PNNL

Abstract

Van der Waals ferroelectrics have captured significant interest for applications in novel memory and neuromorphic computing devices. Within these materials, interfaces between regions of different ferroelectric polarization (i.e. domain walls) exhibit distinctive behavior. Characterization of this behavior remains as challenge, as domain wall thickness resides at the nanoscale. Techniques commonly used to perform such characterization – particularly transmission electron microscopy (TEM) and atomic force microscopy (AFM) – achieve limited practical success at domain wall imaging. Atomically resolved TEM mapping involves invasive sample preparation that renders materials unusable, and electrical AFM methods are resolution limited by electrostatic interactions between tip and surface. We demonstrate tip-enhanced nonlinear spectroscopy as a powerful alternative for nanoscale imaging. Using tip-enhanced second harmonic generation (TESHG), we image domain walls in 2D ferroelectric a-In2Se3. With a narrow-band near-IR laser, we take advantage of high-wavelength, off-resonant signal enhancement to generate robust and reproducible TESHG. Through TESHG, we noninvasively identify changes in crystal lattice orientation with ~ 16 nm spatial resolution. Interestingly, we identify spectral features suggesting that non-local effects from domain walls persist nanometers into adjacent domains. Our results emphasize the need to further understand the properties of domain walls and their impact on the overall characteristics of ferroelectrics.