Single adatoms are expected to participate in many processes occurring at solid surfaces, such as the growth of graphene on metals. We demonstrate, both experimentally and theoretically, the catalytic role played by single metal adatoms during the technologically relevant process of graphene growth on nickel (Ni). The catalytic action of individual Ni atoms at the edges of a growing graphene flake was directly captured by scanning tunneling microscopy imaging at the millisecond time scale, while force field molecular dynamics and density functional theory calculations rationalize the experimental observations. Our results unveil the mechanism governing the activity of a single-atom catalyst at work.

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.

The layered oxide Na 2 Zn 2 TeO 6 is a fast Na + ion conductor and a suitable candidate for application as a solid-state electrolyte. We present a detailed study on how synthesis temperature and Na-content affect the crystal structure and thus the Na + ion conductivity of Na 2 Zn 2 TeO 6 . Furthermore, we report for the first time an O′3-type phase for Na 2 Zn 2 TeO 6 . At a synthesis temperature of 900 °C, we obtain a pure P2-type phase, providing peak performance in Na + ion conductivity. Synthesis temperatures lower than 900 °C produce a series of mixed P2 and O′3-type phases. The O′3 structure can only be obtained as a pure phase by substituting Li on the Zn-sites to increase the Na-content. Thorough analysis of synchrotron data combined with computational modeling indicates that Li enters the Zn sites and, consequently, the amount of Na in the structure increases to balance the charge according to the formula Na 2+ x Zn 2– x Li x TeO 6 ( x = 0.2–0.5). Impedance spectroscopy and computational modeling confirm that reducing the amount of the O′3-type phase enhances the Na + ion mobility.

Sodium orthosilicates NaMSiO (M = Mn, Fe, Co and Ni) have attracted much attention due to the possibility of exchanging two electrons per formula unit. They are also found to exhibit great structural stability due to a diamond-like arrangement of tetrahedral groups. In this work, we have systematically studied the possible polymorphism of these compounds by means of density functional theory, optimising the structure of a number of systems with different group symmetries. The ground state is found to be Pc-symmetric for all the considered M = Mn, Fe, Co, Ni, and several similar structures exhibiting different symmetries coexist within a 0.3 eV energy window from this structural minimum. The intercalation/deintercalation potential is calculated for varying transition metal atoms M. Iron sodium orthosilicates, attractive due to the natural abundance of both materials, exhibit a low voltage, which can be enhanced by doping with nickel. The diffusion pathways for Na atoms are discussed, and the relevant barriers are calculated using the nudged elastic band method on top of DFT calculations. Also in this case, nickel impurities would improve the material performances by lowering the barrier heights. Notably, the ionic conductivity is found to be systematically larger with respect to the case of lithium orthosilicates, due to a larger spacing between atomic layers and to the non-directional bonding between Na and the neighbouring atoms. Overall, the great structural stability of the material together with the low barriers for Na diffusion indicates this class of materials as good candidates for modern battery technologies.

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.

In this work, we examine the distribution of Na+ ions in the interlayer of the super-ionic conductor Na2Zn2TeO6 by means of atomistic first-principle modeling based on density functional theory. This layered structure presents a variety of partially occupied interstitial Na sites forming a triangular prismatic coordination group with the neighboring O atoms. We examine the energetics of Na periodic arrangements, considering distinct occupation patterns of these interstitial sites. We then simulate high-temperature annealing of the system by means of ab initio molecular dynamics simulations, finding that the Na sublattice prefers a disordered distributions along the available interstitial sites rather than a honeycomb arrangement conformal to the periodicity of the Zn–Te layers. Suitable supercells are constructed, reproducing the arrangement patterns observed in the most favorable configurations obtained by simulated annealing. We report a structure with rhomboidal reconstruction of the Na sublattice as the most energetically favorable, and note that the associated occupation of the interstitial sites is compatible with experimental data. We also report the tendency for the formation of a pentagon–triangle reconstruction, predicted to become increasingly favorable as the system is compressed.

The layered super-ionic conductor Na2Zn2TeO6 is a promising material for applications as electrolyte in solid-state batteries. Independent of synthesis route and conditions, a significant peak broadening of (1 0 1) is evident in collected X-ray diffraction data, which is not compatible with the standard structure model (space group P6322). In this work, we describe sodium disorder and stacking faults in Na2Zn2TeO6 and identify the nature of the faults that are manifested by this peak broadening. First principle modeling was applied to describe the Na-distribution and support models on stacking faults. We describe the dominant fault as an in-plane shift causing local alignment of Te atoms along the layer stacking direction. This was experimentally verified by first constructing a supercell model of Na2Zn2TeO6 and simulating X-ray diffraction patterns for different densities of stacking faults, to help identifying a structural model that best matches experimental data. The Rietveld and in particular the stacking fault refinements, were found to perfectly describe the crystal structure and also the peak-broadening features. The stacking fault density in Na2Zn2TeO6 is ∼3%. This value appeared not to be susceptible to differences in synthesis conditions, including quenching and long-term annealing, as furthermore confirmed by in-situ X-ray diffraction data at temperatures between 25–600°C.

We present detailed comparisons between the results of embedded atom model (EAM) and density functional theory (DFT) calculations on defected Ni alloy systems. We find that the EAM interatomic potentials reproduce low-temperature structural properties in both the γ and γ′ phases, and yield accurate atomic forces in bulk-like configurations even at temperatures as high as ∼1200 K. However, they fail to describe more complex chemical bonding, in configurations including defects such as vacancies or dislocations, for which we observe significant deviations between the EAM and DFT forces, suggesting that derived properties such as (free) energy barriers to vacancy migration and dislocation glide may also be inaccurate. Testing against full DFT calculations further reveals that these deviations have a local character, and are typically severe only up to the first or second neighbours of the defect. This suggests that a QM/MM approach can be used to accurately reproduce QM observables, fully exploiting the EAM potential efficiency in the MM zone. This approach could be easily extended to ternary systems for which developing a reliable and fully transferable EAM parameterisation would be extremely challenging e.g. Ni alloy model systems with a W or Re-containing QM zone.