We present a study of a sub-nanometer interlayer of crystalline silicon nitride at the Ni/Si interface. We performed transmission electron microscopy measurements complemented by energy dispersive X-ray analysis to investigate to what extent the nitride layer act as a barrier against atom diffusion. The results show that discontinuous silicide areas can form just below the nitride layer, whose composition is compatible with that of the nickel disilicide. The Ni-Si reaction is tentatively attributed to the thermal strain suffered by the interface during the deposition of Ni at low temperature.

When an organic film is deposited on a metal surface, charge layers are formed at the interface. These are an important feature of the interface electronic structure and play a crucial role as separation layers between electrodes and active layers in organic devices. Here, we report on a study of the interface between diruthenium phthalocyanine, (RuPc)2, and the Ag(001) surface. The molecules form two different commensurable arrangements on the substrate, a low density one for a coverage well below the first monolayer and a high density one up to the completion of the monolayer. The focus of this study is on the interface states evolution with the molecular density on the metal surface and the charge distribution in the thin interfacial layer between molecules and substrate. From this investigation, conducted by low energy electron diffraction, scanning tunneling microscopy/spectroscopy, photoemission spectroscopy, and density functional theory, we have found that, even if individual molecules are characterized by a quite similar surface-to-molecule charge transfer pattern, the two molecular arrangements present different valence band structures and, more interestingly, different modulations of the interface charge. These charge modulations are governed by interfacial states energetically resonant with the molecular states, localized at the position of the molecules as well as by a reaction of the electronic cloud of the metal surface to the molecular adsorption due to a Pauli pushback effect. This complex, spatial charge modulation makes the (RuPc)2/Ag(001) an interesting case of interaction intermediate between physisorption and chemisorption.

Understanding howa spin current flows across metal-semiconductor interfaces at pico- and femtosecond time scales is of paramount importance for ultrafast spintronics, data processing, and storage applications. However, the possibility to directly access the propagation of spin currents, within such time scales, has been hampered by the simultaneous lack of both ultrafast element-specific magnetic sensitive probes and tailoredwell-built and characterized metal-semiconductor interfaces. Here, by means of a novel free-electron laser-based element-sensitive ultrafast time-resolved Kerr spectroscopy, we reveal different magnetodynamics for the Ni M2;3 and Si L2;3 absorption edges. These results are assumed to be the experimental evidence of photoinduced spin currents propagating at a speed of 0.2 nm/fs across the Ni/Si interface.

In this paper, we report on an in-depth study on the growth of nickel silicides, either on a clean Ni(111) substrate or in the presence of a previously-grown epitaxial single graphene (Gr) layer, by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). We demonstrate that two different nickel silicides, namely Ni3Si and Ni2Si, progressively form as the annealing temperature is increased from 450 °C to 600 °C. The presence of the Gr layer does not change the nature of the two silicide phases but rather affects the morphology of the silicide overlayer. Indeed, in the presence of Gr, the deposited silicon atoms intercalate by passing through the Gr defects or domain boundaries and accumulate on specific sample areas, resulting in the formation of multilayer silicide islands. In the absence of Gr, the deposited silicon atoms react uniformly with the nickel substrate, resulting in the formation of homogeneous large scale silicide layers.

We present a theoretical study of the thermodynamical factors that determine epitaxial formation of single layer 2D pnictogen (P, As, Sb, Bi) allotropes on substrates of any type. The interplay of substrate-adlayer interaction and strain induced by epitaxial matching is analyzed in terms of the phase diagram describing growth during the gaseous deposition stage. The necessary conditions to favor particular allotrope growth (in particular, the alpha and phases) are determined. We show that buckled Sb and Bi layers can overcome large tensile strain and form flat honeycomb layers even on common metal surfaces. An alternative strategy for controlled allotrope formation via thermally induced phase transformations between alpha and phases is examined in detail, including important methodological analysis. All four elements follow reconstructive transition pathways, whose activation barrier correlates with the bond dissociation energy. If nucleation is considered, the barrier can further reduce by about 13% and thus becomes quite accessible under typical annealing conditions. The role of van der Waals and spin-orbit corrections in the stability of several allotropes is carefully addressed. The theoretical insight gained is evaluated in light of experimental reports and strategies for controlling growth are outlined.

The "two-step" growth technique has been used to grow atomically uniform Ag films on 7 × 7 Si(111) and 8 × 8 ?-Si3N4(0001)/Si(111) surfaces. Angle-resolved photoemission spectroscopy reveals the formation of sp quantum well states in the Ag films with distinct properties in the two cases. It is shown that the valence electrons in silver can be confined in the fundamental gap of a less than 1-nm-thin nitride layer, effectively decoupling the Ag and Si states.

Here, we report on the conceptual design, the hardware realization, and the first experimental results of a novel and compact x-ray polarimeter capable of a single-pulse linear polarization angle detection in the extreme ultraviolet photon energy range. The polarimeter is tested by performing time resolved pump-probe experiments on a Ni80Fe20 Permalloy film at the M2,3 Ni edge at an externally seeded free-electron laser source. Comparison with similar experiments reported in the literature shows the advantages of our approach also in view of future experiments.

Silicene, the 2D silicon allotrope analogue of graphene, was theoretically predicted in 1994 as a metastable buckled honeycomb silicon monolayer. Similarly to its carbon counterpart it was predicted to present an electronic structure hosting Dirac cones. In the last decade a great deal of work has been done to synthesize silicene and exploit its properties. In this paper we will review our research group activity in the field, dealing in particular with silicon-substrate interaction upon silicon deposition, and discuss the still debated silicene formation starting from the chemistry of silicon unsaturated compounds.

Topological surface states usually emerge at the boundary between a topological and a conventional insulator. Their precise physical character and spatial localization depend on the complex interplay between the chemical, structural and electronic properties of the two insulators in contact. Using a lattice-matched heterointerface of single and double bilayers of ?-antimonene and bismuth selenide, we perform a comprehensive experimental and theoretical study of the chiral surface states by means of microscopy and spectroscopic measurements complemented by first-principles calculations. We demonstrate that, although ?-antimonene is a trivial insulator in its free-standing form, it inherits the unique symmetry-protected spin texture from the substrate via a proximity effect that induces outward migration of the topological state. This "topologization" of ?-antimonene is found to be driven by the hybridization of the bands from either side of the interface.

In building up molecular circuits and molecule-based devices, it is central to determine how molecules interact with the surface. Here, we explore the behaviour of a bifunctional molecule, i.e. phenol, on Si(111)7 x 7 by high resolution photoemission, comparatively in the C1s, Si2p, VB and O1s energy ranges. We found out that the adsorption is more complex than originally hypothesized with a mixture of dissociative and molecular adsorbates at low exposures, and a preeminence of dissociative adsorbates at higher exposure. The co-existence of these states is different from the Si(100) surface, where the molecular states are readily converted into dissociative ones.

This work reports the first experimental study of graphene transferred on ?-Si3N4(0001)/Si(111). A comprehensive quantitative understanding of the physics of ultrathin Si3N4 as a gate dielectric for graphene-based devices is provided. The Si3N4 film is grown on Si(111) under ultra-high vacuum (UHV) conditions and investigated by scanning tunneling microscopy (STM). Subsequently, a graphene flake is deposited on top of it by a polymer-based transfer technique, and a Hall bar device is fabricated from the graphene flake. STM is employed again to study the graphene flake under UHV conditions after device fabrication and shows that the surface quality is preserved. Electrical transport measurements, carried out at low temperature in magnetic field, reveal back gate modulation of carrier density in the graphene channel and show the occurrence of weak localization. Under these experimental conditions, no leakage current between back gate and graphene channel is detected.

The air-exposed hydrogenated diamond surface has been studied by carrier transport measurements and density functional theory. Our results have allowed us to understand the close relationship between the physisorbed water molecules and the electrical resistance. We have therefore been able to show that the evolution of the resistance over time and temperature can be related to the extent of the hole accumulation layer originating at the surface of the hydrogenated diamond. The method has allowed us to estimate the desorption energy of a single water molecule by means of resistance measurements alone.

Graphene (Gr) is known to be an excellent barrier preventing atoms and molecules to diffuse through it. This is due to the carbon atom arrangement in a two-dimensional (2D) honeycomb structure with a very small lattice parameter forming an electron cloud that prevents atoms and molecules crossing. Nonetheless at high annealing temperatures, intercalation of atoms through graphene occurs, opening the path for formation of vertical heterojunctions constituted of two-dimensional layers. In this paper, we report on the ability of silicon atoms to penetrate the graphene network, fully epitaxially grown on a Ni(111) surface, even at room temperature. Our scanning tunneling microscopy (STM) experiments show that the presence of defects like vacancies and dislocations in the graphene lattice favor the Si atoms intercalation, forming two-dimensional, flat and disordered islands below the Gr layer. Ab-initio molecular dynamics calculations confirm that Gr defects are necessary for Si intercalation at room temperature and show that: i) a hypothetical intercalated silicene layer cannot be stable for more than 8 ps and ii) the corresponding Si atoms completely lose their in-plane order, resulting in a random planar distribution, and form strong covalent bonds with Ni atoms.

We report the discovery of a temperature induced phase transition between the alpha and beta structures of antimonene. When antimony is deposited at room temperature on bismuth selenide, it forms domains of alpha-antimonene having different orientations with respect to the substrate. During a mild annealing, the beta phase grows and prevails over the alpha phase, eventually forming a single domain that perfectly matches the surface lattice structure of bismuth selenide. First principles thermodynamics calculations of this van der Waals heterostructure explain the different temperature-dependent stability of the two phases and reveal a minimum energy transition path. Although the formation energies of free-standing alpha- and beta-antimonene only slightly differ, the beta phase is ultimately favoured in the annealed heterostructure due to an increased interaction with the substrate mediated by the perfect lattice match.

Silicon has been grown on the (8×8)-reconstructed ?-Si3N4(0 0 0 1) surface at 350 °C temperature. The pure Volmer-Weber growth mode has induced the formation of nano-sized silicon clusters randomly distributed on the surface. Synchrotron radiation photoelectron spectroscopy and scanning tunneling microscopy have been employed to study the system. A fit to the photoemission spectra, complemented by topographic information, has allowed us to assign each Si2p and N1s component to the different non equivalent sites of silicon and nitrogen atoms.

We report a study of the interface between antimony and the prototypical topological insulator Bi2Se3. Scanning tunnelling microscopy measurements show the presence of ordered domains displaying a perfect lattice match with bismuth selenide. Density functional theory calculations of the most stable atomic congurations demonstrate that the ordered domains can be attributed to stacks of beta-antimonene.

The surface electronic structure of Si(1 1 1)-$7\times 7$ has been studied by angle-resolved photo electron spectroscopy. Replicas of the S 1 surface state are found in correspondence with several $7\times 7$ unit cells in the reciprocal space. This observation resolves in a direct way the long-standing dichotomy between the structural and electronic properties of the system previously discussed on the basis of the $2\times 2$ or $\sqrt{3}\times \sqrt{3}$ R30° surface models.

Monolayer silicene and multilayer silicon films on Ag(111) have been the subject of many investigations within the last few years. For both systems, photoemission data have been interpreted in terms of linearly dispersing bands giving rise to the characteristic Dirac cone features, similar to graphene. Here we demonstrate, on the basis of angle-resolved valence band and core level photoemission data that this assignment is not correct. The bands previously attributed to states with Dirac fermion character are shown to derive from Ag(111) interface and bulk states in the silicene monolayer and from the well-known Ag-(3×3)R30-Si(111) structure in Si multilayers. These results question the validity of the claim that graphene-like silicene and silicene multilayers are in fact formed on Ag(111).

The possibility to intercalate noble gas atoms below epitaxial graphene monolayers coupled with the instability at high temperature of graphene on the surface of certain metals has been exploited to produce Ar-filled graphene nanosized blisters evenly distributed on the bare Ni(111) surface. We have followed in real time the self-assembling of the nanoblisters during the thermal annealing of the Gr/ Ni(111) interface loaded with Ar and characterized their morphology and structure at the atomic scale. The nanoblisters contain Ar aggregates compressed at high pressure arranged below the graphene monolayer skin that is decoupled from the Ni substrate and sealed only at the periphery through stable C-Ni bonds. Their in-plane truncated triangular shapes are driven by the crystallographic directions of the Ni surface. The nonuniform strain revealed along the blister profile is explained by the inhomogeneous expansion of the flexible graphene lattice that adjusts to envelop the Ar atom stacks.

Silicon multilayers on Ag(111) have been suggested to exhibit the structure of silicene, a material that has been heralded as a novel basis for microelectronic applications. However, our angle-resolved photoemission spectra (ARPES) from silicon multilayers on Ag(111) and of the silver-induced reconstruction of Si(111) demonstrate, from the close match in the valence level band structures, that the films exhibit a sp3 diamondlike structure. This refutes the interpretation of silicon multilayers on Ag(111) as silicene, a conclusion that is strengthened by the observation from core level photoemission that significant silver segregation occurs to the surface of these layers.