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The influence on the growth of cobalt (Co)-based nanostructures of a surface carbide (Ni2C) layer formed at the Ni(100) surface is revealed via complementary scanning tunneling microscopy (STM) measurements and first-principles calculations. On clean Ni(100) below 200 degrees C in the sub-monolayer regime, Co forms randomly distributed two-dimensional (2D) islands, while on Ni2C it grows in the direction perpendicular to the surface as well, thus forming two-atomic-layers high islands. We present a simple yet powerful model that explains the different Co growth modes for the two surfaces. A jagged step decoration, not visible on stepped Ni(100), is present on Ni2C. This contrasting behavior on Ni2C is explained by the sharp differences in the mobility of Co atoms for the two cases. By increasing the temperature, Co dissolution is activated with almost no remaining Co at 250 degrees C on Ni(100) and Co islands still visible on the Ni2C surface up to 300 degrees C. The higher thermal stability of Co above the Ni2C surface is rationalized by ab initio calculations, which also suggest the existence of a vacancy-assisted mechanism for Co dissolution in Ni(100). The methodology presented in this paper, combining systematically STM measurements with first-principles calculations and computational modelling, opens the way to controlled engineering of bimetallic surfaces with tailored properties.

Under near-ambient pressure conditions, carbon monoxide molecules intercalate underneath an epitaxial graphene monolayer grown on Ni(111), getting trapped into the confined region at the interface. On the basis of ab-initio density functional theory calculations, we provide here a full investigation of the intercalated CO pattern, highlighting the modifications induced on the graphene electronic structure. For a CO coverage as low as 0.14 monolayer (ML), the graphene layer is spatially decoupled from the metallic substrate, with a significant C 1s core level shift towards lower binding energies. The most relevant signature of the CO intercalation is a clear switching of the graphene doping state, which changes from n-type, when strongly interacting with the metal surface, to p-type. The shift of the Dirac cone linearly depends on the CO coverage, reaching about 0.9 eV for the saturation value of 0.57 ML. Theoretical predictions are compared with the results of scanning tunnelling microscopy, low-energy electron diffraction and photoemission spectroscopy experiments, which confirm the proposed scenario for the nearly saturated intercalated CO system. This result opens the way to the application of the graphene/Ni(111) interface as gas sensor to easily detect and quantify the presence of carbon monoxide.

In the last decade, hydrogen (H-)tautomerization, that is, a reaction that involves simple intramolecular proton transfer, has been studied in single phthalocyanine, porphyrin, and porphycene derivatives as a prototypical single molecular conductance switch. Here, by means of low-temperature scanning tunneling microscopy and density functional theory calculations, we report a binary H-tautomerism and stereoisomeric conformational switch in (amino-functionalized) porphyrins assembled in molecular chains on a gold surface. We show that the formation of the chain is crucial for the binary tautomeric switch mechanism as the single molecule switches differently. Our findings suggest that the (amino-)functionalization of molecules can be exploited not only to drive the formation of molecular self-assemblies but also to steer their switching properties.

We engaged some high school Physics teachers in a deep revision of their PCK (Pedagogical Content Knowledge) to contrast the trend of a diffuse disaffection and negative attitude of the students towards Physics studies. Starting from the analysis of the Math/Phys interplay prevalent patterns of the teachers, we observed that the application type is the most common, which adversely affects students’ learning. For this reason, we suggested that they adopt a new approach, based on conceptual fragmentation and multiple representations reasoning tools. This approach stimulates teachers to adopt different Math/Phys interplay patterns. Here we present the main features of some of the tested activities, which also allowed teachers to better understand and recognise the learning challenges students have to cope with.

L’interazione tra Matematica e Fisica è intrinseca in un approccio quantitativo alla Fisica: il “come” essa viene intesa da parte dei docenti emerge potentemente nella spiegazione e razionalizzazione di un concetto e del suo contesto fenomenologico attraverso l’uso di formule ed equazioni e ancora di più nella proposta di esercizi e di problemi. Nello sviluppo della professionalità docente è quindi fondamentale che i docenti diventino consapevoli di quale sia il loro schema prevalente adottato nella costruzione dei percorsi didattici. Tale consapevolezza favorisce lo sviluppo del proprio PCK. Lo studio condotto su un campione di docenti di fisica di diverse scuole secondarie di secondo grado mette in luce che, indipendentemente dalla tipologia della scuola e dalla formazione di base del docente, lo schema prevalente adottato è di tipo applicativo. Si propongono alcune strategie di accompagnamento ai docenti nella loro azione quotidiana come processo permanente di formazione in servizio. Questa forma di accompagnamento favorisce nel docente lo sviluppo nel proprio PCK anche di altri schemi di interazione tra Matematica e Fisica, necessari per l’integrazione tra le due discipline ma anche per una loro distinzione epistemologica. The interaction between mathematics and physics is intrinsic in a quantitative approach to physics: the way in which it is understood powerfully emerges in the explanation and rationalization of a concept and its phenomenological context through the use of formulas and equations and even more in the proposed exercises and problems. In the development of teaching professionalism, it is therefore essential that the teachers become aware of what is their prevalent scheme adopted in the construction of educational paths. This awareness improves the development of their own PCK. The study conducted on a sample of physics teachers of different secondary schools highlights that, regardless of the type of school and the teacher curriculum, the prevailing scheme adopted is an applicative one. Here some strategies are proposed to accompany the teachers in the process of their daily action as a permanent process of in-service training. This form of support develops new forms of teachers’ own PCK and also promotes other schemes of interaction between mathematics and physics that are necessary both for the integration between the two disciplines and for their epistemological distinction.

The widely investigated oxygen reduction reaction (ORR) is well-known to proceed via two competing routes, involving two or four electrons, and yielding different reaction products, respectively. Both pathways are believed to share a common, elusive intermediate, namely, the hydroperoxyl radical. By exploiting a cobalt single-atom biomimetic model catalyst, based on a self-assembled monolayer of Co-porphyrins grown on an almost free-standing graphene sheet, we identify, in situ at room temperature in O2+H2O atmosphere, a hydroperoxyl-water cluster that is stabilized at the Co single metal atom catalytic site. We show that the interplay between charge transfer, dipole and H bonding, and water solvation behavior actually determines the hydroperoxyl-water complex stability, the Co-OOH bonding geometry, and, prospectively, opens to the engineered control of the selectivity of ORR pathways.

Nature determines selectivity and activity in biological reactive centers, based on single metal atom macrocycles, by properly tuning the primary coordination sphere and the surrounding protein scaffold. In a biomimetic approach, we show that activation of carbon dioxide at a 2D crystal of phthalocyanines supported by graphene can be controlled by chemical tuning of the position of the Dirac cones of the support through oxygen adsorption. The room temperature stabilization of the CO 2 -Fe chemical bond, detected in situ and confirmed by computational density-functional theory simulations, is obtained by governing the charge transfer across the graphene-metallorganic layer interface upon oxidation of graphene at close-to-ambient conditions. In this way, we can turn a weakly binding site into a strong one in an artificial structure that mimics many features of complex biological systems.

Direct incorporation of Ni adatoms during graphene growth on Ni( 111) is evidenced by scanning tunneling microscopy. The structure and energetics of the observed defects is thoroughly characterized at the atomic level on the basis of density functional theory calculations. Our results show the feasibility of a simple scalable method, which could be potentially used for the realization of macroscopic practical devices, to dope graphene with a transition metal. The method exploits the kinetics of the growth process for the incorporation of Ni adatoms in the graphene network.

Recent experiments indicate that the reactivity of metal surfaces changes profoundly when they are covered with two-dimensional (2D) materials. Nickel, the widespread catalyst choice for graphene (G) growth, exhibits complex surface restructuring even after the G sheet is fully grown. In particular, due to excess carbon segregation from bulk nickel to surface upon cooling, a nickel carbide (${\mathrm{Ni}}_{2}\mathrm{C}$) phase is detected under rotated graphene (RG) but not under epitaxial graphene (EG). Motivated by this experimental evidence, we construct different G/Ni(111) interface models accounting for the two types of G domains. Then, by applying density functional theory, we illuminate the microscopic mechanisms governing the structural changes of nickel surface induced by carbon segregation. A high concentration of subsurface carbon reduces the structural stability of Ni(111) surface and gives rise to the formation of thermodynamically advantageous ${\mathrm{Ni}}_{2}\mathrm{C}$ monolayer. We show the restructuring of the nickel surface under RG cover and reveal the essential role of G rotation in enabling high density of favorable C binding sites in the Ni(111) subsurface. As opposed to RG, the EG cover locks the majority of favorable C binding sites preventing the build-up of subsurface carbon density to a phase transition threshold. Therefore we confirm that the conversion of C-rich Ni surface to ${\mathrm{Ni}}_{2}\mathrm{C}$ takes place exclusively under RG cover, in line with the strong experimental evidence.

Sviluppare un atteggiamento positivo nei confronti della Fisica è il primo passo per il superamento delle difficoltà di apprendimento che persistono nello studio di questa disciplina. Attraverso la somministrazione di un questionario a un campione di 467 studenti di una scuola secondaria di secondo grado di Trieste nell’a.s. 2019/20 si è sondata la dimensione cognitiva, emotiva e comportamentale costituenti l’atteggiamento individuale. La scala proposta è stata validata e se n’è verificata la consistenza con la determinazione del coefficiente alfa di Cronbach che sui quesiti proposti è risultato 0,9 (considerato quindi di valore “eccellente”). Vengono riportati gli esiti dell’indagine sia per gruppi d’età (biennio e triennio) sia per genere, con particolare attenzione ad individuare la correlazione tra atteggiamento e fattori che lo possono influenzare quali la metodologia didattica piuttosto che le caratteristiche intrinseche disciplinari e la struttura epistemologica talvolta sottintesa nelle prassi di insegnamento. Si propone l’uso di tale strumento per un’indagine più estesa al fine di evidenziare in modo quantitativo quali atteggiamenti emergono e favorire la promozione di attività curricolari e di orientamento maggiormente calibrate allo sviluppo e alla promozione di un atteggiamento positivo.

Two-dimensional materials on metallic surfaces or stacked one on top of the other can form a variety of moire superstructures depending on the possible parameter and symmetry mismatch and misorientation angle. In most cases, such as incommensurate lattices or identical lattices but with a small twist angle, the common periodicity may be very large, thus making numerical simulations prohibitive. We propose here a general procedure to determine the minimal simulation cell which approximates, within a certain tolerance and a certain size, the primitive cell of the common superlattice, given the two interfacing lattices and the relative orientation angle. As case studies to validate our procedure, we report two applications of particular interest: the case of misaligned hexagonal/hexagonal identical lattices, describing a twisted graphene bilayer or a graphene monolayer grown on Ni(111), and the case of hexagonal/square lattices, describing for instance a graphene monolayer grown on Ni(100) surface. The first one, which has also analytic solutions, constitutes a solid benchmark for the algorithm; the second one shows that a very nice description of the experimental observations can be obtained also using the resulting relatively small coincidence cells.

An operando investigation of graphene growth on (100) grains of polycrystalline nickel (Ni) surfaces was performed by means of variable-temperature scanning tunneling microscopy complemented by density functional theory simulations. A clear description of the atomistic mechanisms ruling the graphene expansion process at the stepped regions of the substrate is provided, showing that different routes can be followed, depending on the height of the steps to be crossed. When a growing graphene flake reaches a monoatomic step, it extends jointly with the underlying Ni layer; for higher Ni edges, a different process, involving step retraction and graphene landing, becomes active. At step bunches, the latter mechanism leads to a peculiar 'staircase formation' behavior, where terraces of equal width form under the overgrowing graphene, driven by a balance in the energy cost between C-Ni bond formation and stress accumulation in the carbon layer. Our results represent a step towards bridging the material gap in searching new strategies and methods for the optimization of chemical vapor deposition graphene production on polycrystalline metal surfaces., Comment: 23 pages including title page and references, 4 figures

Cronaca

Molecules intercalate at the graphene/metal interface even though defect-free graphene is impermeable to any atomic and molecular species in the gas and liquid phase, except hydrogen. The mechanism of molecular intercalation is still a big open question. In this Letter, by means of a combined experimental (STM, XPS, and LEED) and theoretical (DFT) study, we present a proof of how CO molecules succeed in permeating the graphene layer and get into the confined zone between graphene and the Ni(111) surface. The presence of N-dopants in the graphene layer is found to highly facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude, through the stabilization of multiatomic vacancy defects that are the open doors to the bidimensional nanospace, with crucial implications for the catalysis under cover and for the graphene-based electrochemistry.

We show that, for the formation of a metallorganic monolayer, it is possible to artificially divert from substrate- to laterally-driven self-assembly mechanisms by properly tailoring the corrugation of the potential energy surface of the growth template. By exploiting the capability of an ultrathin alumina film to host metallic nanoparticle seeds, we tune the symmetry of a iron phthalocyanine (FePc) two-dimensional crystal, thus showing that it is possible to switch from trans to lateral dominating interactions in the controlled growth of an organic/inorganic heterostack. Finally, by selecting the size of the metallic clusters, we can also control the FePc-metal interaction strength.

Lateral quantum confinement is of great interest in tuning the electronic properties of graphene-based nanostructures, making them suitable for technological applications. In principle, these properties might be controlled through the edge topology: for example, zigzag nanoribbons are predicted to have spin-polarized edge states. The practical realization of these structures is of utmost importance in fully harnessing the electronic properties of graphene. Here, the formation of regular, 1.4 nm wide ribbon-like graphene structures with zigzag edges are reported, showing 1D electronic states. It is found that these “pseudo-ribbons” embedded in single-layer graphene supported on Ni(100) can spontaneously form upon carbon segregation underneath 1D graphene moiré domains, extending hundreds of nanometers in length. On the basis of both microscopy/spectroscopy/diffraction experiments and theoretical simulations, it is shown that these structures, even though seamlessly incorporated in a matrix of strongly interacting graphene, exhibit electronic properties closely resembling those of zigzag nanoribbons.

The recent discovery of the role of adenosine-analogues as neuroprotectants and cognitive enhancers has sparked interest in these molecules as new therapeutic drugs. Understanding the behavior of these molecules in solution and predicting their ability to self-assemble will accelerate new discoveries. We propose a computational approach based on density functional theory, a polarizable continuum solvation description of the aqueous environment, and an efficient search procedure to probe the potential energy surface, to determine the structure and thermodynamic stability of molecular clusters of adenosine analogues in solution, using caffeine as a model. The method was validated as a tool for the prediction of the impact of small structural variations on self-assembly using paraxanthine. The computational results were supported by isothermal titration calorimetry experiments. The thermodynamic parameters enabled the quantification of the actual percentage of dimer present in solution as a function of concentration. The data suggest that both caffeine and paraxanthine are present at concentrations comparable to the ones found in biological samples.

Here we show how time-resolved broadband optical spectroscopy can be used to quantify, with femtometer resolution, the oscillation amplitudes of coherent phonons through a displacive model without free tuning parameters, except an overall scaling factor determined by comparison between experimental data and density functional theory calculations. $WTe_2$ is used to benchmark this approach. In this semimetal, the response is anisotropic and provides the spectral fingerprints of two $A_1$ optical phonons at $\sim$8 and $\sim$80 $cm^-$$^1$. In principle, this methodology can be extended to any material in which an ultrafast excitation triggers coherent lattice modes modulating the high-energy optical properties., Comment: Main Text 5 pages 4 figures, Supplemental Material 20 pages 18 figures

Interaction with the substrate strongly affects the electronic/chemical properties of supported graphene. So far, graphene grown by chemical vapor deposition (CVD) on catalytic single crystal transition metal surfaces - mostly 3-fold close-packed - has mainly been studied. Herein, we investigated CVD graphene on a polycrystalline nickel (Ni) substrate, focusing in particular on (100) micrograins and comparing the observed behavior with that on single crystal Ni(100) substrate. The symmetry-mismatch leads to moiré superstructures with stripe-like or rhombic-network morphology, which were characterized by atomically-resolved scanning tunneling microscopy (STM). Density functional theory (DFT) simulations shed light on spatial corrugation and interfacial interactions: depending on the misorientation angle, graphene is either alternately physi- and chemisorbed or uniformly chemisorbed, the interaction being modulated by the (sub)nanometer-sized moiré superstructures. Ni(100) micrograins appear to be a promising substrate to finely tailor the electronic properties of graphene at the nanoscale, with relevant perspective applications in electronics and catalysis. Fil: Zou, Zhiyu. The Abdus Salam. International Centre for Theoretical Physics; Italia Fil: Carnevali, Virginia. Università degli Studi di Trieste; Italia Fil: Jugovac, Matteo. Università degli Studi di Trieste; Italia. Helmholtz Gemeinschaft. Forschungszentrum Jülich; Alemania. The Abdus Salam. International Centre for Theoretical Physics; Italia Fil: Patera, Laerte L.. Helmholtz Gemeinschaft. Forschungszentrum Jülich; Alemania. Vienna University of Technology; Austria Fil: Sala, Alessandro. Università degli Studi di Trieste; Italia Fil: Panighel, Mirco. Helmholtz Gemeinschaft. Forschungszentrum Jülich; Alemania. Università degli Studi di Trieste; Italia Fil: Cepek, Cinzia. Università degli Studi di Trieste; Italia Fil: Soldano, Germán. Helmholtz Gemeinschaft. Forschungszentrum Jülich; Alemania Fil: Mariscal, Marcelo. Università degli Studi di Trieste; Italia Fil: Peressi, Maria. Universitat Regensburg; Alemania Fil: Comelli, Giovanni. Laboratorio Nazionale Tasc; Fil: Africh, Cristina. Università degli Studi di Trieste; Italia

We report the formation of dimethyl sulfoxide (DMSO) molecular complexes on Au(111) enabled by native gold adatoms unusually linking the molecules via a bonding of ionic nature, yielding a mutual stabilization between molecules and adatom(s). DMSO is a widely used polar, aprotic solvent whose interaction with metal surfaces is not fully understood. By combining X-ray photoelectron spectroscopy, low temperature scanning tunneling microscopy, and density functional theory (DFT) calculations, we show that DMSO molecules form complexes made by up to four molecules arranged with adjacent oxygen terminations. DFT calculations reveal that most of the observed structures are accurately reproduced if, and only if, the negatively charged oxygen terminations are linked by one or two positively charged Au adatoms. A similar behavior was previously observed only in nonstoichiometric organic salt layers, fabricated using linkage alkali atoms and strongly electronegative molecules. These findings suggest a motif for anchoring organic adlayers of polar molecules on metal substrates and also provide nanoscale insight into the interaction of DMSO with gold.

Atomic-scale description of the structure of graphene edges on Ni(111), both during and post growth, is obtained by scanning tunneling microscopy (STM) in combination with density functional theory (DFT). During growth, at 470 °C, fast STM images (250 ms/image) evidence graphene flakes anchored to the substrate, with the edges exhibiting zigzag or Klein structure depending on the orientation. If growth is frozen, the flake edges hydrogenate and detach from the substrate, with hydrogen reconstructing the Klein edges.

The adsorption of CO on Pt nanoclusters grown in a regular array on a template provided by the graphene/Ir(111) Moire was investigated by means of infrared-visible sum frequency generation vibronic spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy from ultrahigh vacuum to near-ambient pressure, and ab initio simulations. Both terminally and bridge bonded CO species populate nonequivalent sites of the clusters, spanning from first to second-layer terraces to borders and edges, depending on the particle size and morphology and on the adsorption conditions. By combining experimental information and the results of the simulations, we observe a significant restructuring of the clusters. Additionally, above room temperature and at 0.1 mbar, Pt clusters catalyze the spillover of CO to the underlying graphene/Ir(111) interface.

Finite-size effects of nanometric metallic particles can be exploited to obtain tailored and novel electronic and chemical properties. At variance with traditional metal-dispersed supported heterogeneous catalysts widely employed for today's applicative purposes, a thorough control of size, shape, and alloying, together with a careful choice of the templating support, may yield to a bottom-up approach to the design of novel materials. However, challenges show up when dealing with realistic systems. Indeed, proper recipes to grow well-ordered arrays of equally sized supported clusters are not straightforward, even if self-assembly and seeding strategies are adopted or if cluster sources are employed. In addition, with respect to model ultrahigh-vacuum conditions under which these systems are generally investigated, realistic working conditions require thermal stability, in order to avoid sintering effects, while interaction with an ambient pressure gas phase may influence the shape, composition, and chemical state of the nanoclusters yielding restructuring, poisoning, and deactivation. In this essay, we review up-to-date experimental and theoretical approaches to investigate these issues, providing examples of selected systems, and outline a perspective direction to progressively close the material and pressure gaps for the effective transferability of modern surface science modeling to potentially applicative conditions.

The adsorption energy of carbon monoxide on Ni ad-islands and ultra-thin films grown on the Cu(110) surface can be finely tuned via a complex interplay among diffusion, site change mechanisms, and coverage effects. The observed features of CO desorption can be explained in terms of migration of CO molecules from Cu to Ni islands, competition between bridge and on-top adsorption sites, and repulsive lateral adsorbate-adsorbate interactions. While the CO adsorption energy on clean Cu(110) is of the order of 0.5 eV, Ni-alloying allows for its controlled, continuous tunability in the 0.98-1.15 eV range with Ni coverage. Since CO is a fundamental reactant and intermediate in many heterogeneous catalytic (electro)-conversion reactions, insight into these aspects with atomic level detail provides useful information to potentially drive applicative developments. The tunability range of the CO adsorption energy that we measure is compatible with the already observed tuning of conversion rates by Ni doping of Cu single crystal catalysts for methanol synthesis from a CO2, CO, and H2 stream under ambient pressure conditions.

The mechanisms of seeding and nucleation of Cu nanoclusters onto an ultrathin alumina template supported on Ni3Al(111) has been investigated by means of ab initio calculations. Single Cu ad-atom diffusion on the oxide film is effective at room temperature, allowing preferential occupation of the defective sites of the so-called ‘‘dot’’ structure, where the adsorption is much stronger than in the ‘‘network’’ or any other surface site of the oxide. After the adsorption of the first Cu atom, further nucleation at the ‘‘dot’’ sites proceeds with the formation of multi-atomic seeds (with up to 6 atoms contained in the defect) that offer stiff anchoring for larger clusters. The whole process is thermodynamically favoured. We therefore clearly confirm and rationalize some experimental evidence showing that the ultrathin Al2O3/Ni3Al(111) is an efficient template for the growth of highly ordered arrays of small Cu nanoparticles.

Through a combined scanning tunneling microscopy (STM) and density functional theory (DFT) approach, we provide a full characterization of the different chemisorbed configurations of epitaxial graphene coexisting on the Ni(111) single crystal surface. Top-fcc, top-hcp, and top-bridge are found to be stable structures with comparable adsorption energy. By comparison of experiments and simulations, we solve an existing debate, unambiguously distinguishing these configurations in high-resolution STM images and characterizing the transitions between adjacent domains. Such transitions, described in detail through atomistic models, occur not only via sharp domain boundaries, with extended defects, but predominantly via smooth in-plane distortions of the carbon network, without disruption of the hexagonal rings, which are expected not to significantly affect electron transport.

We report the observation of spontaneous chiral symmetry breaking within ordered, racemic monolayers of theophylline, manifesting itself as extended, nanoscale unichiral stripes at the interface between molecular domains. Theophylline is a xanthine derivative playing an important role in several biochemical processes. Molecular chirality is induced by adsorption on the Au(111) surface, resulting in extended domains with two different racemic, ordered structures, coexisting with a disordered phase. By combining low-temperature scanning tunneling microscopy (LT-STM) and ab initio density functional theory calculations, we first provide a detailed picture of the interactions within the ordered assemblies, and we uncover the origin of the distinct contrast features in STM images. Secondly, experiments reveal the existence of nanoscale stripes of unichiral molecules separating racemic domains of one of the two ordered phases, giving rise to a local enantiomeric imbalance. Systematic theoretical investigation of their structure and chiral composition confirm their unichirality, with the specific handedness related to the registry between the two ordered domains facing the stripes. These findings can open the way to new insights into the elusive mechanisms leading to local chiral imbalances in racemic systems, possibly at the origin of biomolecular homochirality, as well as suggest novel approaches for stereoselective heterogeneous catalysis.

Mn5Ge3 thin films epitaxially grown on Ge(111) exhibit metallic conductivity and strong ferromagnetism up to about 300 K. Recent experiments suggest a non-collinear spin structure. In order to gain deep insights into the magnetic structure of this compound, we have performed fully unconstrained ab-initio pseudopotential calculations within density functional theory, investigating the different magnetic states corresponding to Collinear (C) and Non-Collinear (NC) spin configurations. We focus on their relative stability under pressure and strain field. Under pressure, the C and NC configurations are degenerate, suggesting the possible occurrence of accidental magnetic degeneracy also in Mn5Ge3 real samples. We found a continuous transition from a ferromagnetic C low-spin state at small volumes to a NC high-spin state at higher volumes. Remarkably, the degeneracy is definitely removed under the effect of uniaxial strain: in particular, NC spin configurations is favoured under tensile uniaxial strain.

The adsorption and the nucleation of different transition metals (Fe, Co, Ni, Cu, Pd, Ag, and Au) on alumina/Ni3Al(111) have been studied to shed light on the first stages of the synthesis of supported nanoparticles, focusing in particular on the possibility of producing ordered arrays. Affinity for oxygen, atomic radii, electronic properties and kinetics have been taken into account to rationalize the different behavior. In agreement with empirical findings, Pd is confirmed to be the best choice for a highly ordered nucleation following the "dot" superstructure of the alumina, due to a remarkable preference for the corresponding adsorption sites (holes) with respect to others, and for a rather strong binding. Atom by atom nucleation of this material has been studied, for seeds up to 6 atoms that offer a stiff anchoring of nanoparticles to the support.

Correction for ‘Study of structures and thermodynamics of CuNi nanoalloys using a new DFT-fitted atomistic potential’ by Emanuele Panizon et al., Phys. Chem. Chem. Phys., 2015, DOI: 10.1039/c5cp00215j.

The interaction of carbon monoxide with an ordered array of copper nanoclusters was investigated under ultrahigh vacuum conditions by means of in situ X-ray photoelectron spectroscopy in combination with density functional theory calculations. The Cu clusters were supported on an alumina template grown on the Ni3Al(111) termination. Adsorption and dissociation of carbon monoxide occur at the copper clusters, yielding accumulation of carbidic carbon at the metal particles through the Boudouard process. The involved mechanisms are investigated at the atomic level, unveiling the effects of cluster finite size, reconstruction, support, and of local CO coverage. It is found that the high coverage of CO at the cluster surface, which considerably exceeds that achievable on single crystal surfaces, facilitates the metal restructuring and the reaction, yielding carbon incorporation into the bulk of the particles.

The adsorption and the nucleation of different transition metals (Fe, Co, Ni, Cu, Pd, Ag, and Au) on alumina/Ni3Al(111) have been studied to shed light on the first stages of the synthesis of supported nanoparticles, focusing in particular on the possibility of producing ordered arrays. Affinity for oxygen, atomic radii, electronic properties and kinetics have been taken into account to rationalize the different behavior. In agreement with empirical findings, Pd is confirmed to be the best choice for a highly ordered nucleation following the “dot” superstructure of the alumina, due to a remarkable preference for the corresponding adsorption sites (holes) with respect to others, and for a rather strong binding. Atom by atom nucleation of this material has been studied, for seeds up to 6 atoms that offer a stiff anchoring of nanoparticles to the support.

Shape, stability and chemical ordering patterns of CuNi nanoalloys are studied as a function of size, composition and temperature. A new parametrization of an atomistic potential for CuNi is developed on the basis of ab initio calculations. The potential is validated against experimental bulk properties, and ab initio results for nanoalloys of sizes up to 147 atoms and for surface alloys. The potential is used to determine the chemical ordering patterns of nanoparticles with diameters of up to 3 nm and different structural motifs (decahedra, truncated octahedra and icosahedra), both in the ground state and in a wide range of temperatures. The results show that the two elements do not intermix in the ground state, but there is a disordering towards solid–solution patterns in the core starting from room temperature. This order–disorder transition presents different characteristics in the icosahedral, decahedral and fcc nanoalloys.

Mn 5 Ge 3 thin films epitaxially grown on Ge(1 1 1) exhibit metallic conductivity and strong ferromagnetism up to about 300 K. Recent experiments suggest a non-collinear (NC) spin structure. In order to gain deep insights into the magnetic structure of this compound, we have performed fully unconstrained ab initio pseudopotential calculations within density functional theory, investigating the different magnetic states corresponding to collinear (C) and NC spin configurations. We focus on their relative stability under pressure and strain field. Under pressure, the C and NC configurations are degenerate, suggesting the possible occurrence of accidental magnetic degeneracy also in Mn 5 Ge 3 real samples. We found a continuous transition from a ferromagnetic C low-spin state at small volumes to a NC high-spin state at higher volumes. Remarkably, the degeneracy is definitely removed under the effect of uniaxial strain: in particular, NC spin configuration is favoured under tensile uniaxial strain.

We study different configurations of the (110) cross-sectional surface of Si-doped GaAs, from the isolated Si donor up to an entire donor-acceptor Si bilayer embedded along the (001) growth direction. Electronic potentials, density of electronic states, cross-sectional scanning tunneling microscopy (XSTM) images are calculated using first-principles numerical simulations. Doping configurations with compensating Si impurities in cationic and anionic sites, such as the donor-acceptor bilayer, are characterized by XSTM images with bright signal at negative bias, strongly attenuated when the bias is reversed. These features are characteristic of real samples above the onset of self-compensation. The comparison of the experimental images with the numerical simulations allows to shed light on the microscopic picture of self compensation hitherto associated to a variety of mechanisms - including the formation of complexes of Si with native defects - and to uniquely attribute the observed experimental features to Si donor-acceptor configurations. (c) 2005 Elsevier B.V All rights reserved.

The role of average composition and local compositional fluctuations on the structural properties of Ga1−yInyAs1−xNx quaternary alloys and related ternary alloys was investigated by means of accurate density functional pseudopotential calculations. For the quaternary alloy, the authors focus particularly on the case with x=0.5 and y=0.5. They study the different anion–cation bond length distributions and the macroscopic average structural parameters.

We determined the properties of Zn1−xMgxSe semiconductor alloys through a combination of optical and photoelectron spectroscopy, x-ray diffraction, Rutherford backscattering spectrometry, and ab initio pseudopotential calculations. The complementary character of the techniques and the good agreement between calculated and experimental trends allowed us to explain some of the discrepancies between the reported properties of these wide band gap alloys.

We present first-principles calculations of NiMnSb/GaAs(001) junction within the framework of density functional theory (DFT) by using the plane wave pseudopotential method. After optimization of the atomic positions, we have investigated the main electronic and magnetic properties. We extract the band alignments for the majority and the minority spin channels. We found that the half-metallicity, which characterizes the NiMnSb in its bulk and epitaxial phase, is lost at the junction.

The MnAs compound is presently of increasing interest for spintronic applications. By means of accurate first-principles pseudopotential calculations, we investigate its structural, electronic and magnetic properties under different strain/stress conditions, starting from its stable bulk free standingphase which has the hexagonal structure of NiAs. The other structures considered here simulate the pseudomorphic growth on substrates with different orientation and lattice mismatch. The effects of stress on the magnetic properties are linear over a limited range, which is typically accessible to measurements, but sizeable and strongly nonlinear for higher stress, suggesting a magnetic phase transition.

We study the physical properties of MnAs under strain by using accurate first-principles pseudopotential calculations. Our results provide new insight on the physics of strained multilayer that are grown epitaxially on different lattice mismatched substrates and which are presently of interest for spintronic applications. We compute the strain dependence of the structural parameters, electronic bands, density of states and magnetization. In the region of strain/stress that is easily directly accessible to measurements, the effects on these physical quantities are linear. We also address the case of uniaxial stress inducing sizeable and strongly non linear effects on electronic and magnetic properties.

We investigate the electronic structure of over-coordinated defects in amorphous silicon via density-functional total-energy calculations, with the aim of understanding the relationship between topological and electronic properties on a microscopic scale. Maximally localized Wannier functions (MLWF)are computed in order to characterize the bonding and the electronic properties of these defects. The five-fold coordination defects give rise to delocalized states extending over several nearest neighbors (NNs), and therefore to very polarizable bonds and anomalously high Born effective charges for the defective atoms.

We present ab-initio studies of band offsets, formation energy, and stability of (100) heterojunctions between (Zn,Be)(Se,Te) zincblende compounds, and in particular of the lattice-matched BeTe/ZnSe interface. Equal band offsets are found at Be/Se and Zn/Te abrupt interfaces, as well as at mixed interfaces, in agreement with the established understanding of band offsets at isovalent heterojunctions. Thermodynamical arguments suggest that islands of non-nominal composition may form at the interface, causing offset variations over about 0.8 eV depending on growth conditions. Our findings reconcile recent experiments on BeTe/ZnSe with the accepted theoretical description., RevTeX 5 pages, 3 embedded figures

We study by means of the first-principles pseudopotential method the coordination defects in amorphous silicon and hydrogenated amorphous silicon, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ‘electron localization function’ allows us to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly threefold (T3) and fivefold (T5) coordinated defects. We found that electronic states in the gap can be associated with both kinds of defect and that in both cases the interaction with hydrogen can reduce the density of states in the gap.

Understanding the mechanisms which determine the band offsets and Schottky barriers at semiconductor contacts and engineering them for specific device applications are important theoretical and technological challenges. In this review, we present a theoretical approach to the band-line-up problem and discuss its application to prototypical systems. The emphasis is on ab initio computations and on theoretical models derived from first-principles numerical experiments. An approach based on linear-response-theory concepts allows a general description of the band alignment for various classes of semiconductor contacts and predicts the effects of various bulk and interfacial perturbations on the band discontinuities.