Magnetism of Cr10 wheels on Au(111) and Cu(111) surfaces

Trabajo presentado al X Meeting de GEFES (División de Física de la Materia Condensada de la Real Sociedad Española de Física), celebrado en Valencia del 24 al 26 de enero de 2018We report on the magnetic properties of {Cr10} wheels (Cr10(OMe)20(O2CCMe)10) UHVsublimated on Cu(111) and Au(111) single-crystals investigated by means of by XAS & XMCD, SQUID magnetometry and STM. We discuss the magnetic anisotropy and magnetic moment field-dependence of the grafted {Cr10} molecules, as a function of the metallic substrate and layer thickness, and compare it with bulk material. Our results demonstrate that evaporated molecules show negligible anisotropy and weak dependence with the substrate. A combination of XMCD and SQUID magnetometry show that Mono- and Multilayer {Cr10} samples exhibit a magnetic behavior stemming from a S = 9 ground state, ruling out some previous results. Montecarlo simulations show that the magnetic evolution can be explained by {Cr10} molecules magneticaly consisting of two semi-crowns containing 4 Cr ions interacting ferromagnetically, separated by 2 Cr ions with antiferromagnetic interactions, giving rise to the ground S = 9 state, in agreement with previous EPR data in literature. Evaporation of {Cr10} onto a substrate producing subtle structural changes induce strong changes in the Cr-Cr interactions and distinct magnetic behavior from the bulk.Peer Reviewe

Tunable Band Alignment with Unperturbed Carrier Mobility of On-Surface Synthesized Organic Semiconducting Wires

The tunable properties of molecular materials place them among the favorites for a variety of future generation devices. In addition, to maintain the current trend of miniaturisation of those devices, a departure from the present top-down production methods may soon be required and self-assembly appears among the most promising alternatives. On-surface synthesis unites the promises of molecular materials and of self assembly, with the sturdiness of covalently bonded structures: an ideal scenario for future applications. Following this idea, we report the synthesis of functional extended nanowires by self-assembly. In particular, the products correspond to one-dimensional organic semiconductors. The uniaxial alignment provided by our substrate templates allows us to access with exquisite detail their electronic properties, including the full valence band dispersion, by combining local probes with spatial averaging techniques. We show how, by selectively doping the molecular precursors, the product\u2019s energy level alignment can be tuned without compromising the charge carrier\u2019s mobility

Tuning the magnetic moment of high density FePc/Ag(110) phases by oxygen dosing

Resumen del trabajo presentado a la 64th Annual Conference on Magnetism and Magnetic Materials (MMM), celebrada en Las Vegas (USA) del 4 al 8 de noviembre de 2019.Peer reviewe

Ultrathin V films on Pt(111): a structural study by means of X- ray photoelectron spectroscopy and diffraction

A detailed structural study of V growth on Pt (111) has been performed mainly by means of angle resolved X-ray photoelectron spectroscopy and diffraction, with the aid of multiple scattering cluster-spherical wave simulations of the diffraction curves. Vanadium on Pt (111) grows with a two-domain bulk-like bcc (111) stacking largely incoherent with the substrate, at least up to a thickness of eight equivalent monolayers. For thicker layers, an orientational transition is observed, leading to a six-domain bulk-like bcc (110) structure. Although bcc (111)-oriented V is preserved in the thicker layer, we cannot exclude the fact that some of the initially (111) ultrathin film has been partially restructured to the (110) orientation when the critical thickness associated with the transition has been exceeded. Strong three-dimensional clustering of the overlayer is observed for any investigated thickness which supports a Volmer\u2013Weber growth of the V film. Our findings are compared to literature data concerning the growth of Cr on the same substrate

Reactive deposition of NiO ultrathin films on Pd(100)

NiO ultrathin films have been grown on Pd(100) following a reactive deposition procedure. Ni has been dosed at room temperature on the substrate surface in an oxygen partial pressure of 4 · 10-6 mbar. The electronic and structural evolution of the resulting NiO(100) ultrathin films has been followed by means of X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), low energy electron diffraction (LEED), and scanning tunnelling microscopy (STM). XPS, XPD and STM data indicate a 2D growth of the first NiO monolayer, while further growth leads to the nucleation of 3D islands, in a Stranski–Krastanov growth scheme. Combined XPD and LEED data indicate an initially pseudomorphic growth, characterised by in-plane compressive tetragonal strain of the NiO film, with a consequent out-of-plane interlayer expansion. Partial strain relaxation occurs abruptly, very likely between the second and the third atomic layer of the 3D islands, while a completely bulk-like cubic environment is reached only gradually as a function of thickness. NiO(100) films even ca 50 equivalent monolayers thick can be grown with good long-range order, as shown by (1 · 1) LEED images

Strain analysis of epitaxial ultrathin films on Pt(111)

X-ray photoelectron diffraction (XPD) measurements have demonstrated that Ni ultrathin films on Pt(111) exhibit a single domain fcc stacking pseudomorphic to the substrate, with a consequent trigonal distortion of the Ni unit cell in order to accommodate the in-plane expansion of the Ni lattice parameter of about 11% with respect to its bulk value. We show that the amount of interlayer contraction and the strain energy resulting form the trigonal distortion are very well predicted by a strain analysis in the framework of simple linear elasticity theory. Strain analysis also allows to discuss in some detail the controversial case of Co growth on the same substrate. Finally, we discuss the dependence of the strain energy of the overlayer on the substrate crystal orientation and its effects on chemisorption experiments performed on heteroepitaxial monolayers (HML)

Polarization effects to enhance surface sensitivity of angle- scanned X-ray photoelectron diffraction in synchrotron- radiation-based experiments

We show that the surface sensitivity of angle-scanned photoelectron diffraction applied to bulk crystals or epitaxial multilayers can be enhanced by exploiting the angular anisotropy of the photoelectron primary wave excited by linearly polarized light, combined with the possibility of performing measurements in two different light polarization/electron detection geometries. The obtained relative photoelectron diffraction patterns of the form I1(k)/I2(k), where 1 and 2 denote the two different geometric set-ups discussed in the present paper, show an enhanced structural surface sensitivity, which should allow the determination of the presence and the extent of surface relaxation by comparing the experimental data to suitable theoretical simulations. In a set of preliminary model calculations, the best results are obtained for emission from an s-initial state. High angular resolution of the electron detectors is an additional requirement

Angle scanned photoelectron diffraction: Probing crystalline ultrathin films

Electrons emitted from the core levels of a photon-irradiated crystalline sample undergo scattering by atoms in the vicinity of the emitting species. Subsequent interference phenomena between the electron wave portions propagating to the detector produce intensity modulations as a function of the direction of detection. This process constitutes the physical basis of the angle-scanned X-ray photoelectron diffraction (XPD) technique. The resulting modulations, properly interpreted, are rich in structural information concerning the near-surface atomic layers. In this review, after an introduction to the principles of XPD, some selected results in the field of ultrathin epitaxial films will be reported in order to outline the merits of the technique. Qualitative structural information (e. g., growth modes and lattice distortions) is directly obtained from the experimental raw data without the need for theoretical simulation. On the other hand, quantitative structural parameters, as well as the presence of stacking faults and other structural defects, may be deduced by using a trial-and-error fitting procedure based on simple scattering models