Perimeter Atoms Research Articles

PorphyStruct: A Digital Tool for the Quantitative Assignment of Non-Planar Distortion Modes in Four-Membered Porphyrinoids.

PorphyStruct, a new digital tool for the analysis of non‐planar distortion modes of different porphyrinoids, and its application to corrole structures is reported. The program makes use of the normal‐coordinate structure decomposition technique (NSD) and employs sets of normal modes equivalent to those established for porphyrins in order to describe the out‐of‐plane dislocation pattern of perimeter atoms from corroles, norcorroles, porphycenes and other porphyrinoids quantitatively and in analogy to the established terminology. A comparative study of 17 porphyrin structures shows very similar results to the original NSD analysis and no systematic error. Application to corroles is successful and reveals the necessity to implement an extended basis of normal modes for a large share of experimental structures. The results frequently show the concomitant occurence of several modes but remain interpretable. For group XI metal corroles the phenomenon of supersaddling was unravelled, allowing for more in‐depths discussions of structure‐function correlations.

Hubbard model analysis of the double photoionization of aromatic hydrocarbons

Using the one-dimensional Hubbard model as a guide, the peaks in the double photoionization in a variety of aromatic hydrocarbons and related molecules are analyzed. In all the aromatic hydrocarbons that have been studied, there are peaks in the 2[Formula: see text] ion concentration that are 40 eV above threshold. These peaks arise from the emission of electrons from a Hubbard state associated with carbon atoms on the perimeter of the molecule. Pyrene and coronene also have low energy peaks 10 eV above threshold that are associated with the carbon atoms inside the perimeter. The linear behavior of the 2[Formula: see text] ion concentration above the two-electron ionization energy that is found in organic molecules lacking periodic symmetry in their perimeter atoms is associated with the simultaneous emission of two photoelectrons with approximately equal kinetic energies moving in opposite directions.

Atomistic simulation of finite-temperature magnetism of nanoparticles: Application to cobalt clusters on Au(111)

We developed a technique to determine suitable spin models for small embedded clusters of arbitrary geometry by combining the spin-cluster expansion with the relativistic disordered local moment scheme. We present results for uncovered and covered hexagonal Co clusters on Au(111) surface, and use classical Monte Carlo simulations to study the temperature dependent properties of the systems. To test the new method we compare the calculated spin-model parameters of the uncovered clusters with those of a Co monolayer deposited on Au(111). In general, the isotropic and DM interactions are larger between atoms at the perimeter than at the center of the clusters. For Co clusters covered by Au, both the contribution to the magnetic anisotropy and the easy axis direction of the perimeter atoms differ from those of the inner atoms due to reduced symmetry. We investigate the spin reversals of the covered clusters with perpendicular magnetic anisotropy and by measuring the magnetization variance in the easy direction we suggest a technique to determine the blocking temperature of superparamagnetic particles. We also determine the N\'eel relaxation time from the Monte Carlo simulations and find that it satisfies the N\'eel--Arrhenius law with an energy barrier close to the magnetic anisotropy energy of the clusters.

DFT/QTAIM analysis of the effect of late transition metal doping on methane selectivity in Fischer–Tröpsch catalysis

The effect of late transition metal substitution into Fe(100), Ni(111), and Co(0001) surface analogs has been investigated density functional theory (DFT) methods. The surface was modeled using a 7-atom cluster, with perimeter atoms saturated with hydrogen atoms to approximate surface coordination and mitigate dangling bond artifacts. All calculations were performed at the B3PW91 level of theory with the LANL2DZ basis. Eight surface adsorbates were studied: C, CH, CH2, and CH3 represented the hydrogenating steps on surface carbide, while C+CH, CH+CH, C+CH3, and CH2+CH2 represented four competitive coupling pathways. A review of the effect of Cu, Ag, Au, and Pd on the reaction energies and barriers associated with these critical steps is discussed. QTAIM is employed to develop a picture of the electronic environment associated with methane selectivity. Attention is focused on the charge trends for the involved surface atoms and coupling species. Our results suggest that promising candidates for the reduction of FT methane selectivity include Au and Pd on Ni, Au and Ag on Co, and Cu, Ag, and Pd on Fe.

Magnetization reversal mechanism of ramified and compact Co islands on Pt(111)

We report on the magnetization reversal mechanism of Co islands on Pt(111) as a function of their size and shape. We measure the zero-field susceptibility chi(T) and low-temperature magnetization curves M(H) with in situ magneto-optical Kerr effect. Together with the island morphology deduced from scanning tunneling microscopy, this creates sufficient information to determine both the magnetization reversal mechanism and the distribution of anisotropy energies between perimeter and surface atoms. We find a transition from quasicoherent rotation to domain wall nucleation and propagation with a critical size of 350 atoms for ramified, and of 600 atoms for compact islands.

Unconventional magnetic properties in transverse Ising nanoislands: Effects of interlayer coupling

The phase diagram and magnetizations in two transverse Ising nanoislands with different (hexagonal and square) structures which are consisted of the two layers are examined by using the effective-field theory with correlations. The effects of interlayer coupling and two transverse fields at the center and the perimeter atoms on them are examined. We present some characteristic phenomena in the magnetic properties which come from the frustration induced by them.

Remarkable features of magnetic properties in transverse Ising nanoislands

Abstract The phase diagram and magnetizations in two transverse Ising nanoislands with the same structures which are consisted of the two layers are examined by using the effective-field theory with correlations. The effects of interlayer coupling and two transverse fields at the center and the perimeter atoms on them are examined. We find remarkable features in them which come from the frustration induced by the interlayer coupling and two transverse fields.

Catalytic oxidation of cyclohexane by size-selected palladium clusters pinned on graphite

We report the catalytic oxidation of cyclohexane to CO and CO2 over size-selected palladium clusters (Pd N clusters, N = 10–120) supported on graphite as a function of cluster size. The stability of the pinned clusters (nanoparticles) under reaction conditions is investigated by scanning tunnelling microscopy measurement both before and after reaction. Temperature-programmed reaction experiments at 800 Torr show that the turnover rates (per surface Pd atom) for both CO and CO2 increase significantly as cluster size decreases and correlate with the number of Pd perimeter atoms at the graphite interface. Under oxygen-rich conditions, the activity of the clusters increases by a factor of 3 while the product ratio CO:CO2 rises by an order of magnitude.

Edge atoms effects on the perpendicular anisotropy of ultrathin magnetic layers

The present work reports experimental and theoretical results for electrodeposited Co/Au(111) ultrathin layers with very specific magnetic behavior. We show that the observed two peaks in the out-of-plane magnetization versus deposition time variation could be explained by the remarkably high perpendicular anisotropy of the perimeter atoms of low-dimensional islands formed during the layer-by-layer growth, as compared to that of the surface atoms. Our results indicate that it is possible to sustain high anisotropy in very small grains without coming across the superparamagnetic limit, opening excellent opportunities for materials engineering.

Stochastic coarsening model for Pb islands on a Si(111) surface

The coarsening behavior of individual Pb islands on Si(111) surface has been studied by scanning tunneling microscopy. Traditionally island decay follows a smooth power-law dependence on the time until disappearance. In Pb/Si(111), some unstable islands are inactive for a long time but once their decay is triggered they suffer a ``sudden death.'' Four-layer islands are found to decay rapidly, increasing the area covered by seven-layer islands. All islands, decaying or otherwise, are accompanied by island size fluctuation which involve a large number of perimeter atoms moving collectively as a ``quantized'' unit. A stochastic model is developed to elucidate the mechanism behind this coarsening behavior of Pb islands. The distinct evolution of the islands with different heights is correctly predicted, and the size fluctuations of islands and the sudden death behavior observed in island coarsening are also recovered. The key ingredients are incorporation of accurate non-Gaussian statistics of the size fluctuations and also accounting for size changes in large quantized bursts.

Characterizing low-coordinated atoms at the periphery of MgO-supported Au islands using scanning tunneling microscopy and electronic structure calculations

The perimeter of oxide-supported metal particles is suggested to be of pivotal importance for various catalytic processes. To elucidate the underlying effects, the electronic properties of edge and corner atoms of planar Au clusters on MgO/Ag(001) thin films have been analyzed with scanning tunneling microscopy and electronic structure calculations. The low-coordinated perimeter atoms are characterized by a high density of $s$-derived states at the Fermi level. Those states accommodate transfer electrons from the MgO/Ag substrate, which render the perimeter atoms negatively charged. In contrast, the inner atoms of the island are not affected by the charge transfer and remain neutral. This combination of charge accumulation and high state-density explains the specific relevance of the cluster perimeter in adsorption and reaction processes.

Au/ZnO as catalyst for methanol synthesis: The role of oxygen vacancies

Gold catalysts supported on zinc oxide with Au loadings of 1, 2, and 3 wt% were prepared by the colloidal deposition method and applied in methanol synthesis in CO 2-free (CO + H 2) and in CO 2-containing (CO + CO 2 + H 2) synthesis gas. The characterization by transmission electron microscopy and X-ray diffraction before and after the catalytic high-pressure tests demonstrated a very narrow and uniform Au particle size distribution and a high stability against sintering. Reactive frontal chromatography (RFC) experiments with N 2O were performed aiming at the titration of oxygen vacancies. With increasing Au loading, the amount of consumed N 2O increased in good correlation with the number of Au perimeter atoms present in the Au/ZnO catalysts suggesting an enhanced formation of oxygen vacancies at the Au/ZnO interface. In both synthesis gas mixtures the presence of the Au particles led to an increased activity compared with pure ZnO. All Au/ZnO samples exhibited higher catalytic activity in the absence of CO 2, as had been observed for pure ZnO with similar apparent activation energy. It is concluded that oxygen vacancies in ZnO are also the active sites in methanol synthesis over Au/ZnO, and that the presence of the Au particles enhances the number of exposed oxygen vacancies in ZnO, presumably located at the interface region.

Geometry and bonding in the ground and lowest triplet state of D6h symmetric crenellated edged C 6[3m(m−1)+1]H 6(2m−1) ( m = 2, … , 6) graphene hydrocarbon molecules

Ab initio plane wave all valence electron based DFT calculations were used to explore the dichotomy of perimeter vs. interior in the electronic and geometric structure of the D 6 h singlet ground state and D 2 h lowest triplet state of planar graphene hydrocarbon molecules with crenellated (arm chair) edges and the general formula C 6[3 m ( m −1)+1] H 6(2 m −1) where m = 2, … , 6. The largest molecule C 546H 66 was 4.78 nm across and contained 2250 valence electrons. These molecules are nominally “fully benzenoid hydrocarbons”. However with increasing size, the core of central atoms abandoned any fully benzenoid geometry they had in small systems and organized into single layer graphite (graphene) structure. The perimeter atoms of the crenellation adopted a conjugated geometry with unequal bonds and between core and perimeter there were some C 6 rings retaining remnants of aromatic sextet-type properties. Compared to a zigzag edge the crenellated edge conferred stability in all the systems studied as measured by the singlet homo–lumo level gap BG 0 and the singlet-lowest triplet energy gap Δ E ST. For the largest crenellated system ( m = 6) BG 0 and Δ E ST were approximately 0.7 eV, larger in value than for similarly sized hexagonal graphenes with zigzag edges. Triplet states were identified for all the molecules in the series and in the case of the m = 2 molecule hexabenzocoronene C 42H 18, two conformations with D 2 h symmetry were identified and compared to features on the triplet state potential energy surface of benzene.

Geometry, bonding and magnetism in planar triangulene graphene molecules with D3h symmetry: Zigzag [formula omitted] ( m = 2, …, 15)

Ab initio plane wave based all valence electron DFT calculations with geometry optimization are reported for the electronic structure of planar zigzag edged triangular shaped graphene molecules C m ∗ ∗ 2 + 4 m + 1 H 3 m + 3 where the zigzag ring number m = 2, …, 15. The largest molecule C 286H 48 has a 3.8 nm side length and retains D 3 h symmetric geometry. The zone in the middle of the molecules, where the geometry and electronic properties resemble infinite single sheet graphite (graphene), expands with increasing ring number m, driving deviations in geometry, charge and spin to the perimeter. If a molecule is viewed as a set of nested triangular rings of carbon, then the zone where the lattice resembles an infinite sheet of graphene with CC = 142 pm, extends to the middle of the penultimate ring. The radial bonds joining the perimeter carbon atoms to the interior are long CC = 144 pm, except near the three apexes where the bonds are shorter. Isometric surfaces of the total charge density show that the two bonds joined at the apex have the highest valence charge. The perimeter CC bonds establish a simple pattern as the zigzag number increases, which shares some features with the zigzag edges in the D 2 h linear acenes C 4 m +2H 2 m +4 and the D 6 h hexangulenes C 6 m ∗ ∗ 2 H 6 m but not the D 6 h symmetric annulenes ( CH ) ( 2 m - 1 ) ∗ 6 . The two CC bonds forming each apex are short (≈139 pm), next comes one long bond CC ≈ 142 pm and a middle region where all the CC bonds have length ≈141 pm. The homo–lumo gap declines from 0.53 eV at m = 2 to approximately 0.29 V at m = 15, the latter being larger than found for linear or hexagonal shaped graphenes with comparable edge lengths. Across the molecule the charge on the carbon atoms undergoes a small oscillation following the bipartite lattice. The magnitude of the charge in the same nested triangle decreases monotonically with the distance of the row from the center of the molecule. These systems are predicted to have spin polarized ground states with S = ½( m − 1), in accord with the theorems of Lieb for a bipartite lattice with unequal numbers of sub-lattice carbon atoms. The magnitude of the spin on the atoms increases monotonically from the center to the edges, this effect being greatest on the majority A-sub lattice atoms. The spins are delocalized, not confined to specific atoms as might result in geometries stabilized by islands of aromatic resonance. In the largest systems the magnetic non-bonding levels (NBL) occur as a narrowly distributed set of homos close to the Fermi level, separated from the lower lying valence bond manifold by a gap of about 1 eV. The NBL are a set of disjoint radical orbitals having charge only on atoms belonging to the A-lattice and this charge is concentrated on the perimeter and penultimate row atoms.

Electron correlation effects in Co nanoscale islands on a nitrogen-covered Cu(001) surface

The electronic states of cobalt nanoislands on a nitrogen-saturated Cu(001) $c(2\ifmmode\times\else\texttimes\fi{}2)\text{-N}$ surface have been investigated by means of x-ray absorption spectroscopy. In the case of low coverages, when the surface is covered by small Co islands, a shoulder structure in the $\text{Co}\text{ }L$-edge absorption spectrum is observable at an energy about 3 eV higher than the absorption edge. By considering the unoccupied $\text{Co}\text{ }3d$ state calculated by first-principles method, x-ray photoelectron spectra and the multiple scattering during the absorption process, the shoulder is attributed to the correlation among $3d$ electrons. The satellite intensity decreases with the increase in the Co island size. The perimeter Co atoms of the island are assigned as the origins of the correlation satellite by comparing its intensity with the morphology of the islands observed using scanning tunneling microscopy. The polarization dependent satellite intensity suggests that the electron correlation in the in-plane $3d$ orbital of the perimeter atoms is larger than that in the out-of-plane one.

Bonding and Magnetism in High Symmetry Nano-Sized Graphene Molecules: Linear Acenes C<SUB>4<I>m</I>+2</SUB>H<SUB>2<I>m</I>+4</SUB> (<I>m</I>=2,…25); Zigzag Hexangulenes C<SUB>6<I>m</I><SUP>**</SUP>2</SUB>H<SUB>6<I>m</I></SUB> (<I>m</I>=2,…10); Crenelated Hexangulenes C<SUB>6(3<I>m</I><SUP>**</SUP>2−3<I>m</I>+1)</SUB>H<SUB>6(2<I>m</I>−1)</SUB> (<I>m</I>=2,…6); Zigzag Triangulenes C<SUB><I>m</I><SUP>**</SUP>2+4<I>m</I>+1</SUB>H<SUB>6<I>m</I></SUB> (<I>m</I>=2,…15)

Plane wave based ab initio DTF all valence electron spin polarized calculations are reported for the electronic structure and geometry of differently shaped graphene molecules. These polycyclic hydrocarbon molecules comprise four series: D 2h symmetric linear polyacenes C 4m+2 H 2m+4 (m = 2,,..,25); D 6h hexagulenes with zigzag edges C 6m**2 H 6m (m = 2,..., 10); D6h hexagulenes with crenelated edges C 6(3m**2-3m+1) H 6(2m-1) (m = 2,..., 6); D 3h zigzag edged triangulenes C m**2+4m+1 H 6m (m = 2,... 15). The systems variously display ground states that are spin paired singlet (S = 0), singlet anti-ferromagnetically ordered diradical and spin polarized S = 1/2(m - 1). Molecules with zigzag edges show evidence of electron delocalization along the perimeters with some bond alternation at the comers. In the acenes the spin paired singlet state of the short members switches to a singlet diradical at m ≈ 7-8 and this remains as the ground state for larger m. In contrast the triangulenes are magnetic and the atomic charge and spin density changes monotonically with distance from center to perimeter. In the hexagonal systems the development of a graphene core region, where the C-C bonds are 142pm, extends to within a few C-C bonds of the perimeter atoms. Zigzag hexagulenes have a spin paired singlet ground state for m ≤ 8 and a singlet diradical ground state for larger m. When hexagulenes are substitutionally doped with boron or nitrogen, acceptor or donor levels appear that track the valence or conduction band edges with increase in zigzag number. This result suggests the possibility of building several semiconductor device structures into the same graphene molecule.

Magnetic anisotropy from single atoms to large monodomain islands of Co/Pt(111)

By studying the magnetic behavior of self-assembled Co islands on a single-crystal metal surface, Pt(111), we show how the magnetic anisotropy evolves from isolated atoms to monolayer islands and films. Single Co adatoms are found to have a giant magnetocrystalline anisotropy energy of E a = 9.3 ± 1.6 meV / atom arising from the combination of partly preserved orbital moments ( m L = 1.1 μ B ) and strong spin-orbit coupling induced by the Pt substrate. Combined scanning tunneling microscopy and X-ray magnetic circular dichroism experiments performed for differently sized small two-dimensional Co islands establish a clear connection between E a and m L , both quantities decrease sharply with the lateral coordination of the magnetic atoms. In accordance with this, Kerr magnetometry experiments, again performed in conjunction with scanning tunneling microscopy, reveal that the anisotropy energy of large two-dimensional Co islands is almost entirely determined by the relatively low number of perimeter atoms, having E a = 0.9 ± 0.1 meV / atom . These results confirm theoretical predictions and are of fundamental value the understanding of how the magnetic anisotropy develops in finite-size magnetic particles. Identification of the role of perimeter and surface atoms opens up new opportunities for engineering the anisotropy and the moment of a magnetic nanostructure. To cite this article: P. Gambardella et al., C. R. Physique 6 (2005).

Atomic view of cluster diffusion on metal surfaces

Field ion microscope measurements show a strong correlation between the mobility and shape of small clusters on fcc(100) metal surfaces. For self-diffusion on Rh(100) this correlation leads to an oscillatory behavior in the activation energy of surface diffusion as a function of cluster size. Comparison of the measured activation energies to previous theoretical calculations indicate that the mechanism of cluster diffusion involves individual displacements of edge atoms (i.e., perimeter diffusion). The rate-determining step in the migration of clusters is the partial detachment of one of the perimeter atoms. The relative ease of adatom motion along straight edges of stationary clusters also permits measurements of diffusion barriers at steps — measurements useful in the interpretation of fractal vs. compact island growth on fcc metal surfaces.

Evidence for oxygen-island formation on Al(111): Cluster-model theory and x-ray photoelectron spectroscopy.

The O 1s x-ray photoelectron spectroscopy spectrum for Al(111)/O at 300 K shows two components whose behavior as a function of time and variation of detection angle are consistent with either (a) a surface species represented by the higher binding-energy (BE) component and a subsurface species represented by the lower BE component, or (b) small close-packed oxygen islands with the interior atoms represented by the lower BE component and the perimeter atoms by the higher BE component. We have modeled both situations using ab initio Hartree-Fock wave functions for clusters of Al and O atoms. For an O atom in a threefold site, it was found that a below-surface position gave a higher O 1s BE than an above-surface position, incompatible with interpretation (a). This change in the O 1s BE could arise because the bond for O to Al may have a more covalent character when the O is below the surface than when it is above the surface. We present evidence consistent with this view. An O adatom island with all the O atoms in threefold sites gives calculated O 1s BE's which are significantly higher for the perimeter O atoms. Further, the results for an isolated O island without the Al substrate present also give higher BE's for the perimeter atoms. Both these results are consistent with interpretation (b). Published scanning-tunneling-microscopy data supports the suggestion that the chemisorbed state consists of small, close-packed islands, whereas the presence of two vibrational modes in high-resolution electron-energy-loss spectroscopy data has been interpreted as representing surface and subsurface oxygen atoms. In light of the present results, we suggest that a vibrational interpretation in terms of interior and perimeter adatoms should be considered.

X‐ray photoelectron spectroscopy of oxygen adsorbates on Al(111): Theory experiment

We present the result of polar angle resolved x‐ray photoemission spectroscopy on Al(111)/O and cluster calculations of the O(1s) binding energy (BE) for various model situations. In the experimental data two O(1s) peaks are observed, separated by 1.3 eV. The angular behavior (depth‐resolution) could indicate that the lower BE peak is associated with an O atom under the surface, and the higher BE peak with an O atom above the surface. Equally, it could indicate oxygen islands on the surface where the perimeter atoms have a higher O(1s) BE than the interior atoms. The cluster calculations show that the former interpretation cannot be correct, since an O ads below the surface has a higher calculated O(1s) BE than one above. Cluster calculations simulating oxygen islands are, however, consistent with the experimental data.