Ultrafast Dynamics of 2D Materials The Case of MoS2 and Black Phosphorus

Understanding materials dynamics at the fundamental level requires a thorough investigation of the materials properties along all crystallographic directions and the effect of an external variable on these properties, where an example of these variables can be the applied pressure, the system temperature, or the substrate dielectric constant for a thin film etc. In the case of van der Waals materials, most thin film growth techniques and exfoliation techniques lead to a horizontally stacked layers making the study of the transient changes along certain crystallographic orientations challenging or inaccessible even with some of the most advanced probing techniques. Moreover, using van der Waals materials in making or developing a technological device necessitate the evaluation of the effect of the dielectric medium which is in contact with the van der Waals materials. In this work, ultrafast electron diffraction in reflection geometry is the method of choice to probe the structural dynamics in the out-of-plane direction of MoS2 and black phosphorus in chapter 3 and 4 respectively. In the case of MoS2 we were able to probe the monolayer dynamics at a low incidence angle and the supporting substrate at a larger incidence angle under similar excitation conditions. We found that the dynamics in the out-of-plane direction of a supported MoS2 monolayer are comparable to those in-plane and that MoS2 thermalizes in about 12 ps with a weak thermal boundary conductance at the interface which results in slower dynamics at the MoS2-sapphire interface. In chapter 4, we found that the long-lived carriers in black phosphorus (bp) trigger a lattice contraction at early time that is concurrent with a coherent lattice movement in the out-of-plane direction and most likely corresponds to a carrier coupling with the Ag phonon modes that have a large component in the out-of-plane direction. Thermalization in bp is slower comparted to MoS2 and is reached in about 50 ps evident through the Debye-Waller analysis of several orders of diffraction spots and through the lattice thermal expansion in the out-of-plane direction. These findings highlight the importance of using ultrafast electron diffraction in reflection geometry to capture a more complete picture of the materials dynamics and relaxation pathways. This work also brings to light the importance of the interlayer coupling in 2D materials and the role of the long-lived carriers in the observed ultrafast dynamics.