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Submission, work flow for evaluation, procedures and ceiling for user’s travel and subsistence reimbursement, user’s and provider’s feedback., WP2 - Pilot scheme for the management of a distributed research infrastructure offering harmonised, interoperable and integrated services

Negli ultimi dieci anni, dopo l'isolamento del grafene e lo studio delle sue eccezionali proprietà, la comunità scientifica ha rivolto la propria attenzione ai cristalli bidimensionali (2D), ed in particolare ai dicalcogenuri dei metalli di transizione (TMDCs). Sebbene molto sforzo sia stato profuso per portare la loro produzione su scala industriale e integrarli all'interno dei futuri dispositivi elettronici ed optoelettronici, un metodo che porti alla scalabilità di questi materiali è ancora una sfida. Questa tesi è focalizzata sul meccanismo di crescita dei TMDCs e studia gli aspetti legati al processo di nucleazione e le fasi che portano alla formazione del cristallo, in modo da identificare le condizioni ottimali per promuovere l'estensione dei domini dei layer. Inoltre la tesi investiga i metodi che permettono la produzione di campioni TMDCs con una singola orientazione, consentendo di ottenere una migliore qualità dei layer, grazie alla riduzione del numero di bordi di grano formati dalla coalescenza di isole con diversa orientazione e aprendo strade nuove verso l'applicazione di concetti come la spin-valletronica ai dispositivi di nuova generazione. La prima parte della tesi è dedicata alla crescita di MoS2 su diversi substrati metallici, come Au(111), Ag(111) e Ag(110), con lo scopo di investigare l'effetto dell'interazione tra il layer sovrastante e il substrato, e l'effetto della simmetria del substrato sulla formazione di layer estesi di MoS2. Per ottenere layer altamente cristallini formati da domini orientati allo stesso modo, è stato implementato un nuovo protocollo di crescita su Au(111) ed adattato poi per gli altri substrati, sebbene esso su questi ultimi producesse due orientazioni, pur mantenendo alta la cristallinità. E’ stata caratterizzata la struttura elettronica del MoS2/Au(111), con particolare interesse alla polarizzazione di spin fuori dal piano delle bande vicino a K e -K della zona di Brillouin. Abbiamo successivamente esplorato la crescita di WS2 su Au(111) per valutare la versatilità del metodo di crescita da noi sviluppato. Mediante la calibrazione dei parametri di crescita, sono stati ottenuti dei campioni altamente cristallini di WS2 con una singola orientazione, che sono stati successivamente utilizzati per studiare il coupling elettrone-fonone, derivando quindi preziose informazioni per le applicazioni sui device. L'ultima parte della tesi è dedicata all'intercalazione dell'ossigeno al di sotto del grafene cresciuto su Ir(111), con lo scopo di verificare se la presenza del grafene possa cambiare la geometria degli atomi di ossigeno sull'Ir(111). Comparando i risultati ottenuti dall'adsorbimento dell'ossigeno sull'Ir(111) pulito con quelli in presenza del grafene, è stato possibile vedere che le strutture dell'ossigeno formate sotto il grafene possono essere riprodotte sull'Ir(111) dosando delle specie fortemente ossidanti come l'NO2, concludendo che il grafene non interferisce in maniera apprezzabile con l'interazione adsorbato-substrato. In the past ten years, after the isolation of graphene and the discovery of its outstanding properties, the scientific community has aimed its attention on a new class of materials, the two-dimensional (2D) atomic crystals, and particularly on the transition metal dichalcogenides (TMDCs). Though much effort is spent in the perspective of scaling up the production and integrating these materials in future electronic and optoelectronic devices, so far, a way towards sizable growth of 2D-TMDCs is still a challenge. This thesis is focused on the growth mechanism of TMDCs and studies aspects related to the nucleation process and the steps that lead to the formation of the crystals, in order to identify the optimal conditions to promote the increase of the domain size of the layers. In addition, the thesis investigates on the methods to produce TMDCs samples with a single domain orientation that allows to obtain better quality layers, thanks to the reduce presence of grain boundaries formed by the coalescence of islands with different orientations, and allows to open the path towards novel device concepts such as those based on the spin-valleytronic. This thesis is mainly devoted to the growth of MoS2 on different metallic substrates, namely Au(111), Ag(111) and Ag(110), with the aim of investigating the effect of the interaction between adlayer and substrate, and the effect of the substrate symmetry in the formation of extended layers of MoS2. In order to obtain a single domain orientation with respect to the substrate, a new growth protocol was developed on Au(111) and adapted also for the other substrates. The electronic structure of the MoS2/Au(111) sample was characterized, with a special attention to the out-of plane spin polarization of the bands near K and -K points of the Brillouin zone. We have subsequently explored the growth of single layer WS2 on Au(111) in order to evaluate the versatility of the developed growth method. By finding the proper growth parameters, we obtained high quality WS2 samples with a single orientation, that has been subsequently used to study the electron-phonon coupling thus deriving useful information for electronic device applications. The last part of the thesis is dedicated to the oxygen intercalation under graphene grown on Ir(111), with the aim of verifying whether the presence of graphene changes the adsorption geometry of the oxygen atoms on Ir(111). By comparing the results of oxygen adsorption on the bare Ir surface with those in the presence of graphene, it was possible to see that the oxygen overstructures formed under graphene can be reproduced on bare Ir(111) by dosing strongly oxidizing species such as NO2, concluding that graphene does not affect the adsorbate-substrate interaction.

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.

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