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Creating a good contact between electrodes and graphene nanoribbons (GNRs) has been a long-standing challenge in searching for the next GNR-based nanoelectronics. This quest requires the controlled fabrication of sub-20 nm metallic gaps, a clean GNR transfer minimizing damage and organic contamination during the device fabrication, as well as work function matching to minimize the contact resistance. Here, we transfer 9-atom-wide armchair-edged GNRs (9-AGNRs) grown on Au(111)/mica substrates to pre-patterned platinum electrodes, yielding polymer-free 9-AGNR field-effect transistor devices. Our devices have a resistance in the range of 106-108 Ω in the low-bias regime, which is 2-4 orders of magnitude lower than previous reports. Density functional theory calculations combined with the non-equilibrium Green's function method explain the observed p-type electrical characteristics and further demonstrate that platinum gives strong coupling and higher transmission in comparison to other materials, such as graphene., Applied Physics Letters, 122 (17), ISSN:0003-6951, ISSN:1077-3118

A recently developed bottom-up synthesis strategy enables the fabrication of graphene nanoribbons with well-defined width and non-trivial edge structures from dedicated molecular precursors. Here we discuss the synthesis and properties of zigzag nanoribbons (ZGNRs) modified with periodic cove-cape-cove units along their edges. Contrary to pristine ZGNRs, which show antiferromagnetic correlation of their edge states, the edge-modified ZGNRs exhibit a finite single particle band gap without localized edge states. We report the on-surface synthesis of such edge-modified ZGNRs and discuss tunneling conductance dI/dV spectra and dI/dV spatial maps that reveal a noticeable localization of electronic states at the cape units and the opening of a band gap without presence of edge states of magnetic origin. A thorough ab initio investigation of the electronic structure identifies the conditions under which antiferromagnetically coupled, edge-localized states reappear in the electronic structure. Further modifications of the ribbon structure are proposed that lead to an enhancement of such features, which could find application in nanoelectronics and spintronics.

Machine learning is changing how we design and interpret experiments in materials science. In this work, we show how unsupervised learning, combined with ab initio random structure searching, improves our understanding of structural metastability in multicomponent alloys. We focus on the case of Al-O-N alloys where the formation of aluminum vacancies in wurtzite AlN upon the incorporation of substitutional oxygen can be seen as a general mechanism of solids where crystal symmetry is reduced to stabilize defects. The ideal AlN wurtzite crystal structure occupation cannot be matched due to the presence of an aliovalent hetero-element into the structure. The traditional interpretation of the c-lattice shrinkage in sputter-deposited Al-O-N films from X-ray diffraction (XRD) experiments suggests the existence of a solubility limit at 8 at % oxygen content. Here, we show that such naive interpretation is misleading. We support XRD data with accurate ab initio modeling and dimensionality reduction on advanced structural descriptors to map structure-property relationships. No signs of a possible solubility limit are found. Instead, the presence of a wide range of non-equilibrium oxygen-rich defective structures emerging at increasing oxygen contents suggests that the formation of grain boundaries is the most plausible mechanism responsible for the lattice shrinkage measured in Al-O-N sputtered films. We further confirm our hypothesis using positron annihilation lifetime spectroscopy.

Noble metal nanostructure synthesis via seed-mediated route is a widely adopted strategy for a plethora of nanocrystal systems. Ag@Au core–shell nanostructures are radiolytically grown in real-time using in situ liquid-cell (scanning) transmission electron microscopy. Here we employ a capping agent, dimethyl-amine (DMA) and a coordinating complex, potassium iodide (KI) in an organic solvent (methanol) in order to (1) slow down the reaction kinetics to observe mechanistic insights into the overgrowth process and (2) shift the growth regime from galvanic-replacement mode to direct synthesis mode resulting in the conventional synthesis of Ag@Au core–shell structures. A theoretical approach based on classical simulations complements our experiments, providing further insight on the growth modes. In particular, we focus on the shape evolution and chemical ordering, as currently there is an insufficient understanding regarding mixed composition phases at interfaces of alloys even with well-known miscibilities. ...

Inelastic electron tunneling through single molecules located in the tunnel junction between a surface and the tip of a scanning tunneling microscope may cause molecular surface dynamics. Vibrations excited by inelastically tunneling electrons may couple to other vibrational modes and finally induce processes like hopping or rotation. The reaction yield Y(V) as a function of a bias voltage V depict quasi an action spectrum of a certain dynamic event. Experimental results of action spectra are reproduced for rotation and hopping of a single propene molecule on a stepped Cu(211) surface. The good agreement between calculated and experimental Y(V) identify the respective vibrational modes excited by tunneling electrons. These induce then motions via intermode-coupling with the reaction coordinate modes. A general formula of a reaction rate R(V) in a two-electron process, exciting two different vibrational modes for enantiomer conversion of a single propene molecule is derived and the expected R(V) is calculated.

Machine learning is changing how we design and interpret experiments in materials science. In this work, we show how unsupervised learning, combined with

The production of enantiopure materials and molecules is of uttermost relevance in research and industry in numerous contexts, ranging from nonlinear optics to asymmetric synthesis. In the context of the latter, dehalogenation, which is an essential reaction step for a broad class of chemical reactions, is investigated; specifically, dehalogenation of prochiral 5-bromo-7-methylbenz(a)anthracene (BMA) on prototypical, chiral, intermetallic PdGa{111} surfaces under ultrahigh vacuum conditions. Asymmetric halogen elimination is demonstrated by combining temperature-programmed X-ray photoelectron spectroscopy, scanning probe microscopy, and density functional theory. On the PdGa{111} surfaces, the difference in debromination temperatures for the two BMA surface enantiomers amounts up to an unprecedented 46 K. The significant dependence of the dehalogenation temperature of the BMA surface enantiomers on the atomic termination of the PdGa{111} surfaces implies that the ensemble effect is pronounced in this reaction step. These findings evidence enantiospecific control and hence promote intrinsically chiral crystals for asymmetric on-surface synthesis.

arXiv:2006.10837v2, Generating and detecting radiation in the technologically relevant range of the so-called terahertz gap (0.1–10 THz) is challenging because of a lack of efficient sources and detectors. Quantum dots in carbon nanotubes have shown great potential to build sensitive terahertz detectors, usually based on photon-assisted tunneling. A recently reported mechanism combining resonant quantum dot transitions and tunneling barrier asymmetries results in a narrow linewidth photocurrent response with a large signal-to-noise ratio under weak THz radiation. That device was sensitive to one frequency, corresponding to transitions between equidistant quantized states. In this work we show, using numerical simulations together with scanning tunneling spectroscopy studies of a defect-induced metallic zigzag single-walled carbon nanotube quantum dot, that breaking simultaneously various symmetries in metallic nanotube quantum dots of arbitrary chirality strongly relaxes the selection rules in the electric dipole approximation and removes energy degeneracies. This leads to a richer set of allowed optical transitions spanning frequencies from 1 THz to several tens of THz, for a ∼10 nm quantum dot. Based on these findings, we propose a terahertz detector device based on a metallic single-walled carbon nanotube quantum dot defined by artificial defects. Depending on its length and contacts transparency, the operating regimes range from a high-resolution gate-tunable terahertz sensor to a broadband terahertz detector. Our calculations indicate that the device is largely unaffected by temperatures up to 100 K, making carbon nanotube quantum dots with broken symmetries a promising platform to design tunable terahertz detectors that could operate at liquid nitrogen temperatures., This work has been partially supported by the Spanish Ministry of Science and Innovation with PID2019-105488GB-I00 and PCI2019-103657 (A.A.) and FIS2017-82804-P (D.B.). The work of D.B. is partially supported by by the Transnational Common Laboratory QuantumChemPhys. The Basque Government supported this work through Project No. IT-1246-19 (A.A.). J.W.G. acknowledges financial support from FONDECYT: Iniciación en Investigación 2019 Grant N. 11190934 (Chile). A.A. acknowledge financial support by the European Commission from the NRG-STORAGE project (GA 870114). K.E., C.P. and D.P. acknowledge the Swiss National Science Foundation under Grant No. 200020_182015 and No. 200021_172527, and the NCCR MARVEL funded by the Swiss National Science Foundation (51NF40-182892). The Swiss National Supercomputing Centre (CSCS) under project ID s746 and s904 is acknowledged for computational resources.

In this work, we propose an efficient computational scheme for first-principle quantum transport simulations to evaluate the open-boundary conditions. Its partitioning differentiates from conventional methods in that the contact self-energy matrices are constructed on smaller building blocks, principal layers (PL), while conventionally it was restricted to have the same lateral dimensions of the adjoining atoms in a channel region. Here, we obtain the properties of bulk electrodes through non-equilibrium Green’s function (NEGF) approach with significant improvements in the computational efficiency without sacrificing the accuracy of results. To exemplify the merits of the proposed method we investigate the carrier density dependency of contact resistances in silicon nanowire devices connected to bulk metallic contacts.

Localized basis sets in the projector augmented wave formalism allow for computationally efficient calculations within density functional theory (DFT). However, achieving high numerical accuracy requires an extensive basis set, which also poses a fundamental problem for the interpretation of the results. We present a way to obtain a reduced basis set of atomic orbitals through the subdiagonalization of each atomic block of the Hamiltonian. The resulting local orbitals (LOs) inherit the information of the local crystal field. In the LO basis, it becomes apparent that the Hamiltonian is nearly block-diagonal, and we demonstrate that it is possible to keep only a subset of relevant LOs which provide an accurate description of the physics around the Fermi level. This reduces to some extent the redundancy of the original basis set, and at the same time it allows one to perform post-processing of DFT calculations, ranging from the interpretation of electron transport to extracting effective tight-binding Hamiltonians, very efficiently and without sacrificing the accuracy of the results., 15 pages, 13 figures

While supramolecular chemistry is a firmly established research field in laboratory conditions, the experimental study of non-covalent interactions, such as hydrogen bonding and piinteractions, between different molecules in chemically rich, non-crystalline matter remains highly challenging. We demonstrate that soft ion bombardment can trigger the joint desorption of weakly interacting adjacent molecules, and that this in turn allows molecular interaction probabilities, surroundings, and arrangement in organic matter to be probed. Assemblies of organic molecules linked by hydrogen bonds or dipole- interactions are extracted here with preservation of chemistry and structure as single-charged supramolecular secondary ions. Among the examples shown is the desorption of stable 12-molecular clusters of the amino acid L-proline, which suggest an icosahedral on-surface self-assembly, and the desorption of supramolecular linear oligomers of 2,5-piperazinedione. The second half of the study lays down the statistical framework for the reconstruction of a molecular interactome based on the relative abundances of supramolecular dimers of different compositions within a mass spectrum acquired on organic matter containing more than one type of molecule.

This paper presents a joint theoretical and experimental study of the electronic structure of pristine and Ni-substituted LaFeO${}_{3}$. The authors analyze the peaks in the x-ray spectra using density-functional theory combined with extended Hubbard functionals

Extended planar defects found in epitaxially grown $\mathrm{SrFe}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}$ thin films are expected to exhibit distinct conductivity properties. Here, we use a combination of scanning transmission electron microscopy techniques and electron energy-loss spectroscopy (EELS) to uncover the peculiar structure of these planar defects and to explore their electronic properties. We find that the defects are formed by $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{2+\ensuremath{\alpha}}$ layers consisting of $\mathrm{Fe}{\mathrm{O}}_{5}$ polyhedra alternating with SrO and $\mathrm{Fe}{\mathrm{O}}_{2}$ perovskite-type layers, analogous to the $\mathrm{S}{\mathrm{r}}_{4}\mathrm{F}{\mathrm{e}}_{6}{\mathrm{O}}_{12+\ensuremath{\delta}}$ crystal structure. Our experimental and theoretical EELS data, combined with projected density of states calculations, reveal peak width changes and energies shifts, which suggest an increased electron doping of the Fe $3d{e}_{g}$ band in the $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{2+\ensuremath{\alpha}}$ layers as compared to the $\mathrm{SrFe}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ film. Thus we show that the presence of $\mathrm{F}{\mathrm{e}}_{2}{\mathrm{O}}_{2+\ensuremath{\alpha}}$ planar defects indeed can effectively modify the electron-hole conductivity in $\mathrm{SrFe}{\mathrm{O}}_{3\text{\ensuremath{-}}\ensuremath{\delta}}$ films.

Two approaches to simulations of phonon properties of solids beyond the harmonic approximation, the self-consistent

The azide-alkyne Huisgen cycloaddition has a key role in click chemistry and is configured as a powerful tool in pharmaceutical and medicinal chemistry. Although this reaction has already been largely studied, there is an ongoing debate about its mechanism. In this work we study the dynamical aspects of the process using metadynamics computer simulations. We focus on the conformational aspects that determine the course of the reaction and characterize its free energy landscape. To properly capture the thermodynamics of the process we select optimal collective variables using harmonic linear discriminant analysis. The results qualitatively confirm and explain the experimental evidence and give insights into the role of the substituents and the possible transition mechanisms.

We unravel the origin of the recently observed striking enantioselectivity of the PdGa(111) surface with respect to the adsorption of a small organic molecule, 9-ethynylphenanthrene, using first-principles calculations. It turns out that the key ingredient to understand the experimental evidence is the appropriate description of van der Waals interactions beyond the widely employed atomic pairwise approximation. A recently developed van der Waals-inclusive density functional method, which encompasses dielectric screening effects, reveals the origin of the experimentally observed enantioselectivity and provides conclusive evidence of chiral recognition on a bimetallic surface driven by dispersion interactions. The incorporation of dielectric screening leads to a renormalization of the dispersion interaction range, allowing for the appropriate weighting of the molecule-substrate interactions at intermediate distances between 2.5 and 5 Å. Our findings have implications for the structure and stability of complex organic/inorganic systems where dielectric screening effects are expected to be of general importance.

Graphene nanoribbons are attractive candidates for future technological applications due to exceptional electronic and magnetic properties. We investigate the stability of edge magnetism in zigzag graphene nanoribbons with indene-type functionalized edges, recently synthesised on Au(111) in our laboratory. Using density functional theory (DFT) calculations, we show that the functionalized nanoribbons preserve most electronic properties of the pristine ribbon, with band modifications making edge magnetism sensitive or unstable to environmental influences. The tendency towards such instability already emerges when the system is investigated with a simple mean field Hubbard model where the on-site Coulomb repulsion can be parametrically tuned. It becomes fully evident in DFT calculations when the mostly dispersive interactions between the nanoribbon and the metal substrate are modeled using different van der Waals correction schemes. One of such schemes gives results in agreement with spectroscopy experiments in which the highest occupied and lowest unoccupied states locate on the unmodified and modified zigzag edge sections, respectively. When a different dispersion correction approach is considered, theory predicts, despite a very similar adsorption geometry, an inversion of occupied/unoccupied states and a 60% reduction in absolute magnetization. Such sensitivity with respect to small perturbations is discussed.

The validation of classical potentials for describing multicomponent materials in complex geometries and their high temperature structural modifications (disordering and melting) requires to verify both a faithful description of the individual phases and a convincing scheme for the mixed interactions, like it is the case of the embedded atom scheme. The present paper addresses the former task for an embedded atom potential for copper, namely the widely adopted parametrization by Zhou, through application to bulk, surface and nanocluster systems. It is found that the melting point is underestimated by 200 degrees with respect to experiment, but structural and temperature-dependent properties are otherwise faithfully reproduced.

Noble metal nanostructure synthesis

Little is known about metallic clusters consisting merely of a dozen of atoms or even less, despite of their importance in catalysis and crystal nucleation. Scanning transmission electron microscopy (STEM) provides direct atomic structure information but has inherently suffered from limited time resolution. We employ fast dynamic STEM combined with a spatio-temporal image denoising algorithm to explore the structure and stability of Pt clusters on carbon, which represents a highly relevant catalysis system. At room temperature, dynamic amorphous 2D structures are found, while above ≈300 °C, the clusters transform into a crystalline state. Our experimental and theoretical data reveal an unexpected strong trend of the crystalline clusters to exhibit the face-centered cubic, bulk structure of Pt with cuboidal geometries being most prominent.

Nonbenzenoid carbocyclic rings are postulated to serve as important structural elements toward tuning the chemical and electronic properties of extended polycyclic aromatic hydrocarbons (PAHs, or namely nanographenes), necessitating a rational and atomically precise synthetic approach toward their fabrication. Here, using a combined bottom-up in-solution and on-surface synthetic approach, we report the synthesis of nonbenzenoid open-shell nanographenes containing two pairs of embedded pentagonal and heptagonal rings. Extensive characterization of the resultant nanographene in solution shows a low optical gap, and an open-shell singlet ground state with a low singlet-triplet gap. Employing ultra-high-resolution scanning tunneling microscopy and spectroscopy, we conduct atomic-scale structural and electronic studies on a cyclopenta-fused derivative on a Au(111) surface. The resultant five to seven rings embedded nanographene displays an extremely narrow energy gap of 0.27 eV and exhibits a pronounced open-shell biradical character close to 1 (

Edge dislocations are frequently found in epitaxial BiFeO3 multiferroic thin films and are expected to exhibit distinctive and localized magnetoelectric properties. However, an exhaustive characterization of these dislocations at the atomic level has to date been largely overlooked. Here, we use a combination of scanning transmission electron microscopy techniques, atomistic simulations obtained from classical molecular dynamics calculations, and real-space multiple-scattering theory to explore the chemical properties and the bonding characteristics of the atoms located at and near the dislocation cores. We find that in addition to Bi, small amounts of Fe atoms are present in the BiFeO3 dislocation cores which result in uncompensated Fe spins along the dislocations and give rise to a magnetic signal. Our results suggest that edge dislocations in BiFeO3 films could be efficiently used for realizing BiFeO3-based magnetic devices.

We study the band gap of finite $N_A=7$ armchair graphene nanoribbons (7-AGNRs) on Au(111) through scanning tunneling microscopy/spectroscopy combined with density functional theory calculations. The band gap of 7-AGNRs with lengths of 6 nm and more is converged to within 0.1 eV of its bulk value of 2.3 eV, while the band gap opens by several hundred meV in very short 7-AGNRs. The termination has a significant effect on the band gap, doubly hydrogenated termini yielding a lower band gap than singly hydrogenated ones., Comment: Submitted version (preprint, pre-reviewing) to ChemPhysChem (An invited contribution to a Special issue on On-SurfaceSynthesis)

Two approaches to simulations of phonon properties of solids beyond the harmonic approximation, the self-consistent ab initio lattice dynamics (SCAILD) and decoupled anharmonic mode approximation (DAMA) are critically benchmarked against each other and molecular dynamics simulations using a density-functional-theory description of electronic states, and compared to experimental data for fcc aluminium. The temperature-dependence of phonon dispersion and the phonon density-of-states, heat capacity, and the mean atomic displacement for fcc aluminium are examined with these approaches at ambient pressure. A comparison of results obtained with the harmonic approximation to the ones predicted by SCAILD and DAMA reveal a negligible anharmonic contribution to phonon frequencies, a small, but significant influence on heat capacity, and a strong effect on atomic mean-square displacement. The phase space accessed with SCAILD and DAMA is reduced relative to molecular and harmonic lattice dynamics simulations. In particular the DAMA results are in good agreement with displacement amplitudes determined by the Debye–Waller factor in x-ray diffraction experiments.

Cloud platforms allow users to execute tasks directly from their web browser and are a key enabling technology not only for commerce but also for computational science. Research software is often developed by scientists with limited experience in (and time for) user interface design, which can make research software difficult to install and use for novices. When combined with the increasing complexity of scientific workflows (involving many steps and software packages), setting up a computational research environment becomes a major entry barrier. AiiDAlab is a web platform that enables computational scientists to package scientific workflows and computational environments and share them with their collaborators and peers. By leveraging the AiiDA workflow manager and its plugin ecosystem, developers get access to a growing range of simulation codes through a python API, coupled with automatic provenance tracking of simulations for full reproducibility. Computational workflows can be bundled together with user-friendly graphical interfaces and made available through the AiiDAlab app store. Being fully compatible with open-science principles, AiiDAlab provides a complete infrastructure for automated workflows and provenance tracking, where incorporating new capabilities becomes intuitive, requiring only Python knowledge., Comment: Manuscript: 25 pages, 6 figures. Supplementary information: 15 pages, 10 figures

Hydrogen (H) interaction with metals and alloys is of intense interest in a span of topics in materials science and engineering. Distinct H absorption behavior of two similar intermetallic compounds, namely, Zr2Pd and ZrPd2, is not clearly elucidated in terms of their common MoSi2-type structure and hydride-forming constituents. This work addresses that extended structures (electron density topologies) of these compounds are actually different, which causes the different properties they possess. Pseudoatoms manifested by non-nuclear maxima of electron density are uncovered in Zr2Pd's Zr tetrahedral interstices, whereas atoms in ZrPd2 are bonded through straight bond paths. Interstitial pseudoatoms, which are characterized by a weak localization nature revealed by Laplacian calculations, readily lose excess charges to H. Therefore, these properties favor formation of covalent–ionic Zr–H bonding and concurrent H absorption. During Zr2PdH2 formation, the entire topological structure of Zr2Pd is retained; by contrast, ZrPd2H2 formation requires charge redistribution of the stable host lattice of ZrPd2 to change electron density topology and hence is endothermic and unfavorable. The present work reveals a concrete and visualized origin of the distinct H absorption behaviors of the two similar compounds, which has implications in the search for new H-related materials.

The relative stability of different realizations of the Ag(111)/Alumina interfaces with varying oxygen partial pressures is investigated by means of ab initio density functional theory (DFT) simulations. Previous theoretical studies of similar systems always involve oversimplified geometries like stoichiometric Al-terminated, Al-rich, or O-terminated alumina interfaces. Such framework cannot explain the experimental behavior observed at intermediate oxygen partial pressure. Our approach, instead, suggests that the oxygen at the interface can play an important role at intermediate concentrations, leading to a more realistic interpretation of the experimental data.

Local measurements of the oxidation state of vanadium in monoclinic BiVO4 particles by means of electron energy-loss spectroscopy in scanning transmission electron microscopy reveal a pronounced surface reduction: within a 5-nm-thick shell, the oxidation state of vanadium is reduced from +5 to about +4. Thus, charge neutrality near the surface demands for ∼15% oxygen vacancies. Our results provide direct evidence for the segregation of oxygen vacancies at the surface of BiVO4. This observation is confirmed by X-ray photoelectron spectroscopy. The experimental findings are complemented with all-electron density functional theory based WIEN2k calculations of the density of electron states and of the electron energy-loss near-edge structure. The theoretical results provide further information on the electronic changes induced by the experimentally verified oxygen vacancies.

The evolution of the composition of tungsten carbide and silicon surfaces initiated by the bombardment with Zr and Cr ions has been investigated as a function of the substrate bias voltage. Surface composition profiles were measured by Rutherford backscattering and have been compared with the results obtained by the TRIDYN simulation program. It is found that the general dependence of film thickness on substrate bias is satisfactorily reproduced by this model. Deviations between experiment and simulation are attributed to possible partial oxidation of the surface or uncertainties in the charge state distribution of metal ions. The results confirm that TRIDYN facilitates the predictability of the nucleation of metallic vapor at substrate surfaces.

High-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) has been performed along the low-index zone axes of the o-Al4(Cr,Fe) complex metallic alloy to obtain a real-space representation of the crystal structure and to elucidate the material's inherent structural disorder. By comparing experiments with multislice STEM simulations, the model previously suggested by X-ray diffraction is further refined to provide a new set of positions and occupancies for the transition metal atoms.Pmnbis suggested as the new space group for the o-Al4(Cr,Fe) phase. A nonperiodic layer-type modulation, averaged out in bulk diffraction methods, is detected, corroborating the need for complementing bulk diffraction analysis with real-space imaging to derive the true crystal structure of Al4(Cr,Fe).

The atomic structure and composition of a catalyst's surface have a major influence on its performance regarding activity and selectivity. In this respect, intermetallic compounds are promising future catalyst materials, as their surfaces exhibit small and well-defined ensembles of active metal atoms. In this study, the active adsorption sites of the 3-fold-symmetric surfaces of the PdGa interrnetallic compound were investigated in a combined experimental and computational approach using CO as a test molecule. The PdGa(111) and (-1-1-1) surfaces exhibit very similar electronic structures, but have Pd sites with very different, well-defined atomic coordination and separation. They thereby serve as prototypical model systems for studying ensemble effects on bimetallic catalytic surfaces. Scanning tunneling microscopy and Fourier transform infrared spectroscopy show that the CO adsorption on both surfaces is solely associated with the topmost Pd atoms and Ga acts only as an inactive spacer. The different local configurations of these Pd atoms dictate the CO adsorption sites as a function of coverage. The experimental results are corroborated by density functional theory and illustrate the site separation and ensemble effects for molecular adsorption on intermetallic single crystalline surfaces.

The surface of a crystal made of roughly spherical molecules exposes, above its bulk rotational phase transition at T= T$_r$, a carpet of freely rotating molecules, possibly functioning as "nanobearings" in sliding friction. We explored by extensive molecular dynamics simulations the frictional and adhesion changes experienced by a sliding C$_{60}$ flake on the surface of the prototype system C$_{60}$ fullerite. At fixed flake orientation both quantities exhibit only a modest frictional drop of order 20% across the transition. However, adhesion and friction drop by a factor of $\sim$ 2 as the flake breaks its perfect angular alignment with the C$_{60}$ surface lattice suggesting an entropy-driven aligned-misaligned switch during pull-off at T$_r$. The results can be of relevance for sliding Kr islands, where very little frictional differences were observed at T$_r$, but also to the sliding of C$_{60}$ -coated tip, where a remarkable factor $\sim$ 2 drop has been reported.

Local controlled deposition of graphene on a moving polycrystalline substrate, in the presence of a fast-flowing buffer gas, allows the evolutionary selection of the fastest-growing crystallites, forming macroscopic graphene monocrystals.

The ability of amphiphilic polymers to self-assemble and form a gel or gel-like layer has been investigated by means of both experimental and theoretical studies on alkylated derivatives of poly(acrylic acid). Experiments were performed to determine the relationship between amphiphilic polymer chemistry, structure, water retention, and friction in the presence of hydrophobic substrates. The results indicate that the amphiphilic polymer forms a water-enriched, friction-reducing adsorbed layer on hydrophobic surfaces. The shear moduli and viscosities of the adsorbed layers, as determined by fitting the Voigt model to QCM-D data, were consistent with the presence of a gel. Computational studies on HPAA-12 were performed and are consistent with the presence of adsorbed conformations, in which the lowest free energy in the model corresponded to a partially adsorbed molecule, with a small fraction of hydrophobic side chains being compelled, for configurational reasons, to point into the bulk water. This would support the possibility of the formation of either a gel-like layer or surface aggregation. However, because the adsorption experiments showed no evidence of aggregation, this strongly suggests the formation of a gel.

Here, it is demonstrated, using high-resolution X-ray spectroscopy and density functional theory calculations, that 2H-tetraphenyl porphyrins metalate at room temperature by incorporating a surface metal atom when a (sub)monolayer is deposited on 3d magnetic substrates, such as Fe(110) and Ni(111). The calculations demonstrate that the redox metalation reaction would be exothermic when occurring on a Ni(111) substrate with an energy gain of 0.89 eV upon embedding a Ni adatom in the macrocycle. This is a novel way to form, via chemical modification and supramolecular engineering, 3d-metal-organic networks on magnetic substrates with an intimate bond between the macrocycle molecular metal ion and the substrate atoms. The achievement of a complete metalation by Fe and Ni can be regarded as a test case for successful preparation of spintronic devices by means of molecular-based magnets and inorganic magnetic substrates.

The structural and electronic properties of a series of SrMnO3 thin films grown on different substrates leading to different biaxial strains are examined by means of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and electron energy-loss spectroscopy (EELS). Here, we report a comprehensive theoretical investigation on the impact of oxygen nonstoichiometry and strain on the O-K and Mn-L-2,L-3 electron energy-loss near-edge structures of SrMnO3 with the aid of the all-electron density functional theory code WIEN2K. Our experimental and theoretical EELS data reveal pronounced peak-height changes and energy shifts as a function of strain, which are ascribed to differences in the oxygen vacancy density rather than to a direct effect of lattice distortions. In addition, we find that the formation of oxygen vacancies in SrMnO3 is likely to be enhanced by tensile strain. Compared to the unstrained material, the vacancy formation energy is reduced by 0.25 eV under 3.78% tensile strain.

We develop a formalism (decoupled anharmonic mode approximation, DAMA) that allows calculation of the vibrational free energy using density functional theory even for materials which exhibit negative curvature of the potential energy surface with respect to atomic displacements. We investigate vibrational modes beyond the harmonic approximation and approximate the potential energy surface with the superposition of the accurate potential along each normal mode. We show that the free energy can stabilize crystal structures at finite temperatures which appear dynamically unstable at T = 0. The DAMA formalism is computationally fast because it avoids statistical sampling through molecular dynamics calculations, and is in principle completely ab initio. It is free of statistical uncertainties and independent of model parameters, but can give insight into the mechanism of a structural phase transition. We apply the formalism to the perovskite cryolite, and investigate the temperature-driven phase transition from the P21/n to the Immm space group. We calculate a phase transition temperature between 710 and 950 K, in fair agreement with the experimental value of 885 K. This can be related to the underestimation of the interaction of the vibrational states. We also calculate the main axes of the thermal ellipsoid and can explain the experimentally observed increase of its volume for the fluorine by 200-300% throughout the phase transition. Our calculations suggest the appearance of tunneling states in the high temperature phase. The convergence of the vibrational DOS and of the critical temperature with respect of reciprocal space sampling is investigated using the polarizable-ion model.

Herein the formation of water molecules in the intermediate step of the redox reaction of porphyrins self-metalation on O/Cu(111) is demonstrated. Photoemission measurements show that the temperature on which porphyrins pick-up a substrate metal atom on O/Cu(111) is reduced by about 185 +/- 15K with respect to the pure Cu(111). DFT calculations clearly indicate that the formation of a water molecule is less expensive than the formation of H-2 on the O/Cu(111) substrate and, in some cases, it can be also exothermic.