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Interfaces between antiferromagnetic CoO and ferromagnetic Fe are typically characterized by the development of Fe oxides. Recently, it was shown that the use of a proper ultra-thin Co buffer layer prevents the formation of Fe oxides [Brambilla et al., Appl. Surf. Sci. 362, 374 (2016)]. In the present work, we investigate the magnetic properties of such an interface, and we find evidence for an in-plane uniaxial magnetic anisotropy, which is characterized by a multijump reversal behavior in the magnetization hysteresis loops. X-ray photoemission spectroscopy and element-sensitive hysteresis loops reveal that the occurrence of such an anisotropy is a phenomenon developing at the very interface.

The functionality of low dimensional phases of porphyrins in optical, chemical, electrical, and multimodal combinational devices is strictly related to the control of molecular orientation within the produced solid layers. A promising strategy to drive the growth of adlayers with predictable structural properties relies on the template effect exerted by the substrate. Tetraphenyl porphyrins, being disc-shaped objects, can be adsorbed on a crystal surface by taking on different geometries. An edge-on configuration is adopted when the interactions among molecules overtake those between molecules and substrate, whereas a flat-on configuration is adopted when molecule-substrate interaction is dominant, with the weaker intermolecular interaction driving a close-packed geometry in the adlayer. For this latter reason, square and/or hexagonal lattice symmetries of physisorbed porphyrin layers are disclosed on highly interacting metal substrates such as Au(111). Unfortunately, metal substrates modify the intrinsic properties of porphyrins by suppressing many of their functionalities. To overcome this drawback, here we report the selective growth of porphyrins in a flat-on arrangement on the chiral (110) cleavage surface of the mixed molecular organic crystal formed by 2,5-diketopiperazine and fumaric acid in a 1:1 mole ratio. The energetic advantage ensured by the interaction with the insulating substrate drives the prevalent formation of domains with a square symmetry, which is retained from monolayer to multilayers. However, rare domains with a hexagonal symmetry are revealed and analyzed by high-resolution scanning probe microscopic techniques. The experimental structural analysis performed at the nanoscale, combined with ab initio calculations, allowed us to demonstrate that the molecular architectures we found arise from the simultaneous fulfillment of site adsorption energy maximization driven by peculiar molecular motifs of the selected substrate, close-packing criteria, and epitaxial locking to the substrate surface by weak van der Waals interactions.

Porphyrins are an extremely valuable class of molecules engaged in a variety of roles spanning from biology to optoelectronics. Manipulation of the chemical and physical properties of the inner cavity of porphyrins has been recognized as crucial for the exploitation of these systems in organic devices, particularly when porphyrins self-organize at the interface with a flat-on orientation of the macrocycle. Such an orientation has been mostly observed on metallic surfaces. Unfortunately, the physical-chemical properties of the molecules result in being largely perturbed due to the molecule-metal interaction. In addition, conducting substrates are unsuited to exploit electrically driven devices based onorganic layers. To overcome these issues, we performed a topology-based analysis of insulating organic single crystal structures to identify a surface which (i) ensures easy exfoliation through mechanical methods, (ii) ensures epitaxial match with an overlayer of close-packed flat-on porphyrin molecules, and (iii) displays chirality. The outcome of this work is represented by a unique crystal of mixed 2,5-diketopiperazine and fumaric acid in a 1:1 ratio. We demonstrate that the (110) surface of this crystal fulfills the aforementioned requirements and, thanks to its peculiar subnanometric corrugations, allows one to grow uniaxially aligned monolayers of flat-on porphyrin molecules assembled through van der Waals interactions

When light is reflected by a solid sample, the collected signal includes the optical response of all the matter within the light penetration depth. Studying films thinner than this length, the substrate thus contributes to the reflected beam, and affects optical spectroscopy measurements. This is particularly true when polarized light is used, as in reflectance anisotropy spectroscopy (RAS), where substrate symmetry properties play a crucial part. A detailed comprehension of the substrate role in the optical response of a sample has important consequences for correctly extracting the optical contribution of the layer grown on it. This problem becomes delicate in organics, where the choice of the substrate is often fundamental for obtaining an overlayer with a desired structure. In particular, the use of organic single crystal substrates, often structurally anisotropic, is a powerful tool for obtaining highly ordered crystalline films by organic epitaxy. To achieve the film optical properties from an experimental RAS spectrum, an interpretative key is necessary: but the most used model describing the interaction of light with the layered system (the so called three-layer model) fails when the substrate is anisotropic. This care in the application of RAS to organics has not been discussed in details yet, in particular to understand the contribution of the substrate to the total anisotropy signal. Within a phenomenological approach to RAS, we have here weighed the substrate contribution, by considering five different and carefully chosen samples, i.e. a crystalline overlayer of the same material and thickness but on five different substrates. The data show that the substrate clearly influences the readability of the spectra by influencing their lineshape, in some complex cases limiting the possibility of extracting the film properties. Consequently, the correct choice of the substrate - that is a fundamental task for obtaining specific properties of the growing layer - is also a delicate element for a correct interpretation of RAS results.

A drop-casting technique designed for getting a high control over thin layers of small organic molecules is assessed for the deposition of Pt-octaethyl porphyrin on highly oriented pyrolytic graphite. We demonstrate that by a proper choice of solvent, namely anisole, it is possible to obtain an almost homogeneous 2D film at the micrometric scale in few minutes (evaporation time of the solvent). This growth strategy represents a reasonable alternative for Pt-octaethyl molecules deposition to longer and more expensive techniques in many applications and could be probably extended to other porphyrins and porphyrinic compounds. © 2014 Elsevier B.V.

A major challenge in molecular electronics is to develop logic devices based on a truly intramolecular switching mechanism. Recently, a new type of molecular device has been proposed where the switching characteristic is mediated by the bistability in the position of the two hydrogen atoms which can occupy different, energetically equivalent positions (tautomerization) in the inner cavity of porphyrins and naphthalocyanines. Up to now, such a reaction has only been exploited at low temperatures and induced or detected through atomic scale manipulation. In addition, the unpredictability of the tautomer orientation currently excludes molecular interconnection to functional electronic circuits. Here, full evidence is provided that, following a newly proposed growth strategy, 2D layers of metal-free tetraphenylporphyrins (H 2TPP) show frozen tautomerization even at room temperature on macroscopic domains, with the H atoms aligned along a direction settled a priori. This behavior is ascribed to the buckling of the molecule, anchored to the substrate, which removes the degeneracy between the two tautomer alignments. On this basis, a new way to exploit uniaxially oriented H2TPP tautomers in a first elementary logic device is proposed. Frozen tautomers at room temperature are exploited in a first logic device prototype. Following a newly proposed growth strategy, a purely 2D ordered array of metal-free porphyrins shows all the tautomers aligned along a direction settled a priori. This occurrence characterizes the anisotropic optical property of the system. The quenching/renewing of the porphyrin anisotropy defines two distinct logic states (0/1) easily readable by optics. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Reflectance anisotropy spectroscopy (RAS) is demonstrated to be particularly suitable for studying the deposition of organic epitaxial layers in ultrahigh vacuum by organic molecular beam epitaxy, thanks to its high sensitivity and applicability in situ. In the case of α-quaterthiophene, both homoepitaxy and heteroepitaxy have been monitored, demonstrating the crystallinity of the films up to tens of monolayers and the epitaxial relation to the substrate. In both cases, optical RAS data are compared to the results of ex situ characterization of the same samples by atomic force microscopy.© 2009 American Vacuum Society

For organic molecular materials the definition of the condition of epitaxial growth has been subjected to several misconceptions and a great debate, leading in any case to less restrictive requirements than for inorganic materials. Here, the deposition of oligothiophene films by molecular beam epitaxy on properly grown organic single crystals is discussed and all-organic rigorous epitaxy demonstrated for both sub-monolayer and several-monolayer thick films. The possibility of growing all-organic nanostructures directly follows from these results. © 2006 Elsevier B.V. All rights reserved.

Reflectance anisotropy spectroscopy (RAS) has recently been used to monitor in situ and in real time the growth of thin organic layers in ultra high vacuum (UHV) (Goletti et al 2003 Appl. Phys. Lett. 83 4146). In this paper, after a short discussion of RAS, with particular attention to the application to organics, we present recent results on the deposition of ordered oligothiophene films by organic molecular beam epitaxy (OMBE), namely α-sexithiophene and quaterthiophene films onto potassium acid phthalate substrates.

The oxygen-saturated Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface has been used as a template to stabilize two-dimensional Cr oxides on Fe(001). Cr deposition at 400 ${}^{\ensuremath{\circ}}$C leads to two different well-ordered phases, depending on the amount of Cr deposited. In the submonolayer regime a novel $c(4\ifmmode\times\else\texttimes\fi{}2)$ overlayer self-assembles on the Fe(001)-$p(1\ifmmode\times\else\texttimes\fi{}1)$O surface, saturating for a coverage of about $0.75$ monolayers. This phase becomes unstable for higher coverages, when a $(\sqrt{5}\ifmmode\times\else\texttimes\fi{}\sqrt{5})R{27}^{\ensuremath{\circ}}$ superstructure emerges. The structural and electronic details of the two one-layer-thick oxides are studied by combining high-resolution scanning tunneling microscopy, low-energy electron diffraction, Auger electron spectroscopy, and density functional theory.

Organic molecular beam epitaxy (OMBE) is the growth technique assuring high quality films and properly designed multilayers of organic molecular materials in view of both their fundamental studies and applications. To monitor in-situ the OMBE growth, we have applied reflectance anisotropy spectroscopy (RAS), which is particularly effective and sensitive, being non destructive for organic materials. In this work, RAS is used to monitor, in-situ, the growth of organic heterostructures. The experiment is carried out using organic single crystals as substrates and oligothiophenes as molecular materials to build a stack of layers. A layer-by-layer growth of the films is demonstrated, together with the formation of interfaces with peculiar optical behavior and structure. © 2008 Elsevier Ltd. All rights reserved.

The growth process of thin films and multilayers of quaterthiophene and sexithiophene onto molecular single crystals has been monitored in situ and in real time during deposition by organic molecular beam epitaxy, measuring the anisotropy of the optical reflectivity. The evolution of the spectra with thickness provides the signature of an epitaxial growth of the films. © 2007 Elsevier B.V. All rights reserved.