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We deeply investigated the properties of submonolayer distributions of isolated molecular Cr7 Ni rings deposited on Au(111) by liquid phase. X-ray absorption spectra measured at the Cr and Ni L2,3 edges show that the grafting of the Cr7 Ni rings onto the gold surface does not affect the oxidation state and the local symmetry of the Cr and Ni sites. The circular dichroism shows a change in sign of the Ni magnetic moment. This is due to a reduction in the exchange coupling constants that, however, preserves the structure of the low-energy levels of the grafted rings, as corroborated by spin-Hamiltonian simulations and comparison with measurements on bulk sample. Density-functional theory calculations show that the Ni-Cr bond gets weaker with slight ring distortion suggesting possible explanation for the observed magnetic behavior. These results show that complex magnetic molecules can be grafted onto surfaces, and that changes in their magnetic behavior must be examined in individual cases. © 2009 The American Physical Society

Single molecule magnets (SMMs) form a new class of magnetic materials consisting of identical nanoscale particles that can show magnetization in the absence of a magnetic field. We have experimentally and theoretically investigated the low-spin (S=4) member of the Mn6 SMM family, properly functionalized with two 3-thiophenecarboxylate (3tpc) ligands in order to graft it on to a Au(111) surface. We report the theoretical density of states calculated within the local density approximation (LDA) scheme accounting for the on-site Coulomb repulsion (LDA+U) for U values ranging from 0 to 8 eV. On the experimental side, by exploiting resonant photoemission at the Mn 2p edge, we were able to single out the Mn 3d derived states in the valence band energy region for a submonolayer distribution of Mn6-3tpc deposited on Au(111). From the comparison between the experimentally derived 3d density of states and the theoretical one, we found that the best agreement occurs for a U value of 4 eV. From the binding energy of Mn 2 p3/2 core line, measured in situ, we also derived a value for the 2p-3d correlation energy of about 5 eV -in agreement with previous determination. © 2008 The American Physical Society

We investigated the electronic and magnetic properties of thick films of molecular Cr8 and Cr7 Ni antiferromagnetic rings by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD). We determined the local symmetries, the electronic configuration, and the values of orbital and spin moments at the Cr and Ni sites of the molecular rings. XMCD measurements show that the correlation between the Cr and Ni spins in the Cr7 Ni molecular ring switches from antiferromagnetic to ferromagnetic with increasing temperature. Experimental data are interpreted using XMCD sum rules that allow the separate evaluation of the spin and the orbital contributions to the total magnetic moment of the ring as a function of temperature and magnetic field. The magnetic behaviors experimentally observed are compared with the results of spin-Hamiltonian calculations, based on microscopic parameters derived by inelastic-neutron scattering and low-temperature specific-heat measurements. The very good agreement between experimental data and calculations is a clear indication of the integrity of molecules. The temperature dependence of the ion magnetic moments results from the interplay between Zeeman and isotropic-exchange contributions, and is well captured by the theoretical model. © 2008 The American Physical Society

Single molecule magnets (SMMs) form a new class of magnetic materials consisting of identical nanoscale particles that can show magnetization in the absence of a magnetic field. We have experimentally and theoretically investigated the low-spin (S=4) member of the Mn6 SMM family, properly functionalized with two 3-thiophenecarboxylate (3tpc) ligands in order to graft it on to a Au(111) surface. We report the theoretical density of states calculated within the local density approximation (LDA) scheme accounting for the on-site Coulomb repulsion (LDA+U) for U values ranging from 0 to 8 eV. On the experimental side, by exploiting resonant photoemission at the Mn 2p edge, we were able to single out the Mn 3d derived states in the valence band energy region for a submonolayer distribution of Mn6-3tpc deposited on Au(111). From the comparison between the experimentally derived 3d density of states and the theoretical one, we found that the best agreement occurs for a U value of 4 eV. From the binding energy of Mn 2p(3/2) core line, measured in situ, we also derived a value for the 2p-3d correlation energy of about 5 eV-in agreement with previous determination.

Organic molecules with sulfur-containing headgroups interacting with noble metal surfaces represent a fruitful class of systems with potential relevance in new device technology. Within this context, the room temperature adsorption of 2-mercaptobenzoxazole (MBO) on the Cu(100) surface has been studied by high-resolution electron energy-loss spectroscopy (HREELS) as a function of molecular exposure. At low coverage, the HREEL spectra present only one mode at 93 meV (750 cm-1), which is attributed to the out-of-plane C–H bending gCH. Its off-specular angular dependence indicates that the molecules lie almost flat on the surface. At higher coverage, the MBO layer orders in a p(2¥2) superstructure, and energy loss peaks at higher energies appear. These features can be assigned to in-plane vibrations indicating that the molecular axis tilts toward a more vertical position, thus allowing a closer molecular packing. Assignment of the HREELS peaks is confirmed by estimation of the gas phase normal modes of the free molecule, evaluated by ab initio methods. The absence of the N–H stretching mode in the HREEL spectra indicates that the H atom bonded to nitrogen in the gas-phase molecule is lost upon adsorption, suggesting that MBO molecule adsorbs on the Cu surface as a thiolate.

We present high energy-resolution photoemission measurements of the spectral density at the discrete quantized electronic levels of a two-dimensional (2D) electron gas. The dynamical 2D electron gas has been obtained by generating a strong accumulation layer at the (110) surface of narrow-gap III–V semiconductors. Exploitation of a number of cases generating band bending (metallic chains or clusters, atomic structure, defects) demonstrates the generality of 2D electron gas formation at charge-accumulated semiconductor surfaces. A self-consistent solution of the Poisson and Schrodinger equations gives the potential well shape, the sub-band energy level position and the accumulated charge density, in excellent agreement with the present experimental data.