Preparation and characterisation of optical and optoelectronic devices based in two-dimensional semiconductors

In the Materials Science field, two-dimensional materials have gained the scientific community attention in recent years. The change and the appearance of novel properties when their thickness is reduced to nanometric scale has special interest for its fundamental properties study for, from this base, the design and its implementation in devices. The wide variety of materials with the possibility of being exfoliated at the two-dimensional level opens the field to different applications, from optoelectronic devices, detection and sensing, energy storage, catalysis, medical applications and quantum information technologies, among others. This thesis gathers results in both directions: a fundamental science level study in two-dimensional materials less explored by the scientific community and its implementation in optoelectronic devices focused on different applications. In a first part, optical and electrical properties of III-VI semiconductors are studied. Photoluminescence (PL) of Gallium Selenide when thickness is reduced as well as its oxidation mechanism are discussed before its implementation. After this, electrical properties of 2D Indium Selenide (InSe) will be the focus of the study. This thesis demonstrates the usage of multiterraced nanosheets as p-n heterojunctions without joint defects. In addition, the thickness - work function dependence is studied by means of Kelvin-probe force microscopy. Finally, its usage for gas sensing due to the PL change in 2D samples is proposed and demonstrated. After its presentation, a second part of the thesis will focus on its implementation to take advantage of or to optimise the previous properties, comparing the results with monolayers of transition metal dichalcogenides (TMD) such as Tungsten Selenide or Molybdenum Selenide. Specifically, the recently demonstrated out-of-plane dipolar nature of the InSe nanosheets, which hampers its usual vertical excitation-collection performance, will be discussed. First, using silicon oxide microspheres on the 2D nanosheets, the PL collection will be enhanced. The behavior as an out-of-plane dipole will be demonstrated in 2D InSe and the low energy contribution of TMD PL emission due to the whispering gallery modes that occur in the microspheres. After this, these 2D materials will be studied in vertical heterostructures with perovskite nanocrystals, obtaining an enhancement in the PL collected in InSe nanosheets compared to that obtained with TMD, where PL detected is reduced by depositing a nanocrystalline perovskite on top, due to the orientation between perovskites emission and 2D dipolar natures. Closing this part, InSe nanosheets and TMD monolayers on photonic waveguides are studied, allowing excitation and collection in both horizontal and vertical directions. In a third and final part of the thesis two two-dimensional materials not explored in the literature will be presented: Bismuth Sulfide and Molybdenum Oxide. In the case of Bismuth Sulfide, a semiconductor material is presented that, in addition to its anisotropy between the exfoliation plane and the vertical direction, presents an unusual optical and structural anisotropy within the plane, demonstrated by several optical techniques such as Raman spectroscopy, photoluminescence, optical contrast, differential reflectivity and transmittance. The application of such anisotropy in optical fibers as a Fabry-Perot cavity in its core is demonstrated, from which results its birefringence is obtained in comparison with other reported laminar materials. Finally, Molybdenum Oxide is presented as a two-dimensional insulating material, uncommon in the two-dimensional materials field, where hexagonal Boron Nitride is the only one considered. The exfoliability of Molybdenum Oxide and its advantages over hexagonal Boron Nitride are demonstrated: its absence of low temperature defects and its almost-zero nuclear spin compared to hexagonal Boron Nitride, which hampers its usage in nuclear spintronics oriented devices. The usage of this new material as a two-dimensional insulator is demonstrated by the encapsulation of transition metal dichalcogenides monolayers within this material and studying its behavior at low temperature, from its single photon emitters behavior to the narrowing in the emission and absorption of the encapsulated semiconductor, obtained by photoluminescence and differential reflectivity at low temperature, respectively.