Strain-tunable robust ferroelectricity in two-dimensional monochalcogenide heterostructures

Low-dimensional layered ferroelectric van der Waals heterostructures (vdWHs) are of tremendous interest for novel nanoscale electronic applications, including nonvolatile memories, transistors, and sensors. Here, we design SnS/GeSe multilayer vdWHs and show that the construct is ferroelectric with a spontaneous polarization of 3.71 × 10^-10 Cm⁻¹. We computationally analyze SnS and GeSe monolayers (MLs) and SnS/GeSe heterostructures using density functional theory (DFT) with van der Waals (vdW) correlations. Specifically, we determine structural parameters, formation energies, polarization, and phonon modes. Negative formation energies with real frequencies of phonon spectra confirm the (relative) stabilities of SnS and GeSe monolayers (MLs) and SnS/GeSe heterostructures. The electronic properties of both monolayers are retained in the SnS/GeSe construct, which exhibits a direct band gap with a value of 1.13 eV. The calculations of phonon spectra and double-well potential of monolayers unveil a Pnm21 ferroelectric – Pnmm paraelectric phase transformation. The SnS/GeSe multilayer shows the largest spontaneous polarization of 3.71 × 10^-10 Cm⁻¹ compared to 2.78 × 10^-10 Cm⁻¹ and 3.69 × 10^-10 Cm⁻¹ for SnS- and GeSe-MLs, respectively. Additionally, we demonstrate that the spontaneous polarization, band gap, dynamic stability, and band gap types of SnS/GeSe heterostructures can be tuned through the application of biaxial strains. For an in-plane 5 % tensile strain, SnS/GeSe has a spontaneous polarization of 3.90 × 10^-10 Cm⁻¹. The band gap of the SnS/GeSe heterostructure widens with increasing tensile biaxial strain, maintaining its characteristics as a direct band gap semiconductor up to 2 % tensile strain. These findings demonstrate that SnS/GeSe heterostructures are promising materials with significant potential for applications in nanoelectronics.