Visualizing Moiré ferroelectricity by plasmons and nano-photocurrent in twisted WSe₂ structures

Ferroelectricity, with spontaneous electric polarization, in two dimensional (2D) materials enriches assembled van der Waals heterostructures with intriguing properties. Recently, the already wide spectrum of ferroelectric physics and applications has further broadened following the discovery of moiré ferroelectricity at the interfaces of stacked 2D materials. In these moiré ferroelectric materials, the polarization amplitude and its switching are expected to be governed by the local stacking configurations. In addition, it has been extensively predicated that such ferroelectric domains could influence both optical and optoelectronic responses. However, probing these properties remains challenging because the moiré scale is far below the optical diffraction limit. Here, we employ scattering-type scanning near-field optical microscopy (s-SNOM) and photocurrent nanoscopy to study the ferroelectricity in marginally twisted WSe₂ at the nanometer scale. The ferroelectric domains are visualized via the plasmonic excitation. Ferroelectric polarization direction and magnitude can be extracted from the plasmonic excitation spectra. Moreover, the photocurrent nanoscope demonstrates that the optoelectronic properties can be non-volatilely modulated by the ferroelectric domains. These findings offer a new paradigm for studying ferroelectricity and open up new prospects for optoelectronic devices.