| Popis: |
The discovery of graphene marked the beginning of a new research field devoted to study and exploit the properties of materials confined at the two-dimensional limit. Among this class of materials, metallic single layer transition metal dichalcogenides were discovered to exhibit exotic physical phenomena, such as charge density waves or unconventional superconductivity. In this thesis, the charge carriers ultrafast dynamics of two-dimensional tantalum disulphide grown in the 1H structural polymorph are investigated by means of time- and angle-resolved photoemission spectroscopy. The time-dependent photoemission intensity is fitted to an energy- and momentum-resolved model in order to track the intriguing interplay between the band structure modifications and the changes in the electronic temperature. It will be shown how the combination of temperature-induced shifts of the chemical potential and changes in static screening contribute to the temperature-dependent energy shift of the valence band. Moreover, the energy exchange between the charge carriers and the lattice is addressed resulting in the observation of electronic temperature-dependent electron-phonon interaction.The physical properties of the tantalum-based two-dimensional compounds drastically change upon modification of the chemical composition. For instance, bulk tantalum monopnictides, a class of material comprised of a tantalum and a group V element, were recently found to host Weyl fermions, a novel state of quantum matter. In this thesis, an original growth method is presented for the unprecedented epitaxial growth of single layer tantalum phosphide compounds on low-index gold crystal surfaces. The structural and spectroscopic characterization of the compounds is carried out by a combination of scanning tunnelling microscopy, low-energy electron diffraction and x-ray photoelectron spectroscopy techniques. Intriguingly, several by-products of the reaction are found at the surface, including allotropes of phosphorous and phosphorous-driven reconstruction of the (110) gold surface. The selective growth of an isolated phase of single layer tantalum phosphide compounds could permit the unparalleled study of the electronic properties of a two-dimensional Weyl fermion and open the way to future technologies for the production of green energy |
