M. Goodman
Texas State University,
United States
Keywords: twisted hexagonal boron nitride, kelvin probe force microscopy, domain switching
Summary:
Twisted bilayers of hexagonal boron nitride (t-hBN) exhibit moiré patterns that give rise to ferroelectric-like states at the interface, characterized by alternating out-of-plane polarizations. This unique heterostructure has many distinct properties that make it an emerging material for many advanced electronic and optoelectronic applications. Unlike conventional ferroelectrics, "moiré ferroelectrics" display unique switching characteristics arising from a relative sliding or shifting between layers that can be controlled with an external electric field. Herein we image the interfacial domain switching dynamics by applying a local electric field using a biased atomic force microscopy (AFM) tip and observe the dynamic evolution of the domain network using Kelvin probe force microscopy (KPFM) in subsequent scans. The t-hBN heterostructure samples were prepared with varying twist angles using mechanical exfoliation from bulk crystals and then transferred onto conductive Pt substrates, a requirement for KPFM measurements. By varying the twist angles, the domain supercell size and shape can be tuned to influence the extracted potential modulation measured at the top surface of the t-hBN layers. The KPFM measurements were performed using a two-pass frequency-modulation KPFM (FM-KPFM) mode using a conductive cantilever probe kept at about 2 nm above the sample. Polarization switching was accomplished by biasing an AFM tip in electrostatic force microscopy (EFM) mode and raster scanning across the sample. The polarization switching is evident from changes in the surface potential captured subsequently with KPFM. Our results exhibit this switchable polarization behavior, and we observed non-local switching of the domain network that extended over large areas when an external electric field was applied. Specifically, the external electric field was applied to an area of the moiré pattern significantly smaller than the larger moiré pattern, often only a few supercells across and had a non-local effect across the large moiré pattern. In many cases, rather than inducing a switch in the polarization states, a non-local charge accumulation effect occurred that screened KPFM imaging. This charge accumulation effect slowly dissipated over time for our grounded sample but was observed to saturate with increasing voltage before a sample breakdown voltage occurred. Other imaging techniques that were used with varying success to support the findings include piezoresponse force microscopy (PFM), tortional force microscopy (TFM), and a conductive AFM (c-AFM) point by point method. These experimental findings provide some insight into the fundamental mechanisms governing t-hBN polarization switching behavior which appears to be non-local phenomena. It has been theorized that proximity coupling from adjacent layers with a moiré pattern can periodically modulate electronic structures such as graphene, so the ability to tune whole heterostructures with a local applied voltage could have profound implications. Furthermore, investigations were conducted analyzing the effect of the thickness of the top and bottom hBN layers and the effect of the substrate on the measured KPFM signal.