Electronic states and optical properties of quantum well heterostructures with strain and electric field effects

The aim of this work was to develop an envelope function method to calculate the electronic states and optical properties of complex quantum well heterostructures, and to demonstrate its effectiveness by application to some device structures of topical interest. In particular, structures have been considered which might form the basis of intensity modulators and polarization insensitive amplifier devices for light at a wavelength of 1.55 µm. The modulator structures considered all have the general form of two coupled quantum wells of different widths as the active region. The application of an electric field in the growth direction is intended to result in a shift in the energy and spatial localisation of the confined states and produce an increase in the absorption coefficient at longer wavelengths than the zero field absorption edge. The effectiveness of certain structures is examined in terms of field induced absorption increase at 1.55 µm. A system which shows a significant increase in absorption coefficient at this wavelength on application of a practical electric field has been identified as a possible candidate for an intensity modulator. In the case of the amplifier, the active region of the most promising structure considered consists of a stepped well which comprises two layers, one with tensile and one with compressive strain. It is known that the presence of the two oppositely strained layers can result in the TE and TM gain peaks appearing at similar photon energies. Our calculations show that a suitable choice of strain and layer widths can result in a small or zero difference between the TE and TM gains at 1.55 µm, which can be important for the polarization insensitive operation of devices in optical communications applications. In order to predict the optical properties of quantum well devices it is necessary to calculate the electron and hole states for a range of in-plane wavevectors. The calculations developed and carried out in this work are based on a multi-layer (eight band) k.p model including strain effects. The interfacial boundary conditions which result from approximations to Burt's exact envelope function theory are included in the model. The effect of an electric field is modelled by including a potential energy term in each layer Hamiltonian which is equal to the average energy shift across the layer in question due to the presence of the field. The model has been developed with flexibility in mind and has applications beyond the specific devices considered in this thesis

Simulation of Quantum Infrared Photodetectors

The topic of research is concerned with modelling and simulation of high temperature long wavelength infrared quantum photodetectors using advanced finite element methods. The aim is to devise novel designs based on quantum well structures to improve quantum efficiency, and operating temperature. These new designs rely on quantum confinement of electrons and holes inside a mixture of materials within which the energies of the carriers become discrete and differ from those observed in bulk materials. Type II InAs / GaSb superlattices is one of these meta–materials which offer a large flexibility in the design of infrared photodetectors, including the possibility to adjust the detected wavelength over a very wide range and to realize a suitable absobers’ unipolar barriers to suppress dark current while maintaining a significant portion of photocurrent at high temperatures. In order to validate this interest, A set of rigorous modelling tools based on multi-band k· p band structure theory and Boltzmann transport theory has been developed, which provide a better understanding of the electronic structure and transport in these heterostructures. The framework takes into account in particular the effect of the intrinsic strained property of the unintentional interfaces on the electronic structure and the optical properties. First, the finite element method is used to solve 8 × 8 k · pHamiltonians for InAs/GaSb superlattices with type II alignment to compute the optical and materials’ characteristics. For InAs and AlAsSb and alloys based detectors, An optical material library has been developed to generate all the needed bulk material properties. Secondly, the transfer matrix method or the Beer-Lambert law is used to compute the optical generation profiles in the device. Finally, the the finite volume method has been employed to solve the transport equations to compute the dark- and photo- currents, quantum efficiency among other device properties. Using this tools, new structures based on nBn and nBp architectures have been designed, with optimized design, which contribute to the realization of mid- and long- wave infrared photodetector based on Type-II superlattices InAs / GaSb material system as well as InAs/AlAsSb alloy mterial system. The developed model allows to study the underlying physics of these devices and to explain the factors limiting the device performances. Based on the simulation results, detectors involving absorbers with period composed of 14 Mono-Layer (ML) of InAs and 7 ML of GaSb was found to have a band gap wavelength close v to 11 μm and exhibit a lower dark current than those with period mainly composed of GaSb. The designed LWIR barrier device consists of a 4 μm thick p-type InAs-rich 14 ML InAs / 7ML GaSb LWIR T2SL absorber, a 200 nm thick p-type InAs/AlSb SL barrier and an n-type InAs-rich 14 ML InAs / 7ML GaSb LWIR T2SL contact layer. The 16.5ML InAs / 4ML AlSb superlattice of the BL is designed to give a smooth conduction band alignment and a large VBO of nearly 400 meV with the AL. The optimum doping level of absorber, barrier and contact layer are found to be 1 × 1016cm3, 5 × 1015cm3 and 1 × 1016cm3 respectively. This nBp detector design exhibits at 77 K a diffusion limited dark-current down to -300 mV with a dark-current level plateau as low as 8.5 × 10−5A/cm2 which is more than one order of magnitude lower compared to a similar PIN photodiode. Furthermore, this value is near the level of the MCT ‘rule 07’ demonstrating that InAs/GaSb SL detectors may provide new opportunities to replace the MCT technology in the LWIR spectral window given the MCT material instability problem at longer wavelengths. Moreover, we have demonstrated that the presence of the majority carriers’ barrier improves the current performances and the operating temperature over the standard PIN device. A temperature improvement of 20 K was found for a given current density of 2x10−4 A/cm−2 compared to a similar LWIR PIN device working at 60 K

Modelling and analysis of hydrogenated and dilute nitride semiconductors

Dilute nitride alloys, containing small fractions of nitrogen (N), have recently attracted research interest due to their potential for application in a range of semiconductor optoelectronic devices (e.g. lasers, light emitting diodes and single photon sources). Experiments have revealed that dilute nitride alloys such as GaAs1−xNx, in which a small fraction x of the arsenic (As) atoms in the III-V semiconductor GaAs are replaced by N, exhibit a number of unusual properties. For example, the band gap energy decreases rapidly with increasing N composition x, by up to 150 meV per % N replacing As in the alloy. This provides an electronic band structure condition which is indeed promising for the development of highly efficient and temperature stable semiconductor optoelectronic devices based on GaAs. We develop a fundamental understanding of this unusual class of semiconductor alloys and identify general material properties which are promising for application in light sources such as light emitting diodes and single photon sources. By performing detailed k·p calculations, we investigate the electronic band structure of nitrogen-free and dilute nitride III-V semiconduc tors. We reinforce our theoretical investigations by comparing our calculations to the results of experimental measurements. We first analyse the optical properties of type-I InAs1−xSbx/AlyIn1−yAs quantum wells (QWs) grown on relaxed AlyIn1−yAs metamorphic buffer layers (MBLs) using GaAs substrates, using a theoretical model based on an eight-band k·p Hamiltonian. The theoretical calculations, which are in good agreement with experiment, identify that the observed enhancement in PL intensity with increasing wavelength is associated with the impact of compressive strain on the QW valence band structure. Via a systematic analysis of strain-balanced quantum well structures we predict that growth of narrow (≈ 4-5 nm) strained wells could lead to a further doubling in optical efficiency for devices designed to emit at 3.3 µm. Analysing the properties and performance of strain-balanced structures designed to emit at longer wavelengths, we rec ommend the incorporation of dilute concentrations of nitrogen (N) to achieve emission beyond 4 µm. We confirm the benefits of growth on relaxed AlyIn1−yAs MBLs, with an Al composition y = 12% providing significantly improved band offsets and optical characterisics compared to a MBL with y = 6%. In the next part of the thesis, we investigate the design of type-II GaAsSb/GaAs quantum ring based (QR) intermediate band solar cells. We present an analytical solution of Schr¨odinger’s equation for a cylindrical QR of infinite potential depth to describe the evolution of the QR ground state with QR morphology, and then undertake 8-band k·p calculations for more de tailed analysis. The calculated electronic properties demonstrate several benefits, including (i) large hole ionisation energies, mitigating thermionic emission from the intermediate band, and (ii) electron-hole spatial overlaps exceeding those in conventional GaAs1−xSbx/GaAs quantum dots. Finally, we turn our attention to modelling hydrogenated InGaAsN/GaAs nanostructures for application as single photon sources at telecommunication wavelengths. The longest wavelength emission achieved to date from such structures is at 1.2 µm. By analysing their electronic band structure and comparing with existing literature data for InGaAsN/GaAs QW structures, we identify a range of QW compositions and well widths for which it should be possible to achieve hydrogenated InGaAsN/GaAs nanostructures emitting at 1.31 µm

Two-Dimensional Electronics and Optoelectronics

The discovery of monolayer graphene led to a Nobel Prize in Physics being awarded in 2010. This has stimulated further research on a wide variety of two-dimensional (2D) layered materials. The coupling of metallic graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) and black phosphorus have attracted a tremendous amount of interest in new electronic and optoelectronic applications. Together with other 2D materials, such as the wide band gap boron nitride nanosheets (BNNSs), all these 2D materials have led towards an emerging field of van der Waal 2D heterostructures. The papers in this book were originally published by Electronics (MDPI) in a Special Issue on “Two-Dimensional Electronics and Optoelectronics”. The book consists of eight papers, including two review articles, covering various pertinent and fascinating issues concerning 2D materials and devices. Further, the potential and the challenges of 2D materials are discussed, which provide up to date guidance for future research and development

Two-dimensional electronics and optoelectronics

The discovery of monolayer graphene has led to a Nobel Prize in Physics in 2010. This has stimulated research on a wide variety of two-dimensional (2D) layered materials. The coupling of metallic graphene, semiconducting 2D transition metal dichalcogenides (TMDCs) and black phosphorus has attracted tremendous amount of interest in new electronic and optoelectronic applications. Together with other 2D materials such as the wide band gap boron nitride nanosheets (BNNSs), all these 2D materials have led towards an emerging field of van der Waal 2D heterostructures. This book is originally published in Electronics (MDPI) as a special issue of “Two-Dimensional Electronics and Optoelectronics”. The book consists of a total of eight papers, including two review articles, covering important topics of 2D materials. These papers represent some of the important topics on 2D materials and devices. Promises and challenges of 2D materials are discussed herein, which provide a great recent guidance for future research and development

Theory of Linear and Nonlinear Optical Susceptabilities of Semiconductor Quantum Wells

In this thesis the optical properties of semiconductor quantum well structures suitable for optical integration, particularly those based on the GaAs/GaAlAs material system, are studied. The linear optical properties due to interband transitions, absorption and refractive index change, of semiconductor square, disordered and asymmetric step quantum wells are studied theoretically. The model used for the calculations is the density matrix formalism with intraband relaxation taken into account. The electronic properties of the different quantum well structures are obtained from the envelope function approximation including the valence band mixing model

HALL EFFECTS IN TOPOLOGICAL INSULATORS

Ph.DDOCTOR OF PHILOSOPH

Ferromagnetism and interlayer exchange coupling in then metallic films

Die vorliegende Arbeit befasst sich mit dem ferromagnetischen Kondo-Gitter-Modell (s-d-, s-f-Modell) für Filmstrukturen. Die Spin-Fermion-Wechselwirkung des Modells kommt in Materialien vor, in denen lokalisierte Spins mit beweglichen Ladungsträgern wechselwirken, wie etwa in (verdünnten) magnetischen Halbleitern, Manganaten, oder Seltene-Erd-Verbindungen. Die durch die Ladungsträger vermittelte, indirekte Wechselwirkung zwischen den lokalisierten Spins reicht von der langreichweitigen, oszillierenden RKKY-Austauschwechselwirkung im Falle schwacher Kopplung bis zur kurzreichweitigen Doppelaustausch-Wechselwirkung bei starker Spin-Fermion-Kopplung. Beide Grenzfälle werden in dieser Arbeit durch die Abbildung des Problems auf ein effektives Heisenberg-Modell erfasst. Der Einfluss von reduzierter Translationssymmetrie auf die effektive Austauschwechselwirkung und auf die magnetischen Eigenschaften des ferromagnetischen Kondo-Gitter-Modells wird untersucht. Curie-Temperaturen werden für verschiedene Parameterkonstellationen berechnet. Die Auswirkungen von Ladungstransfer und von Gitter-Relaxation auf die magnetische Oberflächenstabilität werden betrachtet. Die Diskussion bezieht sich auf die Modifizierungen der Zustandsdichte und der kinetischen Energie im dimensionsreduzierten Fall, da die effektiven Austauschintegrale eng mit diesen Größen verknüpft sind. Die Bedeutung von Spinwellen für den Magnetismus dünner Filme und an der Oberfläche wird gezeigt. Die Interlagen-Austauschkopplung stellt ein besonders interessantes und wichtiges Beispiel der indirekten Wechselwirkung zwischen lokalisierten Momenten dar. Im Rahmen einer RKKY-Behandlung wird die Kopplung zwischen Monolagen in dünnen Filmen untersucht. Sie wird entscheidend durch die Art der ebenen und senkrechten Ladungsträgerdispersion bestimmt und ist jenseits eines kritischen Wertes der Fermi-Energie stark unterdrückt. Schließlich wird die temperaturabhängige magnetische Stabilität von interlagen-gekoppelten dünnen Filmen behandelt und die Bedingungen für einen temperaturgetriebenen magnetischen Reorientierungsübergang werden diskutiert.This thesis is concerned with the ferromagnetic Kondo lattice (s-d, s-f) model for film geometry. The spin-fermion interaction of this model refers to materials in which localized spins interact with mobile charge carriers like in (dilute) magnetic semiconductors, manganites, or rare-earth compounds. The carrier-mediated, indirect interaction between the localized spins comprises the long-range, oscillatory RKKY exchange interaction in the weak-coupling case and the short-range double-exchange interaction for strong spin-fermion coupling. Both limits are recovered in this work by mapping the problem onto an effective Heisenberg model. The influence of reduced translational symmetry on the effective exchange interaction and on the magnetic properties of the ferromagnetic Kondo lattice model is investigated. Curie temperatures are obtained for different parameter constellations. The consequences of charge transfer and of lattice relaxation on the magnetic stability at the surface are considered. Since the effective exchange integrals are closely related to the electronic structure in terms of the density of states and of the kinetic energy, the discussion is based on the modifications of these quantities in the dimensionally-reduced case. The important role of spin waves for thin film and surface magnetism is demonstrated. Interlayer exchange coupling represents a particularly interesting and important manifestation of the indirect interaction among localized magnetic moments. The coupling between monatomic layers in thin films is studied in the framework of an RKKY approach. It is decisively determined by the type of in-plane and perpendicular dispersion of the charge carriers and is strongly suppressed above a critical value of the Fermi energy. Finally, the temperature-dependent magnetic stability of thin interlayer-coupled films is addressed and the conditions for a temperature-driven magnetic reorientation transition are discussed

Semiconductor Spintronics

Spintronics refers commonly to phenomena in which the spin of electrons in a solid state environment plays the determining role. In a more narrow sense spintronics is an emerging research field of electronics: spintronics devices are based on a spin control of electronics, or on an electrical and optical control of spin or magnetism. This review presents selected themes of semiconductor spintronics, introducing important concepts in spin transport, spin injection, Silsbee-Johnson spin-charge coupling, and spindependent tunneling, as well as spin relaxation and spin dynamics. The most fundamental spin-dependent nteraction in nonmagnetic semiconductors is spin-orbit coupling. Depending on the crystal symmetries of the material, as well as on the structural properties of semiconductor based heterostructures, the spin-orbit coupling takes on different functional forms, giving a nice playground of effective spin-orbit Hamiltonians. The effective Hamiltonians for the most relevant classes of materials and heterostructures are derived here from realistic electronic band structure descriptions. Most semiconductor device systems are still theoretical concepts, waiting for experimental demonstrations. A review of selected proposed, and a few demonstrated devices is presented, with detailed description of two important classes: magnetic resonant tunnel structures and bipolar magnetic diodes and transistors. In most cases the presentation is of tutorial style, introducing the essential theoretical formalism at an accessible level, with case-study-like illustrations of actual experimental results, as well as with brief reviews of relevant recent achievements in the field.Comment: tutorial review; 342 pages, 132 figure

Addressing and tailoring the electronic properties of semiconductor nanostructures: nanowires and transition metal dichalcogenides

Semiconductor materials played and still play a pivotal role in the technological development of modern life. From personal computer to data storage (e.g. solid-state disk-drives), from solar cells and cell phones to LEDs and biological sensors, there has always been a new system to study, a novel application to develop, and a solution to an otherwise unsolvable problem in which semiconductors play an essential role. All those technological goals have been achieved thanks to the synergic works carried out by basic researches in materials science, in particular in the semiconductor field. In the last decades, tremendous efforts have been made to miniaturize semiconductor devices at a nanometer scale, aiming at obtaining more compact devices with optimized speed and reduced power consumption. Unfortunately, or fortunately, the physical properties of any material dramatically change when the material dimensions are reduced to nanometer-lengths. Therefore, many efforts are required to understand the properties of any desired nanostructure, if they have to play a central role in technological applications. In recent years, a great interest has grown in the investigation and applications of nanowires (NWs). NWs are several micron-long filamentary-crystals whose diameters range from few to hundreds of nanometers. Their dimensions make NWs suitable to bridge the gap between the microscopic and the nanoscopic world in both research and technology fields. Although several types of materials can be grown in a NW form, e.g. metals, insulators, and semiconductors, the latter are the most interesting and promising materials. As a matter of fact, owing to their peculiar shape and dimensions, semiconductor NWs are valuable candidates for novel nanoscale devices, in which they act as both functionalized components and interconnects. Moreover, semiconductor NWs represent nanostructured systems for which some key parameters in device engineering, e.g. chemical composition, size, and crystal phase, are well controlled nowadays. This is mainly due to the technique used to grow NWs. NWs are usually fabricated via the vapor-liquid-solid (VLS) technique, in which metal nanoparticles are used as catalyst seeds to induce a one-dimensional crystal growth. This well-controlled process allows for the synthesis of a wide range of semiconductor systems in the NW form, ranging from IV-IV to II-VI, with a high degree of manageability of both the chemical composition and morphology. In addition, under suitable VLS conditions, non-nitride III-V NWs can crystallize in the hexagonal wurtzite (WZ) structure in materials that, instead, are notoriously stable in the cubic zinc-blende (ZB) structure. The opportunity to controllably grow NWs in different crystal phases, namely, the polytypism, adds a new degree of freedom in device engineering. The presence of a WZ crystal phase in many III-V NWs offers also the opportunity to address the electronic band structure of this poorly known structure, vii viii whose presence itself is a subject of fundamental interest in materials science and chemistry. As an example, there is no experimental information concerning the variation of the spin and transport properties, i.e. gyromagnetic factors and carrier effective-masses, respectively, when the phase transition from ZB to WZ occurs. Even the fundamental band-gap value of some WZ semiconductor materials have not been determined, yet. Therefore, a comprehensive study aimed at the investigation of the correlation between the NW electronic properties and NW crystal structure is mandatory nowadays. A great interest has grown also in the field of layered materials. Since the discovery of graphene in 2004, it has been understood the great potential of layered systems for advanced-technological applications. As a matter of fact, layered materials thinned to their physical limits -and usually referred to as two-dimensional (2D) materials- exhibit properties quite different from those of their bulk counterparts. A very wide spectrum of 2D materials has been then investigated. The most studied material is graphene because of its exceptional electronic and mechanical properties. Group VI transition metal dichalcogenides (TMDs) have also attracted the attention of researchers involved in the semiconductor field. TMDs have a crystal structure similar to that of graphite. Their layered structure, X-M-X, where M is the transition metal and X is the chalcogen atom, is characterized by weak interlayer van der Waals bonds and strong intralayer covalent bonds. That structure allows for an easy mechanical exfoliation, as in the graphene case, which is a major advantage of 2D materials, together with their synthesis techniques, cheap and easy as compared to the molecular-beam-epitaxy or metal-organic chemical-vapor-deposition techniques used for the fabrication of other nanostructured systems. The most surprising feature observed in 2D TMDs is the transition from an indirect band-gap in the infrared region to a direct band-gap in the visible region when they are thinned to the mono- layer limit. That feature, coupled with the TMD extremely high flexibility, elasticity, and resistance, makes TMDs suitable in the field of low-dimensional optoelectronic devices. In addition, the TMD high surface-to-volume ratio is valuable in biological fields, as they can be used as highly reactive sensors. Besides, the TMD unique properties in the single-layer limit of valley-valley coupling and valley-spin coupling render TMDs the suitable candidates for novel technologies based on valleytronic and spintronic. However, almost all these aforementioned properties are at the early stage of investigation and systematic studies are necessary before TMDs could be exploited in future applications. In this thesis, the electronic properties of InP NWs and MX2 TMDs, with M=Mo or W and X=S or Se, are thoroughly investigated mainly by means of optical spectroscopy, in particular photoluminescence (PL) in combination with external perturbations, e.g. high magnetic fields. The response of semiconductor TMDs to hydrogen irradiation is studied, too. The thesis is therefore structured in two parts, the first one, from chap. 1 to chap. 3, is devoted to InP NWs, the second one, from chap. 4 to chap. 6, is devoted to 2D TMDs. • In the first chapter, the high degree of freedom achieved in NW fabrication is presented and accounted for by the VLS technique, which is also discussed in details together with its recent development: the selective-area-epitaxy technique. Then, the differences between the structural, electronic, and optical ix properties of WZ and ZB crystal phases are discussed. The striking variation induced in the band structure by the crystal phase-transition is highlighted, too. Moreover, the different optical anisotropies of the two crystal phases are summarized. The chapter is concluded by a review of the technological applications of semiconductor NWs in the fields of optoelectronic, energy conversion, biosensoring, and as probes of elusive quantum effects. • The second chapter comprehends a systematic investigation of InP NWs in both the ZB and WZ crystal-phases. The morphological characteristics of the investigated samples as accessed through scanning-electron-microscopy, transmission-electron-microscopy, and selective-area-diffraction patterning are also presented. The basic optical properties of InP in both crystal phases are assessed by either PL or μ-PL experiments as a function of lattice temperature and power excitation. Polarization-resolved measurements are shown, too. The three lowest-energy critical points of the WZ band-structure are investigated by PL excitation (PLE) as a function of lattice temperature. A quantitative reproduction of those spectra allows for establishing the temperature depen- dence of the A, B, and C inter-band transitions. A comparison with ZB results is made, too. Finally, the hot-carrier effect in NWs is found and its dependence on NW morphology is investigated. • In the third chapter, the transport and spin properties of WZ InP are assessed by PL spectroscopy under high magnetic fields (up to 28 T ). A brief review of the effects that a magnetic field has on the energy and symmetry of exciton recombinations and of free-electron-to-acceptor and donor-to-acceptor transi- tions in WZ crystal is presented. Both diamagnetic shift and Zeeman splitting depend on the magnetic-field direction with respect to the NW symmetry-axis, namely the WZ cˆ-axis. That dependence has been investigated by applying the magnetic field either parallel or orthogonal to the NW axis. The obtained results are compared with the literature of both theoretical models of WZ InP and experimental results in other WZ compounds, such as GaN, InN, and ZnO. Finally, the non-linearity observed in the Zeeman splitting for magnetic fields above 10T and parallel to the NW axis is compared to a theoretical prediction. • In the fourth chapter, the lattice, electronic, and vibrational properties of 2D TMDs are described. In particular, the lattice structures of several polytypes are shown, with special emphasis on the 2H polytype, whose electronic and vibrational properties are investigated and its different properties in the bulk and single-layer regimes highlighted. Then, several methods aimed at reaching the mono-layer limit are presented and top-down exfoliations from bulk mate- rials are singled out from bottom-up syntheses. The chapter ends with a brief review of the technological applications of semiconductor 2D TMDs in the fields of optoelectronic, energy conversion and storage, and molecular sensing. • The fifth chapter comprehends a systematic investigation of the effects of hydrogen irradiation on the emission properties of single- and bi-layer TMDs, such as MoSe2 and WSe2. Firstly, a wide variety of experimental results con- x cerning MX2 optical band-gaps and vibrational mode-energies are summarized. A brief description of the investigated samples is presented, too. The optical properties of pristine samples are assessed by means of either μ-Raman or μ-PL experiments whose room- and low-temperature results agree well with the existing literature. Then, the pristine flakes are irradiated with progressively increasing doses of hydrogen and the results thus obtained are reported. In the single-layer regime, a worsening of the material optical quality is observed together with the appearances of very sharp peaks below the band-gap energy. Conversely, a small improvement in the PL efficiency is obtained in the bi-layer regime. Finally, a solution to the worsening of the optical quality observed in hydrogenated single-layer flakes is provided. • In the sixth chapter, the effects of hydrogen irradiation on the morphological and optical properties of multi-layer TMDs are discussed. Surprisingly, hy- drogenation favors unique conditions for the production and accumulation of molecular hydrogen just one or few layers beneath the crystal surface of all the multi-layer MX2 compounds investigated. That turns into the creation of atomically-thin domes filled with hydrogen molecules. The results of an atomic-force-microscopy and optical investigation of these new fascinating nanostructures are discussed. Finally, the possibility to tailor the dome posi- tion, size, and density is demonstrated, which provides a tool to manage the mechanical and electronic structure of 2D materials. • The main results obtained in this work are summarized in the conclusive remarks. • In the appendix, the theoretical basis of the optical-spectroscopy techniques here used, such as PL, PLE, magneto-PL, and Raman spectroscopy, are provided. PL and PLE are complementary techniques that enable a complete characterization of the electronic states of any optically-efficient material. Indeed, PL is an extremely sensitive probe of low-density electronic states, such as impurities or defects, while PLE can address the full density of states, i.e, it mimics absorption measurements, at least under certain approximations. On the other hand, PL spectroscopy under magnetic field allows for the determination of carrier effective-masses and g-factors, while Raman allows for getting information about the lattice properties of solids. A description of all the used experimental setups is also given. Finally, a description of the experimental apparatus used for hydrogen irradiation and atomic-force- microscopy measurements is provided. • Finally, a list of the publications to which the author of this thesis has contributed is provided, along with a list of poster/oral contributions to international conferences given by the author of this thesis during his PhD studies