In Situ Imaging of Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe₂

Phase transformations in 2D materials have been an emerging area of research for potential applications as monolithic 2D electronics and phase-change memories [1-2]. In particular, molybdenum ditelluride (MoTe₂) exhibits multiple structural phases which are close in energy. As a result, MoTe₂ can be easily switched between its 2H, 1T', and T_d[3] phases through the application of electric fields [4], strain [5], heat [6], and laser irradiation [7]. Here, we use graphene (Gr) encapsulation to create an enclosed, in-situ reaction cell for transmission electron microscopy (TEM) studies of MoTe₂ on a MEMS based heating holder. The graphene cells prevent MoTe₂ from evaporation induced by annealing or electron beam irradiation [8] while being highly electron transparent, allowing atomic resolution studies of MoTe₂ down to a single unit cell thick. We utilized these cells to image the phase transitions of few-layer MoTe₂ from the micro- to atomic scales using a combination of dark-field transmission electron microscopy (DFTEM) and aberration-corrected scanning transmission electron microscopy (STEM). We find that the propagation of the phase transition can be highly anisotropic: below a threshold temperature, we observe that T_d-to-2H phase fronts travel primarily along the b-axis of the T_d phase, producing a layer-by-layer phase transformation. These results indicate new potential methods to direct the phase transition of 2D materials at the nanoscale.<br/>To investigate the phase-change mechanisms of MoTe₂, we first fabricate a stack of hBN/Gr/MoTe₂/Gr/hBN by encapsulating a few-layer exfoliated MoTe₂ flake between two layers of graphene and hBN. The encapsulated MoTe₂ was then irradiated by a laser to locally convert part of the 2H phase into a metastable T_d phase. The thick hBN layers were subsequently removed by XeF₂ dry etching, leaving the Gr-encapsulated MoTe₂. We then transfer the phase-engineered, Gr-encapsulated MoTe₂ stacks onto MEMS chips for in-situ heating. Aberration-corrected STEM was used to visualize the crystal and interface structures before and after heating. In order to capture the dynamics of T_d-to-2H phase transition we apply heat pulses as short as 0.5s to "freeze" the growth front of 2H phase and enable detailed studies of intermediate structures. By selecting diffracted beams from the 2H phase, we collected dark-field TEM images that highlight the 2H phase region after each heat pulse. We observe the T_d-to-2H phase transition at temperatures as low as 275 °C, where it initiates at 2H-T_d interfaces. Our images clearly indicate that the phase transition occurs anisotropically: 2H regions propagate layer-by-layer, within the basal plane of each MoTe₂ layer. At low temperatures, the phase propagation is predominantly along the b-axis of the T_d phase and is stopped at rotational T_d grain boundaries. We individually measure and compare the phase front of each separate layer and find that the linear growth rate of the 2H phase at 275 °C varies from 5 to 14 nm/sec between different layers. The T_d-to-2H phase transition at low temperatures is best described by a kinetic control model, i.e., the growth is intermittent, with some pulses producing rapid growth while others do not produce any observable changes. In summary, we have measured the kinetics and unveiled the anisotropic nature of T_d-to-2H phase transition of few-layer MoTe₂ using in-situ pulsed heating and DFTEM.<br/><b>Reference:</b><br/>1. X. Zhu et al., Nat. Mater. 18 (2019), p. 141-148<br/>2. X. Zhang et al., Nat. 566 (2019), p. 368-372<br/>3. R. Sankar et al., Chem. Mater. 29 (2017), p. 699-707<br/>4. F. Zhang et al., Nat. Mater. 18 (2019), p. 55–61<br/>5. S. Song et al., Nano Lett. 16 (2015), p. 188-193<br/>6. D. H. Keum et al., Nat. Phys. 11 (2015), p. 482-486<br/>7. S. Cho et al., Sci. 349 (2015), p. 625-628<br/>8. H. Ryu et al., Adv. Funct. Mater. (2021), p. 2107376