Characterization of wide band gap semiconductors and multiferroic materials

Structural, optical and electrical properties of zinc oxide (ZnO), aluminum nitride (AlN), and lutetium ferrite (LuFe2O4) have been investigated. Temperature dependent Hall Effect measurements were performed between 80 and 800 K for phosphorus (P) and arsenic (As) doped ZnO thin films grown on c-plane sapphire substrate by RF magnetron sputtering. These samples exhibited n-type conductivity throughout the temperature range with carrier concentration of 3.85 × 10 16 cm-3 and 3.65 × 10 17 cm-3 at room temperature for P-doped and As-doped ZnO films, respectively. The Arrhenius plots of free electron concentration of those doped samples showed double thermal activation processes with a small activation energy of about 0.04 eV due to shallow donors and a large activation energy of about 0.8 eV due to deep donors. The deep donor level could be related to oxygen vacancy. For undoped ZnO layer, growth condition was optimized to use as low background electron buffer layer. Hall Effect measurements showed that the resistivity and background electron concentration of the films decreases as the substrate temperature increases. The film deposited at 900 oC has more than two orders less background electron concentration than that deposited at 300 °C. Based on photoluminescence and Transmission Electron Microscopy (TEM) analysis, the ZnO grown under this condition is formed to be a greatly reduced density of stacking faults. Transmission electron microscopy (TEM) was employed to investigate dislocations in aluminum nitride (AlN) epilayers grown on sapphire substrate using three-step growth method by metal organic chemical vapor deposition (MOCVD). AlN epilayers grown by this method have smooth surfaces, narrow width of X-ray rocking curves, and strong band edge photoluminescence (PL) emissions with low impurity emissions. Transmission electron microscopy revealed that most of the threading dislocations are annihilated within 300 nm. Stacking faults are greatly reduced in the epilayers grown by this method resulting in very low screw type threading dislocation density. Dominant threading dislocations in the AlN epilayers are edge type originated from misfit dislocations (MD). The electro-optical and temperature-dependent electrical-transport properties of LuFe2 O4 (LFO) thin films have been investigated. The LFO thin films at 78 K showed the electro-optical effects of size up to 5% near the Fe2+ d to d on-site electronic transition. In the three-dimensional charge-ordered state of LFO, we observed hysteresis in dc voltage-current measurements and nonlinear voltage-current relationship in transient response of voltage under current pulses. The electro-optical and electrical properties of LFO thin films are interpreted in terms of the field-induced changes of the charge-ordered state mediated by the spin-charge-lattice coupling effect. We also discuss possible mechanisms of the complex electrical properties and electro-optical effects in conjunction with the Maxwell-Wagner effects

Second harmonic generation from ZnO films and nanostructures.

Zinc oxide ZnO is a n-type semiconductor having a wide direct band gap (3.37 eV) as well as a non-centrosymmetric crystal structure resulting from hexagonal wurtzite phase. Its wide transparency range along with its second order nonlinear optical properties make it a promising material for efficient second harmonic generation processes and nonlinear optical applications in general. In this review, we present an extensive analysis of second harmonic generation from ZnO films and nanostructures. The literature survey on ZnO films will include some significant features affecting second harmonic generation efficiency, as crystalline structure, film thickness, surface contributes, and doping. In a different section, the most prominent challenges in harmonic generation from ZnO nanostructures are discussed, including ZnO nanowires, nanorods, and nanocrystals, to name a few. Similarly, the most relevant works regarding third harmonic generation from ZnO films and nanostructures are separately addressed. Finally, the conclusion part summarizes the current standing of published values for the nonlinear optical coefficients and for ZnO films and nanostructures, respectively

Spinodal nanodecomposition in magnetically doped semiconductors

This review presents the recent progress in computational materials design, experimental realization, and control methods of spinodal nanodecomposition under three- and two-dimensional crystal-growth conditions in spintronic materials, such as magnetically doped semiconductors. The computational description of nanodecomposition, performed by combining first-principles calculations with kinetic Monte Carlo simulations, is discussed together with extensive electron microscopy, synchrotron radiation, scanning probe, and ion beam methods that have been employed to visualize binodal and spinodal nanodecomposition (chemical phase separation) as well as nanoprecipitation (crystallographic phase separation) in a range of semiconductor compounds with a concentration of transition metal (TM) impurities beyond the solubility limit. The role of growth conditions, co-doping by shallow impurities, kinetic barriers, and surface reactions in controlling the aggregation of magnetic cations is highlighted. According to theoretical simulations and experimental results the TM-rich regions appear either in the form of nanodots (the {\em dairiseki} phase) or nanocolumns (the {\em konbu} phase) buried in the host semiconductor. Particular attention is paid to Mn-doped group III arsenides and antimonides, TM-doped group III nitrides, Mn- and Fe-doped Ge, and Cr-doped group II chalcogenides, in which ferromagnetic features persisting up to above room temperature correlate with the presence of nanodecomposition and account for the application-relevant magneto-optical and magnetotransport properties of these compounds. Finally, it is pointed out that spinodal nanodecomposition can be viewed as a new class of bottom-up approach to nanofabrication.Comment: 72 pages, 79 figure

Nanoestruturas baseadas em ZnO e GaN para aplicações optoeletrónicas : síntese e caracterização

Wide bandgap semiconductors, such as GaN and ZnO, are materials with a wide range of applications in several important technological areas including lighting, transparent electronics, sensors, catalysis or photovoltaics. This thesis focuses on the study of GaN and ZnO, including related compounds. In the first case, the emphasis is given to the incorporation of rare-earth (RE) ions (4fn) into the nitride hosts envisaging to contribute for the development of “all-nitride” solid state lighting devices. GaN and related III-nitrides ternary alloys appear as excellent hosts for the incorporation of these ions. The use of RE ions is motivated by the electromagnetic widespread spectral range (from the ultraviolet to the near infrared) covered by the intraionic radiative relaxation of the trivalent charged ions. Ion implantation appears as an alternative approach to doping since it allows the introduction of impurities in a controlled way and without solubility limits. GaN samples with different dimensionalities were analysed and their influence in the luminescence properties of the RE3+ was investigated. Photoluminescence (PL) measurements revealed that after thermal annealing a successful optical activation of the RE3+ was achieved for the samples implanted with the different RE3+. A detailed spectroscopic analysis of RE3+ luminescent tarnsitions is presented by using temperature dependent steady-state PL, room temperature PL excitation and time resolved PL. This thesis also aims to the growth and characterization of ZnO micro and nanostructures, through a new growth technique designated by laser assisted flow deposition (LAFD). LAFD is a very high yield method based on a vapour-solid mechanism that enables the growth of ZnO crystals in a very short timescale. LAFD was used in the growth of wurtzite micro/nanocrystalline ZnO with different morphologies (nanoparticles, tetrapods and microrods) as revealed by the extensive morphological characterization. Moreover, structural analysis evidenced the high crystalline quality of the produced crystals. The optical properties of the as-grown ZnO crystals were fully investigated by luminescence techniques, which revealed a high optical quality of the LAFD produced ZnO. In addition to the unintentionally doped micro/nanocrystals, ZnO/Ag and ZnO/carbon nanotubes (CNT) composite structures were also synthesized by LAFD. Silver-related spherical particles were found to be inhomogeneously distributed at the microrods surface, accumulating at the rods tips and promoting the ZnO nanorods re-nucleation. For the case of the ZnO/CNT composites two main approaches were adopted: i) a direct deposition of ZnO particles on the surface of vertically aligned multi-walled carbon nanotubes (VACNTs) forests without employing any additional catalyst and ii) ZnO/CNT buckypaper nanocomposites. It was found that the use of the LAFD technique carried out in framework of the first approach preserves the CNTs structure, their alignment, and avoids the collapse of the VACNTs array, which is a major advantage of this method. Additionally, taking into account that a crucial step in designing modern optoelectronic devices is to accomplish bandgap engineering, the optical properties of CdxZn1-xO alloy were also evaluated. A tuning of the ZnO bandgap towards the visible spectral region was accomplished by alloying this semiconductor with CdO. Finally, the potential application of the LAFD produced ZnO structures in the photocatalysis and photovoltaic fields was tested.Os semicondutores de elevado hiato energético, como é o caso do GaN e do ZnO, são materiais com aplicações em diversas áreas tecnológicas, que incluem, por exemplo, iluminação, eletrónica transparente, sensores, catalisadores ou fotovoltaicos. Esta tese é dedicada ao estudo de materiais baseados em GaN e ZnO, sendo dada ênfase à incorporação de iões terras-raras (RE) nas matrizes de nitretos, com a finalidade de contribuir para o desenvolvimento de dispositivos de iluminação de estado sólido. O uso dos iões RE é motivado pelas suas emissões intraiónicas abrangendo uma ampla gama espectral (do ultravioleta ao infravermelho próximo), quando estão no seu estado de carga trivalente. A implantação iónica surge como uma alternativa para a dopagem destes materiais, uma vez que permite a introdução de dopantes de uma forma controlada e independente dos limites de solubilidade dos iões nas matrizes. Amostras de GaN com diferentes dimensionalidades foram analisadas e a sua influência nas propriedades luminescentes dos RE3+ foi investigada. Medidas de fotoluminescência (PL) revelaram que, depois de um tratamento térmico, a ativação ótica dos iões foi bem-sucedida para as amostras implantadas com os diferentes iões. Uma análise espectroscópica detalhada das transições luminescentes dos RE3+ foi realizada usando técnicas como a PL em estado estacionário e transiente e excitação da fotoluminescência. Outro objetivo desta tese foi o crescimento e caracterização de micro e nanoestruturas de ZnO, recorrendo a uma nova técnica de crescimento designada por deposição de fluxo assistida por laser (LAFD). Este é um método com elevado rendimento, baseado num mecanismo sólido-vapor, que permite o crescimento de ZnO com diferentes morfologias (nanopartíclulas, tetrapodes e microfios). A sua análise estrutural pôs em evidência a excelente qualidade cristalina do ZnO produzido por esta técnica. As propriedades óticas foram também investigadas através de fotoluminescência, revelando a sua elevada qualidade ótica. Para além dos cristais não dopados intencionalmente, foram ainda preparados compósitos com prata e nanotubos de carbono (CNTs). No primeiro caso, foram observadas partículas esféricas de prata distribuídas de uma forma não uniforme na superfície dos microfios, mostrando uma tendência para se acumularem no topo destes e promovendo a sua renucleação. No caso dos compósitos ZnO/CNTs, foram usadas duas abordagens: i) deposição de partículas de ZnO diretamente no topo dos CNTs alinhados verticalmente, sem a utilização de nenhum catalisador adicional, e ii) produção de ZnO/CNTs buckypapers. No primeiro caso, a técnica de LAFD provou manter o alinhamento dos CNTs, evitando o seu colapso, sendo esta uma vantagem do método usado. Adicionalmente, tendo em consideração a importância do controlo do hiato energético dos materiais a ser aplicados em dispositivos optoelectrónicos, foram também estudadas as propriedades óticas da liga CdxZn1-xO. Devido ao aumento da fração molar de CdO na liga ternária observou-se um desvio do hiato do ZnO para a região visível do espetro eletromagnético. Finalmente, as estruturas de ZnO crescidas por LAFD foram testadas em dispositivos fotovoltaicos e em estudos de fotocatálisePrograma Doutoral em Nanociências e Nanotecnologi

La doped SrTiO3 Based Oxide Thermoelectrics

In this project, the thermoelectric properties of La-doped Sr3Ti2O7, Ca3Ti2O7 and SrTiO3 ceramics sintered in air and N2/5%H2 have been investigated. Different defect chemistry models were studied in an attempt to improve the thermoelectric performance of La-doped SrTiO3 and related systems. For La-doped Sr3Ti2O7 Ruddlesden-Popper (RP) ceramics, the three starting nominal compositions, (Sr1-xLax)3Ti2O7 (electronic donor-doping), (Sr1-3y/2Lay)3Ti2O7 (A-site vacancies) and (Sr1-zLaz)3Ti2-z/4O7 (B-site vacancies) were sintered under air and flowing N2/5%H2 at 1773 K. The La-doped air sintered ceramics were all off-white/yellow in appearance and electrical insulators with low bulk conductivity and a high activation energy, Ea, confirming that solid solubility of La was small and that electronic (donor-doping) compensation does not exist for La-doping of ceramics sintered in air. Processing ceramics under reducing atmosphere is sufficient to form dark single-phase samples for the x series (electronic donor-doping series) up to (Sr0.95La0.05)3Ti2O7 (x = 0.05), indicating that reducing conditions and oxygen-loss from the Sr3Ti2O7 lattice are conducive towards electronic La-doping in Sr3Ti2O7-δ ceramics and to extend solid solubility. In all N2/5%H2 sintered samples, an insulating surface layer associated with SrO volatilization and oxygen up-take (during cooling) from the sintering process occurred that, unless removed, masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that were phase mixtures containing highly conductive (Sr, La)TiO3-δ, ST. This highlights the superior power factor properties of reduced perovskite-type ST compared to reduced RP-type Sr3Ti2O7 and serves as a warning for the need to identify low levels of highly conducting perovskite phases when exploring rare-earth doping mechanisms in RP-type phases. For La-doped SrTiO3, the favoured mechanism for doping was through the formation of A-site vacancies independent of P_(O_2 ). Samples with A-site vacancies (Sr1-3y/2LayTiO3) had the highest electrical conductivity for the same La content (i.e. 10 at. %) sintered at 1773 K, independent of P_(O_2 ). In the Sr1-3y/2LayTiO3 system, air sintered ceramics were metrically cubic for 0.1 ≤ y < 0.30, tetragonal with short range strontium vacancy, VSr, ordering for 0.30 ≤ y < 0.50, then orthorhombic with long range ordering of VSr for y ≥ 0.50 by X-ray powder and electron diffraction at room temperature. For samples reduced in N2/5%H2, compositions with 0.1 ≤ y ≤ 0.50 were metrically cubic. Short range VSr ordering and an orthorhombic structure with long range VSr ordering were observed for y = 0.50 and 0.63, respectively. Samples with y = 0.15 sintered in N2/5%H2 revealed the largest dimensionless thermoelectric figure-of-merit (ZT = 0.41 at 973 K) reported for n-type SrTiO3 based materials, suggesting that the accommodation of La through the formation of VSr accompanied by reduction in N2/5%H2 represents a new protocol for the development of oxide-based thermoelectrics

Defect-rich Titanium (IV) Oxide and Zirconium (IV) Oxide Nanostructures for Ultra-efficient Photocatalyst and High-Tc Dilute Ferromagnetic Semiconductor Applications

In transparent conductive oxide nanostructures, oxygen vacancy defects (neutral, singly charged, and doubly charged defects) are found to be one of the most important and prevalent defects due to the enhancement of light absorption and charge transport properties, improved performance in photoelectrochemical water splitting reaction driven by visible light, and the introduction of ferromagnetism. However, the traditional methods of creating of oxygen vacancies, including hydrogen thermal treatment, high energy particle bombardment, and thermal annealing under oxygen depletion condition, generate oxygen vacancies mostly at the surface of the nanostructures. The performance of these nanostructures is therefore limited to surface oxygen vacancies. More importantly, the surface oxygen vacancies are found to be highly susceptible to oxidation upon long-term exposure to air. In addition, the dependence of optical, photoelectrochemical, and magnetic properties on the surface morphology and oxygen vacancy defect composition of the one-dimensional transparent conductive oxide nanostructures are not well understood. For these reasons, there is a great interest in the development of a novel method to create oxygen vacancies both at the surface and in the bulk of transparent conductive oxide nanostructures. As two of the most important functional transparent conductive oxides, TiO2 and ZrO2 are specially preferred catalysts for photoelectrochemical water splitting reaction because of their suitable band edge positions for hydrogen evolution and exceptional stability against photocorrosion upon optical excitation. In the present work, highly oxygen-deficient TiO2 and ZrO2 nanostructures including nanobricks, nanopopcorns, nanowires and nanosheets are prepared on Si substrates by a one-step catalyst-assisted pulsed laser deposition method. The use of a high vacuum system and Ar flow, and precise control of the gold-nanoisland catalyst size, interfacial SiO2 layer thickness, and growth temperature have enabled us to produce oxygen-deficient single-crystalline nanostructured films with different morphologies and different composition of oxygen vacancy defects. The oxygen-deficient TiO2 nanostructures have been chosen as the starting point of the present study. For TiO2 nanowires reported to date, the oxygen vacancies have been found to form just within a few tens of nanometers at the outer surface of these nanowires, and the photocurrent density is significantly reduced by two to three orders of magnitude when ultraviolet light (<430 nm) is filtered out from the AM 1.5G simulated sunlight. Here, we demonstrate, for the first time, that by manipulating the thickness of the SiO2 buffer layer, together with appropriately optimized growth temperature and growth environment, it is possible to synthesize TiO2 nanobelts, and corrugated nanowires, straight nanowires, and tapered TiO2 nanowires decorated with TiO2-nanocrystallites using a one-step catalyst-assisted pulsed laser deposition method. We further show that the amount of oxygen vacancy defects depends on the growth temperature, while our electrochemical impedance measurement confirms the lower charge transfer resistances at the depletion layer of the decorated nanowires. Photoelectrochemical measurement under simulated sunlight (100 mW/cm2) shows that the photocurrent density measured at 0.5 V (vs Ag/AgCl) for the decorated nanowires (1.5 mA/cm2) is found to be significantly higher than those of nanobelts (0.18 mA/cm2), nanobricks (0.25 mA/cm2), straight nanowires (0.6 mA/cm2), and corrugated nanowires (0.94 mA/cm2). More importantly, the photocurrent density of defect-rich decorated nanowires is reduced only slightly from 1.5 mA/cm2 to 1.4 mA/cm2 when the ultraviolet light (<430 nm) is filtered out, which represents 87% of the overall photocurrent. The high activity in the visible region can be attributed to a larger amount of oxygen vacancy defects in decorated nanowires, and to the enhanced charge transfer from the nanocrystallites to the cores of the decorated nanowires. To extend the aforementioned method to other transparent conductive oxides, ZrO2 nanowires with different morphologies and compositions of oxygen vacancy defects have been prepared by tuning the gold-nanoisland catalyst size and growth temperature. The as-grown hierarchical ZrO2 nanowires (12.1 mA/cm2), consisting of individual ZrO2 nanowires decorated with ZrO2 nanoplates, have shown 1.9 times more photocurrent density than that of as-grown regular nanowires (6.4 mA/cm2). The photoelectrochemical performance of as-grown nanostructures has been further improved by partial delamination or flaking of the as-grown nanostructured film by a simple hydrofluoric acid treatment. The photocurrent density of the partially delaminated hierarchical nanowires, obtained after the HF treatment, is found to increase remarkably to 42.4 mA/cm2, i.e. nearly 3.5 times that of the as-grown hierarchical nanowires due to improvement of the composition of oxygen vacancy defects, charge carrier transport resistance, and specific surface area of the as-grown single-crystalline hierarchical nanowires. More importantly, the HF-treated partially delaminated hierarchical nanowire film electrode provides the highest cathodic photocurrent of 32.2 mA/cm2 (at −0.8 V vs reversible hydrogen electrode) in the visible light (>400 nm) region reported to date. The variation of the pulsed deposition growth temperature also produces ZrO2 nanostructures with different specific surface areas and amounts of oxygen vacancy defects, including nanobricks, nanopopcorns, nanospikes, and nanowires. The presence of different types of oxygen vacancies (neutral, singly charged, and doubly charged defects) and their correlation to the Zrx+ oxidation states (4>x>1) are found to affect the exchange interactions and the ferromagnetic properties of these nanostructures. The saturation magnetization measured at 2000 Oe for the nanowires (5.9 emu/g) is found to be significantly greater than those of nanospikes (2.9 emu/g), nanopopcorns (1.2 emu/g), and nanobricks (0.6 emu/g), while the coercivity for the nanowires (99 Oe) is approximately twice that of the nanobricks (50 Oe). More importantly, a Curie temperature (Tc) considerably above room temperature has also been observed for these ZrO2 nanostructures, including nanowires (700 K), nanospikes (650 K), nanopopcorns (550 K), and nanobricks (400 K). We also provide the first experimental evidence that it is the amount of defects in and not the phase of ZrO2 that controls the ferromagnetic order in undoped ZrO2 nanostructures. The present work therefore provides, for the first time, a direct correlation between the surface morphology and the composition of oxygen vacancy defects with the photoelectrochemical and ferromagnetic properties of the TiO2 and ZrO2 nanostructures

Doping and its effect on ZnO properties

Ph.DDOCTOR OF PHILOSOPH

From Dopant to Source: The Use of Zinc as an Enabler in the Synthesis of Nanostructures by Metalorganic Vapour Phase Epitaxy

As conventional methods of semiconductor fabrication approach fundamental physical limits, new paradigms are required for progress. One concept with the potential to deliver such a paradigm shift is the bottom-up synthesis of semiconductor nanostructures. Beyond further scaling, bottom-up methods promise novel geometries and heterostructures unavailable by conventional top-down methods. This is particularly true in the case of nanostructure growth by the vapour-liquid-solid (VLS) method. Commonly realised using existing vapour phase epitaxy techniques, a range of high-performance VLS devices have now been demonstrated including photovoltaic cells, lasers and high-frequency-transistors. In this dissertation, selected applications of diethylzinc (DEZn) are used to step through a range of opportunities and challenges arising from the VLS synthesis of semiconductor nanostructures by metal-organic vapour phase epitaxy (MOVPE). These applications are broadly grouped into four chapters focusing on the use of zinc firstly as a dopant and then morphological agent, internal quantum efficiency (IQE) enhancer and finally, source. In the context of doping, relatively high DEZn flows are shown to alter the morphology of GaAs nanowires by introducing planar defects, kinking and seed-splitting. Growth studies are used to establish the threshold for these effects and thus the range of DEZn flows suitable for doping. Successful incorporation of up to 5 x1020 Zn/cm3 is demonstrated through atom probe tomography (APT) and electrical characterisation. Building on these results, DEZn is then used to generate periodic twinning in GaAs nanowires. The morphology and overgrowth of these twinning superlattice (TSL) nanowires is studied. Unlike for other III-V materials, twin spacing is found to be a linear function of nanowire diameter. By analysing the probability of twin formation, this result is related to the relatively high twin plane and solid-liquid interface energies of GaAs. Values for the wetting angle and supersaturation of the seed particle during growth are also extracted. In addition to acting as a dopant, zinc is also shown to produce an orders of magnitude increase in the IQE of GaAs nanowires. Performance gains are quantified by measuringthe absolute efficiency of individual nanowires. This increase in IQE with doping enables room-temperature lasing from unpassivated GaAs nanowires. The performance of doped nanolasers, including the transition to lasing, is fully characterised. In addition to increasing radiative efficiency, Zn doping also increases differential gain while reducing the transparency carrier density. The threshold pump power of a Zn doped nanowire is thus shown to be less than that of an equivalent AlGaAs passivated structure. In the final chapter, DEZn is used as a source for the growth of ZnAs, ZnP and ZnSb nanostructures by MOVPE. A range of growth conditions, substrates and seed materials are investigated. Individual nanostructures of both ZnAs and ZnP are shown to exhibit excellent optoelectronic performance with emission from individual nanostructures at 1.0 and 1.5 eV respectively. Overall, this thesis underlines the vast range of possibilities offered by VLS growth and opens to the door to both a variety of new techniques and new family of semiconductor nanomaterials

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