Modulation of Polarization and Metallicity in Janus Sliding Ferroelectrics

Abstract

Sliding ferroelectricity is emerging as a distinct and promising mechanism for realizing ferroelectricity in low-dimensional systems, offering new design principles beyond the conventional ferroelectric mechanism. Further, the coexistence of the out-of-plane polarization with in-plane conductivity induced by electrostatic charge doping makes these systems strong candidates for realizing ferroelectric metals. Using density functional theory calculations, we analyze the transition metal dichalcogenides (TMDs) based Janus sliding ferroelectric bilayers XMY (M = Mo, W; X, Y = S, Se, Te; X $\neq$ Y). In addition to exhibiting switchable interlayer polarization, Janus sliding ferroelectrics possess an intrinsic electric field within each monolayer, arising from the electronegativity difference between the chalcogen atoms. We discover that the intrinsic electric field of the monolayers can be used to modulate the interlayer ferroelectric polarization and the electronic band structure. We identify the decrease in the interlayer distance due to a particular stacking of the Janus bilayers as a major contributor to increasing polarization and reducing the bandgap. The direction of the intrinsic electric field within the Janus monolayers plays a significant role in the modulation of layer-wise contribution in the valence and conduction bands, which influences the polarization reduction due to extrinsic charge dopants. Extending this concept to Janus trilayers, we observe further enhancement in polarization and additional bandgap reduction compared to their bilayer counterparts. These results highlight the tunability of TMD-based Janus sliding ferroelectrics and suggest a pathway for designing low bandgap ferroelectrics and potential ferroelectric metals.

Figures (7)

• Figure 1: Crystal structure of the bilayer Janus sliding ferroelectrics.(a) Top and side view of the AB-stacked XMY bilayers. The interlayer distance, $d$, and the interface separation, $t$, are represented by the red and blue colors in the side view, respectively. The black arrow depicts the direction of polarization. The region enclosed by dashed black lines represents the unit cell in the top view. The dashed black line in the side view allows for better visualization of the AB stacking. Here, $c$ is along the stacking direction ($z$-axis). The $a$ and $b$ are along the in-plane lattice vectors. (b) Different stacking of the intrinsic electric field, $E$, of the Janus monolayers, with AB-stacked parent bilayer shown for reference. Note that the direction of $E$ is from a lower electronegativity atom to a higher electronegativity atom.
• Figure 2: Polarization and interlayer charge density modulation in bilayer Janus sliding ferroelectrics.(a) Comparison of $P_{berry}$ (polarization via Berry phase method) for the Janus E-in (left) and E-out (right) bilayers. (b) $P_{berry}$ versus interlayer distance $d$ for XMoY bilayers. (c) Charge density difference profile for MoS$_2$ bilayer. The yellow and blue colors represent the electron and hole accumulation, respectively. The dashed red lines represent the vertical location of the atoms. Black arrows denote the parameters $l_h$ (difference of hole accumulation peaks) and $l_e$ (electron accumulation peak) of the charge density profile. The variation of (d) $l_h$ and (e)$l_e$ parameters of the charge density profiles for the XMoY bilayers.
• Figure 3: Polarization and interlayer charge density variation for fixed interlayer distance. Variation of polarization calculated from charge density, $P_{CD}$, (a) without relaxation, and (b) constrained relaxation of other structural parameters. (c) Charge density profiles for XMoS bilayers. The purple circle highlights the variation in electron accumulation in the regions. (d) Comparison of the parameter $l_e$ of the charge density profiles of XMoY bilayers.
• Figure 4: Band structure modulation in bilayer Janus sliding ferroelectrics.(a) Comparison of electronic bandgap, $E_{gap}$, for the Janus E-in (left) and E-out (right) bilayers. Layer-contribution-projected band structure of (b) TeMoS and (c) SMoTe bilayers. The blue and red circles represent the region around the valence band maximum (VBM) and the conduction band minimum (CBM), respectively. The blue and red colors represent the contribution of the bottom and top layers to the particular band, respectively. Here, Q is the midpoint between K and $\Gamma$. The zero of the energy scale is at the Fermi level. The illustrations on the top right (red color box) and on the bottom right (blue color box) are the partial charge density at the CBM and VBM, respectively. (d) VBM ratio and (e) CBM ratio for the Janus E-in (left) and E-out (right) bilayers.
• Figure 5: Modulation of doping-induced depolarization in bilayer Janus sliding ferroelectrics. Comparison of (a) Depol$_{hole}$ and (b) Depol$_{electron}$ for the Janus E-in and E-out bilayers. The Depol$_{hole}$ crossing 100% (red dotted line) in (a) for SeWTe suggests reversal of polarization with high doping.