Graphene and beyond: development of new two-dimensional materials

During the three years of my PhD project, I explored part of the world of two-dimensional materials. My activity has been focused on the growth and analysis of two-dimensional materials by means of Surface Science techniques. For the growth both chemical methods, such as decomposition of gaseous precursors, as well as physical methods, such as evaporation of metals under ultra-high vacuum conditions, were used. The main method for studying the properties of these materials was photoemission spectroscopy from core levels and valence band. The materials were mostly grown and analysed directly in-situ, avoiding air exposure, which is known to alter their properties. Taking the cue from the results on single materials, I further widened my investigation toward complex heterostructures, i.e. artificial architectures of two-dimensional materials. Systems stemming from different combinations among graphene, hexagonal boron nitride and two-dimensional chalcogenides were produced and investigated with the aim to unravel the structure-activity relationships in heterostructures. The thesis is divided into four main chapters. The first is an introduction to the world of two-dimensional materials and summarized the main themes and the general structure of the thesis. The second chapter is dedicated to the growth and study of graphene, which is the archetype of this class of materials. After an introduction on its electrical properties and on its growth on conventional metal single crystals, the chapter is divided into four sections that cover specific issues. Paragraphs 2.1.1 and 2.1.2 examine the properties of graphene and nitrogen doped graphene in contact with ultra-thin layers of iron. The section 2.2 studies the reaction of water with graphene grown on nickel single crystal, for the production of hydrogen. The paragraph 2.3 describes the growth of graphene on an unconventional substrate: platinum nickel alloy (Pt3Ni). The third chapter is devoted to the study of other two-dimensional materials firstly introducing the studied materials: hexagonal boron nitride, transition metals dichalcogenides, other layered chalcogenides and heterostructures. Afterward, this chapter continues with three specific sections: paragraphs 3.1.1 and 3.1.2 are dedicated to two innovative methods for preparing heterostructures under ultra-high vacuum conditions. The section 3.1.1 presents a new strategy to synthesize monolayer in-plane heterostructure composed by graphene and hexagonal boron nitride, the 3.1.2 discusses a versatile route to create vertically stacked heterostructures of various two-dimensional materials. The last paragraph, 3.2, reports a detailed investigation of the electronic and chemical properties of a bulk layered chalcogenide, indium selenide. The fourth chapter summarizes the main conclusions of the work

Missing data patterns in runners' careers: do they matter?

Predicting the future performance of young runners is an important research issue in experimental sports science and performance analysis. We analyse a data set with annual seasonal best performances of male middle distance runners for a period of 14 years and provide a modelling framework that accounts for both the fact that each runner has typically run in three distance events (800, 1500 and 5000 meters) and the presence of periods of no running activities. We propose a latent class matrix-variate state space model and we empirically demonstrate that accounting for missing data patterns in runners' careers improves the out of sample prediction of their performances over time. In particular, we demonstrate that for this analysis, the missing data patterns provide valuable information for the prediction of runner's performance

A perspective on the application of spatially resolved ARPES for 2D materials

In this paper, a perspective on the application of Spatially- and Angle-Resolved PhotoEmission Spectroscopy (ARPES) for the study of two-dimensional (2D) materials is presented. ARPES allows the direct measurement of the electronic band structure of materials generating extremely useful insights into their electronic properties. The possibility to apply this technique to 2D materials is of paramount importance because these ultrathin layers are considered fundamental for future electronic, photonic and spintronic devices. In this review an overview of the technical aspects of spatially localized ARPES is given along with a description of the most advanced setups for laboratory and synchrotron-based equipment. This technique is sensitive to the lateral dimensions of the sample. Therefore, a discussion on the preparation methods of 2D material is presented. Some of the most interesting results obtained by ARPES are reported in three sections including: graphene, transition metal dichalcogenides (TMDCs) and 2D heterostructures. Graphene has played a key role in ARPES studies because it inspired the use of this technique with other 2D materials. TMDCs are presented for their peculiar transport, optical and spin properties. Finally, the section featuring heterostructures highlights a future direction for research into 2D material structures

Impact of Sb and Na Doping on the Surface Electronic Landscape of Cu2ZnSnS4 Thin Films

Open-circuit voltage deficiency is the key limiting factor in Cu2ZnSnS4 (CZTS) thin-film solar cells, which is commonly associated with band tails and deep gap states arising from elemental disorder. The introduction of dopants such as Na and Sb has led to improvement in device performance, yet their effects on the optoelectronic properties of CZTS are yet to be fully elucidated. In this Letter, we unraveled the effect of Sb and Na:Sb co-doping on the surface energy landscape of solution-processed CZTS films employing energy-filtered photoelectron emission microscopy. In the absence of the additives, 150 nm resolution photoemission maps reveal oscillations in the local effective work function as well as areas of low photoemission energy threshold. The introduction of dopants substantially reshapes the photoemission maps, which we rationalize in terms of Cu:Zn and Sn disorder. Finally, we establish unprecedented correlations between the photoemission landscape of thin films and the performance of over 200 devices

Anodization study of epitaxial graphene:insights on the oxygen evolution reaction of graphitic materials

The photoemission electron microscopy and x-ray photoemission spectroscopy were utilized for the study of anodized epitaxial graphene (EG) on silicon carbide as a fundamental aspect of the oxygen evolution reaction on graphitic materials. The high-resolution analysis of surface morphology and composition quantified the material transformation during the anodization. We investigated the surface with lateral resolution amp;lt;150 nm, revealing significant transformations on the EG and the role of multilayer edges in increasing the film capacitance.Funding Agencies| [EP/K035746/1]; [EP/M000605/1]</p

Aerosol Synthesis of N and N-S Doped and Crumpled Graphene Nanostructures

Chemically modified graphene-based materials (CMG) are currently attracting a vast interest in their application in different fields. In particular, heteroatom-doped graphenes have revealed great potentialities in the field of electrocatalysis as substitutes of fuel cell noble metal-based catalysts. In this work, we investigate an innovative process for doping graphene nanostructures. We optimize a novel synthetic route based on aerosol preparation, which allows the simultaneous doping, crumpling, and reduction of graphene oxide (GO). Starting from aqueous solutions containing GO and the dopant precursors, we synthesize N-and N,S-dual-doped 3D graphene nanostructures (N-cGO and N,S-cGO). In the aerosol process, every aerosol droplet can be considered as a microreactor where dopant precursors undergo thermal decomposition and react with the GO flakes. Simultaneously, thanks to the relatively high temperature, GO undergoes crumpling and partial reduction. Using a combination of spectroscopic and microscopic characterization techniques, we investigate the morphology of the obtained materials and the chemical nature of the dopants within the crumpled graphene sheets. This study highlights the versatility of the aerosol process for the design of new CMG materials with tailored electrocatalytic properties.</p

Experimental Studies of Electron Affinity and Work Function from Aluminium on Oxidized Diamond (100) and (111) Surfaces

none6Three different procedures are used to deposit aluminium onto O-terminated (100) and (111) boron-doped diamond, with the aim of producing a thermally stable surface with low work function and negative electron affinity. The methods are 1) deposition of a > 20 nm film of Al by high-vacuum evaporation followed by HCl acid wash to remove excess metallic Al, 2) deposition of <3 Å of Al by atomic layer deposition, and 3) thin-film deposition of Al by electron beam evaporation. The surface structure, work function, and electron affinity are investigated after annealing at temperatures of 300, 600, and 800 °C. Except for loss of excess O upon first heating, the Al + O surfaces remain stable up to 800 °C. The electron affinity values are generally between 0.0 and −1.0 eV, and the work function is generally 4.5 ± 0.5 eV, depending upon the deposition method, coverage, and annealing temperature. The values are in broad agreement with those predicted by computer simulations of Al + O (sub)monolayers on a diamond surface.openM. C. James, M. Cattelan, N. A. Fox, R. F. Silva, R. M. Silva, P. W. MayJames, M. C.; Cattelan, M.; Fox, N. A.; Silva, R. F.; Silva, R. M.; May, P. W

First-Principles Estimation of Core Level Shifts for Hf, Ta, W, and Re

A simple first-principles approach is used to estimate the core level shifts observed in X-ray photoelectron spectroscopy for the 4f electrons of Hf, Ta, W, and Re; these elements were selected because their 4f levels are relatively close to the Fermi energy. The approach is first tested by modeling the surface core level shifts of low-index surfaces of the four elemental metals, followed by its application to the well-studied material TaSe2 in the commensurate charge density wave (CDW) phase, where agreement with experimental data is found to be good, showing that this approach can yield insights into modifications of the CDW. Finally, unterminated surface core level shifts in the hypothetical MXene Ta3C2 are modeled, and the potential of XPS for the investigation of the surface termination of MXenes is demonstrated