ELECTRONIC AND STRUCTURAL THERMALLY INDUCED MODIFICATIONS ON 2D MoS2.

The growing demand for electronic components of ever-reducing dimensions,driven by the crisis of transistor miniaturisation, has shifted considerable attentiontowards nanomaterials, which represent promising candidates for alternativeelectronic and optoelectronic components offering high efficiency,low power consumption, and nanometric dimensions by their very nature [1].Within this landscape, monolayer molybdenum disulphide MoS2 has emergedas a leading candidate among two-dimensional transition metal dichalcogenidesfor next-generation optoelectronic and electronic device applications, owing toits direct bandgap, strong excitonic effects, and sensitivity to the surroundingenvironment [2, 3]. Despite significant progress in the field, a comprehensiveunderstanding of how synthesic routes, substrate interaction, and post-growthtreatments collectively determine the optical and electronic properties of MoS2monolayers remains an open challenge and constitutes the central motivationof the present work. In this thesis, the optical and electronic properties ofsingle-layer MoS2 are systematically investigated as a function of three keyvariables: method of synthesis, underlying substrate, and thermal treatmenteffects. Three distinct growth approaches are compared, namely gold-assistedexfoliation at room temperature (GAE), gold-assisted transfer (GAT), andchemical vapour deposition with liquid precursors (CVD), with the aim of establishinga direct correlation between the production route and the structuraland optical characteristics of the resulting monolayers. The structural, electronic,and optical properties of the samples are characterised by means ofRaman and photoluminescence spectroscopy, providing direct information on the structural quality and carrier density experienced by the monolayer. Therole of the substrate is examined by studying MoS2 monolayers deposited onthree representative surfaces, namely semiconductive gallium nitride (GaN),insulating silicon dioxide (SiO2), and conductive gold (Au). These materialsare all widely employed in current electronic and optoelectronic device technologies,and their selection is motivated by the aim of providing direct insightinto the prospective integration of MoS2 with existing device architectures andindustrially relevant substrates. They span a wide range of dielectric polarisabilitiesand charge-transfer characteristics that have been shown to exert asignificant influence on the structural, optical, and electronic properties of themonolayer, modulating them through substrate-induced doping and dielectricscreening effects. The main contribution of this thesis focuses on the systematicstudy of the effects of thermal annealing under controlled atmosphericconditions on the optical and electronic properties of MoS2 monolayers acrossthe different substrates investigated. The thermal treatment is shown to reducecontaminants and the density of sulphur vacancies, which represent theprimary intrinsic defect in MoS2 and act as sources of unintentional n-typedoping and non-radiative recombination centres [4, 5]. The resulting passivationof the material leads to a reduction in charge-carriers concentration,a narrowing of the photoluminescence linewidth, and an overall improvementin the optical quality of the samples, consistent with a suppression of defectmediatednon-radiative recombination pathways. The treatment emerges asgeneral in character, driving charge-carrier concentration and strain towardsequivalent conditions across samples prepared by different synthesic routes anddeposited on different substrates, independently of their initial state. Whilstthis convergence is robustly achieved in terms of doping and strain, the emissiveproperties tell a more nuanced story: although the photoluminescenceresponse broadly improves following the treatment, the magnitude and characterof this improvement are strongly modulated by the nature of the substrateand its interaction with the monolayer, reflecting the central role ofthe MoS2/substrate interface in determining the final optical quality of thematerial. Furthermore, electron irradiation treatments were carried out toinvestigate the role of sulphur vacancies as a source of unintentional dopingin monolayer MoS2 [6, 7], offering a controlled and reproducible means of introducing a well-defined defect density and quantifying its impact on thecharge-carriers concentration and optical response of the material. Strikingly,the monolayer exhibits a marked resilience to electron irradiation, a findingthat points towards its potential suitability for operation in extreme radiationenvironments and opens prospective avenues for applications in contextswhere conventional semiconductor technologies are known to suffer significantperformance degradation. Taken together, the results presented in this thesisdemonstrate that thermal annealing under controlled atmospheric conditionsconstitutes an effective and substrate-compatible strategy for the controlledtuning of the optical and electronic properties of MoS2 monolayers, providinga physically grounded framework for the optimisation of this material in viewof its integration into future device architectures. This thesis is organised intosix chapters:• Chapter 1 provides an introduction to two-dimensional materials andto MoS2 from the bulk down to the monolayer limit, presenting thestructural, electronic, and optical characteristics that have established itas a central material within this class.• Chapter 2 examines the intrinsic and extrinsic factors that influence theproperties of monolayer MoS2, surveying a range of post-preparation processesthrough which these properties may be systematically modified.• Chapter 3 describes the production methods employed for the preparationof the monolayers studied in this work, together with the fundamentalprinciples of the main characterisation techniques used, including adescription of the experimental setups and the data analysis procedures.• Chapter 4 presents a systematic study of the properties of monolayerMoS2 as a function of preparation route and substrate, establishing areference framework for interpreting the property variations induced bythe subsequent treatments.• Chapter 5 reports an in-depth investigation of the effects of thermal annealingperformed under controlled atmospheric conditions, namely activeand inert gas atmosphere, examining the impact of these treatmentson the material properties and structural integrity of the monolayer. • Chapter 6 presents a study of the exposure of monolayer MoS2 to electronirradiation, characterising the effects on its structural integrity andelectronic and optical properties.• Chapter 7 summarises the principal findings of the thesis and outlinesthe future perspectives and research directions that naturally arise fromthis work.