Photo-induced Structural Dynamics in Transition Metal Dichalcogenides

Abstract

Ultrafast electron microscope (UEM), a combination of transmission electron microscopy and laser-based pump-probe techniques, facilitates ultrafast imaging, diffraction, and electron-spectroscopy with high spatial resolution. The unique advantages of UEM enable local ultrafast dynamic studies in materials, nano-system, and biology. The performance of UEM, such as its temporal and energy resolutions and coherence, is largely determined by the quality of electron beam. In this thesis, the beam dynamics in our UEM with a thermionic gun was studied. The influence of cathode geometry and Wehnelt bias voltage on the electron pulse dynamics is determined through experiments and finite element simulations. A guard ring cathode can effectively address the problem of shank-emitted electrons in traditional truncated tip geometries, allowing UEM operation at minimum Wehnelt bias and improving the temporal resolution under realistic conditions. A sub-ps temporal resolution can be reached with few electrons in one pulse. Compared to the 300 fs laser pulse width, the temporal duration of the electron pulse is nevertheless elongated during the propagation in the UEM column. The simulations show that the initial energy spread and the angular distribution from the photoemission process are the dominant factors in this temporal dispersion.

Utilizing our UEM, the structural dynamics including photo-induced phase transitions and coherent phonon excitation were studied in two transition metal dichalcogenides (TMDs), 1T-TaSe₂ and Td-WTe₂. 1T-TaSe₂ is a room temperature commensurate charge density wave (C-CDW) material. The C-CDW phase undergoes a phase transition to an incommensurate charge density wave (IC-CDW) at 473 K featured by a rotation of the superstructure. Under photoexcitation, the C-CDW in 1T-TaSe₂ can be suppressed within sub-ps time scale. A recovery time-constant of ~0.7 ps is observed for the commensurate periodic lattice distortion (PLD) at a pump fluence insufficient to drive a phase transition into the IC-CDW phase. At higher pump fluence, sufficient to drive nucleation of the IC-CDW phase, there is a ~1 ps delay between the extinction of the C-CDW phase and the onset for formation of the IC-CDW phase. Within the ~1 ps, a transient unreconstructed state may exist. The ~1 ps delay time for the nucleation of the IC-CDW phase implies that a phononic thermalization is involved in the decay of this highly perturbed photoinduced transient state. During the nucleation of the IC-CDW phase, a face-centered cubic (FCC) like stacking order is observed already at ~4 ps after photoexcitation. Such rapid stacking order formation indicates that the nucleation of the IC-CDW phase in the adjacent layers is not independent but coupled together. We can infer that the nucleation of the IC-CDW is inherently 3-dimensional (3D). The highly 3D feature of CDW in 1T-TaSe₂ indicates a strong interlayer interaction that establish long range out-of-plane stacking order.

Both in 1T-TaSe₂ and Td-WTe₂, a coherent shear phonon is observed by photoexcitation. In 1T-TaSe₂, the coherent shear mode is along the stacking direction of the C-CDW phase. We analyze the launching mechanism in terms of hot/cold spots on the Se-sublattice that result from the rapid melting of the PLD. During the melting, a difference in Se-phonon amplitudes results in shear forces between the layers. For a perfect trigonal stacking, the force will be compensated. However, there always remain uncompensated restoring forces in stacking-order direction because of the domain structure in out-of-plane direction. The excitation of a coherent shear phonon is even stronger in Td-WTe₂. The shear direction is along the b axis where there is a stacking displacement for the adjacent layers.

In Td-WTe₂, a photo-induced phase transition from orthorhombic Td to orthorhombic T* phase is observed which involves a stacking order change in the out-of-plane direction by a layer shear displacement along the b axis direction. Upon photoexcitation with pump fluence higher than a critical value, the change in interlayer potential results in the formation of a new metastable phase with a ~4 ps time constant. The shear displacement of the adjacent layers increases linearly with the increase of pump fluence and stabilize at ~ 8 pm when the pump fluence is higher than ~2 mJ/cm². The photo-induced phase transition in Td-WTe₂ can be influenced by local defect structures. In a ripple defect rich sample, a new phase transition from orthorhombic T* to monoclinic T’ phase will occur following the Td to T* phase transition. It can be inferred that strain fields in the sample can modulate the photo-induced phase stability. This effect has potential application in strain engineering of 2 dimensional TMDs.

The observed photo-induced phase transition and coherent shear phonon in 1T-TaSe₂ and Td-WTe₂, demonstrate the importance of inter-layer interaction in TMDs. 

urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-279167