V. De Renzi

Grafting Derivatives of Mn6 Single-Molecule Magnets with High Anisotropy Energy Barrier on Au(111) Surface

We study the magnetic properties of two new functionalized single-molecule magnets belonging to the Mn 6 family (general formula [Mn (III)6O2(R-sao)6(O2C-th)2L(4-6)], where R=H (1) or Et (2), HO2C-th=3-thiophene carboxylic acid, L=EtOH, H2O and saoH2 is salicylaldoxime) and their grafting on the Au(111) surface. Complex 1 exhibits spin ground-state S=4, as the result of ferromagnetic coupling between the two antiferromagnetic Mn (III) 3 triangles, while slight structural changes in complex 2, switch the dominant magnetic exchange interactions from anti- to ferromagnetic, enhancing the spin ground-state to S=12 and, consequently, the effective energy barrier for the relaxation of magnetization. Direct-current and alternating-current magnetic susceptibility measurements show that the functionalized complexes preserve the main magnetic properties of the corresponding not-functionalized Mn 6 clusters (i.e., total spin value and magnetic behavior as a function of temperature), though a reduction of the anisotropy barrier is observed in complex 2. For both complexes, the -O2C-th functionalization allows the direct grafting on Au(111) surface by liquid-phase deposition. X-ray photoemission spectroscopy demonstrates that the stoichiometry of the molecular cores is preserved after grafting. Scanning tunneling microscopy (STM) reveals a sub-monolayer distribution of isolated clusters with a slightly higher coverage for complex 1. The cluster stability in the STM images and the S-2p energy positions demonstrate, for both derivatives, the strength of the grafting with the gold surface.

Growth dynamics of L-cysteine SAMs on single-crystal gold surfaces: a metastable deexcitation spectroscopy study.

We report on a metastable deexcitation spectroscopy investigation of the growth of L-cysteine layers deposited under UHV conditions on well-defined Au(110)- (1 × 2) and Au(111) surfaces. The interaction of He(*) with molecular orbitals gave rise to well-defined UPS-like Penning spectra which provided information on the SAM assembly dynamics and adsorption configurations. Penning spectra have been interpreted through comparison with molecular orbital DFT calculations of the free molecule and have been compared with XPS results of previous works. Regarding adsorption of first-layer molecules at room temperature (RT), two different growth regimes were observed. On Au(110), the absence of spectral features related to orbitals associated with SH groups indicated the formation of a compact SAM of thiolate molecules. On Au(111), the data demonstrated the simultaneous presence, since the early stages of growth, of strongly and weakly bound molecules, the latter showing intact SH groups. The different growth mode was tentatively assigned to the added rows of the reconstructed Au(110) surface which behave as extended defects effectively promoting the formation of the S-Au bond. The growth of the second molecular layer was instead observed to proceed similarly for both substrates. Second-layer molecules preferably adopt an adsorption configuration in which the SH group protrudes into the vacuum side.

Spin-communication channels between Ln(III) bis-phthalocyanines molecular nanomagnets and a magnetic substrate.

Learning the art of exploiting the interplay between different units at the atomic scale is a fundamental step in the realization of functional nano-architectures and interfaces. In this context, understanding and controlling the magnetic coupling between molecular centers and their environment is still a challenging task. Here we present a combined experimental-theoretical work on the prototypical case of the bis(phthalocyaninato)-lanthanide(III) (LnPc2) molecular nanomagnets magnetically coupled to a Ni substrate. By means of X-ray magnetic circular dichroism we show how the coupling strength can be tuned by changing the Ln ion. The microscopic parameters of the system are determined by ab-initio calculations and then used in a spin Hamiltonian approach to interpret the experimental data. By this combined approach we identify the features of the spin communication channel: the spin path is first realized by the mediation of the external (5d) electrons of the Ln ion, keeping the characteristic features of the inner 4 f orbitals unaffected, then through the organic ligand, acting as a bridge to the external world.

HREELS study of the adsorption mechanism and orientational order of 2-mercaptobenzoxazole on Cu(1 0 0)

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(1 0 0)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 cCH. 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. � 2003 Elsevier B.V. All rights reserved.

Substrate reconstruction and electronic surface states: Ag(0 0 1)

We present the result of a combined ab initio theoretical study and ultraviolet photoemission spectroscopy experiment of the electronic surface states of the clean and of the oxygen covered, unreconstructed and missing-row reconstructed, Ag(0 0 1) surface. For the clean and for the unreconstructed oxygen covered surface, theory and experiment find an electronic surface state at about 0.4 eV below the Fermi energy (EF) at the X point of the surface Brillouin zone. At the same point, theory predicts the existence of an empty state at +3 eV. Upon reconstruction, the photoemission signal of the filled state is strongly reduced and, according to calculations, the state shifts in energy above EF. The surface interband transition (SIT) between the two states exists thus only for the unreconstructed surfaces and nearly matches the surface plasmon energy. Our investigation confirms therefore the correlation between the presence of this SIT and the anomaly of the surface plasmon dispersion curve of unreconstructed Ag(0 0 1).

Ultra-high-vacuum single-layer formation of α-hexathienyl on the (1 × 2) Au(110) surface

Abstract The formation of an α-hexathienyl single layer on the (1×2) Au(110) surface has been investigated by means of thermal He atom scattering (THAS) in ultra-high-vacuum (UHV) conditions. The monolayer (ML) structure has a rectangular surface unit cell with the T6 molecules lying flat on the surface and aligned parallel to the Au rows. The periodicity perpendicular to the Au rows is doubled with respect to the bare Au surface, while along the rows it is 27.4 ± 0.5 A, corresponding to the molecular length. The correlation length of the overlayer structure depends upon substrate temperature during deposition and increases when the system is heated up to 653 K. The surface structure is irreversibly lost upon heating to temperatures higher than 673 K, but T6 molecules do not completely desorb up to about 773 K.

A high-resolution spectroscopy study on bidimensional ordered structures: the (1 ? 1) and (1 ? 2) phases of Bi/InAs(110)

The room-temperature growth of bismuth on the InAs(110) surface and the Bi-(1 ? 1) and Bi-(1 ? 2) ordered phases have been studied by means of high-resolution ultra-violet photoemission and high-resolution electron-energy-loss spectroscopy. A modified Stransky-Krastanov growth mode at room temperature and the stability of the (1 ? 2)-symmetry phase in the 480-580 K annealing temperature range are deduced by quantitative analysis of the core-level data and Auger spectra. A Bi 5d and In 4d core-level analysis is presented, and discussed according to a recently proposed model for the (1 ? 2) overlayer reconstruction. The formation of Bi-derived electronic states has been followed during the growth of the (1 ? 1)-symmetry phase, showing a semiconducting behaviour at the monolayer coverage, with a well-defined gap state at 0.39 eV binding energy related to the Bi p-like dangling bonds. The (1 ? 2)-Bi phase is metallic, as indicated by the well-defined Fermi edge and by the metallic-like quasi-elastic peak tail in the energy-loss spectra. The metallicity and the electronic structure of the (1 ? 2)-Bi phase are discussed in relation to the available geometric structure.

Thin Film SiC Epitaxy on Si(111) from Acetylene Precursor

A promising route for the production of SiC on Si surfaces exploits the use of small πbonded hydrocarbons, like acetylene, as carbon source. This choice in fact, a llows maintaining the growth temperatures well below 1000 C. Starting from the hydrogenated Si(111)(1x1) surface can be a way to keep the Si substrate always below 800°C. In this way, how ever, to obtain a SiC film composed of large crystallites is not straightforward. We report on he epitaxial growth of 3C-SiC thin films on the Si(111)-(7x7) surface, obtained by dosing acetylene at 750°C. The growth was been followed by means of LEED, X-ray photoemission and Auger spectrosc opy, as well as by high-resolution electron-energy loss spectroscopy. HREEL spectra show very strong features related to the SiC Fuchs-Kliewer phonon (and its replicas), XPS peaks show l osses associated to the SiC bulk plasmon, while the LEED pattern is that of the 3C-SiC, confirming the crystalline character of the epitaxial film. The mean film thickness, as measured by ex-situ TEM, is of f ew hundreds Å.