Room-Temperature Ferroelectricity in $1{\mathrm{T}}^{\prime }$-${\mathrm{ReS}}_2$ Multilayers

van der Waals materials possess an innate layer degree of freedom and thus are excellent candidates for exploring emergent two-dimensional ferroelectricity induced by interlayer translation. However, despite being theoretically predicted, experimental realization of this type of ferroelectricity is scarce at the current stage. Here, we demonstrate robust sliding ferroelectricity in semiconducting $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$ multilayers via a combined study of theory and experiment. Room-temperature vertical ferroelectricity is observed in two-dimensional $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$ with layer number $N\ge 2$. The electric polarization stems from the uncompensated charge transfer between layers and can be switched by interlayer sliding. For bilayer $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{ReS}}_2$, the ferroelectric transition temperature is estimated to be $\sim 405\mathrm{K}$ from the second harmonic generation measurements. Our results highlight the importance of interlayer engineering in the realization of atomic-scale ferroelectricity.

Figure 1

(a) Top and side views for the crystal structures of monolayer ${\mathrm{ReS}}_2$. (b) Top and side views of the two energy-degenerate ferroelectric structures of bilayer ${\mathrm{ReS}}_2$ (states A and $A^{\prime }$) and the high-symmetry nonpolar structure (state B). The arrows indicate the polarization direction. (c) The energy contour plot of bilayer ${\mathrm{ReS}}_2$ versus the sliding distance $(l_a,l_b)$. The contour colors illustrate the total energy of the unit cell relative to the energy of ferroelectric states (red dots). (d) The energy pathway of ferroelectric transition. (e) The charge density difference of bilayer ${\mathrm{ReS}}_2$ in state A. The yellow and blue colors represent the positive and negative values, respectively.

Figure 2

(a),(b) Ferroelectric switching of ${\mathrm{ReS}}_2$ in its trilayer (a) and tetralayer (b) form. (c) Layer dependence of polarization values and transition barriers.

Figure 3

(a) Optical image of ${\mathrm{ReS}}_2$ mechanically transferred onto $\mathrm{Si}/{\mathrm{SiO}}_2$. Inset: the synthesized ${\mathrm{ReS}}_2$ bulks. (b) XRD pattern of ${\mathrm{ReS}}_2$ bulk crystals. (c),(d) TEM images of ${\mathrm{ReS}}_2$. Inset in (d): corresponding SAED pattern, showing only one set of diffraction spots. (e) Excitation angle dependence of the Raman spectra for ${\mathrm{ReS}}_2$. (f) Polar plots of three representative Raman features. The dashed lines indicate the orientation of the principal axis for each mode.

Figure 4

(a) Optical image of a ${\mathrm{ReS}}_2$ flake transferred onto Au-coated quartz substrate. The sample surface is atomically flat with occasional terraces from the cleaving process. (b),(c) AFM images corresponding to the region marked by red and green dashed squares shown in (a), respectively. (d),(e) PFM phase hysteresis (d) and butterfly-shaped PFM amplitude-voltage loops (e) for 1L–5L ${\mathrm{ReS}}_2$. (f),(g) PFM phase (f) and amplitude (g) images for 4L ${\mathrm{ReS}}_2$ on graphene, with a written box-in-box pattern acquired by opposite dc bias. (h),(i) Cross-sectional profiles of the PFM phase (h) and amplitude (i) signals along the red dotted lines as shown in (f) and (g).

Figure 5

(a) Temperature dependence of the SHG intensity of 2L ${\mathrm{ReS}}_2$. (b) Schematic illustration of an FTJ based on 2L ${\mathrm{ReS}}_2$ and 1L graphene. (c) Optical image of the ${\mathrm{ReS}}_2$-based FTJ. (d),(e) $I\text{−}V$ characteristics of the FTJ device, exhibiting tunneling electroresistance between the ON and OFF states, set by $\pm 4\mathrm{V}$ poling voltage. (f) Resistance as a function of the poling voltage with reading pulses of 0.1 V, measured by applying the voltage pulse train. Inset: the voltage pulse train, composed of writing pulses following a triangular profile between $\pm 4\mathrm{V}$.