Strain-dependent near-zero and negative Poisson ratios in a two-dimensional <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>CuI</mml:mi><mml:mo>)</mml:mo><mml:mi mathvariant="normal">P</mml:mi></mml:mrow><mml:mn>4</mml:mn></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:math> monolayer

Abstract

The unique mechanical properties of auxetic materials are widely used in specialized fields, such as medicine, defense security, and aerospace. Here, a potential two-dimensional (2D) auxetic (CuI)${\mathrm{P}}_{4}{\mathrm{Se}}_{4}$ material is predicted by first-principles calculations. The 2D (CuI)${\mathrm{P}}_{4}{\mathrm{Se}}_{4}$ monolayer is a quasidirect-band-gap semiconductor with a relatively large band gap of 2.66 eV, which can be regulated by applying uniaxial strain. In particular, the unique coordinate structure of polymeric ${\mathrm{P}}_{4}{\mathrm{Se}}_{4}$ strands and ${\mathrm{Cu}}_{2}{\mathrm{I}}_{2}$ units endows the (CuI)${\mathrm{P}}_{4}{\mathrm{Se}}_{4}$ monolayer with fascinating mechanical flexibility and anisotropy. The calculated Young's modulus of 8.62 (41.81) N/m and ideal fracture strength 0.66 (1.35) N/m in the $x$ ($y$) direction are one to two orders of magnitude smaller than those of other previously reported 2D materials. More remarkably, the (CuI)${\mathrm{P}}_{4}{\mathrm{Se}}_{4}$ monolayer exhibits the strain-dependent near-zero Poisson's ratio (ZPR) and negative Poisson's ratio (NPR) behaviors. Both the in-plane and out-of-plane Poisson's ratios can translate from positive value to negative value under suitable in-plane uniaxial strain. Our calculations show that the in-plane NPR characteristic is mainly controlled by the distances and angles of the neighbor Cu atoms and the out-of-plane NPR behavior is mainly dominated by the rotation of ${\mathrm{Cu}}_{2}{\mathrm{I}}_{2}$ units. In addition, the electronic and geometric response analyses further determine that the difference of strain-response capabilities between ${\mathrm{P}}_{4}{\mathrm{Se}}_{4}$ strands and ${\mathrm{Cu}}_{2}{\mathrm{I}}_{2}$ units should be responsible for the strain-dependent mechanical properties. Our results reveal a 2D intrinsic near ZPR and NPR material (CuI)${\mathrm{P}}_{4}{\mathrm{Se}}_{4}$ and explore the origin of its strain-dependent mechanical behaviors.

Topics & Concepts