We report on the magnetic properties of individual Fe atoms deposited on MgO(100) thin films probed by x-ray magnetic circular dichroism and scanning tunneling spectroscopy. We show that the Fe atoms have strong perpendicular magnetic anisotropy with a zero-field splitting of 14.0 AE 0.3 meV=atom. This is a factor of 10 larger than the interface anisotropy of epitaxial Fe layers on MgO and the largest value reported for Fe atoms adsorbed on surfaces. The interplay between the ligand field at the O adsorption sites and spin-orbit coupling is analyzed by density functional theory and multiplet calculations, providing a comprehensive model of the magnetic properties of Fe atoms in a low-symmetry bonding environment.

We report on the self-assembly of Fe adatoms on a Cu(111) surface that is patterned by a metal-organic honeycomb network, formed by coordination of dicarbonitrile pentaphenyl molecules with Cu adatoms. Fe atoms landing on the metal surface are mobile and steered by the quantum confinement of the surface state electrons towards the center of the network hexagonal cavities. In cavities hosting more than one Fe, preferential interatomic distances are observed. The adatoms in each hexagon aggregate into a single cluster upon gentle annealing. These clusters are again centered in the cavities and their size is discerned by their distinct apparent heights. Self-assembly of adatoms or small clusters at surfaces enables the bottom-up fabrication of well-defined nanoscale structures. Well ordered nanostructure superlattices can be created by the nucleation and growth on template surfaces exhibiting long period adatom binding energy variations, such as equidistant pinning sites [1] or networks of repulsive line defects [2]. Besides surface reconstructions and stress relief patterns of epitaxial thin films, nanoporous metalorganic networks [3,4] are potential candidates for such templates. So far, deposition of metal atoms on the latter systems resulted in the decoration of the organic molecules themselves or of the coordination nodes, but not in equidistant clusters on the substrate [5].An additional source of order can be introduced by the quasi-two-dimensional (2D) electron gas of a surface state mediating long-range adsorbate interactions [6][7][8]. On homogeneous surfaces, they stabilize atomic superlattices [6][7][8][9]. Surface state confinement by static scatterers results in local density of state (LDOS) patterns that influence the adsorbate binding energy. In 1D structures formed by substrate steps or strings of atoms or molecules, this leads to 1D confinement of adsorbed atoms [10][11][12][13]. The surface state LDOS patterns formed in a network of hexagonal molecular cavities have been demonstrated to influence the binding sites of adsorbed CO molecules [14].Here, we demonstrate strong surface state confinement by a metal-organic network, preferred adatom locations due to the LDOS pattern created in each cavity, and aggregation of these atoms to a single cluster per network cavity, thus giving rise to a cluster superlattice with the period of the metal-organic template. Our system is a honeycomb network with % 5 nm period formed by dicarbonitrile pentaphenyl (NCÀPh 5 ÀCN) molecules and Cu atoms on Cu(111), and the steered adatoms are Fe.The Cu(111) substrate has been prepared by Ar þ sputter and annealing cycles. The NCÀPh 5 ÀCN molecules [3] were evaporated from a molecular effusion cell at 230 C.

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We present the results of temperature-dependent self-assembly of dicarbonitrile-pentaphenyl molecules (NC-Ph 5 -CN) on Cu(111). Our low-temperature scanning tunneling microscopy study reveals the formation of metal-organic and purely organic structures, depending on the substrate temperature during deposition (160-300 K), which determines the availability of Cu adatoms at the surface. We use tip functionalization with CO to obtain submolecular resolution and image the coordination atoms, enabling unequivocal identification of metal-coordinated nodes and purely organic ones. Moreover, we discuss the somewhat surprising structure obtained for deposition and measurement at 300 K. C 2015 AIP Publishing LLC. [http://dx

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We have studied Er(trensal) single-ion magnets adsorbed on graphene/Ru(0001), on graphene/Ir(111), and on bare Ru(0001) by scanning tunneling microscopy and X-ray absorption spectroscopy. On graphene, the molecules self-assemble into dense and well-ordered islands with their magnetic easy axes perpendicular to the surface. In contrast, on bare Ru(0001), the molecules are disordered, exhibiting only weak directional preference of the easy magnetization axis. The perfect out-of-plane alignment of the easy axes on graphene results from the molecule-molecule interaction, which dominates over the weak adsorption on the graphene surface. Our results demonstrate that the net magnetic properties of a molecular submonolayer can be tuned using a graphene spacer layer, which is attractive for hybrid molecule-inorganic spintronic devices.

We report on low-temperature scanning tunneling microscopy and spectroscopy measurements on NC-Ph 3 -CN molecules adsorbed at 300 K on a Cu(111) surface. Upon cooling, the molecules form chain and honeycomb structures, incorporating Cu adatoms supplied by the substrate as metal linkers. In these assemblies, the molecules align along two main directions, with a relative abundance that depends on the coordination number and on the bond length. We show spectroscopic data about the unoccupied molecular orbitals and investigate the patterns obtained by depositing different amounts of molecules. Comparison of these results with the ones obtained for NC-Ph 5 -CN molecules on the same substrate enables us to establish a hierarchy of the different interaction forces at work in the system. ■ INTRODUCTIONSupramolecular chemistry at surfaces is a topic of widespread interest because low-dimensional architectures with specific functionalities can be created. 1−4 A detailed understanding of the mechanism underlying the self-assembly of the various organic and metal−organic networks is important to realize the desired morphology, chemical composition, and eventually functionality. In this respect, regular porous networks are especially promising: They act as templates for the positioning of molecules or metal atoms and clusters onto well-defined sites within the cavities 5−8 as well as on the ligand molecules. 9 Moreover, depending on the metal atoms used for the coordination nodes, regular arrays of transition-metal 10,11 or rare earth atoms 12 can be created.Molecular adsorption and assembly are controlled by several competing interactions. 13−16 For the case of metal−organic porous networks, there are the van der Waals forces between the organic molecules and the surface and the corrugation of this potential energy surface upon translation and rotation of the molecules. A second ingredient is the potential energy surface felt by the metal coordination atoms, that is, their preferred adsorption sites and their diffusion barriers. Finally, there are the bond angle, distance, and coordination number of the metal−organic coordination bond.To shed light on the hierarchy of these energies, we report here on the self-assembly of NC-Ph 3 -CN molecules on Cu(111). When deposited on the substrate kept at room temperature, upon cooling NC-Ph 3 -CN molecules form chain and honeycomb structures and the molecules align with orientations that deviate from the crystallographic high symmetry directions of the surface. The comparison with the motifs formed by NC-Ph 5 -CN molecules on the same substrate 17 enables us to estimate which interaction dominates in each case, depending on the number of phenyl rings in the system. To complement the characterization of the observed metal−organic structures, we report spectroscopic data for the molecular orbitals. Moreover, the dependence of the obtained structures on the molecular coverage is discussed.

The observation of sharp atomiclike multiplet features is unexpected for individual 3d atoms adsorbed on transition-metal surfaces. However, we show by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism that individual Fe atoms on Cu(111) exhibit such features. They are reminiscent of a low degree of hybridization, similar to 3d atoms adsorbed on alkali-metal surfaces. We determine the spin, orbital, and total magnetic moments, as well as magnetic anisotropy energy for the individual Fe atoms and for small Fe clusters that we form by increasing the coverage. The multiplet features are smoothened and the orbital moment rapidly decreases with increasing cluster size. For Fe monomers, comparison with density functional theory and multiplet calculations reveals a d 7 electronic configuration, owing to the transfer of one electron from the 4s to the 3d states.