Raman Studies of 2-Dimensional van der Waals Materials

Presented herein are results of optical studies with emphasis on the Raman response, providing significant contribution to the knowledge of the field. In Mox W(1−x) S2 , confirmation of the behaviour of the excitonic properties is made. Raman measurements performed in this system allow investigation with unprecedented resolution, highlighting deviations in the high frequency A1g optical phonon mode from theoretical predictions, and previous experimental studies. In the low frequency, data confirms the trends in the shear and breathing interlayer modes in alloys between WS2 and MoS2 are well described by the modification in the square density. Resonant excitation for [Mo] < 0.4, highlights new evidence for the understanding of the hitherto unexplained ‘Peak X’ resonant feature. Diverse indium-selenium compounds isolated by novel means are studied. The ULF Raman modes of PDMS exfoliated InSe are documented for the first time, demonstrating the ε-phase with ABA stacking, with flake of thickness N manifesting (N − 1) shear modes owing to resonant excitation of few layer samples. InSe flakes encapsulated in hexagonal boron nitride manifest different stacking orders to those of PDMS exfoliated InSe, and were found to have significant contamination, with crystalline degradation of the monolayer flake, and peaks corresponding N2 & O2 rotational modes present. In2Se3 films grown epitaxially on GaSe display substrate-selective polymorphism, where α-, β-, & γ- phases are identified, in addition to regions of InSe. Laser photo-annealing is shown to drive a phase change from the groundstate β → α phase, which is against the thermodynamic gradient. MoSe2/WS2 twisted hetero-bi-layer structures are studied, where shear modes showing a linear softening from AA′ stacking towards the AB at 60° indicating reduced interlayer coupling, as expected from the difference in interlayer spacing of AA′ and AB ordering. High frequency modes in the heterobilayer also demonstrate some sensitivity in the relative angle, and are analysed in detail.