Impact of Stacking Sequence on Multi-Layer Ferroelectrics—A Combined AFM and TEM Analysis

Multi-Layer Ferroelectrics have garnered significant interest as rhombohedral stacked two-dimensional ferroelectrics exhibit unique sliding ferroelectricity due to lateral shifts between layers, generating out-of-plane electric polarization, termed ‘slide-tronics’. Despite efforts to develop 2D ferroelectric devices, current strategies predominantly rely on indirect methods to correlate stacking sequence with ferroelectric properties, with limited experimental evidence directly linking stacking sequence to polarization and interlayer interactions. The comprehensive understanding of ferroelectricity in multi-layered materials remains incomplete, particularly regarding its relation to cyclic and acyclic stacking. Furthermore, while the use of polarized pulses for ultrafast ferroelectric switching is a growing field, experimental data on interlayer shear modes across different stacking sequences are still sparse.<br/>In this study, we performed an in-depth combined analysis using Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), and Raman Spectroscopy on the same tungsten diselenide flakes, yielding comprehensive characterization through these cohesive techniques. This approach elucidates the correlations between stacking sequence, polarization, and interlayer shear mode, essential for applications in multi-layer ferroelectrics. First, using a 4D-STEM framework in TEM, we identified stacking sequences by obtaining diffraction patterns for each position and decomposing the diffraction stack into the overall intensity and diffraction polarity of the first-order peak. Next, we measured surface potential variations using Kelvin Probe Force Microscopy (KPFM), compiling a database to investigate differences in polarization among anti-parallel, cyclic, and acyclic stacks. Lastly, low-frequency Raman Spectroscopy was used to detail how the interlayer shear mode evolves with accumulating layers.<br/>We explored a variety of stacking scenarios in tungsten diselenide, demonstrating how cyclic, acyclic, and anti-parallel stacking induce property changes in multi-layer situations when these various stacking configurations are mixed. We have identified a trend in KPFM where surface potential increases or decreases based on cyclic and acyclic stacking. Building on this, the study progressed to complex cases where parallel and antiparallel stacking are mixed, investigating the impact of hexagonal stacking by focusing on surface potential changes in cyclic and acyclic stacking that occur after hexagonal stacking. Additionally, low-frequency Raman spectroscopy results indicated that the observed peaks are characteristic of each stacking sequence rather than the number of layers, providing insights into how the stacking sequence induces interlayer shear modes.