Atomic-Resolution Real-Space Tracking of Structural Phase Transformations in 2D Quantum Materials

The distinct thickness- and stacking-dependent properties of two-dimensional (2D) materials provide a unique handle for tuning quantum effects through layer control or heterostructuring [1-3]. Accordingly, there is a need to understand the implications of concomitant structural features, such as van der Waals stacking order, on novel physics at the temperatures where they occur. While atomic-resolution scanning transmission electron microscopy (STEM) has become almost routine for radiation-hard inorganic materials, thermal drift poses substantial challenges when imaging at low temperatures where unique phases frequently emerge. We use a continuously variable temperature (CVT) cryogenic-STEM system that couples liquid nitrogen cooling with fast, local heating to investigate van der Waals stacking transformations 2D Nb₃Br₈ with atomic resolution at temperatures between ~175-640 K.<br/>Here, niobium halides of form Nb₃X₈ are a model 2D material as they exhibit a low-temperature magnetic transition attributed to a structural transformation from a 2-layer (α-phase) to a 6-layer (β-phase) unit cell [4,5]. We studied the structure of exfoliated Nb₃Br₈ with atomic resolution while cycling the temperature through the full structural transition (~175 K to ~640 K to ~175 K). This revealed several significant insights into the nature of the structural phase transition, including an unexpected gradual transition loop through numerous distinct and metastable stacking sequences with transition phases of varying degrees of order. Surprisingly, the full structural transition exhibits a wide hysteresis (&gt;400 K) with a strong thickness dependence. We have shown CVT cryo-STEM to be an effective local probe for revealing novel structural phenomena in 2D layered materials within phase transitions. This imaging tool holds promise for understanding the complex stacking orders influencing low-temperature topological phases in 2D materials such as MoTe₂. [6]<br/> <br/>References<br/>[1] K.S. Novoselov, <i>et al</i>., Science <b>353</b> (2016), p. 461.<br/>[2] K.F. Mak, <i>et al.</i>, Science <b>344</b> (2015), p. 1489.<br/>[3] B. Huang, <i>et al.</i>, Nature <b>546</b> (2017), p.270.<br/>[4] J.P. Sheckelton, <i>et al.</i>, Inorg. Chem. Front. <b>4</b> (2017), p. 481.<br/>[5] C. Pasco, <i>et al.,</i> ACS Nano <b>13</b> (2019), p. 9457.<br/>[6] This work is supported by PARADIM, an NSF-MIP (DMR-1539918), and NSF DMR 1429155 & DMR-1719875.