STEM Imaging of AC Electric Field Driven Non-Centrosymmetric Atomic Displacements in The 2D TMD WSe₂

Simultaneous characterization by (S)TEM and application of electrical bias is a powerful platform to study the real-space dynamic electronic couplings at nanoscales: the length scale of interest for many electronic, quantum, and electrochemical materials and processes. Minimum acquisition times across all STEM modes favor in situ electrical bias experiments that are quasi-static for a good signal/noise. Exploring dynamic responses at faster time scales necessitates multi-cycle summation to overcome the shot-noise limits for a coherent source. In this work we utilize this approach to characterize moderate frequency (25kHz) AC electric-field induced response on the atomic structure of a 2D semiconductor WSe₂.<br/><br/>As tunable 2D direct-gap semiconductors, WSe₂ and similar Transition Metal Dichalcogenides show promise for applications in nano- and opto-electronics. Their potential lies in part with a wide library of 2D materials, TMD and otherwise, and a correspondingly large design-space for heterogenous multi-layer engineering. Here we applied an in-plane electric field across a free-standing monolayer WSe₂ by biasing across a gap in an overlaid graphene layer. Rolling series short-dwell HAADF images were collected with a custom scan coil controller¹ while an AC 25kHZ sinusoidal bias was applied. Atomic structure fluctuations vs applied voltage were determined form a workflow of phase-detection -&gt; averaging -&gt; binning -&gt; and atom position finding using a machine-learning model <i>AtomAI</i>². Collective atomic motion was removed, observed to be ~0.5 Å/V, owing to beam deflection by long range stray-fields. We observe a small but robust centrosymmetry breaking counter-motion between the W and Se atoms. This displacement has a total delta of ~0.5pm/V up to 4 volts (maximum applied) and can be attributed to field-induced electrical polarization. Notably, this effect was only observed for datasets collected within the electrode gap and not within the graphene electrode itself. The nominal field concentration within the former and absent in the latter strongly supports this is direct real-space observation of local field induced dynamics, specifically a fixed-frequency optical-phonon type dielectric response.<br/><br/>[1] Sang, X. et al. <i>Dynamic scan control in STEM: spiral scans</i>. Adv. Struct. Chem. Imag. 2, <b>6</b> (2016).<br/>[2] Ziatdinov, M., et al. <i>AtomAI: A Deep Learning Framework for Analysis of Image and Spectroscopy Data in (Scanning) Transmission Electron Microscopy and Beyond</i>. arXiv:2105.07485 (2021)<br/>[3] This work was supported by the U.S Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division