Photo-oxydation et spectroscopie Raman de couches minces de phosphore noir

Thin films of black phosphorus are recognized as anisotropic lamellar materials promising for their optical and electronic properties. From a chemical point of view, thin films of black phosphorus represent a challenge due to a fast degradation in air. Since black phosphorus has the potential to be produced on a large scale, optical characterization must be developed in order to ensure its integrity. To this end, Raman spectroscopy is a versatile tool for probing crystal vibration, which depend explicitly on inter-atomic interactions. The thickness of the CMs, the presence of defects or stresses should have an influence on the Raman spectra of black phosphorus, which behaviors can be isolate through a complete study. This thesis deals with the study of the mechanisms behind the instability of black phosphorus in air and with the characterization by Raman spectroscopy. The aim of this thesis is to develop a useful theory to understand the oxidation of black phosphorus and to provide a complete characterization of all Raman signatures observed, which in turn shows that Raman spectroscopy allows to quantify the integrity of thin films of black phosphorus. In a first part, we explore the degradation of black phosphorus. Assisted by Raman spectroscopy and transmission electron microscopy (TEM), we isolate the ingredients responsable for the degradation observed and identify the process of photooxidation in a wet and oxygenated environment. The modeling of the phenomenon with the help of Marcus-Gerischer theory allows to include the effect of the three key elements of degradation: oxygen, water and light and more importantly, it predicts the strong dependence on the thickness of black phosphorus observed in the experiments. The latter is associated with an effect of the quantum confinement and is shown experimentally using TEM imaging. In a highly corrosive environment, a strong degradation of the black phosphorus is accompanied by a decrease in the intensity of the Raman signal which follows the expected dependence as a function of the fluence of light under which the material is exposed. In oxidative conditions, we identify the Raman signature of the degradation using the integrated intensity ratio between the modes A1g and A2g . We also present one vii of the first studies of oxidation-free Raman spectra for atomic thicknesses (n) ranging from 1 to 5. A dependence of the phonon energy is observed and allows the thickness to be identified using Raman spectroscopy. Several new modes are presented without being able to be assigned to a particular process. One of these modes is assigned to a separation of Davydov leading to a conversion of an infrared mode into a Raman mode. In the second part, the origin of the other new modes is traced. Four modes are identified in a Raman study samples for n = 1 to 18 doing degradation experiments. They are associated to phonon-defects modes, which are second-order Raman processes involving the emission of a phonon with a non-zero quasi-momentum and a defect in the crystal. The analysis shows that the modes A1g and A2g can participate in the process and each of them are separated in two responses due to the anisotropic response of black phosphorus in the lamellar plane. Using simulations of the electronic and phononic bands structure, a modeling of the phonon-defect spectral response for the monolayer are constructed and used to the identification of the modes. A direct consequence of these modes is that it is possible to quantify the defects density in black phosphorus with Raman spectroscopy

Keyphrases: photooxydation, Marcus-Gerischer, Raman spectroscopy, Transmission electronic microscopy, Black phosphorus, Phonon-defect modes, Second order Raman