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My research activities focused on the growth and characterisation of two-dimensional materials. Initially Physical Vapour Deposition (PVD) growth of single layer (SL) MoS2 on Au(111) was performed by dosing Mo in H2S atmosphere at room temperature followed by annealing (Temperature Programmed Growth (TPG)) and we studied the evolution of intermediate species during the transformation by measuring fast-XPS. Thereafter by careful tuning the growth parameters, we developed a new procedure that leads to the direct growth of high quality SL MoS2 by dosing Mo in H2S atmosphere at high temperature (High Temperature Growth (HTG). Besides the improved quality of the layer the HTG growth method results in just one single orientation of the layer, leading to complete out-of plane spin polarisation in Spin-resolved ARPES measurements, whereas TPG produced a mixture (70:30) of two opposite orientations. We studied the growth of SL WS2 on Au(111) by following the HTG method. In-situ fast XPS allowed to find the proper growth parameters leading to singly oriented WS2 high quality layer. The absence of partially sulfided peaks in the W 4f and S 2p core levels and sharp LEED spots indicated the high quality of the grown layer that was also reflected in the ARPES measurements that presented sharper bands than previously reported values and allowed the observation of moirè-induced mini gaps and the quantification of branch-dependent electron-phonon coupling strength. MoS2 growth on Ag(111) was also performed by HTG method and resulted in the formation of two 180° rotated domains in equal proportion, as seen with LEED and XPD. LEED pattern indicated a very high structural order of the layer supported by the STM measurements. The Mo 3d core level presented a larger asymmetry than that measured for MoS2/Au(111) which is likely due to a semiconductor to metal transition of the MoS2 layer due to the strong interaction with the Ag(111) substrate. HTG growth of SL MoS2 on Ag(110) was performed to explore the effect of the different substrate geometry on the properties of MoS2. The resulting layer was of very high quality, as determined with XPS, LEED, STM and ARPES measurements. The LEED and XPD patterns showed the presence of equal proportions of two mirror domains. A structural model for the relative arrangement of the hexagonal MoS2 lattice with respect to the rectangular lattice of the substrate was obtained based on LEED and STM measurements. In order to study the reported growth of Silicene on Ir(111), the adsorption of Si on Ir(111) was studied for Si coverages up to more than one monolayer and characterized by high-resolution XPS and LEED. Experiments and DFT calculations showed dominant Si adsorption at the hollow sites together with segregation into the surface and even bilayer formation at high coverage, thereby implying the instability of Si/Ir(111) interface towards Si-Ir alloys, rather than formation of silicene layer.

In this work we prove that ordered single-layer MoS2 can be grown epitaxially on Ag(110), despite the different crystalline geometry of adsorbate and substrate. A comprehensive investigation of electronic and structural features of this interface is carried out by combining several techniques. Photoelectron diffraction experiments show that only two mirror crystalline domains coexist in equal amount in the grown layer. Angle-resolved valence band photoelectron spectroscopy shows that MoS2 undergoes a semiconductor-to-metal transition. Low-energy electron diffraction and scanning-tunneling microscopy experiments reveal the formation of a commensurate moiré superlattice at the interface, which implies an anisotropic uniaxial strain of the MoS2 crystalline lattice of ca. 3% in the [110] direction of the Ag(110) surface. These outcomes suggest that the epitaxial growth on anisotropic substrates might be an effective and scalable method to generate a controlled and homogeneous strain in MoS2 and possibly other transition-metal dichalcogenides.

Ultrathin films of iron oxide form a two-dimensional (2D) FeO layer on Ir(111). Because of difference in lattice constant between 2D-FeO and Ir(111), a moiré superstructure is formed. The 2D-FeO/Ir(111) structure is examined by soft X-ray photoelectron spectroscopy, X-ray photoemission diffraction, scanning tunneling microscopy, and low-energy electron diffraction. A 2D-FeO layer may also be grown by iron intercalation and subsequent oxidation underneath a graphene layer on Ir(111). Thus, the graphene can be decoupled from the metal by the 2D-FeO layer. Changes in the graphene C 1s binding energy can be mainly explained by shifts in the Fermi level of graphene as a consequence of interface band alignment for weak interactions between graphene and the substrate. A shift of C 1s to lower binding energy, for graphene supported on FeO/Ir(111), is a consequence of the dipole moment in the 2D-FeO layer normal to the Ir(111) surface. Broadening of the C 1s peak is consistent with a locally varying 2D-FeO dipole within the moiré structure and thus implies a modulated charge doping of the graphene.

We present a complete characterisation at the nanoscale of the growth and structure of single-layer tungsten disulfide (WS$_2$) epitaxially grown on Au(111). Following the growth process in real time with fast x-ray photoelectron spectroscopy, we obtain a singly-oriented layer by choosing the proper W evaporation rate and substrate temperature during the growth. Information about the morphology, size and layer stacking of the WS$_2$ layer were achieved by employing x-ray photoelectron diffraction and low-energy electron microscopy. The strong spin splitting in the valence band of WS$_2$ coupled with the single-orientation character of the layer make this material the ideal candidate for the exploitation of the spin and valley degrees of freedom., 6 figures, 1 supplementary material PDF file

The adsorption of Si atoms on a metal surface might proceed through complex surface processes, whose rate is determined differently by factors such as temperature, Si coverage, and metal cohesive energy. Among other transition metals, iridium is a special case since the Ir(111) surface was reported first, in addition to Ag(111), as being suitable for the epitaxy of silicene monolayers. In this study we followed the adsorption of Si on the Ir(111) surface via high resolution core level photoelectron spectroscopy, starting from the clean metal surface up to a coverage exceeding one monolayer, in a temperature range between 300 and 670 K. Density functional theory calculations were carried out in order to evaluate the stability of the different Si adsorption configurations as a function of the coverage. Results indicate that, at low coverage, the Si adatoms tend to occupy the hollow Ir sites, although a small fraction of them penetrates the first Ir layer. Si penetration of the Ir surface can take place if the energy gained upon Si adsorption is used to displace the Ir surface atoms, rather then being dissipated differently. At a Si coverage of ∼1 monolayer, the Ir 4f spectrum indicates that not only the metal surface but also the layers underneath are perturbed. Our results point out that the Si/Ir(111) interface is unstable towards Si–Ir intermixing, in agreement with the silicide phase formation reported in the literature for the reverted interface.

We present a study on the growth and characterization of high-quality single-layer MoS$_2$ with a single orientation, i.e. without the presence of mirror domains. This single orientation of the MoS$_2$ layer is established by means of x-ray photoelectron diffraction. The high quality is evidenced by combining scanning tunneling microscopy with x-ray photoelectron spectroscopy measurements. Spin- and angle-resolved photoemission experiments performed on the sample revealed complete spin-polarization of the valence band states near the K and -K points of the Brillouin zone. These findings open up the possibility to exploit the spin and valley degrees of freedom for encoding and processing information in devices that are based on epitaxially grown materials., Comment: 25 pages, 10 figures

The absence of inversion symmetry leads to a strong spin-orbit splitting of the upper valence band of semiconducting single-layer transition-metal dichalchogenides such as ${\mathrm{MoS}}_{2}$ or ${\mathrm{WS}}_{2}$. This permits a direct comparison of the electron-phonon coupling strength in states that only differ by their spin. Here, the electron-phonon coupling in the valence band maximum of single-layer ${\mathrm{WS}}_{2}$ is studied by first-principles calculations and angle-resolved photoemission. The coupling strength is found to be drastically different for the two spin-split branches, with calculated values of ${\ensuremath{\lambda}}_{K}=0.0021$ and 0.40 for the upper and lower spin-split valence band of the freestanding layer, respectively. This difference is somewhat reduced when including scattering processes involving the Au(111) substrate present in the experiment but it remains significant, in good agreement with the experimental results.