide, respectively. Herein, using a comprehensive range of experimental and theoretical methods, we raise evidence that nickel tetraphenyl porphyrins deposited in vacuo on a copper surface are reactive towards nitric oxide disproportionation at room temperature. The interpretation of the measurements is far from being straightforward due to the high reactivity of the different nitrogen oxides species (eventually present in the residual gas background) and of the possible reaction intermediates. The picture is detailed in order to disentangle the challenging complexity of the system, where even a small fraction of contamination can change the scenario.

The sensitivity of photoemission tomography (PT) to directly probe single molecule on-surface intramolecular reactions will be shown here. PT application in the study of molecules possessing peripheral ligands and structural flexibility is tested on the temperature-induced dehydrogenation intramolecular reaction on Ag(100), leading from CoOEP to the final product CoTBP. Along with the ring-closure reaction, the electronic occupancy and energy level alignment of the frontier orbitals, as well as the oxidation state of the metal ion, are elucidated for both the CoOEP and CoTBP systems.

Due to its unique magnetic properties offered by the open‐shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d‐states of FePc and the sp‐band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature., By exploiting the FePc/Cu(100) interface, the possibility to control, via nitrogen dioxide (NO2) dissociation at the interface, the magnetic properties of the chelated ion the FePc array is demonstrated. Three different spin configurations have been stabilized within the same metalorganic/metal interface.

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

Transport phenomena in molecular materials are intrinsically linked to the orbital character and the degree of localization of the valence states. Here we combine angle-resolved photoemission with photoemission tomography to determine the spatial distribution of all molecular states of the valence band structure of a ${\mathrm{C}}_{60}$ thin film. While the two most frontier valence states exhibit a strong band dispersion, the states at larger binding energies are characterized by distinct emission patterns in energy and momentum space. Our findings demonstrate the formation of an atomic crystal-like band structure in a molecular solid with delocalized $\ensuremath{\pi}$-like valence states and strongly localized $\ensuremath{\sigma}$ states at larger binding energies.

In recent years, the method of orbital tomography has been a useful tool for the analysis of a variety of molecular systems. However, the underlying plane-wave final state has been largely expected to be applicable to planar molecules only. Here, we demonstrate on photoemission data from the molecule ${\mathrm{C}}_{60}$ adsorbed on Ag(110) that it can indeed be a valid approximation for truly three-dimensional molecules at a metal-organic interface. A comparison of the experimental data supported by density functional theory (DFT) calculations of the full interface and simulations of the photoemission process with a more exact final state enables the determination of the adsorption geometry and orientation of the ${\mathrm{C}}_{60}$ molecules in a monolayer on the Ag(110) surface. Additionally, charge transfer into the molecules is used to confirm the lifting in degeneracy of the ${t}_{\text{1u}}$ molecular orbitals as predicted by DFT calculations.

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.

Diffusion of impurities along grain boundaries of compound materials is of high relevance for multiple practical applications. In this work, we perform a case study of Cu-impurity diffusion along the special coincident site lattice model Σ 5 (012)[100] GB in stoichiometric and off-stoichiometric TiN. The study includes a thorough investigation of the GB structure, formation of point defects and their interaction with a Cu-impurity at the interface as well as the evaluation of energy barriers for a few plausible impurity migration pathways along TiN grain boundaries. The results show that one of the most probable diffusion mechanisms (with an activation energy of 0.77 eV) is Cu-interstitial migration along a grain boundary channel. For a non-stoichiometric case when nitrogen vacancies can be present at GB, the results show that they can act as trapping centers for Cu atoms and increase the migration energy barrier to 1.2 eV. However, our results also indicate that at equilibrium conditions the effect of trapping may be reduced due to slightly unfavored segregation of N vacancies at TiN grain boundaries.

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.

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.

Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. Here, we study nickel porphyrin molecules on a copper surface as an archetypal system containing a metal center whose oxidation state can be controlled through the interaction with the metal substrate. We demonstrate that the strong molecule-surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin to the substrate. Upon thermal treatment, the molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification. This journal is

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 influence of all transition element dopants on grain boundary (GB) cohesion in both W and Mo is investigated by means of high throughput density functional theory (DFT) calculations. We present site specific segregation energies to GBs and free surfaces (FS) of W and Mo and evaluate the strength of embrittlement of the individual solutes. The segregation properties for a consistent set of solutes allow an unprecedented comparison to previous DFT data and phenomenological models for segregation. Our DFT data show that the trends are very similar in W and Mo for the different solutes, while the comparison with available phenomenological models highlights the limits of band-filling and elastic models for GB segregation and strengthening.

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.

Two-dimensionally ordered monolayers of water on MgO(001) have been extensively studied in the past using diffraction and spectroscopic and computational methods, but direct microscopic imaging has...

Understanding the interactions of solutes in a host material is a key factor for successful alloy design. In this work, we focus on the effect of C, B, O, Fe, and Hf interactions in Mo on grain boundary (GB) segregation and cohesion within an ab initio framework. We show that segregation energies reveal only a weak coverage dependence for the interstitial solutes while for substitutionals a pronounced minimum occurs at 1 ML coverage. For Fe, segregation energies remain negative up to very high coverages of up to 8 ML, which correspond to thick segregation films whose geometric structure and magnetic properties are explored. As a next step we evaluate the binding energy for different solute combinations and show that interactions are mainly repulsive at the GB, while strong attractive interactions occur in the bulk. Origins for this phenomena are discussed and cosegregation energies are extracted for different scenarios. The coverage dependence on cohesion is nearly linear for all investigated solutes and increases for all solutes except for O and Hf.

We investigate grain boundaries (GBs) in W–25 at.% Re alloys with special focus on the segregation of Re to the GBs and the changes in their cohesive properties that arise therefrom. Our simulations rely on density functional theory and a mean-field approximation, the virtual crystal approximation, to model the highly alloyed system. Based on a gamma-surface approach, the geometry of a wide range of GBs, including tilt, twist and mixed GBs, is obtained. Segregation energies are found to vary strongly between different segregation sites, with the strongest segregation energy amounting to −0.75 eVs. We show that bond-order parameters are able to identify strong segregation sites based on purely geometric information only. With a thermodynamic model based on a Bragg–Williams approach, the concentration of Re atoms is calculated as a function of GB character at 1913 K and compared with atom probe experiments. The segregation levels are similar, with a trend towards higher segregation levels observed in the calculations for certain misorientation angles. Finally, Re alloying is found to lead to a general increase in the work of separation of the GBs, suggesting a reduction in the tendency for intergranular fracture.

We present a combined experimental and theoretical study of electronic and optical properties of dihydro-tetraaza-acenes (DHTAn). Using solvent-free condensation, we are able to synthesize not only DHTA5 but also the longer DHTA6 and DHTA7 molecules. We then investigate their gas-phase electronic structures by means of ab initio density functional calculations employing an optimally tuned range-separated hybrid functional. By comparing with the parent linear oligoacenes (nA) and based on computed ionization potentials and electron affinities, we predict DHTAn molecules to be more stable than acenes of the same length, where we expect DHTAn molecules to be persistent at least up to n = 7 rings. We further exploit the analogy with nA by analyzing the entire intramolecular π-band structure of the DHTAn molecules. This clearly reveals that the additional two electrons donated by the dihydropyrazine group are delocalized over the entire molecule and contribute to its π-electron system. As a consequence, the symmetry of the frontier orbitals of DHTAn differs from that of the parent nA molecule. This also affects the UV–vis absorption spectra which have been measured for DHTA5, 6, and 7 dissolved in dimethyl sulfoxide and analyzed by means of excited state calculations within a time-dependent density functional theory framework.

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 analyzed quantitatively . Recently angle-resolved UPS 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 when a plane wave final state is assumed. This approach, becoming known as orbital or photoemission tomography, relates the emission distribution to the Fourier transform of the ground state orbitals. The examples given will introduce photoemission tomography and demonstrate its potential as a technique to determine both electronic and geometric structures not just complementary but also competitive to methods as diverse as scanning tunneling microscopy and near-edge X-ray absorption spectroscopy.

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.

We calculate the coefficient of thermal expansion (CTE) in tungsten-rhenium random alloys for Re concentrations between 0% and 50% and for temperatures up to 2400 K by employing the quasiharmonic approximation within the ab initio framework of density functional theory. We treat chemical disorder by the virtual crystal approximation and compute the phonon density of states at two levels of sophistication. While the traditional Debye-Gr\"uneisen (DG) model fails to account for the experimentally observed increase in CTE upon Re addition for concentrations above 10% Re, explicit phonon calculations within density functional perturbation theory lead to an overall good agreement with experiment. Thereby we identify the pronounced phonon softening and anisotropy between transversal and longitudinal modes in W-Re to be responsible for the breakdown of the DG model.

This study focuses on hexagonal boron nitride as an ultra-thin van der Waals dielectric substrate for the epitaxial growth of highly ordered crystalline networks of the organic semiconductor parahexaphenyl. Atomic force microscopy based morphology analysis combined with density functional theory simulations reveal their epitaxial relation. As a consequence, needle-like crystallites of parahexaphenyl grow with their long axes oriented five degrees off the hexagonal boron nitride zigzag directions. In addition, by tuning the deposition temperature and the thickness of hexagonal boron nitride, ordered networks of needle-like crystallites as long as several tens of micrometers can be obtained. A deeper understanding of the organic crystallites growth and ordering at ultra-thin van der Waals dielectric substrates will lead to grain boundary-free organic field effect devices, limited only by the intrinsic properties of the organic semiconductors.