Optical and vibrational properties of anisotropic layered materials

Abstract : Atomically thin layered materials with low symmetry which leads to in-plane anisotropy of electronic and optical properties are particularly interesting from the point of view of applications. In this thesis, the optoelectronic properties of three layered materials with in-plane anisotropy, black phosphorus, rhenium disulfide and franckeite are investigated using optical spectroscopy. Black phosphorus (BP), with a puckered orthorhombic structure, has significant in-plane anisotropy and a direct bandgap which varies strongly with the number of layers. A significant hindrance from the point of view of the realization of BP-based devices is its degradation in ambient conditions, which can be prevented by encapsulation, for example using hexagonal boron nitride (h-BN). We have investigated the vibrational properties of h-BN encapsulated black phosphorus using Raman spectroscopy and report the first experimental observation of a theoretically predicted new peak slightly above the A1g mode. We explain its appearance by a difference of the mode frequencies in the inner and surface layers and discuss this effect in the context of the strong interlayer coupling in BP and the influence of the encapsulation on the lattice properties. The second part of this thesis presents the results of photoluminescence studies of few-layer rhenium disulfide. The optical response of ReS2 is dominated by two energetically nondegenerate anisotropic excitons X1 around 1.55 eV and X2 around 1.57 eV which can be observed in polarized absorption and emission. A controversy exists regarding the nature of the fundamental bandgap of ReS2 , in particular the presence of a crossover from indirect to direct bandgap with changing thickness. We have studied the polarized emission related to the excitonic states at low temperature as a function of the number of layers and compared the experimental results with the predictions of a kinetic model describing the radiative recombination and scattering between excitonic states. We conclude that the features observed in photoluminescence arise due to hot emission from direct excitonic states and that a smaller, indirect bandgap and a related excitonic state are responsible for the depopulation of the direct states, the relatively weak emission intensity and the observed X1 and X2 emission ratio. Franckeite, a complex sulfide mineral, has attracted attention as a natural type-II heterostructure composed of alternating pseudo-hexagonal and pseudo-tetragonal layers. The incommensurate character of the two lattices leads to one-dimensional rippling. The last part of this thesis describes our observation of anomalous polarization dependence of the Raman modes in exfoliated franckeite flakes, which suggested the presence of optical anisotropy and could be used as a rapid and non-invasive method to identify the rippling direction.
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Joanna Urban. Optical and vibrational properties of anisotropic layered materials. Physics [physics]. Universite Toulouse 3 Paul Sabatier (UT3 Paul Sabatier), 2019. English. ⟨tel-02194732⟩

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