Temperature-driven disorder–order transitions in 2D copper-intercalated MoO₃ revealed using dynamic transmission electron microscopy

We demonstrate two different classes of disorder–order phase transitions in two-dimensional layered nanomaterial MoO₃ intercalated with ∼9–15 atomic percent zero-valent copper using conventional in situ electron diffraction and dynamic transmission electron microscopy. Heating to ∼325 °C on a time scale of minutes produces a superlattice consistent with the formation of a charge density wave stabilized by nanometer-scale ordering of the copper intercalant. Unlike conventional purely electronic charge-density-wave states which form, reform, and disappear on picosecond scales as the temperature is changed, once it forms the observed structure in Cu–MoO₃ is stable indefinitely over a very large temperature range (30 °C to the decomposition temperature of 450 °C). Nanosecond-scale heating to ∼380–400 °C produced a completely different structure, replacing the disordered as-fabricated Cu–MoO₃ with a much more crystallographically ordered metastable state that, according to a precession electron diffraction reconstruction, resembles the original MoO₃ lattice apart from an asymmetric distortion that appears to expand parts of the van der Waals gaps to accommodate the copper intercalant. Control experiments in Cu-free material exhibited neither transformation, thus it appears the copper is a necessary part of the phase dynamics. This work shows how the combination of high-density metal atom intercalation and heat treatment over a wide range of time scales can produce nanomaterials of high crystalline quality in unique structural states that cannot be accessed through other methods. (paper)