Single-Layer MX₂ (M = Zn, Cd and X = Cl, I): Auxetic Semiconductors with Strain-Tunable Optoelectronic Properties

Abstract

The emergence of novel two-dimensional (2D) materials with multifunctional properties provides new possibilities for nanoscale applications. Here, based on first-principles calculations, we identify a class of 2D auxetic materials with a special structural buckling, namely, MX2 monolayers (M = Zn, Cd and X = Cl, I). The calculated results indicate that four monolayers all exhibit an abnormal negative Poisson’s ratio (NPR) with a maximum value of −0.24, which is fairly rare in 2D transition metal halides (TMHals). The origin of NPR can be explained by the unique local corner-sharing tetrahedral structural motif of MX2 monolayers under the low-dimensional effect. More importantly, ZnCl2 monolayer can be exfoliated from its bulk counterpart with an ultralow exfoliation energy of 8.37 meV Å–2 (0.1341 J m–2), suggesting the feasibility of its experimental preparation. In addition, strain engineering studies on MX2 monolayers show effective band gap modulation, indirect-to-direct band gap transitions, and significant enhancement of light absorption in the visible and near-ultraviolet regions. Our findings enrich the large family of versatile 2D TMHals and highlight the promising applications of MX2 monolayers in nanomechanical and optoelectronic devices.

Multifunctional PbS2 Monolayer with an In-Plane Negative Poisson Ratio and Photocatalytic Water Splitting Properties

“…However, the low-dimensional effect impels the PbS 2 monolayer to construct its lattice along the 2-fold axis, which causes the NPR behavior of the PbS 2 monolayer (see Figure c). The similar NPR behavior of this kind of tetrahedral framework is found in our prior reports of WN 4 and transition metal halide monolayers, as well as other square lattice 2D materials. ,, …”

“…Nanoscale materials with excellent electronic and mechanical properties are ideal candidates for engineering applications. Recently, negative Poisson ratio (NPR) materials have drawn an enormous amount of attention due to their unconventional mechanical behavior, which is expanded perpendicularly to tensile strain under tensile deformations. − They are also called auxetic materials. Materials with a NPR are proven to have desirable resistance to shear fracture and vibration absorption. , The NPR materials possess broad applications in practice, in sensors, piezo electronics, vanes for aircraft, national defense, and biomedicine. ,, Moreover, the 2D NPR materials have been designed for the high-resolution sequencing of DNA molecules .…”

“…The similar NPR behavior of this kind of tetrahedral framework is found in our prior reports of WN 4 and transition metal halide monolayers, as well as other square lattice 2D materials. 17,18,36 The electronic density of states (DOS) and band structure of the PbS 2 monolayer are calculated by using both PBE and HSE06 methods. The results are shown in Figure 3 and Figure S4.…”

Two-dimensional AlXY (X = S, Se, and Y = Cl, Br, I) monolayers: promising photocatalysts for water splitting with high-anisotropic carrier mobilities

“…It is known that strain engineering is one of the most efficacious methods to tune the electronic band structure. 51,52 This method has also been validated by its successful application in regulating the electronic structure to satisfy the photocatalytic water splitting of many 2D materials. 10,22,28,50,53 Thus, we further studied the band edge positions of the AlXY monolayers by utilizing strain engineering to explore whether they can meet the photocatalytic water splitting at higher pH conditions.…”

MgXN2 (X = Hf/Zr) Monolayers: Auxetic Semiconductor with Highly Anisotropic Optical/Mechanical Properties and Carrier Mobility

“…Notably, an abnormal in-plane NPR emerges in the MgHfN 2 (MgZrN 2 ) monolayer around the diagonal direction with a relatively broad range (14.5∼75.5° for MgHfN 2 , 16.7∼73.3° for MgZrN 2 ), reaching a maximum negative value of −0.21 at 40∼41°, which suggests the MgXN 2 monolayer is an in-plane auxetic material. Note that this in-plane NPR value (in magnitude) is larger than those of pentagraphene (−0.068), 2D δ-silica (−0.11), and 2D Ag 2 S (−0.12) while comparable to that of 2D ZnI 2 (−0.24) …”