Screening Vertically Polarized 2D Layered Materials with Giant Negative Longitudinal Piezoelectricity by Comprehensive Calculations and Experimental Characterizations

The counterintuitive negative longitudinal piezoelectric effect (NLPE) occurs in those piezoelectric compounds whose electric polarizations are enhanced under the compression stress(strain) applied along the polar axis. Especially, piezoelectric materials with sizable NLPE responses suitable for electromechanical applications are highly desirable. Here, based on comprehensive first-principles calculations and theoretical modeling, NLPE and substantial negative piezoelectric responses are demonstrated to be prevalently attained in various synthetic heteroanionic 2D van der Waals (vdW) layered materials with intrinsic out-of-plane polarizations. Depending on their interlayer spacing percentages and interlayer electronic coupling magnitudes, NLPE in most polar layered materials arise either from the domination of negative “internal-strain” over positive “clamped-ion” contributions, or negative “clamped-ion” over “internal-strain” contributions. As an exception, Sb₂TeSe₂ is predicted to have both negative “clamped-ion” and “internal-strain” terms in considerable magnitudes, contributing cooperatively to overall piezoelectric response and yielding a giant negative longitudinal piezoelectric coefficient d₃₃ up to –32.666 pC N⁻¹. Furthermore, piezoresponse force microscopy (PFM) measurements performed on Sb₂TeSe₂ crystal verify its non-centrosymmetric structural character and sizable piezoelectric responses. Combining the comprehensive simulations and experimental characterizations, this work deepens the understanding regarding the unusual NLPE, and demonstrates a practical route for screening the polar vdW layered materials with substantial NLPE.