Trialogenuri dei metalli di transizione bidimensionali: dai fondamenti alle applicazioni ​

The discovery of graphene, with its unique properties, laid the groundwork for the exploration of materials in the two-dimensional (2D) flatland. After considerable research efforts in the last two decades, an extended library of 2D materials has been synthesized and investigated beyond the semimetallic graphene extending the landscape to semiconductive and insulating 2D materials with further technological appeal. To enrich the scenario, the existence of 2D magnets, such as transition metal trihalides (MX3), having shown long-range magnetic order has inspired the quest for new physics at the nanoscale. However, all the MX3 compounds are generally soluble in water, undergoing immediate degradation hampering their integration in real applications. The only promising exception is the CrCl3. Indeed, the micro-structural and chemical stability of CrCl3 is comprehensively investigated in this thesis. We prove systematically the isolation of single and few-layer flakes onto SiO2 substrate, which allows the identification of each multilayer, exhibiting long-term stability at the microscale under ambient conditions. Instead, at the atomic scale CrCl3 flakes show the formation of an extrinsic long-range ordered semi-oxidized O-CrCl3 surface structure and the existence of Cl vacancies, whose concentration can be tuned by thermal annealing. In combination with first-principles calculations, the electromagnetic properties of the pristine, oxidized, and Cl-defective CrCl3 monolayer phases are unveiled. From the combined studies we clarify that the external surface 2D phase of CrCl3 is subject to charge transfer. 2D CrCl3 is tested for gas sensing applications, exploiting its charge-transferred surface phase. We demonstrate that few-layer CrCl3 exhibits excellent capabilities as chemoresistive 2D gas sensor to humidity and NO2, H2, NH3 gases. Few layers of CrCl3 are produced with high-yield liquid phase exfoliation and with high reproducibility and long-term stability of the p-type electrical response. The gas sensing mechanism in real humid air conditions is also unveiled for different operating temperatures, passing from an ionic regime (@25 °C) to an electronic conduction regime (@100 °C). Exploiting the structural and chemical instability of MX3 compounds, an alternative and facile route to fabricating nanoporous vanadium oxide flakes through the spontaneous in-ambient degradation VI3 crystals is proposed for optoelectronic applications. VI3 proved to be a precursor for the production of nanoporous vanadia with intense and stable visible photoluminescence. Finally, the complicated magnetism and experimental anomalies of VX3 class are clarified for the indicative case of VCl3. Indeed, MX3 crystals have unfilled d-shells of the transition metal that make up an octahedral crystal field splitting into eg and t2g orbitals. In the case of VX3, the t2g states are unfilled inducing a Jahn-Teller symmetry breaking, which destabilizes the ground state. By synthesizing VCl3 in single crystalline form for the first time and studying its structural and electromagnetic properties, we discovered the polaronic mechanism that causes the lattice distortions and mixed valence. The polarons strongly affect the electromagnetic properties of VCl3 promoting the occupation of dispersion-less spin-polarized V-3d a1g states and band inversion with e'g states, and explain the different experimental anomalies reported in the literature for the entire VX3 class.