M.S. Colloquium: Modulation of Polarization and Metallicity in Janus Sliding Ferroelectrics

In recent years, sliding ferroelectricity has emerged as a promising mechanism for designing novel two-dimensional (2D) ferroelectric materials [1,2]. Due to its interfacial nature, the out-of-plane polarization can coexist with electrostatically doped in-plane conductivity [3,4], opening avenues for engineering ferroelectric metals. Sliding ferroelectricity has also been demonstrated in MoS2 trilayers [3,5,6] where the presence of multiple polarization states offers prospects for designing multistate ferroelectric devices [5]. More recently, first-principles calculations have predicted sliding ferroelectricity in Janus transition metal dichalcogenide (TMD) bilayers[7,8], further expanding the design landscape for 2D ferroelectric materials by leveraging the unique properties of Janus structures.

In this thesis, we investigated the rhombohedral-stacked bilayers of Janus TMDs (XMY; M = Mo, W; X, Y = S, Se, Te; X ≠ Y) which exhibit switchable out-of-plane polarization [9]. Our first-principles analysis reveals that the direction of the intrinsic electric field, E, within the Janus monolayers can significantly modulate the polarization and band structures of the bilayer sliding ferroelectrics. The polarization and the bandgap modulation are predominantly driven by the modulation of the interlayer distance, which is achieved by the redistribution of the accumulated interlayer charge via the E fields of the Janus monolayers. This highlights a novel approach for designing lower bandgap and higher polarization 2D ferroelectrics.

In addition to the bandgap, the E fields can also modulate the layer-wise contribution of the valence and conduction bands in the Janus sliding ferroelectrics. This results in the modulation of the depolarization (polarization reduction) caused by the extrinsic charge dopants and thus suggests a new mechanism to modulate the unique existence of metallicity with ferroelectric polarization in TMD-based sliding ferroelectrics. In addition, we analyzed the effect of the variation of the interlayer distance on the tuning of the properties of the TMD-based sliding ferroelectric bilayers, which suggests the possibility of designing novel ferroelectric metals using the design principles of Janus sliding ferroelectrics. In the end, we proposed a material design for a similar modulation of polarization and electronic bandgap in the trilayer Janus sliding ferroelectrics, expanding the playground for designing new sliding ferroelectrics.

Overall, our study provides comprehensive insights into tuning the ferroelectric and electronic properties of TMD-based sliding ferroelectrics and highlights Janus monolayers as promising building blocks for designing next-generation low-bandgap ferroelectrics and ferroelectric metals.