In operando cryo-STEM of pulse-induced charge density wave switching in TaS2.

The charge density wave material 1T-TaS2 exhibits a pulse-induced insulator-to-metal transition, which shows promise for next-generation electronics such as memristive memory and neuromorphic hardware. However, the rational design of TaS2 devices is hindered by a poor understanding of the switching mechanism, the pulse-induced phase, and the influence of material defects. Here, we operate a 2-terminal TaS2 device within a scanning transmission electron microscope at cryogenic temperature, and directly visualize the changing charge density wave structure with nanoscale spatial resolution and down to 300 µs temporal resolution. We show that the pulse-induced transition is driven by Joule heating, and that the pulse-induced state corresponds to the nearly commensurate and incommensurate charge density wave phases, depending on the applied voltage amplitude. With our in operando cryogenic electron microscopy experiments, we directly correlate the charge density wave structure with the device resistance, and show that dislocations significantly impact device performance. This work resolves fundamental questions of resistive switching in TaS2 devices, critical for engineering reliable and scalable TaS2 electronics.

Accuracy of Local Polarization Measurements by Scanning Transmission Electron Microscopy.

Accurately determining local polarization at atomic resolution can unveil the mechanisms by which static and dynamical behaviors of the polarization occur, including domain wall motion, defect interaction, and switching mechanisms, advancing us toward the better control of polarized states in materials. In this work, we explore the potential of atomic-resolution scanning transmission electron microscopy to measure the projected local polarization at the unit cell length scale. ZnO and PbMg1/3Nb2/3O3 are selected as case studies, to identify microscope parameters that can significantly affect the accuracy of the measured projected polarization vector. Different STEM imaging modalities are used to determine the location of the atomic columns, which, when combined with the Born effective charges, allows for the calculation of local polarization. Our results indicate that differentiated differential phase contrast (dDPC) imaging enhances the accuracy of measuring local polarization relative to other imaging modalities, such as annular bright-field or integrated-DPC imaging. For instance, under certain experimental conditions, the projected spontaneous polarization for ZnO can be calculated with 1.4% error from the theoretical value. Furthermore, we quantify the influence of sample thickness, probe defocus, and crystal mis-tilt on the relative errors of the calculated polarization.