Exploring Dynamical Heterogeneity in Layered 2D Materials Within an Operando Ultrafast Electrochemical Setup

The unique layer-stacking degree of freedom in 2D layered materials facilitates the formation of nearly degenerate and exotic phases of matter that can be easily switched using a variety of static and dynamic external stimuli, thus opening novel routes for the design of low-power reconfigurable devices. Leveraging the gaps formed between stacked layers, electrochemical ion intercalation has emerged as a promising approach to further stabilize metastable phases in these layered materials across the entirety of the bulk and explore the effects of extreme carrier doping and strain on the functionality of the system. However, the prevailing scarcity of opportunities to characterize the in-situ structural evolution of the material during electrochemical intercalation has limited the direct visualization of key transient physical processes that directly affect the material functionality. Here we introduce a novel experimental platform capable of performing electrochemical lithium-ion intercalation, while allowing multimodal ultrafast characterization of the lattice using, for example, ultrashort, femtosecond-long electron bunches and near-infrared light pulses. Centering the study on the layered semimetal WTe2 as a model system, we demonstrate that our unique experimental approach uncovers a previously unnoticed dynamically fluctuating polymorphic structure at room temperature, which can be subsequently homogenized by the introduction of electronic carriers accompanying the positively charged lithium ions. This leads to significant changes in the interlayer shear mode, including increases in the damping time and mode amplitude. We complement and contrast these findings with x-ray structural studies and theoretical calculations, and find that dynamic structural heterogeneity originates from the low energetic separation between metastable states and the presence of thermal fluctuations.