Photo-induced Structural Dynamics of Molybdenum Disulfide

Abstract

In this dissertation, the photo-induced structural dynamics of molybdenum disulfide (MoS2) have been imaged over seven orders of magnitude in time – picoseconds (ps, 10-12 seconds) to tens of microseconds (10-5 seconds) – and in real space with up to 2 nanometer (nm, 10-9 meter) spatial resolution. Molybdenum disulfide, a heavily studied layered material due to its tunable electronic bandgap, is of interest for a variety of device applications and due to the tunability of many of its properties with strain (e.g., thermal properties and electronic bandgap). Ultrafast electron microscopy (UEM) combines the nanometer spatial resolution of transmission electron microscopy with the femtosecond temporal resolution of ultrafast pump-probe spectroscopy and was used to directly image the photo-excited dynamics in a freestanding, micrometer-size, multilayer MoS2 flake viewed along the [001] direction. Following photo-excitation with a 515-nm, 700-femtosecond laser pulse, individual wave trains were observed to emerge from nanoscale morphological features, particularly the vacuum-specimen interface. The strain waves propagated in-plane along or near contrast features at approximately the speed of sound (7 nm per ps) and at frequencies in the tens of gigahertz (GHz), which identified them as Lamb modes (quasi-shear plate waves). Using the dispersion relation for 50-nm thick MoS2 plate and the cut-off frequencies of an anisotropic plate, the waves were identified as second-order anti-symmetric or symmetric Lamb modes, which are differentiated by their symmetry with respect to the mid-plane of the specimen. The change in contrast was attributed to the tilting of planes into or out of the diffraction condition for anti-symmetric waves or a change in in-plane inter-planar spacing for the longitudinal wave coupled to the symmetric waves. The strain waves were then imaged on the nanosecond (ns, 10-9 seconds) timescales and found to undergo phonon-phonon scattering. This scattering resulted in the damping of the high-GHz oscillations and the emergence of incoherent oscillations with frequencies on the order of single GHz. On microsecond timescales, three megahertz (MHz, 106 hertz), whole-flake mechanical resonances were observed. These mechanical modes were found to have microsecond lifetimes and were simulated using finite element modeling. This work provides insight into the photoexcitation of high-velocity elastic strain waves and the subsequent mode-coupling processes and whole-flake oscillations. Work to understand the factors needed for high resolution ultrafast electron imaging is also presented. High-resolution ultrafast imaging first requires that the specimen drift is low in the requisite acquisition time at high magnifications. Thus, average specimen drift rates and the required acquisition times at these magnifications were measured. To produce the high temporal resolution in UEM, electron packets rather than a continuous beam of electrons are produced via the photoelectric effect upon irradiation with laser pulses. Next, the stability in the number of electrons in the packets was probed. The lattice fringes of tungsten disulfide nanoparticles (6 Å spacing) were successfully imaged.